The Reversed Uptake of Oxygen and Its Different Sources

Another way of creating mixed valence systems is by introducing into the A site of perovskite structure heteroatoms of almost similar size but of different valence, as shown in the following examples  

Mother compound Lc Fe Os. Partial substitution of La with Sr leads to compounds Laf+ Sr +Fef+ Fe 03+5 with mixed valence pairs (Fe /Fe ). Tentatively, in the compound Lao.75Sro.25Feg] Feg 03+, 50% of the iron cations have an heterovalence similar to neighbor-forming (Fe /Fe ) pairs. The other 50% of Fe cations do not have a neighboring Fe .  

Such mixed valence systems, as will be discussed in the next section, result in labile oxygen species on the surface, as well as in the bulk of perovskites, which is a prerequisite for enhanced catalytic activity. 

Let us be more precise about the reasons for the high activity in such systems. In redox catalysis, especially during the oxidation of hydrocarbons, oxygen species are adsorbed on the surface and the rate-determining step is the transfer of a partial charge (3) to - and from - the catalytic site, usually a metal cation M, as follows M + -I- 0.502 M + -I- O.5O2 and so 

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