Degradation of N-containing compounds by pure and doped titania: study of the reaction in dark conditions, under UV light and after incubating the catalysts in ascorbic acid

A sample of mesoporous TiO2 (MT, specific surface area = 150 m2 g-1) and two samples of MT doped with 2.5 wt.% Fe were prepared by either direct synthesis (Fe2.5-MTd) or impregnation (Fe2.5-MTi). Commercial TiO2 (Degussa P25, specific surface area = 56 m2 g-1) was used both as benchmark and as support for the impregnation with either 0.8 or 2.5 wt.% Fe (Fe0.80-IT and Fe2.5-IT). The powders were characterized by X-rays diffraction, N2 adsorption isotherms at -196 °C, Energy Dispersive X-ray Spectroscopy, Diffuse Reflectance UV-Vis and Mössbauer spectroscopy. Degradation of the azo dye Acid Orange 7 (AO7) by H2O2 was used as a test reaction: effects of dark-conditions versus UV irradiation and two H2O2 concentrations (i.e. stoichiometric and excess) were considered. In dark conditions, MT provided higher AO7 conversion with respect to Degussa P25; Fe3+ surface species reacted through a (slow) Fenton-like reaction. The same Fe3+ species were reduced by ascorbic acid to Fe2+ species reacting very lively with H2O2 through Fenton reaction. Under UV illumination, MT was as active as Degussa P25; the photocatalytic activity of Fe2.5-MTd was enhanced with respect to MT, and also Fe-impregnated samples resulted very active, likely due to the occurrence of a photo-Fenton process.