Absorption of Hydrogen by Iridium/Palladiuni Substitutional Alloys

Abstract

Thermodynamic parameters for absorption of hydrogen by Ir/Pd alloys have been determined by p, c, T techniques in an u.h.v. system and by electrochemical measurements. The equilibrium solubility of hydrogen in these alloys (1 atm, 25°C) decreases to low values at small contents of substituted iridium in comparison to other palladium-rich alloys which have been investigated. Formation of a second, non-stoichiometric hydride phase ceases above approximately 8 atomic percent of substituted iridium (25°C). Somewhat anomalous behaviour is observed for the values of the heat of solution at infinite dilution: ΔH°H=–10.04 kJ (g-atom H)–1 for Pd; –8.03 kJ (g-atom H)–1 for Ir(2.82 %)/Pd; –8.36 kJ (g-atom H)–1 for Ir(5.77 %)/Pd and –8.78 kJ (g-atom H)–1 for Ir(8.86 %)/Pd. Lattice parameters of the hydrogen-containing alloys are reported. The electrical resistances of these alloys have been measured as a function of their hydrogen-contents and the observed increases are greater than those reported for other palladium alloy systems.