Palladium-silver alloys hydrogen solubility

G.L. Holleck, Diffusion and solubility of hydrogen in palladium and palladium>silver alloys, J, Phys. Chem, 74 503 (1970). 

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]. 

Holleck, G.L. (1970) Diffiision and solubility of hydrogen in palladium and palladium-silver alloys. The Journal of Physical Chemistry, 74, 503-511. 

Silver and gold have an interesting influence. Addition of silver to palladium at first increases the solubility of hydrogen despite the fact that hydrogen is insoluble in pure silver. The maximum solubility is reached with 40 per cent, of silver, after which it falls. At 138° C. an alloy containing 40 per cent, of silver and 60 per cent, of palladium absorbs four times as much hydrogen as pure palladium. With 70 per cent, of silver the solubility of hydrogen is reduced to zero. 

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. 

A. Sieverts, E. Jurish, A. Metz, Solubility of hydrogen in sohd alloys of palladium with gold, silver, platinum, Z. Anorg. Chem. 91 (1915) 1 5. 

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