Modelling metal centres, acid sites and reaction mechanisms in microporous catalysts

We discuss the role of QM/MM (embedded cluster) computational techniques in catalytic science, in particular their application to microporous catalysis. We describe the methodologies employed and illustrate their utility by briefly summarising work on metal centres in zeolites. We then report a detailed investigation into the behaviour of methanol at acidic sites in zeolites H-ZSM-5 and H-Y in the context of the methanol-to-hydrocarbons/olefins process. Studying key initial steps of the reaction (the adsorption and subsequent methoxylation), we probe the effect of framework topology and Brønsted acid site location on the energetics of these initial processes. We find that although methoxylation is endothermic with respect to the adsorbed system (by 17–56 kJ mol−1 depending on the location), there are intriguing correlations between the adsorption/reaction energies and the geometries of the adsorbed species, of particular significance being the coordination of methyl hydrogens. These observations emphasise the importance of adsorbate coordination with the framework in zeolite catalysed conversions, and how this may vary with framework topology and site location, particularly suited to investigation by QM/MM technique

Magnetic coupling constants for MnO as calculated using hybrid density functional theory

The properties of MnO have been calculated using generalised gradient approximation (GGA-) and hybrid (h-) density functional theory (DFT), specifically variants of the popular PBE and PBESol exchange–correlation functionals. The GGA approaches are shown to be poor at reproducing experimental magnetic coupling constants and rhombohedral structural distortions, with the PBESol functional performing worse than PBE. In contrast, h-DFT results are in reasonable agreement with experiment. Calculation of the Néel temperatures using the mean-field approximation gives overestimates relative to experiment, but the discrepancies are as low as 15 K for the PBE0 approach and, generally, the h-DFT results are significant improvements over previous theoretical studies. For the Curie–Weiss temperature, larger disparities are observed between the theoretical results and previous experimental results

A computational study of direct CO₂ hydrogenation to methanol on Pd surfaces

The reaction mechanism of direct CO2 hydrogenation to methanol is investigated in detail on Pd (111), (100) and (110) surfaces using density functional theory (DFT), supporting investigations into emergent Pd-based catalysts. Hydrogen adsorption and surface mobility are firstly considered, with high-coordination surface sites having the largest adsorption energy and being connected by diffusion channels with low energy barriers. Surface chemisorption of CO2, forming a partially charged CO2δ−, is weakly endothermic on a Pd (111) whilst slightly exothermic on Pd (100) and (110), with adsorption enthalpies of 0.09, −0.09 and −0.19 eV, respectively; the low stability of CO2δ− on the Pd (111) surface is attributed to negative charge accumulating on the surface Pd atoms that interact directly with the CO2δ− adsorbate. Detailed consideration for sequential hydrogenation of the CO2 shows that HCOOH hydrogenation to H2COOH would be the rate determining step in the conversion to methanol, for all surfaces, with activation barriers of 1.41, 1.51, and 0.84 eV on Pd (111), (100) and (110) facets, respectively. The Pd (110) surface exhibits overall lower activation energies than the most studied Pd (111) and (100) surfaces, and therefore should be considered in more detail in future Pd catalytic studies

Methanol loading dependent methoxylation in zeolite H-ZSM-5

We evaluate the effect of the number of methanol molecules per acidic site of H-ZSM-5 on the methoxylation reaction at room temperature by applying operando diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS), which capture the methoxylation reaction by simultaneously probing surface adsorbed species and reaction products, respectively. To this end, the methanol loading in H-ZSM-5 (Si/Al ≈ 25) pores is systematically varied between 32, 16, 8 and 4 molecules per unit cell, which corresponds to 8, 4, 2 and 1 molecules per Brønsted acidic site, respectively. The operando DRIFTS/MS data show that the room temperature methoxylation depends on the methanol loading: the higher the methanol loading, the faster the methoxylation. Accordingly, the reaction is more than an order of magnitude faster with 8 methanol molecules per Brønsted acidic site than that with 2 molecules, as evident from the evolution of the methyl rock band of the methoxy species and of water as a function of time. Significantly, no methoxylation is observed with ≤1 molecule per Brønsted acidic site. However, hydrogen bonded methanol occurs across all loadings studied, but the structure of hydrogen bonded methanol also depends on the loading. Methanol loading of ≤1 molecule per acidic site leads to the formation of hydrogen bonded methanol with no proton transfer (i.e. neutral geometry), while loading ≥2 molecules per acidic site results in a hydrogen bonded methanol with a net positive charge on the adduct (protonated geometry). The infrared vibrational frequencies of methoxy and hydrogen bonded methanol are corroborated by Density Functional Theory (DFT) calculations. Both the experiments and calculations reflect the methoxy bands at around 940, 1180, 2868–2876 and 2980–2973 cm−1 which correspond to ν(C–O), ρ(CH3), νs(C–H) and νas(C–H), respectively

Controlling structural transitions in AuAg nanoparticles through precise compositional design

We present a study of the transitional pathways between highsymmetry structural motifs for AgAu nanoparticles, with a specific focus on controlling the energetic barriers through chemical design. We show that the barriers can be altered by careful control of the elemental composition and chemical arrangement, with core@shell and vertex-decorated arrangements being specifically influential on the barrier heights. We also highlight the complexity of the potential and free energy landscapes for systems where there are low-symmetry geometric motifs that are energetically competitive to the high-symmetry arrangements. In particular, we highlight that some core@shell arrangements preferentially transition through multistep restructuring of lowsymmetry truncated octahedra and rosette-icosahedra, instead of via the more straightforward square-diamond transformations, due to lower energy barriers and competitive energetic minima. Our results have promising implications for the continuing efforts in bespoke nanoparticle design for catalytic and plasmonic applications

