Direct evidence of a triple-path mechanism of formate electrooxidation on Pt black in alkaline media at varying temperature. Part I: The electrochemical studies

Abstract

Electrooxidation of formate on high-surface Pt black in alkaline media has been studied at varying temperature by means of cyclic voltammetry and stripping voltammetry. In the positive-going scans from 0.10 to 1.2V vs RHE, the formate oxidation produces three oxidation current peaks: (i) peak I (at potentials where the coverages of both surface hydrogen and oxygen-species are very low), (ii) peak II (exhibiting obvious potential shift from 0.66 to 0.51V upon increasing temperature from 20 to 80°C), and (iii) peak III (at higher potentials where a considerable formation of surface oxygen species commences). Both peaks I and II are closely correlated but they are independent of peak III. Among the three peaks, the temperature dependence of peak II is well in agreement with that of the stripping peak of a CO adlayer. These results suggest a triple-path reaction mechanism. Adsorption of formate onto Pt surfaces may result in formation of precursor adsorbates with different reactivity. Analogous to the reported dual-path mechanism, active precursor adsorbate is responsible for (i) a direct path involving the formate oxidation to CO2 (leading to peak I), and (ii) an indirect path involving the formation of surface CO and its further oxidation to CO2 (leading to peak II). An independent third path via oxidation of less-active precursor adsorbate to CO2 with adsorbed HCOO as the most likely intermediate accounts for peak III. All the oxidation reactions involved in the triple paths are accelerated by increasing reaction temperature with different apparent activation energies. At elevated temperature, diffusion-limited oxidation currents are attained. It is suggested that both the activities of surface OH and precursor adsorbates play a major role in mediating the reaction mechanism as well as participating in the formate oxidation.