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Operando Investigation of Intermediate Jongyoon Bae,1,2 Do-Young Hong,1,3 Maeum Lee,1 Sunyoung Park,2 Yong-Ki Park,2 Young Kyu Hwang1,3* 1Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology(KRICT), Daejeon 305-600, Korea, 2Center for Convergent Chemical Process, Korea Research Institute of Chemical Technology(KRICT), Daejeon 305-600, Korea, 3Department of Green Chemistry, University of Science and Technology, 217 Gajeongro, Daejeon 305350, Korea Abstract: 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 operando 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∙), was reversely related to the extent of methane conversion due to active reactions among the methanium and C2 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. Direct non-OCM in RF plasma Operando Plasma Diagnostic System Catalytic Process Plasma Process Reaction Conditions . Gas: High purity CH4, . High selectivity for . Limited . High reaction desired products selectivity High purity Ar temp. Low operating cost . High . Heater Temperature: (250-800ºC) . Low reaction temp. operating cost 30-600ºC (room temp.) . Flow rate: 3-30sccm . Pressure: 40-180mTorr Catalytic Plasma Process . RF power: 2-90W 1. To better understand the RF methane plasma and to study the formation of C2 products by monitoring reaction intermediates (radicals, electrons, ions) ICCD: Intensity distribution of plasma 2. To investigate how active/key reaction intermediates interact with catalyst surfaces in plasma Langmuir Probe: Electron Energy Distribution(EEDF) 3. To design optimal catalyst for plasma direct non-OCM and to come up with plasma-catalyst EQP: Ionic/Neutral species distribution, Cationic energy distribution, Radical detection hybrid system Properties of Ion Species During Plasma Reaction Methandium Density vs. Plasma Reaction Conversion and Selectivity Methane Conversion 15 40 10 C H + 2 5 40 CH + (a) (b) 30 5 12 + 8 C2H5 30 C H + (%) 30 3 7 + 9 + + + C H on CH C H C H 4 9 6 20 + 5 2 5 3 5 iti C H 2 3 + + 20 CH C H 20 3 2 3 6 4 + + ompos CH 5 C H C 10 3 5 + 10 10 on CH Mean Ion Energy (eV) Energy Mean Ion I 3 3 50% CH Composition (%)Ionic 2 4 Methane Conversion (%) Conversion Methane 100% CH Methane Conversion (%) 4 0 0 0 0 0 36912 10 20 30 40 50 10 20 30 40 50 60 0 1020304050607080 60 70 80 90 100 110 120 Ion Mass (amu) Ion Mass (amu) Methane Conversion (%) RF Power (W) Pressure (mTorr) 15 Ionic composition as a function of 20 Methane plasma ionic species distribution Energy level of ions in 50% CH4/Ar and (c) (d) methane conversion for the flow rate and composition (RFpower=5W, P=40mTorr, 100% CH4 discharge variation results. 15 F=9sccm, Telectrode=25°C) 10 + + 10 Plasma Property vs. Methane Conversion CH5 + C2H6 → C2H5 + CH4 + H2 + + 5 CH5 + C2H4 → C2H5 + CH4 Plasma reaction conditions and plasma diagnostic results at Telectrode =25°C 5 + + Methane Conversion (%) Mean Mean C2H5 + C2H6 → C3H7 + CH4 Methane Conversion (%) Varied Fixed Methane Electron Electron Ion Energy Parameter Parameters Conv. Conc. + + 0 0 + C2H5 + C2H6 +CH4 → C4H9 + H2 + CH4 0 100 200 300 400 500 600 Energy (C2H5 ) 5101520 Flow Rate (sccm) Electrode Temperature (C) 50% CH /Ar 3.3%* 16.8eV 0.71ppm 13.9eV 4 5W, 40mTorr, 1 Methane conversion changes with parameter variations of (a) RF power, (b) flow 5sccm 100% CH4 3.8%* 14.0eV 0.69ppm 10.9eV rate, (c) pressure, and (d) electrode temperature. 10W 5.2% 14.4eV 1.37ppm 7.3eV 40 RF Power 20W 8.6% 14.7eV 2.84ppm 11.0eV 6 60 RF Power 60mTorr, 20scc 1x10 60mTorr Flow Rate Flow Rate 80mTorr Pressure 30W m 10.4% 14.0eV 3.41ppm 12.0eV 50 Pressure 100mTorr 30 Electrode 8x105 Electrode 120mTorr Temperature 40W 12.9% 13.3eV 3.89ppm 12.6eV 40 Temperature 5sccm 15.0% 13.8eV 1.44ppm 8.4eV 6x105 30 20 10sccm 10.0% 13.6eV 1.49ppm 8.2eV 10W, 60mTorr 4x105 20 15sccm 7.3% 13.7eV 1.36ppm 8.1eV 10 5 10 Ethylene Selectivity (%) 20sccm 5.2% 14.4eV 1.37ppm 7.3eV 2x10 Ethane Selectivity (%) Methanium Intensity (c/s) 0 (a) (b) *Values are normalized with methane concentrations 0 0 0 5 10 15 20 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 Methane Conversion(%) Ion Energy (eV) Methane Conversion(%) 14 16 30 RF power Flow rate RF Power 100 RF Power Methanium ion energy distributions for Flow Rate Flow Rate 14 12 Pressure Pressure different chamber pressures. Electrode 80 Electrode Temperature 12 20 Temperature 10 Plasma Spatial Intensity 60 10 8 40 8 10 6 Methane Conversion (%) 6 (%) Cabon Balance 20 Methane Converison (%) Converison Methane Acetylene Selectivity (%) Selectivity Acetylene (c) (d) 4 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4 0 0 30 60 90 120 150 180 210 0 5 10 15 20 25 30 35 40 Electron Concentration (ppm) 0 5 10 15 20 25 30 35 40 Estimated Residence Time (ms) Methane Conversion (%) Methane Conversion(%) Methane conversion as a function of electron concentration (left) and estimated Intense plasma zone in the sheath near Product selectivity and carbon balance as a function of methane conversion. (a) ethane residence time (right) the bottom RF electrode selectivity, (b) ethylene selectivity, (c) acetylene selectivity, and (d) carbon balance. Different process parameters were changed to selectively induce desired changes on plasma property and reaction condition. It was found that when the electron energy in plasma is sufficient to activate methane molecule, additional energy did not lead to an increased conversion. Likewise, the increase in ion energy did not improve the conversion. Both the electron concentration, which represents the degree of ionization, and molecule residence time in plasma were linearly correlated with the methane conversion. The methanium density was expected to be a quantitative representation of methyl radical formation, but the correlation between the methane conversion and ion density showed that the methanium was actively consumed through reactions with C2 product molecules. Thus, the methyl radical density could not be measured by the methanium density. The reaction product distribution was not influenced by the plasma property and was mainly determined by the conversion level for the plasma conditions in this study This research was supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016M3D3A1A01913257).
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