镧对镁吸氢和放氢动力学及热力学性能的影响

Effect of lanthanum on kinetic and thermodynamic properties of magnesium for hydrogen absorption and desorption

摘要:

研究纳米La2O3对Mg颗粒的助磨作用以及吸氢和放氢的催化作用，对克服Mg吸氢和放氢动力学差的问题有重要意义。Mg粉中添加1.0 mol.%的La2O3于球磨后，XRD分析表明Mg晶粒为49.7 nm，而纯Mg球磨后的晶粒为51.6 nm，La2O3使球磨后Mg晶粒的更小，这有利于提高Mg的吸氢和放氢速率。SEM观测表明，球磨后La2O3纳米晶粒与Mg晶粒紧密接触，呈镶嵌结构，这有利于La2O3对Mg吸氢和放氢发挥催化作用。吸氢和放氢动力学测试表明，添加La2O3使Mg吸氢和放氢速率加快，吸氢反应活化能降低49.8 kJ/mol，放氢反应活化能降低23.1 kJ/mol，即显著地了改善Mg吸氢和放氢的动力学性能。p-c-T等温吸附平衡测试表明，添加La2O3球磨后Mg的吸氢活性更高，平衡氢压较低。DSC放氢分析表明，添加La2O3后MgH2放氢温度有所降低。硬度比Mg大的La2O3纳米颗粒在球磨过程中起助磨作用，不与Mg反应；在加氢和放氢过程中La2O3起催化作用，不与H2反应。

Abstract:

It is significant to investigate the effect of La2O3 as milling aid and hydrogen sorption catalyst for magnesium in order to solve its problem of low kinetic performance. The XRD analysis shows that after milling with 1.0 mol.% of La2O3, the crystallite size of Mg decreases to 49.7 nm, while it is 51.6 nm without La2O3. The decrease in crystallite size from the addition of La2O3 is in favor of the improvement of hydrogen sorption rate of Mg. The SEM observation shows that La2O3 nanocryatallites contact with crystalline Mg closely and form mosaic structure, which is in favor of La2O3 playing the role of catalysis for hydrogen sorption of Mg. The kinetics test shows that with the addition of La2O3, the hydrogen sorption rate of Mg increases obviuosly. With the addition of La2O3, the activated energy of hydrogen absorption decreases by 49.8 kJ/mol, and the activated energy of hydrogen desorption decreases by 23.1 kJ/mol. So, the kinetic performance of hydrogen sorption of Mg is substantially improved with the addition of La2O3. The p-c-T isothermal absorption analysis shows that the hydrogen absorption activity of Mg increases and results in lower equilibrium hydrogen pressure. The DSC analysis shows that the hydrogen desorption rate of MgH2 decreases with the addition of La2O3. In the process of milling, nanoscale La2O3 play the role of milling aid with its higher hardness than that of Mg, and it does not react with Mg; in the process of hydrogenation and dehydrogenation, La2O3 plays the role of catalyst, and it does not react with H2.