New materials based on modified synthetic Nb2O5 as photocatalyst for oxidation of organic contaminants
Introduction
In heterogeneous catalysis, main applications involve Nb compounds as promoters and support for other metals mainly due to the increase of catalytic activity and stability of the catalyst. Recently, the photocatalytic properties of niobia compounds have been explored by many authors [1], [2]. These properties appear when electron–hole pairs are created on the semiconductor surface by irradiation. Thus, a charge will transfer between electron–hole pairs and species adsorbed onto semiconductor surface, resulting in the photodegradation of contaminants. As it is well known, TiO2 is believed to be one of the most promising candidates as a photocatalytic material for degradation of organic compounds because of its photoelectrochemical durability. However, TiO2 has a large band gap of approximately 3.2 eV [3]. Thus, the use of Nb2O5 as a catalyst can be a new alternative for the photodegradation of contaminants. Even though Nb2O5 presents a high band gap value similar to that of TiO2, it is possible to produce a material with low band gap value by modification of niobia [4]. Moreover, the application of modified Nb2O5 for the photodegradation of contaminants is not well explored in the literature. In this direction, this present paper reports the application of Nb2O5 in the photodegradation of methylene blue dye (MB) after two modifications: (i) doping with W or Mo, and (ii) treatment with hydrogen peroxide. The synthetic niobia was modified with hydrogen peroxide in order to improve the photoelectronic properties of the catalyst by the decreasing of the band gap. A previous work [5] showed that the catalytic activity by the synthetic niobia can be due to the surface groups, known as niobium peroxo complexes, formed by the treatment with H2O2. The reaction intermediates were monitored by electrospray ionization mass spectrometry (ESI-MS).
Section snippets
Synthesis and characterization
Niobia was prepared by slow dropping of an 1 mol L−1 NaOH solution in a 500 mL Teflon beaker containing 100 mL 0.26 mol L−1 solution of NH4[NbO(C2O4)(H2O)](H2O)n, kindly donated by CBMM – Companhia Brasileira de Metalurgia e Mineração (Araxá, state of Minas Gerais, Brazil) at 70 °C under vigorous stirring. The solids obtained were washed with distilled water until neutral pH. The Mo or W substituted niobia were prepared from NH4[NbO(C2O4)(H2O)](H2O)n (1.7 mol L−1) and Na2MoO4 · 2H2O or Na2WO4 · 2H2O
Niobia characterization
The FTIR spectra of Nb2O5 after modification with W and Mo (Fig. 1a) and treatment with hydrogen peroxide (Fig. 1b) presented peak characteristic for this catalyst: a peak at 3518 cm−1 assigned to the OH stretching of Nb–OH and a typical signal at 3160 cm−1, due to the bulk hydroxyl stretching. A broad band at 1696 cm−1 related to adsorbed water on the Nb2O5 surface, a peak at 873 cm−1 assigned to Nb–O, Mo–O or W–O stretchings, and bands between 500 and 950 cm−1 corresponding to Nb–O–Nb, W–O–W and
Conclusion
Nb2O5 presented photocatalytic activity to oxidize methylene blue dye. After modifications (doping with W, Mo or H2O2 treatment), this catalyst presented an improvement in its catalytic activity. These modifications are responsible for producing materials with lower band gap, diminishing the energy of the electron/hole pair formation and consequently increasing catalytic activity. ESI-MS indicated that an oxidation mechanism may occur by attack of the free OH radical over the molecule giving
Acknowledgments
The authors are grateful to CNPq, Finep and FAPEMIG by financial support and CBMM (Brazil) for samples.
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