How to Make Soot and Stardust

The Science

Everyone’s seen soot, but nobody could find the mechanism that generated it. It was a mystery how gaseous hydrocarbon fuel and small soot precursor molecules could form large molecular networks and thus, soot particles. Now, researchers have identified rapid reactions that can explain how soot particles form. Soot forms when reactive molecules found in flames set off a chain reaction. Reactive molecules form and quickly bind with other hydrocarbons. The reaction forms the seed of the soot particle, but the process isn’t done. The binding creates reactive molecules. These reactive products bind with other hydrocarbons and cause the soot particle to grow.

The Impact

Soot is ubiquitous. It can harm human health, agricultural output, and energy-consumption efficiency; on the other hand, it is also critical for some industrial processes, such as glass and tire production. This study solves the longstanding mystery of the fundamental physics of how soot forms. It may also explain interstellar-dust formation. Understanding the physics may give scientists a better chance of controlling particle generation. That could mean reducing soot emissions from combustion sources, controlling soot formation during glass production and other industrial processes, or making novel carbon-based materials for other applications.

Summary

The chemical reactions behind soot particle formation have long been a mystery. The reactions, which turn gaseous hydrocarbons into a solid that resembles graphite, were difficult to measure and model. Why? The reactions occur extremely quickly inside a flame; most measurement techniques perturb the conditions under which the chemistry proceeds, and the complexity of the problem makes modeling and theoretical approaches difficult or intractable. Now, scientists have obtained detailed measurements from flames produced by burning different fuels under various conditions. Their work revealed sequences of resonance-stabilized radicals. The radicals quickly react with various hydrocarbons to form stable molecular complexes and generate more radicals, perpetuating the chain reaction. The result? The radical-based chain reaction leads to covalently bound clusters of hydrocarbons that would otherwise be too small to condense at high temperatures. One of the main breakthroughs of this work is the realization that soot inception is not a typical particle-nucleation process. Instead of a thermodynamically driven condensation process, soot inception occurs through rapid formation of strong chemical bonds (covalent bonds). This discovery provides a fundamentally new insight into the physics of the high-temperature inception of small carbonaceous particles.

Funding

The Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences funded this research. Work was done at the Advanced Light Source, a DOE Office of Science user facility at Lawrence Berkeley National Laboratory.

Publication

K.O. Johansson, M.P. Head-Gordon, P.E. Schrader, K.R. Wilson, and H.A. Michelsen, “Resonance-stabilized hydrocarbon-radical chain reactions may explain soot inception and growth.” Science 361(6406), 997 (2018). [DOI: 10.1126/science.aat3417]