Reactivity Enhancement of 2-Propanol Photocatalysis on SO₄²⁻/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.