Yuan Hongliang, Kong Wenjun, Xia Jun
Department of Mechanical and Aerospace Engineering & Institute of Energy Futures, Brunel University London, Uxbridge UB8 3PH, UK.
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China.
Phys Chem Chem Phys. 2021 Sep 15;23(35):19590-19601. doi: 10.1039/d1cp01019k.
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 (A-A and 2 A) 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 (C), even A 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 (C) and a simulated soot (CHO), respectively, at 1500 K, but not at higher temperatures over 1800 K.
多环芳烃（PAHs）如何影响烟灰核并随后通过PAH缩合生长的机制仍不清楚。使用引导分子动力学（SMD），利用二聚化和缩合过程中的自由能分布对PAHs与烟灰之间的非键相互作用进行了定量研究。结果表明，在1000 K时只有两种二聚体（A-A和2A）保持稳定。模拟结果表明，在烟灰质量增长过程中，PAH在富勒烯上的缩合不容忽视。对于直径不小于1.8 nm的富勒烯（C），即使在1500 K的温度下A也会缩合，而即使在2000 K时A10也能稳定地缩合在富勒烯表面。多层和氢化富勒烯对PAH缩合自由能的影响不同。通过自由能和化学平衡讨论了PAH二聚体和PAH缩合对的稳定性。结果表明，在火焰温度下，较大的二聚体比较小的更稳定。在火焰温度下，缩合在质量增长中比成核重要得多。此外，PAH越大，其在烟灰上缩合的转化率越高，因此缩合产物越稳定。最后，缩合上限温度的稳定性分析和ReaxFF-MD模拟结果相互印证，芘分别在1500 K时稳定地缩合在简化的初生烟灰（C）和模拟烟灰（CHO）上，但在超过1800 K的更高温度下则不会。
