Principal Limitations in Homogeneous Gas Phase Chemistry in Non-Thermal Plasmas

This study highlights the oxidation of H₂, CH₄, and HCl present in the range of some volume percent in a homogeneous O₂ or air phase in a flow through glass barrier discharge reactor. The oxidation of all three compounds is highly exothermic and exergonic at ambient temperature and proceeds at sufficiently high temperatures as radical chain reaction. The conversion of each compound was below 10% in a non-thermal oxygen plasma under various reaction conditions. Increasing concentrations of H₂ and CH₄ above the lower explosion limit did not lead to higher conversion degrees. It is assumed that only initial radical formation by electron impact dissociation and exothermic steps within the chain process run in a sufficiently fast manner at ambient temperature. For endothermic steps within the radical chain, the necessary activation energy is not available and the chain reaction aborts, most likely, after formation of peroxyl (hydro- or methyl-peroxyl) radicals.