熔铁催化剂还原过程的原位XRD研究及活性相的形成机理

选择Fe1-xO基和Fe3O4基氨合成熔铁催化剂ZA-5和A110, 采用XRD原位反应器模拟真实的还原条件进行原位XRD实验. 通过对还原过程的物相跟踪分析和对前驱体、活性相的XRD微结构动态演化分析表明, ZA-5和A110的还原温度区间分别为300~362 ℃和343~450 ℃, 前者比后者具有更快的还原速度及更低的还原温度; ZA-5和A110在(211)和(110)方向的晶粒度比值(D₍₂₁₁₎/D₍₁₁₀₎)分别为0.7014和0.8631, ZA-5催化剂具有更好的高活性(211)晶面, 其活性相微观应力明显比A110大, 且随着温度升高逐渐减小; 采用Rietveld全谱拟合结构分析和Popa模拟技术对活性相晶形进行模拟, 得到A110活性相晶形为凹陷立方体, ZA-5为立方体和球形的混合晶形, ZA-5的高活性晶面(111)和(211)晶面比A110生长更好; ZA-5催化剂活性相晶形从低温的类八面体向高温球形演化, 在355 ℃时有比较完善的八面体晶形, 此温度下(111)晶面生长得最好; ZA-5催化剂活性相与前驱体的八面体晶形结构匹配性好, 比A110更容易还原.

In situ X-ray Diffraction Investigation on Reduction Process of Ammonia-synthesis Fused-iron Catalysts and the Formation Mechanism of Its Active Phase

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.