Adsorbed formate: the key intermediate in the oxidation of formic acid on platinum electrodes.

The electrooxidation of formic acid on Pt and other noble metal electrodes proceeds through a dual-path mechanism, composed of a direct path and an indirect path through adsorbed carbon monoxide, a poisoning intermediate. Adsorbed formate had been identified as the reactive intermediate in the direct path. Here we show that actually it is also the intermediate in the indirect path and is, hence, the key reaction intermediate, common to both the direct and indirect paths. Furthermore, it is confirmed that the dehydration of formic acid on Pt electrodes requires adjacent empty sites, and it is demonstrated that the reaction follows an apparently paradoxical electrochemical mechanism, in which an oxidation is immediately followed by a reduction.