Activating dinitrogen for chemical looping ammonia synthesis: nitridation of manganese

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 μm.