Permeability hydrogen transport, metallic

In yet another variation, composite membranes are fabricated by sintering together powders of highly hydrogen permeable metals, Pd, Nb, Ta, Ti, V, Zr and their alloys, with powders of a second metal or alloy that is non-permeable to hydrogen [12]. The function of the non-permeable metal is to provide mechanical support for the hydrogen transport materials, especially if the latter are to be... 

In fact, although, niobium (Nb), vanadium (V) and tantalum (Ta) offer higher hydrogen permeability than palladium in a temperature range between 0 and 700°C, as shown in Fig. 2.8, nevertheless these metals give a stronger surface resistance to hydrogen transport than the palladium (Pd). For this reason, dense palladium membranes are preferentially used. 

Metallic membranes, (Pd-Ag) alloys, are typically used for separation of H2, either as an unsupported foil or a supported thin film. In these membranes, the hydrogen transport is by adsorption and atomic dissociation on one side of the membrane, dissolution in the membrane, followed by diffusion, and finally desorption (on the other side). Due to the H2 dissociation step, H2 separation is driven by a transmembrane difference of the square roots of the hydrogen partial pressures. The preparation technologies of both unsupported and supported Pd-Ag membranes are well developed and such membranes are commercially available. Since the membrane reformer performance is limited by separation capability, optimization of membrane permeability is one of the important issues. 

Since these hydrides are thermodynamically stable in the metal, the passive oxide can only be considered as a transport barrier, not as an absolute barrier. Various electrochemical techniques including EIS and photoelectrochemical measurements have been used to identify the mechanism by which the Ti02 may be rendered permeable to hydrogen, and to identify the conditions under which absorption is observable (31). These determinations show that H absorption into the Ti02 (and hence potentially into the metal) occurs under reducing conditions when redox transformations (Ti1 —> Tim) in the oxide commence. However, the key measurement, if H absorption is to be coupled to passive corrosion, is that of the absorption efficiency. 

Issues Regarding Metal Cation Transport in Hydrogen-permeable Membrane Materials... 

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