Synthesis Of Hollow MnO_x@PrO_x Core-shell Catalyst And The Catalytic Performance For Low Temperature NH₃-SCR

Selective catalytic reduction with NH₃?NH₃-SCR?is the most widely used and mature denitration technology for the removal of NO_xfrom stationary sources at home and abroad,the core of which is catalyst.Low-temperature NH₃-SCR technology is that the catalyst can remove NO_xin the flue gases under low temperature,its reaction temperature lower than250?and has a wide reaction temperature window,in addition the low temperature SCR reaction device can be directly placed behind desulfurization and dust removal device,no need to retrofit the existing equipment and heat the flue gas,greatly reduce the cost,based on these advantages the technology has wide application prospect.In recent years,many low-temperature NH₃-SCR catalysts are continually being developed.However,the problem that the residual SO₂and H₂O in flue gas are easily to poisoning low-temperature SCR catalyst has not been solved.Therefore the development of catalysts with high resistant to SO₂and H₂O capability is the key to the application of low-temperature NH₃-SCR technology.Mn O_xis often used as an active species in low temperature NH₃-SCR catalytic system due to its rich valence state and redox capacity.While low-temperature NH₃-SCR catalysts which doped rare earth metals have excellent resistance to sulfur and water.In addition,the catalyst with a special morphology and structure is also an important idea to improve its anti-poisoning ability.In this paper,a low temperature NH₃-SCR core-shell structure catalyst with high activity and high toxicity resistance was prepared with Mn O_xas the core and Pr O_xas the shell,and further explore its anti-SO₂poisoning mechanism.Firstly,carbon spheres were prepared by hydrothermal method,and the effects of different hydrothermal temperature and hydrothermal time on the morphology and particle size of CSs were investigated.The results show that the hydrothermal temperature and hydrothermal time have a great influence on the morphology and particle size of CSs,the hydrothermal temperature mainly affects the CSs particle size,and the hydrothermal temperature mainly affects the morphology of carbon sphere.Using glucose as the carbon source,CSs was synthesized by thermal reaction at 180?for 24 h with a particle size of about 520 nm,spherical uniform,smooth surface,and basically no cross-linking.Secondly,a series of hollow Mn O_xcatalysts were prepared by secondary hydrothermal method,and the effects of different Mn/CSs ratio and calcination temperature on the performance of low-temperature NH₃-SCR were investigated.The results show that when Mn/CSs is 15%,the catalyst prepared by roasting at 400?has the optimal catalytic activity.The catalyst is rich in Mn O₂with high crystallinity,whose surface Mn⁴⁺ratio is the highest,which can rapidly oxidize NO to NO₂and is conducive to rapid SCR reaction.Thirdly,the hollow Mn O_x@Pr O_xcatalyst was prepared by chemical precipitation method,and the effect of Pr/Mn ratio on the low-temperature NH₃-SCR catalytic activity was emphatically studied.Studies have shown that the catalyst with Pr/Mn ratio of 0.3 has the more optimal low-temperature SCR activity,its NO conversion is close to 99%at 120?,and the NO conversion is above 90%within 100-240?,and it also has excellent N₂selectivity and stability.The characterization results show that it is beneficial for the shell layer to coat the core layer tightly under this condition,increase the contact surface of the two oxides,and at the same time,it is also conducive to the full diffusion of the reaction gas.Finally,by comparing the SCR catalytic performance of hollow Mn O_x,hollow Mn O_x@Pr O_xand hollow Mn O_x-Pr O_x,we found that the hollow Mn O_x@Pr O_xcatalyst not only has the optimal catalytic activity,but also has the optimal sulfur and water resistance.Through TEM,NH₃-TPD,FTIR,TG,etc.characterization analysis,its good catalytic performance is attributed to the high concentration of Mn⁴⁺,Pr³⁺ratio and the chemical adsorption oxygen O_?on its surface,the appropriate surface acidity and acid amount,and the strong redox performance.The reason for the more optimal anti-SO₂poisoning ability is that the catalyst has a special core-shell structure,whose shell layer will protect the core,what's more Pr O_xshell can inhibit the formation of surface ammonium sulfate and sulphate,and accelerate the decomposition of these species.