Electrochemical oxidation of CO on Pt in acidic and base solutions

Abstract The effect of surface roughening of Pt by potential cycling and of the initial adsorption potential on the CO oxidation mechanism was studied by monitoring the potential dependence of the infrared spectra of the adsorbed CO in the course of the electrooxidation. The most interesting phenomenon has been observed in the course of oxidation of CO adsorbed in the hydrogen region on moderately roughened Pt: the infrared spectra split into two bands, of which the one at the lower wavenumber disappears first at less positive potentials than the other band, at the higher wavenumber, with a concomitant large band shift to lower wavenumber, while the other band shows no such band shift when it disappears at more positive potentials. It is concluded that two oxidation mechanisms are operating on Pt at the same time in such a case: an island mechanism, in which the oxidation proceeds from the edges of the CO islands, and random oxidation which proceeds homogeneously in the CO layer. This implies that there are two CO domains on Pt. Irrespective of the degree of surface roughening, an adsorbed CO layer formed in the hydrogen region is oxidized around 0.5 V/RHE, while CO adsorbed in the double layer region, 0.4 V, is oxidized around 0.6 V/RHE, i.e., the former is more electroactive. However, even for the same adsorption potential, 0.4 V, the previous history of the Pt (i.e., whether the Pt has experienced a hydrogen region or an oxygen region before 0.4 V) plays an important role in determining the reactivity of the adsorbed CO layer.