Oxydation de l'acide formique sur électrode d'or

Abstract

The anodic oxidation of formic acid on gold electrodes has been investigated in aqueous sulphate electrolyte as a function of pH (0.2 to 8.0), and temperature (30°C to 80°C). The faradaic yield was found to be 100% according to the overall reaction HCOOH=2H aq + +2 e − +CO 2 . Voltage sweep measurements give current vs. potential curves independent of sweep rates (from 10 mV s −1 to 10 V s −1 ). Mechanistic criteria have been deduced from the transfer coefficient, the heat of activation, and the reaction orders with respect to formic acid and H + ion. Around the pH corresponding to the p K (≈3.8), the transfer coefficient varies varies sharply from 0.20 to 0.50, and the reaction order with respect to H + from −0.65 to 0.15. These results have been attributed to a change in the reaction mechanism due to a change in the electroactive species: undissociated formic acid HCOOH for pH<p K and formate ion HCOO − for pH>p K . In acid medium a mechanism involving the dissociation of formic acid into an adsorbed formate anion and a solvated proton is proposed. This equilibrium is followed by a rate determining one electron transfer step. In neutral medium the overall reaction is controlled by the adsorption of the HCOO aq − anion.