Dr Benedikt Lassalle-Kaiser

Ms Ms Anna Chen Mroszczak

Mr Xu

Labeesan

Dr Ms Chanjuan Zhang

Zakaria Halime

Dr Gary Moore

Dr Edgar Reyes Cruz

Dr Silvia Duran

Dr Cédric Tard Took

Dr Jean-Blaise Brubach

Ms Chen Xu

Dr Frédéric Kanoufi

Dr Yuanyuan Liao

Dr Stephanie Blanchandin

Ms Karine Chaouchi

Keywords: Renewable energy, CO2 reduction, heterogeneous catalysis, molecular macrocycles CO 2 reduction reaction, Electrocatalysis, X-ray spectroscopy, In situ/ operando techniques

Titre : Formation de multiples liaisons C -C au cours de la réduction électrocatalytique du CO2 par des macrocycles moléculaires de métaux de transition Mots clés : Énergie renouvelable, réduction du CO2, catalyse hétérogène, macrocycles moléculaires Résumé : Les macrocycles moléculaires possèdent des sites actifs bien définis et sont des électrocatalyseurs prometteurs pour la réduction du dioxyde de carbone en produits chimiques à valeur ajoutée. Jusqu'à présent, la plupart de ces catalyseurs ne peuvent produire que des produits de type C1. Nous avons étudié l'utilisation de la phtalocyanine de fer, une molécule disponible commercialement à base d'éléments abondants, pour générer des hydrocarbures légers lors de la réduction électrocatalytique du CO2 dans des conditions aqueuses et à pH neutre. Une cellule spectroélectrochimique à écoulement imprimée en 3D a été développée en interne et connectée à un système de caractérisation des gaz en ligne. Lorsqu'un potentiel électrochimique est appliqué, des hydrocarbures linéaires saturés et insaturés de type C1 à C4 sont détectés. Des expériences de marquage isotopique ont montré sans ambiguïté que ces produits sont issus du CO2. Des expériences de contrôle et une analyse par spectroscopie des rayons-X en conditions operando ont démontré l'intégrité structurelle du catalyseur moléculaire pendant la catalyse et sa responsabilité dans la réaction. Une espèce terminale de type carbène terminal est proposée comme intermédiaire clé pour l'étape de formation de la liaison C -C sur la base d'expériences avec des substrats alternatifs. Nous avons ensuite exploré les paramètres d'électrocatalyse influençant les performances de la phtalocyanine de fer, notamment le débit et les concentrations de catalyseur, d'électrolyte et de support conducteur. Enfin, une série de phthalocyanines de différents métaux de transition, ainsi que de phthalocyanines de fer substituées et de porphyrines de fer ont été criblés pour leur capacité à former des produits avec des liaisons C -C ors de la réduction du CO2.

Nous avons optimisé les paramètres d'électrolyse en faisant varier le débit et les concentrations d'électrolyte, de catalyseur et de support carboné pour augmenter la sélectivité des produits C2+. Bien que les résultats montrent des améliorations modestes, aucune percée significative n'est obtenue. Cependant, la corrélation entre la production élevée d'hydrogène et la production élevée d'hydrocarbures est observée. Enfin, nous étudions la formation de liaisons C -C à partir du CO2 de macrocycles métalliques apparentés : les phtalocyanines métalliques, la phtalocyanine de fer substituée par différents groupes fonctionnels, et les porphyrines de fer et leurs dérivés. Nous avons constaté que le centre métallique joue un rôle essentiel dans la production des hydrocarbures, alors que les effets des ligands ne sont pas évidents.

Nous concluons la thèse en fournissant des perspectives futures sur les travaux et des commentaires généraux sur l'état du processus de transition énergétique pour faire face au changement climatique.

LIST OF ABBREVIATIONS AND ACRONYMS

ACKNOWLEDGEMENT

First and foremost, I would like to express my heartfelt gratitude to my supervisor, Dr. Benedikt Lassalle-Kaiser. He did not only guide me through my scientific journey but also shared with me invaluable experience and lessons that I will hold dear in the future. He always believed in me, even when I had doubts about myself. Unlike many PhD students having a team of supervisors, he carried the responsibility of guiding a naïve early-career researcher all by himself. His job was made order-of-magnitude more challenging since the world was going through a once-in-a-lifetime pandemic at the beginning of my PhD, and yet he exceeded all my expectations. He went out of his way to ensure all my scientific and everyday needs were met, from sending me to the best conferences in the field to taking care of me when I caught Covid. I consider him my second family in France, someone I know I can count on in all circumstances. In the future, I will undoubtedly miss discussing politics, culture, and every other topic with him during our daily lunch. I could not have asked for a better person to guide me through this journey.

I have the distinct honor and privilege to have a group of excellent scientists on my jury: Dr. Jennifer Peron, Pr. Cyrille Costentin, Pr. Pedro de Oliveira, Pr. Elena Savinova, Dr. Audrey Bonduelle, and Dr. Stafford Sheehan. I sincerely thank them for taking the time to evaluate my work and attend my defense. The discussion we had was lively and insightful, and the questions and comments they posed significantly improved the quality of my thesis as well as opened new directions for future work.

I would like to express my sincere gratitude to all the collaborators I have worked with during my PhD. I thank Pr. Gary Moore and Dr. Edgar Reyes Cruz for providing the iron porphyrin samples and the exciting discussion that followed afterward. I thank Ms. Chanjuan Zhang and Dr. Zakaria Halime for helping me measure NMR samples in their laboratory. I am grateful to Dr. Jean-Blaise Brubach for his help in characterizing my samples using infrared spectroscopy. I thank Dr. Frédéric Kanoufi for the valuable advice regarding finite element analysis. I am grateful for the considerable support I received from the members of LUCIA and SAMBA beamlines, the microfluidics lab, and the chemistry lab in Synchrotron SOLEIL.

I want to thank all the colleagues and friends I have worked with during my time as a PhD student. To Dr. Daniela Mendoza, you were my mentor at the beginning of my PhD, and you are still a role-model researcher to me. Your pure love for science and chemistry is admirable. Our rants about all the stress during our PhDs indeed helped calm me down from time to time. To Dr. Yuanyuan Liao, I am thankful for your scientific advice, and I value all the experiences you shared with me. To Anna, Chen, and Labeesan, it was my great pleasure to work with you, I learned a lot from you, and your hard work has contributed significantly to my thesis. I appreciate all the members of the young researchers' group in SOLEIL for the opportunity to share our scientific ix journey, as well as countless other activities.

I want to thank Dr. Cédric Tard, who was keeping track of my PhD as a member of my following committee. I enjoyed every discussion I had with him, and he always offered excellent guidance to improve the science I was doing. His advice about a scientific career certainly helped guide me through my scientific journey, which is undoubtedly less arduous because of him. I am also grateful to him and Dr. Silvia Duran for helping me with experiments in their laboratory.

I am grateful to Prof. Ally Aukauloo, Prof. Winfried Leibl, and Dr. Zakaria Halime, for allowing me the chance to participate in their groups' weekly bibliography meetings, especially Prof. Winfried Leibl for driving me to the meeting from time to time. The discussions certainly enhanced my scientific knowledge and transferable skills and better me as an early-career researcher. I also valued all the contributions made by the participants of the meetings.

I want to thank Dr. Jean Daillant, the director of SOLEIL, for allowing me the opportunity to do my PhD in Synchrotron SOLEIL. In addition, I would like to thank all the administrative employees at SOLEIL for their professionalism, which made my stay in France as a foreigner a much more welcoming experience.

I could not be the person I am today without my girlfriend, Mai Vuong. It was a unique experience for her and me to do our PhDs together, and we even managed to publish a paper together (mostly her work). She helped me scientifically, physically, and emotionally. She is there for me when I am unsure whether I can make it. She showed me my good side when I felt overwhelmed with negativity. I cherish every moment I was with her, from all of our vacation trips to all the times we complained about how hard it was to do a PhD. A scientist usually avoids using absolute statements, but I am undoubtedly sure that I am a better person because of her. I cannot overstate how lucky I am to be with her, and I look forward to our future together.

Finally, I dedicated everything I have achieved to my family. I always appreciate them, but being away in a foreign country makes me truly realize how much they mean to me. Back in Vietnam, I sometimes got annoyed by my mother caring a bit too much for me. And yet when I am not with her, I always look forward to answering her questions about the weather in France, whether I have enough sleep, or what I eat every day. Her constant updates about what is going on in our family make me feel like I am home despite being thousands of kilometers away. My brother always cheers me with silly memes in our group chat, and yet he goes out of his way to help whenever I need something from him. One of my greatest regrets is not being there to welcome his beloved daughter, my niece, into this world. My father, who I don't talk to very often, but I know how much he cares about me, and I can still feel his support deep down. I miss you all, and I cannot wait to see you again in person.

Chapter 1 1 Energy sources have been pivotal in all eras throughout human history. The discovery of fire transformed humanity and made biomass the world's primary energy source until the Industrial Revolution in the 18 th century. Since the Revolution, the world has relied heavily on fossil fuels to satisfy its energy demand. As shown in Figure 1-1, in 2021, the combination of oil, coal, and gas accounts for more than 80% of the world's total energy consumption. Population boom and rising standard of living increase energy demand significantly (Figure 1-1). The rising trend is expected to continue, and total energy demand is projected to increase by 50% by 2050, with fossil fuels remaining the largest share of the energy mix. 1 Figure 1-1. Energy consumption by sources, measured in TWh 2,3 The extensive combustion of fossil fuels since the Industrial Revolution has released millions of years' worth of carbon dioxide into the atmosphere over a few hundred years. 4 Consequently, CO2 concentration in the air has risen dramatically in the last 200 years to approximately 400 ppm in 2022, the highest value in the previous 650 000 years. 5 CO2 is a greenhouse gas that can trap solar thermal energy in the atmosphere by the infrared absorption of sunlight reaching Earth. This effect results in a warmer planet compared to one without an atmosphere, thus keeping the temperature on Earth livable for humans and other species. However, as Arrhenius first mentioned in 1896, 6 the release of an excessive amount of CO2, a long-lasting greenhouse gas, in the atmosphere in such a short period would contribute significantly to the quick increase in the average temperature on Earth, as shown in Figure 1-2. It is a fact that this rise in temperature is the result of human activity. 5 Figure 1-2. Evolution of atmospheric CO2 concentration (NOAA) 4 and temperature anomaly (Hadley Centre -HadCRUT4) 7 from 1850 to 2020

CHAPTER 1: INTRODUCTION

The sudden change in climate has negatively affected humanity and, if left unchecked, will have catastrophic consequences for humans in particular and the whole biosphere of Earth in general. A higher frequency of extreme weather events, including drought, wildfire, heavy precipitation, and flood, has been attributed to human-induced climate change with high confidence. 8 These weather events worsen the world's food and water security, disproportionately affecting vulnerable people in developing countries. Moreover, the rising temperature has contributed to the rising sea levels, mainly by melting ice caps at the two poles. The rising sea levels will disrupt the livelihoods of a large portion of the human population, as nearly 4 billion people live within 100 km of the coast. 9 The consequences of climate change are cascaded and intertwined, so the more they happen and the higher the magnitude of their effects, the more complex the solutions will be. 8 Therefore, it is in humanity's best interest to mitigate the impacts of climate change and ensure sustainable development in the shortest possible timeline.

While the impacts of climate change will be more pronounced in a relatively long-term timescale, the reliance on fossil fuels of the world economy has been the cause of short-term conflicts and instability in the past. The 1970s and early 1980s saw multiple events in the Middle East disrupting the oil supply, causing a recession and economic stagflation in Western countries and other parts of the world. In 2022, the Russian invasion of Ukraine interrupted the supply of oil and natural gas to the world, threatening a worldwide recession amid spiraling inflation and economic downturn. The impacts on European countries have been extreme, as Europe heavily relies on Russian gas for its industry and consumer uses. Therefore, the world's dependence on fossil fuels not only leads to the disastrous consequences of climate change but also gives authoritarian regimes a leveraging tool to achieve their purposes and cause instability in the world.

The 1970s oil crises sparked initial attempts to develop fuels saving technologies and alternative energy sources. 10 Since then, awareness of fossil fuel reliance and climate change impacts has been slowly but steadily gaining traction among the public and political leaders. The first wide-ranging international effort to curb greenhouse gas emissions was the Kyoto protocol 11 in 1997, in which it made binding emission requirements for developed countries and encouraged the reduction of greenhouse gases where it is most cost-effective. In 2015, the Paris Agreement -the most significant effort to combat climate change to date -was adopted by 195 parties. 12 The agreement aims to keep the rise of global average temperature in the 21st century to well under 2 o C, limiting the increase to 1.5 o C compared to the pre-industrial period.

The parties to the agreement are encouraged to establish independent road maps to reach the temperature targets while facilitating low-emission and climate-resilient development and adapting to the impact of global warming. The agreement calls for reducing the amount of CO2 emission, increasing the percentage of renewable energy in the total energy sources, and increasing the energy efficiency to achieve greenhouse gas emission peaking.

Renewable energy sources to mitigate CO2 emissions

In the past few decades, there have been tremendous research and development efforts for economical solutions to generating energy from sustainable sources, mainly solar and wind. The share of renewable energy generation has increased significantly in the last ten years, accounting for less than 1% of the total energy consumption in 2011 to 5% in 2021. 2 The price of electricity generated by solar and wind power has decreased dramatically in recent years, making them the cheapest sources of energy in certain places. 13 As the renewable energy share in the total energy supply mix increases, the intermittency of renewable sources will become more pronounced. Since power generated by solar and wind varies depending on weather conditions, energy generated from other sources and storage devices needs to be modulated accordingly to meet immediate demand. Natural gas power plants are the favored method to accompany renewable energy, as their capacity can be quickly adjusted with high power and energy density. 14 However, although natural gas burns cleaner than coal and oil, it still is a finite fossil fuel that emits greenhouse gases. And depending on the origin of the gas, relying on it for a short-term fossil-to-renewable transition period can still be problematic, 14 with Germany's dependence on Russian natural gas serving as a cautionary tale. A net-zero emission scenario with renewable being the primary energy source must come with practical and economical means of storing extra energy to smooth out the intrinsic supply fluctuation.

Storing electrical energy is a difficult task. While batteries can store energy for relatively low-power applications, large-scale energy storage technology is limited. Currently, pump-storage hydroelectricity is the most widely-used technology, accounting for most grid energy storage. However, as it involves pumping a large volume of water between two reservoirs at different elevations, this technology is suitable for a limited number of sites, making scaling up a challenge. 15 Lithium-ion battery, propelled by the rise of electric vehicles, is a fast-growing technology for grid energy storage. While some pilot plants using the technology are operating, the requirement for limited minerals and the degradation of the battery renders the future of lithium-ion batteries in large-scale energy storage uncertain. 16 Other technologies, including different types of batteries, compressed air, and flywheel, are in the early developing stages. Overall, large-scale energy storage is currently a diverse field without a dominant technology, thus calling for research and development efforts in varied directions.

Artificial photosynthesis

In nature, photosynthesis has been the energy storage solution of the biosphere since it started more than two billion years ago. Plants and other organisms use solar energy to convert atmospheric carbon dioxide and water into carbohydrates and oxygen through complex biochemical pathways, storing solar energy in chemical bonds of carbohydrates molecules. The stored energy is released in reverse when animals breathe in oxygen, consume the biomass directly or indirectly, and emit CO2 and water, completing the natural carbon cycle (Figure 1-3 -left half). All biomass on Earth is produced via photosynthesis, directly or indirectly, and a portion of it is transformed into fossil fuels. 17 However, relying on natural photosynthesis to store renewable energy and tackling humanity's problem of excess CO2 is not a suitable approach. As the process is optimized by evolution for the survival of the photosynthetic organisms and not for storing energy, the solar-to-chemical efficiency of photosynthesis is only a few percent. 18 Moreover, integrating natural photosynthesis to store energy within our current infrastructure is an improbable challenge. While natural photosynthesis is not suitable to address the world's energy problem, an artificial version of photosynthesis is a promising energy storage solution. An ideal artificial photosynthesis device, the so-called artificial leaf, would harvest sunlight and use it to turn water and CO2 into desired fuels and chemicals. 18 A tandem system is another approach where the energy harvesting unit is decoupled with the chemicals production unit. In particular, a solar photovoltaic plant or a wind farm can produce electricity to power a plant, generating fuels and chemicals from CO2 and water using electrochemistry. 19 The generated products, either by artificial leaves or by a tandem system, store the renewable energy used to produce them. A reverse reaction can release the stored energy to account for renewable energy's intermittency or be used in other applications. This reverse process turns the chemical products back into CO2 and water, completing the industrial carbon cycle without emitting net greenhouse gases into the atmosphere (Figure 1-3, right half).

The most straightforward product of artificial photosynthesis is hydrogen generated by splitting water. While the process does not consume CO2, hydrogen production by water splitting using renewable energy does not emit CO2 or other greenhouse gas. This type of hydrogen is named green hydrogen to differentiate it from hydrogen produced by natural gas sources that emit greenhouse gases. Green hydrogen can act as an energy carrier similar to fossil fuels, with its emission being water and oxygen instead of potent greenhouse gases. Therefore, green hydrogen has the potential to significantly facilitate the transition to a net-zero emission economy with its acting as an energy carrier and applications to other sectors, including transportation and materials and chemicals production. 20

CO2 reduction as energy storage and chemical-producing technology

Another approach of artificial photosynthesis mimics the whole natural process: using renewable energy to reduce carbon dioxide and split water to produce fuels and chemicals and release oxygen (Figure 1-3, right half). Like green hydrogen, renewable energy is stored in the fuels and chemicals generated by this process. No net greenhouse gases are emitted to the atmosphere when the fuels are converted back to CO2 and water to release the stored energy. But unlike green hydrogen, CO2 reduction uses CO2 as the carbon source, combining them with water to synthesize chemical products. Therefore, if the generated chemicals are used as feedstocks to produce long-lasting materials such as plastic or asphalt, the utilized carbon dioxide will be kept away from the atmosphere for an extended time without absorbing heat from sunlight and contributing to global warming. Using CO2 as a carbon source is especially appealing as society is slowly moving away from extracting finite sources of fossil fuels. This process can potentially turn CO2 from a waste product to a carbon source with excellent economic opportunities.

The CO2 feedstock for CO2 reduction can come from concentrated and atmospheric Chapter 1 sources. Concentrated CO2 sources usually include flue gas of fossil fuels energy plants or steel and cement production plants, where CO2 concentration is around 10%. 21 Capturing CO2 from concentrated sources requires less energy than CO2 from the atmosphere, making reducing CO2 from concentrated sources more economical than reducing CO2 captured from the air. However, in the near term, the highest priority of the decarbonization process is to stop using carbon-intensive technologies and replace them with cleaner ones. As this process plays out, the importance of carbon capture, storage, and utilization to offset the emissions of carbon-intensive technologies that are challenging to replace will gradually grow. 22 During this transition, the number of concentrated CO2 sources will fall, leading to CO2 captured from these sources becoming more limited, and CO2 captured directly from air becoming more available. Moreover, even when a net-zero economy is achieved, the current CO2 concentration in the atmosphere will continue to warm the planet above the pre-industrial level. 23 Therefore, in the long run, the CO2 feedstock for CO2 reduction would mainly come from the air, as CO2 becomes the alternative carbon source to fossil fuels.

Overall, CO2 reduction, under the context of artificial photosynthesis, would turn CO2 from a waste product into an economic opportunity. The utilization of CO2 incentivizes against releasing carbon dioxide into the atmosphere, getting humanity closer to a net-zero emission economy in the medium term. In the long run, CO2 reduction can potentially expand the carbon sink to decrease the concentration of CO2 in the atmosphere.

Overview of CO 2 reduction

History and basics of CO2 reduction

Reports of electrochemical CO2 reduction to formic acid on mercury and other inert electrodes dated back to 1870, and similar observations were reported consistently in the following decades. 24 Throughout this period, CO2 reduction reports did not involve the applications of generating fuels and chemicals; hence there were no significant efforts to improve the reaction's performance or analyze its mechanism. Since the oil crisis in the late 1970s and early 1980s, CO2 reduction has gained traction as an alternative way to synthesize fuels and chemicals.

Table 1-1 shows standard CO2 reduction reaction (CO2RR) products. 25 Although the reduction processes are generally facile in terms of thermodynamics, a significant activation barrier needs to be overcome to perform CO2RR. Without a catalyst, most CO2RRs go through the one-electron transfer step to create a bent radical anion (CO2 •-) with high energy loss. The stability of CO2 comes from it being the end product of exothermic reactions, making it a very inert molecule. In addition, the similarity of thermodynamic levels of different CO2 reduction products causes making a particular product selectively a challenging task, especially with products requiring multiple Chapter 1 electrons transfer. Therefore, catalysts are essential for a selective CO2RR with little to no waste energy. Typical catalysts for CO2 reduction are based on transition metals in metallic form, molecular complex, or composite materials. Several sources can provide the energy needed to reduce CO2, including electricity, heat, and light. Electrochemical and thermochemical reductions of CO2 are currently the most advanced methods, while photochemical and other processes are at the early development stage.

Products of CO2 reduction

The two most simple products of CO2 reduction are CO and HCOOH. As they only require the transfer of two electrons, there are few intermediate steps and competitive reactions, making the problems of activation energy and selectivity less complicated to address. When more than two electrons are transferred to a CO2 molecule, further reduced products are obtained, including formaldehyde, methanol, and methane. Transferring multiple electrons is generally complicated, making producing more reduced products less efficient than CO and HCOOH. The reduction of CO2 can involve more than one molecule of CO2, forming products with C -C bonds. This intricate process requires not only the generation of highly reduced intermediates but also the precise positioning of these intermediate species for the C -C coupling. As a result of their simplicity, under current economic conditions, the technology to reduce CO2 to CO and HCOOH is closest to becoming market-ready. 27,28 However, the market potential of formic acid is relatively small, and the toxicity and flammability of gaseous CO make its storage and transportation a logistical challenge. 28 In addition, these two C1 molecules (molecules containing only one carbon atom) do not reflect the potential range of chemicals that can come from CO2 reduction. Therefore, despite the intrinsic difficulties, producing long-chain carbon products from CO2, especially in liquid form, is an appealing approach. For example, ethanol and n-propanol hold great market potential, while ethylene can serve as a chemical feedstock to make plastic materials (Figure 1234). Overall, simple C1 products are closer to becoming economically viable, and C2+ products have high market potential but require substantial research and development efforts to compete with the current production methods. 26 Figure 1234. Comparison of cost of products between traditional and CO2 reduction reproduced from reference 26 1.2.3 CO2 reduction methods CO2 reduction is an endothermic process; therefore it needs an energy supply to occur. CO2 reduction is categorized into several methods depending on the energy sources used. As discussed above, electrochemical and thermochemical are the two most mature ones, while photochemical reduction of CO2 has also been extensively studied. Biochemical and chemical approaches can also be used to reduce CO2, though their applications are more limited than other methods. Combinations of any two of these methods have also been studied.

Photochemical reduction of CO2 using sunlight closely resembles the process of natural photosynthesis. Like the electrochemical method, CO2 photoreduction can happen under homogeneous or heterogeneous conditions. Standard products of CO2 reduction, including CO, HCOOH, and CH4 [29][30][31] have been generated using photocatalysis. Another approach to the photoreduction of CO2 includes tandem devices, where photovoltaic solar cells are coupled to electrolysis systems. This device, often called an artificial leaf, combines the electricity produced by photovoltaic solar cells to carry out the electrochemical reduction of CO2. Thus, it is more industrially ready than direct photoreduction. There are devices demonstrating CO2 reduction and water-splitting processes using sunlight in real-world conditions. 32,33 However, the field of photochemical CO2 reduction has many intrinsic challenges to overcome. First, capturing sunlight and using it to drive the transfer of electrons and protons to CO2 and water requires a system of intricate molecular machinery consisting of rare metals such as ruthenium or rhenium. While these systems can carry out the reaction in Chapter 1 laboratory scale, scaling up a complex chemical system built around scarce materials is a challenging task. The intricate photochemical system is also often subject to premature degradation that is incompatible with industrial standards. Secondly, any system that uses sunlight as an energy source requires a large area to harvest sufficient energy due to the inherently low energy density of sunlight. However, unlike solar photovoltaic, a solar fuel plant needs a dedicated system to transfer and process the reactants and products, further complicating its implementation. Overall, despite the appealing aspects of the photochemical reduction of CO2, the inherent challenges of the method limit its industrial application. Still, the insights gained from developing photochemical CO2 reduction prove beneficial to develop catalysts for CO2 reduction using other energy sources.

Reducing CO2 using biological species, such as enzymes, is another approach toward CO2 conversion. In nature, enzymatic reduction of CO2 is a multi-step process employing three enzymes: formate hydrogenase, formate dehydrogenase, and alcohol dehydrogenase. 34 Artificial versions of biochemical CO2 reduction include the works of Ai and co-workers, 35 and Lee and colleagues, 34 where they built a multi-enzyme system to reduce CO2 to methanol. The biochemical reduction of CO2 often involves intricate biochemistry processes, but the active species are often subject to rapid degradation, rendering them impractical for large-scale application. However, the chemistry of these complex biological processes can serve as inspiration to design and improve other catalysts utilizing different energy sources. 36,37 The cascade approach has been explored by combining electrochemical or photochemical with chemical synthesis. Skrystrup and colleagues used CO generated electrochemically from CO2 to carry out carbonylation reactions on a Pd catalyst. 38 The same approach was reported by Aukauloo and co-workers, using photochemical instead of electrochemical methods to produce CO from CO2. 39 However, these methods of CO2 reduction are more suitable for small-scale applications than industrial-scale CO2 utilization. 40 These small-scale applications demonstrate the versatility of the cascade catalysis approach for CO2 reduction.

Thermochemistry is one of the major routes to reduce CO2. Thermochemical reduction of CO2 involves heating CO2 at appropriate temperatures and pressure over catalysts to facilitate the conversion of CO2. This process mainly yields CO, as the hightemperature requirement is unsuitable for producing more complicated products. 41 Combination of thermochemistry and electrochemistry has been studied to make CO from CO2 using molten carbonate or solid oxide electrolyzers. 42,43 Concerning CO production, high-temperature solid oxide electrolysis has the highest technological readiness level. 42 The Perseverance rover on Mars also employs this method to split CO2 into CO and O2, experimenting with providing breathable oxygen in the Martian atmosphere. 43 Fischer-Tropsch (FT) process is a notable thermochemical process that converts a Chapter 1 mixture of carbon monoxide and hydrogen, known as syngas, into hydrocarbons. 44 Developed in the late 1920s, it has recently gained considerable attention as a method to synthesize clean fuels and chemicals. 45 In particular, the scheme involves cascade catalysis, where CO2 reduction and water splitting produce the syngas feedstock for the FT process. 46,47 When the syngas mixture is heated at high temperature and pressure, typically 150 -300 o C, and several tens of atmosphere, CO and H2 are converted to hydrocarbon in a process resembling chain growth polymerization, where the chain is initiated on metal catalysts. Common catalysts FT process include iron and cobalt, while ruthenium is an active catalyst but less widely used due to its scarcity. 45 The hydrocarbons resulting from the FT process follow the Flory-Schulz distribution, which favors shorter hydrocarbons than longer ones. 48 Three main polymerization schemes have been proposed regarding the formation of C -C bonds in FT synthesis: carbide, CO insertion, and hydroxycarbene mechanism (Figure 12345). In the carbide mechanism, the hydrocarbons are formed by adsorbed M-CH2 species reacting with each other. Figure 1-5B shows the propagation step in the CO insertion mechanism, where chains are grown by the insertion of carbon monoxide into the metal-alkyl bonds, followed by the reduction of the acyl group. The mechanism based on the formation of the hydroxycarbene species is shown in Figure 1-5C. The chemisorbed hydrogen hydrogenates the chemisorbed carbon monoxides to form hydroxycarbene. After eliminating water, a C -C bond is formed between the two hydroxymethylene groups. Electrochemical reduction at room temperature is another standard method to reduce CO2. Under this condition, the CO formation rate is not as high as the high-Chapter 1 temperature electrolysis. However, the low temperature allows more intricate chemistry to occur. Consequently, high-value products such as ethylene or ethanol can be generated by room-temperature CO2 electrolysis with specific catalysts. In the next section, the electrocatalytic reduction of CO2 will be discussed in detail.

Electrocatalytic reduction of CO 2

Overview and basic concepts

The electrocatalytic reduction of CO2 can be classified as homogeneous, where the catalysts and the reactants are in the same phase, or heterogeneous, where the components exist in different phases (Figure 123456). Homogeneous catalysis electrochemical reduction of CO2 involves both CO2 and the catalyst dissolved in the working electrolyte, which can either be an organic solvent or water. In this approach, the catalyst acts as a shuttle to facilitate the electron transfer between the electrode and the substrate, lowering the activation barrier. On the other hand, heterogeneous electro-reduction of CO2 involves solid catalysts immobilized on the electrode surface.

The electrode material itself can be the catalyst for the reaction by providing a favorable surface for the substrate to bind, facilitating electron transfer, and lowering the activation barrier. Other immobilization strategies include physical bonding (dropcasting) or chemical bonding (covalent bonding) of the catalyst onto the surface of the electrode. In the homogeneous reduction of CO2, inert working electrodes (glassy carbon, platinum) are typically used in a static cell. The cell can contain one compartment (Figure 1-7A) or two compartments with a membrane or a frit separating the working and the counter sides (Figure 1-7B). A similar setup can be used with heterogeneous conditions. Under this configuration, diffusion is the primary mode of mass transport between the electrode surface, the catalysts, and the substrates. In addition, the available CO2 is confined by the amount of CO2 dissolved in the electrolyte. As a result, when catalysis is fast, the lack of supplied CO2 or the slow rate of mass diffusion can be the limitation of the reaction. A flowing configuration can overcome these limits: electrolyte flow induces the convection mass transport mode, and CO2 can be supplied Chapter 1

externally and flown into the cell where the reaction occurs (Figure 1-7C). To further increase the availability of CO2, a flowing setup with a gas diffusion electrode can be employed (Figure 1-7D). Gaseous CO2 and liquid electrolytes are flown on opposite sides of a gas-permeable electrode, and CO2 reduction happens at the interface of the gaseous CO2, the liquid electrolyte, and the solid catalyst. While a flowing configuration can be employed both heterogeneously and homogeneously, its primary use is for a heterogeneous system, as diffusion limit and CO2 supply are more pronounced problems in fast and active heterogeneous catalysis. A benchmarking process is necessary to accurately characterize an electrocatalyst for CO2 reduction and compare it with different species. Overpotential (𝜂) and turnover frequency (TOF) are related parameters that reflect the intrinsic properties of a catalyst. Overpotential is defined as the difference between the thermodynamic potential of a CO2 conversion process and the applied potential. The lower the overpotential, the more efficiently the catalyst lowers the activation barrier. TOF generally refers to the number of substrates reacting per active site per unit of time. The TOF represents the activity of a catalyst, so the higher the TOF, the faster one active site can convert reactants. Overpotential and TOF are usually linked together, and their relationship will be discussed in more detail in the following section. While the two parameters help assess a catalyst, they only represent a specific reaction at specific active sites. Therefore, non-selective catalysts with heterogeneous active sites are challenging to evaluate using only overpotential and TOF. Two critical global properties to evaluate an electrocatalytic system are its selectivity and current density. The selectivity of an electrocatalyst toward a product is represented by the Faradaic efficiency (FE), which is the ratio between the number of electrons involved in generating a product used and the number of total electrons passed through the system. Proton reduction to hydrogen is the most common competing side reaction in CO2 reduction, causing the Faradaic efficiency to be less than unity. In addition, converting CO2 to highly-reduced molecules with multiple electron transfers may result in a non-selective distribution with a wide range of products. While the TOF represents the intrinsic activity of a catalyst, the current density can be used to evaluate the global activity of an electrocatalysis system. While a highly-active catalyst is desirable, it usually comes with a stability tradeoff. Therefore, catalyst stability is critical in judging a catalyst and is a crucial factor in practical applications.

Metal catalysts

Pure metals are a common type of catalyst for CO2 reduction. This type of catalyst is often employed under heterogeneous conditions with water-based electrolytes. As solid catalysts, the activity of metal catalysts follows Sabatier's principle: 50 to work effectively, the catalyst must adsorb the substrate neither too strongly nor too weakly.

In other words, the binding energy between the metal and the key intermediates of the CO2 reduction process (CO* and H*) plays an essential role in the activity of the catalyst. Transition metals generally possess the appropriate binding energy, and their selectivity is summarized in Figure 1-8. 51 When the metals bind weakly with CO*, depending on the interaction with H*, they either favor the production CO (Ag, Au, Zn) or HCOOH (Sn, Pb, Hg). If the metals bind with too CO* strongly, the production of CO* from CO2 can poison the catalyst, resulting in low CO2-reducing activity, as in the case of Pt, Ni, or Pd. Cu is unique since it interacts with CO* and H* with intermediate binding energy. As a result, the intermediates can stay on the metal surface long enough to form products with multiple C -C bonds without poisoning the catalyst.

Chapter 1

From the first work by Hori and co-workers in the 1980s 52,53 where they observed C2+ molecules as products of CO2 reduction on metallic copper, copper-based catalysts have been studied extensively. 54 Multiple C2+ species were identified as products of the electrochemical reduction of CO2, including ethylene and propylene as the majority species. 55 A variety of strategies were employed to improve the performance of Cu, including tuning the chemical composition of Cu to improve ethanol selectivity, 56 current density, overpotentials, and stability. While copper can produce C2+ products efficiently, silver and gold remain among the most active metal catalysts for CO production from CO2. 57 However, the limited supply and high cost of noble metals hinder gold and silver from becoming the catalysts of choice to produce CO on an industrial scale. Despite the excellent performance of metal catalysts, the exact mechanism of CO2 reduction remains elusive. The concept of active sites in heterogeneous catalysis is not as straightforward as molecular catalysts, 58 so establishing a detailed mechanism for a reaction is an intricate task. The challenge of determining the reaction mechanism is exacerbated in the case of copper producing C2+ products, as the process involves multiple intermediates step. In particular, reducing CO2 to C2+ molecules on copper is a dynamic process as it consists of restructuring copper during catalysis. [59][60][61] This evolution of the catalytically active sites renders constructing a comprehensive mechanistic model a complicated task. Therefore, detailed mechanisms derived from catalysts' well-defined actives sites, such as molecular catalysts, could better the understanding of metal catalysts and inspire the rational design of efficient metal catalysts for CO2 reduction.

Homogeneous molecular catalysts

Appeals for homogeneous molecular catalysts

Transition metal molecular complexes were one of the first known species to catalyze CO2 reduction efficiently under homogeneous conditions. 62 Unlike metals or composite materials, the structure of molecular catalysts can be accurately characterized, and their active sites are well-defined. The electrochemical responses of molecular catalysts under homogeneous conditions can be described precisely with advanced electrochemical methods. 63 Coupled with spectroscopy techniques to monitor the oxidation state, structure, and intermediate species, homogeneous molecular catalysts provide the ideal platform for the detailed study of CO2 reduction mechanisms. Establishing structure-activity relations allows the rational design of catalysts with appropriate substituents to tune the catalytic performance. 64 Moreover, the well-defined metal active sites of molecules maximize the utilization of metal atoms since all metal atoms can be catalytically active during operation. In addition, some of the most active molecular catalysts are made of Earth-abundant, such as iron, carbon, and hydrogen, making large-scale implementation less challenging. Therefore, the study of molecules in solutions is valuable for the development of electrochemical CO2 reduction catalysts. 65 Many molecular families, including pyridines, 66 cyclam, 67 porphyrin 65 , and phthalocyanine (Pc) 68 can efficiently catalyze the CO2 reduction reaction. However, porphyrin (Figure 1-9A) and phthalocyanine (Figure 1-9B) are the two macrocycles that stand out due to their exceptional performance. 68 Iron tetraphenyl porphyrin (FeTPP), where four phenyl rings are attached to the R positions indicated in Figure 1-9A, is one of the most efficient and extensively studied species. In the following section, FeTPP and its derivatives are discussed to demonstrate various aspects of homogeneous molecular catalysts for CO2 reduction.

Catalyst benchmarking: overpotential, and turnover frequency calculation

By definition, the TOF at a specific overpotential can be calculated by performing electrolysis at the desired potential, quantifying the obtained products after a certain period, and determining the number of catalytically active sites. However, this meticulous process is prone to errors from secondary phenomena, including ohmic losses, substrate consumption, catalyst self-inhibition, and catalyst deactivation. 69 Savéant and colleagues have developed and proposed the foot-of-the-wave analysis based on non-destructive cyclic voltammetry (CV) to estimate the TOF of a catalyst. 63,[START_REF] Costentin | Benchmarking of Homogeneous Electrocatalysts: Overpotential, Turnover Frequency, Limiting Turnover Number[END_REF] At a sufficiently low current and high scan rate, the analysis can avoid side phenomena, giving values close to those obtained from bulk electrolysis experiments. 64,71 The rate constant of the reaction can be derived from the linear relation between the catalytic current 𝑖 and the overpotential (𝜂 = 𝐸 -𝐸 𝑐𝑎𝑡 0 ):

𝑖 𝑖 𝑝 0 = 2.24 √ 𝑅𝑇 𝐹𝜈 𝑘 𝑐𝑎𝑡 𝐶 𝑐𝑎𝑡 0 1 + exp [ 𝐹 𝑅𝑇 (𝐸 -𝐸 𝑐𝑎𝑡 0 )]
where 𝑖 is the catalytic current in the presence of CO2, 𝑖 𝑝 0 is the reduction peak current without CO2, 𝜈 is the scan rate, 𝐶 𝑐𝑎𝑡 0 is the initial concentration of the reactants (CO2), 𝐹 is the Faraday constant, 𝑅 is the gas constant, 𝑇 is the absolute temperature, 𝐸 is the applied potential, and 𝐸 𝑐𝑎𝑡 0 is the standard potential of the reversible reduction peak of the active form of the catalyst. From the rate constant, the maximum TOFmax can be calculated:

𝑇𝑂𝐹 𝑚𝑎𝑥 = 𝑘 𝑐𝑎𝑡 𝐶 𝑐𝑎𝑡 0
The maximum TOF only depends on the rate constant and the initial concentration of CO2. This value is reached when 𝜂 ≥ 𝐸 𝐶𝑂 2 /𝐶𝑂 0 -𝐸 𝑐𝑎𝑡 0 . In the catalytic Tafel plot (logTOF vs. 𝜂 -Figure 1-10A), this region is represented by the horizontal part, where logTOF equals logTOFmax and is independent of overpotential. The linear region of the plot can then be constructed following the equation:

log 𝑇𝑂𝐹 = log 𝑇𝑂𝐹 𝑚𝑎𝑥 - 𝐹 𝑅𝑇 ln 10 (𝐸 𝐶𝑂 2 /𝐶𝑂 0 -𝐸 𝑐𝑎𝑡 0 -𝜂)
The minimum logTOF0 when 𝜂 equals zero can be derived from this equation. From the catalytic Tafel plot, a catalyst can be assessed using two factors: the logTOFmax and effective overpotential at which the logTOFmax is achieved (𝜂′ = 𝐸 𝐶𝑂 2 /𝐶𝑂 0 -𝐸 𝑐𝑎𝑡 0 ). The higher logTOFmax and the lower 𝜂 ′ , the more active and more efficient the catalyst is (Figure 1-10B).

CO2-to-CO performance and mechanism of FeTPP

Study on metal porphyrins for the electrochemical reduction of CO2 date back to the late 70s, 72 and Savéant and co-workers has contributed extensively to the understanding and development of FeTPP as a catalyst for CO2RR. In their early study, the authors pointed out that in an aprotic solvent like dimethylformamide (DMF), the active species was the triply reduced Fe("0")-TPP, as the catalytic wave coincided with the Fe(I)-Fe(0) wave in the cyclic voltammogram (Figure 1-11A and B). 73,74 Proton source is a crucial factor in homogeneous CO2 reduction by FeTPP, and weak Bronsted acids, including water, trifluoroethanol, and phenol (PhOH), significantly enhanced catalysis. 74,75 Figure 1-11B demonstrates this phenomenon, where without CO2, the Fe(II) species got reduced to Fe(I) at -0.8 V vs. standard hydrogen electrode (SHE), and the Fe(I) species subsequently got reduced to Fe(0) at a more negative potential (-1.4 V vs. SHE). In the presence of CO2, and phenol as a proton source, a substantial increase in current and a change in the wave's shape were observed at the same position of the Fe(I)/Fe(0) wave. The electrolysis experiment at -1.46 V vs. SHE (𝜂 ′ = 0.74 V) showed the product to be CO, with a logTOFmax of 2.75, and the Faradaic efficiency ranging from 100% to 94%, depending on the amount of PhOH. 64 Subsequent studies provided a detailed analysis of the CO2-to-CO reduction mechanism with FeTPP using cyclic voltammetry. 63,76 By performing CV at various PhOH concentrations, the authors determined the reaction of CO2 was first-order, while the reaction order with PhOH was first at low concentrations and second at high concentrations.

Influences of ligands on FeTPP: proton source

A significant appeal of molecular catalysts is the ability to precisely tune the metal active site's chemical environment. The beneficial effect of phenol addition to the organic electrolyte inspired the synthesis of a FeTPP-(OH)8 species, where eight -OH groups were attached to the ortho positions of the four phenyl rings. 78 The -OH functional groups serve as the internal proton source for the CO2-to-CO catalysis, delivering considerably higher local proton concentration than adding acid to the bulk electrolyte. The electrolysis in CO2-saturated DMF/H2O (2 M) showed over 90% Faradaic efficiency for CO with a current density of 0.31 mA/cm 2 at -1.16 V vs. normal hydrogen electrode (NHE). The FeTPP-(OH)8 catalyst proved efficient and robust with the logTOFmax of 5.97 and an overpotential of 0.64 V while maintaining the selectivity with no observable degradation. Control experiments with FeTPP-(OCH3)8 and unmodified FeTPP with phenol addition were performed to further characterize the -OH substituents' contribution. FeTPP-(OCH3)8 showed a lower logTOFmax of 2.7 and a higher overpotential of 1.00 V compared to the -OH counterparts, emphasizing the importance of the -OH groups. Another study by Warren and a co-worker showed when there is only one -OH functional group, the activity of FeTPP-(OH) considerably decreases, with a logTOFmax of 2.1 and overpotential of 0.91 V. 79 Comparing the reaction rate of FeTPP-(OH)8 with unmodified FeTPP with 4 M PhOH addition showed that the eight phenolic groups produced an equivalent of around 150 M PhOH in the bulk electrolyte.

