CATALYTIC ROLE OF MG2CU IN THE HYDRIDING AND DEHYDRIDING OF MG

The eutectic alloy Mg/Mg2Cu was studied at H2 pressure such that only the Mg phase is hydrided and the Mg2Cu phase had a catalyzer role only. This alloy was chosen as a conve ient model system : a two phase alloy with one phase a catalyzer and the second a hydride former. From the t, T, P, behavior of the hydriding/dehydriding kinetics it is concluded that hydriding is rate-limited by the diffusion of through a growing MgH2 layer; dehydriding by the diffusion of through a growing Mg layer. These slow, intrinsic kinetics are shown to be primarily due to small concentration gradients because of limited solubility rather than to small diffusivities. It is concluded that the catalytic role of the Mg2Cu phase is the providing of an unoxidized external surface because its oxides are reduced by the H2. It is speculated that an additional catalytic role may be the providing of an unhydrided external surface. This catalytic mechanism is generalized to explain the activation process in the alloys Mg2Ni, TiFe and LaNi5. © by Akademische Verlagsgesellschaft