Mitigation of C-deposits in plasma-assisted non-oxidative methane coupling using a water-cooled double dielectric barrier discharge reactor

The non-oxidative coupling of methane into hydrogen and light hydrocarbons is a promising route for methane chemical valorisation and efficient production of building blocks for the chemical industry. Making this reaction at ambient conditions and using renewable energy to avoid CO2 emissions is a challenge compared to current solutions. We show here that using a non-thermal plasma double dielectric barrier discharge (DDBD) reactor with a water-circulating ground electrode makes it possible to realise a stable and selective process at ambient conditions. A systematic investigation is performed to determine how the reactor configuration influences plasma discharge characteristics, CH4 conversion, selectivity, and energy efficiency. The impact of varying the applied power, the discharge gap, and the gas mixture composition is studied using optical emission spectroscopy (OES) to clarify the mechanism. The results reveal radically different plasma chemistry when reducing the discharge gap to the micrometric scale (0.5 mm), resulting in a plasma confinement near the surface of the electrode, enhancing the selective activation of the C-H bond. The proposed system exhibits outstanding stability over 50 h of continuous testing, achieving a carbon balance of almost 100 % with the micrometric gap and mitigating the formation of undesired coke byproducts.