Hydrocarbon adsorption in an aqueous environment: A computational study of alkyls on Cu(111)

Non-Monotonic Variation of Potential-Dependent Surface Diffusion at Electrochemical Interfaces in the Presence of Coadsorbates

The influence of coadsorbed ions on adsorbate diffusion, an inherent effect at solid-liquid interfaces, was studied for adsorbed sulfur on Ag(100) electrodes in the presence of bromide or iodide. Quantitative in situ high-speed scanning tunnelling microscopy (video-STM) measurements were performed both in the potential regime of the c(2×2) halide adlayer at its saturation coverage and in the regime of a disordered adlayer where the halide coverage increases with potential. These studies reveal a surprising non-monotonic potential dependence of Sad diffusion with an initial increase with halide coverage, followed by a decrease upon halide adlayer ordering into the c(2×2) structure. Density functional theory (DFT) and Monte Carlo (MC) simulations only qualitatively reproduce the rise in Sad mobility with halide coverage, suggesting that many-adsorbate interactions and the presence of the electrolyte need to be considered.

Computational Design of Catalysts with Experimental Validation: Recent Successes, Effective Strategies, and Pitfalls

Computation has long proven useful in understanding heterogeneous catalysts and rationalizing experimental findings. However, computational design with experimental validation requires somewhat different approaches and has proven more difficult. In recent years, there have been increasing successes in such computational design with experimental validation. In this Perspective, we discuss some of these recent successes and the methodologies used. We also discuss various design strategies more broadly, as well as approximations to consider and pitfalls to try to avoid when designing for experiment. Overall, computation can be a powerful and efficient tool in guiding catalyst design but must be combined with a strong fundamental understanding of catalysis science to be most effective in terms of both choosing the design methodology and choosing which materials to pursue experimentally.

Differences in solvation thermodynamics of oxygenates at Pt/Al2O3 perimeter versus Pt(111) terrace sites

A prominent role of water in aqueous-phase heterogeneous catalysis is to modify free energies; however, intuition about how is based largely on pure metal surfaces or even homogeneous solutions. Using multiscale modeling with explicit liquid water molecules, we show that the influence of water on the free energies of adsorbates at metal/support interfaces is different than that on pure metal surfaces. We specifically compute free energies of solvation for methanol and its constituents on a Pt/Al2O3 catalyst and compare the results to analogous values calculated on a pure Pt catalyst. We find that the more hydrophilic Pt/Al2O3 interface leads to smaller (more positive) free energies of solvation due to an increased entropy penalty resulting from the additional work necessary to disrupt the interfacial water structure and accommodate the interfacial species. The results will be of interest in other fields, including adsorption and proteins.

Water-Mediated Reduction of Aqueous N-Nitrosodimethylamine with Pd

Pd-catalyzed reduction has emerged as a promising treatment strategy to remove the recalcitrant disinfection byproduct N-nitrosodimethylamine (NDMA). However, the reaction pathways remain unexplored, and questions remain about how water solvent influences NDMA reduction mechanisms and selectivity. Here, we compute the energies and barriers of all relevant elementary steps in NDMA reduction by H₂ on Pd(111) using density functional theory. We further calculate water-assisted H-shuttling for all hydrogenation reactions explicitly and include water solvation for all elementary reactions implicitly. We parametrize microkinetic models to predict product formation rates and selectivities over a wide range of NDMA concentrations. We show that H₂O-mediated H-shuttling lowers the reaction barriers for all hydrogenation reactions involved in NDMA reduction while implicit solvation has negligible impact on the reaction and activation energies. We further conduct batch experiments with SiO₂-supported Pd nanoparticles and compare them with the microkinetic models. The predicted rates, selectivity, and apparent activation energy from the model parametrized with both explicit H₂O-mediated H-shuttling and implicit solvation correspond well with experimental observations. Models that ignore water as an H-shuttle or solvent fail to recover the experimental rates and apparent activation energy. We identified the rate-determining steps of the reaction and show the reaction flow pathways of the complicated reaction network. Finally, we demonstrate that water-mediated H-shuttling changes the rate-determining steps and reaction flows of elementary reactions.

Theoretical Understanding of Mechanisms of Proton Exchange Membranes Made of 2D Crystals with Ultrahigh Selectivity

Recent reports on proton conduction across pristine graphene and hexagonal boron nitride (h-BN) provide a new avenue for the design of proton exchange membranes. The uniform pores formed by the electron clouds of two-dimensional (2D) crystals can effectively block the undesired transportation of other species thus ultrahigh selectivity can be achieved. With the aid of first-principles calculations, we investigate the proton conduction process across six kinds of intact 2D crystals, namely graphene, h-BN, β₁₂ boron sheet, χ₃ boron sheet, phosphorene, and silicene. To clarify the proton conduction mechanism, three proton penetration modes are proposed: dissociation-penetration, adsorption-penetration, and direct penetration. Based on our calculation results, for graphene and h-BN without atomic defects, they are unlikely to provide sufficient proton conductivity at room temperature when no bias potential is applied. By contrast, the β₁₂ boron sheet, χ₃ boron sheets, and silicene exhibit relatively lower proton penetration energy barriers, making them prospective candidates for future proton exchange membrane applications.