Influence of iron leaching and oxidizing agent employed on solar photodegradation of phenol over nanostructured iron-doped titania catalysts

Highlights

• Structure/activity relationships over nanostructured iron-doped titania photocatalysts for solar degradation of phenol.

• Iron leaching shown to be most relevant for the photoactivity.

• Results compared in terms of the oxidizing agent, either H₂O₂ or oxygen from air.

Abstract

Iron-doped TiO₂ catalysts with two different iron contents (0.7 and 3.5 wt.%) as well as the corresponding undoped system, prepared by a combined sol–gel/microemulsion method and calcined at 600 °C, have been examined with respect to their behaviour for photocatalytic degradation of aqueous phenol with H₂O₂ under solar light. The activity results are complemented with structural/morphological and electronic characterization analysis achieved by XRD, TEM, Raman, S_BET, UV–vis DRS and XPS techniques. A detrimental effect of the presence of iron on the photoactivity is detected for the sample with 0.7 wt.% iron. In contrast, some activity enhancement is produced for the sample with highest iron loading. Such irregular catalytic behaviour with respect to iron loading is analyzed on the basis of the presence of additional catalytic contributions from new photoactive species. These are created as a consequence of surface modifications produced under reaction conditions related to the existence of phenomena of iron leaching from the catalysts. Differences in the specific nanostructure present in each case can also play a role on explaining the differences observed in the photoactivity. Finally, an analysis of the photoactivity as a function of the oxidizing agent employed, either H₂O₂ or oxygen from air, is also performed.

Introduction

Polycrystalline titanium dioxide presents unique properties as heterogeneous photocatalyst considering its good stability in aqueous environment under ambient pressure and temperature condition [1], [2]. However, its photocatalytic efficiency can be limited by stabilization of the charge photocarriers in the bulk of the material, by fast electron–hole recombination either at the bulk or at the surface of the oxide or by its relatively large band gap which limits light absorption in the visible region [1], [2], [3], [4], [5], [6]. Among strategies aimed to enhance such properties, in particular extending light absorption to the visible region, it is common to employ methods based on doping the titania catalyst with transition metal cations [7], [8], [9], [10]. Fe³⁺ is considered among most interesting dopants in this sense since it originates a localized narrow band above the valence band of titania which makes the catalyst sensitive to visible absorption [11], [12]. However, the catalytic role of the Fe³⁺ dopant during photooxidation processes remains controversial [8], [9], [10], [13], [14], [15]. Thus, discrepancies appear with respect to its role in enhancing electron/hole recombination properties since the presence of iron in the catalyst formulation has been reported to be detrimental to the photoactivity in a great number of cases [10], [13]. In turn, the photoactivity of iron-doped titania under visible light can be limited because the oxidizing power and mobility of holes in the Fe³⁺-derived localized narrow band can be lower than that of holes in the valence band of TiO₂ [11].

In any case, optimum photocatalytic properties have been apparently achieved in an important number of cases upon homogeneous doping at a relatively low level around 0.5–1 at.%, at which the distance between dopant cations in the titania lattice could optimize dynamical characteristics of the recombination process [1], [3], [10]. However, doping above such optimum level typically results detrimental to the photoactivity [10], [15], [16], [17]. In turn, the specific reaction conditions (solution pH or type of oxidant employed) can strongly affect the photocatalytic properties of titania-based materials [10], [18], [19]. In this sense, it is generally concluded that the use of H₂O₂ as oxidizing agent can significantly enhance, as compared with dissolved oxygen, the UV- or visible-photoassisted contaminants oxidation rate [18], [20], [21], [22], in spite of its possible hydroxyl radical scavenger role [18], [23], [24]. Other interrelated aspects that could affect the photoactivity concern modifications of the surface properties induced by doping as well as the possibility that species originated from iron lixiviates can participate in the reaction mechanism [11], [12], [17], [25], [26], [27], [28].

With respect to this latter, we have proposed in a recent work the involvement of iron species leached under reaction conditions from iron-doped titania in the mechanism of solar contaminants photodegradation employing oxygen as oxidant under semi-pilot plant conditions [27]. A similar study has been performed when H₂O₂ is employed as oxidant over nanostructured anatase TiO₂ (crystal size between about 10 and 12 nm, as obtained when using calcination at 450 °C as final preparation step) systems of this kind, also indicating an important role of such leached iron species [17]. The present work aims to extend the latter analysis, employing H₂O₂ as oxidant, to more crystalline catalysts (obtained by calcination at 600 °C) in order to explore whether the mechanism involving iron leached species could still prevail in the presence of more crystallized and likely more stable iron oxide and titania entities, and considering also the effects of crystallinity on phenol photooxidation activity [22], [27], [28], [29], [30]. In this respect, it must be noted that our previous results comparing samples calcined at 450 °C (anatase) and 600 °C (mixed anatase–rutile) when using O₂ (from air) as oxidant showed a generally higher photoactivity for the latter despite their appreciably lower specific surface area [27]. Within this context, pure titania and iron-doped titania catalysts prepared by sol–gel/microemulsion method and (as final preparation step) calcined at 600 °C have been tested for phenol photodegradation with H₂O₂ under solar light irradiation in a pilot plant system. Information achieved by employing different structural and electronic characterization techniques (XRD, TEM, Raman, S_BET, as well as UV–vis and XPS spectroscopies) has been employed to complement the discussion of the catalytic activity results.