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The treatment of the solvation charges using Gaussian functions in the polarizable continuum model results in a smooth potential energy surface. These charges are placed on top of the surface of the solute cavity. In this article, we study the effect of the solute cavity (van der Waals-type or solvent-excluded surface-type) using the Gaussian charge scheme within the framework of the conductor-like polarizable continuum model on (a) the accuracy and computational cost of the self-consistent field (SCF) energy and its gradient and on (b) the calculation of free energies of solvation. For that purpose, we have considered a large set of systems ranging from few atoms to more than 200 atoms in different solvents. Our results at the DFT level using the B3LYP functional and the def2-TZVP basis set show that the choice of the solute cavity does neither affect the accuracy nor the cost of calculations for small systems (< 100 atoms). For larger systems, the use of a vdW-type cavity is recommended, as it prevents small oscillations in the gradient (present when using a SES-type cavity), which affect the convergence of the SCF energy gradient. Regarding the free energies of solvation, we consider a solvent-dependent probe sphere to construct the solvent-accessible surface area required to calculate the nonelectrostatic contribution to the free energy of solvation. For this part, our results for a large set of organic molecules in different solvents agree with available experimental data with an accuracy lower than 1 kcal/mol for both polar and nonpolar solvents.

The shape control of nanoparticles constitutes one of the main challenges in today’s nanotechnology. The synthetic procedures are based on trial-and-error methods and are difficult to rationalize as many ingredients are typically used. For instance, concave nanoparticles exhibiting high-index facets can be obtained from Pt with different HCl treatments. These structures present exceptional capacities when are employed as catalysts in electrochemical processes, as they maximize the activity per mass unit of the expensive material. Here we show how atomistic simulations based on density functional theory that take into account the environment can predict the morphology for the nanostructures and how it is even possible to address the appearance of concave structures. To describe the control by etching, we have reformulated the Wulff construction through the use of a geometric model that leads to concave polyhedra, which have a larger surface-to-volume ratio compared to that for nanocubes. Such an increase makes these sorts of nanoparticles excellent candidates to improve electrocatalytic performance

Nanoparticles are applied in a variety of industrially relevant transformations as heterogeneous catalysts typically with the help of an external force (pressure, temperature, or voltage) to steer the chemistry. The modification of platinum nanoparticles by a phosphate–amino surfactant enables catalysis without external energy supply in the hydrogenation of nitrobenzene to aniline. This can be attributed to the complex surfactant/metal interface which is able to split hydrogen into protons and electrons. The subsequent hydrogenation process mimics the electrochemical reduction described by Haber. The surfactant decorated Pt catalyst is two orders of magnitude more active than the state-of-the-art Pb-poisoned Pt catalyst. Our study provides a new approach to understand the functionality of emerging catalytic systems and can be applied to design new materials with optimal interfaces.

Continuum solvation models have become a widespread approach for the study of environmental effects in Density Functional Theory (DFT) methods. Adding solvation contributions mainly relies on the solution of the Generalized Poisson Equation (GPE) governing the behavior of the electrostatic potential of a system. Although multigrid methods are especially appropriate for the solution of partial differential equations, up to now, their use is not much extended in DFT-based codes because of their high memory requirements. In this Article, we report the implementation of an accelerated multigrid solver-based approach for the treatment of solvation effects in the Vienna ab initio Simulation Package (VASP). The stated implicit solvation model, named VASP-MGCM (VASP-Multigrid Continuum Model), uses an efficient and transferable algorithm for the product of sparse matrices that highly outperforms serial multigrid solvers. The calculated solvation free energies for a set of molecules, including neutral and ionic species, as well as adsorbed molecules on metallic surfaces, agree with experimental data and with simulation results obtained with other continuum models.

