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 ν̃_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⁻¹⋅mol⁻¹) and of deuterium (6.91 K⁻¹⋅mol⁻¹) is in excellent agreement with the value calculated from ν̃_H and ν̃_D=ν̃_H/√2.