Keywords: réduction du CO2, catalyse photochimique, catalyse supportée, systèmes hybrides, catalyseur moléculaire bimétallique, complexes de cobalt CO2 reduction reaction, photochemical catalysis, supported catalysis, hybrid systems, bimetallic molecular catalysts, cobalt complexes

La production photocatalytique de carburants solaires est un moyen efficace de stockage chimique de l'énergie solaire et offre une option potentiellement fructueuse pour parvenir à un système énergétique à zéro émission de carbone. La pierre angulaire d'un processus pratique de production de carburant solaire est de concevoir et d'optimiser des photocatalyseurs stables, efficaces et déployables à grande échelle, et comprenant un matériau semi-conducteur permettant l'absorption de photons, la génération efficace de porteurs de charge, leur transport jusqu'à la realization de réactions catalytiques.

La catalyse moléculaire joue un rôle essentiel dans la photosynthèse naturelle et artificielle. Dans ce travail de doctorat, j'ai étudié (i) un complexe bimétallique pour la réduction sélective du CO2, illustrant le fait que la catalyse moléculaire est l'un des moyens prometteurs pour mettre en oeuvre la coopérativité des métaux avec des activités intrinsèques exceptionnelles ; (ii) des matériaux inorganiques (nitrure de carbone semi-conducteur et acide graphitique) pouvant être fonctionnalisés par une molécule (complexe quaterpyridine de cobalt) par le biais d'une liaison amide covalente. Une hétérogénéisation efficace des deux composants a été réalisée. Ces systèmes hybrides ont montré des performances catalytiques élevées vis-à-vis de la photoréduction au CO2. Ils illustrent que le développement de catalyseurs moléculaires vaut la peine d'être exploré afin de parvenir à des systèmes catalytiques évolutifs qui sont nécessaires pour la production pratique de carburant solaire à grande échelle. Dans le mécanisme de fonctionnement des catalyseurs moléculaires supportés par des matériaux, des défis subsistent en ce qui concerne à la fois la stabilité de l'unité moléculaire, les techniques d'hétérogénéisation et le contrôle de l'interaction électronique entre les composants.

With industrial progress and economic development, more than 80% of global energy consumption comes from fossil energy, an source that has insufficient reserves, is unequally available and emits pollutants. 1 This requires us to find clean and renewable alternative resources and corresponding carbon-neutral technologies.

The most abundant renewable energy source available on the planet is solar energy: solar illumination on Earth every hour is greater than the worldwide energy consumption for a whole year. 2 The intrinsic carrier of solar energy is a photon traveling at light speed. It is only by efficiently collecting these energy carriers, then producing separated electrons-holes (i.e. achieving charge separation) to achieve its conversion into "electronic" energy that solar energy can be used on a larger scale. the most common method to utilize solar energy for generating electricity is through photovoltaic cells in which photogenerated electrons and holes can be collected to produce potential and current, i.e. the harvested solar energy is directly converted into electricity.

However, electricity cannot readily be stored on a massive scale, is not easy to transport without important lossess and cannot realize a complete replacement of fossil fuels, because the energy density of batteries is far below that of chemical fuels regarding both weight and volume. 3 Therefore, electricity is not an ideal energy source for applications such as large oversea vessels and long-distance air transport at present. Another strategy is to take inspiration from photosynthesis in Nature which can complete the conversion of solar energy to chemical energy in the form of chemical bonds and which can produce energy containing compounds such as carbohydrates. In this way, solar energy can be directly converted into chemical fuels via photoelectrocatalytic reactions, or it can be converted into fuels indirectly by using the electricity generated from solar energy to drive electrocatalytic reactions (Scheme 1.1). Scheme 1.1 Two routes for solar energy-to-chemical fuels conversion.

However, it is impossible and quite unnecessary to completely replicate natural photosynthesis in living organisms, so the concept of artificial photosynthesis (AP), a multistep reaction process, was suggested. Specifically, current researches mostly imitate only a part of the photosynthetic reaction, namely water decomposition, CO2 reduction, nitrogen fixation and oxygen reduction.

Generally speaking, artificial photosynthesis mainly includes four steps: photon absorption (possibly through a dedicated agent called photosensitizer); charge separation; electron transfer from the photoabsorber to the catalytic site; and finally catalytic reactions. Light absorber and catalyst are the two most important parts of an artificial photosynthetic system.

The former requires light absorption, a long-lived excited state and a high current-carrying ability. The latter requires high catalytic activity and stability. Most reactions involve the transfer of multiple electrons and protons, which slows the reaction kinetics and requires large overpotentials. Thus, it is required that the catalyst can overcome high reaction energy barrier by lowering the overpotential and thus accelerating the reaction.

Research and development for CO2 reduction with molecular catalysts 1.2.1 Molecular catalysts

Molecules have attracted wide attention because of their well-defined properties, controllable synthesis, and tunable structures. As a consequense current researches on AP systems and devices involving molecular catalysts advocate that they indeed are promising candidates in the development of efficient AP systems. 4 Molecular complexes can work as efficient catalysts because the electronic structure of their metal cores can be precisely tuned by the coordination environments created by the ligands. Thus, a molecular catalyst usually comprises an active metal center and specific ligands which favor controlling the electronic structure of the active site. Thanks to this configuration, molecular catalysts possess several advantages regarding their use in AP:

a. Identifying active site and improving structure

Resolved structures of molecules are essential for studying catalytic activity and unraveling catalytic mechanisms. FT-IR spectroscopy, XRD spectroscopy as well as X-ray crystallography have revealed many molecular catalysts with their active sites and coordination structures. With the help of spectroelectrochemistry, stopped-flow UV-vis spectroscopy, in situ NMR spectroscopy, in situ IR spectroscopy, in situ Raman spectroscopy and in situ mass spectrometry, several catalytic mechanisms implying molecular catalysts have been proposed. As a consequence, the electronic structures and steric configuration can be adjusted by ligand modification. All these studies can further contribute to the researches on catalytic mechanisms. Because of the easy-to-identify and adjustable active sites in molecular catalysts, the product selectivity for CO2-reduction reaction and other reactions can be improved and tuned.

b. Ajustable intrinsic activity

Catalytic performance of catalysts in AP systems can be enhanced either by increasing the number of active sites or by improving the intrinsic activity of each active site. 5 The increase of active sites density can be achieved through higher loading of catalysts or by designing catalyst structures to expose more active sites per gram (Fig. 1.1). Nevertheless, these two ways are physically limited by the influence of other processes, such as charge and mass transport or catalyst aggregation. 5 Therefore, a plateau effect is usually obtained in practice with a high loading of molecules. In contrast, the improvement in the intrinsic activity of catalysts boosts the catalytic performance, avoiding transport issues resulting from high loadings. Additionally, the difference between high loadings and low loadings usually is one to three orders of magnitude, whereas the difference between "good" and "poor" catalysts can reach more than ten orders of magnitude. 6

c. Metal-atom economy

From the perspective of practical applications, the efficient use of metal ions is necessary, especially the use of some precious/toxic metals should be avoided as much as possible.

Molecular catalysts are undoubtedly more advantageous than inorganic materials. In homogeneous catalysis, most of the metal atoms are theoretically accessible to work as active sites. In contrast, in nanostructures, only metal sites present on the surface, edges, corners, or heterojunctions can successfully catalyze reactions.

After working on large and localized nanostructures, material scientists have started investigating single-atom catalysts. 7,8 At the same time, bi-metallic heterogeneous catalysts have also been reported. 9 The development of catalytic materials and systems is now more and more focused on the molecular level, one major difference between homogeneous and heterogeneous catalysts being that the active site coordination environment is a molecular ligand in the former case and an anchoring matrix in the latter.

Theoretical concepts a. Molecule-based photocatalytic cycles

Photocatalytic molecular systems for CO2 reduction comprise three components, i.e. a catalyst (CAT), a redox photosensitizer (PS), and a sacrificial electron donor (SD). The metal complex serves as a catalyst capable of accumulating multiple electrons needed to reduce CO2. Light-driven catalytic routes can be classified as oxidative (Fig. 1.2a) or reductive quenching (Fig 1 .2b.) according to the quenching reaction of the photosensitizer. After excitation (Fig. 1.2a, step (1)), the lowest excited state of the PS (PS*) is a strong reductant and gives an electron to the electron acceptor (EA, being the CAT or an electron mediator).

Emission of PS* (Fig. 1.2a, step (2)) is oxidatively quenched to give rise to the corresponding one electron-oxidized form PS + (Fig. 1.2a, step (3)). When the standard redox potential of SD + /SD is more negative than that of PS + /PS, one electron can be donated to PS + (Scheme Fig. 1.2a, step (4)) to recover PS. Alternative possibility involves the reductive quenching of PS* (Fig. 1.2b). In this case, PS* accepts an electron directly from SD (Fig. 1.2b, step ( 3)).

The one-electron-reduced form PS -is thus produced. PS is regenerated through electron transfer from PS -to EA (Fig. 1.2b, step (4)). Favorable conditions for oxidative quenching include long lifetime and high reducing power of PS* as well as high concentration of EA because electron transfer between PS* (PS -) and EA is a bimolecular reaction. Quantum yield (QY) and turnover number (TON) are typically employed to evaluate CO2 reduction efficiency. b. Quantum yield Quantum yield includes internal quantum yield (IQY) and apparent quantum yield (AQY), which are crucial parameters for assessing the performance of a photocatalyst or a photocatalytic system. IQY is calculated as the ratio between the moles amount of product multiplied by the number of electrons necessary for the product formation and the moles amount of adsorbed photons (Equation 1.1), while AQY is calculated relatively to the molar number of incident photons (Equation 1.2). 11 Back electron transfer (BET) may decrease quantum yield. For example, in the case of oxidative quenching of PS* ( Irradiation intensity and area, the concentration of CO2, photocatalyst and sacrificial donor (SD), as well as the difference of standard redox potential between PS and SD may significantly influence the product yield.

c. Turnover number (TON)

Turnover number (TON) is defined by the ratio between the moles amount of product and catalyst once the reaction has stopped. In some cases, high turnover numbers have been obtained at a very low concentration of catalyst (nM range), leading to a very low absolute yield of CO2 reduction products. In such cases, not only the real nature of the catalyst becomes doubtful but also performances may be overestimated.

The number of active sites number is a crucial parameter in evaluating the catalytic efficiency. In the case of molecular-supported systems, it will depend on the catalyst loading and active sites accessibility. A crude estimation of the catalyst content in supported systems can be made by calculating the amount of catalyst used in the synthesis process. Control of the remaining catalyst left in solution after preparation (through UV-Vis spectroscopy for example) can help to refine the previous estimation. Inductively coupled plasma atomic emission spectroscopy (ICP-OES, after treatment of the material with a strong acid solution)

and X-ray photoelectron spectroscopy (XPS) can be employed to more precisely quantify the amount of active species.

1.3 Homogeneous photocatalytic systems involving transition-metal-based molecular catalysts for 2-electron CO2 reduction

The homogeneous catalysts we discuss here refer to molecular complexes that are homogeneously dispersed in solution. Common homogeneous catalysts for CO2 reduction include metal-complexes, small organic molecules or enzymes.

1.3.1

Molecular-based photocatalystic system for CO2-to-CO or to-HCOO - conversion Transition metal-based complexes have been gradually discovered in recent decades to photocatalyze the reduction of carbon dioxide. Fe-, Co-based cyclams, polypyridines, porphyrins, phthalocyanines, corroles, and others have been studied in the reduction of CO2.

While studying their catalytic activity, corresponding catalytic mechanism, have been gradually revealed. Regarding CO2-to-formate conversion, a classical mechanism is that the reduced catalyst center generates a hydride that can nucleophilically attack CO2 to produce a metal-formyl complex. The reduction of CO2 to CO also faces the challenge of inhibiting the kinetic and thermodynamic competitive hydrogen evolution reaction (HER), which can be achieved upon fastbinding CO2 in the presence of a proton source whose acidity is sufficient to promote subsequent C-O bond cleavage.

In the following paragraphs, I will describe a few examples of molecular CO2 reduction photocatalysts. Table 1. With Ru(bpy)3 2+ (PS1, Fig. 1.5) as a photosensitizer, CO was produced with maximum selectivity of 32% (Table 1.1, entry 1-4). 12 Moderate selectivity (57%) with a Ni II cyclam (Ni2, Fig. 1.4.) has been reported in similar aqueous conditions, while its corresponding dimer (Ni21, Fig. 1.7) showed 89% selectivity and a TON 6 times higher than the monomer (Table 1.1, entry 5). 13 At the same time, Brunschwig and Fujita et al. have tried to unveil possible intermediates during the photocatalysis with Co cyclams (Co3-Co5, Co7, Fig. 1.3). [14][15][16] 17,18 The catalytic mechanism for CO2 reduction has thus been studied through a series of Cobased cyclams (Co3-Co8, Fig. 1.3) with interesting performances that led to mechanistic studies, including the identification of some reaction intermediates involved in the processes.

Some aromatic organic molecules were applied as photosensitizers. The main catalytic product is CO with p-terphenyl (PS2, Fig. 1.5 and Table 1.1, entries 6-13) 19,20 while it changes to formate with phenazine (PS3, Fig. 1.5. and Table 1.1, entry 14). 21 The reason for this difference is that phenazine can donate both electron and proton during catalysis. The formation of a Co-H hydride species is enhanced by proton transfer from the phenazine sensitizer, which was proposed to lead to formate upon CO2 insertion. Different selective reduction has also been found with molecular catalysts containing different metals. 22 Two complexes with cobalt and iron, (Co9, Fe1, Figs. 1 .3,) were also found to be active CO2 catalysts (Table 1.1, entries 21-23). Fe3 (Fig. 1.4) was first used for the photoreduction of CO2 in 1997 (Table 1.1, entries 24, 25). 26,27 With the exploration of the catalytic activity, its possible mechanism has been gradually revealed. In a DMF/TEA solution of Fe3 (Fig. 1.4), photoinduced ligand-to-metal charge transfer generated Fe II from initial Fe III species. Fe II was further converted to Fe I upon light excitation and reductive quenching by TEA. Finally, the catalytically active species "Fe 0 " was proposed to be formed by the disproportionation of two Fe I species (see Scheme 1.2). The structural optimization of iron porphyrin led to catalytic activity in recent years. A modified Fe simple tetraphenyl porphyrin with OH groups in all ortho, ortho' positions of the four phenyl groups (Fe7, Table 1.1, entry 32) were studied by electrochemical methods. 27 Electrochemistry confirmed that the hydroxyl group is versatile in the porphyrin molecule.

On the one hand, the -OH stabilizes the Fe-CO2 adduct through hydrogen bond. At the same time, the -OH acts as a transfer station for protons to accelerate the cleavage of the C-O bond. 28 This complex exhibits interesting activity in the photocatalytic reduction of carbon dioxide. A good selectivity for CO (85%) was already obtained with Fe7 without the aid of photosensitizer, significantly higher than that with Fe8. In the presence of an organic sensitizer (9-CNA, PS5, Fig. 1.5) and a strongly reducing Ir complex (Ir(ppy)3 3+ , PS6, Fig. In addition to -OH, porphyrins have also been modified with trimethylamine (Fe9), four positively charged trimethylammonium groups equipped at all para positions of each phenyl rings of the porphyrin. Under similar conditions with Fe7, BIH was employed as sacrificial donor for a non-sensitized CO2 reduction with Fe9, showing an enhanced efficiency and selectivity (100%) towards CO evolution (Table 1.1, entry 35). 30 Moreover, the catalyst was able to convert CO2 to CO in aqueous solutions (Table 1.1, entry 36). 31 Replacing purpurin with Ir(ppy)3 as a sensitizer, Fe9 can trigger the reduction of CO2 to CH4 production in a CH3CN solution containing up to 70% H2O (Table 1.1, entries 37, 38). 32,33 This is of great significance for using iron-based molecular complexes to reduce CO2 to CO and then to CH4 under mild conditions in visible-light-derived systems.

Fe-and Co-based phthalocyanines (Fe4, Co12, Figs. 1.3 and 1.4 and Table 1.1, entries 26-28) 23 and corroles (Co13, Fe5, Fe6, Table 1.1, entries 29-31) 34 have also been introduced in photocatalytic reduction of CO2. The Fe-and Co-phthalocyanines, sensitized by pterphenyl, can reach comparative TONCO to those of porphyrin analogs, such as Fe3 and Co10. 25 The high activity indicates the unlimited potentiality of these catalysts in the catalytic reduction of CO2. In addition, a series of Fe-carboxyl complexes also show catalytic activity in the reduction of CO2. Fe-carboxyl complexes are also active catalysts for CO2 reduction to CO and formate. Applying Ir(ppy)3 and TEOA to N-methyl-2-pyrrolidone (NMP) solution, a commercially Fe3(CO)12 (Fe31, Fig. 1.4) was reported to produce CO with a 50% selectivity and TON ca. 100 (Table 1.1, entry 39). 35 Subsequently, the same group reported another carboxyl complex, (cyclopentadienone)iron-tricarbonyl (Fe(cpd)(CO)3, Fe10, Fig. 1.4), that could accomplish CO2 reduction with a TONCO of 600 and selectivity of 50% in 1 hour (Table 1.1, entry 40). 36 Further, with a Cu complex as sensitizer and BIH as the sacrificial donor, CO could be obtained in an enhanced selectivity of 99% with a high quantum yield of 13.3%

(Table 1.1, entry 41). 37 It is also worth mentioning that Ishitani et al. have reported a Cu-based complex sensitized efficient photocatalytic system using a Fe-containing complex as the catalyst in which the metal is chelated by two di-methyl 1,10-phenanthroline molecules (Fe(dmp)2(NCS)2, Fe11, Fig. 1.4). The visible-light-drived photocatalytic reduction can convert CO2 to CO with a TON of 273 and a quantum yield of 6.7% (Table 1.1, entry 42). 2+ and Ru(phen)3 2+ showed even better catalytic performance for CO production (Table 1.2, entry 4). 42 The failed attempt did not depress the researchers. Another dyad RuNi2 was synthesized successfully by Aukauloo et al., including a ruthenium trisbipyridyl-like unit covalently attached to a nickel cyclam via a triazole ring. In aqueous ascorbate solution, RuNi2 was able to afford CO with a small TONCO of 5.2 under 450 nm illumination, as a sacrificial donor under 450 nm illumination with a few TON (Table 1.2, entry 5). 43 Instead of nickel, cobalt-based catalysts were also linked to ruthenium photosensitizer for CO2 reduction (RuCo1 and RuCo2, Fig. 45 All investigations illustrate that conjugation between the PS center and catalyst can promote catalytic performances due to the desirable electronic interaction during the subunits, which is possible to generate higher selectivity and better stability of CO2 reduction system. In addition, some multimetallic complexes such as Fe21, Cu21, Cu22, and Fe41-Fe43 (Fig. 1.7) also exhibit good activity in electrochemical catalytic carbon dioxide reduction reactions.

