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 Aun 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.