The electro-oxidation of methanol on smooth polycrystalline platinum in the presence of a sulfuric acid solution was investigated using impedance spectroscopy. The impedance response of both the electro-oxidation of methanol and the adsorption of hydrogen were measured directly at potentials where these processes are active. The time dependence of the impedance of the adsorption of hydrogen in the presence of methanol was used as an in situ probe of the formation of methanol adsorption products. Monitoring the formation of methanol adsorption products by this method yielded kinetic information for the electro-oxidation of methanol at low potentials that previously has been difficult to obtain. The fitting of the impedance of the electro-oxidation of methanol to an appropriate model provided the values of impedance parameters whose potential dependence reveal information about the kinetics of the reaction, changes in the coverage of the adsorbed intermediate, and the effect of coadsorbed oxygen containing species. The results indicate the presence of five different potential regions between 40 and 800 mV vs RHE where a change in the reaction mechanism occurs. The mechanistic changes are interpreted as being due to the presence of oxygen containing species with different reactivities at different potentials.

We measured the time dependence of the impedance of the electro-oxidation of methanol on polished polycrystalline platinum immersed in a solution of sulfuric acid. The impedance data were fit to a model for a reaction containing one adsorbed intermediate to obtain four physical parameters. The time dependence of these parameters illuminates how a change in the coverage of an adsorbed intermediate and the buildup of other poisons affects the kinetics and mechanism of the electro-oxidation of methanol. These results further support our conclusions based on the potential dependence of the impedance, that is, four potential regions exist between 200 and 800 mV vs RHE where a change in the mechanism for the electro-oxidation of methanol occurs (Melnick, R. E.; Palmore, G. T. R. J. Phys. Chem. B 2001, 105, 1012.). We hypothesize that changes in the mechanism are caused by the presence of four different species that contain oxygen, each belonging to a specific potential region with the more reactive species at higher potentials. We have combined impedance data from potential-dependent studies with impedance data that are time dependent to construct a qualitative model of this reaction. This model describes the rate of charge transfer for the electro-oxidation of methanol as a function of potential and coverage of adsorbed intermediate. The result is a model that predicts the behavior of this reaction at a platinum electrode, which is in agreement with that observed experimentally by impedance spectroscopy, chronoamperometry, and cyclic voltammetry.

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