A Middle Way: A Review of Physical + Chemical Pathways to Soot Inception

tl;dr Our new preprint "Carbonaceous nanoparticle formation in flames" is out.

The paper is now published in Progress in Energy and Combustion Science

 

A middle way can refer to many things. In common usage it refers to a comprise between two positions. In philosophy or religion, it can refer to a rejection of extremes as exemplified by Aristotle’s golden mean that “every virue is a mean between two extremes, each of which is a vice”. In logic it can refer to a fallacy - halfway between a lie and a truth is still a lie and therefore some care is required in proposing such compromising positions. In science it has been used for a variety of justifiable and unjustifiable positions. One famous example being the middle way between physical scales and another being a position we recently put forward for the formation of the pollutant soot.

In the influential paper “The middle way” published in the Proceedings of the National Academy of Sciences of the USA in 2000, Laughlin et al. discussed the challenge in probing the scale between the atomic and macroscopic dimensions. In this mesoscopic region significant gaps exists in our understanding of how atoms and molecules interact, organise and form complex structures. This intermediate scale is too large to be measured by analytical chemical approaches and too small to be approached from the macroscale. Examples include protein folding, high temperature superconductors and disordered or topologically frustrated materials.

Our recent study on the formation of the pollutant soot illustrates the challenges probing the mesoscopic scale nicely. Figure 1 below shows a schematic of the transformation of fuel molecules into the pollutant soot. Only in the last 5 years have experimental techniques allowed for the aromatic soot precursor molecules as well as the earliest nanoparticles to both be directly imaged. Mass spectrometry has also allowed for the mass of the clustering molecules to be measured during soot formation. However, the mechanism by which these molecules cluster continues to baffle combustion scientists. The prize sought is the ability to understanding and potentially halt the emission of these toxic pollutants from internal combustion engines that damage almost every organ in our bodies as well as contribute to climate change. 

Figure 1 – Schematic for the transformation of fuel into soot inside a flame with insets showing the experimental results from which the schematic is derived. High resolution atomic force microscopy (HRAFM)from Commodo et al. 2019, Helium ion microscopy (HIM) from Schnek et al. 2013, high resolution transmission electron microscopy (HRTEM) from Martin et al. 2018 and scanning electron microscopy (SEM) from Orion carbons. 

Our modelling efforts also struggle to traverse the molecule to nanoparticle transition in soot formation. There are two main classes of models that have been proposed for soot formation. The first is physical nucleation where aromatic molecules grow until the intermolecular interactions between the molecules allows them to stick together and condense. The second is chemical inception where bonds form between the molecular systems. Only recently have accurate computational approaches been developed to explore these suggestions.

Concerning physical nucleation, Prof. Kraft’s group worked with the physical chemist Prof. Alston Misquitta (Queen Mary University) in the 2010s to accurately compute the intermolecular interactions between aromatic species (using a symmetry adapted perturbation with a hybrid density functional approach). From these results it was clear that the clustering species seen in the flame are far too small to possess the significant intermolecular energies required for physical nucleation mechanism. For my PhD, I explored electrical enhancements to physical nucleation that arise from curved aromatic species that possess a strong electric polarisation. While this electrical effect may help explain the electrical control of soot formation it alone cannot justify a nucleation mechanism either.

Concerning chemical inception, we recently undertook a systematic study of the bonds that could form between reactive aromatic soot precursors with Prof. Xiaoqing You’s group at Tsinghua University (made possible by the CARES programme). This was only possible due to the direct imaging of the reactive aromatics in 2019 (see Figure 1) and the recent advances in density functional computational techniques optimised for radicals (the meta hybrid GGA density functional method M06-2X). Figure 2 shows the systematic comparison that was possible with such an approach for small aromatic molecules. The green coloured grid squares correspond with thermally stable species. Mr Angiras Menon was recently able to compute the rate at which each of these crosslinks forms and compared them with the speed of soot formation. We found that for these small species none of the crosslinks formed sufficiently fast enough to explain the rapid clustering of molecules into soot nanoparticles.

Figure 2 – Bond energy between various reactive aromatic soot precursors. Green indicates bonds that have enough thermal stability to be considered as important in flames.

