Production of ammonia via a chemical looping process based on metal imides as nitrogen carriers

Ammonia is a promising carbon-free energy carrier, but is currently synthesized industrially under harsh conditions. Synthesizing ammonia using lower temperatures and pressures could therefore improve its prospects as a chemical means to store and transport energy. Here we report that alkali and alkaline earth metal imides function as nitrogen carriers that mediate ammonia production via a two-step chemical looping process operating under mild conditions. Nitrogen is first fixed through the reduction of N 2 by alkali or alkaline earth metal hydrides to form imides and, subsequently, the imides are hydrogenated to produce NH 3 and regenerate the metal hydrides. The oxidation state of hydrogen therefore switches between −1 (hydride), 0 (H 2 ) and +1 (imide and NH 3 ). Late 3d metals accelerate the reaction rates of both steps. The chemical loop mediated by BaNH and catalysed by Ni produces NH 3 at 100 °C and atmospheric pressure.