Preparation,Characterization And Functional Applications Of Ceria-Based Hollow Spheres

CeO₂,as an important rare earth oxide,has achieved considerable research in recent years and found widespread applications in many important fields such as catalysts,fuel cells,oxygen sensors and luminescence due to the high oxygen storage capacity and quick redox response relying on the ready reduction and oxidation of Ce between its two oxidation states(Ce⁴⁺,Ce³⁺).Many kinds of ceria materials,such as different morphology of ceria or ceria based oxide,such as nanotube,nanowire,nanosphere,nanorod,and hollow structure,have been successfully fabricated.Among the various kinds of ceria materias,hollow structure materials have attracted special attention due to their high surface area,low density and hierarchical porous structure.In this thesis,we have focused on preparation,characterization and their applications of ceria based hollow spheres,as well as some rudimentary mechanism analysis and design.The results were summarized as follows:We developed a facile colloidal SiO2 template based approach to prepare Ce O2 hollow spheres employing NaOH as etching agent to remove the hard templates.The colloidal SiO2 templates were synthesized by a typical St?ber method.Hollow spheres with different shell thickness and pore structure could be easily obtained by controlling the concentration of the feeding cerium source.On the basis of the experiment,we found that,by adjusting the kinds and concentrations of precipitant,it could combine the formation of shell and the remove of template into one step,which greatly simplified the preparation process.Well-dispersed CeO₂–CuOx composite hollow spheres have been successfully synthesized through a facile reflux method using carbon spheres as sacrificial templates.The shells of the hollow spheres,40 nm in thickness,consisted of self-assembled 10–15 nm sized nanoparticles.Moreover,most of the copper species were highly concentrated on the surface of CeO₂ shell support.The CeO₂–CuOx composite hollow spheres exhibited enhanced catalytic activity and stability for CO oxidation in that complete CO conversion could be obtained at 112oC;and after five reaction cycles,the sample still maintained high catalytic activity with 100% CO conversion at 125 oC.On the basis of the above work,we designed an extremely facile method to prepare well-defined MnO₂@CeO₂-MnO₂ ball-in-ball binary oxide hollow spheres by employing carbon spheres（CSs）as sacrificial templates.The synthesis process involved a novel self-assembled approach to prepare core-shell CSs@CeO₂ precursor,which would directly reacted with KMnO4 aqueous solution to form yolk-shell CSs@MnO₂/CeO₂-MnO₂ precursor in the following step.Well-dispersed Ce-Mn binary oxide with double shelled hollow sphere structure could be achieved after annealing the precursor in air.The evolution process and formation mechanism of this novel structure were thoroughly studied in this thesis.Especially the as-prepared double-shell MnO₂/CeO₂-MnO₂ hollow spheres exhibited enhanced catalytic activity for CO oxidation compared with the pure MnO₂ hollow spheres and pure Ce O2 hollow spheres.Most importantly,this method could be extended to prepare other transition metal oxides.As an example,Co-Mn composite oxide hollow spheres assembled by ultrathin nanoplates were successfully prepared.