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 (0001) 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(0001) 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(0001). 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 0.58eV at the equilibrium position 1.98Å. Our results point to a large contribution of Coulomb correlations and relaxation effects in the adsorption processes on alumina surfaces and clusters.