Simulation On The Fe/Pt Synergistic Catalytic Mechanism Of Bi-functional ASC Catalyst For Diesel Engine

Urea Selective Catalytic Reduction?Urea-SCR?technology is widely used to reduce the NO_x emission of diesel engines.However,when the NO_x is reduced on the SCR catalyst surface,conditions that are encountered in practice for which the slip of the reductant NH₃ is unavoidable include release of the stored NH₃ during an exhaust temperature increase,overdosing of urea,and SCR catalyst deactivation due to aging.Therefore,in order to meet the more stringent NO_x limit and NH₃ slip limit requirements of emission regulations,it is necessary to develop an advanced bi-functional ammonia slip catalyst?ASC?.The application of the bi-functional catalyst in diesel aftertreatment can effectively remove NH₃ emission from SCR catalyst and assist upstream SCR catalyst to achieve higher NO_x conversion.A thorough understanding of the reaction mechanism on the catalyst surface is a prerequisite for improving and utilizing the performance of catalyst.In this paper,the global reaction kinetic models of NH₃-SCR reaction over Fe-ZSM-5 catalyst and NH₃ oxidation over Pt/Al₂O₃ catalyst were established respectively.By optimizing the kinetic parameters of each catalytic reaction step,these models were calibrated with experiments.The results show that the developed models accurately predict the trend of species concentrations and conversion.The results show that the NH₃ storage capacity of Fe-ZSM-5 catalyst is rather small,and it is significantly affected by temperature.The oxidation activity is also low,and the NH₃conversion is only 80%at 600?.The catalyst shows higher SCR activity at high temperatures.The inhibition effect of NH₃ on Fe-ZSM-5 catalysts leads to a lower NO_x conversion in standard SCR reaction at low temperatures,and this effect is weakened when the temperature rises.Pt/Al₂O₃ catalyst exhibits strong NH₃ oxidation activity.The light-off temperature of Pt catalyst is 213?,and NH₃ has been completely converted when the temperature exceeds 230?.The main product of NH₃ oxidation is N₂ at low temperatures,and a great amount of N₂O and NO_x are produced in the middle and high temperature range.In order to make full use of the catalytic performance of Fe-ZSM-5 catalyst and Pt/Al₂O₃catalyst,the two catalysts were combined in the form of Fe?top?-Pt?bottom?dual-layer and Fe+Pt single layer to improve the yield of N₂ product and the total NO_x conversion of SCR-ASC system.The performance of bi-functional combined catalyst was studied by using the developed reaction mechanism model of Fe-Pt combined catalyst.The results show that Fe-Pt combined ASC catalyst can simultaneously remove NH₃ and NO_x emissions from SCR catalyst.The conversion of NH₃ over Fe-Pt combined bi-functional catalyst is still close to95%at 250?.At higher temperatures?>300??,the emission of NO_x decreases sharply and the yield of N₂ increases greatly.The Fe-Pt combined catalyst achieves the expected catalytic performance,and the optimal combination structure is the arrangement of Fe?top?-Pt?bottom?dual-layer.The thickness ratio of Fe and Pt washcoat in the combined catalyst was optimized.It is confirmed that the dual-layer combined catalyst with Fe?50%?and Pt?50%?has the highest N₂ yield under the studied conditions.It is concluded that NO_x formed by NH₃ oxidation on the surface of Pt catalyst diffuses to Fe zeolite catalyst and react with stored NH₃ for SCR reaction.The internal SCR?i-SCR?reaction is the key to realize the online coupling of Fe and Pt catalysts and the selective catalytic oxidation of NH₃ over Fe-Pt combined catalyst.