Insights into Acetone Photochemistry on Rutile TiO2(110). 2. New Photodesorption Channel with CH3 Ejection along the Surface Normal

Abstract

Angle-resolved photon-stimulated desorption (PSD) combined with infrared reflection–absorption spectroscopy and temperature-programmed desorption reveal two distinct channels in the photochemistry of acetone on rutile TiO2(110). During UV irradiation of coadsorbed oxygen and acetone molecules, nonthermal methyl radicals (CH3) are ejected in two different directions: (i) normal to the surface and (ii) off-normal at ∼±66° to the surface normal in the [11̅0] azimuth (i.e., perpendicular to the O and Ti rows). The direction of the “off-normal” PSD component is consistent with the orientation of the C–CH3 bonds in the η2-acetone diolate—a photoactive form of acetone chemisorption on the oxidized TiO2(110) surface proposed in earlier studies. The direction of the “normal” PSD component requires an orientation of a C–CH3 bond which is not consistent with the η2-acetone diolate structure. The “off-normal” PSD component dominates at lower acetone coverage (<0.2 ML), but does not increase at higher coverages in accord with the acetone diolate peak intensity in the infrared reflection–absorption spectra. The “normal” PSD component grows with the acetone coverage up to 0.6 ML. Based on the H/D exchange experiments, the newly discovered “normal” PSD channel is tentatively assigned to a photoproduced η2- acetone enolate as a precursor.