Adsorption of dimers and trimers of Cu, Ag, and Au on regular sites and oxygen vacancies of the MgO(001) surface:: a density functional study using embedded cluster models

We studied the adsorption of small coinage metal aggregates M-n ( M = Cu, Ag, Au; n = 2, 3) on the ideal MgO( 001) as well as on oxygen vacancies, F-S and F-S(+) using a first-principle computational approach. We applied a generalized-gradient density functional in combination with cluster embedding in an elastic polarizable environment, which provides an accurate description of substrate relaxation. In the same way as single adsorbed atoms, metal moieties on regular O2- surface sites are polarized and interact mainly by electrostatic attraction, counteracted by Pauli repulsion. However, adsorption on vacancies involves some transfer of electron density to the adsorbate, particularly for Au-2. Our cluster results for Cu-2 and Cu-3 on regular O2- sites agree quantitatively with previous results of periodic slab models. The adsorption energy per atom decreases from dimers to trimers in line with the fact that metal-metal cohesion dominates over metal-oxide interaction. Compared to regular sites on flat terraces, dimerization is not particularly favorable on F-S sites and it is unfavorable on F-S(+) sites.