Molecular chemisorption on TiO2(110):: A local point of view

The molecular adsorption of some probe molecules (CO, H2O, and H2S) on a 5-fold-coordinated Ti Lewis acid site of the TiO2 rutile (110) surface is studied within the density functional theory. The substrate is modeled with a small Ti7O9 cluster terminated with pseudo-hydrogens. This is found to describe the electronic and structural properties of the clean surface in good a,agreement with both experimental and periodic slab calculations. Adsorption energies (6.7, 19.3, and 7.0 kcal/mol for CO, H2O, and H2S, respectively) and adsorbate stretching frequencies compare favorably with available experimental data. The agreement is particularly good for the C-O stretching frequency. shift (+56 cm(-1)), as was found in previous investigations carried out with the same theoretical approach on other oxides. In contrast, the HOH scissoring mode is poorly reproduced, suggesting that molecularly adsorbed H2O is actually involved in weak bonds with other adsorbed species. This agrees with predictions of very recent slab calculations by Lindan et al. (Phys. Rev. Lett. 1998, 80, 762). Differences with respect to calculations carried out by embedding the cluster in point-charge arrays are discussed.