(288c) Sulfur Resistant Pd-Ag-Au Alloys for Hydrogen Separation
Separation of hydrogen from mixed gas streams is a key unit operation in the generation of carbon-neutral fuels and electricity from fossil- and bio-derived feedstocks. Pd-based membranes have received significant attention for the separation application in advanced coal gasification processes because of their near-perfect selectivity for hydrogen. In practice, pure Pd suffers from several limitations, including high cost, poor mechanical strength, and deactivation by minor components, most notably H2S. H2S can compromise membrane performance by reacting with Pd to form a low-permeability Pd4S scale on the membrane surface. Pd has been alloyed with minor components to improve structural properties, improve permeability (i.e., Ag) and impart resistance to H2S (i.e., Au). To achieve both high permeability and H2S tolerance, we are investigating PdAgAu ternary alloys.
In this work, we synthesized Pd83Ag2Au15 and Pd74Ag14Au12 alloys by sequential (Pd/Ag/Au) electroless plating onto stainless steel. After deposition, samples were annealed in H2 at 600°C for 48 hr to obtain homogeneous composition and achieve complete formation of the FCC solid solution. Both alloys and a 25 µm thick Pd foil reference sample were characterized by XRD, SEM-EDS and XPS before and after treatment in 1000 ppm H2S and H2 at 350°C for 3 and then 30 hours. As expected, the Pd reference sample displayed clear XRD evidence of bulk sulfide formation upon H2S exposure. A Pd4S phase started to form at 3 hr and grew with increasing exposure time. The alloys, in contrast, displayed no evidence of bulk sulfide formation. EDS measurements of bulk composition were consistent with the structural characterization results: after 30 hours of H2S exposure, the S content of the Pd foil was 20 at% (i.e., complete conversion to Pd4S), and no S was detected in either alloy. XPS depth profiling experiments revealed very weak S signals at the terminal surfaces of the H2S-exposed alloys, but those disappeared before a depth of 10 nm. The depth profiles also revealed co-segregation of Au and Ag to the surface of the alloy; preferential location of Au at the alloy surface may be related to its resistance to bulk sulfidation.
In a preliminary permeation test, a Pd75Ag12.5Au12.5 alloy delivered H2 fluxes comparable to those of pure Pd. This alloy foil was fabricated by flame melting pure metals and then rolling to a thickness of 100 µm. The permeance was measured over the range of 350 to 750°C using pure H2. Near future experiments will measure the alloy permeance in 1000 ppm H2S / H2 at several temperatures.