(415g) Molecular Simulation Study of How the Structure of Liquid Water Affects the Free Energies of Adsorption and Reaction in Aqueous Phase Heterogeneous Catalysis

Authors: 
Zhang, X., Clemson University
Getman, R., Clemson University
There are many applications of aqueous phase heterogeneous catalysis, including aqueous phase reforming (APR), Fischer-Tropsch synthesis, electrocatalytic oxygen reduction, etc., and a grand challenge in catalysis research is understanding how water influences the chemistry, mechanism, thermodynamics, and kinetics of reactions occurring on metal catalyst surfaces under liquid H2O. Recent work by our group and others has shown that H2O molecules form hydrogen bonds (HB) with reaction intermediates, alter their adsorption energies, co-catalyze certain reactions, affect kinetic barriers and reaction mechanisms, amongst other phenomena. We are interested not only in the roles that liquid H2O molecules play, but also how these phenomena influence the rates and equilibria of catalytic surface reactions. For example, we previously found that hydrophilic adsorbates form stronger and more stable HB with H2O molecules than hydrophobic adsorbates, which we postulated would lead to greater degrees of order in the interfacial water structure around hydrophilic adsorbates than for hydrophobic ones. We further postulated that this would give rise to a free energy effect in reactions where hydrophilic adsorbates are converted into less hydrophilic adsorbates or even hydrophobic ones, e.g., in APR reactions where sugar alcohol molecules are sequentially deprotonated down to CO2 and H2 (and other analogous reactions). In this work, we use density functional theory (DFT) and methods in molecular dynamics (MD) and statistical mechanics to study these effects in the context of the decompositions of CH3OH and NH3 over Pt catalysts. Specifically, we compute the free energies of different surface intermediates along the pathways for these reactions under liquid H2O using the following methods: potential of mean force (PMF), calculated from the radial distribution function (RDF), which gives the free energy due to configurational fluctuations in the liquid H2O structure, and free energy perturbation (FEP), thermodynamic integration (TI), and implicit solvation method for surfaces (iSMS), which give the free energies of solvation for the different adsorbates. A key step in the PMF/RDF, FEP, and TI methods is producing an explicit structure of liquid H2O at the catalyst interface, in order to identify how it influences catalytic thermodynamics. We provide values for these interfacial free energy contributions relative to the free energies of reaction and discuss how they influence the thermodynamics of adsorption to the surface from aqueous phase and reaction on the surface under liquid H2O. We summarize by discussing how these contributions influence the equilibrium and rate constants in aqueous phase heterogeneously catalyzed reactions.