MECHANISM OF HYDROGEN ABSORPTION BY LANI5

A model is presented for the mechanism of absorption (or release) of hydrogen by LaNi5. The surface of LaNi5 is extensively coated by La2O3. The surface oxide forms in two ways: (1) by reaction of gaseous oxygen with LaNi5 or (2) by diffusion of dissolved oxygen to the surface where it reacts with LaNi5 to form oxide and precipitate Ni, some of which is also at the surface. Hydrogen reaches the underlying LaNi5 by diffusion in monatomic form along the La2O3-Ni interface. The dissociation of H2 at the point of entry into the interfacial region is rate controlling for absorption. Release of hydrogen involves diffusion of monatomic hydrogen along the interface followed by recombination to form molecular hydrogen at the top of the interfacial region. The latter is rate controlling. The model is in accord with the known surface features of the intermetallic and is consistent with the observed kinetics of absorption and desorption of H2 by LaNi5. It is also consistent with the kinetics of hydrogenation of C2H4 over LaNi5 and LaNi5H2.4. In the latter, interfacial diffusion of monatomic hydrogen is rate controlling. The present model differs from earlier models which regarded either the phase transformation or mass transport through macroscopic cracks or fissures as the rate-controlling step. © 1979 American Chemical Society.