(220b) Chemical Looping Ammonia Synthesis: Thermal, Microwave, and Plasma Approaches

Ammonia synthesis by Haber-Bosch is responsible for 1% of global energy use per year, 2.5% of global CO2 emissions per year, and for supporting approximately 4 billion additional persons’ nutritional requirements ^1–3. The importance of Ammonia to the world economy is not in doubt, but ammonia may also prove to be a future energy carrier due to its high hydrogen content. The chemical looping ammonia synthesis (CLAS) reaction seeks to decouple the high energy dinitrogen cleavage reaction from the ammonia synthesis reaction. The ammonia synthesis reaction may proceed as a hydrogenation or hydrolysis reaction, depending on how the loop is constructed. Due to the passive nature of this process, nitrogen may be stored and retrieved later at the site where it will be used.

Fixed bed thermal and microwave reactors were used to evaluate the three-cycle ammonia synthesis productivity of CLAS materials; Fe and CoMo. These materials were physically mixed with the insulating microwave sorbent, SiC, in a 1:1 ratio to enhance their microwave heating capability. Microwave heated samples were found to be more productive but deactivate faster than thermally heated samples. Regeneration strategies and deactivation modes were considered and analyzed.

During thermal fixed bed experiments CoMo was found to evolve ammonia during nitridation, in the absence of hydrogen ⁴. A dynamic surface, a shrinking core and counter diffusion were proposed as the source of hydroxyl ions stabilized by nitride ions, reacting at the Co₃Mo₃N surface detected with XPS ⁴. The existence of an unreacted oxide core containing molybdenum bronzes, H_xMoO₃ was confirmed using Raman spectroscopy. This OH^-1 diffuses outward as the core is stabilized by N^-3, resulting in the observation of gas phase H₂O and NH₃⁴.

The productivity and kinetics of common CLAS candidates was experimentally evaluated in thermal and thermal-plasma conditions. Common materials include Mn, Fe, and CoMo, as well as promoters and oxides are considered in both systems. Plasma can pre-activate the N≡N system and lower the energy required for reaction. Plasma treatment can also interact with the surface in different ways, perhaps circumventing some of the deactivation modes encountered with thermal only CLAS processes. Spent materials are characterized with SEM, BET, and XRD. Time on stream reactions of outlet gases are analyzed and OES is used to determine the species present in the plasma and evolving from the surface.

References:

(1) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Kilmont, Z.; Winiwarter, W. How a Century of Ammonia Synthesis Changed the World. Nat. Geosci. 2008, 1, 636–639. https://doi.org/10.1038/ngeo325.
(2) Pfromm, P. H. Towards Sustainable Agriculture: Fossil-Free Ammonia. J Renew. Sustain. Energy 2017, 12.
(3) Appl, M. Ammonia. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006.
(4) Brown, S. W.; Jiang, C.; Wang, Q.; Caiola, A.; Hu, J. Evidence of Ammonia Synthesis by Bulk
Diffusion in Cobalt Molybdenum Particles in a CLAS Process. Catalysis Communications 2022, 106438. https://doi.org/10.1016/j.catcom.2022.106438.