Preparing Highly-Efficient Bi₂M₂O₇（M=Sn,Ti,Zr） Pyrochlore Catalysts For Soot Combustion

While the gradual popularizing of diesel motor vehicles brings convenience to industry production and people’s life,the environmental pollution caused by soot particles is becoming more and more serious.Catalytic combustion is one of the effective means to eliminate soot particles from the exhaust of diesel vehicles.Therefore,it has become a hot topic in this field to develop low-cost and highly efficient catalysts for soot particles elimination.A₂B₂O₇ compounds generally possess excellent thermal stability,inherent oxygen vacancies and good oxygen mobility,thus meeting the requirements of soot combustion catalyst.In this study,according to the feature of soot particles released from diesel vehicle exhaust,a series of highly active and stable A₂B₂O₇ compounds with Bi as the A-site have been designed and prepared for soot combustion.By using a variety of characterization methods,the structural properties and active surface centers of the catalysts,and their influence on the soot combustion performance has been investigated and discussed.The main content and results are summarized here:Part I:A series of Bi₂M₂O₇(M=Sn⁴⁺,Ti⁴⁺and Zr⁴⁺)catalysts with Bi³⁺as the A-site and Sn⁴⁺,Ti⁴⁺and Zr⁴⁺as the B-site were prepared by hydrothermal method for the soot combustion.The XRD,Raman,HRTEM,STEM-mapping,O₂-TPD,soot-TPR,EPR,XPS and in-situ DRIFTS characterization results have proven that the crystalline phases of the Bi₂M₂O₇ compounds vary from well-ordered pyrochlore（Bi₂Sn₂O₇）to disordered pyrochlore（Bi₂Ti₂O₇）and finally disordered defect fluorite structure（Bi₂Zr₂O₇）by changing the B-site cation,hence the disorder degree of the crystal structure increases.As a consequence,more abundant active surface oxygen species such as superoxide O₂^-and peroxide O₂^2-species,which are beneficial to the soot combustion reaction,have been formed.DFT calculations and oxygen ion conductivity results have shown that the surface oxygen vacancy formation energies of the catalysts become lower with the increasing of the disorder degree.Therefore,the ability to adsorb and activate gas-phase O₂ molecules becomes stronger.The ¹⁸O₂isotopic tracing and oxygen exchange results have demonstrated that both the surface oxygen vacancies and lattice oxygen can adsorb gas phase O₂ molecules dissociatively,thus unravelling the oxygen transfer process during the reaction.It is revealed that the synergistic actions of surface oxygen vacancies,active oxygen and lattice oxygen species determine the reaction performance of Bi₂M₂O₇ catalysts,and the disorder degree of the catalysts is the intrinsic factor for this.PartⅡ:Based on the work in part I,the Bi₂Zr₂O₇ compound was thus prepared by four different methods:co-precipitation,glycine combustion,sol-gel and solution combustion,and used for soot catalytic combustion.Various methods were used to characterize the active surface sites as well as redox properties,and their effects on the reaction performance.The activity of the catalysts obeys the order of Bi₂Zr₂O₇-CP>Bi₂Zr₂O₇-GNC>Bi₂Zr₂O₇-SG>Bi₂Zr₂O₇-SC.XRD,Raman and N₂ adsorption-desorption results have indicated that all the Bi₂Zr₂O₇ compounds synthesized by the four methods have formed a defective fluorite structure.The Raman,H₂-TPR,O₂-TPD,soot-TPR and XPS results have testified that the redox properties and oxygen vacancies on the catalyst surface are the key factors affecting the soot combustion activity.If a Bi₂Zr₂O₇ catalyst owns more surface oxygen vacancies,it is able to generate more abundant active surface oxygen species,such as superoxide O₂^-,peroxide species O₂^2-.This explains that the Bi₂Zr₂O₇-CP catalyst prepared by co-precipitation method displays the best activity for soot combustion.PartⅢ:To study the effect of partial A-site substitution on the structure and reactivity of A₂B₂O₇ compounds,a series of Bi RZr₂O₇ catalysts(R=La³⁺,Pr³⁺,Sm³⁺,Y³⁺)with half amount of A-site Bi³⁺cations replaced by these rare earth cations were designed and synthesized,and adopted for soot combustion.XRD,Raman and N₂adsorption-desorption results have proven that all the catalysts possess pure defect fluorite phase structure.In comparison with the unsubstituted Bi₂Zr₂O₇ compound,the substituted Bi RZr₂O₇ catalysts have a smaller unit cell side lengths and higher specific surface areas.The soot-TPR results have shown that the content of active surface oxygen species is the decisive factor for the combustion performance of the catalysts.The FTIR results have demonstrated that the radius of the A-site substitution cations can affect the strength of the Zr-O bond in the structure.The larger the radius of the substitution cation,the weaker the Zr-O bond strength.Among all the catalysts,Bi La Zr₂O₇ owns the most abundant active surface oxygen species and the weakest Zr-O bond strength,hence it shows the highest soot combustion activity.