Study On Performance Of Cu-SSZ-13 Molecular Sieve Catalyst For NH₃-SCR

Selective catalytic reduction?SCR?system becomes a necessary part of diesel engine after-treatment system complied to China?standard.The choice of catalyst is of great importance for SCR system,which determines the catalytic efficiency and hydrothermal stability of diesel vehicle aftertreatment system.In the future,the emissions will foucus more on low-speed conditions of diesel engine,and the imporvement of thermal efficiency of heavy diesel engine leds to the low temperature of exhaust gas of engine,which require higher low-temperature activity of the catalyst.Meanwhile,in order to give consideration to active regenerative diesel particulate filter?DPF?,which is possibly located upstream of SCR in the aftertreatment system and will regenerate periodically,there will be a higher requirement for anti-hydrothermal aging ability for SCR catalyst.This study focuses on Cu-SSZ-13 catalyst,a chabazite structure molecular sieve,and investigates the activity at low temperature and anti-hydrothermal ability of catalysts of different composition.Catalysts of different Si/Al ratios and Cu loadings are made with liquid phase ion exchange method,and the effect of Si/Al ratio,Cu loading on the performance of Cu-SSZ-13 catalyst is studied,including NO_x conversion,N₂ selectivity,and absorption ability of NH₃ and NO+O₂.Moreover,hydrothermal aging process are carried out on the catalyst at different temperature,so as to investigate the effect of high temperature hydrothermal aging on Cu-SSZ-13 catalyst,and comparison is made with fresh catalyst of the same parameter.In addition,Ce modification was studied to make understanding of the effect of the second active element on the performance of fresh and aged catalysts.The effect of Si/Al ratio on the performance of catalysts showed that catalyst with smaller Si/Al ratio in the range of this study had more B acid sites,which was conducive to NH₃ adsorption in the form of NH₄⁺on the surface of the catalyst.At the same time,H-SSZ-13 with more B acid sites could introduce more isolated Cu ions in during ion exchange process,which was conducive to the formation of Cu active sites of the catalyst and avoided the formation of Cu_xO species,and avoided the non-selective oxidation of NH₃.In the range of this study,catalyst with smaller Si/Al ratio had better catalytic activity and N₂ selectivity at high temperature.The effect of Cu loading on the performance of Cu-SSZ-13 catalysts showed that the catalysts with the same Si/Al ratio but a higher Cu loading had more active sites and better catalytic activity.Catalyst with small Cu loading had more residual B acid sites and more NH₄⁺ions were adsorbed when exposured to NH₃,but the overall catalytic activity is poorer due to the lack of Cu active sites.The effect of hydrothermal aging on catalysts showed that the activity of Cu-SSZ-13 catalyst decreased with the increase of hydrothermal aging temperature,and the decrease of catalyst activity became more obvious when temperature was higher.The process of hydrothermal aging at high temperature could destroy the B acid sites on catalyst surface,reduce the NH₃ adsorption performance of the catalyst,and reduce the total NH₃adsorption amount of the catalyst,and strongly absorbed NH₃.The NH₄⁺attached to the catalyst decreases with the increase of hydrothermal aging temperature.After high temperature hydrothermal aging,the structure of catalyst collapsed,crystallinity became worse,specific surface area and pore volume decreased;the collapse of catalyst skeleton also made its micro-morphology disordered,particles more rough and uneven distribution,and Cu_xO species formed on the surface,which made the catalytic efficiency lower and selectivity worse,but the high temperature hydrothermal aging did not cause the catalytic mechanism of the catalyst in SCR reaction.The effect of Ce modification on the catalytic performance of Cu-SSZ-13 catalyst was investigated.The results showed that the introduction of Ce element could enhance the B acid site of the catalyst and thus facilitate the adsorption of NH₃ on the catalyst surface and promote the reaction process,and improve the SCR catalytic activity of fresh catalyst.At the same time,the introduction of Ce element can reduce the formation of Cu_xO species on the catalyst surface and thus reduced the non-selective oxidation rate of NH₃,and finally improved the activity and N₂ selectivity of the catalyst at high temperature.After hydrothermal aging process,the activity and N₂ selectivity of the catalyst modified by Ce element still decrease and the non-selective oxidation of NH₃ still occured,but the N₂selectivity decreases slighter than catalyst without Ce modification.The introduction of Ce element is beneficial to improving the hydrothermal stability of the active site of the catalyst.After hydrothermal aging process at high temperature,the catalyst modified by Ce element is better than that of the catalyst without Ce element.The catalyst retained more active sites and adsorbs more NH₃ in the form of NH₄⁺ions,which proved Ce modification still enhanced the B acid site of Cu-SSZ-13 catalyst after hydrothermal aging,Ce-modified catalyst could partly reduce the inactivation of active sites and the formation of Cu_xO species by reducing the process of Cu aggregation after hydrothermal aging,but it could not completely prevent this process.