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

Abstract

Abstract We calculated the structures of and analyzed the bonding in adsorption complexes of small gold species Au n on α ‐Al 2 O 3 (0001), n = 1–6, and γ‐Al 2 O 3 (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 ‐Al 2 O 3 (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‐Al 2 O 3 (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.