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

Abstract

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 (V_N), 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 V_N facilitates the adsorption and activation of dinitrogen, a process that is further supported by the replenishment of V_N 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 g_Fe⁻¹ h⁻¹, surpassing the performance of state-of-the-art Fe-based catalysts at 300 °C 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.