Nitrogen Reduction to Ammonia on a Fe16 Nanocluster: A Computational Study of Catalysis.

Abstract

The sequence of elementary steps leading to reductive ammonia formation from N2 and H2 catalyzed by a Fe16 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 N2 to Fe16 is considered, followed by exploration of the pathways leading to distal (Fe16-N-NH2) and enzymatic (NFe16-NH2) formation of an amino group. Next, the pathways leading to NH3 formation in both distal and enzymatic cases are examined. Two mechanisms for NH3 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 Fe16 + N2 + 2H2 → Fe16NH + NH3 is found to be exothermic by 1.02 eV (93.8 kJ/mol).