(583ae) Adsorption of Pd and Au Over Fe3O4(001) Surface and Effect of CO Molecule

In order to understand the catalytic activity of magnetite supported palladium and gold catalysts, the adsorption configurations and energies of single Pd and Au atoms on the Fe₃O₄(001) surfaces were studied using density functional theory based GGA+U calculations. Various adsorption sites for Pd and Au atoms on 9-layer and 17-layer slabs were considered, from which two strongly favored geometries emerged. Gold atoms prefer to be linearly coordinated to two surface oxygen atoms (as in the case of Cu₂O). This is facilitated in a tilted geometry, in which one surface oxygen atom relaxes into the surface and one relaxes outward; gold is accommodated in the surfaca plane. Palladium is also twofold coordinated to surface oxygens in a bent molecular geometry, therefore protrudes from the surface; the oxygen atoms remain in the surface plane. Both adsorbates are positively charged, resulting in reduced magnetic moments and altered orbital occupancies on iron atoms. In addition, the effect of CO addition on these surfaces was investigated, and found to differ significantly for two precious metals. CO molecule binds weakly to Au, but strongly to Pd, having a strong effect on the Pd-surface geometry and the electronic structure. The activation barriers that clarify possible diffusion pathways were calculated for both adsorbates. It was found that CO has a 'skyhook' effect on Pd adatoms and is responsible for Pd mobility in the system. In case of Fe₃O₄-supported Au system, CO does not affect the electronic structure and adsorption configurations of Au since the surface is inert toward CO molecule.