MORPHOLOGY AND CRYSTALLOGRAPHIC RELATIONSHIPS IN REDUCED MAGNETITE - A COMPREHENSIVE STRUCTURAL STUDY OF THE POROUS IRON AMMONIA-SYNTHESIS CATALYST

Ammonia synthesis catalyst has been examined by optical microscopy and transmission electron microscopy. In reduced magnetite grains four di fferent pore morphologies with characteristic crystallographic orienta tions exist. These structures are several micrometers in size and may be described as porous single crystals of bcc-iron. The dominant struc ture (roughly 80% of the catalyst volume) contains a nearly random net work of pores and the crystallographic orientation of the porous iron is the same as that of the original magnetite. Another structure (ca. 5% by volume) has sheets of iron along the three {100} planes of the f ormer magnetite and a [100] axis of iron normal to the sheets. A third structure (ca. 3%) with weakly sheet-like pores along the twelve {112 } planes of the original magnetite has a [221] direction of iron norma l to the pores. The most complex structure (very rare in the industria l catalyst) displays strongly sheet-like pores along the four {111} pl anes of magnetite and has a [100] axis of iron normal to the sheets. T he particular orientation relations between the original magnetite and the resulting iron suggests that epitaxy plays an important role duri ng reduction. Alumina, which is added as a promoter, reduces the latti ce parameter of magnetite. The local Al3+ concentration in the magneti te may influence which structure occurs by changing the likelihood of the possible epitaxy relations. Differences in activity among the vari ous structures may exist. However, activity measurements are not a par t of this structural study of the porous iron ammonia synthesis cataly st. (C) 1997 Academic Press.