Temperature dependence of hydrogen isotope effect in LaNi5-H(D) system

To study inverse effect of hydrogen isotopes in LaNi₅ for hydrogen isotope separation at ambient temperature, temperature dependence of hydrogen isotope effect in LaNi₅－H(D) system was investigated in the temperature range -70 °C to 3 °C. Isotherms of hydrogen isotopes were tested in an automated Sieverts apparatus with a thermostat, and pressure differences were studied between H₂ and D₂. Isotope exchange process was carried out and H₂－HD－D₂ equilibrium was judged by online chromatography, separation factors were calculated to investigate the thermodynamic isotope effect. In the H₂, D₂ isotherms, average time interval (0.5h) between each equilibrium point was relatively short due to high precision temperature control. From the plateau pressures at a constant hydrogen (or deuterium) concentration in LaNi₅ (2D or 2H atoms/mole of LaNi₅) the partial molar enthalpies, ⊿H, and entropies, ⊿S of formation were calculated. ⊿H and ⊿S kept good accordance with the values from references, which indicated good performance of the Sieverts apparatus. The isotope exchange equilibrium was reached within 1h. The results showed that ratio between H₂ pressure and D₂ pressure, r(H－D) = P(D₂)/P(H₂), dropped as temperature decreased and the inverse effect was found (r(H－D)<1.0) below -5 °C according to the absorption branch of isotherms, whereas the effect was estimated to appear below 60 °C from the desorption branch. The separation factor of the H₂－HD－D₂ mixture, decreased as temperature increased and the inverse effect happened below -20 °C. The inverse effect was affected by the isotherm hysteresis and HD existence.