C14H10 polycyclic aromatic hydrocarbon formation by acetylene addition to naphthalenyl radicals observed

Abstract

The formation of polycyclic aromatic hydrocarbons (PAHs) during combustion has a substantial impact on environmental pollution and public health. The hydrogen-abstraction-acetylene-addition (HACA) mechanism is expected to be a significant source of larger PAHs containing more than two rings. In this study, the reactions of 1-naphthalenyl and 2-naphthalenyl radicals with acetylene (C₂H₂) are investigated using VUV photoionization time-of-flight mass spectrometry at 500 to 800 K, 15 to 50 torr, and reaction times up to 10 ms. Our experimental conditions allow us to probe the Bittner–Howard and modified Frenklach HACA routes, but not routes that require multiple radicals to drive the chemistry. The kinetic measurements are compared to a temperature-dependent kinetic model constructed using quantum chemistry calculations and accounting for chemical-activation and fall-off effects. We measure significant quantities of C₁₄H₁₀ (likely phenanthrene and anthracene), as well as 2-ethynylnaphthalene (C₁₂H₈), from the reaction of the 2-naphthalenyl radical with C₂H₂; these results are consistent with the predictions of the kinetic model and the HACA mechanism, but contradict a previous experimental study that indicated no C₁₄H₁₀ formation in the 2-naphthalenyl + C₂H₂ reaction. In the 1-naphthalenyl radical + C₂H₂ reaction system, the primary product measured is C₁₂H₈, consistent with the predicted formation of acenaphthylene via HACA. The present work provides direct experimental evidence that single-radical HACA can be an important mechanism for the formation of PAHs larger than naphthalene, validating a common assumption in combustion models.