ADSORPTION OF NITROGEN ON RUTILE (110) - AB-INITIO CLUSTER CALCULATIONS

Results of quantum-chemical ab initio cluster calculations for the end -on and side-on adsorption of N-2 On the coordinately unsaturated Ti a toms of the (110) surface plane of TiO2 (rutile) are reported. A large number of cluster models for the Ti adsorption site is investigated; the TinOm clusters are either saturated by hydrogen atoms or embedded in point-charge arrays. The dependence of the adsorption energy on the size and composition of the cluster and on the ionicity of the embedd ing point charge field is discussed in detail, as is the influence of the basis set and of electron correlation. Because of the strong coval ency of the Ti-O bond in bulk TiO2, it needs clusters of at least the size Ti9O18, in which the electronic structure of the Ti adsorption si te as well as of the five adjacent oxygen atoms is described correctly , in order to obtain adsorption energies which are independent of the composition of the cluster and the embedding point charge field. The e nd-on adsorption of N-2 on Ti is attractive with a binding energy of ( 35+/-5) kJ/mol (equilibrium Ti-N distance 2.40+/-0.05 Angstrom), which is in good agreement with experiment, while the side-on adsorption is purely repulsive. A detailed analysis of the bonding mechanism shows that the N-2/TiO2 (110) adsorption can be considered as a typical phys isorption in which electrostatic as well as inductive attraction and P auli repulsion are the predominant contributions to the binding energy . [S0163-1829(98)05007-3].