Trimethyl acetate acid (TMAA) adsorption and photoinduced decomposition at room temperature on a model rutile TiO2(110) surface have been examined in–situ with STM, combining the molecular beam dosing and UV irradiation on the STM stage. TMAA ((CH3)3CCOOH) molecules adsorb dissociatively through deprotonation to form TMA group, which bridged over two Ti4+ sites of the same Ti row through a carboxylate end. While most of the initial oxygen vacancies are intact at low coverages, in a small number of cases a minor second TMAA adsorption/dissociation channel likely exists, with one O bonding at the vacancy site and the other one at a regular Ti4+ site. While it has been implicitly assumed that upon TMAA deprotonation the acid proton adsorbs on an adjacent bridging O2- site (forming an OH- group) [1], there is no unambiguous evidence in the literature for the presence of these hydroxyl species. We have found no evidence of atop H bonding and OH- formation on the basis of joint STM and HREELS analysis. We suggest that an uncommon proton bonding configuration is favored due to electrostatic pairing with neighboring TMA groups in which the proton bonds, possibly dynamically, to a pair bridging oxygen ions. Such a configuration in turn, through steric alignment, is the main factor in the formation of the highly ordered (2x1) TMA structures observed by STM. Similar proton bonding may be expected for other carboxylates on TiO2(110) surface.
After UV light irradiation, TMA photodepletion takes place rather homogeneously across the surface; hence, each photodecomposition event occurs independently from previous events and/or remaining TMA species. Photolysis probability has been found to decrease with coverage increase. Such photo-activated process efficiency dependence on TMA coverage illustrates dynamics change of a typical photo-induced reaction on TiO2 surface.
This work was supported by the Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences and performed at Environmental Molecular Science Laboratory, a DOE national user facility.
[1] M.A. Henderson, J.M. White, H. Uetsuka, H. Onishi, J. Am. Chem, Soc. 125 (2003)
14974.
[2] I. Lyubinetsky, Z. Q. Yu, and M. A. Henderson, J. Phys. Chem. C (2007) in press.
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