(145g) Plasma-Assisted Upgrading of Methane: Mechanistic Insights from in-Situ PM-IRAS and OES Spectroscopy

Non-thermal plasmas (NTPs) produce reactive chemical environments including electrons, ions, radicals, and vibrationally-excited molecules and drive chemistry at temperatures at which such species are thermally inaccessible. The integration of a catalyst with reactive NTPs can promote novel chemical transformations. In particular, we are interested in upgrading of light hydrocarbons (e.g. methane) by direct coupling with nitrogen to produce value-added liquid chemicals (e.g. pyrrole and pyridine) in a plasma-assisted catalytic process. To design effective catalysts and plasma-catalytic processes, we need comprehensive understanding of the plasma catalytic processes including characterization of plasma-catalyst interactions. However, to date, the fundamental understanding of plasma-catalyst interactions is still in its infancy. In this work, we use a newly developed multi-modal polarization-modulation infrared-reflection absorption spectroscopy (PM-IRAS) tool with optical emission spectroscopy (OES) to probe plasma-catalyst interactions of CH₄/N₂ plasma species with different model catalytic surfaces (Ni, Pd, and Au). CH₄/N₂ feed gas mixture is introduced in three different ways: (1) CH₄-N₂ in sequence, (2) N₂-CH₄ in sequence, and (3) simultaneous with varying CH₄/N₂ ratio. We characterize excited species in the plasma-phase using OES, detect surface-adsorbed species using PM-IRAS, and detect gas-phase products using mass spectrometry. We find that different sequences in the feed gas introduction result in different surface-adsorbed species, and the presence of different surfaces dictates the surface-adsorbed species present. We also find that varying CH₄/N₂ feed ratio affects the macroscopic properties of the plasma and the production of intermediates such as cyanide (CN).