(185b) Density Functional Theory Methods for Modeling Electrocatalysis: Application to Borohydride Oxidation Over Au and Pt(111)

Difficulties associated with experimental characterization of elementary kinetics for complex electrocatalytic reactions, such as the eight electron oxidation of borohydride species, motivates the application of density functional theory (DFT) methods to mechanism determination. A series of quantum-mechanical model system configurations will be presented, involving sequential addition of the complexities of the electrode-electrolyte interface, and the utility of each in evaluating elementary reaction energetics of both redox and chemical reaction steps will be discussed.

In this study, multiple approaches are discussed to approximate the electrochemical nature of the catalytic surface with the DFT model. The first method uses a metal-vacuum slab interface to determine the interaction of individual species with the electrode surface. The energy of ion or electron species produced or consumed in individual reaction steps is dictated by their energy in the bulk electrolyte or by the electrode potential. This method approximates that ion adsorption occurs with integral electron transfer and neglects solvation of surface bound species and the potential dependent interaction of surface species with an interfacial electric field. Corrections for dipole-field and solvation interactions may be added to energetics determined with the vacuum-slab model. The second method includes solvation and electric field variation within the model system in determining the energy of adsorbed species. This is done at increased cost in computational resources, and is used for a subset of calculations performed.

The ion adsorption energies as well as energetics and activation barriers of initial steps of borohydride oxidation over Au(111) and Pt(111) surfaces was calculated and will be discussed. Specifically, comparison between ?vacuum slab? and solvated, variable potential methods in calculating ion adsorption equilibrium constants and surface reaction activation barriers will be presented.