Subsequent studies showed that the reduction mechanism on FeTPP-(OH)8 differs from the unmodified FeTPP. [START_REF] Costentin | Pendant Acid-Base Groups in Molecular Catalysts: H-Bond Promoters or Proton Relays? Mechanisms of the Conversion of CO2 to CO by Electrogenerated Iron(0)Porphyrins Bearing Prepositioned Phenol Functionalities[END_REF][START_REF] Costentin | Ultraefficient Homogeneous Catalyst for the CO2-to-CO Electrochemical Conversion[END_REF] In particular, the phenolic groups play two roles: a proton relay and an H-bond donor. The first adduct Fe(0)-CO2 is stabilized by forming an H-bond with the -OH groups. Then the groups act as a proton relay in subsequent protonation and cleavage step. Unlike the mechanism of the unmodified FeTPP, the second electron transfer takes place before the C -O bond cleavage due to the strong stabilizing effect the -OH groups have on the intermediate adduct. Interestingly, external proton sources play an essential role in catalysis. When 3 M PhOH was used, the observed catalytic enhancement (logTOFmax = 3.8 and 𝜂′= 0.66 V) was more modest compared to when H2O was used (logTOFmax = 5.97 and 𝜂′ = 0.64 V). This implies competition happening between the phenolic functional groups and the PhOH molecules in the bulk electrolyte.

In addition to the through-space effects (H bond and proton donors), the electronwithdrawing nature of the -OH groups contributes to lowering the overpotential. This inductive effect was further explored by adding ten fluorine atoms to two opposite phenyl rings of FeTPP, keeping the -OH groups in the ortho positions of the two other rings (Figure 1-12B). [START_REF] Costentin | Ultraefficient Homogeneous Catalyst for the CO2-to-CO Electrochemical Conversion[END_REF] Compared to FeTPP-(OH)8, the addition of ten fluorine atoms decreases the overpotential (0.59 V vs. 0.64 V), but also reduces the logTOFmax (5.97 vs. 4.0). The inductive effect moves the standard potential to the positive direction, but it also lowers the electron density around the metal center, slowing down the electron transfer process between CO2 and the catalyst. The intermediates' stabilization effect can be exploited by designing functional groups capable of forming hydrogen bonds with metal-CO2 adduct or with external proton sources. Attaching an amine group in the ortho position of the phenyl ring of a FeTPP molecule resulted in a higher log(TOFmax) (4.02) but also higher overpotential (0.76 V) compared to an unmodified FeTPP (log(TOFmax) = 3.01 and 𝜂 ′ = 0.74 V). [START_REF] Liu | Construction of Secondary Coordination Sphere Boosts Electrochemical CO2 Reduction of Iron Porphyrins[END_REF] However, when a -NH2 group was placed in the meta position instead of ortho, its effect on both overpotential (0.82 V) and activity (logTOFmax = 3.4) was worse than its ortho counterpart, highlighting the importance of the functional groups positioning. Another study by Nocera and co-workers presented an FeTPP species modified with a more intricate guanidinium group to stabilize the adduct. [START_REF] Margarit | Carbon Dioxide Reduction by Iron Hangman Porphyrins[END_REF] Although theoretical calculation showed a thermodynamically favored stabilization effect of the functional group, the catalytic activity was poorer (logTOFmax = 2.47 and 𝜂 ′ = 0.77 V) compared to a simple amino function. The authors suggested that the dibenzofuran part did not provide an appropriate distance for the guanidinium to stabilize the intermediate.

A comprehensive study by Chang and co-workers underlined the impact of the positioning of functional groups. [START_REF] Nichols | Positional Effects of Second-Sphere Amide Pendants on Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins[END_REF] They compared four versions of FeTPP derivatives bearing amide pendants at various positions of the metal center, as shown in Figure 1-13A. All candidates improve the catalytic activity, while there are only slight changes with the overpotential. The functional groups in the ortho position performed better than in the para position, and the amide groups at the more distanced place were more active than the ones nearer to the phenyl rings of the FeTPP. Therefore, the best catalyst in the study was Fe-ortho-2-amide, with logTOFmax of 6.74 and overpotential of 0.80 V. In the same approach, Dey and co-workers synthesized FeTPP derivatives with four amide functional groups on the same side. [START_REF] Sen | Role of 2 nd Sphere H-Bonding Residues in Tuning the Kinetics of CO2 Reduction to CO by Iron Porphyrin Complexes[END_REF] This catalyst had higher activity (logTOFmax = 5.71) and lower overpotential (0.62 V) when performing catalysis in acetonitrile with 3 M PhOH. This result indicated that the number of functional groups could also influence the catalytic system.

Inspired by this approach, Aukauloo and colleagues employed a FeTPP derivative with four urea groups on the opposite sides of the porphyrin plane (FeTPP-Ur, Figure 1-13B), providing two points for hydrogen bond formation. This species considerably decreased the overpotential (0.43 V) while inducing a modest increase in the log(TOFmax) (3.83). A control experiment was done with a FeTPP species containing amide groups, which serve as single-point hydrogen bond donors. A similar log(TOFmax) (3.85) was obtained, but the overpotential (0.63 V) was higher than that of the urea species, underlying the crucial role of the multiple points hydrogen bond donors.

Influences of ligand on FeTPP: electrostatic interaction

Introducing charged functional groups to a FeTPP molecule can induce electrostatic interactions between the charged groups and the reaction intermediates. Regarding this approach, Savéant and co-workers reported FeTPP derivatives grafted with four trimethylammonium groups. [START_REF] Azcarate | Through-Space Charge Interaction Substituent Effects in Molecular Catalysis Leading to the Design of the Most Efficient Catalyst of CO2-to-CO Electrochemical Conversion[END_REF] In accordance with the result from FeTPP with hydrogen bond donors, the charged groups performed best in ortho positions, substantially lowering the overpotential (0.25 V) and increasing the catalytic activity of the catalyst (logTOFmax = 6). Catalysis was still enhanced when the charged groups were in para positions (overpotential = 0.57, logTOFmax = 4.4), though the effectiveness was lower than the ortho counterparts. Aukauloo and colleagues corroborated the positive effects of charged substituents. They found that a FeTPP species having four combined charged groups (methylimidazolium) and H-bond donor groups (amido) improved both the overpotential (0.37 V) and the logTOFmax (6.0). [START_REF] Khadhraoui | Local Ionic Liquid Environment at a Modified Iron Porphyrin Catalyst Enhances the Electrocatalytic Performance of CO2 to CO Reduction in Water[END_REF] The cationic functional groups also make the iron complex soluble in water, which can serve as an electrolyte and a proton source. In a water-based electrolyte, both iron porphyrin derivatives from the groups of Saveant [START_REF] Costentin | Efficient and Selective Molecular Catalyst for the CO2-to-CO Electrochemical Conversion in Water[END_REF] and Aukauloo 89 resulted in high CO-selectivity (over 90%), low overpotential, and relatively good stability. However, incorporating negatively charged functional groups, such as sulfonate, increased the overpotential (0.74 V) and decreased the catalytic activity (3.6). [START_REF] Azcarate | Through-Space Charge Interaction Substituent Effects in Molecular Catalysis Leading to the Design of the Most Efficient Catalyst of CO2-to-CO Electrochemical Conversion[END_REF] Another study using FeTPP with sulfonic acid groups confirmed this observation. [START_REF] Margarit | Carbon Dioxide Reduction by Iron Hangman Porphyrins[END_REF] This result was attributed to the repealing interaction between the negatively charged function groups and reaction intermediates.

Bi-metallic complexes

While ligand modification is the typical strategy to tune the activity of a molecular catalyst, the nuclearity can also be modified to influence the catalysis. Inspired by the carbon monoxide dehydrogenase enzyme, Naruta and colleagues linked two FeTPP molecules using a phenyl linker, creating a dimer species with two metal centers facing each other (Figure 1-14). The Fe-Fe distance depends on whether the two monomers are connected via ortho or meta positions on the phenyl linker. The authors found that the ortho configuration, having a Fe-Fe distance of 3.2 to 4.0 Å, showed the best catalytic behaviors (logTOFmax = 4.3, FECO = 95%, and overpotential = 0.71 V).

Limitations of homogeneous molecular catalysts

As mentioned earlier, homogeneous molecular catalysts, having well-defined active sites and established electrochemical methods, are straightforward for studying the structure-reactivity relationships. This relationship allows the determination of detailed mechanisms, enabling precise chemical tuning around the active center. However, the current densities in most homogeneous systems are usually in the range of a few milliamperes per centimeter square, which is two orders of magnitude lower than the industry-relevant value. [START_REF] Shin | Techno-Economic Assessment of Low-Temperature Carbon Dioxide Electrolysis[END_REF] In addition, common molecular complexes, such as porphyrin and phthalocyanine, are generally only soluble in an organic solvent, complicating the scaling-up process. While water-soluble species may alleviate this problem, in a homogeneous system, only a minor amount of catalysts near the electrode surface is active -an inefficient use of catalysts. Moreover, despite the high selectivity, homogeneous molecular catalysts are mostly limited to producing CO and HCOOH. 25 Coupled with technical challenges like product separation, CO2 conversion efficiency, and catalyst stability, homogeneous molecular catalysis is an unlikely candidate for industrial-scale CO2 utilization solutions. Nonetheless, the fundamental insights gained from studying molecular species can play an essential role in better understanding and optimizing other catalytic systems.

1.3.4 Heterogeneous reduction of CO2 by molecular catalyst

Appeals for heterogeneous molecular catalyst

The heterogenization of molecular macrocycles helped to increase the catalytic activity to meet the requirements of industrial applications. [START_REF] Ren | Molecular Electrocatalysts Can Mediate Fast, Selective CO2 Reduction in a Flow Cell[END_REF][START_REF] Corbin | Heterogeneous Molecular Catalysts for Electrocatalytic CO2 Reduction[END_REF][START_REF] Wang | CO2 Electrochemical Catalytic Reduction with a Highly Active Cobalt Phthalocyanine[END_REF][START_REF] Zhang | Molecular Engineering of Dispersed Nickel Phthalocyanines on Carbon Nanotubes for Selective CO2 Reduction[END_REF] Like homogeneous conditions, porphyrin and phthalocyanine are the two most notable families of molecular catalysts under heterogeneous conditions. The heterogenization strategy includes physical adsorption by drop-casting a mixture of catalyst, support, and binder on the electrode surface; or chemical grafting of molecules directly on an electrode. Since the catalysts are immobilized on the surface of the electrode, catalyst diffusion is eliminated, minimizing the mass transport limitation. The immobilization of catalysts also removes the need for the separation of products and catalysts, simplifying the product collection step. As molecular catalysts are generally not soluble in water, an aqueous electrolyte can be used without the risk of catalysts leaching into the electrolyte. Compared to homogeneous conditions, heterogenization can lead to changes in the nature of active sites, where the behavior of molecules resembles a metal species. [START_REF] Kaminsky | Adsorbed Cobalt Porphyrins Act like Metal Surfaces in Electrocatalysis[END_REF] Overall, the same catalyst under heterogeneous conditions can operate with a different activity, stability, and reaction mechanisms compared to under homogeneous conditions. [START_REF] Corbin | Heterogeneous Molecular Catalysts for Electrocatalytic CO2 Reduction[END_REF]

Influences of metal center on phthalocyanine species

The nature of the metal center in porphyrins and phthalocyanines plays a crucial role Chapter 1 in the performance of the catalysts, similar to metals and metal complexes in homogeneous conditions. For phthalocyanine, early research [START_REF] Meshitsuka | Electrocatalysis by Metal Phthalocyanines in the Reduction of Carbon Dioxide[END_REF][START_REF] Kapusta | Carbon Dioxide Reduction at a Metal Phthalocyanine Catalyzed Carbon Electrode[END_REF][START_REF] Tanabe | Electrocatalysis of Metal Phthalocyanine Thin Film Prepared by the Plasma-Assisted Deposition on a Glassy Carbon in the Reduction of Carbon Dioxide[END_REF] found that CoPc showed the highest current density at reducing CO2, with CO being the primary product. Other minor products were observed under specific conditions, including oxalic acid and glycolic acid in quaternary ammonium electrolyte; 97 methanol at pH lower than 4. [START_REF] Kapusta | Carbon Dioxide Reduction at a Metal Phthalocyanine Catalyzed Carbon Electrode[END_REF] NiPc, FePc, and CuPc were active in producing CO, but their current densities were lower than that of CoPc. These observations were corroborated and expanded by two comprehensive studies of metal center effects on phthalocyanine CO2 reduction activity, using gas diffusion electrodes in 0.5 M KHCO3. 100,101 In particular, Furuya and co-workers found CoPc and NiPc to be highly selective at producing CO, reaching Faradaic efficiency of 100%. Other CO-selective catalysts included FePc and PdPc, with maximum CO Faradaic efficiency of 80% at -1.5 V vs. reversible hydrogen electrode (RHE). Phthalocyanines with metal centers from group IIIA and IVA, such as Al, Sn, Pb, and In, were selective to formic acid, with the best performance of 70% at -1.6 V vs. RHE. Methane was the main product from the reduction of CO2 on CuPc, GaPc, and TiPc, with Faradaic efficiency ranging from 30% to 40%. The authors justified their findings using electron configurations of the metal center. Specifically, they argued that depending on the nature of the electrons in the HOMO and LUMO, the mode of CO binding could differ, resulting in different product distributions ranging from HCOOH to CO and CH4.

In the group of CO-selective metal phthalocyanines, only CoPc can reach a high current density of 50 mA/cm 2 at around -1.3 V vs. SHE. To get the same current density with NiPc, a potential of -1.7 V vs. SHE must be applied. FePc can produce CO selectively at -1.2 V vs. SHE with a modest current density of 15 mA/cm 2 , but the high CO faradaic efficiency quickly decreases at more negative potential. 101 These findings suggest CoPc is the most active CO-producing catalyst, while NiPc and FePc are promising candidates if their overpotential and selectivity could be enhanced. Recently, Berlinguette, Robert, and co-workers demonstrated the potential of CoPc by reducing CO2 to CO at an industrially relevant current density of 150 mA/cm 2 in a zero-gap membrane flow reactor operating in 1M KOH. [START_REF] Ren | Molecular Electrocatalysts Can Mediate Fast, Selective CO2 Reduction in a Flow Cell[END_REF] In a microfluidic reactor with 0.5 M KHCO3, the catalysts lasted for more than 100h while keeping the current density at 50 mA/cm 2 and CO selectivity at 80%.

Influences of metal center on porphyrin species

For porphyrin macrocycles, the effects of metal centers on CO2 reduction catalysis activity were reported in a comprehensive study by Sakata and co-workers. 102 The reactions were carried out with the metal tetraphenyl porphyrins (TPPs) immobilized on gas diffusion electrodes in 0.5 M KHCO3. A fixed current density of 100 mA/cm 2 was applied, and the catalysts were in a CO2 atmosphere at 1 or 20 atm. Similar to phthalocyanines, at atmospheric pressure, CoTPP exhibits the highest selectivity for CO at 74.8% at -0.96 V vs. Ag/AgCl, followed by FeTPP at 42.4%. ZnTPP and CuTPP showed some activity for CO reduction, while HCOOH was also observed with CuTPP. Unlike NiPc, NiTPP hardly showed any activity for CO2 reduction and mainly produced H2. Other porphyrins, including MgTPP, MnTPP, and H2TPP, showed no activity for CO2 reduction.

A recent study looked at the influences of metal centers of different protoporphyrins (PPs) on CO2 reduction to formate. 103 Protoporphyrins are a derivative of a general porphyrin macrocycle, having two vinyl, two propionic acids, and four methyl groups attached to the ring. Rh, In, and Sn protoporphyrins were the most active species in producing formate, while Cr, Mn, Co, and Fe yielded no formate. In-protoporphyrin had the best performance, with a Faradaic efficiency of 70% in a pH of 9.6 under an applied potential of -1.9 V vs. RHE. The authors showed a reverse correlation between formate production and hydrogen evolution reaction.

Chapter 1 105 The findings suggest that both the ligands and the metal centers play essential roles in the functioning of the molecular catalyst.

Further-than-CO reduction

While Co-based macrocycles are very selective toward CO, recent studies showed that cobalt protoporphyrin and cobalt phthalocyanine could reduce CO2 further than CO to make CH4 or CH3OH. In 2015, Koper et al. used a cobalt protoporphyrin catalyst to reduce CO2, obtaining CO as the primary product, methane as a side product, and methanol in trace amounts. 106 The maximum selectivity for methane reached ca. 4% at high pressure (p = 10 atm) and low pH (pH = 1). In 2019, Wang and co-workers obtained more than 40% faradaic efficiency of methanol using cobalt phthalocyanine highly dispersed on carbon nanotubes (CNTs). 107 The authors attributed the high performance to the highly dispersed nature of the catalyst on suitable carbon nanotube supports. Similar observations of methanol produced from CO2 using CoPc catalysts were reported, albeit with lower methanol selectivity. [START_REF] Kapusta | Carbon Dioxide Reduction at a Metal Phthalocyanine Catalyzed Carbon Electrode[END_REF]108 To form further reduced species with C -C bonds, Cu macrocycles proved the most effective. However, as discussed in more detail in section 1.3.6, the molecular nature of the utilized copper species was not preserved during catalysis. These results showed that depending on the metal center and the specific experimental conditions, metal phthalocyanines and porphyrins can catalyze the multi-electron reduction of CO2 to highly reduced products.

Influences of ligand on porphyrins and phthalocyanines species

As in homogeneous conditions, ligands surrounding the molecular macrocycles can be precisely tuned to control the binding of intermediates to optimize reaction rate and selectivity. Two early studies by Kaneko and co-workers compared the effect electron donating groups (butoxy), and electron-withdrawing groups (cyano) have on the CO2 reduction activity of CoPc. Under applied potential of -1.3 V vs. Ag/AgCl and in phosphate electrolyte with pH of 4.4, CoPc(BuO)8 reduces CO2 to CO with higher turnover frequency (306 s -1 vs. 15 s -1 ) and higher CO/H2 ratio (4.34 vs. 0.69) compared to CoPc. 109 Under a similar electrolyte at -1.15 V vs. Ag/AgCl, CoPc(CN)8 was also more selective at reducing CO2 to CO without significantly impacting the turnover frequency. 110 The authors suggested that the electron-donating BuO improves the electron density around the metal center, facilitating the binding of CO2 and increasing activity and CO selectivity. Regarding the effect of the electron-withdrawing group, the authors argued that the rate-determining step was changed from the catalyst's second reduction to the desorption of CO from CoPc. Other recent studies also found increased activity and CO selectivity when electron-withdrawing groups, including CN 111 and F, 112 are attached to CoPc catalysts. The activity enhancement results from both electron-withdrawing and electron-donating groups did not give a clear trend of the effect of substituents. The conflicting observations suggest that other factors resulting from the functional groups' addition may play a non-negligible role in CO2 reduction activity.

In 2019, Manthiram and co-workers reported a comprehensive study on the influences of different substituents (Figure 1-17A) on cobalt porphyrin activity of CO2 to CO reduction. 113 The authors used a low catalyst loading of 0.8 nmol/cm 2 to minimize aggregation and quantified the electron-withdrawing character based on Hammett parameters, and Mulliken charges. With cationic substituents, the log(TOFCO) reversely correlated to the Hammett 𝜎 values of the functional groups (Figure 1-17B). The correlation indicates that the stronger the electron-donating character of the substituents, the higher the turnover frequency of the catalysts. The same effect was observed with neutral and anionic functional groups. However, the trendline drastically differed from cationic species (Figure 1-17B), with catalysts having a much lower log(TOFCO) despite the strong electron-withdrawing character. This phenomenon suggests that the electrostatic effect of the substituents strongly affects the performance of the catalysts, which has been observed under homogeneous conditions. 69 Chapter 1

29 Besides induction and electrostatic effects, the steric effect of ligands of a molecular macrocycle can change how the catalyst interacts with the electrode surface. Officer and co-workers reported in 2019 that bulky alkoxy groups grafted on cobalt phthalocyanine reduce the aggregation of catalysts on the electrode surface, resulting in the increase of the TOFCO compared to unmodified CoPc. 114 Depending on the ligands, a molecular macrocycle can also serve as a precursor to creating highly dispersed metal catalysts. In 2012, Kenis et al. studied the CO2 to CO catalytic activity of organometallic silver with different ligands. 115 While no organometallic species perform better than the 70-nm-silver-nanoparticles, AgPc, Ag pyrazole, and Ag 3,5diamino-1,2,4-triazole showed increased CO current density and selectivity. This improved performance happened at a much lower mass loading of nanoparticle silver, demonstrating a much more efficient use of the metal. Another study obtained active copper electrode materials via electrodeposition from a copper cyclam solution. 116 When a fluorine-doped tin oxide electrode was subjected to multiple cyclic voltammetry cycles from -0.3 V to -2.0 V vs. Fc + /Fc in a DMF/H2O (97/3 v/v) containing 1.3 mM Cu(cyclam), a nanoparticle composite Cu materials with the organic ligand attached was deposited on the electrode. This functionalized electrode can produce HCOOH at 90% faradaic efficiency in a similar CO2-saturated DMF solution without the Cu(cyclam) complex, demonstrating a viable pathway for tuning the catalysis properties of metal catalysts using organic ligands.

Aggregation effects and TOF calculation

Similar to the homogeneous system, the turnover frequency of a catalyst represents its intrinsic activity, providing a reliable benchmark to compare different catalysts. [START_REF] Costentin | Benchmarking of Homogeneous Electrocatalysts: Overpotential, Turnover Frequency, Limiting Turnover Number[END_REF] Typical cyclic voltammograms in heterogeneous molecular catalysts are not electrochemically reversible, making foot-of-the-wave analysis unable to derive the TOF value. Therefore, performing electrolysis, quantifying the resulting products, and determining the number of active sites are necessary steps to accurately calculate the TOF of a heterogeneous catalyst. While the concept of active sites in heterogeneous catalysis is an intricate topic, 58 for heterogenized molecular complexes, a lower limit of the TOF can be estimated by considering all immobilized catalysts on the electrode to be active. The precise number of active sites is complicated to determine due to catalyst aggregation, which could result from external factors like loading, metal center, ligands, and electrode preparation.

The aggregation effect is more pronounced in the case of phthalocyanine and porphyrins, which are planar complexes with favorable 𝜋 -𝜋 stacking mode. 117 Porosity, double layer capacitance measurement, and integrating catalytic redox peak from cycle voltammetry are standard techniques to estimate the number of active sites. 118 However, porosity and electrochemically active area do not directly correlate to the catalytically active sites, and redox peak integration can induce uncertainty from baseline correction. In some cases, CV peaks of heterogenized species are not reliable. [START_REF] Corbin | Heterogeneous Molecular Catalysts for Electrocatalytic CO2 Reduction[END_REF] One study demonstrated the effect of catalyst aggregation on the performance of CoPc on CO2 reduction by varying the catalyst loading. When CoPc surface concentration increased from 10 -11 to 10 -7 mol/cm 2 , aggregate can be seen from the scanning electron microscopy (SEM) images (Figure 1-18), and the turnover frequency increased three orders of magnitudes. 119 Under aggregation-free conditions, the current density should be proportional to the catalyst loading, and the TOF should be constant at sufficiently high overpotential. 120 Therefore, to obtain accurate TOF of heterogenized molecular catalyst, the catalyst should be investigated at low concentration with all the influencing conditions adequately declared. 121 In addition to using low catalyst loading, depositing the molecular species in a highly solubilizing solvent can better disperse the catalyst. In all cases, the lower bound TOF should be reported as the aggregate-free conditions are not the desired conditions to achieve high current density and selectivity. 119 The interaction between the electrode and the immobilized catalyst layer significantly influences their CO2 reduction performance, a critical difference between heterogeneous and homogeneous catalysis. A wide range of factors affects the electrode-catalyst interaction: the nature of the electrode, the conductive support materials, the nature of the catalyst, and the immobilization method. Koper and coworkers conducted a systemic study assessing the impact of different carbon-based electrodes on CO2-to-formate reduction using indium protoporphyrin (InPP). 122 Out of the three studied materials, pyrolytic graphite substrate showed higher selectivity and reactivity than glassy carbon and boron-doped diamond, while the boron-doped diamond was the most stable. Since pyrolytic graphite has the highest electrochemically active area, it had less aggregation and was not saturated with InPP at high loading, unlike glassy carbon and boron-doped diamond. Although this study demonstrated the importance of electrode materials, the reasoning behind the observed activity difference was not clear.

The standard method of molecular catalyst immobilization is physical adsorption by drop-casting a mixture of molecular complex, conductive support, and binder onto the electrode surface. Commonly-used supports are carbon-based materials, including carbon nanotubes (CNTs), carbon black, and graphene, with CNTs generally outperforming the other materials. One study found that CoTPP displayed significantly higher current density (2.9 vs. 0.028 mA/cm 2 ) and selectivity (91% vs. 28%) towards CO compared to the dissolved species. 123 When carbon black was used instead of CNTs, higher selectivity (97%) but lower current density (1.8 mA/cm 2 ) were observed. Multiple studies corroborate the positive effect of CNTs towards CO2 reduction with molecule macrocycles, including the increased formate current density with FeTPP, 124 increased CO current density with Co protoporphyrin 125 and CoPc, with CoPc/CNTs outperforming CoPc supported on reduced graphene oxide or carbon black. 112 The studies attributed the increased CO2 reduction activity to the 𝜋 -𝜋 interaction between the macrocycles and the CNTs, since CNTs have the highest degree of sp 2 carbon in the studied species. Other beneficial factors include the high surface area and high electrical conductivity of CNTs. 112 In the studies of InPP, 122 CoPP, 125 and CoPc, 112 the authors pointed out the influence of catalyst aggregation on the performance of the catalysts. This observation emphasizes the importance of decoupling the effects of increased dispersity and the intrinsic electronic interactions between the support and the catalyst. Besides physical adsorption, molecular complexes can be bonded to the electrode surface via covalent grafting to improve the catalytic performance and catalyst stability. 126 In addition to conductive carbon materials, polymer encapsulation is another strategy Chapter 1

to support catalysts on an electrode. Unlike conductive carbon materials, catalysts are typically chemically attached to the polymer support. Therefore, the support can influence the second-sphere interaction and the electronic structure of the active site, as well as control the local environment surrounding the catalysts. Poly(4vinylpyridine) (P4VP) is a widely studied encapsulating agent for molecular macrocycles. An early study showed the CO2-to-CO reaction was more selective and active when the CoPc catalyst was encapsulated by P4VP. 127 The authors attributed the enhancement effects of P4VP to the electron-donating nature of the axial pyridine ligand, the local proton source of the P4VP, and the increase in CO2 local concentration. Later work by the same group of authors further optimized the reaction conditions: they found a pH of 4.4 to be best for CO production and a pH of 5 for optimal CO/H2 selectivity. 128 The authors also found that Poly(2-vinylpyridine) (P2VP), a non-coordinating version of P4VP due to steric hindrance, performed worse than P4VP, suggesting the importance of the pyridine coordination effect. A more recent study provides a comprehensive assessment of the impact of polymer coordination on the CO2 reduction performance of CoPc (Figure 1-19A). 105 The pyridine (py)coordinated CoPc(py) and the non-coordinating polymer P2VP underperformed the coordinating polymer P4VP, indicating the combined effect of axial ligand and polymer backbone on the increased catalytic activity. When CoPc(py) was mixed with the noncoordinating P2VP, they obtained similar TOFCO and FECO, suggesting that the combined effect was not synergistic.

Another study tested various polymers encapsulating InPP for formate production, including DDAb, P4VP, PEDOT/PSS, and Nafion (Figure 1-19B). 122 Except for Nafion, all species enhanced the selectivity and current density compared to unmodified InPP. And while the P4VP species provided the highest formate selectivity, the current density was the lowest compared to the other three polymers. However, a control experiment with only P4VP showed a Faradaic efficiency of 20% for formate, which suggested the observed increase in activity may result from the independent catalysis on the polymer and the InPP.

Co-catalytic systems

Heterogenized molecular complexes on catalytically active metal electrodes can work in tandem to increase the selectivity and activity of specific products. Grafting molecular ligands on metal electrodes can help stabilize and promote specific intermediates in the catalytic cycle. The highly selective nature of the molecular macrocycle can also be employed to reduce CO2 to CO, which then gets further reduced on metal catalysts in a cascade manner. Concerning the first approach, metalfree tetra(4-aminophenyl) porphyrin ligand was electrochemically polymerized on gold substrates to enhance the catalysis of CO2 to CO. 129 At -0.7 V vs. RHE, the modified electrode had significantly higher CO selectivity (95% vs. 25%) and current density (2.5 mA/cm 2 vs. 0.3 mA/cm 2 ) compared to the unmodified gold substrate. Interestingly, when the ligand monomers were attached to the gold foil, no noticeable catalysis enhancement was observed, suggesting the importance of polymerization. The polymer layer thickness appears critical for the catalytic performance, with the optimal value at 60 nm. A similar strategy was used on gold nanoparticles with attached porphyrin ligands, and enhancement in CO production was also observed. 130 In addition to gold, porphyrin ligands can also tune the reduction of CO to C2+ products on copper. 131 The porphyrin ligands, lying flat on the copper substrate by thiol linkers at the ortho position, increased the production of oxygenated species, with ethylene selectivity remaining unchanged. Interestingly, the ligand binding mode was crucial to the observed C2+ selectivity. When the thiol linkers were changed to the para position, the ligands were arranged perpendicular to the electrode, and C2+ products were diminished.

Regarding the approach of using metal complexes to produce CO selectively, Sargent and co-workers reported a tandem system of iron porphyrin sprayed on a copperbased gas diffusion electrode. The increase in local CO concentration near the copper sites steers the CO2 reduction activity towards ethanol. 132 Using a similar strategy, Buonsanti and colleagues found that a mixture of iron porphyrin and copper nanocubes adsorbed on a glassy carbon electrode produces more ethylene than the nanocubes alone. 133 The iron porphyrin species having the highest CO turnover frequency induces the most significant increase in ethylene generation on the copper catalyst, emphasizing the crucial role of the CO local concentration.

Limitations of heterogeneous molecular catalysts

Heterogeneous molecular catalysts generally reflect the benefits of the homogenous counterpart: having well-defined active sites, allowing for fine chemical tuning around the catalytic center. The determination of in-depth reaction mechanisms is, nonetheless, more complex. First, unlike in a homogeneous system, the cyclic voltammograms of adsorbed species often display broad, sometimes irreversible peaks, making determining the exact oxidation states of the molecules challenging.

The cyclic voltammetry response can also vary strongly depending on external factors such as the conductive carbon support, the catalyst concentration, or the solvent used for drop-casting. Furthermore, heterogeneous catalysis usually employs water-based electrolytes. While water is preferred in applied systems, it offers less control over the catalysis process since protons are typically available from the water reduction side reaction. Finally, evaluating the exact number of active sites in a heterogeneous catalysis system remains a question of debate. 58,119 Therefore, a precise estimation of the turnover frequency is hard to achieve. In a homogeneous system, cyclic voltammetry usually exhibits well-defined and reversible waves, enabling the oxidation states of the investigated species to be determined precisely. Foot-of-the-wave analysis can make use of well-defined CV features to quickly and accurately estimate the turnover frequency. The use of organic solvents as electrolytes also offers control over the number of protons and the nature of their sources, allowing for complex analysis of a molecule's behaviors.

The heterogeneous nature means some issues related to the homogeneous system are not present, including inefficient catalyst use, water solubility, and product separation from catalysts. Heterogeneous molecular catalysts have also demonstrated industrially relevant current density and the ability to produce highly-reduced molecules like methanol and methane with modest selectivity. However, creating highvalue products with C -C bonds and catalyst stability remain the two critical limitations. These two topics will be discussed in detail in the following section. Overall, heterogeneous molecular catalysts provide a bridge between fundamental study and more applied systems of metal and composite catalysts. Whether or not it applies to the industrial scale, the knowledge from studying heterogeneous molecular catalysts will be valuable for developing CO2 utilization technologies.

Reduction of CO2 to C2+ products by catalysts with well-defined active sites

As discussed earlier, in the scope of the CO2 reduction reaction, C2+ products such as ethylene or ethanol hold special interests as they have both high value and a large market. 26 Forming C2+ products on catalysts with well-defined actives would provide a platform for a detailed mechanistic study of the process. Generating C2+ products primarily uses Cu-based species, but other transition metals catalysts, 134 or metal-free catalysts 135,136 , can reduce CO2 to C2+, albeit with much lower activity.

While most molecular catalysts for CO2 reduction produce only C1 products, there have been several reports of C -C bond formation from CO2 using a molecular catalyst. In 2002, Fujishima and colleagues reported the presence of C2H4 and C2H6 after the electrochemical reduction of CO2 with cobalt and copper phthalocyanine. 137 However, since the Faradaic efficiency of these highly reduced products was lower than 1%, they did not further characterize and study this aspect of their catalysts. The following Chapter 1 discussion focuses on examples of typical C2+ products being formed with molecular catalysts and single-atom catalysts.

Oxalate

Oxalate is the least reduced product with a C -C bond from one electron CO2 reduction. It can be formed by an un-catalyzed process involving two radical CO2 Another pathway includes multi-nuclear molecular complexes that form oxalate at a lower overpotential due to the synergistic effect of two or more binding sites in close proximity. Examples include polymerized rhenium complex, 144 bi, and tri-metal complex based on Co, Rh, Ir, 145,146 and a binuclear ruthenium catalyst 147 . One notable example consists of the work of Bouwman and co-workers, 148 where they used a dinuclear copper complex as a catalyst to reduce CO2 to oxalate with Faradaic efficiency of 96% at -0.03 V vs. NHE. Although the catalyst possesses two binding sites, the Cu-Cu distance is too far for the two sites to interact.

Ethylene

Copper-based molecular catalysts have been reported to reduce CO2 to ethylene. In 2016, the groups of Brudvig and Wang 149 showed that CH4 and C2H4 are generated on a carbon fiber electrode modified with Cu-porphyrin complex with a hydroxyl functional group. The total current density at -0.97 V vs. RHE is 7.5 mA/cm 2 , with CH4 and C2H4 faradaic efficiency of 25% and 17%, respectively. Control experiments with metal-free porphyrin ligand and Cu-porphyrin without hydroxyl functional group showed CO is the main product of CO2 reduction. The experiments demonstrated the role of the Cu metal center and the -OH functional group in forming the C -C bond.

The authors postulated the reactivity of the Cu center and the stabilizing effect of the -OH group facilitate the formation of the observed multi-carbon products, but a detailed mechanism was not provided. Finally, using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and inductively coupled plasma mass spectrometry (ICP-MS), they showed the nature of the Cu molecular catalyst stayed unchanged before and after electrolysis.

Following the work of the groups of Brudvig and Wang, Yotsuhashi and co-workers 150 Chapter 1 synthesized a crystalline copper phthalocyanine and studied its electrocatalytic activity for CO2 reduction, as they argued that the distance between Cu atoms plays a vital role in forming C -C bond. Indeed, at -1.6 V vs. Ag/AgCl, they obtained 25% FE for C2H4 with a partial current density of 2.8 mA/cm 2 . With a non-crystalline CuPc, CO was the main product of CO2 reduction, demonstrating the role of the catalyst's crystallinity in forming the C -C bond. However, the selectivity for C2H4 was not maintained in electrolysis longer than 12h, indicating the poor stability of the crystal. Based on these findings, Nam and co-workers 151 reported the activity of a di-nuclear copper complex with a Cu-Cu distance of 3.77Å for reducing CO2 to C1 products and C2H4. They demonstrate the dinuclear species yield maximum C2H4 at 42% Faradaic efficiency at -1.27 V vs. RHE, compared to 22% at -1.37 V vs. RHE with the mononuclear counterpart. However, they also show the effects of other parameters such as electrolyte salt and concentration, carbon support type, and electrode rotating speed affect the production of C2H4 at the same order of magnitude compared to monomer and dimer species.

Following the same theme, Liao et al. prepared a metal-organic framework (MOF) based on copper phthalocyanine ligands with CuO4 node. 152 The MOF exhibits a Faradaic efficiency of 50% at -1.2 V vs. RHE with a current density of 7.3 mA.cm -2 . They attribute the good selectivity to the synergistic effect between CuO4 and CuPc, where the former has favor *CO adsorption energy to generate CO for C -C dimerization on the latter.

Ethanol and acetic acid

Although most C -C bond formations are reported on Cu-based molecular catalysts, there are a few examples of non-Cu molecular catalysts for CO2 to C2 products. In 2019, Roy, Schöfberger, and co-workers synthesized a fluorinated cobalt corrole molecular complex with a triphenylphosphine axial ligand and three polyethylene glycol (PEG) groups (Figure 1-20A) capable of reducing CO2 to C2H5OH. 153 Using the cobalt-corrolemodified carbon paper in 0.1 M NaClO4, CO2 was reduced to ethanol with 48% Faradaic efficiency for five hours at -0.8 V vs. RHE. Other reduced products included methanol, formate, and acetate, with a small amount of formaldehyde and glyoxal. CO was not presented, leading the authors to propose a pathway of ethanol formation through formic acid and formaldehyde intermediates instead of the frequent CO pathway. The proposed pathway was partly demonstrated using formic acid, glyoxal, formaldehyde, and methanol as substrates, along with density functional theory (DFT) calculation.

Ethanol was not generated in control experiments with similar cobalt corrole species without the polyethylene glycol substituents, demonstrating the PEG groups' crucial role in forming ethanol. Using a similar corrole ligand with a manganese metal center instead of cobalt (Figure 1-20B) as a catalyst for CO2 reduction, the groups of Roy and Schöfberger were able to obtain acetic acid. 154 At -0.724 V vs. RHE, they observed 60% acetate with a specific current density of 0.570 mA.cm -2 and a small amount of methanol. The proposed mechanism involved the binding of CO2 and the subsequent proton-coupled electron transfer step, converting it to an Mn-COOH species. The C -C bond formation step creates an intermediate oxalate species, which is converted to acetate by additional reduction, protonation, and dehydration steps.

Single-atom catalysts

Since the work of Zhang and co-workers on CO oxidation using single-atom catalysts (SACs), 155 this family of catalysts has gained considerable interest in the field of CO2 reduction due to their hybrid nature between molecular and materials catalysts. Similar to most molecular catalysts in single and separated active sites, the main product of CO2 reduction on transition metals SACs is CO with high selectivity and current density, 156-161 with a few works reporting higher reduced products. CH3OH has been generated using Cu-based SACs, 162 and CH4 has been formed on Cu-doped CeO2, 163 ZnN4 164 and FeN4 165 species. Indeed, DFT calculation showed SACs with porphyrin-like structures having different binding energy with radical species of CO and H than their metal counterparts, enabling reduction further than CO with MnN4 and FeN4, which is not favorable on purely metallic species. 166 Moreover, C2+ products, including ethanol, 167 acetone, 168 have been demonstrated as the result of the reduction of CO2 on Cu-based SACs.

As most SACs are prepared by pyrolyzing a variety of starting materials, precisely controlling the material's structure is challenging. [169][170][171] Extended X-ray absorption fine structure (EXAFS) spectroscopy and TEM are the most common techniques to characterize a SAC, mainly its coordination number and the nature of the neighboring species to the metal center. However, EXAFS will provide only the average coordination number, and different adjacent atoms can fit in one EXAFS model. At the same time, TEM images only represent specific spots and not the whole sample. Therefore, unlike molecular catalysts, whose structures are well characterized, SAC materials' exact nature is undetermined. The structure uncertainty makes both modeling and mechanism study a complicated task. In addition, similar to a molecular species, the structure of SACs is subject to changes during catalysis, making in situ characterization a necessity in studying the activity of SACs for CO2 reduction.

Chapter 1

Limitations of producing C2+ products from molecular and single-atom catalysts

Despite significant efforts, producing C2+ products from CO2 using molecular species achieved limited success. The obtained current density and selectivity are often lower than their metal counterpart. More importantly, the structural integrity and the nature of the active species are a matter of debate. However, producing C2+ products from catalysts with well-defined active sites remains a great challenge and opportunity. Due to their well-defined structure and easy tunability, molecular catalysts provide a platform for the in-depth study of the mechanism of the CO2 reduction catalysis process, both theoretically and experimentally. First, a detailed mechanistic cycle can be established by varying experimental parameters, i.e., substrates, electrolyte concentration, and proton sources. Based on the mechanism, spectroscopy techniques can reveal the local and electronic structures of intermediates. The rate-determining step can also be established from the catalytic cycle. Analyzing molecular catalysts will provide great insight into the development of other families of catalysts for CO2 reduction

Importance of in situ characterization and isotopic labeling

While investigating the formation of the C -C bond from CO2 using molecular catalysts, confirming the nature of the catalyst during its operation and identifying the origin of the obtained products are crucial. When investigating different copper complexes generating C2H4 from CO2, Wang and co-workers discovered that the catalysts transformed into copper nanoparticles under catalytic conditions. 172 Using Xray absorption spectroscopy, they obtained the signature signal of metallic copper from their molecular catalysts under applied reduction potential (Figure 1 -21). The molecule-to-metal transformation was further corroborated by EXAFS fitting showing the coordination number and bond nature of a copper species and XPS and TEM signals. Interestingly, the CuPc and the previously reported Cu porphyrin 149 exhibited reversible nature, demonstrated by the similarities of X-ray absorption spectroscopy (XAS) and XPS spectra of the initial species and the species after applying an oxidative potential. This behavior was not observed on the Cu-based MOF and Cu(cyclam), where the application of oxidative potential did not reverse their structures. Using a similar operando XAS technique, Fontecave and co-workers showed similar reversible behavior of a Cu-N-C material 173 and an immobilized CuPc on carbon nanotubes 174 .

These examples showed the importance of in-situ and operando characterization, as ex-situ analysis does not reveal the nature of the catalytically active species. However, in the work of Nam et al., 151 ex-situ characterization by XPS revealed changes in their copper-based catalyst before and after electrolysis, and XAS and TEM images confirmed metal nanoparticle formation. Due to the low amount of products generated by laboratory-scale reduction of CO2, it is crucial to verify the products' origin. The detected carbon products may arise from a non-catalytic reaction involving the catalyst and/or other species in the system.

