Small Metal Clusters on MgO OOl

The interaction of TM metal clusters with the surface of MgO has been studied with ab initio methods with both cluster [201-204] and slab [205-207] models but mostly small supported clusters, containing less than 10 atoms, have been considered. The theoretical study of these systems is rather complex because of the very many isomers that one can have. The number of these isomers increases exponentially with the cluster nuclearity making an accurate evaluation with ab initio methods of the relative stabilities very difficult and extremely time consuming. The metals studied so far are Co, Ni, Cu, Pd, Ag, and Au. Some of isomers studied for Co, Ni, Cu, Pd, Ag tetramers are shown in Fig. 2.10. They can be classified into three main groups Planar or nearly planar structures with the cluster plane parallel to the MgO surface, Fig. 2.10a-c, planar structures with the cluster plane normal to the surface, Fig. 2.10d-f and tetrahedral or distorted tetrahedral structures (Fig. 2.10g-i). 

The results show that nanoclusters adsorbed on the regular MgO(OOl) surface do not necessarily tend to adhere to the surface with the largest possible number of metal atoms (surface wetting) but rather that they keep some bond directionality. This results from the balance of various terms, the energy gain due to the bond formation with the O anions, the Pauli repulsion with the surface, and the loss of metal-metal bonding within the cluster due to distortions of the metal frame. 

When the metal-oxygen bond is sufficiently strong, e.g., as for Ni and Pd, the formation of new interface bonds may compensates the loss of metal-metal bonds and the cluster distorts from its gas-phase geometry (Fig. 2.10i). For weaker metal-oxide bonds, as found for Co, Cu, and Ag, the metal-metal interactions prevail over the metal-MgO ones so that the cluster maintains, to a large extent, its electronic and geometric structure (Fig. 2.10f). Ni4 and Pd4, which form relatively strong bonds with MgO, tend to adhere with more metal atoms to the surface leading to a distorted tetrahedron while in the 

The magnetization on the Ni and Co clusters is largely unchanged also in the supported species. In some cases, however, there is a partial quenching of the magnetic moment which is generally restricted to the metal atoms in direct contact with the oxide anions [203]. Thus, despite the relatively strong MgO/M4 bonds (C04 is bound on MgO by 2.0eV, Ni4 by 2.4eV), the electronic structure of supported transition metal moieties is only moderately perturbed. These conclusions are valid only for an ideal defect-free surface  

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