A comparison of the hydriding and dehydriding kinetics of LaNI 5 hydride

Abstract

A comparison of the hydriding and dehydriding reactions of LaNi 5 hydride, in the two-phase region, has been made under essentially identical conditions. In both the absorption and desorption processes, the sample was maintained at approximately isothermal conditions by mixing it with a heat buffer. In addition, the hydrogen pressure in the system was adjusted such that the ratio of the equilibrium plateau pressure to the opposing pressure was the same for both the hydriding and dehydriding processes. Pressure ratios of 2, 3, 4 and 5 were used in each case. Since these factors, as well as the number of times that the sample had been subjected to hydriding-dehydriding cycles, were kept constant, it was possible to make valid comparisons of the intrinsic rates of hydriding and dehydriding. The rates of hydriding and dehydriding were found to differ substantially under the conditions used. In the case of hydriding, the reaction rate started out slowly, increased to a maximum, then decayed exponentially. In the case of dehydriding, the reaction rate decreased exponentially throughout. This suggests that in the early stages, absorption proceeds by a nucleation and growth process in which β-phase nuclei form on the surface. Once the nuclei have grown to the extent where overlapping occurs, the β phase forms a contracting envelope around the α phase and grows inwardly with a corresponding decrease in the α-β interface area. Dehydriding, however, can best be described throughout the entire reaction by a shrinking core model in which α-phase forms on the surface and grows inwardly. In both processes, the reaction rate seems to be controlled by phase transformation at the α-β interface. Activation energies were found to be 27 and 37 kJ mol −1 for the absorption and desorption process respectively, at a pressure ratio of 2. Also, since the first order rate constants for absorption and desorption were found to vary linearly with the logarithm of the pressure ratio, it is suggested that pressure ratios should always be specified when reporting kinetic data.