Catalytic effect of niobium oxide on hydrogen storage properties of mechanically ball milled MgH₂

We examined a catalytic effect of niobium oxide (Nb₂O₅) on the hydrogen storage properties of MgH₂ prepared by mechanical ball milling method. The MgH₂ composite doped with 1 mol% Nb₂O₅ by ball milling for 20 h desorbed hydrogen up to ∼6 mass% in the temperature range from 200 to 250 °C at the heating rate of 5 °C/min under a purified helium flow. After dehydrogenation at 200 °C, the product showed remarkable hydrogen absorption kinetics. A large amount of gaseous hydrogen up to ∼4.5 mass% was absorbed even at room temperature under 1 MPa hydrogen pressure within 15 s and finally its capacity reached up to 5 mass%. Furthermore, the valence state of Nb₂O₅ doped in MgH₂ was examined by X-ray absorption near edge structure (XANES) measurement. The results indicated that additive Nb₂O₅ was reduced by MgH₂ during mechanical milling. This suggests that the Nb compound, in which the valence state of Nb atom is less than 5+, acts as a catalyst for the hydrogen absorbing/desorbing kinetics.