High-temperature thermodynamics of palladium-hydrogen. I. Dilute solutions of H2 and D2 in Pd at 555 K

The thermodynamic properties of palladium-hydrogen and palladium-deuterium solutions have been studied by a combined calorimetric-equilibrium method at 555 K for n_H/n_Pd<0.01. For these dilute solutions the relative partial enthalpies and excess entropies of hydrogen are found to vary strongly with compostion, in a way which is not consistent with the known thermodynamic behavior of more concentrated alloys. From the limiting values of the partial enthalpies of solution for hydrogen (-2.27 kcal mol^-1) and for deuterium (-1.80 kcal mol^-1) we calculate from the Einstein model a vibrational frequency ν∼_H for hydrogen in palladium of 695 cm^-1, in reasonable agreement with the neutron scattering result for dilute solutions of Kley et al. (∼600 cm^-1), but differing significantly from the much lower value of 420 cm^-1 for PdH_0.6 found by Bergsma and Goedkoop. The difference between the limiting values of the partial excess entropy of hydrogen (5.15 cal K^-1ṡmol^-1) and of deuterium (6.91 K^-1ṡmol^-1) is in excellent agreement with the value calculated from ν∼_H and ν∼_D=ν∼_H/√2.