Traditional ammonia synthesis proceeds via the Haber-Bosch process, which operates at high temperatures and pressures. In the chemical looping conception of ammonia synthesis, dinitrogen gas is first reacted with a metallic catalyst to form a reactive nitride species. Next, the resulting nitride is reacted with gaseous hydrogen to form ammonia. The material undergoes nitride formation-hydrogenation-ammonia desorption-nitride regeneration with dinitrogen, this process is analogous to the more familiar concept of chemical looping combustion. This overall process is deemed chemical looping ammonia synthesis. The benefits of the chemical looping ammonia synthesis system are lower pressures, and optimized materials for each step of the reaction. Most candidates readily form nitrides below 900 °C and belong to alkali, alkaline, or transition metal groups, some are intermetallic or metal hydride compounds. Manganese sits between the highly stable nitride chromium and the well-studied Haber-Bosch support iron which forms a weak nitride. Manganese nitride has received attention due to its ability to form Mn4N, Mn2N and Mn3N2 at relatively low temperatures. The compound has also been studied in physical mixtures with BaH2 and other promoters to enhance its activity. While a detailed understanding of the fluid-solid reaction is lacking, some studies have characterized the bulk thermodynamics and the utility of this material for ammonia synthesis and the storage and transfer of N3-. The aim of this work is to elucidate the rate limiting steps in the gas-solid chemical looping ammonia synthesis reaction under hydrogen and dinitrogen. Unlike traditional heterogeneous catalysis, there may be both surface reactions and bulk reactions occurring simultaneously. Additionally, gas diffusion and internal diffusion all contribute to the activity of Mn nitrides. It is proposed that particle size, surface area, morphology, degrees of oxidation, nitridation, phase, inclusion of oxide supports, and temperature are critical factors that will be considered. The experimental strategies and methods may be applicable to other novel materials for ammonia synthesis.

Kinetic study of chemical looping ammonia synthesis candidates: Manganese nitride as a model material

ACS Spring 2022

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