Calorimetric enthalpies of absorption and desorption of protium and deuterium by palladium

Abstract

In this research enthalpies were measured calorimetrically for the reaction of gaseous hydrogen (protium or deuterium) with palladium over a range of hydrogen contents from [H(D)] [Pd] = 0 to [H(D)] [Pd] ≈ 0.77 (at 298 K). A twin-cell, heat leak differential calorimeter was employed for these measurements. Results for the hydride formation enthalpies are − 19.1 kJ (mol H) −1 and − 17.3 kJ (mol D) −1 at 298 K. Enthalpies of essentially the same magnitudes, i.e. ± 0.1 kJ, are obtained from the decomposition of these hydrides. Entropies were determined from the calorimetrically measured enthalpies and the geometric mean of the plateau pressures for hydride formation and decomposition. The magnitudes of these entropies are 46.3 J (mol H) −1 K −1 and 46.7 J (mol D) −1 K −1 using the calorimetric enthalpies of absorption; they agree very well with values determined in the literature from van't Hoff plots of the plateau pressures. For the first time relative partial molar enthalpies have been obtained by calorimetry for the β-phase of Pd-D. The magnitudes of these enthalpies decline markedly with increasing deuterium content. This largely accounts for the marked increase in μ D with deuterium content and the concomitant difficulty in obtaining high values of [D] [Pd] .