Exploring the relationship between N-radical and nitride formation in plasma-assisted ammonia synthesis*

In order to gain an understanding of the reaction pathways present in plasma assisted ammonia synthesis compared to the traditional thermal process, it is critical to quantify the variety of excited and ionized species produced in the plasma and study how these species interact in both the gas phase and on catalyst surfaces. One of the critical elementary steps in this system is the dissociation of N₂ into two N radicals, which adsorb on the catalyst surface resulting in the formation of metal nitrides. The role of nitride in ammonia synthesis remains unclear. This work utilizes femtosecond two-photon absorption laser-induced fluorescence(fs-TALIF) to measure the atomic densities of nitrogen radicals formed in an AC-driven, N₂/H₂, dielectric barrier discharge (DBD) plasma. TALIF and online-sampling gas chromatography (GC) data showed the presence of N and H radicals in addition to NH₃ formation, however, there is a lack of understanding on the relationship between N radicals and surface nitride formation. Ex-situ surface characterization detected the presence of metal nitride for several nanometers within a nickel film following treatment with an N₂/H₂ plasma. This work will couple the gas phase N radical concentration and surface nickel nitride formation to help lay the groundwork for designing more efficient catalysts for ammonia production.