Effects of nano size mischmetal and its oxide on improving the hydrogen sorption behaviour of MgH2

This paper reports the catalytic effects of mischmetal (Mm) and mischmetal oxide (Mm-oxide) on improving the dehydrogenation and rehydrogenation behaviour of magnesium hydride (MgH2). It has been found that 5 wt.% is the optimum catalyst (Mm/Mm-oxide) concentration for MgH2. The Mm and Mm-oxide catalyzed MgH2 exhibits hydrogen desorption at significantly lower temperature and also fast rehydrogenation kinetics compared to ball-milled MgH2 under identical conditions of temperature and pressure. The onset desorption temperature for MgH2 catalyzed with Mm and Mm-oxide are 323 °C and 305 °C, respectively. Whereas the onset desorption temperature for the ball-milled MgH2 is 381 °C. Thus, there is a lowering of onset desorption temperature by 58 °C for Mm and by 76 °C for Mm-oxide. The dehydrogenation activation energy of Mm-oxide catalyzed MgH2 is 66 kJ/mol. It is 35 kJ/mol lower than ball-milled MgH2. Additionally, the Mm-oxide catalyzed dehydrogenated Mg exhibits faster rehydrogenation kinetics. It has been noticed that in the first 10 min, the Mm-oxide catalyzed Mg (dehydrogenated MgH2) has absorbed up to 4.75 wt.% H2 at 315 °C under 15 atmosphere hydrogen pressure. The activation energy determined for the rehydrogenation of Mm-oxide catalyzed Mg is ∼62 kJ/mol, whereas that for the ball-milled Mg alone is ∼91 kJ/mol. Thus, there is a decrease in absorption activation energy by ∼29 kJ/mol for the Mm-oxide catalyzed Mg. In addition, Mm-oxide is the native mixture of CeO2 and La2O3 which makes the duo a better catalyst than CeO2, which is known to be an effective catalyst for MgH2. This takes place due to the synergistic effect of CeO2 and La2O3. It can thus be said that Mm-oxide is an effective catalyst for improving the hydrogen sorption behaviour of MgH2.