Measurement of Palladium Hydride and Palladium Deuteride Isotherms Between 130 K and 393 K

Abstract

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.

Keywords:

• Palladium
• hydrogen
• deuterium
• hydride

Acknowledgments

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under award number DE-NA0003856 and the University of Rochester and the New York State Energy Research and Development Authority. This report was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.