(80a) Reaction Path Analysis of Ethanol Electro-Oxidation On Pt(111): Role of Water

Reaction Path Analysis of Ethanol electro-oxidation on Pt(111): Role of Water

Quang Thang Trinh, Mark Saeys

Dept. of Chemical and Biomolecular Engineering, National University of Singapore

Water plays an important role in aqueous phase catalytic reactions, such as electro-oxidation in direct alcohol fuel cells. Water not only affects the activity and selectivity hydrogen-bonding with reactants and transition states, it can also act as a source of active hydroxyl groups [1]. In this presentation, the effect of water on the activity and selectivity of the electro-oxidation of methanol and ethanol on Pt catalysts was studied using the recently developed RPBE-vdW functional [2,3]. This functional accurately describes hydrogen-bond interactions [3], which greatly affects activity and selectivity. Indeed, the presence of water increases the barriers for both C-H and O-H activation because water molecules stabilize the reactants more than the transition states. Hydrogen bonding has a larger effect for the C-H pathway than for the O-H pathway, and hence reverses the selectivity as compared to the gas-phase catalytic reaction. At higher potentials, the presence of surface hydroxyl groups opens new hydrogen-abstraction pathways, with very low barriers for O-H activation. Surface hydroxyl groups also facilitate C-H activation, yet only for surfaces that are less reactive than Pt. For ethanol electro-oxidation, the selectivity between Cα and Cβ C-H activation determines the dominant reaction products. Our study suggests that this selectivity is governed by the adsorption site of the ethoxy species. For ethoxy species at top sites, the preferred sites on Pt, the orientation favors Cα-H deprotonation to CH₃CHO. On Rh, the hollow site is the preferred adsorption site and the ethoxy group is well-oriented for Cβ-H activation to CH₂CH₂O. Surface hydroxyl group can again participate in this C-H activation step; however, hydrogen abstraction barriers are higher than direct dehydrogenation barriers on both Pt and Rh.

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