Preparation Of A New Type Of Copper Molecular Sieve And Research On Catalytic Elimination Of Nitrogen Oxide Tail Gas

NH₃-SCR technology is currently one of the mainstream technologies for the catalytic elimination of NO_xfrom mobile sources such as diesel exhaust.Due to the advantages of low-temperature SCR activity,wide temperature window and resistance to CH_xpoisoning,Cu-SSZ-13 has received more and more attention and applications in the catalytic purification of NO_xexhaust gas from mobile sources such as diesel vehicles.However,the low-temperature activity and temperature window of Cu-SSZ-13 molecular sieves prepared by traditional methods still need to be improved,especially the resistance to SO₂poisoning and poor hydrothermal stability are important factors limiting its wide application.Therefore,in this thesis,we explored the effect of coating a mesoporous oxide shell layer on copper-based small pore molecular sieve to construct core-shell catalysts and investigated the effect of mesoporous oxide shell layer coating on catalytic activity,hydrothermal stability and sulfur resistance of NH₃-SCR catalysts.The catalyst structure and physicochemical properties were characterized in detail by XRD,BET,SEM,TEM,H₂-TPR,NH₃-TPD,UV-vis DRS and In-situ DRIFTS,and the pathways and mechanisms of the effect/enhancement of the shell layer cladding on the catalytic performance of copper-based small pore molecular sieves were elucidated.The main findings are as follows.（1）The catalyst Cu-SSZ-13@meso-Ce O₂,which was successfully prepared by the self-assembly method,has better SCR catalytic performance than Cu-SSZ-13.The reaction temperature window（175°C～470°C）of the core-shell catalyst Cu-SSZ-13@meso-Ce O₂was broadened by 110°C compared with that of the Cu-SSZ-13 catalyst,and the hydrothermal stability was significantly improved.A detailed characterization analysis revealed that the superior catalytic performance of Cu-SSZ-13@meso-Ce O₂over Cu-SSZ-13 could be attributed to the increased acidic sites and isolated Cu²⁺on Cu-SSZ-13@meso-Ce O₂,the synergistic interaction between reducible species and the simultaneous participation of Lewis acid sites and Br?nsted acid sites in the SCR reaction.In addition,the core-shell catalyst has a better crystal structure and pore structure after hydrothermal treatment,resulting in less loss of acidic sites and copper species,which may be the reason for the superior hydrothermal stability of Cu-SSZ-13@meso-Ce O₂catalyst compared to Cu-SSZ-13.（2）The sulfide poisoning performance of Cu-SSZ-13@meso-Ce O₂catalyst was investigated in detail to probe the mechanism of the effect of mesoporous Ce O₂oxide shell coating on the sulfur poisoning resistance of Cu-SSZ-13.The results showed that Cu-SSZ-13@meso-Ce O₂exhibited better sulfur resistance performance compared with Cu-SSZ-13.The characterization results revealed that the reduction of Cu（OH）⁺-Z species on Cu-SSZ-13-S and Cu-SSZ-13@meso-Ce O₂-S catalysts sulfided under reaction conditions may be due to the binding of Cu²⁺to sulfate.Meanwhile In-situ DRIFTS analysis revealed that the introduction of SO₂inhibited the adsorption capacity of NH₃and NO_xspecies on the catalysts.Comparing the two catalysts,the effect of SO₂on Cu-SSZ-13@meso-Ce O₂was much less than that of Cu-SSZ-13,and the generation of cerium sulfate on Cu-SSZ-13@meso-Ce O₂-S after sulfidation indicated that the cerium oxide shell layer might play a protective role for the catalyst.It was also found that the E-R reaction mechanism was followed on the Cu-SSZ-13 catalyst,while the Cu-SSZ-13@meso-Ce O₂catalyst followed both the E-R mechanism and the L-H mechanism.This may also be one of the reasons for the better sulfur resistance of Cu-SSZ-13@meso-Ce O₂.（3）Based on the work in the previous two chapters,Cu-SSZ-13 core-shell catalysts coated with different mesoporous metal oxides（Fe₂O₃,Ti O₂and Si O₂）were further prepared.The effects of the metal oxide shell layers on the reaction temperature window,hydrothermal stability and sulfur resistance of the catalysts were investigated.It was found that the coating of mesoporous Fe₂O₃and Ti O₂had positive effects on the widening of the reaction temperature window,hydrothermal stability and sulfur resistance of the catalysts,while the Si O₂-coated catalysts showed poor high-temperature performance and narrowed effective reaction temperature window.Combined with the characterization results,it was found that the coating of Fe₂O₃and Ti O₂shell layers increased the acidic sites and redox ability of the core-shell catalysts,which was beneficial to the widening of the reaction temperature window.And the cladding of oxide shell layer protects the acidic sites and reducible species of the core-shell catalyst during hydrothermal aging,which may play a role in promoting the stability of the catalyst.Meanwhile,the iron and titanium oxide shell layers will block the contact of SO₂with the active center of the catalyst,which makes the catalyst exhibit better sulfur resistance.