In 2014, Maverick et al. reported a binuclear copper complex reducing CO2 to oxalate. 175 However, after further studies, Maverick, Beller and co-workers demonstrated that the previously reported oxalate resulted from ascorbate oxidation instead of CO2 reduction. 176 Notably, when isotopic labeling using 13 CO2 was reperformed in a more controlled way, unlike the original study, the result showed only an unlabelled product. These discoveries resulted in the article's retraction. In addition, when studying water oxidation using cobalt porphyrin, which is also a common catalyst for CO2 reduction, Sakai's group characterized the degradation of their catalyst. 177 Using electrospray ionization -mass spectrometry, they identified several degraded porphyrins with fewer carbon atoms on the ligand that could form molecules similar to the products of CO2 reduction.

Thesis outline

This thesis aims to use molecular catalysts, namely molecular transition metal macrocycles, to carry out the electrochemical reduction of CO2 beyond CO.

In chapter 2, we explore the cascade catalysis approach to achieve our aims by developing a series of microfluidic electrochemical flow cells. The idea would be for the first cell to reduce CO2 to CO, which then travels to the subsequent cell to get further reduced. Upon encountering difficulties with the electrochemical performance of the microfluidic cell caused by bubbles generation, we fabricated a 3D-printed electrochemical flow cell with the channel in millimeter dimension. Laminar flow and the electrocatalytic performance of the cell were tested. We then used finite element analysis to build a model and calculate the concentration profiles of the products and reactants inside the flow channel of the cell during CO2 electrolysis.

In chapter 3, we first tested the electrochemical reduction of CO2 on an iron phthalocyanine catalyst using the 3D-printed millifluidic cell. Although our initial goal was the selective production of CO, upon detecting hydrocarbon products from the catalysis with just one reduction stage, we focused on the catalytic behavior of FePc further instead of building a sequential reduction system as the original aim. Therefore, we incorporate a gas chromatograph into the flow system to perform inline characterization of the gas products. The electrochemical cell was also modified to add a window for performing operando X-ray spectroelectrochemistry experiments. We then carried out a series of control experiments to confirm the origin of the detected carbon. Spectroscopic techniques (X-ray and infrared) were used to investigate the molecular nature of the FePc catalyst before, during, and after catalysis. A mechanism for the C -C bond formation step was proposed based on experiments with FePc, where CO2 was replaced with other reactants, namely formaldehyde and CO.

In chapter 4, we first explored the influences of external parameters, namely the electrolyte flow rate and the concentrations of catalyst, electrolyte, and conductive carbon support, on the reduction of CO2 on FePc. Then we studied the impact of substituent groups on FePc, as well as the effects of changing the metal center of the phthalocyanine ligands. Finally, we performed preliminary tests on iron porphyrin and its derivatives, focusing on their ability to produce C2+ products.

In the last chapter, we restate the novelty and impact of forming products with C -C bonds on a single-site catalyst. The summary of each chapter was provided, as well as future experiments and strategies to understand the CO2-reducing activity of molecular macrocycles. We end the thesis by discussing the general role of CO2 reduction and other electrochemical energy storage technologies in tackling climate change.

Introduction

Cascade catalysis for CO2 reduction

Cascade catalysis is an appealing approach toward CO2 reduction. As discussed in chapter 1, reducing CO2 often involves multiple electron transfer steps, with various pathways the reaction could follow, especially when high-value products with C -C bonds are concerned. The complexity of the reaction can result in a non-selective process that yields many side products along with the desired one, lowering the overall efficiency. The cascade catalysis approach separates a complex reaction into a series of simpler processes. 1 Cascade catalysis is common in nature. One example includes the Calvin cycle, where carbon dioxide is reduced to glucose in a stepwise fashion. 2 When the reaction steps happen sequentially, the number of undesirable pathways the reaction can follow is minimized, resulting in higher selectivity for the desired products. Moreover, in a sequential system, each step can be tuned independently, leading to a more efficient reaction since all intermediate processes happen under the optimized conditions.

Many studies have applied cascade catalysis to the field of CO2 reduction. Two common strategies involve utilizing a system of reactors to carry out a series of reactions and tuning the catalyst to incorporate different active species responsible for different reaction steps. Concerning the former approach, Xiang and co-workers calculated that a two-step conversion from CO2 to C2H6O would result in higher solarto-fuel efficiency than a one-step conversion. 3 The role of cascade catalysis was highlighted, as each reactor could have a specific catalyst and potential optimized to convert CO2 to CO and CO to C2H6O. Spurgeon and colleagues studied the reduction of CO2 to ethanol in a series of two flow reactors. 4 The first reactor consisted of a silver cathode to generate CO. The resulting CO was transferred to the second reactor, which was converted to ethanol on an oxide-derived copper catalyst. The sequential system yielded ethanol with an average faradaic efficiency of 11% at an average applied potential of -0.52 V vs. RHE. Although this result was not inherently better than a direct conversion of CO2, the study demonstrated the sequential concept and proposed a better reactor design to improve the CO conversion. A similar approach was reported by Ager and co-workers, where two plates of silver and copper form an electrochemical flow cell cathode. 5 The tandem electrode exhibited a higher CO conversion rate with increased oxygenated products having C -C bonds.

Upon reducing CO2 to CO, chemical conversion of CO to high-value products is an alternative strategy instead of electrochemical reduction. 6 Skrystrup and colleagues developed a CO2 reduction system consisting of a two-chamber glass cell connected via a glass bridge. The first chamber electro-reduced CO2 to CO with FeTPP catalyst in TBABF4/DMF electrolyte. The next chamber contained a Pd catalyst and other reagents to perform carbonylation reactions with CO. The amount of synthesized organic species correlated with the amount of CO generated in a linear relationship. They also demonstrated that the same strategy worked with atmospheric CO2 using a threechamber cell, with the additional cell containing CO2 binder to capture CO2 from the atmosphere. Aukauloo and co-workers employed a similar approach to valorize CO via carbonylation reaction on a Pd catalyst. 7 The same two-chamber glass cell was used, but the electroreduction was replaced by the photoreduction of CO2 using rutheniumbased photosensitizer and rhenium-based catalyst with DMF and water (9:1) as solvent and proton donor, respectively.

The catalyst-tuning approach was partly discussed in the co-catalytic systems in Chapter 1 with the work of Sargent and co-workers, where they functionalized a copper surface with iron porphyrin to increase the local concentration of CO. 8 The group of Ager also patterned a gold and silver electrode with copper sites for cascade CO2 reduction. 9 In both cases, the high local concentration of CO steers copper's catalytic activity toward oxygenate products, agreeing with the above observation made by Ager's group. Tandem catalyst design is not limited to the electrochemical reduction of CO2. Yang and co-workers designed a nanostructured core-shell catalyst, CeO2-Pt@mSiO2-Co, to convert CO2 to C2 -C4 hydrocarbon via thermochemical reduction. 10 The Pt/CeO2 part carried out the reverse water-gas shift reaction, converting a mixture of CO2 and H2 to CO. The product mixture was the feedstock for the Fischer -Tropsch process happening on the Co-SiO2 site to form the hydrocarbon products. Under optimum temperature and pressure, the process had a 60% selectivity toward the hydrocarbon products. Likewise, Lee and co-workers used a system of three different enzymes to perform a stepwise conversion of CO2 to methanol via formate and formaldehyde. 11

Microfluidics and its application in electrochemical energy devices

Microfluidics deals with manipulating fluids in systems with at least one dimension from tens to hundreds of micrometers. In a microscopic channel, the role of viscosity dominates that of inertia, resulting in laminar flow, a notable characteristic of a microfluidic device. In a laminar flow, fluids move in parallel layers without any turbulence and eddies; thus the diffusion of molecules between the layers is the only mode of mass transport. 12 Since microfluidic devices offer greater control in fluid handling due to laminar flow, manipulating reaction intermediates and implementing multi-step synthesis using these devices can be advantageous compared to a batch system. 13 In electrochemical energy systems, the microscopic dimension of the fluid channel means the electrode distance is similar to the diffusion layer's thickness, potentially leading to better energy efficiency and improved power density. Moreover, the limited mixing action in laminar flow enables the design of membraneless electrochemical devices, where their flow separates the catholyte without needing a membrane. 14 Fuel cells are the most notable energy conversion device currently being developed under microfluidic conditions; flow batteries and electrolyzers are other examples. 14 Microfluidic fuel cells have been demonstrated to have high power density and shorter refueling times. 15,16 Hydrogen bromide flow battery also showed improved performance under microfluidic conditions. 17 Membraneless water splitting electrolyzer developed by Modestino and co-workers showed promising performance and excellent product separation at the device's output. 18 Microfluidic CO2 electrolyzers often employ a gas diffusion electrode, where a thin layer (1 -3 μm) of flowing electrolyte separates the anode and the cathode. 19 Kenis and colleagues developed a microfluidic CO2 reactor using a gas-diffusion electrode to reduce CO2 to HCOOH. 20 The versatility of the microfluidic reactor allows for rapid investigation of catalysts under different operating conditions. Although these devices function in microscopic dimensions, their uses in large-scale applications are possible by areal scaling or parallelization. 14

Scope of the chapter

In this chapter, we present the development of a polydimethylsiloxane (PDMS) microfluidic reactor aiming to carry out CO2 reduction sequentially. The performance of the cell was evaluated using cobalt phthalocyanine as a catalyst. We then discussed the limitations of the cell design and envisioned a different microfluidics design to address some of the problems encountered.

Subsequently, we fabricated a millifluidic electrochemical flow cell by 3D printing. The cell can perform an operando spectro-electrochemical experiment to study CO2reducing catalysts using X-ray absorption spectroscopy. A mixing test demonstrated the dimensions of the millifluidic cell can still induce a laminar flow in its channel. An iron phthalocyanine catalyst was used to assess the CO2-reducing behavior in the cell.

The distribution of products inside the flow channels was calculated using finite element analysis. Finally, we discussed the advantages and drawbacks of the cell and provided some perspectives to improve the design.

PDMS microfluidic flow cell

A microfluidic PDMS flow cell was designed and fabricated to investigate the prospect of CO2 reduction by membrane-free electrolysis based on laminar flow separation. The PDMS material was chosen since it is the standard material for building microfluidic devices. The flow cell would serve as a reactor in a sequential system to reduce CO2 to high-value products.

Fabrication

A PDMS microfluidics electrochemical flow cell was made following the design scheme shown in Figure 2-1A. The process started with fabricating a mold for the PDMS casting. A 250 µm thick SU-8 negative photoresist was spin-coated onto a 3-inch diameter, 360 µm thick, (111) silicon wafer. The photoresist layer thickness defined the thickness of the microfluidic channel, which in turn defined the distance between the working and the counter electrodes. Afterward, the coated wafer was loaded into the Kloe Dilase 250 laser writer lithography machine, which shined a laser onto the photoresist according to a pre-designed pattern. Finally, the wafer was washed with the SU-8 photoresist developing solution. Since a negative photoresist was used, the photoresist parts not shined on by the laser would be dissolved after the developing step. The final structure of the mold on the silicon wafer is illustrated in Figure 2-1B. It consists of the main microfluidic channel, two circles for the input and output holes, and two support blocks for the placement of the counter electrode.

The counter electrode of the cell was made by attaching a 10 x 10 x 0.3 mm glassy carbon SIGRADUR® plate (HTW) to a piece of copper tape. A copper wire was soldered to the copper tape, and the assembly was glued to a 3D-printed support plate made with Veroclear® plastic to increase the mechanical strength of the glassy carbon plate. The electrode assembly was then placed on the mold indicated by the transparent black square in Figure 2-1B, in which the two support blocks ensured that the surface of the glassy carbon plate lay flat with the channel. A mixture of PDMS and curing agent (10/1 weight ratio, respectively) from Sylgard was mixed, and bubbles were removed by putting the mixture in a vacuum chamber. The silicon wafer mold was placed into a petri dish, and the bubble-free PDMS was slowly poured onto the mold until the thickness of the PDMS was approximately 5 mm. The container was loaded into a vacuum chamber to evacuate the bubbles formed in the pouring process. When the PDMS was bubble-free again, the dish was placed inside an oven at 73 o C for at least one hour. The excess was cut when the PDMS was fully cured, forming a 2.5 x 3 cm rectangular PDMS chip. Three holes were punched to accept 1/16 inch outer diameter, 0.2 mm inner diameter Teflon tubing. Two holes at the ends of the channel are for the inlet and outlet of the working solution, while the hole indicated by the grey circle in Figure 2-1B is for reference electrode connection. placement of counter electrode assembly, the circle represents the hole for the reference electrode), the scheme is scaled 300% for clarity.

A 25 x 10 x 0.3 mm glassy carbon SIGRADUR® plate was used as the working electrode.

The plate was masked with a 25-µm-thick Kapton tape with a 2 x 4 mm 2 rectangle opening (Figure 2-2A). The mask made the electro-active area of the working electrode five times smaller than the counter electrode. The electrical connector was made by sticking a piece of copper tape on the other side of the glassy carbon plate. The reference electrode was prepared by inserting a 0.3 mm diameter silver wire (99.99%, Goodfellow) into a short section of a 1/16 inch outer diameter, 0.5 mm inner diameter PTFE tubing. The silver wire was adjusted so that one end was flush with the tubing while the other was glued to the tubing with Torr Seal epoxy, leaving about 2 cm protruding outside the tubing.

The actual photographs of the PDMS cell are shown in Figure 2-2B-D. The working electrode was placed under the PDMS chip to assemble the cell, so the channel was covered entirely, as shown in Figure 2-2D. Then the chip and the working electrode were placed inside a 3D-printed holder, also made of Veroclear®, consisting of a bottom and a top cover (Figure 2-2E). The two parts were held together by six M3 nuts and bolts, which pressed the PDMS chip tightly against the working electrode, creating a leak-free seal between the channel and the working. After that, the input, output, and reference electrode tubing were connected to the chip using 1/16 inch outer diameter and 0.2 mm inner diameter Teflon tubing (Figure 2-2E). suppressed the water reduction activity. The broad peak near the -0.9 V point was still visible, signifying that oxygen was still present in the system.

Figure 2-3B shows the electrolysis current profiles of three electrodes in similar conditions to the cyclic voltammetry experiments. The trend of these curves was analogous to the cyclic voltammetry results, where a CoPc-modified electrode exhibited the highest activity in CO2-saturated electrolyte. Without the presence of CoPc or CO2, little to no electrochemical activity was observed. The noise level in experiments involving CO2 was significantly higher than in experiments under Ar. Whenever CO2-saturated electrolyte was used, the bubbles formed in the pumping process tended to disturb the electrochemical technique when they passed by the electrode's surface, causing the observed noise. This effect intensified in the case of electrolysis, where a high amount of gas products was formed. These gases formed at the surface of the electrode in a constraint channel in a much more considerable amount compared to a cyclic voltammetry experiment, in combination with their low solubility in aqueous media, contributing significantly to the noise in these curves. These gas bubbles also interrupted the laminar flow inside the microfluidic chip, causing the catholyte and the anolyte to mix in the middle of the cell, resulting in unstable electrolysis. Nevertheless, the cyclic voltammograms and the electrolysis curves showed that the PDMS microfluidic cell can perform electrochemistry experiments and that CoPc was responsible for the CO2 reduction catalytic activity. The electrolyte is 0.1 M KCl, the applied potential is -1.6 V vs. pseudo-Ag, and the flow rate is 15 µL/min.

Evaluation and perspectives

Despite these initial electrochemistry results, using a PDMS chip as an electrochemical cell for electrocatalysis applications presented several drawbacks. First, the laminar separation of the catholyte and anolyte between the working and counter electrodes was not constantly achieved due to the gas bubbles generated from CO2 and water reduction. In addition, using a pseudo reference electrode made converting the applied potential to an absolute potential scale a challenging task, given the changing chemical composition of the electrolyte. Finally, PDMS is a soft and permeable material. Therefore, gas products could escape the chip, and the geometry of the channel was changed when the chip was pressed tightly against the working electrode.

The permeability of PDMS might also be responsible for oxygen contaminating the electrolyte; hence the oxygen reduction activity observed in cyclic voltammetry experiments. Due to these reasons, a sequential system to reduce CO2 to high-value products has not been built. A different design for an electrochemical flow cell is necessary to address these problems.

Inspired by the work of Jensen and co-workers, 21 we fabricated a microfluidic cell where the gasket itself acts as the flow channel. A fluorinated ethylene propylene gasket (127 μm thickness) with a 15 x 5 mm 2 rectangular cutout was sandwiched between two glassy carbon plates (25 x 10 x 0.3 mm 3 ), as shown in Figure 234. The inlet and outlet holes were etched through the glassy carbon plate by applying an oxidative potential of 4V in a 1 M KOH solution. The plate was masked with a piece of Kapton tape (25 μm thickness), leaving the locations of the two holes exposed to the electrolyte solution. The reference electrode was a silver wire glued to a cutout on the side of a polyether ether ketone (PEEK) tubing. The silver wire was flush with the end of the tube, ensuring constant contact between the reference electrode and the electrolyte. This microfluidic electrochemical cell was not made of PDMS; hence the geometry of the channel is well-defined, and the gas permeability is limited. However, a reliable seal between the gasket and the electrodes was not always achieved. Consequently, the cell was prone to leakage, and electrochemistry was not tested. Overall, this microfluidic cell is still in an early stage of development, and more optimization is necessary.

3D-printed millifluidic flow cell

Design and fabrication

The electrochemical cell design was inspired by the work of Ager and co-workers. 5 The two flow plates shown in Figure 2-5A were the main components where the reactions occurred. The two pieces had similar designs and thicknesses (2.5 mm), except the cathodic one had an additional part with a hole on its side to mount the reference electrode. The compartments in both pieces had the dimensions of 15 x 5 x 2.5 mm 3 . The two covers held the two flow plates and other cell components (Figure 2-5B). The bottom cover contained a window for spectro-electrochemical experiments. This window had a 45-degree taper to avoid interfering with the beam when it hit the working electrode at an angle (Figure 2-5C). The top cover had a smaller window for accessing the counter electrode when the cell was fully assembled. The slots and holes in the two covers were for mounting the cell and electrical connection, respectively. The plates and the covers had eight holes, with four inner holes for the bolts and four outer ones for alignment during the assembly.

The flow plates and covers were 3D printed using the Objet30 Pro Polyjet 3D printer (Stratasys), with VeroClear printing materials and SUP706 support materials. After printing, the cell was cleaned and sonicated in a diluted solution of NaOH (~2%) to remove the support material. The inlet and outlet of the cell were made by gluing PEEK tubings (ID 0.75 mm, OD 1/16 inch) to the designated hole with Torr-seal epoxy, with the end of the tubes being flush with the flow channel. The reference electrode was held in place with a 3D-printed piece, an O-ring, and two bolts. The assembly of the electrochemical cell is shown in Figure 1. The four layers were pressed tightly together by four M3 bolts. Two silicone gaskets helped create the seals between the electrodes and the flow plates. A 212 Nafion membrane was sealed against the two flow plates using two other silicon gaskets that separated the catholyte and the anolyte. The annex of the thesis contains more details regarding the cell assembly procedure.

Flow characterization

The Reynold number predicts whether a flow system is in the laminar or turbulent flow region. At a Reynold number lower than 2000, the fluid flow stays in the laminar region, where fluid particles stay in parallel lines, with diffusion as the only mass transport mode. 22 When the Reynold number is large, turbulent flow dominates, where the flow pattern is chaotic, and both convection and diffusion mass transport modes occur. As discussed above, if the flow pattern in the electrochemical cell is laminar, membraneless electrolysis can be carried out. The Reynolds number associated with our electrochemical flow cell when the total aqueous electrolyte flow rate at 2 mL/min is calculated as follows: , where A is the cross-section of the flow and P is the perimeter of the wetted area. The flow speed is calculated by dividing the flow rate by the cross-section of the flow channel. Since the electrolyte is a diluted salt solution, the density and dynamic viscosity of the electrolyte solution can be considered similar to water. As laminar flow occurs at Re < 2000, the Reynold number in the electrochemical cell system indicates that the flow is strictly in the laminar regime.

𝑅𝑒 = 𝜌𝜇𝐿 𝑢 = 997 𝑘𝑔 𝑚 3 × 8.
A mixing test was performed to verify the laminar flow result. The cell was assembled as previously described, except without a membrane between the plates. Solutions of water and green food coloring dye were flown at the same rate (1 mL/min) through the cathodic and anodic plates using a peristaltic pump. As shown in Figure 23456, there was little to no solution mixing after flowing through the cell. This result indicated that the laminar flow condition is achieved in the cell. Although the preliminary mixing test shows the solutions were well separated, membrane-less electrolysis was more complicated. Firstly, the flow rate of the two channels in the peristaltic pump tended to deviate slightly, resulting in mixing as the flow rate in the two flow plates was not the same. More importantly, the reduction of CO2 (and water) on the cathode side and water oxidation on the anode side generated a significant amount of bubbles. These bubbles could disrupt the cell's laminar flow, causing mixing to occur. Therefore, a membrane was still necessary for a standard electrochemical experiment. Chapter 2 lower noise level. The larger dimensions of the flow channel allowed the bubbles generated during electrolysis to be evacuated faster. Consequently, fewer bubbles interfered with the reference electrode, reducing the noise experienced. The Nafion membrane also prevented the crossing over of the catholyte and the anolyte, as the laminar flow was not achieved. The electrochemistry experiments using the 3D-printed cell and the resulting products will be discussed in more detail in the following chapters.

Electrochemistry performance

Reactants and products concentration profiles

To characterize the distribution of the reactant and products during the electrolysis experiment described above, we performed a finite element analysis of the fluid flow inside the cell.

The geometry of the flow cell: 2D model and meshing

A two-dimensional model of our electrochemical flow cell was built with the COMSOL software version 5.2. The model consisted of the working electrode half of the cell, shown in Figure 2-8A. Figure 2-8B overlays the model on a picture of the actual electrochemical cell. Table 2-1 summarizes the geometric parameters of the model. The distance from the working electrode to the Nafion membrane included the thickness of the flow channel and the thickness of the two silicone gaskets between the electrode and the cell and between the membrane and the cell. The width of the channel was not used to build the model, but it was used to derive the active area of the electrode, which will be described in more detail in the following section. The model was meshed automatically using Physics-control meshed sequence with Finer We based the simulation of our electrochemical flow cell on the model established by Gatrell et al. 23 and Ager et al. 5 Chapter 2

As the Reynold number indicates the flow stayed in the laminar region, the laminar flow physics module was used to simulate the flow of electrolytes in our cell. This physics module uses the Navier-Stokes and the continuity equations to model the behavior of the fluid. The Inlet boundary condition was applied to the bottom boundary of the bottom tubing. Normal inflow velocity mode was used, and the velocity of the fluid was calculated by dividing the flow rate by the cross-section area of the tubing. The outlet boundary condition was applied to the top boundary of the top tubing, with outlet pressure equal to zero, and the suppress backflow option was enabled. No slip boundary condition was applied to all other boundaries. Initial values of velocity and pressure across all domains were set to zero. Under the abovementioned conditions, the Navier-Stokes and the continuity equations are as follows:

𝜌(𝑢. ∇) = ∇. [-𝜌𝐼 + 𝜇(∇u + (∇u) 𝑇 ) ] + 𝐹 𝜌∇. (u) = 0
where 𝜌, 𝑢, 𝜇, and 𝑇 are the density (kg/m 3 ), the velocity (m/s), the viscosity (Pa.s), and the temperature (K) of the fluid, respectively. 𝐹 is the external forces and ∇ is the divergence operator.

The transport of dilute species physics module was used to calculate the consumption rate of CO2 and the generation rate of CO. Two modes of mass transport were considered: diffusion and convection. The mass balance equation is the governing equation of this physics module:

∇. (-𝐷 𝑖 ∇𝑐 𝑖 ) + 𝑢. ∇𝑐 𝑖 = 𝑅 𝑖 where for species i, D is the diffusion coefficient (m 2 /s), c is the concentration (mol/m 3 ), u is the velocity (m/s), R is the reaction rate (mol/(m 3 .s)), and ∇ is the divergence operator.

The velocity field resulting from the laminar flow physics calculation was used in the model inputs. A flux boundary condition was applied at the working electrode, with one flux accounting for CO2 and the other accounting for CO. The equation describes the flux as follows:

𝑁 𝑖 = -𝐷 𝑖 ∇𝑐 𝑖 + 𝑢𝑐 𝑖
where for species i, 𝑁 is the flux (mol/m 2 .s), D is the diffusion coefficient (m 2 /s), c is the concentration (mol/m 3 ), u is the velocity (m/s), and ∇ is the divergence operator.

The flux of CO2 (mol/(m 2 .s)) was calculated based on the current density resulting from electrolysis at -1.1 V vs. RHE in CO2 and the faradaic efficiencies of all carboncontaining species as follows: where j (A/m 2 ) is the electrolysis current density at -1.1V vs. RHE in CO2, F is the Faraday constant (C/mol), and FE is the Faradaic efficiencies of the corresponding products. The Faradaic efficiencies were divided by the number of electrons needed to be injected into one molecule of CO2 to reduce it to the corresponding species.

𝐹𝑙𝑢𝑥 = - 𝑗 × 𝐹𝐸 𝐹 = - 𝑗 𝐹
Similarly, the flux of CO (mol/(m 2 .s)) was calculated based on the electrolysis current density at -1.1 V vs. RHE in CO2 and the faradaic efficiencies of CO:

𝐹𝑙𝑢𝑥 = - 𝑗 × 𝐹𝐸 𝐶𝑂 𝐹
where j (A/m 2 ) is the electrolysis current density at -1.1 V vs. RHE in CO2, F is the Faraday constant (C/mol), and FECO is the Faradaic efficiency of CO. The current density and the Faradaic efficiency were obtained from electrolysis experiments that will be discussed in more detail in the next chapter.

Like the inlet and outlet boundary conditions, inflow and outflow boundary conditions were applied to the bottom-most and top-most boundaries of the tubing, respectively. The concentration of the inflow was set to be the saturated concentration of CO2 at 25 o C (33.4 mM). No flux boundary condition was applied to all other boundaries. Initial values of CO2 concentration in all domains were set to 33.4 mM, and that of CO was to be zero. Table 2-2 summarizes the parameters used in the two physics modules. The concentration profiles of CO2 and CO in the electrochemical cell are illustrated in Figure 23456789, which shows the conversion of CO2 to CO only took place close to the electrode surface, and the majority of CO2 passing through the cell was not consumed. This agrees with the gas chromatograph result, as the amount of CO2 after 2-hour electrolysis at -1.1 V vs. RHE stayed relatively unchanged. However, since the electrolyte flow near the corners of the cell was significantly slower, one can observe the decrease of CO2 and buildup of CO in these regions, which can exceed the saturated concentration of CO in an aqueous solution.

Millifluidic cell evaluation and perspective

Although the laminar flow inside the cell was not achieved, overall, the 3D-printed millifluidic flow cell was a reliable device for lab-scale electrochemical experiments. The 3D-printing fabrication method enabled quick changes to be made, making modifying the cell to adapt to specific needs (e.g., operando spectro-electrochemistry) a more straightforward task. Moreover, the versatile design allowed different electrode and membrane materials to be used.

However, several aspects of the cell are subject to improvement. First, the cell was made of a plastic material only compatible with water-based electrolytes in mild conditions. Therefore, experiments in harsh conditions, such as at high temperatures and pressure or with organic solvents as the electrolyte, were out of the cell's capability. Another drawback of the cell was the liquid connections with a PEEK tubing glued to the 3D-printed flow plates. These connections could degrade and form cracks over repeated use of the cell, which was prone to leakage. Finally, while 3D printing could produce a cell quickly, it often results in the pieces having rough surfaces, making sealing and cleaning the cell a complicated task. In fact, a cell with incorporated flat-bottom fittings liquid connection was fabricated to avoid the glue joint, but the rough surface prevented the connectors from forming a good seal. A cell fabricated with PEEK or Teflon, with dedicated liquid connection ports built into the flow plates, would address the mentioned drawbacks. PEEK or Teflon are robust materials that can withstand experiments in demanding conditions. And a smooth surface finish with precise built-in liquid connections can be achieved from the machining process required to fabricate this thermoplastic.

Conclusion

In this chapter, we presented the design and fabrication of a PDMS microfluidic cell to carry out membraneless electrolysis and to build a system for the cascade reduction of CO2. Using a CoPc catalyst, we demonstrated that CO2 was reduced to CO in the cell. However, the PDMS cell behaved poorly in terms of electrochemistry, as the small dimensions of the cell prevented the gas bubbles from the products and the reactant from being evacuated quickly, leading to unstable electrochemistry. Moreover, while PDMS is the standard material for microfluidic devices, it was unsuitable for this electrochemical application due to its inherent flexibility and gas permeability. We then proposed a different design for the microfluidic cell without using PDMS. Although the bubble problem proved complicated, reducing CO2 in a thin channel is still an attractive idea, as it allows CO2 to be consumed completely, which is challenging to achieve in a cell employing a gas diffusion electrode due to the high CO2 flux.

In parallel, we presented the development of a 3D-printed millifluidic electrochemical flow cell capable of performing operando X-ray absorption spectroscopy. The laminar flow conditions were achieved in the cell, but gas bubbles in a CO2-reducing experiment caused the turbulent flow to occur in the cell's flow channels. Therefore, a membrane was necessary for the performance of the cell. With a membrane employed, CO2 electrolysis using an FePc catalyst could last at least two hours with a lower noise level than the PDMS cell. The concentration profiles of the reduced products were characterized using finite element analysis based on laminar flow and transport of diluted species physics. The calculation showed CO concentration existed in a supersaturated concentration near the electrode surface, but the CO2 conversion rate was poor. Finally, an improved version of the cell was proposed.

Introduction

As discussed in Chapter 1, molecular macrocycles are promising electrocatalysts for reducing carbon dioxide to value-added chemicals. Amongst them, transition metal phthalocyanines and porphyrins are the most noticeable since they exhibit high catalytic activity. However, most of these catalysts produced CO as the main product, with a few studies reporting the formation of further reduced C1 products such as methanol 1,2 and methane. 3 In the phthalocyanine family, only CuPc has produced species containing C -C bonds with faradaic efficiencies greater than 1%. 4 However, the molecular nature of the catalyst was not preserved during catalysis, and the C -C bond formation likely happened on copper metal nanoparticles. 5,6 Ethylene and ethane were reported to form upon the reduction of CO2 on a CoPc catalyst. Still, their selectivity was less than 1%, and no characterization of the catalyst after the catalysis experiment was performed. 7 In this chapter, we report that iron phthalocyanine (Figure 3-1) could produce light hydrocarbons upon electrocatalytic reduction of CO2 in aqueous conditions and neutral pH. Using the 3D-printed millifluidic electrochemical flow cell described in the previous chapter, C1 to C4 saturated and unsaturated linear hydrocarbons were evolved under suitable applied electrochemical potential. Isotopic labeling experiments unambiguously showed that these products stem from CO2. Cyclic voltammetry was employed to estimate the number of active catalytic sites. Control experiments and operando X-ray spectroscopic analysis demonstrated that the molecular catalyst remained intact during catalysis and was responsible for the reaction. Based on experiments with alternate substrates, a mechanism is proposed for the C -C bond formation step.

Products distributions based on applied potentials

Electrocatalysis and products identification

Using the electrochemical cell previously reported in Chapter 2, we performed electrocatalytic experiments using FePc-modified electrodes with the catalyst loading of 80 nmol/cm 2 . The electrolysis potentials ranged from -0.7 V vs. RHE (-1.3 V vs. Ag/AgCl) to -1.1 V vs. RHE (-1.7 V vs. Ag/AgCl). The lower limit of -1.1 V was the most negative potential at which the current was stable enough for extended electrolysis, while the upper potential limit of -0.7 V is the least negative potential at which there are significant products of CO2 reduction. The chapter and the thesis annex contain more details about the experiments. Control experiments were performed to ensure that FePc catalyzed the reduction of CO2 into the products observed. When a FePc-modified electrode was set under catalytic conditions (-1.7 V vs. Ag/AgCl) in an Ar-saturated KHCO3 electrolyte, hydrogen was the main product detected after two hours of electrolysis, with a trace amount of CO and hydrocarbons, as shown in Figure 34. The observed carbon products can be attributed to the small amount of CO2 in equilibrium with the KHCO3 electrolyte. In fact, a similar FePc-modified electrode, at the same potential, under a CO2-free environment (Ar-saturated KCl electrolyte), produces only H2 (Figure 3-2B, C, and D). The current density observed is higher than under CO2, indicating a high activity of FePc towards hydrogen evolution in a CO2-free environment at a high overpotential. When an H2Pc-modified electrode was poised at -1.1 V in a CO2saturated KHCO3 electrolyte, a twenty-fold decrease in current density was observed compared to an FePc-modified electrode, indicating negligible catalytic activity without the Fe center. In agreement with the low catalytic activity, the main product detected was H2, with a minuscule amount of CO (Figure 3-2C). These results strongly indicate CO2 as the source of the carbon products observed and underline the essential role of the metal center in the catalytic process. V vs. Ag/AgCl for two hours, after which a GC samples the resulting gaseous products.

Origin of the products

To confirm the origin of the carbon atoms in the C2+ products observed in the chromatograms, we performed isotopic labeling experiments using 13 CO2. The GC was coupled with a mass spectrometer perform gas chromatography-mass spectrometry (GC-MS) analysis of the products, with details in the annex of the chapter.

The major fragment of 12 CO in a mass spectrum is at m/z = 28, overlapping with major fragments of ethane and ethylene. Since the dominant product of the electrochemical reduction of CO2 in our system was CO and the amount of C2 products detected was orders of magnitude lower, the signal from the CO fragment overwhelmed the signals of other species in the region. Although the gas chromatograph could separate CO and C2+ products somewhat effectively, the difference in signal-to-noise ratio made detecting mass signals of C2+ and some C3 products with masses in the vicinity of m/z = 28 impossible with our setup. Similarly, the major fragment of 12 CO2 and 13 CO2 at m/z equal 44 and 45, respectively. These signals overlapped with some fragments of the C3 and C4 products detected. The catholyte was saturated with CO2, resulting in a high background signal around C3 and C4 products' main fragments near m/z of 44 and 45, making detecting these fragments challenging. Finally, the m/z value of 16 could come from the minor fragment of CO2 and the mono-oxygen peak from the residual dioxygen, overlapping with the major signal of CH4. Therefore, the signal of CH4 is also undetectable with the current setup due to signal interference with more dominating species. The peaks that were not subjects of background interference are m/z = 41 ( 12 C3H5), m/z = 56 ( 12 C4H8), and m/z = 60 ( 13 C4H8), which are shown in Figure 3-5. The detail of the GC-MS procedure is shown in section 3.7.2, along with the reference mass spectra of the concerning species.

Figure 3-5A and B show the GC-MS chromatograms corresponding to the retention time of C4 products (190-240 s). We focused on the C4H8 signal because it had the highest intensity of all C4 products without overlapping the signal of CO2 or CO. The m/z = 56 mass, which corresponds to a C4H8 species containing four 12 C atoms, showed a peak when 12 CO2 was used as the reactant while not appearing in the presence of 13 CO2. The opposite behavior was observed with the m/z = 60 mass, corresponding to a C4H8 species with four 13 C atoms. The GC-MS chromatogram of the m/z = 41 mass at the region of the retention time of C3 and C4 products is shown in Figure 3-5C. The m/z = 41 mass, which corresponds to a 12 C3H5 fragment (originating from both C3 and C4 products), showed two distinguishable peaks when 12 CO2 was used, which disappeared with 13 CO2. The first peak represented the signal of C3 products since it was in the region of 130 -170 s, which corresponds to the elution time of the C3 species. Similarly, the second peak appeared in the retention time region from 190 to 240 s, corresponding to the elution time of the C4 products. This observation agreed with the result of other C4 fragments shown in Figure 3-5A andB. Overall, the isotopic labeling experiments demonstrated that the carbon atoms in the observed hydrocarbons come from CO2 molecules. Although no signals of CH4 and C2 were obtained, the result of C3 and C4 products, which were present in lower concentrations, showed that the same behaviors are expected with all carbon products.

Products quantifications

Figure 3-6 shows the electrolysis curves of FePc-modified electrodes with the catalyst loading of 80 nmol/cm 2 at applied potentials of -0.7 to -1.1 V vs. RHE. As expected, the current density decreased with less negative potentials. Figure 3-7 shows product distribution in terms of amount and faradaic efficiency (FE) for each product obtained after a 2-hours electrocatalytic experiment under CO2 at different potentials. Between -0.7 V and -0.8 V, CO and H2 were the only products reliably observed, with CO being the majority product (FE > 85%). When the potential turned more negative, the selectivity of CO decreased, and hydrogen gradually became the main product, with FE equaling 65 % at -1.1 V. This increase in hydrogen came from the fact that with more negative potentials, the amount of CO stayed relatively constant, while H2 amount increased, as shown in Figure 3-7A. A noticeable amount of hydrocarbons could be observed starting from -0.9 V, methane being the major species. At -1.1 V, the selectivity of CH4 reached 4%, and ethylene, ethane, propene, propane, and nbutane were produced with faradaic efficiencies of 0.1 -0.25% each. The total faradaic efficiency of hydrocarbon reached 5%, with C2+ products accounting for around 1%. It should be noted that the amount of ethylene was underestimated because of the high background generated by CO2 in the chromatograph column. While the amount of CH4 and C2+ products increased with more negative potential, their selectivity was generally constant from -1.0 V. This suggested the peak of hydrocarbons generation lies around the -1.0 V and -1.1 V vs. RHE, as more negative potential would mostly drive hydrogen evolution. Despite the low amounts of products generated, their distribution was reproducible, as shown by the replicates performed on three different electrodes.

FePc catalyst characterization: stability and active site estimation

X-ray absorption spectroscopy

To determine the nature of the catalyst under operating conditions and check its integrity, we performed in situ X-ray absorption spectroscopy (XAS) experiments at the iron K-edge in the same electrochemical cell described in Chapter 2. Figure 3-8 shows the X-ray absorption near-edge structure (XANES) spectra of the FePc-modified electrode under open circuit potential (OCP) and catalytic conditions in a CO2 atmosphere. The spectrum of the electrode under resting state presented the same features as the pure, starting compound, showing the chemical robustness of the FePc catalyst after drop-casting and in the presence of an electrolyte. When poising the electrode at a potential of -1.1 V (at which C2+ products are observed), the spectrum showed a shift to lower energies (-2.7 eV at half-edge jump) while keeping the features that are characteristic of phthalocyanines, 8 i.e., two pre-edge peaks at 7114 eV and 7118 eV. A XANES spectrum was also recorded in situ after performing electrocatalytic experiments under a CO2 atmosphere, and it was remarkably similar to the one collected on the starting electrode. The comparison between the spectra collected under in situ conditions and those of reference compounds (metallic iron, Fe3O4) ruled out the presence of metallic iron or iron oxide nanoparticles that the decomposition of the molecular catalyst would produce. Linear combination fitting of the operando spectrum using Fe, Fe2O3, and Fe3O4 as standards yielded a poor fit with FePc content to be 100%, suggesting FePc was the only species existing under catalytic conditions. Figure 3-8B shows the raw spectra without normalization. The spectrum of the FePcmodified electrode under oxidative potential after an electrolysis experiment experienced a decrease of 10% in intensity compared to the electrode under open circuit potential. The decline showed that the majority of the catalyst layer was robust enough to resist the mechanical erosion induced by the flowing electrolyte. Altogether, these spectroscopic data collected under in situ conditions clearly demonstrated the molecular nature and the stability of the catalyst throughout the experiment.

Infrared spectroscopy

Infrared spectra were recorded on the FePc catalyst powder and FePc-modified electrodes before and after electrolysis at -1.1 V vs. RHE (Figure 3456789). We could observe from the spectra the characteristic vibrational peaks of the phthalocyanine ligand in a typical metal phthalocyanine species: stretching modes of C=C, C-N=C, isoindole, and pyrrole groups from 1800 to 1330 cm -1 ; as well as scissoring modes of C-H and isoindole groups from 1330 cm -1 to 800 cm -1 . 9,10 The peaks in the stretching region of pure FePc powder were more well-defined since the powder was not affected by the presence of carbon black and Nafion as in the electrodes. However, the IR spectra recorded with modified electrodes before and after electrolysis were identical, agreeing with the XAS results about the high stability of the catalyst during electrolysis.

Cyclic voltammetry and active site estimation

Figure 3-10A shows the cyclic voltammograms (CVs) of an FePc-modified and catalystfree modified electrode under CO2 and Ar. Under CO2, no catalytic activity was observed in the CV without the presence of FePc. On the contrary, the presence of FePc under Ar facilitated hydrogen evolution at a highly negative potential, starting from -1.1 V vs. RHE. When both CO2 and FePc were presented, a current increase started around -0.6 V, forming a catalytic wave. The rise in current indicated the reduction of CO2, as the onset potential was less negative than the potential needed for the hydrogen evolution reaction. Overall, the CVs corroborated the result obtained from electrolysis and product characterization, where an FePc-modified electrode exhibited a high CO selectivity at low overpotential, and low selectivity at high overpotential. However, the heterogenization of FePc on a large electrode and the use of a flow cell prevented its redox feature from being observed, as there were no visible redox peaks in the CVs.

We prepared a modified circular working electrode to obtain the redox peaks of FePc and performed cyclic voltammetry in a heart-shaped glass cell in Ar-saturated H2SO4 0.05 M. When the redox feature is reliably observed, we can integrate a peak to obtain the amount of charge transferred in the reaction. We can deduce the fraction of the electrochemically active species by comparing the measured charge versus the theoretical charge needed to reduce or oxidize all the catalysts on the electrode surface. The chapter annex contains more details about the experimental setup. In Figure 3-10B, a reversible wave could be observed at 0. Wavenumber (cm -1 )

Chapter 3 91 peak was stable after ten cycles of cyclic voltammetry, and the reversible wave was observed with three independent experiments (Figure 3-24), indicating the repeatability of the feature. This feature represented a one-electron redox process that happened on FePc, as shown in a previous study with immobilized FePc in acidic conditions. 11 A reduction peak appeared in the range of 0.2 to 0.4 V in the first cycle, but it was not visible in subsequent repetitions, indicating that it was not an intrinsic feature of the catalyst layer (Figure 3-10B). As observed earlier, when an electrode was modified with a FePc-free ink, its CV exhibited a broad peak with no reversible feature (Figure 3-10C). The lack of the reversible feature without FePc indicates that the observed reversible wave in Figure 3-10B comes from FePc. Comparing the charge obtained by the peak versus the charge needed to reduce Fe(II) to Fe(I), the percentage of the active catalyst in the drop cast layer is 7.8 ± 0.8%.

