Hydrogen Cycling of Niobium and Vanadium Catalyzed Nanostructured Magnesium

The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage
purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using
nanostructured powders. In situ neutron diffraction on MgNb0.05 and MgV0.05 powders give a detailed insight
on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During
the early stage of hydriding (and deuteriding), a MgH1<x<2 phase is observed, which does not occur in bulk
MgH2 and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this
phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of
nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst
is present as NbH1. Second, a hitherto unknown Mg−Nb perovskite phase could be identified that has to
result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the
diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2−20 nm.
Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature
variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH2
layers.