Mechanism of simulated solarlight-induced photocatalytic oxidation of 5-hydroxynaphthalene-1-sulphonic acid on sulfur doped TiO2

The photodegradation of 5-hydroxynaphthalene-1-sulphonic acid (HNSA) was investigated with the TiO2 and sulfur doped TiO2 (S/TiO2) photocatalysts under the irradiation of the ultraviolet light (UV) and the simulated solar light. The S/TiO2 photocatalyst was found to be highly effective for the HNSA photodegradation both under the UV light or the simulated solar light irradiation. Especially, using various analysis methods, the photodegradation mechanism of HNSA on the S/TiO2 photocatalyst under the simulated solar light irradiation was investigated. Our results indicated that desulfonation and hydroxylation of the starting compound in the degradation of HNSA is the initial step, leading to formation of naphthol and hydroxyl-naphthol. Subsequently oxygenation takes place to formation naphthaquinone and hydroxyl naphthaquinone, which are the main intermediates in the HNSA photodegradation. Naphthaquinone can be further destructed to yield straight-chain esters. Finally, these intermediates are further oxidized to small compounds, such as CO2 and H2O.