ON THE ROLE OF PROMOTERS IN PROMOTED IRON CATALYSTS USED IN THE INDUSTRIAL SYNTHESIS OF AMMONIA

The surface structure and atomic composition of triply promoted (K2O/Al2O3/CaO) fused iron catalysts, used in the industrial synthesis of ammonia, have been examined by temperature-programmed desorption (i) after having been reduced in hydrogen and (ii) after having synthesised ammonia from a hydrogen/nitrogen (3:1) stream at 723 K. The nitrogen desorption spectrum from a clean, or a hydrided surface (produced by reduction in hydrogen), showed only one peak maximum at 120 K, corresponding to a desorption activation energy of 8 kcal mol-1. Regardless of the composition of the catalyst, the higher energy state, desorbing at ≈ 210 K, seen in potassium-doped single-crystal studies was not observed. Indeed the same low-temperature (≈ 120 K) nitrogen desorption spectrum was observed from a nitrided/hydrided surface produced during ammonia synthesis, the peak at ≈ 210 K being signally absent. The notion that the role of the potassium promoter in ammonia synthesis on promoted fused iron catalysts is to promote a higher energy state for nitrogen adsorption leading to a lowering of the activation energy to dissociation should now be re-examined. The hydrogen desorption spectrum obtained after reduction in hydrogen or after ammonia synthesis indicated that the surface was a mixture of the three low-index faces. No extensive reconstruction to the (111) surface appeared to have occurred during the interaction of the iron with the water produced during reduction. Here again the idea that the role of the alumina promoter is to stabilise this reconstruction must also be re-examined. © 1990.