Charge transfer on an atomic scale

It is generally accepted that valency transitions of cations are connected with the redox mechanism. It is obvious therefore, that activity and selectivity demand that the cation in the active site has the right oxidation state before the hydrocarbon is adsorbed, and that it is effectively reoxidized afterwards. 

Accordingly, correlations are often found between activity, selectivity and the concentration of cations in specific oxidation states, e.g. V4+ in V205. The improvement of selective catalytic qualities of metal oxides by addition of modifiers, or by combination in mixed oxides may hence be explained by stabilization of the essential cation in the proper oxidation state. In some cases, stabilization of a (partially) reduced cation appears to yield the most effective catalyst however, more often it is the higher oxidation state that should be maintained, and accordingly the role attributed to a second cation often concerns facilitating the reoxidation, for instance, by direct electron transfer between the cations or, in general, by increasing electron conductivity. 

Techniques that enable the observation of specific valencies of cations include E.S.R., 7-resonance spectroscopy and ESCA, and have been considerably improved in the recent years. 

Iron molybdates were investigated by several authors. It is generally observed that iron is reduced first (Fe3+ - Fe2+), while deeper reduction is required to reduce the molybdenum ions as well. Both cations occur in partially reduced states during the reaction with butene. Pernicone [254] concludes from his ESR work that under stationary reaction conditions the iron ions stay in the reduced state and that the redox process only involves Mo6+ and Mos+. However, Trifiro and Pasquon [318] and Matsu ura and Schuit [207] are of the opinion that reoxidation initially may lead to Fe3+ which in turn (rapidly) oxidizes the Mos+ ions at the hydrocarbon reaction sites of the catalyst. However, direct evidence is not provided. 

The amount of Mo5 on the surface of Mo- Ti—O and Mo—Te—O catalysts has been assessed with ESR techniques by Akimoto and Echigoya [13,15,17] and Andrushkevich et al. [27]. These workers find a strong correlation between the maximum intensity of the Mo5 signal with maximum activity in the oxidation of propene to acrolein (at 8 at. % Te) and conversion of butadiene to maleic anhydride (75 at. % Ti). 

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Studies have also been carried out which are more specifically aimed at charge transfer on an atomic scale and deal with the atomic situation within the lattice. This is especially so in the case of binary oxides. Many authors assume that, in these systems, both types of cation participate in electron transfer. The reactivity of the binary oxides is then explained by the hypothesis that the cation on the active site obtains an electron supply from the second type. 

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