The formation of polycyclic aromatic hydrocarbons (PAHs), especially fused-ring compounds, represents an essential step in the mechanisms of soot formation. In particular the second-ring species, naphthalene, plays a key role as a building block for the subsequent growth to larger PAHs. Nevertheless the pathways leading to naphthalene are still uncertain requiring further experimental and theoretical investigations.
The objective of this work was to investigate the pyrolytic reactions of the phenyl radical in the presence of acetylene as a possible pathway to the formation of the second-ring species. In addition, the oxidation of m-xylene, an abundant single-ring component for typical fuels and surrogate mixtures, was investigated.
The experimental work was conducted using the single-pulse high-pressure shock tube (HPST) at the University of Illinois at Chicago. A new experimental set-up, for use with the HPST, was studied and developed for accurate measurement of large compounds. The major stable species obtained from the experimental work, including the heavy polycyclic aromatic hydrocarbons, were identified and measured using gas chromatography/mass spectrometry techniques. The experiments were performed over a wide range of high-pressures (25 – 50 atm) and temperatures (900 – 1800 K) which encompass typical conditions in modern combustion chambers.
The experimental results on the phenyl pyrolysis (preliminary to the experiments with acetylene), the phenyl + acetylene reactions, and the m-xylene oxidation provide unique data about these reaction systems. In fact, for the first time, it has been possible to detect and accurately measure a variety of PAH compounds, including the fused-ring species, for which mole fraction profiles have been obtained. Such species profiles were utilized to develop and validate comprehensive chemical kinetic models which helped clarify aspects related to the mechanisms involved in the formation of large polycyclic aromatic hydrocarbons at high pressures.
In addition, in order to explore new possible pathways for the formation of condensed structures, a theoretical study of the radical/π-bond addition reactions between single-ring aromatic hydrocarbons was performed using ab-initio quantum mechanics calculations. Several pathways leading to the formation of PAH compounds have been addressed as potentially relevant for typical combustion environments, especially in relation to the formation of the second-ring aromatic hydrocarbon, naphthalene. The calculations are also supported by the experimental results obtained on the phenyl pyrolysis, which confirmed the presence of the nonconventional pathway to naphthalene involving the reaction between o-benzyne and benzene.
The calculations were also extended to a different radical/π-bond addition reaction between o-benzyne and cyclic C5 hydrocarbons, cyclopentadiene and cyclopentadienyl radical. The results indicated the presence of alternative pathways for the formation of another important building block for soot, indene.
The experimental, modeling, and theoretical work performed under the program provided novel information about the processes involved in the incipient stages of soot formation. The chemical kinetic models developed and validated against the experimental results constitute the base for the development of comprehensive soot models for predicting emissions in order to address regulatory and legislative concerns.