CH₃ Radical Generation in Microplasmas for Up-Conversion of Methane

The conversion of methane, CH₄, into higher value chemicals using low temperature plasmas is challenged by both improving efficiency and selectivity. One path toward selectivity is capturing plasma-produced methyl radicals, CH₃, in a solvent for aqueous processing. Due to the rapid reactions of methyl radicals in the gas phase, the transport distance from the production of the CH₃ to its solvation should be short, which then motivates the use of microplasmas. The generation of CH₃ in Ar/CH₄/H₂O plasmas produced in nanosecond pulsed dielectric barrier discharge microplasmas is discussed using results from a computational investigation. The microplasma is sustained in the channel of a microfluidic chip in which the solvent flows along one wall or in droplets. CH₃ is primarily produced by electron-impact of and dissociative excitation transfer to CH₄, as well as CH₂ reacting with CH₄. CH₃ is rapidly consumed to form C₂H₆ which, in spite of being subject to these same dissociative processes, accumulates over time, as do other stable products including C₃H₈ and CH₃OH. The gas mixture and electrical properties were varied to assess their effects on CH₃ production. CH₃ production is largest with 5% CH₄ in the Ar/CH₄/H₂O mixture due to an optimal balance of electron-impact dissociation, which increases with CH₄ percentage, and dissociative excitation transfer and CH₂ reacting with CH₄, which decreases with CH₄ percentage. Design parameters of the microchannels were also investigated. Increasing the permittivity of the dielectrics in contact with the plasma increased the ionization wave intensity, which increased CH₃ production. Increased energy deposition per pulse generally increases CH₃ production as does lengthening pulse length up to a certain point. The arrangement of the solvent flow in the microchannel can also affect the CH₃ density and fluence to the solvent. The fluence of CH₃ to the liquid solvent is increased if the liquid is immersed in the plasma as a droplet or is a layer on the wall where the ionization wave terminates. The solvation dynamics of CH₃ with varying numbers of droplets was also examined. The maximum density of solvated methyl radicals CH_3aq occurs with a large number of droplets in the plasma. However, the solvated CH_3aq density can rapidly decrease due to desolvation, emphasizing the need to quickly react with the solvated species in the solvent.