Synergistic Photothermal Catalysis of 3d Transition Metal-Lithium Hydride Composites for Low-Temperature Ammonia Synthesis.

Abstract

Ammonia synthesis under mild conditions remains a critical yet formidable challenge in sustainable energy research. In this study, we report a novel photothermal ammonia synthesis strategy employing synergistic 3d transition metal-lithium hydride (TMs-LiH) composites. The TMs-LiH system (comprising V-, Cr-, Mn-, Fe-, Co-, and Ni-LiH composites) exhibits universal catalytic performance at low temperatures (100-200 °C), forming a characteristic "plateau curve" that indicates ammonia synthesis activity is less dependent on the specific TM component. Crucially, catalytic activity collapses entirely under identical thermal conditions in the absence of light, underscoring the pivotal role of photothermal effects in activating the reactants. Notably, compared to conventional thermochemical ammonia synthesis, the TMs-LiH catalysts under light illumination demonstrate a significant reduction in activation energies and reaction orders for H₂ and NH₃, revealing enhanced hydrogenation efficiency and ammonia desorption kinetics. Among the tested composites, Fe-LiH exhibits the highest catalytic activity, prompting further investigations into its unique reactivity. Fe-LiH induces a strong interplay with N₂ with the assistance of light, driving the reductive elimination of hydridic hydrogen to H₂, which likely creates a reduced Fe-LiH_1-x interface that favor nitrogen fixation to form lithium amide species (LiNH₂). Subsequent hydrogenation of LiNH₂ on Fe-LiH_1-x proceeds facilely to produce NH₃.