N₂/Ar Mixed Discharge for Efficient Nitrogen Activation toward Green Ammonia Synthesis Application

In ammonia synthesis, a new reaction system that does not use hydrogen (H2) as a raw material, such as the plasma/liquid (P/L) reaction, contributes to creating a sustainable chemical industry. The P/L reaction is intended to abstract hydrogen atoms from water molecules to synthesize ammonia under ambient conditions without any catalysts but using electrically activated nitrogen species in the plasma. Therefore, the energy transfer process leading to nitrogen activation is key to the P/L reaction. Argon-admixed nitrogen plasma can drastically alter nitrogen activation processes by energizing electrons and pooling energy into its excited states. This study examines the efficacy of activated nitrogen in N2/Ar mixed plasma and the lifetime of atomic nitrogen (Natom) produced in gaseous plasma. A qualitative assessment of the electron temperature using optical emission spectroscopy indicated that the argon admixture enhanced the electron collision reaction process and energy pooling. Quantification of Natom using vacuum ultraviolet spectroscopy revealed that argon admixture extended the apparent lifetime of Natom, thereby improving the transfer efficiency of activated nitrogen to the liquid surface for the P/L reaction and increased ammonia synthesis. Thus, this study contributes to the development of green synthetic processes of ammonia.