| In recent years,with the rapid development of social economy,the number of motor vehicles powered by diesel engines has been increasing,and the problem of atmospheric pollution caused by nitrogen oxides in diesel exhaust is becoming more and more serious.According to the requirements of the Ministry of Ecology and Environment of the People’s Republic of China,stricter emission standards have been gradually implemented in all parts of the country,which not only imposed stricter requirements on NOx emmissions,but also put forward higher requirements on NH3-SCR catalysts.Cu-based zeolites with CHA structure has been widely used in field of NH3-SCR due to its advantages of large specific surface area,excellent hydrothermal stability and wide reaction temperature window.However,in the actual reaction process,due to the existence of internal diffusion limit,the mass transfer rate of reactants and products in the zeolites was reduced,resulting in poor low temperature activity of the Cu-based zeolite.In this paper,Cu-SSZ-13-Nano was prepared by FAU trans-crystallization and hierarchical Cu-S SZ-13-Meso was prepared by adding a hard template.The effect of zeolite diffusion limitation on the DeNOx performance of diesel exhaust was investigated systematically,which provided guidance for the preparation of more excellent NH3-SCR catalysts.The researches of this paper are follows:Firstly,the nano H-SSZ-13 was synthesized in one step through the FAU trans-crystallization strategy,and even the crystallization time was shorten to one day or the amount of template agent was reduced to half of that of the traditional synthesis method,which nanoscale H-SSZ-13 also could be successfully synthesized.After the same Cu ion exchange,the nanocale Cu-SSZ-13-Nano(120 nm)exhibited better NH3-SCR performance at low temperature:its NOx conversion could reach 90%at 175℃ and could be maintained to 480℃,with a wide reaction temperature window.However,the traditional large-particle Cu-SSZ-13-Conv(29.8 μm)needed over 220℃ to achieve NOx conversion of 90%,and only maintain to 440℃.At the same time,Cu-SSZ-13-Nano also had better H2O/SO2 resistance.Even after hydrothermal aging at 800℃ for 16 h,it still showed better hydrothermal stability than traditional zeolite.Through in-depth internal diffusion calculation,it was confirmed that there was internal diffusion limitation in the traditional Cu-SSZ-13-Conv with large particle,which limited the mass transfer rate of reactants and products and maked the reaction rate of NH3-SCR slower at low temperature.On the other hand,Cu-SSZ-13-Nano could effectively reduce the influence of internal diffusion limitation on mass transfer rate,so it has excellent DeNOx at low temperature.Then,the high crystallinity hierarchical SSZ-13 zeolite was synthesized by using cheap and easily available carbon black as hard template.According to N2 adsorption-desorption,it can be seen that the hierarchical SSZ-13 zeolite was rich in mesoporous structure and had a larger external specific surface.After the same ion exchange,the hierarchical Cu-SSZ-13-Meso zeolite and traditional microporous Cu-SSZ-13-Trad were prepared for NH3-SCR test.The results showed that the Cu-SSZ-13-Meso had a higher NOx conversion than Cu-SSZ-13-Trad in low and high temperature range.In addition,a series of more stringent tests on water resistance,sulfur resistance and hydrothermal stability were performed,and the Cu-SSZ-13-Meso always showed better NH3-SCR activity than Cu-SSZ-13-Trad.Through kinetic test,under the condition of excluding the internal and external diffusion,the activation energy Ea of Cu-SSZ-13-Meso and Cu-SSZ-13-Trad were the same.But with the temperature increase,the NOx conversion of Cu-SSZ-13-Meso is significantly higher than Cu-SSZ-13-Trad under the condition of internal diffusion limitation.This indicated that there was a serious internal diffusion limitation in the Cu-SSZ-13-Trad,which affected the mass transfer rate of reactants and products in the NH3-SCR reaction,thereby deteriorating its NH3-SCR performance.At last,the In situ DRIFTS experiment results proved that NH4NO3 would generate on Cu-SSZ-13 and even(NH4)2SO4 crystal would be generated in the presence of SO2.Due to the internal diffusion limitation,they would cause the pores of the traditional microporous Cu-SSZ-13 catalyst to block,while the hierarchical Cu-SSZ-13-Meso could diffuse NH4NO3 and(NH4)2SO4 particles to the outer surface or promote its decomposition,which further proved that the hierarchical porous can reduce the internal diffusion limitation and improve the mass transportation of the zeolite. |
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