These detailed studies left us with the uncomfortable conclusion that the two main routes proposed for soot formation were unable to describe it. However, something did catch our attention crosslinks that allowed the molecules to both bond and stack, see Figure 2 B), C) and D) sites. This opened up another possibility that both physical and chemical mechanisms could cooperatively contribute to soot formation. Upon exploring these possibilities, we found that π-radicals on five membered rings, site B), formed highly localised states that did not become deactivated as the molecule grew in size, unlike their hexagonal ring equivalent, thereby remaining highly reactive. This allowed for an additive contribution between the physical interactions and the chemical bond only in these so-called aromatic rim-linked hydrocarbons (ARLH). Figure 3 shows the various mechanisms placed on a C/H versus molecular weight schematic to show the middle way suggested.

Figure 3 – A middle way is schematically shown between physical and chemical mechanisms for soot formation. 

As mentioned at the beginning of this article claims to middle ways are poor arguments unless they can be justified. Currently, we have shown that the addition of physical interactions and chemical bonding considerably increases the thermodynamic stability of aromatic rim-linked hydrocarbons. However, we have yet to show that such species can explain the rapid formation of soot in the flame. This requires the collision efficiency between these species and the concentration of the localised π-radicals on five-membered rings to be determined. Experiments are underway in the community to probe such species and close this missing gap between the micro and mesoscale of soot formation.

Unraveling the complex tangle of atoms in charcoal, glassy carbon and activated carbons

tl;dr: Scientists, myself included, were having trouble figuring out the nanostructure of disordered carbon (BBQ charcoal, or the material in your water filter). The structure is kind of like a graphite pencil, with layers of carbon, but these layers were tangled in a mess. We were able to use computers to reproduce this tangle of atoms and find out how they're connected. It turns out that the atoms are connected by warped, curved sheets that connect in 3D to resemble a foam. Stacking of the sheets, we think, is due to them being twisted together like a corkscrew. I've been trying to figure this out for a while and was very excited to work with researchers at Curtin University to shed some light on this long-standing problem in science.

Disordered 3D graphene network (1.5 g/cc similar density to charcoal). Shown as a surface mesh constructed from the graphene rings with the curvature coloured saddle-shape red, bowl-shape blue.

Unraveling the complex topology of disordered 3D graphenes

Disordered 3D graphenes may sound exotic but they are ubiquitous. They are the carbon materials found in BBQ charcoal, batteries' electrodes, water filters, gas masks, high-temperature ceramics, electrochemical sensors and insulation, and were even used to protect the Parker solar probe spacecraft from burning up on its approach to the sun. 

Rosalind Franklin, the scientist who would later deduce the helical geometry of DNA, first discovered this class of materials in 1951. Most carbon-containing materials develop small layered regions of graphene when heated. Upon further heating, to thousands of degrees, she found (to her surprise) a complete reluctance of the carbons to convert to the most stable form of carbon graphite - making it supremely metastable. 

Explanations for this reluctance to graphitise have centred around the integration of non-hexagonal rings which warp the network into either bowl-shaped fullerene or theoretically explored saddle-shaped schwarzite nanoforms of carbon, which are foam-like carbon networks. However, the nanostructures were unable to be resolved from experiments.

Researchers from Curtin University and the University of Cambridge this week published a possible solution to Franklin's problem in Physical Review Letters. They turned to large scale simulations using Australia’s Pawsey supercomputer to self-assemble the largest and most accurate networks of disordered 3D graphene networks to date.

Curtin Carbon group visualising a large scale carbon network using the Curtin Hive immersive display Twitter.

Working with researchers at the University of Cambridge they developed a new metric for the global curvature of the networks, they found that for all structures an excess of saddle-shaped graphene sheets are present. These saddle shapes are caused by the integration of 7- or 8-membered rings within the hexagonal graphene network. This warping allows it to connect in 3D and the researchers suggest it is the cause for the material's resistance to convert into graphite.

New nanostructure proposed for disordered 3D graphenes with bowl-, saddle- and ribbon-like graphene sheets. With increasing density, screw dislocations allow for winding up and layering of the network.

How about Franklin’s small regions of layered graphene? The researchers found that upon increasing the density of the material, the graphene sheets wound up like a spiral staircase. This screw or helix defect is well known in graphite but has not been suggested in these disordered materials. A variety of other defects were discovered, which resolve many issues of the graphene network being both curved and layered.

