Interplay between Reaction Mechanism and Hydroxyl Species for Water Formation on Pt(111)

Predicting the reaction mechanism of water and hydrogen peroxide formation on a platinum catalyst is a crucial step toward the understanding of the corresponding selectivity in polymer electrolyte membrane fuel cells. In this perspective, the environment of the catalytic active site should play an important role; however, its explicit description at the atomic scale is an ongoing challenge for theoretical approaches. In this study, we propose to model three effects of the environment: surface hydroxyl coverage, temperature, and reactant pressure. A detailed investigation of the reaction mechanism of water and hydrogen peroxide formation on a platinum surface is reported on the basis of density functional theory (DFT) calculations and Gibbs free energy diagrams. In standard conditions of reaction (1 atm and 353 K), the selectivity toward water and hydrogen peroxide depends on the competition between two reaction paths (molecular oxygen direct dissociation and hydrogenation), which can be tuned by the partial coverage of OH intermediate. At a low coverage of 1/12 ML, the catalyst activity is expected to be low due to a preferential but activated direct oxygen dissociation. When the OH partial coverage increases, the hydroperoxyl route becomes favorable, hence leading to hydroxyl and water by the nonactivated OOH dismutation. The direct oxygen dissociation and the whole reaction mechanism are sensitive to the hydroxyl partial coverage. Our gas/metal model opens the way to new elementary mechanisms in the presence of aqueous electrolyte and electric field that would explain how water can be produced at the beginning of the reaction (at low coverage).