Thermodynamic properties for solution of hydrogen in Pd-Ag-Ni ternary alloys

Abstract

The hydrogen absorption characteristics of well-annealed Pd 100−2 x Ag x Ni x ternary solid solution alloys with x (at.%) = 1.5, 3.0, 4.5, 6.0 and 7.5 have been investigated at temperatures between 273 and 433 K at hydrogen pressures up to 1000 Torr (133.32 kPa) by means of pressure-composition isotherm measurements and X-ray measurements of the lattice parameters of hydrogen-free and hydrogenated alloys. The room temperature lattice parameters of hydrogen-free ternary alloys decrease slightly with increasing solute metal content. The relative partial molar enthalpy ΔH o H at infinite dilution for the Pd-Ag-Ni alloys becomes more exothermic with solute metal content, despite the lattice contraction of the alloys. The behaviour is near the mid-point between those of Pd-Ag and Pd-Ni binary alloys. The partial molar entropy ΔS° H( β = 1) at infinite dilution decreases with solute metal content. The exceptional behaviour of the ΔH° H values in the ternary alloys for the hydrogen absorption of Pd alloys on the basis of the lattice ‘expanded’ or ‘contracted’ categories may be attributed to the preferential occupation of Pd-rich interstices near Ag atoms rather than near Ni atoms in the alloy solid solutions. The β-hydride in Pd-Ag-Ni ternary alloys becomes more unstable with increasing solute metal content, and the standard free energy change ΔG° plat at 298 K for the hydride formation is nearly between that for each binary alloy.