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The carbon dioxide reforming of methane to synthesis gas was investigated in a dielectric barrier discharge reactor at room temperature. The influence of dilution of reactants by helium was studied. We showed that, at a fixed contact time, the conversions of CH 4 and CO 2 increase when the amount of helium in the gas mixture increases. This result is attributed to the ''penning ionization'' phenomenon, which corresponds to an energy transfer from excited He to molecules in ground state (CH 4 , CO 2 ). The selectivity to products is affected by the dilution factor. As soon as helium is present in a large amount the formation of products resulting from recombination of methyl radicals (such as C 2 , C 3 and C 4 ) is less favourable due to the lowest probability of collisions to proceed. A kinetic model is proposed based on the assumption that the reactant molecules CH 4 or CO 2 are attacked by active species produced by the plasma discharges, and the production of this active species are function of the plasma power. This model which takes into account the dilution by helium fits particularly well the experimental data we obtained.

Methane partial oxidation was investigated using a plasma microreactor. The experiments were performed at 5 and 300 °C. Microreactor configuration allows an efficient evacuation of the heat generated by methane partial oxidation and dielectric barrier discharges, allowing at the same time a better temperature control. At 5 °C, liquid condensation of low vapour pressure compounds, such as formaldehyde and methanol, occurs. 1H-NMR analysis allowed us to demonstrate significant CH3OOH formation during plasma-assisted partial oxidation of methane. Conversion and product selectivity were discussed for both temperatures. In the second part of this work, a numerical simulation was performed and a gas-phase chemical mechanism was proposed and discussed. From the comparison between the experimental results and the simulation it was found that CH3OO· formation has a determinant role in oxygenated compound production, since its fast formation disfavoured radical recombination. At 5 °C the oxidation leads mainly towards oxygenated compound formation, and plasma dissociation was the major phenomenon responsible for CH4 conversion. At 300 °C, higher CH4 conversion resulted from oxidative reactions induced by ·OH radicals with a chemistry predominantly oxidative, producing CO, H2, CO2 and H2O.