6 + 4H 2 O -0.27 Scheme1.1 Two possible mechanisms for the reduction of CO 2 to methane

Keywords: iron porphyrin, CO 2 reduction, photochemistry, homogeneous catalysis, molecular catalyst, solar fuels

Photocatalytic reduction of CO 2 into value-added chemicals and fuels could not only contributes to mitigating CO 2 emissions, but also provides valuable fuels and thus enhances energy security in light of the depletion of fossil resources and the strong fluctuation of oil prices.

At the same time, it will provide a new approach for the storage of renewable energy. It is like killing three birds with one stone.

Iron porphyrins have been shown to be the most efficient electrochemical CO 2 -to-CO conversion molecular catalysts in our lab. We introduced four trimethylammonio groups on the para position of the phenyl rings of an iron porphyrin which makes the complex active under visible light for the photochemical reduction of CO 2 to CO, without additional photosensitizer.

When combining with a simple organic dye (namely purpurin) as the photosensitizer, CO is produced with a catalytic selectivity of 95% and turnover number up to 120 in aqueous solutions under visible light irradiation. The catalysis is ultimately limited by progressive sensitizer degradation but it could be restarted by adding fresh sensitizer. This work opens new perspectives for developing efficient CO 2 reduction photocatalysis in water. On the other hand, if the same catalyst is sensitized by a much stronger sensitizer, such as Ir(ppy) 3 (ppy = 2-phenylpyridine), not only the absolute amount of CO increases, but CO 2 can also can be converted into CH 4 . This complex complicated eight-electron and eight-proton catalytic process is efficiently driven by visible light irradiation at ambient temperature and pressure. CO 2 could be selectively reduced into CO and the later gas is transformed to CH 4 with an overall 82% selectivity, the production of which was sustained over several days. As far as we know, it's the first efficient homogenous photochemical CO 2 -to-CH 4 catalytic system. When Ir(ppy) 3 was replaced by Ir(ppy) 2 (bpy)PF 3 in a aqueous solution, the process of CO 2 -to-CH 4 conversion continued, while the process of CO-to-CH 4 conversion was inhibited thanks to the deactivation of the photosensitizer. Finally, a phenoxazine-based organic photosensitizer was found to drive the whole catalysis. Above all, the absolute mol number of methane produced in this non-noble-element system is even 2 times higher than the Ir sensitized system.

II

Résumé

La réduction photocatalytique du CO 2 en composés à haute valeur ajoutée ou en carburants pourrait non seulement contribuer à la limitation des émissions de CO 2 mais également sécurises une ressource énergétique et pallier la baisse des ressources naturelles et les fluctuations de prix. Dans le même temps, cela ouvrirait une nouvelle voie pour le stockage des énergies renouvelables. En résumé, cela reviendrait à faire d'une pierre trois coups. Il a été montré au sein du laboratoire que les porphyrines de fer étaient les catalyseurs moléculaires les plus efficaces pour la conversion électrochimique de CO 2 en CO. Nous avons introduit quatre groupements triméthylammonium dans les positions para des cycles phényles d'une porphyrine ce qui permet de la rendre activable par la lumière visible et donc pour la réduction photostimulée de CO 2 en CO, sans l'aide d'un sensibilisateur externe. Associée à un colorant organique simple (la purpurine) en guise de sensibilisateur, CO est obtenu avec une sélectivité catalytique de 95% et un nombre de cycles catalytique de 120 en solution aqueuse sous lumière visible. La catalyse est cependant limitée par la dégradation progressive du sensibilisateur mais elle peut être restaurée par un ajout de sensibilisateur frais. Ce travail ouvre de nouvelles perspectives pour le développement de la réduction catalytique photochimique de CO 2 dans l'eau. Par ailleurs, nous avons montré que ce même catalyseur, associé à un sensibilisateur plus réducteur, à savoir un complexe d'iridium (Ir(ppy) 3 ), permettait d'obtenir un nombre de cycles catalytiques en CO plus élevé, mais également de parvenir à la réduction de CO 2 en CH 4 . Ce processus complexe, impliquant le transfert de 8 électrons et 8 protons, a été conduit de manière efficace sous lumière visible et à pression et température ambiantes. CO est d'abord obtenu sélectivement puis transformé en CH4, de manière durable sur plusieurs jours, avec une sélectivité globale de 82%. A ce jour, ceci constitue la première démonstration de faisabilité de cette transformation en catalyse photochimique homogène. Lorsque le complexe d'iridium est remplacé par un complexe analogue soluble en solution aqueuse, la conversion de CO 2 en CH 4 est également observé mais pas celle de CO en CH 4 par dégradation du sensibilisateur. Enfin, de nouveaux sensibilisateur organiques de type phénoxazine ont été utilisés avec succès pour ce processus, permettant d'obtenir des quantités absolues de méthane 2 fois plus élevées tout en n'employant plus d'élément rare et cher.

Mots-clés: porphyrine de fer; réduction du CO 2 ; photochimie; catalyse homogène; catalyseur moléculaire; carburants solaires. Fossil fuels, as one of the most important primary energy resources, make great contribution to the development of human beings since Industrial Revolution. However, due to the worldwide increasing demand and the accompanying excessive emissions of carbon dioxide (one of the main greenhouse gases), energy shortage and global warming present to be two main crises for the sustainable development of human society in the 21 st century. The consumption of abundant fossil fuels, particularly coal and oil, becomes more accessible with the developing technology. Two hundred years ago, global energy consumption was almost exclusively in the form of biofuels such as crop straw and wood; however, the use of coal first and then oil and natural gas has skyrocketed, particularly in the past century. Fossil fuel usage accounts for the vast majority due to several factors, including abundance, availability, energy density, storability and transportability. In the International Energy Outlook 2017 (IEO2017)

Reference case, world consumption of marketed energy from all fuel sources increased, except coal, for which demand is essentially flat through 2040 (Figure 1.1). [START_REF]International Energy Outlook Executive Summary[END_REF] Renewable energies are the world's fastest-growing sources, with consumption increasing by an average 2.3%/year between 2015 and 2040. The world's second fastest-growing source of energy is nuclear power, with consumption increasing by 1.5%/year over that period. Although consumption of non-fossil fuels is expected to grow faster than fossil fuels, the latter still account for 77% of energy use in 2040.

In other words, fossil fuels will still be the main energy sources for quite some time.

At the very beginning, the natural carbon cycle driven by sunlight, together with the water and nitrogen cycles, is one of the main processes allowing life to take place. On earth, carbon exists in various reservoirs. The main atmospheric reservoir is in the form of gaseous CO 2 . But carbon is also bound on land in living vegetation, in soils and in carbon containing minerals, whereas in the oceans, carbon exists in equilibrium between dissolved CO 2 , bicarbonates and carbonates.

The carbon cycle links these reservoirs by various fluxes, as shown in Figure 1.2. [START_REF] Ciais | Carbon and other biogeochemical cycles[END_REF] However, the red numbers greatly impacted the fluxes in the earth's carbon cycle and have thereby significantly disturbed its equilibrium. In particular, the widespread utilization of fossil fuels has led to a drastic increase in carbon dioxide emissions. a drastic increase of ~45.7%. [START_REF] Solar | Oxidation of Tryptophan and N-Methylindole by N 3 -, Br 2 .-, and (SCN) 2 .-Radicals In Light and Heavy-Water Solutions: A Pulse Radiolysis Study[END_REF] In the past 60 years, it increased about 25% and it is currently rising by about 2 ppm/year. What is worse, this growth trends will keep going even faster if we do nothing. The massive quantities of carbon dioxide, about a quarter of the total emitted since the start of the Industrial Revolution, have entered the oceans. While this dissolution of CO 2 has mitigated the change in atmospheric carbon, it has led to detectable acidification of the oceans because it forms carbonic acid, threatening the vast biodiversity of that ecosystem. In addition to ocean acidification, the rising of atmospheric CO 2 levels have been associated

with the strengthening of the greenhouse effect and have been implicated greatly in climate change. [START_REF] Hartmann | Observations: atmosphere and surface[END_REF] The increase of carbon dioxide, which will absorb and re-emit infrared radiation strongly, leads to a huge decrease in the net amount of energy radiated out of the atmosphere by the Earth. As a consequence, the global temperature has been trending upward relative to longterm patterns, particularly in the past several decades, with a surface temperature deviation of 1.3 °C from the pre-industrial levels already. [START_REF] Amenda | Solubilities of the common L-alpha-amino acids as a function of temperature and solution pH[END_REF] The most optimistic of predicted scenarios, in which CO 2 emissions peak immediately and subsequently decline, anticipates a total temperature change of about 1.0 °C by the end of the twenty-first century. [START_REF] Collins | Chapter 12: Long-term Climate Change: Projections, Commitments and Irreversibility-Final Draft[END_REF] According to the International Panel on Climate Change (IPCC), a CO 2 level above 450 ppm carries a high risk of causing global warming by more than 2 °C. If the temperature change can be limited to less than 2 °C, there is a good chance that human society can adapt. [START_REF] Oppenheim | Climate change and the economy: myths versus realities[END_REF] Unless we are able to radically reduce CO 2 emissions within the next 20 years, exceeding the 450 ppm level seems unavoidable. The most pessimistic projection, but unfortunately the most politically and economically likely at present, in which CO 2 emissions continue to rise throughout the century as energy demands increase, predicts a mean temperature change of over 4 °C by 2100. Such drastic global temperature shifts would cause massive changes in climate, ice coverage, sea levels, and plant and animal diversity.

A lot of low elevation countries and regions will disappear permanently from the earth. Human society, faced with the numerous agricultural, economic crises and geopolitical that would unfold because of these transformations, would be forced to adapt quickly to the rapidly changing world.

Figure 1.4 Land-ocean temperature indexes, 1880 to present, with base period 1951-1980. The solid black line is the global annual mean and the solid red line is the five-year lowest smooth. The blue uncertainty bars (95% confidence limit) account only for incomplete spatial sampling. [START_REF] Amenda | Solubilities of the common L-alpha-amino acids as a function of temperature and solution pH[END_REF] Human being's production and living cause these issues because it does not form a closed cycle but instead simply consists in the rapid depletion of fossil carbon reservoirs. If the same fuels, however, had been generated by fixing carbon from the atmosphere, namely no net increase of atmospheric carbon dioxide concentration would occur, bringing about the system entirely carbon neutral. The production of fuels from CO 2 and water is practically feasible but requires energy. If the invested energy derives from renewable resources, e.g. solar energy, the production of fuels from CO 2 opens the door to a closed and sustainable anthropogenic carbon cycle, the Holy Grail to solving the issues of global warming and energy crisis. The conversion of carbon dioxide into small molecules using solar energy would thus be like killing two birds with one stone.

Solar energy and artificial photosynthesis

The generalized solar energy is the source of much energy on earth, such as wind energy, chemical energy, water potential energy and so on. Fossil fuels can be called ancient solar energy.

Solar energy is extremely rich, environmentally friendly and omnipresent. Solar irradiation gives rise to biological processes taking place on Earth all the time. Through a complex cyclic process, solar energy is absorbed by chlorophyll molecules in green plants and then used to drive the transformation of water and carbon dioxide into carbohydrates, which serve as the source of energy for all life. This process is called photosynthesis, in which molecular oxygen is released as a byproduct, allowing for aerobic life forms to exist. Inspired by this, many research efforts have contributed to replicating the efficient natural process of photosynthesis by using mimetic or bioinspired model systems under ambient conditions. Solar energy is of great potential and it is considered to be the only source that can meet all of our energy needs. Total solar power reaching earth's surface is estimated to be 120, 000 TW. [START_REF] Lewis | Powering the planet: Chemical challenges in solar energy utilization[END_REF] To generate 20 TW of power from the Sun, the area to be covered by 10% efficient solar cells is about 816, 000 km 2 , which corresponds to an area of about 900 × 900 km [START_REF] Ciais | Carbon and other biogeochemical cycles[END_REF] and accounts for 0.16% of earth's surface. While this appears relatively small when projected onto Africa (Figure 1.5), it actually is equal to the total area of France and Germany combined. If these solar cells work 4 h every day for 365 days per year, they will produce 2.9×10 13 kWꞏh/yr electricity, which is much more than the whole world electricity consumption in 2014 (2.2×10 13 kWꞏh/yr). [START_REF] Bartocci | Iron mesotetra(2,6-dichlorophenyl)porphyrin as a very efficient catalyst for the photoreduction of carbon tetrachloride by alcohols[END_REF] As the contribution of solar energy to the total energy mix increases, it will become difficult for electricity network operators to cope with the intermittent nature of solar power (day/night cycle, clouds, and seasons). At a certain point, grid-based storage capacities will be exceeded and large-scale energy storage solutions will need to be implemented. One of the more attractive possibilities is to store solar energy in chemical bonds, such as in molecular hydrogen, methane and so on. The energy of a visible-light photon range between 1.5 and 3.1 eV, or 150-310 kJ/mol, which is more than sufficient for many chemical synthesis routes. Compared to, i.e., batteries and mechanical or gravity-based storage systems such as flywheels and pumped water reservoirs, chemical fuels combine the advantages of high energy storage densities and ease of transportation.

Inspired by green plants, artificial CO 2 chemical recycling, as a hot research topic at the turn of the century, plays an essential role in preventing CO 2 release in the atmosphere and provides a novel energy source simultaneously through efficient CO 2 reduction.

However, the solar spectrum spreads from photons in the UV range where the wavelengths start at ~250 nm (equals to photon energy of 4.96 eV) to photons in the IR range where the wavelengths can be 2500 nm (equals to photon energy of 0.496 eV). Photon wavelengths, their frequencies and their energies relationship follow Max Planck's equation [START_REF] Planck | On the law of distribution of energy in the normal spectrum[END_REF] :

E p = hν = (1)
where E p is the energy of photon, h is Planck's constant and equals to 6.63 × 10 -34 Jꞏs, ν is photon frequency, λ is photon wavelength and c is the speed of light which approximately equals to 3.00 × 10 8 m/s.

UV light is strong enough to activate the photocatalyst (A substance is able to achieve chemical catalysis under illumination.) while it only accounts for about 5% of the total solar irradiance (Figure 1.6). [START_REF] Faunce | Energy and environment policy case for a global project on artificial photosynthesis[END_REF] IR light accounts for about 50%, but its energy does not suffice to activate photocatalysts. Visible light, which accounts for about 45%, will be an eclectic choice to be used for artificial photosynthesis. Its photon energy ranging from 1.63 eV to 3.26 eV is strong enough and the energy intensity is also appropriate. In a typical solar simulated system, a Xenon lamp, with a cut-off filter to shield UV light and a water filter to shield IR light, is usually employed as the light source.

Figure 1.6 Solar spectrum with the x-axis in wavelength (nm) unit and in energy (eV) unit. Data was obtained from National Renewable Energy Laboratory (NREL) at link http://rredc.nrel.gov/solar/spectra/am1.5/astmg173/astmg173.html. [START_REF] Faunce | Energy and environment policy case for a global project on artificial photosynthesis[END_REF] Thomas Edison once said, "I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that." In the past decades, a range of ever-evolving technologies such as solar heating, photovoltaics, solar thermal energy, solar architecture, molten salt power plants and artificial photosynthesis have been developed. Artificial photosynthesis is commonly used to refer to any scheme for capturing and storing the energy from sunlight in the chemical bonds of a fuel, mainly including photocatalytic water splitting and light-driven carbon dioxide reduction. Solar energy capture and storage in chemical bonds are the key process in artificial photosynthesis. The first investigations towards the development of artificial photosynthesis aimed at directly mimicking the natural system by using components from biology within synthetic structures to effectively produce fuels. A significant development in achieving an artificial photosynthetic system based on a copy of the natural machinery was developed in the late 90s by Thomas A. Moore at Arizona State University. [START_REF] Steinberg-Yfrach | Light-driven production of ATP catalysed by F 0 F 1-ATP synthase in an artificial photosynthetic membrane[END_REF] The conception was directly inspired by Nature through photo-induced adenosine triphosphate (ATP) synthesis, using a triad consisting in a central porphyrin photosensitizer (PS) positioned between an electron donor (ED), a carotenoid, and an electron acceptor (EA), a quinone. It was the first example in which the sunlight capturing strategy was a direct mimic of the natural system, but the energy conversion step to create a useful fuel, i.e. sugar in natural photosynthesis, was not developed. (1) Chemical reduction by metals which occurs at relatively high temperatures [START_REF] Quinn | Carbon dioxide[END_REF] 2Mg + CO 2 → 2MgO + C Sn + 2CO 2 → SnO 2 + 2CO

(2)Thermochemical chemical conversion 14 (3) Radiochemical method [START_REF] Getoff | Reduction of carbon dioxide in aqueous solutions under the influence of radiation[END_REF] CO 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ HCOOH, HCHO (4) Biochemical conversion [START_REF] Jee | Continuous CH4 Production from H2 and CO2 by Methanobacterium thermoautotrophicum in a fixed-bed reactor[END_REF] CO 2 + 4H 2 ⎯⎯⎯⎯⎯⎯⎯ CH 4 + 2H 2 O

(5) Electrochemical conversion [START_REF] Amatore | Mechanism and kinetic characteristics of the electrochemical reduction of carbon dioxide in media of low proton availability[END_REF] CO 2 + xe -+ yH + ⎯⎯⎯⎯ CO, HCOOH (6) Photochemical conversion [START_REF] Lehn | Photochemical Generation of Carbon Monoxide and Hydrogen by Reduction of Carbon Dioxide and Water under Visible Light Irradiation[END_REF] CO 2 ⎯⎯⎯⎯ CO, HCOOH

The last approach is very attractive because the breaking of C=O bonds in the very stable CO 2 molecule needs a very high supply of energy that can be easily obtained with the light quanta of UV and visible spectral range. The reduction of CO 2 , which is one the most stable forms of C (IV)

requires also the presence of electron-supplying agents, typically water or other sacrificial and abundant electron donors, such as ascorbic acid, oxalate, thiols, amines and so on. Among the advantages of the photocatalytic CO 2 conversion are the relatively mild conditions, the simultaneous mitigation of climatic changes caused by ever-increasing anthropogenic CO 2 emission, the possibility of formation of C-C bonds in the form of hydrocarbons, ethers and carbonic acids and others.

Theoretical concepts 1.2.1 Thermodynamics and kinetics of carbon dioxide reduction

Carbon dioxide molecule contains two C=O bonds with a linear configuration, in which oxygen atoms show a weak Lewis basicity, while the carbon atom is electrophilic. Because it is centrosymmetric, the molecule has no electrical dipole, and only two vibrational bands are observed in the IR spectrum: an antisymmetric stretching mode at 2349 cm -1 and a bending mode near 666 cm -1 . There is also a symmetric stretching mode at 1388 cm -1 in its Raman spectrum. Equation Reaction ΔH 0 (kJ mol -1 ) ΔG 0 (kJ mol -1 ) ΔE 0 (V)

The single electron reduction of CO 2 to CO 2 ꞏ-, which has been well recognized as the first step to activate CO 2 for subsequent reduction steps, occurs at -1.90 V versus the normal hydrogen electrode (NHE) at pH 7, due to a large reorganizational energy between linear molecule and bent radical anion. This step has also been determined as the rate determining step (RDS) for CO 2 reduction.

The thermodynamic redox potentials for water splitting and various CO 2 reduction products are listed below. [START_REF] Hong | Photocatalytic reduction of CO 2 : a brief review on product analysis and systematic methods[END_REF] The first step of CO 2 reduction may undergo the one-electron reduction to CO 2

.

intermediate, however, this reaction is extremely thermodynamically unfavorable due to the highly negative redox potential of -1.90 V as mentioned above and is generally regarded as the RDS. In contrast, redox potentials of proton-assisted multiple electron reduction processes are very close to the one of H 2 formation. These processes, involving the participation of protons, are called proton-coupled electron transfer (PCET) processes. [START_REF] Huynh | Proton-coupled electron transfer[END_REF] PCET processes can bypass the formation of thermodynamically unfavorable CO 2 .

-intermediate and own a much lower energy barrier to break. However, except the thermodynamic barrier, multiple electron transfer reactions usually suffer from kinetic limitations depending on the concentration of available protons in the reaction solution. [START_REF] Costentin | Catalysis of the Electrochemical Reduction of Carbon Dioxide[END_REF] For instance, although CH 4 formation has a more positive redox potential (E 0 = -0.24 V) than H 2 production (E 0 = -0.41 V), implying that the formation of CH 4 is more thermodynamically favorable, eight electrons and eight protons are needed to participate in this reaction, which is actually much more difficult than the two-electron process of H 2 production.

This implies that a preferred catalyst probably entails catalytic sites transferring electrons which are close from sites providing protons. Furthermore, PCET processes always include multistep reactions. [START_REF] Habisreutinger | Photocatalytic reduction of CO2 on TiO2 and other semiconductors[END_REF] Despite the yet ambiguous mechanism, two plausible pathways for CO 2 reduction to CH 4 are extensively studied and partially verified: formaldehyde pathway [START_REF] Ji | Theoretical study on the mechanism of photoreduction of CO2 to CH4 on the anatase TiO2 (101) surface[END_REF] and carbene pathway 26 (Scheme 1.1). The intermediates could desorb into solvent to form the side products during the reaction processes. For instance, HCOOH and HCHO could be generated in the formaldehyde pathway (Scheme 1.1), while CO is the main side-product in the carbene pathway.

In addition, some other pathways involving C-C coupling (e.g. glyoxal pathway) were also reported and investigated, which might induce more complex products. [START_REF] Kang | Photocatalytic reduction of carbon dioxide by hydrous hydrazine over Au-Cu alloy nanoparticles supported on SrTiO 3 /TiO 2 coaxial nanotube arrays[END_REF] The hydrogenation of adsorbed C 1 intermediates is kinetically easier than the formation of C-C bonds, which limits the rate and selectivity of C 2 or higher hydrocarbon production. The maximum faradic efficiency for C 2 H 2 , namely the simplest C 2 product, was reported to be 60%, [START_REF] Mistry | Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene[END_REF] whereas that of C 3 product (C 3 H 7 OH) is no higher than 30%. [START_REF] Lee | Electrocatalytic production of C 3 -C 4 Compounds by conversion of CO 2 on a chloride-induced bi-phasic Cu 2 O-Cu catalyst[END_REF] The products are sensitively influenced by catalysts and external environments during the processes of reduction, forming a grand challenge for optimizing the selectivity of CO 2 reduction.

Possible CO 2 photoreduction reaction pathways with reduction potentials:

Photooxidation reactions

Water oxidation/decomposition E 0 redox /V vs. NHE

H 2 O + 2h + → 1/2O 2 + 2H + -0.82
Hydrogen peroxide formation

2H 2 O + 2h + → H 2 O 2 + 2H + -0.695

Photoreduction reaction

Hydrogen formation E 0 redox /V vs. NHE

Common terms about photochemical reduction of carbon dioxide

The turnover number (TON) is defined as the number of catalytic cycles (i.e. number of CO 2 reduction into one of its product) that occur per catalyst unit over the catalyst's lifetime. The turnover frequency (TOF) is a measure of the instantaneous efficiency of a catalyst, calculated as the ratio of the TON over time per active site. [START_REF] Kozuch | Turning over" definitions in catalytic cycles[END_REF] TON and TOF are pieces of evaluation for the activity of catalytic active centers, and they have been extensively applied to metal nanoparticles and homogeneous metal complex catalysts. The larger the TON and TOF, indicates the greater the catalytic activity. These definitions for photocatalytic CO 2 reduction are listed below: [START_REF] Zhang | Recent progress on advanced design for photoelectrochemical reduction of CO 2 to fuels[END_REF]

TON =

Total moles product (2) TOF (in t -1 ) = Catalytic selectivity (CS) is defined as the molar ratio of one of the CO 2 reduction products to that of total amount of products as follows:

CS (%) = × 100%. (4)
Any catalyst that reduces CO 2 to carbon monoxide, formate or other products is also thermodynamically capable of proton reduction to hydrogen. In fact, depending on the number of electrons and protons transferred, CO 2 can be reduced to at least 16 different kinds of products mentioned previously. [START_REF] Sun | Fundamentals and Challenges of Electrochemical CO 2 Reduction Using Two-Dimensional Materials[END_REF] The competition with protons reduction is always present. CS values provide quantitative data on the efficiency of CO 2 reduction relative to all products in specific experimental conditions (i.e. pH, solvent, temperature, pressure). Synthetic modification of the transition-metal catalysts provides the opportunity to tune relative reactivity through inductive and steric effects. The ability to control reactivity at the molecular level represents a significant advantage of homogeneous complexes over heterogeneous materials.

On account of various products, the selectivity is an indispensable parameter for the evaluation of CO 2 reduction. In terms of photoelectrocatalysis, the Faradaic Efficiency (FE) is typically used to assess the efficiency of producing selective products and is calculated by Equation ( 5). [START_REF] Yang | Surface faceting and reconstruction of ceria nanoparticles[END_REF] FE (%) = × 100% (5) where n is the number of needed electrons for product evolution (e.g., 2 for CO and 8 for CH 4 ), F is the Faraday constant, and Q is the total passed charge.

Quantum yield (QY) is a crucial parameter for assessing the performance of a photocatalyst or photocatalytic system. The internal and apparent quantum yields are defined by Equations 6 and 7, respectively. [START_REF] Chen | Semiconductor-based photocatalytic hydrogen generation[END_REF] Internal quantum yield (%) = × 100% (6) Apparent quantum yield (%) = × 100% (7) where n is the number of needed electrons for product evolution. The apparent quantum yield is estimated to be smaller than the internal quantum yield because the number of absorbed photons is usually smaller than that of incident light. In addition to the quantum yield, the solar energy conversion efficiency that is usually used for evaluation of solar cells is also sometimes reported in the literature. It is defined by Equation 8.

Solar energy conversion (%) =

Output energy of carbon dioxide reduction evolved Energy of incident solar light × 100% (8)

Photocatalytic reduction of carbon dioxide

Photocatalysis is a reaction which uses light to activate a substance which modifies the rate of a chemical reaction without being involved itself. A photocatalyst is a substance which can modify the rate of chemical reaction using light irradiation. In Nature, chlorophyll of plants is a typical photocatalyst: it captures sunlight to turn water and carbon dioxide into oxygen and glucose. As inspired by the natural photosynthesis, [START_REF] Concepcion | Chemical approaches to artificial photosynthesis[END_REF] the nature has been evolved in catalyzing CO 2 reduction with perfect activity and selectivity, and the research area of artificial photosynthesis has grown exponentially over the past decades. [START_REF] Brudvig | Introduction to a themed issue of Chemical Society Reviews on artificial photosynthesis[END_REF] Furthermore, compared with other methods for CO 2 reduction, photocatalytic conversion of CO 2 has a unique advantage of direct using solar energy. Since solar energy is completely inexhaustible and has zero carbon emission, photocatalytic reduction of CO 2 for the formation of synthetic fuels and fine chemicals has a great potential to cut CO 2 emissions and to alleviate our dependence on fossil fuels.

