Surface dependent nitrogen reduction pathways on transition metals unveiled through adsorption energies

Electrocatalytic nitrogen reduction reaction (NRR) offers a sustainable alternative to the Haber-Bosch process but is hindered by competing hydrogen evolution and sluggish triple bond activation in N₂. Here, density functional theory (DFT) calculations are performed to systematically evaluate the adsorption energies of key NRR intermediates (*N₂,*N, *NNH, *NHNH, and *NNH₂) on representative transition-metal surfaces, including FCC(110), FCC(111), and HCP(0001). By comparing the adsorption energies of 2*N and *NNH, and those of *NNH₂ and *NHNH, we establish a simple adsorption-energy framework to identify the likely dissociative or associative mechanism and, for associative pathways, the distal or alternating route. The framework predicts dissociative pathways for Ni(111), Rh(111), Ir(111), Ni(110), Ru(0001), and Co(0001), whereas Pd(111) and Cu(111) favor the associative distal pathway and most other surfaces favor the associative alternating pathway. Analysis of Bader charge transfer, N-N bond elongation, and minimum metal-N distances provides further insight into the structural and electronic factors governing NRR intermediate adsorption. Overall, these results demonstrate that adsorption-energy differences, complemented by electronic-structure analysis and representative transition-state calculations, provide an efficient framework for screening and understanding NRR mechanisms across diverse transition-metal surfaces.