Electrocatalytic Oxidation of Formate and Formic Acid on Platinum and Gold: Study of pH Dependence with Phosphate Buffers

摘要

The effect of electrolyte pH on the electrooxidation of formic acid/formate is studied using cyclic voltammetry on polycrystalline rotating disk and single-crystalline Pt and Au electrodes in phosphate-based solutions over a wide range of pH (1-12). A non-linear relationship between oxidation current and pH at constant overpotential is identified for both metals. Surface structure influences the reaction for both Pt and Au electrodes. The results in terms of pH dependence are in agreement with those reported in literature. However, experimental evidence shows that adsorbed phosphates cause dramatic changes in the behaviour of the oxidation of formic acid on Pt and Au electrodes due to site blocking and competitive adsorption. The pH dependence on the catalytic activity for formic acid oxidation on Pt is more complex, due to the poisoning of the electrodes by adsorbed CO in addition to intricate anion adsorption effects. The role of the phosphates in the electrocatalyzed reaction is more than maintaining the pH of the system. Rather, various phosphate anions strongly adsorb on the surface, block reactive surface sites and quantitatively decrease oxidation currents. The blocking effect of the phosphate anions increases with increasing pH value. A more considerable blocking effect is established for Au. In addition, a strong pH dependence on overpotential is identified for Pt. In general, oxidation kinetics of formic acid depends strongly on pH, the nature of the adsorbed phosphate species and the electrode potential.