THE EFFECT OF ANNEALING PRETREATMENT OF PD-RH ALLOYS ON THEIR HYDROGEN SOLUBILITIES AND THERMODYNAMIC PARAMETERS FOR H-2 SOLUTION

Pressure-composition hydrogen isotherms in the temperature range 273 l ess-than-or-equal-to T less-than-or-equal-to 343 K have been determine d 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 ra tes: spin casting, fast quenching, slow quenching, furnace cooling (25 0-degrees-C h-1) and prolonged annealing at 873 K; this order of cooli ng leads to an increasing extent of inhomogeneity. The dilute phase hy drogen solubilities are found to increase with increasing alloy inhomo geneity. The isotherms in the dilute region for the spin-cast alloy ar e the same as for the fast-quenched one. The partial molar enthalpy wi th DELTAH(H)-degrees at infinite dilution is less exothermic for the f ast-quenched alloys as compared with the slow-quenched and furnace-coo led ones, but the enthalpy change for the plateau reaction, DELTAH(pla t), does not change. Generally, for all these Pd-Rh alloys the hydroge n solubilities decrease with increasing Rh content and the relative pa rtial molar enthalpy at infinite dilution, DELTAH(H)-degrees, and the enthalpy for hydride formation, DELTAH(plat), become less exothermic w ith increasing Rh content.