Mechanism analysis of methane conversion under the action of simple plasma

• Categories:Technical Support
• Author:plasma cleaning machine-surface treatment equipment-CRF plasma machine-Sing Fung Intelligent Manufacturing
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• Time of issue:2020-11-21
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(Summary description)At present, most researchers believe that the mechanism of plasma activated methane conversion is the free radical reaction process. Plasma discharge excites a large number of high-energy electrons. These high-energy electrons inelastic collide with methane molecules, splitting stable methane molecules into different active groups, which are coupled with each other to form C2 hydrocarbon products.   From the perspective of energy, under the action of plasma, the energy of high-energy electrons (1 ~ 20 eV) is enough to break the C-H bond of CH4 molecules (the average c-H bond energy is 4.3eV, and the dissociation energy of CH3-H is 4.5eV), thus forming CHx(x=0~3) free radicals in the gas phase. The CHx radical is then directionally recombined on solid surfaces such as the wall and electrode to form products that are desorbed from the surface. In the plasma system, the main role of plasma is to activate methane molecules to form CHx free radicals. The type and concentration of free radicals are determined by the plasma source and its energy-related parameters. The surface properties of free radicals were used to regulate the directed compound reaction of free radicals on the surface and transfer energy for the compound reaction of free radicals.   The emission spectrum in situ diagnosis technology can be excited state of atmospheric pressure plasma methane diagnosis to active species, in 250 ~ 800 nm wavelength range, can be concluded that under the action of plasma conversion of methane generated in the process of the main active species for: CH (430.1 ~ 438.7 nm), C (563.2 nm and 589.1 nm) and C2 (512.9 nm and 516.5 nm) and H (434.1 nm and 486.1 nm and 656.3 nm).   In plasma discharge areas, high-energy electrons are first produced. These high-energy electrons collide inelastic with methane molecules, thus generating a large number of active species and active free radicals, which further collide and combine to form new substances.   CH4+e*—>CH3+H+e               （3-1） CH3+e*—>CH2+H+e               （3-2） CH2+e*—>CH+H+e                 （3-3） CH+e*—>C+H+e                      （3-4） CH4+e*—>CH2+2H(H2)+e      （3-5） CH4+e*—>CH+3H(H2+H)+e    （3-6） CH4+e*—C+4H(2H2)+e            （3-7）   Coupling reactions between free radicals occur and the following products are generated (M is the third body, reactor wall, etc.) :   CH3+CH3+M—>C2H2+M          （3-8） CH2+CH2+M—>C2H4+M          （3-9） CH3+CH2+M—>C2H4+H+M     （3-10） CH +CH +M—>C2H2+M            （3-11） CH +CH2+M—>C2H2+H+M      （3-12） CH3+C+M—>C2H2+H+M         （3-13）   Since the particles with high concentration in the system are methane molecules, it is also an important way that methane molecules collide with various methyl radicals to initiate new radicals and generate various C2-hydrocarbon products.   CH2+CH4+M—>C2H6+M       （3-14） CH+CH4+M—>C2H4+H+M    （3-15） C+CH4+M—>C2H4+M            （3-16） C+CH4+M—>C2H2+H2+M    （3-17）   At the same time, the presence of C2 species in the emission spectra of methane plasma suggests that acetylene can also be generated by the following pathways:   C2+H+M—>C2H+M            （3-18） C2H+H+M—>C2H2+M        （3-19）   In atmospheric pressure pulsed corona plasma, high-energy electrons have a wide energy distribution range, so the concentration of various free radicals in methane plasma is different. The main products of the reaction are acetylene and hydrogen, while the secondary products are ethylene and ethane. CH and C were the main components of CHx free radical distribution in methane plasma, followed by CH3 and CH2.   C2H6 is the primary product of methane dehydrogenation coupling reaction, and C2H4 and C2H2 are the secondary products of further dehydrogenation of C2H6 and C2H4, respectively. Therefore, the following reaction pathways exist:   CH4→C2H6→C2H4→C2H2    （3-20）   To this end, we respectively investigates the pure ethane, ethylene in pulse corona plasma in the dehydrogenation reaction, the results showed that the pure ethane dehydrogenation reaction is the main product of the C2H4 and C2H2, pure ethane dehydrogenation reaction is the main product of C2H2, shows that under the action of plasma methane dehydrogenation coupling reaction do exist such as type (3-20) as shown in the reaction.   In the plasma, C2H6 and C2H4 generated by methane dehydrogenation will further interact with high-energy electrons to form radicals such as C2H5 and C2H3. Therefore, it can be speculated that trace C3 and C4 products are generated by methane dehydrogenation mainly in the following ways:   CH3+C2H5 +M→C3H8 + M         （3-21） CH2 +C2H6 +M→C3H8 + M       （3-22） CH3+C2H3 +M→C3H6 + M         （3-23） CH2+C2H4 +M→C3H6 + M        （3-24） C2H5 +C2H5 +M→C4H10 +M    （3-25）   The results of spectroscopic analysis show that the dehydrogenation of methane is mainly a free radical process under the action of plasma. However, in the methane d