Hydrogen Abstraction/Addition Reactions in Soot Surface Growth
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The hydrogen abstraction (HB) and addition reactions (HD) by H radicals are examined on a series of polycyclic aromatic hydrocarbon (PAH) monomers and models of quasi-surfaces using quasi-classical trajectory (QCT) method. The QCT results reproduce the rate constants of HB reactions on PAH monomers from density function theory (DFT) in the range of 1500-2700 K. The PAH size has a minor impact on the rates of HB reactions especially at temperatures beyond 2100 K. By contrast, HD reactions have a clear size dependence and a larger PAH yields a higher rate. It is also found that the preferred reaction pathway changing from HB to HD reactions at ~1900 K. The rates of surface HB and HD reactions exceed those in the gas phase by nearly a factor of magnitude. Further analysis on the detailed trajectory of QCT method reveals that about 50% of the surface reactions can be attributed to the events of surface diffusion, which depends on the local energy transfer in the gas-surface interactions. However, this phenomenon is not preferred in PAH monomers as expected. Our finding here highlights the misinterpretation of surface reactions as the product of the first collision between gaseous species and particle surface, and surface diffusion induced reactions should be accounted for in the rates of surface HB and HD reactions. Rate constants of HB and HD reactions on each reactive site (surface zig-zag, surface free-edge and pocket free-edge sites) are calculated by QCT method, which are recommended for the further development of surface chemistry models in soot formation.