Catalytic effect on hydrogen desorption in Nb-doped microcrystalline MgH2

Mg and Nb-doped Mg films were deposited by rf magnetron sputtering. Morphological and structural analysis were performed by scanning electron microscopy and x-ray diffraction. The desorption kinetics has been investigated by using a Sievert-type apparatus. The overall activation energy and the reaction order controlling desorption are (141+/-5) kJ mol(-1) H and napproximate to4 for Mg and (51+/-5) kJ mol(-1) H and napproximate to1 for Nb-doped (5 at. %) Mg. It is suggested that Nb atoms dispersed in the MgH2 grains catalyzes the dissociation of the hydride phase and that the rate limiting step in the H-2 desorption is given by the H atomic migration through interconnected transformed domains of h-Mg. (C) 2004 American Institute of Physics.