Palladium Alloys for Hydrogen Diffusion Membranes

Palladium holds a unique position among the metallic elements in being able to take into solution large quantities of hydrogen while simultaneously retaining a high degree of ductility. These attributes coupled with the high mobility or rate of diffusion of hydrogen in the lattice have been exploited in the use of palladium and subsequently of palladiumbased alloys as hydrogen diffusion membranes. Earlier articles in this Journal (I, 2) have described the application of this principle to commercial diffusion units for the production of high purity hydrogen, and more recently to a hydrogen generator for military requirements (3). In these applications the permeability of the palladium alloy to all other gases at the operating temperatures is so low as to be negligible in practice, and the alloy therefore functions as a highly specific filter for the production of ultra-pure hydrogen, or for removing hydrogen from mixed process gases. A limitation to the use of pure palladium for hydrogen diffusion has its basis in the pressure-concentration isotherms for the palladium-hydrogen system shown in Figure I (4). At temperatures below 300°C and pressures below 20 atm, increasing the hydrogen concentration leads to the formation of the p phase which can coexist with the 01 phase. The (3 phase has a considerably expanded lattice compared with u, for example a hydrogen/palladium ratio of 0.5 results in an expansion of about 10 per cent by volume. Nucleation and growth of the in the a matrix therefore sets up severe strains in the material resulting in distortion, dislocation multiplication and hardening. After a few hydrogenation/dehydrogenation cycles splitting of the diffusion membrane may occur. One method whereby the phase change can be avoided is to ensure that the palladium diffusion membrane is always operated in the single phase region of the Pressure-Composition-Temperature diagram (5). This may be achieved by maintaining the temperature above the critical value of 300°C as long as the membrane is in a hydrogen atmosphere, or by ensuring that cooling is allowed only when it is in a dehydrogenated condition with the hydrogen pumped from the system. Such expedients do not, of course, avoid the volume changes that inevitably occur, but in a single phase region the composition varies smoothly and the distortion phenomena associated with nucleation and growth are circumvented. With these limitations in mind it is nevertheless possible to operate pure palladium membranes successfully for the large scale separation of hydrogen from mixed gases. Diffusion installations in the Union Carbide Corporation with individual outputs of 9 million cu ft/day have been described (6).