Photochemical CO 2 reduction can be catalyzed heterogeneously or homogeneously. In the heterogeneous approach, molecules of the gaseous or liquid reactants are adsorbed on the surface of a solid catalyst. In the homogeneous one, a molecular catalyst is uniformly dispersed in a liquid phase. In most of the existing industrial chemical transformations, catalysts are insoluble solids. Homogeneous catalysts are also feasible because their reduction potentials can be controlled through structural modification to match the potential required for CO 2 reduction and provide selectivity toward specific target products from CO 2 molecules. Moreover, catalysts are well characterized in terms of their chemical composition, structure, optical and redox properties at a molecular level. Since all molecules of a given homogeneous catalyst have the same structure, they facilitate breaking, forming, and reorganization of chemical bonds of the reactants in an identical manner. Both heterogeneous and homogeneous catalysts operate by reducing the energy required to achieve the reorganization of molecular structures of the reactants.

Semiconductor-based carbon dioxide reduction

Since the seminal investigation by Halmann in 1978, the photocatalytic CO 2 reduction into fuels with H 2 O, using solar energy as the driving force has drawn much attention. [START_REF] Halmann | Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells[END_REF] The first studies of the photocatalytic CO2 reduction were performed on GaP, ZnO, ZnS, then a variety of semiconductor (SC) photocatalysts was extended to TiO Nanocrystalline semiconductors as photocatalysts of CO 2 conversion. Zinc sulfide has relatively negative CB potential (around -1.8 V vs. NHE) [START_REF] Harriman | Energy Resources through Photochemistry and Catalysis[END_REF] and therefore this SC is one of the most viable photocatalyst candidates for CO 2 reduction. Nanocrystalline ZnS-based photocatalysts can be produced by a variety of methods, including controlled precipitation [START_REF] Kanemoto | Semiconductor photocatalysis. Part 20.-Role of surface in the photoreduction of carbon dioxide catalysed by colloidal ZnS nanocrystallites in organic solvent[END_REF] (Figure 1.9), ion-exchange and hydrothermal synthesis, the latter producing the most photo-active materials. [START_REF] Chen | Photocatalytically reducing CO 2 to methyl formate in methanol over ZnS and Ni-doped ZnS photocatalysts[END_REF] Zinc sulfide NPs attached to the surface of montmorillonite were used as a working photosensitive body in reactors of various geometry for CO 2 conversion to methanol, methane, and CO. [START_REF] Kočí | Influence of reactor geometry on the yield of CO 2 photocatalytic reduction[END_REF] Figure 1.9 Photocatalytic CO 2 reduction with ZnS NPs. 39b

Cadmium sulfide NPs with a more positive CB potential (E CB = -0.8 V vs. NHE) can also be used for CO 2 reduction provided a suitable cocatalyst is introduced into the photocatalytic system.

In particular, the efficient visible-light induced conversion of CO 2 into CO was observed for CdS NP assemblies with carbon monoxide dehydrogenase 42 (Figure 1.10). Hexagonal colloidal CdS NPs in N, N'-dimethylformamide (DMF) were found to be visible-light-sensitive photocatalysts for CO 2 reduction with CO as a main product. [START_REF] Fujiwara | Effect of surface structures on photocatalytic CO 2 reduction using quantized CdS nanocrystallites[END_REF] Figure 1.10 Photocatalytic reduction of CO 2 with CdS nanoparticle assemblies. [START_REF] Chaudhary | Visible light-driven CO 2 reduction by enzyme coupled CdS nanocrystals[END_REF] Recently, open-framework zeolite-like structures consisting of nanosized metal-chalcogenide nanoclusters were tested as visible-light-sensitive photocatalysts of the CO 2 reduction (Figure 1. 11). The mixed zinc-germanium-sulfide-based frameworks were found to photocatalyze the carbon dioxide reduction with water to methane and the conversion efficiency is affected by the incorporation of third metal cations (Au 3+ , Pd 2+ ) into the framework. [START_REF] Sasan | Open framework metal chalcogenides as efficient photocatalysts for reduction of CO 2 into renewable hydrocarbon fuel[END_REF] Figure 1.11 Photocatalytic reduction of CO 2 with open framework metal chalcogenides. [START_REF] Sasan | Open framework metal chalcogenides as efficient photocatalysts for reduction of CO 2 into renewable hydrocarbon fuel[END_REF] Photocatalytic CO 2 reduction was extensively studied in Ti-based systems. In particular, three nanocrystalline TiO 2 polymorphs anatase, rutile, and brookite were compared both in the pristine form and after a treatment with ionized helium flow creating oxygen vacancies on the NP surface (Figure 1.12 (A)). [START_REF] Liu | Photocatalytic CO2 reduction with H 2 O on TiO 2 nanocrystals: Comparison of anatase, rutile, and brookite polymorphs and exploration of surface chemistry[END_REF] Such treatment resulted in a remarkable (up to 10 times) increase of the rate of photocatalytic CO 2 reduction to CO and CH 4 on anatase and brookite, while rutile retained a low activity in these processes even after the treatment. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that the enhancement effect originates from a higher efficiency of CO 2 .

-anion radical formation on oxygen vacancies and from the formation of Ti 3 + sites of the plasma-treated titania NPs. The photoactivity of titania in CO 2 conversion can be increased by introducing co-catalysts, such as metallic NPs which can collect photogenerated electrons and favor multi-electron processes. As shown in Figure 1.12 (B), [START_REF] Xie | MgO-and Pt-promoted TiO 2 as an efficient photocatalyst for the preferential reduction of carbon dioxide in the presence of water[END_REF] the rate of photocatalytic CO 2 reduction into methane with water vapor increases from Ag to Rh, Au, Pd and Pt in line with an increase of the electron-accepting capability of the metal NPs. For a given metal, CO 2 conversion increases with a decrease of the NP size and typically shows a domeshaped dependence on the metal content. CO 2 reduction efficiency is limited by the competing water reduction to H 2 reaction of which can be suppressed by the deposition of an additional protective MgO layer on the photocatalyst surface. In-doped nanocrystalline TiO 2 was applied as a photo-active phase for the photocatalytic CO 2 conversion in microchannel monolith photoreactors. [START_REF] Tahir | Photocatalytic CO2 reduction and kinetic study over In/TiO 2 nanoparticles supported microchannel monolith photoreactor[END_REF] These systems generate a broad range of reduction products with a population decreasing in the following sequence: CO > CH 4 > C 2 H 6 > C 2 H 4 > C 3 H 6 . After a multi-parameter optimization of the photoreactor performance, quantum yields of CO and CH 4 reached 0.1 and 0.022%, respectively. Doping of the mesoporous titania with In was also reported to change the CO 2 reduction product from CO to CH 4 increasing the light harvesting efficiency by a factor of ca. 8. [START_REF] Tahir | Indium-doped TiO 2 nanoparticles for photocatalytic CO 2 reduction with H 2 O vapors to CH4[END_REF] Nanocrystalline ZnS doped with Ni(II) showed a high selectivity toward the formation of methyl formate as a result of the photocatalytic CO 2 reduction in methanol. [START_REF] Chen | Photocatalytically reducing CO 2 to methyl formate in methanol over ZnS and Ni-doped ZnS photocatalysts[END_REF] The highest yields were observed at ~0.3 mol% dopant content.

Sensitized systems for CO 2 photoreduction. Nanocrystalline titania sensitized with a typical Ru(II)-bipyridyl dye (N719) coupled with a Pt counter electrode was used for the visible-lightdriven photoreduction of CO 2 to HCOOH, HCHO and CH 3 OH in a two-vessel geometry (Figure 1.14 (A)). [START_REF] Qin | Photocatalytic reduction of carbon dioxide to formic acid, formaldehyde, and methanol using dye-sensitized TiO 2 film[END_REF] The functionalization of TiO 2 NPs with various aminosalicylic acids (ASA) results in the formation of surface charge transfer complexes that extend the absorption range of titania far into the visible range (Figure 1.14 (B)). [START_REF] Finkelstein-Shapiro | CO 2 preactivation in photoinduced reduction via surface functionalization of TiO 2 nanoparticles[END_REF] The photoinduced charge transfer in such complexes occurs directly from the highest occupied molecular orbital (HOMO) of ASA molecules into the CB of TiO 2 NPs and then to adsorbed CO 2 molecules. As a result, the absorption band edge of a ternary TiO 2 -ASA-CO 2 system shifts even further into the visible range as compared to the binary TiO 2 -ASA charge transfer complex. A tandem principle was realized for a Nickel (II) oxide photocathode sensitized with a supramolecular Ru(II)-Re(I) complex assembly and coupled to a tantalum oxynitride photoanode modified with a cobalt oxide co-catalyst (Figure 1.15 (A)). [START_REF] Sahara | Photoelectrochemical Reduction of CO 2 Coupled to Water Oxidation Using a Photocathode with a Ru (II)-Re (I) Complex Photocatalyst and a CoO x/TaON Photoanode[END_REF] The nickel oxide-based photocathode showed selectivity toward the formation of CO. The system also used water as an electron donor and a low external bias of ~0. photocathode. [START_REF] Sahara | Photoelectrochemical Reduction of CO 2 Coupled to Water Oxidation Using a Photocathode with a Ru (II)-Re (I) Complex Photocatalyst and a CoO x/TaON Photoanode[END_REF][START_REF] Kumagai | Hybrid photocathode consisting of a CuGaO 2 p-type semiconductor and a Ru (ii)-Re (i) supramolecular photocatalyst: non-biased visible-light-driven CO 2 reduction with water oxidation[END_REF] In tandem systems comprising graphitic carbon nitride (g-C 3 N 4 ) and Ru(II)-bipyridyl complexes, the rate and pathway of CO 2 reduction depend on the bipyridyl substituent X in the 4position. [START_REF] Kuriki | Unique solvent effects on visible-light CO 2 reduction over ruthenium (II)-complex/carbon nitride hybrid photocatalysts[END_REF] In particular, for tandems based on X = -COOH and X = -PO 3 H 2 , the main product was HCOOH, while for X = -CH Titania coupling with g-C 3 N 4 allows the spatial charge carriers separation since photogenerated electrons are collected in the lower-positioned titania CB and VB holes (photogenerated in both SCs) are supplied to the reactants through the VB of carbon nitride. As a result, titania/ g-C 3 N 4 composites displayed an enhanced photocatalytic activity. [START_REF] Reli | Novel TiO 2 /C 3 N 4 photocatalysts for photocatalytic reduction of CO 2 and for photocatalytic decomposition of N 2 O[END_REF] A similar enhancement effect in the CO 2 reduction to methane was also observed for g-C 3 N 4 /KNbO 3 60 and g-C 3 N 4 /NaNbO 3 61 .

Figure 1.16 Scheme of a "Moorella thermoacetica-CdS/TiO 2 -Mn phthalocyanine" tandem system for the CO 2 conversion. [START_REF] Sakimoto | Cysteine-Cystine Photoregeneration for Oxygenic Photosynthesis of Acetic Acid from CO 2 by a Tandem Inorganic-Biological Hybrid System[END_REF] In metal chalcogenide/titania composites with proper CB and VB level positions, CO 2 reduction and donor (for example, water) oxidation occur on the surface of titania and metal chalcogenide NPs, respectively. Both branches of the photoprocess can be additionally separated in space to avoid the re-oxidation of CO 2 reduction products and to promote the formation of C-C bonds. For example, by combining two subsystems -the Moorella Thermoacetica bacteria decorated with CdS NPs and TiO 2 NPs loaded with Mn(III) phthalocyanine into a Z-system -CO 2 reduction to acetic acid can be achieved (Figure 1.16). [START_REF] Sakimoto | Cysteine-Cystine Photoregeneration for Oxygenic Photosynthesis of Acetic Acid from CO 2 by a Tandem Inorganic-Biological Hybrid System[END_REF] Both subsystems are connected by a donor/acceptor cysteine/cystine couple. The cysteine gets oxidized on the surface of CdS NPs to cystine and then cystine is regenerated to cysteine on the surface of the phthalocyaninefunctionalized titania.

The efficiency of photocatalytic reduction of CO 2 with water vapor over CdSe/Pt/TiO 2 nanoheterostructures was found to depend on the CdSe NP size as a result of a size-dependence of the CdSe CB position (Figure 1.17). [START_REF] Wang | Visible light photoreduction of CO2 using CdSe/Pt/TiO 2 heterostructured catalysts[END_REF] The main reduction product was methane with CH 3 OH, CO and H 2 present as secondary admixtures. To sum up, in recent years, lots of working groups put intensive efforts in the development of SC photocatalysts for CO 2 reduction. In these simple systems, namely powder in water, SC photocatalysis has a big potential for the wide use in CO 2 reduction. Not only were these materials able to achieve superior efficiency, but their activity was also demonstrated as being amenable to fine-tuning through the control of size and morphology. [START_REF] Chen | Aqueous CO 2 reduction at very low overpotential on oxide-derived Au nanoparticles[END_REF] In this way, materials increase their surface area, thus effectively improving their activity, while at the same time widening their photoelectronic properties. It would be preferable to control each of these independently. Another aspect of heterogeneous catalysis that must be put forth is the fact that these materials are difficult to rationally control on the molecular level and the most active systems rely mainly on the use of precious and/or rare metals. Thus, tuning the selectivity at the molecular level remains difficult. Its application in commercial use has still far to go. ) is promoted to an excited state PS* (Equation 9). Thanks to its strong absorption in the visible range and to internal heavy atom effect (triplet metal-to-ligandcharge-transfer (MLCT)), high quantum efficiency in photocatalysis can be achieved. [START_REF] Koike | Visible-light radical reaction designed by Ru-and Ir-based photoredox catalysis[END_REF] The excited state PS * is then reductively quenched by a SD, typically triethylamine (TEA) or triethanolamine (TEOA), to yield the reduced form of the sensitizer, PS -, and the oxidized form of the donor, SD ꞏ+ as shown in Equation 10. It is important to note that the oxidized amines are quite reactive and are known to undergo hydrogen atom abstraction and radical rearrangements which result in the formation of highly reduced carbon-centered radical species, Et 2 NC ꞏ HCH 3 or (HOC 2 H 4 ) 2 N(C ꞏ HCH 2 OH), which can, under certain conditions, reduce a second PS (Equation 13and 14). [START_REF] Whitten | Photoinduced electron transfer reactions of metal complexes in solution[END_REF] We denote this carbon-centered radical as SD ꞏ . The reduced sensitizer, PS In order to avoid the use of precious metal, such as Ru complex as PS, organic dyes have been employed. In 1991, Yanadida et al. reported photocatalytic systems using p-terphenyl as PS, cobalt cyclams complexes as CAT, and TEA or TEOA as SD in mixed ACN/CH 3 OH solutions. [START_REF] Matsuoka | Enhanced p-Terphenyl-Catalyzed Photoreduction of CO 2 to CO through the Mediation of Co (III)-Cyclam Complex[END_REF] Irradiation with UV light (λ > 290 nm) of these solutions produced CO and HCOO -(CO/HCOO - = 1.5) along with H 2 evolution. These systems showed good selectivity (CS (CO + HCOO -) = 86%~99%) and high QY (Φ(CO + HCOO -) = 25 % with λ ex = 313 nm). When using CoCl 2 instead of Co cyclams as CAT, CO 2 was not transformed. However, these systems can only use UV light because of the limited visible light absorption of p-terphenyl. Interestingly, if phenazine was used as the PS, a CS of 93% for the formation of HCOO -was observed (Φ(HCOO -) = 7 % with λ ex = 313 nm), 69b which indicates that PS can not only affect the photocatalytic efficiency of the system but also its catalytic selectivity. The PS was able to supply efficiently not only an electron but also a proton at the N atom of the CAT. The formation of the corresponding hydride complex (Co-H) was thus enhanced and followed by CO 2 insertion, resulting in the HCOO - acting as a ligand in the Co complex. The external quantum yield of 3.4 ± 0.3% at 400 nm is comparable to state-of-the-art precious metal photocatalysts. Transient absorption spectroscopy showed that band-gap excitation of ZnSe QDs is followed by rapid hole scavenging and very fast electron trapping in ZnSe. The trapped electrons transfer to NiCycP on the ps timescale, explaining the high performance for photocatalytic CO 2 reduction. [START_REF] Guo | Highly efficient and selective photocatalytic CO 2 reduction by iron and cobalt quaterpyridine complexes[END_REF] With Ru(bpy) 3 2+ as PS and 1,3-dimethyl-2-phenyl-2,3dihydro-1H-benzo [d]imidazole (BIH) as the SD reductant in ACN/TEOA solution under visiblelight excitation (blue light-emitting diode), a TON CO as high as 2660 with 98% selectivity can be achieved for the Co catalyst. In the case of the Fe catalyst, the TON was > 3000 with up to 95% selectivity. More significantly, when Ru(bpy) 3 2+ was replaced by the organic dye sensitizer purpurin, TONs of 790 and 1365 were achieved in N, N-dimethylformamide for the cobalt and iron catalysts, respectively. Visible light irradiation of an ACN/TEOA solution containing PS, CAT and BIH as SD produced a TON CO of 270, with a selectivity of 78% with respect to CO. Quantum yields of 6.7%

(mononuclear) and 2.6% with a TON CO > 54 (dimeric) were reached. The catalytic process was initiated by reductive quenching of the exited state of the Cu complex by BIH. Another type of catalysis consists in one compound both acting as the light absorber and the catalyst. The "all in one" molecular photocatalysis lowers the number of required components and intermolecular electron transfer events, which may be more efficient for catalytic CO 2 reduction. The photocatalyst, PCAT, is reduced by excited state reductive quenching as described in the first type catalysis (Equations 15 and16). Similarly, the oxidized amine donors undergo subsequent reactivity to produce SD ꞏ radical, which can continue reducing the PCAT (Equations 18 and19). The reduced PCAT -is a catalyst active state and can directly interact with CO 2 and then form products (Equation 17). Very few catalysts are capable of both absorbing solar spectrum irradiation and reducing CO 2 . It should be noted that all these type of catalysts could also be used as CATs for the first type approaches with an appropriate PS.

PCAT + hv → PCAT * ( 15)

PCAT * + Et 3 N → PCAT -+ Et 3 N ꞏ+ (16)
PCAT -+ CO 2 → PCAT + products ( 17)

Et 3 N ꞏ+ + Et 3 N → Et 3 NH + + Et 2 NC ꞏ HCH 3 (18) Et 2 NC ꞏ HCH 3 + PCAT → Et 2 N + =CHCH 3 + PCAT - (19)
Re(CO) 3 (bpy)X-based Complexes. Hawecker et al. were the first to investigate Re I (L)(CO) 3 X, where X = Cl -or Br -and L = 4,4'-R2-2,2'-bipyridine (R = H or CH 3 ) or 1,10phenanthroline, for photocatalytic CO 2 reduction. [START_REF] Morris | Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels[END_REF][START_REF] Hawecker | Efficient photochemical reduction of CO 2 to CO by visible light irradiation of systems containing Re (bipy)(CO) 3 X or Ru (bipy) 3 2+ -Co 2+ combinations as homogeneous catalysts[END_REF] Upon light excitation, Re I (L)(CO) 3 X is promoted to a MLCT excited state with a Re II metal center and an electron located on the L ligand. In the presence of a SD, the excited state catalyst is reductively quenched to form [ReI(L ꞏ )(CO) 3 X] -species (where L ꞏ is an anion radical in Scheme 1. formation and increased the yield of CO production.

Scheme 1.3. Proposed mechanistic steps in the reduction of CO 2 by Re(CO) 3 (bpy)X. Hydrogen (blue), formate (red), CO (brown and pink) and bicarbonate formation (pink), as well as putative intermediates and electron sources (green). [START_REF] Morris | Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels[END_REF] While formate production is reasonably understood, the mechanism of CO formation is still an area of active investigation and debate. Upon protonation of a putative Re-CO 2 adduct, a metal carboxylate intermediate was observed that undergoes acid-promoted hydrolysis to yield H 2 O and CO. Gibson et al. characterized Re(L)(CO) 3 (COOH) and proposed that proton-promoted dehydroxylation of Re(L)(CO) 3 (COOH) could yield Re(L)(CO) 4 + which would release CO under irradiation (or when it reacts with X -) and then regenerate the initial compound. [START_REF] Gibson | Synthesis and Reactions of fac-Re (dmbpy)(CO) 3 X (dmbpy= 4, 4 '-dimethyl-2, 2 '-bipyridyl; X= COOH, COOMe, H, OH, and OCHO)[END_REF] As all CO in Re(L)(CO) 4 + may exchange with X -, this pathway explains prior results which demonstrated that the use of [START_REF] Quinn | Carbon dioxide[END_REF] CO 2 resulted in the formation of the fully labeled fac-Re I (L)( 13 CO) 3 X.

Recently, Grätzel et al. were the first to report a molecular Re(CO) 3 (bpy)X-based complex coupled to protected Cu 2 O photocathodes for CO 2 -to-CO conversion (Figure 1.22 (A)). [START_REF] Schreier | Efficient and selective carbon dioxide reduction on low cost protected Cu 2 O photocathodes using a molecular catalyst[END_REF] The device produced a high photovoltage of 560 mV and a photocurrent density of 2.1 mA cm -2 , corresponding to the highest photocurrent observed towards CO 2 reduction on an oxide material.

The protection of the Cu 2 O photocathode by TiO 2 enabled stable and selective reduction of CO 2 over several hours, and they found that the protic electrolyte additives were needed to overcome the unexpected charge transfer limitations on the protected photocathode surface. The discovery of charge transfer limitations from the surface of TiO 2 to the Re(CO) 3 (bpy)X-based molecular catalyst and its elimination by the use of protic additives is highly relevant to the field of photoelectrochemistry where there is a general trend towards the use of protective overlayers.

More recently, they reported a similar system with the Re(CO) 3 (bpy)X-based complex was covalently immobilized on the photocathode with phosphonate linking groups (Figure 1.22 (B)). [START_REF] Schreier | Covalent immobilization of a molecular catalyst on Cu2O photocathodes for CO 2 reduction[END_REF] The direct link between the CAT and the SC is not a strict requirement for high electron injection efficiency and as a positive side effect, the utilization of a spacing group between the phosphonic acid and the bipyridine ligand allows for positioning the catalytic redox center at a considerable distance away from the SC surface, making it more accessible for the reaction. This led to a 40-fold enhancement of the catalytic photocurrent as compared to planar devices, resulting in the sunlight-driven evolution of CO at large current densities and with high selectivity. However, it should be noted that Re is a precious and rare metal. A recent development in this field has been the successful application of analogous Mn(CO) 3 (bpy)X-based complexes as CO 2 reduction catalysts, taking advantage of the much more abundant and economical first row transition metal, manganese. These series of catalysts covert CO 2 into both CO and HCOO -and their catalytic selectivity is flexible. For example, Ishitani et al. recently reported fac-Mn(CO) 3 (bpy)Br as a CAT for photocatalytic CO 2 reduction. [START_REF] Schreier | Covalent immobilization of a molecular catalyst on Cu2O photocathodes for CO 2 reduction[END_REF] A DMF/TEOA (4:1 v/v) solution containing the Mn complex (0.05 mM), Ru(dmb) 3 2+ (0.05 mM, bmb = 4,4'-dimethyl-2,2'bipyridine) as PS, and 1-benzyl-1,4-dihydronicotinamide (BNAH) as SD was irradiated at λ ex = 480 nm under a CO 2 atmosphere, producing HCOOH after a short induction period with a Φ HCOOH of 5.3%. A TON HCOOH of 149 was obtained after 12 h irradiation. Such performance is comparable to the photocatalytic system using fac-Re(dmb)(CO) 3 Cl, under similar reaction conditions (Φ CO = 6.2%). [START_REF] Gholamkhass | Architecture of Supramolecular Metal Complexes for Photocatalytic CO 2 Reduction: Ruthenium-Rhenium Bi-and Tetranuclear Complexes[END_REF] Note that the reaction product was HCOOH and not CO, although CO was the main product in the electrochemical CO 2 reduction using the same Mn catalyst. [START_REF] Bourrez | Pulsed-EPR Evidence of a Manganese (II) Hydroxycarbonyl Intermediate in the Electrocatalytic Reduction of Carbon Dioxide by a Manganese Bipyridyl Derivative[END_REF] During the induction period of HCOOH formation, a Mn dimer ([Mn(bpy)(CO) 3 ] 2 ) with a Mn-Mn bond was formed via a one-electron reduction of the original Mn complex. During this period, CO was the main product. Reduction of the Mn dimer by the reductive PS might proceed giving [Mn(bpy)(CO) 3 ] -, along a slow process. After the Mn dimer was no longer detected in the reaction solution, the production of HCOOH proceeded more rapidly, and formation of CO just stopped. This suggests that the Mn dimer does not lead to HCOOH formation but might produce CO. andϕ 2 (Scheme 1.4). [START_REF] Morris | Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels[END_REF][START_REF] Dhanasekaran | P-terphenylsensitized photoreduction of CO 2 with cobalt and iron porphyrins. Interaction between CO and reduced metalloporphyrins[END_REF] More specifically, a donor molecule (here TEA) binds axially to [M III P] + to form an adduct (Equation 20) which could yield [M II P] under light excitation, through an intramolecular reductive quenching step (Equation 21). While TEA binds to [M III P] + , the UV-Vis spectrum of the resultant [M II P] is not dependent on the TEA concentration, indicating that TEA does not bind to it. However, given the extremely short lifetime of exited state of MPs, like hemes, a weak TEA adduct must form prior to absorption of the second photon to account for the observed formation of [M I P] -. [START_REF] Dhanasekaran | P-terphenylsensitized photoreduction of CO 2 with cobalt and iron porphyrins. Interaction between CO and reduced metalloporphyrins[END_REF] The quantum yield for the second quenching step is typically an order of magnitude lower than for the first quenching step due to the weaker ground state adduct formed. It should be noted that for each of these catalysts, further reduction is needed to generate the catalytically active state. (green). [START_REF] Morris | Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels[END_REF][START_REF] Dhanasekaran | P-terphenylsensitized photoreduction of CO 2 with cobalt and iron porphyrins. Interaction between CO and reduced metalloporphyrins[END_REF] The formation of the catalytically active state [M 0 P] 2-by direct light excitation has not been observed. Alternatively, it is proposed that this state is formed via disproportionation (Equation 22).

Metal

2[M

I P] ↔ [M 0 P] 2-+ [M II P] (22)
Kinetic evidence for a disproportionation reaction of [Fe I P] -, where P = tetrakis(N-methyl-2pyridyl)porphyrin, was indeed reported. 84a While the kinetic data was complicated by the presence of different porphyrin isomers, the rate law was clearly second-order in [Fe I P] -, consistent with Equation 22.

