Photo- and thermo-catalytic mechanisms for elemental mercury removal by Ce doped commercial selective catalytic reduction catalyst (V2O5/TiO2)
Introduction
Mercury is widely regarded as a global pollutant that poses a serious threat to the human environment. The combustion of coal is one of the main causes of atmospheric mercury pollution in China (Pirrone et al., 2010). There are three main forms of mercury in coal-fired fiue gas mainly: oxidized mercury (Hg2+), particulate mercury (HgP) and elemental mercury (Hg0). Hg0 is particularly more difficult to control by existing contaminant removal devices than Hg2+ and HgP because of its inertness, high volatility and low solubility in water (Zhang et al., 2018; Mei et al., 2020). Hence, gaseous elemental mercury capture from coal-fired flue gas has been a great challenge to in the field of environmental pollution prevention and control. According to the existing experimental research, the most cost-effective commercial SCR catalysts (V2O5/TiO2) not only have remarkable NOx reduction performance, but also have the capability of promoting Hg0 oxidation at low temperature (Li et al., 2020a, 2021a, 2021b; Meng et al., 2019). Therefore, using commercial SCR catalyst for synergistic catalytic removal of elemental mercury is a promising and practical method. However, the integrated removal of NOx and mercury has been mainly based on the principle of thermal catalysis.
Up to date, photocatalytic oxidation technology has been deemed to have a wide application prospects for pollutant control and for energy utilization (Li et al., 2021c; Zhang et al., 2021; Cheng et al., 2019). Photocatalytic oxidants, with the advantages of strong oxidation and no secondary pollution, have the capability of removing elemental mercury more efficiently and environmentally. The photocatalytic behavior is largely dependent on the type of photocatalyst. TiO2 based photocatalysts are widely used in the field of photocatalysis because of its low preparation cost, good photostability and nontoxicity (Wang et al., 2020; Chen et al., 2020a; Li et al., 2021d). Therefore, Ti based catalysts were expected to be low-cost, environmentally friendly, and regenerable sorbents for simultaneous thermal and photocatalytic mercury removal in this paper. In recent years, the modification of TiO2 by cerium (Xu et al., 2002) and its oxides (Lin and Jimmy, 1998) to improve the photocatalytic performance have been reported in removing various environmental pollutants. CeO2 was judged to be a suitable choice for modifying TiO2 owing to its narrow band gap and Ce4+/Ce3+ reversible redox couple (Xie et al., 2021; Li et al., 2020b; Wang et al., 2013). The research showed that the reversible conversion between Ce(IV) and Ce(III) could realize efficient electron transfer between CeO2 and TiO2 (Yu et al., 2010), so as to improve the photocatalytic capability.
Few studies have revealed the effect of Ce on the thermo- and photo-oxidation performance of Hg0 on SCR catalysts through the combination of experiments and calculations. In this study, the Ce-doped V2O5/TiO2 catalysts were synthesized by the impregnation method and their catalytic properties were tested under the UV irradiation at 30–160 °C. In addition, density functional theory (DFT) calculations can be combined with the traditional experimental and characterization methods to better analyze the catalytic mechanism of elemental mercury.
Section snippets
Catalyst preparation
In this study, a V2O5/TiO2 catalyst (hereafter referred to as V-Ti) was used as the base catalyst, and Ce-modified catalysts were prepared by the impregnation method. First, V-Ti catalyst powder was stirred in 30 mL deionized water for 1 h. Then, the modified catalysts (0.4 % Ce, 0.8 % Ce, 1.2 % Ce, 1.6 % Ce and 2.0 % Ce) were prepared by Ce(NO3)3·6H2O solution. The V-Ti samples were filtered and dried for 24 h and calcined at different temperatures (400, 500 and 600 °C) for 2 h. Ce doped
BET analysis
The specific results are shown in Table S1. The larger the BET specific surface area is, the stronger the physical adsorption capacity is (Meng et al., 2019). The specific surface areas of V-Ti and Ce-V-Ti were 25.6–60.6, and 59.7–63.5 m2/g, respectively, and the pore volumes were 0.18–0.23 and 0.23–0.24 cm3/g, respectively, suggesting that there was a marked drop in the specific surface area of V-Ti according to increasing calcination temperatures from 400 to 600 °C, which was confirmed by the
Conclusion
V2O5/TiO2 catalysts were modified by cerium (Ce) through impregnation method. The mechanisms of Hg0 catalytic reaction were explained based on catalyst characterization. The catalysts displayed the highest mercury removal efficiency under UV light at 120 °C, revealing both of the photo-catalytic and thermo-catalytic mechanisms of Hg0 removal over V-Ti and Ce-V-Ti samples. The addition of Ce played a greatly promoting role in the mercury removal capability of the catalyst, resulting from that Ce
Credit author statement
Yili Zhang constructed the overall idea and innovation of the article. Yili Zhang did DFT related calculations and carried out the experiment. Jing Liu helped analyze calculation parts. Rihong xiao and Tian Gao helped experiment parts. Pengfei Liu and Bengen Gong helped with partial characterization analysis. Yili Zhang, Yongchun Zhao, Zhuo xiong and Junying zhang co-write the paper. Yongchun Zhao and Junying Zhang proposed and supervised the project. All authors revised the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Key R&D Program of China (2019YFC1907000), the Program for HUST Academic Frontier Youth Team (2018QYTD05), the Key Research and Development Program of Hubei Province (2020BCA076) and the National Nature Science Foundation of China (NSFC) (42030807 and 51806076).
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