Palladium metal alloy membranes

F. N. Berseneva, 1. A. Mikhaylova, N. 1. Timofeyev, The influence of hydrogen on the phase composition and physico-mechanical properties of the membrane palladium-based alloy V-1, Phys. Met. Metall. 1992, 72(2), 151-156. 

MetaHic membranes. Among the metallic membranes, palladium and its alloys offer the most promise and are used and suidied most extensively. Metallic membranes often suffer from the problem of structural degradation after repeated cycles of adsorption and desoiption at an elevated temperature above, say, 200X. 

Composite metal membranes are most often the structure of choice when a reactive group 3-5 metal or alloy is the principle constituent of the membrane. The relative chemical reactivity of these metals dictates that an inert coating must be applied to at least the feed surface of the membrane. Palladium, or better yet a palladium alloy, customarily serves as the coating layer. If it can be guaranteed that the permeate side of the membrane will never be exposed to reactive gases (e.g., water, carbon oxides, and hydrocarbons), then a two-layer composite membrane is a satisfactory choice. However, normal operating procedures and the potential for process upsets typically favors the selection of a three-layer composite structure. 

If a thinner membrane is required, then one must choose a supported membrane. The permselective metal layer may be palladium or, more commonly, palladium-silver alloy, palladium-copper alloy, or other alloy of palladium. The permselective layer ranges in thickness from about 2-25 /an thinner than 2/rm is very difficult to achieve without introducing pin holes and other adverse defects into the permselective layer. The support layer is porous and is composed of either metal (such as sintered stainless steel or tightly woven wire cloth) or an inert ceramic alumina is very common. Since all of the mechanical strength is derived from the support layer, consideration must be given to its shape and thickness. 

The other approach to increase the electronic conductivity of these perovskite-based membranes is to add a metal phase (10-40 vol%). The metal phases studied include palladium, niobium, tantalum, vanadium and zirconium or their binary mixtures [69-72]. In order to nrmirtiize the stress at internal interfaces that can lead to the formation of dislocations and initiation of cracks, the ceramic support materials were chosen so as to be lattice matched to the metals and metal alloys [73]. 

The physical properties of the specific metal alloy used for the hydrogen permeable membrane strongly influence the sequence of steps required to roll foil and draw thin-waUed tube. If the alloy is either too hard or too soft, it will be difficult to work if the alloy work-hardens quickly, frequent annealing steps will be necessary. Pure palladium, Pd-23Ag, and Pd-40Cu have all been fabricated into thin foil and drawn tubes. The palladium alloys are somewhat easier to fabricate into... 

Fig. 2.10 Hydrogen flux through palladium alloy membranes against metal content [58]...

Non-palladium-based metal alloy materials are under development for reducing the cost, increasing the flux, and improving the durability of H2 separation membranes. 

Hydrogen dissolution in metals (group V metals, palladium, Pd-alloys) is a dissociative process leading to the formation of surface hydrogen adatoms that diffuses across the membrane with a diffusion coefficient Dh. Assuming that hydrogen behaves as an ideal gas, its chemical potential is ... 

Barbieri et al. [523] prepared palladium membranes by solvated metal atom deposition, a deposition method that created a 0.1-pm thick film of palladium on an alumina tube. Another membrane was prepared from a palladium/silver alloy (21 wt.% silver), which had a thickness of 10 pm. Both membranes suffered from pinholes and cracks and thus did not show infinite hydrogen selectivity. However, conversion exceeding the thermodynamic equilibrium could still be achieved for methane steam reforming at temperatures exceeding 400 °C. 

F.A. Lewis, K. Kandasamy, B. Baranowski, The uphill diffusion of hydrogen—strain-gradient-induced effects in palladium alloy membranes. Platinum Metals Review, 32... 

Knapton A. G., Palladium alloys for hydrogen diffusion membranes, Platinum Metals Rev., 21 (1977) 44-50. 

Alternatives to palladium in membranes for hydrogen separation nickel, niobium and vanadium alloys, ceramic supports for metal alloys and porous glass membranes... 

Metallic Palladium films pass H9 readily, especially above 300°C. Ot for this separation is extremely high, and H9 produced by purification through certain Pd alloy membranes is uniquely pure. Pd alloys are used to overcome the ciystalline instability of pure Pd during heat-ing-coohng cycles. Economics limit this membrane to high-purity apphcations. 

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

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