Identification Of Active Sites In Copper Oxide Clusters Supported On Ceria Catalysts For CO Oxidation

Copper-ceria as one of the typical catalysts for the preferential oxidation of carbon monoxide?CO-PROX?reaction,as well as the CO oxidation,has been widely studied.However,the absence of the homogeneously structured catalyst system and the limited availability of in-situ sophisticated techniques notably hinders the mechanistic understanding on the reactive site at atomic level like mesoscopic decentralized state,the micro coordination environment as well as the copper-ceria interactions.In this paper,based on the shape-controllable ceria nanocrystals,highly dispersed CuOx supported on CeO₂ were synthesized by a deposition-precipitation?DP?method.Then the different pretreatment and ceria supports with different morphologies were adopted to manipulate the surface structure of copper oxide and the catalytic reactivities were also investigated.Using in-situ techniques,the change of the active species were identified at atom-scale,and the relationship between the active species and catalytic reactivities was explored,so as to guide the design of novelty catalytic materials.This thesis includes three main parts:?1?Identification of active sites for highly dispersed copper-ceria catalysts in CO-PROX reactionIn order to identify of active sites for copper-ceria catalysts,a series of copper-ceria catalysts with different Cu loading supported on CeO₂ nanorods were synthesized by a deposition-precipitation?DP?method.The as-prepared samples were characterized by various structural and textural detections including XRD,Vis-Raman,TEM,ex-situ/in-situ XAFS,and H2-TPR.It has been confirmed that the highly dispersed copper oxide?CuOx?clusters,as well as the strong interaction of Cu-[Ox]-Ce structure,were the main copper species deposited onto the ceria surface.The XANES profiles,by the aid of linear combination analysis,identified the oxidized Cu???as the dominant copper species in both O2-and H2-pretreated samples after CO-PROX at 80 ?.Furthermore,EXAFS fitting results,together with the corresponding H2-TPR data distinctly determined that the highly dispersed CuOx?x =0.2-0.5?cluster,rather than the Cu-[Ox]-Ce?x = 0.7-3.2?structure,were the crucial active species for the CO-PROX reaction.?2?Exploration of the intrinsic support effect for copper-ceria catalysts in CO oxidationIn the project to explore the importance of crystal plane effect on the metal-support interaction in copper-ceria catalysts,highly dispersed CuOx?1 wt%Cu loading?supported on both ceria nanorods?CeO₂-NR-{110}/{100}?and nanospheres?CeO₂-NS-{111}/{100}?have been prepared by deposition-precipitation?DP?method for CO oxidation reaction.Although 1CuCe-NS sample exhibited an OSC value of 296 umol g^-1,which is lower than that of 1CuCe-NR(349 umol g^-1),the CO reaction rate on 1CuCe-NR and 1CuCe-NS surface are 1.8 ×10^-6 molCO gcat^-1 s^-1?118 ??and 5.7×10^-6molCO gcat^-1 s^-1?104 ??,respectively.By aid of XANES/EXAFS,H2-TPR,assisted with in-situ DRIFTS,we have demonstrated that strongly bound of Cu-[Ox]-Ce structure in CeO₂-{110} adverse to the formation of Cu+ species,On the contrary,CuOx clusters on the surface of CeO₂-{111} was easily reduced to Cu+species when subjected to the interaction with CO.The stronger reduction trend of Cu2+?Cu+ in CeO₂-{111} should be the intrinsic reason that ensures the high activity of the CuCe-NS catalysts.?3?Effect of surface reconstruction of highly dispersed copper-ceria catalysts under high temperature on their catalytic performanceIn the studying on the surface reconstruction for copper-ceria catalysts,the CeO₂ nanorods exposing {110}/{100} were calcined at different temperatures.A{111?-type faceting instead of {110}-terminated surface undergo extensive restructuring,which may induce the change of copper-ceria interaction.CO/H2-TPR results show that the facile reduction of the catalyst and the formation of more Cu+species is directly related to the activity.Interestingly,for the calcinated copper-ceria catalysts,a decrease in the surface area is accompanied by the increase in the activity.It is shown that the CuCe?800-4?sample calcinated at 800 ? is most active in CO oxidation,which could be due to the surface reconstruction and the appearance of defect sites that cause the change of copper-ceria interaction.These results open up new possibilities in the design of highly active ceria-based catalysts with controlled surface structures.