Although the percentage of active sites was relatively low, some cautions should be made when interpreting this result. First, the experimental conditions of the cyclic voltammetry experiments in a glass cell with visible redox features were different from the conditions of standard electrolysis in the flow cell. Moreover, the accuracy of the active sites deduced from CV peaks integration was a question of debate due to the uncertainty in baseline correction, 12 and the unreliability of the peaks themselves. 13 Furthermore, the XAS spectrum during catalysis significantly changed compared to the spectra obtained before and after electrolysis. Given that the loss of the material was limited, the change in 8% of the catalyst would unlikely lead to the observed change.

Overall, the percentage of the active sites determined electrochemically could serve as a lower limit value, but it might not reflect the most accurate percentage of the active catalyst.

Control experiments with FePc and Fe 3 O 4 nanoparticles

In the previous section, operando X-ray absorption spectroscopy was used to demonstrate the nature of the FePc catalyst during its operation. The spectrum during catalysis did not resemble that of iron or iron oxide species, and the spectra before and after electrolysis closely overlapped, proving the molecular nature of FePc. However, since not all of the catalysts were active, a small portion of the catalyst could degrade and form an iron-based material that might be responsible for the formation of products with C -C bonds. These iron materials could exist in a small quantity below the detection limit of X-ray absorption spectroscopy. To test this hypothesis, we experimented with 5-nm Fe3O4 nanoparticles (Sigma-Aldrich) as a catalyst for CO2 reduction. The nanoparticles are stabilized by modified polyethylene glycol (PEG) ligands covalently bonded to the particle by a hydroxamic acid on the end of the PEG chain. The PEG concentration is less than 1 % in weight. First, we examined the characteristic features of electrodes containing a mixture of Fe3O4 nanoparticles and FePc using ex situ XAS. We then studied the catalytic behavior of solely the particles as well as mixtures of the nanoparticles and FePc. Finally, we performed operando X-ray absorption spectroscopy with an electrode having a mix of FePc and Fe3O4. In all cases, we kept the total surface concentration of iron to 80 nmol/cm 2 , similar to the amount of iron on a standard FePc-modified electrode described in the previous section. The drop casting procedure was identical to the FePc casting process, except the solution of Fe3O4 suspension was added after adding FePc to the ink mixture. The electrolysis was carried out in a KHCO3 0.1 M electrolyte with a flow rate of 1 mL/min.

Ex situ X-ray absorption spectroscopy

Figure 3-11 shows the XAS spectra collected on the Fe3O4/FePc electrodes, as well as reference spectra collected with only FePc on an electrode and Fe3O4 powder. The Fe3O4 sample possessed a characteristic pre-edge peak at 7114 eV, which is visible in all samples containing Fe3O4. Interestingly, the XAS spectrum of the electrode where Fe3O4 contributed only 5% of the iron content (black curve) showed a strong signal of Fe3O4, while the two characteristics pre-edge peaks of FePc at 7114 and 7118 eV (purple curve) were hardly visible. When the theoretical iron content of Fe3O4 nanoparticles accounted for 50%, its signal (red curve) closely matches that of the 100% Fe3O4 sample (blue curve). spectra. Using the spectra from the two electrodes containing FePc only and Fe3O4 nanoparticles only as references, the electrode supposedly with 5% of iron content coming from Fe3O4 yielded 22.1% Fe3O4 and 77.9% FePc (Figure 3-12A). Similarly, the electrode where FePc and Fe3O4 supposedly have equal iron content actually showed 81.4% Fe3O4 and 18.6% FePc, with a better fitting result (Figure 3-12B). Although the fit with the 5% nominal concentration of Fe3O4 was poor, both fit results indicated that the Fe3O4 content in each sample was higher than the calculated values. The significant deviation between the calculated concentrations and the observed concentrations meant the drop-casting process was poorly controlled. Subsequently, we will refer to the electrodes containing mixtures of FePc and Fe3O4 with their concentrations derived from XAS instead of their calculated iron concentrations. Despite the substantial difference between the theoretical and the actual Fe concentration, the XAS result demonstrated that considerable changes would be observed if a small fraction of Fe3O4 was presented on the measured electrodes.

Electrolysis and product distribution

Figure 3-13A shows the electrolysis curves of electrodes modified with a mixture of FePc and Fe3O4 nanoparticles. We examined two electrodes with only Fe3O4 nanoparticles and only FePc, as well as one electrode containing a mixture of FePc and Fe3O4, with 5% of the iron contents theoretically contributed by Fe3O4. This number was actually 22.1%, based on the result of XAS linear combination fitting; thus, we referred to the corrected number in our analysis.

Concerning the electrochemistry performance, the highest current density obtained was the electrode having 22.1% of the Fe content from Fe3O4. The sample with 100% Fe3O4 appeared unstable as its electrolysis curve was trending downward. Since the nanoparticles did not adhere well to the glassy carbon surface, mechanical erosion of the drop-casted layer may lead to the observed instability. This phenomenon will be discussed in more detail in the following section. product on the electrode having no FePc, with CO accounting for less than 1% of the faradaic efficiency. This poor result is expected, as metallic iron or iron oxide are not known to be a good catalyst for CO2-to-CO reduction.

CH4 and C2+ products were observed after catalysis at -1.1 V vs. RHE on electrodes containing only Fe3O4 and a mixture of FePc and Fe3O4. The faradaic efficiencies of individual C2+ products are shown in Figure 3-14. The production of these hydrocarbons was likely the result of the PEG stabilizing ligands degradation, as the breaking of PEG chains yields various carbon-containing products. 14 Therefore, the generation of hydrocarbons on Fe3O4 nanoparticles was not the result of CO2 reduction.

Operando X-ray absorption spectroscopy

Figure 3-15 compares the spectra obtained with a 22.1% Fe3O4 iron sample before and after 2-hour electrolysis at -1.1 V vs. RHE. The normalized data in Figure 3-15A demonstrated that the overall shape of the spectra before and after electrolysis remained similar, albeit with a lower signal-to-noise ratio and other minor differences. These changes were likely due to the consequences of the loss of the materials, as shown in the raw spectra in Figure 3-15B. The signal intensity decreased by 95%, indicating a significant amount of materials loss, much more than what was observed with an FePc-modified electrode, as discussed in the previous section. Indeed, visible damage to the catalyst layer could be seen after electrolysis, with some black particles recovered in the catholyte bottle after the experiment finished. This result indicates that the presence of Fe3O4 nanoparticles decreased the adhesion between the glassy Chapter 3 carbon surface and the catalyst layer, and nanoparticles are more likely to be removed by the electrolyte flow.

Discussion

Overall, the result from experiments with Fe3O4 nanoparticles showed that when FePc contained a small amount of Fe3O4 nanoparticles, the resulting XAS spectra exhibited significant changes. Although this experiment could not completely rule out the formation of every iron species, the reversibility of the XAS spectra on an electrode with only FePc demonstrated that the degradation of FePc was highly improbable. Moreover, the presence of a Fe3O4 species significantly alters the adhesion between the catalyst layer and the glassy carbon plate. Therefore, the outcome of this experiment provided strong evidence of the structural integrity of the catalyst.

C -C bond formation mechanism

The demonstration that a molecular electrocatalyst can form several consecutive C -C bonds poses the question of their formation mechanism. Since the main product observed upon the reduction of CO2 was CO, we postulated that the first intermediate for forming C -C bonds could be an FePc-CO species (see Figure 3-19). Indeed, electrolysis with CO as substrate showed the formation of similar hydrocarbon products observed for CO2, albeit with faradaic efficiencies approximately twice as low (see Figure 345678910111213141516). We attributed this lower Faradaic efficiency to the low solubility of CO in water (1 mM) compared to that of CO2 (33 mM). As observed in other studies, 15,16 we surmised that the difference in initial concentration of substrate was responsible for this difference in efficiency. According to the finite element analysis calculating the Chapter 3 concentration profiles upon CO2 reduction in Chapter 2, a supersaturated concentration of CO was observed near the electrode. Specifically, the calculation showed the concentration of CO generated by CO2 reduction within five μm from the working electrode ranges from 1.5 to 4.5 mM, while the saturated concentration of CO in water is around 1 mM. The CO supersaturation supported a relationship between the amount of C2+ products and the local concentration of CO. By analogy with what happens during the Fischer-Tropsch synthesis of long-chain hydrocarbons, we hypothesized that a terminal FePc=CH2 carbene species could be the precursor to all C2+ products. This transient species would be formed, via a hydroxycarbene species, by reacting an FePc-CO adduct with either H2 formed locally or with protons and electrons available in large amounts. This species would then further react either with more H2 (or protons and electrons) to form methane. The putative iron carbene species undergo CO insertion similar to the Fischer-Tropsch mechanism to create a C -C bond. This CO insertion pathway is different from the carbene coupling involved in forming ethylene on metallic copper since there are no adjacent active sites in iron phthalocyanine. The process could continue a few times following various pathways and lead to the formation of longer saturated or unsaturated hydrocarbon chains (see Figure 345678910111213141516171819).

To verify this hypothesis, we used formaldehyde as a substrate, expecting it to form an FePc=CH(OH) hydroxycarbene species by direct reaction with the activated form of the catalyst (see Figure 3-19). One of the polymerization schemes in the Fischer-Tropsch process is initiated by forming metal-hydroxycarbene intermediates from the hydrogenation of the chemisorbed carbon monoxide. 17 By using HCHO instead of CO2 as the reactant, creating the hydroxycarbene species would not require the reduction of CO2 to CO and the hydrogenation of the adsorbed CO. Consequently, we surmised that the hydroxycarbene intermediate would be readily formed on the reduced iron center. With 30 mM of HCHO as the reactant, the product distribution showed similar multi-carbon products as with CO2 or CO as substrates, although in much lower amounts (Figure 3-16B), as well as methane (2%) and traces of methanol (Figure 3-16A and Figure 3 -17). The observed methanol was likely the result of the Cannizzaro reaction between the formaldehyde molecules. 18 While the concentration of formaldehyde (30 mM) was similar to that of saturated CO2 in water at 25 o C (33 mM), the very low faradaic efficiencies observed for C2+ products could be explained by the absence of any CO to be inserted in the FePc=C bond of the putative carbene species.

Without any CO to react further, the carbene species were preferentially reduced into methane, leading to a 6-fold increase in the faradaic yield of methane as compared to the CO substrate.

To confirm the reaction of CO with an FePc carbene species, we performed electrolysis with HCHO in a CO2 atmosphere, as CO2 reduction could provide the necessary CO to increase the C2+ formation with the carbene species generated from HCHO. Indeed, at -1.1 V vs. RHE, the total faradaic efficiency of C2+ products was one order of magnitude higher than observed in HCHO without CO2 (Figure 3-18A and C), thus confirming our hypothesis of a reaction between CO and a carbene species obtained from formaldehyde. At less negative potentials, where the formation of CO from CO2 is more favorable, the total C2+ faradaic efficiency increased further, reaching a maximum value of 1.5% at -0.9 V vs. RHE, ca. three times higher than when only CO2 was used as substrate (Figure 3-18B and C). The higher selectivity of C2+ products after electrolysis of HCHO under CO2 emphasized the crucial role of the CO insertion step while corroborating the hypothesis that the formation of C2+ species involves CO insertion on an intermediate FePc carbene species, as described in Figure 3-19. The existence of an FePc=CH2 intermediate is supported by the fact that several carbene species of iron porphyrin (a macrocycle similar to FePc) have been reported, either as transient 19,20 or stable species, 21 and that FePc itself has been reported as an efficient catalyst for the cyclopropanation of alkenes, 22 which involves FePc-carbene intermediates. Iron porphyrins are also known for catalyzing the insertion of carbenes into N-H, S-H, or C-H bonds. 23,24 These precedents, as well as the experiments described here with alternate substrates, strongly support the involvement of an FePc carbene species on the way to C2+ hydrocarbons. To show that the Fe=CH2 species is Chapter 3 101 the intermediate of the proposed mechanism, such species could be synthesized and isolated in order to study its single turnover reactivity, i.e., whether it would react with CO and H2 to form a C -C bond. Spectroscopic studies of such a species would also be beneficial. However, comparison with operando data would be difficult since it is produced in very low amounts and likely very fugacious during the catalytic cycle. However, the identification of a reaction between the iron carbene species and a mixture of CO and H2 would provide concrete evidence to support the proposed mechanistic pathway for forming C2+ products. The mechanism we propose is based on a CO insertion step to form C -C bonds, which is similar to one mechanism reported for the Fischer-Tropsch process. Consequently, we surmise that the obtained hydrocarbons would follow the Flory-Schultz distribution, analogous to what happens during the Fischer-Tropsch reaction. 25 To verify this hypothesis, we plot the ln (Wn/n) against n, where n is the number of carbon, and Wn is the mole fraction of all products containing n carbon. This plot is known as the quasi-Anderson-Schulz-Flory plot, which yields the same trend as a standard Flory-Schulz plot (common logarithm of mass fraction vs. the number of carbon) but with improved linearity. 26 102 not show a linear relationship. However, the value of C2 products was underestimated due to the C2H4 peak got overlapped with the background of the CO2 peak in the gas chromatograms. Therefore, we removed the point of the C2 species, resulting in Figure 3-20B, which shows a clear linear relationship between ln(Wn/n) and n. The linear trend demonstrated the generation of hydrocarbons was a polymerization process based on the insertion of CO into carbene species. 27 We note that our calculations overestimate the mole fractions of all species due to ethylene underestimation and the amount of some C4 and C5 being below the level of quantification. However, this is a systemic error that applied to all points, thus the trend would not be affected.

Conclusion

In this chapter, we reported the electrocatalytic activity of iron phthalocyanine toward reducing CO2 in our 3D-printed millifluidic electrochemical flow cell. Upon reducing CO2, linear hydrocarbons with up to three C -C bonds were detected, while the main product was CO. The formation of the hydrocarbons depended on the applied potentials, with reliable observations of these products starting from -0.9 V to -1.1 V vs. RHE. Using operando X-ray absorption spectroscopy, we showed that iron phthalocyanine kept its molecular structure throughout the electrocatalytic process.

Experiments with Fe3O4 corroborate the evidence proving the structural integrity of FePc during catalysis. Based on experiments involving carbon monoxide or formaldehyde and the finite element analysis of the product concentration profiles, we showed that the local concentration of CO is key to the formation of C -C bonds. We proposed a terminal carbene as an intermediate on the way to the C2+ products. The hydrocarbon products substantiated the proposed mechanism following the Flory-Schulz distribution. These results indicate that single-site transition metal macrocycles can catalyze the formation of several consecutive carbon-carbon bonds under mild electrochemical conditions. The mechanistic insights we provided open a perspective for designing molecular and material-based catalysts with enhanced selectivity towards C2+ products.

Chapter 3

Chapter 3 annex

NMR characterization

NMR measurement is used to detect liquid products that might be in the electrolyte. An aliquot of catholyte is sampled after each electrolysis. A solution of 50 mM Phenol and 10 mM DMSO in D2O is used as the standard for the NMR measurement. 450 µL of the catholyte solution and 50 µL of the standard solution are thoroughly mixed and dropped into an NMR tube for measurement. A 400 MHz Bruker NMR spectrometer with a water suppression program is used for the measurement. Each measurement contains 128 scans.

Isotopic labeling experiments

The Micro GC FUSION® is coupled to the Omni Star GSD 320, O2 mass spectrometer by connecting the analytical outlet of module B of the gas chromatograph to the capillary of the mass spectrometer. A multiple-ion detection mode with an SEM detector is used to monitor the masses of the theoretical fragments of C2 to C4 products based on the NIST database. 28 The parameters of the detection method are summarized in Table 3-1. A standard electrocatalytic experiment with 12 CO2 is performed as described in section 3. The mass spectrometer measurement is started simultaneously with the gas chromatograph measurement every 30 minutes during the 2-hour electrolysis. For the labeling experiment, the standard 12 CO2 purging gas is changed to 99.9% 13 CO2 from Eurisotop, while other steps in the procedure remain unchanged. Reference mass spectra of species presented in the sample mixture. Data were taken from the NIST chemistry WebBook. 28

Cyclic voltammetry and active site estimation

To perform cyclic voltammetry in the electrochemical flow cell, the experimental setup is similar to the one for constant-potential electrolysis described in the general annex. However, the purging gas (CO2 or Ar) is kept flowing, and the peristaltic pump is stopped during the potential cycling. The cyclic voltammetry is performed using the VSP-300 potentiostat from Biologic.

A homemade glass heart shape electrochemical cell is used to perform cyclic voltammetry in the standard static electrochemical cell. The working electrode is a rotating disk glassy carbon (d = 5 mm) from Pine research, though the electrode was not rotating when performing the experiment. Before each drop-casting, the electrode is polished using diamond slurry solutions (6 µm  3 µm  1 µm, 2 minutes each).

The ink and the drop-casting procedure are similar to the ones described in the general annex. However, since the working electrode is smaller than the glassy carbon plate used in the flow cell, to get the surface concentration of 80 nmol/cm 2 , an 8-µL drop is added to the electrode until the total volume of 83 µL is reached. The electrode is dried in an oven at 100 o C after every drop. The catalyst-free electrode is prepared in a similar fashion using FePc-free ink. The reference electrode is a saturated calomel electrode, and a Pt counter electrode is used. The electrolyte is 0.05 M H2SO4, and the scan rate is 100 mV/s. Before every experiment, the electrolyte is purged with Ar for at least 15 minutes. An Autolab PGSTAT302N potentiostat (Metrohm) is used to perform electrochemical experiments. The charge needed to carry out a 1-electron redox process with all FePc presented on the electrode is:

𝑞 = 𝑐 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 × 𝐹 = 80 × 10 -9 𝑚𝑜𝑙 𝑐𝑚 2 × 96490 𝐶 𝑚𝑜𝑙 𝑞 ≈ 7.7 × 10 -3 𝐶 𝑐𝑚 2 ≈ 7.7 𝑚𝐶/𝑐𝑚 2
The charges exhibited in a cyclic voltammetry experiment can be obtained by dividing areas under the peak of a cyclic voltammogram by the scan rate (0.1 V/s). With three peak areas obtained from three independent experiments shown in Figure 3-24, the average charge is 𝑞 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 = 0.604 ± 0.065 𝑚𝐶/𝑐𝑚 2 . Dividing the obtained charge to the charge needed for a 1-electron redox process with all the catalysts, we got the percentage of active catalyst to be 7.8 ± 0.8%.

Infrared spectroscopy

The infrared measurements were performed at the AILES beamline 29 of Synchrotron SOLEIL. The beamline is equipped with a Fourier Transform infrared spectrometer (IFS 125 from Bruker Instruments) working under a vacuum (10 -4 mbar). The spectra are recorded using a Globar source, a KBr beamsplitter, and a homemade Mercury-Cadmium-Telluride detector cooled at 4.2K 30 . Two different set-ups were used for this study. References samples (pure powder) were analyzed using attenuated total reflectance technique 4 with a single reflection on a 2mm diamond crystal. Modifiedelectrode samples were analyzed using an optical setup (A513-Bruker), allowing reflectivity measurement at grazing angle (70°), the spectrum of the glassy carbon modified with catalyst-free ink being used as a reference.

Introduction

The metal center and the ligands of a molecular catalyst can be tuned to achieve specific properties. As discussed in Chapter 1, the nature of the metal and ligands of many phthalocyanine species have been studied and optimized for CO2 reduction for several decades. Concerning the influence of the metal center on the CO2-to-CO reducing activity, cobalt phthalocyanine is the most active species, followed by iron and nickel phthalocyanine. 1,2 Other metal phthalocyanines could produce CO, but their performances are generally worse than Co, Ni, and FePc. Further reduced products such as methane and methanol have been reported. The formation of species with C -C bonds is much rarer, with only one report mentioning the presence of low amounts of methane, ethane, and ethylene with CoPc and CuPc without further investigation. 3 The porphyrin family behaves similarly, with iron and nickel porphyrin being the most active species for CO2-to-CO reduction. 4 The nature of the ligand also plays a prominent role in the performance of porphyrin and phthalocyanine species. Multiple properties of a substituent group, including its inductive effect, steric hindrance, charge, or proton relaying site, can influence the catalytic activity of a catalyst. 2,5,6 Consequently, the direct substituents-activity relationship is complicated to establish and can differ from case to case.

Besides the nature of the catalyst, many parameters surrounding CO2 electrolysis can significantly affect its performance. Some of these parameters have been discussed in Chapter 1, namely electrolyte flow rate, catalyst concentration, electrolyte concentration, and the nature of the conductive carbon support. In an electrolysis flow system where CO2 is dissolved into the electrolyte, the electrolyte flow rate can change the amount of CO2 coming in contact with the electrode surface, altering the selectivity of species products. 7 The catalyst loading in heterogeneous electrocatalysis affects the dispersity of the active sites, leading to different turnover frequencies and current densities at different loading. 8 For carbonate and bicarbonate electrolytes, their buffer capacity depends on their concentration, influencing the local pH in close proximity to the electrode surface. [8][9][10] Finally, the type and amount of the conductive carbon supports have wide-ranging effects on the current density and selectivity of the CO2 reduction process. 11,12 In the previous chapter, we provided evidence showing that FePc could produce a small amount of CH4 and C2+ products from CO2 reduction. The applied potential significantly affected the product distribution of CO2 electrolysis. In this chapter, we further explore the impacts of other factors on the CO2 reduction activity of FePc, aiming to increase the selectivity of the catalyst towards C2+ products. The investigated parameters include the electrolyte flow rate, the concentrations of catalyst, electrolyte, and conductive carbon support. We then conducted a preliminary study to see the effect of ligands of FePc, and different metal phthalocyanines to produce C2+ species. Finally, we used iron tetraphenyl porphyrin and its derivatives as catalysts for CO2 reduction, focusing on whether other iron macrocycles would yield C2+ products.

Chapter 4

Influence of external factors on FePc

Flow rate

Figure 4-1 shows the electrolysis curves of FePc-modified electrodes at applied potentials of -0.9 to -1.1 V vs. RHE with the electrolyte flow rate of 0.25, 1, and 4 mL/min. Other parameters were similar to experiments with FePc described in Chapter 3 and the annex, with a catalyst loading of 80 nmol/cm 2 and a KHCO3 0.1 M electrolyte. The lower flow rate limit was chosen to ensure the bubbles generated from electrolysis can be evacuated fast enough to avoid disrupting the electrolysis. The higher flow rate limit was the maximum rate where the catalyst layer could adhere well to the working electrode without undergoing significant mechanical erosion.

At both potentials, the fastest flow rate of 4 mL/min displayed a slightly higher current density, while the values at 0.25 and 1 mL/min were similar. The higher current density at a higher flow rate might suggest that the CO2-reducing activity was higher due to more reactants being in contact with the electrode surface. However, the effect was insignificant, as only minor changes were observed. reduction activity and less side reaction with water. This CO2 availability effect played a more prominent role at high overpotentials, as the electrode consumes more CO2 at this condition. Therefore, an electrolysis system with higher CO2 availability, such as a high-pressure or gas diffusion setup, would enhance the selectivity of the FePc catalyst towards CO2 reduction.

Interestingly, the selectivity-flow rate trend appears to reverse concerning the more reduced hydrocarbon species. The most selective condition for CH4 generation was at 1 mL/min at -0.9 V vs. RHE (1.5%) and 0.25 mL/min at -1.1 V vs. RHE (4.5%). The total C2+ products consistently decreased with increasing flow rate at both electrolysis potentials, from 0.35% to 0.23% at -0.9 V and from 0.66% to 0.56% at -1.1 V vs. RHE. Chapter 4

118

The full distribution of individual C2+ products is shown in Figure 4-3. The evolution of the hydrocarbon species is generally correlated with the evolution trend of hydrogen. The correlation indicated that the formation of the highly reduced species was more limited by the hydrogen availability than CO. Therefore, a setup with higher CO2 availability can increase the CO selectivity of FePc, but it may also hinder the formation of products with C -C bonds. Overall, the choice of a flow rate value could considerably alter the product distribution of CO and H2. However, the flow rate effect toward CH4 was marginal. Concerning the C2+ products, C2H6 exhibited the most significant changes, increasing from 0.2 to 0.3% when the flow rate was decreased from 4 to 0.25 mL/min and the potential poised at -1.1 V.

Figure 4-3. C2+ product distribution observed with FePc-modified electrodes as faradaic efficiencies at -0.9 V vs. RHE (A) and -1.1 V vs. RHE (B) as a function of flow rate. The total current density at 60 minutes is plotted as a blue dot for each flow rate. Although no ethylene (C2H4) was plotted, its peak was likely underestimated due to the high background generated

by CO2 in the chromatograph column.

Electrolyte concentration and pH

To study the effect of KHCO3 electrolyte concentration on the CO2-reducing activity of FePc, we perform electrolysis experiments at -1.7 V vs. Ag/AgCl with FePc-modified electrodes under CO2 at KHCO3 concentration ranging from 0.02 M to 2.5 M. Since the bulk pH of the electrolyte is dependent on the salt concentration, the applied potential in the scale of the reversible hydrogen electrode is slightly different depending on the electrolyte concentration. 4-4A, the current density increased with the electrolyte concentration. However, starting from 0.5 M, the electrolysis becomes unstable as the current increased over time. The instability was most substantial at the highest concentration of 2.5 M, as the noise and the rising trend were greatly exacerbated. The increase in current density over time was likely the result of a corrosive side reaction. Therefore, the total faradaic efficiencies under these conditions did not reach one hundred percent. In addition, the high amount of hydrogen generated in these conditions considerably increased the pressure in the headspace. Since the faradaic efficiency calculation assumed the head space pressure to equal atmospheric pressure, the amount of gas products in these conditions was likely underestimated.

When the electrolyte concentration increased, the CO2 reduction activity of FePc drastically decreased. The highest CO selectivity was obtained at 0.02 M KHCO3, which then falls to around 20% at 0.1 M and less than 10% at higher electrolyte concentrations. On the contrary, the hydrogen evolution activity rises with electrolyte concentration, peaking at 80% at 0.5 M. At higher electrolyte concentrations, the hydrogen selectivity was likely to be underestimated, as explained above. At first, these trends seemed counter-intuitive, as hydrogen yield was higher in concentrated bicarbonate electrolytes with higher bulk pH. However, the observed trends were consistent with previous studies in the literature. 9,10 The concentrated bicarbonate electrolyte has a higher bulk pH, but it also has a higher buffer capacity. The high buffer capacity would prevent the rise of local pH caused by CO2 reduction activity consuming protons. Therefore, concentrated electrolytes would have lower local pH than dilute electrolytes, hence the higher hydrogen evolution and lower CO2 reduction activity.

V vs. Ag/AgCl (A) and product distribution observed with FePc-modified electrodes as faradaic efficiencies at -1.7 V vs. Ag/AgCl (B-D) as a function of KHCO3 concentration. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

The high electrolyte concentration also did not favor the formation of hydrocarbon species, as little to no CH4 and C2+ species were observed at 2.5 M concentration. However, instead of a monotonic decrease, as in the case of CO, the evolution of CH4 and C2+ products follow a volcano trend. The total distribution of individual C2+ products is shown in Figure 4-5. CH4 peaks around 4% at 0.1 M, while C2+ peaks around 0.85% at 0.5 M. Nevertheless, controlled electrolysis in an Ar-saturated 1 M KHCO3 electrolyte showed that all the carbon products are present, albeit in much lower amounts compared to a CO2-saturated electrolyte despite the similar current density. In contrast, a control experiment with 0.1 M KHCO3 (Chapter 3) yielded only a trace amount of CO and no traces of CH4 or C2+ products. The presence of these highly reduced species suggests that at high bicarbonate concentrations, the bicarbonate Chapter 4 121 itself is also a feedstock and can be reduced to hydrocarbon species. Therefore, excluding the amount of C2+ products generated from bicarbonate, the optimized concentration to produce species with C -C bonds from CO2 would lie around 0.1 M.

Figure 4-5. C2+ product distribution observed with FePc-modified electrodes as faradaic efficiencies at -1.7 V vs. Ag/AgCl as a function of KHCO3 electrolyte concentration. The total current density at 60 minutes is plotted as a blue dot for each bicarbonate concentration. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in

the chromatograph column.

FePc concentration

Figure 4-6 shows the electrolysis curves of FePc-modified electrodes at applied potentials of -0.7, -0.9, and -1.1 V vs. RHE with different FePc loading. The experimental conditions were similar to experiments with FePc described in Chapter 3 and the annex, with a catalyst loading of 80 nmol/cm 2 , a KHCO3 0.1 M electrolyte, and a flow rate of 1 mL/min. Regardless of the catalyst concentrations, the current density was higher at more negative potential. At -0.7 and -1.1 V vs. RHE, no significant difference in current density was displayed by electrodes with different amounts of FePc. However, at -0.9 V vs. RHE, the highest current density (~1.5 mA/cm 2 ) was observed with the electrode having the highest catalyst loading, while the values of the two others were around one and a half times lower.

0.02 0.1 0. Figure 4-7 shows the faradaic efficiencies (FE) of CO, H2, CH4, and C2+ products obtained after 2-hour electrolysis at -0.9 and -1.1 V vs. RHE with FePc-modified electrodes under CO2 at FePc concentrations ranging from 80 to 450 nmol/cm 2 . At -1.1 V vs. RHE, the selectivity of CO and H2 exhibited a moderate variation at different FePc concentrations, but no clear trend emerged. On the other hand, at -0.9 V vs. RHE, the faradaic efficiency of CO decreased, while that of H2 increased with higher catalyst Chapter 4 123 loading. Since CO2 reduction is more favored than hydrogen evolution at this moderate potential, the amount of CO2 near the electrode might be depleted too quickly with high catalyst concentrations. Therefore, water reduction could happen more readily as the CO2 mass transfer was not quick enough. A similar trend was observed at a potential of -0.7 V vs. RHE (Figure 45678). At this potential, CO and H2 were the only products observed, indicating that a sufficient potential was necessary to reduce CO2 further than CO. When the applied potential was sufficiently negative, CH4 tended to be favored at lower catalyst loading. At -1.1 V vs. RHE, the highest CH4 amount obtained was at the lowest FePc concentration. At -0.9 V vs. RHE, both the highest and the lower FePc concentrations yielded a similar amount of CH4, while the lowest CH4 faradaic efficiency was observed at the moderate FePc loading.

Concerning the C2+ products, at a high overpotential of -1.1 V vs. RHE, the total selectivity increased, but it appeared to reach a saturating value of around 1.2 % at 225 nmol/cm 2 . A similar trend was observed at a less negative potential of -0.9 V vs. RHE as the total C2+ products saturated when the catalyst concentrations increased. The entire distribution of individual C2+ products is shown in Figure 456789. The saturation effect of C2+ products observed with increasing FePc concentrations suggested that catalyst degradation could not be the source of the observed hydrocarbons, as higher catalyst loading did not lead to equally higher observed products.

V vs. RHE (A) and -1.1 V vs. RHE (B) as a function of FePc concentration. The total current density at 60 minutes is plotted as a blue dot for each catalyst concentration.

The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column. The increase in carbon black concentration correlated with the rise in CO and CH4 concentration. However, the evolutions of H2 and C2+ exhibited a reverse trend. Similar to the flow rate series, the high yield of C2+ products coincided with the high yield of H2. The correlation corroborates the hypothesis that C2+ species are more favored in an environment with an abundance of hydrogen. However, in this case, CH4 was not favorable when there was high availability of hydrogen.

Conductive carbon support concentration

We propose a hypothesis concerning the observed behavior of FePc with different amounts of carbon support. The low carbon black concentration means higher proximity of the catalytic centers, potentially forming clusters. The FePc in close proximity leads to a higher local concentration of CO generated, producing more C2+ products. The higher carbon black concentration renders the FePc catalyst more dispersed, leading to a lower local CO concentration to carry out the CO insertion step on the carbene species. Therefore, the observed C2+ products were low, and the obtained methane was high.

Although the selectivity of products with C -C bonds is favored at low concentrations of carbon black, the catalyst layer is not stable as it is peeled off after one electrolysis experiment. Consequently, a sufficient amount of conductive carbon support is necessary to preserve the mechanical stability of the catalyst layer.

Figure 4-10. Product distribution observed with FePc-modified electrodes as faradaic efficiencies at -1.1 V vs. RHE as a function of carbon black concentration. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Chapter 4

Figure 4-11. C2+ product distribution observed with FePc-modified electrodes as faradaic efficiencies at -1.1 V vs. RHE as a function of carbon black concentration. Although no ethylene (C2H4) was plotted, its peak was likely underestimated due to the high background generated

by CO2 in the chromatograph column.

Metal phthalocyanines

In this section, we investigated the influence of metal centers on the CO2-reducing activity of metal phthalocyanines, focusing on the hydrocarbon products with C -C bonds. Then we studied the impact of various substituent groups on the catalytic activity of FePc.

Metal center effects

We examined six first-row transition metal phthalocyanines for their CO2-reducing activity: Mn, Fe, Co, Ni, Cu, and Zn. The experimental conditions were similar to experiments with FePc described in Chapter 3 and the annex, with a catalyst loading of 80 nmol/cm 2 , a KHCO3 0.1 M electrolyte, and a flow rate of 1 mL/min. Figure 4-12 shows the electrolysis curves of the studied metal phthalocyanines at applied potentials ranging from -0.7 to -1.1 V vs. RHE. CoPc exhibited the highest stable current density in all potentials. The high current density came with a considerable noise level, likely due to the large amount of gas bubbles generated. At -0.7 V vs. RHE, all metal phthalocyanines exhibited current densities one order of magnitude lower than CoPc (CuPc was not examined at this potential). At the moderate potential of -0.9 V vs. RHE, CoPc was followed by CuPc, NiPc, and FePc, while MnPc and ZnPc were inactive. The electrolysis curve of NiPc displayed a sharp decline in current density. The same trend can be observed at - show the highest selectivity for CO, greater than 70%, at potentials of -0.7 and -0.9 V vs. RHE. At these moderate potentials, Mn, Cu, and ZnPc produce more H2 than CO. The CO2-reducing activity on these metal Pc is low, resulting in the current density being close to the background current value and the total faradaic efficiencies not reaching close to a hundred percent. Although the liquid products were not characterized in all experiments, based on the literature, we would expect at most a few percent of methanol generated from electrolysis on CoPc. At a more negative potential of -1.1 V vs. RHE, CoPc was the only catalyst maintaining CO as the primary product. Other catalysts mainly produced hydrogen, except for ZnPc, generating CO and H2 at roughly equal selectivity. 128 produced methane. At a more negative potential of -1.1 V vs. RHE, Mn, Fe, and NiPc are the catalysts that yield methane, with the highest methane faradaic efficiency of around 4% coming from FePc. Interestingly, methane was only presented when CuPc was poised at -0.9 V instead of -1.1 V vs. RHE.

Figure 4-14. CH4 distribution observed with metal phthalocyanines electrodes as faradaic efficiencies after 2-hour electrolysis at -0.9 V vs. RHE (A), and -1.1 V vs. RHE (B)

Figure 4-15 shows the faradic efficiencies of all C2+ products coming from the examined metal phthalocyanines. At -0.9 V vs. RHE, only FePc produced the hydrocarbon products with C -C bonds, with their distributions presented in Chapter 3. When the potential is more negative at -1.1 V vs. RHE, Mn, Fe, and CuPc produce hydrocarbon species. C2H4 accounted for the majority of hydrocarbons coming from CuPc. However, as discussed in Chapter 1, the molecular nature of CuPc may not be preserved during catalysis; thus, the actual active sites where the C -C bonds formed was likely metal site. FePc and MnPc generated a range of C2+ products, with the total accounting for 0.76% and 0.21%, respectively.

C2+ product distribution as total C2+ products (A) and individual C2+ products (B) observed with metal phthalocyanine electrodes as faradaic efficiencies after 2-hour electrolysis at -1.1 V vs. RHE. The total current density at 60 minutes is plotted as a blue dot for each metal species. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

The observation that FePc was the most active metal phthalocyanine in reducing CO2 to hydrocarbon products emphasizes the distinct properties of the iron center. Amongst metal catalysts, copper is the unique species that can form products with C -C bonds in high quantities. This specific ability comes from copper being the only metal to possess mild binding energy with both CO* and H* intermediates. Therefore, on a copper surface, a CO* species can remain for a sufficient amount of time to form a C -C bond (through a carbene species) without separating quickly to form CO like silver or binding too strongly, leading to catalyst poisoning like platinum. Although the binding energy of iron metal with CO* and H* do not fall into this sweet spot, this property can change when the iron is complexed as an ion by organic ligands. Using DFT calculations, Rossmeisl and co-workers showed that a porphyrin-like structure of an Fe-N-C species possesses binding energies for CO* and H* similar to metallic Cu. 13 Similarly, Mn-N-C and Cu-N-C are the only other common transition metals to fall into this category. This calculation result completely matches our experimental results, as MnPc, FePc, and CuPc were the only species in the series that could form products with C -C bonds from CO2 reduction. These findings emphasize that the surrounding environment of metal can alter its properties and demonstrate the prospect of a copper-free catalyst for forming products with C -C bonds from CO2 reduction.

Ligand effects on FePc

We studied the substituents' effects on the CO2-reducing activity of FePc using four species shown in Figure 4-16. The Cl and COOH groups are electron-withdrawing, which helps to lower the overpotential needed to reduce CO2. However, these groups also decrease the electron density around the metal center, potentially hindering the Chapter 4 electron transfer to CO2. On the other hand, the NH2 donating group has the opposite effect of increasing the electron density around the metal center while also increasing the overpotential of the reaction. The standard experimental conditions were used, with catalyst loading of 80 nmol/cm 2 , a 0.1 KHCO3 electrolyte, and a flow rate of 1 mL/min. Figure 4-17 shows the electrolysis curves of FePc-substituted electrodes at -0.9 and -1.1 V vs. RHE (FePc-Cl16 was tested at -0.8 V vs. RHE instead). At -0.9 V vs. RHE, the highest current densities are observed for the amino substituted at the ortho position, followed by the meta-amino and the carboxylic acid species, while the unmodified FePc and the chloro-substituted FePc were the least active. The trend was analogous to that at -1.1 V vs. RHE. Figure 4-18 shows the faradaic efficiencies of H2, CO, CH4, and C2+ products from the studied substituted FePc. The non-substituted FePc exhibited the highest CO selectivity at both applied potentials, accounting for 70% and 20% at -0.9 and -1.1 V vs. RHE, respectively. Interestingly, all substituted FePc were not highly active at reducing CO2 to CO. Catalysts with Cl and COOH functional groups produced CO with faradaic efficiency higher than 10%, while the amino-substituted FePc hardly yielded any CO. The poor performance may arise from the drop-casting process and the solubility of these species in DMF. Out of the analyzed catalysts, FePc is the most soluble, followed by FePc-Cl16, and then the rest of the catalyst. In fact, FePc-COOH and FePc-NH2(m) were hardly soluble in DMF. Consequently, the insoluble species were not well-dispersed on the electrode surface and stayed in the form of large particles, leading to the poor performance observed. This result reflects the complication in studying the influence of ligand functional groups on the CO2reducing activity of metal complexes, as secondary phenomena can interfere with the intended effect of the study.

Although the substituted FePc did not produce a considerable amount of CO, CH4, and C2+ products were present in the products of these catalysts at both applied potentials. The complete distribution of individual C2+ products is shown in Figure 4-19. At -1.1 V vs. RHE, the unmodified FePc produced the most significant amount of methane, followed by the chloro, the m-amino, and the carboxylic acid substituted FePc. This order was similar concerning the total selectivity of the C2+ products. Interestingly, FePc-(o-NH2)4 produced a negligible amount of both methane and the C2+ hydrocarbons. However, at the less negative potential of -0.9 V vs. RHE, the trend Chapter 4 132 seemed to be reversed as the non-substituted FePc has the least amount of CH4 and C2+ while the amino-substituted had the most. This result indicates that, despite the low amount of CO produced, when hydrogen is abundant, C2+ products are favored.

Iron porphyrins and derivatives

As discussed in Chapter 1, metal phthalocyanines and porphyrins are the two molecular macrocycles with the highest CO2-to-CO activity. Since we found that iron phthalocyanine produces methane and hydrocarbon products with C -C bonds, iron porphyrin is a promising candidate to study whether it can form C -C bonds from CO2 reduction. We examined four iron porphyrin species shown in Figure 4-21: a commonly used iron tetraphenyl porphyrin (FeTPP), then two FeTPP substituted with electronwithdrawing groups (fluoro -FeTPP-F20), and electron-donating group (methyl -FeTTP). Finally, we studied a bimetallic iron porphyrin species where the porphyrin rings are fused on the same plane (Fe2FP). The last two porphyrins were provided by the group of Prof. Gary F. Moore from Arizona State University in the United States of America. The idea behind the two adjacent iron sites is that their proximity may facilitate the formation of products with C -C bonds in a mechanism similar to the coupling of the carbene species on metallic copper that yield ethylene.

The experimental conditions were similar to those described above, with a catalyst loading of around 80 nmol/cm 2 , a 0.1 KHCO3 electrolyte, and a 1 mL/min flow rate. Since the fused porphyrin has two iron per molecule, the iron content doubled that of the other catalysts. Other catalysts all produced more H2 than CO at this highly negative potential. However, under these conditions, all catalysts produced more reduced hydrocarbon products. Methane was present with all iron porphyrin catalysts at around 1.5%, with FeTTP having the highest selectivity of 2%.

Only FeTPP-F20 and FeTTP yielded C2+ products, with FeTPP-F20 having the highest total C2+ faradaic efficiency of around 0.6%. The distribution of individual C2+ products is shown in Figure 4-23.

