Thermal Reverse Water-Gas Shift Catalysis

Scheme 27: Oxidized and reduced state of the active site of [NiFe] CODH and [Mo/W] FDH.

Formate dehydrogenases (FDH) can be separated into two classes. Metal-independent formate dehydrogenases make use of the ability of the NAD⁺/NADH couple to mediate hydride transfer and irreversibly oxidize formate.^[167] In contrast, anaerobic molybdenum and tungsten based FDH are capable of the reversible interconversion of CO2 and formate.^[169] In the reduced form, the active site consists of a single metal oxide or sulfide coordinated by two bidentate pyranopterin based ligands (Scheme 27).^[170]

Arginine, histidine and (seleno-)cysteine are present in the second coordination sphere of the metal center and crucial for proton transfer to the substrate. CO2 activation proceeds by PCET from multiple sites resulting in a metal formate which forms the oxidized active site after formate dissociation.

2.1.3.2 Thermal Reverse Water-Gas Shift Catalysis

Heterogeneous catalyst systems are more popular than homogeneous complexes in thermal rWGS catalysis.

Research on heterogeneous rWGS catalysis has shown activity of copper and platinum based materials.^[151]

Copper surfaces catalyze the rWGS reaction at low temperature with high selectivity. Unfavorable CO2

dissociation on the copper surface results in the need for surface activation.^[151] This can be achieved by increasing the H2 content of the feed, which however results in formation of methane as byproduct.^[171]

Association of surface-bound hydrogen atoms and adsorbed CO2 is postulated to give formates as intermediates in Cu catalyzed rWGS.^[171] While pure platinum does not catalyze the rWGS reaction, the combination with a support like CeO2 gives an active catalyst. The mechanistic understanding is that deoxygenation of CO2 takes place on the support at a surface vacancy and that carbonates are formed as main intermediates.^[172] Hydrogen is activated on Pt and regenerates the oxygen vacancies by formation of

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water. Accordingly, in both Pt and Cu based systems, oxygen bound species are formed as carbon monoxide selective intermediates.

While several homogeneous catalysts, mainly based on transition metal carbonyls, are reported for the WGS reaction, the microscopic reverse is investigated less thoroughly.^[173] Even though most WGS catalysts should in principle also catalyze the reverse reaction, reports are limited to trimetallic and monometallic ruthenium carbonyls by Tominaga.^[174,175] In both cases harsh conditions (p ≥ 60 atm, T ≥ 160°C) are needed.

While thermodynamic considerations of the rWGS discussed in the previous chapter suggest a thermodynamically uphill reaction under these conditions, the driving force is affected by solvation of the gases.^[156] For the homogeneously catalyzed WGS equilibrium the general accepted mechanism involves formation of a carboxylate as key intermediate (Scheme 28).[155,156,173] This species is formed via the Hieber base reaction of a metal carbonyl in basic aqueous solution. Decarboxylation yields a metal hydride which regenerates the metal carbonyl upon hydrolysis, forming H2 and closing the catalytic cycle.^[176]

Scheme 28: Mechanisms for CO and formic acid selective hydrogenation of carbon dioxide.

Reversal of the reaction following the same mechanism requires formation of the hydroxycarbonyl by reaction of the metal hydride with CO2. This so-called ‘abnormal insertion’ is unprecedented in the literature and Tominaga discusses CO2 coordination instead of insertion in his proposed mechanism, giving a ruthenium hydrido carboxylate as alternative species.^[156,175] The lack of reported catalysts for homogeneous rWGS catalysis can be understood as a consequence of ‘normal’ CO2 insertion to formates which are in general considered intermediates in formic acid selective catalysis (Scheme 28).^[156] While metal hydroxycarbonyl intermediates are generally seen as intermediates in CO selective reduction of CO2 in homogeneous catalysis, hydrogenation of carbon dioxide to CO via metal formates is accepted in heterogeneous catalysis. A metal formate mediated mechanism is also discussed for homogeneous systems, however lacks unambiguous experimental evidence in rWGS catalysis.^[155,173] Notably, the stoichiometric

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dehydration of formic acid at transition metal complexes is reported and the hydration of carbon monoxide in aqueous solvent proceeds via formic acid as intermediate.[154,177,178]

2.1.3.3 (Photo-)Electrocatalytic CO2 Reduction to CO

Since hydrogen gas is produced by steam reforming, hydrogen production by proton reduction from renewable energy sources at economic cost is required to use CO2 as environmentally friendly carbon feedstock for hydrogenation. An alternate approach to carbon dioxide reduction features the use of photo- and electrochemical methods, providing the necessary driving force for the uphill reaction. Electrochemical reduction of CO2 is catalyzed by several transition metal based heterogeneous materials. Electrodes based on Cu, Ag, Au or Zn show high selectivity in CO formation upon electrolysis in KHCO3 aqueous solution.

Further, adsorbed CO is often regarded as intermediate in reduction to lower oxidation states, including C2 products.^[179,180] In a computational study, Nørskov considers surface bound hydroxycarbonyl species as possible intermediate of formation of both, formic acid and carbon monoxide.^[181] A recent report by Züttel on Cu/In nanowires deposited on copper as electrode material rather suggests a surface-bound formate to yield formate as product.^[182] Whether adsorption of the intermediate proceeds by initial adsorption of CO2

or protons, or by a concerted reaction is not addressed in these studies.

