Identifying a New Pathway for Nitrogen Reduction Reaction on Fe-Doped MoS₂by the Coadsorption of Hydrogen and N₂

The electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the Haber-Bosch process with the potential for producing ammonia (NH₃) at ambient temperatures and pressures. Molybdenum disulfide (MoS₂), a layered transition-metal dichalcogenide, has attracted interest as an NRR electrocatalyst but possesses only a limited number of NRR-active sites and, furthermore, displays poor NRR selectivity due to the more favorable thermodynamics of the competing hydrogen evolution reaction (HER). To overcome these two challenges, we dope monolayer (ML) MoS₂with iron (Fe) and employ density functional theory (DFT) calculations to investigate the nature of NRR-active defects and alternative reaction mechanisms. We show that Fe-doping can modify the structure of edges of MoS₂MLs and assist in the formation of sulfur vacancy defects, which, in some cases, can selectively bind N₂over protons. In a departure from current approaches to modeling NRR, we carefully consider the role of coadsorbed H atoms, both at and in the vicinity of adsorption sites, and show how these competing adsorbates can profoundly affect both the preferred NRR pathways and their energetics. Our DFT studies reveal that a single sulfur vacancy on Fe-doped sulfur edges (50% S-coverage) can selectively reduce N₂to NH₃via a hitherto unexplored H-mediated enzymatic NRR pathway at a moderate cathodic limiting potential of 0.42 V. Our proposed H-mediated enzymatic NRR pathway shows that coadsorbed H atoms can assist indirectly in the reduction of N₂prior to the eventual evolution of H_2(g). Our results suggest that Fe-doping of MoS₂MLs is a promising approach for producing catalytic edge sites that are both active and selective for NRR at moderate potentials.