Rational design of metal nitride redox materials for solar-driven ammonia synthesis

Abstract

Fixed nitrogen is an essential chemical building block for plant and animal protein, which makes ammonia (NH 3 ) a central component of synthetic fertilizer for the global production of food and biofuels. A global project on artificial photosynthesis may foster the development of production technologies for renewable NH 3 fertilizer, hydrogen carrier and combustion fuel. This article presents an alternative path for the production of NH 3 from nitrogen, water and solar energy. The process is based on a thermochemical redox cycle driven by concentrated solar process heat at 700–1200°C that yields NH 3 via the oxidation of a metal nitride with water. The metal nitride is recycled via solar-driven reduction of the oxidized redox material with nitrogen at atmospheric pressure. We employ electronic structure theory for the rational high-throughput design of novel metal nitride redox materials and to show how transition-metal doping controls the formation and consumption of nitrogen vacancies in metal nitrides. We confirm experimentally that iron doping of manganese nitride increases the concentration of nitrogen vacancies compared with no doping. The experiments are rationalized through the average energy of the dopant d-states, a descriptor for the theory-based design of advanced metal nitride redox materials to produce sustainable solar thermochemical ammonia.

Electrochemical synthesis of ammonia via Mars-van Krevelen mechanism on the (111) facets of group III–VII transition metal mononitrides

“…Therefore, TMNs have evolved a potential candidate for the noble metal material catalyst and consequently represented better activity i.e. for electrochemical ammonia synthesis [3][4][5] and solar thermochemical ammonia production [6][7][8] when compared with pure metals [9]. For catalytic nitrogen reduction to ammonia, these TMNs have the extra benefit over pure transition metals since nitrogen atoms are already incorporated in their structure.…”

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N2 and Fe–CN Reductions

“…2 Free N 2 exhibits a low electron affinity (−1.9 eV) and proton affinity (118 kcal/mol), 3 and activation by a suitable catalyst is hence required. While growing in number, 4 few well-defined molecular systems mediate catalytic N 2 -to-NH 3 conversion; those that do use a combination of inorganic reductant and acid rather than H 2 .…”

Manipulating the Geometric and Electronic Structures of Manganese Nitrides for Ammonia Synthesis

“…Propelled by the need to reduce energy consumption and fossil fuels dependence, metal nitrides are gaining attention as solid N carriers for intermittent, sustainable NH 3 production . In the past decade, a two‐stage chemical looping scheme, which carries out N 2 activation and NH 3 formation in separate steps, has been demonstrated for NH 3 formation at atmospheric pressure …”

“…For chemical looping processes based on Mn nitrides only, local or global modifications of their chemical compositions via doping or alloying result in altered electronic structures, can also be attempted to optimize NH 3 productivity . The central guiding principle is to sufficiently weaken the Mn−N bonds in nitrides without sacrificing their ability for N 2 activation.…”