Data Mining Practical Machine Learning Tools and Techniques

Solvents are crucial components in specialty chemical and pharmaceutical industries and in electrochemical and photoelectrochemical processes, and are increasingly being used in catalytic reactions. Solvents significantly influence the kinetics and thermodynamics of reactions and can alter product selectivity markedly. While such solvent effects are observed routinely, identification of the root causes of such effects is less frequent. Solvents can influence reaction rates, conversion and product selectivity by 1) directly participating in the reaction steps and opening alternate reaction pathways, 2) competing with the reactant for interaction with the catalysts, 3) changing the relative stabilization of the reactant, the transition state (TS) and/or the product, 4) altering intra-pore diffusion characteristics in porous catalysts, 5) exhibiting entropic confinement effects altering free energy barriers of reactions, 6) changing the solubility of different components in the reaction mixture, and 7) inhibiting undesired reactions. Their indirect influences may be due to 1) changes brought on to active sites on catalysts and 2) altered structure/stability of catalysts. This article discusses these fundamental reasons behind observed solvent effects with suitable examples. Advances in computational chemistry have led to the development of multiple tools and techniques, considering solvents either as implicit or as explicit molecules, providing molecular insights into complex solvent effects in catalysis. This article provides an overview of some of these methods with suitable examples to demonstrate their application and potential. This mapping of the solvent effects and their origins is believed to aid in the rational selection of solvents for catalytic reactions. The description of the computational tools, their application and their potential is likely to encourage widespread use of these techniques to investigate solvent effects.

Origins of complex solvent effects on chemical reactivity and computational tools to investigate them: a review

Solvation effects on DFT predictions of ORR activity on metal surfaces

The liquid water–Pt(111) interface is studied with constant temperature ab initio molecular dynamics to explore the importance of liquid water dynamics of catalytic reactions such as the oxygen reduction reaction in PEM fuel cells. The structure and energetics of hydroxyls formed at the liquid water–Pt(111) interface are found to be significantly different from those of the hydroxyl formed on a bare Pt(111) surface and the hydroxyl formed on a Pt(111) surface with a static water layer. We identify 1/12 ML *OH, 5/12 ML *OH and 2/3 ML *OH as particularly stable hydroxyl coverages in highly dynamic liquid water environments, which – contrary to static water–hydroxyl models – contain adjacent uncovered Pt sites. Atomic surface oxygen is found to be unstable in the presence of liquid water, in contrast to static atomic level simulations. These results give an improved understanding of hydroxide and surface oxide formation from Pt(111) cyclic voltammetry and allow us to draw detailed connections between the electrostatic potential and the interface structure. The study of hydrogen adsorption at the liquid water–Pt(111) interface finds competitive adsorption between the adsorbed hydrogen atoms and water molecules. This does not adhere with experimental observations, and this indicates that the Pt(111) surface has to be negatively charged for a correct description of the liquid water–Pt(111) interface at potentials where hydrogen adsorption occurs.

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OH formation and H 2 adsorption at the liquid water-Pt(111) interface

Implicit solvation is an effective, highly coarse-grained approach in atomic-scale simulations to account for a surrounding liquid electrolyte on the level of a continuous polarizable medium. Originating in molecular chemistry with finite solutes, implicit solvation techniques are now increasingly used in the context of first-principles modeling of electrochemistry and electrocatalysis at extended (often metallic) electrodes. The prevalent ansatz to model the latter electrodes and the reactive surface chemistry at them through slabs in periodic boundary condition supercells brings its specific challenges. Foremost this concerns the diffculty to describe the entire double layer forming at the electrified solid-liquid interface (SLI) within supercell sizes tractable by commonly employed density-functional theory (DFT). We review liquid solvation methodology from this specific application angle, highlighting in particular its use in the widespread {\em ab initio} thermodynamics approach to surface catalysis. Notably, implicit solvation can be employed to mimic a polarization of the electrode's electronic density under the applied potential and the concomitant capacitive charging of the entire double layer beyond the limitations of the employed DFT supercell. Most critical for continuing advances of this effective methodology for the SLI context is the lack of pertinent (experimental or high-level theoretical) reference data needed for parametrization.

Implicit Solvation Methods for Catalysis at Electrified Interfaces

Liquid-metal interfaces occur in a number of surface processes, and it is fundamentally important to accurately model these interfaces. We have systematically determined how the presence of water affects several processes using density functional theory with implicit solvation models. We modeled adsorption of 41 common adsorbates, as well as four catalytic reactions in both vacuum and water over the Pt(111) surface. Our results show that adsorption energies for some species can change significantly in the presence of water (up to 0.44 eV). We further show that solvation effects may be explained and predicted by analyzing simple chemical descriptors such as dipole moment and adsorbate charge. We also report models from artificial neural networks using several potential descriptors, including gas-phase solvation energy, adsorbate charge, dipole moment, and surface area. When water was present reaction energies changed by up to 0.23 eV, although it appeared that water solvent negligibly affected several elementary reaction steps. Our results show that hydrogen bonding can be important for a number of reactions, which are largely absent in the implicit solvation models. Furthermore we also modeled other solvents besides water and found that when a solvent has low dielectric constant, then small solvation effects occurred. Our work provides guidelines on when solvation effects may be important for surface chemistry, and also provides valuable insights on modeling such effects.

Assoc. Prof. N. Aaron Deskins

Papers

Evaluating Solvent Effects at the Aqueous/Pt(111) Interface.