Theoretical Analysis on the Removal of Cyclic Volatile Organic Compounds by Non-thermal Plasma

Abstract

Volatile organic compounds (VOCs) from anthropogenic sources, especially cyclic organics with stable structure and strong toxicity (e.g., cyclohexane and benzene), exert hazards to atmospheric environment and human health, and need to be controlled urgently. As an emerging technology for VOC removal, non-thermal plasma (NTP) with low-energy consumption and high removal efficiency is widely studied. However, in contrary to the abundant experiments, theoretical studies are only few, and the degradation mechanisms and pathways of VOCs are still open questions, which hinders the effective VOC removal. Herein, the density function theory (DFT) calculations of NTP degradations for two typical cyclic organics, cyclohexane and benzene, are performed. The degradations of cyclic organics are mainly resulted by the dehydrogenation, decarburation, elimination, and ring-opening reactions and can be markedly promoted with radical ·OH, H·, and O· produced by background gas (H2O and O2). For cyclohexane degradations, the presence of O· decreases the energy barrier from 81.9 to 8.7 kcal/mol in the initial step, leading to an optimal degradation pathway with minimum plasma energy at around 0.5 eV. For benzene degradations, the presence of ·OH decreases the energy barrier from 118.4 to 5.5 kcal/mol in the initial step, triggering an optimal degradation pathway with minimum plasma energy at around 6 eV. The higher plasma energy required in degradation of benzene is due to its more stable structure than alkanes ring. Moreover, the O2 concentration and plasma energy are suggested to increase for efficient degradation of cyclohexane and benzene, respectively.