Hydriding kinetics of LaNi₅ and LaNi_4.7Al_0.3

The hydriding kinetics of LaNi₅ and LaNi_4.7Al_0.3 were investigated at a constant pressure. In order to avoid the effect of the heat evolved in the hydriding reaction and to obtain the intrinsic kinetic data, the samples were mixed with a large amount of Al powder and a Cu tube reactor with good thermal conductivity was used. The dependence of the hydrogen absorption rate on the reacted fraction, F, and the applied hydrogen pressure shows that the dissociative chemisorption of hydrogen molecules on the sample surface is the rate controlling step up to F = 0.6-0.8 and F = 0.4-0.5 at the low applied hydrogen pressure for the hydriding reaction of LaNi₅ and LaNi_4.7Al_0.3 respectively. At the high applied hydrogen pressure, the rate controlling step is the nucleation and growth of the hydride phase at the site of the hydrogen chemisorption. At a later stage of the hydriding reaction, irrespective of the applied hydrogen pressure, the hydrogen diffusion through the hydride phase is the rate controlling step. In both alloys, the rate controlling step has changed from the dissociative chemisorption of hydrogen at the surface or the nucleation and growth, to the hydrogen diffusion through the hydride phase. The transition of the rate controlling step takes place at a much earlier stage in the hydriding reaction of LaNi_4.7Al_0.3 than LaNi₅. This is caused by the slower diffusion rate of hydrogen atoms through the hydride phase in LaNi_4.7Al_0.3 than in LaNi₅ particles.