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

• Published in: International journal of hydrogen energy 2016-07, Vol.41 (27), p.11709-11715
• Main Authors: Pukazhselvan, D., Otero-Irurueta, Gonzalo, Pérez, J., Singh, Budhendra, Bdikin, Igor, Singh, Manoj K., Fagg, Duncan Paul
• Format: Article
• Language: English
• Subjects: Catalysis; Hydrogen storage; Microscopy; X-ray diffraction
• ISSN: 0360-3199; 1879-3487
• DOI: 10.1016/j.ijhydene.2016.04.029

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. •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.