Research On N₂O Formation Mechanism Over Cu-SSZ-13 SCR Catalyst For Diesel Engine

With the continuous tightening of diesel engine emission regulations,diesel engine after-treatment system is also constantly upgrading and improving.The introduction of DOC+DPF+SCR+ASC multi catalytic collaborative after-treatment system greatly reduces the difficulty of diesel engine meeting the standard,but also brings about the problem of strong greenhouse gas N₂O secondary emission.In this system,N₂O is mainly formed in the process of NO_x selective catalytic reduction.Therefore,the research on N₂O formation mechanism of Cu-SSZ-13 copper-based zeolite SCR catalyst for China VI and higher emission regulations is of great significance for the improvement of SCR catalyst formulation and the optimization and improvement of diesel engine after-treatment system,and also helpful to improve the control of diesel engine greenhouse gas emission.This thesis focused on the N₂O formation mechanism and mitigation over Cu-SSZ-13 copper-based SCR catalysts for diesel engine.To explore the N₂O formation pathway and influencing factors over Cu-SSZ-13 catalyst,the standard SCR,fast SCR,NO oxidation,NH₃ oxidation and N₂O formation pathway decoupling test were conducted on a flow reactor.Then,XRD,H₂-TPR,EPR and in situ DRIFTS were employed to investigate N₂O formation over Cu-SSZ-13 with different copper loadings,and the mechanism of Cu species effect on N₂O formation were analyzed.Based on the mechanism of N₂O formation,a new SCR reaction kinetic model of Cu-SSZ-13 catalyst including N₂O formation and decomposition process was established,and the parameters of the model were estimated and validated by flow reaction experiments.Finally,the feasible measures to inhibit N₂O formation over Cu-SSZ-13 catalyst were proposed based on the results of model simulation and experiments.The main work of the thesis are as follows:(1)Study on N₂O selectivity and its influencing factors over Cu-SSZ-13 catalyst.In this paper,N₂O selectivity over Cu-SSZ-13 catalyst under standard and fast SCR conditions were studied.It was found that the N₂O formation mechanism on the surface of Cu-SSZ-13 catalyst was different at high and low temperatures.Temperature programmed surface reaction(TPSR)experiments and decoupling test of N₂O formation show that N₂O formation is mainly ascribed to the decomposition of surface intermediate NH₄NO₃ at low temperatures,while it is mainly related to the non-selective catalytic oxidation of NH₃ at high temperatures.O₂,NO+O₂ and NO₂can act as oxidants,and the amount of N₂O is increased at high temperature with the increase of oxidation of oxidants.In this paper,the influence factors of N₂O selectivity were also studied.The results show that the ammonia nitrogen ratio(ANR),NO₂/NO_x ratio,space velocity,O₂ and H₂O concentration can affect the N₂O formation process,and the effects on the two mechanisms are totally different at low and high temperatures.(2)Study on the N₂O formation mechanism over Cu-SSZ-13 catalyst.Based on the results of N₂O selectivity over Cu-SSZ-13 with different Cu loadings and combining with the characterization of catalysts,it can be found that the content of Cu in the catalyst played a decisive role in the formation of N₂O.At low temperatures,when there is only NO in the gas,NO can react with NH₃ on the surface of Cu-SSZ-13 catalyst to form NH₄NO₂,which is an important intermediate of SCR reaction.In the presence of O₂,Cu(OH)⁺and Cu O can further oxidize NH₄NO₂ to NH₄NO₃.When NO₂ exists in the gas,NO₂ disproportionation leads to the formation of NO₃^-on the surface of the catalyst,which can react with adsorbed NH₃ to form NH₄NO₃.What's more,as a strong oxidant,NO₂ can oxidize NH₄NO₂ on the surface of the catalyst to NH₄NO₃.The direct decomposition of NH₄NO₃ leads to N₂O formation at low temperatures,but excessive NH₄NO₃ will block the CHA pore and inhibit its decomposition process.Nevertheless,Cu²⁺is active for the reaction between NH₄NO₃ and NO,the increase of Cu²⁺allows more NH₄NO₃ consumed and the self-inhibition of NH₄NO₃ alleviates,which allows NH₄NO₃decompose to N₂O at lower temperatures,and N₂O formation peak shift to lower temperatures.At high temperatures,Cu(OH)⁺and Cu O is active for the formation of N₂O.On the one hand,Cu(OH)⁺and Cu O can oxidize NH₃ to N₂ or NO_x,leading to the reducing of ammonia nitrogen ratio during SCR reaction and the improving of N₂O selectivity.On the other hand,Cu(OH)⁺and Cu O can oxidize NO to NO₂,and the increment of strong oxidant NO₂ will further promote N₂O formation.(3)Study on kinetic model of N₂O formation and decomposition over Cu-SSZ-13catalyst.A new multi-site Cu-SSZ-13 reaction kinetic model was established on MATLAB/Simulink platform.The model not only includes conventional ammonia adsorption and desorption,ammonia oxidation,NO oxidation,standard SCR reaction,fast SCR reaction and slow SCR reaction,but also includes the formation and decomposition process of N₂O and the inhibition of NH₄NO₃ on low-temperature SCR reaction based on the research conclusions and discoveries of this thesis.The model parameters were estimated and validated based on the flow reactor experiments.The results show that the model can accurately predict NO_x conversion,NH₃conversion and N₂O formation under the conditions with different space velocity,temperatures,ammonia nitrogen ratio and NO₂/NO_x ratio.In the SCR system operating temperature range,the maximum error of NO_x and NH₃ conversion between simulation and experiment is less than 5%,and N₂O formation is less than 2 ppm.It provides research basis for the follow-up simulation research and SCR control system optimization and upgrading.(4)Study on N₂O formation mitigation methods during SCR process over Cu-SSZ-13catalyst.Based on the experimental results,it was found that when the Cu loading of Cu-SSZ-13catalyst is about 2.5%,the catalyst can reduce N₂O emission during SCR process on the premise of ensuring NO_x conversion.Based on the results of model simulation,in order to reduce N₂O emission as much as possible under the premise of ensuring high NO_x conversion and low NH₃slip of the after-treatment system,the NO oxidation characteristics of DOC+DPF should be optimized to decrease the NO₂ content in SCR inlet NO_x;the urea injection control strategy with variable ammonia nitrogen ratio should be adopted,the ammonia nitrogen ratio is 1 at low temperatures,and the ammonia nitrogen ratio is gradually increased when the temperature is higher than 350?,and the ammonia nitrogen ratio is even higher than 1.2 at 550?;the NH₃oxidation performance of ASC system at low temperature should be optimized to reduce NH₃ slip from SCR system;the exhaust gas thermal management system should be adopted to make SCR system work above 250?as far as possible and avoid the high N₂O emission temperature range of 200?-250?.