While the reason why only FeTTP and FeTPP-F20 produced C2+ is unclear, the fact that not all iron macrocyclic compounds produced molecules with C -C bonds indicated that the ligand plays a crucial role in the CO2-reducing activity of these catalysts. Moreover, the complexes with adjacent sites did not yield C2+. This result indicates that the proximity of two potential active sites is insufficient (at least in this geometric configuration) to promote the formation of C -C bonds. However, the mechanistic route proposed in Chapter 3 suggested that only one metal active site is involved in forming multiple C -C bonds by CO insertion into an iron carbene species. The absence of C2+ products from the Fe2FP compound supports this single-site hypothesis.

Conclusion

In this chapter, we first reported on the influences of the electrolyte flow rate and the concentrations of catalyst, electrolyte, and carbon support on the CO2-reducing activity of FePc. The flow rate influenced the amount of CO2 reaching the electrode, altering the product distribution of the catalysis. The high catalyst loading leads to the depletion of CO2 near the electrode, hindering CO2 reduction while benefiting hydrogen evolution. Higher catalyst loading did not significantly increase the C2+ production, confirming catalyst degradation was not the source of the observed hydrocarbons. The high electrolyte concentration provides high buffer capacity, keeping the local pH low near the electrode and thus facilitating hydrogen evolution. The highly reduced products were detected at sufficiently high concentrations, suggesting that bicarbonate may be a reactant in appropriate conditions. Finally, the amounts of carbon black support affected the proximity of the catalyst active site, favoring C2+ products with less carbon while promoting CH4 with higher amounts of carbon.

In the second part, we investigated the behavior of different metal macrocycles modified with several functional groups for their activities in reducing CO2 to hydrocarbon products. Six metal phthalocyanines were tested, and except for CuPc producing C2H4, only FePc and MnPc yielded C2+ products. The favored binding energy with CO* and H* intermediates of the Fe-N-C and Mn-N-C species could facilitate the formation of C -C bonds, emphasizing the role of the metal center. No clear trend emerged regarding the impact of electron-donating and electron-withdrawing groups on the performance of FePc, although some substituents clearly showed better performances toward C2+ formation. Analogous iron porphyrin counterparts only produced C2+ products when substituent groups were presented. Finally, a bi-metallic center fused iron porphyrin did not yield C2+ products, supporting the single-site mechanism to form C -C bond proposed in Chapter 3 rather than a dimerization similar to what happens in metallic copper.

Chapter 4

CHAPTER 5: CONCLUSIONS AND PERSPECTIVES

Chapter 1

In the first chapter, we looked at the importance of energy storage regarding the transition from fossil fuels to renewable energy. Inspired by natural photosynthesis, we explored why carbon dioxide reduction is a promising method to store renewable energy and produce valuable fuels and chemicals. By reviewing the literature focusing on metal phthalocyanines and porphyrins, we made a case for studying molecular catalysts to understand better the detailed mechanisms of the CO2 reduction reaction.

We then laid out the strategy of the thesis in designing and fabricating an electrochemical flow system in an attempt to generate highly reduced products from CO2 reduction on molecular catalysts

Chapter 2

In chapter 2, we first presented a strategy for using a cascade catalysis system to reduce CO2 to highly reduced products using a series of microfluidic devices. We reported on designing and fabricating a PDMS microfluidic flow cell prototype. We learned that despite the apparent benefit of the microfluidics conditions for electrochemical energy applications, a microfluidics device was not the appropriate choice for handling a fluid with high amounts of dissolved gas. The microscopic channel prevented the gas bubbles from being evacuated quickly, leading to unstable electrochemistry. To alleviate the problem, we 3D-printed a millifluidic electrochemical flow cell, anticipating the larger dimension of the flow channel would allow bubbles to escape faster. The 3D printing process allows for quick modifications, enabling us to adapt the cell for spectroelectrochemistry applications with X-ray absorption spectroscopy. Although having larger dimensions, laminar flow conditions were still achieved in the cell, similar to the microfluidic conditions. However, a CO2-saturated solution still caused the turbulent flow in the cell's channels by generating bubbles. A membrane was then employed, and CO2 electrolysis using an FePc catalyst could last at least two hours with a lower noise level than the PDMS cell. The concentration profiles of the reduced products were characterized using finite element analysis based on laminar flow and transport of diluted species physics. The calculation showed that CO concentration existed in a supersaturated concentration near the electrode surface, but the CO2 conversion rate was relatively poor.

For perspectives, we propose an alternate design for the microfluidic cell. The cell uses a Teflon gasket instead of PDMS to form the flow channel. Although the bubble problem proves tricky to solve, reducing CO2 in a thin channel is still an attractive idea. First, under appropriate conditions (i.e., flow rate, CO2 pressure, and electrolyte), the total consumption of CO2, which is challenging to achieve in a cell employing a gas diffusion electrode, could be achieved. Under these conditions, a small amount of pure CO can be generated in place and on demand. This pure stream of CO can serve as a feedstock in the subsequent reactor of a sequential reduction system. Moreover, creating a controlled amount of CO in a safe concentration is helpful in multiple applications, as handling CO in a compressed tank is complicated and risky due to its toxicity. Finally, the complete conversion of reactant is not desirable to only CO2. In Xray spectroelectrochemistry for homogeneous catalysis, the total transformation of a reactant is required to obtain a signal solely from the intended species. The milifluidic 3D-printed flow cell could be fabricated with more robust materials such as PEEK or Teflon to accommodate more extreme experimental conditions. The cell could also be converted to use in a gas-diffusion setup.

Chapter 3

In chapter 3, we reported the electrocatalytic activity of iron phthalocyanine toward reducing CO2 in our 3D-printed millifluidic electrochemical flow cell. We learned that upon reducing CO2, in addition to CO, a small amount of linear hydrocarbons with up to three C -C bonds were detected at appropriate potentials. Control experiments and GC-MS characterization confirmed that CO2 was the source of carbon atoms in the detected hydrocarbons. We demonstrated the integrity of the iron phthalocyanine molecular nature throughout the electrocatalytic process using operando X-ray absorption spectroscopy. Based on experiments involving carbon monoxide or formaldehyde and the finite element analysis of the product concentration profiles, we showed that the local concentration of CO is key to the formation of C -C bonds. We proposed a terminal carbene as an intermediate on the way to C2+ products.

These results indicate a novel finding that single-site transition metal macrocycles could catalyze the formation of several consecutive carbon-carbon bonds under mild electrochemical conditions. We propose a strategy to validate the hypothesis of the C -C bond formation mechanism by CO insertion into an iron carbene intermediate. The Fe=CH2 species could be synthesized and isolated to study its single turnover reactivity, i.e., whether it would react with CO and H2 to form a C -C bond. Spectroscopic studies of such a species would also be beneficial. However, matching those spectra with operando data would be challenging since the intermediate existed in low amounts during the catalytic cycle. However, the e of a reaction between CO and H2 on an iron carbene species would provide concrete proof to support the proposed mechanistic pathway for forming C2+ products. In addition to experimental data, DFT calculation could provide valuable information regarding the thermodynamics favorability of each intermediate species, as well as the kinetic pathway of the transformation between each step. The effect of external factors such as conductive carbon support, electrolyte, and flow rate is tricky to incorporate into the DFT model. Overall, the fact that C -C bond formation could happen on singlesite species opens a perspective for the design of both molecular and material-based catalysts with enhanced selectivity towards C2+ products.

Chapter 4

In chapter 4, we learned about the influences of external factors on the CO2-reducing activity of FePc: the electrolyte flow rate and the concentrations of catalyst, electrolyte, and carbon support. The higher the flow rate, the higher the amount of CO2 reaching the electrode, and the more CO2 is reduced to CO. The higher the catalyst loading, the lower the CO produced due to CO2 being depleted near the electrode. The amount of C2+ hydrocarbon increased, but not in a linear relationship with the catalyst concentration, thus proving that catalyst degradation was not the source of the observed hydrocarbons. Hydrogen evolution was favored at high electrolyte concentration, as the local pH was kept low near the electrode surface due to the high buffer capacity of the electrolyte. The highly reduced products were detected even without CO2 at sufficiently high bicarbonate concentrations, indicating that bicarbonate may be a reactant in appropriate conditions. Finally, the increasing amounts of carbon black support correlated with the rising amount of CH4 and the decreasing of C2+ products. We attributed the observation to the proximity of the catalyst's active site induced by different amounts of the carbon support. In all the series tested, the high yield of C2+ products correlated with high hydrogen selectivity. This observation suggests that the availability of hydrogen plays a role in forming C -C bonds. Therefore, a sequential setup where the amount of CO and H2 could be precisely tuned may prove beneficial to produce C2+ products more selectively.

We then investigated the behavior of different metal macrocycles modified with several functional groups for their activities in reducing CO2 to hydrocarbon products. Six metal phthalocyanines were tested: Mn, Fe, Ni, Co, Cu, and Zn. Except for CuPc that produced C2H4, C2+ products were detected only on FePc and MnPc. We attribute the activity of FePc and MnPc to their unique binding energy with CO* and H* intermediates. Like metallic copper, FePc and MnPc can bind with CO* and H* not too strongly or weakly, providing the ideal condition for forming C -C bonds. Chloro and amino substituent groups were grafted on FePc, but their influence on the production of C2+ was unclear. Analogous iron porphyrin counterparts only produced C2+ products when substituent groups were present, signifying the importance of the ligands in molecular catalysts. Finally, a bi-metal center fused iron porphyrin did not yield C2+ products, supporting the single-site mechanism forming the C -C bond proposed in Chapter 3.

General discussion of the thesis

As discussed in Chapter 1, molecular catalysts may not be the ideal candidate to reduce CO2 on an industrial scale. Still, they provide a platform for studying detailed mechanisms of the CO2 reduction reaction. Previously, this application of molecular catalysts was limited to only C1 products. The discovery of C2+ products on a singlesite molecular macrocycle, even in low amounts, helps to inspire the development of molecular catalysts to form products with C -C bonds. While the formation of the C2+ products could begin with CO2, starting from a mixture of CO and H2, or CO and HCHO with the optimum ratio, it would be much more efficient. In this case, cascade catalysis would be an appealing approach. Based on the reactivity we have shown for several metal macrocycles and iron phthalocyanine in particular, a sequential catalytic system could be constructed. The first reactor will consist of a highly active catalyst like CoPc to produce an optimum mixture of CO and H2 to feed to the second reactor, which contains catalysts (FePc or MnPc) capable of forming hydrocarbon products. Alternatively, a tandem catalyst containing multiple active species can be designed and synthesized. One species would be responsible for generating CO and H2, which serve as the feedstock for a neighboring species to convert into C2+ products.

While we identified several influencing trends of electrolysis parameters and catalysts' nature on the generation of C2+ products, the optimum conditions have not been combined. The best conditions of all investigated parameters should give several percent of C2+ products, which could be higher if a cascade system is employed. The high selectivity toward the hydrocarbon products would allow the collection of operando spectroscopic data with a higher signal-to-noise ratio. The high-quality spectroscopy data could then be compared with data obtained from single-turnover experiments with the metal carbene intermediate, providing robust evidence for the proposed mechanism. Even without industrial applications, the mechanistic insights gained from studying molecular species would benefit the design and optimization of more applicable candidates. Finally, a gas-diffusion setup could help to enhance the activity of the investigated catalysts.

The big picture

Developing and implementing energy storage systems are the crucial missing links in the energy transition from fossil fuels to renewable sources. Presently, fossil fuels are the standard choice to fill the intermittent gap between wind and solar power. As the share of wind and solar increases in the energy mix, energy storage systems must replace gas-fired power plants as the primary method to make up for renewable sources' intermittency. In immediate terms, batteries are the most matured electrochemical storage devices and should be heavily invested to accommodate society's rapid electrification. In the medium term, green hydrogen from water electrolysis will be a substantial factor in energy storage, helping to decarbonize the economy. Hydrogen can make use of the existing infrastructure for natural gas, and it could be a solution to fields that are challenging to electrify, such as aviation and heavy industry. In the long run, carbon capture, utilization, and storage will play a significant role in reaching a net-zero society. As the share of carbon-emitting technology falls, capturing carbon, either from the source or the atmosphere, will become an attractive solution. When the fossil fuels industry shrinks, carbon dioxide may become the alternative carbon source to produce essential fuels, materials, and chemicals. Carbon utilization represents both an environmental solution and an economic opportunity.

Chapter 5

146

Climate change, the most significant challenge humanity has ever faced, requires an all-hands-on-deck approach to tackle. A silver bullet to solve global warming does not and will not exist. Every decarbonization solution should be impartially considered and studied, from employing energy storage and nuclear power to redesigning transportation infrastructure and changing everyday behaviors. Technical solutions are just one piece of the bigger puzzle, as it takes political will to implement sensible technologies and policies to combat climate change. Therefore, people living in democratic systems should rally around the leaders with the most pragmatic approaches to enact rational environmental laws and regulations. As Russia, Saudi Arabia, and other authoritarian regimes continue to wield oil and gas as weapons, the democratic nations should accelerate the decoupling of fossil fuels from the global economy. It will not be a trivial task, but it is possible, and it must be done.

Chapter 5 12. The cell is now ready. The experimental setup is described in detail in Chapter 3. After each experiment, the cell is sonicated in water purified by reverse osmosis to 18 MΩ to remove any possible contaminants.

Supplementary figure

Ink and electrodes preparation procedure

8 mg of carbon black (Vulcan XC72 -Fuel Cell Store) is dispersed in 6 mL of dimethylformamide (DMF -99.8% -VWR) by sonicating the mixture for 30 minutes at high power (FB11209 Fischer brand sonicator). 0.74 mg of FePc (Porphychem) dissolved into 2 mL of DMF is added to the carbon black dispersion, and the mixture is then sonicated for 10 minutes at low power. 80 µL of Nafion D-520 solution (Alfa Aesar) is added, with another 10 minutes of sonication at low power. This standard ink solution is made for electrodes having the FePc surface concentration of 80 nmol/cm 2 and the carbon black concentration of 1 mg/mL. All the phthalocyanines and porphyrins species were purchased from Porphychem, except for FeTTP and Fe2FP, which Prof. Gary Moore provided. The Fe3O4 nanoparticles were purchased from Sigma Aldrich. The nanoparticles are stabilized by modified polyethylene glycol (PEG) ligands covalently bonded to the particle by a hydroxamic acid on the end of the PEG chain. The PEG concentration is less than 1 % in weight. Their density is 1 mg/1 mL of H2O, and their molecular weight is 231.53 g/mol.

The drop-casting procedure for an electrode is shown in Figure 6-5. The working electrodes are cast right after the ink is made to avoid the aggregation of the carbon black. A glassy carbon plate (HTW) with a dimension of 25 x 10 x 0.3 mm 3 is kept on a hotplate at 80 o C. A 70-µL drop of ink is cast onto the glassy carbon plate, covering the whole surface of the electrode. The total volume cast on one electrode is 1167 µL, yielding a surface concentration of the catalyst around 80 nmol/cm 2 . After adding the ink, the excess catalyst that would not be in contact with the electrolyte is removed. Then the electrode is kept in a vacuum desiccator at 80 o C overnight to evaporate the DMF solvent fully.

Constant-potential electrolysis experimental setup

The assembly of the electrochemical flow cell is described in detail in chapter 2. Figure 6-6 shows the setup of the electrolysis and product characterization system. PEEK tubing (1/16'' OD, 0.5 mm ID from Cluzeau) connects the electrolyte bottles with the peristaltic pump and the electrochemical cell. The PEEK tubings of the fully-assembled cell are connected to the PEEK tubings of the system using flexible silicone peristaltic pump tubing (1.02 mm ID, TYGON 2001 from Ismatec). The catholyte and anolyte bottles contain 25 mL of 0.1 M KHCO3 (VWR) electrolyte. A peristaltic pump (Ismatec) circulates the electrolyte solution from the bottles to the cell and back to the bottles. Before an experiment, the catholyte solution is bubbled with CO2 (Alphagaz 1 -Air Liquide) for at least 15 minutes while the solution is circulating. The pump's flow rate is 1 mL/min, which is calibrated before each experiment. When the gas purging finishes, the vent valve is closed while the purge valve is kept open for around 5 minutes, keeping the system with a slight overpressure to check for leakage. When no leakage is detected, the purge valve is closed, and the vent valve is quickly opened to depressurize the system, then closed immediately afterward. After that, constantpotential electrolysis is performed using the VSP-300 potentiostat from Biologic.

In control experiments in a CO2-free environment, Ar (Alphagaz 2 -Air Liquide) is Annex purged instead of CO2 before the electrolysis. In experiments with CO, instead of CO2 as the reactant, CO (Air Liquide) is purged. When HCHO is used as the reactant, a solution of 30 mM HCHO in 0.1 M KHCO3 is prepared using 16% HCHO (methanolfree, Thermo Scientific). The solution is used as the catholyte, and either Ar or CO2 is purged in the HCHO/KHCO3 solution for at least 15 minutes before the experiments.

A gas chromatograph (Inficon MicroGC) is connected directly to the catholyte bottle using similar PEEK tubing to analyze the gas products. The chromatograph analyzes the gas composition in the bottle's headspace. The headspace volume of the catholyte container is derived from the mass difference of water between a filled versus a 25-mL-filled container, yielding 9.5 mL.

Gas chromatograph characterization

This study uses a 2-module Micro GC FUSION® Gas analyzer to characterize gaseous products. Module A has a 10-meter Rt-Molsieve 5A column, while module B has a 12meter Rt-Q-bond column. Module A analyzes "light" gases, while module B analyzes "heavier" gases and some volatile organic compounds. Both modules use a thermal conductivity detector. The parameters of each module are shown in Table 6-2. While the exact elution time of a gas depends on a method's parameters, the appearance order in both modules is fixed. For module A, the order of appearance is H2/He → O2/Ar →N2 → CH4 →CO. Module B can analyze more species, so the order of appearance is more complicated as there are more chances for peaks to overlap. However, common species in module B follow a general trend: module A mixture → CO2 → C2 → H2O → C3 →C4 → Cn. For multi-carbon species with isomers, unsaturated isomers generally elute before saturated isomers. If H2 and CH4 are presented in high amounts, the peak representing the module A mixture will have a negative peak at the beginning corresponding to H2 and a shoulder at the end corresponding to CH4. The chromatograms of both modules exhibit a peak at the beginning due to the injection process. The direction of the peak depends on the carrier gas, and the peak in module A is more pronounced due to the backflush process to prevent unwanted gas from entering the column.

The gas chromatograph is calibrated using mixtures of gases with known concentrations. A 2.9% (v/v) mixture of H2 in Ar and a 0.5% (v/v) mixture of CO in He from Air Liquide are used to calibrate CO and H2. For CH4, C2H4, C2H6, C3H6, and C3H8, a mixture of 100 ppm (v/v) of each gas in N2 from Air Products are used. For n-C4H10, a pure sample of the gas from Messer is used. A linear forced through zero regression is performed to establish the relationship between the peak area and the volume concentration for each gas product, as recommended by the chromatograph manufacturer. The peak corresponding to pentane (C5H12) was identified by sampling the headspace of a flask containing liquid pentane. However, it was not calibrated due to the difficulties in calibrating volatile liquid organic products with the gas chromatograph in the low-concentration region observed in our experiments.

Faradaic efficiency calculation and electrode potential conversion

A gas chromatograph measurement is performed every 30 minutes during the 2-hour electrolysis. The volume percentages of each gas are derived from the calibration line obtained from the procedure described above. The mole numbers of each product are calculated using the ideal gas law:

𝑛 = 𝑃 × (%𝑉 × 𝑉) 𝑅 × 𝑇
where P, V, and T are the headspace pressure, volume, and temperature, of the catholyte container, respectively. %V is the volume percentage measured by the gas chromatograph, and R is the gas constant (8.314 J.mol -1 .K -1 ). Due to the low amount of generated products compared to the headspace volume, as well as the climatecontrolled condition of the laboratory, the values of the pressure and the temperature in the headspace are assumed to be the values of atmospheric pressure (1.013 hPa) and room temperature (298K).

The Faradaic efficiency of all gaseous products is then calculated using the formula:

%𝐹𝐸 = 𝑛 × 𝑒 × 𝐹 𝑄
where n is the mole number of the corresponding products, e is the number of electrons needed to reduce CO2 (or H2O in the case of H2) to that product, F is the Faraday constant (96490 C.mol -1 ), and Q (C) is the total charge passed through the system at the time of the gas sampling, given by the potentiostat.

The procedure for calculating the Faradaic efficiency of liquid products is similar, except that the mole number is calculated directly using the molar concentration measured by NMR with a DMSO internal standard.

The conversion of potential from 3.4 M Ag/AgCl reference electrode to reversible hydrogen electrode is performed using the formula:

𝐸 𝑅𝐻𝐸 = 𝐸 𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.205 𝑉 + 0.0591 × 𝑝𝐻
where the pH of 0.1 M KHCO3 is 6.8 when saturated with CO2 and 8.5 when saturated with Ar; the pH of 0.1 M KCl is 6.3 when saturated with Ar. All potential values are rounded to the first decimal place.

X-ray absorption spectroscopy experiments

XAS experiments were performed on the LUCIA 1 beamline of Synchrotron SOLEIL, with a ring current of 500 mA and nominal energy of 2.3 GeV. On LUCIA, The beamline energy was selected using a Si (111) double-crystal monochromator, and the beam size was 2 x 2 mm 2 . Figure 6-1B shows the electrocatalytic flow cell in the experimental chamber under vacuum, with a 25° outgoing angle with respect to the detector, which is located at a 90° angle from the incident beam path. The experimental setup is similar to the one described in Section 6.4. However, a silver wire glued in the cell was used as the pseudo reference electrode since the mini Ag/AgCl reference electrode is not vacuum-compatible. In addition, the gas is kept bubbling during the whole measurement, as the gas products were not characterized during the spectroelectrochemical measurement. XANES data were collected as fluorescence excitation spectra using an SDD Bruker detector. The reference spectra of FePc, Fe metal, and Fe3O4 are measured with 6-mm round pellets containing 1 mg of sample and 40 mg of graphite window materials. All XAS data were normalized to the intensity of the incoming beam and reduced using the Athena software. 2

Introduction

Atmospheric carbon dioxide (CO 2 ) has been used as a feedstock to produce all biomass, 1 including fossil fuels, since photosynthesis developed on Earth more than two billion years ago. Industrialized societies are eagerly searching for such a solar-tochemical conversion process, which would help mitigate their carbon dioxide emissions while providing alternative ways to produce staple chemicals. 2 The electrochemical CO 2 reduction reaction (CO 2 RR) has been proposed as a viable technology to convert this gas into valuable chemicals. At present, the most common products of CO 2 reduction are carbon monoxide (CO) and formic acid (HCOOH). Although these C 1 products are near the market-ready level, they only reect a few of the chemicals that can be generated from CO 2 . 3 Great efforts have been directed to further reduce CO 2 to higher value products, particularly those including C-C bonds. Starting from the work of Hori and co-workers 4 in the 80s, copper-based catalysts have shown remarkable activity towards the formation of light hydrocarbons, such as methane and ethylene. 5 Despite these performances, the precise nature of the active sites involved remains debatable, 6 partly because of the restructuring of copper during catalysis. 7,8 Recently, single-atom catalysts (SACs) have gained signicant attention in the eld of CO 2 reduction due to their low metal content and high activity. 9 While the reduction of CO 2 on SACs yields mostly CO, other more reduced products have been obtained, 10,11 including C 2+ products such as ethanol 12 and acetone. 13 Although SACs properties are expected to be favorable to the reduction of CO 2 beyond CO, 14 the heterogeneity of their structures 9 prevents establishing detailed reaction mechanisms. Molecular complexes catalyze the reduction of CO 2 to CO with high efficiency and stability, 15,16 and the homogeneity of their chemical structure allows establishing precise reaction mechanisms. 17,18 Transition metal macrocycles such as porphyrins 19 or phthalocyanines [20][21][22][23] have shown very promising results in the electrochemical reduction of CO 2 , both as homogeneous 23 and heterogeneous systems. 24 Beside their efficiency and robustness, these molecular catalysts also offer chemical tunability, which allows orienting their reactivity towards higher turnover numbers (TONs), 25 higher selectivity, 26 or the synthesis of a specic compound. 27 While most molecular species catalyzed the CO 2 to CO reduction, a handful of examples have shown that C 1 products such as methane 28,29 or methanol 21,30 can be produced by catalysts with M-N 4 structure. Ethanol and acetic acid were found as a product of CO 2 reduction on cobalt and manganese corrole species, respectively. 31,32 There is, however, no example of C 2+ compounds produced from CO 2 reduction by a catalyst for which the molecular nature during catalysis is demonstrated.

We report here on the electrocatalytic formation of saturated and unsaturated light hydrocarbons (C 1 -C 4 ) from CO 2 using a heterogenized molecular iron phthalocyanine catalyst in a owing system (see Fig. 1 andS1 †). The analysis of the products formed under applied electrochemical potential shows that, besides CO and H 2 , small amounts of methane, ethylene, ethane, propene, propane, and n-butane are detected along with traces of C 5 products. Electrocatalytic experiments using alternate substrates such as carbon monoxide (CO) or formaldehyde (HCHO) support a mechanism involving a terminal carbene as a critical species on the way to C 2+ compounds.

Results and discussion

Electrocatalysis and distribution of products

Based on the design of Ager and co-workers, 33 CO 2 reduction experiments were performed in an electrocatalytic ow cell consisting of two chambers for the cathodic and anodic reactions, separated by a Naon® membrane (see Fig. 1 and S1 †). The cathode consists of a glassy carbon electrode, on which an ink containing iron phthalocyanine (FePc) and a carbon powder is drop-casted with a surface concentration of around 80 nmol cm -2 (see ESI † for electrode preparation procedure). A reference electrode is arranged in the cathode chamber, and the anode consists of a nickel plate. Two independent reservoirs contain the catholyte and anolyte (KHCO 3 0.1 M), which are own independently in the anodic and cathodic chambers. The headspace of the cathodic compartment is analyzed during the experiment by an online gas chromatograph that performs automatic sampling every thirty minutes (see Fig. S2A †).

Fig. 2A shows the chronoamperometric curves of constant potential electrolysis under Ar or CO 2 for two hours. Aer poising an FePc-modied electrode at -1.1 V vs. RHE under a CO 2 atmosphere, peaks corresponding to ethylene (C 2 H 4 ), ethane (C 2 H 6 ), propene (C 3 H 6 ), propane (C 3 H 8 ) and n-butane (C 4 H 10 ) clearly stand out in the chromatogram (see Fig. 2B). Other peaks corresponding to unsaturated C 4 as well as npentane and unsaturated C 5 products can also be observed, although with a lower intensity. Methane (CH 4 ) is also clearly present on another module of the gas chromatograph (see Fig. S3 †). The catholyte was analyzed by 1 H NMR to check for liquid products, but none was detected (see Fig. S4 †). Control experiments were performed to ensure that FePc catalyzed the reduction of CO 2 into the products observed. When a FePc-modied electrode was set under catalytic conditions in an Ar-saturated KHCO 3 electrolyte, hydrogen was the major product detected aer two hours of electrolysis, with a trace amount of CO and hydrocarbons (see Fig. S5 †). The observed carbon products can be attributed to the small amount of CO 2 that exists in equilibrium with the KHCO 3 electrolyte. In fact, a similar FePc-modied electrode, at the same potential, under a CO 2 -free environment (Ar-saturated 0.1 M KCl electrolyte), produces only H 2 (see Fig. 2B and S3 †). The current density observed is higher than under CO 2 (see Fig. 2A), indicating a high activity of FePc towards hydrogen evolution in a CO 2 -free environment at high overpotential. When an H 2 Pc-modied electrode was poised at -1.1 V in a CO 2 -saturated KHCO 3 electrolyte, a twenty-fold decrease in current density was observed as compared to an FePc-modied electrode, indicating WE, CE, and RE stand for "working electrode", "counter electrode", and "reference electrode", respectively. The scheme is not drawn to scale. a negligible catalytic activity without the Fe center. In agreement with the low catalytic activity, the main product detected was H 2 , with a miniscule amount of CO (Fig. S3 †). These results strongly indicate CO 2 as the source of the carbon products observed and underline the essential role of the metal center in the catalytic process.

To conrm the origin of the carbon atoms in the C 2+ products observed in the chromatograms, gas chromatography coupled mass spectrometry (GC-MS) experiments were performed. Fig. 3 shows the GC-MS chromatograms corresponding to the retention time of C 4 products (190-240 s). We focus on the C 4 H 8 signal since it has the highest intensity of all C 4 products without overlapping the signal of CO 2 or CO. The m/z = 56 mass, which corresponds to a C 4 H 8 species containing four 12 C atoms, shows a peak when 12 CO 2 is used as the substrate, while it does not appear in the presence of 13 CO 2 . The opposite behavior is observed with the m/z = 60 mass, which corresponds to a C 4 H 8 species with four 13 C atoms. The GC-MS chromatogram of the m/z = 41 mass at the region of the retention time of C 3 and C 4 products is shown on Fig. S6. † The m/z = 41 mass, which corresponds to a C 3 H 5 fragment (originating from both C 3 and C 4 products) shows two distinguishable peaks when 12 CO 2 is used, which disappear with 13 CO 2 . These isotopic labelling experiments demonstrate that the origin of the carbon atoms in the hydrocarbons observed is indeed CO 2 .

Fig. 4 shows the product distribution in terms of faradaic efficiency (FE) for each of the products obtained aer a 2 hours electrocatalytic experiment under CO 2 . The potential range is chosen where there is signicant CO 2 reduction activity without generating hydrogen bubbles that would make the system unstable. Between -0.7 V and -0.9 V, CO and H 2 are the only products observed. Although the current density at high overpotential slightly decreases over time, the selectivity of all products remains constant (see Fig. S7 †). The amount of hydrogen increases steadily from -0.7 V to -1.1 V up to 65% FE at the expense of CO. Noticeable amounts of hydrocarbons can be observed starting from -0.9 V, methane being the main of them with up to 4% at -1.0 and -1.1 V. Ethylene, ethane, propene, propane and n-butane are produced with faradaic efficiencies of 0.1-0.25% each, making the total of C 2+ products to be around 1%. It should be noted that the amount of ethylene is underestimated because of the high background generated by CO 2 in the chromatograph column. Despite the low amounts of products generated, their distribution was reproducible, as shown by the replicates performed on three different electrodes.

Catalyst characterization

In order to determine the nature of the catalyst under operating conditions and check its integrity, we performed in situ and operando X-ray absorption spectroscopy (XAS) experiments at the iron K-edge in the exact same electrochemical cell (see Fig. 1, S1 and S2B †). Fig. 5 shows the X-ray absorption near-edge structure (XANES) spectra of the FePc-modied electrode under open circuit potential (OCP) and under catalytic conditions, in a CO 2 atmosphere, with the corresponding non-normalized spectra in Fig. S8. † The spectrum of the electrode under resting state presents the same features as the pure, starting compound, showing the chemical robustness of the FePc catalyst aer dropcasting and in the presence of an electrolyte. When poising the electrode at a potential of -1.1 V (at which C 2+ products are observed), the spectrum shows a shi to lower energies (-2.7 eV at half-edge jump) while keeping the features that are characteristic of phthalocyanines, 34 i.e. two pre-edge peaks at 7114 eV and 7118 eV. A XANES spectrum was also recorded in situ aer performing electrocatalytic experiments under a CO 2 atmosphere, and it was remarkably similar to the one collected on the starting electrode. The non-normalized spectra indicate a 10% material loss aer 4.5 hours of continuous owing, which is standard in this type of experiment. 35 The comparison between the spectra collected under in situ and operando conditions and those of reference compounds (metallic iron, Fe 3 O 4 ) clearly rule out the presence of metallic iron or iron oxide nanoparticles that would be produced by the decomposition of the molecular catalyst. Altogether, these spectroscopic data collected under in situ conditions clearly demonstrate the molecular nature and the stability of the catalyst throughout the experiment.

In addition, infrared (IR) spectra were recorded on the FePc catalyst powder and FePc-modied electrodes before and aer electrolysis at -1.1 V vs. RHE (see Fig. S9 †). The spectra recorded show close similarity, agreeing with the XAS results concerning the stability of the catalyst during electrolysis.

C-C bond formation mechanism

The demonstration that a molecular electrocatalyst can form several consecutive C-C bonds poses the question of their formation mechanism. Since the major product observed upon reduction of CO 2 is CO, we postulated that the rst intermediate for the formation of C-C bonds could be an FePc-CO species (see Fig. 6). Indeed, electrolysis with CO as substrate shows the formation of similar hydrocarbon products as observed for CO 2 , albeit with faradaic efficiencies approximately twice as low (see Fig. S10 †). We attribute this lower faradaic efficiency to the low solubility of CO in water (1 mM) as compared to that of CO 2 (33 mM). As observed in other studies, 33,36 we surmised that the difference in initial concentration of substrate was responsible for this difference in efficiency and performed nite element calculations to simulate concentration proles upon CO 2 reduction. The results show that the concentration of CO generated by CO 2 reduction within 5 mm from the working electrode ranges from 1.5 to 4.5 mM (see ESI †), supporting a relationship between the amount of C 2+ products and the local concentration of CO. These experiments conrm that FePc-CO is indeed an intermediate on the way to C 2+ products. By analogy with what happens during the Fischer-Tropsch synthesis of long-chain hydrocarbons, we hypothesized that a terminal FePc]CH 2 carbene species could be the precursor to all C 2+ products. This transient species would be formed, via a hydroxycarbene species, by reaction of an FePc-CO adduct with either H 2 formed locally or with protons and electrons available in large amounts. This species would then further react either with more H 2 (or protons and electrons) to form methane. To form a C-C bond, the iron carbene species undergo CO insertion similar to Fischer-Tropsch mechanism. This CO insertion pathway is different from the carbene coupling involved in the formation of ethylene on metallic copper since there are no adjacent active sites in iron phthalocyanine. The process could continue a few times following various pathways and lead to the formation of longer saturated or unsaturated hydrocarbon chains (see Fig. 6).

To verify this hypothesis, we used formaldehyde as a substrate, expecting it to form an FePc]CH(OH) hydroxycarbene species by direct reaction with the activated form of the catalyst 37 (see Fig. 6). In these conditions, the product distribution shows similar multi-carbon products as with CO 2 or CO as substrates, although in much lower amounts (Fig. S10 †), as well as methane (2%) and traces of methanol (Fig. S10 andS13 †). While the concentration of formaldehyde (30 mM) is similar to that of saturated CO 2 in water at 25 °C (33 mM), the very low faradaic efficiencies observed for C 2+ products can be explained by the absence of any CO to be inserted in the FePc]C bond of the putative carbene species. Without any CO to react further, the carbene species is preferentially reduced into methane, leading to a 6-fold increase in the faradaic yield of methane as compared to the CO substrate. In order to conrm the reaction of CO with an FePc carbene species, we performed electrolysis with HCHO in a CO 2 atmosphere, as CO 2 reduction could provide the necessary CO to increase the C 2+ formation with the carbene species generated from HCHO. Indeed, at -1.1 V vs. RHE, the total faradaic efficiency of C 2+ products is one order of magnitude higher than observed in HCHO without CO 2 (see Fig. S11 and S12 †), thus conrming our hypothesis of a reaction between CO and a carbene species obtained from formaldehyde. At less negative potentials, where the formation of CO from CO 2 is more favorable, the total C 2+ faradaic efficiency increases further, reaching a maximum value of 1.5% at -0.9 V vs. RHE, ca. three times higher than when only CO 2 is used as substrate (see Fig. S12 †). The higher selectivity of C 2+ products aer electrolysis of HCHO under CO 2 clearly supports the hypothesis that the formation of C 2+ species involves CO insertion on an intermediate FePc carbene species, as described in Fig. 6.

The existence of an FePc]CH 2 intermediate on the way to C 2+ products is supported by the fact that several carbene species of iron porphyrin (a macrocycle similar to FePc) have been reported, either as transient 38,39 or stable species, 40 and that FePc itself has been reported as an efficient catalyst for the cyclopropanation of alkenes, 41 which involves FePc-carbene intermediates. Iron porphyrins are also known for catalyzing the insertion of carbenes into N-H, S-H, or C-H bonds. 42,43 These precedents, as well as the experiments described here with alternate substrates, strongly support the involvement of an FePc carbene species on the way to C 2+ hydrocarbons.

Conclusions

We reported in this paper on the electrocatalytic activity of iron phthalocyanine, a molecular macrocycle based on earth-abundant elements, towards the reduction of CO 2 into light hydrocarbons with up to three C-C bonds. Using operando X-ray absorption spectroscopy, we showed that the catalyst keeps its molecular structure throughout the electrocatalytic process. Based on experiments involving carbon monoxide or formaldehyde, we showed that the local concentration of CO is key to the formation of C-C bonds and propose a terminal carbene as an intermediate on the way to C 2+ products. These results indicate that the formation of several consecutive carboncarbon bonds can be catalyzed by single-site transition metal macrocycles under mild electrochemical conditions. The mechanistic insights that we provided open a perspective for the design of both molecular and material-based catalysts with enhanced selectivity towards C 2 + products.

In situ/operando X-ray spectroscopy applied to electrocatalytic CO 2 reduction: Status and perspectives Daniela Mendoza, Si-Thanh Dong and Benedikt Lassalle-Kaiser Abstract X-ray spectroscopy is a powerful tool to understand the electronic and local structure of electrocatalysts under operating conditions. Although catalysis itself is performed by electroactive species, other, non-electroactive ones play a critical role in electrocatalytic reactions as well. Cations, anions, solvents or reactants all have significant effects in the efficiency of electrocatalytic reactions. Focusing on the electrochemical reduction of carbon dioxide, we give a short summary of the current status of the in situ/operando X-ray spectroscopy experiments performed on electroactive species, as well as a perspective on the techniques and setups that could be used to probe non-electroactive ones.

Introduction

The electrocatalytic CO 2 reduction reaction (CO 2 RR) is a promising process to convert this industrial byproduct into value-added chemicals [1]. Synthetic building blocks such as carbon monoxide, formic acid, formaldehyde or methanol are used industrially at the megaton level and can be produced through this process. The CO 2 RR actually entails several reactions, which can occur in parallel (and therefore compete with each other), or in series (also competing with each other). CO 2 can be reduced by two electrons into carbon monoxide (CO) or formic acid (HCOOH), CO can be reduced to formaldehyde (HCHO) and formaldehyde can be further reduced to methanol (CH 3 OH) or directly to methane (CH 4 ). A few catalysts are also able to form C 2þ products, such as ethylene [2] or ethanol [3]. Out of these products, only carbon monoxide [4] and formic acid [5] are obtained with a very high selectivity (>95%), while other products are usually obtained as mixtures, preventing their large-scale use and industrial application (among other constraints). The production of hydrogen concomitantly to carbonbased products is also a very frequent phenomenon that is deleterious to applications requiring high purity products. The improvement of the efficiency (the total current density) and selectivity (the faradaic yields) of the CO 2 RR requires a fundamental understanding of the parameters that influence this reaction. One aspect of these fundamental studies is the field of spectroelectrochemistry, which aim is to provide spectroscopic information on species that appear, disappear or evolve under an electrochemical stimulus. These techniques provide information on the chemical nature, the local and electronic structure and the long-range order of the species that are present in the vicinity of the electrodeeelectrolyte interface. They are therefore complementary to the thermodynamic and kinetic information that can be obtained from electrochemical techniques and allow building structureeactivity relationships that can be used for the optimization of a reaction. In this paper, we will focus on the use of X-ray spectroelectrochemistry for the study of the CO 2 reduction reaction, with a special emphasis on nonelectroactive species. A simplified scheme of an in situ/ operando X-ray absorption spectroelectrochemical setup is shown in Figure 1a. A very exhaustive review of X-ray absorption spectroscopy applied to electrocatalysis in general was recently published by Timoshenko and Roldan Cuenya [6], we invite the readers to consult it for a more complete view of the topic.

The nature of the catalyst used in the CO 2 RR is obviously critical for both the efficiency and selectivity of the reaction, depending on the nature of the metal used as the primary catalytic site, its composition, its morphology and crystallinity as well as many other electronic and structural parameters. In the first part of this article, we will give an overview of the works that used in situ/operando X-ray spectroscopies to probe the structure of electroactive species involved in the CO 2 RR.

Other components such as the solvent [7], the electrolytes [8,9], the reactants or other additives [10] can also have a significant impact on its outcome. For example, the product distribution in the electrocatalytic reduction of CO 2 by heterogeneous copper catalysts was shown to vary as a function of the cations used in the electrolyte [10e12] (see Figure 1b). The detailed effects of these and other non-electroactive components on the CO 2 RR have been reviewed recently elsewhere [13]. In the second part of this paper, we will present a selection of X-ray-based photon-in/photon-out techniques that could be used to probe non-electroactive species involved in the CO 2 RR and provide valuable information on the reaction mechanism.

State of the art: understanding the structure of CO 2 -reducing electrocatalysts under operating conditions with in situ/operando X-ray absorption spectroscopy Principles of in situ/operando XAS applied to CO 2 RR Among spectroelectrochemical techniques, X-ray-based spectroscopies have received a lot of attention in the last decade, particularly in the field of electrocatalysis [6]. This is due to advances in instrumentation that made possible experiments with strong experimental constraints, but also very likely to the electrocatalysis community realizing that these tools were the ones that could actually answer their questions. XAS is a very complete technique that provides both structural (from the picometer to the nanometer range) and electronic (oxidation and spin states) information on electrocatalysts, with the ability to probe them under in situ or operando conditions thanks to the high penetration depth of the photons used. This property of X-rays leads to probing a significant volume of the electrode/electrolyte assembly, which can be an advantage to have an average view of the sample, but also a drawback when it comes to probing only the solid or liquid part of the solideliquid interface [14].

In the vast majority of X-ray spectroelectrochemical studies, electroactive species have been the point of focus, because they are those primarily involved in electron transfer processes, and also because they are the most easily observed. Changes in oxidation states or structural rearrangements usually have obvious signatures, with shifts in absorption edge positions in the XANES spectra and changes in the peak positions and intensities of the Fourier transform spectra of the EXAFS signal. It is therefore possible to determine the chemical nature of an electrode material under applied electrochemical bias, which can significantly differ from their starting structure. The same is true for electroactive species in solution (homogeneous catalysis), in which chemical structure upon oxidation or reduction can be probed in situ under applied potential. These information are extremely important to determine the nature of the species that are responsible for catalysis.

