Water dissociation on well-defined platinum surfaces: The electrochemical perspective

Abstract

This paper discusses three important discrepancies in the current interpretation of the role of water dissociation on the blank cyclic voltammetry of well-defined single-crystalline stepped platinum surfaces. First, for H adsorption both H-terrace and H-step contributions have been identified, whereas for OH adsorption only OH-terrace has been identified. Second, different shapes (broad vs. sharp) of the H-terrace and H-step voltammetric peaks imply different lateral interactions between hydrogen adatoms at terraces and steps, i.e. repulsive vs. attractive interactions. Third, the H-step peak exhibits an unusual pH-dependent shift of 50 mVNHE/pH unit. We propose here a model that can explain all these observations. In the model, the H-step peak is not due to only ad- and desorption of hydrogen, but to the replacement of H with O and/or OH. The O:OH ratio in the step varies with step geometry, step density and medium. In alkaline media relatively more OH is adsorbed in (or on) the step than in acidic media, under which conditions more O is adsorbed in (or on) the step. This would explain the anomalous pH dependence and would provide a possible explanation for the higher catalytic activity of alkaline media for electro-oxidation reactions. Although the model certainly still contains speculative elements, we believe it provides the most consistent interpretation of platinum single-crystal electrochemistry currently available, and presents an important and significant improvement over previous interpretations. In situ spectroscopic data are ultimately needed to confirm or disprove some of the assumptions of the model.