TYPES OF ACTIVE CENTERS ON SURFACE OF PROMOTED IRON OXIDE CATALYST

Abstract

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