Small gold species supported on alumina. A computational study of α-Al₂O₃(0001) and γ-Al₂O₃(001) using an embedded-cluster approach

We calculated the structures of and analyzed the bonding in adsorption complexes of small gold species Au_n on α-Al₂O₃(0001), n = 1-6, and γ-Al₂O₃(001), n = 1-5. We applied a scalar-relativistic gradient-corrected density functional (DF) method to cluster models of the support that were embedded in an extended elastic polarizable environment (EPE). The shortest Au-O distances, 204-211 pm, are consistent with extended X-ray adsorption fine structure (EXAFS) data for gold clusters on alumina surfaces. The calculated total adsorption energies increase with cluster nuclearity, up to n=4, but drop for larger adsorbed species. In the gas phase, these small gold clusters exhibit a planar structure which they keep, oriented parallel to the surface, as adsorbates on α-Al₂O₃(0001). Unfavorable energy contributions result for larger clusters as their planar shape is notably distorted by the interaction with the support which amounts to 0.5-1.5 eV. On γ-Al₂O₃(001), also the larger gold clusters retain their intrinsic planar structure as they adsorb oriented perpendicular to the surface. The corresponding adsorption energies are slightly smaller, 0.3-1.2 eV.