| Nitrogen oxides(NOx) as one of the main air pollutants, contribute to the formation of acid rain, photochemical smog and the fine particles as an important precursor, and do great harm to human health and ecological environment. Therefore, the control of NOx has been concerned by both domestic and oversea researchers. At present, selective catalytic reduction with ammonia(NH3-SCR) is the most widely used and maturest technology for eliminating NOx emitted from stationary sources. As the crucial factor for the SCR technology, series of catalysts have been studied. The commonest commercial catalysts(V2O5/TiO2) show their optimum performance in the high temperature range 300~400°C. In order to avoid reheating the flue gas, the SCR system must be located upstream of desulfurizer and electrostatic precipitator device, which accelerates catalysts deactivation due to exposure to high concentrations of particulate matter and SO2. Thus, to save cost and avoid device transformation, the development of highly efficient catalysts for low-temperature NH3-SCR has become an important research direction in the field of flue gas denitration.With variable valence state and high redox abilities, and containing various types of labile oxygen which are necessary to complete the catalytic cycle in SCR reactions, MnOx show excellent low-temperature SCR activity. SAPO molecular sieve possess the channel structure range from six-membered ring to twelve-membered ring and the pore diameter is between 0.3~0.8nm, so they can adapt to different sizes of molecular adsorption and diffusion. And their ion exchange characteristics and appropriate surface acidity, make them become the potential research material as catalyst or catalyst carrier in catalytic field. In this paper, a series of MnOx supported on SAPO molecular sieve catalysts were prepared by ethanol dispersion method. And through doping transition metal oxides, as well as optimizing doping amount and calcination temperature, we obtained the denitration catalyst with excellent low-temperature SCR activity.Firstly, a series of MnOx supported on SAPO-n(n=5,11,34) catalysts were prepared and tested for low-temperature NH3-SCR activity. MnOx/SAPO-34 catalyst showed the highest NH3-SCR activity at low temperature. The surface reactive species and surface acidity of the catalysts were characterized by XRD, N2 adsorption-desorption, XPS, H2-TPR, NH3-TPD and NH3 FT-IR. The results indicated that MnOx were mainly dispersed on the supports in amorphous state, and the amounts and valence of Mn species were quite different on different support surface. Excellent catalytic performances of catalysts at low temperature were due to the rich Lewis acid sites and high surface concentration of surface Mn4+ species.Secondly, in order to improve SCR activity of MnOx/SAPO-34 catalysts at lower temperature, the catalysts were modified by doping transition metal(Fe, Co, Ni, Cu) oxides. The results showed that the addition of transition metal oxides could enhance the dispersion of MnOx on the surface of SAPO-34 as well as the redox abilities of catalysts. The addition of Fe, Ni, Cu could improve low-temperature SCR activity of the catalysts, and the addition of Cu showed the most significant effect.Finally, further researches of MnOx-CuOy/SAPO-34 catalysts showed that the doping amount of Cu and calcination temperature had great influences on the SCR activity. The MnOxCuOy/SAPO-34 catalyst with molar ratio of n(Cu)/n(Mn)=0.2 and calcination temperature of 400℃ had the highest SCR activity. The addition of Cu could change the distribution of acid sites on the catalyst, which made the catalyst have suitable surface acidity and medium strong acid site ratio, as well as increased Lewis acidity of catalyst effectively. These were conducive to promote the adsorption and activation of NH3 on the catalyst surface, thus the lowtemperature SCR performance of catalyst was significantly improved. |
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