Novel Solid State Hybrid QM/MM Embedding Investigation into Methanol Synthesis over Cu Supported on ZnO Catalysts
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Novel Solid State Hybrid QM/MM Embedding Investigation into Methanol Synthesis over Cu Supported on ZnO Catalysts Samuel A. French a, S. T. Bromley a, A. A. Sokol a, C. R. A. Catlow a, J. Kendrick b, S. Rogers b and P. Sherwood c a Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, W1X 4BS, UK. b Imperial Chemical Industries PLC, Wilton, Teeside, TS90 8JE, UK. c CLRC Daresbury Laboratory, Warrington, WA4 4AD, UK. ABSTRACT A new solid-state embedding approach has been developed which focuses on modelling the surfaces of polar materials. The method is applied to investigate the chemisorption of premethanol species on the polar (000-1) surface of zincite (a major phase of zinc oxide having the wurtzite structure). Initial results include the geometries of active sites and adsorbates in different charge states. INTRODUCTION The study of industrial physical and chemical processes at a microscopic level initiated recent developments in hybrid QM/MM techniques, which allows one to apply different levels of approximation at different length scales. In particular the nature of active sites in catalysts and characterisation of chemical processes on these sites are well suited for investigation with such an approach. We have chosen the long-standing industrial catalytic process of methanol synthesis from the feed gas CO2/CO/H2 over the multicomponent Cu/ZnO/Al2O3 catalyst as a test for our new surface solid state embedding method. We aim to provide a detailed atomistic model of a possible active site and mechanism of synthesis. This modern industrial catalyst has been shown to have an analogous mechanism to pure ZnO (see Table I), which has been used as a simplified model in this study [1]. Table I. Comparison of model and industrial catalyst. ZnO Cu/ZnO/Al2O3 • Active surface is ZnO polar (0001) or • Cu metal/oxide surface (000-1) • Methanol formed via CO2 + H2 and not CO + H2 • Co-adsorption of CO2 and H2 leads to chemisorption / reaction • Involvement of O vacant site on surface • Rate depends on Cu surface area I • CO adsorption involves a surface anion • Involvement of Cu0/Cu sites in partially oxidised Cu surface • Important intermediates: • Important intermediates: –Formate HCO2 –Formate HCO2–Methoxy CH3O–Methoxy CH3O–Carbonate CO3? • Higher T reaction over ZnO has similar • ZnO maybe an important reservoir for adsorbed mechanism to Cu reactants AA9.3.1
We have, therefore, concentrated on the polar (OOO-1) oxygen terminated surface. The polar character of the surface necessitates a charge transfer between opposing polar surfaces, in order to quench surface induced polarisation of the material, which may be achieved in zincite by abstracting ca. 25% of oxygen ions from the surface layer, thus creating vacant oxygen interstitial surface sites. The presence of such sites has been confirmed by optical and ESR spectroscopies. They have, moreover, been suggested as the active catalytic sites for methanol synthesis. We have studied the adsorption on this site of the importa
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