Adsorbed Formate is the Last Common Intermediate in the Dual-Path Mechanism of the Electrooxidation of Formic Acid

We report a study using Pt(111) and Pt(100) electrodes of the role of adsorbed formate in both the direct and indirect pathways of the electrocatalytic oxidation of formic acid. Cyclic voltammetry at different concentrations of formic acid and different scan rates, as well as pulsed voltammetry, were used to obtain a deeper insight into the effect of formate coverage on the rate of the direct pathway. Pulsed voltammetry also provided information on the effect of the concentration of formic acid on the rate of formation of adsorbed CO on Pt(100). At low to medium coverage, increasing formate coverage increases the rate of its direct oxidation, suggesting that decreasing the distance between neighboring bidentate adsorbed formate favors its interconversion to and/or stabilizes monodentate formate (the reactive species). However, increasing the formate coverage beyond approximately 50% results in a decrease of the rate of the direct oxidation, probably because bidentate formate is too closely packed for its conversion to monodentate formate to be possible. Cyclic voltammetry at very high scan rates reveals the presence of an order-disorder phase transition within the bidentate formate adlayer on Pt(111) when the coverage approaches saturation. The dependence of the potential of maximum rate of dehydration to adsorbed CO, and of the rate at the maximum, on the concentration of formic acid is in good agreement with predictions made for a mechanism in which adsorbed CO is formed through the adsorption of formate followed by its reduction to adsorbed CO, thus confirming that monodentate adsorbed formate is the last intermediate common to both the direct and indirect pathways.