Influence of promoter and Fe~(2+)/Fe~(3+) ratio on microstructure of fused iron catalysts for ammonia synthesis

摘要

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 Fe~(2+)/Fe~(3+) ratio and addition of promoters on the microstructure of the catalysts have been investigated by X-ray diffraction, high-rcsolulion 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 wustile. SEM and EDS measurements show that the Fe~(2+)/Fe~(3+) ratio significantly influences the microstructure of the catalyst as well as the distribution of the promoters K and Ca. The catalyst with high Fe~(2+)/Fe~(3+) ratio is reduced more easily than that with low Fc~(2+)/Fc~(3+) ratio. The promoter in the sample with high Fe~(2+)/Fe~(3+) ratio is distributed homogeneously on the surface of particles as needle-like films. After being reduced, the sample with low Fe~(2+)/Fe~(3+) ratio has a compact structure with the promoter being distributing heterogeneous on the catalyst surface in an amorphous state, while the sample with high Fe~(2+)/Fe~(3+) ratio had a porous structure and on the surface of particles the promoter is crystalline.