THE EFFECT OF THE PARTIAL REPLACEMENT OF LANTHANUM IN LANI5-H WITH CERIUM, PRASEODYMIUM, AND NEODYMIUM ON ABSORPTION AND DESORPTION-KINETICS

A study was done to compare the absorption and desorption kinetics of a series of La0.8R0.2Ni5 hydrides (R ≡ Ce, Pr, Nd) with those of LaNi5-H. Measurements were done under essentially isothermal and isobaric conditions. Experiments were also designed so that the ratio of the equilibrium plateau pressure (Peq) to the opposing pressure (Pop) was the same in all cases. Since it was determined that the difference in chemical potential between the metal hydride phase and the gaseous hydrogen is directly proportional to ( Peq Pop), it was concluded that these experiments were carried out under constant thermodynamic driving forces. It was found that under identical reaction conditions, partial substitution of the lanthanum in LaNi5 with cerium, praseodymium, or neodymium generally increases reaction rates. It was also found that the hydriding rates of the L.a0.8R0.22Ni5 alloys varied with R as follows: ≡ Ce > Nd > Pr > La. Since the stability of these hydrides is in the reverse order, it appears that in alloys having similar crystallographic and electronic properties, reaction rate is inversely proportional to hydride stability. © 1990.