Radical Contribution in Methane Plasma-Catalysis

Radicals are widely assumed to drive surface reactions in plasma-catalysis, but their mechanistic contribution remains poorly defined. In this study, we investigate the contribution of radicals relative to vibrationally excited CH₄ in a dielectric barrier discharge (DBD) reactor by systematically varying plasma power and CH₄:Ar dilution. Optical emission spectroscopy (OES) characterizes excitation conditions, while GC-MS is used to quantify product distributions on Ni and Cu catalysts. We observe that increasing plasma power elevates radical density and overall CH₄ conversion; however, this is accompanied by the accumulation of carbonaceous deposits on the catalyst surface. These species exhibit limited chemical reactivity, suggesting they are weakly bound or physisorbed rather than participating in sustained surface chemistry. At low-to-moderate plasma powers, where vibrational excitation is more prominent relative to radical formation, we achieve higher CH_x* surface coverage and enhanced C–C coupling with improved catalyst stability to form C₂ hydrocarbons. To interpret these trends, we develop a DFT-informed microkinetic model incorporating both Langmuir–Hinshelwood (L-H) and Eley–Rideal (E-R) mechanisms. We probe whether E-R reactions involving gas-phase radicals contribute to CH_x* formation and subsequent C-C coupling and compare this to the contributions from vibrationally activated CH₄. Preliminary modeling efforts suggest that radical-driven pathways are limited under most conditions. These findings highlight the need to critically assess the catalytic relevance of radicals and point toward whether vibrational excitation and intermediate surface retention act as more effective levers for achieving selective plasma-catalytic upgrading.