| The temperature of vehicle emissions is much lower than operating temperature ranges of aftertreatment catalysts during the cold start period and most of the emissions are emitted into the atmosphere.Pd-Beta catalysts can solve cold start problem from trapping NOx and C3H6 at low temperature and releasing them at high temperature that was higher than the operating temperature of aftertreatment catalysts.Therefore,Pd-Beta catalysts get widespread attention and become research focus.Pd-Beta catalysts with different Si/Al ratios were prepared in this research.The influence of Si/Al ratios on the dispersity of Pd was investigated and the relationship of Pd species and adsorption performance was evaluated.Meanwhile,the reaction process of adsorbed HC and NOx was discussed.Finally,the applicable Si/Al ratio of Pd-Beta catalysts was selected.In this research,synthesis technology of Beta zeolites was optimized.The results indicated that the Beta zeolite synthesized with 5%crystal seed and 4 days’crystallization time possessed better physicochemical properties.Besides,the influence of Si/Al ratios on NO and C3H6 adsorption and desorption performance of Pd/Beta catalysts was investigated.The results revealed that the amount of dispersed Pd ions decreased in parallel with the increase of the Si/Al ratio,which led to the decrease of NO and C3H6 adsorption amounts.Besides,there are NO reduction and C3H6 oxidization reactions in desorption stages.Finally,the formation of an additional dispersed Pd cationic species,Pd（OH）+,led to earlier NO desorption in low Si/Al ratios sample.Therefore,an optimum Si/Al ratio of Pd/Beta for cold start application was found at around 55 due to the balanced performance among the adsorption amounts,reaction activity and desorption temperature. |
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