Coupling of Methane in a DBD Plasma Reactor: Effect of H2 as Co-Feed

The coupling of methane to C2 hydrocarbons by means of non-thermal plasma is a recognized process as it is performed at room temperature, through the generation of radical species, but it still presents a major disadvantage of carbon deposition. The main goal of the study is to better control the chemistry and avoid C deposit. For that purpose, the addition of hydrogen as co-feed in a Dielectric Barrier Discharge plasma reactor was proposed. Experiments were carried out at a fixed deposited power of 8 W in the presence and absence of helium. As expected the conversion of methane was low without helium (6.5% for pure CH4 against 22.3% for He/CH4: 30/10 mL min−1). A high selectivity into hydrocarbons was achieved by adding H2 to CH4 (75% for H2/CH4 = 1 against 62% without H2), the main product being ethane (selectivity of 60%). An increase of the deposited power from 8 to 21 W favoured methane transformation and the hydrocarbons yield reached 9.1%. Surprisingly, the carbon deposition was limited at high deposited power. It could result from the hydrogenation of coke precursor (ethylene, acetylene) or reaction of H2 with C deposit. The beneficial action of hydrogen on the reaction to reach high C2 selectivity has been demonstrated and a reaction mechanism is proposed.