A theoretical investigation in heterogeneous gold catalysis

Phala, Noko Simon

A theoretical investigation in heterogeneous gold catalysis

Thesis

2004

Publisher

University of Cape Town

Department

Centre for Catalysis Research

Faculty

Faculty of Engineering and the Built Environment

Abstract

Despite the nobleness of bulk gold metal in air, small supported gold particles have been shown experimentally to be active in a wide range of chemical reactions. The objective of this work was to study, theoretically, some of the fundamental aspects of the reactivity of gold catalysts. Using activation of CO, CO2 and H2 as a test case, periodic and cluster density functional theory (DFT) calculations, within the generalized-gradient approximation (GGA), were performed to investigate the change in nobility of gold from the extended surface to small clusters. Potential methanol synthesis intermediates were optimized on the Au(111) surface. It was found that the molecules that are stable as gasphase species generally adsorbed weakly on the surface. Surface hydrogenation of CO-derived species appeared to be easier than surface hydrogenation of CO2- derived species. On an AU13 cluster, the energetics of CO2 adsorption and hydrogenation remain unfavourable. The cluster-size dependency of hydrogen and carbon monoxide adsorption was investigated. It was found that small gold clusters (1 to 13 atoms in size) can bind both H and CO strongly. Due to the changes in the orbital spatial symmetries and the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with cluster size in this small size range, the adsorption energies depend very strongly on the number of gold atoms present, i.e. each atom makes a difference. For H adsorption, there is a very marked oscillation in adsorption energies, with the clusters with an odd number of gold atoms (with lower LUMO energies) being generally more reactive than the even clusters, up to about 10 atoms when the HOMO-LUMO gap ceases to fluctuate strongly. The role of the support material in activating gold atoms was studied. A hybrid quantum mechanics/molecular mechanics (QMlMM) electronic embedding technique was employed to model the ZnO(000l) surface of zincite. The QM region of the surface, treated by density functional theory, consisted of a total of 13 zinc and oxygen atoms for the zinc-vacant site, and 14 atoms for the bulk-terminated island site. It was found that Au0 and Au+ could be stabilized at the zinc vacant site of this surface. The higher oxidation states are unstable with respect to auto-reduction by the ZnO surface (i.e. their LUMO energies were below the HOMO of a bare ZnO surface. However, gold hydroxyls, where gold has + 1 to +3 oxidation states, can be stabilized at the vacancy. As zinc-substitutional impurities on the bulk-terminated island site, Au+, Au2+ and Au3+ oxidation states can be stabilized. CO was used as a test molecule to probe the chemical reactivity of the gold atoms in different adsorption sites and oxidation states. It was found that supported Au+ was more reactive than Au0, Au2+, or Au3+. Furthermore, CO binds more strongly to supported Au0 than the free Au0 atom. This implies that the support does not simply disperse gold particles, but it also modifies their electronic properties. It was also found that the nucleation of gold atoms to clusters can be affected by the support. Supported charges Au clusters have shorter Au-Au distances than their gas-phase counterparts.

Description

Includes bibliographical references.

Keywords

Chemical Engineering

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