| The selective oxidation of alkanes or aromatics to gain value-added aldehydes and ketones products is of great significant. Currently, there exist some main drawbacks of researches in this area, including low catalytic efficiency, narrow catalysts, and using of ultraviolet irradiation or noxious solvent. Bismuth tungstate (Bi2WO6) as a low cost and easily prepared photocatalyst, has attracted a lot attention due to its good visible light response, suitable band location and unique layered structure. Recently, Bi2WO6 was found displaying a good photocatalytic activity in the oxidation reactions of alcohols. In this paper, on the one hand, a flower-like Bi2WO6 was synthesized by a simple hydrothermal method, and it was investigated for selective oxidation of toluene derivatives. On the other hand, through adjusting the preparation conditions of Bi2WO6, a series of catalysts were synthesized with different structures and properties. The effects of different synthesis conditions on the catalyst's properties such as morphology structure were explored. Furthermore, we tried to reveal the structure-activity relationship between the properties and selective oxidation activity of the as-prepared Bi2WO6. The main research contents and innovative results of the paper are summarized as follows:1. A flower-like Bi2WO6 consisting of nanosheet was prepared by a simple hydrothermal method. High selective oxidation of toluene into benzaldehyde was achieved (sel.96%) with the flower-like Bi2WO6 under mild conditions, e.g. room temperature and pressure, solvent-free, visible light irradiation. The results from substrate expansion showed that flower-like Bi2WO6 performs all good activity to toluene derivatives, and only the a-carbon of the C-H bond could be activated in this reaction system. In addition, the investigation of reaction mechanism in the photocatalytic system has demonstrated that the main active species are photogenerated holes, while hydroxyl radicals and superoxide radicals are little involved in, which is different from most reported studies. The plausible mechanism for the photocatalytic selective oxidation of toluene has also been proposed base on the results: toluene is directly oxidized to benzal raidicals by holes as a rate-determining step, then O2 reacts with the benzal radicals to form peroxy intermediate, and benzaldehyde is produced after dehydration finally.2. Through adjusting the preparation conditions of Bi2WO6, e.g. hydrothermal temperature, time, pH value, addition amount of HNO3, a series of Bi2WO6 photocatalysts were obtained with different properties. Power X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), X-ray photoelectron spectroscopic (XPS), UV-vis diffuse reflectance spectra (UV-vis DRS) were utilized for analysing the influences of synthesis conditions on the structure and properties. The characterization results showed that high temperature (>140?) or under neutrality, alkaline condition are beneficial to synthesize Bi2WO6 with high crystallinity and strong visible light absorption ability; the addition of HNO3 could change the composition of surface atoms by affecting the solubility of ions in the crystal growth process. The selective oxidation of toluene into benzaldehyde has been performed to explore the structure-activity relationship of Bi2WO6. Results showed that with the basic properties of good crystallization and visible light absorption, the selective oxidation activity of Bi2WO6 may be concerned with (200) crystal plane and the concentration of oxygen atoms on the catalyst surface, the defects of W-O, Bi-O and the adsorbed oxygen is favourable to improve the photocatalytic activity for toluene oxidation reaction. |
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