Characterization of unreduced fused iron catalyst for ammonia synthesis

Abstract A procedure for the characterization of iron catalysts is proposed. For this purpose, some model iron catalysts (obtained by fusion) for ammonia synthesis, in unreduced form, were studied. In order to characterize the catalysts, the following properties have to be established: (i) the content of major and minor cations, including Fe 2+ , and the total amount of iron; (ii) phase composition, insofar as the major phases are considered; (iii) homogeneity on the macro- and micro-scale, insofar, at least, as the major cations and major phases are considered; (iv) non-stoichiometry of the major iron oxides. In order to check the proposed procedure, five samples of the model catalysts (unpromoted, potassium oxide-promoted, alumina-promoted and potassium oxide/alumina-promoted) were prepared under industrial conditions and analysed according to the programme outlined above. It was found that the Fe 2+ content varies significantly on either the macro- or micro-scale, this property being reflected in the wustite abundance. Wustite is present in two separate phases, one being rich in iron. This feature is due to slow cooling of the fused material. On the other hand, the magnetite phase is almost stoichiometric and well crystallized. The magnitude of the effective fields acting on iron nuclei at room temperature is a measure of the amount of alumina built into the magnetite phase. Mossbauer spectroscopy was found to be a very useful tool for the determination of the composition of the major phases and of the stoichiometry of iron-containing phases. The contents of impurities can be well approximated by the content of unreducible oxides at their highest degree of oxidation.