A sorption rate hypothesis for the increase in H_2 permeability of palladium-silver (Pd-Ag) membranes caused by air oxidation

摘要

Hydrogen permeation measurements were performed at 300 ℃ for 25-jim 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 trans-membrane 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.