Catalytic Reactivity Of Rare Earth Mn-Based Perovskite-Type Oxides For CO And C₃H₈ Oxidation

With the increasingly stringent emission standards of automobile exhaust,the catalytic elimination of carbon monoxide（CO）and propane（C₃H₈）,which are the main pollutants in automobile exhaust,is of great significance,and the rational design of such catalysts is the focus of research.Perovskite-type oxide ABO₃ have attracted much attention because of its high catalytic activity and thermal stability.The results show that the catalytic activity of ABO₃ perovskite-type oxides is mainly related to the B-site cations of the active component,and the A-site cations can also be adjusted by doping,substituting,etching and so on,which can also affect the catalytic performance.Therefore,this the sit focused on the rare earth perovskite-type oxides LnMnO₃.A adjusting A/B site cation stoichiometric ratio and A site rare earth ion,were used for the catalytic oxidation of CO and C₃H_8.The structure-activity relationship of the catalysts was revealed by various characterization methods.The main results are as follow:（1）By changing the A/B cation stoichiometry ratio strategy,we synthesized La_xMnO_3±δ（x=0.8,0.85,0.95,1.05,1.15,1.20）catalysts with different cation defects using glycine-nitrate combustion method.Compared with the stoichiometric LaMnO₃meeting the stoichiometric ratio,the La_0.84MnO_3-δcatalysts with lanthanum cationic defects have larger specific surface area,smaller crystalline size and weaker Mn-O bond,and its catalytic activity for CO and C₃H₈ is significantly higher than that of stoichiometric LaMnO₃ and La_xMnO_3+δ（x>1）with Mn cationic defects.Specifically,La_0.84MnO_3-δwith La cation defects has a specific activity R_s that is 6.6 times higher than LaMnO₃ and 10.1 times higher than La_1.15MnO_3+δfor CO oxidation reaction.For the C₃H₈ combustion reaction,the specific activity R_s of La_0.84MnO_3-δis 2.4 times and3.3 times higher than those of LaMnO₃ and La_1.15MnO_3+δ,respectively.La_0.84MnO_3-δwith La cation defects exhibits the best CO and C₃H₈ catalytic performance.According to H₂-TPR,FITR,O₂-TPD and other characterization techniques,the formation of La defects increases the number of surface acidic sites,the content of Mn⁴⁺and surface active oxygens,which is beneficial to the adsorption and activation of CO and C₃H₈.（2）We prepared LnMnO₃（Ln=La,Nd,Sm）catalysts with different A-site cations by changing the A-site rare earth nitrate precursor for CO oxidation and C₃H₈ combustion reactions.These LnMnO₃ with different A-site cations formed different crystal structures,leading to differences in the strength of the Mn-O bonds.This further resulted in trends of changes in the catalytic activity of CO and C₃H₈ activation,which were NdMnO₃>SmMnO₃>LaMnO₃.At 110℃,the specific rate of CO oxidation on NdMnO₃ was 6.2 times higher than that on LaMnO₃.At 270℃,the specific rate of C₃H₈ combustion on NdMnO₃ was twice that on LaMnO₃.According to various characterization techniques such as H₂-TPR,XPS,O₂-TPD,CO-TPD,and C₃H₈-TPD,different A-site cations led to differences in the surface area of the catalysts,the strength of the Mn-O bonds,the number of surface acidic sites and activated oxygen,as well as the adsorption and activation performance of CO and C₃H₈.These differences further determined their order of catalytic activity.According to EPR,NdMnO₃ had the most O₂^-,which also played a role in the oxidation reaction.NdMnO₃with moderate strength of Mn-O,moderate activity of adsorbed oxygen,and the strongest adsorption and activation ability of CO and C₃H₈ showed the best catalytic performance.