Supported molecules CO2 reduction catalysts

Compared to inorganic materials, molecular catalysts are usually characterized by a relatively mediocre stability, non-recyclability, and the need for both electron donors and photosensitizers limit their practical applications. 51 Appending molecular catalysts to inorganic matrices has recently become a research hotspot since it may overcome the above shortcomings, at least partially. For example, these supported molecular catalysts show promising activity in photocatalytic reduction of CO2. 52 It is worth mentioning that recently some inorganic materials with atomically dispersed catalytic metal sites have been synthesized. 53 Such systems share with appended molecular catalysts the possibility to fine tune the environment at the metal-binding site, but with a higher degree of electronic conjugation between the metal and the supporting matrix.

Supported molecular catalysts can be obtained through a variety of linkage strategies.

A first type is through non-covalent grafting, which mainly includes simple physical mixing and in-situ growth of complexes on matrices. A second type is through covalent grafting via a covalent bond between the inorganic material and the molecular catalyst.

This can also be extended to conjugated grafting which utilizes aromatic structures with faster electron transport.

The selection of supporting substrate also requires careful consideration to minimize undesirable characteristics of the hybrid, such as resistive electron transfer towards the catalyst, occurrence of side reactions, and instability of the heterogeneous catalyst in different reaction conditions. On one hand, semiconductors, including carbon-based materials as well as metal chalcogenides and metal oxides (TiO2 and ZrO2), [54][55][56][57] have been extensively used in constructing heterogeneous electro-and photocatalysts. In these catalytic systems, the semiconductor works as photosensitizer, as co-catalyst and/or as an electronic conductor. On another hand, scaffolds such as inorganic materials (metalorganic frameworks (MOFs), covalent organic frameworks (COFs)) or carbon-based materials (e.g. single or multiwall carbon nanotubes, graphene or fullerene), can be used both as conductive support and as platform for molecular immobilization.

In general, supported molecule catalysts can be divided into three parts: (i) the catalytic center-molecular complex, (ii) the anchoring group on the surface of the material which can achieve the chemisorption of molecules, and (iii) a linker to permit electron transfer. During the catalysis, the size and the electronic inductive effect (electron-withdrawing or -donating) of the ligand linker will both affect spatial and electronic properties of the catalyst by adjusting electron transfer kinetics, which may trigger significant changes in the overpotential required in catalytic activity. Because the electron transfer between the surface and the catalyst proceeds either "through-bond" (via the linker orbitals) or "through-space" (via direct orbital overlap or superexchange). 58 1.5.1 Hybrid catalysts assembled by non-covalent effect

The simplest approach to modify inorganic material is absorbing or grafting molecules to scaffold matrices, such as silicon, zeolite, kaolin. One strategy successfully combined [Mn(CO)5Br] with bipyridyl-containing periodic mesoporous silica (bpy-PMO, The Royal Society of Chemistry.

In the case of carbon material derivatives, noncovalent strategies are mainly employed in carbon materials through π-π stacking or hydrophobic interactions, allowing the intrinsic properties of the substrate to be maintained during the chemical synthetic process. [60][61][62][63][64] In another example, a polymeric cobalt phthalocyanine (Co6, CoPPc) was coupled with mesoporous carbon nitride (mpg-CNx) through in situ polymerization (Fig. 1.10). 65 In CH3CN solution with TEOA as electron donor, CO was generated with a TON of 84 and a selectivity of 85% under full solar spectrum irradiation (> 300 nm), whereas under visible light, TONCO = 51 and selectivity decreased to 76% with identical catalyst loading. KGaA.

Hybrid catalysts assembled by covalent bonds

As discussed above, the hybridization of carbon materials is usually achieved derivatization sp 2 carbon on the surface before hybridizing with molecules. One of the functinalizing method is to generate reactive functional groups by applying strongly chemically or electrochemically oxidization to create ketones, quinones, or carboxylic acid groups. [66][67][68][69] For example, surface carboxyl groups and ortho-quinone can be linked to molecules through amide/ester bonds and the formation of pyrazine, respectively.

However, Control of the oxidation process is difficult and it may lead to unexpected changes in the physical properties (such as conductivity and stability) of the material due to the formation of defects.

Reisner et al. have used a nickel terpyridine and phosphonic acid-functionalized Ni (cyclam) (Ni1, Figure 1.4) to garft with CdS QDs and ZnSe QDs, respectively. And these molecule supported catalyst can convert CO2 to CO in aqueous solution, althrough with a low selectivity and modest stavility (TONCO = 121, selectivity 8% after 20 hours) 70 An interesting strategy associating polymeric carbon nitride (PCN) with cobalt bipyridine [Co(bpy)3 2+ ] was followed through the formation of in molecular junctions with 2,2 ' -biisonico-tinic acid (BINA), and it led to interfacial electron transfer (Fig. 1.12). 71 The binary photocatalyst [Co(bpy)3 2+ /BINA2-CN] showed photocatalytic activity for H2 and CO2 reduction with a rate of 8.8 µmolh -1 and 8.1 µmolh -1 , respectively. While the rates with samples obtained by physical mixing of Co(bpy)3 2+ and PCN were 12.5 and 16 times smaller than that of binary photocatalysts , respectively. These observation highlights the importance of controlling the connection between solid materials and molecular catalysts to accelerate the interfacial charge-carrier mobility and, consequently, improve the photocatalytic activity.

Motivation and outline of the thesis

Unlike hydrogen production which has been widely studied, there is still a long way to go in the research on CO2 reduction into useful compounds. Molecular catalysts have significant advantages in terms of activity and selectivity, which are worth digging deeply.

In particular, the photocatalytic CO2 reduction with molecular catalysts takes its inspiration from natural photosynthesis to give rise to an artificial analog. Studying the activity and mechanisms of the processes generating energy-rich substances is a crucial step for the development of an alternative to fossil energy.

Few work has been devoted to multimetallic complexes. In addition, hybrid systems, combing active molecular catalysts and absorbing photo materials have been equally barely investigated, although such systems would be interesting in view of developing devices. My work aims to contribute filling in these two gaps.

Chapter 2 will describe a new bimetallic (Co-Co) biquaterpyridine complex for the photoreduction of CO2 towards CO or formate. We will show that metal cooperativity is a key to the reactivity.

Chapter 3 will describe two Cobalt-quaterpyridine supported photocatalysts grafted on semiconducting carbon nitride and graphitic acid, respectively. Chapter 4 will discuss preliminary results obtainedpreliminary studies on a graphiteconjugated cobalt catalyst for electrochemical carbon dioxide reduction.

Chapter 5 will give a brief summary of the completed work, with an overview of ongoing projects.

Chapter 6 will describe the experimental methods including chemicals, material synthesis and various characterization techniques.

Chapter 2 -Homogeneous photochemical reduction of CO2 under visible light with dinuclear complexes

Multi-metallic complexes as catalytic centres of metalloenzymes ubiquitously exist in nature, which inspiring us to design of bimetallic complexes as catalysts for CO2 reduction. One prominent example is the Ni-Fe-based carbon monoxide dehydrogenases (CODHs) that reversibly transform CO2 into CO. In these enzymes, a Ni I centre binds to the C atom of the CO2 while a Fe II binds to one O atom, playing the role of a Lewis acid to assist cleavage of the C-O bond. 73,74 Previous studies have fully demonstrated that the two metals act in synergy for high selectivity. However, only a few reported examples of molecular multimetallic catalysts can efficiently mimic the enzymatic CO2 reduction. 47,75,76 Since the 1996 report from Constable about the successful synthesis of 2, 2':6,2''quaterpyridine (qpy) ligand, 77 transition metal complexes with this ligand as molecular catalysts have been extensively studied. 78,79 For CO2 reduction reaction, both homogeneous and heterogeneous catalysis in photocatalysis and electrocatalysis have been reported. 22,80,81 In this chapter, we will report a binuclear cobalt complex bearing a bi-quaterpyridine ligand which can selectively reduce CO2 to formate or CO under visible light in ACN solution. Sensitized by a ruthenium complex, by an organic compound, or by carbon nitride, electron transport from PS to CAT can be completed and result in high product selectivity. In basic ACN solution, the selectivity of formate evolution can reach a maximum of 97% with a TON of up to 821. Replacing a base (such as TEOA) with a weak acid (such as phenol), CO becomes the dominant product with high selectivity (maximum of 99%) and a maximum TON of 829. Applying cyclic votammetry, spectroelectrochemistry (SEC), together with DFT calculations, a catalytic mechanism has been proposed, highlighting that the two cobalt in the binuclear complex are not completely independent but act synergistically to complete the conversion of CO2-to-CO or CO2-toformic acid, and the reactivity can be steered towards the desired product by simply changing the acid co-substrate.

Quaterpyridine complexes in homogeneous conditions

The complex was first reported in a homogeneous solution for the visible-lightdriven photocatalytic CO2 reduction reaction. In a mixed solution of acetonitrile and TEOA, using BIH as sacrificial donor and Ru(bpy)3 2+ as photosensitizer, iron quaterpyridine was shown to catalyze the reduction of CO2 to CO with a TONCO of 2660 and a selectivity of 98%. 82 Copper quaterpyridine exhibited even higher catalytic activity.

In the same conditions than the previous example, with the addition of 3% (v/v) water, a high TONCO of 12400 has be reached with a selectivity of 97%. 80 with a max TONH2 of 1730. 79 Fe-and Co-quaterpyridine complexes can also achieve electrochemical CO2-to-CO conversion in the presence of weak Brønsted acids. For example with phenol, CO2 reduction occurs at an overpotential as low as 140 mV with Coqpy, while the catalyst keeps active over hours of bulk electrolysis. 83 In the case of Feqpy, CO can be afforded in the presence of phenol with a selectivity of 95% at an overpotential of 240 mV. Although the molecular catalyst is not after stable after hours of bulk electrolysis, cyclic voltammetry studies, spectroelectrochemistry, controlled-potential electrolysis as well as 32,33,93 Even if CO is indeed a useful chemical, for example in the Fischer-Tropsch reaction, HCOOH has recently attracted some attention as a hydrogen storage substrate which can be employed in liquid formic acid fuel cells, or as feedstock for bacteria to produce alcohols as liquid fuels. 94 The mononuclear [Co(qpy)] 2+ (qpy=2,2′:6′,2′′:6′′,2′′′-quaterpyridine) has been shown

as an efficient and selective catalyst for both photo-and electrocatalytic reduction of CO2. 82 . The product is predominately CO (selectivity > 98%), with only trace amounts of formate or H2. Althrough other Co-based 20 and Fe-based 22 molecular catalysts are able to generate formate, the selectivity or yield is very low. Mn-based diamine complexes have resulted in improved catalytic activity, together with a complex fac-Mn(bpy) (CO)3Br (bpy=2,2′-bipyridine) which converts CO2 to formic acid with a TON of 149 and a selectivity of 85% in DMF/ TEOA with BIH as a sacrificial reductant. 95 A similar system catalyzed by fac-Mn(CN)(bpy)(CO)3 illustrates that the product selectivity is related to the nature of solvent 91 . The HCOOH/CO ratio is 18 and 0.42 in DMF/TEOA and ACN/TEOA, respectively. (2), molecular sensitizers ([Ru(phen)3] 2+ , Pheno) and sacrificial reductant BIH.

Photocatalytic performance for CO2 reduction

During the photochemical reduction of CO2, formate could be selectively and efficiently produced in the absence of additional acid. Conversely, when phenol was used as a co-substrate, the selectivity is switched from formate to CO. It appears that

[Co2biqpy] 4+ is a highly efficient and selective molecular catalyst for the visible-lightinduced reduction of CO2, with two cooperative metals controlling catalytic activity and selectivity.

a. Visible-light-driven reduction of CO2 into formate.

Photocatalytic CO2 reduction experiments were initially performed in a threecomponent system containing 1 (Fig. 2.1) as a catalyst, Ru(phen)3Cl2 (phen=1,10phenanthroline) as photosensitizer, BIH as sacrificial donor and ACN as solvent.

Ru(phen)3Cl2 was employed since of [Ru(phen)2(H2O)2] 2+ was photochemically inactive due to the photolabilization of the phen ligand of [Ru(phen)3] 2+ *. Note that [Ru(bpy)2(H2O)2] 2+ originated from the decomposition of [Ru(bpy)3] 2+ * may be itself a catalyst for CO2 reduction. Regarding the solvent, DMF was not used because it can produce some formate during photocatalysis through hydrolysis, especially in the presence of trace amounts of amine or water, 96,97 thus ACN was preferred.

Triethylamine (TEA) was first used as electron donor. In a typical run, a glass flask containing a mixture of 2 mL of CO2-saturated ACN, 1 (0.05 mM), Ru(phen)3Cl2 (0.2 mM) and TEA (20% vol) was irradiated using a blue light-emitting diode (LED) strip (460 nm) at 24°C. Adapted with permission from ref. 49 Copyright © 2019, Springer Nature. As shown in Fig. 2.2 and Table 2.1 (entry 1), formate was produced with a TON of 110 and selectivity of 92% with traces of H2 (TON of 9) obtained (Fig. 2.2a). Only H2

(TON of 22) was generated under Ar, while no products were detected in the absence of catalyst (Table 2.2, entries 2 and 3) as well as when the non-metallated ligand was used instead of 1 (Table 2.2, entry 8). Note that the amount of formate using 2 as catalyst was only about one-third of that with 1 (Fig. When catalyst was replaced by a cobalt salt, namely CoCl2, neither formate nor CO was detected after irradiation for 23 h (Table 2.2, entry 7), indicating that the catalytic activity is not result from demetallation of the Coqpy. With additional BIH as an efficient two-electron donor, catalytic performances could be optimized further. In the presence of 0.025 M BIH, the TON and selectivity for formate were boosted to 386 and 96.5% respectively (Fig. 2.2b and Table 1, entry 2). Only trace amounts of CO and H2 were obtained (TONs of 8 and 6 for CO and H2, respectively). Control experiments conducted under Ar only gave trace amounts of formate (TON of 7) and H2 (TON of 1), while a negligible amount of formate was detected in the absence of catalyst. The TON for formate could be further increased to 821 when TEA/BIH was replaced by TEOA/BIH (Fig. 2.4 and Table 1, entry 3). However in this case the selectivity for formate then dropped to 75.9% while more CO and H2 were produced (TONCO of 221 and TONH2 of 40). Under optimized photocatalytic conditions (Fig. 2.2), a dynamic light scattering (DLS) experiment ruled out the possibility of particle existence (Fig. 2.5), in line with a mercury poisoning test illustrating that the photocatalytic system is mainly homogeneous (Fig. 2.6). The ESI-MS spectrum of the product solution for isotope labelling experiments, i.e. when saturating the solution with 13 CO2, led to H 13 COO -formation, verifying that formate is generated from CO2 reduction exclusively (Fig. 2.7). As can be seen from Fig.

2.2b, catalysis stopped after 18 h. Upon re-addition of 1, Ru(phen)3Cl2 or TEA to the inactive solution, only 5%, 21% and 22% of the catalytic activity was restored, respectively. However, 91% of activity could be recovered on adding BIH, indicating that the loss of activity is mainly due to the consumption of the sacrificial agent BIH (Fig. 2.8).

Finally, the quantum yield for formate production was determined to be 2.6% from ferrioxalate actinometry (at λ = 460 nm). with permission from ref. 49 Copyright © 2019, Springer Nature. b. Photochemical experiments with non-metallic sensitizers (Pheno, g-C3N4).

The expensive and rare Ru-based sensitizers could suitably be replaced by an organic dye as well as by the semiconducting g-C3N4. For example, the organic chromophore Pheno (Fig. 2.2), which is based on a phenoxazine motif, was used as a sensitizer thanks to its excellent photon absorption in the visible region, its high triplet quantum yield combined with a long triplet lifetime, as well as redox reversibility and strong reducing ability (the triplet state excited standard reduction potential E 0 ( 2 Pheno•+/ 3 Pheno*)=-1.80

V vs. SCE in DMA) 98 . In the general reaction conditions with 15 μM 1, 0.4 mM Pheno, 20% TEOA and 0.05 M BIH, formate was obtained with a high TON of 565 (Table 1, entry 5).

g-C3N4 is a well-known semiconductor which has visible-light absorbability and negative conduction band energy able to transfer electrons to 1 (the estimated flat band potential is ~-1.35 V vs. SCE) [99][100][101] . With a system containing 20 μM 1, 2.5 mg g-C3N4 and 20% TEOA +0.025 M BIH, CO2 was reduced to formate with a TON of 493 (Fig. By comparing with the single-electron reversible wave of monomeric complex 2, the number of electrons in each wave of 1 can be calculated (Fig. 2.9). 81,83,102 In a CO2 saturated solution, a multi-electronic catalytic wave occurred on the wave at -1.75 V vs. SCE (Fig. 2.13), while the other waves remain unchanged. The glassy carbon diameter was 3 mm at scan rate of 100 mV/s. Adapted with permission from ref. 49 Copyright © 2019, Springer Nature.

