Relationship of O₂Photodesorption in Photooxidation of Acetone on TiO₂

Organic photooxidation on TiO 2 invariably involves the coexistence of organic species with oxygen on the surface at the same time. In the case of acetone and oxygen, both species exhibit their own interesting photochemistry on TiO 2, but interdependences between the two are not understood. In this study, a rutile TiO 2 (110) surface possessing 7% surface oxygen vacancy sites is used as a model surface to probe the relationship between O 2 photodesorption and acetone photodecomposition. Temperature programmed desorption and photon stimulated desorption (PSD) measurements indicate that coadsorbed oxygen is essential to acetone photodecomposition on this surface; however, the form of oxygen (molecular or dissociative) involved is not known. The first steps in acetone photodecomposition on TiO 2 (110) involve thermal activation with oxygen to form an acetone diolate ((CH 3 ) 2 COO) species followed by photochemical decomposition to adsorbed acetate (CH 3 COO) and an ejected CH 3 radical that is detected in PSD. Depending on the surface conditions, O 2 PSD is also observed during the latter process. The time scales for the two PSD events (CH 3 and O 2 ) are quite different, with the former occurring at ∼10 times faster than the latter. By varying the preheating conditions or performing preirradiation on an O 2 -exposed surface, it becomes clear that the two PSD events are uncorrelated. That is, the O 2 species responsible for O 2 PSD is not a significant participant in the photochemistry of acetone on TiO 2 (110) and likely originates from a minority form of O 2 on the surface. The CH 3 and O 2 PSD events do not appear to be in competition with each other, suggesting either that ample charge carriers exist under the experimental conditions employed or that different charge carriers or excitation mechanisms are involved.