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 I mol% Nb2O5 by ball milling for 20 h desorbed hydrogen up to similar to 6 mass% in the temperature range from 200 to 250 degrees C at the heating rate of 5 degrees C/min under a purified helium flow. After dehydrogenation at 200 degrees C, the product showed remarkable hydrogen absorption kinetics. A large amount of gaseous hydrogen up to similar 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. (c) 2006 Elsevier B.V. All rights reserved.