Thermodynamics of H in Disordered Pd−Ag Alloys from Calorimetric and Equilibrium Pressure−Composition−Temperature Measurements

In this research, the thermodynamics of H₂ solution and hydride formation in a series of disordered Pd−Ag alloys has been determined using both reaction calorimetry and equilibrium P_H2−composition−T data. Trends of ΔH_H and ΔS_H with both H and Ag concentration have been determined. For the Pd_0.76Ag_0.24 alloy, which does not form a hydride phase, ΔH_H and ΔS_H both exhibit minima with H/(Pd_0.76Ag_0.24) followed by a linear increase of the former. A linear increase of ΔH_H is found for all of the alloys in the high H content region beyond the two-phase region or, if, there is no two-phase region, in the high H content region. ΔH_H° at infinite dilution of H decreases with atom fraction Ag, X_Ag, up to about 0.40 and then increases. Enthalpies for hydride formation/decomposition, ¹/₂H₂(g) + dilute ↔ hydride, have been determined calorimetrically for alloys which form two phases (303 K). The enthalpies for hydride formation become more exothermic with X_Ag while the corresponding entropy magnitudes are nearly constant, 46 ± 2 J/K mol H.