O 2 reduction and CO oxidation at the Pt-electrolyte interface. The role of H 2O and OH adsorption bond strengths

Abstract

The nature of surface sites on which OH(ads) forms in acid and basic electrolytes is discussed based on the analysis of experimental results in the literature and the results of quantum calculations. Theoretical evidence is given for OH(ads) forming in acid solution on Pt surfaces by the reaction Pt OH 2⋯ OH 2( OH) 2↔ Pt OH⋯ H + OH 2( OH 2) 2+ e −( U) ( i) and an analysis of experimental results suggests that in base it forms by the reaction Pt OH 2⋯ OH −( HOH) 2↔ Pt OH⋯ H OH( HOH) 2+ e −( U) ( ii) The reversible potential for reaction (i) is calculated to be 0.62 V, which is essentially the onset potential for OH(ads) formation in weak acid electrolytes. It is suggested that OH(ads) forms at potentials as low as ∼0.17 V in weak basic electrolyte by reaction (ii) where H 2O molecules bond by lone-pair donation to unblocked Pt δ+ sites of the hydrided electrode surface, and that as the potential is increased to the double layer region, beginning at 0.4 V, H 2O no longer bonds with the surface to participate in this reaction. This behavior would explain the ∼0.3 V prewave in CO(ads) oxidation by OH(ads). At the potential of zero charge, ∼0.6 V, H 2O again adsorbs and the reaction resumes. It is concluded based on experimental and theoretical evidence that four-electron O 2 reduction on Pt electrodes at low overpotentials requires a special site characterized by a relatively small ratio of OH to H 2O adsorption bond strengths and or a higher activation energy for OH(ads) formation by reaction (i), as well as a weak ability to adsorb anions.