Pd and Ag dimers and tetramers adsorbed at the MgO(001) surface:: a density functional study

We have studied computationally the adsorption properties of small Ag and Pd clusters deposited on the MgO(001) surface. The calculations were carried out employing a gradient-corrected density functional approach; the oxide surface was represented by model clusters embedded in a large array of point charges. Supported Ag and Pd dimers and tetramers were investigated in order to identify the preferred adsorption sites and the modifications induced in the cluster by the interaction with the substrate. All metal clusters adsorb in proximity of oxygen centers. An adsorption mode with the molecular axis parallel to the surface is the most stable one for Pd-2 while Ag-2 prefers an upright adsorption mode. Various isomer structures of the supported metal tetramers have been considered. In general, the most stable gas phase structure is also the preferred one upon adsorption. This suggests that the metal-metal bonding prevails over the metal-MgO interaction.