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Kinetics and Electrocatalysis of Methanol Oxidation on Electrodeposited Pt and Pt70Ru30 Catalysts

Most relevant aspects of the reaction kinetics and the mechanism of methanol electrooxidation on electrodeposited Pt and Pt70Ru30 catalysts in 1 mol dm^-3 HClO_4 solution are discussed on the basis of cyclic voltammetry, quasipotentiostatic, chronoamperometry and electrochemical impedance spectroscopy (EIS) measurements. The exact chemical composition of Pt-Ru catalysts has been determined using X-ray photoelectron spectroscopy (XPS). The deduced values of kinetic parameters: the Tafel slope (b_a), the exchange current density (j_0) and the rate constant (k_a) as well as the order of the reaction (n) indicate significantly faster kinetics of the anodic oxidation of methanol on the Pt70Ru30 catalyst in comparison with the electrodeposited Pt. The maximum rate of oxidation is obtained at potentials at which the fractional coverage (theta) of adsorbed species is governed by a Langmuir adsorption isotherm ( delta G_ads = -30 kJ mol^-1) and majority of OH_ads species is consumed for oxidative removal of CO_ads in an autocatalytic step in the Langmuir-Hinshelwood type of reaction. Kinetic analysis and the obtained value of the diffusion coefficient of the order of 10^-7 cm^2s^-1 suggest that the rate-limiting interaction mechanism between CO_ads and OH_ads species involves fast surface diffusion of CO_ads from Pt sites to active Ru sites.

Pt-catalysts; Pt-Ru-catalysts; Langmuir adsorption isotherm; diffusion coefficient of COads; surface diffusion; poisoning; ac impedance;

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