Inhibition by gaseous impurities of hydrogen absorption by Pd and by internally oxidized Pd–Al alloys

Abstract

Absorption of H₂ is a sensitive technique for the study of the effects of impurity gases on H₂ chemisorption by Pd because chemisorbed is an intermediate for absorption. Since large amounts of hydrogen are absorbed by Pd, the absorption can be readily followed experimentally. The rates of H₂ absorption by Pd foil in the presence of He, CO₂ and CO gases are, in the order of increasing inhibition: He:CO₂:CO. Surprisingly CO, which is strongly chemisorbed on Pd, was not as strong a poison for H₂ chemisorption as anticipated. The effect of CO pre-adsorption followed by evacuation at 323 K did not affect the subsequent rates of H₂ absorption nearly as much as absorption from (CO + H₂) mixtures. For Pd foil, the simultaneous presence of H₂ and CO led to inconveniently slow rates, but for high surface area Pd such as Pd-on-kieselguhr or Pd powder, the rates from (H₂ + CO) mixtures were reasonably fast. After internal oxidation, Pd–Al alloys are converted into composites of alumina precipitates within Pd matrices. The rates of H₂ absorption have been shown to be faster for these composites than for Pd foil. In the presence of gaseous impurities the rates of H₂ absorption by internally oxidized Pd–Al alloys are faster than for pure Pd foil. The rates of H₂ absorption from (H₂ + CO) mixtures increase with % Al in the internally oxidized alloys and for alloys with Al greater than about 3%, the rates are quite fast.