Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert O2 in presence of hydrocarbons into CO or CO2 on demand. The underlying reaction mechanisms of plasma conversion of O2 in the presence of hydrocarbons are
analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of O2 and of CH4. The plasma process is analysed with infrared absorption spectroscopy to monitor CH4 as well as the reaction products CO, CO2 and H2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from O2 by electron.
Oxygen atoms are eciently converted into CO, CO2 and H2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced O2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of O2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion effciency is enhanced by 15% to 20% already at temperatures of 430 K.