Adsorption of transition metal atoms on oxygen vacancies and regular sites of the MgO(001) surface

Adsorption of Cu, Ni, Ag, and Pd atoms on F_s and F_s⁺ oxygen vacancy sites as well as on regular O^2- centers of the MgO(001) surface has been studied by means of gradient-corrected density functional calculations using cluster models embedded in a matrix of model potentials and point charges. Scalar relativistic effects have been taken into account for adsorbed Ag and Pd species. The electronic structure, geometric parameters, and binding energies of the adsorption complexes have been calculated and analyzed with reference to the electronic properties of the vacancy sites and the metal atoms in question. For all adsorbates considered, adsorption is found to be stronger on F_s sites by 1-2.4 eV compared with regular O^2- sites, with Pd and Ni forming the most stable complexes. On the F_s⁺ site the single valence electron of Cu and Ag atoms couples with an unpaired electron of the vacancy forming a covalent bond. As a result the adsorption energy of these atoms on F_s⁺ is by more than 1 eV stronger than on the F_s sites; on the other hand, the adsorption energies of Ni and Pd are reduced on F_s⁺ by 0.5 eV and 1.3 eV respectively. The whole series of M/F_s⁺ complexes is characterized by rather uniform values of adsorbate-substrate distances (1.5-1.7 angstroms) and adsorption energies (2.2-2.6 eV).