Catalytic role of binary oxides (CuO and Al₂O₃) on hydrogen storage in MgH₂

Hydrogen sorption characteristics of magnesium hydride (MgH2) were enhanced by ball milling for 12 h under an inert atmosphere with the addition of 20 wt% Al2O3 and as prepared CuO. X-ray diffraction (XRD) characterization and thermal gravimetric analyzer coupled with a mass spectrometer (TGA-MS) of the ball milled samples were carried out to determine the H-2 sorption reactions. Additionally, XRD data were analyzed using Rietveld method to identify the crystallographic properties. Ball milled MgH2 with beta and gamma allotrope mixtures were found with broad peaks in XRD indicating the as prepared samples are amorphous and strain, alongside bare additives. H-2 sorption in MgH2 is greatly enhanced by the presence of CuO and Al2O3 binary oxides. TGA experiment demonstrated fast desorption kinetics for ball milled samples with the oxide additives. Particularly, MgH2 milled with CuO showed a more remarkable effect on desorption kinetics rather than Al2O3. It takes less than 10 min to desorb more than 5 wt% H-2 at 623 K. The activation energy of H-2 desorption was estimated for the mixtures by Kissinger method and the lowest value (65 kJ mol(-1)) was obtained for the MgH2/CuO mixture. A kinetic model has been proposed in the context to illustrate the complex mechanism of dehydrogenation of MgH2 with the additives. Finally, the addition of metal oxides influences H-2 sorption characteristics of MgH2 through lowering the activation energy and reducing the waste of energy for diffusing the thin layer of Mg that creates a pathway to store H-2 within the system.