Proton and CO2 Reduction

In this chapter, we survey recent efforts to produce biologically inspired catalysts for proton and CO2 reduction. First, we introduce the biological catalysts that underpin this research hydrogenases, carbon monoxide dehydrogenases, and formate dehydrogenases. Then, we describe some of the most catalytically successful synthetic molecules that have been inspired by these biological catalysts, paying particular attention to catalytic properties and mechanism. 

Table 5.1 Selected single- and multielectron formal reduction potentials for water oxidation and proton and CO2 reduction reactions, in water at pH 7. (P. M. Wood, Biochem. J., 1988, 253, 287 E. Fujita, Coord. Chem. Rev., 1999, 185-186, 373.)...

A newly-designed photoelectrocatalytic (PEC) reactor for CO2 reduction, which combines photocatalysis by Ti02 and electrocatalysis by carbon nanotubes (CNT), has recently been proposed (Fig. 7) [152]. A proton-conductive Nafion membrane connects the Ti02 and CNT. Irradiation of the combined system of nano-structured Ti02 deposited on a metal Ti electrode with Pt modified CNT deposited on carbon sheet caused water splitting to H2 and O2. A half-cell for the cathodic electrode, i.e., Pt or Fe modified CNT electrode, produces various organic molecules such as 2-propanol due to electrocatalytic reduction of CO2 on the electrode. The proposed PEC reactor is incomplete in its present state. However, these systems are expected to couple water splitting and CO2 reduction, and thus it may establish a new artificial photosynthetic system. 

In this section we describe dark catalysis (water oxidation, proton reduction and CO2 reduction) and photoexcited state electron transfer carried out in polymer matrixes towards artificial photosynthesis. 

Scheme 8.4 Competitive coordination to a metal centre and metal-catalysed reduction reactions implying the proton and CO2. The formation of H2, CO and HCO2H are highlighted...

While one-electron reduction of a proton and CO2 (eqs 1 and 6, respectively) and oxidation of water (eq 2) take place at very negative and positive potentials, respectively, the coupled multi-electron and multi-proton reactions occur at relatively modest potentials (eqs 3-5,7-12). 

The process is in fact just the reverse of a fuel cell for methanol oxidation, and the two photocatalytic processes (water oxidation by WO3 and CO2 reduction Zn-Cu-Ga LDH) require light irradiation and should occur at the same rate transferring electrons (through the metallic conductor) and protons (through the membrane). 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

Carbon dioxide reduction is thought to proceed via metallocarboxylate intermediate (s) formed by coordination of CO2 to the electron-rich Re center, although discrete steps in the process cannot be unambiguously assigned. The timing of Cl displacement from and CO2 adduction to the Re(bpy) (CO)3 unit are important mechanistic parameters. Most interpretations are based on a one-electron pathway, involving the interaction of CO2 with the product of Eq. (5) a two-electron pathway, involving interaction of CO2 with the product of Eq. (6) or a combination of these steps. Additional mechanistic considerations are the role dimeric rhenium intermediates and likely proton sources. 

Reaction (1) is one of the most elementary chemical reactions and is of central importance for energy research as it provides H2, which either serves as a fuel by itself or acts as a precursor for further reduction reactions (e.g. in the activation of O2, CO2, and N2 - reactions [3], [4], and [6]). Ideally, the protons and electrons required for... 

Although hydrogenase linked H2 production does not require ATP utilization, normal aerobic fixation of atmospheric CO2 does. As will be discussed below, when CO2 fixation does not occur (as is the case under anaerobic, sulfur-deprived condi tions), the accumulation of ATP molecules in the stroma inhibits ATPase function. This results in the non dissipation of the proton gradient and causes the build-up of the proton motive force. It has been shown that, under these conditions, photosynthetic electron transport is down regulated917 and consequently reductant is not available for efficiently producing H2.140... 

Both electrochemical and photochemical reduction of Re(bpy)(CO)3X (X = halide), yield [Re(bpy)(CO)3X] [54]. It has been suggested that [Re(bpy) (CO)3X]- reacts directly with CO2 after loss of a CO [56] however, electrochemical results [54, 61] indicate that CO2 attachment only takes place after loss of X". The labilization of X produces either a five-coordinate [Re(bpy)(CO)3] or the six-coordinate [Re(bpy)(CO)3S]. The five-coordinate species can dimerize but under photolysis conditions their low concentration makes this reaction slow. The intermediates may react with CO2 or pick up a second electron and a proton to give Re(bpy)(CO)3H. The formation of Re(bpy)(CO)3H and Re(bpy)(C0)3(02CH) has been reported for Re(bpy)(CO)3X (X = Cl and Br) systems [38, 54, 55, 57, 58, 66]. 

CH3-S-C0M is reduced to methane via the heterodisulfide of H-S-CoM and H-S-HTP. The reduction of the heterodisulfide has been shown to be coupled with ATP synthesis according to a chemiosmotic mechanism (see above). The electrons required for the reduction are derived from the oxidation of enzyme-bound CO ([CO]) which is oxidized to CO2 via CO-DH. It is assumed that electron transport from [CO] to the heterodisulfide is coupled with the generation of an electrochemical proton potential which then drives ATP synthesis. Possible eleetron transport components, a cytochrome b and a membrane-bound hydrogenase, have been identified [232]. Probably two H" -translocating sites are present in electron transport from CO to the heterodisulfide the oxidation of CO to CO2 and H2, and the reduction of the heterodisulfide (or methyl-CoM) by H2. Both H2 and... 

This result indicates that HjO acted as a proton donor in an AN solution for hydrocarbon production. A naked p-Si electrode gave mainly H2 together with small amounts of CO, HCOOH and CH4, indicating that the Si surface has low catalytic activity for CO2 reduction. The particulate-Cu/p-Si... 

It has been reported that the concentration of proton and adsorbed hydrogen can be controlled by adjusting the anodic and cathodic bias in the pulsed method [7]. The hydrogen adsorbed on the electrode surface seems to interrupt the reaction for the electrochemical reduaion of COj. The CO2 coverage on the electrode surface may be increased by the elimination of adsorbed hydrogen during anodic period. In the subsequent cathodic period, the electron transfer to CO2 was promoted, yielding CO2 radical anions. The selectivity of products for the electrochemical reduction of CO2 was determined in association with electrode material and CO2 radical anion [10,11]. CO is intermediate species in the reaction process of hydrocarbonization [8]. 

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