Heterogeneous catalyst based on peroxo-niobium complexes immobilized over iron oxide for organic oxidation in water

https://doi.org/10.1016/j.apcatb.2011.07.017Get rights and content

Abstract

Nb-doped iron oxides were used as heterogeneous catalyst to oxidize organic compounds in aqueous medium containing hydrogen peroxide. XRD data reveal that the composites contain hematite (α-Fe2O3), maghemite (γ-Fe2O3) and FeNb2O6. The H2O2 pretreatment of the solid catalyst promotes important surface and structural changes of the iron oxides mainly by peroxo-niobium complexes formation, which expressively enhances the catalytic properties of the composite. Transmission electron microscopy images show that the H2O2-treatment tends also to decrease the mean particle size of grains of the composite. Nb-doped iron oxides were found to impart an important role to the solid catalyst towards H2O2 reactions. The developed composites were confirmed to have remarkable catalytic activity on the oxidation of organic substrates and can be regenerated for several successive reaction cycles for degrading polluting organic in water.

Graphical abstract

After six reaction cycles, the Nb-NCBT catalyst presented catalytic activity loss, but after H2O2 solution added into de system the catalytic activity was recovered suggesting that the oxidizing sites are regenerated by the addition of H2O2.

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Highlights

► We produced Nb-doped iron oxide with high catalytic capacity for organic oxidation. ► H2O2-pretreatment of the solid catalyst promotes important surface and structural changes. ► We obtained peroxo-niobium complexes, which expressively enhances the catalytic properties of the materials.

Introduction

In heterogeneous catalysis, several applications involve niobium-bearing compounds as promoters or support for other metals, mainly due to their relatively high catalytic activity and chemical stability. During the last decades, peroxo and hydroperoxo complexes with different transition metals, including W, V, Mo, Ti and Nb, have drawn attention due to their outstanding catalytic activity in several processes, such as oxidation of alkenes, aliphatic and aromatic hydrocarbon compounds [1], [2], [3], [4], [5], [6], [7], [8], [9]. These complexes can be used either in stoichiometric or as catalysts in oxidation reactions involving H2O2, which is able to cyclically regenerate the active sites [10], [11].

Catalysts based on niobium(V)-peroxo complexes for oxidation of organic and inorganic substrates and for epoxidation reactions have been reportedly documented [12], [13], [14]. However, these complexes were found to operate only an in homogeneous system, which makes the catalytic process more costly and difficult to execute, especially on a large scale [15], [16]. As an alternative to the homogeneous systems, we have recently reported a novel heterogeneous catalyst based on niobium-peroxo complexes immobilized on niobia (Nb2O5) with high chemical activity for oxidation reactions of organic compounds [17], [18], [19], [20]. However, the use of single niobium oxides as a catalyst for the oxidation of organic compounds in aqueous medium is scarcely reported. A study has been reported with niobium oxides as a catalyst for oxidation, but combined with an inorganic cation operating at high temperature [21]. In general, research has been focused on the study of the catalytic performance of Nb2O5-rich materials impregnated with metals [22]. Some known properties of niobium such as, more specifically, chemical activity in redox and catalytic systems, photosensitivity, acidity behavior constitute enough evidence to motivate understanding and an attempt to use niobium for catalytic purposes [23]. The Nb oxides are mainly used as acid catalysis due to its surface characteristics. In other recent research works of our group, we could report changes of Nb oxides with hydrogen peroxide, to enhance oxidation rates, as a chemical effect mediated by these materials [17], [18], [19], [20]. Catalysts based on niobium(V)-peroxo complexes for oxidation of organic and inorganic substrates and for epoxidation reactions have also been reportedly documented elsewhere [12], [13], [14].

Herein, we report, for the first time, a highly reactive system designed to oxidize organic compounds based on Nb-peroxo complexes immobilized on iron oxides produced in situ by pretreatment of Nb-doped iron oxides composites with H2O2. The use of iron oxides as support for niobium is interesting for the following main raisons: (i) they are relatively stable in the solution pH range commonly used in oxidation reactions of organics in water; (ii) the ionic radius of Nb5+ is comparable to that of Fe3+, and this dimension similarity favors niobium to isomorphically replace iron in the iron oxides structure; (iii) iron oxides are efficient Fenton-like catalysts, (iv) relatively cheap and (v) of wide natural availability, as iron is one of the most abundant elements on the Earth crust.

Section snippets

Materials preparation

Nb-doped iron oxides composites were prepared by first reacting Fe(NO3)3·6H2O (1.7 M) with 0.88 M NH4[NbO(C2O4)2(H2O)](H2O)n (used as supplied by CBMM, which stands for Companhia Brasileira de Metalurgia e Mineração, Brazil), and precipitating with concentrated ammonium hydroxide (pH 9.5–10). The precipitates were washed with ammonium acetate solution (2.60 M), dried at 100 °C for 12 h and thermally treated under N2 atmosphere at 400 °C for 2 h. The Nb-iron oxides composites were calcined at 500 °C for

Composites characterization

57Fe Mössbauer measurements were carried out to trace the physical and chemical changes on the pure and Nb-doped iron oxide nanocomposites, following the pretreatment of the material with H2O2. Mössbauer spectrum of pure iron oxide nanocomposites before treatment with H2O2 (PNCBT) (Fig. 1) shows a magnetic hyperfine field (Bhf) distribution with maximum probability value at 50.5 T (relative subspectral area for this spectral contribution, RA = 73%), which is a characteristic value assignable to

Conclusions

This work present a highly active iron oxides composite obtained by incorporation of niobium and treatment with H2O2. The innovative material produced presented a high activity for oxidation of organic dye in liquid phase. The analysis of the product with mass spectrometry showed that successive hydroxylations take place to produce several intermediate compounds. The characterization showed that the materials are constituted mainly by hematite, maghemite and FeNb2O6. The H2O2-treatment promotes

Aknowledgements

This work was supported by the FAPEMIG, CNPq, CAPES, Department of Chemistry of the University of California at Riverside and US National Science Foundation, CBMM (Brazil) for the samples and National Laboratory of Synchrotron Light (Brazil).

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