Computational Investigation of Aqueous Phase Effects on the Dehydrogenation and Dehydroxylation of Polyols over Pt(111)

Abstract

Prediction of solvation effects on the kinetics of elementary reactions occurring at metal−water interfaces is of high importance for the rational design of catalysts for the biomass and electrocatalysis communities. A lack of knowledge of the reliability of various computational solvation schemes for processes on metal surfaces is currently a limiting factor. Using a multilevel quantum mechanical/molecular mechanical (QM/MM) description of the potential energy surface, we determined characteristic time and length scales for typical free-energy perturbation (FEP) calculations of bond cleavages in ethylene glycol, a sugar surrogate molecule, over Pt(111). Our approach is based on our explicit solvation model for metal surfaces and the repetition of FEP calculations to estimate confidence intervals. Results indicate that aqueous phase effects on the free energies of elementary processes can be determined with 95% confidence intervals from limited configuration space sampling and the fixed charge approximation used in the QM/MM-FEP methodology of smaller 0.1 eV. Next, we computed the initial O−H, C−H, and C− OH bond cleavages in ethylene glycol over Pt(111) in liquid water utilizing two different metal−water interaction potentials. Our calculations predict that aqueous phase effects are small (<0.1 eV) for the C−H bond cleavage and the activation barrier of the C−OH bond cleavage. In contrast, solvation effects are large (>0.35 eV) for the O−H bond cleavage and the reaction free energy of the C−OH bond scission. While the choice of a different Pt−water force field can lead to differences in predicted solvation effects of up to 0.2 eV, the differences are usually smaller (<0.1 eV), and the trends are always the same. In contrast, implicit solvation methods appear to currently not be able to reliably describe solvation effects originating from hydrogen bonding for metal surfaces even qualitatively.

Influence of an electrified interface on the entropy and energy of solvation of methanol oxidation intermediates on platinum(111) under explicit solvation

“…For example, Heyden developed a state-of-the-art QM/MM method (called explicit solvation model for metal surfaces, or eSMS 83 ) to study catalytic chemistry under liquid H 2 O. 84,85 In general, eSMS and MSS arrive at similar conclusions about the influences of liquid H 2 O on surface species free energies 52,84 as well as the ratio of solvation entropy to solvation enthalpy 53,85 for surface species, indicating that errors due to holding the surface species fixed during cMD simulations should not influence our overall conclusions.…”

Solvation Free Energies and Adsorption Energies at the Metal/Water Interface from Hybrid Quantum-Mechanical/Molecular Mechanics Simulations

“…Hybrid quantum-mechanical/molecular mechanics (QM/MM) approaches have also been suggested but have so far not been compared to the rare experimental data. 25,26 Our twostep hybrid QM-MM level, that we had called MM-FEP, 27 consists in describing the surfaceadsorbate interaction via a DFT optimization and accounting for the solvation effects of the frozen DFT geometry through free energy perturbation (FEP) exploiting efficient molecular mechanics. This allows to retrieve all the major solvation effect, including the competition between water and the adsorbate for adsorption on the surface.…”

Dependency of solvation effects on metal identity in surface reactions

“…In this class of simulations, the adsorbate and metal atoms involved in the reaction are considered as a QM sub-system described from first principles, while the bulk of the solvent and metal atoms distant to the active site are considered as an MM sub-system described using classical molecular mechanics force fields. We have previously developed such a hybrid QM/MM model, named eSMS (explicit solvation model for metal surfaces) 54 , which considers the long-range electrostatic interaction of the solvent molecules in the electronic structure calculation of the active site and applied it to the free energy calculation of the initial dehydrogenation and dehydroxylation of an adsorbed ethylene glycol (EG) moiety on Pt (111) in the presence of liquid water 55 .…”

“…To confirm this hypothesis, we have investigated the aqueous-phase effects on the initial C-H and O-H bond cleavages of EG over the (111) facet of six transition metal surfaces (Ni, Pd, Pt, Cu, Ag, Au) using our explicit solvation method, eSMS. The choice of reaction systems is motivated by (i) EG being a commonly studied surrogate molecule of biomassderived polyols, (ii) the selected transition metals are relatively stable and commonly used for aqueous-phase processing of biomass-derived oxygenates 61 , (iii) early dehydrogenation steps of EG over Pt and Ni/Pt catalysts have previously been found to control the overall reaction rate [62][63][64] , (iv) at least over Pt (111) in the vapor phase, initial C-H and O-H bond cleavage are competitive (although O-H bond cleavage is believed to be somewhat favored) 34,64 , and finally (v), explicit solvation approaches have recently been used to demonstrate that for bond cleavage reactions of alcohols over Pt (111), aqueous solvation effects are large and can currently not be described by implicit solvation models 55,65 . Figure 1 illustrates the aqueous-phase effects on the activation free energy barrier of the O-H (CH 2 OHCH 2 OH** + * ↔ CH 2 OCH 2 OH** + H*) and C-H bond cleavages (CH 2 OH-CH 2 OH** + * ↔ CHOHCH 2 OH ** + H*) of EG at 423 K computed by eSMS (see Table 1 for specific numbers).…”