Hydrogenation and dehydrogenation behavior of LaNi5-xCOx (x = 0, 0.25, 2) alloys studied by pressure differential scanning calorimetry

The hydrogenation and dehydrogenation behavior of LaNi5, LaNi4.75CO0.25 and LaNi3Co2 was studied by the pressure differential scanning calorimetry (PDSC) at the hydrogen pressure range of 1 to 5 MPa in the temperature range from 323 to 473 K with the heating and cooling rates of 2 to 30 K min(-1). In the heating run of the hydride of LaNi5, two endothermic peaks were observed. One was the peak for the transformation from the gamma phase (full hydride LaNi5H6) to the beta phase (hydride LaNi5H3). The other was the peak for the transformation from the beta phase to the alpha phase (solid solution). In the cooling run, one exothermic peak for the transformation from the a phase to the gamma phase was observed. These endothermic and exothermic peaks shifted to higher temperatures with the increase in hydrogen pressure. In the heating and cooling runs of the LaN4.75CO0.25-H-2 system the PDSC curves similar to those of the LaNi5-H-2 system were observed. However, in the heating run of the hydride of LaNi3Co2 only one endothermic peak was observed. Using Ozawa's method, the activation energies for dehydrogenation of the hydrides were estimated. The activation energy for the gamma-beta transformation was higher than that for the beta-alpha transformation. Substitution of cobalt for a part of nickel in LaNi5 increased the activation energies for the phase transformations.