Metal Deposition on Oxide Surfaces: A Quantum-Chemical Study of the Interaction of Rb, Pd, and Ag Atoms with the Surface Vacancies of MgO

Abstract

The interaction of Rb, Pd, and Ag atoms with the surface vacancies of MgO, the F s and the V s centers, has been studied by ab initio cluster model wave functions. We have considered the interaction of each atom with F s, F s +, F s 2+, V s, V s -, and V s 2- sites. These sites correspond to the removal of O, O -, O 2-, Mg, Mg +, and Mg 2+ atoms or ions, respectively, from the surface. The bond with the metal atoms, which is found to be very weak on the regular surface sites, can be very different depending of the formal charge of the vacancy. Neutral F s centers are in general rather unreactive as their electronic structure resembles that of the regular surface; F s + paramagnetic centers have a relatively large electron affinity and tend to ionize metal atoms with low ionization potentials, such as alkali-metal atoms or to form covalent polar bonds with the adsorbed metal atoms; F s 2+ centers have a very high electron affinity so that all metal atoms are ionized when interacting with these sites. Neutral V s sites are also electron deficient; here the metal atoms tend to form a dication and to replace the missing Mg ion in the lattice with large gain in the electrostatic energy. On V s - vacancies the metal atoms lose one electron and become singly ionized with formation of strong ionic bonds at the interface. Finally, no charge transfer occurs between the metal atoms and the electronically saturated V s 2- sites; in this case the bond strength is due only to the metal polarizability.