Metal-free catalysis for the reaction of nitrogen dioxide dimer with phenol: An unexpected favorable source of nitrate and aerosol precursors in vehicle exhaust.

Atmospheric reaction mechanism and dynamics of phenol with nitrogen dioxide dimer were explored by the density functional theory and high-level quantum chemistry combined with statistical kinetic calculations within 220-800 K. The nitric acid and phenyl nitrite, the typical aerosol precursors, are the preponderant products because of the low formation free energy barrier (∼8.7 kcal/mol) and fast rate constants (∼10-15 cm3 molecule-1 s-1 at 298 K). Phenyl nitrate is the minor product and it would be also formed from the transformation of phenyl nitrite in NO2-rich environment. More importantly, kinetic effects and catalytic mechanism of a series of metal-free catalysts (H2O, NH3, CH3NH2, CH3NHCH3, HCOOH, and CH3COOH) on the title reaction were investigated at the same level. The results indicate that CH3NH2 and CH3NHCH3 can not only catalyze the title reaction by lowering the free energy barrier (about 1.4-6.5 kcal/mol) but also facilitate the production of organic ammonium nitrate via acting as a donor-acceptor of hydrogen. Conversely, the other species are non-catalytic upon the title reaction. The stabilization energies and donor-acceptor interactions in alkali-catalyzed product complexes were explored, which can provide new insights to the properties of aerosol precursors. Moreover, the lifetime of phenol determined by nitrogen dioxide dimer in the presence of dimethylamine may compete with that of determined by OH radicals, indicating that nitrogen dioxide dimer is responsible for the elimination of phenol in the polluted atmosphere. This work could help us thoroughly understand the removal of nitrogen oxides and phenol as well as new aerosol precursor aggregation in vehicle exhaust.