Adsorption of nitrogen on rutile (110):Ab initiocluster calculations

Results of quantum-chemical ab initio cluster calculations for the end-on and side-on adsorption of ${\mathrm{N}}_2$ on the coordinately unsaturated Ti atoms of the (110) surface plane of ${\mathrm{TiO}}_2$ (rutile) are reported. A large number of cluster models for the Ti adsorption site is investigated; the ${\mathrm{Ti}}_n{\mathrm{O}}_m$ 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 embedding point charge field is discussed in detail, as is the influence of the basis set and of electron correlation. Because of the strong covalency of the Ti-O bond in bulk ${\mathrm{TiO}}_2,$ it needs clusters of at least the size ${\mathrm{Ti}}_9{\mathrm{O}}_{18},$ in which the electronic structure of the Ti adsorption site 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 end-on adsorption of ${\mathrm{N}}_2$ on Ti is attractive with a binding energy of $(35\mathrm{}\pm 5)\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$ (equilibrium Ti-N distance $2.40\pm 0.05\AA$), 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 ${\mathrm{N}}_2{/\mathrm{T}\mathrm{i}\mathrm{O}}_2$ (110) adsorption can be considered as a typical physisorption in which electrostatic as well as inductive attraction and Pauli repulsion are the predominant contributions to the binding energy.