Abstract
Physical dimerization of polycyclic aromatic hydrocarbons (PAHs) has been investigated via molecular dynamics (MD) simulation with the ReaxFF reactive force field that is developed to bridge the gap between the quantum mechanism and classical MD. Dynamics and kinetics of homo-molecular PAH collision under different temperatures, impact parameters, and orientations are studied at an atomic level, which is of great value to understand and model the PAH dimerization. In the collision process, the enhancement factors of homo-molecular dimerizations are quantified and found to be larger at lower temperatures or with smaller PAH instead of size independent. Within the capture radius, the lifetime of the formed PAH dimer decreases as the impact parameter increases. Temperature and PAH characteristic dependent forward and reverse rate constants of homo-molecular PAH dimerization are derived from MD simulations, on the basis of which a reversible model is developed. This model can predict the tendency of PAH dimerization as validated by pyrene dimerization experiments [H. Sabbah et al., J. Phys. Chem. Lett. 1(19), 2962 (2010)]. Results from this study indicate that the physical dimerization cannot be an important source under the typical flame temperatures and PAH concentrations, which implies a more significant role played by the chemical route.
Highlights
It is generally believed that the particle nucleation starts from polycyclic aromatic hydrocarbon (PAH) dimerization as supported by experimental evidences, such as the bimodality in the soot particle size distributions (PSD) of premixed flames, stacked polycyclic aromatic hydrocarbons (PAHs) conformations observed by transmission electron microscopy (TEM)
Previous experimental and theoretical studies found that the reactive PAH fragments formed from ion collisions may interact with intact neighboring molecules by strong covalent bonds or PAHs in charge states (PAHq+–PAH) are thermodynamically stable.36–38. This study investigates both the dynamics and kinetics of the reversible homo-molecular PAH dimerization by
Molecular dynamics (MD) simulations with the ReaxFF force field. By performing both center-to-center and centrifugal collisions, the quantified enhancement factors for different types of PAHs are found to decrease with increasing PAH size and temperature
Summary
Commonly generated from the combustion of hydrocarbon fuels in transportation, jet propulsion, and power plants, is detrimental to the combustion efficiency, atmosphere, and health of human beings. Associated processes, including aromatics formation, particle nucleation, coagulation, surface reaction, and oxidation, have been investigated to develop soot mechanistic models. Among these processes, it is generally believed that the particle nucleation starts from polycyclic aromatic hydrocarbon (PAH) dimerization as supported by experimental evidences, such as the bimodality in the soot particle size distributions (PSD) of premixed flames, stacked PAH conformations observed by transmission electron microscopy (TEM). As a consequence, PAH dimerization is regarded as the source term of the soot particle dynamics in the population balance models.8,9population balance models, it does not fully reflect the underlying physics in the dimerization process due to its independence on temperature. Associated processes, including aromatics formation, particle nucleation, coagulation, surface reaction, and oxidation, have been investigated to develop soot mechanistic models.. Studies from Thomson et al considered the reversibility in modeling soot nucleation and condensation, and the newly developed model was reported to give the best agreement with experimental data.. Studies from Thomson et al considered the reversibility in modeling soot nucleation and condensation, and the newly developed model was reported to give the best agreement with experimental data.9,15 In their models, the forward collision rate is calculated based on the collision theory, with a sticking coefficient in the range of 0.75–1. Some PAH dimers formed by van der Waals (vdW) interactions are assumed to instantaneously convert to chemically bonded dimers, while the rest dimers decompose to PAH monomers, which is lack of theoretical and experimental validations
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