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, F_s and F_s⁺ 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 F_s 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.