Cluster Model Simulations of Metal-Doped Amorphous Silicates for Heterogeneous Catalysis

Abstract

This Feature Article discusses how cluster models can be effectively used for quantum chemistry simulations of metal-doped amorphous silicates. These materials have been successfully used as heterogeneous catalysts for a variety of reactions, including olefin metathesis and polymerization, and alcohol dehydration. The amorphous surface provides a large surface area and distorted metal sites that are very reactive. However, the disordered microscopic nature of these silicates makes them hard to characterize, and progress toward better catalysts relies on a trial-and-error approach. Simulations can in principle provide insights on structure–property relations that may lead to a rational design approach, but again the disordered structure of the active sites makes the formulation of reliable models difficult. Using cluster models, it is possible to create multiple replicas of the same site with different distorted structures and silica environments, such that one can obtain a more realistic picture of the behavior of the material. Simulations based on the METal-doped Amorphous SIlicate Library (METASIL), which includes 70 cluster structures for single-site Zr-, Nb-, and W-doped silica, show very good agreement with a variety of experimental characterization techniques. These calculations validate the models and, more importantly, they allow to identify important structural descriptors of the active sites as well as a number of potential issues in the interpretation of experimental data. The Feature Article also discusses what challenges are still open in the simulation of amorphous materials and how these may be addressed in combination with new approaches such as machine learning algorithms.

Simple Model and Spectral Analysis for a Fluxional Catalyst: Intermediate Abundances, Pathway Fluxes, Rates, and Transients

“…The solutions typically invoke additional approximations, for example, rate-determining steps, quasi-equilibrium steps, and the pseudo-steady-state approximation (PSSA) . Microkinetic models also tacitly assume uniform sites and adopt a mean field description of surface coverages with no spatial resolution. , These assumptions/approximations can err badly, for example, in catalysts with quenched disorder − or when interactions between neighboring adsorbates cause fluctuations in the surface coverage. − In contrast to the other approximations, the PSSA is extremely reliable in catalysis. The simple but quantitative justification, first noted by Aris and co-workers, is that catalyst sites are (almost always) vastly outnumbered by reactants.…”

Modeling Catalyzed Reactions on Metal-Doped Amorphous Silicates: The Case of Niobium-Catalyzed Ethylene Epoxidation

“…DFT simulations are then conducted with either periodic boundary conditions (PBC), [26][27][28][29][30] or by carving out molecular cluster models. [31][32][33][34] PBC simulations preserve the solid nature of the material, but they require large simulation cells to model disorder and they are computationally intensive. Cluster models are computationally cheaper, but one needs to minimize computational artifacts due to the finite cluster size.…”

“…35 We have shown that large cluster models (about 300 atoms in size) can accurately reproduce a variety of experimental characterization techniques. [31][32][33][34] However, small size models are really necessary to conduct efficient quantitative studies of reactivity. 8 The goal of this work is to address a variety of issues related to simulations of reactivity on metal-doped amorphous silicates (although the same issues apply to most amorphous supports) that hinder our ability to evaluate effective reaction rates directly comparable to experiment.…”

Electrolyte clusters as hydrogen sponges: diffusion Monte Carlo simulations

“…This approach allows one to create multiple replicas of adsorption sites with different distorted structures and provide a more realistic picture of the structure–property relations that may lead to a rational design approach. 15–18 In the case of charged clusters, it is also possible to gather information by means of hole-burning tagging techniques. 19,20…”

“…This approach allows one to create multiple replicas of adsorption sites with different distorted structures and provide a more realistic picture of the structure-property relations that may lead to a rational design approach. [15][16][17][18] In the case of charged clusters, it is also possible to gather information by means of hole-burning tagging techniques. 19,20 One needs only consider the extensive exploration of small to medium sized Lennard Jones clusters published to date, [21][22][23] to come to the realization that, by itself, the characterization of Electrolyte Clusters (ECs) remains in its infancy.…”