Steered molecular dynamics and stability analysis on PAH dimerisation and condensation on fullerene and soot surfaces

The mechanism of how a soot nucleus is impacted by polycyclic aromatic hydrocarbons (PAHs) and then grows through PAH condensation remains unclear. Using steered molecular dynamics (SMD), the non-bonding interaction between PAHs and soot was quantitatively studied using the free energy distribution during the dimerisation and condensation. The results showed that only two dimers (A7–A10 and 2 A10) remained stable at 1000 K. The simulations showed that PAH condensation on a fullerene should not be ignored in soot mass growth. For fullerenes with a diameter not less than 1.8 nm (C540), even A4 condenses at temperatures of 1500 K, and A10 condenses stably on the surface of fullerenes even at 2000 K. The effects of multilayers and hydrogenated fullerenes on the free energy of PAH condensation are different. The stability of PAH dimers and PAH condensation pairs was discussed through free energy and chemical equilibrium. The results show that larger dimers are more stable than small ones at flame temperatures. Condensation is far more important than nucleation in mass growth at flame temperatures. Furthermore, the larger the PAH is, the higher the transformation ratio of the PAH in condensation on soot and thus the more stable the condensation product is. Finally, both the stability analysis of an upper temperature limit for condensation and simulation results of ReaxFF-MD cross-confirm that pyrene stably condensates on a simplified nascent soot (C540) and a simulated soot (C658H319O9), respectively, at 1500 K, but not at higher temperatures over 1800 K.