The mechanism of the electrocatalytic oxidation of formic acid on metals

We present a detailed spectrokinetic study of the electrocatalytic oxidation of formic acid on Au and Pt electrodes using ATR-SEIRAS that has allowed us to unveil the mechanisms of both the direct (in which adsorbed CO is not involved) and the indirect (through adsorbed CO) paths of the reaction with unprecedented detail. Au electrodes were used to study the mechanism of the direct path without the interference of the indirect path, and the observed quadratic dependence of the reaction rate on the formate coverage was then shown to apply also to Pt. The direct path consists of three steps, namely, (i) the electroadsorption of formate (corresponding to the first electron transfer), (ii) the purely chemical bimolecular decomposition of adsorbed formate, and (iii) the second electron transfer. The dehydration of HCOOH to adsorbed CO, that is then oxidized to CO2 in the indirect path, was studied on Pt at E < 0.4 V vs the reversible hydrogen electrode (RHE), at which potentials the dehydration reaction is the only one taking place on the Pt surface. Our results show that adsorbed formate is also the intermediate in the dehydration of formic acid to adsorbed CO and is, hence, the key intermediate in the electrocatalytic oxidation of formic acid on metals.