Novel Mn/SAPO-34 Series Catalysts For Low-temperature Selective Catalytic Reduction Of NO_x With NH₃: Catalytic Performance And Reaction Mechanism

Selective catalytic reduction?SCR?technology is widely used domestically and overseas to denitration from flue gas,which is stable and efficient.Catalysts are crucial for SCR technology,which directly determine the relative efficiency of the system.However,the most common commercial catalysts V₂O₅-WO₃?MoO₃?/TiO₂,do not exhibit acceptable catalytic activity at low temperature?< 200 ??and there are several shortcomings for this system,including short service life,low N₂ selectivity at high temperatures and the environmental toxicity of vanadium.Therefore,development of an environmentally-friendly,low-temperature SCR catalysts is an approach to solve these problems.A low temperature SCR catalyst would be easily deactivated by SO₂ and H₂O,and it would not be useful under actual working conditions.Therefore,enhancing the SO₂ and H₂O resistance of these catalysts is significant in the practical application of low-temperature SCR technology.A series of novel Mn/SAPO-34 catalysts were prepared in this study.The improved catalysts were optimized to obtain catalysts with excellent performance at low-temperature with acceptable resistance to SO₂ and H₂O.Firstly,the MnO_x/SAPO-34 catalysts were prepared by conventional impregnation and an improved molecularly designed dispersion method,and the effect of the preparation method on the SCR activity of the MnO_x/SAPO-34 catalysts was investigated.The experimental results showed that the MnO_x was highly dispersed on the surface of a SAPO-34 molecular sieve support and the 20 wt.% MnO_x/SAPO-34-MD catalysts exhibited a strong redox capability and suitable surface acidity,which was the key reason for its excellent low-temperature SCR activity.The MnOx nanoparticles were existed in the channeles of the 20 wt.% MnO_x/SAPO-34-MD catalysts,which was the main reason for better SO₂ resistance of the 20 wt% MnO_x /SAPO-34-MD.Besides,the low-temperature SCR reaction mechanism for 20 wt.% MnO_x/SAPO-34-MD catalysts has been investigated by in situ FT-IR technology,and the results showed that the —NH₂ species and the bridged nitrate species were key active intermediates in the SCR reaction.At 200 ?,both L-H mechanism and E-R mechanisms were probably involved in the SCR reaction,while the E-R mechanism dominated the catalytic activity of MnO_x/SAPO-34.Secondly,based on their relative ionic polarization,praseodymium and cerium were screened as additives to resist catalyst poisoning caused by SO₂ and H₂O.The structural properties of the catalysts were characterized using X-ray diffraction,X-ray photoelectron spectroscopy and Thermogravimetric analysis as well as other characterization methods before and after catalyst poisoning.The results showed that Pr and Ce could improve the activity of catalyst.In addition,it was also found that Pr doping could significantly improve the low-temperature SCR activity of the subject catalyst.The Pr or Ce was proved to exist in the catalysts in the form of amorphous state and preferred to reacting with SO₂ in the sulfur atmosphere.As a result,the sulfation of Mn near to the Ce or Pr was suppressed which could improve the SO₂ resistance of the catalysts.It was also found that Pr and Ce could improve H₂O resistance of the catalysts to some extent.Traditional route to synthesis molecular sieves is difficult to control the amount of manganese,chemical state of manganese and the crystallinity of the molecular sieve in MnSAPO-34 molecular sieve,which could directly influence its low temperature NH₃-SCR performance.In this study,methods for improving the degree of substitution isomorphous have been investigated.In addition,through screening organic ammonium template with different spatial structures,we have controllable synthesis of manganese-rich MnSAPO-34 molecular sieves.Further study showed that the manganese-rich MnSAPO-34 molecular sieves synthesized with the ratios(n?MnO?/n?P₂O₅?= 0.4 and with TEA and DIPA,exhibited the highest activity among all the selected samples.Besides,when the reaction temperature is 220 ?,the MnSAPO-34 molecular sieves presented nearly 100% NO_x conversion and over 95% N₂ selectivity.Stability tests demonstrated that the SCR performance of the catalyst was relatively stable.Through using X-ray photoelectron spectroscopy,Diffuse reflectance UV–Vis measurement and other characterization methods,Mn?IV?species were successfully introduced into the framework of molecular sieve,providing an important activity center for the NH₃-SCR reaction.Finally,a variety of methods were examined to improve the low-temperature SCR activity,anti-sulfur and anti-water performance of the catalysts.The experimental results showed that MnCuSAPO-34 molecular sieves prepared by a one-pot hydrothermal crystallization method exhibited excellent low-temperature NH₃-SCR activity.When the reaction temperature was 180 ?,the MnCuSAPO-34 molecular sieves exhibited nearly 100% NO_x conversion and over 95% N₂ selectivity.Moreover,the ability of this catalyst to resist water poisoning was significantly enhanced.The MnCuSAPO-34 molecular sieves contain mononuclear Cu²⁺ species.Also,there is a synergistic effect between the mononuclear Cu²⁺ species and Mn?IV?species,providing the excellent low-temperature NH₃-SCR performance.