Fundamental Research On SCR Of NO_x By NH₃ Over Cu/CHA Zeolite For Diesel Vehicle Emission Control

With the significant emission of NO_x from diesel engines,the abatement of NO_x from mobile sources has drawn increasing attention.The selective catalytic reduction?SCR?of NO_x with ammonia is considered an effective way to remove nitrogen oxides from the effluent of lean-burn diesel engines.Cu???exchanged zeolites,Cu/SSZ-13 and Cu/SAPO-34,with the chabazite?CHA?structure have been commercialized as NH₃-SCR catalysts in diesel engines for their broad activity window,stable performance,and high selectivity to N2.Two series of Cu/CHA catalysts,Cu/SAPO-34 and Cu/SSZ-13,with the same Cu loading were prepared by ion-exchange method.Comparative investigations were performed on Cu/SAPO-34 and Cu/SSZ-13 with respect to the reaction mechanism,hydrothermal stability and resistance to SO₂.The fundamental mechanisms for the catalyst active center,zeolite framework,and reaction pathes were established.And the key factors influenced the catalysts' SCR activity and stability were confimed.The detailed conclusions are as follows:Firstly,SCR mainly followed the reaction between Lewis acid site-adsorbed NH₃ and Cu-NO₂^-.In the presence of NO,Cu-NO₃^-could reacted with NH₃ adspecies to form NH₄NO₂ which would easily decomposed to N₂ and H₂ O.However,in the simulated NH₃-SCR gases,the pathway involved Cu-NO₃^-was depressed.Cu/SSZ-13 and Cu/SAPO-34 followed the same reaction paths.However,Cu/SSZ-13 was more favorable to the formation and activation of Cu-NH₃,Cu-NO₂^-and Cu-NO₃^-,improving the SCR activity at low temperature.Secondly,SAPO-34 showed stronger stability during hydrothermal aging than SSZ-13 did.During hydrothermal aging process,the variation in framework and active sites were coorelated with each other.The vacancy in the SAPO-34 framework caused by desilication could be healed with the migration of extra-framework Al and P atoms to the defects.And the Cu species showed a certain degree of aggregation with improved redox ability on aged Cu/SAPO-34 zeolite and the acidic property was well maintained.Dealumination occurred in SSZ-13 zeolite,leading to the crystallinity loss and severe decrease of the Br???nsted acid sites.Simultaneously,the detached Al?OH?₃ species deactivated the Cu species by the transformation of isolated Cu²⁺ ions to CuAlOx species.Finally,the formation of sulfate,regarding the amount,the relative ratio and stability were influenced by the temperature,gaseous component and catalyst property.For sulfated Cu/SSZ-13 catalyst,?NH₄?₂SO₄ and NH₄HSO₄ were the main sulfate species and the CuSO₄ was not such stable as that formed on Cu/SAPO-34.The degraded Cu/SSZ-13 could be regenerated at temperatures above 450? in N₂ flow.While,the proportion of highly stable Al₂?SO₄?₃ was much higher on the poisoned Cu/SAPO-34 catalyst,making he SCR activity of Cu/SAPO-34 was difficult to be recovered.Therefore,Cu/SAPO-34 was not suitable for the NO_x removal with SO₂ existed in diesel exhaust gas.