(620a) Properties of Palladium-Gold Alloys for Hydrogen Separation and Purification

Dense palladium membranes are able to separate hydrogen from mixed-gas streams via a solution-diffusion mechanism. While they have high hydrogen permeability and theoretically infinite selectivity, commercial application has been limited due to the metal's sensitivity to poisoning by common feed stream contaminants like sulfur and carbon monoxide, as well as the fact that destructive hydride phase formation can occur at temperatures below 573 K. In order to alleviate these issues, substantial research has gone into alloying other metallic elements with palladium. As early as 1967, it was demonstrated that alloys of palladium with up to 20 wt% gold had higher hydrogen permeabilities than pure palladium, and that membranes containing 40 wt% gold were more sulfur tolerant than copper or silver alloys[1]. The focus of this research is to optimize the fabrication technique and alloy composition for the Pd-Au alloy system.

Two fabrication techniques were used to produce membranes: sequential electroless plating and cold-working. The sequentially plated membranes were produced on porous media to examine poisoning and inhibition, as well as in the form of freestanding foils to examine annealing behavior. Cold-worked self-supported membranes were also studied in order to examine fundamental properties of the Pd-Au system. Sequential electroless plating and cold-working each have different advantages. Cold-worked membranes have precise composition control and a homogeneous initial alloy composition across the membrane thickness, but require large capital equipment investments to produce, and are presently generally limited to >25 microns in thickness for widths greater than 5 cm. Sequential electroless plating can be used to make thinner membranes and requires very little capital investment, but produces membranes with a layered, inhomogeneous structure.

Cold-worked membranes with 0-20 wt% Au were permeation tested at temperatures ranging from 473-773 K, and characterized with XRD and SEM/EDS. Membranes containing 5 wt % gold or more were found to be stable under hydrogen permeation conditions for up to 100 hours at temperatures as low as 473 K. At moderate hydrogen pressure gradients (under 220 kPa), the 15 wt% Au alloy had the highest hydrogen permeability across the range of temperatures tested, whereas at higher pressures, optimum permeability became a strong function of temperature. All gold-containing membranes demonstrated hydrogen permeation-driven lattice expansion on the feed side, with a simultaneous lattice contracture on the permeate side.

Composite membranes were produced by sequential electroless plating on 0.2 micron porous alumina that were able to separate a water-gas shift mixture (51% hydrogen, 26% carbon dioxide, 21% steam, and 2% carbon monoxide) and produce a 99.95% pure hydrogen permeate stream with 76% hydrogen recovery. The PdAu membranes had no CO permeation inhibition and less than half of the flux reduction due to sulfur of PdCu membranes with similar thickness.

Self-supported, sequentially plated foils were examined by XRD, both under ambient conditions and at high temperatures and controlled atmospheres. It was determined that stable gold-enriched surface phases persisted at temperatures up to 1023 K. These phases may act to suppress hydrogen permeability, though the suppressive effect is negligible if the amount of surface gold is small.

References

[1] McKinley, D.L., Metal Alloy for Hydrogen Separation and Purification, U.S. Pat. No. 3,350,845,1969.