Structure of the ammonia synthesis catalyst

Abstract

The synthesis of ammonia from hydrogen and nitrogen over a reduced iron oxide catalyst is so well known1–5 and so widely used (current world capacity is close to 100 × 106 tons per year) that it may seem surprising that the mode of operation of the catalytic remains enigmatic. That both H2 and N2 must first dissociate at the catalyst surface is beyond dispute, but there is debate as to the role of various promoters (Al, K, Ca) which greatly improve the catalytic performance when added to the precursor Fe3O4. Although it is widely accepted that regions of paracrystallinity exist within the catalyst, no one has questioned its overall crystal-linity. In this work, which entails in situ X-ray diffractometry, we provide evidence that the active catalyst contains substantial amounts of a non-crystalline phase. Without promoter, reduction of Fe3O4 in the same conditions yields only crystalline α-iron.

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Rayment, T., Schlögl, R., Thomas, J. et al. Structure of the ammonia synthesis catalyst. Nature 315, 311–313 (1985). https://doi.org/10.1038/315311a0

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