2.2c and

When performing CV under an Ar atmosphere in the presence of 0.5M TEA, the first two reduction waves merged into a single one at a more cathodic potential (-0.92 V vs. SCE, Fig. 2.12), suggesting TEA binding to the Co II centres, which are then reduced with a C+E mechanism (a chemical reaction follow-up electrode electron transfer mechanism):

The binding of complex 1 to the amine may happen considering the disappearance of the [Co II Co II (L)] 4+ oxidation wave of 1 (Fig. 2.12). Once CO2 is introduced into the solution, a 24 mV shift towards positive potential occurs on the first reduction peak, indicative of binding to CO2 (Fig. 2.12).

[ n Bu4NPF6, 0.5 M TEA) under CO2; the potential was set at the first reduction wave (see text). Adapted with permission from ref. 49 Copyright © 2019, Springer Nature.

Co II Co II (L)] 4+ + 2 TEA [Co II Co II (TEA)2L] 4+ (Eq. 2.1) [Co II Co II (TEA)2(L)] 4+ + 4 e - [Co I Co I (L 2-)] + 2 TEA (Eq.
The addition of TEA also encouraged a new catalytic wave at ~-1.5 V vs. SCE (Fig. With increasing the concentration of TEA to 1.44 M (20% in volume ratio), faradaic yield for formate and H2 also increased to 15.8% and 3.8%, respectively, while the generation of CO was inhibited to a faradaic yield 78.3%. That means, formate production could happen in electrochemical conditions with a large concentration of amine, even if CO formation represents a competitive pathway occurring in parallel.

Spectroelectrochemistry (SEC) in the infrared region was performed at various potentials in CO2-saturated solutions and in the presence of 0.5 M TEA (Fig. 2.13b). When the potential was scanned from -0.35 V to -0.85 V (vs. the Ag pseudo-reference), a potential window within which the catalyst is reduced, an absorption band at 1,635 cm -1 built up (Fig. On setting the potential at the foot of the catalytic wave (-1.25 V vs. the Ag pseudoreference electrode), the band at 1635 cm -1 quickly disappeared and new bands at 1649 cm -1 and 1680 cm -1 appeared, showing the formation of bicarbonate (Fig. 2.15). A lowintensity band (shoulder) at ~1616 cm -1 also appeared. Although we do not know exactly its nature so far, this may be related to a formato-complex or free formate. with the parallel formation of bicarbonate (bands at 1680 cm -1 and 1649 cm -1 ) occurring during the first scans. Adapted with permission from ref. 49 Copyright © 2019, Springer Nature.

All these observations are consistent with photochemical experiments for high selective formate production (

Proposed pathway for CO and formate production

All these observations, together with preliminary theoretical calculations (Fig. 2.17 with permission from ref. 49 Copyright © 2019, Springer Nature.

This isomerization step is endowed with a barrier of 18.5 kcal mol -1 and may become competitive in media with low proton availability. [CoCoO2C(L)] -is then quickly protonated at the basic C atom with release of formate, in accordance with experimental results observed under basic conditions. Cooperativity between the two Co centres of reduced catalyst 1 is thus the key for not only CO2 binding but also for driving the reaction towards HCOO -evolution. The alternative formation of a hydride species bridging between the two Co atoms that would further insert CO2 to give HCOO -could not be ruled out, although the absence of formate evolution in acidic conditions and the low amount of H2 produced in all experiments makes this mechanism unlikely. However, a further mechanistic investigation will shed more light on the process, starting from Fig. 2.18, which stands as a working model. -leads to product formation, either CO or formate. Adapted with permission from ref. 49 Copyright © 2019, Springer Nature.

Sub-conclusion and sub-perspectives for bimetallic catalytic system

Various sensitizers have been employed for the photochemical reduction of CO2, including a Ru complex, an organic phenoxazine compound as well as semiconducting g-C3N4 in combination with a binuclear cobalt complex as catalyst. These systems can selectively catalyze visible-light-driven CO2 reduction to CO with high TON in the presence of weak acid phenol. By simply replacing phenol by TEOA in the solution, the catalytic process could be switched to selective formate formation with a maximum quantum efficiency of 2.6%. The synergy between the two cobalt metals guarantees the dual reactivity, which illustrates the fantastic potential of multimetallic molecular catalysts.

Observation in cyclic votammetry, spectro-electrochemistry, and DFT calculations reveal different intermediates that might correspond to new pathways of highly reduced products beyond 2 electrons/ 2 protons. We have used a ligand that consists of two quaterpyridine linked by xanthene moiety to coordinate with two cobalt, obtaining a complex with substrate-depended selectivity towards formic acid or carbon monoxide. Other metalbased complexes have also been reported, including zinc, iron, copper, nickel. 47,48,92 Heterobimetallic catalysts containing two of them can considered for constructing a catalytic assembly with nucleophilic center and electrophilic center, respectively. It is possible to select other metals to complete the conversion of carbon dioxide to highly reduced products.

On the other hand, the link units in bimetallic complexes also have effects on the whole catalytic performance. Stable and efficient electron transfer along the linkage may boost product formation. Also, the structure of the linkage can be adjusted to tune the distance between metal centers. It is interesting to imagine the possibility of containing two or more CO2 molecules to form C-C bond for complex reduced products. In that case, cobalt may not be the best choice of metal center: Coqpy is indeed an efficient catalyst for CO production and but is not known to be able to fix CO for r further. So metals such as Fe and Cu that can keep and utilize CO in other catalytic processes may provide much more suitable active catalytic centers.

Chapter 3 -Heterogeneous photochemical reduction of CO2 with covalently bonded molecular catalysts

Surface modification of carbon materials such as carbon nanotubes, carbon nitride and graphene derivatives has received particular attention, 103, 104 and these materials are now widely used as electrode materials with high effective surface area combined with excellent conductivity (especially multiwalled CNTs -MWCNTs) and mechanical stability. 105 Recently, different techniques have been developed for the chemical functionalization of carbon materials such as graphene. 106 Generally, most reactions reported for the modification of sp 2 carbon in CNTs and graphene can potentially be adapted to other types of π-conjugated carbon surfaces. We chose the widely studied semiconductor materials carbon nitride (C3N4) and graphene as substrate. After morphology improvement and surface functionalization, they were covalently combined with molecular catalysts to study their catalytic performance towards CO2 reduction. 107 3.1 Photochemical reduction of CO2-to-CO with Coqpy covalently grafted on mesoporous graphic carbon nitride ("Coqpy@mpg-C3N4") was washed with ultrapure water several times, following the above procedure. Finally, the precipitate was dried under vacuum to yield the Coqpy@mpg-C3N4 hybrid.

Coqpy@nsg-C3N4 was similarly synthesized by using nsg-C3N4 instead of mpg-C3N4. b. Nitrogen adsorption/desorption experiment From nitrogen adsorption/desorption isothermal test at 77 K, the pore volume and specific surface area of the substrate mpg-C3N4 were calculated to be 0.4993 cm 3 g -1 and 110.7 m 2 g -1 , respectively. After grafting with Coqpy and photoreaction, the pore volume of 0.5219 cm 3 g -1 and specific surface area of 118.8 m 2 g -1 was obtained (see Fig. 3.2), meaning that the morphology was maintained. a Calculated for 6 mg of hybrid material suspended in a 3 mL solution.

f. Cyclic Voltammetry analysis

The CV of Coqpy-Ph-COOH was performed in DMF containing 0.1M Bu4NPF6 as supporting electrolyte and is shown in Fig. 3.5. Coqpy@nsg-C3N4 0.02670 4.53 9.0 -1.1 V vs. SCE and -1.4 V vs. SCE due to combined metal and ligand-based reduction, in line with previous reports. 82,111 Under CO2 atmosphere, an increase is observed on the latter wave, showing that Coqpy-Ph-COOH possesses the ability to catalytically reduce CO2. In the presence of a weak Brönsted acid (phenol), the catalytic current drastically further increases. The onset potential of the catalytic wave is positive to the position of the conduction band (CB) of mpg-C3N4, located at ca. -1.35 V vs. SCE, as reported elsewhere, 112 indicating that the illuminated semi-conductive material may be able to reduce the cobalt molecular catalyst.

Catalytic performance for CO2 reduction under visible-light

Under visible-light irradiation, the catalyst suspension can convert CO2 to CO with a selectivity of 97% (Fig. 3.6). Long-term recycling experiment shows that the catalyst is still active for at least 4 days, reaching a TON for CO of 254 (Fig. 3.12). The origin of carbon was asserted by running isotope-labeled experiments with 13 CO2

giving 13 CO as the product (see Fig. In order to further illustrate the advantages of this covalent bond combination, we performed a series of comparative and control experiments. Table 3.2 reports data obtained after typical 24h irradiation. No formate was identified from the liquid phase during the photocatalysis. Complete blank experiments results are also given in Table 3.

2,

showing that all components of the catalytic system, including CO2 and light irradiation, are needed.

(A) (B)

Molecular catalysts associated with the semiconductor particles may become active once photons lead to the formation of an electronically excited state. Electron transfer occurs either between molecules on the surface or between a surface site and an adsorbed/attached molecule. Similar to the classification of photocatalysis, the electron transfer process can be broadly divided into two categories, "type I" for the adsorbate molecules directly excited by photons and "type II" for the solid excited by photons.

Materials that don't participate in the photo-excited electron transfer process are called "insulator", such as SiO2 and Al2O3. They only provide a two-dimensional environment for the reactive molecules. In the case of insulator Al2O3 for example, electrons transfer occur directly from donor molecules to acceptor molecules.

In our experiments, we employed Al2O3 to further verify the role of carbon nitride (Table 3.2, entry 7). The electrons in the valence bond of C3N4 are first excited into the semiconductor's conduction band (CB) and then transferred into the vacant orbitals of the acceptor. The positively charged holes are left on the band edge of the valence band (VB).

At the same time, another electron recombined with the holes on the valence band edge, and will be provided from the filled orbital of the donor, an easily oxidisable amine in our case.

In order to further prove the catalytic ability of the carbon nitride supported Coqpy, we use some comparative experiments to probe for catalytic role of various components.

Metal cations, individual carbon nitrides, individual molecular catalysts, and insulator

alumina were all tested in a series experiments. Specifically, experiments performed with a mixture of a cobalt chloride salt and carbon nitride or Coqpy-Ph-COOH and Al2O3 suspension only furnishes a very low amount of reduction products (entries 7 and 8 respectively, Table 3.2). Moreover, a simple mixture of the cobalt molecular complex with mpg-C3N4 considerably decreases the amount of CO produced (entries 2 and 3, Table 3.2)

and concomitantly the selectivity for CO2 reduction significantly diminishes. These observations indicate that covalent grafting of the molecular catalyst greatly enhances catalysis. Under light excitation, electrons can be transferred from mpg-C3N4 to the complex Coqpy-Ph-COOH due to electronic interaction between the conduction band and the complex orbital. The complex thus play the role of an oxidative quencher. BIH is a reduction quencher which can supplement electrons in the valence band of the carbon nitride at the same time.

In our measurements, steady-state emission spectrum of suspended mpg-C3N4 shows two peaks of similar intensity at ca. 490 and 540 nm, in accordance with previous studies. 114 We observed that the two emission peaks were quenched linearly at different rates, both with BIH and Coqpy-Ph-COOH which could be explained taking into account these peaks correspond to slightly different excited states.

(A) (B)

Electron transfer rate constant determination through Stern-Volmer (S-V) analysis (Fig. 3.8(B) and 3.9(B)) requires the excited state lifetime of mpg-C3N4. Several studies reported a very short lifetime for C3N4 excited states. [115][116][117] In another study which investigated excited-states and electron transfer dynamics of g-and mpg-C3N4 hybridized with a Ru mononuclear complex, emissive excited states decay was observed with a threecomponent (from 0.7 to 12 ns) kinetics. However, neither electron acceptors nor hole scavengers can quench these excited states. Generally speaking, kq will be used to evaluate the rate of electron transfer in quenching experiments. But the excited-state lifetime (τ0) is needed for calculation (see equation 3.1). Due to the detection limit of the instrument, we chose to use KSV to evaluate the electronic transfer rate, without precisely estimating τ0.

We applied Stern-Volmer analysis with the following equation:

0 SV q 0 1 [Q] 1 [Q]      I K k I (Eq. 3.1)
where I0 and I are the emission intensity without and in the presence of a quencher (respectively), KSV is the Stern-Volmer constant equal to the slope of the linear relationship, kq is the apparent quenching rate constant, τ0 the excited-state lifetime without quencher, and [Q] the quencher concentration. In the quenching experiments, the Stern-Walmer constant of the cobalt complex (Table 3.3) is much larger (by a factor > 3900) than that of BIH. It largely counterbalances the smaller catalyst concentration (by a factor of 16). Therefore, even in cases where the exact rate constant cannot be determined, electron transfer is theoretically effective and is likely to occur from mpg-C3N4 to the cobalt molecular catalyst.

Photocurrent response has been measured to get insight into the electron transfer (Fig. 3.10). The materials mpg-C3N4, Coqpy/mpg-C3N4 (obtained by simple mixing of the Coqpy-Ph-COOH and mpg-C3N4) and Coqpy@mpg-C3N4 were deposited at an FTO electrode, respectively, which was connected to a platinum electrode in a 0.1 M Na2SO4 solution without any bias (see Experimental section for details). Upon running 10 consecutive illumination cycles, the current responses of mpg-C3N4 and Coqpy/mpg-C3N4

were negligible whereas the covalently linked Coqpy@mpg-C3N4 assembly showed a significant, quickly rising and stable current response. A slight attenuation ws observed after ten cycles, which may result from the spalling of traces materials on the FTO electrode. In sample preparation, the hybrid and conductive glass FTO electrode are only connected by small quantity of Nafion polymer. Under the influence of light and current during the test, it is likely to gradually fall off as the test progresses.

The experiments illustrate that the electron transmission between the two parts is enhanced through the covalent chain. Electronic impedance spectroscopy (EIS) response of mpg-C3N4, Coqpy/mpg-C3N4

and Coqpy@mpg-C3N4 were further measured (Fig. 3.11). From these data, the chargetransfer resistance of Coqpy@mpg-C3N4 is smaller than Coqpy/mpg-C3N4 and mpg-C3N4, illustrating half the hybrid Coqpy@mpg-C3N4 possesses the lowest impedance. It confirms that the covalent linking significantly accelerates the photoexcited charge transfer. For each cycle, the quartz cell was irradiated with visible light for 24 hours in standard Coqpy@mpg-C3N4/BIH/PhOH MeCN solution. After each cycle, the hybrid was washed with a large amount of MeCN combined with ultrasonic treatment, and then centrifuged and dried. After that, the hybrid was dispersed again into a fresh ACN containing BIH and PhOH, then saturated with CO2 for 30 min to start a new photocatalytic cycle.

All the four catalytic cycles show similar efficiency and selectivity, which indicating a remarkable stability of CO production and asserting the absence of large hybrid degradation during irradiation (Fig. 3.12). Although the number of catalytic cycles decreases with each cycle, a total number of ca. 500 catalytic cycles was reached after 96 h. Taking into account that CO2-to-CO conversion necessitates two electrons, the apparent quantum yield for CO formation is Φ = 0.25% (see calculation details in Chapter 1). This recycling strategy also indicates the simple recovery of the catalyst from the solution is promising. b. X-ray diffractograms and UV-Vis absorption spectra X-ray diffractograms of complex mpg-C3N4, nsg-C3N4, Coqpy-Ph-COOH, Coqpy@mpg-C3N4 before and after irradiation are shown in Fig. 3.13.

The Coqpy@mpg-C3N4 mixed diffraction peaks before and after irradiation include the characteristic diffraction peaks of the complexes at 7. According to the database search and matching, the structure of the hybrid and the porous aluminum silicate zeolite (JCPDS reference card 00-023-0214) match well, and it partially overlaps with the complex and confirming the retention of mpg-C3N4 porosity. This porous structure was not observed on mpg-C3N4, which may be due to the change of the chemical environment on the surface of the hybrid during the synthesis process.

In the catalyst after the photocatalytic reaction, no characteristic diffraction peak of cobalt oxide was observed. Although the content of cobalt is low, this data cannot provide evidence for the decomposition of the complex into metal oxides or even metal oxides as catalytic centers. Combined with the XPS data in Fig. 3.4, it also confirmed that cobalt is present in Coqpy @ mpg-C3N4 before and after photocatalysis, and cobalt is also present in the adsorbed sample Coqpy-mpg-C3N4.

In order to evaluate the stability of the complex in the photocatalytic process, the absorption spectra of the compounds in the solution before and after the photocatalytic reaction were also examined. The absorption spectra of PhOH, BIH, qpy-Ph-COOH ligand, Coqpy-Ph-COOH complex and the photochemical mixture before and after irradiation are shown in Fig. 3.14. The UV-Vis absorption spectrum retains the same spectral trend as before irradiation, although the intensity is lower. No characteristic absorption peak of the ligand was observed. The Coqpy@mpg-C3N4 hybrid material we synthesized is the first catalytic system made of a molecular catalyst covalently immobilized on carbon nitride for CO2RR. The effective and selective covalent connection of molecular catalysts and polymeric materials seems to be a promising method for catalytic reduction of CO2, leading to good control of the structure of the catalyst and excellent selectivity and activity.

Among reports of earth-abundant metal-based catalysts combined with carbon nitride-based materials, Coqpy@mpg-C3N4 not only shows high selectivity and CO production activity, but also shows long-term stability. Therefore, it inherits the high selectivity of molecular catalysts and the excellent stability of solid materials. This type of combination takes into account the independent adjustment capabilities of the molecular complex structure and inorganic material structure, and there is still a lot of room for adjustment and improvement in the association between the two parts. Such

(A) (B)
excellent performances provide new development ideas for the development of highly active catalytic materials, and not only for the reduction of carbon dioxide. Other catalytic reaction can be envisioned, while a better understanding of change transport and transfer properties with the hybrid materials still needs to be achieved.

