Role of charge transfer in catalytic hydrogen oxidation over platinum

Charge transfer plays the central role in chemisorption and chemical reactions at metal surfaces. The Somorjai group in Nano. Lett. {\bf 9}, 3930 (2009) has reported a chemicurrent, a flux of charge carriers in response to $\mathrm{H_2}$ oxidation by $\mathrm{O_2}$ into $\mathrm{H_2O}$ over a platinum surface. The nature of the chemicurrent has been debated in the literature; explanations both extrinsic and intrinsic to the reaction mechanism have been offered. This article suggests a picture behind the chemicurrent experiments, which is intrinsic to the mechanism of hydrogen oxidation in a high temperature regime. Surface reaction intermediates, $"\mathrm{O^-}"$, $"\mathrm{H^-}"$, $"\mathrm{OH^-}"$, are negatively charged while the product, $\mathrm{H_2O}$, and reactants, $\mathrm{H_2}$ and $\mathrm{O_2}$, are neutral. Hence, charge transfers between the metal and reacting species are inevitable. After electrons are transferred from the metal to the interface in dissociative oxygen and hydrogen adsorption, translationally excited electrons in the metal arise in part as a result of decay of negatively charged transition states in reaction steps connecting surface species of different charges, $"\mathrm{O^-+H^-}" \rightleftarrows "\mathrm{2 e+OH^-}"$ and $"\mathrm{OH^-+H^-}"\rightleftarrows "\mathrm{2 e+H_2O}"$. These transition states are the lowest energy configurations possible for changing the charge of the negatively charged surface intermediates. In particular, the transition state for $"\mathrm{O^-+H^-}" \rightleftarrows \mathrm{"2 e+OH^-"}$ limits the reaction rate. These processes may contribute to the chemicurrent. This picture should also apply to other catalytic chemical reactions on metal surfaces that proceed through formation of charged surface intermediates and their consequent discharge into the metal.