Adsorption properties of Ni4 and Ni8 clusters supported on regular and defect sites of the MgO (0 0 1) surface

Using density functional calculations we have studied the interaction of oxygen and sodium atoms with free and MgO supported Ni4 and Ni8 clusters. The clusters are deposited on regular and defect sites, in particular on neutral and charged oxygen vacancies, Fs and Fs+ centers, of the MgO (1 0 0) surface. The MgO surface has been represented by clusters of ions embedded in arrays of point charges and effective core potentials. The structure of square Ni4 and tetragonal antiprism Ni8 clusters has been optimized in the gas-phase and on the oxide substrate. The two clusters represent simple models of two- and three-dimensional (2D and 3D) metal particles supported on MgO. When the nucleation of the metal cluster occurs in the vicinity of an Fs center, where the interaction with the substrate is considerably stronger, also the characteristics of adsorbed O or Na atoms are affected. This holds in particular for 2D Ni clusters, while 3D Ni8 exhibits an almost complete screening of the charge transfer occurring at the interface on the properties of the supported cluster.