Electrooxidation of formic acid on gold: An ATR-SEIRAS study of the role of adsorbed formate

Formate was found to be the only adsorbed species detected with time-resolved ATR-SEIRAS during the electrooxidation of formic acid on Au in acidic media. No adsorbed CO was detected, since even in CO-saturated acidic solutions the adsorption of CO on gold is negligible. The onset of formic acid electrooxidation coincides with that of formate electroadsorption, pointing to bridge-bonded adsorbed formate as the reaction intermediate. Furthermore, at constant potential the current increases quadratically with the formate coverage, which indicates that the rate-determining step in the oxidation of adsorbed formate to CO2 is a bimolecular reaction between adjacent formate species. The rate of this reaction was unaffected by potential changes, which unequivocally confirms its chemical character.

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► The electrooxidation of formic acid on gold occurs exclusively through the direct path and consists of three steps.
► The steps are the electroadsorption of formate, the decomposition of adsorbed formate, and the second electron transfer.
► The electroadsorption of formate is irreversible (i.e., it is not in equilibrium).
► The second step is the purely chemical bimolecular decomposition of adsorbed formate to CO2.
► The second electron transfer is extremely fast and, hence, does not affect the overall rate of the reaction.