Defects observed in disordered 3D graphenes.

These results open up possibilities for understanding and engineering carbon materials for applications in supercapacitors, carbon fibres and high-temperature ceramics applications. However, more work is needed to experimentally confirm some aspects of the model. 

In terms of new applications, the researchers suggest that carbon materials could be topologically tuned and optimised for a given product. For example, how could you steer a carbon towards becoming graphite (of particular industrial importance for making batteries and electrodes)? This could open up many more materials for transformation into graphite, used in battery anodes, instead of having to mine the graphite.

There is a pleasing connection with Franklin's later work on DNA in that the solution to her earlier problem of non-graphitisability in carbon materials could also lie in topology and the famed helix structure. 

Read the preprint here while the paper is published in Physical Review Letters.

Thanks to Carla de Tomas, Irene Suarez-Martinez and Nigel Marks from the Carbon group at Curtin University for an excellent collaboration!

How are the atoms arranged in charcoal?

I recently published a paper on the structure of charcoal on the nanoscale with Leonard Nyadong, Caterina Ducati, Merilyn Manley-Harris, Alan G. Marshall, and Markus Kraft. Here is a link to the preprint and the published article in the journal Environmental Science & Technology.

In brief

• Charcoal is the black carbon product produced from heating biomass in a low oxygen environment. 

• Why would we be interested in studying charcoal? It has recently been suggested as a potential carbon dioxide storage method to combat climate change (called biochar in this capacity). Instead of the photosynthetically trapped carbon dioxide being released when waste biomass decomposes it is trapped by carbonisation into stable biochar that will not break down for thousands of years. One advantage is that it can be sold as it can improve soil fertility. We need to understand the nanostructure of charcoal in order to understand how long it is stable in the ground and how best to optimise its properties. Charcoal can also be used in electronic applications and
  
• The currently understood nanostructure of charcoal is that it is made up sheets of carbon atoms in a "chicken wire" or hexagonal arrangement. These sheet-like molecules then stack into small graphitic disordered crystals. Below is a picture of some of these stacked regions in a char made from resin.

(Top) Model of stacked ribbons of carbon (Bottom) Ribbon-like graphene structures imaged in char [Guo et al. 2012]. Used with permission from Wiley.

• Some of the highest magnification electron microscopes have found evidence for different nanostructures not planar but curved sheet-like carbon sheets where the curvature arises from non-hexagonal rings that warp the sheets.

  

  Non-hexagonal rings imaged in chars indicating curvature [Guo et al. 2012]. Credit permission granted from Wiley 

• When scientists see curved carbon nanostructures the first thing that comes to our minds is the most famous curved carbon structures - fullerenes which are cages of carbon that form a spherical net. The most well known curved carbon molecule is C60 buckminsterfullerene with atoms arranged in a similar manner to the intersection of seams in a soccer ball with 20 hexagonal rings, and 12 pentagonal rings of carbon. Given the presence of non-hexagonal rings, many suggested the nanostructure should be fullerene-like. 

C60 Buckminsterfullerene Credit

• If charcoal is fullerene-like many researchers expected to see C60 as it was thought to be a stable form of carbon as it is readily produced in high-temperature carbon arcs, but none could be found.
• We produced some high-quality charcoal in a gasifier, see my other blog post on gasification for more information. But for this study, it served to produce high-quality charcoal with a well-defined nanostructure so no tar or soot stuck to the surface.

  

  
  

  Gasifier was based on the Microlab gasifier from Fluidyne Gasification Ltd.

• We used some of the most precise machines in the world to weigh the molecules in charcoal  the Fourier Transform Ion Cyclotron Resonance Mass spectrometer (here is a video if you are curious about how it works from one of the authors Prof. Marshall).  We did not find any C60 or C70 in gasification charcoal as has been found before. We did however found a common ion in many charcoals (mass to charge ratio of m/z 701) which we previously thought could be part of the nanostructure as it is near to that of C60 (m/z 720), but we found this to be an unstable breakdown product and not a molecule that lasted upon heating. 

