A kinetics mechanism of NO_X formation and reduction based on density functional theory

NO_X are serious pollutants emitted during combustion, which are greatly harmful to human health and the environment. However, previous studies have not accurately elucidated the NO_X conversion mechanism in complicated combustion reactions. To reveal the micro-chemical mechanism of NO_X conversion and obtain accurate kinetics data, advanced quantum chemistry methods are employed in this study to systematically explore the pathways of NO_X formation and reduction, and determine the new rate coefficients. An energy barrier analysis revealed that during NO_X formation (N₂ → N₂O → NO→NO₂), NO is primarily produced by a sequence of reactions (N₂ + O → N₂O → NO) rather than the traditional reaction (O + N₂ → NO+N). Meanwhile, NO₂ formation (NO→NO₂) largely depends on the O and HO₂ radicals, while the active O atom can promote both the formation and destruction of NO₂. During NO_X reduction (NO₂ → NO→N₂O → N₂), NO₂ reduction (NO₂ → NO) is closely related to H, CO, and O, whereas CO plays a critical role in NO₂ destruction. However, NO reduction (NO→N₂O) is unfavourable because of a high energy barrier, while N₂O reduction (N₂O → N₂) is strongly affected by the O atom instead of CO. HONO is mainly formed when NO₂ reacts with the HO₂ and H radicals, and when NO reacts with OH radicals; thus, HONO consumption largely depends on OH and H radicals. Based on the transition state theory, we obtained new kinetic parameters for NO_X conversion, which supplement and correct critical kinetics data obtained from the current NO_X model. Performance assessment of the proposed NO_X kinetic mechanism reveals that it can improve the existing NO_X kinetic mode, which is in good agreement with experimental data.