The phase and chemical composition of compounds in the potassium-iron-oxygen system in a wide range of molar ratios of potassium and iron was studied by X-ray phase analysis and atomic absorption spectroscopy. The catalytic properties and the mass fraction of coke deposits on ferritic systems of various composition are determined. It has been shown that at least two types of active sites are present on the surface of the iron oxide catalyst. The dehydrogenation centers include oxygen ion, ions of a promoting alkali metal, and ions of bivalent and triply charged iron, between which electron exchange takes place. Most probably such center is realized in the structure of potassium-polyferrite (K2Fe2+Fe3+10O17). The coke formation centers contain an unpromoted cluster consisting of oxygen ion and iron (III) ion, are realized in Fe3O4 and KFe11O17. Coke deposits on the surface of the catalyst block non-selective active sites and increase the selectivity of action. The probability of realization of clusters corresponding to the dehydrogenation centers is three orders of magnitude higher than in the places of phase contact, the aggregate of which contains the whole set of ions corresponding to dehydrogenation centers, for example, "magnetite + potassium monoferrite". The pure potassium β"-polyferrite provides an optimal concentration of selective centers on the surface of the catalyst, operates highly efficiently in the absence of negative external influences (catalyst re-recovery, corrosion reaction of the reaction medium, poisoning effect). Individual b²-polyferrites, like any catalytically active phase, are unstable, however, being in equilibrium with potassium monoferrate and magnetite, it is able to operate effectively for a long time and to withstand the negative effects of redox properties of the surrounding reaction medium. The presence of potassium monoferrite in the catalytically active system ensures the polyfunctionality of the action of contact, i.e. ability to self-regeneration. It is likely that in the structure of potassium monoferrite, centers for preventing coke formation and annealing of coke are realized, containing oxygen ion, iron ion, and alkaline promoter.Forcitation:Dvoretskii N.V., Anikanova L.G., Malysheva Z.G. Types of active centers on surface of promoted iron oxide catalyst. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 6. P. 61-68

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The effect of chemical composition on the cationic conductivity of mixed alkali metals β"-polyferrites was established. Cationic conductivity is determined by the mobility of alkali metal ion in inter-unit space which in turn depends on the ratio of cation radius and width of inter-unit space. The conductivity increases with z, and at the same z in the range of 0.28 – 0.4 becomes constant (z in the formula Me2-zAdzFe11O17, where Me - the basic alkali metal, Ad - introduced alkali metal ion, which has the larger radius (Cs, Rb)) for all samples. This is due to the structural features of polyferrites.

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In order to clarify the mechanism of charge compensation with the introduction of additives of four-charged ions ions into the composition of catalytically active potassium polyferrites, mixtures with different molar ratios KFeO2: Fe: Fe2O3: MeO2, where Me is Ce, Ti, Zr, were calcined in a muffle furnace in a stream of nitrogen for 4 - 6 h at a temperature of 970 K. As a result, single-phase potassium polyferrites with the β˝-alumina-type structure were obtained, which was confirmed by X-ray diffractometry. Based on the measurement of the electronic conductivity of doped polyferrites, the determination of the content of two-charged and three- charged iron, it has been established that the introduction of four-charged ions into the structure of a β˝-alumina type polyferrite occurs in accordance with the charge compensation mechanism described by the formula K2FeII1+qFeIII10-2qMeqIVO17, where q is a coefficient characterizing the content of the additive of four-charged ion. The above mechanism is implemented by reducing part of the three- charged iron while maintaining the initial content of alkali metal. The effect of the nature of the alloying ion on the composition and the electronic conductivity of the obtained polyferrite is shown. The destabilizing effect of four-charged ions, which is expressed in facilitating the emission of alkali metal from the crystal lattice of polyferrite, has been revealed. It is shown that the ratio of various charge forms of iron can be controlled within wide limits not only by changing the redox properties of the atmosphere, but also by introducing various additives into the potassium polyferrite structure. A violation of single-phase cerium-doped polyferrites was noted when reaching a value of q of more than 0.6. For polyferrites doped with titanium or zirconium, the structure was maintained throughout the entire range of q values studied.