Visible-light driven degradation of ibuprofen using abundant metal-loaded BiVO4 photocatalysts
Introduction
Pharmaceuticals are now considered to be aquatic contaminants because they have the potential to damage our environment due to their extensive production and usage (Zhang et al., 2014). Thousands of tons of pharmaceutical substances, such as antipyretic analgesics, can reach our aquatic environment, mainly from sewage-treatment plants, as a result of their incomplete removal (Trovo et al., 2012). Ibuprofen is one of the most commonly used antipyretic analgesic drugs and has been found at a concentrations level of 0.53 μg L−1 in surface water (Buser et al., 1999). Several methods have been attempted to treat wastewater containing ibuprofen, including biological treatments and the Fenton method. However, these methods require a huge amount of energy and may produce secondary pollution (Matamoros et al., 2009). Because visible light accounts for the largest proportion of the solar spectrum, photocatalysis has been using semiconductor photocatalysts to split water or degrade organic pollutants under visible-light irradiation, which is a promising and energy-sustainable method (Long et al., 2006). Because the band gaps of common photocatalysts such as TiO2 are too large to respond to visible light, many investigators have been focused on the preparation of new catalysts. As a result, bismuth vanadate (BiVO4) is one of the visible-light photocatalysts that have various applications, especially in photochemistry (Zhang et al., 2007).
As an important material in the coatings and plastics industry, BiVO4 possesses many excellent properties, such as ferroelasticity, ionic conductivity, and photocatalytic characteristics that are used for water splitting and oxidative dehydrogenation. It can also be used as a yellow pigment, in gas sensors and as a positive electrode material. There are three main crystalline phases for BiVO4 as a catalyst: the tetragonal zircon, monoclinic scheelite, and tetragonal scheelite structures. Monoclinic scheelite BiVO4 shows much better photocatalytic activity than the other forms (Tokunaga et al., 2001). However, the photophysical and photocatalytic properties are strongly influenced by the recombination of photogenerated electrons and holes. Researchers have indicated that the activity of pure BiVO4 is relatively low. According to certain reports, loading metals onto the surface of BiVO4 can greatly improve the photocatalytic performance by suppressing the recombination of electrons and holes. Xu et al. prepared a Cu–BiVO4 catalyst by a hydrothermal method: the doping with Cu did not change the crystal structure of the BiVO4, but the Cu–BiVO4 catalyst significantly increased the photocatalytic efficiency for degrading methylene blue (Xu et al., 2008). Ge et al. synthesized Pt-BiVO4 and Pd-BiVO4 via the impregnation method and reported that the photocatalysts exhibited significant photocatalytic efficiency in degrading methyl orange (Ge, 2008a, Ge, 2008b). Kudo et al. synthesized an Ag–BiVO4 catalyst using a dispersant, which showed high efficiency in degrading phenolic wastewater (Kohtani et al., 2005). Compared with loading Pd or Pt precious metals, Cu and Ag are much less expensive and are common metals that can be easily found on the earth. Therefore, we study loading Cu and Ag metal onto pure BiVO4 for the degradation of ibuprofen under visible-light irradiation.
Section snippets
Photocatalyst preparation
All chemicals were reagent grade and used without further purification. Monoclinic BiVO4 powders were prepared by a hydrothermal method. The monoclinic BiVO4 powders were synthesized by mixing the appropriate amounts of Bi(NO3)3 and NH4VO3. First, the two precursors were dissolved in a 4 M nitric acid solution. The mixture was stirred for 0.5 h at room temperature in air. Then, a saturated sodium bicarbonate solution was dropped into the mixture until the pH value was approximately 5. This yellow
XRD pattern analysis
The XRD patterns of the as-prepared catalysts, including pure BiVO4 and M-BiVO4 (M = Cu, Ag), are shown in Fig. 1. The diffraction peaks of all of the samples were indexed to the standard card of monoclinic scheelite BiVO4 (JCPDS 14-0133). Characteristic peaks at 15.2°, 18.7°, 28.6°, and 30.5° were observed for these three samples, which were identical to the peaks for the monoclinic scheelite phase of BiVO4. This indicated that all of the samples were monoclinic scheelite BiVO4, and no peaks of
Conclusion
A BiVO4 catalyst was prepared by a hydrothermal method, and two metal-loaded BiVO4 catalysts, Cu–BiVO4 and Ag–BiVO4, were synthesized by the wet-impregnation method. All of the as-prepared samples were characterized by XRD, SEM, UV–vis, XPS, and BET. The results revealed that doping with Cu and Ag did not change the crystal structure of the photocatalyst, that Cu existed as two types of oxides, Cu2O and CuO, on the surface of the pure catalyst and that Ag ions were present as a mixture of
Acknowledgments
This work was supported by the Beijing Higher Education Young Elite Teacher Project (No. YETP0773), the National Natural Science Foundation of China (Grants 51278053 and 21373032), the Beijing Natural Science Foundation (No. 8122031), the Fundamental Research Funds for the Central Universities, and grand-in-aid from Kochi University of Technology, which are greatly appreciated.
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