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

Absorption of H-2 is a sensitive technique for the study of the effects of impurity gases on H-2 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 H-2 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 H-2 chemisorption as anticipated. The effect of CO pre-adsorption followed by evacuation at 323 K did not affect the subsequent rates of H-2 absorption nearly as much as absorption from (CO+H-2) mixtures. For Pd foil, the simultaneous presence of H-2 and CO led to inconveniently slow rates, but for high surface area Pd such as Pd-on-kieselguhr or Pd powder, the rates from (H-2+CO) mixtures were reasonably fast. After internal oxidation, Pd-Al alloys are converted into composites of alumina precipitates within Pd matrices. The rates of H-2 absorption have been shown to be faster for these composites than for Pd foil. In the presence of gaseous impurities the rates of H-2 absorption by internally oxidized Pd-Al alloys are faster than for pure Pd foil. The rates of H-2 absorption from (H-2+CO) mixtures increase with % Al in the internally oxidized alloys and for alloys with Al greater than about 3%, the rates are quite fast.