Revealing the dual role of hydrogen for growth inhibition and defect healing in polycyclic aromatic hydrocarbon formation: QM/MD simulations

Quantum mechanical molecular dynamics simulations are employed to reveal the influence of hydrogen on polycyclic aromatic hydrocarbon (PAH) formation in oxygen-lean combustion. While higher hydrogen concentration leads to the inhibition of PAH growth, it simultaneously facilitates pentagon and heptagon defect healing, leading to thermodynamically more stable PAH fragments with more hexagons. We therefore propose the existence of an optimal H/C ratio that facilitates the growth of all-hexagon-containing PAH species. Analysis of the PAH edge reconstruction in our simulations shows a near-equal ratio of armchair and zigzag edge structures. As armchair edge structures are thermodynamically considerably more stable than zigzag edge structures, the present simulations show that both kinetic and thermodynamic factors are needed to understand PAH/graphene edge reconstruction.