Lithium hydride synergy with Fe/N-co-doped porous carbon for efficient ammonia synthesis

The development of supported metal hydride catalysts with abundant active sites is a highly desirable yet challenging endeavor in ammonia synthesis. In this work, a lithium hydride (LiH)-loaded Fe/N-co-doped porous carbon (Fe-N-C) composite (LiH/Fe-N-C) was synthesized and demonstrated as an efficient catalyst for ammonia synthesis under mild conditions. The deposition of LiH onto Fe-N-C induced a partial reduction of the Fe-N moiety, resulting in the slight aggregation of isolated Fe single-atom sites into nanoparticles. Under reducing conditions (in the presence of H2), the coordinated nitrogen in Fe-N-C can be removed, forming nitrogen vacancies (VN), while surface iron hydride species (Fe-H) are generated through the interaction between LiH and Fe-N-C. The presence of Fe atoms adjacent to VN facilitates the adsorption and activation of dinitrogen, a process that is further supported by the replenishment of VN under reaction conditions. Notably, no lithium imide or amide species (Li2NH or LiNH2) — typically regarded as key intermediates in LiH-mediated ammonia synthesis, were observed on LiH/Fe-N-C, likely due to the fast hydrogenation step. Consequently, LiH/Fe-N-C achieves an ammonia production rate of 872 mmol·gFe-1·h-1, surpassing the performance of state-of-the-art Fe-based catalysts at 300 oC and 1 MPa. This study demonstrates that the synergistic catalysis of LiH, transition metals, and their adjacent nitrogen vacancies offers a promising pathway for efficient ammonia synthesis under mild conditions.