Probing of photocatalytic surface sites on SO42−/TiO2 solid acids by in situ FT-IR spectroscopy and pyridine adsorption

Photocatalytic surface sites on SO42−/TiO2 were investigated by pyridine adsorption and in situ Fourier transform infrared (FT-IR) spectroscopy. Results revealed that the sulfate-modification not only increased the number of strong Lewis acidic sites, but also induced a large amount of strong Brønsted acidic sites on the surface of TiO2. Pyridine molecules were chemically captured on Brønsted and Lewis acidic sites on the SO42−/TiO2 surface. These pyridine molecules were progressively decomposed to final products of CO2 and H2O under actual photocatalytic conditions. The high photocatalytic performance of SO42−/TiO2 can be explained by the improved surface acidities that favor the adsorption of both oxygen and pyridine molecules. Moreover, the Lewis acidic sites could react with H2O and was then converted to Brønsted acidic sites, leading to the activation of the water. This conversion promoted the formation of hydroxyl groups on the catalyst surface, which could also contribute to the high photocatalytic reactivity of SO42−/TiO2.