Disproportionation is highly unfavorable for all the metallomacrocyles discussed, and therefore the concentration of the active state, [M 0 P] 2-, must be small. The observed reactivity has been rationalized by a favorable interaction between the active state and H + or CO 2 (K 3 and K 4 in Scheme 1.4). However, increasing electron density on the cobalt(I) in tetraaza-macrocycles favored the binging of CO 2 ; therefore, it is likely that the active [M 0 P] 2-favors adduct formation relative to more oxidized forms. [START_REF] Fujita | Carbon dioxide activation by cobalt (I) macrocycles: factors affecting carbon dioxide and carbon monoxide binding[END_REF] Protonation of the putative [M 0 P-CO 2 ] 2-to form a metal carboxylic acid intermediate that subsequently decomposes to M II P, CO, and HO -has been proposed. Alternatively, under high CO 2 concentrations, [M 0 P-CO 2 ] 2-may react with CO 2 to yield M II P, CO, and CO 3 2-(cyan in Scheme 1.4). Experimental proof of [M 0 P-CO 2 ] 2-species is lacking due to the reactive nature of these highly reduced compounds; indeed, [M 0 P-CO 2 ] 2-has neither been observed nor isolated. However, electrochemical reduction of CO 2 reported by Hammouche et al. [START_REF] Hammouche | Chemical catalysis of electrochemical reactions. Homogeneous catalysis of the electrochemical reduction of carbon dioxide by iron ("0") porphyrins. Role of the addition of magnesium cations[END_REF] indicates that [Fe 0 P] 2-is the catalytically active species, consistent with photochemical data. Therefore the reaction mechanisms proposed for formate and carbon monoxide production are based on presumed similarities to other catalysts discussed.

MPs and related derivatives are strong light absorbers with extremely large extinction coefficients in the visible spectral region. However, yields of active catalysts formed after light absorption are very low, mainly due to the short (picosecond) lifetimes of their excited states. In addition, MPs and derivatives undergo unwanted photo-Birch reduction of the porphyrin ring, converting it to the corresponding chlorin structure, followed by further photochemical rapid decomposition, and these degradation products seem to catalyze the reduction of protons over CO 2 .

Addition of p-terphenyl as a PS to photocatalytic CO 2 reduction with FeTPP, which made up a sensitized system, was investigated in order to overcome the above-mentioned problems. [START_REF] Dhanasekaran | P-terphenylsensitized photoreduction of CO 2 with cobalt and iron porphyrins. Interaction between CO and reduced metalloporphyrins[END_REF] The standard reduction potential of reduced p-terphenyl was negative enough (-2.45 V vs. SCE in dimethylamine) to reduce the Fe II TPP (-1.05 V vs. SCE) and Fe I TPP species (-1.66 V vs. SCE) so as to form the corresponding Fe 0 TPP active state. This system exhibited 10 times higher photocatalytic efficiency than in the absence of PS. Moreover, Co porphyrin analogue can also be used to produce CO, and its photocatalytic efficiency was about 1.5 times higher compared to the Fe porphyrin.

Recently, Weiss et al. reported a colloidal CuInS 2 /ZnS QDs sensitized FeTPP to reduce CO 2 to CO by using 450 nm light (Figure 1.24). [START_REF] Lian | Powering a CO 2 Reduction Catalyst with Visible Light through Multiple Sub-picosecond Electron Transfers from a Quantum Dot[END_REF] They underlined the crucial role of the sensitization process which has to be highly efficient in the transfer of redox equivalents from the PS to the CAT; in systems with molecular PSs, this transfer is often slow because it depends on diffusion-limited collisions between PS and CAT. The colloidal sensitization efficiency (turnover number per absorbed unit of photon energy) of the QD system is a factor of 18 greater than analogous systems with molecular PS. The selectivity for CO is 84% (16% H 2 ). The QD/FeTPP mixture is stable after 40 h of illumination at 4.5 mW. The sensitization efficiency of the system originates from the ultrafast electron transfer between the QD and FeTPP: the time constants for the dominant electron-transfer processes responsible for the first two sensitization steps (Fe III TPP → Fe IIT TPP, and Fe II TPP → Fe I TPP) are both < 200 fs. This ultrafast photoreduction is enabled by formation of a quasi-static QD/FeTPP complex, probably driven by the interaction between Fe and sulfur on the surface of the QD. The ability of QDs to self-assemble with a co-catalyst is a crucial advantage of the QD-sensitized system over molecular systems, where the rate of charge transfer is limited by occasional collisions between freely diffusing species. The redox property of porphyrin complexes, which are closely related to CO 2 reduction efficiency, could be easily tuned through substituents modification on the four benzene rings. The photocatalytic reaction using iron porphyrin with two -OH groups on the o-positions of the four phenyl groups was investigated in a ACN solution containing the CAT and TEA as tSD.

Irradiation (λ ex > 280 nm) of the solution gave CO with higher selectivity (ca. 85%) compared to unmodified FeTPP. [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] The introduction of -OH groups stabilizes the CO 2 adduct owing to the internal hydrogen bonding between -OH groups and the CO 2 bound to the metal center. It was also shown that -OH groups accelerate the cleavage of the C-O bond. In the photocatalytic CO 2 reduction, the TON CO was ca. 30 (with the production of a small amount of H 2 ), although in electrochemical conditions, the selective CO 2 reduction continued with little degradation, exhibiting a high faradaic yield of CO (> 95%). Robert et al. further improved this photocatalytic CO 2 reduction by adding fac-Ir(ppy) 3 (ppy = 2-phenylpyridine) as PS. Visible-light irradiation with λ > 420 nm light in the presence of TEA as SD led to CO evolution with a higher efficiency and selectivity (ca. 93%). [START_REF] Bonin | Selective and Efficient Photocatalytic CO 2 Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst[END_REF] The TON was also improved to 140, which was significantly higher than in the absence of PS. An organic dye, 9-cyanoanthracene (9CNA), was also used as PS instead of the Ir complex and the new system exhibited visible-light-driven CO 2 reduction, with a high selectivity of nearly 100% for CO formation. The reduced 9CNA exhibits strong reducing power (E 0 = -1.58 V vs SCE), high enough to reduce the Fe porphyrins to Fe 0 active species.

However, the durability still needs to be improved (TON CO = ca. 60). PS fulfills its function as light collector and thanks to its reductive quenching with SD, it generates a highly reducing species able to increase the Fe 0 formation rate and to efficiently assist the C-O bond breaking in the CO 2 -CAT adduct and thus to favor the CO generation at the detriment of the H 2 evolution.

Lowering the excitation energy and separating the light absorption from catalysis roles also prevented catalyst degradation.

Photoreduction of CO 2 can be also realized over inorganic catalytic materials. Whereas molecular catalysts usually exhibit high activity but low stability and often become inactive under long term irradiation, materials has low activity but very good stability. He et al. reported the use of graphitic carbon nitride (g-C 3 N 4 ) nanosheets as PS to integrate with Fe tetra(4carboxylphenyl)porphyrin chloride (FeTCPP) molecular catalyst (Figure 1.25). [START_REF] Lin | Highly efficient visible-light driven photocatalytic reduction of CO 2 over g-C3N4 nanosheets/tetra (4-carboxyphenyl) porphyrin iron (III) chloride heterogeneous catalysts[END_REF] Besides π-π stacking between the tris-triazine unit and the porphyrin, carboxyl group modified Fe-porphyrin is used for CO 2 photoreduction so as to form hydrogen bonding with the rich amino groups in g-C 3 N 4 nanosheets. g-C 3 N 4 /FeTCPP heterogeneous catalysts are prepared via a facile self-assembly approach, in which light harvesting is spatially and temporally separated from catalysis. The obtained g-C 3 N 4 /FeTCPP heterogeneous catalysts exhibit high activity for CO 2 reduction under visible-light irradiation, with a CO yield of 6.52 mmol g -1 in 6 h and selectivity up to 98%. The authors proposed a mechanism which is slightly different from the Fe 0 mechanism in the purely homogenous system. The formation of Fe 0 species being much slower under heterogeneous photochemical conditions, Fe II TCPP-CO 2 intermediates will be directly formed after the oneelectron reduction of the CAT. Since Fe II and Fe I species do accumulate in the solution for several minutes, as evidenced by quasi in-situ electron spin resonance (ESR) and UV-vis measurements, the formation of Fe II TCPP-CO 2 intermediates may be crucial and take precedence for the CO 2 photoreduction in the reported system. Moreover, carboxyl substituents on the phenyl ring can enhance the interaction between g-C 3 N 4 nanosheets and FeTCPP and stabilize CO 2 adduct formed with FeTCPP via intramolecular hydrogen bonding thus improving photocatalytic activity of the g-C 3 N 4 /FeTCPP heterogeneous assembly. Fe and Co MPcs and MCs also exhibit photocatalytic activity for CO 2 reduction. In the case of MPs, TONs (50-90) were not higher than for porphyrin analogues even in the presence of pterphenyl as PS. 84c In the case of MCs, photocatalytic activities (TONs = 40-100) were comparable to those using FeTPP and CoTPP under the same conditions, with p-terphenyl as

PS. 84d

As discussed above, tremendous advances have been achieved in the photochemical reduction of carbon dioxide using either SC or molecular catalysts, in terms of both activity and selectivity.

With the proper structure design of a redox-active catalyst, it will be possible to selectively reduce CO 2 to CO or HCOOH as main products. However, the drawbacks of these catalytic systems such as poor efficiency and low stability under operating conditions, greatly limit the real application of this technology. Most of the reported catalysts will be decomposed after a few hours of reaction, which is a severe impediment to their implementation on a large scale. Hence, the future research efforts in this field should focus on developing strategies to extend the lifetime of such catalysts. In addition, initial steps should be taken towards the integration of these catalysts in tandem cells, where electrons and protons for the reduction of carbon dioxide are provided by the oxidation of H 2 O instead of an external SD, to demonstrate the sustainability of this promising technology.

Motivation and outline of this thesis

Developing photocatalytic systems for CO 2 reduction will provide useful and energy-rich compounds, which will be one of the most important focuses in the field of "artificial photosynthesis" and "solar fuels". In the past few decades, such studies have been conducted

from the perspective of basic science and for solving the shortage of fossil resource. However, it is still substantially lagging behind the astonishing progress that has been achieved for the production of hydrogen from solar energy. It is still a big challenge to develop more efficient, more stable, more selective CO 2 reduction photocatalytic systems.

This thesis discusses serval Fe-porphyrin based homogenous systems for visible light driven photochemical carbon dioxide reduction under ambient conditions. Both spectroscopic and electrochemical methods are used to elucidate the mechanism. The goal of this thesis to develop highly efficient and stable molecular photochemical CO 2 reduction systems which may be industrialized for generating carbon-based solar fuels from solar energy.

Chapter 2 describes a non-sensitized iron porphyrin-based molecular photochemical system for efficient CO 2 -to-CO conversion. Similar catalysis will be enhanced with the addition of an organic dye (purpurin) as the photosensitizer even in aqueous solutions.

Chapter 3 discusses with the introducing of a much stronger Ir-based photosensitizer, the efficiency of CO 2 -to-CO conversion is greatly increased and more marvelously, CH 4 is formed in this system. Actually, this system can also drive CO-to-CH 4 conversion directly, which is very similar to a molecular Fischer-Tropsch synthesis. And then this catalysis is transformed into a aqueous system with a water soluble Ir-based photosensitizer. Finally, we also develop a pure non-noble-element system for both CO 2 -to-CH 4 conversion and CO-to-CH 4 conversion with a phenoxazine-based organic photosensitizer.

Chapter 4 gives a short summary of the work completed during this thesis, with an overview of ongoing projects and possible future developments.

Chapter 2 Photochemical Reduction of CO 2 to CO under Visible Light with Iron(0) Porphyrins

Introduction

The efficient photochemical reduction of carbon dioxide into useful high-energy density fuel will be an important step in the concept of a CO 2 neutral cycle, as it will allow for the direct conversion of solar energy into stored chemical energy, in a very similar way than Nature does.

Green plants fix CO 2 from the atmosphere by photosynthesis with porphyrin-based enzymes which selectively binds CO 2 and inserts it into existing carbon chains by reductive carboxylation, the high energy electrons needed in this process being photogenerated through Photosystem II.

Synthetic catalysts for the photocatalytic reduction of CO 2 , which can facilitate an "artificial photosynthetic" CO 2 neutral cycle, have been investigated for several decades. [START_REF] Tinnemans | Tetraaza-macrocyclic cobalt (II) and nickel (II) complexes as electron-transfer agents in the photo (electro) chemical and electrochemical reduction of carbon dioxide[END_REF][START_REF] Hawecker | Efficient photochemical reduction of CO 2 to CO by visible light irradiation of systems containing Re (bipy)(CO) 3 X or Ru (bipy) 3 2+ -Co 2+ combinations as homogeneous catalysts[END_REF] The use of light energy for breaking and/or making chemical bonds can decrease the temperature and pressure needed to drive the reaction. Although great progress have been made in the past few years, a lot of huge challenges still need our concern. [START_REF] Trickett | The chemistry of metal-organic frameworks for CO 2 capture, regeneration and conversion[END_REF] For example, low efficiency and yield are the major shortcomings of photocatalytic CO 2 reduction. This process indeed involves multielectron steps which produce various products, including carbon monoxide, methanol, methane, formaldehyde, formic acid and higher hydrocarbons, which makes it way more complicated to conduct than proton reduction in photocatalytic water splitting. Except CO 2 reduction itself, there always accompanies with several intermediate processes, such as proton transfer, hydroxylation and so on. Since a wide range of products can be formed, controlling the selectivity for a desired product is a main issue. Additional challenges include the low solubility of CO 2 in water (if the reaction is performed in aqueous solution) as well as the competition of hydrogen evolution in aqueous media. A relatively poor understanding of the process and the difficulty in elucidating mechanisms at the molecular scale still prevent the design of optimal catalysts.

Recent fundamental and theoretical works have reconsidered porphyrin-based molecular catalysts for CO 2 reduction. is produced as minor product from subsequent CO reduction by concerted proton-coupled electron transfers (PCET) assumed for each electrochemical step. Previous works in our team considered ligand modifications of iron-based porphyrins and found that local proton sources built into the porphyrin ring give rise to high activity and good Faradaic efficiency for the electrocatalytic reduction of CO 2 to CO in a mixed DMF-water solvent. [START_REF] Costentin | A Local Proton Source Enhances CO 2 Electroreduction to CO by a Molecular Fe Catalyst[END_REF] With the same catalyst, it could directly photochemically convert CO 2 into CO, with H 2 as a minor product under irradiation. [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] Moreover, when combined with a PS, the catalytic CO 2 reduction efficiency can be greatly enhanced. [START_REF] Bonin | Selective and Efficient Photocatalytic CO 2 Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst[END_REF] In fact, it has been known since the early 1980s that porphyrin based complexes, through either electrochemical or photochemical methods, act as effective catalysts for CO 2 reduction. [START_REF] Kapusta | Carbon dioxide reduction at a metal phthalocyanine catalyzed carbon electrode[END_REF] In this chapter, we will report on a Fe-porphyrin complex which is able both to collect visible light and to selectively catalyze the reduction of CO 2 into CO. This single-molecule approach is rare, especially considering the high stability of the process and the fact that the catalyst is based on earth abundant metal. We will also discuss the possible mechanism based on electrochemical and spectroscopic measurements. Remarkably, this catalyst can also work in an aqueous solution system with the addition of a proper PS. Therefore, we believe that these insights have significant implications for the design of new and improved molecular catalyst for the design of optimized process conditions for efficient photochemical CO 2 reduction in both organic and water solution. In ideal conditions, the energy to drive the catalytic reduction of CO 2 would be derived directly (photocatalysis) or indirectly (solar-driven electrocatalysis) from the Sun. Numerous reports on electrocatalysis since the 1970s have emerged using a host of transition metals including Re, Ru, Co, Fe, Ni and others. [START_REF] Bediako | Proton-Electron Transport and Transfer in Electrocatalytic Films. Application to a Cobalt-Based O 2 -Evolution Catalyst[END_REF] Some of these electrocatalysts have been demonstrated as capable of reducing CO 2 when combined with a visible-light photosensitizer (Figure 2.1). [START_REF] Huckaba | Photocatalytic Reduction of CO 2 with Re-Pyridyl-NHCs[END_REF] In the past several years, we were particularly interested in using metal complexes as homogeneous catalyst based on earth abundant elements such as cobalt or iron, both as electro-and photocatalysts. [START_REF] Bonin | Selective and Efficient Photocatalytic CO 2 Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst[END_REF]100 In the latter approach, the goal is to use visible light as the only incoming energy, since almost 50% of the solar light flux is located in the visible range. We have previously showed that iron porphyrins could be used as photocatalysts for the reduction of CO 2 into CO, but they suffered from photodegradation under near-UV irradiation [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] and were inactive under visible light if not associated with a photosensitizer. [START_REF] Bonin | Selective and Efficient Photocatalytic CO 2 Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst[END_REF] Actually, very few catalysts capable of both absorbing solar spectrum irradiation and reducing CO 2 were reported to date. Sensitization (Figure 2.1) is a common solution to shift and/or extend light absorption of a catalytic system, but it implies the use of an additional molecular partner which is frequently based on noble metal (e.g. Ru, Ir) complexes, even if some examples of cheaper organic sensitizers have been identified. 74, 90, 100a Luminescent tricarbonyl Re(I) diimines complexes have been used as selective catalysts for the CO 2 -to-CO conversion in aprotic solvent without sensitizer, but the solar light utilization is limited in these cases by the weak visible absorption due to the weak tailing in this range. 101 Replacing a CO ligand with an N,O or N,S-NHC containing ligand (weaker π-accepting ligand) led to shifting the MLCT transitions to lower energy and makes the complex able to harvest visible light (λ > 480 nm). 102 Recently Re(I) pyridyl N-heterocyclic carbene (NHC) complexes have also been used to directly absorb visible light and catalyze CO 2 reduction. [START_REF] Huckaba | Photocatalytic Reduction of CO 2 with Re-Pyridyl-NHCs[END_REF] Another series of non-sensitized CO 2 reduction catalysts involving an iridium center, a noble metal, were also reported to have moderate activity under visible light. 74, 89, 98, 100c, 100d, 101-103 Such examples of non-sensitized molecular photochemical conversion of CO 2 however remain scarce. And as alluded above rare examples using an earth abundant metal based complex have been identified so far (Co tetraphenyl porphyrin [START_REF] Dhanasekaran | P-terphenylsensitized photoreduction of CO 2 with cobalt and iron porphyrins. Interaction between CO and reduced metalloporphyrins[END_REF] , Fe phatlocyanine 84c and substituted Fe tetraphenyl porphyrins [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] and they all led to low TONs and poor stability over time, while employing UV light as an excitation source. In this part, a modified tetraphenyl iron porphyrin (Fe-p-TMA, Figure 2.2) which operates under visible light irradiation without the assistance of another light collector was used for directly reduce CO 2 . with no trace of H 2 being detected. This performance was already significantly better than what we observed with hydroxyl-functionalized porphyrins in the absence of PS. [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] The selectivity towards CO formation reaches 100% with Fe-p-TMA, whereas some H 2 formation was systematically noticed in the case of hydroxyl-functionalized porphyrin, along with rapid degradation of the catalyst through photo-Birch reduction of the porphyrin ring, converting it to chlorin-type compounds which were ultimately photodecomposed. [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] We then used another SD, namely BIH (Figure 2.2), since this compound was recently reported 105 to enhance the photocatalytic formation of CO from CO 2 , thanks to its high reductive ability (𝐸 / = +0.33 V vs. SCE), 106 its fast deprotonation to BI˙ once oxidized and its overall two-electron donation capacity (BI˙ could be in turn easily oxidized to BI + ). With BIH as SD, the selectivity and robustness were maintained, and the efficiency was increased by a factor of ca. 2, with the number of catalytic cycles in CO reaching 63 after 47 hours of irradiation (Figure 2.3, blue squares and diamonds).

Blank experiments performed in the absence of the catalyst, light or CO 2 all led to the absence of CO production. As can been seen in species with the competing reaction with CO 2 being much faster. It also shows that the porphyrin ring is not reduced and/or hydrogenated since it would have led to the rapid degradation of the catalyst, as previously observed with other analogs. [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] We think the substituted tertraphenyls may help stabilizing the CAT-CO 2 intermediate and it will be discussed in more details in the next chapter. However, the catalytic reaction rate was only weakly enhanced by the acid addition and the number of catalytic cycles was similar with and without acid, reaching 78 vs. 63, respectively, after 2 days of irradiation. One explanation may be that the catalysis is limited by the reduction of the porphyrin to its active state (from the Fe III starting reactant to the Fe 0 catalytic species) with essentially no influence of the acid content on these charge transfer reactions. Another possibility is that the reaction is limited by the release of CO from the Fe-CO adduct obtained after one C-O bond cleavage occurs.

This point is discussed below based on absorption spectroscopy data. The photocatalytic reaction was pursued until the CO production levels off (Figure 2. Irreversibility and somewhat increased current compared with the blank experiment may reflect some catalysis of H 2 evolution from water reduction (Figure 2.6, blue). Trimethylammonio groups are strong electron-withdrawing substituents which will decrease the electron density of the central Fe atom and thus will make it easier to reduce (and harder to oxidize, conversely).

When the Fe porphyrin catalyst is well stabilized, the second electron transfer becomes more difficult than the electro-generation of the Fe 0 and the mechanism is a electrochemical chemical chemical electrochemical (ECCE) process, which is different from that of FeTPP with a electrochemical chemical electrochemical chemical (ECEC) process. 108 However, through a systematic analysis of the wave obtained under 1 atm CO 2 and in the presence of 3 M phenol according to the foot-of-the wave approach, which aims at minimizing the effects of side phenomena which interfere at large catalytic currents. It assumes that the electrochemical reaction mechanism is of the same type as for FeTPP. 109 Introducing the four trimethylammonio groups leads to a positive shift of the Fe I /Fe 0 potential, i.e. a favorable factor in terms of overpotencial, ca. 200 mV less negative than for FeTPP. More surprising is that this effect, which tends to decrease the electron density on the iron and porphyrin ring at the oxidation state 0, does not slow down the catalytic reaction to a significant extent. It has been described to throughspace electrostatic interactions between the trimethylamonio groups (positively charged) and the oxygen atom (negatively charged) of the CO 2 bound to the Fe center, leaving to a very efficient binding step of CO 2 to the Fe 0 catalytic active species. Deciphering the detailed mechanism for the CO 2 -to-CO conversion will require further spectroscopic investigations; however, based on previous in-depth studies related to variously substituted tetraphenyl Fe-porphyrins [START_REF] Hawecker | Efficient photochemical reduction of CO 2 to CO by visible light irradiation of systems containing Re (bipy)(CO) 3 X or Ru (bipy) 3 2+ -Co 2+ combinations as homogeneous catalysts[END_REF][START_REF] Costentin | A Local Proton Source Enhances CO 2 Electroreduction to CO by a Molecular Fe Catalyst[END_REF] and the electrocatalytic mechanism, we suggest that the generation of the Fe 0 catalytically active species likely occurs upon a series of electron transfer steps (the total porphyrin charge is also indicated for all species):

4 4 h III II • Fe -p-TMA Cl Fe -p-TMA Cl ν               ( 23
) 4 4 h III II Fe -p-TMA Cl SD Fe -p-TMA Cl SD ν                   (23') 4 3 h II I Fe -p-TMA SD Fe -p-TMA SD ν                ( 24
) 3 2 h I 0 Fe -p-TMA SD Fe -p-TMA SD ν                (25)
First, the catalyst obtains three electrons from the SD to form the Fe 0 active state with the help of light. The Fe 0 porphyrin then efficiently binds to CO 2 and the Fe-CO 2 adduct (that could be described both as Fe I CO 2 ꞏ-or Fe II CO 2

2-

) is stabilized by through-space coulombic interaction between the positive charge of the Me 3 N + substituents on the porphyrin ring and the partial negative charge borne by the oxygen atoms of the gas molecule (see the proposed mechanism in As already observed during electrochemical studies, CO formation from the Fe-CO 2 adduct is efficient and fast, 110 so that the catalytic rate under photochemical conditions may be limited by the generation of the active Fe 0 species along the ( 1)-( 3) reactions, even if these reactions are more thermodynamically favorable than in the case of the unsubstituted porphyrin. It is probably the case when TEA is used as SD since the Fe I state gives a major contribution to the absorption spectra upon irradiation (Figure 2.5 A). In the case of BIH as SD, the driving forces for the electron transfer reactions are larger because BIH is more easily oxidized. In that case, the major contribution to the absorption spectra upon irradiation comes from the Fe-CO adduct (Figure 2.5 B), while Fe I species do not accumulate, suggesting that the adduct photo-cleavage or homogeneous reduction by an Fe 0 species is rate determining. Further mechanistic investigation will shed more light on these aspects.

In conclusion, the introduction of four trimethylammonio groups onto the para position of the phenyl rings of an iron porphyrin makes the complex active under visible light for the photochemical reduction of CO 2 to CO, without an additional photosensitizer. Irradiation does not lead to catalyst photodegradation and the competitive hydride formation pathway previously observed with other porphyrins is avoided. Thanks to the stabilizing role of the four positively charged groups, the catalyst is highly selective and robust, generating pure CO from CO 2 with no loss of activity over 2.5 days, albeit at a moderate rate.

Purpurin-sensitized system in aqueous solution for CO 2 -to-CO conversion

As mentioned before, most of the reported molecular catalysts are employed in organic solvent such as in ACN, DMF or THF. Aqueous CO 2 reduction systems are quite rare but certainly more interesting in terms of large scale applications. We already recalled that the direct reduction of CO 2 is a highly unfavored and that a coupled proton transfer reaction is always necessary for facilitating the reduction process above one electron. If water can be used as both the proton source and the reaction solvent, in a similar way that in natural photosynthesis, the whole process will be much more sustainable, scalable and environmentally friendly. However, proton reduction in water-containing systems proton reduction is inevitably competitive with CO 2 reduction due to the thermodynamic stability of CO 2 , leading to a decrease in both efficiency and selectivity for CO 2 -to-CO conversion. [START_REF] Lehn | Photochemical Generation of Carbon Monoxide and Hydrogen by Reduction of Carbon Dioxide and Water under Visible Light Irradiation[END_REF] Moreover, CO 2 solubility in water is much lower than that in organic solvent as shown in Table 2.1. 111 With the introduction of four trimethylammonio groups at the para position of the phenyl ring on the Fe-porphyrin, not only the electron density of the catalyst center and the redox potential were changed, but also the polarity of this molecular was greatly changed. In fact, the solubility of Fe-p-TMA in water is even greater than in ACN. It is also one of our final targets that to do the catalysis of CO 2 reduction in pure water. As a consequence, this section is dedicated to the use of Tertiary amines are widely used as sacrificial electron donors (SDs) in photocatalytic systems.

Several amines were used as SDs in this system. The best results were obtained with triethylamine (TEA). As shown in Figure 2.9 and in Table 2.2 (entry 1), CO is produced upon visible light irradiation (λ > 420 nm). The amount of CO keeps increasing for about two days and then levels off, with a TON of 60. A small amount of H 2 is also produced with a TON of 5.