3.2 Selective reduction of CO2 to CO or HCOO-under visible light with Coqpy covalently grafted on a graphene material (Coqpy@GA)

Graphene consists of a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice, and its derivatives are well-known materials suitable for anchoring molecules due to their adjustable porosity, high electronic and thermal stability, and chemical inertness in most reaction conditions. [118][119][120] Recently, various graphene derivatives have been developed, which are characterized by extremely rich surface chemical properties, affording a wide range of materials for advanced functionalization schemes. Due to the simplicity and scalable synthesis, graphene oxide is the most widely studied matrix. 121 However, the control of the rich oxygen-containing functional groups on the surface is a difficult task, including tertiary alcohols, epoxide, carboxyl and carbonyl groups. 122 Graphene acid (GA) has been synthesized and investigated as a graphene layer with the basal plane densely and homogeneously decorated by carboxylic groups. 123 Owing simple synthesis, easy scalability, high surface area and electronic conductivity, GA can afford both precise control on the type of surface groups and maintenance of the sp 2 network of pristine graphene, which makes this material an interesting substrate for chemical functionalization.

3.2.1

Synthesis and characterization of Coqpy@GA a. Synthesis of graphene acid (GA)

GA was synthetized according to a 2-steps protocol described in the literature. [124][125][126] Briefly, 0.12 g (~4 mmol) of fluorographite was suspended in 15 mL DMF and sonicated under nitrogen atmosphere for 4 h. Then, 1.06 g (16 mmol) of KCN were added to the suspension and let react at 130°C for 24 h, in order to promote the nucleophilic substitution of fluorine atoms with cyano groups. The suspension was diluted with 15 mL of acetone and washed by centrifugation with fresh and hot (80 °C) DMF, dichloromethane, acetone, ethanol and water for 3 times. In a second step, the so formed cyanographene was hydrolysed in 20% HNO3 under reflux at 100°C for 24 h, to convert the -CN groups into -CO2H groups. Finally, the material obtained was washed by following the same centrifugation protocol described above. The preparation of this material was done by ours collaborators at Padova University in Italy.

b. Functionalization of GA with CoqpyNH2

The coupling of GA with the derivatized CoqpyNH2Cl2 molecule was achieved employing carbodiimide chemistry, which is often used for peptide synthesis since it preserves the structure of the organic substrates (Scheme 3.2). In order to activate the -COOH groups of GA, 20 mg of GA were suspended in 20 mL of N,N-dimethylformamide (DMF) and 4 mg (0.03 mmol) of hydroxybenzotriazole (HOBt) and 4 µL (0.03 mmol) of trimethylamine were added. The mixture was kept under stirring for 45 min at 0 °C in an ice bath. Then, the ice bath was removed and 6 mg (0.03 mmol) of dicyclohexyl carbodiimide (DCC) and 5 mg (0.009 mmol) of the amino-functionalized Cobalt quaterpyridine modified with amino phenyl group were added (Scheme 1). The reaction mixture was kept under stirring at room temperature (RT) for 6 days. The resulting solid was separated by filtration using a 0.2 µm polytetrafluoroethylene (PTFE) membrane, and thoroughly washed with 3x50 mL of fresh DMF, 3x50 mL of a 1:1 DMF:water mixture, 3x50 mL of methanol and 2x50 mL of acetone. After each filtration step, the powder was suspended in the fresh washing solvent and sonicated for 5 min in order to remove the eventual physisorbed species. Finally, the sample was dried under vacuum to obtain the GA-Coqpy.

Characterization of GA and GA-Coqpy a. Surface density of -COOH analysis

Graphene acid is a promising oxidized derivative of graphene, whose basal plane is strongly and selectively decorated with -COOH groups. The amount of -COOH groups is around 6 at. %, which is higher than in pristine Graphene oxide (GO). In addition, it has been demonstrated both experimentally and theoretically that GA is also conductive, owning an electrical conductivity very close to pristine graphene. Therefore, it represents a very appealing platform for immobilizing molecular catalysts by covalent functionalization, especially for energy related applications.

b. Raman spectra and FT-IR spectra analysis

In order to confirm the covalent functionalization of GA with the CoqpyCl2 molecule, the preservation of the CoqpyCl2 molecule structure and the interaction between the molecule and GA substrate, different characterization analysis were performed.

Vibrational spectroscopies confirmed the successful formation of amide bonds. Fig. 3.15 shows the Raman spectra for the pristine GA, GA-Coqpy and ammino-modified CoqpyCl2 samples. The Raman spectrum of GA shows the characteristic D (1350 cm -1 ), G (1590 cm -1 ), 2D (2700 cm -1 ) and D+D' (2900 cm -1 ) bands of graphene, whereas the one of the amino-modified CoqpyCl2 molecule shows several bands in the 500-1700 cm -1 range. The GA-Coqpy sample mainly shows the D, G, 2D and D+D' bands characteristic of GA; however, it is also possible to observe the presence of some bands characteristic of the CoqpyCl2 molecule overlapping with those of GA, confirming its immobilization on GA. For example, one can see a broad band between 600 and 700 cm -1 , and an increase of the intensity at around 1500 cm -1 , which perfectly match with the bands associated to the CoqpyCl2 molecule. An increase of the ID/IG ratio for the functionalized sample (ID/IG = 1.07), compared to that of the pristine GA (ID/IG = 1.04), is also observed. This increase is typically associated with the introduction of defects in the structure of the graphene layer and may be interpreted as confirmation of the functionalization. The formation of amide bonds is inferred from the FTIR spectroscopy results reported in Fig. 3.16. The presence of -COOH groups in the GA is confirmed by the bands at 1710 cm -1 and 3421 cm -1 , related to the C=O and C-OH stretching in the carboxylic groups, respectively. After the coupling with the CoqpyNH2Cl2 molecule, the band at 3421 cm -1 decreases due to the reaction between the -COOH groups of GA and the -NH2 groups of the cobalt molecule, with the subsequent formation of the covalent bond. This is also confirmed by the shift of the C=O stretching band from 1710 cm -1 (C=O in acid group) to 1699 cm -1 (C=O in amide group), further confirming the formation of the -CONH-group.

In addition, characteristic bands of the amino-modified CoqpyCl2 molecule are observed in the GA-Coqpy FTIR spectrum, proving the presence of cobalt quaterpyridine in the hybrid sample. Therefore, vibrational spectroscopies strongly suggest that the molecule was incorporated to the GA scaffold via covalent bonds and that the structure of both the substrate and the molecule are preserved by employing our soft functionalization procedure.

c. XPS analysis Fig. 3.17 as well as Tables 3.4 and Table 3.5 show the XPS results for the GA-Coqpy hybrid. The C 1s region of GA is also reported for comparison. The C 1s spectrum of the two samples is centered at 284.4 eV, which is attributed to the graphitic component (sp 2 hybridized carbon atoms). The component centered at 285.4 eV is associated to sp 3 hybridized carbon atoms (C sp 3 ) and constitutes the 11.7 at. % of GA. After the immobilization of the Cobalt complex, it increases up to 22.2 at. % due to the introduction of the Co-complex, since this component overlaps with the C-N one. GA contains oxygen species, like alcohols (286.5 eV), epoxy (287.1 eV), carbonyl (287.8 eV) and carboxylic (288.9 eV) groups (Table 3.4). After the coupling, the amount of the different oxygen groups does not vary significantly, except in the case of the carboxylic groups where a decrease from 5.6 at. % to 3.7 at. % is observed. This is attributed to the fact that these later groups are involved in the covalent coupling with the Co complex, as shown by FTIR spectroscopy.

Regarding the N 1s region, it was deconvoluted into three components centered at 398.6 eV, 399.8 eV and 401.2 eV, which are ascribed to C=N, N-Co and C-N-H bonds, respectively. It is interesting to note that the ratio between the N-Co and C-N-H components is close to 4:1 (Table 3

e. UV-Vis diffuse reflectance spectroscopy analysis

The interaction between light and the samples was investigated by UV-Vis diffuse reflectance spectroscopy and the results are shown in Fig. 3.20. GA exhibits a constant absorption in all the visible range, while the Co complex shows a wide absorption band between 650 and 400 nm. The GA-Coqpy shows the same behaviour as GA in the 400-800 nm range, which could indicate that only GA absorbs the light. Below 400 nm, the decrease of the absorbance is sharper than that for GA but similar to that of the Co complex. The concentration of the complex Coqpy employed in the photocatalytic reaction was optimized to be 15 µM (compare entries 1-3, Table 3.7). The concentration of the reactants was optimized through experiments 5 and 6, and then two sets of photocatalytic experiments were conducted using phenol and trifluoroethanol, respectively. First of all, two long-term photocatalytic reaction of CO2 were carried out. After more than 200 hours of light irradiation, the conversion of CO2 to carbon monoxide can reach turnover number as high as 513 (with Phenol, Table 3.7, entry 5) or 437 (with TFE, Table 3.7, entry 4), corresponding to high selectivities of 97% or 98%, respectively. employed as catalysts. The mixed sample show negligible hydrogen and CO evolution with PhOH (Table 3.7, entry 8) or TFE (Table 3.7, entry 7). While the cobalt salt as catalyst only generates traces of hydrogen (Table 3.7, entries 9-10). Moreover, control experiments in dark or under argon also illustrated the necessity of photoirradiation and CO2 (Table 3.7, entries 11-14).

b2. Photocatalytic CO2 reduction to formate First, triethylamine was chosen to replace phenol to create alkaline conditions. In the presence of BIH, carbon dioxide was reduced to CO with a selectivity of 69% and a TON of 194 upon 79 hours of irradiation (Table 3.8, entry 1). When the triethylamine in the system was replaced with triethanolamine, the catalytic selectivity changed drastically.

The liquid formate became the main product with a TON of 260 and a selectivity of 97%, while the gas-phase product only included a small amount of hydrogen (Table 3.8, entry

2). The amount of catalyst and BIH used in the system also impacted catalysis (Table 3.8, entries 3-5). After optimization, when the amount of the composite catalyst is adjusted to a concentration of 7µM, the turnover number for formic acid reached 330 and the selectivity was 99% (Table 3. In addition to the optimal amount of catalyst, the amount of electron sacrificial agent was also adjusted to achieve the best catalytic efficiency. Control experiments were then conducted under alkaline conditions to evaluate the carbon dioxide conversion effect of the catalyst. Similar to the experimental protocol under acidic conditions, physically mixed GA and Coqpy as well as cobalt salt (CoCl2) can not trigger the CO2 reduction (Table 3.8, entries 6-7). Also, the GA-Coqpy as catalyst only generates traces of hydrogen under argon (Table 3.8, entry 8), and shows no catalytic activity in the dark (Table 3.8, entry 9). a reaction conditions: 0.02 mg catalyst in a CO2-saturated ACN solution containing BIH, TEA, or TEOA.

A 6 mL quartz cell with a septum was used as the reaction vessel; and a solar simulator equipped with a 100 W Xenon lamp and a 400 nm optical filter was used as irradiation source. b relatively to the amount of Coqpy or CoCl2.

c under an argon atmosphere.

d in the dark.

e reduction products are H2 and CO.

c. Investigation on formate generation with Coqpy@GA With addition of isopropanol, no CO has been detected in these conditions (Table 3 entries 2-3). Replacing TEOA with TEA and adding PrOH in the photocatalysis, the production of formate was suppressed and hydrogen increased. (entry 2) As the concentration of PrOH increases, the production of formate decreases, and the yield of hydrogen increases (entry 3). When TEOA and isopropanol were simultaneously added to the solution (entries 2-3), the production of H2 was promoted while the production of formate was not completely suppressed but was weakened. Then, isopropanol (pKa = 16.5) was replaced by phenol (pKa = 9.95, entry 4), the production of formic acid was further reduced and the hydrogen generation was greatly increased. And some CO was also detected. After that, a photocatalysis was conducted with molecular Coqpy in MeCN solution of BIH and acetic acid (pKa = 4.76). Hydrogen generation was boosted to a TON of 118 with a little amount of CO (TON = 18) produced (Table 3.9, entry 6).

Table 3.9 GA@Coqpy photocatalytic CO2-to-formate conversion with additional acid (CH3COOH, PrOH and PhOH) under visible light (λ > 400 nm).

a reaction conditions: 0.02 mg catalyst in a CO2-saturated ACN solution containing BIH and PrOH. A 6 mL quartz cell with a septum was used as the reaction vessel; and a solar simulator equipped with a 100 W Xenon lamp and a 400 nm optical filter was used as irradiation source. b relatively to the amount of Coqpy.

* the optimized conditions of the CO2-to-formate conversion.

c irradiation using blue LED (460 nm), data referenced from our previous work. The number of incident photons was calculated using the classical iron ferrioxalate (K3Fe(C2O4)3) chemical actinometer, following the procedure reported previously and using known parameters for calculations. We determined that the number of incident photons reaching the sample was (2.18 ± 0.17) × 10 19 photons per hour. The CO2-to-CO reduction being a two-electron process, the overall quantum yield Φ of the process was determined using the following equation:

𝐶𝑂% = Number of CO molecules formed × 2 Number of incident photons × 100%
Taking 513 as the turnover number for CO (Table 3.7, entry 8), we obtain an apparent quantum yield Φ of about 0.57 % after 222 h of irradiation.

Taking 330 as the turnover number for formate (Table 3.8, entry 1), we obtain an apparent quantum yield Φ of about 0.37 % after 72 h of irradiation.

e. Isotope labelled experiments

Both CO and formate have been confirmed to be issued from CO2 by using labelled 13 CO2. To further investigate the stability of the hybrid assembly, we re-employed the same hybrid for four successive cycles. Each cycle corresponded to visible-light irradiation of a standard GA@Coqpy/BIH/PhOH or GA@Coqpy/BIH/TEOA ACN solution. After each cycle, the hybrid was washed with ACN, sonicated and then centrifuged. After drying, the hybrid was dispersed again into a fresh ACN containing BIH/PhOH or BIH/TEOA and then saturated with CO2 for 30 min prior to start a new photocatalytic cycle. Brönsted acid, releasing CO in both cases (yellow pathway, Scheme 3.3). Note that the pending free carboxylic acid groups remaining at the surface of the graphene material may also act as a proton donor, even if the concentration is very small. In the presence of triethanolamine (TEOA = L, Scheme 3.3), an equilibrium occurs with M-CO2 adduct, with L acting as an axial ligand to the metal (it is reasonable to postulate that k1 and k2 are large with k1 << k2, so that the equilibrium constant K = k1/k2 is small). It significantly increases electron density at the carbon atom of the partially reduced CO2. 132 In the presence of a low content of acid (residual acid groups at the graphene surface, see e.g. entries 2-5, Table 3.8) or of a very weak acid (e.g. entry 4 in Table 3.9), protonation may occur at the C atom, leading to the release of formate (blue pathway, Scheme 3.3). In such cases, the equilibrium between M-CO2 and L-M-CO2 is shifted to the right and formate is obtained as only product (entries 2-5, Table 3.8) or in competition with H2 production once a weak external acid is added to the solution, due to the concomitant enhancement of the hydricity of the M-H hydride intermediate (entries 4-6, Table 3.9), as illustrated on the red pathway in Scheme 3.3.

Conclusions and perspectives

The Coqpy@mpg-C3N4 hybrid material we synthesized is the first catalytic system made of a molecular catalyst covalently immobilized on carbon nitride for CO2RR. The effective covalent connection of molecular catalysts to polymeric materials seems to be a promising method for catalytic reduction of CO2, leading to good control of the structure of the catalytic site, high selectivity and activity.

After combing earth-abundant metal-based catalysts with carbon nitride-based materials, Coqpy@mpg-C3N4 hybrid not only shows high selectivity and CO production activity, but also shows long-term stability. It combines the high selectivity of molecular catalysts and the excellent stability of solid materials.

Coqpy and the surface-functionalized graphene material may also be connected by covalent bonds to achieve efficient electron transfer. This hybrid can efficiently and selectively achieve the photochemical reduction of CO2 and still maintain stable catalysis after recycling. The target product of CO2 reduction can in this case be switched by changing the reactants. These results demonstrate the promising interaction between inorganic materials and molecular catalysts towards new catalytic activity.

Such covalent combinations are also based on independent adjustment of the molecular complex structure and inorganic material structure, and there is still a lot of room for improvement in the combination between the two parts. The good catalytic performances so far obtained open new pathways for the development of highly active catalytic materials, and not only for the reduction of CO2. Other catalytic reactions may be envisioned, such as OER, ORR and reduction of N2. On a more fundamental side, a better understanding of change transport and transfer properties within the hybrid materials still needs to be achieved.

PCET reactions. The metal-based GCC sites can give rise to redox waves only if they can undergo ion exchange with the solution. After that, the GCE was immersed in 20 mL of Ar sparged DMF containing 0.1 mM of the desired ortho-phenylenediamine derivative. The reaction vessel was subsequently heated under Ar for 24 hours at 60℃. Upon cooling, the electrode was copiously washed with ethanol and subsequently treated with 0.1 M HClO4 for 3 hours to hydrolyze adventitious imine linkages formed on the surface. Following the acid treatment, the electrode was washed with pure water.

An additional procedure is necessary for removing organic physisorbed species. The electrode was then soaked into ACN for 3h to remove organic physisorbed species. Next, after rinsing the electrode with water and soaked in water for an additional hour, the obtained electrode was dried in a 50 ℃ oven. We further tried to oxidize the electrode surface differently. After polishing the glassy carbon electrode, using the platinum wire as the counter electrode, the electrolytic solution was oxidized under stirring in time sequences of 5 seconds. A uniform oxidation surface can be obtained in 25 to 30 seconds. After condensation with molecular catalyst, the electrode surface is smooth and shiny. Moreover, it is still smooth after cyclic voltammetry, without visible changes from simple naked eye (Fig. 4.4). We set the oxidation time until the surface is uniform. From XPS survey of GCC-Coqpy, it can be concluded thay the Co:N ratio is approximately 1:6 (Fig. 4.5; theoretical ratio is 1:6). In high solution XPS, Npyridine:Npyrazine of GCC-Ru is 1.5:1 (Fig. 4.6; theoretical ratio is 2:1). The above XPS analysis is thus consistent with the expected condensation structures.

