Direct Partial Oxidation of Methane at Plasma-Catalyst-Liquid Interfaces

The direct and partial oxidation of methane to value-added chemical fuels, such as methanol, remains a major yet lucrative challenge in catalytic chemistry. Herein, we introduce engineered plasma-catalyst-liquid interfaces (PCLIs) that enable a one-step, ambient-pressure, electrified pathway for methane oxidation to methanol and higher-order hydrocarbons. By integrating a CuO-infused porous glass frit coupled with a nonthermal methane plasma at an aqueous interface, we demonstrate the importance of mass transfer of plasma-activated species to the catalyst surface in controlling oxidative selectivity. Following systematic experiments of reaction conditions, we report an optimized liquid-phase methanol selectivity of 96.8 ± 0.6% (highest total selectivity = 57.9 ± 5.5%) with a simultaneous production rate of 51.8 ± 1.5 mmol_MeOH g_CuO^-1 hr^-1. Additional gas-phase production of H₂ and C₂₊ hydrocarbons (e.g., ethane, ethylene, propane, propylene) was measured with a notable absence of overoxidized products (i.e., CO₂) under optimized reaction conditions. A specific electricity consumption of 46.7 kWh/kg_MeOH indicates competitive efficiency for electrified methane upgrading. Plasma diagnostics, including charge-voltage Lissajous analysis, optical emission spectroscopy, and plasma modeling, reveal a complex mechanistic picture where CuO-stabilized biradical coupling, gas-phase radical recombination, and vibrationally "hot" methane compete for overall reaction selectivity as a function of the pulsed plasma discharge. This study demonstrates the importance of modulating plasma chemistry and transport between plasma, catalyst, and liquid to improve reaction outcomes under complex multiphase environments.