Surface self-exchange coefficients

The lEDP method can be combined with gas-phase analysis (GPA) as alternative methods for measuring the self-diffusion and surface self-exchange coefficients. In this analysis, the sample placed in a flxed bed is first equilibrated with a controlled atmosphere kept at a well-defined 02 partial pressure. After equilibration, the feed stream is switched to 02 at the same partial pressure and the outlet gas composed by 02 and 02 leaving the reactor is monitored (i.e., breakthrough curve analysis). The self-diffusion and surface self-exchange coefficients can be measured using the Pick s second law (i.e.. Crank s equation). 

The lEDP and GEA methods provide self-diffusion and surface self-exchange coefficients (i.e., at zero overall gradient for for O2 permeation). The... 

J and q are the flux and the concentration of ions in the ion-exchange membrane, respectively F, R, T, and are Faraday s constant, gas constant, temperature, and electrical potential, respectively X is the distance in the membrane, perpendicular to the plane of the membrane surface D is the self-diffusion coefficient of the diffusing ions... 

Zelsmann and co-workers [88] have reported tracer diffusion coefficients for water in Nafion membranes exposed to water vapor of controlled activity. These were determined by various techniques, including isotopic exchange across the membrane. They reported apparent self-diffiision coefficients of water much lower than those determined by Zawodzinski et al. [64], with a weaker dependence on water content, varying from 0.5 x 10 cm to 3 x 10 cm /s as the relative humidity is varied from 20 to 100%. It is likely that a different measurement method generates these large differences. In the experiments of Zelsmaim et al., water must permeate into and through the membrane from vapor phase on one side to vapor phase on the other. Since the membrane surface in contact with water vapor is extremely hydrophobic (see Table 7), there is apparently a surface barrier to water uptake from the vapor which dominates the overall rate of water transport in this type of experiment. 

For predominant electronic conductors like, for example, the perovskites Lai xSrxCoi.yFey03 is determined by the ratio of the oxygen self diffusivity and surface exchange coefficient. Both parameters can be measured simultaneously by using isotopic exchange techniques. Calculations show that... 

Covering temperatures from -140 up to 200 C and chain lengths from one to six carbon atoms, the intracrystalline mean life times are found to coincide with the values oi rV 1 r calculated from the nmr self-diffusion coefficients. This clearly indicates that molecular exchange is controlled by intracrystalline self-diffusion, and that for the considered adsorbate-adsorbent systems there are no perceptible surface barriers. 

In order to derive self-diffusion coefficients from the TEX-PEP experiments, a mathematical model is needed to describe the re-exchange process in the zeolite reactor bed. A common way to describe diffusion in packed beds is to use a set of diffusion equations, describing the mass transport in the zeolite bed and inside the crystals [3,38,39]. The model to analyze the TEX-PEP experiment study is basically a modification of the equations by Noordhoek et al. [25]. The process is thought to consist of the transport of molecules via convection and axial diffusion in the space between the crystals, adsorption and desorption at the zeolite crystal surface and diffusion inside the pores... 

Figure 1 shows a schematic diagram of a small globular micelle. The rapid, continuous exchange of monomers with bulk solution means that the micelle is a very mobile, disordered, aggregate. There is a continuous movement of molecules jumping part way in and out of the interface [21] (this has recently been termed protrusion [23,24]). Also, diffusion around the micelle surface is rapid [self-diffusion coefficient (D) = 10 ° s ], as is the exchange be-... 

Interdiffusion gradients produced by annealing at 1100 and 1200C yielded the self-diffusion coefficients that were comparable to those obtained from gas-solid exchange, indicating that the surface exchange reaction was fast enough. The results were interpreted in terms of a defect model in which O diffusion occurred by an... 

The O2 self-exchange and surface exchange coefficients are usually found to be a function of the O2 partial pressure. 

Fielitz, P. and Borchardt, G. (2001) On the accurate measurement of oxygen self-diffusivities and surface exchange coefficients in oxides via SIMS depth profiling. Solid State Ionics, 144,... 

In a tracer experiment, we obtain Dt which is close to the self diffusivity (Dt = f Dr) and then the exchange coefficient should also reflect the tracer exchange coefficient kt = ki and this should be comparable to that calculated as above from measurements of Re the difference should say to what extent the presence of an electrode has changed the kinetics of the rate limiting step of the surface reaction. 

Chater, R.J., Carter, S., Kilner, J.A., and Steele, B.C.H. Development of a novel SIMS technique for oxygen self-diffusion and surface exchange coefficient measurements in oxides of high diffusivity. Solid State Ion. 1992, 53,... 

A cations and one B cation [48]. Furthermore, it has been observed that correlations exist between measured transport parameters, e.g., the relationship between the self-diffusion coefficient and the surface exchange coefficient [11],... 

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