Probing the ozone-assisted low-temperature oxidation chemistry of toluene

Toluene is the simplest alkylated aromatic and an important component of transportation fuel. The low-temperature oxidation kinetics of toluene is crucial for the development of advanced combustion engines. In this work, we studied the low-temperature oxidation of toluene in a jet-stirred reactor (JSR) with ozone addition, from the temperature range of 350 K to 785 K. Key intermediates such as formaldehyde, benzaldehyde, benzene, phenol, furfural, benzyl alcohol and cresols were measured and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The hydroperoxides were also detected, such as hydrogen peroxide, methyl hydroperoxide, and benzyl hydroperoxide. The detailed data were used to evaluate the low-temperature oxidation chemistry of toluene by four models, which were developed by Nancy University, Lawrence Livermore National Laboratory, Politecnico di Milano and RWTH Aachen University. Large deviation between experimental measurement and model prediction was observed, especially for the reaction intermediates. We discussed the differences in predictions by these models, and we tentatively improved the low-temperature combustion kinetics of toluene based on the model from Nancy University, with a particular focus on the reaction network of benzyl and phenyl radical. The reaction of benzyl with ozone was considered in this work, and it is important for benzoxyl radical formation. Additionally, two pathways for the formation of benzyl hydroperoxide were considered, which contributes to a better prediction of benzyl hydroperoxide. This work also focuses on discussing the reaction network of the phenoxy radical with O atom, suggesting that the pathway leading to the C₄H₅ radical and CO is the main reaction channel under our experimental conditions. The improved Nancy model yielded a better prediction for the intermediates and products in the low-temperature oxidation of toluene. This work provides a deeper understanding of the low-temperature oxidation chemistry of alkylated aromatics, which are valuable to improve the reaction network and to develop the low-temperature combustion models of alkylated aromatics.