Reduction of hydrogen desorption temperature of ball-milled MgH2 by NbF5 addition

Enhanced sorption properties of ball-milled MgH2 are reported by adding NbF5. Among various catalyst amounts, 2 mol% of NbF5 reveals to be the optimum concentration leading to significant reduction of the desorption temperature as well as faster kinetics of ball-milled MgH2. At 200 degrees C. temperature at which Mg-H-2 does not show any activity, MgH2NbF5/2 mol% composite desorbs 3.2 wt.% of H-2 in 50 mins. Interestingly, the addition of NbF5 is also associated with an increase in the desorption pressure. At 300 degrees C, MgH2NbF5/2 mol% composite starts to desorb hydrogen at 600 mbar in comparison with 1 mbar for MgH2. Further improvements were successfully achieved by pre-grinding NbF5 prior to ball-milling the catalyst with MgH2. Such pre-ground NbF5 catalyzed MgH2 composite desorbs 3 wt.% of H-2 at 150 degrees C. Improved properties are associated with smaller activation energies down to values close to the enthalpy of formation of MgH2. Finally, the mechanism at the origin of the enhancement is discussed in terms of catalyst stability. MgF2 formation and electronic density localization. (C) 2007 Elsevier B.V. All rights reserved.