The effect of annealing pretreatment of Pd-Rh alloys on their hydrogen solubilities and thermodynamic parameters for H 2 solution

Abstract

Pressure-composition hydrogen isotherms in the temperature range 273 ⩽ T ⩽ 343 K have been determined for some Pd-rich Rh alloys with various degrees of homogeneity as a result of different cooling rates from above the miscibility gap. The following techniques have been employed to obtain different cooling rates: spin casting, fast quenching, slow quenching, furnace cooling (250 °C h −1) and prolonged annealing at 873 K; this order of cooling leads to an increasing extent of inhomogeneity. The dilute phase hydrogen solubilities are found to increase with increasing alloy inhomogeneity. The isotherms in the dilute region for the spin-cast alloy are the same as for the fast-quenched one. The partial molar enthalpy with ΔH H o at infinite dilution is less exothermic for the fast-quenched alloys as compared with the slow-quenched and furnace-cooled ones, but the enthalpy change for the plateau reaction, ΔH plat, does not change. Generally, for all these Pd-Rh alloys the hydrogen solubilities decrease with increasing Rh content and the relative partial molar enthalpy at infinite dilution, ΔH H o, and the enthalpy for hydride formation, ΔH plat, become less exothermic with increasing Rh content.