The Formation of Surface Lithium-Iron Ternary Hydride and its Function on Catalytic Ammonia Synthesis at Low Temperatures.

LiH has strong effect on Fe leading to ca. 3 orders-of-magnitude activity increase in catalyzing ammonia synthesis. The existence of Li-Fe ternary hydride species at the surface/interface of the catalyst were identified and characterized for the first time by gas-phase optical spectroscopy coupled with mass spectrometry and quantum chemical calculation. Those ternary hydride species may serve as active center and can readily activate and hydrogenate N2 forming Fe-(NH2)-Li and LiNH2 moieties possibly through the redox of N2 and hydridic H (LiH) that is mediated by Fe, showing distinct differences from ammonia formation on conventional Fe or Ru based catalysts. Hydrogen associated activation and conversion of N2 are thus discussed.