| The rare-earth elements, with unique 4f electronic configuration, are considered as a treasure house of new materials. They have the widespread applications in industry and in synthesis. Lanthanum oxide has been widely applied in the many fields of industry and technology. For example, it is an important component of automobile exhaust-gas conversion, as a catalyst support in the formation of gas conversion catalyst and as a catalyst of oxidative coupling of methane. It is also used as a refractory oxide for calcium lights, optical glass and in the formation of ceramics as a core for carbon are electrodes. However, as the raw material of these fields, lanthanum oxide's quality, purity and particle size is an emerging issue. Develop the technology of preparation of lanthanum oxide can expand the industrial applications of high abundance rare earth elements, such as lanthanum. Therefore the study of preparation of lanthanum oxide superfine powders in lab is indeed necessary.In the present study, precursor was prepared in the SCISR with different precipitators such as NH4HCO3, (NH4)2C2O4, NH3·H2O and impact of these precipitators on the particle size of the powder is studied. Then the kinetic study of the lanthanum oxide precursor which was prepared by (NH4)2C2O4 was carried out.(1) The lanthanum oxide precursor was prepared by NH4HCO3, La(NO3)3·6H2O and with a little amount of surfactant in the SCISR. After roasting, the superfine powder was ready. The following factors were studied through the orthogonal experiment:concentration ratio, reaction time, reaction temperature, amount of PEG, rotational speed of the propeller, roasting time, roasting temperature. And the variance analysis showed significant F values (0.1,0.05 and 0.01) for roasting temperature, reaction time and concentration ratio. So according to the orthogonal and the single factor experiments the optimized processes parameters were; concentration ratio (6:1), reaction time (30 min), reaction temperature (30℃), amount of PEG (4%), rotational speed of the propeller (1000 r/min), roasting temperature (800℃), roasting time (3 h). Particle size of the product was analyzed by scanning electron microscopy and was found to be 100 nm.(2) The lanthanum oxide superfine powder was prepared by (NH4)2C2O4 in the SCISR, and according to the single factor experiment the optimized processes parameters were; concentration ratio (6:1), reaction time (30min), reaction temperature (30℃), amount of PEG (4%), rotational speed of the propeller (1000 r/min), roasting temperature (800℃), roasting time (3 h). Particle size of the product was analyzed by scanning electron microscopy and was found to be 200 nm with spheroid geometry.(3) The lanthanum oxide superfine powder was prepared by NH3·H2O in the SCISR, and according to the single factor experiment the optimized processes parameters were; the amount of NH3H2O (25 mL), reaction time (15 min), reaction temperature (45℃), amount of PEG (10%), rotational speed of the propeller (2000 r/min), roasting temperature (800℃), roasting time (2 h). Particle size of the product was analyzed by scanning electron microscopy and was found to be 100nm with spheroid geometry.(4) Under the help of the TG-DSC and the Infrared Spectroscopy, the pyrolysis kenetics studies of the lanthanum oxide precursor, which prepared by (NH4)2C2O4, were carried out. The results showed that the precursor was La2(C2O4)3·10H2O and the pyrolysis process was consisted of five steps. The first was losing six crystal waters, the second and third were losing two crystal waters, the fourth was producing an intermediate La2O2CO3 and the last was obtaining the lanthanum oxide. After the analysis of the La2(C2O4)3·10H2O by the FWO and KAS methods, it showed that the activation energy E changed with the turnover ratio a by FWO as well as by KAS method and indicative of multi-step reaction. Because of the characteristics of the model-free and kinetics model, the optimized parameters of a multi-step reaction can not be worked. So the activation energy was obtained by average value and the kinetics model was obtained by multiple nonlinear fitting method. The activation energy values in the five stages of the La2(C2O4)3 pyrolysis process were 68.94,116.76,144.31,181.43,179.24 kJ/mol as investigated by FWO method. The activation energy values in the five stages of the La2(C2O4)3 pyrolysis process were 65.60,114.28,141.49, 179.01,171.79 kJ/mol as investigated by KAS method.(5) The pyrolysis theory of La2(C2O4)3 was analyzed by the multiple nonlinear fitting method and the obtaining results agree with g(α)= [1-(1+α)1/3]2 model. The kinetics parameters in those five stages were as follow:the first E= 65.47 kJ/mol, lgA=-1.6 S-1; the second E= 106.9 kJ/mol, lgA=-0.1 S-1; the third E= 120.9 kJ/mol, lgA=-0.183 S-1; the fourth E= 177.68 kJ/mol, lgA= 2.27 S-1; the last E= 156.4 kJ/mol, lgA=-2.54 S-1. And the fitting TG curve meets well with the experimental curves. |
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