Direct conversion of methane by an atmospheric-pressure dielectric barrier discharge microplasma

Abstract

The conversion of methane to hydrogen and more valuable higher-order hydrocarbons is of great current importance due to the abundance of natural gas and its growing importance as an alternative fuel source. Non-oxidative coupling of methane is particularly attractive to avoid the generation of COx and potentially produce clean hydrogen. Plasma processes enable the direct dissociation of methane at much lower temperatures than a thermal process and avoid coking and deactivation of solid catalysts. Here, we present results for non-oxidative conversion of methane using an atmospheric-pressure, coaxial dielectric barrier discharge (DBD). Critically, we confined the DBD between two grounded electrodes inside the reactor to independently control the plasma volume and power (i.e. power density) and correlate these properties with methane conversion and product selectivity. One of the grounded electrodes could be moved by a micrometer-coupled feedthrough to make systematic changes to the volume. All experiments were performed with a pure feed of methane gas into the DBD operated at 20–40 kHz AC voltage. The gas effluent was continuously sampled by a gas chromatography system equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID) to characterize the gas composition. Our results show that methane conversion increases nearly linearly with plasma volume from ∼1% at 4.0 mm3 to ∼8% at 32 mm3, independent of power which was kept constant at a power density of 32 mW/mm3. Interestingly, the product selectivity changed somewhat significantly as the volume was increased towards longer, saturated hydrocarbons such as ethane and propane. The selectivity was also found to change with power density with unsaturated hydrocarbons being favored at lower power densities. Microkinetic modeling, which takes into account the electron-induced reactions as well as chemical reactions of the plasma-induced intermediates, is carried out in support of these experimental observations.