Formation mechanism of polycyclic aromatic hydrocarbons beyond the second aromatic ring

Area of Science:

• Combustion Chemistry
• Polycyclic Aromatic Hydrocarbons (PAHs)
• Reaction Kinetics

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are significant pollutants formed during combustion.
• The hydrogen abstraction-C2H2 addition (HACA) mechanism is the classical pathway for PAH growth.
• Previous understanding suggested HACA predominantly forms PAHs with only six-member rings.

Purpose of the Study:

• To investigate the formation mechanism of three-ring PAHs from naphthalene.
• To elucidate the role of the HACA mechanism in PAH synthesis.
• To develop a comprehensive kinetic scheme for PAH formation relevant to combustion.

Main Methods:

• Accurate ab initio G3(MP2,CC)//B3LYP/6-311G** calculations were employed.
• Kinetic analysis of various reaction pathways and relative product yield computations were performed.
• The study focused on naphthalene as the initial reactant at combustion temperatures (1000-2000 K).

Main Results:

• The HACA mechanism predominantly yields cyclopentafused PAHs (e.g., acenaphthalene, ∼75%) rather than six-member ring PAHs (anthracene, phenanthrene, 3-6%).
• Formation of six-member ring PAHs like anthracene and phenanthrene is minor at combustion temperatures.
• A new kinetic scheme for HACA buildup to three-ring PAHs from naphthalene was constructed.

Conclusions:

• The HACA mechanism's role in PAH formation is re-evaluated, highlighting the prevalence of cyclopentafused structures.
• The study provides detailed rate constants for inclusion in flame kinetic models.
• Discrepancies in phenanthrene yield suggest potential alternative formation pathways for all-six-member-ring PAHs.