Oxygen vacancies, redox rate

The replenishment of the vacancy can be directly from the gas phase or indirectly from the catalyst. In the latter case, the oxygen mobility within the catalyst is so large that bulk oxygen can diffuse to the vacancy. Then oxygen from the gas phase reoxidizes the lattice on sites which differ from hydrocarbon reaction sites. In a steady state, the rate of catalyst oxidation will be equal to the rate of reduction by the substrate. The steady state degree of reduction, equivalent to the surface coverage with oxygen, is determined by the ratio of these two rates. Kinetic models based on these principles are called redox models, for which the simplest mathematical expression is... 

Evidence for the Fe2+/Fe3+ redox cycle was provided later by ESR measurements [205], while recent experiments with deuterium-labelled butene indicate that C—H cleavage is involved in the rate-controlling step [138]. In agreement with the views of Schuit [281], chemisorption of the olefin on an anion vacancy is assumed, but O- is postulated as the active oxygen species rather than O2-. An argument in favour of O" is that otherwise much more, and rather complicated, elementary reaction steps are required to account for the transfer of charge. 

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