Synthesis Of Ceria-based Nanocomposites And Their Catalytic Properties For Low-temperature CO Oxidation

Air pollution has become one of the most serious challenges facing countries.CO is one of the pollutants with the largest emissions.The CO pollution will pose a threat to the atmospheric environment and human health.Catalytic oxidation reaction is an important means to achieve effective CO removal.As an important rare earth oxide,cerium oxide（CeO₂）has been commonly used in the field of CO catalytic oxidation.This thesis mainly focused on the controllable preparation of CeO₂ based nanomaterials and high-efficiency CO catalytic oxidation research,including metal-support interactions,multicomponent composites with the transition metal ions and noble metal species.The main research contents of this thesis are as follows:（1）CeO₂ nanospheres were synthesized by a solvothermal method.The Au was successfully deposited onto the CeO₂ by a deposition-precipitation method.The sample was calcined in muffle furnace at 300°C,400°C and 600°C for 3h.The effect of the different pretreatments（N₂,O₂ and H₂）and calcination temperatures on the catalytic performance of CO oxidation were investigated.The Au-CeO₂ sample without any subsequent pretreatments has lower CO conversion.However,introducing different pretreatments to the samples caused a significant enhancement in the catalytic activity.In addition,the Au-CeO₂ samples with high temperature calcination still have good catalytic performance after different pretreatments.More surface sites for CO adsorption and active oxygen binding on the catalyst were formed after pretreatment,resulting in the improved activity of Au-CeO₂.（2）In this work,one simple and mild pretreatment with N₂ has been reported to re-activate the Au-CeO₂ catalysts which prepared by deposition-precipitation method and calcination at 600°C.Upon N₂ pretreatment at 200°C,the metal-support interaction between Au nanoparticles（NPs）and CeO₂ has been observed with the evidence of the particular coverage of Au nanoparticles by CeO₂,electronic interactions and CO adsorption changes.The N₂ pretreatment also makes the Au NPs more resistant to sintering at high temperature.Furthermore,this mild pretreatment strategy can provide a potential approach to improve the thermal-stability of other supported noble metal catalysts.（3）A series of Au-CeO₂ catalysts were prepared by deposition–precipitation method and calcination at 600°C.The effects of calcination time and H₂ pretreatment on catalyst activity and thermal stability were investigated.In order to understand the improved activity and thermal stability,a series of techniques were used to characterize the physico-chemical changes of the catalyst samples.H₂ pretreatment may lead to:（i）a strong metal–support interaction（SMSI）between Au nanoparticles（NPs）and CeO₂,evidenced by the particular coverage of Au NPs by CeO₂,electronic interactions and CO adsorption changes.（ii）the production of surface bicarbonates which can accelerate CO oxidation.As a result,the H₂pretreatment makes the Au NPs more resistant to sintering at high temperature and enhances the CO oxidation activity.（4）The Au/doped-CeO₂ catalysts were successfully prepared by deposition-precipitation method and calcination at 600°C.The Ce-based solid solution composite oxide supports exhibit a fluorite cubic CeO₂ phase.Doped ceria samples exhibited higher crystallite parameters,higher specific surface area and more abundant oxygen vacancies than pure ceria.Cu²⁺or Ni²⁺incorporation of CeO₂ exhibited the better CO oxidation activity,which was attributed to the higher specific surface area and more CO adsorption sites.Conversely,La incorporation caused an opposite effect due to the stable chemical properties.However,incorporation of La³⁺into the Au-CeO₂ resulted in high CO oxidation activity attributed to the appropriate ratio of Au^δ+/Au⁰,oxygen vacancies and the number of higher OH^-groups.