Synthesis Of Mn SAPO-34 Molecular Sieves And Its Performance On Selective Catalytic Reduction Of Nitrogen Oxides With Ammonia

Nitrogen oxides(NOx) is one of the major air pollutants, which can cause photochemical smog, production of acid rain, formation of fine particles and many other respiratory problems to ecosystems and human health. Selective catalytic reduction(SCR) of NOx by ammonia is one of the major technologies for reducing nitrogen oxides emitted from stationary sources such as power stations, industrial heaters, and cogeneration and has been successfully commercialized. Vanadium-based catalysts such as V2O5/TiO2(anatase) mixed with WO3 or MoO3 are typical commercial catalysts for this process; however, they are only active within a narrow temperature window of 300~400 oC, and are susceptible to deactivation by dust deposition or SO2 poisoning. Thus, there has been a very strong incentive to develop highly efficient denitration catalysts for low-temperature SCR processes in which these catalysts would be placed downstream of the desulfurizer and electrostatic precipitator in the power generation system. Mn-based catalysts show excellent SCR activities according to many years’ researches, besides, in recent years, SAPO-34 molecular sieves which with a small-pore structure, high thermal stability and suitable acidic sites exhibit better activities during SCR reactions.In this dissertation, MnSAPO-34 molecular sieve were synthesized by hydrothermal method and characterized by a suite of analytical methods(XRD, XPS, SEM, UV-Vis DRS, TPR, TPD et al.). Mn loading, calcination temperature, synthesis time, different templates and Si loading were investigated.(1) The suitable manganese loading can successfully synthesize Mn SAPO-34 which has a cubic structure and a particle size range from 2 μm to 50 μm. N2 adsorption-desorption results reveal that the incorporation of manganese could decrease the specific surface area and pore volume of the catalysts, however, it yielded the opposite results with decreased calcination temperature and shorter synthesis time. Shortening the synthesis time could decrease the particle size(D(50%)) lower than 3 μm, however, when the mixed templates were used in the synthesis process, the decrease of particle size couldn’t be seen in the products, the longer synthesis time and the fierce reaction between TEA and Si/P/Al sources might be the reasons of this results.(2) The MnSAPO-34, which was synthesized in 6 hours and calcined at 550 °C with the ratios(n(MnO)/n(P2O5) = 0.1; n(SiO2)/n(P2O5) = 0.8) and with TEA, exhibits the highest activity among all the samples, with NOx conversion of almost 100 % and N2 selectivity higher than 80 % at the temperature window between 280 °C~400 °C.(3) The results show that a small amount of manganese is in the catalysts and both framework and non-framework manganese species are proven to be in the catalysts. Manganese species of high oxidation state—mostly Mn4+—are shown to be on the catalysts surface after high temperature calcination, and MnO2 is presumed to be the main active manganese species. Besides, the increase ratio of Mn3+ could help to improve the catalytic activity.(4) Under proper synthesis conditions, the incorporation of manganese could improve the adsorption of nitric oxide and ammonia, and the interaction between the strongly absorbed NO and strongly absorbed NH3 might be the reason for the enhancement in their catalytic efficiency.