Study On The Performance And Mechanism Of Ce Modified Cu-SSZ-13 Catalysts For The Selective Catalytic Reduction Of NO With NH₃

Diesel vehicle exhaust is one of the major sources of nitrogen oxides in the atmosphere.Selective catalytic reduction with ammonia is considered to be the one of the most effective method to remove NO_x.The key is to develop a stable and efficient catalyst.Copper-based small-pore zeolite catalyst Cu-SSZ-13 owns many advantages such as the wide temperature operating window,super activity and hydrothermal stability,and strong resistance to water and sulfur poisoning,together with the high durable to GHSV,etc.So,removal of NO_x from the exhaust gas with Cu-SSZ-13 zeolite shows a promising future.The hydrothermal method was used to synthesize Cu-SSZ-13 zeolite in the laboratory.Then,the Ce-Cu-SSZ-13 and Cu-SSZ-13@CeO₂ with Core-shell structure were prepared by ion exchange method and coating method respectively.The NH₃-SCR catalytic activity,hydrothermal stability,resistance to water,sulfur and high GHSV have been tested before and after modification.Meanwhile,XRD,SEM,TEM,H₂-TPR,NH₃-TPD and XPS were used to characterize the physical and chemical properties of the catalysts.In-situ DRIFTS was also used to study its reaction mechanism in the low temperature region.The specific conclusions are as follows:The Cu-SSZ-13 zeolite was synthesized by using hydrothermal mothed with a green and cheap template Cu-TEPA.Which showed high catalytic activity,wide reaction temperature window,strong water and sulfur poisoning and high GHSV resistance,and good hydrothermal stability.A series of characterizations were used and the results showed that,the catalyst owns a regular cubic crystal structure,larger BET specific surface area and pore volume,good redox property and high surface acidity.The Cu²⁺is the active species on the catalyst surface.In the low-temperature region,NH₃ is mainly adsorbed on the Lewis acid sites and Br?nsted acid sites of the catalyst.The adsorption of NO+O₂ on the catalyst surface also could form some important intermediates and then react with NH₃ to produce N₂ and H₂O.After hydrothermal aging and changing the molar ratio of silicon to aluminum,the catalytic activity was reduced in different degree,the main reason is the destruction of the internal structure of the catalyst.The Ce-Cu-SSZ-13 catalyst was prepared by supporting Ce on the Cu-SSZ-13 by using ion exchange method.The 0.075 mol/L Ce（NO₃）₃ is the best ion exchange concentration.The catalyst also exhibited higher activity,hydrothermal stability and resistance to water and sulfur poisoning.Through characterization analysis,it was found that after Ce ion exchanged,the structure of the zeolite was kept in a good condition,the redox property and acidity were significantly enhanced,the active species Cu²⁺,oxygen vacancy and the pore volume were increased.The introduced Ce⁴⁺/Ce³⁺redox couple enhanced the redox capacity of the Cu²⁺/Cu⁺.The Lewis acid sites and Br?nsted acid sites on the catalyst surface are stronger,and then quickly reduced to the intermediate to form N₂ and H₂O.All of those could enhance the NH₃-SCR activity.The Cu-SSZ-13@CeO₂ catalyst with core-shell structure was synthesized by self-assembly method using hydrothermal and introduction of surfactant in an alcohol/water coexistence system.After the catalytic activity was evaluated,it was found that CeO₂ coating can promote the low temperature reaction activity of the catalyst,but detrimental to high temperature reaction activity.The catalytic activity is best when the coating amount of CeO₂ is 36.0 wt%.The resistance to water and sulfur poisoning and the hydrothermal stability was improved significantly.The existence of core-shell structure was proved by using characterizations.The redox capacity and acidity were significantly enhanced.The pore volume and the oxygen vacancies were increased.The introduction of Ce³⁺and Cu⁺together promoted the catalytic activity.