Determination of Fe²⁺/Fe³⁺ mole ratio based on the change of precursor lattice parameters of wustite based iron catalysts for the ammonia synthesis

In the presented article, oxide forms of iron catalysts with the wustite structure and with a R = Fe²⁺/Fe³⁺ molar ratio in the range from 3.78 to 8.16 were investigated. The chemical composition of the tested catalyst precursors was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The X-ray diffraction (XRD) technique was used to determine the phase composition and location of reflections characteristic of the Fe_1−xO phase. The molar ratio of iron ions R = Fe²⁺/Fe³⁺ was determined by manganometric titration. The distribution of promoters in the structure of iron catalyst precursors with different R = Fe²⁺/Fe³⁺ ratio was determined by a selective etching method. The dependence of the lattice parameter a_o value in the crystal structure Fe_1−xO on the molar ratio R = Fe²⁺/Fe³⁺ was determined. On the basis of the determined dependence, R can easily be calculated in catalyst precursors of the wustite structure.