Particulate Formation, Evolution, and Fate- Hope Michelson Lecture Day 1 Part 3

Summary

The script delves into the complex mechanisms of molecular weight growth and particle inception in flames, focusing on the Haka mechanism for modeling these processes. It introduces the concept of hydrogen abstraction and carbon addition, explaining how these reactions contribute to the formation of multi-ring hydrocarbons. The speaker also explores different inception mechanisms, contrasting thermodynamically driven nucleation with kinetically controlled covalent bond formation. The discussion includes recent theories and experimental evidence supporting the role of resonance-stabilized radicals (RSRs) in particle inception through radical chain reactions. The presentation aims to provide clarity on these intricate topics and stimulate further investigation into the underlying chemical processes.

Takeaways

• 🔬 The Haka mechanism, introduced by Michael Frenklach's group, is used to model molecular weight growth and surface growth, involving hydrogen abstraction and acetylene addition to hydrocarbon species.
• 🔍 The Gibbs free energy for the hydrogen abstraction carbon addition mechanism shows sequential growth from benzene to larger hydrocarbons like pyrene through the abstraction of hydrogens and addition of acetylene.
• 🔴 Molecular weight growth occurs at high temperatures in flames, leading to the simultaneous growth of various hydrocarbons, while inception is the process of gas-phase hydrocarbons clumping together to form particles.
• 🌡️ Inception is distinguished from molecular weight growth by the clumping of hydrocarbons into particles, which can be thermodynamically driven (like nucleation) or kinetically controlled (involving covalent bond formation).
• 🌟 The thermodynamically driven inception may involve large species, possibly larger than 11 aromatic rings, which could nucleate at flame temperatures, but their high carbon to hydrogen ratio and low concentration in flames make this less likely.
• 🚫 Kinetically controlled particle inception is favored at lower temperatures and involves reactions that form covalent bonds, leading to the growth of particles that cannot easily be vaporized.
• 🛑 The difference between dimerization and inception is highlighted, with dimerization being a step towards inception but not the complete process, which also involves continued growth and attraction of more species.
• 🧬 The potential role of resonance-stabilized radicals (RSRs) in particle inception is suggested, with the hypothesis that RSRs could lead to rapid polymerization and particle formation through radical chain reactions.
• 🧪 Pyrolysis experiments with ethylene and indene indicate that RSRs can seed particle formation at lower temperatures, supporting the hypothesis that RSRs are involved in the inception process.
• 📉 The mass spectrometry analysis of particles from flames and pyrolysis experiments show a distribution of species with varying carbon to hydrogen ratios, suggesting the presence of aliphatic character and potential branching in the particles.

Q & A

What is the Haka mechanism?

The Haka mechanism, introduced by Michael Frenklach's group, is a multi-step process used to model molecular weight growth and surface growth in flames. It involves the abstraction of a hydrogen atom from carbon, followed by the addition of acetylene, leading to the formation of larger hydrocarbon species.

How does molecular weight growth differ from inception in the context of flames?

Molecular weight growth refers to the process where hydrocarbons grow in size through chemical reactions at high temperatures in a flame. Inception, on the other hand, is the process where gas-phase hydrocarbons clump together to form particles.

What are the two main classes of inception mechanisms?

The two main classes of inception mechanisms are thermodynamically driven mechanisms, such as nucleation and condensation, and kinetically controlled mechanisms, which involve reactions that lead to covalent bond formation.

What is the significance of the carbon to hydrogen (C/H) ratio in understanding particle formation in flames?

The C/H ratio is significant as it helps determine the types of hydrocarbon species present in a flame and their potential to form particles. A high C/H ratio indicates larger, less volatile species that might be more prone to nucleation, while a lower ratio suggests smaller, more reactive species.

What is the role of dispersion forces in the thermodynamically driven inception mechanisms?

Dispersion forces, such as van der Waals forces, play a role in the thermodynamically driven inception mechanisms by providing an attractive force between nonpolar molecules, which can lead to the clumping of gas-phase species into droplets or particles.

How does the kinetically controlled mechanism differ from the thermodynamically driven mechanism in terms of particle formation?

In the kinetically controlled mechanism, particle formation is driven by reactions that lead to the covalent bonding of hydrocarbon species, creating a solid-like structure that cannot be easily vaporized. This is different from the thermodynamically driven mechanism, which relies on the physical clumping of species through dispersion forces.

What is the significance of the experiment where particles were extracted directly from a flame?

The experiment where particles were extracted directly from a flame provides valuable insights into the types of hydrocarbon species present in the particles and their growth patterns. It helps researchers understand the processes of molecular weight growth and inception by analyzing the mass spectrum of the extracted particles.

What evidence suggests that resonance-stabilized radicals (RSRs) may play a role in particle inception in flames?

Evidence from experiments, such as pyrolysis studies and mass spectrometry, show the presence of RSRs in the gas phase and their potential to react with closed-shell hydrocarbons like ethylene. This suggests that RSRs could be involved in radical chain reactions that lead to particle inception.

How do the concepts of molecular weight growth and inception relate to the formation of soot in flames?

Molecular weight growth and inception are precursor steps to soot formation in flames. As hydrocarbons grow in size through molecular weight growth and clump together through inception, they eventually form larger aggregates that can lead to the development of soot particles.

What challenges do researchers face in modeling the inception and growth of particles in flames?

Researchers face challenges such as the lack of kinetic data for the reactions involved in inception, the need to accurately represent the complex chemistry of hydrocarbon growth, and the difficulty in measuring the properties of incipient particles at the onset of formation.