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

Abstract

International audiencePredicting 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)

The puzzle of rapid hydrogen oxidation on Pt(111)

“…The development of theory and simulation has also been important. DFT calculations in particular provide values for enthalpies of reactants, intermediates, and products, and barriers to elementary reaction steps, some of which may not be available from experiment [25][26][27][28][29][30].…”

Mechanisms of Enhanced Electrocatalytic Activity for Oxygen Reduction Reaction on High-Index Platinum n(111)–(111) Surfaces

“…20,21 Density functional theory (DFT) in particular have been a useful tool to predict efficiencies of the ORR by calculating the binding energies and free energy of the intermediate reaction steps. 22,23 For instance, it has been successfully applied to explain the ORR activity enhancement in Pt alloys, 12,21,24,25 Pt skins, 26−29 and core− shell structures, 30−32 as well as to predict possible reaction pathways for the ORR reaction. 5,23,33−35 Although this technique allowed researchers to predict and explain the enhanced ORR process, they have been limited to the simple facets, such as (100) and (111).…”

Performance and degradation of Proton Exchange Membrane Fuel Cells: State of the art in modeling from atomistic to system scale

“…Those models are based on Pt clusters (from a few atoms [30,31] up to hundreds [32][33][34][35][36]) either in vacuum or using polarizable continuum model (PCM) calculations to implicitly account for solvent effects [37]. So far, various theoretical studies have shown the competition between two key elementary steps in the mechanism: the dissociation of O 2 either through a direct route (producing two *O species) 1 or through the formation of *OOH and the subsequent formation of *OH surface species [15,24,31,[38][39][40]. However, the mechanism has not yet been explored directly on a nano-size particle model, neither in vacuum nor in a solvent, although recent studies have started to address particle-size effects on the ORR activity of Pt nanoparticles [41,42].…”

“…Secondly, more systematic studies have been proposed to combine these competitive elementary steps and to conclude about the preferential elementary mechanism [39,48,59]. During the past few years, significant efforts have been devoted to probe the influence of the aqueous solution on the ORR mechanism [37,[60][61][62][63][64][65][66][67], either by considering continuum models [37] or by treating explicitly a small number of water molecules [60,62,[65][66][67] such as ice bilayers [61,63], or confined static water [64].…”

“…Finally, for this coverage, the formation of hydrogen peroxide from *OOH species is predicted to be disfavored due to a slightly larger barrier (0.21 eV) compared to its quasi-spontaneous dissociation into *O + *OH (0.16 eV) [48]. Recently, the influence of *OH surface coverage on the competition between *O 2 dissociation and *OOH formation has been investigated on Pt (111) [39]. For *OH coverage below 0.3 ML, the direct dissociation is preferential, whereas *OOH formation becomes favored for coverage above 0.3 ML.…”