Nonthermal Plasma-Stimulated C–N Coupling from CH₄ and N₂ Depends on the Presence of Surface CHx and Plasma-Phase CN Species

Formation of C–N containing compounds from plasma-catalytic coupling of CH4 and N2 over various transition metals (Ni, Pd, Cu, Ag, and Au) is investigated using a multimodal spectroscopic approach, combining polarization-modulation infrared reflection–absorption spectroscopy (PM-IRAS) and optical emission spectroscopy (OES). Through sequential experiments utilizing CH4 and N2 nonthermal plasmas, we minimize plasma-phase reactions and identify key intermediates for C–N coupling on metal surfaces. Results show that simultaneous CH4 and N2 exposure with plasma stimulation produces surface C–N species. However, N2–CH4 sequential exposure does not lead to C–N species formation, while CH4–N2 sequential exposure reveals the presence of CHx surface species and CN radical species as key precursors to C–N species formation. From further analysis using X-ray photoelectron spectroscopy and liquid chromatography–mass spectrometry, the influence of exposure conditions on the degree of nitrogen incorporation and the nature of C–N species formed were revealed. The work highlights the importance of surface chemistry and exposure conditions in surface C–N coupling with plasma stimulation.