Ab initio calculations of H₂O and O₂ adsorption on Al₂O₃ substrates

We present and discuss the results of ab initio calculations for the adsorption of H₂O and O₂ on different Al₂O₃ substrates, namely the Al terminated (0 0 0 1) surface of corundum, and amorphous-like (Al₂O₃)_n clusters with n = 2-7 formula units. We employ two types of first-principles computer codes, CRYSTAL and SIESTA, which are based on the Hartree-Fock and Kohn-Sham equations, respectively. The former is used for the adsorption calculations on the α-Al₂O₃(0 0 0 1) surface, and the latter is used for the study of adsorption and dissociation of H₂O and O₂ on both kinds of substrates allowing internal relaxation. We have also performed complementary research of adsorption and dissociation of water on (Al₂O₃)_n amorphous-like clusters with n = 2-7. As a next point, we have calculated the binding energy of an O₂ molecule on α-Al₂O₃(0 0 0 1). Using Hartree-Fock (Kohn-Sham) approximation, the adsorption energy of O₂ is 0.12 (0.38) eV at the equilibrium position 2.46 (1.81) Å. Allowing internal relaxation, the Kohn-Sham binding energy of O₂ calculated within the generalized gradient approximation to exchange and correlation effects is 0.58 eV at the equilibrium position 1.98 Å. Our results point to a large contribution of Coulomb correlations and relaxation effects in the adsorption processes on alumina surfaces and clusters.