Small gold species supported on alumina. A computational study of a-Al 2 O 3 (0001) and g-Al 2 O 3 (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 a-Al 2 O 3 (0001), n ¼ 1-6, and g-Al 2 O 3 (001), n ¼ 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 ¼ 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 a-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 g-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.
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