Tuesday, November 10, 2015: 10:10 AM
355B (Salt Palace Convention Center)
Density Functional Theory (DFT) is an integral tool in rational catalyst design. However for heterogeneously catalyzed liquid-phase reactions, accurately and efficiently modeling the effects of the liquid phase remains a challenge due to complexity at the liquid/solid interface. This complexity arises from solvent molecules and adsorbates on the catalyst surface and includes the energetics of chemical and/or physical interactions between these species, as well as their entropic consequences. These effects influence the chemical potentials of adsorbates, and therefore affect things like adsorbate coverages, reaction coordinates, equilibrium constants, and rate constants. Modeling these effects is a challenge in traditional quantum mechanics, which is performed at 0 K and thus does not include thermal fluctuations of the liquid configuration. We aim to capture electronic structure and thermal effects in this work using a combination of DFT and molecular dynamics (MD). We are specifically interested Pt-catalyzed methanol and glycerol oxidation in aqueous conditions, which are relevant to applications in direct methanol fuel cells and biomass reforming. We use classical MD simulations to generate ensembles of configurations of liquid H2O molecules at finite temperatures and pressures, and we choose representative subsets to study with DFT. We interrogate both the energies and entropies of adsorbate--H2O interactions. We find that hydrogen bonding gives rise to strong energies of interaction with larger, more polar adsorbates, as we calculate the energy of interaction relative to the vacuum systems to be -113 kJ/mol for C3H7O3, -41.3 kJ/mol for CH2OH, and -1.0 kJ/mol for CO. Since hydrogen bonding involves strong directional bonds, H2O molecules that participate in hydrogen bonding also exhibit some degree of order, which contributes to the entropies of interaction. We determine the effects of these interactions on the reaction coordinate in methanol and glycerol oxidations and show how they can be incorporated into calculations of the equilibrium and rate constants involved in the catalyzed mechanisms.