The photo-activity of methanol on the rutile TiO2(110) surface was shown to depend on the ability of methanol to diffuse on the surface and find sites active for its thermal dissociation to methoxy and hydroxy species. Temperature programmed desorption results showed that the extent of methanol photodecomposition to formaldehyde was negligible on the clean TiO2(110) surface at 100K due to a scarcity of sites that could convert (photo-inactive) methanol to (photoactive) methoxy. The extent of photo-activity at 100K significantly increased when methanol was co-adsorbed with oxygen, however only those molecules able to absorb near (next to) a co-adsorbed oxygen species were active. Pre-annealing co-adsorbed methanol and oxygen to above 200K prior to UV irradiation resulted in a significant increase in photo-activity. Scanning tunnelling microscopy images clearly showed that the advent of increased photo-activity in temperature programmed desorption correlated with the onset of methanol diffusion along the surface Ti4+ rows at 200K. These results demonstrated that optimizing thermal processes (such as diffusion or proton transfer reactions) could be critical to maximizing photo catalytic reactivity on TiO2 surfaces.

Importance of Diffusion in Methanol Photochemistry on TiO2(110). Sheen, M., Chary, D.P., Dohnálek, Z., Henderson, M.A.: Journal of Physical Chemistry C, 2012, 116[48], 25465-9