Preparation Of Pd/SSZ-13 Zeolite Core-shell Materials And Investigation Of Passive NO_x Adsorption Performance

In recent years,the emission of nitrogen oxides（NO_x）from motor vehicles has shown an increasing trend.NO_x can cause acid rain,smog and photochemical smog,etc.,which are seriously pernicious to the human condition and body.As a result,governments around the world have introduced stricter NO_x emission regulations.At present,the most ubiquitously used denitrification technology is selective catalytic reduction（SCR）.However,it has poorly active during cold start（<180°C）.Passive NO_x adsorber（PNA）can be used upstream of SCR to quickly store NO_x at low temperature（about 100°C）,and release NO_x in the range of high SCR catalyst activity（200-350°C）,so it is expected to become one of the most promising technologies for mitigating low-temperature NO_x emissions.Currently,the most attractive PNA catalyst is the Pd/SSZ-13 zeolite material,in which Pd cation(Pd²⁺or[Pd（OH）]⁺)species act as the active site for fast and efficient storage and release of NO_x.However,under hydrothermal conditions,the activity of Pd/SSZ-13 will inevitably decrease,and the adsorption capacity of the PNA catalyst still needs to be improved.Therefore,preparing materials with higher NO_x storage capacity and better hydrothermal stability has become a top priority to improve the PNA activity.In this paper,effect of the core-shell structure support in Pd/（SSZ-13@Al₂O₃）and the CeO₂ content in Pd/（SSZ-13@CeO₂-n）on key factors of passive NO_x adsorption performance were studied by constructing SSZ-13@metal oxides core-shell structure materials and loading Pd species.1.By using SSZ-13 as the core and Al₂O₃ as the shell,SSZ-13@Al₂O₃with a core-shell structure was constructed,and acted as a carrier to load Pd species to prepare Pd/（SSZ-13@Al₂O₃）,and compared with other related material carriers to load Pd species for passive NO_x adsorption performance research.The study shows that Pd/（SSZ-13@Al₂O₃）had the highest NO_xadsorption activity and desorption efficiency（91.2%）and desorption temperature（273°C）that is more in line with the requirements of PNA materials.Through the characterization of UV-Vis,XPS,NH₃-TPD,H₂-TPR,²⁷Al MAS NMR and STEM-EDX,the existence states of Pd species in all samples were investigated,and it was found that the core-shell structure material Pd/（SSZ-13@Al₂O₃）had the most Pd ions(Pd²⁺or[Pd（OH）]⁺).The results of the hydrothermal aging resistance test show that Pd/（SSZ-13@Al₂O₃）still had improved low-temperature NO_x adsorption activity compared with Pd/SSZ-13 after hydrothermal aging,which was attributed to the effective improvement of the hydrothermal stability of the sample by the core-shell structure carrier.In addition,when SSZ-13@Al₂O₃ was used as a carrier,the high-temperature calcination conditions were more conducive to the dispersion of Pd species,so that the PNA activity of the sample was significantly improved.This study provides a new idea for the improvement and optimization of PNA materials.2.A series of SSZ-13@CeO₂-n core-shell structure materials with SSZ-13 zeolite as the core and different CeO₂ contents were controllably prepared,and these materials were respectively used as supports to load Pd species to prepare Pd/（SSZ-13@CeO₂-n）,the effect of CeO₂ contents on the adsorption performance of PNA was researched.The study shows that the Pd/（SSZ-13@CeO₂-0.3）sample was more conducive to NO_x storage due to its suitable CeO₂ content compared with Pd/SSZ-13,while the samples with higher or lower CeO₂ content had no obvious effect on the change of PNA activity.Through the characterization of SEM,TEM,UV-Vis and XPS,etc.,the change of CeO₂ content in the shell and its influence on Pd species were studied.SEM and TEM results show that the CeO₂ shells coated on the SSZ-13 zeolite were formed by the accumulation of small CeO₂ particles,and an appropriate amount of CeO₂ was more uniformly distributed on the outer surface of SSZ-13.UV-Vis and XPS studies show that the Pd/（SSZ-13@CeO₂-0.3）catalyst with the best PNA activity contained the most divalent Pd ions(Pd²⁺or[Pd（OH）]⁺).In addition,it is worth noting that Pd/（SSZ-13@CeO₂-0.3）-HTA exhibits more excellent hydrothermal stability compared to Pd/SSZ-13-HTA.This study provides a strategy for the controllable preparation of PNA materials with outstanding NO_xadsorption performance.