The Impact of Water Concentration on the Electro-Oxidation of Methanol on Platinum

Abstract

The electro-oxidation of small organic molecules on platinum occurs via a mechanism that includes parallel reaction pathways and steps of adsorption/desorption and oxidation. The oxidation of adsorbed carbonaceous residues usually involves the participation of adsorbed oxygenated species. In this work, the impact of water concentration on the electro-oxidation of methanol on polycrystalline platinum was investigated, using phosphoric acid as supporting electrolyte. The bulk water concentration was varied from 14 to 50 mol L−1, and the system studied under both conventional and oscillatory regimes. Changing the [H2O]/[H3PO4] ratio implies changes in both water concentration and nature and population of electrosorbing anions. The developed strategy allowed at discriminating both effects and study in-depth the contribution of water concentration itself. Bulk water concentration was shown to play a major role on the electro-oxidation of methanol, and the lower the water concentration, the higher the potential at which the reaction sets in. Chronoamperometric data evidenced that water has a stronger impact on the reaction kinetics. The reaction order with respect to the water was estimated at different applied potentials and found to decrease as the potential increases. Concerning the potential oscillations, water concentration was found to affect the oscillatory frequency, waveform and amplitude. In summary, an increase in water concentration led to an increase in the oscillatory frequency; and the amplitude reaches a maximum when [H2O] = 30 mol L−1.