Identifying Surface Reaction Intermediates in Plasma Catalytic Ammonia Synthesis

Ammonia synthesis by plasma catalysis has emerged as an alternative process for decoupling nitrogen fixation from fossil fuels. Plasma activation can potentially circumvent the limitations of conventional thermocatalytic ammonia synthesis; however, the contribution of different reaction mechanisms to the production of ammonia at the catalyst surface remains unclear. Here, we identify the reaction intermediates adsorbed on γ-Al2O3-supported Ni and Fe catalysts during plasma-activated ammonia synthesis under various temperatures and reactor configurations 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.