Hence the selectivity for CO production is 95 %.

Interestingly, in the absence of PP -, a TON of 6 was obtained for CO after 47 h irradiation,

showing that the Fe catalyst is also able to slowly convert CO 2 into CO without any sensitizer.

When the amine was omitted, TONs of 8.4 for CO and 1.6 for H 2 were measured, indicating that water itself could act as a sacrificial electron donor. Figure 2.9 shows the effect of the nature of the amine used. TEA gave slightly better results than triethanolamine (TEOA) and ethylenediamine tetraacetic acid (EDTA), although the TONs are not very different (Table 2.2, entries 2-4), reflecting the fact all three donors have similar redox potentials. 114 It is remarkable that the selectivity for CO formation remained very high even under these aqueous conditions. As already discussed in this thesis, the active species reacting with CO 2 is the reduced Fe 0 porphyrin, implying that the latter compound does not lead to hydride formation followed by hydrogen evolution. It also indicates that the porphyrin ring itself is not reductively hydrogenated, since that would have led to rapid degradation of the catalyst, as previously reported for similar derivatives. [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] These results are also in accordance with recent electrochemical studies, which showed that Fe-p-TMA is one of the few molecular catalysts to keep its high activity towards CO 2 in aqueous solutions. 109 One major reason for the catalytic activity levelling off is photosensitizer instability, as shown in Figure 2.10, where the main absorption peak of PP -(at 510 nm) starts decreasing after a couple of hours, reaching a 60 % lessening after 47 h. PP -photobleaching has otherwise been described. 115 Adding more PP -to the solution mixture after two days of irradiation almost fully restored the catalytic activity (Figure 2.11), with a TON of 120 for CO and retention of 95 % catalytic selectivity at the end of a four-day experiment (Table 2.2, entry 6). When a more concentrated solution of the photosensitizer (0.4 mM PP -) was initially used, more CO was also produced (TON = 71; Table 2.2, entry 2) while the same excellent catalytic selectivity towards carbon dioxide reduction was maintained. Upon adding more catalyst to the solution after 47 h of irradiation, only a slight increase of the TON for CO was observed (Figure 2.11, left, and Table 2.2, entry 3), confirming that sensitizer stability over time is the main factor explaining the levelling off of formation of the reduction product. This observation is also in accordance with recent studies that showed that Fe-p-TMA remains stable even after extensive visible-light irradiation. 104, 116 After 47h / 47h irradiation and addition of one equivalent of PP after the first 47h. 112

In the absence of catalyst no product was detected, while photocatalysis using 13 C-labeled CO 2 led to [START_REF] Quinn | Carbon dioxide[END_REF] CO production as detected by GC-MS, confirming that carbon monoxide was issued from CO 2 reduction (Figure 2.12). where I 0 and I are the emission intensities without and with quencher, respectively, k q is the apparent quenching rate constant, τ 0 is the exited-state lifetime without quencher, and [Q] is the quencher concentration. Based on literature data, 113 the excited-state lifetime of PP is in its monoanionic species was taken as 2600 ps. Quenching experiments of a PP -solution excited at 420 nm showed that PP *-reacts with TEA extremely fast, close to the diffusion limit, the quenching rate constant being estimated at 1.2 × 10 10 L mol -1 s -1 (Figure 2.13). V vs. SCE] species, the corresponding driving forces being largely negative. As PP 2-is a stable species, it may be excited to PP *2-and further quenched by TEA to the doubly reduced species PP 3-. The standard redox potential of the former species was determined by CV to be -0.83 V vs. SCE (Figure 2.16). PP 3-could then both react by electron transfer with Fe II porphyrin and further reduce Fe I to the catalytically active Fe 0 species. The following series of electron transfer steps (the global charge is also indicated for all porphyrins) could thus be proposed:

4 4 III 2 II Fe -p-TMA Cl PP Fe -p-TMA PP Cl                    ( 26
) 4 3 II 3 I 2
Fe -p-TMA PP Fe -p-TMA PP The thermodynamics for reactions ( 27) and ( 28) are still uphill by +0.17 eV and +0.47 eV respectively. However, when taking into account the work terms, these reactions are favorable in the case of Fe II reduction and only slightly endergonic for Fe I reduction. The driving force for electron transfer is given by Equation 29:

                ( 27
)
3 2 I 3 0 2 Fe -p-TMA PP Fe -p-TMA PP                 (28)
3 2 n ( n1 ) 0 0 0 R P PP /PP Fe /Fe ΔG E E w w          (29)
where w R and w P are the work terms required to bring the reactants and products, respectively, from infinity to reacting distance. These terms are assumed to be purely electrostatic in origin and are significant, since both reactants and products are charged. They can be estimated as follows:

w R + w P = (-3 × 4 -2 × 3) × w 0 = -18 × w 0 = -0.47 eV for reaction (27) and w R + w P = (-3 × 3 -2 × 2) × w 0 = -13 × w 0 = -0.338 eV for reaction (6); w 0 = e 0 2 / (4π × ε 0 × ε S × d) = 0.026 eV, where d is the distance between the two reactants (it has been estimated in similar cases to be 7 Å) 121 and ε S is the solvent dielectric constant (78.8 for water). By using Equation ( 29), ΔG 0 = -0.3 eV for ( 27) and 0.13 eV for (28), showing that the two electron transfer reactions are indeed feasible.

In the case of reaction ( 28), the reaction is still slightly unfavorable but it is followed by the fast, favorable binding to CO 2 .

After reduction to the Fe 0 catalytically active species, the porphyrin binds to CO 2 . The Fe-CO 2 adduct [Fe-CO 2 ꞏ-↔ Fe II CO 2 2-] is likely stabilized by a through-space coulombic interaction between the positive charges borne by the phenyl rings and the partial negative charges of the oxygen atoms of the carbon dioxide, as already suggested. 110 C-O bond cleavage may then occur from protonation of the Fe-CO 2 adduct, leading to the release of a water molecule and a Fe-CO adduct. The latter species furnishes CO either upon photoirradiation or by a one-electron homogeneous reduction by the Fe 0 species. 104 In conclusion, introducing trimethylammonio groups on the para position of the phenyl rings of an iron porphyrin provides a water soluble molecular catalyst able to perform photochemical conversion of CO 2 into CO in aqueous solutions, at close to neutral pH. By using visible light and a cheap organic sensitizer, selective production of CO was maintained over two days of irradiation. Catalysis is ultimately limited by progressive sensitizer degradation but it could be started again by adding fresh sensitizer. This work opens new perspectives for developing efficient CO 2 photocatalysis in water. Further studies aiming at this goal will include the development of new sensitizers combining efficient light absorption properties with increased stability over long-term irradiation.

Chapter 3 Photochemical Reduction of CO 2 to CH 4 under Visible Light with porphyrin Molecular Catalysts

Introduction

The production of carbon-based fuels or chemicals using the most abundant carbon source (CO 2 ) requires designing efficient, cheap, selective and sustainable processes able to convert CO 2 into target products. 122 Such processes could be achieved using electrolyzers, the electricity being issued from renewable sources. To sum up, most reports on CO 2 reduction to methane have involved copper-based electrodes or semiconductor materials in a heterogonous photocatalytic system. The development of molecular catalysts for homogeneous photocatalytic CO 2 -to-CH 4 conversion is of far-reaching significance for fundamental understanding of the process. In this chapter, we will discuss the possibility for Fe-porphyrin complexes to achieve the photochemical CO 2 -to-CH 4 conversion.

Ir(ppy) 3 sensitized Fe-porphyrin system for CO 2 reduction in

ACN solution

Choice of the sensitizer and components of the system

As already discussed in Chapter 1, most of the transition metal complex catalysts show poor absorption in the visible region, a photosensitizer is thus very necessary for the utilization of solar light. Systems applied for light-driven CO 2 reduction usually include a catalyst, responsible for the capture of CO 2 molecules and their subsequent reduction; a sacrificial electron donor (SD), which is consumed stoichiometrically; and a photosensitizer (PS) for harvesting light energy and mediating the electron transfer from the sacrificial electron donor to the catalyst. Two wellknown mechanisms by which the excited PS is quenched are illustrated in Figure 3.1. In the oxidative quenching process, a first step consists in an electron transfer from the PS * to the catalyst, whereby the PS is oxidized, and the active, reduced form of the catalyst is generated. In the second step, the original PS is regenerated by donation of an electron from SD to PS + .

Whereas it is different from the reductive quenching mechanism, the excited PS is first reduced by an SD to form PS -and SD + . This electron is then transported to the neutral catalyst to generate the active CAT -. The preference for one mechanism over the other is strongly dependent on the respective redox potentials of the particular species of the CAT, PS and SD. And most of the time, both these two mechanism can happen at the same time. Their relationship is competitive with each other. In particular, substitution of the four para-phenyl hydrogens by trimethylammonio groups 109 led to CO formation with selectivity close to 95% in water at pH 7 as well as in aprotic solvent such as N, N-dimethylformamide (DMF), at low overpotentials and with catalytic activity that could be maintained over several days. The fact that the standard redox potential of the Fe I /Fe 0 is not very negative (E 0 = -1.50 V vs. SCE) combined to the remarkable high intrinsic activity towards CO 2 reduction makes these catalysts good candidates for photochemical reduction of the gas. In the last chapter, we have showed that Fe-p-TMA can photochemically converts CO 2 into CO without any PS or with an organic dye as PS. In this part, we will choose Ir(ppy) 3 (PS-8) as the PS owing to its much stronger redox ability, which may sharply promote the catalytic efficiency.

Experiments under CO 2 atmosphere

In Chapter 2, Fe-p-TMA was used as a photocatalyst without a photosensitizer under visible light irradiation (> 420nm) with triethylamine (TEA, 50 mM) as SD. Illumination of a 1 atm CO 2 -saturated solution of acetonitrile containing 2 μM of Fe-p-TMA at room temperature for 47 h selectively produced CO, with a turnover number in CO relative to catalyst concentration of 33.

No side products were observed, and the linear production of CO with time indicates good stability of the catalytic system. When purpurin was added as PS, the TON of the system increased. Nevertheless, it suffered from the instability of PP under a long-run irradiation. A factor that can potentially limit the catalytic rate of this system [START_REF] Bonin | Selective and Efficient Photocatalytic CO 2 Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst[END_REF]104 is the three-electron reduction of the initial Fe III porphyrin species to generate the active Fe 0 state. Using electron donors with high reducing ability should thus be favourable, and adding 0. 3.4, red), which was much more efficient than the previous systems. More interestingly, besides CO and H 2 , a third product in the gas phase was found through gas chromatography. This peak was preliminarily proved to be methane by injection of standard CH 4 samples (Figure 3.5). The system achieved a TON of CH 4 of 31 with a selectivity of ca. 12% in 47 h. In addition, some hydrogen was formed with a TON of 24 and selectivity of 10%.

To prove that the CO 2 reduction reaction was catalyzed by the CAT, multiple control experiments were conducted. In one control experiment, argon gas was supplied in the solution instead of CO 2 , which resulted in the exclusive production of hydrogen with a TON of 43 (Table 3.1). Without CAT, SD or light, no product could be detected. When using an isotopically labelled [START_REF] Quinn | Carbon dioxide[END_REF] CO 2 gas feed, mass spectrometry showed signals for [START_REF] Quinn | Carbon dioxide[END_REF] CH 4 (m/z = 17), confirming that this product is derived from 13 CO 2 photochemical reduction. Whereas using general 12 CO 2 (Figure 3.6), mass spectrometry showed signals for [START_REF] Steinberg-Yfrach | Light-driven production of ATP catalysed by F 0 F 1-ATP synthase in an artificial photosynthetic membrane[END_REF] CH 4 (m/z = 16).

Experiments under CO atmosphere

As shown in Figure 3.4, the production rate of CH 4 was quite slow at the first few hours and then increased rapidly. Evolution of the different products shows that methane production starts only after a large amount of CO has built up, suggesting that CO is an intermediate in the methane formation process. On the other hand, we have also previously shown by ultravioletvisible spectroscopy (Figure 2.5) that irradiation of a CO 2 saturated solution of CAT without a sensitizer (in that case, only CO is obtained) led to the formation of detectable amounts of Fe II CO species. We may thus hypothesize that this iron-carbonyl adduct is an intermediate for further reduction towards methane in the presence of a strong reducing agent. To explore the influence of CO on methane formation more directly, experiments were then conducted in a 1 atm COsaturated acetonitrile solution under visible light irradiation (λ > 420 nm), with PS-8 as PS and TEA as SD. After 47 h of illumination, CH 4 was found to be the main products through gas chromatograph (Figure 3.7) and this system achieved a turnover number of CH 4 of 89 (Figure

Optimization of the system

Under CO 2 atmosphere, blank experiments in the absence of CAT, SD or light achieved nothing or trace of H 2 (entry 6, 7, and 8 in Table 3 with some loss of selectivity (from 87% to 82%; entry 12 in Table 3.1). The successful methane evolution under these conditions over 102 h with an average rate of 1,467 μmol h -1 g -1 illustrates the robustness, activity and selectivity of the catalytic system. Taking 159 as the highest turnover number for CH 4 , we obtain a quantum yield Φ of about 0.18% after 102 h of irradiation (see experimental section for details of the calculation). When replacing Fe-p-TMA by Fe-o-OH (Table 1), which carries -OH groups at all ortho, ortho' positions of the four phenyl rings [START_REF] Costentin | A Local Proton Source Enhances CO 2 Electroreduction to CO by a Molecular Fe Catalyst[END_REF] instead of trimethylammonio groups at the para positions, methane was also evolved although in slightly smaller amounts (turnover number 26 after 47 h irradiation and 14% selectivity, Table 3.1, entry 9). The standard redox potential E 0 (Fe I /Fe 0 ) = -1.575 V vs. SCE 134 in DMF for Fe-o-OH is only 75 mV more negative than for Fe-p-TMA, and as in the latter case, the substituents on the phenyls may help stabilizing reaction intermediates (through internal H bonds involving the -OH groups). In contrast, the non-substituted tetraphenyl Fe porphyrin (FeTPP, Table 1) only gives CO and H 2 (with turnover numbers/selectivities of 84/79% and 22/21%, respectively) under the same irradiation conditions [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] , probably owing to its much more negative standard redox potentials (for example, E 0 (Fe I /Fe 0 ) = -1.67 V vs. SCE in DMF) 135 and the absence of phenyl ring substituents for stabilizing intermediate species involved in hydrocarbon production. The ability to produce methane is thus not restricted to Fe-p-TMA, and it is probably a more general property of Fe porphyrins that have a sufficiently positive standard redox potential and are functionalized with substituents that can stabilize intermediates involved in the catalytic cycle. As we have shown that increasing the irradiation time increased the amount of CO 2 reduction products generated, a longest irradiation time of 102 h in these series experiments was conducted.

The final TON CH4 reached 79, corresponding to a production rate of 763 μmol per hour per gram of catalyst (μmol h -1 g -1 ), which exceeds the rate of many other catalysts 129, 131a, 131b, 131e, 136 that generate methane from CO 2 . The linear evolution of both CO and CH 4 over more than 80 h and the stable absorption spectrum of the system under irradiation (Figure 3.10), with no evidence for degradation of the sensitizer PS-8 or Fe-p-TMA, illustrate the stability of the catalytic system. Emissions quenching experiments between the excited state of the PS-8 and Fe-p-TMA or TEA were shown in Figure 3.12. PS-8 and TEA revealed very weak quenching with TEA, while it is very efficient and diffusion-controlled with Fe-p-TMA (k q ≈ 1.7 × 10 10 M -1 s -1 ), suggesting that direct electron transfer occurs from the excited PS-8 to the Fe-porphyrin through an oxidative quenching process, which is in line with the above redox potential analysis. In Nature, green plants convert CO 2 into carbohydrates a complex biochemical process occurring in water environment. However, most of the reported photochemical CO 2 reduction systems are in organic solvent, mainly ACN and DMF. In contrast, the photochemical CO 2 -to-CO conversion in water has only been achieved in few cases, with modest turnover number and limited stability over time. 139 The easy occurrence of the photocatalytic proton reduction is competitive with the CO 2 reduction reaction, which may inevitably results in a decrease of the efficiency and selectivity for the latter. Therefore, to develop a highly active and selective watercontaining photochemical CO 2 reduction system is very necessary for constructing an artificial photosynthesis cycle. In addition, water is used not only as the reaction solvent but also both as the proton and electron sources, which is noticeably environmental friendly. We have previously demonstrated the remarkable performance of Fe-p-TMA catalyst (Figure Herein we show the use of PS-9 for photochemical CO 2 reduction with Fe-porphyrin catalyst.

Ir-9 has a relatively intense absorption band even at wavelengths longer than 450 nm, and its excited state has a longer lifetime than that of typical sensitizer [Ru(bpy) 3 ] 2+ . Although the performances remain limited by the instability of the sensitizer upon prolonged irradiation, we have found it opens the door for the use of water as solvent in the generation of solar fuels from CO 2 catalysis in homogeneous systems.

Preliminary experiments in ACN solutions

As both the structure and properties of PS-8 and PS-9 are quite similar, we conducted a series of experiments in ACN with PS-9 to make a straight comparison with previous results. As shown in Figure 3.15, three kinds of tertiary amine (TEA, TEOA and DIPEA) were used as SD.

Similarly to what has been observed with PS-8 sensitizer, CO, H 2 , and CH 4 were produced with CO being the major product (ca. 60% catalytic selectivity with a maximum TON of 178, Table 3.2), while methane was produced with ca. 10% selectivity (TON of 32 with TEA as a sacrificial donor).

Using TEA as SD led to the highest TON for the reduction products (CO, CH 4 ) among the three amines tested (TEA, TEOA and DIPEA), but no major difference could be observed in terms of both efficiency and catalytic selectivity (Table 3.2). We then conducted a series of experiments using a mix of 30% ACN and 70% water (v/v) as the reaction solvent, chosen to maximize the water proportion while allowing a suitable solubility for [Ir(ppy) 2 (bpy)] + to ensure an efficient light absorption. Keeping all other parameters (including concentration) identical, we observed (Figure 3.16) a lower amount of all reduction products, which may be ascribed to a significantly lower solubility of the reactant (CO 2 ) in the reaction mixture. The sacrificial electron donor giving the best efficiency is TEA, once again with no major difference in terms of efficiency and catalytic selectivity for the three SDs that have been employed (Table 3.2). Regarding methane formation, catalytic selectivity was close to 10%, with a maximum TON of 3 obtained with all SDs. Overall, a ca. 75% catalytic selectivity for CO was measured, corresponding to a TON in the range of 20 to 25. At first look, these results are not surprising since all SDs have relatively close oxidation potentials (0.5-0.8 V vs. SCE range); 114, 144 however, TEOA has a lower pK a (7.9) 114 than TEA (10.7) and DIPEA (10.5) 145 so one could have expected a higher generation of H 2 in the former case, which was not observed.

Electron transfer mechanism in this system

To help understanding the reaction mechanism in aqueous solutions, we conducted emission quenching experiments upon exciting the [Ir(ppy) 2 (bpy)] + sensitizer at 420 nm in the presence of increasing concentration of TEA, TEOA or DIPEA (Figure 3.17-3.19). Analysis of emission quenching using the Stern-Volmer analysis and the excited-state lifetime measured by time-resolved laser kinetics (145 ns, Figure 3.20) gives quenching rate constants of 5.8 × 10 6 , 4.6 × 10 6 , and 1.4 × 10 8 M -1 s -1 for TEA, TEOA, and DIPEA, respectively. However, no quenching was detected with Fe-p-TMA (Table 3. These results first revealed that the excited sensitizer is quenched via a reductive pathway from the sacrificial amine and is thus reduced into the anionic form [Ir(ppy) 2 (bpy)] •-(see below), as reported before. 146 Second, the quenching rate constants are well below the diffusion limit, also as already reported 147 so that the initial electron transfer steps leading to the catalytic active species (Fe 0 ) are of low efficiency. It is important to note that these characteristics are in contrast with what we observed before with Ir(ppy) 3 in organic solvent. In that latter case, the quenching process goes through an oxidative electron transfer with the catalyst at a rate constant close to the diffusion limit. 116 On the other hand, the values for the reduction/oxidation potentials for the investigated species of [Ir(ppy) 2 (bpy)] + found in literature and the measured values are summarized Figure 3.22. 147c As we can see, the oxidation potential for the process of * [Ir(ppy) 2 (bpy)] + to [Ir(ppy) 2 (bpy)] 2+ is -0.85 V vs. SCE and the Fe-p-TMA displays the third reduction event at ca. -1.50 V vs. SCE.

Evidently, the oxidative quenching by direct electron transfer from the excited state of PS to CAT is highly thermodynamically unfavorable, which is also in line with the emission quenching experiments. Therefore in the initial stage, the excited state of the iridium PS should be reductively quenched by SD to give a reduced species [Ir(ppy) 2 (bpy)] (E 0 = -1.42 V vs. SCE for [Ir(ppy) 2 (bpy)] + / [Ir(ppy) 2 (bpy)]), which is much more capable to reduce Fe III into Fe 0 active site.

Besides, the concentration of SD is always much higher than that of CAT, which is also in favor of a reductive quenching pathway. Interestingly, even if the quenching rate constant is ca. 25 times faster for DIPEA than for TEA, TON with DIPEA remains lower than for TEA, thus highlighting, as already noticed, the absence of simple rationalization in the choice of a particular amine as SD. 114 The choice of a suitable SD is difficult to rationalize. For instance, Bernhard et al. described the superiority of TEA over TEOA for their photocatalytic system for stability reasons. 148 Chang et al. observed an improved efficiency with TEA rather than TEOA, 149 whereas Zou et al. reported the improved reductive quenching rates with TEOA vs. TEA. 150 The conditions are nevertheless rather different from one experiment to another, depending on the requisites of each property (solubility, activity of the catalysts…) in the considered catalytic systems. There is therefore no clear-cut consensus, TEA and TEOA having very similar properties. Conclusively, picking an organic amine SD over another depends on many parameters, including the property of the reaction components and the overall reaction medium.

Stability of the photochemical system

When employing the neutral Ir(ppy) 3 as sensitizer in organic media, we previously determined that CO was a key intermediate product in the CO 2 reduction process, and that the Fe II CO adduct species was further reduced in a multi-electron multi-proton process to afford CH 4 . 116 Without any addition of acid, residual water and/or SD protonated form could act as the proton source.

Addition of a weak acid such as trifluoroethanol (TFE) accelerates the reaction and as shown in Figure 3.23, optimized conditions consisting of the addition of 0.5 M TFE allow the system to reach, after 102 h irradiation, 195 TON in CH 4 , with a selectivity of 81% (also see in Table 3.3).

Using larger acid concentration or a stronger acid may lead to selectivity decrease and larger amount of hydrogen. We further transposed this approach to the aqueous system containing [Ir(ppy) 2 (bpy)] + as photosensitizer and performed irradiation in CO saturated solutions. We observed in this case a different behavior. After a few tens of minutes of visible-light irradiation, the color of the solution drastically faded and a precipitate finally appeared, as showed in Figure 3.24, whereas no change at all was observed during a similar experiment conducted under CO 2 atmosphere. The observed precipitation could be ascribed to a structural modification of the iridium sensitizer giving an insoluble form. Otherwise, it has been reported 151 that cationic [Ir(ppy) 2 L 2 ] + complexes (L being ancillary ligand such as pyridine, acetonitrile) can bind to CO and/or to acetonitrile and optical signatures of these complexes give peaks close to 450 and 485 nm, respectively, the latter being observable in our experiment (Figure 3.24).

We thus hypothesize that electron transfer from the sacrificial electron donor to the excited sensitizer induces the loss of the bpy ligand

Conclusions

Photochemical reduction of CO 2 has been achieved using an iron porphyrin as catalyst, an iridium complex [Ir(ppy) 2 bpy] + as sensitizer and various amines as sacrificial electron donor.

Upon visible light irradiation, CO was the main product (178 TON in optimized conditions) and a substantial amount of CH 4 was also produced (32 TON, 10% catalytic selectivity) while H 2 was formed as a minor byproduct when acetonitrile was used as a solvent. Among the various amines employed, TEA appeared as the best choice to maximize CO 2 product formation. Remarkably, the CO 2 catalytic reduction could also be realized in aqueous conditions (acetonitrile/water 3:7 v:v). In that case, the main product was again carbon monoxide while methane was produced with selectivity up to 10% (3 TON). Efficiency of the catalytic system is hampered by the limited CO 2 solubility and most importantly by instability of the sensitizer that leads to CO ligandation and further loss of activity. Despite these limitations, this work opens the door to further studies in pure water once more stable sensitizers have been designed.

Organic dyes sensitized visible-light-driven CO 2 -to-CH 4 reaction

Building organic dyes-based photochemical systems

As we have already shown above and also according to literatures, the popular molecular photochemical CO 2 reduction is mostly studied via a multi-component catalytic system. A typical manmade system always contains a photosensitizer and a heterogeneous or homogeneous catalyst. 154 A large majority of these studies has employed precious-metal polypyridyl ruthenium and iridium photosensitizers. These metal complexes exhibit essential characteristics, including strong absorption of visible light by means of spin-allowed metal-to-ligand charge transfer (MLCT), efficient conversion to long lifetime triplet MLCT excited ( 3 MLCT) state because of heavy atom effect, and excellent redox reversibility. 155 Their redox properties of the ground and excited states are easily tailored through ligand and metal modifications. However, both ruthenium and iridium are precious metals and amongst the rarest elements on the earth, escalating their costs and presenting concerns related to sustainability and scalability, driving the need to realize new photosensitizers incorporating non-precious metals [START_REF] Takeda | Photocatalytic CO 2 reduction using Cu (I) photosensitizers with a Fe (II) catalyst[END_REF]156 or develop entirely organic succedaneums. 157 Some of these compounds (PS-1, PS-2, PS-3 and PS-4) have been discussed in the previous chapter. Other common organic sensitizers used for solar fuel conversion are Eosin Y, 158 rhodamine dyes, 159 perylene diimides 160 and carbazolyls, 161 which are excited state oxidants and operate through a reductive quenching cycle. Although a few strong reducing organic photosensitizers exist, many do not efficiently absorb visible light. 162 Photosensitizers obviously have to exhibit both wide and strong visible light absorption ranges and appropriate redox properties. As part of efforts to design and examine new systems based solely on earth-abundant elements, we have tried to design such a new homogenous system for the photocatalytic reduction of CO 2 using highly absorbing organic dyes and an iron-porphyrin molecular catalyst.

We have already shown in Chapter 2 that Fe-p-TMA could convert CO 2 into CO with PP as the sensitizer. However, in this system, no CH 4 could be detected by the GC. The possible reason could be that the reduction potential of PP was too low to further reduce the Fe-CO intermediate into CH 4 . As a consequence, the great challenge is to develop a new sensitizer possessing a stronger reducing power. In general, photosensitizers that possess such strong excited state reducing power are rare, in particular for pure organic systems.

