Adsorption of Methanethiol on Stoichiometric and Defective TiO2(110) Surfaces: A Combined Experimental and Theoretical Study

The interaction of CH 3 SH with TiO 2 (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 TiO 2 (110) substrate there is no dissociation of CH 3 SH. The molecule bonds to Ti sites via its S lone pairs and desorbs at temperatures below 300 K. For CH 3 SH chemisorbed on terraces of TiO 2 (110), the desorption energies for molecular adsorption are ∼10−13 kcal/mol. The desorption energy for CH 3 SH on defects is ∼18 kcal/mol. Photoemission results show that the active sites for the decomposition of CH 3 SH are associated with oxygen vacancies (“Ti δ+ ” sites, δ ≤ 3). These defects induce occupied electronic states above the valence band of stoichiometric TiO 2 that bond well CH 3 S, 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 CH 3 S. The bond between CH 3 S 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 CH 3 S and H recombine and desorb as CH 3 SH into gas phase. For surfaces with a large concentration of O vacancies and defects, the C−S bond in adsorbed CH 3 S breaks in the 250−750 K temperature range with CH 3 or CH 4 desorbing into gas phase and leaving S and CH x fragments on the surface. These results illustrate the important role played by O vacancies in the chemistry of a thiol over an oxide surface.