Effects of heat treatment in air on hydrogen sorption over Pd–Ag and Pd–Au membrane surfaces

Abstract

Abstract The effects of heat treatment in air on H 2 sorption over the free-standing 25-μm cold-worked Pd–Ag25 wt% and Pd–Au10 wt% membrane surfaces were investigated. The equilibrium sorption results demonstrated that H 2 solubility did not change for both Pd–Ag and Pd–Au membranes before and after air oxidation at 300 °C. The sorption kinetics data revealed a much faster H 2 sorption rates for the as-received Pd–Au membrane than the as-received Pd–Ag alloy. The heat treatment in air accelerated the H 2 sorption kinetics for both membranes at small H 2 equilibrium pressure (EP) of 0.16 kPa. However, the oxidation enhancement extent is greater for Pd–Ag and the disparity becomes more significant when EP increases. The sorption rate enhancement for Pd–Ag membrane existed over a wide range of EPs even though the enhancement effect decreased with increasing EPs. For Pd–Au membrane, the enhancement induced by air oxidation decreased sharply when EP increased because the sorption rate is already very fast for the as-received Pd–Au alloy and there is less latitude for further improvement than Pd–Ag membrane. It is found that the variation of H 2 sorption rates over membrane surfaces correlates to the change of H 2 permeability. After heat treatment in air, the Pd–Ag membrane experienced a systematic increase in H 2 permeability with higher permeability increase at lower pressures while there was basically no permeation increase due to air oxidation effect for the Pd–Au membrane within the pressure range of practical H 2 permeation tests, which are consistent with the previously reported sorption rates hypothesis and results.