Studies On The Synthesis And Properties Of Mesostructured, Morphology Controllable Ceria-based Nanomaterials

CeO₂ is an important type of rare earth material, and has widely applications in modern high-tech areas. The application of nanosized CeO₂ is largely depended on its micro-structure and morphologies. In this dissertation, we report the synthesis and characterization of mesostructured and morphology controlled ceria based nanomaterials, as well as their catalytic properties in CO oxidation and degradation of organic pollutants.1. Mesoporous CeO₂ particles with high surface area were synthesized via a modified evaporation-induced self assembly(EISA) method using citric acid as complexing agent, CTAB as surfactant respectively. The effects of Ce³⁺/CTAB molar ratio as well as calcination temperature on the surface area of mesoporous CeO₂ were investigated. It was found that the cerium oxide/surfactant mixture gave rise to pure and fluorite-structured CeO₂ after calcination at 300℃. Moreover, the textural analysis revealed high specific surface area (205 m²Ã‚·g^-1) and mesoporous structure of the sample. The catalytic performances of mesoporous CeO₂ heated at various temperatures for CO oxidation were examined. The catalytic tests exhibited that the product had enhanced catalytic efficiency compared with the decomposed ceria.2. Novel mesoporous mesocrystal CeO₂ were synthesized using acetate salt as inorganic species and P123 as surfactant. Transmission electron microscopy revealed that the wall framework consists of a single phase based on the face-centered cubic CeO₂ and the polycrystalline crystals were highly oriented with the crystal axis [001] parallel to the pore channel if the Zr⁴⁺ molar fraction x was 0.3 or less, In-situ TEM demonstrated that the integrity of the framework could be maintained as high as 800℃on account that the random growth of the grains was suppressed. However, when the Zr⁴⁺ molar fraction was larger than 0.3, a mixture of cubic and tetragonal phases formed and the preferential crystal orientation disappeared as revealed by XRD and Raman measurements. It was proposed that the hydrophilic segments of P123 and the acetate group acted cooperatively and lead to the preferred crystals attachment facilitated by the strong dipole-dipole interactions following the manner of coherent interface. The N₂ sorption measurement suggested that the single phase solid solution at Zr⁴⁺ molar fraction 0.3 had the largest BET surface area and it consequently demonstrated the best catalytic performance for CO conversion due to the unique mesoporous mesocrystal structure with dominant exposure of highly active {200} planes and an enhanced redox property caused by adequate Zr⁴⁺ incorporation. 3.Prism-like mesocrystal CeO₂ was synthesized for the first time via hydrothermal method without any surfactant. The effect of the molar ratios of HMT to Ce(NO₃)₃·6H₂O , reaction temperature and solvent type on the morphology of the product was investigated. A plausible formation mechanism was put forward that the nanocrystals aggregated along with the epitaxial orientation following the manner of coherent interface .The UV-visible adsorption spectrum exhibited the red-shift phenomenon (E_g=3.02eV) compared with bulk CeO₂ particles (E_g=3.19eV) , presumably due to the existence of considerable defects in particular twin boundaries in this unique structure. 4. Hollow spherical CeO₂ were prepared via a layer-by-layer (LBL) method using self-made carbon spheres as sacrificial template, HMT as precipitant respectively. CO conversion was used as a catalytic test reaction. The obtained products exhibit well-defined hollow spherical structure with a diameter of ca. 250 nm as well as the thin shell about ca. 20 nm composed of various-oriented polycrystals. Catalytic test revealed that the activity of the hollow spherical products in CO oxidation were substantially higher than for a non-hollow sample, the hollow structure was believed to provide substantial space for the adsorption and desorption of gas molecules5. CeO₂ nanotubes were synthesized by a hydrothermal reaction assisted by P123. The effects of the reaction time, reaction temperature, and the surfactants concentration on the morphology evolution of the products were investigated. The formation of CeO₂ nanotube can be rationalized by the dissolution-anisotropic growth-rolling up mechanism. The experimental results showed that CeO₂ nanotube had enhanced efficient catalytic activity on degradation of methylene blue compared with nanoparticles and nanorods, which was ascribed to the tubular structure owning excellent adsorption capability with dye moleculars , as well as the dominant exposure of high-energy surface of {220}.6. CeO₂ nanotubes were synthesized facilely with a layer-by-layer deposition route templated by modified attapulgite for the first time. As a natural clay, attapulgite can be completely converted into amorphous SiO₂ nanorods with surface functionalized by-OH groups and acted as promising template. The obtained CeO₂ nanotubes had a uniform diameter ranging from 20-40 nm with a bundle-like structure. Under slow hydrolyzing of HMT, the oppositely-charged OH^- was released in the solution, and consequently Ce³⁺/OH^-/Ce³⁺/OH^-…deposited onto the carbon sphere following a layer-by-layer assembly. Appropriate calcination at 500℃was found to be crucial to solidify the framework on account of the well-fused neighboring crystals caused by the reduction of interfacial energy. This synthesis strategy by taking advantage of natural clay as hard template implies a simple and inexpensive way to prepare oxide nanotubes on a large scale for modern chemical synthesis.