Investigation of intermediates in non-oxidative coupling of methane by non-thermal RF plasma

Non-oxidative coupling of methane by low pressure RF plasma was investigated with real time monitoring tools such as Langmuir probe, and ion mass and energy spectrometer. Process parameters were systematically changed to generate various plasma conditions and thus to identify key factors affecting the methane activation. From real time analysis, it was found that effects of electron energy and ion energy on methane conversion were minimal when the electron energy of plasma is greater than methane activation energy. On the other hand, electron concentration and the residence time in plasma were strongly correlated with the degree of methane conversion. Moreover, it should be noted that the density of methanium (CH5+) which is a byproduct of ion-molecule reaction pathway of methyl radical formation (CH3 center dot), was reversely related to the extent of methane conversion due to active reactions among the methanium and C-2 product molecules. Furthermore, for tested experimental conditions in this study, the product distribution of the methane plasma reaction was more closely related with the methane conversion level than with other plasma parameters. This real time analysis results can be used to derive meaningful insights to help designing an effective synergistic plasma-catalyst system. (C) 2017 Elsevier B.V. All rights reserved.