Structural effects on the oxidation of soot particles by O2: Experimental and theoretical study

Soot particles are composed of polycyclic aromatic hydrocarbons (PAHs), which have either planar or curved structures. The oxidation behaviors of soot particles differ depending on their structures, arrangement of PAHs, and the type of surface functional groups. The oxidation rate of curved PAHs in soot is thought to be higher than that of planar ones. To understand the role that PAH structure plays in soot reactivity towards O2, experimental studies are conducted on two types of commercially produced soot, Printex-U and Fullerene soot, using high resolution transmission electron microscopy, electron energy loss spectroscopy, thermo-gravimetric analysis and elemental analysis. The relative concentrations of active sites, oxygenated functional groups, aliphatics and aromatics present in soots are evaluated. The activation energies for soot oxidation at different conversion levels are determined. The average activation energies of the two soots are found to differ by 26 kJ/mol. To understand the reason for this difference, quantum calculations using density functional (B3LYP) and Hartree–Fock theories are conducted to study the reaction pathways of the oxidation by O2 of planar and curved PAHs using 4-pyrenyl and 1-corannulenyl as their model molecules, respectively. The energetically preferred channels for curved PAH oxidation differ from the planar one. The addition of O2 on a radical site of a six-membered ring to form a peroxyl radical is found to be barrierless for both the model PAHs. For peroxyl decomposition, three pathways are suggested, each of which involve the activation energies of 108, 170 and 121 kJ/mol to form stable molecules in the case of planar PAH, and 94, 155 and 125 kJ/mol in the case of curved PAH. During the oxidation of a five-membered ring, to form stable molecules, the activation energies of 90 kJ/mol for the curved PAH and 169 kJ/mol for the planar PAH relative to the energy of the peroxyl radical are required. The low activation barriers of preferred pathways for the oxidation of six and five-membered rings present on curved PAH (as compared to the planar one) explain and support its experimentally observed high reactivity.