Mechanism of accelerating soot oxidation by NO2 from diesel engine exhaust

NO2 oxidation of soot exhausted from engines is more efficient than O-2 under low-temperature conditions, and is crucial for diesel particulate filter to control soot pollution. To explore the principle behind accelerating soot oxidation by NO2, this paper uses density functional theory to reveal soot oxidation process by NO2. This study contributes to understanding rules of soot oxidation by NO2 and perfecting soot oxidation models to develop soot emission control technologies. Results show that NO2 oxidation of pyrene radical involves three steps. Firstly, NO2 attacks the C* atom to form -C (NO2 ) with reaction energy of 306.3 kJ/mol, which decomposes to produce a -C (O) compound. Secondly, another NO2 molecule climbs over an energy barrier of 8.8 kJ/mol, and changes into a -C (ONO2) intermediate on -C (O). Finally, the N or O atom of NO2 attacks -C (O) for a second time to help open aromatic ring for releasing CO or CO2. Further decomposition of -C (NO2) and -C (ONO2) requires activation energies of 81.6 kJ/mol, 75.7 kJ/mol, and 53.5 kJ/mol, respectively, on preferential pathways. Calculations prove that attacks of O atom from NO2 on C* help open the aromatic ring more efficiently than N atom. (C) 2020 Elsevier Ltd. All rights reserved.