Reaction Scheme in Nonaqueous Electrolyte

As discussed in Section VI.3, COz is not adsorbed on Pb, Hg, and Tl, and is freely present in nonaqueous electrolyte solution as well. If H2O is present in the electrolyte, a COz will react with a H2O molecule at the nucleophilic carbon atom, as shown in Fig. 11(1), further reduced to formate. The H2O molecule reacts as a Lewis acid. If H2O molecule is not available and plenty of CO2 is 

On the electrode surface of CO formation metals Au, Ag, and Zn, CO2 is stabilized by adsorption both in aqueous and nonaqueous electrolytes. In nonaqueous media, a CO2 molecule reacts as a Lewis acid with adsorbed CO2 , and allows a C-O bond of the CO2 to be broken. This process forms CO(ad) and COs as postulated by Saveant and his coworkers for electrochemical reduction of CO2 catalysed by iron porphyrins. CO(ad) thus formed is easily desorbed, as shown in Fig. 11(2). 

and In, of medium CO selectivity, do not strongly adsorb CO2 . CO2 will be mostly freely present in aqueous electrolyte owing to the hydrogen bond stabilization by water molecules high dielectric constant of water molecule will also contribute to the stabihzation of CO2 . CO2 stabihzed in the electrolyte will be further reduced to HCOO . However, CO2 is not sufficiently stabilized in nonaqueous electrolyte due to lack of hydrogen bond formation and low dielectric constant of the solvents. Thus CO2 adsorbed on Cd, Sn, and In may be relatively stabler than CO2 dissolved in the electrolyte. These metals yield CO in nonaqueous media in the same manner as Au. Ag, and Zn in Fig. 11(2). The CO selectivity mentioned above will be closely connected to the stability of adsorbed CO2 on the electrode. 

A similar case is likely to happen in 0.5 M KHCO3 aqueous solution under elevated pressures. Sakata et al. reported that an In electrode yields CO preferentially instead of HCOO at a low current density (1 mA cm ) under 20 atm. CO2 , adsorbed on the electrode with a higher coverage than under 1 atm, may react with CO2 molecules present abundantly in pressurized solution, forming CO as the major product. 

Ikeda et al. showed that addition of small amount of H2O to nonaqueous electrolyte greatly affects the product selectivity the product distributions in 0.8 to 4.2% of H2O (ca. 0.5 to 2.8-M H2O) in 0.1-M TEAP/ PC become similar to those in aqueous electrolytes obtained by the same authors (Table 10). A similar result was obtained with a Sn electrode in 0.1 M-TEAP/ AN-H2O mix-tures. CO formation at Sn electrodes is predominant in AN based electrolytes of H2O below 1000 mM (Fig. 15). HCOO for- 

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