Particle size and catalytic effect on the dehydriding of MgH2

Conventional polycrystalline β-MgH2 was ball-milled under hydrogen-atmosphere to reach a nanocrystalline microstructure (∼10 nm), until the particle- and grain-size attained a constant value. Subsequently, 2 mol.% pre-milled Nb2O5 (∼40 nm) was added to the MgH2 as a catalyst and the milling was continued. The effect of the particle- and grain-size as well as of the milling duration on the temperature of hydrogen desorption and on the dehydriding kinetics of the ball-milled MgH2 + Nb2O5 powders was analyzed. The grain- and particle-size reduction decreases substantially the H-desorption temperature. The size-reduction occurs during the first 120 min of milling MgH2, but subsequent milling with the catalyst results in additional slight MgH2 particle size decrease, due to the harder Nb2O5 particles compared to the MgH2 ones. The decrease of the activation energy of dehydriding caused by the catalyst was observed after only 15 min of ball-milling. The catalytic effect is, however, suppressed at longer milling times, probably due to penetration of the catalytic particles into the MgH2.