A sorption rate hypothesis for the increase in H2 permeability of palladium-silver (Pd–Ag) membranes caused by air oxidation

Abstract Hydrogen permeation measurements were performed at 300 °C for 25-μm cold-rolled Pd–Ag 25 wt% membranes before and after air oxidation at the same temperature as permeation. The air oxidation resulted in enhanced H 2 permeation through the membrane, as well as a roughening of the surface with the formation of surface grains and defects. The protruding grains can be leveled off by exposure to H 2 but the surface defects cannot. These microstructure changes are only on the membrane surfaces and do not create transmembrane defects that would allow permeation for gas species other than H 2 . The H 2 permeability of the oxidized membrane increased by 25–90% compared to that of the as-received film at the same permeation condition, and the membranes retained perfect H 2 selectivity over N 2 . The percent improvement of H 2 permeability decreases with increasing H 2 feed pressure. A new sorption kinetics hypothesis is proposed to elucidate the increase in H 2 permeability of Pd–Ag membranes caused by oxidation. H 2 solubility and sorption rate results were presented to test the new hypothesis. It is found that air oxidation does not change the H 2 solubility in Pd–Ag membranes, but enhances the H 2 sorption kinetics significantly. The extent of kinetics enhancement also decreases with increasing H 2 pressures. The much faster sorption equilibrium implies higher effective H 2 diffusivity at the Pd–Ag membrane surface for the oxidized sample and a higher transfer rate of atomic hydrogen from surface/sub-surface to the membrane bulk that contributes to the increase of H 2 permeability observed in experiments.