Simulation Of NO_x Removal Mechanism Of Copper-based Small Pore Size Molecular Sieve SCR Catalysts

Currently,the problem of air pollution caused by exhaust emissions from motor vehicles is not to be underestimated,especially for heavy-duty diesel vehicles,which contain a large amount of particulate matter（PM）and nitrogen oxides（NO_x）and have a particularly serious impact on the environment.Many scholars are currently working to improve the reactivity of SCR systems to achieve higher NO_x conversion efficiency.It is found that copper-based small pore size molecular sieve catalysts have become a research focus due to their high catalytic activity,stable hydrothermal stability and better resistance to sulpur poisoning.Among them,small pore size molecular sieve catalysts have two configurations,CHA and AEI,represented by SSZ-13 and SSZ-39 respectively.For Cu-SSZ-13 molecular sieve,in the standard SCR reaction with NO₂/NO_x=0,increasing the copper metal loading will effectively improve the NO_x conversion efficiency;For the fast SCR reaction with NO₂/NO_x=0.5,increasing the copper metal loading will improve the NO_x conversion efficiency in the high-temperature section,and the inhibition phenomenon appears in the low-temperature section instead.The Cu-SSZ-39 molecular sieve catalyst has better hydrothermal stability and low temperature NO_x conversion performance than the former,but the standard SCR reaction rate under this catalyst is lower and the NO_x conversion efficiency in the high temperature section is lower.In order to broaden the active temperature window of the copper-based small pore size molecular sieve catalyst and improve the NO_x conversion efficiency,this paper investigates the method to improve the NO_x conversion efficiency by numerical simulation.Firstly,a one-dimensional reactor model and a catalytic reaction kinetic model were developed using GT-SUITE software,and the chemical reactions in the selective catalytic reduction system were investigated using simulation and computational methods.The model involves the adsorption,desorption and oxidation of ammonia（NH₃）,the oxidation of nitric oxide（NO）,the reduction of nitrogen oxides（NO_x）and the decomposition of ammonium nitrate（NH₄NO₃）over a temperature range of 423 K to 673 K.Furthermore,the emissions of an important greenhouse gas,nitrous oxide（N₂O）,are monitored in the exhaust gas.The crystallization of NH₄NO₃ formed by the reaction of NO₂ with NH₃ in the low temperature section below 573 K in the presence of Cu-SSZ-13 molecular sieve catalysts with high loading of copper,and the blocking of pores by crystals to inhibit the reaction.The crystals block the pore channels and inhibit the reaction;As the temperature increases,NH₄NO₃ gradually decomposes and the reaction proceeds normally.In this study,the inlet NO₂ content was controlled to reduce the production of NH₄NO₃,and the catalyst configuration was changed to accelerate the decomposition of NH₄NO₃ at low temperatures to improve the NO_x conversion efficiency.It was found that the NO₂ content influenced the conversion efficiency of the highly loaded Cu-SSZ-13 molecular sieve catalyst,with the highest NO₂ content increasing and then decreasing as the temperature increased,with the highest NO₂percentage not exceeding 40%and the N₂O content below 6 ppm in the corresponding temperature range.The optimum NO₂ content for the highest NO_x conversion efficiency was investigated by varying the ammonia to nitrogen ratio,air velocity,catalyst activity and site-density,but the values obtained were all dynamic.It was found that the NH₃ distribution pattern at the inlet end at different temperatures had a certain influence on the NO_x conversion efficiency,where the more uneven the NH₃distribution,i.e.the smaller the UI,the lower the overall NO_x conversion efficiency.Compared with the Cu-SSZ-13 molecular sieve catalyst,Cu-SSZ-39 can effectively solve the NH₄NO₃ crystallization problem due to its surface acidity and other factors,so the two catalysts were proposed to be compounded and the two were superimposed by numerical simulation.The simulation results showed that the best NO_x conversion efficiency was achieved when Cu-SSZ-13 molecular sieve was placed in the lower layer and the coating thickness was set to Cu-SSZ-39:Cu-SSZ-13=1:9.For the Cu-SSZ-39/13molecular sieve catalyst,increasing the ammonia to nitrogen ratio would significantly improve the NO_x conversion efficiency and effectively reduce the ammonia leakage.