G. Pacchioni et al., CLUSTER AND BAND-STRUCTURE AB-INITIO CALCULATIONS ON THE ADSORPTION OF CO ON ACID SITES OF THE TIO2(110) SURFACE, Surface science, 350(1-3), 1996, pp. 159-175
The interaction of CO with the cationic sites of the TiO2(110) surface
has been investigated with cluster models and band structure calculat
ions. The analysis of Hartree-Fock and correlated wavefunctions has sh
own 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 el
ectrostatic attraction is almost exactly cancelled by the Pauli repuls
ion. The adsorption is accompanied by two important measurable feature
s, a considerable blue shift of the CO vibrational frequency and an in
crease 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 repu
lsion 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 s
hift 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 ch
arge associated with a given orbital, and (c) the CO dynamic dipole mo
ment. The same analyses also rule out the occurrence of a Ti-to-CO bac
k donation.