Research On Controllable Sythesis Of Cu-SSZ-13 Catalyst And Its SCR Performance For The Removal Of NO_x From Diesel Vehicles

With the more stringent NS VI regulations for China being issued,the conventional vandium-based and Cu-ZSM-5 catalysts are not able to meet the requirements of emissions for the much tighter limit of NO_x.As a result,the new designed small-pore Cu-SSZ-13catalyst with a CHA-type is considered as the next generation NH₃-SCR catalyst for the China NS VI due to the superior deNO_x activity,improved hydrothermal stability and good N₂ selectivity.Firstly,our works focus on the choice of the Si/Al ratio for the Cu-SSZ-13.A series of Cu-SSZ-13 with different Si/Al ratios are synthesized by a one-pot method,and the disparity in the catalytic activity is discussed,besides,the relationships among the Si/Al ratio,Cu loading,distribution of Cu species,chemical environment of Si and Al,zeolite acidity and NH₃-SCR performance are established.It concludes that the Cu-SSZ-13 with Si/Al=15 has a suitable Al amount to provide enough exchange sites for isolated Cu²⁺ions location,and the feasible Na⁺content has no negative influence on the hydrothermal stability of Cu-SSZ-13.What's more,the zeolite structure has an appropriate content of Si-O-Al bond,which makes the catalyst is more robust to the hydrothermal treatment.As a result,the Cu-SSZ-13 with Si/Al=15 exhibits excellent NO_x reduction and hydrothermal durability,which is beneficial for its application in the real diesel post-treatment system.Secondly,in consideration that the real environment of post-treatment system consists of highly amount of steam and varies of metals,the resistances of Cu-SSZ-13 to hydrothermal aging and alkali metal poisoning are studied.It illustrates that 750?hydrothermal treatment and deposit of 0.50 mmol/gcatal has little effect on the NO conversion of Cu-SSZ-13.It could be attributed to the well preservation of isolated Cu²⁺ions,zeolite structure and acidity.For the alkaline metal poisoning mechanism,the metal ions with small diameter is easily entering the framework of the zeolite and ion-exchanges with the isolated Cu²⁺ions in the CHA cage firstly.Besides,the metal ions with higher valance could exchange with more Cu²⁺ions.Thus,the excess Cu²⁺ions will aggregate to form CuO,and the generation of CuO may block the channels or even destroy the zeolite framework.As a result,the decline in the Cu²⁺ions content and zeolite acidity,formation of CuO and the destruction of zeolite structure may contribute to the deactivation of catalytic activity.And the extent of alkali metal poisoning effect on the NH₃-SCR performance of Cu-SSZ-13 catalyst can be ordered as:Mg>Ca>Na>K.Thirdly,the China NS VI improves the NO_x emissions reduction during vehicle's cold start phase,as a result,the cold start wetting effect on the Cu-SSZ-13 is studied.It suggests that the OH^-in the alkaline solution could break the Si-O-Si bond in the zeolite,resulting in the desilication phenomenon and the zeolite structure destruction.Moreover,the desilication may lead to the reduction in the number of ion-exchange sites which also makes the Cu²⁺ions extracted from the zeolite structure.And the Cu-SSZ-13 with a low Si/Al ratio?Si/Al=6?exhibits excellent resistance to alkaline solution deactivation,it is probably because the catalyst has more content of AlO₄^-and extra-framework Al,which could prevent the Si atom extraction from the zeolite framework.Taking into consideration of the catalytic activity,hydrothermal stability and resistance to alkaline solution deactivation of Cu-SS-13,the Si/Al ratio could be optimized between 6 and 15.Finally,in order to synthesize the Cu-SSZ-13 in a more economical and effective method,the Cu-TEPA and essentially cheap choline chloride are used as co-template to prepare Cu-SSZ-13 with a moderate Cu content,adjustable Si/Al ratio and satisfactory NH₃-SCR performance.It shows that a suitable choline chloride content is beneficial for the nucleation and crystallization of CHA structure when the Cu-TEPA content is insufficient,which could help to promote the formation of CHA phases and restrain the formation of mixed crystals.In consequence,the new preparation method provides a feasible way for the industrial production of Cu-SSZ-13.