Only preliminary experiments have been conducted so far to study the catalytic performance of GCC-Coqpy and their electrochemical properties. A brief account is described in the next section. Chapter 5 -General conclusions and perspectives

General conclusions

The main objective of this thesis was to explore the use of molecular catalysts into artificial photosynthetic systems working under visible-light illumination, and to integrate these catalysts into hybrid systems upon combing them to a conductive or semiconductive material. The molecular catalysts are capable of triggering catalytic reactions in homogeneous solutions or upon hybridization with other supporting matrices to drive catalysis in heterogeneous conditions. The presented results describe the molecular photocatalytic reduction conducted with a bimetallic cobalt complex bearing a biquaterpyridine ligand in several different conditions, including various photosensitizers (Ru complex, organic dye, semi-conductive C3N4 polymer), in the presence of additional co-substrates (weak acid-phenol, base-TEOA). Further, light-harvesting semiconductor mpg-C3N4 and graphene acid were chose to append with the monomeric quaterpyridine cobalt catalyst. Both the catalytic efficiency and the selectivity as well as direction towards the product showed a significant improvement compared with the freely diffusing complex. These results could be summarized as follows:

1. We have first studied photo-stimulated catalytic reduction of CO2 in homogeneous solutions with binuclear cobalt complex bearing a bi-quaterpyridine ligand. Various All these catalysts that have been investigated in my thesis illustrate the promising efforts for mimicking artificial photosynthesis. The synthesized bimetallic molecular complex biqpyCo2 shows the significance of cooperation between two metal centers.

Moreover, the proposed catalytic mechanism with a molecular catalyst plays a vital role in achieving better catalytic performance. On the other hand, molecule-supported catalytic systems make efforts to improve the stability and reutilization of the catalysts, which is beneficial to the development of molecular catalysts in practical application. Even these above investigations work on different aspects of artificial photosynthesis, they all provide useful inspiration for further study.

Future perspectives

As a perspective, promising molecular catalysts with high intrinsic still need to be discovered for producing chemical fuels from solar energy via AP. Tuning the structure of molecular catalysts after getting insights spectroscopic and mechanistic studies is a rational approach towardsintrinsic catalytic performance. For an AP device based on molecular catalysis, a high intrinsic activity of the molecular catalyst, especially low overpotential, is an essential requirement for obtaining high efficiency. In this connection, bimetallic or multimetallic complexes may show excellent activities and to mimicking the natural photosynthetic process and as noted above may lead to new chemistry in the future.

Because of the complexity of molecule-supported AP catalytic systems, the performance can be controlled by severalsfactors. The present research is only a preliminary contribution toward a practical device. Such device for solar fuels production is highly challenging involving not only light absorption, charge transfer, and catalytic activity aspects but also engineering problems, such as device design and mass transfer/transport control. Considering the advantages of molecular catalysts, the construction of systems based on molecular catalysts is still highly promising.

The following challenges need to be addressed for developing AP systems:

(1) To develop earth-abundant metal based efficient catalysts that are able to work under aqueous, especially acidic conditions. Moreover, the design of catalysts to go beyond the "traditional" of two-electron reduction to produce liquid fuels (or convert N2 into chemicals) is still not achieved.

(2) To expand light absorbers, explore new transparent p-type semiconductors and upgrade the anchoring groups of functional molecules for long-term stable AP devices.

The common anchoring groups of molecules are carboxylic or phosphate groups.

However, such groups cannot tolerate long periods of irradiation in aqueous electrolyte environment. Stronger binding groups should be designed and investigated.

(3) To reveal the deactivation pathways, especially the reasons for the decomposition of modified catalysts. Such understanding is essential to improve the stability and activity of AP devices for practical applications.

In addition, the strategies for immobilization of molecular catalysts on the substrate, the mechanistic understanding of catalytic process, and the determination of the kinetics of electron transfer (notably at photoelectrodes) also need to be investigated throughly.

These are also urgent tasks for the further development of AP devices based on molecular catalysts and future practical applications.

Chapter 6 -Experiments and methods Methanol and dichloromethane were distilled over calcium hydride; THF was dried over sodium then distilled after addition of benzophenone. Solvents were freshly distilled or stored overnight under an argon atmosphere.

Gases

Argon (> 99.998 %), 12 CO (> 99.7 %), and 12 CO2 (> 99.7 %) gases were purchased from Air Liquide. CH4 (> 99 %) was from Fluka. 13 CO (99 % content in atom 13C) and 13 CO2 (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). The longer the reaction period, the higher was the yield of the salt, especially for substances having electron-withdrawing substituents. The brown crude product was decolorized by the activated carbon in hot aqueous ethanol (ethanol:H2O, 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 CH3OH was slowly added with 459 mg NaBH4 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:H2O (2:1 v/v) to give a colorless crystalline product. The yield was over 80% (0. Ru(phen)3Cl2. Ru(phen)3Cl2 used in this study was synthesized according to literature method using 1,10-phenanthroline instead of 2,2'-bipyridine (bpy). A mixture of RuCl3•3H2O (2.5 mmol, 0.66g) and 1,10-phenanthroline (12.5 mmol, 2.25 g) in 95%

ethanol was refluxed for 72 h under argon. The dark orange-red solution was filtered and then evaporated to dryness to give Ru(phen)3Cl2 as an orange-red solid, which was recrystallized in hot water. Yield: 1.20 g (67%). Anal. Calcd. for C36H24Cl2RuN6 were poured into a quartz cell (1 cm length). Typical spectral region ranged from 200-800 nm and it was swept at a rate of 720 nm s -1 .

Solar simulator

The main figure-of-merit for a solar CO2 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. 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 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.

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.

Fluorescence spectroscopy

Emission quenching measurements were conducted with a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), with the excitation wavelength set at 420 nm ([Ru(phen)3] 2+ ) or 400 nm (mpg-C3N4) and the emission intensities used for the Stern-Volmer analysis were taken at 600 nm ([Ru(phen)3] 2+ ), 488 nm (mpg-C3N4), 543 nm (mpg-C3N4), that is, the emission maximum of PS. 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.

6.2.5

Frontier Infrared (FT-IR) 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. 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 13C NMR spectra were recorded on a Bruker Avance III 400-MHz spectrometer and were referenced to the resonances of the solvent used.

6.2.7

Gas chromatography (GC) and Gas chromatography-Mass (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 detector. H2, CO and CH4 production was quantitatively detected using a CP- 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 (IC)

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 ultrapure water before analyzed through IC. Calibration curve for formate was determined separately by injecting known quantities of pure sample.

6.2.9 pH measurements pH measurements were performed with Hanna pH210 and HI221 instruments and 6 mm microelectrodes (Fisher). 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, 13 C NMR and IC. UV-vis analysis was made directly via the home-made glassware.

6.2.14 X-ray powder diffraction (XRD)

X-ray powder diffraction patterns were recorded by using a the PANALYTICAL X'PERT PRO diffractometer equipped with a Co tube and an X'celerator detector at room temperature.

Electrochemical and photoelectrochemical measurements

For photocurrent measurements of Coqpy@mpg-C3N4, 2 mg of each solid material Pellets of Coqpy@GA and CoqpyNH2.Cl2 were measured in transmission mode at the Co K (7709 eV) absorption edge at 298 K using a 36-element Ge detector. The Co foil was measured simultaneously. Calibration of the monochromator was carried out using the Co foil previously to the measurements. The acquired data were processed and analyzed using the Athena and Artemis programs, respectively, which implement the FEFF6 and IFEFFIT codes. These three years full of challenges and rewards will be an important transformation in my life. I hope this paper will not be the end of academic thinking, and I hope that the previous sentence is not just hope.

Notations

  Physique et de Chimie Analytique de Paris Centre THESE DE DOCTORAT PRESENTEE PAR BING MA Catalyse moléculaire de la réduction photochimique du CO2 à l'aide de complexes de cobalt en conditions homogène et supportée Thèse soutenue publiquement le 4 Décembre 2020 devant le jury composé de : Rapportrice : Caroline MELLOT-DRAZNIEKS, Directrice de Recherche, Collège de France Rapportrice : Marie-Noëlle COLLOMB, Directrice de Recherche, Université Grenoble-Alpes Examinateur : Junwang TANG, Professeur, University College London Examinateur : Alexander COWAN, Professeur, University of Liverpool Directeurs de thèse : Julien BONIN, Maître de Conférences, Université de Paris Marc ROBERT, Professeur, Université de Paris Table of contents Résumé .................................................................................................................................... Abstract ................................................................................................................................... Table of contents ..................................................................................................................... Chapter 1 -Introduction ...................................................................................................... 1.1 Development of artificial photosynthesis systems inspired by Nature .................... Chapter 1 -Introduction 1.1 Development of artificial photosynthesis systems inspired by Nature

Fig. 1 . 1

 11 Fig. 1.1 Enhancement of catalytic performance by increasing the number of active sites (left) and/or increasing the intrinsic activity of each active site (right). Adapted with permission from ref. 5. Copyright 2017 American Association for the Advancement of Science (AAAS).

Fig. 1 . 2

 12 Fig. 1.2 Photosensitizer (PS) emission quenching pathways. EA: electron acceptor (catalyst or electron mediator), SD: sacrificial electron donor. Adapted with permission from ref.10 10 copyright 2020 The Royal Society of Chemistry.

Fig 1 .

 1 2a), the reduced acceptor EA˙ˉ may quickly transfer back an electron to the oxidized photosensitizer (PS + ) since the reaction is endowed with a large driving force. A large concentration of donor (SD) may help fighting BET. 𝐼𝑄𝑌 (%) = 𝑚𝑜𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 ×𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑝𝑒𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑚𝑜𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 × 100 (Eq. 1.1) 𝐴𝑄𝑌 (%) = 𝑚𝑜𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 ×𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑝𝑒𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑚𝑜𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 × 100 (Eq. 1.2)

  photocatalysts. Table 1.1 and 1.2 provide an overview of the most efficient catalysts based Ni, Co and Fe, to date.

Fig. 1 . 3

 13 Fig. 1.3 Cobalt-based complexes.

Scheme 1 . 2

 12 Scheme 1.2 Proposed mechanism for the photochemical catalytic reduction of CO2 to a) CO and b) H2 with Fe porphyrin catalysts (L = Cl). For CO formation, subpathway a1-a2-a3 is followed in the case of FeTPP whereas subpathway a1-a'2 is followed in the case of both Fe7 and Fe8 (see Fig. 1.4). Adapted with permission from ref. 27 27 . Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 1 .

 1 Fig. 1.4 Nikel-and iron-based molecular complexes.

Fig. 1 . 5

 15 Fig. 1.5 Typical molecular photosensitizers including metal-based (Ru, Ir) complexes and organic aromatic molecules.

Fig. 1 . 6

 16 Fig. 1.6 Molecular structures of sensitizer-catalyst dyads.

Fig. 1 . 8

 18 Fig. 1.8 Molecular structures of cobalt-containing dinuclear complexes.

Fig. 1 .

 1 Fig.1.9), while Mn-carbonyl centers coordinated to the bipydine functionalities. With the help of [Ru(bpy)3]Cl2 and BIH, the obtained hybrids catalyst triggered a CO2 reduction towards CO with a TON of 239 or towards formate with a TON of 484, respectively.59

Fig. 1 . 9

 19 Fig. 1.9 Synthetic pathway to [Mn(bpyPMO)(CO)3Br]. Reproduced from ref.59 59 with permission from

Fig. 1 .

 1 Fig. 1.10 Schematic illustration of light-driven CO2 reduction catalyzed by mpg-CNx|CoPPc hybrid (potentials reported vs. NHE; VB and CB denote valence and conduction band, respectively). Adapted with permission from ref. 65, copyright © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co.

Fig 1 . 12

 112 Fig 1.12 Schematic representation of the two different interfacial ET methods in the PCN and Co(bpy)3 2+ system connected by a covalent linker (2,2 ' -biisonico-tinic acid, BINA). Adapted with permission of ref. 71. Copyright © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 1 .

 1 Figure 1.13 Scheme of two different graphite-conjugation porphyrin system. Adapted with permission of ref. 72. Copyright © 2019, American Chemical Society.

  photosensitizer is replaced by organic dye purpurin, Feqpy and Coqpy can reach TONCO of 1365 (92%) and 790 (95%) in DMF solution, respectively.

Fig. 2 . 1

 21 Fig. 2.1 Structure of compounds used in this chapter. Cobalt complexes [Co2(biqpy)] 4+ (1) and [Co(qpy)] 2+

Fig. 2 . 2

 22 Fig. 2.2 Evolution with time of CO2 reduction products during photochemical experiments. performed in a CO2-saturated ACN solution (a) in the absence and (b) presence of 0.025 M BIH for a solution containing 50 µM 1, 0.2 mM Ru(phen)3Cl2 and 20% TEA (λ = 460 nm); (c) with 20 µM 1, 2.5 mg g-C3N4, 20% TEOA and 0.05 M BIH (λ > 400 nm); (d) with 50 µM 1, 0.2 mM Ru(phen)3Cl2, 0.1 M BIH and 1 M PhOH (λ = 460 nm). Adapted with permission from ref.49 Copyright © 2019, Springer Nature.

Fig. 2 .

 2 Fig. 2.3 Photocatalytic CO2 reduction performance with time for the CO2-saturated ACN solution containing 0.05 mM 2, 0.2 mM Ru(phen)3Cl2 and 20 % (v %) TEA. The Data points are resulting from two

Fig. 2 .

 2 Fig. 2.4 Photocatalytic CO2 reduction performance for CO2-saturated ACN solution containing 0.05 mM 1, 0.2 mM Ru(phen)3Cl2, 20 % (v%) TEOA and 0.1 M BIH. Data points are resulting from two to four runs, and error bars represent the standard error on the mean. Adapted with permission from ref. 49 Copyright © 2019, Springer Nature.

Fig. 2 . 5

 25 Fig. 2.5 Particle size distribution of CO2-saturated ACN/TEA (4:1, v:v) solution containing 0.05 mM 1, 0.2 mM Ru(phen)3Cl2 and 0.025 M BIH determined by DLS during irradiation by blue LED. Adapted with permission from ref.49 Copyright © 2019, Springer Nature.

Fig. 2 . 6

 26 Fig. 2.6 Photocatalytic CO2 reduction performance in the presence and absence of 0.4 mL Hg in CO2saturated ACN/TEA (4:1, v:v) solution containing 0.05 mM 1, 0.2 mM Ru(phen)3Cl2 and 0.025 M BIH.The error bars represent the standard error on the mean, which was calculated from two to four runs. Adapted

Fig. 2 .

 2 Fig. 2.9 CV of 0.5 mM [Coqpy] 2+ (black) and [BiqpyCo2] 4+ (red) in 0.1 M n Bu4NPF6 ACN solution under Ar at scan rate of 100 mV/s. Adapted with permission from ref.49 49 Copyright © 2019, Springer Nature.

Fig. 2 .

 2 Fig. 2.10 CVs of 0.5 mM 1 in 0.1M n Bu4NPF6 ACN solution in the presence (red) and absence (black) of 0.5 M PhOH under CO2 at scan rate of 100 mV/s. Adapted with permission from ref. 49 Copyright © 2019, Springer Nature.

Fig. 2 .Fig. 2 .

 22 Fig. 2.11 Current (left) and charge (right) versus time curves for bulk electrolysis of CO2-saturated ACN solution containing: Top, 1 mM 1, 0.5 M PhOH and 0.1 M n Bu4NPF6 at -1.4 V vs. SCE; Bottom: 0.5 M phenol and 0.1 M n Bu4NPF6 at -1.4 V vs. SCE (no CO or formate or H2 was detected). Adapted with

2 . 2 )Fig. 2 .

 222 Fig. 2.13 Cyclic voltammograms and infrared spectroelectrochemistry spectra. (a). CVs in ACN solution containing 0.3 mM 1 and 0.1 M n Bu4NPF6 under Ar atmosphere (blue), in the absence (black) and presence (red) of 20% TEA under CO2 saturation. (b). IR-SEC experiment on a 1 mM solution of 1 in ACN (0.1 M

  2.13a), which corresponds to the CO2-catalyst adduct reduction. Bulk electrolysis of the CO2-saturated MeCN solution containing 0.5 mM 1, 0.5 M TEA and 0.1 M n Bu4NPF6 at -1.5 V vs. SCE gave CO, formate and H2 with faradaic yields of 90%, 7% and 0.5%, respectively (Fig. 2.14).

Fig. 2 .

 2 Fig. 2.14 Current (left) and charge (right) evolution with time during bulk electrolysis of CO2-saturated ACN solution containing: Top, 0.5 mM 1, 0.5 M TEA and 0.1 M n Bu4NPF6 at -1.5 V vs. SCE; Bottom, 0.5M TEA and 0.1 M n Bu4NPF6 at -1.5 V vs. SCE (no CO or formate or H2 was detected). Adapted with permission from ref.49 Copyright © 2019, Springer Nature.

Fig. 2 .

 2 Fig. 2.15 IR-SEC experiment on a 1 mM solution of 1 in ACN (0.1 M nBu4NPF6, 0.5 M TEA) under CO2 at the second reduction wave. The inset shows the disappearance of the CO2-adduct (band at 1635 cm -1 )

  ), converge to a possible mechanism for the visible-light-driven reduction of CO2 with catalyst 1 that is shown in Fig. 2.18. Initially, catalyst 1 undergoes 4e -reduction assisted by the photosensitizer to afford the neutral [CoCo(L)] species, which reductively binds to CO2 with one Co centre linked to the C atom of CO2 and the other to an O atom, forming a sandwich type CO2 adduct [CoCoCO2(L)] (Figs. 2.18 and 2.13 b, inset). Remarkably, decrease from 180° to 126° and the C-O bond lengths increase from 1.116 Å to 1.243 and 1.286 Å, in line with CO2 structure modification during reduction. After a one-electron reduction of this adduct, the catalytically active species [CoCoCO2(L)] -leads to product formation. In acidic conditions, protonation of the CO2 adduct takes place efficiently at the O atom stabilized by one of the two Co atoms, and C-O bond cleavage results in the subsequent release of CO as product. A second reaction pathway was identified, involving C to O linkage isomerization of the reduced CO2 adduct to [CoCoO2C(L)] -(Fig. 2.18).

