The Controllable Preparation And Characterization Of Rare Earth Oxides With 3D Micro/Nanostructures

Rare earth materials such as cerium oxide and lanthanum oxide have the unique characteristic of optics, electricity and magnetism, which are widely used in optical, magnetic, catalytic, medical and agricultural fields. The activities of the rare earth materials are influenced by the morphology, size, micro-structure and surface properties. In this thesis, we focused on the controlled synthesis of CeO2 and La2O3 with controllable shape and size. We applied hydrothermal and solvothermal methods to synthesis of 3D micro/nanostructure CeO2 and La2O3 and further evaluated their catalytic activities for toluene catalytic combustion over CeO2, La2O3 and three-way catalytic purification over Pt-Rh-Pd/CeO2.The obtained results are listed as following:(1) CeO2 with different morphologies were synthesized successfully by changing the type and concentration of precursor, solvent composition, type and concentration of carboxylic acid and other conditions:spherical CeO2 was synthesized successfully through hydrothermal method under CeCl3/PVP/acetic acid/distilled water/urea reaction system (HTS); spherical CeO2 was prepared successfully through solvothermal method with a reaction system of Ce(NO3)3/PVP/ethylene glycol/distilled water/propionic acid (STS). The specific surface area of CeO2 obtained form STS is 133 m2/g for fresh sample (calcinated at 500℃), after aging (calcinated at 1000℃), the surface area decrease to ～7 m2/g; flower-like CeO2 with a narrow pore size distribution was synthesized successfully through solvothermal method under a reaction system of CeCl3/PVP/ethylene glycol/distilled water/propionic acid/KOH (STF), of which KCl is considered as hard template and plays a key role in the formation of mesoporous.(2) To further improve the micro-structure of CeO2, we used HNO3 to treat precursors and finally obtained etched ceria. The XRD patterns show that the etched ceria has a weaker diffraction peaks, meaning the reduced crystalline size; The SEM images illustrate that the effect of etching is dependent on the preparation methods above; The TEM images show that the etched nanospheres possess more interior space; The N2 absorption/desorption results display that the specific surface area, pore volume and pore size distribution of ceria can also be greatly enhanced by the etching method.(3) Similar systems can be used to prepare 3D lanthana precursors via hydrothermal and solvothermal methods. After calcination at different temperatures, O3 and La2O3 could be obtained. For solvothermal methods, the acid concentrations and reaction time has vital impact on the morphologies of lanthana precursors. Hydrothermal methods can be used for obtaining disc-like La(OH)CO3 which can maintain a high specific surface area of over 10 m2/g after calcinating at 1000℃.(4) The catalytic combustion of toluene and three-way catalytic purification were evaluated over CeO2, La2O3 and Pt-Rh-Pd/CeO2. The results show that for CeO2, STSAce has the best performance in toluene catalysis. The etched ceria exhibit better activity. Through BET and XPS analysis, we can speculate that the improvement of catalytic activities is related to enlarged specific surface area and the exposure of surface active sites. For La2O3, ST2 has the best performance in toluene catalytic combustion. Pt-Rh-Pd/STSAce has a higher C3H8 and NO catalytic activities in three-way catalysis, however, etched ceria as carriers display poor performances, which could be caused by the reduced acid intensity.