Reduction of CO2 to CO

The reduction of CO2 to CO by molecular hydrogen - that is, the reverse water-gas shift reaction (RWGSR) (Eq. 11.66) - is an important process for using CO2 via CO [4a,b,119]. Methanol (Eq. 11.67) [4b,120] or ethanol (Eq. 11.68) [4b,121] can each be synthesized from CO2 using ruthenium catalysts. 

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Iron porphyrins (containing TPP, picket fence porphyrin, or a basket handle porphyrin) catalyzed the electrochemical reduction of CO2 to CO at the Fe(I)/Fe(0) wave in DMF, although the catalyst was destroyed after a few cycles. Addition of a Lewis acid, for example Mg , dramatically improved the rate, the production of CO, and the stability of the catalyst. The mechanism was proposed to proceed by reaction of the reduced iron porphyrin Fe(Por)] with COi to form a carbene-type intermediate [Fe(Por)=C(0 )2, in which the presence of the Lewis acid facilitates C—O bond breaking. " The addition of a Bronsted acid (CF3CH2OH, n-PrOH or 2-pyrrolidone) also results in improved catalyst efficiency and lifetime, with turnover numbers up to. 750 per hour observed. ... 

In Sch. 1, if the 002 radical anion reacts with CO2 in solution, CO is formed via a dimeric intermediate in which a C—O bond is formed. A second electron transfer to this intermediate from either the electrode or 002 leads to the formation of carhon monoxide. The ultimate result of this pathway is the formation of the two-electron reduction product, carbon monoxide, and carbonate. In this reaction, a second CO2 molecule acts as the acceptor of the oxide ion formed in the two-electron reduction of CO2 to CO. Many of the features of these reactions are common to the catalyzed reactions discussed in the following. Because free C02 is not present in the catalyzed two-electron reductions of CO2 to CO, the reduction potentials can be considerably less negative than that required to form CO2-. 

The multiple steps shown in Sch. 2 and discussed earlier for various catalysts for reduction of CO2 to CO, indicates that... 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

Low-valent rhenium complexes are effective in the catalytic reduction of carbon dioxide. The conversion can be accomplished photolytically or electrochemically and is of interest with regard to fuel production and greenhouse gas remediation [9]. Electrocatalytic reduction of CO2 to CO is initiated by the reduction of fac-Re(bpy)(CO)3Cl or a related complex and can be accomplished in homogeneous solution [54, 55] or on a polymer-modified electrode surface [56]. Catalytic current... 

The use of high pressure photocatalysis resulted in greater turnover number for photocatalytic reduction of CO2 to CO with Re(bpy)(CO)3Cl and [Re(bpy)(CO)3(POiPr)3]+, in addition to greater catalyst stability [97,98]. For example, for sacrificial quenching in DMF solution the former exhibited 5.1 times the turnover number at normal pressure (0.1 Pa) using a pressure of 2.45 Pa of CO2. The latter showed a 3.8 times increase at 3.8 Pa. 

In order to produce higher-order products, there has been a focus on transition-metal-based electrocatalysts containing multiple metal centers to facilitate multielectron transfers. This approach is based on the concept that a multielectron mechanism is required to produce highly reduced species [31], However, while multielectron charge-transfer catalysts have been demonstrated to affect the 2 e reduction of CO2 to CO and formate, more highly reduced products are only sporadically observed. 

Sequestering in a cyclic environment imparts to the metal novel properties and favours redox activity. For instance, [Nin(cyclam)]2+ in 1 M HC1 is oxidised to the indefinitely stable [Nira(cyclam)]3+ complex, at a moderately positive potential (0.72 V vs. NHE) [9]. The acidic medium is required to prevent intramolecular electron transfer processes, leading to decomposition [10]. Moreover, [Nin(cyclam)]2+ can be electrochemically reduced to NiVcyclam) + at a mercury electrode, where it catalyses the reduction of CO2 to CO and HCOO (in an aqueous solution buffered to pH 5) [11]. This is nothing especially new encircling by tetra-aza macrocycles (e.g. porphyrins) is a trick known to Nature for billions of years to favour and control the redox activity of metal ions. 

After treatment with AgBF, [Cp(dppe)FcMgBr] easily reduces carbon dioxide to give the carbonyl complex [Cp(dppc)Fe(CO) BF4 [285]. The reduction of CO2 to CO by bimetallic magnesium compounds seem to be a very characteristic reaction of these types of complexes. 

Carbon monoxide dehydrogenase/acetyl coenzyme A synthase (CODH/ACS) describes two different classes of enzymes carbon monoxide dehydrogenase (CODH) isolated from Rhodospirillum rubrum or Carboxydothermus hydrogenoformans reversibly oxidizes CO to CO2 according to equation (2), and the bifunctional CODH/ACS enzyme from Moorella thermoacetica catalyzes the reversible reduction of CO2 to CO (CODH) and acetyl coenzyme assembly/disassembly (ACS) (equation 3). ... 

