Written by Michael Padilla, Sandia National Laboratories

Chemists at Sandia’s Combustion Research Facility have discovered a new mechanism leading to soot formation, providing new facts to make models of soot formation more predictive, furthering DOE’s goal of reducing soot formation in practical combustion devices.

“This work is important because it shows how the process of molecular weight growth that eventually leads to large carbon-rich soot precursors is not simply a path through the lowest-energy structures,” says Craig A. Taatjes a co-author on the paper. “We discovered that the kinetics is more complex than first imagined, and the preferred route may in fact lead through higher energy isomers.”

The work is featured in the October issue of Journal of Physical Chemistry Letters in a paper titled, “Time- and Isomer-Resolved Measurements of Sequential Addition of Acetylene to the Propargyl Radical.” Authors of the study include John D. Savee, Talitha M. Selby, Oliver Welz, Craig A. Taatjes, and David L. Osborn.
The Sandia team directly measured a reaction sequence in which multiple acetylene molecules (C2H2) react with a propargyl radical (C3H3). In a flame where more fuel is present than is needed, harmful soot can form. Acetylene is a ubiquitous molecule formed in such rich combustion environments, regardless of the fuel being burned. Many modelers have suggested that reactions of small free radicals with acetylene represent an important pathway to soot formation in these rich flames. However, the exact chemical mechanism — the pathways by which the atoms in the molecules form new bonds and rearrange during reaction — had not been measured directly.
Using tunable energy photons from the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory, the Sandia team discovered the reaction mechanism, and the fact that it changes significantly over the relatively small temperature range of 800 – 1,000 K.

Surprisingly, the C7H7 reactive intermediate (i.e., a propargyl that has added two acetylene molecules) in this sequence is not the most stable C7H7 species, the benzyl radical (a six-membered benzene ring with a –CH2 substituent), as normally assumed in combustion models. Rather the authors could show through the photoionization spectrum that the intermediate is the tropyl radical, a seven-membered ring. This finding agrees with very recent, independent theoretical predictions of the reaction that adds the second acetylene. Finally, the team discovered that the reaction sequence terminates, after addition of the third acetylene molecule, in a two-ring aromatic compound known as indene, demonstrating a pathway to a polycyclic aromatic hydrocarbon that does not pass through a six-membered ring intermediate like benzene or the benzyl radical.

David Osborn, the lead author on the study, says one of the biggest challenges in conducting this research was measuring reference spectra of the benzyl and tropyl radicals. The authors needed to independently and reliably create these reactive intermediates by methods other than the propargyl + acetylene reaction.
“Once we proved to ourselves that we had reliable reference spectra, we could use these spectra as fingerprints of each intermediate, like a forensic scientist would do when investigating DNA evidence on alleged suspects of a crime,” David says. “With that information in hand, the result was definitive: we were quite surprised to find that this reaction forms only the tropyl radical, not the ’usual suspect’ benzyl radical.”
The next big challenge, now that this reaction mechanism has been documented over a reasonable temperature range, is to determine how important this pathway is in formation of soot precursors in real fuel-rich flames. There are several other pathways, notably the reaction of two propargyl radicals, that almost certainly form significant soot precursors in flames, but the relative importance of these mechanisms, and how they change with temperature and other conditions in the flames, remains unknown. By unraveling all the details of each pathway, researchers will move closer to a predictive computer simulation that faithfully reproduces reality.