Reaction mechanism of toluene decomposition in non-thermal plasma: How does it compare with benzene?

Abstract

Non-thermal plasma (NTP) catalysis is considered one of the most promising technologies to address a wide range of energy and environmental needs, such as carbon dioxide(CO2) conversion, NH3 synthesis, and volatile organic compounds (VOCs) removal. A systematic approach to optimizing NTP systems benefits from understanding VOCs' fundamental NTP destruction behavior and analyzing the correlations between molecular structures and conversion and selectivity. Herein, the mechanical performance of the toluene destruction in NTP is examined and compared with benzene bearing a similar molecular structure. Different experimental and theoretical techniques are applied, including synchrotron vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS), thermochemistry, and quantum chemistry. Comparatively, toluene is more readily destroyed under the same NTP conditions than benzene. More intriguingly, the distribution of the decomposition species is significantly different. The theoretical calculations reveal that the abundant methyl radicals generated in toluene decomposition mainly lead to the various species distribution. These radicals promote some reactions, such as the decomposition of o-benzoquinone, one of the key intermediates, thus leading to new reaction pathways and products different from benzene. Finally, the critical mechanism steps of toluene decomposition in the current NTP condition are established, including the reactions between toluene and electrons or active radicals, aromatic ring breakup reactions, and reactions of methyl radicals. This study provides a powerful approach to revealing the different fundamental reaction mechanisms resulting from a slight structural difference and brings new insights for probing the underlying plasma chemistry, which is crucial for its various promising environmental and energy applications.