Mechanism of the N₂ Cleavage Promoted by Lithium vs Other Alkali and Alkaline-Earth Metals

The lithium-mediated nitrogen reduction reaction (Li-NRR) currently stands as the most feasible route for producing ammonia (NH3) from atmospheric nitrogen (N2) in a sustainable manner under close-to-ambient conditions. The key step enabling the Li-NRR to occur under such mild conditions is the capability of metallic lithium (Li0) to spontaneously adsorb N2 and cleave the triple nitrogen–nitrogen bond via a redox reaction producing lithium nitride (Li3N)─a well-known but yet to be fully understood process. The present study theoretically explores the distinctive behavior of Li0, Na0, K0, Be0, Mg0, and Ca0 in terms of their N2-philicity and metal–surface interaction, also delving into the mechanistic aspects of diffusion and the cleavage kinetics of the N2 molecule for Li0. In this sense, Li0 exhibits greater N2 chemisorption than the other metals examined, only surpassed by Ca0 which reveals a very high N2-philicity. In addition, the cleavage and splitting of the N≡N bond is promoted by Li0 with activation barriers less than 1.0 eV for the different Li0 facets studied in this work.