Extensive attempts have been made to utilize CO2, which is a nontoxic and readily available raw material, in place of toxic CO. The underlying principle is the reduction of CO2 to CO, that is, the reverse water gas shift reaction (RWGSR). In this reaction, ruthenium cluster anions exhibit high catalytic activity, and the resulting CO further reacts with hydrogen to give the products. 

The hydroformylation of alkenes using CO2 instead of CO is an attractive target reaction. Since ruthenium complexes are active catalysts for the reduction of CO2 to CO and also for hydroformylation, it is expected that the hydroformylation of an alkene with CO2 would be successful. Indeed, Sasaki and coworkers found that Ru4H4(CO)i2/LiCl catalyzed the hydroformylation of cyclohexene to give (hydroxymethyl) cyclohexane in 88% yield [141]. 

Organic synthesis via transition metal complex-catalyzed electrochemical and photochemical reduction of CO2 has been developed [2,122b, 145-147]. Among transition metal complexes, ruthenium bipyridine complexes show high catalytic activity a typical reaction is shown in Eq. 11.79. [Ru(bpy)2(CO)2] and [Ru(bpy)2(CO)Cl] efficiently catalyze the electrochemical reduction of CO2 to CO and HC02. The nature of the products is dependent upon the pH of the solution. A catalytic cycle involving [Ru(bpy)2(CO)]°, ]Ru(bpy)2(C0)(C02 )] and [Ru(bpy)2(C0)C02H] was proposed (Eq. 11.79) [1461]. 

As with Ti02, CdS can be used to photocatalyse reactions other than water cleavage. Oxidation of halide ions " proceeds smoothly at chalcogenide electrodes and n-type CdS can be used to photo-oxidize NO in the presence of iron(n) complexes. Similar studies have described the photoassisted reduction of CO2 to CO and the photo-oxidation of formic acid, formaldehyde, and methanol. ... 

Many 14-membered tetraazamacrocyclic complexes of cobalt and nickel serve as catalysts for electrochemical CO2 reduction to produce CO and H2 in water, acetonitrile-water or organic solvents [8-11]. The structures of the macrocycles are shown in Figure 2. Among these, Ni(cyclam) + is a very effective and selective catalyst for the electrochemical reduction of CO2 to CO [10, 11]. Ni(cyclam)+ ad-... 

Scheme 5. Comparison of enzymatic and chemical reduction of CO2 to CO and fixation of the reduced form.

Electrolysis of monomeric mono-bipyridine bis-carbonyl ruthenium(II) complexes bearing two tram leaving groups (e g. chloride anions or solvent molecules) generate at the working electrode a strongly adherent deep-blue film (Fig. 2A). This modified electrode demonstrate outstanding catalytic activity for the reduction of CO2 to CO (Fig. 2B) and was introduced in an effort to overcome the above limitations [10]. The overpotential was decreased to about 0.8V, and selective and quantitative formation of CO was obtained in aqueous electrolyte. 

To synthesize ethanol more effectively from CO2, the Cu-Zn-Al-K mixed oxide catalyst was combined with the Fe-based catalyst. An F-T type Fe-Cu-Al-K mixed oxide catalyst, which has been developed already in our laboratory [1], converted CO2 to both ethanol and hydrocarbons, while the Cu-based catalyst converted CO2 to CO and methanol with high selectivity. Through the combination of these two catalysts, the three functions were harmonized C-C bond growth, partial reduction of CO2 to CO, and OH insertion to the products. Furthermore, combination catalyst of Fe- and Cu-based ones was modified with both Pd and Ga to maintain the desirable reduced state of the metal oxides during the reaction. As the result, the space-time yield of ethanol was enhanced to 476 g/l-h at SV=20,000 h ... 

Performances of each catalyst is shown in Figure 1. The ethanol synthesis catalyst (Fe-based catalyst. Cat. 1) have both functions of F-T synthesis and alcohol synthesis. The main products were hydrocarbons, ethanol and methanol. With the increase of CO in reaction gas, the yield of ethanol increased[l]. The Cu-based catalyst (Cat. 2) converted CO2 to CO with selectivity more than 70% at a temperature range from 270 to 370°C, and other products were methanol and a slight amount of methane. Ethanol and C2 hydrocarbons were not produced. In order to harmonize the three functions, C-C bond growth, partial reduction of CO2 to CO, and OH insertion to products, the mixed ratio of Fe-based catalyst to Cu-based catalyst was coordinated at the range from Cu/Fe =... 

Conceptually, the simplest way to synthesize an organic molecule is to construct it one carbon at a time (Ragsdale and Pierce, 2008). The Woods—Ljungdahl pathway (Figure 15.3) does just that, synthesising acetyl-CoA from CO2. Carbon monoxide dehydrogenase, CODH and acetyl-CoA synthase, ACS are responsible for the reduction of CO2 to CO and the subsequent formation of acetyl-CoA (Drennan et al., 2004). CODH/ACS... 

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