Catalytic active centers beyond transition metals: atomically dispersed alkaline-earth metals for the electroreduction of nitrate to ammonia

Abstract

Alkaline-earth (AE) metals have rarely been reported to be the active centers in heterogeneous catalysis. However, in nature, Mg cofactors in enzymes exhibit super activity for biochemical reactions. Herein, taking the AE metal single-atom catalyst (AE-SAC) supported on graphene as a representative, we theoretically explore the feasibility of AE metals as active centers in heterogeneous catalysis for the electrocatalytic nitrate (NO₃⁻) reduction reaction (eNO₃RR) to produce NH₃. Intriguingly, the AE metal active centers could strongly adsorb and effectively activate NO₃⁻, and catalyze eNO₃RR efficiently, similar to that of the transition-metal active centers. In particular, Ba and Sr SACs coordinated with three nitrogen and one carbon atoms exhibited super eNO₃RR activity with an ultralow limiting potential of −0.05 V, which also showed desirable selectivity and good stability. Mechanistic investigations indicated that, although AE metal elements are highly ionic, they function as transmitters to assist the charge transfer between the catalyst support and NO₃⁻ as well as effective intermediates, which enables the effective activation of NO₃⁻, via the “donation-back donation” mechanism, and further completes the catalytic cycle. Our work could broaden the ideas and lay a theoretical foundation for the development of heterogeneous catalysts with AE metals as active centers.