Chemical Looping Ammonia Synthesis with High Performance Supported Molybdenum-based Nitrogen Carrier

Ammonia is not only a vital source of fertilizers production, but also a hydrogen carrier with high energy density, which has the potential to replace the traditional fossil fuels. However, the industrial route for ammonia production (Haber-Bosch process) is a highly energy-intensive process owing to the harsh operating conditions requirement. It is important to develop a novel ammonia synthesis process with mild operating conditions. Chemical looping ammonia synthesis (CLAS) is known to be an innovative and environmentally-friendly low-pressure ammonia synthesis technology, which separates the overall ammonia synthesis reaction into nitrogen release and replenish reactions facilitated with a nitrogen carrier (NC). NC with high efficiencies is the key to the the practical feasibility of CLAS technology. In this work, the supported molybdenum-based NCs were prepared with ammonium molybdate, hexamethylenetramine, and ZSM-5 zeolites through a facile pyrolysis method at different temperatures, and the performance of N-release, N-fixation, and the cyclic CLAS of NCs were studied in detail. The results indicate that the supported molybdenum-based NC pyrolyzed at 450 degrees C outperformed other NCs with an average NH3 production rate of ca. 20000 mu mol.g(-1).h(-1), which is two or three orders of magnitude higher than that of the known metal nitrides under similar conditions. Bulk and surface analyses of NCs indicated the migration of lattice nitrogen of NC during the direct hydrogenation step for NH3 formation. At N-fixation stage, the nitrogen vacancy of molybdenum-based NC is expected to be recharged from N-2. However, the low kinetics is an important problem of NC nitridation. With the introduction of H-2, the nitridation kinetics of NC was significantly enhanced, improving the NC regeneration. During a 12-cycle test at 600 degrees C and atmospheric pressure, and the ammonia production rate was stabilized at ca. 1500 mu mol.g(-1).h(-1) for each cycle. This article investigated the preliminary feasibility of the supported molybdenum-based nitride as NC during the process of CLAS and could provide a theoretical basis for the design and development of new types of NCs.

Automated summary

This study investigates the performance of supported molybdenum-based nitrogen carriers (NCs) in chemical looping ammonia synthesis (CLAS). The results show that the supported molybdenum-based NC pyrolyzed at 450 degrees C exhibits the highest ammonia production rate. The findings provide a theoretical basis for the design and development of new types of NCs.