Hydrogen abstraction/addition reactions in soot surface growth

Area of Science:

• Chemical Kinetics
• Combustion Chemistry
• Surface Science

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are key intermediates in soot formation.
• Understanding hydrogen abstraction (HB) and addition (HD) reactions is crucial for modeling combustion processes.

Purpose of the Study:

• To investigate HB and HD reactions of H radicals on PAH monomers and quasi-surfaces.
• To determine the influence of PAH size and temperature on reaction rates and pathways.
• To elucidate the role of surface diffusion in surface reactions.

Main Methods:

• Quasi-Classical Trajectory (QCT) method was employed to simulate reactions.
• Calculations were performed for a range of temperatures (1500-2700 K).
• Density Functional Theory (DFT) was used for comparison of HB reaction rates.

Main Results:

• QCT results align with DFT for HB reactions on PAH monomers.
• PAH size has minimal effect on HB rates at high temperatures (>2100 K).
• HD reaction rates show a clear dependence on PAH size.
• Reaction pathway shifts from HB to HD around 1900 K.
• Surface HB and HD reactions are significantly faster (nearly 10x) than gas-phase reactions.
• Surface diffusion accounts for ~50% of surface reactions, influenced by local energy transfer.

Conclusions:

• Surface diffusion is a critical factor in surface reactions, challenging the 'first collision' model for soot formation.
• Calculated rate constants for HB and HD reactions on specific surface sites are recommended for soot formation models.