Palladium-silver alloys hydrogen permeability

Uemiya, S., T. Matsuda, and E. Kikuchi, Hydrogen permeable palladium-silver alloy membrane supported on porous ceramics, /. Membr. Sci., 56, 315,1991b. 

The palladium-silver alloy membrane system was successfully commercialized in the early 1960s [12], but the reduction of palladium content by the addition of silver would still not be a cost-effective alternative for large-scale processes [42] unless micron-scale films could be prepared, a goal currently being addressed by many researchers. In recent years, the Pd-Cu system has been the most heavily investigated alloy for hydrogen membrane applications due to the high permeability of select alloys [67, 90, 91], enhanced mechanical properties [92] and reported chemical resistance. The elevated permeability identifled for select Pd-Cu alloys is attributed to an increase in both the solubility and diffusivity of the B2 crystalline phase [86-88] as compared to the face-centered-cubic (fee) phase that exhibits permeability values proportional to the Pd-content [89, 91, 93]. 

For the preferred composition of coatings of palladium-silver alloys, the patent of Hunter [36] is referenced by Makrides et al.. In the Hunter patent, maximum hydrogen permeability through membranes of Pd-Ag was obtained using a composition containing 27 wt% silver. 

The palladium-silver alloys have attractive enhanced permeability compared to pure Pd, while the palladium-copper alloys are more resistant to sulfnr. (The hydrogen... 

The membrane module has a plate-type structure 40 mmW x 460 mmL x 8 mmT in size. Figure 12.2 illustrates the configuration of the membrane module, and Fig. 12.3 shows a view of the membrane modules. The membrane modules consist of palladium-rare earth alloy thin film with thickness of less than 20 pm and a porous structural support. The hydrogen permeability of the membrane is several times higher than that of the widely used conventional palladium-silver alloy membrane (Sakamoto, 1992). 

In palladium-silver alloys, larger amounts of hydrogen can be solubilised because the metal lattice has already been expanded by the silver atoms, and thus it is less brittle than the pure Pd lattice. Based on these considerations, the ideal silver content of the Pd cathode is between 20 wt% and 30 wt%. In order to obtain high flow rates of pure hydrogen, the maximum permeability of the Pd-Ag cathode must be realized. The hydrogen permeability P through the lattice of the Pd-Ag tubular electrode follows Sieverts law. The H2 permeating flow rate Qp (mol s ) can be derived from Equation [16.2] ... 

The most usable binary palladium alloy disclosed up to now is that of palladium with 25 at.% silver. Some other metals of groups IB, IV, and VIII of the periodical table and the rare earth elements have been studied as a second component of palladium alloys for hydrogen separation. Table 2 [6] lists the hydrogen permeabilities of miscellaneous binary palladium alloys at 623 K as compared with that of palladium. 

McKinley studied the effects of poisoning by H2S on membrane materials of pure palladium, and alloys of palladium with silver, nickel, copper and gold [74]. Only the Pd-Au alloys showed reasonable resistance to poisoning by hydrogen sulfide according to McKinley [74]. An alloy of 60 wt% Pd and 40 wt% Au retained 80% of its initial (unpoisoned) permeability upon exposure to 4 ppm H2S and retained 40% of its permeability during exposure to 20 ppm H2S at 623 K (350 °C). In contrast, pure palladium was poisoned to 30% of its initial, unpoisoned value after 6 days exposure to only 4 ppm H2S. An alloy of 73 wt% Pd and 27 wt% Ag was completely poisoned within hours after exposure to only 4 ppm... 

The introduction of other metals to form palladium based alloys has had promising results. In particular doping of the palladium with silver has been shown to improve the stability of the film and increase the solubility of hydrogen. Further, the temperature above which the a palladium hydride occurred was lowered with increasing silver content (Uemiya et al.,1991 Kikuchi Uemiya, 1991). The hydrogen permeability was optimized when the silver content of the alloy was aroimd 23 wt%. Silver occupies interstitial sites in the palladium lattice and so moderates the lattice expansion and contraction due to hydrogen absorption/desorption. 

Dense metal membranes exhibit an absolute permeability to specific species. Clear examples are given by palladium and palladium-alloy membranes, which are exclusively permeable to hydrogen, and by silver membranes. 

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