The mechanism of formation of ammonia synthesis catalyst

Abstract

The formation of ammonia synthesis catalyst by the reduction of magnetite by hydrogen at 450 °C has been studied using electron microscopy, X-ray energy dispersive and electron energy loss analysis, and including in situ reduction experiments in a gas reaction cell in an electron microscope. The reaction is characterised by a cellular mechanism of reduction leading to the formation of a periodic network of iron crystallites and pores without any significant volume change. The iron nucleates epitaxially on the oxide, and minor elements, notably aluminium, initially in solution in the oxide, partition to the iron crystallite surfaces. Elements present in intercrystalline ferrite and glassy phases remain essentially immobile during reduction with the exception of potassium which migrates rapidly to the iron surfaces.