Short communicationInhibition effect of HBr over a commercial V2O5-WO3/TiO2 catalyst in a NH3-SCR process
Graphical abstract
Introduction
Hg0 oxidation is the key problem in mercury control in coal-fired power plants. It has been reported that the SCR catalyst (V2O5-WO3/TiO2), which is widely used for coal-fired flue gas DeNOx in power plants [1], has a synergistic Hg0 oxidation activity [2]. However, the Hg0 oxidation efficiency over the SCR catalyst varies with the HX (X = Cl or Br) flue gas concentration. In general, higher HX concentration results in higher Hg0 oxidation efficiency over the SCR catalyst [3]. Thus, extra HX was added in flue gases to enhance Hg0 oxidation, and this method has been widely studied [4], [5]. Moreover, it has been widely accepted that HBr is far more effective than HCl in this process [6], [14].
Nevertheless, the method may present a potential problem. Studies on the mercury oxidation mechanism between HX and Hg0 over V2O5/TiO2 suggest that HX firstly dissociatively adsorbed on the vanadium active sites; Hg0 then reacted with the surface Br or Cl generated from HBr or HCl dissociation to form the intermediate product [2], [7], [8]. Coincidentally, the vanadium sites are also critical active sites in NH3-SCR reactions. Therefore, the adsorption of HX had a great chance to affect the catalytic activity of the SCR catalyst (V2O5-WO3/TiO2) [9], [10]. Evidence of HCl adsorption on the catalyst surface was obtained by different surface analysis methods [5], [11], [12], and it has been reported that HCl inhibits the DeNOx activity of the SCR catalysts [9], [13]. Since bromine sits right beneath chlorine in the periodic table, HCl and HBr share the same chemical properties, thus raising the question of whether the similar inhibition effect would appear on HBr. The latest studies have found that HBr dissociatively adsorbs on the surface of the V2O5/TiO2 catalyst using a density functional theory (DFT) calculation [1], [8]. However, little direct evidence of the interaction between HBr and the SCR catalyst has been reported, and the related reaction sites are unclear.
In this paper, lab scale experiments were conducted to investigate the effect of HBr on the DeNOx activity of a commercial V2O5-WO3/TiO2 catalyst. The catalysts were characterized by XRD, BET, and XPS. Moreover, in-situ FTIR experiments were conducted to reveal the inhibition mechanism on the active sites.
Section snippets
Experimental system and conditions
The schematic experimental setup is shown in Fig. S1. The experimental conditions are available in supporting information.
Material and methods
A commercial V2O5-WO3/TiO2 catalyst was used in this study, essential information are summarized in Supporting information. The catalysts were characterized by BET, XPS. And the NH3 and HBr absorption on V2O5-WO3/TiO2 were studied by a FTIR spectrometer. Related information are summarized in supporting in formation.
Effects of HBr on NO conversion
Fig. 1 shows the NO conversion over the V2O5-WO3/TiO2 catalyst under different conditions. The catalyst has good DeNOx activities, the NO conversion increases with the increase in temperature, and the 90% conversion temperature (T90) occurs at approximately 300 °C. However, the NO conversion decreases in the low temperature range (i.e., < 300 °C) in the presence of HBr, and the decreasing rate increased with the increasing HBr concentration. No obvious changes are observed at higher temperatures.
Effects of HBr on textural properties
Conclusions
It is confirmed that HBr inhibits the DeNOx activity for V2O5-WO3/TiO2. NO conversion sharply decreases by 10% and 20% at 200 °C and 250 °C, respectively, in an HBr involved SCR reaction. The results of BET and XPS analyses indicate that HBr does not cause a loss of surface nor pore diameter of the catalyst while it reacts with vanadium components. Thus, more V4 + species are generated, which brings about a regular increase of the surface V4 +/V5 + ratio. Further investigation by in-situ FTIR
Acknowledgements
This work was supported by the National Basic Research Program (973) of China (No.2013CB430005) and the National Key Research Program Pilot Project of China (2016YFC0204103).
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