The data collected in the present work extend the measured phase diagram for palladium hydride and palladium deuteride to a region that has been sparsely reported in open literature. Absorption isotherms were measured using a 2.5-g bed of palladium powder at temperatures between 130 and 393 K and pressures less than 1.3 × 105 Pa. Such low-pressure and low-temperature measurements are useful for characterizing palladium beds used for tritium pumping and storage. For tritium storage, pressures are kept below a few millibars for safety reasons. Low temperatures increase the tritium storage capacity of palladium.
The measured absorption isotherms show the well-documented, two-phase behavior for this system: two solubility regions and a mixed, hydride-forming region. The isotherms show that an increased quantity of hydride is formed at lower temperatures, as marked by an increase in the hydride-forming region. This region exceeds hydrogen-to-metal ratios of 0.75 for T ≤ 273 K. Equilibrium pressures in the mixed region decrease with decreasing temperatures until a critical temperature is reached for each isotope. Below these critical temperatures, the rate of pressure decrease with decreasing temperature is significantly reduced. This change in trend suggests hydrogen isotopes are adsorbed onto the palladium surface, rather than forming a hydride. Using the equilibrium pressures recorded at temperatures between 236 and 393 K for protium and between 211 and 354 K for deuterium, the van’t Hoff constants were calculated to be A = −36 ± 1 kJ/mol and B = 88 ± 3 J/K for protium and A = −32 ± 2 kJ/mol and B = 88 ± 9 J/K for deuterium. These constants agree favorably with literature in the range where the temperatures of the measured isotherms overlap.