The CO 2 reduction reaction has been studied in the past decade with various spectroscopic tools, in order to identify the intermediate species involved in the reaction [15]. X-ray spectroscopy has been a tool of choice for this type of studies, especially for heterogeneous systems which can be observed under operating conditions with very few techniques. Pure metals, such as copper, metal/nitrogen-doped carbon (MNC) materials or discrete molecules have all been used as heterogeneous electrocatalysts for the reduction of CO 2 and to some extent studied with X-ray techniques. The next paragraphs give an overview of the most essential work done in this field, with a focus on the fate of the electroactive species.

Pure metals

A large variety of metals can reduce CO 2 [16], mostly into formic acid (such as Sn, Cd or In) or carbon monoxide (such as Ag, Au or Zn). In a few cases, the structure of these metals has been probed by XAS under functioning conditions [17,18]. We will, however, focus our attention on the case of copper, which has been thoroughly investigated with X-ray spectroscopy.

Pure copper is a very interesting electrocatalyst for the reduction of CO 2 , mostly because it is able to produce significant amounts of products beyond CO, such as methane or ethylene, as well as other more reduced products [19]. For this reason, it has been widely investigated with various spectroelectrochemical techniques [15]. To summarize briefly, the use of XAS at the copper K-edge has been used extensively [20e22] to assess the oxidation state of copper under catalytic conditions, the conclusion being that it is essentially purely metallic, while a small fraction of oxide still exists in some cases. On the basis of X-ray photoelectron Spectroscopy (XPS) and theoretical calculations, the presence of remaining oxygen under reducing conditions has been assigned to a sub-surface layer of oxygen [23], which could play a role in tuning the reactivity of copper towards ethylene. This hypothesis is, however, the subject of debates [24], because the presence of traces of oxide at the surface of copper that could contribute to the signal (rather than sub-surface oxygen) is extremely challenging to avoid experimentally. The presence of oxygen at the surface of copper under electrocatalytic conditions has also been evidenced with operando XAS [25], the presence of oxides favoring the formation of ethylene. It is, however, not clear yet whether this reactivity is directly linked to the chemical nature of the catalyst or to the nanostructuring induced by the in situ reduction of the oxide phases [26,27].

Understanding the nature of the catalytic sites of metals or materials-based catalysts remains challenging with Xray absorption, because the reactivity only occurs at the solideliquid interface, while XAS probes the whole material. Complementary techniques more sensitive to the surface or to specific species are often required, such as XPS [28].

Molecular systems

Transition metal molecular complexes have been known to reduce CO 2 efficiently for several decades, mostly under homogeneous conditions [29,30]. For example, polypyridyl complexes of rhenium [31] or manganese [32] were shown to reduce CO 2 in organic solvents and water as early as the 80's [33]. Macrocyclic complexes of transition metals such as cyclams [34,35], porphyrins [36] or phthalocyanines [37] were also shown to be very efficient CO 2 reduction catalysts in the same period, with a surge of interest in recent years. Although industrial application of electrochemical reactions in homogeneous conditions is very unlikely, molecules in solutions are ideal systems to study the mechanism of a chemical reaction [38]. Indeed, their structure can be precisely determined as compared to metals or inorganic materials that present a heterogeneity of sites. Changes in oxidation state and/or structure as well as interactions with reactants, solvent or products can therefore be precisely followed. Recently, molecular systems (particularly transition metal macrocycles) have been heterogenized to meet the requirements of large-scale applications [4,39e41]. Incidentally, the possibility of performing in situ/operando X-ray spectroscopic experiments has increased [41e43] because the diffusion of molecules to/from the electrode is suppressed in these systems and simplifies the data collection and analysis processes. One of the main lessons learned from X-ray spectroscopic experiments is that these molecules seem to be robust enough to withstand the electrocatalytic conditions of CO 2 reduction for a few hours at least [44]. There are, however, exceptions to this statement, with a few molecular systems for which the active species were proposed to be nanoparticles that are reversibly formed during catalysis. For example, copper molecular species were shown to evolve into nanoparticles under CO 2reducing catalytic conditions. When the original molecular complex used was based on a phthalocyanine ligand, the Cu clusters were shown to revert to their original structure after catalysis, as shown by in situ X-ray absorption spectroscopy [45]. Indeed, both the Cu Kedge XANES and EXAFS features showed an evolution from a molecular species to a metallic one (edge position shift and appearance of metalemetal interactions) upon application of cathodic potentials. These changes were reversible upon the application of anodic potentials. It should be noted that a fraction of the initial metal amount was not recovered eventually (as seen by the fluorescence raw signal), indicating a partial dissolution/ decomposition of the copper ions. This unexpected behavior has been observed in a few other cases [46,47] has been the matter of debates [48,49], but in all instances, it shows that the stability of CO 2 RR molecular systems needs to be carefully monitored for each specific case. Perspective articles were recently published by Rooney et al. [50] and Creissen et al. [51], describing how the degradation/restructuring of molecular catalysts should be carefully monitored under operating conditions, and how X-ray spectroscopy is an ideal tool for this purpose.

Apart from these critical stability issues, many questions still need to be answered on the local and electronic structure of the intermediates involved, mostly because of a lack of detailed spectroscopic information on the local and electronic structure of the reduced forms of these molecular species. Although it was not performed under conditions directly relevant to catalysis, the study of Ro ¨melt et al. on the electronic structure of reduced forms of Fe tetraphenyl porphyrin in the solid state by Fe K-edge XANES is a first and important step in this direction [52].

Metal/nitrogen-doped carbon hybrid materials

A class of materials that has emerged in the last decade is metal/nitrogen doped carbon (MNC), which lies somewhere between materials and molecules. These systems are usually obtained by calcination of a matrix containing both metallic salts and organic components, yielding a graphitic-like material with nitrogensubstituted coordination pockets that are filled with metallic ions [53]. The coordination sites are very heterogeneous, with a combination of pyrollic and pyridinic nitrogen sites that can form cavities with 2e6 potential ligands and the presence of metallic clusters or nanoparticles is very difficult to rule out [54,55]. Similarly to the molecular species described above, it was shown (using X-ray absorption spectroscopy) that, in some cases, their structure is reversibly going from a molecular-like species to metallic nanoparticles under applied potential [55]. These nanoparticles would recover their starting molecular structure when the potential is left at open circuit potential or left at air. This behavior once again raises the question of the actual active sites and stresses the importance of operando measurements to obtain a mechanistic understanding. One study, however, showed that the number of coordination sites around a cobalt ion could regulate the selectivity towards CO [56], giving credit to a purely molecular (i.e. free of nanoclusters) behavior of MNCs.

Although XAS proved very useful in determining the presence or absence of metallic clusters under applied potential, it has not allowed yet to determine the structure of the species that are present under catalytic conditions. Besides their tendency to evolve into metallic clusters, MNCs are heterogeneous material in nature with a variety of coordination sites, geometries and nuclearities, which can hardly be disentangled with XAS [57]. Unless a rationale is found for the synthesis and stability of MNCs and their structure are properly described (and XAS can help for that), the mechanism by which these materials are able to reduce CO 2 will not be understood.

Perspective: probing non-electroactive species in CO 2 -reducing electrocatalysis with in situ/operando X-ray spectroscopies As briefly described in the introduction, the role of the solvent, the electrolytes (cations and anions) as well as other non-electroactive components can be critical in the efficiency and selectivity of the CO 2 RR [13]. These species are essentially composed of light elements (C, N, O, Li, K, Na, Mg, Ca), with the exception of heavy alkali (Rb and Cs) and alkali-earth (Sr and Ba). The reactant (CO 2 ) and products (which can in turn become reactants in cascade reactions) are also composed of light elements only. Probing these elements can provide critical information on their oxidation state, geometry, or binding mode and their implication in non-covalent bonds. The next paragraphs are dedicated to describing advanced X-ray techniques [58,59] that can be used to probe light elements under electrocatalytic conditions.

Tender X-rays XANES

The definition of the tender X-ray range is far from being accurate. It could be summarized as "too high for a soft X-ray beamline and too low for a hard X-ray beamline", i.e. from 1000 to 4000 eV. Which essentially means that it is not easily accessible and that only a few beamlines are specifically optimized for it. Alkali metals and alkali earth metals can have a drastic effect on the efficiency and selectivity of the CO 2 RR, which makes it worth studying their structure. The XANES and EXAFS spectra at the K-edge of several of these elements (Na, K, Mg, Ca) can be collected at tender X-ray beamlines, while those of others (Rb, Cs, Sr, Ba) can be obtained at hard X-rays beamlines. The Ledges of the high Z (Rb, Cs, Sr, Ba) elements can also be collected at tender X-ray beamlines. These data can provide information on the geometry of these elements under operating conditions, their ligands, coordination number, and first and second shell neighbors. Such information can help determine the actual role of these ions, whether they act as Lewis acids, as structural templates or as docking stations for reactants (reactant, solvent, or other electrolytes). As an example, the role of cations (in particular calcium) in natural [60] and artificial [61] water oxidizing systems was largely determined with the help of X-ray absorption spectroscopy in the tender X-ray range. The application of tender XAS to CO 2 RR under operating conditions is, however, not trivial. The penetration depth is indeed too high (from a few mm at 1000 eV to ca. 100 mm at 4000 eV in water) to probe only the electrocatalytic material in the case of a heterogeneous system. The contribution of interfacial Valence-to-Core X-ray emission X-ray emission spectroscopy is another technique based on inelastic X-ray scattering and requires an instrumentation similar to XRS (see Figure 3a). Multiple analyzer crystals located on the so-called Rowland circle are aligned to collect the fluorescence photons emitted from the sample with a high energy resolution [58]. The resolution obtained of the spectra obtained (ca. 1 eV) allows differentiating the different recombination processes that occur during the fluorescence phenomenon. One of these processes corresponds to the recombination of valence electrons (3d in the case of a first raw transition metal) into the core hole (1s) orbitals, the socalled valence-to-core (V2C) emission process [77]. A specific feature of the V2C emission spectra is the socalled crossover peak, located at a lower energy than the main feature. While the main feature corresponds to the recombination of electrons located in the 3d block, the crossover peak corresponds to the recombination of electrons located in the ligand's orbitals into the core hole of the metal excited. As such, it is a direct probe of the nature, the valence and protonation state of the ligands that are bound to the metal (see Figure 4b,c) [78]. Unlike soft X-ray XANES or XRS, this technique provides information on light elements that are directly bound to the element probed, hence avoiding the contribution of these elements that are not in contact with the metal involved in catalysis. It has proved extremely powerful in determining the nature or protonation state of ligands in bioinorganic [79] or biomimetic [80] active sites.

As for XRS, V2C XES is very appropriate for in situ experiments applied to catalysis [81], since it is based on hard X-ray photons for both excitation and fluorescence processes. As far as we know, this technique has only been applied once to understand the structure of a CO 2reducing catalyst, although not under catalytic conditions [82]. With a combination of V2C XES and theoretical calculations, Sun et al. managed to identify H 2 O/OH ligands bound to a nitrogen-doped carbon material with iron sites. This type of information obtained under operating conditions would help decipher the binding mode of CO 2 and related species to catalytic sites and understand the structure of critical intermediates.

Conclusion

We have given an overview of the current situation of in situ/operando X-ray spectroscopy techniques applied to the electrocatalytic reduction of CO 2 . The large majority of the studies performed so far focus their attention on the fate of the catalytic centers involved, which are essentially transition metals. There is an increasing interest in the influence that non-electroactive components such as the solvent, electrolytes or reactants can have on the efficiency and product selectivity of the CO 2 RR [9,13]. These effects have not been studied so far with X-ray spectroscopy, although a lot of information could be extracted from it. We therefore provided a perspective on potential evolutions of this field and described a few X-ray techniques that are either well established or still developing and can be used to extract information specifically from non-electroactive species. This type of information is complementary to those that are typically collected on electrocatalysts and can shed new light on the parameters that influence the efficiency and product distribution of the electrochemical CO 2 reduction reaction. associated with in situ UV-Vis, resonance Raman and EPR spectroelectrochemistry, [20][21][22] led to the conclusion that the three-electron reduced species should be considered as an Fe(0) complex. More recently, Römelt et al. proposed, on the basis of X-ray Absorption Near Edge Spectroscopy (XANES) performed on solid-state samples and Density Functional Theory (DFT) calculations, that the second and third electrons injected in [Fe III TPP] + are localized on the porphyrin ring rather than on the metal. [23] They, therefore, formulated the two-and three-electron reduced species as [Fe II TPP•] À and [Fe II TPP••] 2À .

As shown from this work and many others, XANES is an ideal tool to probe the local and electronic structure of transition metals. [24][25][26][27] It provides information on the oxidation state (through the main edge position) and on the geometry and spin state (through the shape and position of the pre-edge) of the element being probed. In combination with electrochemical techniques, it allows for observing species that would otherwise be difficult to isolate. Such in situ measurements, however, require specific instrumentation to couple the two techniques. Up to now, spectroelectrochemical cells for X-ray spectroscopies were mostly developed for heterogeneous systems. [28][29][30][31][32] Unfortunately, in situ/operando analysis on immobilized materials lacks precision when it comes to identifying intermediates. Indeed, the high catalyst loading leads to a signal that can be dominated by inactive material rather than the active one. [33] In addition, the electrochemical analysis of immobilized catalysts by cyclic voltammetry is challenging, with electron transfer processes being hidden by electrode effects. The stepwise generation of reduced intermediates on the way to catalytic potentials is therefore hampered, jeopardizing the identification of the catalytic species involved. On the contrary, the study of molecular catalysts in homogeneous conditions allows a better correlation with electrochemical measurements, thus providing more accurate information on the chemical state of the catalysts as a function of the electrode applied potential.

Herein, we present an in situ spectroelectrochemical study of the CO 2 -reducing molecular electrocatalyst [Fe III TPP(Cl)] in a DMF solution. We describe a custom-made vacuum-compatible X-ray spectroelectrochemical flow cell for the in situ study of homogeneous systems in organic solvents and provide XANES and UV-Vis spectra relative to the four oxidation states involved in the redox process, both under argon and carbon dioxide atmospheres (as identified in Table 1).

Results

A vacuum-compatible X-ray spectroelectrochemical flow cell for the study of homogeneous organic solutions

A handful of cells have been reported for the study of electroactive species in solution with a thin-layer configuration, [34][35][36][37] with the possibility to flow the sample for some of them. [38][39][40][41][42][43] In order to study the FeTPP complex in DMF, we have developed a novel thin layer spectroelectrochemical flow cell suitable for X-ray spectroscopic experiments (see Figure 2 and Figures S1 andS2), inspired by the work of Jiang et al. [44] This cell allows performing in situ measurements in homogeneous conditions with organic solvents under primary vacuum.

Briefly, the cell consists of a main body made in polyether ether ketone (PEEK), a solvent-resistant polymer, to which two polytetrafluoroethylene (PTFE) tubes are connected as liquid inlet and outlet. The electrochemical chamber of the cell consists of a thin layer (200 μm) of liquid that can be circulated between a working (a 60 μm-thick glassy carbon film) and a counter (platinum film) electrode. A reference electrode (silver pseudo-reference) is inserted from the side in the liquid path. The small cell volume ensures the total electrochemical conversion of the species of interest within a short time (< 5 min), preventing excessive X-ray dose and subsequent damage. It ensures a laminar flow of the circulating liquid, preventing the formation of bubbles and allowing a complete renewal of the sample.

To assess the behavior of the X-ray spectroelectrochemical cell, an initial study in the presence of iron chloride was carried out. Iron(III) chloride hexahydrate (Fe III Cl 3 • 6H 2 O) and iron(II) chloride (Fe II Cl 2 ) reference samples were considered due to previously reported XANES data in the solid state and aqueous solutions for these compounds. [41,45,46] Therefore, samples were prepared under anaerobic conditions in DMF and tetrabutylammonium hexafluorophosphate (TBAPF 6 , 0.1 M) as supporting electrolyte.

Fe K-edge XANES spectra were recorded in situ on both the Fe III and Fe II reference solutions. As shown in Figure 3, the spectrum of the Fe III sample (blue line) presents a low-intensity white line (main edge peak top) as well as a pronounced preedge peak at 7114.6 eV (all pre-edge positions are measured at the peak top) and a main edge at 7122.3 eV (all main edge energies are measured at the half-edge jump, where the normalized intensity is equal to 0.5). The ferrous compound (Figure 3 -red line) presents a spectrum with a main edge that is shifted by 2 eV towards lower energies in comparison with the spectrum of the ferric compound (Figure 3 -blue line). Such a downshift in edge energy position is indicative of a reduction of the element probed. In addition, the intensity of the pre-edge peak clearly decreases and is also shifted towards lower energies to 7113.0 eV. The main edge energy shift, as well as the changes observed in the pre-edge intensities for the solutions of Fe III Cl 3 and Fe II Cl 2 are in agreement with previously reported data [46] and confirm an oxidation state difference of one unit between these two species. A cyclic voltammogram (CV) was performed in the spectroelectrochemical cell under an Ar atmosphere (Figure 3, insert) on the Fe III Cl 3 solution. The CV shows an anodic peak at À 0.10 V and a cathodic peak at À 0.22 V vs. Ag/Ag + . When Fe III Cl 3 is electrochemically reduced by applying a constant potential of À 0.6 V vs. Ag/Ag + during approximately 15 minutes, the main edge energy of XANES spectrum shifts by 2 eV towards lower values (Figure 3 -dotted line). The spectrum obtained matches closely the one of the Fe II Cl 2 reference sample. Linear combination fittings (LCF) were performed on the XANES spectrum of the final species, using the spectra of the Fe III Cl 3 and Fe II Cl 2 reference solutions, indicating a conversion of 96 % from Fe III to Fe II (see Figure S3).

In situ XANES and UV-Vis spectroelectrochemistry of [Fe III TPP(Cl)] and its reduced species under Ar atmosphere

The Fe K-edge XANES spectrum of an Ar saturated 2 mM solution of [Fe III TPP(Cl)] (named 1) in DMF/TBAPF 6 (0.1 M) was recorded and compared with the one of the solid sample (see Figure S4). The two spectra are very similar, indicating that, under these conditions, the chloride ligand remains bound to the metal center.

The cyclic voltammogram (CV) of the initial [Fe III TPP(Cl)] complex recorded in the X-ray spectroelectrochemical cell (Figure 4A) in DMF/TBAPF 6 under Ar exhibits three well-defined reversible waves. They correspond to three consecutive oneelectron reduction processes, ascribed to the Fe III /Fe II , Fe II /Fe I and Fe I /Fe 0 formal couples. [6,22] The initial UV-Vis spectrum of the [Fe III TPP(Cl)] species presents a Soret band at λ = 416 nm and Q-bands at λ = 509 nm and 575 nm, which correspond to previously reported values in the same solvent. [22] Controlled potential electrolysis (CPE) were performed at the potentials indicated by arrows in the CV shown in Figure 4A, i. e. À 0.8 V, À 1.6 V and À 2.0 V vs. Ag/Ag + , corresponding to the first, second and third electron reductions, respectively. Figure 4B shows the normalized Fe K-edge XANES spectra of the starting [Fe III TPP(Cl)] species in DMF/TBAPF 6 0.1 M and of the species obtained under various applied potentials. Measurements were performed when the electrolysis currents were Figure 2. 3D scheme of the spectroelectrochemical cell. PEEK stands for "polyethylene ether ketone", WE for "working electrode", GC for "glassy carbon", EPDM for "ethylene propylene diene monomer", CE for "counter electrode" and RE for "reference electrode". stabilized (after ca. 2-5 min) and the possible occurrence of radiation-induced damages was carefully monitored.

The XANES spectra analysis of iron porphyrins may be divided into two different sections. First, a pre-edge region extending from 7111 to 7120 eV corresponds to transitions from the 1s to 3d/4p orbitals, while the main edge region from 7120 to 7140 eV corresponds to transitions from the 1s to the 4p orbitals.

One electron reduction species. The first reduction step was performed by applying a constant electrode potential of ca. À 0.8 V vs. Ag/Ag + . No energy shift was observed in the main edge of the Fe K-edge XANES spectrum upon reduction of [Fe III TPP(Cl)] by one electron (named complex 2 a), as can be seen in Figure 4B. On the contrary, the UV-Vis spectroelectrochemical data recorded upon one-electron reduction of [Fe III TPP(Cl)] leads to a shift of the Soret band from λ = 416 nm for the [Fe III TPP(Cl)] complex to a sharp Soret band at λ = 432 nm for the one-electron reduced species as well as to the evolution of the Q-band region into three peaks at λ = 530, 565 and 603 nm respectively (see Figure 4C and Figure S5 for intermediate spectra).

As shown in the work of Wilson et al., [47] Fe II TPP can adopt three different spin states, namely, high spin (HS, S = 2), intermediate spin (IS, S = 1) and low spin (LS, S = 0). These three species exhibit very different main edge and pre-edge features in their Fe K-edge XANES spectra.

In order to confirm the nature of the species electrogenerated at À 0.8 V vs. Ag/Ag + , we synthesized the [Fe II TPP] species by chemical reduction of [Fe III TPP(Cl)], as previously described, generating 2 b. [23,48] When compared with the spectrum of the initial [Fe III TPP(Cl)] complex, the main edge position of the chemically reduced solid sample (see Figure S6) shows a 2 eV shift towards lower energies (from 7123.0 to 7121.0 eV). It also presents a high-intensity shoulder at around 7118.0 eV, which corresponds to a low-lying 1s to 4p z transition, as previously suggested by Römelt and co-workers. [23] These features indicate that 2 b in the solid state is in a fourcoordinated, square planar environment with an intermediate spin (S = 1), as previously discussed. [23,47] The spectrum of this chemically reduced species is very different from that of complex 2 a, indicating that the two species have different spin states arising from the presence of DMF solvent molecule(s) in the coordination sphere of the complex generated in homogeneous conditions. Indeed, the loss of the feature at around 7118.0 eV in the electrogenerated species shows that the 4p z orbital is shifted towards higher energies due to the axial coordination of DMF. To confirm this interpretation, we dissolved the chemically reduced [Fe II TPP] solid sample in DMF under anaerobic conditions (named complex 2 c) and got a new spectrum that closely matches the one of the electrochemically reduced species (2 a). These results illustrate that DMF plays a critical role in the electronic structure of the [Fe III TPP(Cl)] species. Consistent with previously reported data, [47] Two and three electrons reduced species. The second reduction step was performed by applying constant electrode potential of ca. À 1.6 V vs. Ag/Ag + to obtain the [Fe III TPP(Cl)] + 2e À species (named 3). As observed in Figure 4B, the main edge energy is slightly shifted to lower values (0.7 eV) when compared to the initial sample, while the pre-edge feature increases in intensity. In addition, a low-intensity shoulder appears at ca. 7118.0 eV. The presence of this feature, arising from a 1s to 4p z transition (similar to the one found in the intermediate spin [Fe II TPP] species), points towards a square planar environment around the Fe center.

The UV-Vis spectroelectrochemical data recorded upon reduction of [Fe II TPP(Cl)] by a second electron leads to a decrease in intensity and splitting of the Soret band, with two peaks at λ = 392 nm and λ = 425 nm (see Figure 4C and S5 for intermediate spectra). The Q-bands of 3 present three bands located at λ = 510 nm, 575 nm and 608 nm, respectively. These results are in line with previously reported data on complex 3. [21] Finally, a third reduction step is carried out by applying a constant electrode potential of ca. À 2.0 V vs. Ag/Ag + (formation of the species named 4). The spectrum (see Figure 4B) shows a shift of the main edge position of ca. 0.3 eV towards lower values when compared with the 2e À reduced species, indicating a further reduction of the system. The pre-edge feature at 7113.6 eV decreases in intensity with respect to 3, while the shoulder at ca. 7118.0 eV grows significantly. This increased intensity indicates a higher contribution of 4p z character in the final molecular orbitals of the 1 s!4p z transition. The UV-Vis spectroelectrochemical data recorded on complex 4 leads to slight shifts of the Soret bands to λ = 362 nm and 433 nm (see Figure 4C and Figure S5 for intermediate spectra). More important modifications are observed in the Q-band region, with the presence of a broad band at λ = 514 nm. These values are in line with those previously reported for 4 in DMF. [21,22] In situ/operando XANES and UV-Vis spectroelectrochemistry of [Fe III TPP(Cl)] and its reduced species under CO 2 atmosphere One electron reduced species. Figure 4A shows the comparison between the CVs obtained for the initial [Fe III TPP(Cl)] complex in DMF/TBAPF 6 (0.1 M) under Ar (black line) and CO 2 (red line) atmospheres. Under CO 2 , the third reduction wave presents a current increase and a loss of reversibility as compared to the one in Ar atmosphere, indicating a catalytic process. Although a proton source is usually required to catalyze the CO 2 RR, such as protons from residual water, CO 2 itself can play the role of a Lewis acid and lead to catalysis. [9,15] Figure 5A shows the normalized Fe K-edge XANES spectra recorded in situ on [Fe III TPP(Cl)] and its reduced species under CO 2 atmosphere, while Figure 5B shows the corresponding UV-Vis spectra (see Figure S7 for intermediate spectra). A comparison of the XANES spectrum of the initial [Fe III TPP(Cl)] complex and its one-electron reduced species under CO 2 atmosphere (5 and 6 respectively) with the ones obtained in the presence of Ar (see Figures S8-9) shows that they are identical, indicating that there are no interactions between CO 2 and the Fe metal center at these oxidation states. This is confirmed by the UV-Visible spectra of [Fe III TPP(Cl)] and its one-electron reduced species, which are comparable to their counterparts in Ar.

Two and three electrons reduced species. Further reduction of 6 with a second electron under CO 2 atmosphere (complex 7) yields a XANES spectrum that is almost identical to the one of 6. It presents, however, clear differences with the one of 3, obtained under Ar, as can be seen in Figure 5C. The shoulder around 7118.0 eV is absent in the spectrum recorded under CO 2 , which indicates that the square planar geometry observed under Ar has changed, hence pointing towards an interaction between CO 2 and the Fe center. It is worth mentioning that an interaction between CO 2 and an Fe(I) porphyrin was recently reported, [49] although in a different solvent (acetonitrile). The UV-Vis spectra of complex 7 present a similar Soret band splitting as the one observed under Ar, with peaks at λ = 392 nm and 425 nm. Q-bands were also located at the same position as those obtained under Ar, i. e. at λ = 510 nm, 575 nm, 608 nm, 667 nm and 712 nm.

A XANES spectrum was recorded after the catalytic wave, at À 2.0 V vs. Ag/Ag + , where [Fe III TPP(Cl)] is reduced with three electrons to yield complex 8. Significant changes are observed in the pre-edge region when compared with the two electrons reduced species 7 (Figure 5A), but also with the three-electron reduced species under Ar, 4 (Figure 5E). The pre-edge region of the XANES spectrum is clearly different, not only with the disappearance of the shoulder at 7118.0 eV, but also with a shift of the pre-edge peak from 7113.6 eV eV (under Ar) to 7115.0 eV (under CO 2 ). It indicates an important structural modification around the metal center at this potential, pointing towards the coordination of CO 2 to the iron center. Moreover, the main edge position shifts to higher energies by ca. 0.7 eV when compared with the two-electron-reduced species in CO 2 . This shift is even more pronounced when the spectra of the threeelectron reduced species obtained under Ar (7122.0 eV) and CO 2 (7123.7 eV) are compared, with a shift to higher energies of 1.7 eV for the species obtained in CO 2 . These data further support a coordination of CO 2 to the Fe center, which donates part of its electronic density to the incoming coordinating molecule and therefore appears as being less electron rich. The UV-Vis spectra of the species obtained after reduction of [Fe III TPP(Cl)] with three electrons look almost identical to the one obtained for the two-electron reduced species. The spectrum is, however, significantly different from its counterpart observed under Ar atmosphere (see Figure 5F).

Discussion

The pre-edge and main edge positions of the XANES data, as well as the peak positions of the UV-Visible Soret and Q-bands regions of compounds 1-8 are summarized in Table 2.

Electronic structure under Ar. The XANES data collected on the two and three electrons-reduced species of [Fe III TPP(Cl)] under Ar show slight shifts of the main edge energy toward lower values as more electrons are injected into the complex. These shifts are notably small (À 0.7 and À 0.3 eV) and cannot be assigned to full oxidation state changes, suggesting a partial localization of the electrons both on the metal and on the porphyrin ligand. As previously mentioned, the pre-edge feature observed at 7118.0 eV can be attributed to a 1 s!4p z transition. This peak appears in the [Fe II TPP] complex in the solid state, which does not have apical ligands and adopts a square planar geometry, leading to an intermediate spin configuration. When dissolved in solution, one or two DMF molecule(s) coordinate the metal, and the feature at 7118.0 eV disappears due to a square (bi)pyramidal structure. When the complex is reduced with a second electron, the shoulder at 7118.0 eV appears with a low intensity, which can be explained by the repulsion of the DMF molecule(s) by the charge (À 1) appearing on the Fe porphyrin. Further reduction by a third electron and increase of the charge (À 2) lead to the removal of the DMF apical ligands, thus generating a square planar geometry similar to what that observed for the Fe II complex in the solid state, as attested by the presence of the 1 s!4p z transition.

Taken together, this in situ dataset indicates that the oneelectron reduced species of [Fe III TPP(Cl)] in DMF is a low spin Fe(II) complex, while the two and three electrons reduced species have an electronic structure where the additional electrons are shared between the metal and the porphyrin.

Electronic structure under CO 2 . Under CO 2 atmosphere, significant differences are observed in the XANES spectra with respect to the Ar atmosphere conditions. The two-electron reduced species (complex 7) presents a XANES spectrum that is different from the one in Ar, with a pre-edge region that misses the shoulder at 7118.0 eV, indicating a deviation from the square planar geometry and thus suggesting an interaction with CO 2 . The UV-Vis spectrum obtained under these conditions is, however, almost identical to the one obtained under Ar. This difference may be explained by the lower sensitivity of UV-Vis to valence-level electronic changes. It also suggests that the interaction between 7 and CO 2 is very loose and that no bond is formed with the metallic center.

When reduced with a third electron, more prominent differences can be observed, both by XANES and UV-Visible spectroscopy, when compared to the Ar atmosphere conditions (complexes 8 vs. 4, Figure 5E-F). The pre-edge region of the XANES spectrum is clearly different, not only with the disappearance of the shoulder at 7118.0 eV, but also with a shift of the pre-edge peak from 7113.6 eV (under Ar) to 7115.0 eV (under CO 2 ). Moreover, the main edge position lies 1.7 eV higher than under Ar. The UV-Visible data show clear differences with the one obtained under Ar, but looks actually very similar to those obtained with 3 and 7, the doubly reduced species observed under Ar or CO 2 , respectively. Therefore, both the XANES and UV-Visible data for complex 8 show differences with respect to those obtained under Ar (complex 4), indicating that CO 2 is bound to the iron center. The disappearance of the peak at 7118.0 eV in the pre-edge region of the XANES spectrum supports a deviation from the square-planar geometry, consistent with a square pyramidal geometry with CO 2 bound to Fe center and sitting on top of the porphyrin plane. The UV-Visible spectrum of 8 is very similar to that 3, which is only reduced with two electrons (vs. three electrons for 8). This suggests an electronic structure for 8 where part of the electronic density is delocalized between the metalloporphyrin system and the CO 2 substrate.

CO 2 reduction mechanism. The cyclic voltammetry experiments shown in Figure 4A indicate that CO 2 reduction catalysis occurs at the third reduction wave (ca. À 1.8 V vs. Ag/Ag + ), thanks to the presence of a residual source of protons (and/or with a second CO 2 molecule acting as a Lewis acid to help CÀ O bond breaking). In such low proton availability conditions, the rate-determining step of the CO 2 reduction reaction is the cleavage of the OCÀ O bond (see Figure 6). [18] In other words, the species expected to be observed under these catalytic [a] The pre-edge peak positions are measured at the peak maximum.

[b] The main edge positions are measured at the half-edge jump (normalized intensity = 0.5). conditions is the one accumulating before the rate-limiting step, i. e. the triply reduced FeTPP species with CO 2 bound to the metal. The comparison of the in situ XANES and UV-Visible data collected under Ar or CO 2 atmospheres provide direct evidence for the coordination of CO 2 to the Fe in the triply reduced form of [Fe III TPP(Cl)]. These results are consistent with the CO 2 -to-CO mechanism considered so far (see Scheme S1) and unambiguously demonstrate that, despite an electronic density partly localized on the porphyrin ligand, the reactivity of the triply reduced iron porphyrin towards CO 2 indeed occurs at the metallic center. It implies that the localization of the electronic density is partly shifted from the metalloporphyrin organic backbone to the metal center upon substrate coordination. This behavior may explain the ability of iron porphyrins to store electrons without reacting until a substrate molecule is activated.

Conclusion

Using a custom-made in situ X-ray spectroelectrochemical cell, we collected the XANES spectra of the [Fe III TPP(Cl)] complex in a DMF homogeneous solution, together with its one, two and three electrons reduced species. These data were collected both under Ar and CO 2 atmospheres and correlated with the corresponding UV-Vis spectroelectrochemical data. We showed that, when dissolved in DMF, the one-electron reduced species is an Fe(II) complex with a low spin electronic configuration, due to the coordination of DMF to the central Fe ion. The two and three-electrons reduced species generated under Ar are proposed to have electronic structures where the electron density is shared between the metal and the porphyrin ring. Under CO 2 atmosphere, the XAS and UV-Vis spectra collected in situ show new features, which indicate that (i) CO 2 is loosely bound to the two electrons reduced species and (ii) CO 2 is coordinated to the iron center in the triply reduced iron porphyrin species, which is the active form of the catalyst. This result is consistent with a rate-limiting step of the reaction corresponding to the cleavage of the OCÀ O bond and indicates that reactivity towards CO 2 occurs on the metallic center rather on the ligand. Overall, these findings provide a better understanding of the electrocatalytic mechanism for CO 2 reduction with molecular catalysts.

RESEARCH ARTICLE

Catalytic CO 2 reduction: In situ X-ray absorption spectroscopy was used to observe the binding of CO 2 on reduced forms of CO 2 -reducing iron porphyrins in a homogenous solution. Interactions can be observed on the doubly and triply reduced species of the starting iron(III) complex. Electronic structures are proposed for these species, and implications for the catalytic reduction of CO 2 are discussed.

INTRODUCTION

The concept of sustained drug release has been around since the 1960s; 1 the quest to search for the most appropriate drug carriers for the delivery and controlled release of therapeutic molecules to a targeted location in the patient has never stopped. Since the first report in 2010, 2 metal-organic framework nanoparticles (nanoMOFs) as drug carriers have gained increasing attention as they combine the advantages of both their inorganic and organic constituents. Metal-organic frameworks (MOFs) are porous materials made of metal-ligand coordination units assembled as an ordered crystalline structure with high permanent porosity. [3][4][5] This high porosity enables MOFs to capture a large amount of drug within a small amount of carrier, thus potentially reducing the administered dose and related toxicity for the patients. Among the most studied nanoMOFs for drug delivery to date is nanoMIL-100(Fe), belonging to the MOF family of MIL-100. It consists of trivalent metal cations such as Al(III) or Fe(III) and benzene-1,3,5-tricarboxylate ligands, also called trimesate or BTC. 6,7 Figure 1 illustrates the structure of MIL-100. Its secondary building unit (SBU), the Fe or Al(III) metal trimer, consists of three metal cations M(III) coordinated with one central bridging oxygen and six carboxylate groups. The 6-fold coordination sphere is completed with hydroxyl and water ligands. Two metal cations coordinate with one water molecule each, and one cation coordinates with one hydroxyl group. Every four SBUs connect through trimesate linkers, creating tetrahedra. These tetrahedra build the MTN-type MIL-100 framework, which possesses two kinds of pores: large mesoporous cages of 29 Å diameter, accessible through hexagonal (8.6 Å) and pentagonal windows (4.8-5.5 Å), and small mesoporous cages of 25 Å diameter, accessible exclusively through pentagonal opening windows. The generally accepted formula is M 3 O(H 2 O) 2 OH(C 6 H 3 (CO 2 ) 3 ) 2 (M is Fe or Al). 7,8 In addition, the MIL-100 MOFs consist of coordinative unsaturated metal sites (CUS) with strong Lewis acid properties produced upon thermal activation and departure of labile ligands. 9,10 The high drug payload of nanoMIL-100(Fe) has been demonstrated for various drugs, 12,13 namely, anticancer, 2,14,15 antibiotic, 16,17 antiretroviral, 18 and a combination of antiinflammatory and antibiotics. 19 For example, the anticancer drug busulfan was loaded in nanoMIL-100(Fe) with a payload of 25 wt %, five and sixty times higher than its payload in polymer nanoparticles and in liposomes, respectively. 2,20,21 Recently, the aluminum-trimesate nanoMIL-100(Al) was also explored for drug delivery. The MIL-100(Al) gel loaded with the anticancer drug doxorubicin (DOX) showed a remarkably high payload of 62 wt %. 22 More recently, DOX was efficiently encapsulated (74% encapsulation efficiency) in core-shell nanoMIL-100(Al) coated with γ-cyclodextrin (CD) citrate and was released progressively depending on the initial drug loading. 23 Both nanoMIL-100(Al) and nanoMIL-100(Fe) demonstrated high loading capacity for active molecules bearing phosphate moieties because of the strong interaction between Al(III) or Fe(III) CUS and phosphate group. 14,18,24 Additionally, to improve the nanoMOF's colloidal stability and to modulate their interaction with biological media and cells, the surface of nanoMIL-100(Fe) and nanoMIL-100(Al) was coated covalently with polymers 25 or non-covalently with biopolymers, lipids, and carbohydrates. 15,[26][27][28][29][30][31] More importantly, for biomedical applications, nanoMIL-100(Fe) has been shown non-toxic and biodegradable in vitro 2,32,33 and in vivo. 2,21,34,35 NanoMIL-100(Al) has also been shown non-toxic in vitro. 33 Still, many questions remain about the nanoMIL-100 carrier stability, for instance, toxicity might come from nanoMOF's degradation products 32 or drug encapsulation can modify nanoMOF's degradation kinetics. 21 To further progress in the clinical translation of nanoMOFs, a critical aspect that needs to be addressed is deciphering the degradation mechanism of the nanoMIL-100 family.

However, there is little literature on the important topic of nanoMIL-100 degradation in biological-relevant media and the effects of drug encapsulation and surface coating. A popular hypothesis is that the degradation reaction of MIL-100(Fe) produces free iron(III) ions and trimesic acid. Early studies found that the culprit of degradation is the phosphate component of the cell medium. 18,36 The higher the phosphate concentration, the higher the trimesate release, the more degraded the nanoMOF. 16 Li et al. discovered that despite witnessing more than 30% trimesate ligand loss, the MIL-100(Fe) nano-and micro-particles maintained their sizes and shapes upon exposure to phosphate buffer. 37 Using Raman and Mossbauer spectroscopy, the authors suspected the formation of an amorphous shell of iron phosphate and/or iron oxide surrounding an intact crystalline core. In addition, using in situ atomic force microscopy and high-resolution transmission electron microscopy, Christodoulou et al. also found that the nanoMIL-100(Fe) and microMIL-100(Fe) degraded without collapsing or changing their dimension. 11 Degradation of MIL-100(Fe) started at the particle surface, where the crystal planes at the surface disappeared within 2 h of incubation in phosphate buffer saline (PBS) at pH 7.4. However, none of these techniques have given evidence at the atomic scale for the changes in iron coordination.

To study the MOF structure at the atomic scale, solid-state NMR (ssNMR) spectroscopy is a non-destructive and valuable technique to get information about both the long-range crystal structure (when combined with XRD and periodic firstprinciples calculations) 38 and short-range local structure or interactions in MOFs. Deviations from periodic structures in MOFs can happen after chemical modification or functionalization, which are often not detected by routine characterization techniques such as X-ray diffraction and infrared spectroscopy. In this case, ssNMR spectroscopy is a technique of choice for locally probing atoms, their neighboring environments, and chemical bonds while keeping the MOF intact. 39,40 It has been used extensively to determine the structure of new MOFs 7,41,42 and the adsorption-desorption mechanism of liquid and gases. 43,44 For porous drug delivery systems, ssNMR spectroscopy has been mainly used to study mesoporous silica [45][46][47] and MOFs. 48,49 For MIL-100(Fe), ssNMR spectroscopy gave indirect proof about the interaction between the strong paramagnetic Fe(III) and the drug azidothymidine triphosphate or the coating cyclodextrin phosphate (CD-P). 18,26 For diamagnetic MIL-100(Al), ssNMR spectroscopy provided insights about the host-guest interaction and location of the drugs adenosine triphosphate (ATP) and doxorubicin as well as the surface coating cyclodextrin-phosphate (CD-P). 23,24 In this work, we further use solid-state NMR spectroscopy to investigate the degradation mechanism at the atomic scale of this nanoMIL-100(Al) in PBS, revealing a stepwise degradation process. We also study the impact of ATP drug loading and CD-P surface modification on the nanoMOF degradation. ATP and CD-P were chosen based on their well-known interaction between molecules bearing phosphate moieties and the Al(III) cations of nanoMIL-100(Al). 24,26 The ensemble of ssNMR data ( 27 Al, 31 P, and 31 P-27 Al) reveals the evolution of the metal coordination sphere in MIL-100 during the degradation reaction as a function of several parameters: MIL-100 structure, concentration of the degradation medium, interaction with surface coating, and loaded drug. To check whether our proposed degradation mechanism applies to the iron MIL-100 analog, we studied the degradation of nanoMIL-100(Fe) in PBS by XANES spectroscopy, which was selected because it can provide information about the local structure of iron(III) in MOFs. 50,51 This paper shows that using a combination of ssNMR and XANES spectroscopy allows a better understanding of the chemistry of nanoMIL-100 drug carriers and their degradation.

EXPERIMENTAL SECTION

Herein, the most important experimental methods and techniques are summarized. Details are given in the Supporting Information.

2.1. Synthesis of NanoMIL-100(Al) and NanoMIL-100(Fe). NanoMIL-100(Al) and nanoMIL-100(Fe) were synthesized by microwave-assisted synthesis following a published procedure. 8,52 All reagents and solvents were used without further purification.

