Mechanism of the N<sub>2</sub> Cleavage Promoted by Lithium vs Other Alkali and Alkaline-Earth Metals
Luis Miguel Azofra, J.M. Doña-Rodrı́guez, Douglas R. MacFarlane, Alexandr N. Simonov
Abstract
The lithium-mediated nitrogen reduction reaction (Li-NRR) currently stands as the most feasible route for producing ammonia (NH 3 ) from atmospheric nitrogen (N 2 ) 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 (Li 0 ) to spontaneously adsorb N 2 and cleave the triple nitrogen–nitrogen bond via a redox reaction producing lithium nitride (Li 3 N)─a well-known but yet to be fully understood process. The present study theoretically explores the distinctive behavior of Li 0, Na 0, K 0, Be 0, Mg 0, and Ca 0 in terms of their N 2 -philicity and metal–surface interaction, also delving into the mechanistic aspects of diffusion and the cleavage kinetics of the N 2 molecule for Li 0 . In this sense, Li 0 exhibits greater N 2 chemisorption than the other metals examined, only surpassed by Ca 0 which reveals a very high N 2 -philicity. In addition, the cleavage and splitting of the N≡N bond is promoted by Li 0 with activation barriers less than 1.0 eV for the different Li 0 facets studied in this work.