| Summary: | 碩士 === 國立成功大學 === 化學工程學系 === 88 === Abstract
In order to increase the specific surface area, the hydrothermal and sol-gel methods were employed to prepare the nanosized Perovskite-type LaBO3 (B= Fe, Co, Mn) catalysts. The effects of alkalinity (R), reaction time, temperature and pressure on the formation of LaBO3 powders were investigated. PAA and nitrates were chosen as the reactants and the pH value was controlled at 2 when the sol-gel method was applied. High molecular weight of PAA was preferred for the sake of high coiling. These powders were characterized by the techniques such as XRD, SEM, TEM, BET and TGA methods. The following results were obtained:
In the preparation of the La-Fe series oxide, the powder is amorphous when no hydrothermal treatment was applied. However, when the hydrothermal treatment was utilized, the powders, with crystalline La(OH)3, having a specific surface area in the range of 54~100 m2/g was obtained first. Note that the treatment was undertaken at 150℃ and the reaction became equilibrium after 2 hours. When the powders were calcined at 800℃, very pure LaFeO3 Perovskite crystallines were obtained. The higher the alkalinity, the purer LaFeO3 is. The crystalline size of LaFeO3 was about 20~40nm and the specific surface areas were 14~23 m2/g. The favorable conditions for preparing LaFeO3 Perovskite-type oxides were high alkalinity, high temperature and high pressure.
For preparing LaCoO3 Perovskite-type oxides by the hydrothermal method, the optimal conditions were at low pressures and 6<R<31. The specific surface areas of the oxide powders synthesized were 7~40 m2/g. After calcining at 800℃ the powders still had some impurities. They had an average size of 20~40nm and specific surface areas in the range of 12~18 m2/g.
The proper conditions for preparing LaMnO3 Perovskite-type oxides were at low pressures and R≦6. The specific surface areas of the oxide powders synthesized were 14~34 m2/g. The powders still had some impurities after calcining at 700℃. They had an average size of 20~100nm and specific surface areas of 18~25 m2/g.
When the La-Co oxides were prepared by the sol-gel method and calcined at 700℃ and LaCoO3 obtained were porous because PAA was destructed through calcination. The TEM image shows that its average size is in the range of 20~40nm and has a specific surface areas of 7~12 m2/g.
In the preparation of LaMn Perovskite-type oxides, LaMnO3.15 crystallines would be formed because of the La cation vacancy should be made up due to electroneutrality. A high purity of LaMnO3.15 could be obtained when the powders were calcined at 500℃.
In LaFe series, a high purity of LaFeO3 could be obtained when the powders were calcined at 700℃. The TEM image shows spherical crystallites with an average size of 40~60nm, and they differed from the porous ones.
Comparing the properties of Perovskite-type catalysts prepared by the coprecipitation, hydrothermal and sol-gel methods shows that the utilization of the sol-gel method can result in a higher purity though the specific surface areas of catalysts are slightly lower than those prepared by the hydrothermal method. In conclusion, the catalysts prepared by the sol-gel method are superior to those prepared by the hydrothermal method.
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