Solvents can affect the intrinsic activity and selectivity of active sites in heterogeneous catalysis. For modeling a condensed phase, the potential energy surface should describe accurately the part of the system involving bond forming or breaking process, a significant portion of configuration space has to be sampled and, dynamic fluctuations of solvent molecules has to be taken into account. One way of describing the aqueous phase effects is to use the continuum solvation models, where solvent is replaced by a continuum with an appropriate dielectric constant and the solute is placed in cavities constructed within this continuum. Our implicit solvation calculations indicate that although it provides a faster way to approximate free energies of reaction in solution, the results obtained depend largely on the cavity radii of the transition metals used, which needs further careful investigation itself. Another approach to circumvent the problems associated with continuum solvation models is to use hybrid QM/MM solvation models.<br/><br/>In this study, we present an implicit and explicit, hybrid QM/MM solvation scheme for the study of catalytic reactions at solid-water interfaces. Next, we identify optimal parameters for our hybrid QM/MM free energy calculations to (i) reduce the computational cost and (ii) determine if full QM calculations could become feasible in the foreseeable future. To test the reliability of our models, we investigated the free energy of activation and reaction for initial C-H and O-H bond cleavage in ethylene glycol over Pt(111) in an aqueous reaction environment with both our iSMS (Implicit Solvation for Metal Surfaces) and eSMS (Explicit Solvation for Metal Surfaces) solvation models. We find that implicit solvation models are (largely independent of the parameters) challenged by their inability to describe the site-specific interactions and hydrogen bonding accurately. For example, our implicit solvation model fails to capture any significant solvent effect in the O-H bond cleavage of ethylene glycol while our eSMS model predicts a 0.3 eV reduction in the activation barrier and a 0.14 eV reduction in the reaction free energy.

Implicit and explicit solvation models for computing free energies of activation and reactions at solid-liquid interfaces

253rd National Meeting (2017)

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