Reduction of CO2 to formic acid

The cationic complex [Rh(nbd)(PMe2Ph)3](BF4) catalyzes the oxygenation of several ethers (Eq. 7) by O2/CO2 mixtures with moderate turnovers (1,000 in 8 days at 60 °C for y-butyrolactone) [36], accompanied by the reduction of CO2 to formic acid. In the absence of CO2, 2-hydroperoxidetetrahydrofuran is the major product. 

The solubility of carbon dioxide in aqueous and non-aqueous solutions depends on its partial pressure (via Henry s law), on temperature (according to its enthalpy of solution) and on acid-base reactions within the solution. In aqueous solutions, the equilibria forming HCO3 and CO3 depend on pH and ionic strength the presence of metal ions which form insoluble carbonates may also be a factor. Some speculation is made about reactions in nonaqueous solutions, and how thermodynamic data may be applied to reduction of CO2 to formic acid, formaldehyde, or methanol by heterogenous catalysis, photoreduction, or electrochemical reduction. 

Since the overall reaction is a two-electron reduction, it is desirable for the oxidized metal complex to undergo a reversible two-electron reduction as shown in Step 4 of Scheme 1. At pH 7 the standard potential for the reduction of CO2 to formic acid is -0.61 vs NHE ( ) or -0.85 vs SCE. Since it would be desirable if the reduction of CO2 could ultimately be carried out in aqueous... 

Since that time there have been many reports of intramolecular hydridic-protonic bonds [78-86]. Recently an intermediate with an intramolecular RuH—HN interaction has been implicated in the catalytic asymmetric reduction of ketones [87] and another, in the reduction of CO2 to formic acid [83]. In the last process, the RuH—HN bond is proposed to form via the heterolytic splitting of dihydrogen (see Scheme 4, Section 1.9). 

GDEs have been used successfully in the electroreduction of CO2 to formic acid on tin or lead cathodes, in aqueous solution [57,135,136]. Hence, Kwon and Lee [136] reported the efficient use of nanolayered lead electrodes, prepared by stepwise potential deposition. By this technique, a nanostructured Pb layer forms, which consists of particles and platelets in hexagonal and cubic crystalline form. In electroreductions conducted in aqueous 0.1 mol/ dm KHCO3 supporting electrolyte, at 10 mA/cm current density and a temperature of 5°C, cubic lead surface secured the highest Faradaic yield (94.1%), while polycrystalline smooth Pb films enabled only 52.3% yield. The authors suggested that indirect reduction of CO2 by adsorbed hydrogen atoms (shown in Equations 1.16a, 1.16b and 1.16c) is more likely than direct electroreduction of CO2 molecules. 

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

Other model reactants are simple organic molecules, for example, formic acid [381, 382]. Pt(lll) exerts lower catalytic influence on HCOOH oxidation than do Pt(lOO) and Pt(llO) faces. However, in the presence of Pb adatoms on Pt(lll) a strong catalytic influence has been observed [383]. The poisonous species production in HCOOH oxidation is then inhibited. Electrochemical reduction of CO2 to glycolate/glyoxylate and oxalic acid has been studied [384]. Other products such as formic acid accompanied by CO and methane have also been detected [385]. In the latter case, the efficiency of the competing process of hydrogen evolution has been suppressed to less than 3.5%. 

Of considerable importance are the reports on the photochemical fixation of CO2 and the photoreduction of N2 that have appeared this year. In many respects, these reduction processes offer more potential for the storage of solar energy than does the photoreduction of water to H2. Several developments in this area have been noted recently. Thus, it has been reported that certain zinc porphyrins can fix CO2 upon irradiation with visible light. It has been claimed that reduced will reduce CO2 to formic acid with an overall quantum... 

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. ... 

CO2 reduction proceeds readily to formic acid on most metal electrodes, and formic acid reduction proceeds most rapidly on electrodes with high hydrogen overvoltage such as lead, tin, and indium this appears to be related to the stability of intermediates (22, 23). 

The involvement of the M°P species in the mechanism was also confirmed by van Behar et Co P species formed from reduction of Co porphyrin by electrochemical, photochemical, and radiation methods was found to be unreac-tive towards reduction of C02. But one electron reduction of the Co P resulted in a species which bound CO2 and reduced it with the formation of CO and formic acid as products However, Riqelme et al. reported that the Co°/Co couple (rather than the M°/M couple) was responsible for the catalytic reduction of CO2 to CO and formic acid by polymeric CoTAPP (on GCE). The monomeric form of the catalyst did not catalyze the reductionGrodkowski et al. found the Co and Fe complexes to be the active species when Co and Fe corroles electrocat-alyzed the reduction of C02 . The Fe corroles showed better catalytic activity than the Co corroles. The catalytic behavior of the corroles towards the reduction of CO2 is different from that of MP complexes in that the latter do not react with CO2 until they are reduced to beyond state . 

