Adsorption and dissociation of water on relaxed alumina clusters: A first principles study

Using previous results for the equilibrium geometries of stoichiometric (Al₂O₃)_n clusters as models for non ideal alumina surfaces, we obtain, from ab-initio total energy LCAO calculations, the corresponding relaxed structures of the complexes H₂O-(Al ₂O₃)_n with n ≤ 7. Depending on the initial position of the water molecule relative to the cluster site, the complex evolves to different equilibrium structures, with and without dissociation of H ₂O, whose energetic, bond lengths, and charge transfer trends are studied as the morphology and size of the initial cluster change. Dissociation of H₂O with the radical OH^- bound on top of an Al atom and the proton H⁺ bound to the second nearest neighbour O, is the dominant process for the reaction (Al₂O₃)_n + H₂O with n = 4, in agreement with the one observed for the adsorption of H₂O on the extended real surface.