Computational Design of Active Site Structures with Improved Transition-State Scaling for Ammonia Synthesis

Abstract

The Haber–Bosch process for the reduction of atmospheric nitrogen to ammonia is one of the most optimized heterogeneous catalytic reactions, but there are aspects of the industrial process that remain less than ideal. It has been shown that the activity of metal catalysts is limited by a Brønsted–Evans–Polanyi (BEP) scaling relationship between the reaction and transition-state energies for N2 dissociation, leading to a negligible production rate at ambient conditions and a modest rate under harsh conditions. In this study, we use density functional theory (DFT) calculations in conjunction with mean-field microkinetic modeling to study the rate of NH3 synthesis on model active sites that require the singly coordinated dissociative adsorption of N atoms onto transition metal atoms. Our results demonstrate that this ”on-top” binding of nitrogen exhibits significantly improved scaling behavior, which can be rationalized in terms of transition-state geometries and leads to considerably higher predicted activity. While synthesis of these model systems is likely challenging, the stabilization of such an active site could enable thermochemical ammonia synthesis under more benign conditions.

Identification and elimination of false positives in electrochemical nitrogen reduction studies

“…The most insightful theoretical studies in the NRR context, from our perspective, are those that build upon the theory in the first place. This can be either an overarching analysis of the general trends, plausible mechanisms and key limiting steps for the NRR catalysed by a broad family of materials 2,[40][41][42][43] , or a more specific investigation of one particular highly promising system [44][45][46][47][48] . Ideally, this kind of theoretical work should be followed by experiments to confirm the predicted activity, though unfortunately, we are not aware of any NRR work that has successfully followed that pathway.…”

Atomically Dispersed Single Ni Site Catalysts for Nitrogen Reduction toward Electrochemical Ammonia Synthesis Using N2 and H2O

“…For example, Choi et al indicated that NRR selectivity on single metal sites is significantly higher than that on bulk metals surfaces due to the effective suppression of the HER with the help of ensemble effect . Nørskov and co‐workers found that “on top” binding of nitrogen that is possible on single metal sites can enhance the rate of NH 3 synthesis . Moreover, it has been found that N species can promote the dissociation of N 2 , especially in the presence of single‐metal Lewis acid ions .…”

Ammonia Plasma-Catalytic Synthesis Using Low Melting Point Alloys

“…The use of a simple metal catalyst opens the probability of understanding the complex interactions of catalyst and plasma experimentally by studying the point-emission spectra at the metal plasma interface. Pure transition metals have been quite widely studied using molecular simulation for ammonia synthesis [25,26]. However, a novel class of metals and alloys known as low-melting point alloys has been ignored for catalytic applications in experimental as well as simulation reports.…”