TEM studies of Nb2O5 catalyst in ball-milled MgH2 for hydrogen storage

In this paper we report microstructural studies by transmission electron microscopy (TEM) of magnesium hydride nanocomposites produced by ball milling. The work employed high angle annular dark field (HAADF) imaging, high resolution TEM (HRTEM), and electron energy loss spectroscopy (EELS). MgH2 samples had been prepared by ball-milling with 17 wt% of Nb2O5. Samples milled for a short time (30 h) show a decrease in particle size in comparison to MgH2 milled alone for the same amount of time. After only 30 h milling, finely dispersed Nb2O5 was detected within the MgH2 matrix. After milling for longer times (300 h) the amount of Nb2O5 dispersed within the Mg matrix increases. Analysis of material that had been milled for 300 h and then subjected to hydrogen desorption by heating showed that MgH2 and Nb2O5 interact during the thermal desorption forming both a partially reduced niobium oxide phase and a mixed Nb-Mg-O phase.