Investigation And Regulation Of Fe,Cu Modified SSZ-13 Zeolite Catalysts In NH₃-SCR Reaction

Diesel vehicles with good fuel economy,high thermal efficiency and less CO₂ emissions are widely used.Under typical lean burn combustion conditions,diesel exhaust is characterized by rich nitrogen oxides and particulate matter.NO_x poses threats to the environment such as acid rain,photochemical smog and ozone layer depletion,and harms human life and health by reducing atmospheric visibility and inducing respiratory diseases.Ammonia selective catalytic reduction（NH₃-SCR）is the optimum method to purify NO_xfrom diesel engine.The commercial diesel SCR catalyst is based on Cu or Fe modified small-pore chabazite（CHA）zeolite.After the revelation of low-temperature hydrothermal deactivation that seriously affects the lifespan of Cu-SAPO-34,the research interests are more focused on SSZ-13 material.In NH₃-SCR reaction,the iron-zeolite catalysts show the advantages of excellent high-temperature activity,less N₂O by-products and good sulfur resistance.However,disadvantages such as poor low-temperature activity and hydrothermal stability exist.The copper-zeolite catalysts have the advantages of high low-temperature activity,wide activity window and good N₂ selectivity,accomplied by the disadvantages of being susceptible to SO₂ poisoning and reduced activity at high temperatures.In this thesis,the SSZ-13 zeolite is taken as the research object.The structure-activity relationship of Fe-SSZ-13 is investigated and zirconium is doped to improve its hydrothermal stability.As for Cu-SSZ-13,the sulfur resistance is promoted by adjusting the sodium content.Thus the exploration and regulation of NH₃-SCR performance over metal modified SSZ-13 is achieved.The main contents throughout the full text are as follows:Firstly,the effects of calcination temperature on the iron species distribution and NH₃-SCR performance of Fe-SSZ-13 were studied.The results show that the NO conversion of Fe-SSZ-13 calcined at 450℃and 500℃increases first and then decreases with the increase of reaction temperature between 250-575℃,and reaches the maximum value at 400℃.The NO conversion of Fe-SSZ-13 calcined above 500℃increases firstly and then remains basically unchanged with the increase of reaction temperature.With increasing calcination temperature,low-temperature activity decreases but high-temperature activity increases.Three kinds of iron species coexist in Fe-SSZ-13,i.e.isolated Fe³⁺,oligomer Fe_xO_y cluster and large Fe₂O₃ particles.Higher calcination temperature results in more oligomer Fe_xO_y and less isolated Fe³⁺,with the decrease of acidity simultaneously.Isolated Fe³⁺and oligomeric Fe_xO_y clusters are the main low-temperature and high-temperature active species,respectively.Catalyst acidity plays a considerable role in low-temperature SCR and promote NO conversion by inhibiting the low-temperature ammonia inhibition effect of iron-zeolites.Secondly,to overcome the hydrothermal deactivation issue exposed to Fe-SSZ-13,the0.3Zr/Fe-SSZ-13 catalyst with high hydrothermal stability was prepared by in-situ Zr doping during the one-pot synthesis of Fe-SSZ-13.The promoting mechanism of Zr was fully revealed from the aspects of micropore properties,zeolite structure,active sites,acidity and reaction intermediates.The results show that after hydrothermal aging in wet air at 750℃for12h,the NO conversion of 0.3Zr Fe-F containing 0.31wt.%Zr is about 30%higher than that of Fe-F after aging,and the N₂ selectivity is maintained above 95%.The declined activity of aged Fe-SSZ-13 is mainly attributed to the aggregation of active iron ions and the loss of acid sites.The 0.3Zr/Fe-SSZ-13 catalyst with moderate zirconium can maintain high specific surface area,pore volume and crystallinity during hydrothermal treatment.On the basis of ensuring the stability of support structure,Zr promotes the dispersion of Fe species,thus inhibits the migration and aggregation of isolated Fe³⁺ions,and ensures the number of active sites.The new Br?nsted and Lewis acid sites introduced by Zr doping also assist in the more efficient activation of NH₃ and NO,facilitating the formation of unstable NH₄NO₂ and NH₂NO intermediates,which eventually decomposed into N₂.Finally,aiming at the issue that the sulfur resistance of Cu-SSZ-13 is worse than that of Fe-SSZ-13,the effects of Na content on the de NO_x activity and sulfur resistance of Cu-SSZ-13 were investigated based on the inevitable presence of Na in the hydrothermal synthesis of SSZ-13,and the effect of Na content on the sulfur resistance mechanism of Cu-SSZ-13 was proposed.The results show that Cu-SSZ-13 with 0.86 wt.%Na not only exhibits more than90%NO conversion in 225-575℃,but also has the highest activity after repeated sulfur poisoning and regeneration and best low temperature cumulative sulfur poisoning resistance.H₂-TPR reveals that moderate Na results in abundant Z₂Cu and minor ZCu OH sites in Cu-SSZ-13.In the case of SO₂ poisoning,Z₂Cu is poisoned by easily decomposed ammonium sulfate,and ZCu OH is poisoned by stable copper sulfate,which jointly lead to the loss of low-temperature activity.Nevertheless,the interaction between SO₂ and Cu_xO_y inhibits the ammonia oxidation side reaction,promoting high-temperature NO conversion.When treated with cumulative sulfur poisoning at 200℃,F-0.25Na with the highest activity is deposited by more（NH₄）₂SO₄ and the least Cu SO₄ due to its abundant Z₂Cu and less ZCu OH,and there remains the maximum isolated Cu²⁺in S-0.25Na,contributing to the optimal activity after poisoning.The SO₂ poisoning degree of Na/Cu-SSZ-13 is mainly dominated by the severe poisoning of Cu SO₄,while the poisoning of（NH₄）₂SO₄ is moderate.