Mechanism Study And Numerical Simulation Of Hydrothermal Aging Of Urea-SCR Catalyst For Diesel Engine

Urea Selective Catalytic Reduction?Urea-SCR?is one of the most promising technologies for reducing NOx emissions from diesel engine.The low NOx reduction performance and NH₃ slip can be caused by hydrothermal aging in application process.In order to make it a better application prospect in automotive exhaust aftertreatment system,many researches focus on clarifying the aging mechanism and improving the hydrothermal stability of SCR catalysts.To study the aging characteristics of reactions over the surface of SCR catalyst,this paper was based on the SCR reaction model of catalyst degreened at 500°C,then the gas concentrations at the outlet of catalysts aged at 700,800 and 900°C were predicted by developing an aging model.The aging factor of the aging model was divided into two f₁ and f₂according to the level of Cu ions loading in the active site,and the aging factors were multiplied by the pre-exponential factor of the corresponding reaction.In addition,a reaction mechanism for N₂O formation was developed.The simulation results of catalyst degreened at500°C and catalyst aged at 700°C obtained by the aging model predicted well.Since there were new active sites formed after aging at 800°C,the activation of standard SCR was adjusted and relevant reaction was added to the model.Several parameters needed to be retuned due to that the zeolite structure completely collapsed after 900?aging.The results showed that NH₃ storage capacity,desorption activation energy and low-temperature N₂O formation decreased and the high-temperature N₂O formation increased with the increase of aging temperature.The reactions over SCR catalyst surface were affected by hydrothermal aging at different deactivamtion rate.With the increase of the NO₂ content in feed,the formation of N₂O in SCR reactions increased.The standard SCR reaction is more deactivated during aging compared to fast and slow SCR reactions.After high temperature aging,there were new active sites such as Cu oxides which may be responsible for the following phenomenons:the increase of NO oxidation activity,the reduction of NO₂ to NO and the oxidation of ammonia to NO after hydrothermal aging at 900°C and the sensitivity of high temperature N₂O formation to hydrothermal aging.Since NH₃ storage capacity has an important influence on the NOx conversion efficiency and transient response characteristic of SCR catalyst,the transient response characteristic of the catalyst,the dynamic NH₃ storage capacity of the catalyst and the relationship between the NOx conversion efficiency and the dynamic NH₃ storage capacity after hydrothermal aging were researched.The results showed that the transient response accelerated and the dynamic NH₃ storage capacity decreased with the increase of aging temperature and reaction temperature.The NO conversion rate had the same variation trend along the axial position of the catalyst as the dynamic NH₃ storage capacity.The dynamic NH₃ storage capacity increased only in the SCR reaction zone,and the change rate of dynamic NH₃ storage capacity in the non-SCR reaction zone is zero.