Study On The Simultaneous Removal Of PM And NO_x From Diesel Engines Over 3DOM Perovskite-like Complex Oxide Catalysts

Nitrogen oxides（NO_x）and soot particles（PM）are the main pollutants of diesel engine exhaust,which have caused serious environmental pollution and human health hazards.The simultaneous elimination of PM and NO_x is an efficient method for tail gas purification.The traditional process is to combine DOC,DPF,CDPF,NSR and SCR technology and then take the advantage of each stage to finally eliminate harmful substances.However,the development of simultaneous elimination is restricted by the complex process,large occupied space and high cost of the combined purifier,and the catalytic activity needs to be further improved.In this dissertation,the integration of SCR and CDPF as an integrated（SCRPF）technology is proposed to replace the traditional tail gas-purified process,so that one catalyst can be used to remove soot particles and nitrogen oxides in the tail gas in one step in a relatively low temperature range.Among the SCRPF technology,the design of efficient and simultaneous elimination catalyst is the core of SCRPF technology.The design of the catalyst with a three-dimensional ordered macroporous structure（3DOM）solves the problem of contact efficiency between the catalyst and soot and reduces the mass transfer resistance.At the same time,NH₃ is introduced into the reaction mixer to make up for the deficiency of reducing agent under lean burn conditions.Furthermore,a series of perovskite and perovskite-like catalysts were studies and developed,and alkali metal or/and alkaline earth metal elements were introduced into manganese-based perovskite and copper-based perovskite-like to tune them with chemical modification,i.e.by adjusting the electronic valence configuration and geometric space effect of the catalysts,the activity and selectivity of the catalysts for the simultaneous catalytic elimination reaction can be fundamentally improved by increasing the oxygen defect density,high valence ion content and acid site strength of the catalyst.While improving the activity of the catalyst,the mechanism for the simultaneous elimination of PM and NO_x reaction over perovskite oxide catalysts used was explored,and the source of the product was analyzed to further guide the study and development of perovskite-like catalysts with higher activity and selectivity.In this thesis work,3DOM La BO₃（B=Cr,Mn,Fe,Co,Ni,Cu）,3DOM La_1-xK_xMnO₃,3DOM La_1-xSr_xMnO₃ and 3DOM La_2-xSr_xCu O₄ catalysts were designed,prepared and characterized by means of XRD,Raman,FT-IR,SEM,TEM,BET,XPS,H₂-TPR,O₂-TPD,NH₃-TPD,NO-TPD,NO-TPO and others,combined with DFT theoretical calculation to study the physical and chemical properties of the catalyst surface and explore the general rules and propose the possible reaction mechanism for the simultaneous elimination reaction.It can provide favorable theoretical guidance and techological supporting for the follow-up pilot study and industrial mass production of diesel engine exhaust simultaneous elimination reaction.The main research contents and results are as follows:（1）A series of 3DOM La BO₃（B=Cr,Mn,Fe,Co,Ni,Cu）catalysts were prepared by common colloidal crystal template method（CCT）.The three dimensional ordered macroporous structure increases the effective bonding efficiency between catalyst and soot particles.The difference of B-site ions has a great influence on the simultaneous elimination of catalytic performance.The order of catalytic activity for soot oxidation is La MnO₃>La Ni O₃>La Fe O₃>La Co O₃>La Cu Oδ^*>La Cr O₃,and the order of catalytic reduction efficiency for nitrogen oxides is La MnO₃>La Cr O₃>La Co O₃>La Ni O₃>La Cu Oδ^*>La Fe O₃.When the B-site ion is Mn,the catalytic performance is the best,the peak temperature for catalytic soot combustion is T_m=423℃,the highest conversion of NO is 69%,and the selectivity of N₂ is about 60%.