Yet another interesting application of complex 51 [109] was described by us in the context of transfer hydrogenation processes. Compound 51 was found to be active in the reduction of CO2 to formate using isopropanol as the hydrogen source (Equation 10.1) [110]. This unprecedented reaction is interesting because it uses an inexpensive and environmentally friendly hydrogen source and provides an easy access to formic acid and sodium formate. 

Single electron reduction of CO2 to C02 (112) is the first, rate determining, step in multielectron electrochemical reduction of CO2 to other valuable species, such as formic acid. 

The tendency of a transition metal hydride to transfer H to a substrate is called hydricity [ 12]. It is possible to determine the Gibbs free energy of the splitting of the covalent polar M-H bond to afford a metal cation and the hydride ion in solution. The hydricity is not parallel to the polarity of the M-H IxMid, nor can it be predicted on the basis of the electronic structure of the metal atom. It is a complex property that can be modeled for transition metal hydrides using multiparameter approaches. The hydricity concept applies to the interaction of M-H bonds with CO2 as well [13]. The reactivity of M-H bonds toward CO2 is linked to reactions that may have industrial interest, such as the hydrogenation of CO2 to afford formic acid (4.2) and the electrochemical reduction of CO2 to other Cl or C1+ molecules (4.3). 

Since 1994, pyridinium and its substituted derivatives have been identified as effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of CO2 to various products, such as formic acid, formaldehyde and methanol. Particularly high Faradaic yields were reached in the reduction of CO2 to methanol in both electrochemical and photoelectrochemical systems under energetically advantageous conditions [149]. 

As already mentioned before the elaitrochemical reduction of CO2 at a metal electrode leads only to the formation of formic acid. Recently it has been reported by Ogura et al. (see and literature cited therein), however, that at a Pt-electrode coated by a layer of Everitt s salt (ES), K2Fe(II)[Fe(II) (CNg)], CO2 is selectively reduced to methanol in the presence of metal complexes as homogeneous catalysts and a primary alcohol. The overall reaction is given by... 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

On the other hand, in two other papers, the formation of hydrogen gas was not mentioned, whereas carbon monoxide and formic acid were both observed as products. In studies carried out by Ogura and coworkers [123], electrogenerated [Co(PPh3)2L] (where L is a substituted quinoline, bipyridine, or phenan-throline moiety) was employed as a catalyst for the reduction of CO2 in anhydrous organic solvents, conditions for which the current efficiency for production of CO (the main product) was 83%. Similarly, in an investigation done by Behar et al. [124], who used cobalt porphyrins as catalysts in an acetonitrile medium, the formation of both carbon monoxide and formic acid was noted however, the catalytic species did not appear to contain cobalt(I), but rather a cobalt(O) species complexed with carbon dioxide. 

Other metal complexes such as 2,2 -bipyridine complexes of Rh and Ir are efficient electrocatalysts for the reduction of C02 in acetonitrile.134 In the production of formate the current efficiency is up to 80%. Electrochemical reduction catalyzed by mono- and dinuclear Rh complexes affords formic acid in aqueous acetonitrile, or oxalate in the absence of water.135 The latter reaction, that is, the reduction of C02 directed toward C-C bond formation, has attracted great interest.131 An exceptional example136 is the use of metal-sulfide clusters of Ir and Co to catalyze selectively the electrochemical reduction of C02 to oxalate without the accompanying disproportionation to CO and CO2-. 

Numerous studies have been made on the electrochemical reduction of CO2 under high pressure on various electrodes in an aqueous electrolyte. Productivity increases substantially at 30 bar of CO2 with respect to that at 1 bar of CO2. However, the total cathodic current barely increases with increasing CO2 pressure. In terms of products, CO, H2, and formic acid are mainly observed. A fast deactivation is also typically present. 

Figure 5.1.6 Comparison of the energy efficiencies and current densities for C02 reduction to formic acid, syngas, and hydrocarbons (methane and ethylene) reported in the literature with those of water electrolyzers. Efficiencies of electrolyzers are total system efficiencies, while the CO2 conversion efficiencies only include cathode losses and neglect anode and system losses. Adapted from [17],...

A nice example of a homogeneous catalytic hydrogenation is the reduction of carbon dioxide to formic acid, carried out in SC-CO2 in the presence of a soluble ruthenium(II)-trimethylphosphane catalyst and triethylamine at 50 °C and 21 MPa... 

---