  

  Ultra high resolution mass spectrometer

• Using a different mass spectrometer that used a laser beam to ablate the sample and create charge molecules we could look at some heavier species and consider the nanostructure. We found a collection of molecules (peaks) that matched what we had found previously in a very curved carbon prepared from C60 arc-carbon that had been heated (see my previous post on these experiments).

  

  Mass spectrum from charcoal showing oxygenated fragments

Mass spectrum from heated and oxygenated fullerene arc-carbon showing similar oxygenated species.

• We found oxygen was present in all of these structures and a very similar set of molecules were found, which we could not reproduce repeating the experiment with graphite. This indicated that charcoal shares a curved oxygenated nanostructure with heat treated arc-carbon.
• A model was developed to explore the presence of non-hexagonal rings in a 3D graphene network. 

Stacked fulleroid-like model of the surface of charcoal showing the integration of non-hexagonal rings

• We are now working on understanding how this curvature is integrated into the structure and what  the topology (shape) of these sheets are. We also want to apply this understanding to improve technologies that rely on these materials such as carbon capture using biochar, water purification with activated carbons and energy storage applications like electrodes in batteries and supercapacitors.

This project spanned a decade and involved the help of many others. I want to thank Mr Doug Williams (Fluidyne Gasification Limited) for his advice in designing and building the gasifier and Mr Peter Wilkinson (Wilkinson Transport Engineers) for allowing me access to the workshop to construct the gasifier. Prof. Brian Nicholson (University of Waikato) for allowing me access to the laboratory space and instruments. I would also like to thank Prof. Robert Curl (Rice University) for putting me in contact with the late Prof. Harry Kroto who arranged for the application of the FT-ICR MS experiments with the group at Florida State University. Finally, I would like to thank Assoc. Prof. Nigel Marks, Dr Irene Suarez-Martinez and Dr Carla de Toma ́s (Curtin University) for providing the annealed molecular dynamics models online, which were used and modified to construct the model seen above. 

Carbon conference 2017 and the "Queen of carbon"

I recently attended and presented a poster at the Carbon 2017 conference in Melbourne, Australia. This yearly conference brings together around 800 carbon scientists and engineers from around the world to talk about the many forms of carbon such as fullerenes, nanotubes, graphene and various carbon materials like activated carbons, glassy carbons, nuclear graphite and carbon catalysts.

Something interesting about Carbon Conference this year was that it was a joint conference comprised eight other chemistry related conferences as part of the centenary celebration of the Royal Australian Chemical Institute (RACI). This meant we could attend any one of the many talks in fields from physical chemistry through to green chemistry.

3D printed molecules and electrically polarised carbon

The poster session was also run with the other chemistry conference. While viewing some other posters, I saw this 3D printed model of a metal organic framework which I thought was a creative idea.

I presented a poster on the impact of curvature in aromatic molecules which causes a significant charge polarisation leading to a molecular dipole of about 2 debye per pentagon (compare this with 1.85 debye for water). I had some good conversations with people about this; the activated carbon community was particularly interested in how polar molecules could adsorb onto these carbon structures and how the curved structures should carbonise.

Click on the poster for a larger image.

We recently uploaded a preprint of this work online if your interested further.

The Queen of Carbon

Something quite special about the carbon conference was a memorial session commemorating the life of Mildred Dresselhaus (or Millie to those in the community) who passed away in February this year. Her impact on the field of carbon research cannot be overexpressed. Starting from the elucidation of the electronic structure of graphite (the location of holes and electrons in the first Brillouin zone using magneto-optical spectroscopy) she went on to study the intercalation of ions in graphite (leading to the development of the lithium ion battery). Later, while working on fullerene she suggested the elongation of a fullerene into a tube and studied the electronic properties of these tubes in 1992, before single wall nanotubes had been discovered in 1993, suggesting they could be conducting or semiconducting depending on the nanotubes twist (chirality). She then turned her attention to graphene nanoribbons and graphene, working on tuning the electronic properties of graphene by confining them into nanoribbons as early as 1996.

Modified from presentation 

The "Queen of carbon science" as she was known, was also the first woman to gain a professorship at MIT and was an advocate for women in STEM, she also served in government. What I was struck by, was her character as a scientist. She really filled her life with research but always had space to talk with students, review papers and write textbooks. A truly inspirational woman; I'm sad I did not get an opportunity to meet her.