Controlled Synthesis And Studies On Structure-Activity Relationship Of Ceria-supported Nano-catalytic Materials

The fundamental comprehending and understanding of factors which effect on catalytic properties such as morphology,size,surface structure and composition is of great importance in the design and exploring of novel efficient catalysts?high selectivity,high activity and thermal stability?.Based on the controlled synthesis of ceria nanomaterials with different structure,the uniform and well-defined structure consist of novel model catalysts without "materials gap" and "pressure gap" in the traditional catalysts.Meanwhile,the current researches on catalysts rely on a variety of characterization techniques,especially in-situ structural investigation which can knock on the catalytic reaction "black box".On the basis of the relevant characterization,the exploration of catalytic reaction mechanism and influencing factors?"Morphological effect","size effect" and "support effect"?not only provided important reference but also revealed "structure–activity" relationship on the preparation of efficient and stable nano-catalysts.In this dissertation,ceria-based nanomaterials doping with gold or copper nanocrystals and bimetallic copper–palladium nanocrystals was chosen as the target system,The metal–support interaction and structure-activity relationship of Au/CeO₂,Cu/CeO₂ and Cu-Pd/CeO₂ model catalysts have been fabricated and systematically investigated,respectively.The obtained innovative research results as follows:1.The characterization techniques combined with in-situ X-ray absorption fine structure?XAFS?and Diffuse Reflectance Infrared Fourier Transform spectroscopy?DRIFTS?was successfully designed,built and debug from system design,component splitting and retrofitting to assembly of combination device.This equipment realized experimental data acquisition of high-quality XAFS and DRIFTS in transmission acquisition mode and controlled the catalytic reaction atmosphere and reaction temperature?RT–350 °C?.This work provides the support of techniques and method for the study of subsequent Au/CeO₂ catalysts,in addition,it accumulates valuable experience for developing in-situ XAFS apparatus on Beamline BL14W1.2.CeO₂ nanorods prepared by hydrothermal method were chosen as the matrix and a series supported Au/Ce catalysts including single-atoms,clusters?< 2 nm?and particles?3-4 nm?anchored to the CeO₂ nanorods whose Au species have been proved by the aid of XAFS and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy?Cs-corrected HAADF-STEM?were synthesized by using colloid-deposition?CP?method,deposition-precipitation?DP?method and reduction treatment.Simultaneously,a variety of in situ characterization techniques?XAFS,DRIFTS?further investigated that metallic Au species?Au0?contribute dominantly than ionic Au species?Au?+?in the catalytic CO oxidation.This work have realized the in-situ detection of structural evolution on various types of gold species and clearly identified the active species and catalytic mechanism during carbon monoxide oxidation reaction.3.Cu/CeO₂ catalysts with different Cu contents?5–40 at.%?were prepared by the coprecipitation method.The effects of different metal precursors,precipitants,dropping velocity and aging conditions on the structure and morphology of the catalysts were also investigated.Finally,the appropriate experimental parameters were obtained.Furthermore,Ammonium carbonate was used as the leaching agent on the as-calcined samples to remove the weakly bound copper species.When the Cu doping amount reached 30 at.%,the stable and constant CO-PROX reactivity,including CO conversion and O₂ selectivity,was presented for both parent and ammonium carbonate leached catalysts.The fresh and used samples have been carefully characterized by various techniques such as power X-ray diffraction?XRD?,XAFS,transmission electron microscopy?TEM?,X-ray Photoelectron Spectroscopy?XPS?and temperature-programmed reduction by hydrogen?H2-TPR?.It was confirmed that there are two different types of copper species,i.e.weakly bound CuOx clusters and strongly bound Cu–[Ox]–Ce structure,in both parent and leached catalysts.Furthermore,we have demonstrated that the weakly bound CuOx clusters removed by ammonium carbonate can be recovered by the migration of Cu–[Ox]–Ce species under the reduction or reaction conditions.The strongly bound copper species in CeO₂ have been identified to be the reservoir for the active Cu sites for preferential oxidation of carbon monoxide?CO-PROX?.4.Copper-palladium bimetallic oxide clusters anchored to the surface of ceria nanorods was prepared by deposition-precipitation?DP?.It was Revealed by Cs-corrected HAADF-STEM and XAFS techniques that both copper and palladium species are fully oxidized with dominant metal-support interaction contributions by a strongly bound M–[Ox]–Ce?M = Cu or Pd?structure.However,no direct bond between copper oxide and palladium oxide clusters,i.e.Cu–[Ox]–Pd,has been verified by experimental evidences,and thus no synergistic effect on the catalytic activity of bimetallic copper-palladium oxide clusters,compared to that of single metal?palladium?oxide clusters,has been demonstrated for the CO oxidation reaction.Above experimental results provide deep insight into the fabrication of gold or copper nanocrystals and bimetallic copper-palladium nanocrystals anchored on well-defined ceria nanomaterials and structure-activity relation of supported catalysts.