Influence of promoter and Fe2+/Fe3+ ratio on microstructure of fused iron catalysts for ammonia synthesis

Fused iron catalysts have been prepared by melting a mixture of magnetite powder, reducing agent (iron) and promoters (K, Ca and Al). The effects of the Fe2+/Fe3+ ratio and addition of promoters on the microstructure of the catalysts have been investigated by X-ray diffraction, high-resolution scanning electron microscopy and energy disperse spectroscopy techniques. The XRD results indicate that the addition of the promoters restrains the occurrence of disproportionation reaction and favors the formation of wustite. SEM and EDS measurements show that the Fe2+/Fe3+ ratio significantly influences the microstructure of the catalyst as well as the distribution of the promoters K and Ca. The catalyst with high Fe2+/Fe3+ ratio is reduced more easily than that with low Fe2+/Fe3+ ratio. The promoter in the sample with high Fe2+/Fe3+ ratio is distributed homogeneously on the surface of particles as needle-like films. After being reduced, the sample with low Fe2+/Fe3+ ratio has a compact structure with the promoter being distributing heterogeneous on the catalyst surface in an amorphous state, while the sample with high Fe2+/Fe3+ ratio had a porous structure and on the surface of particles the promoter is crystalline.