Elucidating Mechanistic Origin of the Catalytic Activity of the Fe(111) Surface and Nanoclusters toward the Electrochemical Nitrogen Reduction Reaction

The design and development of highly efficient catalysts for the electrochemical reduction of nitrogen into NH3 at ambient temperature and pressure has been an area of major research interest. In this work, electrochemical N2 reduction following Heyrovsky-type associative and dissociative mechanisms is studied on the periodic Fe(111) surface using density functional theory calculations. Interestingly, the associative pathway has not been investigated on the Fe(111) surface in any of the previous studies though it is reported to be one of the best catalysts for ammonia synthesis. Therefore, we have investigated both the nitrogen reduction reaction (NRR) mechanisms on the Fe(111) surface. Free-energy analysis of associative and dissociative reaction pathways has been carried out, and it has been found that the associative mechanism is favorable over the dissociative mechanism with the formation of *NH2NH2 as a potential-determining step. Furthermore, the catalytic activity of cuboctahedral iron nanoclusters (NCs) is also investigated to understand the dimensional dependence of the Fe-based NRR activity. The NC shows a higher NRR activity by following an energetically more favorable ammonia desorption compared to the Fe(111) surface. The observed activity trends are explained from the site-specific interaction and binding energy of reaction intermediates. The surpassing of the high energy-demanding N2 dissociation step by both the catalytic systems implies that NRR can be facilitated in an energetically favorable manner via an electrochemical reduction pathway.