Plasma Catalysis and Chemical Conversion II

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.

Funding acknowledgement

This research is supported by the DOE Fusion Energy Sciences grant number E-SC0024437 with Dr. Nirmol Podder as Program Manager. The work was in part conducted at the Princeton Collaborative Research Facility (PCRF) which is supported by U.S. DOE under the contract No. DE-AC02-09CH11466.