In situ X-ray powder diffraction analysis of the microstructure of activated iron catalysts for ammonia synthesis

The formation of the iron species catalysing the synthesis of ammonia was followed by in situ X-ray powder diffraction. It was shown that a complex sequence of reactions during the activation process (reduction of the magnetite precursor) leads to a metastable form of alpha-iron. A detailed examination of the diffraction lines of iron helped to elucidate the question, why ''ammonia iron'' is so much superior to ''normal iron''. Unusual line profiles were observed which could be controlled by changing the reaction conditions. Platelet crystallites of defective alpha-iron form the outer shell of isotropic bulk iron particles which represent the main body of the catalyst material. A shoulder at higher d values of the Fe(110) reflection points to the formation of an iron/nitrogen phase during catalytic action. The crystallographic observations are in line with previous data on paracrystallinity, but their interpretation is of a different nature. The structural effects of oxygen poisoning were investigated. It is shown, why the poisoning is detrimental for the catalytic performance and how the speculation could arise that this effect may be beneficial for the reaction.