Ferrite-type catalysts

Ferrite-type catalysts may constitute an alternative for perovskites for total oxidation of hydrocarbons as it was demonstrated by ferrites prepared following two routes, that is, the hydrothermal one and the calcination of oxide mixtures with the following compositions Ni >,Fe3 04 (. = 0.5) and Mn Fe3 c04 ( = 0.65) [28]. These studies demonstrated that the structure of the catalysts is very important in providing catalytic active structures. Pure XRD ferrite structures exhibit a poor activity, while mixed-phase ferrite oxide showed a good activity. 

Florea, M., Alifanti, M., Parvulescu, V.I., Mihaila-Tarabasanu, D., Diamandescu, L., Feder, M., Negrila, C., and Fnmza, L. (2009) Total oxidation of toluene on ferrite-type catalysts. Catal. Today, 141 (3-4), 361-366. 

For the investigation of the electronic character of catalytic reactions, it has been found fruitful to study not only the catalytic action of metals and semiconductors of known electron state distribution, but also the changes of this distribution by illumination during catalysis. The War-burg-Barcroft technique of biochemistry has proved to be very suitable for this purpose. It could be shown in a semiquantitative way that promotion of electrons enhances acceptor reactions, that electron traps hinder such action, and that n-type and p-type conductors behave in opposite fashion when employed as photocatalysts. Using ferrites as catalysts in an acceptor reaction, the electron transfer concept was confirmed. 

In order to have more infoimation on the nature of the oxygen species active in partial and total oxidation we investigated the interaction of the hydrocarbons with the pre-oxidized surfaces of oxides where different types of surface oxygen species are formed. In particular we investigated p-type semiconductors like chromia, chromites and cobalt oxide C03O4. Moreover, we studied n-type metal oxides like FezOs, metal ferrites and CuObased catalysts. 

A correlation between the catalytic qualities and the reducibility of this type of catalyst is suggested by Massoth and Scarpiello [ 205]. They performed reduction experiments both with hydrogen and with butene. Reduction may destroy the lattice, and the best catalysts appear to be those that are only superficially reduced. The effect of the introduction of Cr in the ferrites, mentioned above, is shown to be essentially due to an increase of the stability against bulk reduction. 

Tsagaroyannis et al. [24] investigated Fe-Gd ferrites for WGS reaction. These catalysts exhibit remarkable catalytic activity. This activity is attributed to their p-type semi-conductivity due to a limited number of Fe" cations, existing in their crystal lattice and associated with the presence of some cation vacancies. Catalytic activity is considerably higher for the ferrites thermally treated in rather lower temperatures (about 800 °C) as they included more defects in their crystal lattices. 

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