Ab Initio Cluster Calculations for the Adsorption of Small Molecules on Oxide Surfaces - from Single Molecules to Monolayers

Quantum chemical ab initio cluster calculations were performed for the adsorption of small molecules on metal oxide surfaces. Two systems were studied in detail: The adsorption of N₂ on the (110) surface plane of TiO₂ (rutile) and the adsorption of CO on the polar (0001) surface of Cr₂O₃. In both cases a full five-dimensional potential for the interaction of a single molecule with the respective surface was calculated. For N₂/TiO₂ (110) the minimum was found for the end-on adsorption of N₂ atop a coordinately unsaturated surface Ti atom, with an adsorption energy of (35 ± 5) kJ/mol. In the case of CO/Cr₂O₃ (0001) the CO molecule is adsorbed strongly tilted (almost side-on) along a line connecting two Cr³⁺ ions at the surface; the calculated adsorption energy is 22 kJ/mol. In conjunction with empirical pair potentials for the N₂/N₂ and CO/CO interaction in the gas phase, Monte Carlo simulations were carried out to determine adsorption isotherms and the geometric structure of adsorbed monolayers.