Crystal structure, phase stoichiometry and chemical environment of MgxNbyOx+y nanoparticles and their impact on hydrogen storage in MgH2

• Fe free mechanochemical synthesis of MgxNbyOx+y was focused.
• Steel and zirconia milling medium were employed.
• Formation of cubic Mg0.75Nb0.25O monoxide was confirmed.
• Mg0.75Nb0.25O is catalytically active for MgH2.
• Surface study confirms the existence of Nb in two different reduced valance states.

In the present study, we address a few unsolved aspects involved in the hydrogen storage mechanism of the MgH2/Nb2O5 system. The mixture, 6MgH2+Nb2O5, was mechanochemically reacted for 30 h under an inert atmosphere using two different milling medium, steel and zirconia, under identical experimental conditions. The product contained Nb dissolved MgO phase(s) in both cases. A detailed analysis of crystal structure, particle size and chemical environment of MgxNbyOx+y was performed. Rietveld occupancy analysis and elemental chemical analysis confirmed the population of oxygen to be nearly equal to the sum of metal atoms in the product. X-ray photoelectron spectroscopy analyses showed the existence of Nb in two different valance states, both belonging to oxide phases. A combined study of thermogravimetry – differential scanning calorimetry and Sieverts volumetric hydrogen absorption/desorption kinetics measurements confirmed the good catalytic activity of Nb dissolved MgO products for MgH2. A plausible mechanism is explained on the basis of a diffusion barrier cracking model.