Hydrogen isotherms over a wide temperature range for Pd and for Pd/oxide composites formed by internal oxidation of Pd–Al(Y) alloys

Abstract

Isotherms have been measured for internally oxidized Pd–M alloys where M=Al or Y. After internal oxidation the alloys become composites of nanosized oxide precipitates within Pd matrices. Following the internal oxidation (1073 K) of the alloys H 2 isotherms are closely identical to those of Pd–H, however, differences develop after hydriding/dehydriding (cycling) which increase with %Al (Y). H 2 isotherms for the cycled forms of Pd and a Pd/alumina composite have been measured and compared at 323, 473, 513 and 553 K. At the latter two temperatures the characteristic plateaux appear to disappear for the cycled composite although hysteresis is still present indicating hydride formation/decomposition. Large differences are also observed between the isotherms for Pd and those for Pd composites at lower temperatures especially in the dilute phase and the two-phase region where conversion to the hydride phase is <50%. Similar behavior is found for a Pd/yttria composite obtained from internal oxidation of a Pd 0.98Y 0.02 alloy. The differences between H 2 isotherms for cycled Pd and internally oxidized (1073 K), cycled Pd–Al(Y) alloys are due to microstructural changes in the Pd matrix resulting from cycling in the presence of the small and closely spaced precipitates. Since internal oxidation at 1273 K results in larger and more widely-spaced precipitates than internal oxidation at 1073 K, it would be expected that after cycling the former alloys, their isotherms would be more similar to Pd–H than those internally oxidized at 1073 K. This has been confirmed experimentally.