Drug Encapsulation and Surface

Coating of NanoMIL-100(Al). NanoMOFs were loaded with ATP with the input loading 30 wt % of initial nanoMOF following a reported soaking procedure. 14 A 1.5 mg mL -1 aqueous solution of ATP disodium salt hydrate was added to a determined milligram of nanoMOF (m_MOF). The final nanoMOF concentration was 5 mg mL -1 . The weight of drug remaining in the supernatant after drug loading (m_drug_remaining) was determined by HPLC based on a reported procedure. 53 The actual drug payload wt % was calculated as × , where m_drug_input is the weight of drug added to nanoMOF.

NanoMOFs were coated with phosphate-β-cyclodextrin (CD-P) at the theoretical coating 30 wt % of initial MOF by adapting a published method. 26 , where m_coating_input is the weight of drug added to nanoMOF.

2.3. Degradation of NanoMIL-100(Al) and NanoMIL-100(Fe). NanoMOF was incubated in PBS pH 7.4 at 37 °C for 2-5 days following two conditions: 10 mg mL -1 nanoMOF and PBS 10 mM, and 0.5 mg mL -1 nanoMOF and PBS 1 mM. PBS used was Dulbecco's Phosphate-Buffered Saline 1×, i.e., 10 mM, pH 7.0-7.3, composed of 8.1 mM Na 2 HPO 4 , 1.5 mM KH 2 PO 4 , 138 mM NaCl, and 2.7 mM KCl.

Characterization of NanoMIL-100(Al).

The morphology and size of nanoMOF were observed by transmission electron microscopy (TEM) under a 120 kV microscope. A particle size of 0.5 mg mL -1 nanoMOF suspension in EtOH abs or deionized (DI) water was measured by dynamic light scattering (DLS). The specific surface area of MOF was measured by the Brunauer-Emmett-Teller technique (BET) after degassing the MOF powder under vacuum at 100 °C overnight. Crystallinity was evaluated by performing powder Xray diffraction (PXRD) at λ(Cu Kα) of 1.5406 Å. Thermal stability was studied by thermogravimetric analysis (TGA). The amount of trimesate and ATP in the supernatant collected after nanoMOF degradation was determined by high-performance liquid chromatography (HPLC) following a published procedure. 37 The amount of CD-P in the supernatant was determined by NMR spectroscopy, performed at 9.4 T on a Bruker Avance spectrometer equipped with a probe head of 5 mm. The chemical shifts were referenced to tetramethylsilane for 1 H and 13 C. The coordination structure of nanoMIL-100(Fe) was studied by XANES spectroscopy recorded at the K-edge of Fe at 7112 eV at the SAMBA beamline in Synchrotron SOLEIL, France. The experimental details are reported in the Supporting Information (SI).

The coordination structure of nanoMIL-100(Al) was studied by magic-angle spinning (MAS) NMR spectroscopy performed at 11.7 T on a Bruker NEO NMR WB spectrometer equipped with probe heads of 4 and 3.2 mm, except for the 31 P-27 Al 2D NMR measurement, which was recorded at 17.6 T with a 4 mm probe head. The chemical shifts were referenced to tetramethylsilane for 1 H and 13 C, to Al(NO 3 ) 3 1 M solution for 27 Al, and to H 3 PO 4 85 wt % solution for 31 P. Table S1 in the SI reports the details of NMR experiments.

RESULTS AND DISCUSSION

Preparation and Characterization of NanoMIL-100(Al).

The first step of this study was the synthesis of nanoMIL-100(Al) suspensions adequate as drug carriers, i.e., having a diameter lower than 200 nm and uniform size distribution. However, following a published procedure, 52 the obtained MIL-100(Al) suspension had an inhomogeneous size distribution. Therefore, we used an additional size separation step and successfully prepared nanoMIL-100(Al) with a hydrodynamic diameter Z-average of 147 ± 14 nm and polydispersity index PdI of 0.10 ± 0.02 (Figure S1). TEM images (Figure 3a,c and Figure S2) also displayed solid nanoMIL-100(Al) particles with a uniform shape and size (88 ± 25 nm). The nanoparticle size was overestimated by DLS because in DLS, the average nanoparticle diameter is determined from scattered light intensity and large objects scatter more light than small ones. 54 The chemical composition of the obtained nanoMIL-100(Al) was analyzed by TGA (Figure S4). It also showed good crystallinity (XRD pattern in Figure S5) and a BET surface area of 1770 m 2 g -1 (N 2 adsorption isotherm in Figure S3a), which agree with the published studies of MIL-100(Al). 7,52 1 H and 13 C MAS NMR spectra in Figures S6 andS7 show typical features for MIL-100(Al) materials.

A recent study by our group detected an extra 27 Al peak in MIL-100(Al) nanoparticles, which was not detected in the previous study of MIL-100(Al) microparticles. 7 This extra peak represents the surface Al sites in MIL-100(Al) nanoparticles, indicated by high-field NMR experiments 31 P-27 Al D-HMQC and 27 Al 2D MQMAS. 24 These surface species were reproduced in this study (Figure S8) and are expected to play a crucial role in the degradation of nanoMIL-100(Al).

Degradation Mechanism of NanoMIL-100(Al).

In the second step of this study, nanoMIL-100(Al) was incubated in PBS, composed of phosphate monobasic and dibasic salts, at 37 °C to mimic the physiological medium. All measurements were performed ex situ by recovering the nanoparticles from suspension in PBS after a given degradation period.

Chemistry of Materials

As a first step, to choose the suitable conditions for the NMR investigation, we determined the degradation kinetics of nanoMIL-100(Al) and the formation of trimesate by using HPLC. We chose two degradation conditions, "concentrated" (10 mg mL -1 nanoMIL-100(Al) in PBS 10 mM, 2 days) and "diluted" (0.5 mg mL -1 nanoMIL-100(Al) in PBS 1 mM, 2 days). In the "concentrated" condition, the molar ratio of phosphate/Al 3 trimer (P/Al 3 ) is 0.54:1, meaning that nanoMIL-100(Al) is in excess, whereas in the "diluted" condition, P/Al 3 is 11:1, and PBS is in excess compared to nanoMIL-100(Al). The percentage of released trimesate upon degradation in PBS is shown in Figure 2. As expected, the release profile of trimesate from nanoMIL-100(Al) had a similar pattern with MIL-100(Fe) and consisted of two stages. In the first 3 h after incubation in PBS, there was a rapid generation of trimesic acid. Subsequently, the degradation reaction rate decreased gradually in the next 24 h and reached equilibrium after 48-72 h. This plateau of trimesate release after a given period was also observed in other works dealing with MIL-100(Fe). 11,16,18 There could be two explanations for this degradation plateau: (1) insufficient amount of phosphate to react with nanoMIL-100(Al) or (2) another mechanism preventing the reaction from proceeding further. The first explanation was supported by the fact that the trimesate release was higher in the "diluted" degradation, where the ratio P/Al 3 was higher than in the "concentrated" degradation. To investigate further the limitation step of the nanoMIL-100 degradation reaction, the morphology and structure of the degraded nanoMOF were characterized.

The modification of nanoMIL-100(Al) morphology after 48 h degradation is shown in Figure 3b,d and Figure S9. The degraded nanoMIL-100(Al) retained their initial faceted shape and size distribution 98 ± 24 nm, similar to the degraded nanoMIL-100(Fe) in ref 37. However, the degraded nanoMIL-100(Al) had holes on its surface, which is somewhat different from the degraded nanoMIL-100(Fe), whose TEM images displayed particles with rounded edges and without holes at the surface. Meanwhile, the crystal structure of the degraded nanoMIL-100(Al) did not change, shown by PXRD (Figure S10), which is similar to MIL-100(Fe) degradation. 11 More importantly, solid-state NMR spectroscopy demonstrates significant changes in the coordination sphere of nanoMIL-100(Al) after 48 h degradation, indicated by the evolution of 27 Al MAS NMR spectra (Figure 4a). The first noticeable change is the growth of the 27 Al resonance around -2 ppm, a chemical shift characteristic of 6-coordinate Al cations

Chemistry of Materials

coordinated to phosphate ions, coming from the MIL-100(Al) trimer and the degradation medium PBS, respectively. The presence of this new coordination mode for the 6-fold aluminum was confirmed in the 27 Al multiple-quantum MAS (MQMAS) NMR spectrum of the degraded nanoparticles (Figure 5). The formation of the Al-O-P bond was further evidenced by the 31 P-27 Al 2D MAS NMR spectrum (Figure 4b), which showed a correlation peak between this 27 Al resonance at -2 ppm and the 31 P resonances around -10 ppm (Figure S11). This 31 P- 27 Al correlation peak indicates close spatial proximity within 5 Å between these two nuclei, which fits the Al-O-P bond distance. It was also confirmed by 1 H-31 P and 1 H- 27 Al 2D NMR spectroscopy (Figures S13 andS14), showing, in the degraded MIL-100(Al), the proximity between the phosphorus and the protons of the linker and between the new 6-coordinate Al(III) and the trimesate linker. Note that this degraded Al species still remains in the 6-coordination state as in the original MIL-100(Al) trimer, indicating that substitution of an existing ligand such as water or trimesate occurred. Trimesate is a stronger Lewis base than water, and thus the metal-ligand bond Al-O-H between Al(III) and water is more easily broken than the metal-ligand bond Al-O-C between Al(III) and trimesate. As a result, phosphate ions from PBS should first replace the water ligand and form a strong Al-O-P coordination bond with an Al(III) center. Then, due to an increase in interaction between the Al metal center and the newly bonded phosphate ligand, the interaction of the Al center with the trimesate ligands decreases. As the connection Al-O-C between carboxylic groups of the trimesate and Al weakens, phosphate can also attack and replace the position of carboxylate. Free trimesic acid is liberated when all six Al-O-C bonds between a trimesate linker and Al trimers are disconnected.

Notably, the 1D 27 Al MAS NMR spectrum of the degraded nanoMIL-100(Al) (Figure 4) shows another new Al species at 48 ppm, whose chemical shift lies between the range of 4-and 5coordinate aluminum cations. A decrease in the coordination number of aluminum cations in MIL-100(Al) is a known phenomenon observed when heating the sample above 150 °C. As in the study of Haouas et al., 55 we also found that the coordinated water ligands were removed by heating, thus creating 5-coordinate Al cations with a 27 Al chemical shift of 34 ppm (Figure 4). When the dehydrated MIL-100(Al) readsorbed water from ambient air, this 5-coordinate Al cation returned back 6-fold coordination without modifying the framework. However, in the degraded MIL-100(Al), the degraded low-coordinate Al had a higher chemical shift of 48 ppm, and this chemical shift did not change when the sample was exposed to air or heated above 150 °C (Figure S12), implying that this low-coordinate Al site no longer coordinates with water ligands. As Al(III) species bonded with phosphate usually have lower 27 Al chemical shifts than the same coordination number Al(III) species with no phosphate ligand due to a deshielding effect in NMR, 56 we hypothesize that these Al species at 48 ppm are 4-coordinate Al with at least one phosphate ligand. Such 4-coordinate-Al is favorable in basic pH but not in neutral pH of PBS 56,57 (further explanation in the SI), which could explain why these species have very low intensity in the 27 Al NMR spectrum. When the nanoMIL-100(Al) was degraded in excess PBS ("diluted" condition), the intensity of the 4-coordinate 27 Al peak increased slightly, and even a new 5coordinate 27 Al peak was observed around 22-25 ppm (Figure 6). Therefore, we ruled out the possibility of an intermediate product, and we assigned these 4-and 5-coordinate 27 Al resonances to an inorganic aluminum phosphate AlPO 4 . 58,59 As the 4-and 5-coordinate 27 Al peaks only account for 1% of the total Al amount, the amount of these degradation products is low, and their formation may happen only at the surface. If these 4-and 5-coordinate aluminophosphate form a non-porous layer, this layer can passivate the nanoMIL-100(Al) surface, preventing phosphate from reacting with nanoMIL-100(Al), thus slowing down the degradation reaction. Blocked access to the nanoMIL-100(Al) was supported by a 10-time decrease in BET surface area (Figure S3). The existence of this passivation

Chemistry of Materials

layer may explain why degradation and trimesate release reached a plateau after 48 h as determined by HPLC (Figure 2).

The present ssNMR study about nanoMIL-100(Al) so far agrees with the hypothesis of Li et al. about nanoMIL-100(Fe) 37 that the aluminum/iron phosphate complex is the degradation product of nanoMIL-100 reacting with phosphate buffer PBS. While Li et al. could only hypothesize the formation of iron phosphate and iron oxide hematite and goethite, 37 the present study offers concrete evidence about the structure of the degraded nanoMIL-100(Al) by showing the coordination states of Al and the likely formation of the Al-O-P bond in the degradation product.

To summarize, we propose for the first time a molecular mechanism of nanoMIL-100(Al) degradation in the mimicking biological medium PBS as follows (Figure 7): 2) Phosphate ions substitute the carboxylate of a trimesate linker. As the substitution of water and trimesate continues, the formation of 6-fold aluminum atoms with a new coordination sphere (i.e., aluminum bonded with 1 to 2 phosphate groups) continues, producing the main degradation products of nanoMIL-100(Al). The release of free trimesic acid starts when all six Al-O-C bonds between a trimesate linker and Al trimers are broken.

3) New 4-and 5-coordinate Al species are produced, most likely as a layer of inorganic aluminum phosphate AlPO 4 on the nanoMIL-100(Al) surface, due to cleavage of the trimesate linker and the aluminum-oxo center. 4) After 6 h, the insoluble aluminum phosphate species AlPO 4 passivate the MOF pore and pore window. The intact trimers of nanoMIL-100(Al) are no longer exposed to a high amount of phosphate. As a result, the degradation reaction reaches equilibrium, and the release of trimesate reaches the maximum.

Degradation Mechanism of Drug-Loaded and

Surface-Coated NanoMIL-100(Al). In the final step, we analyzed the effect of surface coating and drug loading on the nanoMIL-100(Al) degradation processes. As reported in the literature, the presence of loaded drug and surface coating could alter the physicochemical properties of nanoMOF, 21,26 thus potentially affecting nanoMOF's degradation reaction.

NanoMIL-100(Al) was successfully loaded with adenosine triphosphate (ATP) at 18.6 ± 0.8 wt % and coated with βcyclodextrin-phosphate (CD-P) at 28.3 ± 0.4 wt %. Before studying the degradation, the drug loading and surface coating quality were evaluated. The results of 27 Al and 31 P MAS NMR in Figure S15a,b agree with our previous study. 24 31 P MAS NMR (Figure S17b) showed that the drug ATP is captured inside MIL-100(Al) by chemical adsorption through Al-O-P coordination. 27 Al MAS NMR (Figure S17a) showed that the CD-P coating is located at the surface and does not penetrate the MOF core. The integrity of the trimesate framework was checked by TEM and 13 C MAS NMR (see Figures S16 andS18, respectively). From now on, we denote in the figures below the CD-P-surface-coated nanoMIL-100(Al) as MOF-coated, the ATP-drug-loaded nanoMIL-100(Al) as MOF-loaded, and the ATP-drug-loaded-CDP-surface-coated nanoMIL-100(Al) as MOF-loaded-coated.

Chemistry of Materials

Degradation of the CD-P coated nanoMIL-100(Al) is similar to degradation of the non-coated one. In Figure 8, the 27 Al MAS NMR spectrum of the degraded MOF-CD-P-shows both the hexa-coordinate Al-O-P around -2 ppm and the tetracoordinate aluminophosphate at 48 ppm. Observing a similar degradation product, we speculate that, as CD-P covers approximately 30% of the nanoMIL-100(Al) surface, 26 a degradation reaction still occurs at the nanoMOF's surface not attached to CD-P, following the reaction mechanism proposed in the previous section.

Additionally, the 31 P MAS NMR spectrum of the degraded MOF-CD-P-in Figure 9a displays a shoulder around -10 ppm, which belongs to CD-P, and the dominant signal of the phosphate coordinated with the MIL-100(Al) framework around -15 ppm. The presence of CD-P coating even after 5 days of degradation suggests that CD-P competes with inorganic phosphate ions to complex with Al(III). Consequently, PBS phosphates can only partially displace CD-P.

Meanwhile, the degradation reaction of the ATP-loaded nanoMIL-100(Al) exhibits features distinctive from that of the empty nanoMOF. No sign of low-coordinated Al was detected. 27 Al NMR (Figure 8) still showed the 6-coordinate Al species around -2 ppm as the main degradation product, while the Al species bonded with phosphate ATP was still present. Likewise, the 31 P MAS NMR spectrum in Figure 9b shows both a peak of coordinated PBS phosphate and a shoulder signal of ATP. Both 27 Al and 31 P NMR data consistently suggest that nanoMIL-100(Al) loaded with ATP is less degraded than the empty one, and only a part of ATP is liberated from the framework. Similar to the surface coating CD-P, the loaded drug ATP competes with inorganic PBS phosphates to coordinate the Al(III) of nanoMIL-100(Al). The triphosphate ATP is a much stronger complexing ligand toward Al(III) than the hydrogen phosphate and dihydrogen phosphate ions, 60 meaning that MIL-100(Al) forms a more stable coordination complex with ATP than with phosphate ions. Therefore, it is difficult to break the coordination between ATP and Al(III) by solely phosphate PBS, which explains the presence of ATP in the 31 P NMR spectrum, even after days of degradation. However, the strong Al-O-P bond between Al(III) and ATP makes the Al-O-C bond between Al(III) and the trimesate linker more easily attacked by phosphate. We hypothesize that phosphate attacks the Al-O-C bond with trimesate, forming hexa-coordinate Al-O-P in the region -2 to -5 ppm observed in the 27 Al spectrum. When the Al center forms a new coordination bond with inorganic phosphate, its coordination bond with ATP is weakened. As a result, the probability is higher for phosphate to displace ATP from the framework. Another factor to be considered is the steric effect caused by ATP. Bulky ATP possibly hinders phosphate diffusion in the MIL-100(Al) trimer, thus preventing trimesate substitution.

In addition, the notable absence of the low-coordinate aluminophosphate can be explained by the chelating capability of triphosphate ATP. 61,62 ATP coordinates with MIL-100(Al) through the terminal γ-phosphorus. During the breaking of the Al-O-C bond, the β-phosphorus of ATP can compete with phosphate PBS ions and create a second coordination bond with Al. The bidentate Al-ATP coordination not only halts the formation of low-coordinate Al but also prevents the 100% release of ATP. Consistently, only 60% ATP was released in excessive PBS, while 100% of trimesate ligand was found by HPLC (data not shown).

Finally, combining the effect of drug encapsulation and surface coating, Figure 8 shows that degradation of the ATPloaded CD-P-surface-coated nanoMIL-100(Al) produces both the 6-coordinate aluminophosphate complex and 4-coordinate inorganic aluminophosphate but with a slightly lower proportion of 4-coordinate degraded complex (1.0%) than that in the surface-coated empty nanoMIL-100(Al) (1.6%). Also, the 27 Al peak at 2.5 ppm belonging to the original In summary, the degradation reaction of the modified MOF including the ATP-loaded and the CD-P-surface-coated nanoMIL100(Al) can be summarized as follows:

1) Surface coating with CD-P has a negligible effect on the degradation of nanoMIL-100(Al). 6-Coordinate and 4coordinate aluminum complexes with phosphate are formed as described in the degradation mechanism of the original nanoMIL-100(Al). Consequently, no low-coordinated Al is created, and ATP is not entirely released.

Degradation Mechanism of NanoMIL-100(Fe) in

Comparison with NanoMIL-100(Al). To evaluate whether our proposed degradation mechanism applies to the iron MIL-100 analog, nanoMIL-100(Fe), XANES spectroscopy was used to study the ex situ degradation of the original, CD-P surface-coated, and ATP-loaded nanoMIL-100(Fe). Similar to nano-MIL-100(Al), nanoMIL-100(Fe) was incubated in PBS at 37 °C, and then, the degraded nanoparticles were recovered after 48 h. A "very diluted" condition (0.5 mg mL -1 nanoMIL-100(Fe) in PBS 10 mM) was used to produce as much as possible degraded MIL-100(Fe) species.

The evolution of original nanoMIL-100(Fe) after degradation is shown in Figure 10. The XANES Fe K-edge spectrum of the degraded nanoMIL-100(Fe) is reminiscent of that of iron(III) phosphate dihydrate, a substance having 6-coordinate iron(III) bonds with phosphate through the Fe-O-P bond. 63 This similarity is also observed in the pre-edge region, where the intact MIL-100(Fe) shows two features at 7114 and 7115.4 eV, while the degraded MIL-100(Fe) and FePO 4 •2H 2 O only show a single, broad peak at 7114.9 eV. Moreover, the XANES spectrum of this degraded nanoMIL-100(Fe) is different from those of iron(III) oxide and iron(III) oxyhydroxide, excluding their presence in the degraded nanoMIL-100(Fe) (Figure 11). Therefore, we propose that the first step of nanoMIL-100(Fe) degradation also forms an iron-phosphate-6-coordinate species with the Fe-O-P coordination bond, in the same way as illustrated in Figure 7, where phosphate from PBS replaces the water ligand of nanoMIL-100(Fe) and coordinates with Fe(III). The 4-coordinate iron phosphate species were not detected due to its low amount, probably 1% of iron in the degraded nanoMIL-100(Fe). Note that nanoMIL-100(Fe) also became less porous after degradation (Figure S19) with a smaller decrease (50% decrease) in the BET surface area than in nanoMIL-100(Al) (90% decrease). In addition, after a degradation reaction, the oxidation number of Fe in nanoMIL-100(Fe) does not change and remain as +III, as the degraded and original nanoMIL-100(Fe) have the same edge position (Figure 10a), and the edge position of nanoMIL-100(Fe) is close to the values of Fe(III) compounds (Figure S20). Previously observed by other authors using Mossbauer spectroscopy, 37 this maintenance of Fe oxidation number after nanoMIL-100(Fe) degradation is consistent with our proposed degradation mechanism based on the model nanoMIL-100(Al) (Figure 7), which describes degradation as an acid-base reaction.

The effect of ATP drug loading and CD-P surface coating on the nanoMIL-100(Fe) degradation was also investigated. Figure S21 shows that the ATP-drug-loaded and CD-P-surface-coated nanoMIL-100(Fe) share the same XANES features as the original one, indicating that drug loading and surface coating do not degrade nanoMIL-100(Fe). Figure 12 shows that degradation of drug-loaded and surface-coated nanoMIL-100(Fe) produces similar species as obtained from the degradation of the original one, as all three XANES and preedge spectra have similar shapes and features. Once again, XANES spectroscopy shows that 6-coordinate iron-phosphate was formed during the degradation reaction of nanoMIL-100(Fe).

This complementary study with XANES suggests that the first step in degradation of nanoMIL-100(Fe) likely follows the degradation reaction of its aluminum analog, corroborating the ssNMR study on nanoMIL-100(Al).

CONCLUSIONS

By using solid-state NMR spectroscopy, we obtained novel knowledge about the degradation mechanism of nanoMIL-100(Al) and nanoMIL-100(Fe) in phosphate buffer. For the first time, we find direct evidence about the change of the metal coordination sphere and the limiting reaction step in the nanoMIL-100 degradation mechanism. First, upon substitution of labile water ligands by phosphate, Al-O-P coordination bonds are formed between aluminum species at the surface of nanoMOF and phosphate from the medium, creating 6coordinate aluminophosphate. Second, the substitution of trimesate by phosphate leads to the release of free trimesate and creates 4-coordinate inorganic aluminum phosphate. This 4-coordinate aluminum, which has not been observed before, can passivate the nanoMOF surface and slow down the degradation reaction. Additionally, our study demonstrates that chemical modification such as drug encapsulation and surface coating can impact the nanoMOF degradation and hence its drug release process as a function of metal-ligand bond strength.

To conclude, insights about the degradation mechanism of nanoMOF from this work contribute to the understanding of MOF chemistry and help optimizing the performance of nanoMOF drug carriers. This study also highlights the useful role of ssNMR spectroscopy with the support of XANES spectroscopy in investigating in depth the nanoMOF structure. Using ssNMR spectroscopy, we were able to obtain enough evidence about the coordination sphere and the reaction limiting step of nanoMIL-100(Al) degradation, while XANES spectroscopy provided complementary proof on the reaction of nanoMIL-100(Fe). The obtained knowledge in this study prompts new studies on understanding drug delivery chemistry and developing characterization tools for drug nanocarriers. Comparative studies of MIL-100(Al) and MIL-100(Fe) are being carried out to investigate the behaviors of MIL-100 analogs. Various types of drug compounds can be explored when different host-guest interactions and degradation behaviors are expected. In the future, we will develop a new NMR spectroscopy method to monitor in situ drug delivery processes of nanoMOF.

■ ASSOCIATED CONTENT

* sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.2c01190.

(1) Experimental procedures of nanoMOF preparation and characterization, (2) standard characterization of nanoMIL-100(Al): DLS, TEM, XRD, BET, TGA, and 1 H MAS NMR, 13 C MAS NMR, 27 Al MAS, and MQMAS NMR spectra, and (3) complementary NMR measurements on degraded nanoMIL-100(Al): 31

Figure 1 - 3 .

 13 Figure 1-3. Simplified scheme of natural photosynthesis and artificial photosynthesis

Figure 1 - 5 .

 15 Figure 1-5. Chain growth pattern during the Fischer-Tropsch process of the carbine mechanism (A), CO insertion mechanism (B), and hydroxycarbene mechanism (C). R in panel B representsan H or alkyl group. Reproduced from reference49

Figure 1 - 6 .

 16 Figure 1-6. Homogeneous (A) and heterogeneous (B) electrocatalysis for the reduction of CO2

Figure 1 - 7 .

 17 Figure 1-7. Types of electrochemical cells generally used for CO2 reduction: (A) Onecompartment static cell, (B) Two-compartment static cell, (C) Liquid flow cell, and (D) Flow cell with a gas-diffusion electrode. WE, CE, and RE stand for working electrode, counter electrode,

Figure 1 -

 1 Figure 1-8. *CO and *H adsorption energy of different metals, reproduced from reference 51

Figure 1 - 9 .

 19 Figure 1-9. Structures of metal porphyrin (A) and metal phthalocyanine (B)

Figure 1 -

 1 Figure 1-10. Catalytic Tafel plot with logTOF and overpotential 𝜂, reproduced from reference 71 1.3.3.3 CO2-to-CO performance and mechanism of FeTPP

Figure 1 -

 1 11C illustrates the proposed catalytic cycle of CO2 to CO with all intermediate steps. Upon the reduction of the initial Fe(II)TPP to Fe(0)TPP, the [Fe(0)-CO2] 2-adduct is formed. The majority of this species exists in the form of [Fe(I)-CO2 •-] -, as Fe(I) is the dominant resonant structure, per DFT calculation. The nature of the reduced iron porphyrin is a matter of debate, as DeBeer and co-workers recently proposed that the second and Chapter 1 third electrons injected in [Fe III TPP] + are localized on the porphyrin ring rather than on the metal. 77 Therefore, they formulated the two-and three-electron reduced species as [Fe II TPP•] -and [Fe II TPP••] 2-. A PhOH molecule forms a hydrogen bond with the bound CO2 intermediate, and the C -O bond is broken upon a proton transfer of another PhOH molecule. This step involves a proton-coupled intermolecular electron transfer combined with the C -O bond cleavage. The C -O bond cleavage is the ratedetermining step: at low acid concentrations, the reaction rate is second-order, and at higher concentrations of acid, this step becomes fast enough to change the reaction rate to first-order. A strong acid would not work because the proton reduction would be more favorable, and the strong acid can decoordinate the metal complex itself. The final step consists of a one-electron reduction of the Fe(II)CO species, releasing the bound CO molecule and returning two Fe(I) complexes, completing the catalytic cycle.

Figure 1 -

 1 Figure 1-11. (A) Cyclic voltammogram of Fe III TPP-Cl (1 mM) in DMF + 0.1M NEt4ClO4 on a glassy carbon electrode under inert gas. (B) Cyclic voltammograms of FeTPP (1 mM) in DMF + 0.1 M n-Bu4NBF4, in the absence (blue) and presence of 0.23 M CO2 and increasing amounts of PhOH (red 5 mM, green 50 mM, orange 200 mM) on a carbon electrode at 21 o C. (C) Catalytic cycle of CO2-to-CO conversion. Reproduced from references 64 and 74

Figure 1 - 12 .

 112 Figure 1-12. Structure of (A) FeTPP-(OH)8, (B) FeTPP-(OH)4F10,, reproduced from reference 82 1.3.3.5 Influences of ligands on FeTPP: second sphere interaction

Figure 1 - 13 .

 113 Figure 1-13. Structure of different modified FeTPP catalysts for hydrogen bond stabilization effect, reproduced from reference 85 (A) and reference 87 (B)

Figure 1 -

 1 Figure 1-14. Structure of FeTPP dimer, reproduced from reference 90

Figure 1 - 15 .

 115 Figure 1-15. Products distribution from CO2 reduction by different metal phthalocyanines, reproduced from reference 93 1.3.4.3 Influences of metal center on porphyrin species

Figure 1 -

 1 Figure 1-17. (A) Structures of cobalt porphyrin and its substituents (B) relation between Hammett values of substituents and the logarithm of the CO turnover frequency of substituted cobalt phthalocyanine (-1.02 V vs. RHE, 0.8 nmol/cm 2 ), reproduced from reference 113

Figure 1 -

 1 Figure 1-18. (A) SEM images of the CoPc-modified electrode (loading: 1 × 10 -7 mol/cm 2 ), on which crystals of CoPc are evident. The red oval highlights a region with many crystals. Inset:

Figure 1 -

 1 Figure 1-19. (A) Pyridine-based polymer encapsulation for CoPc -reproduced from reference 105

Figure 1 - 20 .

 120 Figure 1-20. Structure of (A) cobalt-corrole and (B) manganese-corrole, reproduced from references 153 and 154 1.3.5.4 Single-atom catalysts

Figure 1 - 21 .

 121 Figure 1-21. In situ XAS measurements of CuPc under resting and electrocatalytic reaction conditions. (A) Cu K-edge XANES spectra, (B) first-order derivatives of the XANES spectra, and (C) Fourier-transformed Cu K-edge EXAFS spectra for CuPc. Reproduced from reference 172

Figure 1 -

 1 Figure 1-22. Thesis graphical outline

Figure 2 - 1 .

 21 Figure 2-1. (A) Simplified scheme of the PDMS microfluidics electrochemical flow cell (not to scale); (B) Photoresist mold for the PDMS casting of the electrochemical flow cell (L = 20 mm, W = 0.50 mm, Z = 0.25 mm, d = 1.25 mm, the transparent black square represents the

Figure 2 - 2 .

 22 Figure 2-2. (A) Glassy carbon working electrode with a Kapton mask; (B) bottom view of PDMS chip; (C) top view of PDMS chip; (D) PDMS chip on top of GC working electrode; and (E) PDMSchip fully assembled inside the 3D-printed holder2.2.2 Electrochemistry performance

Figure 2 - 3 .

 23 Figure 2-3. (A) Cyclic voltammograms and (B) electrolysis curves of electrodes modified with CoPc-free ink in CO2 environment (black), CoPc ink in Ar (red), and CO2 environment (blue).The electrolyte is 0.1 M KCl, the applied potential is -1.6 V vs. pseudo-Ag, and the flow rate is 15 µL/min.

Figure 2 - 4 .

 24 Figure 2-4. Scheme of the gasket microfluidics electrochemical flow cell (not to scale)

Figure 2 - 5 .

 25 Figure 2-5. (A) Dimensions of the cathode and anode flow plates (B) Exploded view of the cell, with WE, CE, and RE standing for working electrode, counter electrode, and reference electrode, respectively. (C) View the fully assembled cell and beam bath from synchrotron radiation.

Figure 2 - 6 .

 26 Figure 2-6. Laminar flow mixing test

Figure 2 -

 2 Figure 2-7 shows the cyclic voltammograms and electrolysis curves of two FePcmodified electrodes under an Ar and CO2 environment and a FePc-free modified electrode under a CO2-saturated electrolyte. Unlike in the PDMS microfluidic cell, the electrolysis experiments in the 3D-printed flow cell could last at least two hours with a

Figure 2 - 7 .

 27 Figure 2-7. (A) Cyclic voltammograms and (B) electrolysis curves of electrodes modified with FePc-free ink in CO2 environment (black), FePc ink in Ar (red), and CO2 environment (blue).The electrolyte is 0.1 M KHCO3, the applied potential is -1.1 V vs. RHE (-1.7 V vs. Ag/AgCl), and the flow rate is 1 mL/min.

  element size. The model after meshing is shown in Figure2-8C.

Figure 2 - 8 .

 28 Figure 2-8. 2D model of the electrochemical flow cell built in COMSOL 5.2 (A); Overlay of the model geometry on an actual picture of the electrochemical flow cell (B); a zoom in the model after meshing (C)

Figure 2 - 9 . 4

 294 Figure 2-9. Concentration profile (in mmol/L) in the electrochemical cell of CO2 (A) and CO (B) when a constant potential of -1.1 V vs. RHE is applied. The color graph shows the concentrations.

Figure 2 -

 2 Figure 2-10. Concentration profile as a function of distance from the electrode at the entrance (y = 0), in the middle (y = 7.5 mm), and at the end (y = 15 mm) of the electrochemical cell of CO2 (a) and CO (b) when a constant potential of -1.1 V vs. RHE is applied.

Figure 3 - 1 .

 31 Figure 3-1. Iron phthalocyanine structure

Figure 3 -

 3 Figure 3-2A shows three chronoamperometric curves of constant potential electrolysis under Ar or CO2 for two hours. After poising an FePc-modified electrode at -1.1 V vs.RHE under a CO2 atmosphere for two hours, the main products obtained are CO and H2, as shown in the chromatograms of Module A (Figure3-2C, D). In module B, peaks corresponding to ethylene (C2H4), ethane (C2H6), propene (C3H6), propane (C3H8), and n-butane (C4H10) stand out (Figure3-2B). Other peaks corresponding to unsaturated C4 and n-pentane and unsaturated C5 products can also be observed, albeit with a lower intensity. Methane (CH4) is also presented in module A (Figure3-2C). The catholyte was analyzed by 1 H nuclear magnetic resonance (NMR) spectroscopy to check for liquid products, but none was detected (Figure3-3).

Figure 3 - 2 .

 32 Figure 3-2. (A) Chronoamperometric curves, (B) Module B gas chromatograms, (C) Module A

Figure 3 -

 3 Figure 3-3. 1 H NMR spectrum of catholyte after 2-hour electrolysis in CO2 saturated electrolyte at -1.1 V vs. RHE with FePc catalyst. The signal from PhOH and dimethyl sulfoxide (DMSO) are from internal references of the sample.

Figure 3 - 4 .

 34 Figure 3-4. (A) Chronoamperometric curves, (B) Module B gas chromatograms, (C) Module A gas chromatograms -zoomed, and (D) Module A gas chromatogram of FePc-modified electrodes under Ar in 0.1 M KHCO3. Chronoamperometric experiments are performed at -1.7V vs. Ag/AgCl for two hours, after which a GC samples the resulting gaseous products.

Figure 3 -

 3 Figure 3-5. GC-MS chromatograms for mass m/z = 56 of 12 C4H8 (A), mass m/z = 60 of 13 C4H8 (B) at the retention time of C4 products, and (C) mass m/z = 41 of 12 C3H5 at the retention time of C3 and C4 products. Chronoamperometric experiments are performed on FePc-modified electrodes at -1.1 V vs. RHE for two hours, after which the GC-MS samples the resulting gaseous products.

Figure 3 - 6 .

 36 Figure 3-6. Chronoamperometric curves of FePc-modified electrodes under CO2 in 0.1 M KHCO3 performed at different potentials for two hours.

Figure 3 - 7 .

 37 Figure 3-7. Product distribution observed with FePc-modified electrodes as amount (A) and faradaic efficiencies (B) as a function of applied potential. The total current densities at 60 minutes are plotted as a blue dot for each potential. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 3 - 8 .

 38 Figure 3-8. (A) Normalized X-ray absorption spectra collected on an FePc-modified electrode under different conditions and reference samples. The spectra at the top correspond to an FePc-modified electrode under a CO2 atmosphere poised at OCP (black), at -1.1 V vs. RHE (red), and going back at +0.8 V vs.RHE (blue). The spectra at the bottom correspond to ex-situ reference spectra collected on a pellet of FePc (green), on a metallic sheet of iron (gold), and on a pellet of Fe3O4 (purple). The two dashed lines indicate the pre-edge features of FePc at 7114eV and 7118 eV. (B) Corresponding raw X-ray absorption spectra collected on the FePcmodified electrode shown in panel A.

Figure 3 - 9 .

 39 Figure 3-9. Infrared spectra of the FePc catalyst powder (black) and FePc-modified electrodes before (blue) and after (red) a 2-hours electrolysis at -1.1 V vs. RHE.

Figure 3 -

 3 Figure 3-10. (A) Cyclic voltammograms of an FePc-modified electrode under Ar (red) and CO2 (blue) and of a catalyst-free modified electrode under CO2 (black) in 0.1 M KHCO3 in the electrochemical flow cell without flowing; Multiple cyclic voltammograms of FePc-modified electrode (B) and catalyst-free electrode (C) under Ar in 0.05 M H2SO4 in a glass static cell

Figure 3 -

 3 Figure 3-11. X-ray absorption spectra collected on an FePc/Fe3O4-modified electrode with various Fe3O4 content and reference samples.

Figure 3 - 12 .

 312 Figure 3-12. Linear combination fitting of spectra recorded on the electrode with 5% of the nominal iron content coming from Fe3O4 (A) and electrode with FePc and Fe3O4, each contributing 50% of the nominal iron content (B).

Figure 3 -

 3 Figure 3-13. (A) Electrolysis curves of Fe3O4/FePc electrodes at -1.1 V vs. RHE under CO2 in 0.1 M KHCO3 performed for a mixture of FePc/Fe3O4 nanoparticles for two hours. (B-D) Product distribution observed with Fe3O4/FePc electrodes as faradaic efficiencies after the corresponding electrolysis -1.1 V vs. RHE as a function of the iron content of the Fe3O4 nanoparticles. The percentages indicate the iron content (in mole) contributed by the Fe3O4 nanoparticles out of the total iron content (80 nmol/cm 2 ) of FePc and Fe3O4. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 3 -

 3 Figure 3-13B, C, and D show the faradaic efficiencies (FE) of CO and H2, CH4, and C2+ products, respectively, after 2-hour electrolysis experiments at -1.1 V vs. RHE, with electrodes having different amounts of Fe3O4 and FePc. The presence of Fe3O4 hindered the CO2-to-CO reduction activity, as the electrode containing only FePc produced CO most selectively, with a FE of around 20%. Hydrogen was the dominating

Figure 3 -

 3 Figure 3-14. C2+ product distribution observed with Fe3O4/FePc electrodes as faradaic efficiencies after 2-hour electrolysis at -1.1 V vs. RHE. The total current densities at 60 minutes are plotted as a blue dot for each Fe percentage. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 3 -

 3 Figure 3-15. X-ray absorption spectra as normalized data (A) and raw data (B) collected on an FePc/Fe3O4-modified electrode before and after two-hour electrolysis at -1.1 V vs. RHE. Fe3O4 contributed 22.1% of the iron content in the sample.

Figure 3 -

 3 Figure 3-16. (A) CH4 and (B) C2+ products distribution with faradaic efficiencies after 2h electrolysis at -1.7V vs. Ag/AgCl with different substrates using an FePc-modified electrode. The total current densities at 60 minutes are plotted as a blue dot for each reactant. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 3 -

 3 Figure 3-17. 1 H NMR spectra of catholyte after 2-hour electrolysis in CO saturated electrolyte (A), Ar saturated electrolyte with 30 mM HCHO (B) at -0.97 V vs. RHE (-1.7 V vs. Ag/AgCl)

Figure 3 -

 3 Figure 3-18. C2+ products faradaic efficiencies after 2h electrolysis at -0.76 V and -0.97 V vs. RHE (-1.7 V and -1.5 V vs. Ag/AgCl, respectively) with 30 mM HCHO under Ar atmosphere using an FePc-modified electrode (A); at -0.9 V and -1.1 V vs. RHE (-1.7 V and -1.5 V vs. Ag/AgCl, respectively) under only CO2 (B) and CO2 with 30 mM HCHO atmosphere (C) using FePc-modified electrodes. The total current densities at 60 minutes are plotted as a blue dot for each potential. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 3 - 19 .

 319 Figure 3-19. Simplified mechanisms proposed for forming C -C bonds by FePc with either CO2, CO, HCHO, or a mixture of HCHO and CO2 as substrates. The color code for substrates and products is as follows: blue, purple, and red for high, intermediate, and low concentrations, respectively.

Figure 3 -

 3 20A shows our quasi-Anderson-Schulz-Flory, which did Chapter 3

Figure 3 -

 3 Figure 3-20. Quasi Anderson-Schulz-Flory plot of hydrocarbons formed on FePc-modified electrodes by the electrochemical reduction CO2 at different potentials, including C2 products (A) and excluding C2 products (B). The potentials are reported with the RHE scale.

Figure 3 - 21 .

 321 Figure 3-21. Reference mass spectra of C1 to C4 linear hydrocarbons. Data were taken from the NIST chemistry WebBook.28

Figure 3 -

 3 Figure 3-22. Reference mass spectra of species presented in the sample mixture. Data were taken from the NIST chemistry WebBook.28

Figure 3 -

 3 Figure 3-23. Cyclic voltammetry setup in a homemade glass electrochemical cell.

Figure 3 -

 3 Figure 3-24. CVs peak integration of three independent experiments indicate by the panels (A), (B), and (C)

Figure 4 - 1 .

 41 Figure 4-1. Electrolysis curves of FePc-modified electrodes at -0.9 V vs. RHE (A), and -1.1 V vs. RHE (B) under CO2 in 0.1 M KHCO3 performed at different flowrate for two hours.

