In Situ Identification of NNH and N₂H₂ by Using Molecular-Beam Mass Spectrometry in Plasma-Assisted Catalysis for NH₃ Synthesis

Ammonia synthesis at 533 K and atmospheric pressure was investigated in a coaxial dielectric barrier discharge (DBD) plasma reactor without packing and with porous γ-Al2O3, 5 wt % Ru/γ-Al2O3, or 5 wt % Co/γ-Al2O3 catalyst particles. Gas-phase species were monitored in situ using an electron impact molecular-beam mass spectrometer (EI-MBMS). Gas-phase species NNH and N2H2 were first identified under common conditions of plasma-assisted ammonia synthesis and were present at levels comparable to that of NH3 in the plasma discharge. Concentrations of NNH, N2H2, and NH in a reactor packed with γ-Al2O3 or other particles were lower than those observed in an empty reactor, while the concentration of NH3 increased. These observations point to the importance of NNH and N2H2 in plasma-assisted surface reactions in ammonia synthesis. Reaction pathways of direct adsorption of gas-phase NNH and N2H2 on solid surfaces and subsequent reactions were proposed. This study demonstrated that in situ identification of gas-phase species via EI-MBMS provides a powerful approach to study the kinetics of plasma-assisted catalysis.