Adsorption of Nitrogen on Rutile(110). 2. Construction of a Full Five-Dimensional Potential Energy Surface

A full five-dimensional potential energy surface for the interaction of nitrogen molecules with the (110) surface plane of TiO2 (rutile) is generated. In a first step, ab initio SCF cluster calculations are performed for various adsorption geometries of N2 above the TiO2(110) surface, which is described by different stoichiometric clusters, ranging in composition from Ti7O14 to Ti13O26, embedded in extended point charge fields. The N−N distance is fixed to the experimental equilibrium bond length 1.098 Å. In a second step, a simple analytic form for the interaction potential is developed which contains the electrostatic interaction between the charge distribution of N2 and the electric field above the surface, the polarization of the N2 molecule by this field, and the Pauli repulsion between N2 and the surface. By fitting the five parameters in the analytic expression (quadrupole moment and the two polarizability components of N2, repulsive Lennard-Jones parameters between N and the O and Ti atoms of the surface) to the calculated ab initio interaction energies, one can represent the full five-dimensional interaction potential with a mean error of about 3 kcal/mol. The global minimum of the interaction potential is found for the vertical end-on adsorption of N2 on a coordinately unsaturated row A titanium atom; it has an adsorption energy of −46 kJ/mol and a Ti−N distance of 2.39 Å. The side-on adsorption of N2 on the row B oxygen atoms with the N−N axis perpendicular to the row B direction is also slightly attractive with a small adsorption energy of −5.5 kJ/mol.