The selective C2 epimerization of the glucose/mannose pair on a set of Mo-based catalysts was studied by means of density functional theory. The process, known as the Bilik reaction, encompasses a 1,2 C-shift of the C3 centers at the sugars. Molybdic acid was initially proposed as a catalyst in this reaction, and recent experimental studies have shown that the polyoxometalate (POM) Keggin cluster H3PMo12O40 also presents a good performance. In the present work, we propose a reaction mechanism for the epimerization on the Keggin cluster with different heteroatoms and extend it to a larger POM, H6P2Mo18O62, and the continuous α-MoO3(010) surface. We have found that in the transition state corresponding to the 1,2 C-shift the Mo center acts as an electron buffer that promotes the transformation of the aldehyde group in C1 into an alkoxy group and the C2 alkoxy into an aldehyde group. As a consequence, the activity of Mo-containing compounds can be traced back to the reducibility of the Mo center and a simple microkinetic model illustrates that this descriptor generates an activity volcano. This allows the identification of a new POM that shall be 4.7 times more active than the parent compound. We have thus shown that continuum models linking the properties of molecular cluster-like catalysts and oxide surfaces can be derived and this paves the way towards a unified theory in catalysis.

Solvation is crucial in many chemical and electrochemical processes related to alcohol conversion on metal surfaces. Particularly, understanding the dehydrogenation mechanism of methanol on solvated Pd, Pt, and Ru surfaces could allow the design of efficient methanol fuel cells. The large computational cost related to adopting an explicit solvation approach into density functional theory can be reduced drastically by using implicit solvation methods. In this study, we use our recently developed continuum solvation model (MGCM) to elucidate the minimum number of explicit water molecules to add to the solvated methanol/metal surface systems to reproduce experimental data with an optimized balance between time and reliability. Our results stress the importance of adding two explicit water molecules, especially for the case of Ru surfaces. For this later system, we provide a first insight into the decomposition mechanism of methanol using first-principles calculations. Our predictions can be then a useful reference for future studies that aim at designing more efficient heterogeneous catalysts with solvents.

The relationship between the homogeneous and heterogeneous phases of a catalyst is widely neglected in part due to the inherent differences between the experimental and theoretical techniques employed to study them. It is well- known that, under reaction conditions, many homogeneous catalysts deactivate and generate black metals (i.e., nano- particles). Simultaneously, heterogeneous catalysts tend to suffer of leaching processes under harsh conditions, which produce the formation of species in the homogeneous phase (i.e., volatile or organometallic species). To unravel the links between these two types of catalytic species, we have taken PdAu catalysts in the oxidation of crotyl alcohol to crotonaldehyde and investigated the reaction process for both homogeneous and heterogeneous phases. We show that the process is possible in both phases and, essentially, contains the same elementary steps. The results indicate that the homogeneous catalyst is slightly more active; however, the enhanced stability of the heterogeneous phase provides a better performance under relevant reaction conditions. Both catalytic systems are connected through two simple steps that can be computed: oxidative leaching and deposition. The oxidative leaching of the PdAu nanoparticles in the presence of dioxygen can produce Pd(II) monomeric species able to catalyze the alcohol oxidation in homogeneous conditions. After the reaction the reduced Pd(0) homogeneous catalyst is reabsorbed onto the PdAu nanoparticles, preventing the aggregation process. The present work shows that the full homogeneous/heterogeneous catalytic cycle can be analyzed in a holistic manner with computational techniques.

International audience; The adsorption of CO2, CH4, and N-2 at 303 K by MIL-91(Al), one of the few porous phosphonate-based-MOFs, has been investigated by combining advanced experimental and computational tools. Whereas CH4 and N-2 adsorption isotherms exhibit type I behavior, the reversible CO2 isotherm displays an unusual inflection point at low pressure. In situ X-ray powder diffraction and infrared spectroscopy showed structural changes of this small-pore MOF upon CO2 adsorption. Grand canonical Monte Carlo simulations delivered a detailed picture of the adsorption mechanisms at the microscopic level. The so-predicted arrangements of the confined CO2 molecules were supported by analysis of the in situ diffraction and infrared experiments. It was shown that while adsorbed CH4 and N-2 are located mainly in the center of the pores, CO2 molecules interact with the hydrogen-bonded POHN acidbase pairs. This results in a relatively high adsorption enthalpy for CO2 of ca. -40 kJ mol(-1), which suggests that this material might be of interest for CO2 capture at low pressure (postcombustion).