<p>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.</p>