In Situ XRD Study on Promotional Effect of Potassium on Carburization of Spray‐dried Precipitated Fe2O3 Catalysts

Abstract A study of the promotional effect of potassium on the carburization behavior of a series of spray‐dried precipitated Fe 2 O 3 catalysts (100 Fe, 100 Fe/0.52 K, 100 Fe/1.54 K, and 100 Fe/2.4 K) has been performed by using in situ XRD. The average crystallite size evolution for species such as α‐Fe 2 O 3 , Fe 3 O 4 , χ‐Fe 5 C 2 , and θ‐Fe 3 C were followed. The potassium promoter clearly inhibits the reduction of α‐Fe 2 O 3 to Fe 3 O 4 , where the decreasing binding energy in the Fe 2p 3/2 , O 1s and K 2p 3/2 spectra from X‐ray photoelectron spectroscopy (XPS) suggests an electron density increase in Fe and O upon potassium promotion, leading to the enhanced covalency in the Fe−O bond. In terms of crystallite size during carburization, an optimum potassium loading exists in catalyst 100 Fe/0.52 K, which shows the fastest reduction to Fe 3 O 4 with minimum crystallite sizes of around 7 to 15 nm. Potassium has no clear effect in determining the final crystallite size of χ‐Fe 5 C 2 . An arch‐shaped curve in the evolution of crystallite size of the Fe 3 O 4 intermediate was observed, which can be explained by the activation energy difference between the bidirectional steps of the outward oxygen diffusion and the inward carbon diffusion.