T he notion that solvents can affect the chemical reactivity has been prevalent in the homogeneous catalysis community, going back as far as 1863. 1 Remarkable changes in reaction rate have been reported in the seminal work of Menschutkin, who demonstrated a change in reaction rate constant up to a factor of 700 as a function of the solvents employed for the reaction of triethylamine with ethyl iodide at 373 K. 2 It is well-known nowadays that solvents can affect the reaction rate, reaction mechanism, and selectivity of chemical reactions occurring in condensed phase. While solvent effects usually lead to changes in reaction rates of up to 3 orders of magnitude, rate increases of 9 orders of magnitude have been reported. 3,4 In homogeneous metal catalysis such as hydroformylation, hydrogenation, and cross-coupling reactions, solvent effects have been studied systematically and exploited for industrial applications. 5 Substantial solvent effects have also been reported in heterogeneous catalysis for several hydrogenation, 6−9 oxidation, 10−12 and electrochemical reactions (where electric field effects lead in addition to an electric double layer 13−16 ). However, in heterogeneous catalysis, systematic studies of solvation effects are rare, and solvent effects are generally not well understood. In this context, we note that liquid-phase processing is highly desirable for process cost reduction and high product selectivity for the heterogeneously catalyzed conversion of highly functionalized lignocellulosic biomass, because the feedstocks contain significant amounts of water, are produced in aqueous-phase environments, and reactant molecules are highly water-soluble, reactive, and thermally unstable. 17−19 Processing at relatively low temperatures in condensed phase has therefore the potential to (1) minimize undesirable thermal degradation reactions, (2) increase the targeted product selectivity, and (3) facilitate the product separation from excess water because reaction products often contain less oxygen and are therefore less hydrophilic than the feed streams.Computational catalysis has in the last 20 years become an increasingly important tool for understanding catalytic reactions and designing new catalytic materials of industrial relevance. 20−22 However, progress has been limited to vapor-phase catalysis, and theoretical studies 8,11,23,24 of solvent effects in heterogeneous catalysis are still in their infancies. The relative lack of progress in computational catalysis at solid−liquid interfaces can be explained by the added complexity of a reaction system containing both a complex heterogeneous catalyst and a condensed phase and by fundamental modeling challenges of systems for which the harmonic approximation 25 for estimating partition functions and free energies is no longer valid. The latter challenge is a long-standing issue in the molecular simulations community for systems that require extensive configuration space sampling on a high dimensional potential energy surface that cannot be described by ...

Reaction chemistry for vapor- and aqueous-phase reforming of ethylene glycol over Pt(111) is similar with early dehydrogenation steps being rate-controlling.

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.

Solvent interactions with adsorbed moieties involved in surface reactions are often believed to be similar for different metal surfaces. However, solvents alter the electronic structures of surface atoms, which in turn affects their interaction with adsorbed moieties. To reveal the importance of metal identity on aqueous solvent effects in heterogeneous catalysis, we studied solvent effects on the activation free energies of the O–H and C–H bond cleavages of ethylene glycol over the (111) facet of six transition metals (Ni, Pd, Pt, Cu, Ag, Au) using an explicit solvation approach based on a hybrid quantum mechanical/molecular mechanical (QM/MM) description of the potential energy surface. A significant metal dependence on aqueous solvation effects was observed that suggests solvation effects must be studied in detail for every reaction system. The main reason for this dependence could be traced back to a different amount of charge-transfer between the adsorbed moieties and metals in the reactant and transition states for the different metal surfaces.