Figure 4 -

 4 Figure4-2 shows the faradaic efficiencies (FE) of CO, H2, CH4, and C2+ products obtained after 2-hour electrolysis at -0.9 and -1.1 V vs. RHE with FePc-modified electrodes under CO2 at a flow rate ranging from 0.25 to 4 mL/min. When the flow rate increased from 0.25 to 4 mL/min, the CO2 reduction activity increased, and the hydrogen evolution activity was further suppressed. While this phenomenon was observed at both applied potentials, the rise in CO selectivity, from 30% to 50%, was more pronounced at a more negative potential of -1.1 V vs. RHE. The high flow rate enhanced the CO2 mass transport to the electrode surface, resulting in more CO2

Figure 4 - 2 .

 42 Figure 4-2. Product distribution observed with FePc-modified electrodes as faradaic efficiencies at -0.9 V vs. RHE (A) and -1.1 V vs. RHE (B) as a function of flow rate. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 - 4 .

 44 Figure 4-4. Electrolysis curves of FePc-modified electrodes at -1.7 V vs. Ag/AgCl (A) and product distribution observed with FePc-modified electrodes as faradaic efficiencies at -1.7 V vs. Ag/AgCl (B-D) as a function of KHCO3 concentration. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 - 6 .

 46 Figure 4-6. Electrolysis curves of FePc-modified electrodes at -0.7 V vs. RHE (A), -0.9 V vs. RHE (B), and -1.1 V vs. RHE (C) under CO2 in 0.1 M KHCO3 performed at different FePc concentrations for two hours.

Figure 4 - 7 .

 47 Figure 4-7. Product distribution observed with FePc-modified electrodes as faradaic efficiencies at -0.9 V vs. RHE (A) and -1.1 V vs. RHE (B) as a function of FePc concentration. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 - 8 .

 48 Figure 4-8. Product distribution observed with FePc-modified electrodes as faradaic efficiencies at -0.7 V vs. RHE as a function of FePc concentration.

Figure 4 - 9 .

 49 Figure 4-9. C2+ product distribution observed with FePc-modified electrodes as faradaic efficiencies at -0.9V vs. RHE (A) and -1.1 V vs. RHE (B) as a function of FePc concentration. The total current density at 60 minutes is plotted as a blue dot for each catalyst concentration.The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 -

 4 Figure4-10 shows the faradaic efficiencies (FE) of CO, H2, CH4, and C2+ products obtained after 2-hour electrolysis experiments at -1.1 V vs. RHE with FePc-modified electrodes under CO2 at different carbon support concentrations. The standard carbon black concentration was one milligram per milliliter of ink. This concentration results in a thick catalyst layer of a few microns after drop-casting, which adheres well to the surface of the glassy carbon plate. Other conditions were kept similar to experiments with FePc described in Chapter 3 and the annex, with a catalyst loading of 80 nmol/cm 2 , a KHCO3 0.1 M electrolyte, and a flow rate of 1 mL/min.

  their electrolysis curves sloped down over time.

Figure 4 - 12 .

 412 Figure 4-12. Electrolysis curves of different metal phthalocyanines modified electrodes at -0.7 V vs. RHE (A), -0.9 V vs. RHE (B), and -1.1 V vs. RHE (C) under CO2 in 0.1 M KHCO3 for two hours.

Figure 4 -

 4 Figure 4-13 shows the CO and H2 faradaic efficiency of the investigated M-Pc after 2hour electrolysis.In agreement with previous reports in the literature, Fe, Co, and NiPc show the highest selectivity for CO, greater than 70%, at potentials of -0.7 and -0.9 V vs. RHE. At these moderate potentials, Mn, Cu, and ZnPc produce more H2 than CO. The CO2-reducing activity on these metal Pc is low, resulting in the current density being close to the background current value and the total faradaic efficiencies not reaching close to a hundred percent. Although the liquid products were not characterized in all experiments, based on the literature, we would expect at most a few percent of methanol generated from electrolysis on CoPc. At a more negative potential of -1.1 V vs. RHE, CoPc was the only catalyst maintaining CO as the primary product. Other catalysts mainly produced hydrogen, except for ZnPc, generating CO and H2 at roughly equal selectivity.

Figure 4 -Figure 4 -

 44 Figure 4-13. CO and H2 distribution observed with metal phthalocyanines electrodes as faradaic efficiencies after 2-hour electrolysis at -0.7 V vs. RHE (A), -0.9 V vs. RHE (B), and -1.1 V vs. RHE (C) Figure 4-14 shows the methane distribution of the studied metal phthalocyanines. No hydrocarbons were detected at -0.7 V vs. RHE. At -0.9 V vs. RHE, only FePc and CuPc

Figure 4 -

 4 Figure 4-15. C2+ product distribution as total C2+ products (A) and individual C2+ products (B) observed with metal phthalocyanine electrodes as faradaic efficiencies after 2-hour electrolysis at -1.1 V vs. RHE. The total current density at 60 minutes is plotted as a blue dot for each metal species. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 - 16 .

 416 Figure 4-16. Chemical structures of iron phthalocyanine with different substituents: Cl16 (A), (m-COOH)4 (B), (o-NH2)4 (C), and (m-NH2)4 (D)

Figure 4 -

 4 Figure 4-17. Electrolysis curves of FePc-substituted modified electrodes at -0.9 V vs. RHE (A), and -1.1 V vs. RHE (B) under CO2 in 0.1 M KHCO3 for two hours. The potential applied to the FePc-Cl16 catalyst in panel A was -0.8 V vs. RHE. The data file of the FePc-COOH species corrupted after 50 minutes.

Figure 4 -

 4 Figure 4-18. Product distribution observed with FePc-modified electrodes as faradaic efficiencies after 2-hour electrolysis at -0.9 V vs. RHE (A) and -1.1 V vs. RHE (B) as a function of substituents of FePc. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 -

 4 Figure 4-19. C2+ product distribution observed with FePc-modified electrodes as faradaic efficiencies after 2-hour electrolysis at -0.9 (A) and -1.1 V vs. RHE (B) as a function of substituents of FePc. The total current density at 60 minutes is plotted as a blue dot for each substituent. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 - 20 .

 420 Figure 4-20. Chemical structures of iron tetraphenyl porphyrin (FeTPP) (A) and its derivatives: FeTPP-F20 (B), FeTTP (C), and Fe2FP (D). Panel C and D were reproduced from an article byMoore and co-workers.14

Figure 4 -

 4 Figure4-21 shows the electrolysis curves of electrodes modified with FeTPP and its derivatives at -0.8 and -1.1 V vs. RHE. At -0.8 V vs. RHE, Fe2FP exhibited the highest current density, followed by FeTPP and FeTTP, and FeTPP-F20 was the least active species. Interestingly, at -1.1 V vs. RHE, Fe2FP became the species with the lowest current densities, while FeTPP became the most active species. FeTTP and FeTPP-F20 displayed similar electrolysis curves at this potential.

Figure 4 -

 4 Figure 4-21. Electrolysis curves of electrodes modified with various iron-based macrocycles at -0.8 V vs. RHE (A), and -1.1 V vs. RHE (B) under CO2 in 0.1 M KHCO3 for two hours.

Figure 4 -

 4 Figure4-22 shows the faradaic efficiencies of CO, H2, CH4, and C2+ observed with the four iron porphyrin species at -0.8 V vs. RHE and -1.1 V vs. RHE. At the low overpotential of -0.8 V vs. RHE, CO and H2 are the only products detected. Fe2FP was the most selective catalyst toward CO, while other iron porphyrin species also showed high CO selectivity. The substituents did not clearly influence the CO2-reducing activity of these catalysts. At -1.1 V vs. RHE, Fe2FP was still the most CO-selective catalyst, with CO accounting for around 60% of the products. Other catalysts all produced more H2 than CO at this highly negative potential. However, under these conditions, all catalysts produced more reduced hydrocarbon products. Methane was present with all iron porphyrin catalysts at around 1.5%, with FeTTP having the highest selectivity of 2%. Only FeTPP-F20 and FeTTP yielded C2+ products, with FeTPP-F20 having the highest total C2+ faradaic efficiency of around 0.6%. The distribution of individual C2+ products is shown in Figure4-23.

Figure 4 - 22 .

 422 Figure 4-22. Product distribution observed with electrodes modified with various iron-based macrocycles as faradaic efficiencies after 2-hour electrolysis at -0.8 V vs. RHE (A) and -1.1 V vs. RHE (B-D). The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 4 -

 4 Figure 4-23. C2+ product distribution observed with electrodes modified with various iron-based macrocycles as faradaic efficiencies after 2-hour electrolysis at -1.1 V vs. RHE as a function of substituents of FePc. The total current density at 60 minutes is plotted as a blue dot for each catalyst. The amount of ethylene (C2H4) is underestimated due to the high background generated by CO2 in the chromatograph column.

Figure 6 - 1 . 6 . 2 Millifluidic cell assembly procedure 1 .

 61621 Figure 6-1. Actual photos of the 3D-printed electrochemical flow cell (A) and the flow cell in the experimental chamber of the LUCIA beamline (B).6.2 Millifluidic cell assembly procedure

Figure 6 - 2 .

 62 Figure 6-2. Millifluidic cell assembly 1 5. Stack the cathodic flow plate on top of the working electrode, with the gasket pressing down on the glassy carbon plate, and the catalyst layer is centered in the flow channel. Two pins are inserted into two alignment holes to hold the stack together during assembly (Figure 6-3A). 6. The Ag/AgCl mini reference electrode is inserted into the designated hole, protruding about 1 mm in the flow channel. The electrode is marked where it touches the cover so it can sit at the proper depth when assembled. This step is necessary when the electrode is new, or the mark is not visible. 7. Place a silicone gasket on the cathodic plate, similar to step 3. Wash the Nafion membrane with DI water, and put the membrane on top of the gasket, ensuring it is centered with the flow channel (Figure 6-3B). The gaskets that are not touching the working electrode are used in all experiments. 8. Place a silicone gasket on the anodic plate, similar to the previous step (Figure 6-3C).

Figure 6 - 3 .

 63 Figure 6-3. Millifluidic cell assembly 2 9. Place the anodic plate on top of the cathodic plate with the help of the alignment pins, making sure the membrane is sandwiched between the two gaskets. Then add a silicone gasket on top of the anodic plate. A nickel plate, acting as the counter electrode, is then placed on this gasket (Figure 6-4A). 10. Place the top cover on the assembly with the help of the alignment pins. Insert four M3 bolts (16 mm) in the designated holes (Figure 6-4B). Tighten the bolts in a criss-cross pattern to apply pressure evenly and seal the cell. The bolt holes are covered with copper tape to avoid the beam interfering with the iron in the bolts in a spectro-electrochemical experiment (Figure 6-2A). 11. Insert the reference electrode into its designated hole, with the mark in step 6 lined up with the cover. Gradually tighten the M3 (10mm) bolts to apply pressure and seal the connection (Figure 6-4C).

Figure 6 - 4 .

 64 Figure 6-4. Millifluidic cell assembly 3

Figure 6 - 6 .

 66 Figure 6-6. The electrochemical and gas detection setup used in this study

Fig. 1

 1 Fig.1Schematic side view of the electrochemical cell and simplified CO 2 reduction process on the surface of the working electrode. The dark green circles represent iron phthalocyanine (FePc), the molecular catalyst used in this study, whose structure is shown in the right corner. WE, CE, and RE stand for "working electrode", "counter electrode", and "reference electrode", respectively. The scheme is not drawn to scale.

Fig. 2 (

 2 Fig. 2 (A) Chronoamperometric curves and (B) module B gas chromatograms of FePc-modified electrodes under Ar in 0.1 M KCl (red) and CO 2 in 0.1 M KHCO 3 (blue) and an H 2 Pc-modified electrode under CO 2 in KHCO 3 (black). Chronoamperometric experiments are performed at -1.1 V vs. RHE for two hours, after which a gas chromatograph (GC) samples their resulting gaseous products.

Fig. 3

 3 Fig. 3 GC-MS chromatograms for mass m/z = 56 of 12 C 4 H 8 and mass m/z = 60 of 13 C 4 H 8 at the retention time of C 4 products. Chronoamperometric experiments were performed on FePc-modified electrodes at -1.1 V vs. RHE for two hours, after which the GC-MS sampled the resulting gaseous products. The chromatograms were averaged from three independent experiments.

Fig. 4

 4 Fig. 4 Product distribution observed as faradaic efficiencies with FePc-modified electrodes as a function of applied potential. The total current density is plotted for each potential as the blue lines. The amount of ethylene (C 2 H 4 ) is underestimated due to the high background generated by CO 2 in the chromatograph column.

Fig. 5

 5 Fig.5In situ and operando X-ray absorption spectra collected on an FePc-modified electrode under different conditions and on reference samples. The spectra at the top correspond to an FePc-modified electrode under a CO 2 atmosphere poised at of FePc (green), on a metallic sheet of iron (gold), and on a pellet of Fe 3 O 4 (purple). The two dashed lines indicate the pre-edge features of FePc at 7114 eV and 7118 eV. OCP (black), at -1.1 V vs. RHE (red) and going back at +0.8 V vs. RHE (blue). The spectra at the bottom correspond to ex situ reference spectra collected on a pellet.

Fig. 6

 6 Fig. 6 Simplified mechanisms proposed for the formation of C-C bonds by FePc with either CO 2 , CO, HCHO, or a mixture of HCHO and CO 2 as substrates. The colour code for substrates and products is as follow: blue, purple, and red for high, intermediate, and low concentrations, respectively.

Figure 1 (

 1 Figure 1

Figure 4 (

 4 Figure 4

Figure 1 .

 1 Figure 1. Structure (A) and cyclic voltamogram (B) of iron tetraphenyl porphyrin chloride [Fe III TPP(Cl)] in DMF/TBAPF 6 (0.1 M) under Ar (black line) and CO 2 (red line). Colored areas indicate the first (red), second (green) and third (yellow) electron transfer processes. CVs were recorded in a conventional electrochemical cell at room temperature and a scan rate of v = 100 mV s À 1 .

Figure 3 .

 3 Figure 3. Fe K-edge XANES spectra recorded on 2 mM solution of Fe III Cl 3 (blue line) and Fe II Cl 2 (red line) in DMF/TBAPF 6 (0.1 M) under Ar atmosphere. The spectrum of the Fe II Cl 2 species obtained after a 15 min constant potential electrolysis at À 0.6 V vs. Ag/Ag + under the same conditions is also shown (dotted purple line). Insert: Cyclic voltammogram of the 2 mM solution of Fe III Cl 3 • 6H 2 O in DMF/TBAPF 6 (0.1 M) under Ar atmosphere recorded in the X-ray spectroelectrochemical cell (v = 100 mV.s À 1 , room temperature).

2 a and 2 c

 2 are in a low-spin (S = 0) configuration, while 2 b is in an intermediate spin (S = 1) configuration. Such change in spin configuration explains the similarity between the XANES spectra of [Fe III TPP(Cl)] and 2 b (generated electrochemically in situ), the edge shift towards lower energies expected from the reduction of Fe(III) to Fe(II) being compensated by an upshift due to the spin change.

Figure 4 .

 4 Figure 4. A. Cyclic voltammogram of a 2 mM solution of [Fe III TPP(Cl)] in DMF/ TBAPF 6 (0.1 M) under Ar recorded in the X-ray spectroelectrochemical cell at v = 100 mV.s À 1 (room temperature). Reduction potentials applied during spectroelectrochemical measurements are indicated by black arrows. B. Fe K-edge XANES spectra recorded on [Fe III TPP(Cl)] solution, under the conditions described above (blue) and while applying a constant potential of E = À 0.8 V (red), À 1.6 V (green) and À 2.0 V vs. Ag/Ag + (orange). C. UV-Vis spectra (Soret Bands, left, Q-bands, right) for a solution of [Fe III TPP(Cl)] in DMF/TBAPF 6 (0.1 M) under Ar (blue) and electrochemically reduced at E = À 0.8 V (red), À 1.6 V (green) and À 2.0 V (orange) vs. Ag/Ag + respectively.

Figure 5 .

 5 Figure 5. A. Fe K-edge XANES spectra recorded on a 2 mM solution of [Fe III TPP(Cl)] in DMF/TBAPF 6 (0.1 M) under CO 2 (light blue) and while applying a constant electrode potential of E = À 0.8 V (magenta), À 1.6 V (light green) and À 2.0 V (brown) vs. Ag/Ag + respectively. B. UV-Vis spectra (Soret Bands on the left, Qbands on the right) of [Fe III TPP(Cl)] in DMF/TBAPF 6 (0.1 M) under CO 2 (light blue) and while applying a constant electrode potential of E = À 0.8 V (magenta), À 1.6 V (light green) and À 2.0 V (brown) vs. Ag/Ag + . C. Comparison of the Fe K-edge XANES and D. UV-Visible spectra recorded after controlled potential electrolysis at À 1.6 V vs. Ag/Ag + under Ar (green) and CO 2 (light green), respectively. E. Comparison of the Fe K-edge XANES and F. UV-Visible spectra recorded upon applying a constant electrode potential of E = À 2.0 V vs. Ag/Ag + under Ar (orange) and CO 2 (brown).

Figure 6 .

 6 Figure 6. Simplified catalytic cycle for the electrochemical reduction of CO 2 into CO by FeTPP in DMF with species 7 and 8 identified.

  Dr. D. Mendoza, S.-T. Dong, Dr. N. Kostopoulos, V. Pinty, Dr. O. Rivada-Wheelaghan, Dr. E. Anxolabéhère-Mallart*, Prof. M. Robert*, Dr.

Figure 1 .

 1 Figure 1. Structure of the MOF family MIL-100. The Fe(III) or Al(III) trimer is 6-coordinate Fe or Al (III) coordinating with bridging oxygen, water, trimesate, and hydroxyl ligands. Tetrahedra, made of Fe or Al(III) trimers on their vertices and trimesates on their faces, are arranged following the MTN framework, creating two types of mesoporous cages: large cages with 29 Å diameter, accessible through hexagonal (8.6 Å) and pentagonal windows (4.8-5.5 Å), and small cages with 25 Å diameter, accessible only through pentagonal windows. Adapted from Figure 1 from ref 11. Copyright 2021 MDPI.

  A 3 mg mL -1 aqueous solution of phosphated β-cyclodextrin CD-P sodium salt (average 3-4 phosphate groups per cyclodextrin unit) was added to a determined milligram of nanoMOF. The final nanoMOF concentration was 10 mg mL -1 . The amount of coating remaining in the supernatant after surface coating (m_coating_remaining) was determined by quantitative liquid-state NMR spectroscopy. The actual surface coating wt % was calculated as ×

Figure 2 .

 2 Figure2. Kinetics of trimesate release, represented by the proportion of trimesate detected in the supernatant after degradation over the initial trimesate content in nanoMIL-100(Al). Two degradation conditions were named "diluted" (0.5 mg mL -1 MOF in PBS 1 mM) and "concentrated" (10 mg mL -1 MOF in PBS 10 mM).

Figure 3 .

 3 Figure 3. TEM images of intact nanoMIL-100(Al) at (a) 10k and (b) 30k magnification. TEM images of degraded nanoMIL-100(Al) at (c) 10k and (d) 30k magnification. Degradation was carried out with the "diluted" condition: 0.5 mg mL -1 nanoMOF in PBS 10 mM.

Figure 4 .

 4 Figure 4. (a) 27 Al MAS NMR spectra of intact (solid black line), degraded (orange), and dehydrated (black dash line) nanoMIL-100(Al). The inset graph shows the 40 ppm region corresponding to the 4-and 5-coordinate Al(III). (b) 2D 31 P-27 Al 2D MAS NMR spectrum of degraded MOF in excess PBS.

Figure 5 .

 5 Figure 5. 27 Al MQMAS spectra of (a) original and (b) degraded nanoMIL-100(Al).

Figure 6 .

 6 Figure 6. 27 Al MAS NMR of the degraded nanoMIL-100(Al) at the "diluted" and "concentrated" conditions, demonstrating the effect of increasing the P/Al 3 ratio on the nanoMOF degradation reaction. The arrows indicate the position of 4-and 5-coordinate aluminum species.

1 )

 1 Phosphate ions in PBS attack the nanoMIL-100(Al) surface, substitute labile water ligands of MIL-100(Al), and form a coordination Al-O-P bond with MIL-100(Al). Consequently, the Al-O-C coordination bonds between the MIL-100(Al) trimer and trimesate linker are weakened.

Figure 7 .

 7 Figure 7. Proposed degradation mechanism of nanoMIL-100(Al) in phosphate buffer solution.

Figure 8 .

 8 Figure8.27 Al MAS NMR spectra of degraded original and degraded modified nanoMIL-100(Al), presenting the impact of drug encapsulation and surface coating on the nanoMOF degradation.

Figure 9 .

 9 Figure 9. 31 P MAS NMR of (a) degraded CD-P-coated MIL-100(Al) compared to CD-P-coated MIL-100(Al) and (b) degraded ATP-loaded-MIL-100(Al) compared to ATP-loaded-MIL-100(Al). CD-P and ATP are still present in the degraded nanoMOF after 4-5 days of degradation.

2 )

 2 Encapsulation of ATP inside nanoMIL-100(Al) impacts significantly the nanoMOF degradation. Degradation of ATP-loaded nanoMIL-100(Al) starts with the substitution of the trimesate Al-O-C bond, thus producing hexacoordinate aluminophosphate and free trimesate. However, the bulky ATP limits the amount of trimesate release. The coordination of PBS phosphate with the Al trimer facilitates the dissociation of the Al-O-P bond with ATP. Then, phosphate from PBS replaces the bonded ATP, leading to the release of free ATP. During the bond dissociation process, the coordinated ATP can make the second coordination bond with the degraded Al trimer.

Figure 10 .

 10 Figure 10. (a) Normalized Fe K-edge XANES spectra and (b) zoomed in pre-edge region of degraded nanoMIL-100(Fe) compared with original nanoMIL-100(Fe) and iron(III) phosphate dihydrate.

Figure 11 .

 11 Figure 11. (a) Normalized Fe K-edge XANES spectra and (b) zoomed in pre-edge region of degraded nanoMIL-100(Fe) compared with several Fe(III) possible degradation products, such as iron phosphate, iron oxides, and iron oxyhydroxide.

Figure 12 .

 12 Figure 12. (a) Normalized Fe K-edge XANES spectra of degraded original and modified nanoMIL-100(Fe) and (b) zoomed in pre-edge region.

1 The big picture: Climate change, renewable energy, and energy storage solutions

  The big picture: Climate change, renewable energy, and energy storage solutions ..... 1.1.1 Energy sources, greenhouse gas emissions, and climate change ................................... 1.1.2 Renewable energy sources to mitigate CO2 emissions ....................................................... 1.1.3 Artificial photosynthesis ................................................................................................................... 1.1.4 CO2 reduction as energy storage and chemical producing technology ....................... 1.2 Overview of CO2 reduction .................................................................................................................... 1.2.1 History and basics of CO2 reduction ........................................................................................... 1.2.2 Products of CO2 reduction .............................................................................................................. 1.2.3 CO2 reduction methods .................................................................................................................... 1.3 Electrocatalytic reduction of CO2 ...................................................................................................... 1.3.1 Overview and basic concepts ....................................................................................................... 1.3.2 Metal catalysts ...................................................................................................................................

	1.1

1.3.3 Homogeneous molecular catalysts............................................................................................ 1.3.4 Heterogeneous reduction of CO2 by molecular catalyst .................................................. 1.3.5 Reduction of CO2 to C2+ products by catalysts with well-defined active sites ......... 1.3.6 Importance of in-situ characterization and isotopic labeling ......................................... 1.4 Thesis outline ............................................................................................................................................ 1.5 Bibliography .............................................................................................................................................. 1.1.1.1 Energy sources, greenhouse gas emissions, and climate change

  26

Table 1 -

 1 1. Standard CO2 reduction processes and their thermodynamics potentials

	Reduction process	𝑬 𝟎 𝒗𝒔 𝑺𝑯𝑬/𝑽
	𝑪𝑶 𝟐 (𝒈) + 𝒆 -→ 𝑪𝑶 𝟐 •-(𝒈)	-1.90 𝑉
	𝑪𝑶 𝟐 (𝒈) + 𝟐𝑯 + + 𝟐𝒆 -→ 𝑪𝑶(𝒈) + 𝑯 𝟐 𝑶(𝒍)	-0.11 𝑉
	𝑪𝑶 𝟐 (𝒈) + 𝟐𝑯 + + 𝟐𝒆 -→ 𝑯𝑪𝑶𝑶𝑯(𝒍)	-0.25 𝑉
	𝑪𝑶 𝟐 (𝒈) + 𝟒𝑯 + + 𝟒𝒆 -→ 𝑯𝑪𝑯𝑶(𝒍) + 𝑯 𝟐 𝑶(𝒍)	-0.07 𝑉
	𝑪𝑶 𝟐 (𝒈) + 𝟔𝑯 + + 𝟔𝒆 -→ 𝑪𝑯 𝟑 𝑶𝑯(𝒍) + 𝑯 𝟐 𝑶(𝒍)	+0.02 𝑉
	𝑪𝑶

𝟐 (𝒈) + 𝟖𝑯 + + 𝟖𝒆 -→ 𝑪𝑯 𝟒 (𝒈) + 𝑯 𝟐 𝑶(𝒍) +0.17 𝑉 𝟐𝑪𝑶 𝟐 (𝒈) + 𝟏𝟐𝑯 + + 𝟏𝟐𝒆 -→ 𝑪𝑯 𝟐 𝑪𝑯 𝟐 (𝒈) + 𝟒𝑯 𝟐 𝑶(𝒍) +0.06 𝑉 𝟐𝑪𝑶 𝟐 (𝒈) + 𝟏𝟐𝑯 + + 𝟏𝟐𝒆 -→ 𝑪𝑯 𝟑 𝑪𝑯 𝟐 𝑶𝑯(𝒍) + 𝟑𝑯 𝟐 𝑶(𝒍) +0.08 𝑉

  𝐿 (m) is the characteristic linear dimension, and 𝑢 (m/s) is the flow speed of a fluid particle. In a rectangular channel, the characteristic linear dimension is represented by the hydraulic diameter 𝐷 𝐻 =

90 × 10 -4 𝑃𝑎. 𝑠 × 5.1 × 10 -3 𝑚 1.27 × 10 -3 𝑚/𝑠 ≈ 2.6 where 𝜌 (kg/m 3 ) is the density of the fluid, 𝜇 (kg/(m.s)) is the dynamic viscosity of the Chapter 2 fluid, 4𝐴 𝑃

Table 2 -

 2

	Parameters	Value	Unit
	Tubing radius	0.375	mm
	Tubing length	7	mm
	Flow channel width	5	mm
	Flow channel length	15	mm
	Flow channel thickness	2.5	mm
	Gasket thickness	0.127	mm
	2.4.2 Multiphysics: Laminar flow and transport of diluted species

1. Dimension parameters for modeling the electrochemical flow cell

Table 2 -

 2

	Parameters		Value	Unit	Reference
	CO2 diffusion coefficient at 25 o C	1.92 x 10 -9 m 2 /s	Ager et al. 5
	CO diffusion coefficient at 25 o C	2.03 x 10 -9 m 2 /s	Ager et al. 5
	CO2 saturated concentration in	33.4	mM	Ager et al. 5
	water at 25 o C
	Current density at -1.1 V vs. RHE	2.52	mA/cm 2	Experimental
	Faradaic efficiencies at -	CO	23.25	%	Experimental
	1.1 V vs. RHE	CH4	3.81
		C2H6	0.22
		C3H6	0.21
		C3H8	0.11
		C2H4	0.1

2. Physics parameters used for laminar flow and transport of diluted species physics

  75 V vs. RHE. The oxidation

	Powder
	Before
	After
	(a.u.)
	Intensity
	1800 1600 1400 1200 1000 800

Table 3 -

 3

	Parameters	Value
	Inlet temperature	120 o C
	Capillary temperature	150 o C
	Dwell time	200 ms
	SEM voltage	900 V
	Resolution	50
	Pause calibrate	1.00

1. Mass spectrometry method parameters

Table 4

 4

	-1 summarizes the pH and the applied potentials on
	the RHE scale. Other conditions were kept similar to experiments with FePc described
	in Chapter 3 and the annex, with a catalyst loading of 80 nmol/cm 2 and a flow rate of
	1 mL/min.

Table 4 -

 4 1. pH and applied potential on RHE scale as a function of KHCO3 concentration

	KHCO3 concentration	Initial	pH after 15-minute CO2	V vs. RHE at -1.7 V vs.
	(M)	pH	bubbling	Ag/AgCl
	0.02	8.76	5.87	-1.15
	0.1	8.75	6.80	-1.09
	0.5	8.55	7.43	-1.06
	1	9.50	7.76	-1.04
	2.5	9.39	9.08	-0.96
	Figure 4-4 shows the current densities and faradaic efficiencies of CO, H2, CH4, and C2+
	products obtained after the 2-hour electrolysis experiments at different electrolyte
	concentrations. As observed in Figure

Table 6 -

 6 1. summarizes the parameters For electrodes with different parameters: FePc concentration, carbon black concentration, different metal phthalocyanines, different substituted FePc, and FeTPP and its derivatives.

Table 6 -

 6

	2. Gas chromatograph method parameters

  This article reviews the work done by the group of Koper on the effects that electrolytes (including cations, anions and acids) can have on the rate and selectivity of the CO 2 RR. ACS Catal 2020, 10:14871-14876, https://doi.org/10.1021/ acscatal.0c03553. This paper uses in situ IR spectroscopy to show how the nature and concentration of cations can influence the efficiency of the CO 2 RR.

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Table 1 .

 1 Identification of the species generated and studied in this study.

	e À transferred	Ar	CO 2
	0	1	5
	1	2 a, 2 b, 2 c	6
	2	3	7
	3	4	8

Table 2 .

 2 XANES pre-edge and main edge energy positions (in eV) and UV-Visible peak positions (in nm) for compounds 1-8.

	e À transferred	Complex	Pre-edge [a]	Main edge [b]	Soret band(s)	Q-band(s)
	Ar saturated solution
	0	1	7113.9	7123.0	416	509, 575
	1	2 a	7113.6	7123.0	432	530, 565, 603
	2	3	7113.5, 7118	7122.3	392, 425	510, 575, 608
	3	4	7113.6, 7118	7122.0	362, 433	514, 607
	CO 2 saturated solution
	0	5	7113.9	7123.0	417	508, 575
	1	6	7113.6	7123.0	430	535, 567, 609
	2	7	7113.5	7123.0	392, 425	510, 575, 608
	3	8	7115.0	7123.7	395, 424	510, 575, 608

  P, 1 H-31 P, and Institut des Sciences Moléculaires d'Orsay, Université Paris-Saclay, CNRS, 91405 Orsay, France; orcid.org/0000-0002-7869-0908; Email: ruxandra.gref@ universite-paris-saclay.fr Charlotte Martineau-Corcos -Institut Lavoisier de Versailles,

	1 H-27 Al (PDF)
	■ AUTHOR INFORMATION Corresponding Authors
	Ruxandra Gref -

Université Paris-Saclay, UVSQ, CNRS, 78000 Versailles, France; Present Address: CortecNet, 7 avenue du Hoggar, 91940 Les Ulis, France (C.M.-C); orcid.org/0000-0003-1887-1042; Email: ccorcos@cortecnet.com

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https://doi.org/10.1021/acs.chemmater.2c01190 Chem. Mater. 2022, 34, 8178-8189

We performed linear combination fitting with two electrodes containing mixtures of FePc and Fe3O4 to quantify each species' actual contribution to the obtained iron Chapter 3

Acknowledgements

We express our gratitude to Dr Jean-Blaise Brubach of the AILES beamline of Synchrotron SOLEIL for his help during infrared spectra collection. We are indebted to Ms. Chanjuan Zhang and Dr Zakaria Halime at the Institut de Chimie Moléculaire et des Matériaux d'Orsay, Université Paris-Saclay for their assistance in performing 1 H-NMR measurements. The Agence Nationale de la recherche is acknowledged for a young researcher grant to B. L.-K. (grant no. 18-CE05-0007).

Acknowledgements

The Agence Nationale de la Recherche is acknowledged for a starting grant to BLK and STD (n 18-CE05-0007) and a Michem/synchrotron SOLEIL LABEX doctoral fellowship to DM.

Acknowledgements

We are indebted to the Michem Labex and Synchrotron SOLEIL for a PhD grant to D. M. The peer-review committees of Synchrotron SOLEIL are greatly acknowledged for beamtime allocation. M. R. is thankful to the Institut Universitaire de France (IUF) for partial financial support. O. RÀ W acknowledges financial support from the Investissement l'Avenir through the MOPGA call N°ANR-18-MPGA-0012.

Data availability

The authors conrm that the data supporting the ndings of this study are available within the article and its ESI material. † Raw data that support the ndings of this study are available from the corresponding author, upon reasonable request.

Data availability

No data was used for the research described in the article.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

A nickel plate (99.9% trace metal basis, Sigma-Aldrich) with a dimension of 25 x 10 x 0.5 mm 3 is used as the counter electrode. The electrode is polished gradually with 9 µm, 6 µm, and 3 µm liquid diamond suspensions (Bio DIAMANT®) and repolished with 3 µm liquid diamond suspension after each electrolysis experiment.

Author contributions

Conceptualization: STD, BLK investigation: STD, CX funding acquisition: BLK supervision: BLK writingoriginal dra: STD, BLK writingreview & editing: STD, BLK.

Conflicts of interest

There are no conicts to declare.

vs. bulk cations would therefore be very difficult, unless grazing incidence [62] or microfluidic electrochemical cells [63] are used.

Soft X-rays XANES X-ray absorption spectroscopy in the soft X-ray range is the primary tool to study light elements, whose K-edge ionization energy lies approximately between 200 and 1000 eV. As far as CO 2 reduction is concerned, the main elements of interest, i.e. carbon and oxygen, have their K-edges at 284 and 543 eV. At these energies, the penetration depth of X-rays is very low, with an attenuation length of ca 100 nm (at 284 eV) and 250 nm (at 543 eV) in silicon nitride (Si 3 N 4 ), a material that is very often used as a vacuum compatible membrane for liquid or electrochemical cells in the soft X-ray energy range.

In a solvent with a density of 1, these numbers increase slightly but never go above the micron. This is a drawback when it comes to designing and operating in situ electrochemical cells but becomes an advantage when considering the volume of sample probed. Indeed, a thin film of electrocatalytic material deposited on an electrode will contribute to a much larger extent to the signal than the bulk of the solution. By subtracting or comparing the blank signal of a solvent/electrolyte component with the signal of an electrocatalytic component under in situ conditions, it is possible to extract the contribution of light components (e.g. solvent, electrolyte, CO 2 or its reduction products) in contact with the catalyst. This procedure is nevertheless tedious and requires spectroelectrochemical cells specifically designed for the soft X-rays energy range, which implies compatibility to vacuum and a very thin membrane/electrode assembly to allow X-rays to penetrate.

Such cells exist [64e66] but, as far as we know, have only been used once for the study of the CO 2 reduction reaction under operating conditions [67]. The conditions used for such measurements are, however, quite far from potential applications, since they require the use of a very thin (5e50 nm) conductive layer of noble metals (typically Au) to provide conductivity to the X-ray transparent window.

The ability to probe the oxidation state of carbon under catalytic conditions is a very powerful tool that should provide very valuable information concerning the CO 2 reduction reaction. Indeed, the C K-edge signals of CO 2 , CO and more reduced forms of carbon are very distinctive. This technique could therefore be used to probe the presence of specific adsorbed species such as M-CO. There are several hypotheses as to the binding mode of CO 2 on catalytic sites prior to its reduction [68,69], with terminal, bridging, C-bound or O-bound configurations and several combinations of these modes (see Figure 2).

A combination of XAS data at the carbon and oxygen Kedges collected under in situ conditions should provide the information required to discriminate between these species. For example, Yan et al. studied the adsorption of CO 2 on CeO 2 with C K-edge XANES and identified two different adsorbates, i.e. CO 3 2À and CO 2 depending on the nature of the surface [70]. Another study used C Kedge XANES to show that CO binds much more strongly on nanostructured Au than on its flat counterparts [71]. In a somewhat opposite concern to the CO 2 reduction reaction, Knop-Gericke and collaborators studied the oxidation of methanol on copper with in situ soft x-ray spectroscopy [72]. Thanks to the large energy difference observed for carbon species (see Figure 3b for a similar example), it was possible to deconvolute the spectra obtained in situ and quantify each component. Measurements at the O K-edge have never been performed on CO 2 -reducing systems but could prove useful to determine the presence of oxide phases in metallic catalysts and their potential role in catalysis.

Another significant advantage of the soft X-ray range is that it also allows probing the L-edges of transition metals used as catalysts, which contains a wealth of information on their structure. This possibility has been used by De Luna et al. to study the structural and morphological evolution of a copper-based CO 2reducing electrocatalyst [67].

X-ray Raman

Another X-ray technique that can probe light elements while avoiding the experimental constraints imposed by soft X-rays is X-ray Raman Scattering (XRS) [73]. This method uses hard X-ray photons that interact with the sample and are collected after losing an amount of energy corresponding to the light element of interest. It requires a spectrometer (see Figure 3a) that can collect a large fraction of the solid angle around the sample, since the amount of photons emitted through this process is very low. It has been used to probe light elements embedded into complex matrices involving higher elements, such as glasses [74], tissues [75] or catalysts [76].

Because of the high penetration depth of the hard X-rays used, it is very appropriate for in situ experiments in which experimental conditions cannot be adapted to the vacuum environment required for soft X-rays. This type of experiments have been used by Moya-Cancino et al.

to determine the nature of carbon (both as a support, a reactant and a ligand) in FischereTropsch catalysts under operating conditions [76]. Interestingly, these authors used XRS to probe the L 3,2 -edges of cobalt as well, demonstrating the formation of a carburized cobalt species under operating conditions. XRS could therefore be very useful to track the fate of carbon or oxygen (contained in reactants, products or other components of the reaction) in the electrocatalytic reduction of CO 2 , under conditions that are closer to the potential application than those used for soft X-rays experiments.

It should, however, be noted that XRS is a poorly efficient spectroscopic process, thus requiring high catalyst loading and/or measurement times. Notwithstanding the advantages of the experiments mentioned above, they remain challenging and require a significant instrumental and methodological expertise.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

In situ X-ray Absorption Spectroscopy in Homogeneous Conditions Reveals Interactions Between CO 2 and a Doubly and Triply Reduced Iron(III) Porphyrin, then Leading to Catalysis

Daniela Mendoza, [a, b] Si-Thanh Dong, [a] Nikolaos Kostopoulos, [b] Victor Pinty, [a] Orestes Rivada-Wheelaghan, [b] Elodie Anxolabéhère-Mallart,* [b] Marc Robert,* [b, c] and Benedikt Lassalle-Kaiser* [a] In memory of Doris Lexa

Iron porphyrins are attractive catalysts for the electrochemical reduction of carbon dioxide (CO 2 ), owing to their high activity and selectivity while being tunable through ligand functionalization. Iron tetraphenyl porphyrin (FeTPP) is the simplest of them, and its catalytic behavior toward CO 2 has been studied for decades. Although kinetic information is available, spectroscopic signatures are lacking to describe intermediate species along the catalytic cycle. In situ UV-Visible and X-ray absorption near edge spectroscopy (XANES) were used to monitor the local and electronic structure of FeTPP homogeneously dissolved in dimethyl formamide (DMF) under reductive potentials. The Fe(III) starting species was identified, together with its one, two and three electron-reduced counterparts under both argon and CO 2 atmospheres. Under argon, the second and third reductions lead to species with electronic density shared between the metal and the porphyrin backbone. In the presence of CO 2 and with a low amount of protons, the doubly and triply reduced species interact with CO 2 at the metallic site. In light of these results, an electronic structure for a key intermediate along the catalytic cycle of the CO 2 -to-CO reduction reaction is proposed.

Introduction

The massive use of fossil fuels to supply the global energy demand has increased the CO 2 atmospheric levels to alarming values, as recently presented by the Intergovernmental Panel on Climate Change (IPCC). [1] Research has shown that CO 2 can be electrochemically reduced into carbon building blocks such as carbon monoxide (CO), which is the first and most critical step in the CO 2 upcycling process. Because of the high chemical stability of CO 2 , [2] the CO 2 -to-CO conversion is energetically very costly, requiring the use of efficient catalysts to lower energetic barriers.

Several molecular catalyst families based on earth-abundant transition metals have been proposed as efficient CO 2 -to-CO reducing catalysts. They offer good catalytic activities, both in terms of efficiency and selectivity, and provide convenient platforms for molecular engineering. [3][4][5] Among them, iron porphyrins, such as iron tetraphenyl porphyrin (FeTPP, Figure 1A), have been reported as a class of selective, stable, and efficient catalysts for the reduction of CO 2 into CO, both in aprotic solvents and water. [6][7][8][9][10][11][12] Doris Lexa, together with Jean-Michel Savéant and their collaborators, pioneered the electrochemical study of iron porphyrins and their reduced counterparts as early as the'80s. [6,13,14] Using cyclic voltammetry (CV), they showed that, in dimethyl formamide (DMF) and under inert conditions, FeTPP undergoes three consecutive reduction processes (see Figure 1B), which correspond to the reversible storage of three reducing equivalents. In the presence of CO 2 , the third reduction process becomes irreversible and increases in intensity, which is due to the catalytic reduction of CO 2 into CO following Equation (1):

Despite numerous investigations of the CO 2 -to-CO reaction mechanism via kinetic analysis, [13][14][15][16][17][18] the exact nature and structure of the intermediate species involved still need to be deciphered. There is, however, no doubt that understanding the local and electronic structure of such intermediates is required to optimize their catalytic activity. In this regard, early work by Lexa and co-workers showed that the reactivity of the three-electron-reduced species of iron porphyrins is centered on the metal rather than on the ligand. [19] These observations,

Conflict of Interest

The authors declare no conflict of interest.

Degradation Mechanism of

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Financial support from the IR-RMN-THC Fr3050 CNRS for conducting the research is gratefully acknowledged. This work was also supported by the Paris Ile-de-France Region -DIM "Respore". The present work has benefited from Imagerie-Gif core facility supported by l'Agence Nationale de la Recherche (ANR-11-EQPX-0029/Morphoscope, ANR-10-INBS-04/ FranceBioImaging; ANR-11-IDEX-0003-02/ Saclay Plant Sciences). We also thank the beamline scientists and staffs at the SAMBA beamline for assistance. M.P. and C.M.-C thank Dr. Vincent Sarou-Kanian (CEMHTI Orléans) for support during the high-field NMR experiments.