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The earth-abundant transition metal manganese (Mn) has been shown to activate dinitrogen (N 2) and store nitrogen (N) as nitride for subsequent chemical reaction, for example, to produce ammonia (NH3). Chemical looping ammonia synthesis (CLAS) is a practical way to use Mn nitride by contacting nitride with gaseous hydrogen (H2 ) to produce ammonia (NH 3). Here, the dynamic process of N atoms penetrating into solid Mn has been investigated. Nitride layer growth was modeled to quantitate and pre- dict the storage of activated N in Mn towards designing CLAS systems. The N diffusion coefficient (DN ) and reaction rate constant K for the first-order nitridation reaction were estimated at 6.2 ± 5.5 10-11 m2/s and 4.1 ± 3.5 10-4 1/s, respectively, at atmospheric pressure and 700 °C. Assuming spherical particles of Mn with a diameter of < 10 lm, about 56.8 metric tons of Mn is sufficient to produce a metric ton of NH 3 per day using CLAS 

The earth-abundant transition metal manganese (Mn) has been shown to be useful to activate dinitrogen at atmospheric pressure and elevated temperature by forming bulk Mn nitrides. Mn nitrides could then be used, for example, for ammonia (NH3) synthesis in a chemical looping process by contacting nitride with gaseous hydrogen (H2). Here, we present an investigation of the morphology and local time-dependent composition of micrometer-scale Mn plates during nitridation in dinitrogen (N2) near atmospheric pressure at 700 C. The main motivation was to obtain design data for chemical looping synthesis of NH3 and to add to the somewhat sparse literature on nitridation of Mn. The morphology and elemental compositional variation of the nitrided specimens were studied with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), wide angle X-ray diffraction (WAXD), and mass balances. Three possible nitrogen (N) populations that may govern the Mn nitridation and later NH3 synthesis are identified. After four hours of nitridation, the N weight gain was found to be 9.4 ± 0.7 kgN to nMn-1 for the plates used here, resulting in a nitridation depth of 83 ± 8 lm. 

Abstract Affordable synthetic ammonia (NH₃) enables the production of nearly half of the food we eat and is emerging as a renewable energy carrier. Sodium‐promoted chemical looping NH₃synthesis at atmospheric pressure using manganese (Mn) is here demonstrated. The looping process may be advantageous when inexpensive renewable hydrogen from electrolysis is available. Avoiding the high pressure of the Haber‐Bosch process by chemical looping using earth‐abundant materials may reduce capital cost, facilitate intermittent operation, and allow operation in geographic areas where infrastructure is less sophisticated. At this early stage, the data suggest that 0.28 m³of a 50 % porosity solid Mn bed may suffice to produce 100 kg NH₃per day by chemical looping, with abundant opportunities for improvement.