Effect of the Precursor and Synthesis Regime on the Properties of Hematite for Preparing Promoted Iron Oxide Catalysts

The fine crystal structure of hematite samples used for preparing potassium promoted iron oxide catalysts of dehydrogenation is studied via X-ray diffraction and scanning electron microscopy. α-Fe₂O₃ samples are synthesized under non-equilibrium conditions from several precursors in different regimes of thermolysis. The most important characteristic of hematite that causes the activity and selectivity of a hematite-based catalyst is its fine crystal structure (FCS). The fine crystal structure of hematite predetermines the phase composition of the catalyst. The fine crystal structure of hematite forms during its synthesis and is determined by the nature of the precursor, the temperature of synthesis, the temperature gradient, and the rate of the removal of gaseous thermolysis products. The highest activity is displayed by the catalyst prepared on the basis of hematite with mosaic blocks 70–90 nm in size and a minimum SF concentration caused by half and quaternary dislocations. Such hematite was synthesized via the thermolysis of iron sulfate at 950 K under fluidized bed and low temperature gradient conditions. Hematite from iron carbonate is not recommended for use in synthesizing a catalyst due to the high concentration of low-temperature SFs, which result in the formation of catalytically low-active potassium β-polyferrite.