Structure of the ammonia synthesis catalyst

Abstract

The synthesis of ammonia from hydrogen and nitrogen over a reduced iron oxide catalyst is so well known^1–5 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.