Atomically dispersed Fe–TM pairs on g-C6N6: a comparative theoretical insight into heteronuclear dual-atom catalysts for N2 reduction to form ammonia and urea

Abstract

Heteronuclear double-atom catalysts (HDACs) pair two distinct metal atoms in intricately engineered coordination environments, providing a unique combination of atomic precision, synergistic reactivity, and adjustable electronic properties for unlocking reaction pathways that are inaccessible to their single-atom or homonuclear counterparts. Here, we have utilized a computationally efficient multistep screening strategy based on density-functional theory calculations for the robust evaluation of the performance of 29 different Fe–TM-based HDACs in the comparative reduction of N₂ to ammonia and urea. Based on the catalyst stability, N₂ binding energy and the free-energy changes involved in the potential-determining steps for nitrogen reduction to ammonia and urea, nine different promising candidates were identified for ammonia formation, and two were identified for urea formation. Most strikingly, the Fe–V and Fe–Ir pairs embedded in g-C₆N₆ are found to manipulate the binding strength of the target reaction intermediates for N₂ to NH₃ conversion along the distal pathways with record-low limiting potential values of −0.13 and −0.25 V, respectively. Further, a volcano-type relationship between the limiting potential and the free-energy difference between the NH₂ and NH₃ intermediates (ΔG_*NH2 − ΔG_*NH3) with Fe–V at the apex of the plot serves as a powerful predictive tool to rationally design such catalysts for N₂-to-NH₃ formation. With respect to urea formation, Fe–Ti and Fe–Y exhibit promising catalytic activity for N₂-to-urea formation with limiting potentials of −0.84 and −0.62 V, respectively. Moreover, CI-NEB calculations reveal very low activation barriers for NH₃ and urea formation on the above HDACs. The superior N₂ activation on the modelled HDACs is further rationalized in terms of the electronic and geometric properties of the HDACs. The present study not only puts forth a robust and efficient computational protocol for the screening of HDACs as catalysts for N₂ conversion into urea and ammonia but also contributes to elucidation of their structure–activity correlations.