A. Raj, G.R. da Silva, S.H. Chung
Combust. Flame, 159, pp. 3423-3436, (2012)
PAH, Soot, Oxidation, Kinetic mechanism, Density functional theory, Transition state theory
The reaction pathways for the oxidation by O2 of polycyclic aromatic hydrocarbons present in soot particles are investigated using density functional theory at B3LYP/6-311++G(d,p) level of theory. For this, pyrene radical (4-pyrenyl) is chosen as the model molecule, as most soot models present in the literature employ the reactions involving the conversion of 4-pyrenyl to 4-phenanthryl by O2 and OH to account for soot oxidation. Several routes for the formation of CO and CO2 are proposed. The addition of O2 on a radical site to form a peroxyl radical is found to be barrierless and exothermic with reaction energy of 188 kJ/mol. For the oxidation reaction to proceed further, three pathways are suggested, each of which involve the activation energies of 104, 167 and 115 kJ/mol relative to the peroxyl radical. The effect of the presence of H atom on a carbon atom neighboring the radical site on the energetics of carbon oxidation is assessed. Those intermediate species formed during oxidation with seven-membered rings or with a phenolic group are found to be highly stable. 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.