Rings of fire: Carbon combustion from soot to stars

We live in the carbon age. No other element has proven as versatile or indispensable. Carbon has fueled our industries and transport systems and helps make up such useful and ubiquitous materials as plastics and metallurgical composites. Carbon-based oil, coal, and graphite are among the world’s most important commodities.

Despite the importance of that humble element, scientists still have much to learn about the fundamental mechanisms by which carbon interacts with itself and other elements. Even carbon–carbon interactions yield a host of compounds, each with unique properties arising from its unique geometry. Those compounds, composed of the same single element but differing in atomic arrangement, are called allotropes. The best known among the carbon allotropes are diamond and graphite, which are depicted in figure 1 along with some relative newcomers to the list: graphene, fullerenes, and carbon nanotubes, all of which were synthesized in the laboratory. Those novel synthetic allotropes promise to revolutionize technologies and give birth to new industries. To generate them on an industrial scale and to harness their properties, however, researchers will need to understand their formation mechanisms. Thankfully, work directed toward understanding carbon growth mechanisms has long been under way, primarily in the seemingly disparate fields of combustion science and astrophysics.

A burning question

Even before the discovery of the synthetic carbon allotropes, scientists were busy trying to understand the combustion of carbon-based fuels. At first, researchers were primarily motivated to increase the efficiency of combustion; more recently the drive has been to mitigate global warming and the health effects of emissions.

Carbon combustion has a peculiar and important characteristic: It leads to the production of black, powdery soot. Early efforts to understand soot focused on its characterization, and they soon revealed the complexity and multitude of the compounds that make it up. Soot, we now understand, is built from subunits called polycyclic aromatic hydrocarbons (PAHs). As the examples in figure 1h show, PAHs are complicated molecules in which pentagonal and hexagonal hydrocarbons are fused together. They have been observed with as few as 9 C atoms and as many as 222, and they have displayed a seemingly infinite variety of geometries. The “aromatic” adjective is a chemistry term that refers to molecules that achieve enhanced stability by delocalizing their electrons over multiple bonds; the first compounds understood to demonstrate that phenomenon had a distinct and often pleasant aroma. PAHs can form from the burning of many carbonaceous compounds. Those include oil- and coal-based fuels but also such simple fuels as propane—as evidenced by the delicious black soot present on barbecued food. Thus humans have been producing PAHs since our earliest experiments with wood fire. The universe, however, was making PAHs long before we arrived.

The cosmic barbecue

When astronomers look at starlight, they find wavelengths missing from the stellar spectrum because atoms and molecules in the star’s atmosphere absorb energy at specific wavelengths. Indeed, by comparing stellar spectra with the spectra of known compounds measured in the laboratory, astronomers can determine a star’s composition. In time, scientists noticed that some of the compounds absorbing starlight couldn’t possibly be associated with the stars themselves. The explanation for that surprising result was that the absorbers were located in the expansive stretches of space between stars, areas long thought to be devoid of matter. Those compounds make up what is now known as the interstellar medium.

In some cases, the density of the interstellar material is so great that all visible starlight passing through it is absorbed. Evidently, great swaths of the interstellar medium are populated by what astronomers call dust clouds or grain clouds. When astronomers observed the heat emanating from the fringes of those interstellar dust clouds, they found IR emission lines that looked very similar to the IR emission lines that combustion scientists were reporting for PAHs. They even found space peppered with the exotic fullerene allotropes C₆₀ and C₇₀. Suddenly, astrophysicists and combustion scientists were asking the same question: How does carbon come to form large carbon-based structures such as PAHs and fullerenes?

Hydrogen out, acetylene in

The scientific literature is replete with mechanisms leading to PAH formation. One that the combustion-science community finds plausible was first posited in the mid 1980s by Michael Frenklach and colleagues. The researchers enlisted a then powerful mainframe computer—with 32 Mb of memory—to model the rates of more than 50 reactions that could generate the PAH rings. What they found surprised them: One reaction sequence proceeded at a rate that was orders of magnitude faster than any other. What’s more, they found that their experimentalist contemporaries were reporting abundant formation of the reaction precursors that the fast sequence predicted were of fundamental importance. In a later paper, Frenklach and Hai Wang summarized the essential process in the sequence as hydrogen abstraction (removal) followed by acetylene addition (HACA). The C in the acronym stands for C₂H₂, the chemical formula for acetylene; the journal in which Frenklach and Wang published apparently had a strict titular character limit.

As figure 2a shows, HACA is a repetitive growth process that starts when a hydrogen atom pops off an aromatic hydrocarbon to form a radical—that is, a compound with an unpaired electron. The site with the unpaired electron readily bonds with the acetylene molecule; repetitive acetylene addition then grows the PAH ring.

Frenklach and colleagues’ seminal work, and various subsequent studies, provided a solid theoretical justification for the HACA model. In fact, many experiments support the hypothesis, but none have actually observed the fundamental acetylene-addition reaction crucial for PAH formation. Now, some 30 years after HACA was proposed, we at Lawrence Berkeley National Laboratory’s Advanced Light Source (a synchrotron facility) have joined with colleagues at the University of Hawaii in an effort to validate the core reactions of the HACA sequence.

To simulate the blistering temperatures of combustion, we used a custom-built, one-inch-long thermal reactor milled from silicon carbide. By creating a vacuum in the SiC tube and then passing a strong electrical current through it, we could achieve temperatures as high as 2000 K. Our experiments actually operated at about 1020 K. In them, we used a gaseous mixture of acetylene and nitrosobenzene, an effective precursor of the single-ring aromatic hydrocarbon phenyl. If Frenklach and company were right, the reactants in the high-temperature environment would combine to form naphthalene, a two-ringed PAH. And they did, as we confirmed with a technique called photoionization mass spectrometry, as explained in figure 2b.

Our experiments suggest that the HACA mechanism can account for the formation of naphthalene, but they do not address the formation of a third ring or of exotic carbon allotropes such as fullerenes. Only further studies will decide if HACA could also form those more elaborate structures.

A high-temperature thermal reactor is just one means of probing carbon growth. To fully understand the process, several research programs are exploring growth under a variety of conditions, including the high-temperature, high-pressure conditions present in combustion and stellar death throes; the low-temperature, low-pressure conditions of interstellar space; and laboratory vacuum chambers.

Just as carbon growth mechanisms proceed incrementally and additively, so, too, must our understanding of them. What we learn will lead to ever more efficient combustion processes that produce fewer harmful by-products. We may even stumble upon another carbon allotrope or two; after all, fullerenes and carbon nanotubes were serendipitously discovered during investigations of the humble but remarkable element.