Adsorption of Pd and Pt atoms on α-Al2O3(0 0 0 1): density functional study of cluster models embedded in an elastic polarizable environment

Complexes of single Pd and Pt atoms on a clean Al-terminated surface α-Al2O3(0 0 0 1) were calculated with a scalar relativistic density functional approach using the gradient-corrected BP86 exchange-correlation functional. Stoichiometric cluster models of the oxide substrate were designed, employing bare pseudopotential cations located at the cluster boundaries. Embedding of the clusters in an elastic polarizable environment (based on the shell model) accounted for the substrate relaxation in an accurate fashion. This relaxation notably affected structure and stability of the adsorption complexes. Stable complexes, with binding energies of 1.1 eV (Pd) and 1.7 eV (Pt), were calculated for metal atoms adsorbed on top of an O anion and also interacting with a nearby cation. A significant amount of the adsorption energy, 20% (Pd) and 33% (Pt), is due to substrate relaxation. The sites over the centers of equilateral oxygen triangles were not found to be minima of the potential surface; metal atoms in these positions are weakly bound. No indication was found for the oxidation of Pd or Pt atoms in equilibrium surface complexes at α-Al2O3(0 0 0 1).