Optimization Of Zeolite Carrier And Pd Loading In Passive NOx Adsorber

Standard NOx aftertreatment technologies fail to function efficiently at low temperatures,which results in a large proportion of the tailpipe NOx emission.Meanwhile,high efficiency internal combustion engines lead to much lower exhaust temperatures,which makes the problem of NOx emission during cold-start worse.Pd/zeolite served as a Passive NOx Adsorber（PNA）is considered to be the most likely to solve this problem because of its large NOx storage capacity and proper NOx desorption temperature.In this study,Pd/zeolite catalysts with different zeolite were synthesized,and Pd ions on exchange sites of zeolite are demonstrated to be the active sites for NOx storage.Larger pore sizes of zeolites benefit the formation of this Pd species.So,the sample using Beta as a carrier has the largest NOx storage capacity,which can meet the need of cold start technology.Samples with various Pd loadings are prepared to investigate the influence of Pd loadings on active sites for NOx storage.Two active Pd²⁺species,Z^--Pd²⁺-Z^-and Z^--Pd（OH）⁺,on exchanged sites of zeolites,were identified by in situ FTIR using CO and NH₃.Higher NOx storage capacity of Z^--Pd²⁺-Z^-was demonstrated compared with that of Z^--Pd（OH）⁺,which was caused by the different resistance to H₂O.Besides,lower Pd loading led to a sharp decline of Z^--Pd（OH）⁺,which was attributed to the‘exchange preference’for Z^--Pd²⁺-Z^-in Beta.Based on this research,the atom utilization of Pd can be improved by decreasing Pd loading.