PdAu/YSZ composite hydrogen separation membranes with enhanced stability in the presence of CO

Abstract

The effects of gold concentration on the carbon tolerance of palladium-based composite membranes was studied under the presence of high CO concentrations. Membranes with PdAu compositions up to 41% Au by mass were exposed to 50/50H2/CO gas atmospheres and both hydrogen flux and permeate purity were monitored over time. The highest gold concentration (41 wt%) had no measurable degradation in membrane performance in terms of either hydrogen permeate purity or hydrogen permeation flux during the 48 h exposure; in contrast, the pure palladium membrane suffered total degradation during the same exposure time. Furthermore, for lower gold concentrations, X-ray diffraction patterns suggested that carbon entered the membrane film and could be removed by exposure to hydrogen and a non-CO containing gas mixture over time; however, removal of the carbon adversely affected hydrogen permeate purity. Interestingly, some of the observed hydrogen permeate purity decline could be reversed upon re-exposure to the CO gas mixture. It is hypothesized that carbon loading in the palladium films causes lattice expansion and may even fill pore defects, thus lowering non-hydrogen leaks through the Pd membrane films. This study suggests that a membrane exposed to a CO mixture should not be cycled between CO and non-CO environments, but rather run continuously for best performance. Additionally, the choice of Pd-alloy metal and composition should be considered in regard to its carbon solubility to best predict lifetime performance.