Abstract:

Two types of ammonia-synthesis fused-iron catalysts, A110 and ZA-5, which based on Fe₃O₄ and Fe_1-xO precursors respectively, were investigated by simulating actual reduction conditions of their preparation with in situ X-ray diffraction. During in situ X-ray diffraction, the transformation of crystal phase, the structures of their precursors and active phase based on X-ray diffraction data were analysed. The results show that the reduction temperature ranges of ZA-5 and A110 are 300—362 ℃ and 343—450 ℃, respectively; the reduction temperature of Fe_1-xO-based catalyst ZA-5 is lower than that of Fe₃O₄-based catalyst A110; the reduction rate of Fe_1-xO-based catalyst ZA-5 is faster than that of Fe₃O₄-based catalyst A110; crystallite size ratios of D₍₂₁₁₎/D₍₁₁₀₎ estimated from (211) and (110) crystal planes of catalysts ZA-5 and A110 are 0.7014 and 0.8631 respectively; catalyst ZA-5 owns more developed high active (211) crystal plane, the value of the microstrain of its active phase is stronger than that of catalyst A110, and the value decreases gradually as the temperature increasing. The morphology simulation of active phase based on Popa technology by Rietveld ana-lysis show that the morphology of the active phase of catalyst A110 is cupped cubic, and that of catalyst ZA-5 is a mixture of cube and globe; high active crystal planes (111) and (211) of catalyst ZA-5 develop better than that of catalyst A110; the morphology of active phase crystal of catalyst ZA-5 is well developed octahedron at 335 ℃, and compared with at other temperatures, its crystal plane (111) is the most developed at this temperature; the morphology of active phase of catalyst ZA-5 transforms from octahedron to globe as the temperature increasing gradually; the match of octahedral structure catalyst ZA-5 is better than that of its precursors, and catalyst ZA-5 can be reduced more easily compared with catalyst A110.

Key words: Fused-iron catalyst, In situ X-ray diffraction, Active phase, Microstrain, Formation mechanism