| Selective catalytic reduction with NH3(NH3-SCR)is the most widely used technology for the removal of NOx from stationary sources.The commercial vanadium-based catalysts have a relatively narrow active temperature window concentrated on the middle and high temperature ranges.The temperature of the flue gas after desulfurization and dedusting treatment is usually low,and the flue gas needs to be reheated before entering the SCR reactor,which increased energy consumption.Therefore,the development of high-performance low-temperature NH3-SCR catalysts has broad application prospects MnOx exhibits excellent low temperature SCR activity due to its rich variable valences SAPO-34 molecular sieves with well-organized pores,large specific surface area and suitable surface acidity,have been widely investigated as catalysts carriers.In this paper,with MnOx as the active components and SAPO-34 molecular sieves as the carriers,highly dispersed MnOx/SAPO-34 catalysts were prepared,and modified by element doping to enhance the catalytic activity and resistance to SO2 and H2OFirstly,a series of MnOx/SAPO-34 catalysts were prepared by the improved sol-gel method,and the inherent relationships between the structural properties of the catalysts and the low-temperature SCR performance were investigated using various characterization methods.The results show that the highly dispersed 15%-MnOx/SAPO-34-350℃ catalyst exhibited the best low-temperature SCR activity on the preparation conditions of Mn loading of 15%and calcination temperature of 350℃,and reached more than 90%NO conversion and nearly 100%N2 selectivity in the reaction temperature ranges of 120～240℃.MnOx nanoparticles were highly dispersed on the SAPO-34 surface,and exposed abundant active(110)crystal planes of MnO2,which may contribute to the excellent catalytic performance Moreover,the higher Mn4+ratio,more chemisorbed oxygen species,and the appropriate surface acid strength and acid sites also played significant roles in the excellent low-temperature SCR activity of the catalystSecondly,Ce,La,Co,Ni,Cu and Sb were screened as doping elements to modify the low-temperature SCR activity and the resistance to SO2 and H2O of the MnOx/SAPO-34 catalyst.The results indicate that the Co-doped MnOx/SAPO-34 catalyst showed the best low-temperature SCR activity.With the Co/Mn mole ratio of 0.2,the CoOx(0.2)-MnOx/SAPO-34 catalyst reached more than 90%NO conversion at a reaction temperature of 100℃,and the doping of Co enhanced the resistance to SO2 and H2O of the catalyst.The Co-doped catalysts were characterized by various characterization methods,and the structure-activity relationships of the catalysts were discussed.The results illuminate that part of MnOx interacted with CoOx on the surface of the catalysts,and then CoMnOx solid solutions were generated and highly dispersed on the carriers in the form of nanocrystallites,which increased the dispersity of MnOx and exposed more highly active crystal planes.Meanwhile,higher proportion of(Mn4++Mn3+)species,chemisorbed oxygen Oa species,and appropriate surface acid strength and acid sites were also played important roles in better low-temperature SCR activity and the resistance to SO2 and H2O of the catalyst.Finally,the low-temperature SCR reaction mechanism and the sulfur-resistance mechanism over the highly active CoOx(0.2)-MnOx/SAPO-34 catalyst were studied by in situ FT-IR technology.The results reveal that both Br0nsted acid sites and Lewis acid sites existed in the catalyst,the-NH2 species were active intermediates,and most of the adsorbed NOx species could react with the coordinated NH3,NH4 or-NH2 species.The low-temperature SCR reaction of CoOx(0.2)-MnOx/SAPO-34 catalyst follows the E-R mechanism and L-H mechanism simultaneously.In addition,sulfates species were hardly deposited on the surface of the CoOx(0.2)-MnOx/SAPO-34 catalyst,which may also contribute to the superior resistance to SO2 of the catalyst. |
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