Thermodynamics of hydrogen absorption by PdSb and PdBi alloys

Abstract

The thermodynamic parameters for absorption of hydrogen by PdSb and PdBi solid solution alloys containing up to 10.0 at.% Sb and Bi have been determined from measurements of pressure-composition isotherms at temperatures between 273 and 433 K and hydrogen pressures up to 1000 Torr. The hydrogen-free Pd lattice is expanded by additions of Sb and Bi, and at the same alloy composition the lattice expansion of PdBi alloys is larger than that of PdSb alloys. The dilute phase solubilities in both alloys, as relected by the relative chemical potential of dissolved hydrogen at infinite dilution, Δ μ H 0, increases with increasing Sb and Bi contents; where the solubilities in PdBi alloys are larger than those in PdSb alloys. The relative partial molar enthalpy, ΔH H 0, at infinite dilution in both alloy systems increases in exothermicity with increasing Sb and Bi contents, and at the same solute metal content, the ΔH H 0, values for PdBi alloys are more exothermic. The standard free energy change, ΔG plat 0, for β-hydride formation in PdBi alloys decreases with Bi content, whereas the ΔG plat 0 value for PdSb alloys increases with Sb content, although only by a small amount, despite the fact that the alloys have expanded lattices compared to Pd. The higher pressure solubilities in both alloys decrease with increase of the solute metal contents.