To address this challenge, we have brought our interest to the work of Miyake and co-workers who developed a series of visible-light organic photosensitizers, including perylene, 163 N, Ndiaryl dihydropenazines, 164 and N-aryl phenoxazines 165 that mediate electron through an oxidative quenching pathway in photoredox applications. 166 Dihydrophenazine and phenoxazine contain electro-rich chromophore motifs that form stable radical cations upon oxidation and enable them to be strong excited state reductant. 167 In the following part, we will discuss organic

Photochemical CO 2 reduction experiments

As shown in Figure 3.27 (open symbols), in our typical photochemical assembly, now containing 10 μM Fe-p-TMA, 1 mM Phen2, and 0.1 M TEA of CO 2 -saturated DMF solution under visible irradiation (> 435 nm), we found H 2 , CO and CH 4 in the gas phase. DMF was used as the solvent instead of ACN because of a higher solubility of Phen2 in the former one.

Wavelength higher than 435 nm of light source instead of higher than 420 nm was chosen because we hoped to make full use of Phen2 to absorb visible light and avoid the catalyst itself absorbing light energy. After 47 h reaction, CH 4 production with a TON of 8 was observed, alongside with the formation of H 2 and CO (TON of 8 and 50, respectively).

Furthermore, we found that with the addition of 0.1 M TFE as an external acid, the production of CO and CH 4 was noticeably improved (TON of 71 and 14, respectively, see in Blank experiments showed that the omission of any single reactive component (Fe-catalyst, organic sensitizer, SD, CO 2 or light) produced no CH 4 . GC-MS experiments performed under a [START_REF] Quinn | Carbon dioxide[END_REF] CO 2 atmosphere confirmed that the produced CH 4 originated from CO 2 as shown in Figure 3.29.

Moreover, long term irradiation (over 100 h) led to a TON in CH 4 of 29 and CO of 140 (Figure 3.30). The stability of the system was followed by UV-Vis absorption spectroscopy over the 96 entire irradiation course (Figure 3.31) and it showed no significant Fe-p-TMA or Phen2 degradation. It indicates that Phen2 as a PS, which is an organic analogue of Ir(ppy) 3 can be a good candidate in solar fuel conversion.

Photochemical CO reduction experiments

According to our previously demonstrated dual catalysis employing Fe-p-TMA and Ir(ppy) 3 , CO could also be used as the starting substrate for CH 4 production. 116 The rationale was that CO is an intermediary species toward the highly reduced CH 4 product. In Figure 3.32, using Phen2 as a photosensitizer in a CO-saturated solution, we observed the formation of CH 4 with a TON of 9 and a selectivity of 30%, while H 2 was formed as the major product. The addition of 0.1 M TFE, however, significantly boosted CH 4 formation with a TON of 45 and 87% selectivity upon 24 h of irradiation. The nature of the SD had only minor effects on CH 4 production, e.g. similar results

were obtained with TEA, DIPEA and BIH, while TEOA gave lower CH 4 production. GC/MS experiments performed under a [START_REF] Quinn | Carbon dioxide[END_REF] CO atmosphere again confirmed that the produced CH 4 was originated from CO (Figure 3.33). On contrary, the nature of the acid had a marked influence on the CH 4 yield (Figure 3.32).

Water, being a weaker acid than TFE, resulted in a much lower amount of CH 4 even with concentration up to 0.5 M. Conversely, the addition of 0.1 M phenol, which is a stronger proton donor than TFE, resulted in forming H 2 as a major product and a decrease in CH 4 production.

Further, the use of higher concentrations of TFE (greater than 0.1 M) reversed the catalytic selectivity toward H 2 formation. Thus, it showed that 0.

Electron transfer mechanism in this system

To investigate the electron transfer mechanism in this system, emission quenching experiments were conducted upon exciting the Phen2 sensitizer at 435 nm in the presence of increasing concentration of TEA or Fe-p-TMA. As shown in Figure 3.34, there is no noticeable quenching when adding TEA with the concentration ranging from 0 to 200 mM. While with increasing concentration of Fe-p-TMA from 0 to 10 µM, it shows very efficient quenching (Figure 3.35).

Analysis of emission quenching using the Stern-Volmer analysis and the excited-state lifetime that was measured by time-resolved laser kinetics (443  18 µs, Figure 3.36) lead to a quenching rate constant of (1.60 ± 0.07) × 10 8 M -1 s -1 , which is a second order rate constant. There results indicated that electron transfer in this system occurs through an oxidative quenching pathway, which is also very similar to what has been obtained when Ir(ppy) 3 was used as the SD. Formation of the key Fe 0 intermediate is considered to be one of the essential processes in the mechanism for the electro-and photochemical CO 2 reduction catalyzed by Fe porphyrin. [START_REF] Costentin | A Local Proton Source Enhances CO 2 Electroreduction to CO by a Molecular Fe Catalyst[END_REF]134,169 It is already known that the oxidation potential for the oxidation of 3 Phen2 * to 2 Phen2 indicates that the proton source is probably not unique and that proton exchange occurs in the system between TEA, TFE and even traces of water in the solvent. It is also in line with the fact that even without the presence of TFE, CH 4 can also be observed. However, it is still difficult to identify the proton source. More studies are needed to figure out the mechanism of this ironcatalyzed photochemical reaction. Based on the above analysis, Scheme 3.3 highlights our proposed mechanism. We suggest that CO 2 first complexes to the triply-reduced Fe 0 species (in the form of Fe I CO 2

Conclusions

In conclusion, we have shown that the visible-light-driven 8e -/8H + reduction of CO 2 to CH 4 using an organic phenoxazine-based photosensitizer (Phen2) and an earth-abundant iron porphyrin catalyst (Fe-p-TMA) at ambient conditions is possible. In a CO 2 -saturated DMF solution, CO (TON of 140) and CH 4 (TON of 29) were produced after 102 h of light irradiation; whereas in a CO-saturated solution, CH 4 was produced with TON up to 80 and a selectivity of 85%. This work is expected to advance the field of photochemical CO 2 reduction and contribute to the mechanistic understanding of multi-electron and -proton transfer processes in CO 2 reduction. Future development will include in-depth mechanistic studies and achieve similar catalysis in aqueous conditions. 112

3.5 Sub-conclusion and sub-perspectives for photocatalytic CO 2 -to-

CH 4 conversion

In this chapter, we discussed the photochemical reduction of CO 2 into both CO and CH 4 using an iron porphyrin complex combined with noble metal complexes or organic dye as photosensitizer, and different kinds of organic amines as sacrificial electron donor. These catalytic systems are not only efficient, but also can keep stable for nearly one week in the present conditions. Our study demonstrates the possibility of designing and tailoring molecular metal porphyrin complex to promote homogeneous photochemical catalysis of CO 2 reduction.

According to the literature, besides Fe-porphyrin, Cr-, Mn-, Co-, Ni-, Pd-, Cu-, Zn-and Gaporphyrin complexes were also reported for molecular catalysis of CO 2 reduction. 126,170 We have done preliminary experiments (not described in this manuscript) where Fe was replaced by Co or Cu as the porphyrin metal center, and the photochemical CO 2 reduction could still process. Both CO and CH 4 have been detected as products with these alternative catalysts. However, catalytic efficiency and selectivity were poorer than Fe-porphyrin. It thus seems that both metal center and porphyrin ligand indeed strongly affect the process. For a detailed investigation of the mechanism, more work needs to be conducted and the support of theoretical calculations is also desirable.

Chapter 4 Conclusions and Perspectives

General conclusions

In this thesis, the main objective was to design a highly efficient, selective and stable molecular photochemical CO 2 reduction process using visible light as the only energy input. The catalytic system is made up of multi-components, i.e. an iron porphyrin complex as the catalytic molecular center, an organic amine as the sacrificial electron donor and a photosensitizer for absorbing visible light energy and transferring it to the catalytic center. The presented results describe the molecular photocatalytic reduction of CO 2 in several different systems, including non-sensitized CO 2 -to-CO conversion, purpurin sensitized CO 2 -to-CO conversion in water solution, iridium complex sensitized CO 2 -to-CH 4 conversion, iridium complex sensitized CO 2 -to-CH 4 conversion in aqueous solution and organic dye sensitized CO 2 -to-CH 4 conversion. The results could be summarized as follows.

1. Photochemical catalytic CO 2 reduction was studied by choosing Fe-porphyrin complexes as catalysts due to their unique physicochemical properties but also because they were proved, in our team, to be highly efficient molecular catalysts for the electrochemical CO 2 reduction. With the introduction of four trimethylammonio groups on the para positon of their phenyl rings, the photochemical reduction of CO 2 to CO, even without any additional photosensitizer, has been achieved under visible light. In this configuration, the porphyrin catalyst both acts as a visible light harvester and as the catalytic active center. Irradiation does not lead to catalyst photodegradation and the competitive hydride formation pathway previously observed with other porphyrin derivatives is avoided. Thanks to the stabilizing role of the four positively charged groups, the catalyst is highly selective and robust, generating pure CO from CO 2 with a 100% selectivity, a total TON of 101 and no loss of activity over 2.5 days, but at a moderate rate.

2. As natural photosynthesis approached by plants and other organisms is realized in aqueous conditions, it is of great significance to conduct the "artificial" photochemical CO 2 reduction in similar media, making the process even much cheaper and more environmentally friendly. For this purpose, we used the same earth abundant metal based molecular catalyst and choose a water soluble organic and very common dye, i.e. purpurin, as the photosensitizer. The photochemical reaction were sensitizer that leads to CO ligandation and further loss of activity. Despite these limitations, this work opens a door to further studies in (ultimately pure) water once more stable sensitizers will be designed.

Future perspectives

The year 2016 was the first in History reaching carbon dioxide concentration in the atmosphere above 400 ppm, and it is not expected to drop any time soon. Since the increasing content of this greenhouse gas in the atmosphere has been recognized, the development of efficient chemical routes for the conversion of waste CO 2 has attracted much attention. The viability of large-scale CO 2 conversion to value-added products, and especially energy carriers, is still a matter of debate.

The most desired solution to utilize CO 2 emitted from the exhaust gases is to use it as the feedstock for the production of synthetic fuels by energy storage in chemical bonds. In association with renewable energy sources, the production of energy carrier molecules could not only have the advantage of replacing at least in part fossil fuel-based energy sources, but also of reducing CO 2 emissions into the atmosphere. In this thesis, we have used an organic amine as the electron and proton donor, which is unsustainable, non-green and contaminative. Future work will also have to look at photocatalysis without using a sacrificial electron donor. The required electrons and protons could come from the splitting of H 2 O, taking full advantage of the oxidation reaction occurring along with CO 2 reduction, thus mimicking natural photosynthesis.

According to IUPAC, the definition of photocatalysis is as follows: change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible or infrared radiation in the presence of a substance-the photocatalyst-that absorbs light and is involved in the chemical transformation of the reaction partners. Fe-porphyrin itself could absorb light and then use it directly for CO 2 reduction. However, this process is of quite low efficiency and thus an extra component-the photosensitizer-is always needed to increase the efficiency of solar energy utilization. Future work should thus focus on developing more efficient, stable, cheaper and possibly water-compatible photosensitizer. For example, several reports highlighted semiconductor quantum dots as photosensitizer in photocatalysis. Besides, we can introduce a chromophoric moity directly into a photocatalytic center through chemical bonding to form a binary or tandem assembly, where one part is dedicated to light harvesting and the other to the catalysis. In this case, the electron transfer between chromophore and catalytic center may be fast and efficient so as to increase the global energy conversion efficiency.

Despite the impressive progress, the efficiency of most reported systems is far from being acceptable for industrial applications. Hence, the core issue is still to develop an ideal catalyst, which is active enough, stable enough and cheap enough to be scalable. In addition, reaction conditions are also very important issues, for example, increasing the reaction temperature and pressure. Moreover, as the concentration of CO 2 in the atmosphere is quite low and since its capture and reconcentration require a large energy input, it is also very attractive to develop catalysis under low concentration of CO 2 , ultimately using air as the direct substrate.

Photosynthesis basically supplies the carbon source for all organic compounds on Earth. It is an ideal green process for the conversion of light energy into chemical energy, which makes use of CO 2 (equation 31). Inspired by the Nature's strategy, the development of sustainable and straightforward catalytic process not only to C 1 products but also to C-C bond-forming reactions in industrially relevant target molecules will be the 'crown jewel' of organic synthesis and catalysis research in the future. For example, sodium acrylate is an industrially important compound, with a global market volume of about 4 million tons and is commonly used as a monomer in the production of superabsorber polymers. Thus, the direct synthesis of acrylic acid from CO 2 and ethylene represents an economically attractive but also most challenging dream reaction (equation 32 THF was dried over sodium then distilled after addition of benzophenone. Such solvents were freshly distilled or stored overnight under an argon atmosphere.

(1, 3-dimethyl-2-phenyl-2, 3-dihydro-1H-benzo [d]imidazole) (BIH) A solution of N, N'dimethyl-1, 2-phenylenediamine (1.5 g, 7.7 mmol) was treated with 4 g of methyl iodide in 10 mL of methanol containing 0.32 g of NaOH. 172 The reaction mixture was heated at 110 °C overnight in a Pyrex bottle. The longer the reaction period, the higher was the yield of the salt, especially for the substances having the electron-withdrawing substituents. The brown crude product was decolorized by the activated carbon in hot aqueous ethanol (ethanol:H 2 O, 5:1 v/v).

After removal of the solvent, the product was recrystallized from absolute ethanol. The yield was over 80%. Without further purification, to a solution of 1.7 g this product in 60 mL of CH 3 OH was slowly added with 459 mg NaBH 4 in ice-bath. The reaction took place instantaneously to give a cloudy, white suspension. The reaction mixture was stirred vigorously for 1 h under Ar.

After removal of the solvent under reduced pressure, the white solid was recrystallized from ethanol:H 2 O (2:1 v/v) to give a colorless crystalline product. The yield was over 80% (0.87 g). IR The complex anion is photo-sensitive. This means that upon exposure to light of an appropriate wavelength (< 450 nm in this case) the [Fe(C 2 O 4 ) 3 ] 3-undergoes an intramolecular redox reaction in which the Fe(III) anion is reduced to Fe(II) while one of the oxalate groups is oxidized to CO 2 .

[Fe(C 2 O 4 ) 3 ] 3-→ Fe 2+ + 5/2C 2 O 4 2-+ CO 2 (g)

The generated Fe 2+ then can readily be detected by adding a buffered solution of phenanthroline. The absorbance of the solution at 510 nm was recorded and the value was used to calculate the number of Fe 2+ ions generated during the irradiation process.

Methods

Simulated solar light

The main figure-of-merit for a solar CO 2 reduction device is its performance under real sunlight. However, even real sunlight does not have the same intensity and spectral distribution everywhere on earth. To facilitate meaningful comparisons of device performances, the performance characteristics are usually quoted for the so-called AM 1.5G conditions. This stands for "air mass 1. (https://www.itos.de/en/schott-optical-filters/long-pass/gg395-gg400-gg420/)

UV-Visible spectroscopy

UV-visible absorption measurements were done with an Agilent Technologies Cary 60 or an Analytic Jena Specord 600 UV/VIS spectrophotometer. Aliquots of the solution were poured into a quartz cell (1 cm in length). Typical spectral region ranged from 200-800 nm and it was swept at a rate of 720 nmꞏs -1 .

Fluorescence spectra

Emission quenching measurements were conducted with a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), with the excitation wavelength set at 420 nm (PS-8 PS-9, and PP) or 435 nm (Phen2) and the emission spectrum measured between 430 nm and 700 nm (PS-8), 450 nm and 800 nm (PS-9), 500 nm and 800 nm (PP) or 440 nm and 750 nm .

Emission intensities used for the Stern-Volmer analysis were taken at 517 nm (PS-8), 592 nm (PS-9), 605 nm (PP) or 506 nm (Phen2), that is, the emission maximum of PS.

Transient absorption spectroscopy

Transient

Infrared spectroscopy

Infrared spectra were recorded on a PerkinElmer Spectrum BX FTIR spectrometer equipped with a Specac Omni Cell P/N 800 measurement cell. The Omni-Cell™ is a multidisciplinary support allowing the simple analysis in static mode and at ambient temperature of sample in transmission mode (Figure 5.3). This support accepts both rectangular windows for the analysis of liquids and round windows for analysis according to the technique "Mull", the windows for the analysis of liquid being available in removable or seals.

Nuclear magnetic resonance (NMR) spectroscopy

The 1 H NMR and [START_REF] Quinn | Carbon dioxide[END_REF] C NMR spectra were recorded on a Bruker Avance III 400-MHz spectrometer and were referenced to the resonances of the solvent used.

Notably, the metalation of the free base porphyrin is performed with iron(II) species. Even though the reaction occurs under inert argon atmosphere, oxygen traces from ambient air readily oxidizes the metal center to the oxidation state +3. Unfortunately, iron(III) is paramagnetic and prevents the acquisition of clean NMR spectra of such highly conjugated compounds. Some experiments have been attempted to reduce in-situ the porphyrin center to Fe II by adding a reducing metal (zinc) or compounds such as NOBF 4 that bind to the metal and yield Fe II species.

None of these attempts were successful in delivering neat NMR spectra.

Gas chromatography and GC-MS

Gas chromatography analyses of gas evolved in the headspace during the electrolysis were performed with an Agilent Technologies 7820A GC system equipped with a thermal conductivity GC-MS for detecting the labeled gas products were obtained by a ThermoFisher Scientific TRACE Ultra gas chromatograph equipped with a CP 7514 column (Agilent Technologies) and coupled to a DSQ II mass spectrometer in positive ionization mode, using a TriPlus headspace autosampler.

The mass spectra for detecting the organic compounds were recorded on a Microtof-Q of Bruker Daltonics. Injection method was high resolution electrospray (ESI).

Ionic chromatography

Ionic chromatography (IC) was used for the detection and quantification of formate in aqueous and organic samples recovered from the reactor after decompression. The presence of other anions, such as acetate, fluoride, chloride, oxalate, etc. can also be determined using this technique. Ionic chromatography measurements were performed with a Thermo Scientific Dionex ICS-1100 system equipped with a Dionex ERS 500 electrolytically regenerated suppressor. All the samples were diluted 50 folds with ultra-pure water before analyzed through IC. Calibration curve for formate was determined separately by injecting known quantities of pure sample.

pH measurements

pH measurements were performed with Hanna pH210 and HI221 instruments and 6 mm microelectrodes (Fisher). Then, the solution in the cuvette was irradiated perpendicularly with a Xenon lamp through an irradiation window of 5 cm × 5 cm by using a Schott GG optical filter and a 2 cm long glass OS cell filled with water to cut off IR. Gas products were analyzed via GC and liquid products were analyzed via 1 H NMR, [START_REF] Quinn | Carbon dioxide[END_REF] C NMR and IC. UV-vis analysis was made directly via the home-made glassware.

Cyclic voltammetry

Details on the quantum yield calculations

The number of incident photons was measured using the classical iron ferrioxalate Finally, the number of incident photons per second (# photons) is calculated using the following equation:

# photons 𝑠 6.022 10 𝑛 𝐹𝑒 Φ 𝑡 where Φ λ = quantum yield of Fe 2+ formation, 1.07 was taken from the literature 174 t = time of irradiation (s) Using three independent measurements, we determined that the number of incident photons to the sample was (2.18 ± 0.17) × 10 19 photons per hour. The CO-to-CH 4 reduction being a sixelectron process, the overall quantum yield Φ of the process was determined using the following equation: Φ % 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐶𝐻 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑓𝑜𝑟𝑚𝑒𝑑 6 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 100%

For example, taking 159 as the highest turnover number for CH 4 (Table 3 My parents' kindness is higher than the mountain, and longer than the river. In my late twenties, I felt ashamed that I did not support them. I appreciate their love, support and encouragement whenever needed. Without them it would not be possible for me to complete this work.

III" 1

 1 Reducing greenhouse gases requires honesty, courage and responsibility, above all on the part of those countries which are more powerful and pollute the most." Traditional fossil fuels and the increasing atmospheric carbon dioxide concentration

Figure 1 . 1

 11 Figure 1.1 World energy consumption by energy source over time, in quadrillion Btu. Data obtained from International Energy Outlook Executive Summary 2017 1 .

Figure 1 . 2

 12 Figure 1.2 Schematic of the global carbon cycle. Numbers represent reservoir masses (or 'carbon stocks' in PgC (= 10 15 gC) and annual carbon exchange fluxes (in PgC yr -1 ). Source: IPCC. CO 2 concentration in the atmosphere has lately risen far beyond the amplitude of typical historic oscillations due to ice ages at the geologic time scale and to seasonal variations on a yearly basis. As shown in Figure 1.3, the current atmospheric CO 2 concentration has risen up to

Figure 1 . 3

 13 Figure 1.3 Variation of atmospheric CO 2 concentration over the past 58 years.

3

 3

Figure 1 . 5

 15 Figure 1.5 Area of land that needs to be covered with 10% efficient solar cells in order to generate 20 TW of electrical power.

Figure 1 . 7

 17 Figure 1.7 Artificial photo-induced ATP-synthesis system developed at Arizona State University. 12 1.1.3 Routes for carbon dioxide reduction

Figure 1 . 8 B

 18 shows the band-gap positions of some typical SCs and the standard reduction potentials for CO 2 reduction reactions. To be thermodynamically feasible, , the conduction band (CB) edge of SCs is required to be above the standard potential of the targeted reduction reaction, whereas photooxidation reactions can occur only when the valence band (VB) edge of semiconductors is below the standard potential of the targeted oxidation reaction. It can be seen that photogenerated electrons in the CB of all the SCs do not have enough driving force to carry out the one-electron reduction of CO 2 to CO 2 .-. In addition, because redox potentials for CO 2 reduction are close to the one of reduction of H 2 O into H 2 , CO 2 reduction systems always have to compete with H 2 evolution.

Figure 1 . 8 (

 18 Figure 1.8 (A) Photocatalytic CO 2 reduction with H 2 O over a heterogeneous photocatalyst; (B) Conduction band (red) and valence band (blue) positions of some SC photocatalysts and redox potentials versus NHE of CO 2 reduction and water splitting at pH 0. 19 The photocatalytic conversion of CO 2 with the participation of SC materials is a multi-faced phenomenon that can be analyzed and systematized from various aspects. Therefore, the classification of SC-based systems for CO 2 reduction can be established on the composition and the structure of the photoactive SC phase. It allows grasping the versatility of photocatalytic systems and a scope of materials which can be used in the design of solar-light-driven systems for CO 2 fixation.

Figure 1 .

 1 Figure 1.12 (A) Photocatalytic CO 2 reduction with different TiO 2 nanocrystal polymorphs; (B)Schematic illustration of photocatalytic CO 2 reduction with TiO 2 /co-catalyst systems.[START_REF] Liu | Photocatalytic CO2 reduction with H 2 O on TiO 2 nanocrystals: Comparison of anatase, rutile, and brookite polymorphs and exploration of surface chemistry[END_REF][START_REF] Xie | MgO-and Pt-promoted TiO 2 as an efficient photocatalyst for the preferential reduction of carbon dioxide in the presence of water[END_REF] Doped semiconductor as photocatalysts of CO 2 conversion. Doping the wide-bandgap SCs, such as TiO 2 , with metal ions or non-metal atoms is one of the most frequent and productive strategies. Through this method, photoreaction efficiency could be increased thanks to recombination inhibition and extension of the spectral sensitivity range of the photocatalysts as a result of the participation of localized or delocalized dopant levels in the light absorption.

Figure 1 .

 1 Figure 1.13 Photocatalytic reduction of CO 2 with Pd-, Cu-or Mn-doped TiO 2 . 47

Figure 1 .

 1 Figure 1.14 (A) Photocatalytic reduction of CO 2 with dye-sensitized TiO 2 film; (B) Photocatalytic reduction of CO 2 with aminosalicylic acids modified TiO 2 . 50-51

  photoelectrochemical activity in the CO 2 reduction by water even without an additional external bias (Figure1.15 (B)). It is the first self-driven photoelectrochemical cell constructed with a molecular photocatalyst to achieve the reduction of CO 2 by only using visible light as the energy source and water as the reductant.[START_REF] Kumagai | Hybrid photocathode consisting of a CuGaO 2 p-type semiconductor and a Ru (ii)-Re (i) supramolecular photocatalyst: non-biased visible-light-driven CO 2 reduction with water oxidation[END_REF]

Figure 1 .

 1 Figure 1.15 (A) Photoelectrochemical reduction of CO 2 with Ru(II)-Re(I) immobilized on a NiO photocathode; (B) Photoelectrochemical reduction of CO 2 with Ru(II)-Re(I) immobilized on a CuGaO 2photocathode.[START_REF] Sahara | Photoelectrochemical Reduction of CO 2 Coupled to Water Oxidation Using a Photocathode with a Ru (II)-Re (I) Complex Photocatalyst and a CoO x/TaON Photoanode[END_REF][START_REF] Kumagai | Hybrid photocathode consisting of a CuGaO 2 p-type semiconductor and a Ru (ii)-Re (i) supramolecular photocatalyst: non-biased visible-light-driven CO 2 reduction with water oxidation[END_REF]

Figure 1 .

 1 Figure 1.17 (A) Schematic illustration of photoelectrochemical reduction of CO 2 with CdSe/Pt/TiO 2 heterostructured catalysts; (B) Energy diagram of TiO 2 /CdSe heterostructures with bulk and nanocrystalline cadmium selenide. 63

Scheme 1 . 2 .

 12 Scheme 1.2. Proposed mechanistic steps in the reduction of CO 2 by Co/Ni Cyclams. Hydrogen (blue), formate (red) and CO (brown), as well as putative intermediates (green) when M = Co or Ni. SD ꞏ is a highly reduced carbon-centered radical species.[START_REF] Tinnemans | Tetraaza-macrocyclic cobalt (II) and nickel (II) complexes as electron-transfer agents in the photo (electro) chemical and electrochemical reduction of carbon dioxide[END_REF][START_REF] Morris | Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels[END_REF] Recently, Reisner et al. reported a hybrid assembly consisting in ligand-free ZnSe quantum dots (QDs) as visible-light PS combined with a phosphonic acid-functionalised Ni(cyclam) CAT, NiCycP (Figure1.18). 69b This precious metal-free photocatalytic system shows a high activity for

Figure 1 .

 1 Figure 1.18 Photocatalytic system with ligand-free ZnSe QDs (ZnSe-BF 4 ) combined with a molecular catalyst, NiCycP, for aqueous CO 2 reduction to CO. H 2 evolution can be partially suppressed by a surface inhibitor (MEDA). 69b

Figure 1 .

