Directed Synthesis Of Ceria Based Materials By Small Molecule Adsorption And The Study Of Catalytic Performance

Ceria can be used as both active components and supports in heterogeneous catalysis, which has exhibited obvious morphology effect in the catalytic reactions. It is a great challenge to design and control the CeO₂ morphology and to obtain highly reactive ceria catalysts, though a number of synthesis methods have been used to prepare CeO₂ nanomaterials. This work has successfully developed an approach to prepare highly reactive ceria-based catalysts by small molecules adsorption with further studies on the mechanism of orientated synthesis via small molecules. The influence of synthetic atomosphere on the final products in morphology, structure and surface properties has been investigated. The morphology, size, composition and structure of ceria-based catalysts have been integrated designed and the influences of structure, surface state and other properties of the catalysts on the catalytic activities of CO oxidation and water-gas shift （WGS） reaction have been investigated.At first, we proposed a synthesis method of CeO₂ nanocubes （NCs） via CO molecules adsorption. An obvious shape change from CeO>2 nanoparticles （NPs） to NCs took place when CO molecules were introduced in the synthetic system. The TEM and HRTEM analysis showed the morphology and exposed surfaces of CeO₂ NPs and NCs, respectively. The formed species and element distributions on the surface of CeO₂ nanocrystals has been further confirmed by FT-IR and XPS analysis. Based on the above results, we have proposed a probable formation mechanism of ceria nanocubes. This surface selective growth induced by CO specific adsorption on ceria surfaces make it possible to obtain ceria nanocrystals enclosed with more reactive{100} planes.Mesoporous CeO₂ colloidal spheres were obtained by the assembly of CeO₂ nanocrystals in two shapes:NPs and NCs. CeO₂ colloidal spheres assembled by nanoparticles （NPCS） predominantly expose{111} surfaces and that assembled by nanocubes （NCCS） are terminated by {100} planes. Au/NCCS （NCCS） exhibits a better WGS activity than Au/NPCS （NPCS） under the simulated realistic conditions. The size and surface area of NPCS and NCCS are comparable. A comparison of WGS reaction behavior over Au/NPCS and Au/NCCS was taken to clarify the nanoscale crystal plane effect of CeO₂{111} and{100} crystal planes. The XPS and H2-TPR results showed that more amount of Auδ+ species formed on CeO₂{100} planes and it is helpful to increase the oxygen mobility and reducibility of CeO₂ NCCS samples. The in-situ DRIFTS results demonstrated that the formate mechanism is suitable to illustrate the WGS reaction pathway on Au/CeO₂ colloidal spheres. Au^ species are easier formed on NCCS, the formation/decomposition rate of bidentate formates （intermediates） are faster on Au/NCCS, and less monodentate carbonates （inactive species） accumulated over Au/NCCS. The ability of H2O adsorption on Au/NCCS is stronger than on Au/NPCS due to the higher activity of CeO₂{100} surfaces than the{111} ones. The DFT+U calculations also confirmed that the electrons of Au would prefer to transfer to CeO₂ （100） surface compared with （111） surface, and the H2O adsorption on CeO₂-x （100） surfaces are more stable than that on （111） surfaces.Then we introduced CO molecules into another synthetic system and obtained globin-like mesoporous （GLM） CeCO3OH, which further decomposed to CeO₂ through thermal treatment. It provides a novel way to synthesis 3D self-assembly structures. The morphology and structure of GLM CeCO3OH/CeO₂ were characterized by XRD, TEM and BET. Furthermore, FTIR、XPS and UV-vis analysis were used to study the surface properties of GLM CeCO3OH/CeO₂. Based on the above results, we have proposed a probable formation mechanism for GLM CeCO3OH/CeO₂, in which process CO plays an important role, serving as a chelating ligand to form stable complex with Ce3+ and further kinetically control the reaction rate. GLM CeO₂ are more active than the CeO₂-DC obtained by direct thermal decomposition for CO oxidation. In addition, GLM CeO₂-supported gold catalysts show good performance in CO catalytic oxidation at low temperature.Based on the successful attempt to control the morphology of CeO₂ materials via the introduction of CO into the synthetic system, we applied H2 molecules to optimize the ceria-based materials afterwards. Well dispersed CeO₂-Al2O3 of large surface area were obtained in the ethylene glycol system. Small CeO₂ particles with a size of about 3-5 nm are embedded in amorphous alumina, which is probably stabilized by the interaction between hydroxyl groups of ethylene glycol adsorbed onto ceria/alumina surface. H2 molecules were then introduced into the same synthetic system to study the effect of H2 atmosphere on the properties of final products, which was named as CeO₂-Al2O3-H2. There are more amount of active oxygen species on the surface of CeO₂-Al2O3-H2 than that of CeO₂-Al2O3, and the ability of oxygen migration and transfer in CeO₂-Al2O3-H2 is more easier, which were determined by O2-TPD and H2-TPR analysis. Au/CeO₂-Al2O3-H2 exhibits a better WGS activity than Au/CeO₂-Al2O3 under the simulated realistic WGS conditions. The DRIFT results have illustrated that the activation energy for WGS rection over Au/CeO₂-Al2O3-H2 is lower than over Au/CeO₂-Al2O3, and the decomposition rate of intermediates over Au/CeO₂-Al2O3-H2 is more faster.