KINETICS OF HYDROGEN ABSORPTION AND DESORPTION IN LANI(5-X)AL(X) SLURRIES

The kinetics of hydrogen absorption and desorption in LaNi4.8Al0.2, La Ni4.9Al0.1 and LaNi5, suspended in cyclohexane and LaNi5 in ethanol ha ve been investigated. The absorption process can be described in terms of mass transfer and reaction resistances in series. The rate-limitin g steps for this process are dissolution of hydrogen in the solvent an d the reaction of hydrogen with the metal alloy. Over a broad range of hydrogen loadings, the reaction of hydrogen with the metal follows th e rate equation: R(abs) = k(a)a(s) (C(H2,s) - C(eq,a)) This type of ki netic equation indicates that during hydride formation in a slurry a s urface process is the rate-determining step, which was confirmed by th e observed influence of the solvent. Desorption of hydrogen can be des cribed best with a shrinking core relation: 1 - (1 - X)1/3 = k(d)ln(P( eq,d)/P(H2)t The phase transformation at the beta/alpha boundary appea rs to be the rate-determining step. Alloys with higher aluminum conten ts produce a lower reaction rate constant.