Elucidation of structures of surface sulfate species on sulfated titania and mechanism of improved activity

•Structures of six sulfate species on SO42−/TiO2 photocatalysts were elucidated.•When neutralized these photocatalysts showed higher efficiency.•Brønsted acid amounts hinders photocatalytic activity.•Lewis acid sites promoted the activity.•This study sheds light on the mechanism of improved activity of sulfated photocatalysts.

Sulfated SO42−/TiO2 superacid photocatalysts were synthesized and investigated for their photocatalytic activity and the structures of surface sulfate species. Six structures of sulfate species were elucidated using Fourier-Transform Infrared Spectroscopy. Correlation between the photocatalytic activity and structures of surface sulfate species were studied. The neutral Structures (IV) and (V) promoted the photocatalytic activity for the degradation of organic pollutants in reverse osmosis concentrate (ROC). Structure (II) was inactive. The acidic Structure (I) converted to (IV) when calcined at 600 °C or to (V) when neutralized. Brønsted acid amounts did not correlate to the photocatalytic activity while Lewis acid sites promoted the activity. Sulfated species-bonded Brønsted acid sites and Lewis acid sites and their transformation were observed with pyridine adsorption. Impact of surface area, crystal size and sulfate density on activity was also discussed. Mechanism of improved activity due to neutral sulfate species (IV) and (V) was also proposed. This study sheds some light on the mechanism of improved activity of sulfated photocatalysts and may also provide some clues to mechanism of other superacid catalysts.

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