Study On Simultaneous Removal Of Soot Particulate And No_x With Perovskite Catalyst

A large amount of NO_x and soot emitted by diesel locomotives in China not only pollutes atmospheric environment but also endangers human health.With the increasingly strict requirements of national emission standards,the automobile industry also has an urgent need for tail gas reprocessing technology.In the technology of outboard reprocessing,NO_x and soot can be removed simultaneously under the influence of catalyst,and it has the advantages of saving energy and reducing volume,etc.However,the key is to obtain efficient catalyst.Perovskite oxides have a good catalytic activity for removing NO_x and soot,and by optimizing preparation conditions and substituting some elements,their morphology and active sites can be changed,which is more conducive to the adsorption and activation of reactant molecules,and their catalytic performance can be improved.This paper adopts the citric acid sol-gel method corresponding perovskite catalyst,using X-ray diffraction（XRD）,scanning electron microscope（SEM）,nitrogen adsorption-stripping（BET）,X-ray photoelectron spectroscopy（XPS）and hydrogen temperature programmed reduction（H₂-TPR）technologies such as characterization of the physicochemical properties of catalysts,with carbon black soot particles in the simulated exhaust particulates,evaluated the oxidative conversion of soot（the combustion characteristic temperature of soot was used as evaluation data）and NO reaction catalytic activity（the conversion of NO to N₂）at the same time.Based on the data obtained from the two methods,the effects of preparation conditions and the doping of A and B ions on the performance of perovskite catalysts were investigated,the optimal doping elements and the doping substitution ratio were selected,and the causal relationship between the physico-chemical properties and catalytic activity of the catalysts was established.The main research results are as follows.（1）Optimization of catalyst preparation conditionsBy changing two important conditions in the citric acid sol-gel method:Citric Acid to Metal Ions Molar Ratio（CMMR）in catalyst precursor solution and calcination temperature,different La Co O₃ catalyst samples were prepared,and the optimal preparation conditions were selected through catalytic activity testing and characterization analysis.The results showed that the catalyst samples prepared at the calcination temperature of 700℃showed the best catalytic activity:T_10%,T_50%and T_90%（It is corresponding to the reaction temperature when the soot conversion rate reaches10%,50%and 90%）were 293.8℃,365.2℃and 402.8℃,respectively,and the conversion rate of NO increased to 16.4%.The results of characterization analysis showed that the catalyst sample had perovskite structure and small grain size.At the same time,it is concluded that the specific surface area of the catalyst samples prepared at different calcination temperatures is quite different.When the calcination temperature is high,sintering phenomenon will occur in the sample preparation process,and the specific surface area of the catalyst finally obtained will decrease.Based on the comprehensive results,CMMR=1.5 and calcination temperature of 700℃were selected for subsequent preparation of required catalysts.（2）Determination of B-site elements,A-site partially substituted elements and their substitution amountThe alkali metal ions K⁺,Na⁺,and Li⁺are used to dope the A-based La-based ions in each basic catalyst（La Mn O₃,La Co O₃,and La Fe O₃）,and the catalyst La_1-xA_xMn O₃（A=K,Na,Li）,La_1-xA_xCo O₃（A=K,Na,Li）,La_1-xA_xFe O₃（A=K,Na,Li）.Through comparison of catalytic performance tests,it was found that when the doped metal is K ion and the catalyst is a Co-based catalyst,its catalytic performance is the best.The doping and substitution of K ions reduced the particle size of the catalyst sample and the particles became more uniform and loose;the adsorbed oxygen on the catalyst surface and the oxygen vacancies in the perovskite structure were increased,which improved the catalyst performance.When the K⁺substitution amount is 0.2,the characteristic temperature T₁₀ and T₅₀ of soot are the lowest under the action of La_0.8K_0.2Co O₃,which are 271.6℃and 316.8℃respectively,and the reduction rate of NO reaches 42.7%.（3）Determination of the B-substitution amountA series of La_0.8K_0.2Co_1-yMn_yO₃ was prepared by doping and substituting B-site Co-based ions in the perovskite-type composite oxide La_0.8K_0.2Co O₃ with the best performance of Mn³⁺transition metal ion（y=0～1）sample.Through characterization analysis,it was found that the doping substitution of the transition metal ion to the B site does not change the crystal structure of the perovskite type,and the comprehensive catalytic activity is further improved.Among them,when the doping substitution amount of Mn³⁺is y=0.6,the characteristic temperatures T_10%and T_50%of the soot under the action of the catalyst are 291℃and 335℃,respectively.The test and characterization results of the above catalyst samples show that Co-based catalysts are superior in reducing the soot combustion characteristic temperature,and Mn-based catalysts are more prominent on the reverse side of reducing NO.（4）Test of sulfur resistance of catalystThe sulfur resistance of La_0.8K_0.2Co_0.4Mn_0.6O₃,the catalyst with the best comprehensive performance,was tested for activity.The results show that the performance of the catalyst will decrease slightly after stabilization,but from the XRD characterization results,it is seen that no obvious impurity phase appears in the XRD spectrum of the catalyst.Production,so the reduction rate of NO is not much reduced.