Photocatalytic Oxidation of 2-Propanol on TiO₂ Powder and TiO₂ Monolayer Catalysts Studied by Solid-State NMR

The adsorption and photocatalytic oxidation of 2-propanol was studied by solid-state NMR spectroscopy over TiO 2 powder and a TiO 2 monolayer catalyst anchored on porous Vycor glass (TiO 2 /PVG). Two adsorbed 2-propanol species were identified on the TiO 2 powder, a hydrogen-bonded species and a 2-propoxide species, whereas only the H-bonded 2-propanol species was observed on TiO 2 /PVG. The photooxidation of 2-propanol proceeded along two parallel routes. The first occurred through the formation of acetone from the H-bonded 2-propanol species and was followed by the aldol condensation of acetone to form mesityl oxide. The subsequent photooxidation of mesityl oxide progressed slowly. Under dark conditions, mesityl oxide and its fragmentation products, acetaldehyde and isobutylene, were found to be in thermal equilibrium. The second route occurred through the relatively rapid and complete oxidation of 2-propoxide to CO 2 . In addition, the mobility of H-bonded 2-propanol was found to be important in the photooxidation, particularly when dark regions of the catalyst exist. H-bonded 2-propanol could be converted to 2-propoxide at newly vacated active sites. In contrast, the slow photooxidation rate of 2-propanol to CO 2 on TiO 2 /PVG resulted from the availability of only the first, slow reaction route.