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

Abstract

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 N2 by alkali or alkaline earth metal hydrides to form imides and, subsequently, the imides are hydrogenated to produce NH3 and regenerate the metal hydrides. The oxidation state of hydrogen therefore switches between −1 (hydride), 0 (H2) and +1 (imide and NH3). Late 3d metals accelerate the reaction rates of both steps. The chemical loop mediated by BaNH and catalysed by Ni produces NH3 at 100 °C and atmospheric pressure. Reducing the severity of the conditions required to synthesize ammonia would increase the viability of its use as a carbon-free energy carrier. Here the authors use metal imides to mediate ammonia production via a two-step chemical looping process that operates under mild conditions.