Nature of the Active Structure

Experimentally it has been verified that the synthesis of ammonia takes place on the surface of the catalyst [592] rather than in the bulk. However, isotope labeling experiments seems to indicate that small amounts of ammonia may be formed in the bulk at very high temperatures [593]. X-ray photoelectron spectroscopy demonstrates that Fe is present in the surface of the reduced catalyst as Fe and not as Fe oxide [52, 208]. 

Under ammonia synthesis conditions, ultra-high vacuum data demonstrate that a bulk nitride is not formed [594] whereas during NH3 decomposition the kinetics indicate that the reaction may proceed on a completely nitrided surface [595]. The increase in reaction rate observed by the addition of H2 is interpreted as the reaction being faster on Fe metal than on Fe-nitride [595]. 

The coverage of the total surface by catalytically inactive structural promoters has been determined by a number of techniques. The coverage by catalytically inactive material is 60%, calculated from kinetic data for NH3 synthesis [596], 55% [191, 207], 60% [207], or 45% [195] from chemisorption 

Calculated from the activation entropy, the density of active sites is 10 per cm for a porous Fe-catalyst and 2.5 10 per cm for a Fe-film [597]. 

---