Reactivity enhancement of 2-propanol photocatalysis on SO4(2-)/TiO2: insights from solid-state NMR spectroscopy

Solid-state NMR techniques have been employed to investigate the surface acidic properties of TiO2 and sulfated TiO2, as well as their photocatalytic activities toward 2-propanol. The multinuclear MAS NMR experiments clearly revealed that three different types of Brønsted acid sites with much stronger acid strength were generated after the sulfation of TiO2. Due to the enhanced Brønsted acidity, the protonation of 2-propanol can occur more easily, preferentially leading to the formation of Ti-bound 2-propoxy species on the SO42-/TiO2 catalyst The 2-propoxy species can be directly converted to CO2 and thus the photocatalytic activity of sulfated TiO2 catalyst is remarkably enhanced. For comparison, both hydrogen-bonded 2-propanol and Ti-bound 2-propoxy species are present on the TiO2 catalyst with the former being predominant The hydrogen-bonded 2-propanol species are oxidated into acetone molecules that are difficult to further convert into CO2, and the conversion of 2-propoxy species to 2-propanol hampers the direct mineralization of 2-propoxy species on the TiO2 catalyst.