Kinetics surface-exchange

Attention has already been drawn to the importance of the surface exchange kinetics in determining the rate of oxygen permeation through mixed-conduct- 

The mixed-conducting perovskite oxides have attracted particular interest for use as dense ceramic membrane to control partial oxidation of methane to C2 products or syngas. Such a process bypasses the use of costly oxygen since air can be used as oxidant on the oxygen-rich of the membrane. 

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H.J.M. Bouwmeester, H. Kruidhof and A.J. Burggraaf, Importance of the surface exchange kinetics as rate limiting step in oxygen permeation through mixed-conducting oxides. Solid State Ionics, 72 (1994) 185-194. 

With respect to 7O2 obtained for the tape-cast membrane at 1000 °C, note that the value was enhanced only 50% compared to those for the bulk specimen, although the membrane thickness was reduced by a factor of about one-half. This implies that the oxygen-permeation flux was limited by surface exchange kinetics rather than ambi-polar diffusion process in ceramics. If this is the case, the Wagner s equation can be modified by including a characteristic thickness of dc as follows (Bouwmeester, Kruidhof, Burggraaf, 1994) ... 

Coulometric titration techniques were used to measure chemical diffusion at between 700 and lOOOC. The transient current response to a potentiostatic step was transformed from the time domain to the frequency domain. The equivalent circuit which was used to fit the resultant impedance data contained an element which described the finite-length diffusion of O into the sample. Other elements which were included were the gas-phase capacitance, and the sum of the gas-phase diffusion resistance and that which was associated with the limited surface exchange kinetics of the sample. The chemical diffusion coefficient of the perovskite, Laq gSrq 2C0O3,... 

Berenov AV, Atkinson A, Kilner JA, Bucher E, Sine W (2010) Oxygen tracer diffusion and surface exchange kinetics in La0.6Sr0.4CoO3-delta. Solid State Ionics 181 819... 

In Section 14.3 we concentrate our attention on the formulation of equations for modeling gas semipermeation within perovskite membranes for high-tem-perature CO2 capture applications. In such derivations, we assume that external diffusion at both the feed/membrane and permeate/membrane sides of the membrane (steps 1 and 2, respectively) can be neglected. In such a situation, bulk diffusion (step 3) and surface exchange kinetics (steps 2 and 4) are expected to contribute only to the permeation process. 

Surface Exchange Kinetics as Rate-Determining Step... 

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