Synthesis, characterization, and visible-light-driven photocatalytic performance of W-SBA15

https://doi.org/10.1016/j.jcis.2016.01.077Get rights and content

Abstract

A series of tungsten-containing mesoporous SBA15 with different tungsten contents were prepared through a direct co-condensation sol–gel method and then analyzed by various techniques. XRD patterns revealed that the original mesoporous morphology was maintained after introducing tungsten species and forms of tungsten species were changed from incorporated tungsten to isolated tungsten species or low oligomeric tungsten oxide species, and finally to crystalline WO3 with the increase of tungsten precursor content, which were also proven by HRTEM images, UV–Vis DRS, and FT-IR spectra. The W-SBA15 samples exhibited satisfactory photocatalytic performance toward degradation of dye Rhodamine B (RhB) and 2,4-dichlorophenol (2,4-DCP). In particular, the best candidate, sample 10%W-SBA15 showed an apparent reaction rate constant for RhB that was nearly 10 times as high as that of bulk WO3. The enhancement of photocatalytic capability was attributed to the mesoporous morphology with enlarged surface areas, negatively charged surface, and favorable tungsten forms such as incorporated tungsten, isolated tungsten or low oligomeric tungsten oxide species. In addition, active radicals trapping experiments and DMPO spin-trapping ESR spectra indicated that photogenerated holes were major oxidative species during photocatalysis.

Introduction

Photocatalytic technology has attracted massive attention during last dozen years because of the powerful degradation capability for a wide range of undesirable chemical contaminants in water and air by using solar or artificial light irradiation [1], [2], [3]. Although TiO2 is deemed as a low cost and highly efficient photocatalyst with the unique chemical and thermal stability, practical applications of TiO2 are greatly restrained by the low quantum efficiency and irresponsivity to visible light [4], [5]. In contrast, WO3 with a narrow band gap of 2.7 eV can be activated in visible-light region. The major disadvantage of WO3 as a photocatalyst is the low photonic efficiency, as a result of the low conduction band level and the difficult of electrons consumption after activation [6]. In general, structural modification and morphology tailoring are both solutions to overcome the above-mentioned weaknesses [7], [8], [9].

The former method is relevant to the structural modification of WO3 by means of incorporating other semiconductors with appropriate electronic structures, metal and nonmetal elements, and doping with metal ions [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. With suitable ingredients, accumulated electrons on conduction band of WO3 tend to be completely transferred and accordingly, the recombination rate of photogenerated hole–electron pairs is greatly depressed [15]. The latter approach is to modify synthetic routes to tailor morphology and size of nanostructured WO3 that provides not only abundant active sites for catalytic reactions but reduced diffusion distances for charge carriers to active sites [19], [20], [21], [22], [23], [24], [25]. To support or immobilize WO3 into an inert substrate is an efficient way to enhance photocatalytic behavior in terms of controlling the morphology and size of WO3. Mesoporous silicate SBA15 is generally selected as a support for photocatalytic applications because of the highly ordered two-dimensional hexagonal morphology, large specific surface area, adjustable pore size, good hydrothermal stability [26], [27], and most importantly, the electron delocalization capacity by framework in comparison with any other porous silicate substrates [28], [29], [30].

Actually, WO3-containing SBA15 have been synthesized and employed to catalyze a variety of chemical transformations, such as oxidation of cyclopentene [31], [32], desulfurization [33], dehydration [34], and metathesis [35], [36]. The incorporation of tungsten source into SBA15 matrix induces the formation of different tungsten species forms, among which isolated tungsten species or low oligomeric tungsten oxide species with terminal Wdouble bondO groups are deemed as actives sites to initiate above catalytic reactions [31], [32], [33], [34], [35], [36]. Though these tungsten species are powerful to catalyze organic transformations under conventional conditions, as far as we know, there are no reports focusing on the photocatalytic evaluation of tungsten-containing SBA15 under visible-light irradiation. Therefore, in the present investigation, a series of W-containing SBA15 samples were prepared and characterized by a collection of analytical techniques. In this work, we showed that existed tungsten forms were transformed from highly dispersed tungsten species on SBA-15 to crystalline WO3 along with the increase of tungsten content. The as-prepared samples showed improved photocatalytic performance toward degradation of RhB, an industrial dye, in comparison with bulk WO3, mainly owning to the mesoporous morphology with enlarged surface areas, relatively negative charged surface, and suitable tungsten species. In addition, these W-SBA15 samples were also quite efficient in catalytic degradation of 2,4-DCP, a colorless and non-biodegradable organic pollutant. However, from the aspect of potential application, the prepared W-SBA15 series needed further structural modification to obtain materials with excellent photocatalytic performance as well as good reusability.

Section snippets

Materials

Pluronic P123 (PEO20PPO70PEO20) was obtained from Aldrich Chemical Co. Sodium tungstate (Na2WO4⋅2H2O) was purchased from ACROS Organics. Hydrochloric acid (HCl), tetraethyl orthosilicate (TEOS), and other reagents were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All Chemicals were used directly for experiments without any further purification. Deionized water was used throughout the experiment.

Preparation of W-SBA15 series

In a typical synthesis, P123 (1.8 g) was dissolved in an HCl aqueous solution (2

Results and discussion

Low-angle XRD patterns of W-SBA15 series with different content of tungsten species are shown in Fig. 1A. All samples display three pronounced diffraction peaks corresponding to (1 0 0), (1 1 0), and (2 0 0) reflections of mesoporous silicon-based materials, indicating that the involvement of tungsten species into SBA15 didn’t significantly affect the structure of p6mm hexagonal symmetry [39]. The intensity of the (1 0 0) reflection decreases with the increase of tungsten species content, possibly

Conclusion

In this investigation, tungsten-containing mesoporous SBA15 materials were fabricated by means of a direct co-condensation sol–gel route and characterized by a collection of analytical techniques. Original mesoporous morphology was maintained after introducing tungsten resource and the tungsten form was changed from incorporated tungsten species to isolated tungsten species or low oligomeric tungsten oxide species, and finally to crystalline WO3 as long as the tungsten precursor content was

Acknowledgements

We are grateful to the National Natural Science Foundation of China (Grant number 21207089), the project-sponsored by SRF for ROCS, SEM., and the Hujiang Foundation of China (B14003) for financial support.

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