Reaction mechanism for the oxidation of zigzag site on polycyclic aromatic hydrocarbons in soot by O2

Soot particles are composed of polycyclic aromatic hydrocarbons with different types of reactive sites that are involved in their growth and oxidation reactions. In this study, a detailed reaction mechanism for the oxidation of soot particles through their zigzag sites is presented using density functional theory. For this, a model PAH molecule with 10 aromatic rings and one radical on the zigzag site is selected. The addition of O2 on a radical site to form a peroxyl group is found to be barrierless and exothermic with a reaction energy of 215.9 kJ/mol. For the oxidation reaction to proceed further, four pathways are suggested, each of which involve the activation energies of 69.3, 92.4, 195.7 and 53.9 kJ/mol relative to the peroxyl radical for initiation. Several routes for the formation of CO and CO2 are identified. The effects of the presence of H atom on a carbon atom neighbouring the radical site and the type of reactive sites chosen for O2 addition on the energetics of carbon oxidation are assessed. The stable oxygenated intermediate species formed during oxidation are found. The rate constants evaluated using transition state theory in the temperature range of 300-3000 K for the reactions involved in the mechanism are provided. Through kinetic modelling and energetics comparison, the preferred pathways for the oxidation of zigzag site to form CO and CO2 are identified.