Developing Rare Earth Pyrochlore And Perovskite Composite Oxide Catalysts For Oxidative Coupling Of Methane Into Ethylene

Oxidative coupling of methane（OCM）is one of the effective ways to convert methane directly into low carbon olefin,which has important fundamental study significance and potential application value.Due to the stable structure of methane,the activation of its molecules can only occur at a high temperature.Hence,the deep oxidation of reactant methane and products is un-avoidable,thus resulting in low one-way C₂ product yield.As a result,the industrial application of this reaction is restricted.Aiming to solve these problems,in this thesis,the OCM reaction performance difference on A₂B₂O₇ pyrochlore and ABO₃ perovskite composite oxide catalysts with the varied A-site cations（A=La,Nd,Sm）have been compared.It was found that that the active oxygen formation mechanism on the two types of composite oxides is different,thus displaying considerable OCM performance at low or high temperature.Based on this,A₂B₂O₇ composite oxide catalysts with non-stoichiometric ratios have been prepared by adjusting the A-and B-cation ratio,which have different crystalline phase structures and display good low temperature performance.By using multiple characterization means,the effects of phase structure on the surface oxygen vacancies,active oxygen species,alkaline sites and reaction performance have been investigated.The main contents are summarized here:Part 1:A series of Ln₂Zr₂O₇ and Ln Al O₃（Ln=La,Nd,Sm）composite oxides were prepared by glycine combustion method for OCM reaction.XRD and Raman show that Ln₂Zr₂O₇ composite oxides have a disordered defect fluorite structure,and Ln Al O₃composite oxides have a rhombohedral or orthogonal perovskite structure.Compared with Ln Al O₃ catalysts,Ln₂Zr₂O₇ catalysts have better activity and C₂ selectivity at lower temperature.With the increase of reaction temperature,the difference of the reaction performance between the two kinds of catalysts gradually becomes smaller.At high temperature,the reaction performance of the two kinds of catalysts is close to each other.In situ Raman results have shown that the crystal structure of disorder defective Ln₂Zr₂O₇ fluorite composite oxides is not affected by the reaction temperature.Due to the presence of intrinsic oxygen vacancies,Ln₂Zr₂O₇ catalysts can adsorb and activate gaseous O₂ at relatively lower temperature on their surface oxygen defects,thus displaying good low temperature OCM performance.Whereas,Ln Al O₃ catalysts will only experience phase structure transformation at high temperature,which leads to lattice distortion and the formation of oxygen vacancies.Because of this,the Ln Al O₃catalysts show better OCM performance at higher temperature.The results of CO₂-TPD and in situ DRIFTS have testified that the alkaline sites with different strength and contents on the catalyst surface affect the yield of C₂ products.The results of CH₄pulse,CH₄-TPSR and EPR have shown that the OCM active oxygen species on the catalysts is superoxide（O₂^-）anions.In addition,the amount of surface O₂^-anions is intimately related to the alkaline sites with moderate strength.The interaction of these two types of surface sites determines the OCM reaction performance on the catalysts.Part 2:Based on part one,Nd_nCe_2-nO_x composite oxides with non-stoichiometric ratios have been prepared with glycine combustion method by adjusting the A-and B-cation ratio,which have been used for OCM reaction.In all the catalysts,Nd_0.2Ce_1.8O_xand Nd_0.6Ce_1.4O_x have a cubic fluorite structure,and Nd₂Ce₂O₇,Nd_1.4Ce_0.6O_x and Nd_1.8Ce_0.2O_x have a rare earth C-type structure.The Nd_1.8Ce_0.2O_x catalyst shows good reaction performance at low temperature,on which 19.0%methane conversion and 7.7%C₂ yield were obtained at 450 ^oC.The results of CO₂-TPD have testified that the alkaline sites with moderate-strength on the catalysts are critical for OCM reaction.With the increasing of the amount of the moderate alkaline sites,the selectivity of C₂products will be improved.In situ Raman,EPR and XPS results have proven that the amount of OCM active oxygen species O₂^-is positively related to the surface oxygen vacancy concentration.Nd_1.8Ce_0.2O_x catalyst has the largest quantity of oxygen vacancies,which can generate the largest amount of O₂^-species.Moreover,this catalyst possesses the largest amount of moderate alkaline site,thereby displaying the best OCM reaction performance.