Charge Compensation in Potassium Polyferrite at Doping With Four-Charged Ions

Abstract

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.

Formation of a ceramic structure of promoted iron oxide catalyst

“…However, our data correlate with the results obtained by many other researchers [32], and indicate a high carburisation of such catalyst surface and its low selectivity. The introduction of an alkali metal promoter into the system significantly increases the contact activity [1][2][3][4][6][7][8][9].…”

“…Furthermore, alloying additives are usually introduced into the industrial catalyst, improving the performance properties of the final product -the catalyst [2,4,7,[11][12]. We selected zirconium as an alloying additive.…”

“…Dehydrogenation of olefinic and alkylaromatic hydrocarbons is a multi-tonnage process. It is conducted with the use of promoted iron oxide catalysts [1][2][3][4][5][6][7][8][9][10]. The minimum set of parameters determining the effectiveness of the catalyst, in addition to the catalytic characteristics, always includes the size of the available pore surface, the thermal stability of the contact porous structure, and the mechanical strength of the catalyst granules.…”

“…In the promoted iron oxide catalysts for the dehydrogenation of alkylaromatic hydrocarbons, the mass content is: iron oxide -60-70%; potassium compounds in terms of K2O -10-15%. There are also various additives providing the increased selectivity, thermal stability, mechanical strength, and durability [2][3][4][5][6][7][8][9].…”

“…The metal oxides of various nature have a significant effect on the properties of the catalyst, in particular, alkaline and alkaline earth -K, Rb, Cs, Ca, as well as transition elements such as Cr, Mo, Mn, Zn, Ce, La, Y, Sc, etc. [7][8][9].…”