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

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).