Formic acid oxidation on platinum electrodes: a detailed mechanism supported by experiments and calculations on well-defined surfaces

In spite of the fact that the formic acid oxidation reaction on electrode surfaces has been extensively investigated, a detailed mechanism explaining all the available experimental evidence on platinum has not been yet described. Herein, using a combined experimental and computational approach, the key elements in the mechanism of the formic acid oxidation reaction on platinum have been completely elucidated, not only for the direct path, through an active intermediate, but also for the CO formation route. The experimental results suggest that the direct oxidation path on platinum takes place in the presence of bidentate adsorbed formate. However, the results reported here provide evidence that this species is not the active intermediate. Monodentate adsorbed formate, whose evolution to the much more favorable bidentate form would be hindered by the presence of neighboring adsorbates, has been found to be the true active intermediate. Moreover, it is found that adsorbed formic acid would have a higher acid constant than in solution, which suggests that adsorbed formate can be originated not only from solution formate but also from formic acid. The CO formation path on platinum can proceed, also from monodentate adsorbed formate, through a dehydrogenation process toward the surface, during which the adsorbate transitions from a Pt-O adsorption mode to a Pt-C one, to form carboxylate. From this last configuration, the C-OH bond is cleaved, on the surface, yielding adsorbed CO and OH. The results and mechanisms reported here provide the best explanation for the whole of the experimental evidence available to date about this reaction, including pH, surface structure and electrode potential effects.