用XRD和TPR技术考察制备方法和灼烧温度对钙钛矿型结构形成以及性质的影响

本文用柠檬酸络合物分解法、碳酸盐共沉淀分解法、硝酸盐直接分解法和陶瓷法合成了轻、重稀土钙钛矿型氧化物LnMnO_3(Ln=Sm.Er).用X-射线衍射(XRD)和程序升温还原(TPR)技术考察了制备方法和灼烧温度对钙钛矿型结构形成的影响.实验结果表明,用柠檬酸络合物分解法可以在较低的温度下得到单一相的钙钛矿氧化物,而陶瓷法的灼烧温度比其他方法所需的温度要高得多。不同方法得到的产物SmMmO_3对CO氧化反应的活性为:柠檬酸法～碳酸盐法～硝酸盐法>陶瓷法。实验表明,TPR技术在某些体系中可作为灵敏的物相分析辅助手段,它可在制备或反应过程中原位观察体系结构的变化,实验中观察到,稀土氧化物能促进某些过渡金属氧化物在稀土氧化物表面分散,能阻止它们形成晶态.本文还讨论了钙钛矿型氧化物的形成机理,提出了过渡金属氧化物的扩散是形成钙钛矿结构反应速率的决定步骤观点。

Investigation of Synthesis Process of Perovskite- Like Oxides LnMnO_3 by XRD and TPR

Perovskite-like oxides LnMnO3 (Ln = Sm, Er) were prepared from nitrate solutions by citric acid-complexing (method A), carbonate coprecipitating (method B), direct decomposing(method C) and ceramic method(method D). The effects of preparation methods and calcination temperature on perovskite structure were characterized by X-ray diffraction (XRD) and temperature programmed reduction (TPR). Citric acid-complexing gave a single phase perovskite at a lower temperature and ceramic method required a much higher temperature than the others. The order of the catalytic activity for CO oxidation of LnMnO3 prepared by different methods was (the composition of the reaction gases was CO 2.51% O2 8.15% and N2 balence) : method A-B-C>D. The. TPR technique in some cases could be used as a supplementary sensitive phase identification method and be very convenient for judging the changes of structures and properties of some catalysts during the preparation and reaction. The experiment indicated that the rare earth oxides could promote the dispersion of some transition metal oxides and prevent them from forming crystalline state.