Molecular beam mass spectrometry study on plasma-assisted low-temperature oxidation of ethylene

The chemical kinetics of plasma-assisted low-temperature oxidation of ethylene (C2H4) is explored by molecular beam mass spectrometry and kinetic modeling. In the experiments, a dielectric barrier discharge (DBD) flow reactor coupled with photoionization and electron ionization molecular beam mass spectrometry (PI-MBMS/EI-MBMS) is employed to provide detailed speciation information, including ions, radicals, and molecules in C2H4/O2/Ar systems (∼340 K, 30 Torr). A series of hydrocarbons and oxygenated intermediates including ethenol, methyl hydroperoxide, ethyl hydroperoxide, diacetylene, vinylacetylene, and hydroxyacetaldehyde is detected for the first time in this system. A kinetic mechanism that contains both combustion and plasma reaction networks is developed. The path fluxes of fuel consumption and oxygenates formation are obtained based on the rate of production analysis. Based on our experimental observations and modeling analysis, new insights into the kinetics of C2H4/O2/Ar plasmas are provided. The fuel is mainly consumed through OH addition, dissociation after e/Ar* collision, reactions with O/O(1D), and dissociative ionization. H abstraction reactions that are favored in alkane plasmas contribute little to the fuel consumption of ethylene. Formation pathways of C1 oxygenates are mainly initiated by CH3 produced by O/O(1D) reactions with fuel, and the subsequent pathways of OH/H addition to the fuel are responsible for the formation of most C2 oxygenates. Oxygen addition-elimination reactions of C2H4OH isomers are the sources of ethenol formation. The concentration profiles for some of the species under different oxygen mole fractions are measured. The two distinct patterns of species mole fraction profiles with increasing oxygen concentrations indicate the different dominating formation pathways, namely oxidation pathways and non-oxidation pathways. And the competition between oxidation effects and plasma effects is displayed by the non-monotonic mole fractions profiles of oxidation products.