 1 Figure 1.19 Photocatalytic system with Fe/Co quaterpyridine complexes for CO 2 reduction.[START_REF] Guo | Highly efficient and selective photocatalytic CO 2 reduction by iron and cobalt quaterpyridine complexes[END_REF] Chan, Che and co-workers reported a similar type of photocatalytic system using a Co II complex containing a pyridine based tetradentate tripodal ligand as CAT and Ir III based complexes as PS (Figure1.20). When Ir(ppy) 3 as PS, a selectivity for CO formation of 85% was obtained, with a TON CO of 953 based on the CAT over 70 h.[START_REF] Chan | A robust and efficient cobalt molecular catalyst for CO 2 reduction[END_REF] In addition, by employing [Ir(ppy) 2 (bis-NHC)] + (bis-NHC = cis-chelating bis(N-heterocyclic carbene)), a new PS owing better emissivity and photostability, the selectivity and the TON CO for CO formation in this system were successfully improved to > 95% and > 2400 based on CAT in similar reaction conditions.[START_REF] Yang | Stable luminescent iridium (iii) complexes with bis (Nheterocyclic carbene) ligands: photo-stability, excited state properties, visible-light-driven radical cyclization and CO 2 reduction, and cellular imaging[END_REF]

Figure 1 .

 1 Figure 1.20 Photocatalytic system with a cobalt complex supported by a pyridine-based tetradentate tripodal ligand for CO 2 reduction. 72 A large number of non-noble metal complexes as CAT have been reported, but stable and efficient PS containing non-precious metals are still rare, since it is very difficult to compete with the high efficiency of triplet harvesting complexes Ru or Ir. Heteroleptic emissive Cu I PS developed by the Beller group for photocatalytic proton reduction are also suitable for CO 2 reduction in the visible light range. Ishitani et al. reported the first Fe/Cu system for the effective reduction of CO 2 by employing a dimeric macromolecular heteroleptic CuPS with one diamine

Figure 1 . 21 .

 121 Figure 1.21. Photocatalytic system with Fe polypyridine CAT and Cu PS for CO 2 reduction.[START_REF] Takeda | Photocatalytic CO 2 reduction using Cu (I) photosensitizers with a Fe (II) catalyst[END_REF]

  3). Solvent (S) is proposed to replace the halide ligand to yield the putative catalytically active state, [ReI(L ꞏ )(CO) 3 S].[START_REF] Hayashi | Involvement of a Binuclear Species with the Re-C (O) O-Re Moiety in CO 2 Reduction Catalyzed by Tricarbonyl Rhenium (I) Complexes with Diimine Ligands: Strikingly Slow Formation of the Re-Re and Re-C (O) O-Re Species from Re (dmb)(CO) 3S (dmb= 4, 4 '-Dimethyl-2, 2 '-bipyridine, S= Solvent)[END_REF] Halide ligand exchange, Cl -for Br -, was also observed for [ReI(L ꞏ )(CO) 3 X] -. Reactions of [ReI(L ꞏ )(CO) 3 S] with a proton are proposed to result in the formation of a rhenium-hydride bond. Formate production is thought to occur through CO 2 insertion into the Re-hydride intermediate (red in Scheme 1.3), whose behavior is consistent with other catalysts. Sullivan et al. prepared the analogous [Re I (bpy)(CO) 3 H] and tested it with CO 2 . 77 The rate constant of this dark reaction indicated a remarkable solvent dependency which increased with the dielectric constant of the medium. 78 [Re I (bpy)(CO) 3 (OC(H)O)] was formed with a 81% yield in CO 2 saturated THF solution. Addition of excess halide to the solvent would prevent [Re I (bpy)(CO) 3 (OC(H)O)]

Figure 1 .

 1 Figure 1.22 Schematic diagram of a non-covalent (A) and covalent (B) immobilization of a molecularRe(CO) 3 (bpy)X-based complex on Cu 2 O photocathodes for CO 2 reduction.[START_REF] Schreier | Efficient and selective carbon dioxide reduction on low cost protected Cu 2 O photocathodes using a molecular catalyst[END_REF][START_REF] Schreier | Covalent immobilization of a molecular catalyst on Cu2O photocathodes for CO 2 reduction[END_REF]

  porphyrins and their derivates. Metalloporphyrins are well known for their electron transfer roles in a myriad of redox systems in Nature (chlorophyll, heme, vitamin B 12 , A B cytochrome P450, etc.) as well as highly effective photocatalysts thanks to their strong absorption in the 400-450 nm region (Soret band) and weak absorption in the 500-700 nm region (Q-bands). Metalloporphyrins (MP) and related derivatives studied for CO 2 reduction reactivity include metallocorrins (MN), metallophthalocyanines (MPc) and metallocorroles (MC) (Figure 1.23, where M = Fe or Co). 84 Scheme 1.4 illustrates the mechanism with a MP, as representative compound. The catalytic active states as identified by cyclic voltammetry show the metal center is in the formal oxidation state of zero for porphyrins [M 0 P] 2-and corrins [M 0 N] 2-, +1 for corroles [M I C] 2-and +1 with a reduced phthalocyanine ring [M I Pc ꞏ-] 2-. Some MPs previously investigated bear functional groups with negative or positive charges at the meso substituents, which will greatly affect the efficiency of their catalytic properties.

Figure 1 . 23 .

 123 Figure 1.23. Metalloporphyrin and derivatives investigated for CO 2 reduction: (left to right) metalloporphyrin (MP), metallocorrin (MN), metallophthalocyanine (MPc) and metallocorrole (MC). Prolonged illumination of MPs in the presence of TEA as SD results in the formation of [M I P] -, [M I N] -, [M I Pc] -and [M II C] -through stepwise reductive quenching with quantum yields for the sequential steps, ϕ 1 and ϕ 2 (Scheme 1.4). 68, 85 More specifically, a donor molecule (here TEA)

M

  Scheme 1.4. Proposed mechanism in the reduction of CO 2 by metalloporphyrin (M = Fe or Co). Hydrogen (blue), formate (red) and CO formation (brown and pink), as well as putative intermediates (green).[START_REF] Morris | Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels[END_REF][START_REF] Dhanasekaran | P-terphenylsensitized photoreduction of CO 2 with cobalt and iron porphyrins. Interaction between CO and reduced metalloporphyrins[END_REF]

Figure 1 .

 1 Figure 1.24 CuInS 2 QDs sensitized FeTPP system for CO 2 reduction. 88

Figure 1 .

 1 Figure 1.25 g-C 3 N 4 nanosheets/FeTCPP system for CO 2 reduction. 91 Apart from self-assembly approach of g-C 3 N 4 nanosheets/FeTCPP heterogeneous catalyst, Ye et al. reported a covalently linked Co-porphyrin/low-molecular-weight C 3 N 4 hybrids for photochemical CO 2 reduction (Figure 1.26). 92 Covalent hybrid Co-porphyrin-oligomers (Co-POM) was obtained through the condensation between amino-group from oligomer and ketonicgroup from porphyrin. The hybrids possessed 13-fold higher photocatalytic activity (17 μmol/g/h) compared to bulk C 3 N 4 , and more than twice compared to the Co-porphyrin loaded C 3 N 4 heterojunction system. The efficient electron transfer and trapping by the Co active sites as well as the affinity of Co-porphyrin for CO 2 are considered to account for the enhanced activity. This work may open a promising route to modify carbon nitride and provide a feasible approach to immobilize the active site into the light-harvest antenna for efficient electron-hole separation, electron transfer and the following redox reactions in photocatalytic process, which reforms the conventional semiconductor-cocatalyst heterojunction system.

Figure 1 .

 1 Figure 1.26 Co-porphyrin-oligomers of melems (Co-POM) for CO 2 reduction.[START_REF] Zhao | Coporphyrin/carbon nitride hybrids for improved photocatalytic CO 2 reduction under visible light[END_REF]

2. 2

 2 Figure 2.1 Illustration of non-sensitized (left) and sensitized (right) photochemical catalytic CO 2reduction processes.[START_REF] Huckaba | Photocatalytic Reduction of CO 2 with Re-Pyridyl-NHCs[END_REF]

Figure 2 . 2

 22 Figure 2.2 Non-sensitized Fe-p-TMA system for photochemical CO 2 reduction. 104 In a typical photochemical CO 2 reduction reaction, irradiations of acetonitrile (ACN) solutions containing 2 μM Fe-p-TMA is as the catalyst, 0.05 M triethylamine (TEA) or 0.02 M 1,3dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as the sacrificial electron donor (SD), and 0.1 M TFE or phenol as weak acid if necessary.

  Figure 2.4, both electron donors used had no absorption in the irradiation domain, which also indicated that CO 2 reduction needed a photocatalyst to respond light absorption. The effect of an added external weak acid was explored through the addition of 0.1 M trifluoroethanol (TFE) or phenol (PhOH), respectively, as illustrated in Figure 2.3 (red and green triangles).

Figure 2 .

 2 Figure 2.3 CO formation upon visible irradiation (λ > 420 nm) of a CO 2 -saturated ACN solution containing 2 μM Fe-p-TMA and 0.05 M TEA (○), 0.02 M BIH (□, ◊), 0.02 M BIH + 0.1 M trifluoroethanol (TFE, ∆), 0.02 M BIH + 0.1 M phenol (PhOH, ▽). 104

Figure 2 .

 2 Figure 2.4 UV-Visible absorption spectra of TEA and BIH in ACN. 104 Remarkably, the selectivity for CO formation remained equal to 100% and the acid does not lead to any degradation of the catalyst as shown by the linear evolution of the product formation with irradiation time. It indicates that no hydride intermediate is formed from the Fe 0 active

  3), reaching a plateau after 100 hours. A turnover number (TON) value of 101 was obtained after 102 hours of continuous irradiation, again illustrating the robustness of the porphyrin under these conditions. The spectral evolution upon visible irradiation of a CO 2 -saturated solution containing the porphyrin catalyst and a sacrificial electron donor (TEA or BIH) is sketched in Figure 2.5 and confirms catalyst stability. The exact attribution of the absorption bands of porphyrins is difficult owing to the overlapping between the bands related to the different states of the iron center.

Figure 2 . 5

 25 Figure 2.5 Spectral evolution upon visible irradiation (λ > 420 nm) for a CO 2 -saturated ACN solution containing 5 μM Fe-p-TMA and 0.05 M TEA (A) or 0.02 M BIH (B). Inset (A): Spectral evolution upon visible irradiation (λ > 420 nm) for a CO-saturated ACN solution containing 5 μM Fe-p-TMA and 0.05 M TEA. 104Still, the porphyrin characteristic LMCT band with a peak at 325 nm could be observed at all irradiation times, while the Fe III Soret band at 427 nm rapidly evolves with the formation of the Fe II -CO state, whose stationary concentration is large enough to be the dominant contribution to the spectra with BIH as the SD (Figure2.5B). In that case, a slight decrease (15%) of the absorption band occurs after 300 min of irradiation, being either due to the slow degradation of the porphyrin or to a non-stationary concentration of Fe II -CO species because of a faster light induced cleavage of the adduct as compared to its formation rate. When using TEA as SD, the typical Soret band of an Fe I porphyrin peaking at 423 nm became the dominant contribution after

Figure 2 . 6

 26 Figure 2.6 Cyclic voltammetry of 0.5 mM Fe-p-TMA in DMF + 0.1 M TBAPF 6 under inert argon atmosphere. Gray trace is the blank experiment with no catalyst. T = 21℃, scan rate = 100 mV s -1 , work electrode (WE) = 3 mm diameter glassy carbon.When the system is saturated with CO 2 , the two first redox waves remain unchanged while the third increases and the corresponding oxidation wave on the reverse sweep disappear. This irreversibility is fully consistent with an electrocatalytic process, whereby the Fe 0 species formed during the forward sweep is transformed back to Fe I upon reduction of CO 2(Figure 2.7, red).

Figure 2 . 7

 27 Figure 2.7 Cyclic voltammetry of 0.5 mM FeTNT + 0.1 M KCl in H 2 O. Gray trace is the blank experiment with no catalyst. T = 21 ℃, scan rate = 100 mVs -1 , WE = ϕ 3 mm glassy carbon, pH = 5.8 (Argon, unbuffered) and 6.7 (CO 2 , 0.1 M KHCO 3 buffer).

Scheme 2 . 1 )

 21 Scheme 2.1).[109][110] The Fe I /Fe 0 standard redox potential (-1.47 V vs. SCE) measured by cyclic voltammetry in DMF is less negative than the value obtained with the non-substituted tetraphenyl Fe porphyrin (-1.67 V vs. SCE). It is ascribed to the strong electron withdrawing effect of the trimethylammonio groups. As a consequence, the Fe 0 species is less prone to give a hydride by reaction with acids present in the solution in the case of Fe-p-TMA and H 2 evolution is not observed. Moreover, degradation of the catalyst through reductive hydrogenation of the porphyrin ring is also minimized with Fe-p-TMA. C-O bond cleavage may then occur by protonation of the Fe-CO 2 adduct and would lead to the release of a water molecule and an Fe-CO adduct. The latter species further releases CO either upon photo-irradiation and/or by a oneelectron homogeneous reduction by the Fe 0 species.

Fe

  -p-TMA as a selective and stable catalyst for the photochemical conversion of CO 2 -to-CO in association with a cheap organic PS, namely purpurin (PP) under visible light irradiation (λ > 420 nm) and aqueous conditions (Figure 2.8).

Figure 2 . 8

 28 Figure 2.8 Purpurin-sensitized Fe-p-TMA system for CO 2 reduction in aqueous conditions. 112 Photocatalytic CO 2 reduction by Fe-p-TMA was studied in pH 6.7 aqueous solutions containing 0.1 M NaHCO 3 to control the pH value of the reaction mixture and 10% ACN added (but kept miminum) to ensure a sufficient solubility of PP. 0.1 M NaHCO 3 was also added to

Figure 2 .

 2 Figure 2.9 CO (black symbols) and H 2 (red symbols) production upon visible light irradiation (> 420 nm) of a CO 2 -saturated ACN:H 2 O (1:9, v:v) solution containing 2 µM Fe-p-TMA, 0.2 mM PP, 0.1 M NaHCO 3 and 0.05 M SD: TEA (, ), TEOA (, ) or EDTA (, ). 112

Figure 2 .

 2 Figure 2.10 Spectral evolution with time upon visible irradiation (> 420 nm) of a CO 2 -saturated ACN:H 2 O (1:9, v:v) solution containing 2 µM Fe-p-TMA, 0.2 mM PP, 0.1 M NaHCO 3 and 0.05 M TEA. 112

Figure 2 .

 2 Figure 2.11 CO (black squares) and H 2 (red circles) production upon visible light irradiation (> 420 nm) of a CO 2 -saturated ACN:H 2 O (1:9, v:v) solution containing 2 µM Fe-p-TMA, 0.2 mM PP, 0.1 M NaHCO 3 and 0.05 M TEA. After 47 hours irradiation, 1 equivalent of Fe-p-TMA catalyst (left) or of PP photosensitizer (right) was added to the solution. 112

Figure 2 .

 2 Figure 2.12 Gas chromatogram (a) and mass spectrum (b) analysis of the generated gas from the photocatalytic reduction of 13 CO 2 upon visible light irradiation (> 420 nm) of a 13 CO 2 -saturated ACN:H 2 O (1:9, v:v) solution containing 2 µM Fe-p-TMA, 0.2 mM PP, 0.1 M NaHCO 3 and 0.05 M TEA. Since the sodium hydrogenocarbonate contains 12 C, the mass spectrum both includes peaks at m/z 29 and 28. 112

Figure 2 .

 2 Figure 2.13 Emission spectra of a 0.2 mM PP in ACN:H 2 O (1:9, v:v) solution with increasing concentration of TEA. Inset: linear plot (R 2 = 0.996) of the PP normalized emission ratio at 605 nm versus TEA concentration according to the Stern-Volmer equation. 112

Figure 2 .

 2 Figure 2.14 Emission spectra of a 0.2 mM PP in ACN:H 2 O (1:9, v:v) solution with increasing concentration of Fe-p-TMA. Inset: measured (blue diamonds) and predicted (green hexagons) emission intensity at 605 nm of an ACN:H 2 O (1:9, v:v) solution containing 0.2 mM PP as a function of the concentration in Fe-p-TMA catalyst. 112

Figure 2 .

 2 Figure 2.15 Absorption spectrum of a ACN:H 2 O (1:9, v:v) solution containing 0.2 mM PP and increasing concentration of Fe-p-TMA catalyst showing the typical growing of the porphyrin Soret band around 420 nm. 112 The redox potential of PP was obtained through cyclic voltammetry under argon atmosphere (Figure 2.16). The reduced PP 2-species has a standard redox potential (E 0 = -0.66 V vs. SCE) negative enough to reduce the Fe III porphyrin to Fe II , since the onset E 0 (Fe III /Fe II )≈-0.3 V vs. SCE. 109 It can, however, not reduce Fe II [E 0 (Fe II /Fe I ) ≈ -1 V vs. SCE] or Fe I [E 0 (Fe I /Fe 0 ) ≈ -1.3

Figure 2 .

 2 Figure 2.16 Cyclic voltammogram of an argon-saturated ACN:H 2 O (1:9, v:v) solution containing 0.5 mM PP and 0.1 M NaHCO 3 as supporting electrolyte. Scan rate was 0.1 V s -1 . 112

Figure 3 . 1 Figure 3 . 2

 3132 Figure 3.1 Oxidative and reductive quenching mechanisms in photochemical catalyst activation.

Figure 3 . 3

 33 Figure 3.3 Chemical structures of typical sacrificial electron donors used in photochemical catalysis (Redox potential given in V vs. SCE).

Figure 3 .

 3 Figure 3.4 H 2 (black), CO (red) and CH 4 (blue) formation with irradiation (> 420 nm) time of a CO 2saturated ACN solution containing 2 µM Fe-p-TMA, 0.05 M TEA, and 0.2 mM PS-8.

  photosensitizer (E 0 (Ir(ppy) 3 + /Ir(ppy) 3 * ≈ -1.73 V versus the saturated calomel electrode (SCE) and E 0 (Ir(ppy) 3 /Ir(ppy) 3 -≈ -2.19 V vs. SCE) 133 to the solution indeed enhanced the photocatalytic CO 2 reduction so that 47 h of irradiation gave a TON in CO of 198 relative to Fep-TMA (Figure 3.4, red), which was much more efficient than the previous systems. More

Figure 3 . 5

 35 Figure 3.5 Gas chromatogram of the headspace after 47 h irradiation (> 420 nm) of a CO 2 -saturated ACN solution containing 2 µM Fe-p-TMA, 0.05 M TEA and 0.2 mM PS-8.

Figure 3 . 6

 36 Figure 3.6 Mass spectra of methane obtained after 47 h irradiation of solutions feed with 13 CO 2 (left) and 12 CO 2 (right).

3. 8 )

 8 . The slightly lowered H 2 production and increased CH 4 production by almost was a factor of three compared to the experiment using a CO 2 -saturated solution: 83% of product was CH 4 and 17% of product was H 2 , with the CH 4 formation rate of 1, 865 μmol h -1 g -1 .

Figure 3 . 7

 37 Figure 3.7 Gas chromatogram measured after 47 irradiation (> 420 nm) of a CO-saturated ACN solution containing 2 µM Fe-p-TMA, 0.05 M TEA and 0.2 mM PS-8.

Figure 3 .

 3 Figure 3.8 H 2 (black) and CH 4 (red) formation with time during irradiation (λ > 420 nm) of a COsaturated ACN solution containing 2 µM Fe-p-TMA, 0.05 M TEA and 0.2 mM PS-8.

Figure 3 . 9

 39 Figure 3.9 Photochemical reduction of CO 2 upon visible light irradiation. Gaseous reduction products obtained in a 1 atm CO 2 (a, b, c) or CO-saturated (d) 2 µM Fe-p-TMA and 50 mM TEA acetonitrile solution. a, CO generation with no sensitizer. b, Gas products in the presence of 0.2 mM of PS-8 (filled symbols; + 0.1 M TFE, open symbols). c, Extended irradiation in the presence of 0.2 mM of PS-8. d, Gas products under CO atmosphere, in the presence of 0.2 mM of PS-8 (filled symbols; + 0.1 M TFE, open symbols). 116

Figure 3 . 3 2+

 33 Figure 3.10 Evolution of the absorption spectrum with time. The absorption spectrum of a CO 2saturated acetonitrile solution containing 2 μM of Fe-p-TMA, 0.2 mM of PS-8, 0.05 M of TEA upon visible (> 420 nm) light irradiation remains stable over the course of experiments, highlighting the stability of the system. The inset shows the absorption spectrum of 2 μM Fe-p-TMA in acetonitrile (no PS), revealing that in the photocatalytic mix, >90% of photons above 420 nm are absorbed by PS-8. 116 3.2.5 A possible mechanism for CH 4 formation

Figure 3 .

 3 Figure 3.11 Reduction/oxidation potentials for the investigated system in V vs. SCE.

Figure 3 .

 3 Figure 3.12 PS-8 emission quenching after excitation at 420 nm. a, Upon increasing concentration of TEA in a 0.1 mM ACN solution of Ir(ppy) 3 , no emission quenching is observed, as confirmed by the Stern-Volmer analysis (inset). b, Upon increasing concentration of Fe-p-TMA in a 0.2 mM ACN solution of Ir(ppy) 3 , emission quenching is observed corresponding to a diffusion-controlled quenching rate of (1.7 ± 0.1)×10 10 M -1 s -1 as determined by Stern-Volmer analysis (inset). a.u., arbitrary units. I 0 /I is the emission intensity without quencher divided by the emission intensity with a known concentration of quencher. 116

Figure 3 .

 3 Figure 3.14 Ir(ppy) 2 (bpy)PF 6 sensitized Fe-p-TMA system for photocatalytic CO 2 reduction in aqueous conditions.

3. 14 )

 14 photophysical properties such as a long excited-state lifetime (several hundreds of nanoseconds) 105b, 141 and a highly negative redox potential (ca. -1.45 V vs. SCE) 142 in its reduced form. The thermal population of the dissociative 3 d-d state from the[START_REF] Solar | Oxidation of Tryptophan and N-Methylindole by N 3 -, Br 2 .-, and (SCN) 2 .-Radicals In Light and Heavy-Water Solutions: A Pulse Radiolysis Study[END_REF] MLCT excited state can be suppressed in the case of PS-9 because of the high ligand-field stabilization of the strong σ-donor character of the ppy ligand.143 This results in both high photochemical stability of the Ir(III) complexes and a long lifetime of the excited state of the Ir(III) complexes, which promotes efficient electron transfer from an electron donor.

Figure 3 .

 3 Figure 3.15 CO (black squares), H 2 (red circles), and CH 4 (blue diamonds) formation (turnover numbers) upon visible-light irradiation (λ > 420 nm) of a CO 2 -saturated ACN solution containing 2 μM Fe-p-TMA catalyst, 0.2 mM [Ir(ppy) 2 (bpy)] + sensitizer, and 0.05 M TEA (top), TEOA (middle), or DIPEA (bottom) as sacrificial electron donor.

Figure 3 .

 3 Figure 3.16. CO (black squares), H 2 (red circles), and CH 4 (blue diamonds) formation (turnover numbers) upon visible-light irradiation (λ > 420 nm) of a CO 2 -saturated ACN/H 2 O (3:7 v/v) solution containing 2 μM Fe-p-TMA catalyst, 0.2 mM [Ir(ppy) 2 (bpy)] + sensitizer, and 0.05 M TEA (top), TEOA (middle), or DIPEA (bottom) sacrificial electron donor.

Figure 3 .

 3 Figure 3.17 Emission spectra of a 0.2 mM [Ir(ppy) 2 (bpy)] + solution in ACN/H 2 O (3:7 v/v) containing 0 (black), 10 (red), 30 (green), 50 (blue), 100 (cyan), 150 (magenta), and 250 mM (yellow) TEA. The inset shows the linear plot (R 2 = 0.994) of the normalized emission at 592 nm versus TEA concentration according to the Stern-Volmer equation.

Figure 3 .

 3 Figure 3.18 Emission spectra of a 0.2 mM [Ir(ppy) 2 (bpy)] + solution in ACN/H 2 O (3:7 v/v) containing 0 (black), 30 (red), 80 (green), 100 (blue), 150 (cyan), 250 (magenta), and 400 mM (yellow) TEOA. The inset shows the linear plot (R 2 = 0.999) of the normalized emission at 592 nm versus TEOA concentration according to the Stern-Volmer equation.

Figure 3 .

 3 Figure 3.19 Emission spectra of a 0.2 mM [Ir(ppy) 2 (bpy)] + solution in ACN/H 2 O (3:7 v/v) containing 0 (black), 10 (red), 30 (green), 50 (blue), 100 (cyan), 150 (magenta), and 250 mM (yellow) DIPEA. The inset shows the linear plot (R 2 = 0.997) of the normalized emission at 592 nm versus DIPEA concentration according to the Stern-Volmer equation.

Figure 3 .

 3 Figure 3.20 Time-resolved kinetic trace at 592 nm (black, top), monoexponential fit (red, top) and fit residues (bottom) of the [Ir(ppy) 2 (bpy)] + emission after excitation at 420 nm of a 0.2 mM solution in ACN/H 2 O (3:7 v/v) degassed with argon. The excited state lifetime is ca. 145 ns.

  3). As we demonstrated before,112 Fe-p-TMA has a non-negligible absorbance at λ ex = 420 nm (Figure3.21) so that introducing an attenuation factor for emission quenching measurements (Figure3.20) is necessary. The excitation attenuation due to the absorption of Fe-p-TMA at the irradiation wavelength then leads to a lower excitation of [Ir(ppy) 2 (bpy)] + : the measured intensity is thus lower than the real emission. Considering a mean solution thickness of 5 mm and taking into account absorbance values at each Fe-p-TMA concentration, the predicted emission intensity as a function of [Fe-p-TMA] (Inset, Figure3.21) perfectly matches the experimentally measured emission intensity. One can thus conclude that the emission decrease is not due to a quenching reaction between * [Ir(ppy) 2 (bpy)] + and Fe-p-TMA but is in fact due to optical attenuation.

Figure 3 .

 3 Figure 3.20 Emission spectra of a 0.2 mM [Ir(ppy) 2 (bpy)] + solution in ACN/H 2 O (3:7 v/v) containing 0 (black), 1 (red), 2 (blue), 4 (magenta), 6 (olive), 8 (navy), and 10 mM (violet) Fe-p-TMA. The inset shows the linear plot (R 2 = 0.980) of the normalized emission at 592 nm versus Fe-p-TMA concentration according to the Stern-Volmer equation.

Figure 3 .

 3 Figure 3.21 Absorption of a 0.2 mM [Ir(ppy) 2 (bpy)] + solution in ACN/H 2 O (3:7 v/v) containing 0(black), 1 (red), 2 (blue), 4 (magenta), 6 (olive), 8 (navy), and 10 µM (violet) Fe-p-TMA showing the typical Soret band of the porphyrin growing around 415 nm. Inset: measured (black squares) and predicted (blue circles) emission intensity at 592 nm of an ACN/H 2 O (3:7 v/v) solution containing 0.2 mM Ir(ppy) 2 (bpy) + as a function of the concentration of Fe-p-TMA.

Figure 3 .

 3 Figure 3.22 Reduction/oxidation potentials for the investigated system in V vs. SCE.

Figure 3 .

