First-principles study of the role of solvent in the dissociation of water over a Pt-Ru alloy

Self-consistent gradient-corrected periodic density functional theoretical calculations are used to examine the effects of an aqueous environment on the dissociation of water over a Pt-Ru alloy. This reaction is thought to be one of the rate-limiting steps in oxidative removal of CO from the anode surface of both the direct methanol and reformate fuel cells. The reaction leads to the formation of surface hydroxyl (OH) intermediates that can subsequently oxidize adsorbed CO into CO2. We examine the energetics and mechanism for the dissociation of water over Pt66Ru33(111) in the presence of 23 water molecules (per 615 A3 unit cell volume) that act as a solution phase and in the absence of solution (vapor phase). The reaction is endothermic by +53 kJ/mol and has an activation barrier of +105 kJ/mol when carried out in the vapor phase, but was found to be much less endothermic (+26 kJ/mol) and has a significantly lower activation barrier (+27 kJ/mol) when carried out in solution. In the vapor phase, the reaction occurs homolytically whereby the dissociation is activated by insertion of a Ru atom into the O-H bond of water. The products formed are adsorbed hydroxyl and hydrogen intermediates. In contrast, in solution, the dissociation occurs via a heterolytic path whereby the solvent molecules are directly involved in activating the O-H bond. The reaction leads to the formation of a hydroxyl intermediate that is bound to the alloy surface and a proton that is released into the solution phase. Ab initio molecular dynamics simulations were performed at 300 K to establish the sequence of elementary steps that can occur. The simulations show that water dissociates over Ru and that the hydroxyl intermediate that first forms over Ru rapidly diffuses along the metal surface, migrating over Pt as well as Ru sites. We believe that this evidence shows that diffusion occurs as the result of the proton transfer between the coadsorbed water and hydroxyl intermediates in an aqueous environment. This could have important consequences for CO oxidation in PEM fuel cells whereby the diffusion of CO and/or OH intermediates is important for the reaction at the edge of the Pt/Ru boundaries. The work reported herein applies at open-circuit potentials, but may also be appropriate at other potentials as well.