Fe- and V-doped mesoporous titania prepared by direct synthesis: Characterization and role in the oxidation of AO7 by H2O2 in the dark

Abstract

Two metal-doped mesoporous titania samples (Fe-MT and V-MT) have been prepared, and characterized through several techniques (N2 sorption at −196°C, XRD, TEM, XPS, DR-UV–vis and Raman spectroscopies). As reference, pure mesoporous titania (MT) and commercial non-porous P25 (Degussa) were considered, the latter both as such and after impregnation with either Fe- or V-salts (Fe-IT, V-IT). In addition, the catalytic activity of the samples has been measured in the oxidation by H2O2 in water of Acid Orange 7 (AO7), a model azo-dye. Catalytic measurements were run at natural pH of the AO7 solution (=6.8), to avoid adsorption phenomena occurring in acidic conditions, and, contrary to what usually done, in the absence of light. It is inferred that with Fe-MT inclusion of Fe(III) takes place, so that Fe(III) ions are present both in the bulk and at the surface, whereas with V-MT V species are present basically only at the surface. As to AO7 degradation: (i) titania alone is already rather active, with MT more active that P25, likely because of the higher specific surface area (SSA); (ii) impregnated samples are more active than P25, because of a positive catalytic effect of metallic centres; (iii) synthesized samples are more active than corresponding impregnated ones, not only because of the higher SSA, but also because direct synthesis allows a better dispersions of active sites; (iv) Fe-containing samples are more active, in the long run, than V-containing ones, because of a different reaction mechanism (Fenton-like and Mars van Krevelen, respectively). The occurrence of two mechanisms is shown by a different time course in the two cases. Further support comes from XPS data, showing that, after reaction Fe-MT still features only Fe(III) species, whereas V species in V-MT are basically reduced to V(IV), which leads to deactivation of these samples.