Fig. 2 .

 2 Fig. 2.17 Potential energy surface of CO2 reduction by [CoCo(L)] 1 at B3LYP/LanL2DZ(Co)/6-31+G(d)(nonmetals)//B3LYP/LanL2DZ(Co)/6-31G(d)(nonmetals) level. Relative Gibbs free energies at 298 K in ACN are given in kcal mol -1 . The spin density on Co is also given. Relative Gibbs free energies at 298 K in ACN are given in kcal•mol -1 . The spin density (numbers in blue color) on Co is also given. Adapted

Figure 2 .

 2 Figure 2.18 Proposed mechanism for visible-light-driven catalytic reduction of CO2 into CO and HCOO - with catalyst 1. Complex 1 undergoes 4e -reduction by the photosensitizer to afford the neutral [CoCo(L)] species which reductively binds to CO2 with one Co center linked to C atom of CO2 and the other to O atom, forming a sandwich type CO2 adduct [CoCoCO2(L)]. After a one electron reduction of this adduct, the catalytically active species [CoCoCO2(L)]-leads to product formation, either CO or formate. Adapted with

Fig. 3 . 1

 31 Fig. 3.1 Synthesis scheme of Coqpy-Ph-COOH catalyst. Adapted with permission from ref. 107, 107

  Two other types of materials have prepared. First adsorbed sample Coqpy-mpg-C3N4 was prepared by dissolving Coqpy-Ph-COOH (1 mg) into a mpg-C3N4 (25 mg) suspension in 25 mL DMF. After stirring overnight, the final solid was obtained by a similar posttreatment including filtration and washing cycles, but no sonication. In a final step, the precipitate was dried under vacuum to yield the Coqpy-mpg-C3N4, for which the Co complex is physically adsorbed onto the carbon nitride material. Mixed sample Coqpy-Ph-COOH/mpg-C3N4 was prepared as follows: the mpg-C3N4 was dispersed into ACN and sonicated to achieve a uniform suspension. After that, a defined amount of Coqpy-Ph-COOH solution was added into the suspension and the photocatalysis was started immediately.

Fig. 3 .

 3 Fig. 3.2 (A) Nitrogen adsorption-desorption isotherms and (B) the corresponding Barrett-Joyner-Halenda (BJH) pore-size distribution curve of mpg-C3N4; (C) Nitrogen adsorption-desorption isotherms and (D) the corresponding Barrett-Joyner-Halenda (BJH) pore-size distribution curve of mpg-C3N4@Coqpy after photocatalysis in ACN solution with BIH for 24 h. Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

  -Ray Photoelectron Spectroscopy (XPS) analysisAs shown in Fig.3.4, typical XPS signatures originating from C1s, N1s and O1s atoms within the hybrid are observed at binding energies of about 287, 400 and 532 eV.110 The C1s spectrum consists in three components, a first one from NC=N (carbon nitride) at 288.9 eV, a second one at 284.9 eV corresponding to C-C/C-H; a third one at 286.3 eV from C-N attributed to the coupling of C3N4 and Coqpy-Ph-COOH units.The amide C=O can be observed at 532.4 eV in O1s peak. C=N and N(C)3 peaks of the triazine motif were identified in the N1s spectrum and centered at 398.8 eV and 399.9 eV respectively. A weak N1s peak centered at 404.8 eV is attributed to the nitride substrate. The peak centered at 401.1 eV is consistent with C-N-H from covalent amide linkage. On the other hand, weak Co2p signature at ca. 782 eV can be observed, showing the presence of cobalt inside the hybrid material.

Fig. 3 .

 3 Fig. 3.4 (a) High-resolution XPS spectra of the carbon C 1s (left column) and the nitrogen N 1s (right column) of Coqpy-Ph-COOH, mpg-C3N4, Coqpy@mpg-C3N4 before photocatalysis, Coqpy@mpg-C3N4 after photocatalysis, Coqpy-mpg-C3N4 (from top to bottom). Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

Fig. 3 .

 3 Fig. 3.4 (b). High-resolution XPS spectra of the oxygen O 1s (left column) and the cobalt Co 2p (right column) of Coqpy-Ph-COOH, mpg-C3N4, Coqpy@mpg-C3N4 before photocatalysis, Coqpy@mpg-C3N4 after photocatalysis, Coqpy-mpg-C3N4 (from top to bottom). Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.*

Fig. 3 . 5 .

 35 Fig. 3.5. Cyclic voltammetry of Coqpy-Ph-COOH (0.2 mM) in DMF solution (0.1 M NBu4PF6) at ν = 0.1 V/s and T = 298 K under argon (black) and CO2 (blue) atmosphere as well as in the presence of 0.1 M phenol (green), 0.5 M phenol (red) under CO2. Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.Under argon and upon reductive scan, a reversible wave at -0.7 V vs. SCE is observed and is attributed to the Co II /Co I . Further reduction first leads to quasi-reversible waves at

Fig. 3 . 6

 36 Fig. 3.6 Generation of CO (black squares) and H2 (red circles) over 4 days upon visible-light irradiation (λ > 400 nm) of a CO2-saturated ACN solution containing 6 mg Coqpy@mpg-C3N4, 0.05 M BIH, and 0.03 M PhOH. Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

  3.7) as shown by GC-MS measurements (Fig.3.7(B)).

Fig. 3 .

 3 Fig. 3.7 (A) Typical gas chromatogram obtained after long-term irradiation of a suspension containing 6 mg of hybrid Coqpy@mpg-C3N4, 0.05 M BIH and 0.03 M phenol, under 13 CO2 atmosphere.

Fig. 3 .

 3 Fig. 3.7 (B) Corresponding mass spectra of the CO GC peak. Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

3. 1 . 3

 13 Scheme 3.1 schematic illustration of electron/hole separation under visible light illumination of a semi-

Fig. 3 .

 3 Fig. 3.8. (A) Emission spectra of a 6 mg mpg-C3N4 suspension in ACN containing 0 (black), 10 (red), 20 (blue), 30 (pink), 40 (green), 50 (yellow), and 60 mM (magenta) BIH; (B) linear plot of the emission ratio at 490 (left) and 540 nm (right) vs. BIH concentration according to the Stern-Volmer equation. Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

Fig. 3 .

 3 Fig. 3.9 (A) Emission spectra of a 6 mg mpg-C3N4 suspension in ACN containing 0 (black), 1 (red), 2 (blue), 3 (magenta), 4 (olive), 6 (navy), 8 (violet), 10 (purple), 12 (wine) and 14 μM (dark yellow) Coqpy-Ph-COOH; (B) linear plot of the emission ratio at 490 (left) and 540 nm (right) vs. Coqpy-Ph-COOH concentration according to the Stern-Volmer equation. Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

Fig. 3 .

 3 Fig. 3.10 Transient (light on/off) photocurrent responses of mpg-C3N4(black), Coqpy/mpg-C3N4 (blue) and Coqpy@mpg-C3N4 (red) hybrid ink dropped onto a FTO electrode connected to a platinum electrode (no bias, 0.1 M Na2SO4 solution). Adapted with permission from 107, Copyright 2020 American Chemical Society.

Fig. 3 .

 3 Fig. 3.11 EIS (electrochemical impedance spectroscopy) responses of mpg-C3N4 (black), Coqpy/mpg-C3N4 (blue) and Coqpy@mpg-C3N4 (pink) in the dark. Adapted with permission from 107, Copyright 2020 American Chemical Society.

Fig. 3 .

 3 Fig. 3.12 Generation of CO (black squares) and H2 (red circles) during four consecutive 24 h irradiation cycles using the same hybrid material (see text). Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

Fig. 3 .

 3 Fig. 3.13 XRD pattern of Coqpy-Ph-COOH molecule (red), Coqpy@mpg-C3N4 before photocatalysis (blue), Coqpy@mpg-C3N4 after photocatalysis (orange), mesoporous graphic-C3N4 (green) and nanosheet graphic (pink) compared to patterns of a sodium aluminum silicate hydrate (a porous zeolite, JCPDS reference card # 00-023-0241, black). Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

Fig. 3 .

 3 Fig. 3.14 UV-vis absorption spectra of (A) 0.3 mM PhOH (black), 0.36 mM BIH (blue), qpy-Ph-COOH ligand (red) and Coqpy-Ph-COOH (olive) in ACN (concentrations of the ligand and complex cannot be determined due to low solubility) and (B) photochemical solution mixture containing all components (see entry 1, Table 3.1) before (dark yellow) and after (navy) reaction in ACN. These solutions were filtered and diluted 50 times with MeCN. Adapted with permission from ref. 107, Copyright 2020 American Chemical Society.

Scheme 3 . 2

 32 Scheme 3.2 Functionalization route of GA with the amino-functionalized Cobalt quaterpyridine molecule.

Fig. 3 . 15 .

 315 Fig. 3.15. Raman data of pristine GA, GA-Coqpy and ammino-modified CoqpyCl2 (Coqpy-NH2) molecule.

Fig. 3 .

 3 Fig. 3.16 FTIR data of pristine GA, GA-Coqpy and ammino-modified CoqpyCl2 molecule.

Fig. 3 .

 3 Fig. 3.17 C 1s, N1s and Co 2p XPS spectra for GA-Coqpy sample and their corresponding deconvolution into single chemical components. The C 1s region of GA is also shown for comparison.

Fig. 3 .

 3 Fig.3.18 shows the ex situ XANES and Fourier transform (FT) EXAFS spectra for the GA-Coqpy hybrid and the amino-modified Co complex at the Co K edge, as well as for oxides used as reference to establish correlation between the K-edge excitation energy

Fig. 3 .

 3 Fig. 3.19 shows the fitted FT EXAFS spectra fitted by using the model reported by Henke et al. 127 and the corresponding k-space spectra. The fitting results are summarized in Table 3.6. The amino-modified CoqpyCl2 molecule was fitted with a Co-N shell with 4 nitrogen atoms at 1.89 Å, a Co-Cl shell with 2 chlorine atoms at 2.21 Å, and a Co-C shell with 6 carbon atoms at 2.73 Å. After the covalent bonding with GA, some differences were observed in the Coqpy unit. A similar effect has already been reported in the literature for the grafting of organometallic species to carbon supports. So we can speculate on this possibility that the molecule lost the two chlorine atoms and a new interaction between cobalt and oxygen from the -COOH groups of GA substrate was formed. The new Co-O interaction caused a stretching of the Co-N (2.03 Å) and Co-C (2.94 Å) bonds, which could have an effect on the catalytic activity. 128-131

Fig. 3 .

 3 Fig. 3.19 FT EXAFS spectra (left panel) and the corresponding k 2 weighted data at the Co K edge (right panel) for the GA-Coqpy hybrid and the amino-modified CoqpyCl2 molecule. The black curves represent the experimental data and the coloured ones the fit for GA-Coqpy (green) and CoqpyNH2 (yellow). The dotted squares indicate the R and k ranges used in the fits.

Fig. 3 . 2

 32 Fig. 3.20 UV-Vis diffuse reflectance spectra for pristine GA, GA-Coqpy and amino-modified CoqpyCl2 molecule.

Fig. 3 .

 3 Fig. 3.21 Photocatalytic CO2 reduction products and selectivity as a function of time in CO2-saturated ACN solution for reactions performed in the presence of 0.1 M PhOH for a suspension containing GA-Coqpy (Coqpy 15 μM), 0.05M BIH ( λ>400 nm).

  8, entry 5, fig. 3.22).

Fig. 3 .

 3 Fig. 3.22 Photocatalytic CO2 reduction products as a function of time in CO2-saturated ACN solution for reactions performed in the presence of 0.1 M PhOH (blue) or 1.51 M TEOA (red) in a suspension containing GA-Coqpy (Coqpy 15 μM), 0.05M BIH (λ > 400 nm).

Fig. 3 .

 3 Fig. 3.23 Mass spectra of the CO GC peak corresponding to gas chromatogram obtained after long-term irradiation of a suspension containing hybrid GA-Coqpy (Coqpy 15 μM), 0.05 M BIH and 0.1 M phenol under 13 CO2 atmosphere.

Fig. 3 .

 3 Fig. 3.27 XPS spectra of Co for pristine GA-Coqpy and samples collected after photocatalytic reaction.Slight increase of C sp 3 component was observed and was probably due to contamination from the electrolyte. The Co 2p spectra before and after coupling with GA are very similar, indicating that the Coqpy structure is maintained in the hybrid. The decrease of the satellite peak intensity, as well as the narrowing and shift toward lower BE of the main peak, may indicate a slight reduction of Co 2+ after reaction. This effect is more evident for the reaction conducted in basic conditions.

Fig. 3 .

 3 Fig. 3.29 Recycling experiments of photocatalytic reduction to formate: generation of formate (blue circles) and H2 (red circles) during four consecutive 72 h irradiation cycles using the same hybrid material.

Fig. 3 .

 3 Fig. 3.30 Comparison of formate and CO generation during four cycles.

Fig. 4 . 1

 41 Fig. 4.1 Synthesis of GCC-Phenazine (Top), GCC-Ru (Middle), and GCC-Rh (Bottom). Adapted with permission from ref. 133, Copyright 2018 American Chemical Society.

4. 1 . 3

 13 Fig. 4.3 Pictures of synthesized GCC-Coqpy-I from different perspectives. (a), (b) obtained by using Pt gauze as a counter electrode; (c), (d) obtained by using Pt wire as a counter electrode.

Fig. 4 . 4

 44 Fig. 4.4 Pictures of GCC-Coqpy-II obtained from modified procedure (a) as-synthesized; (b) after CV scan.

Fig. 4 .

 4 Fig. 4.5 XPS survey spectra of two regions in GCC-Coqpy on GCP (glassy carbon plate).

Fig. 4 .

 4 Fig. 4.6 High-resolution N 1s XPS spectra of GCC-Coqpy on GCP (glassy carbon plate).

4. 1 . 4 4 . 7 .

 1447 Cyclic voltammetry characterization of GCC-Coqpy electrodeTypical cyclic voltammetry of a GCC-Coqpy-I under Ar and CO2 is shown in Fig.Underthe CO2 atmosphere, the onset potential is around -1.1 V vs. SCE. However, the same phenomenon was not observed in the GCC-Coqpy-II catalyst (Fig.4.8). There is no significant difference between carbon dioxide and argon. After adding 3 M of phenol, an irreversible reduction peak appeared at -1.2 V. After bubbling with Ar, the peak at -1.2 V disappeared. It thus seems that these electrodes are catalytically active and show early onset potential. Detailed catalytic studies will be the next step of this work.

Fig. 4 .

 4 Fig. 4.7 CV comparison of GCC-Coqpy-I under CO2 and Ar in ACN with 0.1 M TBAPF6. v = 0.1 V/s.

( 1 , 3 -

 13 dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) 135 and 4-bromo-2,2':6',2'':6'',2'''-quaterpyridine were prepared according to literature. 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. The reaction mixture was heated at 110 °C overnight in a Pyrex bottle.

Fig. 6 .

 6 Fig. 6.1 ESI-Mass spectra of the Co2LCl4 in ACN.

Fig. 6 . 2 Fig. 6 .

 626 Fig. 6.2 Crystal structure of [Co2(biqpy)Cl(MeOH)(H2O)] 3+ . ORTEP drawing of the complex. Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms (except O-H) and solvent molecules are omitted for clarity.

  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).

6. 2 . 4

 24 photomultiplier tube linked to a Tektronix TDS 3012C 100 MHz oscilloscope.

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 CO2. Conditions allowed detection of H2, O2, N2, CO, and CO2. Calibration curves for H2, CO and CH4 were determined separately by injecting known quantities of pure gas.

6. 2 .

 2 10 Cyclic voltammetry 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.

  was suspended into 250 μL of DMF. The obtained catalytic ink was then dropped on one face of a fluorine-doped tin oxide (FTO) glass electrode (150 μL for a 1 cm 2 ) and allowed to dry under ambient conditions prior to use it as the working electrode. The Coqpy/mpgC3N4 sample was prepared by grinding mpg-C3N4 with Coqpy-PhCOOH. All electro-and photoelectrochemical experiments were performed in a glass cell with a quartz window in 0.1 M Na2SO4 electrolyte by using a conventional three-electrode system with a platinum wire as a counter electrode and a saturated calomel electrode (SCE) as reference (-0.241 V vs. NHE). Light illumination was provided by a 300 W xenon lamp (Oriel Inst.). The amperometric photocurrent was measured for each switch on/off event by using an Autolab PGSTAT 128N potentiostat (Metrohm) with no bias voltage under the UV-vis light irradiation with light chopping every 20 s.Electrochemical impedance spectroscopy (EIS) plots were measured by using a PARSTAT 4000 potentiostat (Princeton Applied Res.) at the corresponding open circuit potential over the frequency ranging from 0.1 to 106 Hz in the dark.6.2.16 Scanning Electron Microscope (SEM)SEM images were taken on a a Zeiss MEB-FEG (Zeiss SUPRA 40) Scanning Electron Microscope with an continuously variable acceleration voltage from 100V to 30kV. The scanning electron microscope equipped with a high efficiency In-Lens secondary electron detector in axial position and an NPGS lithography device allowing the fabrication of nanostructures. 6.2.17 X-ray Absorption Spectroscopy (XAS) X-ray adsorption spectroscopy (XAS) measurements were recorded on beamline B18 at Diamond Light Source (UK) with ring energy of 3 GeV and a current of 300 mA. The monochromator used was Si(311) crystals operating in Quick EXAFS (QEXAFS) mode.

  38

Table 1 .

 1 1 Photocatalytic CO2 reduction using non-noble metal complexes.