 3 Figure 3.23 Gaseous products formation (turnover numbers) upon visible-light irradiation (> 420 nm) of a CO-saturated ACN solution containing 2 μM Fe-p-TMA catalyst, 0.2 mM Ir(ppy) 3 sensitizer, 0.05 M TEA as sacrificial electron donor, and 0.5 M TFE as proton source.

Figure 3 .

 3 Figure 3.24 Spectral evolution upon visible-light irradiation (> 420 nm) for a CO 2 -saturated (A) and CO-saturated (B) ACN/H 2 O (3:7 v/v) solution containing 2 μM Fe-p-TMA as catalyst, 0.2 mM [Ir(ppy) 2 (bpy)] + as sensitizer, and 0.05 M TEA as electron donor.

Figure 3 .

 3 Figure 3.25 Differential infrared spectrum of 1mM [Ir(ppy) 2 (bpy)] + and 0.05 M TEA in ACN/H 2 O (3:7 v/v) under CO atmosphere after 60 min irradiation (> 420 nm).At the same time, cyclic voltammetry (Figure3.26) also revealed the apparition of an irreversible reduction wave at ca. -2 V vs. SCE under CO, as previously observed.151 All these evidences showed the instability of bpy ligand binding to Ir center under irradiation.

Figure 3 .

 3 Figure 3.26 Cyclic voltammetry (ν = 0.1 Vs -1 in ACN, 0.5 mM [Ir(ppy) 2 (bpy)] + , 0.1 M TBAPF 6 ) at a 3 mm glassy carbon electrode under Ar (black), under CO in dark (red), under CO after 60 min λ > 420 nm irradiation (blue), under CO + 0.05 M TEA after 1 min (pink) and under CO + 0.05 M TEA after 60 min irradiation (green).

Finally

  Scheme 3.1 Possible reactions leading to the decomposition of [Ir(ppy) 2 (bpy)] + in the presence of CO as substrate.

  Scheme 3.2 Molecular structures of the investigated phenoxazine-based organic photosensitizers (Phen1, Phen2).

Figure 3 . 27 ,

 327 solid symbols), whereas the formation of H 2 remained almost unchanged. The presence of TFE could play a role through hydrogen bonding as shown in Figure3.28, in lowering the activation energy of CO 2 reduction and in enabling the direct proton transfer from acid to the activated CO 2 substrate. Similar effects were reported in the literature that the introducing of TFE will greatly enhance the catalysis of CO 2 reduction.168 They use both experimental and calculated methods to show that a proton involving step is rat-limiting for the overall catalytic cycle. The second protonation is always less favorable than the first one. The intermolecular proton transfer via non-cooperative hydrogen bonds to acid in solution provides a relevant model for biological systems and homogeneous catalysts for small molecule activation.

Figure 3 .Figure 3 .

 33 Figure 3.27 Generation of CO (black squares), H 2 (red circles) and CH 4 (blue diamonds) with time upon visible irradiation (> 435 nm) of a CO 2 -saturated DMF solution containing 10 µM Fe-p-TMA, 1 mM Phen2 and 0.1 M TEA (open symbols); addition of 0.1 M TFE as an external acid is indicated by "filled symbols". Arrows indicate increase in product formation upon addition of an external acid.

Figure 3 .

 3 Figure 3.29 Typical gas chromatogram of the sample headspace after several hours of visible light irradiation of a solution containing 10 μM Fe-p-TMA, 1 mM Phen2, 0.1 M TEA and 0.1 M TFE in DMF and saturated 13 CO 2 . The inset shows the corresponding mass spectra of methane.

Figure 3 .

 3 Figure 3.30 CO (black squares), H 2 (red circles) and CH 4 (blue diamonds) generation with time upon visible light irradiation (> 435 nm) of a CO 2 -(filled symbols) or CO-saturated (open symbols) DMF solution containing 10 µM Fe-p-TMA, 1 mM Phen2, 0.1 M TEA and 0.1 M TFE.

Figure 3 .

 3 Figure 3.31 Evolution of the absorption spectrum upon visible light (> 435 nm) irradiation with time of a CO 2 -saturated DMF solution containing 10 μM Fe-p-TMA, 1 mM Phen2, 0.1 M TEA and 0.1 M TFE. The double arrow shows the irradiation threshold (435 nm). Inset: evolution of the absorption spectrum in the first six hours of irradiation.

Figure 3 .

 3 Figure 3.32 TON in H 2 (red) and CH 4 (blue) measured after 47 h of visible light irradiation (> 435 nm) of a CO-saturated DMF solution containing 10 µM Fe-p-TMA, 1 mM Phen2, in the presence of various SDs and added acids.

Figure 3 .

 3 Figure 3.33 Typical gas chromatogram of the sample headspace after several hours of visible-light irradiation of a solution containing 10 μM Fe-p-TMA, 1 mM Phen2, 0.1 M TEA and 0.1 M TFE in DMF and saturated labelled 13 CO. The inset shows the corresponding mass spectra of methane.

Figure 3 .

 3 Figure 3.34 Emission spectra of a 0.2 mM Phen2 DMF solution containing 0 (black), 10 (red), 20 (blue), 50 (magenta), 100 (olive), 150 (navy) and 200 (violet) mM TEA. Inset: linear plot of the normalized emission at 506 nm versus TEA concentration according to the Stern-Volmer equation.

Figure 3 .

 3 Figure 3.35 Emission spectra of a 0.2 mM Phen2 DMF solution containing 0 (black), 1 (red), 2 (blue), 4 (magenta), 6 (olive), 8 (navy) and 10 (violet) µM Fe-p-TMA. Inset: linear plot (R 2 = 0.994) of the normalized emission at 506 nm versus Fe-p-TMA concentration according to the Stern-Volmer equation.

Figure 3 .

 3 Figure 3.36 Time-resolved kinetic trace at 390 nm (black open circles, top), monoexponential fit (blue line, top) and fit residues (red open circles, bottom) of the Phen2 excited state after excitation at 420 nm of a 5 µM solution in DMF degassed with argon. The corresponding excited state lifetime is ca. 443  18 µs, in accordance with previously reported value. 155 Low concentration and low excitation energy were used to limit triplet-triplet annihilation (TTA).

Figure 3 .

 3 Figure 3.37 Typical GC-MS experiment of the sample headspace after several hours of visible light irradiation of a solution containing 10 μM Fe-p-TMA, 1 mM Phen2, 0.1 M TEA and 0.1 M CF 3 CD 2 OD in DMF and saturated CO 2 solution.

Figure 3 .

 3 Figure 3.38 Typical GC-MS experiment of the sample headspace after several hours of visible light irradiation of a solution containing 10 μM Fe-p-TMA, 1 mM Phen2, 0.1 M TEA and 0.1 M D 2 O in DMF and saturated CO 2 solution.

(

  Scheme 5.1 Synthesis routes of BIH K 3 [Fe(C 2 O 4 ) 3 ]ꞏ3H 2 O Weigh approximately 9.0 g of hydrated potassium oxalate, K 2 C 2 O 4 ꞏH 2 O into a 250 mL beaker. Add 30 mL of distilled water and heat to dissolve the solids (do not boil). In a second small beaker dissolve 4.4 g of FeCl 3 ꞏ6H 2 O in a minimum amount of cold water (10-15 mL). Add the FeCl 3 ꞏ6H 2 O solution to the warm oxalate solution and stir with a glass rod. Allow the product to crystallize (away from strong sunlight) by cooling the solution in an ice-water mixture. Collect the green crystalline product, K 3 [Fe(C 2 O 4 ) 3 ]ꞏ3H 2 O, by filtration and dry it under vacuum.

  5 global", and refers to the spectral distribution and intensity of sunlight on a 37 ̊ south-facing tilted surface after it has traveled through 1.5 times the thickness of the earth's atmosphere. The latter corresponds to a solar zenith angle of 48.19. The AM 1.5G spectrum includes both the direct and the diffuse contributions of the incident sunlight, and has a total integrated intensity of 1000 W/m 2 . The most recent and widely used AM 1.5G reference spectrum is published by the American Society for Testing and Materials as the ASTM-G173-03 standard, and represents a reasonable average for the 48 contiguous states of the USA over a period of 1 year [ASTM G173-03(2012), Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface, ASTM International, West Conshohocken, PA, 2012, https://www.astm.org/Standards/G173.htm]. The corresponding international standard is IEC 60904-3 (3nd edition, 2016).Since real AM 1.5G sunlight is not readily available at all times and at all locations, solar simulators are used. Making a reasonably good solar simulator is far from trivial. One cannot use halogen lamps since their maximum color temperature is limited to ~3200 K, which is much lower than the color temperature of the sun (5800 K). Xenon lamps are widely accepted to give the best match to the solar spectrum. Newport LCS-100 solar simulator (see Figure5.1) is usually used in photovoltaic cell research requiring a certified system for small illumination areas. The 38 × 38 mm simulators meet Class ABB as defined by ASTM, IEC and JIS standards. All electronics are built into the lamp housing and are factory preset to run the 100 W xenon bulb with integrated reflector at the proper current and voltage. For replacement assemblies, a "drop in" design eliminates the need for lamp alignment. An integrated attenuator allows partial sun irradiance.

Figure 5 . 1

 51 Figure 5.1 Photo of Newport LCS-100 solar simulator. In addition, optical filters are used to optimize the overall spectral shape and to remove (to some extent) the sharp lines in the emission spectrum of xenon. Schott long-pass filters (also known as edge filters) possess a steep absorption range. This property allows for a sharp separation between the blocking and transmission range. Moreover, the filters are practically insensitive to the angle of installation. In this thesis, both Schott GG420 and GG435 long pass filters are used and their transmittance spectroscopies are illustrated in Figure 5.2.

Figure 5 . 2

 52 Figure 5.2 Transmittance spectroscopies of Schott GG420 and GG435 long pass filters.(https://www.itos.de/en/schott-optical-filters/long-pass/gg395-gg400-gg420/)

  absorption kinetic was measured with an Edinburgh Instruments LP920-KS laser flash photolysis spectrometer. The solutions were excited at 420 nm (5 ns pulse, 2 to 4 mW per pulse) via a Continuum SLOPO Plus OPO pumped by a frequency-tripled Continuum Surelite II-10 Nd:YAG laser. Right-angle probe light was provided by an Osram XBO 450 W ozone free pulsed xenon lamp and then collected into a spectrograph. Excited state lifetime of Phen2 was determined by measuring kinetics of the ground state recovery at 390 nm, i.e. the absorption maximum of Phen2, thanks to a Hamamatsu R928 photomultiplier tube linked to a Tektronix TDS 3012C 100 MHz oscilloscope. Temperature of the sample port was set at 20°C by a Quantum Northwest TC125 Peltier effect controller. The Edinburgh Instruments L900 software ensured the control and the synchronization of the whole setup.

Figure 5 . 3

 53 Figure 5.3 Specac Omni Cell ® P/N 800 measurement cell for liquid IR spectroscopy.

detector. H 2 ,

 2 CO and CH 4 production was quantitatively detected using a CP-CarboPlot P7 capillary column (25 m in length and 25 μm internal diameter). Temperature was held at 150 °C for the detector and 35 °C for the oven. The carrier gas was argon flowing at 7.95 mL/min at constant pressure of 0.6 bars. Injection was performed via a 250 μL gas-tight (Hamilton) syringe previously degassed with CO 2 . The GC analysis was shown in Figure 5.4. Conditions allowed detection of H 2 , O 2 , N 2 , CO, and CO 2 . Calibration curves for H 2 , CO and CH 4 were determined separately by injecting known quantities of pure gas (see Figure 5.5).

Figure 5 . 4

 54 Figure 5.4 Gas chromatography analyses. Products concentration in the electrolysis cell is derived from the area of each peak.

Figure 5 . 5

 55 Figure 5.5 Calibration curves for gas chromatography quantification for H 2 (left panel), CH 4 (middle panel) and CO (right panel) and corresponding coefficients of linear correlation (R 2 ).

  Cyclic voltammograms were obtained in a three-electrode cell by use of a Metrohm Autolab potentiostats/galvanostat interfaced with Nova software. The working electrode was a 3 mm diameter glassy carbon disk carefully polished with 1 μm diamond paste (DP-PasteM) then thoroughly rinsed in acetone then absolute ethanol before use. The counterelectrode was a platinum wire and the reference electrode was an aqueous SCE electrode. All experiments were carried out either under argon or carbon dioxide atmosphere at 21 °C, the double-wall jacketed cell being thermostated by circulation of water. The cell was sealed during measurements and the solution was saturated with argon or carbon dioxide between measurements. Ohmic drop was compensated through the positive feedback compensation method implemented in the instrument.5.2.10 Photochemical experiments setupEach solution was prepared just before the experiment. The solvent was degassed with Ar for approximately 10 min, and then TEA and/or TFE was added. After keeping the solution approximately 5 more minutes under Ar atmosphere, it was poured with a syringe onto the solid porphyrin and photosensitizer previously placed under inert atmosphere. Then, the solution (3.5 mL) was transferred to a sealable fluorescence 1 × 1 cm quartz Suprasil cuvette (Hellma 117.100F-QS) equipped with a home-made glassware, leaving a headspace of approximately 23 mL (see Scheme 5.1). The cuvette was degassed previously through a vial filled with the solvent, TFE, and Et 3 N (if present) for approximately 20 min with Ar, CO 2 or CO to prevent any evaporation. The typical composition of the sample was 2-10 μM porphyrin, 0.05-0.1 M sacrificial electron donor, 0.2-0.1 mM photosensitizer and 0-1 M TFE (see figures for details of each experiment).

(K 3

 3 Fe(C 2 O 4 ) 3 ) chemical actinometer.173 Thus, 3mL (V 1 ) of an aqueous solution containing the iron actinometer (0.15 M) and H 2 SO 4 (0.05 M) was irradiated with a solar simulator (Newport LCS-100) equipped with band pass filters, and using the same experimental setup as in the case of the photocatalytic reactions. The time of the irradiation was kept as short as possible to avoid more than 10% decomposition of the actinometer. Before each measurement, the power output of the lamp was measured using a laser & power energy meter (Gentec Solo 2). After irradiation, an aliquot of 0.180 mL (V 2 ) was taken and 2 mL of a buffered solution of phenanthroline (0.015 M / 0.5 M H 2 SO 4 ) were added together with distilled H 2 O to give a final volume of 25 mL (V 3 ). The absorbance of the solution at 510 nm was recorded and the value was used to calculate the number of Fe 2+ ions (n[Fe 2+ ]) generated during the irradiation process through the following equation: Rubber septum for GC sampling Headspace (conique glassware, ca. 23 mL) PTFE connector with double screw caps Quartz optical cuvette (1 1 cm) with four transparent windows 𝑛 𝐹𝑒 𝑉 𝑉 𝐴 𝐴 1000 𝑉 𝜀 where V 1 = volume of actinometer solution irradiated (mL) V 2 = volume of aliquot taken for analysis (mL) V 3 = final volume to which the aliquot V 2 is diluted (mL) A = measured optical density at 510 nm A 0 = measured optical density at 510 nm of a non-irradiated sample ε 0 = experimental value of the molar extinction coefficient of the Fe 2+ complex (11100 L mol -1 cm -1 )

I

  really hope to express my heartfelt thanks to my supervisors, Prof. Julien Bonin and Prof.Marc Robert for their entire academic and personal supports during the past three years. They taught me how to think creatively and approach scientific explorations in the right way, which will benefit the rest of my life. I would also like to acknowledge the contributions of Prof. Jean-Michel Savéant, Prof. Cyrille Costentin, Prof. Miyake, Dr. Elodie Anxolabéhère, Dr. Benoit Limoges, Dr. Cédric Tard, etc. Together, they have brought significant advice and promoted diverse, profitable discussions all along these three years.Many of my colleagues who were present in the beginning of my Ph.D. are now gone and others have arrived since then. To all of them, Iban, Antoine, Arnaud, Hachem, Charlie, Niklas, Cheng as well as Claudio, Min, Etienne, Alexis, Kristian Yee-Seul, Khalil, Martin, Célia, Hussein, Jérémy, Bing, Lydia, Benjamin, Justine, Telmo, Rabia, Fumiaki, etc. I wish to express my appreciation. All the daily empowering interactions contributed all long these three years. I'm also grateful for the experimental assistance and data analysis I received from various people working within the Lavoisier building. Thank you for providing me a challenging environment for my studies.

  The orbitals of C atom are sp-hybridized, forming two σ bonds with O atoms, while the other two non-hybridized p orbits in C atom form two π bonds with the p orbits in O atoms. As a result, C=O bonds are extremely stable in the atmosphere and have a dissociation energy as high as ~750 kJꞏmol -1 , which indicates that a high activation barrier must be overcome for CO 2 activation and C=O bond cleavage. The generation of useful fuels from their highly thermodynamically stable combustion products, CO 2 and H 2 O, is necessarily endergonic.

	Moreover, given the highest chemical state (+4) of C atom in CO 2 , the reduction of CO 2 can
	result in various and complex products including carbon monoxide (CO), formic acid (HCOOH)
	(HCOO -, basic medium), oxalic acid (H 2 C 2 O 4 ) (C 2 O 4	2-

, basic medium), formaldehyde (HCHO), methanol (CH 3 OH), methane (CH 4 ), carbon (C), ethylene (C 2 H 4 ), ethanol (C 2 H 5 OH), ethane (C 2 H 6 ), n-propanol (C 3 H 7 OH), etc.

[START_REF] Chang | CO 2 photo-reduction: insights into CO 2 activation and reaction on surfaces of photocatalysts[END_REF]

Standard enthalpy (ΔH 0 ), Gibbs free energy (ΔG 0 ) and redox potential (E 0 ) of CO 2 reduction are summarized in Table

1

.1.

[START_REF] Bolton | Solar fuels[END_REF]

All the CO 2 reduction processes are more endothermic than those of water splitting owing to the positive ΔG 0 values, especially for hydrocarbon fuels such as CH 3 OH and CH 4 , indicating that CO 2 reduction is a much more thermodynamically unfavorable process in ambient conditions. Table 1.1. Thermodynamic reactions of CO 2 reduction 20

  CH 3 OH or another product, whereas photogenerated holes can oxidize H 2 O into O 2 .

2 , CdS, SiC, as well as various niobates, tungstates and germinates. It is generally accepted that SC-based photocatalysis for CO 2 reduction with H 2 O involves three main steps

[START_REF] Chang | CO 2 photo-reduction: insights into CO 2 activation and reaction on surfaces of photocatalysts[END_REF]

(Figure

1

.8 A). In the first step, activated electron-hole pairs are generated when a SC photocatalyst is illuminated by an appropriate light source with its energy equal or greater than the bandgap energy (E g ) of the semiconductor. Then, the generated electrons and holes migrate to the surface of the SC or a co-catalyst in contact with the SC, with only a fraction of carriers actually reaching the surface. A larger fraction of electronhole pairs recombine again, with energy released in the form of heat or photons. In the third step, photogenerated electrons can reduce CO 2 which is adsorbed on catalyst surface into CO, CH 4 ,

Binary semiconductor nanoheterostructures for CO 2 photoreduction. The photocatalytic conversion

  of CO 2 in the presence of methanol over a CuO/TiO 2 heterostructures results in the preferential formation of methyl formate.[START_REF] Qin | Photocatalytic reduction of CO 2 in methanol to methyl formate over CuO-TiO 2 composite catalysts[END_REF] In this case, methanol serves both as reactant and as VB hole scavenger, while CO 2 is reduced by the photogenerated CB electrons. Deposition of

	Cu 2 O NPs onto titania was reported to result in enhanced adsorption of CO 2 and simultaneous
	inhibition of water adsorption. 57 Simultaneously, Pt NPs deposited onto TiO 2 acts as electron
	"pools" promoting multi-electron photoinduced reactions. The summary effect of Cu 2 O and Pt
	NPs results in complete suppression of the water reduction pathway on titania surface and in the
	selective reduction of CO 2 with CH 4 as a sole product. Titania and Cu 2 O can also be separated in
	space and used as photoanode and cathode, respectively, in a photoelectrochemical system for the
	CO 2 reduction. In this way, Cu 2 O can be protected against oxidative photocorrosion by
	photogenerated VB holes. 58
	2 PO 3 H 2 the photoprocess yielded CO and HCOOH with a
	relatively high selectivity toward the former (40-70%). The difference arises from a photoinduced
	transformation of the -CH 2 PO 3 H 2 -substituted Ru-bipyridyl complex into a polymeric species
	specifically active toward CO formation. Polymeric Ru-bipyridyl complexes were also used as
	sensitizers/electrocatalysts to increase the rate of photocatalytic CO 2 reduction to formate over
	indium phosphide. 55

1.3.2 Molecular metal-based complex for carbon dioxide reduction

	Compared to semiconductor-based CO 2 reduction catalysts, transition metal complexes are also
	at the forefront of potential catalysts. Such compounds can have multiple and accessible redox
	states which have been shown to promote multiple electron transfer reactivity. Furthermore, the
	formal reduction potentials can be systematically tuned through ligand modification to better
	match the redox potential required for CO 2 reduction. A large number of transition metal
	complexes have been studied for potential CO 2 reduction applications. Some of the typical
	examples are reviewed below.
	Molecular photocatalytic systems for CO 2 reduction typically comprise a catalyst (CAT), a
	redox photosensitizer (PS) which can harvest light energy and then mediate the transfer of
	photoelectrons, and a sacrificial donor (SD) which offer electrons and/or protons. Generally, this
	type of catalytic system is called a three component homogenous photochemical system. First,
	upon light excitation, the molecular light absorber, such as the typical ruthenium (II)
	trisbipyridine ([Ru(bpy) 3 ] 2+

Transition metal polypyridine complexes. Metal polypyridine complexes are often used as a main component, i.e., as redox PS and/or as CAT, in various photocatalytic systems for CO 2 reduction. 70 In particular, the design of highly efficient and selective photocatalytic systems which are based on non-expensive materials is a great challenge for chemists. Lau et al. reported

	the photocatalytic reduction of CO 2 by [Co(qpy)(OH 2 ) 2 ] 2	+ (qpy = 2,2′:6′,2″:6″,2‴-quaterpyridine)
	and [Fe(qpy)(OH 2 ) 2 ] 2	+ (Figure 1.19).

  that cobalt porphyrins could efficiently electrochemically reduce CO 2 into renewable energy fuels, and formic acid can be produced as minor product through a [Co(P)-(OCHO)] intermediate, while CO is the main product through a decoupled proton-electron transfer.95 CH 4

Tripkovic et al.

have performed extensive density functional theory (DFT) calculations of metal-functionalized porphyrin-like graphene surfaces and predicted the potential formation of methane and methanol from CO 2 .

[START_REF] Tripkovic | Electrochemical CO 2 and CO reduction on metal-functionalized porphyrinlike graphene[END_REF]

Similar results reported by

Shen et al. showed

Table 2 .

 2 1 Solubility of CO 2 in common solvents at 298 K and low pressure.Photocatalytic CO 2 -to-CO in water with inexpensive molecular catalysts has thus been reported only in a few cases and confronted with dire challenges. Previous examples include the use of[Co(cyclam)] at pH 4 with a maximum TON of 22, 67 and the use of [Ni(cyclam)] at pH 4-6 with a maximum TON of4.4, 84c, 85, 103c, 105 with [Ru(bpy) 3 ] 2+ as PS. Recently, a Ni complex bearing a 2,2':6',2"-terpyridine ligand and anchored at the surface of CdS quantum dots was reported to reach a maximum TON of 20 at pH 6.5, with a catalytic selectivity of 90 % after few hours irradiation, progressively decreasing to produce mainly H 2 after 24 h. 106 As a consequence, for a scalable water-containing system for artificial photosynthesis combining CO 2 reduction with water oxidation, it is of great importance to develop catalysts which can exhibit high activity and selectivity for a desired product even in water solutions.

	Solvent	Solvent solubility parameter (atm 1/2 )	Gas solubility *
	ACN	24.1	7.1
	acetone	19.8	6.6
	THF	19.1	6.2
	MeAc	19.4	6.0
	DMF	24.0	4.1
	PC	26.0	3.9
	MeOH	29.5	3.6
	CHCl 3	19.0	3.6
	DMSO	26.3	2.9
	n-hexane	15.0	2.1
	Water	48.0	0.76

Table 2 .

 2 2. Catalytic performance (TON) and selectivity (CS) of a 2 µM Fe-p-TMA CO 2saturated ACN:H 2 O (1:9, v:v) solution after 47 h of visible-light (> 420 nm) irradiation in the presence of 0.1 M NaHCO 3 , 50 mM of different SDs and 0.2 mM PP.

	entry	SD	H 2	TON	CO	H 2	CS (%)	CO
	1	TEA	3		60	5		95
	2 [a]	TEA	4		71	5		95
	3	TEOA	4		42	9		91
	4	EDTA	4		46	8		92
	5 [b]	TEA	3 / 5		58 / 78	5 / 6		95 / 94
	6 [c]	TEA	3 / 6		63 / 120	5 / 5		95 / 95

[a]

0.4 mM PP.

[b]

After 47h / 47h irradiation and addition of one equivalent of Fe-p-TMA after the first 47h.

[c]

  21b, 123 Photochemical processes in which visible light would be directly harvested by an appropriate sensitizer which could furnish electrons to a competent catalyst for the CO 2 reduction are also conceivable. SCE in water at pH ~7, 126 although in that case demetalation of the catalyst and further formation of Cu particles, which are highly active towards CO 2 reduction, 127 should not be ruled out. More recently, Wang et al. reported the 4 and C 2 H 4 with a cobalt porphyrin adsorbed onto graphene, although no labelling experiment under[START_REF] Quinn | Carbon dioxide[END_REF] CO 2 was performed to assess the origin of the hydrocarbons while their production rate was modest (1.7×10 -2 µmol per hour and per cm 2 of catalytic material).129 An alternative approach involves the chemical reduction of CO 2 , with the noticeable tandem hydrosilylation of HCOOSiEt 3 with mixed organo-Lewis acid catalysts,

	Al(C 6 F 5 ) 3 and B(C 6 F 5 ) 3 , 130 leading to high yield of methane at moderate temperature (80°C) with
	a catalytic turnover frequency of a few per hour for more than a day. Finally, semi-conductive
	solid materials possibly doped with co-catalysts have also been recently developed for the CO 2 -
	to-CH 4 reduction, with hydrocarbon production rate not exceeding a few µmol per hour and per
	gram of catalyst. 131
	copper(II) phthalocyanine based material that exhibited by far the highest activity for yielding
	methane with a Faradaic efficiency of 66% and a partial current density of 13 mA cm -2 at the
	potential of -1.06 V versus the reversible hydrogen electrode. 128 The highest activity and
	selectivity of CuPc for catalyzing CO 2 -to-CH 4 conversion among the three structures has been

124

In these photoinduced approaches, metal complexes have been used in conjunction with a photosensitizer (being itself a metal complex or a solid material as e.g. a large band gap semi-conductor) and a sacrificial electron donor. The use of earth abundant metal (Fe, Mn or Cu) based catalysts remains a challenge, and the design of selective and stable photo-or electro-catalyst has been only achieved in few cases.

