Diffusivity of Oxide Ion

The diffusion of oxide ions may also dominate an oxidation reaction. In a similar fashion to that described for cations, the anions can be accompanied by a flow of holes or a counterflow of electrons (Fig. 5.22c and 5.22d). The arriving O2-anions extend the oxide film at the metal-metal oxide boundary. Once again, which of these mechanisms is theoretically possible will depend upon whether the oxide is naturally rc-type or p-typc. 

The concept proposed by us is pictured simply in Fig. 23. The level of water represents the chemical potential of active oxygen involved in the oxidation of propylene, and the vessels connected on the tank are involved in two kinds of active sites that activate molecular oxygen to atomic species and oxidize propylene to acrolein. If active sites expressed by vessels are isolated from each other, each site must do everything by itself to convert propylene to acrolein. This situation is less convenient than the preparation of the active catalyst system. When active species of oxygen can migrate rapidly through the bulk diffusion of oxide ion as shown in Fig. 23, equal-... 

III. Stability of the Multicomponent Bismuth Molybdate Catalyst Depending on the Bulk Diffusion of Oxide Ion... 

In the preceding section, we explained that the bulk diffusion of oxide ion plays an important role in the enhancement of the catalytic activity of the multicomponent bismuth molybdate systems. Here, another important role of the oxide ion migration in increasing the stability of the catalyst system is introduced. 

T. Ishigaki, S. Yamauchi, J. Mizusaki, K. Fueki and H. Tamura, Diffusion of oxide ions in LaFeOa single crystal. Solid State Chem., 55 (1984) 50-53. 

The diffusion of oxide ion vacancies is affected hy the elastic strain energy, which is related to the size mismatch between the host and dopant cations [9]. Based on the earlier observations of Nowick [10], and Kilner and Steele [11], the importance of the defect pairs formed due to interaction between the oxide ion vacancies, Vo, and alivalent cations, Mce, in Ce02 should he emphasized. 

Ishigaki, T., Yamauchi, S., Kishio, K., Mizusaki, J. Fueki, K. Diffusion of oxide ion vacancies in perovskite-type oxides. Journal of Solid State Chemistry 73, 179-187, doiiDoi 10.1016/0022-4596(88)90067-9 (1988). 

Figure 6.22. The principle of EVD on both sides of a porous support the two different precursors are introduced. Initially they react to form zirconia in the pores of the substrate. When the pores are clogged and then closed the two reactants can react with each other only via oxide ions through the deposit. The growth reaction in this second phase becomes limited by diffusion of oxide ions in the solid deposit and the layer grows fastest where the deposit is thinnest.

Ishigaki T, Yamauchi S, Kishio K, Mizusaki J, Fueki K (1988) Diffusion of oxide Ion vacancies in perovskite-type oxides. J Solid State Chem 73 179-187... 

The material cost of lanthanum is high so that the thickness of the oxide interconnects should be thin enough to reduce the amount used in cells [31]. On the other hand, the LaCrOs-based intercoimects should be thick enough to prevent the electrochemical oxygen permeation which takes place as the bipolar diffusion of oxide ions and holes inside the interconnect. For such a purpose, Ca-doped LaCrOs is not appropriate because those ceramics are highest in gas permeation among the alkali earth-doped LaCrOs [36, 37]. 

Structural disorder and diffusion of oxide ions in Ce ri j02 solid solutions... 

Diffusion of oxide ions is an important step in the oxygen storage/ release process of ceria-based catalysts. We investigated the crystal structure and structural disorder of compositionally homogeneous Ce Zri. 02 and CeOg by synchrotron and neutron powder dififrac-tometry, the Rietveld method, and The isotropic... 

The observed atomic displacement parameters include not only the dynamic component but also the static one. Static disorder can lead to lower oxygen diffusivity, while dynamic disorder increases ionic conductivity (Fig. 1.33). The ionic conductivity and diffusivity of oxide ions in Ceo.sRo.gOi.g decreases with an increase of the atomic displacement parameter (Fig. 1.34). As discussed in Section 1.5.3, further structural studies of the atomic displacement parameters in... 

Additional conduction mechanisms appear in doped ceria at low temperatures (<200°G). Lee et investigated the reduction and oxidation of Geo.sGdg gOi g using conductive tip atomic force microscopy (AFM). Their work indicated that the surface diffusion of oxide ion vacancies dominates ionic transport in Geo sGdg gOi g at temperatures up to at least 200 °G. They estimated the activation... 

The diffusivity of oxide ions in LSGM was further studied by traeer diffusion measurements [37]. Diffusion of oxide ion in perovskite oxide is explained in detail in Chapter 5. LSGM exhibits large values of diffusion eoeffieient, and the observed fast diffusion in LSGM originates from the higher mobility of oxide ions in the perovskite strueture as compared with the fluorite structure (Table 4.2), presumably due to a large free volume in the lattice. 

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