(434c) ?-Al2O3-Supported Mo2n As a Highly Active and Regenerative Nitrogen Carrier for Chemical Looping Ammonia Synthesis

Ammonia is an essential chemical for fertilizer production and is also recognized as a promising carbon-free fuel with high energy density. Currently, approximately 50% of global ammonia is produced via the energy-intensive Haber–Bosch process, which operates under harsh high-pressure conditions. As an alternative, the chemical looping ammonia synthesis (CLAS) process, which proceeds through the cyclic oxidation and reduction reactions of a metal-based nitrogen carrier (NC), has attracted significant attention due to its high product selectivity and energy efficiency under ambient pressure conditions. For the efficient implementation of CLAS, the development of redox-active NCs with excellent nitrogen fixation and release capability is crucial. Metal nitrides are considered strong candidates for this role owing to their facile synthesis, high thermal stability, and selective nitrogen adsorption capacity. Among them, molybdenum nitride (Mo₂N) is particularly notable due to the inherent ability of Mo to promote N₂ dissociation. However, for practical application in CLAS systems, Mo-based NCs must possess a high surface area and exhibit optimized desorption of NHₓ intermediates, along with stable regeneration of lattice nitrogen to maintain long-term cycle performance.

In this work, supported molybdenum-based nitrogen carriers (NCs) were synthesized using ammonium molybdate, hexamethylenetetramine, and γ-Al₂O₃ as precursors via controlled pyrolysis at various temperatures. Their hydrogenation performance, lattice nitrogen regeneration behavior, and cyclic stability in chemical looping ammonia synthesis (CLAS) were systematically investigated. The results demonstrated that all NCs, except for the sample subjected to pyrolysis at 850 °C, exhibited an average ammonia production rate of approximately 6 mmol·g⁻¹·h⁻¹. Surface characterization revealed that the introduction of γ-Al₂O₃ support facilitated uniform dispersion of NCs, contributing to enhanced hydrogenation activity and nitrogen regeneration. Furthermore, the formation of nitrogen vacancies following hydrogenation played a critical role in promoting N₂ dissociation, ultimately leading to improved overall ammonia productivity. Among the synthesized NCs, the Mo₂N/γ-Al₂O₃ prepared at 650 °C exhibited the most favorable hydrogenation performance and maintained a stable ammonia production rate of around 6 mmol·g⁻¹·h⁻¹ over 12 cycles at 550 °C and atmospheric pressure. This work effectively mitigated the limitations associated with conventional molybdenum-based nitrides in terms of hydrogenation and regeneration efficiency and is expected to provide valuable insights and design guidance for the development of next-generation nitrogen carriers for efficient CLAS processes.