How Potentials of Zero Charge and Potentials for Water Oxidation to OH(ads) on Pt(111) Electrodes Vary With Coverage

Abstract

Self-consistent potential-dependent quantum chemical calculations on the formation of OH(ads) from H2O(ads) oxidation on the Pt(111) surface in acidic solution have been performed using a unified theory for the electrochemical interface. It is found that the potentials of zero charge (PZC) are relatively insensitive to H2O(ads) coverage and, when adsorbed OH is present, are very sensitive to the OH(ads)/H2O(ads) ratio. The PZC generally increases as the degree of oxidation of the surface is increased by higher OH(ads) coverage. Standard reversible potentials for the formation of 1/6 monolayer (ML) OH(ads) from oxidation of H2O(ads) for initial H2O coverage values of 1/6, 1/3, 1/2, and 2/3 ML were calculated and found to be respectively 0.71, 0.59, 0.65, and 0.63 V on the SHE scale. These calculated reversible potentials agree well with experimental measurements of onset potentials for H2O oxidation, but it is argued using experimental measurements of the groups of Orts and Watanabe that the 0.59 V value corresponds to the room temperature interface. The reversible potentials for oxidizing the last 1/6 ML of H2O(ads) at 1/3, 1/2, and 2/3 ML initial H2O coverage are calculated to be respectively 1.25, 1.29, and 1.21 V. The high reversible potentials are attributed to the loss of stabilizations from hydrogen bonding when H2O(ads) is oxidized to OH(ads) and suggest that high coverage for OH(ads) beyond 1/3 ML can exist only at high electrode potentials. However, the experimental results of Watanabe and our preliminary theoretical studies show that the OH(ads) is oxidized to O(ads) at about 0.8 V, well before such high potentials are reached.