In situ X-ray diffraction study of reduction processes of Fe3O4- and Fe1-xO-based ammonia-synthesis catalysts

The temperature-programmed reduction process of two types of industrial ammonia-synthesis catalysts, A110 and ZA-5, which are, respectively, based on Fe3O4 and Fe1-xO precursors, were studied by in situ X-ray power diffraction (XRD). It has been found that the ZA-5 has lower reduction temperature and faster reduction rate, and its active phase alpha-Fe possesses a higher value of lattice microstrain than A110. The simulation based on Rietveld refinement has also shown that the shape of alpha-Fe grain of ZA-5 has a mixed shape of cube and sphere with more exposing (111) and (211) planes, while that of A110 looks like a concave cube with more exposing (110) planes. Based on the results obtained, a growth model of alpha-Fe during the reduction of Fe3O4- and Fe1-xO-based ammonia-synthesis catalysts is proposed, and the origins for the activity difference has been also discussed. (C) 2009 Elsevier Inc. All rights reserved.