Mixed-conducting oxide membranes

Fig. 3. Oxygen transport in solids. 02 is dissociated and ionized at the reduction interface to give O2 ions, which are transferred across the solid to the oxidation interface, at which they lose the electrons to return back to 02 molecules that are released to the stream, (a) In the solid electrolyte cell based on a classical solid electrolyte, the ionic oxygen transport requires electrodes and external circuitry to transfer the electrons from the oxidation interface to the reduction interface (b) in the mixed conducting oxide membrane, the ionic oxygen transport does not require electrodes and external circuitry to transfer the electrons to the reduction interface from the oxidation interface, because the mixed conductor oxide provides high conductivities for both oxygen ions and electrons.

Figure 1 Working principle of a mixed-conducting oxide membrane...

J.E. ten Elshof, H.J.M. Bouwmeester and H. Verweij, Oxidative Coupling of Methane in a Mixed-Conducting Perovskite Membrane Reactor , Appl. Catal. A, 130 195-212 (1995). 

The ITM Syngas process involves the direct conversion of methane to synthesis gas (see Fig. 22.7). The process utilizes a mixed, conducting ceramic membrane and partial oxidation to produce the synthesis gas.40 The goal of this advanced reformer technology is to reduce the cost of hydrogen production by over 25 percent.59,79... 

Fig. 10.1. Different membrane concepts incorporating an oxygen ion conductor (a) mixed conducting oxide, (b) solid electrolyte cell (oxygen pump), and (c) dual-phase membrane. Also shown is the schematics of an asymmetric porous membrane (d), consisting of a support, an intermediate and a barrier layer having a graded porosity across the membrane.

As the mixed proton and electron conductive oxide membrane becomes sufficiently thin, surface kinetics will become important, and difiusion of protons across the membrane will no longer be rate deterrriining for the overall hydrogen flux. Bouwmeester et al. [10] defined a characteristic thickness, L, for membranes where surface kinetics and bulk kinetics are equally important to the flux. Decreasing a membrane s thickness below gives essentially no increase in the flux. 

Tan, X. and Li, K. (2009) Design of mixed conducting ceramic membranes/reactors for the partial oxidation of methane to syngas. AIChE Journal, 55 (10), 2675-2685. 

Jin, W. Q., Zhang, C., Chang, X. F., Fan, Y., Xing, W., Xu, N. (2008). Efficient catalytic decomposition of CO2 to CO and O2 over Pd/mixed-conducting oxide catalyst in an oxygen-permeable membrane reactor. Environmental Science and Techrwlogy, 42, 3064—3068. 

Yamamoto, O. (2009) C02-tolerant mixed conducting oxide for catalytic membrane reactor. Solid State Ionics,... 

C02-tolerant mixed conducting oxide for catalytic membrane reactor. 

The separation layer may be dense (non-porous), such as Pd or Pd-alloy membranes for hydrogen separation and mixed (electronic, ionic) conducting oxide membranes for oxygen separation, or porous, such as metal oxides, silicalite, or zeolite membranes. Inorganic membranes are generally named for this separation layer, since it determines the properties and application of the membrane. The flux and selectivity of inorganic membranes are mainly determined by the quality of the separation layer, which is required to be defect-free and as thin as possible. 

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