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.