Density functional embedded cluster study of Cu₄, Ag₄ and Au₄ species interacting with oxygen vacancies on the MgO(001) surface

Cu₄, Ag₄, and Au₄ species adsorbed on an MgO(001) surface that exhibits neutral (F_s) and charged (F _s⁺) oxygen vacancies have been studied using a density functional approach and advanced embedding models. The gas-phase rhombic-planar structure of the coinage metal tetramers is only moderately affected by adsorption. In the most stable surface configuration, the plane of the tetramers is oriented perpendicular to the MgO(001) surface; one metal atom is attached to an oxygen vacancy and another one is hound to a nearby surface oxygen anion. A very similar structural motif was recently found on defect-free MgO(001), where two O^2- ions serve as adsorption sites. Following the trend of the interactions with the regular MgO(001) surface, Au₄ and Cu ₄ bind substantially stronger to F_s and F_s ⁺ sites than Ag₄. This stronger adsorption interaction at oxygen vacancies, in particular at F_s, is partly due to a notable accumulation of electron density on the adsorbates. We also examined the propensity of small supported metal species to aggregate to adsorbed di-, tri- and tetramers. Furthermore, we demonstrated that core-level ionization potentials offer the possibility for detecting experimentally supported metal tetramers and characterizing them structurally with the help of calculated data.