Cluster and band structure ab initio calculations on the adsorption of CO on acid sites of the TiO2(110) surface

The interaction of CO with the cationic sites of the TiO2(110) surface has been investigated with cluster models and band structure calculations. The analysis of Hartree-Fock and correlated wavefunctions has shown that CO adsorbs at a distance of 4.4-4.5 bohr and a binding energy of 0.7-0.8 eV at low coverage. The bond strength is determined by the CO polarization and the CO sigma-donation to the surface while the electrostatic attraction is almost exactly cancelled by the Pauli repulsion. The adsorption is accompanied by two important measurable features, a considerable blue shift of the CO vibrational frequency and an increase of the CO 5 sigma ionization potential. Both these effects have been analyzed in detail. We found that the CO omega shift is largely due to a combination of the electrostatic Stark effect and of the repulsion occurring when the CO molecule stretches in the presence of the rigid surface. The change in the 1 pi-5 sigma binding energies, on the other hand, has an entirely electrostatic origin. Neither the omega shift nor the 5 sigma binding energy shift are determined by the sigma-donation mechanism. Nevertheless, the occurrence of a charge transfer from CO to the empty levels of the Ti centers is well documented by (a) the energy and dipole moment change associated to this mechanism, (b) the expectation value of a projection operator which measures the charge associated with a given orbital, and (c) the CO dynamic dipole moment. The same analyses also rule out the occurrence of a Ti-to-CO back donation.