Influence of Anion Adsorption on the Parallel Reaction Pathways in the Oscillatory Electro-oxidation of Methanol

We report the experimental study of the impact of anion adsorption on the two parallel pathways of CO₂ formation during the electro-oxidation of methanol on platinum. The effect of nature of the supporting electrolyte (HClO₄ and H₂SO₄) was investigated at two methanol concentrations. Voltammetric profiles and oscillatory time series of the electrode potential were registered in conjunction with the evolution of the production of carbon dioxide and methylformate, as measured by means of <i>on line</i> differential electrochemical mass spectrometry (DEMS). In all conditions studied, the production of CO₂ was higher in the presence of HClO₄ rather than that in H₂SO₄. Importantly, the inhibition caused by anion adsorption was generally more pronounced in the direct pathway, i.e., the non-CO_ad pathway. Furthermore, we have noted an additional peak commonly observed at high potentials during the oscillatory electro-oxidation of small organic molecules can be generally attributed to the oxidative removal of CO_ad. Altogether, the effect of anion adsorption is discussed in connection with both the oscillatory kinetics and the dynamics of adsorbed species, contributing thus to a comprehensive physical-chemistry description of the surface processes