Kinetic and Mechanistic Investigation of Methanol Oxidation on a Smooth Polycrystalline Pt Surface

The rotating disk electrode technique was employed to investigate the methanol electrooxidation reaction (MOR) on a polycrystalline Pt surface in acidic medium to elicit quantitative kinetic information and to investigate the MOR reaction mechanism. The Tafel slope was 125 mV/decade, suggesting that the first electron transfer step was the rate-determining step in the potential range from 0.3 to 0.6 V vs. RHE. An isotope exchange study performed to elucidate the mechanism unequivocally indicated the C-H bond cleavage was the rate-determining step. The number of electrons transferred increased gradually with the applied electrode potential, evidencing a change in the MOR mechanism, from the indirect pathway to the direct pathway involving the formation of adsorbed CO intermediate. The apparent methanol diffusion coefficient was evaluated using chronoamperometry and was found to be 5.37 X 10(-12) cm(2)/s -6 orders of magnitude lower than literature values for methanol/acid solutions. However, this value was deemed to be accurate as it yielded much more accurate estimations of the number of electrons transferred. A thin film diffusion model, involving the counter diffusion of gaseous CO2 and other intermediates in a thin film adjacent to the electrode surface, was proposed to explain the observed methanol diffusion coefficient. (C) 2013 The Electrochemical Society. All rights reserved.