Turning to heterogeneous photoelectrocatalytic carbon monoxide generation, the most relevant systems are semiconductor based photoelectrodes.^[183] While the semiconductor generates an electric field upon illumination, an external potential is usually applied. Photocathodes based on boron-doped p-Si are reported to catalyze selective reduction to CO in a DMF/water mixture.^[184] The system needs potentials close to E⁰(CO/CO^) to operate, indicating reduction of carbon dioxide in solution and subsequent decomposition giving CO. Photocathodes based on III/V semiconductors are able to form CO as well. Research on p-GaP shows CO formation in the presence of water.^[184] Computational and experimental analysis of the interaction of the semiconductor with water indicates formation of surface-bound hydrides which are discussed to be relevant for CO2 activation.^[185–187] Finally, n-type TiO2 based photoelectrodes of different morphology reduce CO2 to CO by substrate coordination at oxygen-vacant sites and subsequent electron transfer to the substrate which induces C-O bond dissociation.^[188,189] Notably, for both p-Si and TiO2

photoelectrodes, the mechanism of photoelectrocatalytic CO2 reduction to CO is characterized by the one-electron reduction of CO2 and subsequent dissociation of the carboxylate.

Cobalt and nickel complexes containing 1,4,8,11-tetraazacyclotetradecane (cyclam) based ligands are known to catalyze electrochemical CO2 reduction in acidic media upon electron transfer from an electrode or a photosensitizer.^[190,191] Importantly, in both platforms the formation of hydrides is not assumed to contribute to CO selective reduction of carbon dioxide.^[192] After initial reduction of the Co^II precursor, Co^I complexes readily bind CO2 via the carbon atom. Investigation of the electronic structure suggests one

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electron transfer from the metal to the substrate upon binding and a more reducing metal center results in an increase in the CO2 binding constant.^[192] Coordination to the metal center greatly facilitates one electron reduction of CO2 as the Co^II/Co^I couple is found at up to E⁰(Co^II/Co^I)^aq = -0.1 V vs. NHE for certain derivatives (E⁰(CO2)^aq = -1.90 V vs. NHE, pH = 7).^[192] Upon axial coordination of a solvent molecule, the dxz molecular orbital (MO) rises in energy, participating in  backbonding from the metal to the carboxylate.

As consequence, an additional intramolecular electron transfer results in twofold CO2 reduction. Slow conversion of [Co(CO2)L4]⁺ to carbonyl [Co(CO)L4]⁺ and carbonate is observed, alternatively generation of [Co(CO)L4]⁺ can be accelerated by addition of acid. Metal centered protonation of Co^I complex [CoL4]⁺ is possible, resulting in production of H2 and formic acid as side product by insertion of CO2 into the Co-H bond (Scheme 29).

Scheme 29: Selectivity in substrate activation at reduced metal cyclam complexes relevant for the product distribution in (photo-)electrocatalytic CO2 reduction.

Nickel cyclam complexes in contrast show high selectivity in CO2 over proton reduction.^[191] In general, CO is the favored product of CO2 reduction by [NiL4]²⁺, however depending on the solvent, formate is produced in significant amounts.^[193] As in case of cobalt as metal center, initial electron transfer gives a reduced metal center. However, Ni^I complex [NiL4]⁺ shows lower CO2 binding constants compared to the cobalt complexes and less intramolecular electron transfer to the substrate.^[192] Computational analysis shows ¹ -CO2 coordination as thermodynamic product of substrate activation at the Ni^I oxidation state.^[194] Formate production is attributed to the unfavorable  ¹-OCO coordination, therefore no hydride is involved here (Scheme 29). Neese proposes proton-coupled electron transfer as second reduction step for both isomers, giving a Ni^II hydroxycarbonyl [Ni(CO2H)L4]²⁺ as intermediate for carbon monoxide selective reduction.

Carbon bound carboxylate species are common intermediates in (photo-)electrochemical CO2 reduction by metal cyclam complexes and related systems like iron porphyrins.^[195,196]

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The lack of H2 formation by [NiL4]²⁺ is attributed to unfavorable hydride formation due to low basicity of the nickel center.^[190] The outstanding selectivity compared to the related cobalt complexes is discussed by Rodgers.^[197] In both cases, the one electron reduced precatalyst is active in substrate binding. While the initial reduction potential E⁰(M^II/M^I) is nearly invariant for both metals, the relevant cobalt complexes are oxidized to Co^III much easier than analogous nickel complexes.[192,197,198] Accordingly, protonation of the active species in case of cobalt gives a highly reactive Co^III hydride, while protonation the Ni^I species is unlikely. Speaking more generally, one electron reduction of the M^II precatalyst gives a one-electron reductant in case of nickel, which is able to coordinate CO2, however requires another reducing equivalent for turnover. In case of cobalt, a two-electron reductant results which efficiently reduces both carbon dioxide and protons. Bullock and DuBois have shown, that efficient nickel based electrocatalysts for proton reduction can be designed by including basic amines in the second coordination sphere, leading to ligand based protonation after reduction to Ni^I.^[199,200] Turning to Fe^II porphyrins as precatalyst for electrochemical CO2 reduction, the active formal Fe⁰ species is formed by two electron reduction of the precursor. As a result, slightly more cathodic potentials are needed for catalysis compared to the Ni and Co complexes discussed previously. Since the active catalyst is a two electron reductant, proton reduction is a side reaction and in case of strong acids, protonation is observed even at the Fe^II/Fe^I potential.^[195]

Photoelectrocatalytic systems are mostly based on a homogeneous catalyst immobilized on an (photo-)electrode surface. While immobilization is an active field of research in (photo-)electrocatalysis, these systems will not be discussed here, given the similarities in the CO2 activation mechanism compared to the parent homogeneous catalysts.