Adsorption properties of Ni4 and Ni8 clusters supported on regular and defect sites of the MgO (001) surface

Using density functional calculations we have studied the interaction of oxygen and sodium atoms with free and MgO supported Ni-4 and Ni-8 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 (100) 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 Ni-4 and tetragonal antiprism Ni-8 clusters has been optimized in the gas-phase and on the oxide substrate. The two clusters represent simple models of two- and three-dimensional (213 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 0 or Na atoms are affected. This holds in particular for 2D Ni clusters, while 3D Ni-8 exhibits an almost complete screening of the charge transfer occurring at the interface on the properties of the supported cluster. (C) 2001 Elsevier Science B.V. All rights reserved.