(168b) A Combined Density Functional Theory and Monte Carlo Approach for Quantifying Catalytic Energies in a Water Environment

One of the biggest challenges in computational catalysis is modeling activity under realistic reaction environments. The main way that the reaction environment influences activity is by “dosing” the catalyst with atoms and molecules, which become chemically and/or physically adsorbed to the surface. These atoms and molecules alter binding and reaction energies through at least two effects: 1) molecules that form strong chemical bonds with the catalyst can accumulate on the catalyst surface, inducing coverage effects, and 2) molecules in condensed phase reaction environments can interact physically with adsorbates, inducing solvation effects. Several computational methods have been developed to treat these phenomena. For example, first-principles thermodynamics (FPT) is used to predict catalyst compositions under real reaction conditions, and a number of solvation models have been used to predict the energies of interaction between static fluid phases and solid catalyst environments. In this work, we extend these methods to more realistic, dynamic fluid environments. Specifically, we demonstrate a method to include entropic contributions. This method takes fluidity in the reaction environment and its influence on the free energies of relevant reactants, intermediates, and products into account. We use a combination of density functional theory (DFT) and Monte Carlo to quantify the different contributions to the overall free energies and make conclusions about how they influence catalytic activity. Our present focus is on reduction of nitrate over nanoparticulate Au in water. We chose Au because it is relatively non-reactive, so we can largely neglect coverage effects and instead focus on interactions between the reaction environment and the catalyst environment. Catalysts are modeled using 13-atom gold icosahedral clusters, which are computationally efficient yet comprise a number of typical adsorption sites for typical metal catalysts. We find that the water environment has a noticeable influence on reaction energetics. Ongoing work involves expanding these methods to more reactive transition metals and more realistic supported catalyst models.