Investigation of Key Intermediate Species in Non-Thermal Plasma-Activated Carbon-Nitrogen Coupling on a Catalytic Surface from Methane and Nitrogen

The integration of reactive non-thermal plasmas (NTPs) with heterogeneous catalysis can promote novel chemical transformations that neither plasma nor catalysis could deliver individually, such as direct carbon-nitrogen (C-N) coupling from methane (CH₄) and nitrogen (N₂). To unveil the full potential of plasma-catalyst coupling, it is important to understand the interaction between the NTP-activated species and a catalytic surface at the interface. In this work, we utilized multi-modal spectroscopy combining polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS), optical emission spectroscopy (OES), and mass spectrometry (MS) to investigate the key intermediate species that lead to C-N coupling on model catalytic surfaces (Ni, Pd, Cu, Ag, and Au). Our system consists of an argon (Ar) plasma jet propagating through a controlled CH₄ and N₂ environment and impinging upon a substrate. We investigated C-N coupling on a catalytic surface exposed to (1) 1:1 CH₄:N₂, (2) CH₄-N₂ in sequence, where pure CH₄ feed followed by pure N₂ feed, and (3) N₂-CH₄ in sequence, where pure N₂ feed followed by pure CH₄ feed. Our results show that surface CH_x and plasma-phase CN species correspond to C-N coupling on a catalytic surface. The effects of (1) different surfaces and (2) different procedures on the nature of surface-adsorbed C-N coupled products are further investigated with post-experiment X-ray photoelectron spectroscopy (XPS) and liquid chromatography-mass spectrometry (LC-MS).