Unlocking the potential of alkaline-earth metal active centers for nitrogen activation and ammonia synthesis: the role of s–d orbital synergy

Abstract

Transition metals (TM) have been widely studied for electrocatalytic nitrogen fixation, while main-group, especially alkaline-earth (AE), metals remain largely unexplored. Herein, we theoretically investigated the capability and mechanism of AE metal's active centers to activate N₂ and electrocatalytically convert it to ammonia. Specifically, the constructed AE metal double-atom catalysts, embedded in TM dichalcogenide monolayers, can strongly adsorb N₂ with exothermic energy up to −0.62 eV. Especially, N₂ has been activated via s–d orbital synergy of AE metal active centers. AE metal's orbitals share electrons with their empty d orbitals, and the partially occupied d orbitals can exchange electrons with the σ and π* orbitals of N₂ to enable its adsorption and activation. Furthermore, an activity volcano map was established for electrocatalytic N₂ reduction, where CaBa@MoSe₂ was observed to achieve a minimum limiting potential of −0.60 V, which is comparable with the TM counterparts. This work offers new insights into the physical mechanism by which the active centers of AE metal activate inert N₂ molecules and could inspire the design of AE metal-based catalysts.