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

Highlights

• Mesoporous TiO₂ NPs with a much larger specific surface area than commercial P25 were synthesized.

• Fe-doped TiO₂ NPs show surface Fe³⁺ sites active in a Fenton's reaction.

• V-doped TiO₂ NPs show surface V⁵⁺ sites active in a Mars van Krevelen mechanism.

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.

Introduction

TiO₂, both as such and after inclusion of transition metals [1], [2], [3], [4], [5], [6], [7], is widely used for photocatalytic reactions, because of its limited band-gap and relatively small hole/electron recombination rate: these include the removal of organic pollutants from water and the splitting of water, a reaction storing solar energy into HH bonds [8], [9], [10].

Of the three TiO₂ polymorphs, anatase is the most active, the redox potentials of valence and conduction bands being 3.0 and −0.2 V, respectively, which indicates that TiO₂ has indeed good oxidation ability for the degradation of organic compounds and water splitting [11], [12]. Anatase is, however, metastable and undergoes transition to rutile at 600–700 °C [13], which may take place with TiO₂ nanocrystals even at lower temperatures [14]. On the other side, smaller particle size leads to higher surface area and hence higher number of surface reactive sites, which is expected to improve efficiency [15]. This could be due, on the one hand, to a change in the electronic structure of NPs: considering, however, quantum-size effects on the absorption edge shift of TiO₂ NPs, no clear correlation with particles size can be drawn and better catalytic performances of smaller NPs have therefore more often been attributed to surface phenomena, like the presence of (very reactive) penta-coordinated Ti sites [16].

Mesoporous TiO₂ NPs have recently received interest because of their large specific surface area (SSA) and decreased internal diffusion resistance, which favour adsorption during catalytic reactions, and play a role also in processes occurring in solar cells, electric and photocatalytic systems, separation/purification and immobilization processes [13], [14], [15], [16], [17], [18]. Indeed, sometimes better performances were obtained with respect to Degussa P-25, the most used commercial titania [19] and a reference system also in the present work.

Sol–gel methods are usually employed to prepare NPs of mesoporous TiO₂ with large surface area and pore volume [13], [17], [18], [20] with surfactants, e.g. Pluronic-type copolymers, acting as structural directing agents [21], [22]. Besides other possible modifications, addition of transition metal ions, such as V, Fe, Ag, Co and Cu, has been used to alter the photocatalytic activity of TiO₂ [23], [24]. The overall picture is, however, unclear, as several studies claim that doping with metal ions leads to an enhanced photo-activity [25], and others reported the opposite [26], [27], [28], [29], [30], [31], [32], [33], [34].

In the present work mesoporous titania both as such and Fe or V doped have been prepared and characterized. This choice stems from the fact that Fe³⁺ ions, with one valence electron more than Ti⁴⁺, are expected to inject electrons into titania conduction band, whereas V⁵⁺ ions, with one valence electron less than Ti⁴⁺, are expected to inject holes into titania valence band.

The catalytic activity was tested towards the degradation with H₂O₂ in water of Acid Orange 7 (AO7, Scheme 1), a “model molecule” for the many azo-dyes found as contaminants in both wastewater and groundwater. Contrary to what usually done, the reaction was studied in dark conditions, because the aim of the investigation was in the first instance the reactivity of surface metal centres, and not the photoactivity of the solid. Such type of study is under way.