Nitrogen fixation to ammonia-based fertilizers forms the basis of modern agriculture and mediates global food insecurity. However, conventional ammonia synthesis has an unsustainable energy and carbon footprint. A competitive and modular alternative to conventional Haber-Bosch ammonia synthesis (HB) could reduce CO2 emissions, improve the global distribution of fertilizer to promote food security, and facilitate storage and transportation of hydrogen generated from renewable energy in nitrogen-based fuels. Emerging plasma-activated catalytic N2 fixation processes may potentially circumvent the limitations of conventional thermocatalytic ammonia synthesis and enable decentralized ammonia production using CO2-free energy and renewable feedstocks. However, fundamental understanding of the reaction mechanisms and plasma-catalyst interactions is lacking. We identify the reaction intermediates adsorbed on γ-Al2O3-supported Ni and Fe catalysts during plasma-activated ammonia synthesis using FTIR spectroscopy, steady-state flow reactor experiments, and computational kinetic modeling. Ammonia yield can be influenced by plasma-derived intermediates and their interactions with catalyst surfaces, which lead to different reaction pathways: Ni/γ-Al2O3 enhances plasma-promoted NH3 production and favors surface-adsorbed NHx species, while Fe/γ-Al2O3 shows the presence of N2Hy and a lower overall concentration of N-containing adsorbates. Plasma-catalyst interactions are probed to reveal that elevated temperature and plasma irradiation of the surfaces promote NH3 desorption. The direct evidence of catalytic surface reactions occurring during a plasma-activated process provides mechanistic insight into plasma-activated ammonia synthesis. Furthermore, we compare the energy requirements, CO2 emissions, and commodity prices associated with plasma-activated ammonia synthesis to those for HB. The threshold efficiency required for a plasma process to become competitive is established for various scenarios of H2 sources, energy sources from natural gas to CO2-free renewable energy, and carbon pricing policies.

Decarbonizing ammonia using plasma catalysis: From surface reactions to techno-economic analysis

ACS Spring 2022

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