Abstract
Polycyclic aromatic hydrocarbon (PAH) dimers are important intermediates in combustion and soot formation. The scattering dynamics of gaseous molecules colliding with PAH dimers and the subsequent PAH dimer stability are investigated by performing molecular dynamics (MD) simulations. Effects of properties of the surrounding gaseous molecules and PAH dimers as well as temperature are investigated in this study. Depending on the residence time of N2 molecules trapped by the PAH dimers, two scattering types, that is, specular scattering and inelastic scattering, have been observed, which is correlated to the temperature and the type of the PAH dimer. Specifically, specular scattering preferentially takes place at high temperatures on small PAH dimers, while inelastic scattering tends to happen at low temperatures on large PAH dimers. During collision, energy transfer between the gaseous molecule and the PAH dimer changes the thermodynamic stability of the PAH dimer. Statistical analysis indicates that the decomposition rate of a PAH dimer to PAH monomers is sensitive to temperature and the PAH dimer type. Furthermore, effects of the gaseous molecule type on the PAH dimer stability are considered. The molecular mass of the colliding gaseous molecule is a key factor in determining the PAH dimer stability, as heavier gaseous molecules are more effective in promoting the PAH dimer decomposition. Results from this study indicate that collisions with gaseous molecules decrease the PAH dimer stability, while increasing the PAH dimer size and decreasing the collision temperature both decrease the decomposition rate of the PAH dimer.
Highlights
Collisions between soot nuclei and surrounding gaseous moelcules are quite common in combustion, which are of great importance to particle formation especially at high pressures.[16,17] Serving as the soot nuclei, Polycyclic aromatic hydrocarbon (PAH) dimers should survive the collision from surrounding gaseous molecules and be stable enough to grow with other PAH monomers or dimers
Due to energy transfer between the N2 molecule and the PAH dimer after collision, the total energy of the N2 dimer is smaller than that before collision. This leads to the increased total energy of the PAH dimer as the binary collision is performed in the NVE ensemble
This paper presents a systematic study on the binary collision between the surrounding gaseous molecules and the PAH dimer by molecular dynamics (MD) simulations
Summary
Collisions between soot nuclei and surrounding gaseous moelcules are quite common in combustion, which are of great importance to particle formation especially at high pressures.[16,17] Serving as the soot nuclei, PAH dimers should survive the collision from surrounding gaseous molecules and be stable enough to grow with other PAH monomers or dimers. As the collision time between N2 molecules in engines (1600 K, 10 bar) is comparable to the PAH dimer lifetime, Schuetz and Frenklach[18] stated that collisions with gaseous molecules is sufficient for PAH dimers to survive long enough for soot. The effects of gaseous collisions on the PAH dimer stability have not yet been established with certainty. This results partly from the difficulty in taking experimental measurements with sufficient spatial and temporal resolutions to detect PAH dimers at low concentrations. Since higher peak pressures are becoming the norm in internal combustion engines, studying the interplay between the gaseous molecules and soot nuclei is ever more demanding.[17,19]
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