Full Length ArticleEnhanced low-temperature NH3-SCR performance of MnOx/CeO2 catalysts by optimal solvent effect
Graphical abstract
Introduction
Nitrogen oxides (NOx), which emitted from stationary and mobile sources, caused serious atmospheric pollution and destroyed the ecological environment [1], [2], [3], [4]. Therefore, it is an urgent task to control the emission of NOx. In the past decades, many purification techniques have been developed to satisfy this purpose. In which, the selective catalytic reduction of NOx by NH3 (i.e., NH3-SCR) is considered as the most cost-effective method to remove NOx from the flue gas of thermal power plants [5], [6], [7], [8]. It is well known that the commercial denitration catalysts are V2O5-WO3/TiO2 or V2O5-MoO3/TiO2, which exhibit excellent catalytic performance between 300 and 400 °C, but not suitable for low-load operating condition and low-temperature denitration (below 300 °C) [9], [10], [11].
However, in order to slow down the deactivation of denitration catalysts caused by dusts and SO2, the denitration unit is planned to be installed downstream of the precipitator and desulfurization device, where the concentration of dusts and SO2 has been decreased greatly [10]. Simultaneously, with regard to the transformation of the old thermal power plants, the denitration unit can only be added downstream of the precipitator and desulfurization device, because there is no enough space to install upstream of the precipitator and desulfurization device. However, the temperature of flue gas is remarkably lower than 300 °C after dusts removal and desulfurization. Therefore, the development of low-temperature denitration catalysts to satisfy this operating condition becomes a hot research topic.
Recently, MnOx/CeO2 catalysts have been widely investigated in low-temperature NH3-SCR reaction due to their excellent catalytic performance [12], [13], [14], [15], [16]. However, the operating temperature window and water resistance need to be further improved to satisfy the practical denitration application. Therefore, many methods have been adopted to broaden the operating temperature window and enhance the water resistance, such as the introduction of modifiers, change of preparation methods, modulation of synthesis parameters, and so on, which can remarkably affect the physicochemical properties of MnOx/CeO2 catalysts [10], [17], [18], [19], [20]. Liu et al. [21] prepared a series of MnOx-CeO2 catalysts with the acetic-acid-chelated titania support, and used for low-temperature NH3-SCR reaction. They found that their catalytic performance is obviously better than that of commercial TiO2-supported MnOx-CeO2 catalysts due to the high dispersion of active species. Furthermore, Ge et al. [22] reported that the dispersion of CeO2 on the surface of γ-Al2O3 support is highly dependent on the solvents (water and acetic acid), which leads to the different physicochemical properties and catalytic performance of CuO-CeO2/γ-Al2O3 catalysts for NO reduction by CO. However, as far as we know, the solvent effect on the dispersion of active species and the catalytic performance of MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction has not been reported.
Therefore, in the present work, we prepared a series of MnOx/CeO2 catalysts by modulating the solvents (deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and oxalic acid (OA) solution) with the purpose of improving the low-temperature NH3-SCR performance, broadening the operating temperature window, and enhancing the H2O + SO2 resistance. The prepared catalysts were characterized by means of N2-physisorption, XRD, EDS mapping, Raman, XPS, H2-TPR, NH3-TPD, in situ DRIFTS, and NH3-SCR model reaction to explore the effect of solvents on the physicochemical properties, catalytic performance, and H2O + SO2 resistance of MnOx/CeO2 catalysts.
Section snippets
Catalysts preparation
CeO2 was obtained by thermal decomposition of Ce(NO3)3·6H2O at 500 °C for 5 h in the flowing air, and then, used as a support to prepare supported MnOx/CeO2 catalysts. In detail, CeO2 was wet impregnated with required amount of Mn(NO3)2 solution in different solvents of deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and 0.1 M oxalic acid (OA) solution, respectively. The suspension was kept in magnetic stirring for 1 h and evaporated to remove the solvents at 110 °C during an oil
Catalytic performance and H2O + SO2 resistance (NH3-SCR model reaction)
Catalytic performance of these MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction is exhibited in Fig. 1. It can be seen from Fig. 1(a) that all of these catalysts present the same change trend during the heating process: firstly, NO conversion increases rapidly with the elevation of reaction temperature; secondly, the growth of NO conversion begins to slow with further elevation of reaction temperature; finally, NO conversion declines gradually when the temperature is above 200 °C due to
Conclusions
In summary, we have carefully investigated the solvent effect on the physicochemical properties and catalytic performance of MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction. The obtained results indicate that MnOx/CeO2 catalyst with oxalic acid as a solvent (i.e., Mn/Ce-OA catalyst) exhibits the best catalytic performance and good H2O + SO2 resistance. The reason may be that oxalic acid is beneficial to the dispersion of MnOx and enhances the electron interaction between MnOx and CeO2,
Acknowledgements
The financial supports of the National Natural Science Foundation of China (21507130), the Chongqing Science & Technology Commission (cstc2016jcyjA0070, cstc2014pt-gc20002, cstc2014yykfC20003, cstckjcxljrc13), the Open Project Program of Chongqing Key Laboratory of Catalysis and Functional Organic Molecules from Chongqing Technology and Business University (1456029), and the Open Project Program of Jiangsu Key Laboratory of Vehicle Emissions Control (OVEC001, OVEC007) are gratefully
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