Embedded-Cluster Calculations in a Numeric Atomic Orbital Density-Functional Theory Framework

We integrate the all-electron electronic structure code FHI-aims into the general ChemShell package for solid-state embedding (QM/MM) calculations. A major undertaking in this integration is the implementation of pseudopotential functionality into FHI-aims to describe cations at the QM/MM boundary through effective core potentials and therewith prevent spurious overpolarization of the electronic density. Based on numeric atomic orbital basis sets, FHI-aims offers particularly efficient access to exact exchange and second order perturbation theory, rendering the established QM/MM setup an ideal tool for hybrid and double-hybrid level DFT calculations of solid systems. We illustrate this capability by calculating the reduction potential of Fe in the Fe-substituted ZSM-5 zeolitic framework and the reaction energy profile for (photo-)catalytic water oxidation at TiO2(110).Comment: 12 pages, 4 figure

Investigation of the Pd(1−x)Znx alloy phase diagram using ab initio modelling approaches

The identification of the stable phases in alloy materials is challenging because composition affects the structural stability of different intermediate phases. Computational simulation, via multiscale modelling approaches, can significantly accelerate the exploration of phase space and help to identify stable phases. Here, we apply such new approaches to understand the complex phase diagram of binary alloys of PdZn, with the relative stability of structural polymorphs considered through application of density functional theory coupled with cluster expansion (CE). The experimental phase diagram has several competing crystal structures, and we focus on three different closed-packed phases that are commonly observed for PdZn, namely the face-centred cubic (FCC), body-centred tetragonal (BCT) and hexagonal close packed (HCP), to identify their respective stability ranges. Our multiscale approach confirms a narrow range of stability for the BCT mixed alloy, within the Zn concentration range from 43.75% to 50%, which aligns with experimental observations. We subsequently use CE to show that the phases are competitive across all concentrations, but with the FCC alloy phase favoured for Zn concentrations below 43.75%, and that the HCP structure favoured for Zn-rich concentrations. Our methodology and results provide a platform for future investigations of PdZn and other close-packed alloy systems with multiscale modelling techniques

QM/MM study of the reactivity of zeolite bound methoxy and carbene groups

The conversion of methanol-to-hydrocarbons (MTH) is known to occur via an autocatalytic process in zeolites, where framework-bound methoxy species play a pivotal role, especially during catalyst induction. Recent NMR and FT-IR experimental studies suggest that methoxylated zeolites are able to produce hydrocarbons by a mechanism involving carbene migration and association. In order to understand these observations, we have performed QM/MM computational investigations on a range of reaction mechanisms for the reaction of zeolite bound methoxy and carbene groups, which are proposed to initiate hydrocarbon formation in the MTH process. Our simulations demonstrate that it is kinetically unfavourable for methyl species to form on the framework away from the zeolite acid site, and both kinetically and thermodynamically unfavourable for methyl groups to migrate through the framework and aggregate around an acid site. Formation of carbene moieties was considered as an alternative pathway to the formation of C–C bonds; however, the reaction energy for conversion of a methyl to a carbene is unfavourable. Metadynamics simulations help confirm further that methyl species at the framework acid sites would be more reactive towards formed C2+ species, rather than inter-framework migration, and that the role of carbenes in the formation of the first C–C bond will be via a concerted type of mechanism rather than stepwise

Computational QM/MM investigation of the adsorption of MTH active species in H-Y and H-ZSM-5

The transformation of methanol-to-hydrocarbons (MTH) has significant potential as a route to synthesise low-cost fuels; however, the initial stages of the zeolite catalysed MTH process are not well understood. Here, we use hybrid quantum- and molecular-mechanical (QM/MM) embedded-cluster simulations to develop our understanding of the interaction between methanol and the zeolite catalysts H-ZSM-5, and for comparison, the larger pore H-Y. Energies and structures, calculated using hybrid-level density functional theory (hybrid-DFT) and higher-level correlated methods, are compared with previous experimental and computational results. We show that hydrogen-bonds between methanol adsorbates, formed through polarizable O–H bonds, substantially influence the adsorption energetics, structural parameters and vibrational frequencies. Our observations are extended by considering polar solvent molecules in the environment, with the presence of both water or methanol around the adsorption site leading to barrier-less transfer of the zeolite proton to an adsorbed methanol, which will significantly influence the reactivity of the adsorbed methanol

QM/MM study of the stability of dimethyl ether in zeolites H-ZSM-5 and H-Y

The methanol-to-hydrocarbons (MTH) process transforms C1 carbon sources to higher hydrocarbons, but details of the mechanism that leads to the formation of the first carbon-carbon bond remain unclear. Here, we present a computational investigation of how a crucial intermediate, dimethyl ether (DME), interacts with different zeolite catalysts (H-ZSM-5, H-Y) to gain insight into the initial stages in the MTH process. We use QM/MM computational simulations to model the conversion of methanol to DME in H-ZSM-5, which is a well characterised and important reaction intermediate. We analyse and compare the stability of DME on several acid sites in H-ZSM-5 and H-Y, and show that the more acidic and open “intersection sites” in the H-ZSM-5 framework are able to bond strongest with DME, with complete deprotonation of the acid site occurring. The conversion of methanol to DME in H-ZSM-5 is observed as requiring a higher activation energy than framework methoxylation, which indicates that a stepwise (indirect) mechanism, through a methoxy intermediate, is the most likely route to DME formation during the initiation of the MTH process