Decoupling Plasma, Thermal, and Catalyst Effects for Selective Methane Conversion in Plasma-Catalysis

Nonthermal plasmas offer a low-temperature route for methane conversion to C₂ hydrocarbons and H₂ across a wide class of catalysts, but selective control on product distribution remains challenging due to coupled effects from plasma excitations, thermal energy, and catalyst surface chemistry. We present a mechanistic framework that independently tunes vibrational excitation (T_vib), surface temperature (T_sur), and catalyst binding strength to control both product speciation (e.g, C₂H₂, C₂H₄, C₂H₆) and catalytic inactivation (surface coverage in the form of CH_x*). Experiments are conducted in a dielectric barrier discharge reactor with independent heating control, using Ni and Cu on Al₂O₃ to represent strong and weak metal-carbon binding regimes. We show T_sur and T_vib influence different steps of the reaction mechanism: T_sur affects the stability and removal of surface CH_x* species, while T_vib controls the rate of CH₄ dissociative adsorption by tuning the relative population of vibrationally excited molecules and radicals. On Ni, increasing T_sur from 373 K to 523 K shifts C₂ product distribution: C₂H₆ is the primary upgraded hydrocarbon at lower T_sur, while C₂H₄ is the primary C₂ hydrocarbon at higher T_sur ( > 473 K). This shift aligns with reduced CH_x* coverage, indicating that higher temperatures promote CH_x* desorption or decomposition and reduce surface buildup. Operando DRIFTS confirms that Ni retains more CH_x* than Cu under similar conditions, consistent with its higher tendency for carbon accumulation. We employ a Langmuir–Hinshelwood microkinetic model, informed by DFT-based linear scaling relationships that correlate reaction energetics with catalyst binding energy, to validate trends in CH_x* coverage and product selectivity.