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Aqueous solvation free energies of adsorption have recently been measured for phenol adsorption on Pt(111). Endergonic solvent effects of ~1 eV suggest solvents dramatically influence a metal catalyst’s activity with significant implications for the catalyst design. However, measurements are indirect and involve adsorption isotherm models, which potentially reduces the reliability of the extracted energy values. Computational, implicit solvation models predict exergonic solvation effects for phenol adsorption, failing to agree with measurements even qualitatively. In this study, an explicit, hybrid quantum mechanical/molecular mechanical approach for computing solvation free energies of adsorption is developed, solvation free energies of phenol adsorption are computed,more » and experimental data for solvation free energies of phenol adsorption are reanalyzed using multiple adsorption isotherm models. Explicit solvation calculations predict an endergonic solvation free energy for phenol adsorption that agrees well with measurements to within the experimental and force field uncertainties. Computed adsorption free energies of solvation of carbon monoxide, ethylene glycol, benzene, and phenol over the (111) facet of Pt and Cu suggest that liquid water destabilizes all adsorbed species, with the largest impact on the largest adsorbates.« less

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 formore » 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.« less

The reaction mechanism of the hydrodeoxygenation (HDO) of levulinic acid (LA) to γ-valerolactone (GVL) has been investigated over a Ru(0001) model surface by a combination of plane-wave density functional theory (DFT) calculations and mean-field microkinetic modeling. Catalytic pathways involving the direct hydrogenation of LA to GVL with and without formation of the experimentally proposed 4-hydroxypentanoic acid (HPA) intermediate have been considered. In the low reaction temperature range of 323–373 K, the activity of the model Ru(0001) surface is low, owing to a very small number of free sites available for catalysis. As an effect, it is unlikely that Ru(0001) ismore » the active site for the experimentally observed catalysis at low temperatures. In contrast, in the medium- to high-temperature range (423–523 K), the HDO of LA is facile over Ru(0001) and we predict at 423 K a turnover frequency, apparent activation barrier, and forward reaction orders that are fairly close to prior experimental observations, leading us to suggest that Ru(0001) sites might constitute the active site for high-temperature catalysis. Lastly, our microkinetic analysis indicates that the HDO of LA occurs by LA adsorption, hydrogenation of LA to an alkoxy intermediate, surface ring closure, and –OH group removal: i.e., it does not occur via HPA production as previously suggested. The first hydrogenation step of LA toward the formation of an alkoxy intermediate is the most rate controlling step over Ru(0001).« less

First-principles, periodic density functional theory (DFT) calculations and mean-field microkinetic modeling have been used to investigate the decomposition of ethylene glycol for hydrogen production on Pt(111) in vapor and aqueous phases. All dehydrogenated species derived from ethylene glycol (C₂H_xO₂, x = 0–6) and methanol (CH_yO, y = 0–4), and all elementary C–C, C–H, and O–H bond breaking steps are included in the microkinetic model. Reaction path analysis in vapor phase indicates that both initial C–H and O–H dehydrogenation steps are kinetically relevant at all temperatures (470–530 K). Initial O–H bond cleavage is reversible at low temperatures but accounts for anmore » increasingly dominant fraction of the total reaction flux at higher temperatures. C–C bond scission is observed only after significant dehydrogenation (x ≤ 3) and is more likely to happen in surface intermediates where one of the cleavage products is CO. Here, the process is highly selective to the production of H2 compared to methanol. For aqueous-phase model development, free energies of solvation were computed for all surface intermediates and transition states using a continuum solvation approach. Our aqueous-phase microkinetic model predicts a 0.4 eV lower apparent activation energy and an order of magnitude larger turnover frequencies. Initial C–H bond cleavage becomes more important but the general trends are similar to the vapor phase, suggesting that the reaction chemistry is similar in both vapor and aqueous phases.« less

We report the development of a quantum mechanics/molecular mechanics free energy perturbation (QM/MM-FEP) method for modeling chemical reactions at metal-water interfaces. This novel solvation scheme combines planewave density function theory (DFT), periodic electrostatic embedded cluster method (PEECM) calculations using Gaussian-type orbitals, and classical molecular dynamics (MD) simulations to obtain a free energy description of a complex metal-water system. We derive a potential of mean force (PMF) of the reaction system within the QM/MM framework. A fixed-size, finite ensemble of MM conformations is used to permit precise evaluation of the PMF of QM coordinates and its gradient defined within this ensemble.more » Local conformations of adsorbed reaction moieties are optimized using sequential MD-sampling and QM-optimization steps. An approximate reaction coordinate is constructed using a number of interpolated states and the free energy difference between adjacent states is calculated using the QM/MM-FEP method. By avoiding on-the-fly QM calculations and by circumventing the challenges associated with statistical averaging during MD sampling, a computational speedup of multiple orders of magnitude is realized. The method is systematically validated against the results of ab initio QM calculations and demonstrated for C–C cleavage in double-dehydrogenated ethylene glycol on a Pt (111) model surface.« less

The hydrodeoxygenation of organic acids is often found to be a rate-controlling process during upgrading of biomass feedstocks into fuels. We developed a microkinetic model based on data obtained from density functional theory calculations for the decarboxylation and decarbonylation mechanisms of propanoic acid (CH₃CH₂COOH) over a Ru(0 0 0 1) model surface. The model predicts that the decarbonylation mechanism is two orders of magnitude faster than the decarboxylation mechanism. The most favorable decarbonylation pathway proceeds via removal of the acid –OH group to produce propanoyl (CH₃CH₂CO) followed by C–CO bond scission of propanoyl to produce CH₃CH₂ and CO. Finally, CH₃CH₂more » is hydrogenated to CH₃CH₃. Dehydrogenation reactions that have been observed to be important over Pd catalysts play no role over Ru(0 0 0 1), and a sensitivity analysis indicates that removal of the acid –OH group is the rate-controlling step in the deoxygenation. Altogether, our results suggest that to improve the Ru catalyst performance for the decarbonylation of organic acids, the free site coverage needs to be increased by, for example, adding a catalyst promoter that decreases the hydrogen and CO adsorption strength (without significantly affecting the C–OH bond scission rate), or by raising the reaction temperature and operating at relatively low CO and H₂ partial pressures.« less

The effects of liquid water, n-octane, and n-butanol on the hydrodeoxygenation of propanoic acid over Pd(111) model surfaces have been studied from first principles. We developed a microkinetic model for the hydrodeoxygenation and studied the reaction mechanism at a temperature of 473 K. Our model predicts that for all reaction media, decarbonylation pathways are favored over decarboxylation pathways. However, in the presence of polar solvents like water, decarboxylation routes become competitive with decarbonylation routes. The activity of the Pd surface varies as a function of the environment as follows: water > n-butanol > octane ≈ gas phase. Lastly, a sensitivitymore » analysis of our models suggests that both C–OH and C–H bond cleavages control the overall rate of the catalyst in all environments and are likely to be activity descriptors for the hydrodeoxygenation of organic acids.« less