Effect of Doubly Charged Ion Additives on the Activity and Chemical Stability of Catalytically Active Potassium Ferrites

Abstract

Potassium polyferrites K₂${\text{Fe}}_{{1 - q}}^{{{\text{II}}}}$M_q${\text{Fe}}_{{10}}^{{{\text{III}}}}$O₁₇ (where M is Mg and Zn) and K_2–2qFe^IIM_q${\text{Fe}}_{{10}}^{{{\text{III}}}}$O₁₇ (where M is Ca and Sr; q = 0–0.4) are synthesized to determine the effect of the size of a doubly charged dopant ion on their catalytic properties. The mechanism of the effect of doubly charged cation additives on the activity, selectivity, and corrosion resistance of potassium β'' polyferrite is identified. It is shown for the first time that doubly charged cations are distributed differently in the structure of β'' polyferrite depending on their size: either they are in a spinel-like block and then replace Fe²⁺ ions, or they replace potassium in interblock space. The Mg²⁺ and Zn²⁺ ion additives sharply reduce the catalytic activity and selectivity of β'' polyferrite. Such additives are undesirable. Small amounts of Ca²⁺ and Sr²⁺ cations greatly improve the corrosion resistance of the catalysts due to a drop in the mobility of potassium ions within a cation-conducting layer.