Elucidating the reaction mechanism of water and hydrogen peroxide formation on platinum catalysts is a crucial step the improvement of the activity and stability of polymer electrolyte membrane fuel cells. In this context, the liquid environment around the catalytic active sites plays an important role, as it modifies the adsorption properties due to solvent-surface interactions and coverage effects. Although essential for the creation of realistic models, the explicit description at the atomic scale of solvents is challenging for theoretical approaches. In this study, we will use two different approaches to explore the effects of liquid environments: surface hydroxyl coverage and solvent effects. In the first approach, a detailed investigation of the reaction mechanism of H2O and H2O2 formation on platinum is reported using DFT calculations and Gibbs free energy diagrams. In standard conditions, the selectivity depends on the competition between two reaction pathways, namely the direct or hydrogen-assisted dissociation of O2, which can be tuned by the partial coverage of *OH. At a low coverage of 1/12 ML, the catalyst activity is low due to preferential but activated direct oxygen dissociation. When the *OH partial coverage increases, the hydroperoxyl (*OOH) route becomes favorable, leading to hydroxyl and water formation by non-activated OOH dismutation [1-2]. In the second approach, the influence of solvation on adsorption properties is explored at the interface between a liquid made of 698 H2O molecules and a platinum nanoparticle made of 201 atoms by means of ab initio molecular dynamics simulations at 350 K. The water solvation layer in contact with the Pt201 is composed of physisorbed and chemisorbed molecules in a ratio of 2:1. Chemisorption occurs only at corners and edges of the nanoparticle and the corresponding surface coverage reaches 0.29 ML. The solvent stabilizes significantly the interaction energy of a single chemisorbed water molecule at the nanoparticle surface. Approximately 66% of the total energy gain is provided by hydrogen bonds created with some physisorbed water molecules, while the remaining 34% is due to the rest of the liquid [3]. References: [1] de Morais, R. F.; Franco, A. A.; Sautet, P.; Loffreda, D. ACS Catal. 2015, 5, 1068–1077. [2] de Morais, R. F.; Franco, A. A.; Sautet, P.; Loffreda, D. Phys. Chem. Chem. Phys. 2015, 17, 11392–11400. [3] de Morais, R. F.; Kerber, T.; Calle-Vallejo, F.; Sautet, P.; Loffreda, D. submitted.

DFT study of liquid environment effects on water and hydrogen peroxide formation on platinum

251st National Meeting (2016)

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