Effects of adsorbed CO on hydrogen ionization and CO oxidation reactions at Pt single-crystal electrodes in acidic solution

Abstract CO was adsorbed on the three low index planes of Pt single-crystal electrodes, and the hydrogen ionization and the dissolved CO oxidation reactions on the CO-covered electrodes were investigated in acid solution. The oxidation wave of CO adsorbed at 50 mV vs. a reversible hydrogen electrode had two peaks; a small prepeak at around 0.5 V and a main oxidation peak at around 0.7 V. Fourier transform IR measurement and the number of electrons per site for adsorbed CO indicated that these oxidation peaks could not be assigned individually to the linear and the bridged CO species. These peaks reflect the difference in the kinetic stability among the adsorbed CO. The stripping of the prepeak CO (CO 1 (a)) initiated both the hydrogen ionization and the dissolved CO oxidation reactions to a value close to the diffusion-limited values. These reactions required only a very small number of free sites which are provided in the present case by the removal of the kinetically unstable CO 1 (a). CO 1 (a) was absent in the adsorbed CO at 0.4 V. The voltammograms for the dissolved CO oxidation on the CO-covered electrodes revealed the presence of other two adsorbed states, CO II (a) and CO III (a), where CO II (a) was most stable and gave the main peak of the oxidation wave of the adsorbed CO. CO III (a) was very sensitive to the experimental conditions and was responsible for the hump that appeared in the dissolved CO oxidation.