Solid-state NMR study of photocatalytic oxidation of acetaldehyde over the flame-made F-TiO₂ catalyst

Solid-state ¹³C high-power proton decoupling (HPDEC), cross-polarization (CP), and 2-dimensional (2D) ¹³C-¹³C homonuclear correlation NMR techniques were used to study the adsorption of acetaldehyde on the flame-made fluorinated TiO₂ (F-TiO₂) catalyst, subsequent photocatalytic oxidation, and complete photo-decomposition at the presence of oxygen under UV light irradiation. Crotonaldehyde is formed via the acid-catalyzed aldol condensation through the adsorption of acetaldehyde on the F-TiO₂ catalyst at room temperature without light illumination. At the presence of molecular oxygen as an electron acceptor, surface bonded acetaldehyde and crotonaldehyde are completely oxidized by the transient holes in the oxygen framework generated by UV light irradiation. This work provides clear evidence for the formation of crotonaldehyde, acetic acid, and formic acid, along with acetate and formate complexes. The identified surfaces complexes participate actively in the photocatalytic oxidation and do not play a significant role in the poisoning of the active sites, at least in such concentrations. The reaction pathways are therefore established.