100d

Most of them are related to the two electrons and two protons reduction of CO 2 into CO or HCOOH. Rare electrochemical examples have been identified where highly reduced compounds like hydrocarbons are produced. One example concerns a cobalt proto-porphyrin adsorbed onto a graphite electrode and led to a 2.5% faradaic yield of methane under 10 atm CO 2 at pH 1.

125

Another one deals with a Cu porphyrin also absorbed onto a carbon electrode and led to 30% faradaic yield in methane upon electrolysis at -1.63 V vs.

explained by its reversible restructuring to form ~2 nm metallic Cu nanoclusters, which are identified as the active sites for the electrocatalysis. In photochemistry, one recent example involves the catalytic production of CH

  .1), while a longer irradiation time of 102 h gave TON CO of 367, TON CH4 of 79 and TON H2 of 26, respectively. With the addition of a weak acid (0.1 M TFE), the efficiency of the system somewhat increased, i.e. TON CO of 198 to 240, TON CH4 of 31 to 66 and TON H2 of 24 to 73 in 47 h irradiation (entry 2 and 3 in Table3.1 and Figure3.9 b and c). Under CO atmosphere, blank experiments in the absence of CAT or in the absence of light did not give any reduction product (entries 13 and 14 in Table3.1), while a longer irradiation time of 102 h enhanced the selectivity for methane further to 87% (entry 11 in Table3.1 and Figure3.9d). Addition of a weak acid in moderate concentration (trifluoroethanol 0.1 M) slightly increased the methane formation rate (from a TON of 140 to 159)

Table 3 .

 3 1 Catalytic performance and structures of catalysts and sensitizer.116

		[Fe-p-TMA]		[PS-8]	[TEA]	λ	Time		TONs
	Entry	µM	Gas	mM	mM	nm	h	CO CH 4 H 2
	1	2	CO 2	-	50	> 420	47	33	-	-
	2 3 *	2 2	CO 2 CO 2	0.2 0.2	50 50	> 420 > 420	47 47	198 240	31 66	24 73
	4	2	CO 2	0.2	50	> 420	102	367	79	26
	5	2	Ar	0.2	50	> 420	47	-	-	43
	6	-	CO 2	0.2	50	> 420	47	3	-	1
	7	2	CO 2	0.2	-	> 420	23	5	-	-
	8 9 †	2 2	CO 2 CO 2	0.2 0.2	50 50	dark > 420	23 47	-139	-26	-15
	10	2	CO	0.2	50	> 420	47	-	89	18
	11	2	CO	0.2	50	> 420	102	-	140 28
	12 *	2	CO	0.2	50	> 420	102	-	159 34
	13	-	CO	0.2	50	> 420	47	-	-	-
	14	2	CO	0.2	50	dark	23	-	-	-

* in the presence of 0.1 M TFE, † with Fe-o-OH as catalyst.

Table 3 .

 3 2 Turnover number (TON,  5%) and catalytic selectivity (CS) of a 2 µM Fe-p-TMA Under CO 2 atmosphere 116 , Table3.1, entry 2. b Under CO atmosphere 116 , Table3.1, entry 10. c Under CO atmosphere + 0.5 M TFE, this study. d Under CO atmosphere + 0.5 M TFE, this study (102 h irradiation).

	CO 2 -saturated solution in the presence of 0.2 mM PS and 0.05 M SD after 47 h of visible (> 420
	nm) light irradiation.
	PS	Solvent	SD		TON			CS (%)
	Ir(ppy) 2 (bpy) +	ACN	TEA	H 2 103	CO 178	CH 4 32	H 2 33	CO 57	CH 4 10
			TEOA	67	134	23	30	60	10
			DIPEA	77	151	24	31	60	9
	Ir(ppy) 2 (bpy) +	ACN/H 2 O	TEA	5	24	3	16	75	9
			TEOA	4	19	3	15	73	12
			DIPEA	4	20	3	15	74	11
	Ir(ppy) 3	A C N a	TEA	24	198	31	10	78	12
		A C N b	TEA	18	-	89	17	-	83
		A C N c	TEA	19	-	100	16	-	84
		A C N d	TEA	45	-	195	19	-	81
	a

Table 3 .

 3 3 Rate constant of the emission quenching of * [Ir(ppy) 2 (bpy)] + (0.2 mM in ACN/H 2 O (3:7 v/v)) by TEA, TEOA and DIPEA sacrificial electron donors and by Fe-p-TMA catalyst.

		TEA	TEOA	DIPEA	Fe-p-TMA
	K q (M -1 s -1 )	(5.78  0.14)  10 6	(4.55  0.18)  10 6	(1.36  0.04)  10 8	0

Table 3 .

 3 4 TON of gaseous products measured after 47 h of visible light (> 435 nm) irradiation of DMF solution containing 10 µM Fe-p-TMA, 1 mM Phen2 and various components (SD, CO 2 /CO, acid).

	TON

  1 M TFE provided proper acidity and concentration to maximize CH 4 production and suppress H 2 evolution. By employing 0.1 M TFE in a CO-saturated DMF solution irradiated for 102 h, we were able to produce CH 4 in 80 TON and 85% selectivity. As a comparison, the noble metal Ir(ppy) 3 catalyzed CH 4 production in 195TON and 81 % selectivity under similar reaction conditions (with 0.5 M TFE at optimized conditions). It is important to note that the Fe catalyst concentration in the Ir(ppy) 3 case was 5 times less as compared to this study, and thus the absolute mol number of methane produced by the catalytic system comprising Fe-p-TMA + Phen2 is in fact 2 times larger. Table3.4 below summarizes key results from Figures 3.27, 3.30 and 3.32. Notably, replacing Phen2 by the less reducing Phen1 led to the exclusive formation of CO (TON 60, selectivity 90%) and H 2 (TON 6) upon irradiation of a CO 2 -saturated solution. The lower energy conversion efficiency may be due to the much lower visible light extinction coefficient and faster triplet life time of Phen1 compared with Phen2. On the other hand, irradiation of a CO-saturated solution with Phen1 only furnished H 2 as detectable product. These results indicate that the slightly lower reducing ability of Phen1 led to the inability to further reduce CO.

  •+ is -1.80 V vs. SCE, 165d which is more negative than all the redox couples related to the Fe porphyrin (Fe III /Fe II , Fe II /Fe I and Fe I /Fe 0 ).100c The oxidative quenching by direct electron transfer from the excited state of the Phen2 photosensitizer to the Fe catalyst is thermodynamically favorable, thus allowing the generation of potential catalytically active Fe II , Fe I , and Fe 0 species upon light irradiation. Interestingly, this experiment probes the singlet excited-state of Phen2 (which has a 90% triplet quantum yield), and some reactivity likely occurs from the triplet excited-state.Considering that the formation of a CH 4 molecule requires the transfer of 8 protons, we conducted GC-MS experiments with the addition of labeled CF 3 CD 2 OD or D 2 O. As shown inFigure 3.37 and 3.38, MS results in these two parallel experiments are very similar. CD 4 is not the main component and products are made up of CD 4 , CD 3 H, CD 2 H 2 , CDH 3 and CH 4 , which

  Fe II CO intermediate. We note that the electron-rich Fe 0 species can react with 2H + to form Fe II and the undesired H 2 as byproduct. The Fe II CO intermediate can eliminate the CO product upon proposed 1e -reduction and alternatively participate in further 6H + /6e -reduction to produce the ultimate CH 4 . Notably, the 2 Phen2 •+ / 3 Phen2 * redox couple fulfills the crucial role of providing highly reducing electrons to enable subsequent multi-electron and-proton transfer processes that convert CO to CH 4 . After transferring an electron, the oxidized Phen2 •+ species is reduced by the SD (e.g. TEA) to regenerate a neutral Phen2 molecule. Scheme 3.3 Proposed mechanism for the 8e -/8H + reduction of CO 2 to CH 4 by tandem catalysis of Phen2 and Fe-p-TMA.

		100	OH, CHD 2 H 2 O, CH 2 D 2
	Relative abundance	25 50 75	CH 3 , CHD CH 4 , CD 2 CH 2 , CD CH	HDO,CHD 3 D 2 O, CD 4
		0
	m/z 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 m/z H 2 O, CH 2 D 2 protonation 104 and elimination of water, generates a 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 100 0 50 75 OH, CHD 2 Relative abundance 25 CH CH 2 , CD CH 3 , CHD CH 4 , CD 2 HDO,CHD 3 D 2 O,CD 4	•-), which upon

  5.It is also of great significance too is to find cheaper organic dyes instead of the noble iridium complexes used as sensitizer. In this purpose, a phenoxazine-based organic photosensitizer, i.e. Phen2, was demonstrated to help catalyzing the 8e -/8H + visible-light-driven reduction of CO 2 to CH 4 . It was the first noble metal free molecular photochemical system for CO 2 -to-CH 4 conversion in ambient conditions. In a CO 2 -saturated DMF solution, CO (TON of 140) and CH 4 (TON of 29) were produced after 102 h of light irradiation, whereas in a CO-saturated solution, CH 4 was produced with TON up to 80 and a selectivity of 85% and a quantum yield of ca. 0.4%. This work is expected to advance the field of photochemical CO 2 reduction and contribute to the mechanistic understanding of multi-electron and multi-proton transfer processes in CO 2 reduction.Generally, this thesis confirmed the great specificity of the modified iron porphyrin with four strong electron withdrawing substituent groups to catalyze CO 2 conversion under ambient conditions. Integrating different catalytic components, the active porphyrin catalyst can not only convert CO 2 into CO, but also convert CO 2 directly into CH 4 , which is a milestone in the field of molecular photochemical CO 2 reduction. Moreover, these reactions could be conducted not only in organic solutions, but also in aqueous conditions. Both CO 2 and CO could be used as the substrates for CH 4 generation. In addition, not only noble metal sensitizers, but also cheap commercial organic sensitizer could drive these reactions. This study opened up a perspective strategy towards obtaining efficient, selective, inexpensive and stable molecular photochemical CO 2 reduction in the future research.

  ). Ethylene glycol is another important raw material in the manufacture of polyester fibers and polyethylene terephthalate resins used in bottling. The direct hydrogenation of CO 2 to produce this chemical in bulk scale might be a prime example for selective reductive coupling using carbon dioxide and could substitute its traditional synthesis from ethylene oxide (equation 33). In addition, the reductive methylation of much more widely available arenes, for example, benzene (from biogas), to generate toluene and xylenes is also highly interesting

		Chapter 5 Experiments and methods
	5.1 Materials
	5.1.1 Commercial reagents
	Solids	NaOH (99.5%), KOH (99.5%), NaHCO 3 (99%), KCl (99.5%), KHCO 3 (99.5%)
	K 2 C 2 O 4 (99.5%) and FeCl 3 ꞏ6H 2 O (99.5%) were purchased from Merck; KH 2 PO 4 (97%) was
	purchased from Prolabo; K 2 HPO 4 (99.5%) was purchased from Labosi; fac-(Tris(2-
	phenylpyridine))iridium(III) (99%) was purchased from Sigma-Aldrich. FeCl 3 ꞏ6H 2 O (99%),
	FeBr 2 (99%) and phenol (99.5%) were purchased from Alfa Aesar; (2, 2'-bipyridine)bis(2-
	phenylpyridinato)iridium(III) hexafluorophosphate (Ir(ppy) 2 (bpy)PF 6 , 90%) was purchased from
	TCI. (NH 4 ) 2 -Fe(SO 4 ) 2 ꞏ6H 2 O (99%), purpurin (90%), benzaldehyde (99%), N, N'-dimethyl-1, 2-
	(equation 34). phenylenediamine (99%) and 2-phenylbenzimidazole (99.5%) were purchased from Sigma
	6 CO 2 + 6 H 2 O Aldrich; NH 4 PF 6 (99.5%) was purchased from Acros Organics; NBu 4 PF 6 (99%) and ./ ⎯⎯⎯⎯⎯⎯⎯ C 6 H 12 O 6 + 6 O 2 (31) Ethylenediaminetetraacetic acid disodium salt (EDTA, 99%) were purchased from Fluka; free
	CO 2 + CH 2 =CH 2 base 5,10,15,20-tetra(N,N,N-trimethyl-4-anilinium)porphyrin tetrachloride was purchased from . ⎯⎯⎯⎯ CH 2 =CHCOOH (32) Frontier Scientific, Phen1 and Phen2 were obtained from Miyake lab.
	2 CO 2 + H 2 All chemicals were used as received without further purification. . ⎯⎯⎯⎯ HOOC-COOH ., ⎯⎯⎯⎯⎯⎯⎯ HOCH 2 -CH 2 OH + 2 H 2 O	(33)
	CO 2 + 3 H 2 + C 6 H 6 (benzene) Liquids and solvents Acetone (99.8%), methanol (99.8%), ethanol (99.9%), ethyl acetate . ⎯⎯⎯⎯ C 7 H 8 (toluene) + 2H 2 O (34)
	(99.5%), dichloromethane (99%), 2-propanol (99.0%), and hydrochloric acid (37%) were
	"Dream" reactions driven by artificial photosynthesis. (31) Natural photosynthesis; (32) Direct purchased from VWR. Methyliodide (99%) and acetic acid (99.5%) were purchased from Alfa
	synthesis of acrylic acid from ethylene and CO 2 ; (33) Ethylene glycol synthesis via reductive Aesar. Electrolyte solutions were prepared with ultra-pure water (TKA MicroPure, 0.055 μSꞏcm -1 )
	or electrochemical grade acetonitrile, dimethyl sulfoxide, N, N-dimethylformamide (Acros, > coupling of CO 2 ; (34) Reductive methylation of benzene using H 2 and CO 2 . 99.8%, over molecular sieves and stored under Argon atmosphere). Triethylamine (TEA, Acros
	It is also my hope that future efforts toward more efficient photochemical conversion of CO 2 Organics, 99%), triethanolamine (TEOA, Sigma, >99%), N, N-diisopropylethylamine (DIPEA,
	and other small molecules (such as H 2 O, N 2 …) will, as a welcome byproduct, also lead to an Fisher Scientific, >99%) and 2, 2, 2-trifluoroethanol (TFE, Romil Ltd., >99.5%) were used
	improved understanding of the fundamental processes involved in photocatalysis. This without further purification. Methanol and dichloromethane were distilled over calcium hydride;
	knowledge should prove widely useful in a large amount of fields relevant to humanity.

  .1, entry 12), we obtain a quantum yield Φ of about 0.18% after 102 h of irradiation. Maybe we will miss each other if we meet in the street someday. It was three years ago that he fortuitously recommended me to LEM for a Ph.D. under the supervision of Prof. M. Robert. Life is really like a box of chocolate and you never know what you are going to get. I was born in a small, poor and unenlightened village in the center of China, where most of the young even have no chance to go to college. Before that, I had never thought I could have the chance to study in Paris. I guess Alain must be a very gracious and warmhearted man. It was him that changed my life path. I will remember him and be grateful to him forever in my deep heart.

	Notations quencher concentration speed of light total passed charge potential redox potential conduction band potential bandgap energy energy of photon valence band potencial Faraday constant Planck's constant emission intensity with a quencher emission intensity apparent quenching rate constant number of needed electrons for product evolution reaction time temperature scan rate work terms required to bring the product work terms required to bring the reactant γ-Radiation Gibbs free energy Standard enthalpy solvent dielectric constant photon wavelength excited wavelength photon frequency exited-state lifetime quantum yield oxalic acid triplet-triplet annihilation Terms and Abbreviations formaldehyde ultra violet formate ultraviolet-visible International Energy Outlook 2017 formic acid valence band 9-cyanoanthracene highest occupied molecular orbital work electrode ascorbic acid Intergovernmental Panel on Climate Change acetonitrile infrared amino salicylic acid turnover number graphitic carbon nitride turnover frequency gas chromatograph tetrahydrofuran 5, 10, 15, 20-Tetraphenyl-21H,23H-porphine iron(III) chloride 2, 2, 2-trifluoroethanol meso-tetraphenylporphine-4,4',4'',4'''-tetracarboxylic acid, iron (III) chloride triethanolamine Fe-p-TMA 5, 10, 15, 20-tetra(4'-N, N, N-trimethylanilinium) porphyrin iron(III) pentachloride [Q] c C E E 0 E CB E g Ep E VB F h I I 0 k q n t T v w P w R γ ΔG 0 ΔH 0 ε S λ λ ex ν τ 0 Φ H 2 C 2 O 4 TTA IEO2017 HCHO UV HCOO -UV-Vis 9CNA HCOOH VB AA HOMO WE ACN IPCC ASA IR ATP BIH BNAH bpy C 2 H 4 C 2 H 5 OH C 2 H 6 C 3 H 7 OH CAT CB CH 3 OH CH 4 CO CO 2 Co-POM CS CV cyclam DFT DIPEA dmb DMF DRIFTS EA ECCE ECEC ED EDTA FE FeTCPP FeTPP GC g-C 3 N 4 TON TOF THF TFE TEOA TEA triethylamine faradaic efficiency SD sacrificial electron donor ethylenediamine tetraacetic acid SCE saturated calomel electrode electron donor SC semiconductor electrochemical chemical electrochemical chemical RDS rate determining step electrochemical chemical chemical electrochemical QY quantum yield electron acceptor qpy 2, 2′: 6′, 2″: 6″, 2‴-quaterpyridine diffuse reflectance infrared Fourier transform spectroscopy QD quantum dot N, N'-dimethylformamide PS photosensitizer 4, 4'-dimethyl-2, 2'-bipyridine ppy 2-phenylpyridine N, N-diisopropylethylamine PP purpurin density functional theory PhOH phenol 1, 4, 8, 11-tetraazacyclotetradecane Phen2 3,7-di(4-biphenyl)-1-naphthalene-10-phenoxazine cyclic voltammetry Phen1 5,10-di(2-naphthyl)-5,10-dihydrophenazine catalytic selectivity PCET proton-coupled electron transfer Co-porphyrin-oligomers PCAT photocatalyst carbon dioxide NREL National Renewable Energy Laboratory carbon monoxide NP nanoparticle methane NHE normal hydrogen electrode methanol NHC N-heterocyclic carbene conduction band N719 Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) catalyst MS mass spectrum n-propanol MPc metallophthalocyanine ethane MP metalloporphyrin ethanol MN metallocorrin ethylene MLCT metal to ligand charge transfer 2, 2'-bipyridine MEDA 2-(dimethylamino)ethanethiol 1-benzyl-1,4-dihydronicotinamide MC metallocorrole 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole LUMO lowest unoccupied molecular orbital adenosine triphosphate LMCT ligand to metal charge transfer known his features.
	123

* Gas solubility in units of cm 3 (STP)/(cm 3 atm), where STP is standard temperature and pressure.111

Acknowledgements

I would like to express my appreciation to all the people who have appeared in my life, for their love and harassment, to make me happy and strong.

First of all, I would like to thank Prof. Alain Deronzier. Actually I had never seen him, nor This work is supported under LEM at Université Paris Diderot and China Scholarship Council.

I express my gratitude to my financial sponsors.

Lastly, I would like to thank this enlightened time of progress that we live in, giving us the opportunities to realize our dreams and find our self-worth.

In experiments done without a PS, the catalyst was directly exited by visible light and then reduced to its active state Fe 0 by three oxidative quenching reactions with TEA. The oxidation pathway of TEA has been investigated previously. 117 Oxidation and degradation of TEA may be represented as follows (Scheme 2.3). In these processes, different radical and cationic species are involved, and they, together with products from their decomposition, can potentially influence the structures and reactivities of photosensitizers and catalysts. In the first step, the tertiary amine reacts with H 2 O to form a quaternary cationic species and HO -.

Hydroxide ion then attacks CO 2 to form the bicarbonate anion. Then the last step is the ionic association of the protonated amine and bicarbonate. Liu et al. also shed light on the chemical interactions between CO 2 and N-containing organic bases through density functional theory (DFT). 120 The CO 2 molecule can be directly activated by organic bases in polar solvent to give a carbamate zwitterion intermediate. The distance between the CO 2 molecule and the Lewis base center in this intermediate being short (1.61-1.67 Å), it results in an increase in the C=O bond length from 1.16 to 1.22 Å and in a distortion of the linear O=C=O bond angle to ~137°. This suggests that although the chemical interactions between CO 2 and organic bases are relatively weak, the CO 2 molecule can still be well activated through a non-bicarbonate route in a polar solvent environment.

After electron transfer, the oxidized PS-8 is reduced by TEA upon irradiation, thereby closing the catalytic cycle and generating the protonated triethylamine TEAH + that could then act as proton donor, as seen before. [START_REF] Bonin | Homogeneous photocatalytic reduction of CO 2 to CO using iron (0) porphyrin catalysts: mechanism and intrinsic limitations[END_REF] Figure 3.13 sketches a plausible mechanism based on these considerations, which involves a postulated formyl intermediate 138 that may be stabilized by through-space interactions between the positive charges of the trimethylammonio groups and the partial negative charge on the CHO species bound to the metal. CO 2 was reduced into CO with two electrons and then followed with complete reduction of the Fe II CO adduct necessitating six electrons. Overall, the starting Fe III porphyrin (top left) is reduced with three electrons to the catalytically active Fe 0 species. The Fe 0 species reduces CO 2 , with the resultant Fe I regenerated through electron transfer from the excited photosensitizer (right-hand side cycle). The CO produced binds to Fe II and is further reduced with a total of six electrons (transferred from the excited sensitizer) and six protons to generate methane, via a postulated Fe I To summarize, Fe-p-TMA is able to reduce CO 2 into CO and then to CH 4 under visible light irradiation with PS-8 as sensitizer. In the absence of added sensitizer, the catalyst reduces selectively CO 2 to CO. The pure CO could be catalytically transformed by the same catalyst into CH 4 with 82% catalytic selectivity and 159 TON over several days, under the same mild conditions (ambient temperature and pressure, visible light). New roads towards solar fuels using earth abundant Fe-based molecular complex are thus opened. conducted in aqueous solutions (acetonitrile:water 1:9 v:v) under visible light irradiation. CO was produced with a catalytic selectivity of 95% and turnover number up to 120 after two days. Catalysis is ultimately limited by progressive sensitizer photodegradation, but it could be restarted by adding fresh sensitizer. Photochemical CO 2 reduction in water-containing system still remains rare in literatures.

This work is a good example of developing meaningful efficient CO 2 photocatalysis in water.

3. Photochemical CO 2 reduction process was also studied using a noble metal photosensitizer, i.e.

Ir(ppy) 3 . As the excited state of Ir(ppy) 3 is a strong reductant, which may further reduce the Fe-CO intermediate, CO was found to be the main products while a certain amount of CH 4 was also produced.

Interestingly, CO could also be the substrate for photochemical CH 4 production, which indicates that CO is an intermediate in the full CO 2 -to-CH 4 cycle. Under optimized conditions, the TON of the system for CO 2 -to-CH 4 conversion reaches 367 for CO, 79 for CH 4 and 26 for H 2 in 102 h, with the corresponding catalytic selectivity of 78%, 17% and 5%, respectively. The TON for CO-to-CH 4 conversion is 159 for CH 4 and 34 for H 2 , with the corresponding catalytic selectivity of 82% and 18%.

Unfortunately, we still failed to detect possible intermediates, such as HCHO or CH 3 OH, most probably due to their very low instantaneous concentrations and their short lifetimes during the process.

An intense work is necessary to better understand the mechanism. This example was the first reported molecular photocatalyst for 8e -/8H + reduction of CO 2 into methane. It could be beneficial to develop CO 2 -to-CH 4 conversion at the surface of semiconductors since it may also open new research strategies for molecular catalysis of multi-electron/proton reduction of CO 2 .

4. CO 2 reduction in pure water solution is always much more attractive than in organic solvent. Ir(ppy) 2 (bpy)PF 6 was introduced instead of Ir(ppy) 3 as the photosensitizer in an analogous system for CO 2 reduction in aqueous solution as solvent. To make a comparison with the former system, experiments were conducted in acetonitrile first. Upon visible light irradiation, CO was the main reduction product (TON of 178 in optimized conditions) and a substantial amount of CH 4 was also produced (TON of 32, 10% catalytic selectivity) while H 2 was formed as a minor byproduct when acetonitrile was used as a solvent. Among the various amines employed, TEA appeared as the best choice to maximize CO 2 reduction products. Remarkably, the process could also be realized in aqueous conditions (acetonitrile/water 3:7 v:v). In this case, the main product was still carbon monoxide while methane was produced with selectivity up to 10% (TON of 3). Efficiency of the catalytic system is hampered by the limited CO 2 solubility and most importantly by instability of the Gases Argon (> 99.998%), [START_REF] Steinberg-Yfrach | Light-driven production of ATP catalysed by F 0 F 1-ATP synthase in an artificial photosynthetic membrane[END_REF] CO (> 99.7%), and 12 CO 2 (> 99.7%) gases were purchased from Air Liquide. CH 4 (> 99%) was from Fluka. [START_REF] Quinn | Carbon dioxide[END_REF] CO (99% content in atom 13 C) and [START_REF] Quinn | Carbon dioxide[END_REF] CO 2 (99% content in atom 13 C) were purchased from Sigma Aldrich. High purity hydrogen was generated from an F-DBS Hydrogen Generator (WM-H2 500). min; the mixture was stirred at reflux under inert atmosphere for 7 d. After methanol was removed, the resulting solid was sonicated in THF and filtered. The brown solid on the glass frit was washed with dichloromethane and dissolved in methanol; after the mixture was concentrated under reduced pressure, a small amount of concentrated HCl was added with a large quantity of THF to give crude product as a dark red powder.

To purify the sample, the crude samples are dissolved in ultra-pure water. The porphyrin is precipitated by adding 10 equivalents NH 4 PF 6 (155 mg, 9.5 × 10 -4 mol). The resulting suspension is centrifuged for 10 minutes at 1.0 × 10 5 rpm to allow for better separation of the liquid and solid phases. The solid is collected and washed with 25 mL ultra-pure water and 10 equivalents NH 4 PF 6 (155 mg, 9.5 × 10 -4 mol). Traces of initial free-base porphyrin are removed by washing the solid with 15 mL acetone/CH 2 Cl 2 (1:1), then 5 mL CH 3 OH, and eventually five times 5 mL acetone/CH 2 Cl 2 (1:1). Then, the residue is treated by 22.5 mL acetone and 2.5 mL concentrated HCl (37%) is added dropwise to exchange both iron ligand and anilinium counter-ions. The resulting suspension is sonicated for 10 minutes then centrifuged for 10 minutes at 1.0 × 10 5 rpm.

After the supernatant is removed, the solid phase is collected and washed with 25 mL acetone.

The residue is then dissolved in a minimum of methanol and precipitated with ethyl acetate. The solvent is eventually removed in vacuo to obtain Fe-p-TMA as a dark red powder (49.9 mg, 44%).