Abstract

Absorption isotherms for activated LaNi 5 H 2 have been determined in detail at 273 and 195 K. The variation of the two-phase plateau pressure with temperature has been measured from 174 to 273 K and extrapolates well with higher temperature data determined elsewhere; these data yield ΔH α β = −15.0 kJ/mol of H. Hysteresis loops have been examined in the two-phase coexistence region. Excess hydrogen sorption has been determined as a function of the equilibrium hydrogen pressures by differences between sorption by activated and unactivated (low surface area) samples. The enthalpy for the excess sorption has been found to be approximately −30 kJ/mol of H and becomes less exothermic with extent of sorption. The amounts of the excess sorbed hydrogen are larger than anticipated from the measured surface area (BET, krypton, 77.4 K) in order for excess sorption to correspond to chemisorption; nonetheless, arguments are presented to suggest this correspondence.

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