Investigation of Plasma Surface Interactions: Methane and Nitrogen Plasma on Model Catalytic Surfaces

Atmospheric pressure non-thermal plasmas (NTPs) produce a highly reactive chemical environment, including electrons, ions, radicals, vibrationally- and electronically-excited atoms and/or molecules, at low bulk temperatures at which such species are thermally inaccessible. The integration of NTPs and heterogeneous catalysis has recently gained substantial interest to carry out chemical transformations that are difficult using conventional thermal catalysis (e.g., methane (CH4) and nitrogen (N2) activation). The synergy between plasma and a catalytic surface originates from their interactions at the interface. Thus, to better control the plasma-catalyst coupling, it is essential to understand how the reactive chemical environment produced in the plasma phase interacts with catalytic materials. However, progress in plasma catalysis is hindered by the lack of fundamental understanding of plasma-catalytic surface interactions due to the complexity of plasma catalysis which includes co-existence of plasma-phase and surface chemistry, and heterogeneity of catalytic materials. In this work, the interactions between NTP-activated methane and nitrogen and various model catalytic and non-catalytic surfaces (Ni, Pd, Cu, Ag, Au, SiO2, and KBr) are studied using a newly developed multimodal spectroscopic tool combining polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS), optical emission spectroscopy (OES), and mass spectrometry (MS). First, this work describes the development of the novel multimodal spectroscopic tool and demonstrates its utility by probing plasma-assisted non-oxidative coupling of CH4 over various surfaces. The results unveil the activation of carbonaceous surface species by an argon NTP on Ni and SiO2, forming hydrogen and C2 hydrocarbons, through distinct pathways. Following this study, the interactions between NTP-activated N2 and various model catalytic surfaces were investigated thoroughly to identify the surface-adsorbed nitrogen-containing species. The results show the potential of non-thermal plasmas to access thermally inaccessible adsorbates and the challenges posed by ppm-level impurities to corrupt the interpretation of plasma catalytic chemistry. Finally, the formation of C-N containing compounds from the plasma-catalytic coupling of CH4 and N2 together over model catalytic surfaces was investigated. C-N coupled products formed were further analyzed with X-ray photoelectron spectroscopy and liquid chromatography-mass spectrometry. The results show the importance of surface chemistry and exposure conditions in surface C-N coupling with plasma stimulation.