Further Insights into the Formic Acid Oxidation Mechanism on Platinum: pH and Anion Adsorption Effects

The influence of the electrolyte pH in the formic acid oxidation reaction on a polyoriented Pt electrode in the presence of different anions, including sulfate, perchlorate, phosphate and chloride, in different concentrations has been investigated, using cyclic voltammetric measurements. The curves of the peak currents in the negative scan direction vs. the pH in pure sulfate and perchlorate solutions are very similar. For these solutions, the maximum oxidation currents increase steadily with increasing pH up to pH ≈ 5, followed by a plateau until pH 10. This suggests that the reaction proceeds via a similar reaction mechanism, in which the concentration of HCOO− anions in solution plays a key role. For phosphate or chloride containing solutions, in contrast, the maximum oxidation currents show a bell-shaped pH-oxidation current correlation, whose exact shape depends on the anion concentration. We suggest that in these cases the pH-current relation is modified by competing specific adsorption of anions which act as site blocking spectator species. These results will be discussed in relation with compatible mechanistic proposals.This work has been financially supported by the MICINN (Spain) (projects CTQ2013-44083-P and CTQ2013-48280-C3-3-R) and Generalitat Valenciana (project PROMETEOII/2014/013, FEDER), as well as by the Deutsche Forschungsgemeinschaft via the project BE 1201 / 17-1