	Entry CRC	PS	Conditions	Illumination	Irrad. time Product (TON)	Selectivity	QY(%)	Ref
	1	Co1 0.08 mM	[Ru(bpy)3] 2+ 0.5 mM	0.5/0.5 M H2A/HA -H2O (pH=4)	Daylight lamp	18 h	CO (1.2)	< 1% (CO)	-
	2	Co2 0.08 mM	[Ru(bpy)3] 2+ 0.5 mM	0.5/0.5 M H2A /HA -H2O (pH=4)	Daylight lamp	18 h	CO (28.7)	32% (CO)	-
	3	Ni1 2 mM	[Ru(bpy)3] 2+ 0.5 mM	H2A H2O (pH=5)	Xe lamp 1000 W, λ=440 nm	24 h	CO (0.1)	-	0.06% (CO) (HCOO -) 0.003%	13
	4	Ni1 0.1 mM	[Ru(bpy)3] 2+ 0.5 mM	H2A H2O (pH=4)	Xe lamp 1000 W 340 <λ <600 nm	8 h	CO (4.8)	12% (CO) 88% (H2)	-
	5	Ni21 0.25 mM	[Ru(bpy)3] 2+ 0.5 mM	H2A H2O (pH=4)	Hg lamp 460 W, λ=435 nm	1 h	CO (4.4)	89%	-
	6	Co12 1.7 mM	p-terphenyl 2 mM	TEOA in CH3CN/ CH3OH (4:1 v:v)	Hg lamp 500 W high-pressure λ > 290 nm	1 h	CO (10.2) HCOO -(6.7)	-	15 (CO) λ=313 nm 10 (HCOO -)	19, 20
	7	Co3 1.7 mM	p-terphenyl 2 mM	TEA in CH3CN/ CH3OH (4:1 v:v)	Hg lamp 500 W high-pressure λ > 290 nm	1 h	CO (4.7) HCOO -(2.3)	60.5% (CO) 31.8% (HCOO -)	-	19, 20
	8	Co13 1.7 mM	p-terphenyl 2 mM	TEOA in CH3CN/ CH3OH (4:1 v:v)	Hg lamp 500 W high-pressure λ > 290 nm	1 h	CO (5.3) HCOO -(3.5)	51.6% (CO) 34.4% (HCOO -)	-	19, 20

Table 1 .

 1 2 Photocatalytic CO2 reduction using molecular dyads and bimetallic complexes.

	Entry	CRC	PS	conditions	illumination	Irrad. time	Product (TON)	Selectivity	QY(%) Ref
	1	Ni2 0.25 mM	[Ru(bpy)3] 2+ 0.5 mM	H2A in H2O (pH=4)	Hg lamp 460 W 1 h	CO (0.7)	-	-
	2	Ni22 0.25 mM	[Ru(bpy)3] 2+ 0.5 mM	H2A in H2O (pH=4)	Hg lamp 460 W 1 h	CO (4.4)	94% CO	-
	3	Co21 10 μM	fac-Ir(ppy)3 0.4 mM	TEA in CH3CN/H2O (5:1 v:v)	LED λ = 450 nm 60 h	CO (56.9) H2 (181.6) HCOO -(64.2)	18.8% CO 60% H2 21.2% HCOO -	-
	4	RuNi1 0.5 mM	-	H2A in H2O (pH=4)	Xe lamp 500 W λ > 450 nm	44 h	CO (<1)	72% CO	-
	5	RuNi2 0.035 mM	-	H2A in H2O (pH=6.5)	Xe lamp 500 W λ > 450 nm	60 h	CO (5.2) H2 (2)	-	-
	6	RuCo1 0.5 mM)	-	TEOA in DMF:H2O (3:1 v/v)	Xe lamp 500 W 400 < λ < 750 nm	29 h	CO (3) H2 (1) HCOO -(31)	9.3% CO	-
	7	RuCo2 0.5 mM	-	TEOA in DMF:H2O (3:1 v:v)	Xe lamp 500 W 400 < λ < 750 nm	29 h	CO (5) H2 (1) HCOO -(34)	12.7% CO	-
	8	[Co(bpy)3] 3+ 0.5 mM	[Ru(bpy)3] 2+ 0.5 mM	TEOA in DMF:H2O (3:1 v:v)	Xe lamp 500 W 400 < λ < 750 nm	29 h	CO (9) H2 (16) HCOO -(28)	17% CO	-
	9	RuCo3 0.1 mM	-	BIH in CH3CN:TEOA (3:1 v:v)	Xe lamp 300 W λ > 415 nm	8 h	CO (51) H2 (13)	79.6% CO	-
	10	RuCo4 0.1 mM	-	BIH in CH3CN:TEOA (3:1 v:v)	Xe lamp 300 W λ > 415 nm	8 h	CO (54) H2 (8)	87.2% CO	-
	11	RuCo5 0.1 mM	-	BIH in CH3CN:TEOA (3:1 v:v)	Xe lamp 300 W λ > 415 nm	8 h	CO (70) H2 (39)	64.3% CO	-
	12	Co22 0.025 μM	[Ru(phen)3] 2+ 0.4 mM	TEOA in CH3CN:TEOA (4:1 v:v)	LED λ = 450 nm 10 h	CO (16896)	98% CO	0.04% CO
	13	CoZn1 0.025 μM	[Ru(phen)3] 2+ 0.4 mM	TEOA in CH3CN:TEOA (4:1 v:v)	LED λ = 450 nm 10 h	CO (65000)	98% CO	0.15% CO

Table 1 .

 1 2, entry 13).48 The superior photocatalytic performance can be ascribed to enhanced dinuclear metal synergistic catalysis effect between Co II and Zn II . With a stronger binding affinity to OH -, Zn II can facilitate the cleavage of C-O in O=C-OH intermediate, Reducing the activation barrier.

Surendranath et al. reported a supported catalyst made of cobalt porphyrins strongly electronically coupled to carbon electrodes by condensing diaminoporphyrin derivatives

	onto o-quinone moieties native to graphitic carbon surfaces (Fig. 1.13). Probing the
	material for O2 reduction reaction (ORR), it was found that the graphite-conjugated Co
	porphyrin, GCC-CoTPP, displays an onset current density of 10 μA/cm 2 at 0.72 V vs. RHE,
	whereas a nonconjugated amide linked Co porphyrin gives an onset at 0.66 V vs. RHE,
	which illustrates that conjugation significantly enhances the rate of catalysis. 72

  DFT calculations have been employed to unveil the catalytic mechanisms. Based on these studies, Robert et al. speculated the contrasting catalytic pathways and gave a prediction on the modification direction of the ligand and the promising catalytic performance.2.2 Quaterpyridine complexes in supported conditionsCometto et al. have employed Feqpy for heterogeneous photocatalysis together with semiconductor mesoporous carbon nitride as light absorber. In a mixed solution of acetonitrile and TEOA, a TON of 155 for CO evolution has been achieved with 97% selectivity and an apparent quantum yield of ca. 4.2%.

84

Wang et al. have reported a Co(qpy) appended at the surface of multi-walled carbon nanotubes, which can convert CO2 into CO in water at pH 7.3 with 100% catalytic selectivity and 100% faradaic efficiency, at low catalyst loading and at reduced overpotential. Electrolysis at 340 mV overpotential indeed showed a stable current density of 9.3 mAcm 2 and excellent catalytic performances (89095 catalytic cycles in 4.5 h).

81

2.3 Selective reduction of CO2-to-CO or to-formate through metal synergy Known molecular catalysts include noble metal complexes such as Ru

85,86

, Ir

87

and Re

88

and earth-abundant metal complexes such as Co 89 , Ni 90 , Fe

29

, Mn 91 and Cu

92

. Among all earth-abundant molecular photocatalysts reported to date, most of them can obtain CO from CO2 while few of them can produce HCOO -with high yield, and there are even fewer examples of complexes which can generate hydrocarbons beyond these 2H + /2e - reduction reactions.

Table 2 .

 2 1 Photoinduced CO2 catalytic reduction with catalyst 1 and [Ru(phen)3] 2+ or Pheno or g-C3N4 as photsensitizer.

a a in a typical run, 2 mL of CO2-saturated ACN solution containing 1 (50 µM [entries 1, 2, 3, 6], 20 µM [entry 4], 15 µM [entries 5, 7]), Ru(phen)3Cl2 (0.2 mM) or Pheno (0.4 mM) or g-C3N4 (2.5 mg), TEA (20%, v%) or TEOA (20%, v%), BIH (0.025 M [entry 2], 0.05 M [entries 4, 5, 7], 0.1 M [entries 3,6]), PhOH (1 M)

was irradiated using blue LED (460 nm, entries 1-3, 6) or with a lamp equipped with a 400 nm optical filter (Xe-Hg lamp for entry 4, solar simulator [entries

5, 7]

). TON values are ± 5%.

  2.3 and Table 2.3, entry 1).

	Entry	Sensitizer	conditions	Time (h)	TON (selectivity, %) HCOO -CO	H2
	1	[Ru(phen)3] 2+	TEA	20	110 (92.4)	0 (0)		9 (7.6)
	2	[Ru(phen)3] 2+	TEA+BIH	23	386 (96.5)	8 (2)		6 (1.5)
	3	[Ru(phen)3] 2+	TEOA+BIH	60	821 (75.9)	221 (20.4)	40 (3.7)
	4	g-C3N4	TEOA+BIH	75	493 (91.1)	0 (0)		48 (8.9)
	5	Pheno	TEOA+BIH	5	565 (58.3)	284 (29.3)		119
								(12.4)
	6	[Ru(phen)3] 2+	PhOH+BIH	1	12 (1.5)	829 (96)		22 (2.5)
	7	Pheno	PhOH+BIH	2.5	0 (0)	518 (89.3) 62 (10.7)

Table 2

 2 Phen (0.4 mM in entries 12-19, 1 mM in entries 20-23) or g-C3N4 (2.5 mg), TEA (20%, v%) or TEOA (20%, v%), BIH (0.025 M [entries 4-8], 0.05 M [entries 29-33, 12-23], 0.1 M [entries 9-11, 34-37] )was irradiated using blue LED (460 nm, entries1-11, 33, 34-36) or with a lamp equiped with a 400 nm optical filter (Xe-Hg lamp [entries24-32], solar simulator [entries12-23]). TON values are ±5%.

	1	--	--	--

a in a typical run, in 2 mL of CO2-saturated ACN solution containing 1

(50 µM [entries 1, 2, 4, 5, 9, 10, 34,35,37]

, 20 µM [entries

24, 25, 29, 30, 33]

, 15 µM [entries

12, 13, 16, 17, 20, 21]

), Ru(phen)3Cl2 (0.2 mM) or b : under Ar c : without catalyst d f : in the presence of 1 M PhOH g : in DMF as solvent a In a typical run, 2 mL of CO2-saturated ACN solution containing 2 (50 µM [entries 1-3, 5], 20 µM [entries 6-7]), Ru(phen)3Cl2 (0.2 mM) or g-C3N4 (2.5 mg) , TEA (20%, v%) or TEOA (20%, v%), and BIH (0.025 M [entries 2-4], 0.1 M [entry 5], 0.05 M [entry 7]) was irradiated using blue LED (460 nm, entries 1-5) or 400 W Xe-Hg lamp equipped with a 400 nm optical filter (entries 6-7) at 24 o C. TON values are ±5%. b : under Ar. c : without catalyst.

Table 2

 2 minor product with a TON of 48. No CO and no hydrocarbons were detected.Similarly, when replacing catalyst 1 by CoCl2 only obtained small amounts of H2 and CO (Table2.2, entries30-32). With only TEOA as sacrificial donor, a similar result reached

	out with slightly lower formate selectivity (74.5%, Table 2.2, entries 24-28). This
	observation illustrates that an earth-abundant metal complex associated with g-C3N4 as
	photoabsorbing material can selectively trigger the reduction of CO2 to formate.
	c. Switching catalytic selectivity to CO
	The addition of phenol (PhOH) can improve the reactivity and switch the selectivity
	for CO2 reduction to CO, with a maximum TON of 829 and catalytic selectivity ranging
	from 90% to 99%, depending on the catalytic conditions. In the presence of 1 M PhOH,
	0.1 M BIH, Ru(phen)3

.1, entry 4) and a selectivity of 91% (1.7% quantum yield). H2 was obtained as a 2+ and 1, CO was obtained with 96% catalytic selectivity (TON of 829) upon 1 h irradiation (Fig.

2

.2d and Table

2

.1, entry 6). In similar conditions, with Pheno as sensitizer, a TON of 518 (89.3% selectivity) in ACN (Table

2

.1, entry 7) and a TON of 380 (99% selectivity) in a DMF solution were obtained (Table

2

.2, entry 20). At the same time, the faster rate for CO production than that for HCOO -formation suggest different catalytic pathways with the same catalyst (see for example entries 2-3 and 6 in

Table 2 .

 2

	1).
	2.3.2	Investigation of Co2biqpy by electrochemistry
	Electrochemical experiments including cyclic voltammetry (CV), electrolysis and
	spectro-electrochemistry as well as density functional theory (DFT) calculations were
	conducted to shed light on the catalytic mechanisms.
	As shown in Fig. 2.8, the first CV reversible wave of 1 is observed at ~-0.65 V vs.
	SCE. It is a two-electron, metal-centered wave that can be assigned to [Co I Co I L] 2+
	formation. The second wave at -0.81 V vs. SCE results from the reduction of each ligand
	moiety, leading to [Co

I Co I L

2-

]. Then the more negative waves may correspond to ligandcentered reaction. Fig. 2.8 CV of 0.5mM 1 in 0.1M n Bu4NPF6 ACN solution under Ar. v = 0.1 V/s. Adapted with permission from ref.

49

Copyright © 2019, Springer Nature.

  SCE for 3 hours (Fig.2.11), in agreement with the results ontained in photochemical conditions (Table2.1, entries 6 and 7).

	vs.	0.05	the first peak
		04	the first two peaks scan by -1.17 V
	Current / mA		scan by -1.44 V scan by -2.0 V
			Potential / V vs SCE

  While an adduct with the C atom from CO2 binding to one Co atom, and one O atom interacting with the second Co atom (Fig. 2.13b, inset), gives a C-O stretching frequency in good agreement with the experimental value.

	Natural bond orbital (NBO) analysis of the adduct between [CoCo(L)] and CO2
	(noted [CoCoCO2(L)]) shows that the Co atom bound to the C of the CO2 has
	approximately nine electrons in its valence orbitals (oxidation state 0) while the Co bound
	to the O atom has only eight electrons in its valence orbitals (oxidation state +1) (Tables
	2.4 and 2.5). This indicates that in the four-electron-reduced compound [CoCoCO2(L)],
	DFT calculations were carried out at B3LYP and M06 levels on two different
	coordination modes of CO2 with [Co I Co I (L 2-)] (noted [CoCo(L)] in the following):. (1)
	CO2 sandwiched between two Co centres and (2) CO2 coordinated to one of the Co only.
	For (1), the scaled vibrational frequencies56 of asymmetric C=O stretching are 1612
	(B3LYP) and 1615 (M06) cm -1 . For (2), the scaled vibrational frequencies of asymmetric

2.13b), which suggest a stable [CO2-4 electrons reduced complex] adduct. C=O stretching are 1,866 (B3LYP) and 1,987 (M06) cm -1 . These results show that it is very unlikely that CO2 molecules bound to only one Co from the outside of the complex cavity. two electrons are located on one Co atom, which acts as a nucleophile on binding to the C of CO2, one electron is on the second Co centre, which behaves as an electrophile to stabilize one O atom of the CO2, and the fourth electron is delocalized over the ligand core. The two metals thus act synergistically to reductively bind and stabilize the CO2.

Table 2 .

 2

	Co1	[core]4s (0.28) 3d(7.29) 4p(0.40)
	Co2	[core]4s (0.31) 3d(7.98) 4p(0.56) 5s(0.01) 4d(0.01) 5p( 0.01)

4. Natural electron configuration of [CoCoCO2(L)].

Table 2 .

 2

	Co1	Qpy1	Co2	Qpy2
	2.71	1.18	0.03	0.02

5 Calculated spin density (e -) of [CoCoCO2(L)] summed for Co atoms and quaterpyridine fragments.

Table 2

 2 the catalyst concentration to 150 µM, the selectivity for formate increased to 50%, with CO and H2 being produced in 33% and 17% yield, respectively (Fig.2.16a and Table2.6). From 5 to 150 µM of catalyst, the CO/HCOO -ratio was reduced from 18.1 to 0.65, following a mechanism for formate production involving two Co centers for binding CO2, which is favored at a high concentration of 2.

	Fig. 2.16 Yields of CO and HCOO	-vs. [Coqpy(ClO4)2] for the photocatalytic CO2 reduction of : Top,
	[Coqpy] 2+ / 0.2 mM Ru(phen)3Cl2 / 20% TEOA / 0.1 M BIH system; Bottom, [Coqpy] 2+ / 0.2 mM
	Ru(phen)3Cl2 / 20% TEA / 0.025 M BIH system. Adapted with permission from ref. 49 Copyright © 2019,
	Springer Nature.

.1, entries 1-5). Comparing with photocatalytic experiments conducted with monomeric complex 2, the reactivity is a direct consequence of metal cooperativity is further illustrated by. We found that at low catalyst concentration (5 µM) and in the presence of [Ru(phen)3] 2+ and TEOA, CO was produced with high selectivity (91%) and TON (1989), as we have already observed with various sensitizers, but on increasing

Table 2 .

 2 6 Photoinduced CO2 catalytic reduction with various concentrations of catalyst 2.

	a

a In a typical run, 2 mL of CO2-saturated ACN solution containing various concentrations of 2, Ru(phen)3Cl2 (0.2 mM), TEOA (20%, v%) and BIH (0.1 M) was irradiated using blue LED (460 nm) at 24 0 C. The TONs of products leveled off after 6.5 h, 16.5 h, 24 h and 26.7 h for entry 1, 2, 3 and 4 respectively. TON values are ±5%.

Table 3 .

 3 1 Amount of cobalt in hybrid materials determined by ICP, and equivalent Coqpy concentration in solution.

