Nitrogen Reduction to Ammonia on a Fe 16 Nanocluster: A Computational Study of Catalysis.

The sequence of elementary steps leading to reductive ammonia formation from N 2 and H 2 catalyzed by a Fe 16 cluster is studied using generalized gradient approximation density functional theory and an all-electron basis set of triple-ζ quality. The computational methods are validated by comparison to experimental data such as binding energies where possible. First, the associative and dissociative attachment of N 2 to Fe 16 is considered, followed by exploration of the pathways leading to distal (Fe 16 -N-NH 2 ) and enzymatic (NFe 16 -NH 2 ) formation of an amino group. Next, the pathways leading to NH 3 formation in both distal and enzymatic cases are examined. Two mechanisms for NH 3 detachment have been discovered. An interesting peculiarity of the pathways is that they often proceed with total spin fluctuations, which are related to the rupture and formation of bonds on the surface of the catalyst over the course of the reactions. The reaction Fe 16 + N 2 + 2H 2 → Fe 16 NH + NH 3 is found to be exothermic by 1.02 eV (93.8 kJ/mol).