Trace metal assisted polycyclic aromatic hydrocarbons fragmentation, growth and soot nucleation

Abstract

The present work demonstrates the role of trace metal, namely Fe, on the polycyclic aromatic hydrocarbons (PAHs) fragmentation, growth and soot nucleation through performing reactive molecular dynamics (MD) simulations with the ReaxFF force field. The structure of the Fe–PAH complex and the diffusivity of the Fe atoms on the PAH surface are scrutinized at different temperatures. A series of binary collisions between a Fe atom and a PAH molecule are systemically investigated to clarify the influence of Fe collision energy, collision orientation and equilibrium temperature of the PAH molecule on the PAH fragmentation. The simulation results indicate that Fe collision on PAH molecules at a typical flame temperature, i.e., 1500 K, is sufficient to break the CH and/or CC bonds by forming PAH radicals with Fe bonded. In contrast to the largest PAH clusters composed of PAH dimers or trimers in pure PAH systems at flame temperatures, presence of Fe atoms dramatically enhances the PAH growth and soot nucleation rate as the Fe atoms significantly lower the PAH fragmentation temperature. Moreover, the soot nucleation pathway is found to be dependent on temperature. At 1500 K, Fe atoms preferentially break the CH bonds and connect the PAHs through dehydrogenation. Eventually, Fe atoms merge with the aromatic rings and facilitate the formation of embedded five/seven-membered rings during PAH growth at elevated temperatures. Results from this study contribute to further understanding of the dynamic physicochemical processes in soot formation during more realistic combustion with trace metals.