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

Lithium hydride (LiH) has a strong effect on iron leading to an approximately 3 orders of magnitude increase in catalytic ammonia synthesis. The existence of lithium-iron 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 calculations. The ternary hydride species may serve as centers that readily activate and hydrogenate dinitrogen, forming Fe-(NH2 )-Li and LiNH2 moieties-possibly through a redox reaction of dinitrogen and hydridic hydrogen (LiH) that is mediated by iron-showing distinct differences from ammonia formation mediated by conventional iron or ruthenium-based catalysts. Hydrogen-associated activation and conversion of dinitrogen are discussed.