（2）Disordered macroporous and three-dimensional ordered macroporous K-modified manganes-based perovskite catalysts were prepared by solution combustion method and CCT method,respectively.This series of catalysts have regular and orderly three-dimensional penetrating macroporous structure and periodic pores.The effective contact efficiency between the catalyst and soot particles was obviously enhanced,and the activity of the reaction was improved.The activity of 3DOM La MnO₃ catalyst is obviously higher than that of nano-powder p-La MnO₃ catalyst.The doping of K element into perovskite obviously decreased the peak temperature for catalytic soot combustion,and the catalytic performance of 3DOM La_1-xK_xMnO₃（0<x≤0.35）was better than that of3DOM La MnO₃ catalyst.When the amount of K doping is 25%,the total catalytic efficency is the highest,the peak temperature of 3DOM La_0.75K_0.25MnO₃ for soot combustion is 357℃,and the NO conversion is 45-63%in the temperature range of 112-265℃.DFT model was used to calculate the molecular adsorption energy changes of O₂,NH₃ and NO reactants when K was introduced into the perovskite lattice for replacing La.（3）Three-dimensional ordered macroporous Sr-substituted manganese-based perovskite catalysts were prepared by carboxyl modified colloidal crystal template（CMCCT）method.Compared with K⁺ions,the radius of Sr²⁺ions is relatively smaller,the charge is closer to La³⁺,and the perovskite phase can be maintained with higher strontium doping content.The results show that the doping modification of Sr can obviously reduce the peak temperature of soot combustion and increase the conversion of nitrogen oxides.However,the higher the amount of strontium doping is not always better.The 3DOM La_0.6Sr_0.4MnO₃ catalyst has the best simultaneous elimination performance,the peak temperature for PM catalytic combustion was 426℃,and the temperature range for complete conversion of NO is 397-458℃.The unique three-dimensional ordered macroporous structure,sufficient surface active oxygen and abundant acid sites are the reasons for the high performance of 3DOM La_0.6Sr_0.4MnO₃catalyst in simultaneously removing PM and NO_x.（4）We successfully prepared 3DOM La₂Cu O₄ catalyst for the first time by using a novel colloidal crystal template method（SCMCCT）modified by carboxyl with small size.Furthermore,it is easier to obtain three-dimensional ordered macroporous structure of Cu-based perovskite with partial substitution of La by Sr.It was found that 3DOM La_2-xSr_xCu O₄ catalyst played a good role in promoting the simultaneous elimination of PM and NO_x,and showed excellent N₂ selectivity.The catalytic reduction ability of nitrogen oxides and the selectivity to nitrogen are better than those of manganese-based perovskite catalysts.Among them,3DOM La_1.5Sr_0.5Cu O₄ catalyst has the lowest peak temperature of 416℃ for soot combustion.3DOM La Sr Cu O₄ catalyst has the highest denitration efficiency,NO conversion temperature range of more than 95%is 312-356℃,and N₂selectivity is 95.2-99.7%The reduction ability of low valent Cu²⁺ions,abundant oxygen defect density and surface acid sites,strong NO adsorption sites and three-dimensional ordered macroporous structure all promoted the performance of 3DOM La_2-xSr_xCu O₄catalyst.（5）Oxygen vacancy of K-modified La_1-xK_xMnO₃ catalyst increases obviously,especially when the amount of K doping is high.This phenomenon was verified through DFT theoretical calculation,appropriate model and method.The same method was emplyed to testify that the perovskite surface introduced by K enhanced the adsorption capacity of small gas molecules such as O₂ and NO.The infrared gas cell quantitative method was used to cetiify that soot produced N₂O during the reduction of NO₂,and N₂O came from NO₂ rather than NO.In view of this,we propose a“double reductant”mechanism for the simultaneous elimination reaction,that is,soot can participate in the reduction of NO_x as well as NH₃.Finally,based on the characterization information of XPS and O₂-TPD and the activity test results,we speculated that in the simultaneous catalytic soot combustion and NO_x reduction catalyzed by manganese-based perovskite involved in NH₃,the active oxygen mechanism,NO₂-assisted mechanism,L-H mechanism and E-R mechanism may exist simultaneously.