Reduction to CH3OH and Alcohols

There has been much interest in reducing C02 to methanol for use in fuel cells. In one of the earliest reports, when Summers et al. used molybdenum electrodes in C02-saturated acidic solutions (pH 4.2.) at-0.7 to -0.8 V (versus SCE), methanol was formed with faradaic efficiencies approaching 100% [62]. Such efficiencies were observed to increase to 370% when the electrode was cycled at-1.2 to +0.2 V (versus SCE), indicating that the electrode was being corroded and becoming unstable. 

In an aqueous C02-saturated Na2S04 electrolyte, using electroplated Ru electrodes, Frese and Leach observed faradaic efficiencies of up to 42% for methanol production at a temperature of 333 K at a potential of only -0.55 V (versus SCE) [54]. Faradaic yields of up to 30% were likewise obtained for methane. When Popic et al. examined Ru02 electrodes, either alone or with Cu and Cd adatoms [64], in 0.5 M NaHC03 at a potential of-0.8 V (versus SCE), they were able to reduce C02 to methanol with faradaic efficiencies of 17%, 41%, and 38% after 480 min of electrolysis for Ru02, Ru02/Cu, and Ru02/Cd electrodes, respectively. 

When Bandi and Kuhne studied the reduction of C02 to methanol at mixed Ru02 + Ti02 electrodes (ratio 3 1) produced by coating titanium foil [65], in a C02-saturated KHC03 solution at a current density of 5 mA cm 2, only minimal C02 reduction was observed. However, the addition of electrodeposited Cu led to faradaic efficiencies of up to 30% for methanol at potentials of approximately -0.972V (versus SCE). Trace amounts of formic acid and ethanol were also observed. In this case, the rate-limiting step was surmised to be the surface recombination of adsorbed hydrogen and C02 to yield adsorbed COOH . 

More recently, Qu et al. examined composite Ru02/Ti02 nanotube and nanoparticle platinum electrodes [66], In a 0.5 M NaHC03 solution at -0.8V (versus SCE), the nanoparticle-based electrodes yielded faradaic efficiencies for methanol of 40%, compared to 61% for the nanotube composites However, no explanation was offered as to why the nanotube-based electrodes provided an increased catalytic activity. 

In general, few electrochemical systems have been shown to reduce C02 to higher-order alcohols, as most operate at fairly large overpotentials and produce low faradaic yields and selectivity. Details of these systems are provided elsewhere [42]. 

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