Carbon black and soot formation

Carbon blacks are a black pigment that has been used for millennia as a dye. Today over 13 million tonnes are produced each year for reinforcing rubber tyres, inks and conductive fillers in the cathode of lithium-ion batteries.

Recently, there has been interest in producing hydrogen alongside the carbon blacks through a process called methane pyrolysis. In this process, renewable energy is used to thermally crack the hydrocarbon forming hydrogen and solid carbon instead of the gas carbon dioxide.

Carbon black can be considered a highly pure form of soot that has been heat-treated to remove toxic aromatic molecules. 

However, soot from incomplete combustion contains significant toxic aromatics is a significant health risk to us as well as contributes to global warming. 

We currently do not know how soot forms. This inhibits our ability to control carbon black synthesis as well as stop soot pollution formation. 

In our recent review, we have highlighted the molecule-to-particle transition as the key step that remains unexplained. 

We are currently working on kinetic mechanisms for methane pyrolysis and the fundamental nature of carbon radicals in collaboration with the Hydrogen Storage Research Group, Future Energy Export CRC as well as international collaborators.

Some more specific scientific insights we have made into this problem include:

• revealing fullerene-like curved aromatic molecules are electrically polarised (flexoelectric) that can couple electrical and chemical soot mechanisms,

• mechanical properties of soot particles reveal there are chemical crosslinks in early soot particles,

• computationally screened the reactivity of aromatic soot precursors to determine which reactions are too slow to explain the rapid formation of soot (most of them),

• reviewed the literature on possible routes for soot formation and highlighted a middle way that includes molecules physically condensing followed by chemically crosslinking.

• Imaged the molecules in the flame and found a molecule with two reactive sites that can polymerise at the rate required to explain soot formation.

Workshop notes prepared with Prof. Markus Kraft on the history of soot and carbon black as part of the Tsinghua Combustion Workshop 2022

Martin, J. W., Salamanca, M., & Kraft, M. (2022). Soot inception: Carbonaceous nanoparticle formation in flames. Progress in Energy and Combustion Science, 88, 100956.

Selvakumar, P. K., Martin, J. W., Lorenzo, M. D., Paskevicius, M., & Buckley, C. E. (2023). Role of π-Radical Localization on Thermally Stable Cross-Links Between Polycyclic Aromatic Hydrocarbons. The Journal of Physical Chemistry A, 127(33), 6945-6952.

Lieske, L. A., Commodo, M., Martin, J. W., Kaiser, K., Benekou, V., Minutolo, P., ... & Gross, L. (2023). Portraits of Soot Molecules Reveal Pathways to Large Aromatics, Five-/Seven-Membered Rings, and Inception through π-Radical Localization. ACS nano, 17(14), 13563-13574.

Martin, J. W., Pascazio, L., Menon, A., Akroyd, J., Kaiser, K., Schulz, F., ... & Kraft, M. (2021). π-Diradical aromatic soot precursors in flames. Journal of the American Chemical Society, 143(31), 12212-12219.

Martin, J. W., Hou, D., Menon, A., Pascazio, L., Akroyd, J., You, X., & Kraft, M. (2019). Reactivity of Polycyclic Aromatic Hydrocarbon Soot Precursors: Implications of Localized π-Radicals on Rim-Based Pentagonal Rings. The Journal of Physical Chemistry C, 123(43), 26673-26682.

Menon, A., Martin, J. W., Akroyd, J., & Kraft, M. (2020). Reactivity of Polycyclic Aromatic Hydrocarbon Soot Precursors: Kinetics and Equilibria. The Journal of Physical Chemistry A.

Menon, A., Martin, J., Leon, G., Hou, D., Pascazio, L., You, X., & Kraft, M. (2020). Reactive localized π-radicals on rim-based pentagonal rings: Properties and concentration in flames. Proceedings of the Combustion Institute.

Menon, A., Dreyer, J. A., Martin, J. W., Akroyd, J., Robertson, J., & Kraft, M. (2019). Optical band gap of cross-linked, curved, and radical polyaromatic hydrocarbons. Physical Chemistry Chemical Physics, 21(29), 16240-16251.

Pascazio, L., Martin, J. W., Bowal, K., Akroyd, J., & Kraft, M. (2020). Exploring the internal structure of soot particles using nanoindentation: A reactive molecular dynamics study. Combustion and Flame, 219, 45-56.

Botero, M. L., Sheng, Y., Akroyd, J., Martin, J., Dreyer, J. A., Yang, W., & Kraft, M. (2019). Internal structure of soot particles in a diffusion flame. Carbon, 141, 635-642.

Pascazio, L., Martin, J. W., Botero, M. L., Sirignano, M., D’Anna, A., & Kraft, M. (2019). Mechanical properties of soot particles: the impact of crosslinked polycyclic aromatic hydrocarbons. Combustion Science and Technology, 1-21.

Martin, J. W., Slavchov, R. I., Yapp, E. K., Akroyd, J., Mosbach, S., & Kraft, M. (2017). The polarization of polycyclic aromatic hydrocarbons curved by pentagon incorporation: the role of the flexoelectric dipole. The Journal of Physical Chemistry C, 121(48), 27154-27163.

Martin, J. W., Bowal, K., Menon, A., Slavchov, R. I., Akroyd, J., Mosbach, S., & Kraft, M. (2019). Polar curved polycyclic aromatic hydrocarbons in soot formation. Proceedings of the Combustion Institute, 37(1), 1117-1123.

Martin, J. W., Botero, M., Slavchov, R. I., Bowal, K., Akroyd, J., Mosbach, S., & Kraft, M. (2018). Flexoelectricity and the formation of carbon nanoparticles in flames. The Journal of Physical Chemistry C, 122(38), 22210-22215.