Table 3 .

 3 2 Visible light-driven CO2 reduction with Coqpy catalyst covalently attached to mpg-C3N4.

	Recently, a mixed catalyst of polymerized cobalt phthalocyanine and mesoporous
	carbon nitride has been reported to generate CO from CO2 with ca. 90 TON and a
	selectivity of 80%. 65 In this case, light adsorption of C3N4 substrate may by shielded after
	loading with the polymers. In another study, Co 2+ has been introduced into C3N4 substrate
	as single sites and resulted in CO2-to-CO conversion with a TON of 200 and a selectivity
	of 80%. 113 However, the formation of CoO oxide was observed, causing decrease
	photocatalytic activity.

a reaction conditions: 6 mg catalyst (3 µM Coqpy) in a CO2-saturated MeCN solution containing 0.05 M BIH and 0.03 M PhOH. A 6 mL quartz cell with a septum was used as the reaction vessel; and a solar simulator equipped with a 100 W Xenon lamp and a 400 nm optical filter was used as irradiation source.

Reaction time: 24 h. Amount of Coqpy in catalyst: covalently linked at mpg-C3N4 (Coqpy@mpg-C3N4), 3 µM; mixed with carbon nitride (Coqpy/mpg-C3N4), 3 µM; adsorbed at the carbon nitride surface (Coqpympg-C3N4), 2.5 µM; mixed with Al2O3 (Coqpy/Al2O3), 3 µM; covalently linked at nsg-C3N4 (Coqpy@nsg-C3N4), 9 µM. b relative to the amount of Coqpy or CoCl2. c under an argon atmosphere. d in the dark. e covalent linkage with different loading amount.

Table 3 .

 3

	3. Stern-Volmer constants for mpg-C3N4 emission quenching analysis.
	Quencher	KSV @ 490 nm	KSV @ 540 nm
	BIH	3.253  0.049	1.826  0.048
	Coqpy-Ph-COOH	12650  286	9379  165

  1 o , 10.1 o , 12.5 o , 16.1 o , 21.6 o , 23.9 o , 27.3 o , 29.9 o . Peak of C3N4 is always seen at 27.5 o , which is due to the strong diffraction peak of the (002) plane of g-C3N4 of heptazine (conjugated aromatic ring).

Table 3 .

 3

	Sample	C sp 2	C sp 3 /C-N	C-OH	Epoxy	C=O	COOH
	GA	284.4 eV	285.4 eV	286.4 eV	287.1 eV	287.9 eV	288.8 eV
		63.7 %	11.7 %	8.8 %	5.0 %	5.2 %	5.6 %
	Coqpy-	284.4 eV	285.4 eV	286.4 eV	287.1 eV	287.9 eV	288.8 eV
	GA	53.0 %	22.2 %	10.5 %	5.4 %	4.9 %	3.9 %

4

Analysis of the C 1s region for GA and GA-Coqpy. For each component, the binding energy and the amount (at. %) is provided.

Table 3 .

 3 5 Analysis of the N 1s region for GA-Coqpy. For each component, the binding energy and the amount (at. %) are provided.

	Sample	C=N	N-Co	C-N-H
	Coqpy-GA	398.6 eV	399.8 eV	401.2 eV
		26.2 %	58.1 %	15.7 %
	d. XAS analysis

Table 3 .

 3 7 CO2 reduction with Coqpy catalyst covalently attached to GA in acidic conditions under visible light (λ > 400 nm).

	a reaction conditions: 0.02 mg catalyst in a CO2-saturated ACN solution containing BIH and PhOH. A 6 mL
	quartz cell with a septum was used as the reaction vessel; and a solar simulator equipped with a 100 W
	Xenon lamp and a 400 nm optical filter was used as irradiation source.
	b relatively to the amount of Coqpy or CoCl2.

c under an argon atmosphere. d in the dark. Control experiments were designed to further evaluate the catalytic performance of GA supported Coqpy catalyst. Physically mixed GA and Coqpy and cobalt salt have been

Table 3 .

 3 8 CO2 reduction with Coqpy catalyst covalently attached to GA in basic conditions under visible light ( λ > 400 nm).

  more efficient with the [Ru(phen)3] 2+ , corresponding to a turnover number of 821 (selectivity 75.9%). At the same time, 20.4% of CO can be obtained. By employing a phenoxazine derivative (pheno) as photosensitizer, CO2 was can be converted to formate

	fluorescence emission quenching illustrated that fast and efficient electron transport
	occurred from the excited material to the catalyst.
	and CO with turnover numbers of 565 (selectivity 58%) and 284 (selectivity 29%), 3. Functionalizing carboxyl groups at the surface of graphene provided another
	respectively. While CO2 reduction affords CO with almost perfect selectivity in the option to graft the amino-modified Coqpy. In that case, the obtained Coqpy@GA can
	presence of a weak acid, phenol for example. Sensitized by [Ru(phen)3] 2+ , CO2 was convert CO2 to CO with a high selectivity of 97% (TON ca. 513) in the presence of phenol.
	converted to CO and formate in a turnover number of 829 (selectivity 96%) and 12, When replacing phenol with TEOA, CO2 can be reduced to formate in a turnover number
	respectively. Organic compound pheno can also be used as photosensitizer, resulting a of 330 and 99% selectivity. This may be related to specific coordination properties at the
	turnover number of 518 for CO in a selectivity of 89.3% with a trice amount of H2 Co metal immobilized at the surface material. Interestingly, this phenomenon bears
	produced. Through investigation conducted with cyclic voltammetry, infrared spectro-similarity to what we have observed in homogeneous photocatalysis with bimetallic
	electrochemistry and DFT calculation, we have demonstrated that such control of 2 complex. After comparing with other experimental results, a possible scheme of H2, CO,
	electrons/2 protons reduction of CO2 results from cooperativity between the 2 cobalt sites. formate formation was proposed to illustrate the different pathways, especially for CO and
	formate.
	2. By creating covalent amide bonds, solid-supported Coqpy photocatalytic systems
	have been designed and evaluated. It includes the grafting the complex with mesoporous
	carbon nitride or graphene acid, respectively. These cheap and relatively easily prepared
	photosensitizers, including Ru complex, organic compound, and semiconductive carbon materials are also excellent solid materials in the design of artificial photosynthetic
	nitride, can sensitize the bimetallic biqpyCo2 and furnish reduction products through two systems.
	electrons/two protons transfers. The catalyst can achieve selective CO2 to CO or HCOO -
	conversion upon visible light irradiation. Remarkably, by changing the acid co-substrate Grafting Coqpy with mpg-C3N4 through an amide bond led toconstructs a molecule
	present in the solution, the two synergistic cobalt atoms can steer the reactivity towards supported photocatalyst for photoreduction of CO2. Using BIH as sacrificial donor, CO
	the desired product. Selective formate production was obtained in basic ACN solution was selectively generated with a TON of 128 (selectivity 98%) in ACN solution
	containing phenol, showing an enhanced catalytic activity compared with samples
	obtained by physical mixing or absorption, along with excellent stability over time.
	Results of photocurrent response, electrochemical impedance spectroscopy and

with TEOA. The most selective formation of formate was sensitized by g-C3N4, which produced 91% formate (TON ca. 493) and 9% H2 (TON ca. 48). Meanwhile, formate can be generated It is one of the first time that such cooperativity is demonstrated in molecular multimetallic catalysts. It opens stimulating perspectives for the activation of CO2. Upon tuning the nature of the two metals: enhanced reactivity may be obtained. Upon tuning the linker between the two sub-units chelating the two metals, possible coordination of several molecules (CO2, CO) could lead to new reactivity and C-C coupling. 4. Conjugating molecular complexes with conductive materials have been proved to be beneficial for electron transfer during catalytic reaction. A preliminary study has been conducted including the preparation of Coqpy conjugated glassy catbon electrode and cyclic voltammtery test, showing catalytic activity with an early onset potential.

  absorption measurements were done with an Agilent Technologies Cary 60 or an Analytic Jena Specord 600 UV/VIS spectrophotometer. Aliquots of the solution

	6.2 Characterization Methods	364.0	3 +
							364.0	364.3
		6.2.1 4 + [BiqpyCo 2 ] 264.3 264.6 UV-visible		3 5 8 [B iq p yC o 2 (O H )] 3 + UV-visible spectroscopy 358.3 358.6 3 5 9	364.7 365.0 365.3 365.6	373.0 373.3 373.7
				264.8
								m /z
				265.1
				265.3
	2 6 2	2 6 3	2 6 4	2 6 5	2 6 6	2 6 7	2 6 8
				m/z
						358.1
							372.8	595.4
	264.3						586.4	609.5
							m/z

  Dr. Mathieu Branca and Dr. Claire Fave for their instruction and generous support during these years. Also, I owe many thanks to my laboratory colleagues Prof. Elodie, Niklas, Orestes, Heng, Claudio, Etienne, Thomas, Kristian, Tamal, Thamires, Alexis, Geordie, Min, Lydia, Benjamin, Martin, Ruwen, Marie, Dorian, Fuhuan, Daniela, Nikolaos, Anandi, Julian, who have taught me a lot in this university, for leading me into a challenging yet fascinating field of academic research. The profit that I gained from them will be of ever lasting significance to my future research. What's more, I would like to present my thanks to my friends in LEM, Justine, Hussein, Esther, Daniel, Sihem and Telmo, from whom I get tremendous love and encouragement as well as technical instruction.Above ground, I am indebted to my family members and friends, whose value to me only grows with age. They helped me and shared with me my worries, frustrations, and happiness. Choosing to start doctoral research is a decision worthy of my lifelong benefit.

	MEDA MLCT	2-(dimethylamino)ethanethiol Terms and Abbreviations metal-to-ligand charge transfer
	MS	mass spectrum
	[Q] c C E E 0 ECB Eg Ep EVB F h I I0 kq n t T v wP wR γ ΔG 0 ΔH 0 εS λ IEO2017 NHC 9CNA NHE AA NP ACN PCAT ASA PCET ATP PhOH BIH PP BNAH PS bpy QD C2H4 qpy C2H5OH QY C2H6 RDS C3H7OH SC CAT SCE CB SD CH3OH CH4 CO CO2 Coqpy CS CV cyclam DFT DIPEA WE VB UV-Vis UV TON TOF THF TFE TEOA TEA thanks go to	quencher concentration speed of light total passed charge potential redox potential conduction band potential bandgap energy energy of photon valence band potential Faradaic 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 N-heterocyclic carbine International Energy Outlook 2017 normal hydrogen electrode 9-cyanoanthracene nanoparticle ascorbic acid photocatalyst acetonitrile proton-coupled electron transfer amino salicylic acid phenol adenosine triphosphate purpurin 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole photosensitizer 1-benzyl-1,4-dihydronicotinamide quantum dot 2, 2'-bipyridine 2, 2′: 6′, 2″: 6″, 2‴-quaterpyridine ethylene quantum yield ethanol rate determining step ethane semiconductor n-propanol saturated calomel electrode catalyst sacrificial electron donor conduction band N, N-diisopropylethylamine work electrode density functional theory valence band 1, 4, 8, 11-tetraazacyclotetradecane ultraviolet-visible cyclic voltammetry ultra violet catalytic selectivity turnover number cobalt quaterpyridine turnover frequency carbon dioxide tetrahydrofuran carbon monoxide 2, 2, 2-trifluoroethanol methane triethanolamine methanol triethylamine
	λex DMF	excited wavelength N, N'-dimethylformamide
	ν EA	photon frequency electron acceptor
	τ0 EDTA	exited-state lifetime ethylenediamine tetraacetic acid
	Φ FE	quantum yield faradaic efficiency
	GC	gas chromatography
	g-C3N4	graphitic carbon nitride
	H2C2O4	oxalic acid
	HCHO	formaldehyde
	HCOO -	formate
	HCOOH	formic acid
	HOMO	highest occupied molecular orbital
	IR	infrared
	LMCT	ligand to metal charge transfer
	LUMO	lowest unoccupied molecular orbital
	MC	metallocorrole

Acknowledgements

My deep gratitude goes first to Prof. Marc Robert and Dr. Julien Bonin, who expertly guided me through my Ph.D. study and who shared the excitement of three years of discovery. Marc's early insights launched the greater part of this thesis, and Julien's mentoring and encouragement have been especially valuable. Their unwavering enthusiasm for science kept me constantly engaged with my research, and their personal generosity helped make my time at Paris enjoyable. My appreciation also extends to all the professors, researchers, and engineers in Laboratoire d'Electrochimie Moléculaire for their kindly help and training. Thanks also go to the experimental assistance work in Lavoisier and Lamark building. Particular

c. Infrared spectra IR spectra of mpg-C3N4, Coqpy-Ph-COOH and of the hybrid assembly are presented in Fig. 3.3. Both bare material and hybrid assembly show typical features of mpg-C3N4, including N-H stretching in the 3200 cm -1 region with -NH2 specific stretching around 3280 cm -1 , interchain NH•••O-C bonding around 3170 cm -1 , triazine features around 810 cm -1 as well as a broad band extending in the 1100-1600 cm -1 region that can be assigned to in-plane C-N stretching and bending vibrations of the graphitic layers. 110 In the hybrid spectrum, despite weak signals due to low catalyst loading, characteristic peaks at 1682 and 1558 cm -1 attributed to C=O stretching of the amide link and to in plane N-H deformation coupled with C-N stretching were observed.

(A) (B)

After saturating the solution with argon, the isotope experiment was conducted under 13 CO2 atmosphere. In addition to the argon signal, overwhelmingly dominant peak is 13 CO.

The peak at m/z 44 in MS spectra is 12 CO2 resulting from the air entrance during a longterm irradiation. And the 12 CO2 also generated trace amount of 12 CO at m/z 28. A small peak of formate (ca. 8.4 ppm) was observed in NMR data before catalysis, which may be issued from graphitic acid surface. Visible-light irradiation were performed under 12 CO2 and 13 CO2, respectively. After photocatalysis, CO2 was converted into formate in the presence of TEOA. Under 12 CO2, the intensity of formate peak increased; while under 13 CO2, the intensity of original peak at 8.4 ppm didn't change, but two symmetrical peaks (ca. 8.15 ppm, 8.65 ppm) were obtained, indicative of H 13 COO -.Formate is then generated through the CO2 reduction under visible-light irradiation.

Chapter 4 -New approach towards strong conjugation of molecular catalysts with (semi)conductive support

4.1 Electrochemical reduction of CO2 through strong electronic coupling of Coqpy to graphitic electrode

In our previous work, we studied molecular catalysts supported by inorganic materials, showing that the covalent bond-linked hybrids catalysts have high catalytic efficiency and excellent selectivity. To achieve efficient electron transfer between the light sensitive material and the catalytic sites, we need to have electronic interaction between the molecular orbitals of the catalyst and the conduction band of the semi-conductor.

Modulation and control of this interaction may be a way to study more thoroughly the impact of this factor and then to design even more efficient catalytic systems. Naturally, we began to seek other ways to combine the Coqpy molecule with solid material. For that, we choose glassy carbon electrode surfaces. This chapter accounts for the preliminary work we have so far done along these lines. It is a short one but since it may open original perspectives for my thesis topics, thus I decided to include it in this manuscript.

In this connection, we became interested in the original and stimulating work of Cyclic voltammetry tests illustrate that in protic electrolytes, both surface-bounded GCC-phenazine units and dissolved molecular phenazine undergo chemically reversible This simple "bottom-up" method for constructing molecularly well-defined active sites on ubiquitous graphitic solids enables the rational design of tunable heterogeneous catalysts.

The same group recently reported the condensation of fac-Re(5,6-diamino-1,10phenanthroline)(CO)3Cl to o-quinone edge defects on graphitic carbon surfaces, leading to a graphite-conjugated rhenium (GCC-Re) catalysts that is highly active for CO2 reduction to CO in acetonitrile electrolyte. 69 In this work, the well-defined and tunable electrocatalysts were again obtained on graphitic carbon surfaces, an additional illustration of this promising strategy for constructing heterogeneous catalysts. Following these early studies, we used an earth-abundant metal cobalt, that is, to conjugate the catalytically active complex Coqpy with glassy carbon electrode (GCE) and started studying its electrochemical behavior and catalytic performance.

Perspectives

The preliminary cyclic voltammetry data have sparked our interest in having a thorough look at these modified electrodes. Preliminary data are promising both in organic solvent and protic medium (not shown in this manuscript). The synthesis of conjugated systems has also been proven to be successful and it may be widely applied to many molecular catalysts. One may also envision to use different types of materials (both conductive and semi-conductive, 2D and 3D materials). Thus, further studies will help in characterizing the catalysis and getting insights into the catalytic mechanisms. This may lead to extremely stable catalytic systems, and maybe to new catalytic properties.

X-ray photoelectron spectroscopy (XPS)

The composition (contents in oxygen, nitrogen, carbon, and cobalt elements) of samples were determined by X-ray photoelectron spectroscopy (XPS) recorded on an ESCALAB 250 spectrometer (Thermo Sci.) accompanied by a microfocalized X-ray source (Kα Al 1486.6 eV) and a double monochromator.

6.2.12 Inductively Coupled Plasma -Optical Emission Spectroscopy (ICP-

OES)

Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was obtained with an iCAP 6300 ICP-OES CID spectrometer (Thermo Sci.) to determine the concentration in Co in the different hybrid materials. A test was performed with a RACHID detector including a diode array, a peristaltic pump, and argon plasma (analyzes present at 50 rpm, 1150 W). Heterogeneous samples were preprocessed by acid digestion with concentrated nitric acid. In the present case, 2.0 mg of solid sample was introduced into 1.0 mL of concentrated nitric acid and heated on a hot plate. After cooling to room temperature, the reaction solution was filtered. Then the filtrate was diluted into a 10.0 mL volumetric flask of ultrapure water for testing.

Photochemical experiments setup

Each 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 Et3N (if present) for approximately 20 min with Ar, CO2 or CO to prevent any evaporation. Then, the solution in the cuvette was irradiated perpendicularly with a Xenon lamp through an irradiation