Kinetics of hydrogen absorption and desorption in LaNi 5‐x Al x slurries

The kinetics of hydrogen absorption and desorption in LaNi 4.8 Al 0.2 ,LaNi 4.9 Al 0.1 and LaNi 5 , suspended in cyclohexane and LaNi 5 in ethanol have been investigated. The absorption process can be described in terms of mass transfer and reaction resistances in series. The rate‐limiting steps for this process are dissolution of hydrogen in the solvent and the reaction of hydrogen with the metal alloy. Over a broad range of hydrogen loadings, the reaction of hydrogen with the metal follows the rate equation:\documentclass{article}\pagestyle{empty}\begin{document}$$ R_{{\rm abs}} = k_a a_s \left({C_{{\rm H}_{2,S}} - C_{{\rm eq,}a}} \right) $$\end{document}This type of kinetic equation indicates that during hydride formation in a slurry a surface process is the rate‐determining step, which was confirmed by the observed influence of the solvent. Desorption of hydrogen can be described best with a shrinking core relation:\documentclass{article}\pagestyle{empty}\begin{document}$$ 1 - \left({1 - X} \right)^{1/3} = k_d ln\left({\frac{{P_{eq,d}}}{{P_{H_2}}}} \right)t $$\end{document}The phase transformation at the β/α boundary appears to be the rate‐determining step. Alloys with higher aluminum contents produce a lower reaction rate constant.