Adsorption of methanethiol on stoichiometric and defective TiO2(110) surfaces:: A combined experimental and theoretical study

The interaction of CH3SH with TiO2(110) has been studied with a combination of synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy, and first-principles density functional slab calculations. On the Ti and O sites of a perfect TiO2(110) substrate there is no dissociation of CH3SH. The molecule bonds to Ti sites via its S lone pairs and desorbs at temperatures below 300 K. For CH3SH chemisorbed on terraces of TiO2(110), the desorption energies for molecular adsorption are similar to10-13 kcal/mol. The desorption energy for CH3SH on defects is similar to18 kcal/mol. Photoemission results show that the active sites for the decomposition of CH3SH are associated with oxygen vacancies ("Tidelta+" sites, delta less than or equal to 3). These defects induce occupied electronic states above the valence band of stoichiometric TiO2 that bond well CH3S, S, and C. Thus, the presence of O vacancies in the oxide surface allows the cleavage of the S-H bond in methanethiol and the deposition of CH3S. The bond between CH3S and O-vacancy sites is mainly covalent, but the bonding interactions are very strong and can induce the migration of O vacancies from the bulk to the surface of the oxide. In systems with a limited number of O vacancies, adsorbed CH3S and H recombine and desorb as CH3SH into gas phase. For surfaces with a large concentration of O vacancies and defects, the C-S bond in adsorbed CH3S breaks in the 250-750 K temperature range with CH3 or CH4 desorbing into gas phase and leaving S and CHx fragments on the surface. These results illustrate the important role played by O vacancies in the chemistry of a thiol over an oxide surface.