The effect of doubly charged ion additives on the activity and chemical stability of catalytically active potassium ferrites

To reveal the effect of the dopant doubly charged ion size on the catalytic properties, samples of potassium polyferrites of β′′-alumina type with the composition K₂Fe^II_1-q M_qFe^III ₁₀O₁₇, where M = Mg, Zn, and K_2–2qFe^IIMqFe^III ₁₀O₁₇, where M = Ca, Sr, and q = 0.4, were synthesized. The mechanism of action of the introduced doubly charged cations on the activity, selectivity and corrosion resistance of potassium β′′-polyferrite was elucidated. It was shown for the first time that the size of doubly charged cations determines their position in the structure of β′′-polyferrite: they either enter the spinel-like moiety and substitute Fe²⁺ ions, or substitute potassium in the intermoiety space. The introduction of Mg²⁺ and Zn²⁺ ions dramatically reduces the catalytic activity and selectivity of β′′-polyferrite. The application of such additives is undesirable. Small amounts of Ca²⁺ and Sr²⁺ cations significantly improve the corrosion resistance of the catalyst by decreasing the mobility of potassium ions within the cation-conducting layer.