Dense non-porous membranes and surface reaction limitation

6 Dense (non-porous) membranes and surface reaction limitation 

These types of membranes are currently only suitable for oxygen separation — mainly with oxidic layers, or hydrogen separation — mainly with Pd(Ag) alloys. As discussed in Chapter 6, reasonable fluxes (permeation values) can be obtained at high (for oxygen) to intermediate (for hydrogen) temperatures. The production of defect-free thin layers of these materials remains problematic. 

The best prospects seem to be present for deposition of Pd(Ag) plugs into the pores of a porous support material as e.g. reported by Morooka et al. [21]. Further research to confirm the reproducible s)mthesis of this system and of its long-term stability for hydrogen separation is worthwhile. 

In the case of oxygen transport the best prospects at this moment are the use of metal-oxide composites with high electronic conductivity, or separation with perovskite-derived membranes as reported by Balachandral et al. [22]. These latter membranes are thick (0.5-1.0 mm) and have long-term stability at high temperature. 

The use of thinner membranes of this t)q)e increases the permeance but to a lesser extent the thirmer the membrane. With a thickness around 0.3 mm (depending on the precise system) surface reactions which transfer oxygen from the gas phase to solid material completely become rate determining. This phenomenon limits the much higher permeances which are potentially based on the very high bulk permeation (see Chapters 6 and 8). 

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