Ab initio molecular dynamics of solvation effects on reactivity at electrified interfaces

Abstract

Using ab initio molecular dynamics as implemented in periodic, self-consistent (generalized gradient approximation PerdewBurke-Ernzerhof) density functional theory, we investigated the mechanism of methanol electrooxidation on Pt(111). We investigated the role of water solvation and electrode potential on the energetics of the first proton transfer step, methanol electrooxidation to methoxy (CH 3 O) or hydroxymethyl (CH 2 OH). The results show that solvation weakens the adsorption of methoxy to uncharged Pt(111), whereas the binding energies of methanol and hydroxymethyl are not significantly affected. The free energies of activation for breaking the C−H and O−H bonds in methanol were calculated through a Blue Moon Ensemble using constrained ab initio molecular dynamics. Calculated barriers for these elementary steps on unsolvated, uncharged Pt(111) are similar to results for climbing-image nudged elastic band calculations from the literature. Water solvation reduces the barriers for both C−H and O−H bond activation steps with respect to their vapor-phase values, although the effect is more pronounced for C−H bond activation, due to less disruption of the hydrogen bond network. The calculated activation energy barriers show that breaking the C−H bond of methanol is more facile than the O−H bond on solvated negatively biased or uncharged Pt(111). However, with positive bias, O−H bond activation is enhanced, becoming slightly more facile than C−H bond activation.ethanol presents a promising fuel alternative to hydrogen for low-temperature polymer electrolyte membrane fuel cells (1, 2). However, the slower reaction kinetics, compared with hydrogen, requires high overpotentials, which reduces the overall energy efficiency of these fuel cells. Additionally, CO molecules that are formed during the reaction poison Pt catalysts, limiting their activity (2). To address this problem, improved electrocatalysts must be developed (3, 4). Toward this goal, it is essential to understand the sequence of elementary steps that comprise the reaction mechanism. Through knowledge of the detailed mechanism on a particular catalyst, one can determine the ratedetermining step(s). In turn, this information can be used to design improved catalysts that are targeted toward facilitating the rate-determining step.First-principles density functional theory (DFT) calculations have become invaluable for working toward understanding these detailed reaction mechanisms at the atomic scale. In particular, these methods have successfully been used to calculate the thermochemistry (reaction energies) and kinetics (activation energy barriers) of a variety of different catalytic reactions in the vapor phase. However, modeling electrocatalytic reactions poses a greater challenge due to the complexity of the reaction environment at the active site. That is, the reaction occurs on a charged substrate that is surrounded by solvent (and electrolyte). A variety of methods have been developed to model the effects of solvent and charged surfaces on the thermo...

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