Research On Modified Synthesis By Na Ions And Mechanism Of Deactivation For Pd/SSZ-13 As Passive NO_x Adsorbers

The National VI emission standards for vehicle pollutants have been gradually implemented,which have put forward higher requirements to alleviate the emission of nitrogen oxides（NO_x）for after-treatment systems in vehicles.Up to now,selective catalytic reduction（SCR）is considered as the most efficient technology for NO_x abatement.However,the high efficiency of SCR is only achievable at temperatures above 200°C,most of NO_xis emitted during vehicle operation at a temperature below 200°C（i.e.,cold-start period）.Passive NO_x adsorber（PNA）has proven to effectively alleviate NO_x emission at low temperatures.Due to the excellent adsorption performance and sulphur tolerance,Pd/SSZ-13 is widely favored and studied by researchers.However,similar to most after-treatment catalysts,PNA can be subject to severe hydrothermal aging and chemical poisoning,leading to the degradation in adsorption performance.This study mainly focused on the adsorption performance and hydrothermal stability of Pd/SSZ-13,as well as the mechanism of deactivation after chemical poisoning.Firstly,Pd-based Na-SSZ-13（Pd/Na-13）was synthesized,and the effects of Na ions on the hydrothermal stability and adsorption performance of Pd/SSZ-13 were investigated.Compared to Pd-based NH₄-SSZ-13（Pd/NH₄-13）,Pd/Na-13 exhibited superior hydrothermal stability and adsorption performance after mild hydrothermal aging treatment.²⁷Al MAS NMR demonstrated that Na co-cations protected framework Al,thus alleviating the dealumination of Pd/SSZ-13 during hydrothermal aging treatment.NO-and CO-DRIFTS studies proved that the Z^-Pd²⁺Z^-sites are dominant in aged Pd/Na-13,while the Pd/NH₄-13 has more Z^-[Pd（Ⅱ）OH]⁺sites.Strikingly,the aged Pd/Na-13 exhibited excellent adsorption capacity with additional NO adsorption at above 100°C.Secondly,three introduction methods were simulated to investigate the introduction methods on the activity and hydrothermal stability of Pd/SSZ-13:residual,ion-exchanged and impregnated.The ²⁷Al MAS NMR results showed that Na ions introduced by three ways protected the framework Al of zeolite from dealumination during hydrothermal aging treatment.NO-and CO-DRIFTS studies showed that the residual Na ions affected the variety of Pd active sites in Pd/SSZ-13.The ion-exchanged Na ions increased the amount of Z^-Pd²⁺Z^-sites.However,impregnated Na ions induced the formation of a large amount of Pd O,resulting in the severe deactivation of Pd/SSZ-13 due to alkaline metal poisoning.Based on the investigation on the introduction methods of Na ions,which has proved that impregnated Na ions caused the deactivation of Pd/SSZ-13.Here,Pd/SSZ-13 was impregnated with alkaline metals（Na and K）and alkaline earth metals（Mg and Ca）,respectively.And then the effects of Na,K,Mg,and Ca on the adsorption performance of Pd/SSZ-13 by combining with hydrothermal aging treatment were investigated.The results showed that the impregnated metals deposited on the surface of zeolite and blocked the pores,while the framework structure of SSZ-13 was not destroyed.However,the impregnated metals were extremely destructed the Pd active sites,leading to the sintering of Pd ions and thus forming large Pd O particles.The poisoning effect of alkaline metals and alkaline earth metals on the adsorption performance of Pd/SSZ-13 was shown as follows:Ca>Mg>Na>K.In addition,phosphorus is also one of the typical poisons that cause irreversible deactivation of PNA materials.Here,the effects of phosphorus on Pd/SSZ-13 were investigated by combining with hydrothermal aging treatment to gain deeper insights into phosphorus-induced deactivation.It was found that phosphorus decreased the micropore volume and acidity of Pd/SSZ-13,the deactivation behavior mainly resulted from the decrease of active sites.Specifically,the Pd sites were poisoned by phosphorus species,leading to the transformation of Pd active sites to Pd O.After hydrothermal aging treatment,the phosphorus induced the extensive formation of Al PO₄ that significantly accelerated the dealumination of zeolite framework,thus leading to the decrease in adsorption activity and hydrothermal stability.Finally,combining the promotion of Na ions on hydrothermal stability with the activation of hydrothermal aging treatment,a regeneration strategy after phosphorus poisoning was proposed.It was found that Na ions greatly enhanced the hydrothermal stability of phosphorus-poisoned Pd/SSZ-13 during hydrothermal aging,thereby providing abundant ion exchange sites for Pd ions.After hydrothermal aging treatment,some Pd O particles were redispersed.And the Pd ions were combined with two adjacent Al sites and formed new Z^-Pd²⁺Z^-sites,leading to a significant increase of NO_x adsorption capacity.For instance,the adsorption capacities of the poisoned Pd/SSZ-13 with phosphorus content of 0.2,0.4,and 0.8 mmol/g recovered more than 90%.This study aims to provide guidance for the design and synthesis of passive NO_xadsorbers with high activity and stability.On one hand,this study reveals the importance of Na ions on Pd/SSZ-13,expecting to provide guidance on the catalyst modification or optimization for low-temperature NO adsorption.One the other hand,this study sheds light on the deactivation mechanism of Pd/SSZ-13 from the perspectives of phosphorus poisoning and hydrothermal aging,expecting to provide guidance on the design of Pd-based zeolites with high stability for cold-start application.