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

Abstract

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 ∼10−13 kcal/mol. The desorption energy for CH3SH on defects is ∼18 kcal/mol. Photoemission results show that the active sites for the decomposition of CH3SH are associated with oxygen vacancies (“Tiδ+” sites, δ ≤ 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.