A multi-functional composite nitrogen carrier for ammonia production via a chemical looping route

Abstract

Chemical looping has gained increasing attention as an alternative approach for sustainable ammonia synthesis. Nitrogen carriers with high efficiencies are the key to the practical feasibility and competitiveness of this approach. Herein, a multi-functional composite nitrogen carrier composed of Mn nitrides and alkali or alkaline earth metal imides (Li₂NH or BaNH) was identified, and it mediated a two-step chemical loop through N₂ fixation by the N-poor Mn nitride and alkali or alkaline earth hydride, and the hydrogenation of the N-rich Mn nitride and alkali or alkaline earth metal imides to ammonia. The distinguishing features of this nitride–imide composite nitrogen carrier lie in: 1. the Mn nitride not only acts as a nitrogen carrier but also functions as a catalyst significantly enhancing the kinetics of N₂ fixation by hydride to form imide and the subsequent hydrogenation of the imide to ammonia and the hydride; and 2. the hydrides, on the other hand, also facilitate the kinetics of N₂ fixation and subsequent hydrogenation of the Mn nitride. These features make the loop highly effective under mild conditions, i.e., at ambient pressure and temperatures below 300 °C; the composite nitrogen carrier Mn₂N–BaNH can mediate a loop with an ammonia production rate more than one order of magnitude higher than that mediated by neat BaNH and Mn₂N, respectively. Such an ammonia production rate can only be attained by a thermo-catalytic process catalyzed by Cs-promoted Ru/MgO at higher temperature and pressure. This work opens opportunities for the design and development of new types of nitrogen carriers with multiple components and functionalities.