Kinetics of hydrogen absorption of Al-doped MmNi5

Using recently published models for the formation of metal hydrides with one rate determining partial reaction step together with absorption kinetics measurements performed on MmNi(4.7)Al(0.3) samples, we identified the limiting process of the reaction. Absorption kinetic was measured at several temperatures using the standard Sieverts technique on activated (powdered), surface-treated (Pd-covered) and intentionally oxidized samples and compared to the modeled pressure-time dependence to find the limiting reaction step in each case. For the activated and Pd-covered samples, we fmd diffusion to be the limiting process during the formation of metal hydride, whereas for the oxidized samples a surface limited partial reaction step seems to better explain the experimental results. This suggests that surface treatments only improve the kinetics up to a stage in which it is diffusion-limited The use of the proposed expressions to fit absorption experimental data showed to be a very useful tool to determine which is the true Limiting step in the absorption kinetics of a given system.