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.

Are any reactions fast enough for soot formation?

tl;dr In order to stop soot pollution, we need to know what reactions cause soot to form. We used the computer to work out how fast a variety of different reactions between soot molecules to see what reactions could be forming soot. Most of the reactions are too slow and suggest larger molecules are required. 

Soot continues to be a problem for our climate by warming the atmosphere and melting ice. It also damages our bodies and causes significant health impacts. Some recent studies are coming out showing a strong relationship between polluted areas and places where the COVID-19 virus has taken many lives. For example in 66 administrative regions in Italy, Spain, France and Germany, 78% of COVID-19 deaths occurred in the five most polluted regions. (Ogen 2020). With a recent study based in the USA finding that for every 1 microgram per cubic metre of PM2.5 soot pollution is associated with an 11% increase in COVID-19 death rate? (Wu et al. Sci. Adv. 2020). Frustratingly we are still unable to describe how soot forms at the molecular scale and this is inhibiting our ability to reduce the emission of these toxic pollutants.

In this paper, my coworkers and I were able to run a series of calculations on the computer to systematically compare the speed of many reactions thought to happen in the flame. We refined a table of bond energies that we proposed in a previous paper (see this blog post) and by reordering the grid we found that we could categorise reactions into four main classes depending on the type of reactive site involved. 

Next we computed the reaction rate between each of these bonds using transition state theory. This involved computationally stretching the bonds until they were about to break and then determining the likelihood of a collision between these molecules leading to that transition state and ultimately the product with the bond formed. This allowed for a map of reaction rates versus temperature to be plotted and for the various reactions to be compared. 

Surprisingly we found that for all of the reactions between these small aromatics the reaction rates are too low to explain soot formation. This includes all of the mechanisms proposed to date involving small aromatic molecules found in flames. 

So we looked for various effects that could stabilise and enhance the reactions as the molecules enlarge. We found that for the localised pi-radicals the dispersion forces could enhance the equilibrium constant for dimerisation. It is unknown how this effect will impact the forward and backward rate constants but it is suggestive of an enhancement to the forward rate. 

There is more work to be done to work out whether this stabilisation of the larger localised pi-radical dimers will speed up the reactions to explain soot formation and whether they are in high enough concentration. However, we think the main contribution of this paper is being able to rule out a large number of possible reactions that have previously been proposed for soot formation. This is discussed in more detail in the review article that is currently online as a preprint. 

Combustion webinar - Carbonaceous nanoparticle formation in flames

At the end of last month Prof. Kraft, my PhD supervisor, presented the recent work on soot formation from the Computational Modelling group. I helped plan out the talk with colleagues and it is a very nice overview of the research we have been doing recently on the formation of soot. It is definitely for a more technical audience so be warned. For a less technical description, I recommend the webinar I recently gave at Churchill College. 

Are new reactive molecules present in flames?

tl;dr

Which reactive molecules lead to the formation of soot? Recently we showed that adding hydrogen to the edges of soot molecules makes the edges reactive. In this paper, we showed that these sites are very common in flames, making them likely to be important for soot formation.

In my previous blog post, I talked about our systematic comparison of possible crosslinks between reactive aromatic molecules that had recently been detected using atomic force microscopy. This gave the bond energies and allowed us to work out which bonds could be stable at flame temperature. It showed that adding hydrogen to a pentagonal ring gives a reactive localised π-radical that allows crosslinking and stacking.  

In our recent paper that just got accepted in the Proceedings of the Combustion Institute, "Reactive localised π-radicals on rim-based pentagonal rings: properties and concentration in flames" (see preprint here), we showed that these reactive sites are present in the flame in significant concentrations. 

To show what we mean by localisation the electron's spin density is shown below. The spin density shows where the reactive electron is likely to be able to form a bond with higher values indicating higher reactivity. We find two classes of π-radicals, those that 1) delocalise in 6-membered ring aromatic molecules and 2) reactive localised π-radical for pentagonal rings or methylene (–CH2) that does not delocalise across the molecule. 

While these reactive sites have been seen in molecules sampled from the flame it was not clear whether they also exist in the flame. For example, sampling these molecules from the flame could lead to hydrogen being added while the molecules in the flame could actually be lacking this hydrogen i.e. it could be an artifact of sampling. 

In order to determine if these species are present in the flame, we calculated all of the reactions that could allow hydrogen to be added or removed (thanks to Angiras and Dingyu for this). We could then consider, given the concentrations of hydrogen species in the flame, what sort of concentration we would expect. The reactions are shown below (those barrierless reactions do not make computing the rates very easy but it can be done with sufficient approximations).

We found that between 1-10% of the molecules contained a localised π-radical on their rim-based pentagon (for between 1400-1500K which are temperatures within a flame where soot begins to form). Comparing these results with the HR-AFM structures recently imaged we found a consistent frequency of rim-based pentagonal sites with a ratio of 27:12:4 for the unsaturated, saturated and partially saturated rim-based pentagonal rings, showing that these species are present in the flame in significant concentrations.

We explored another exciting possibility - that multiple localised π-radicals are present on a single molecule. These species are also likely to be present in reasonable fractions (thanks to Gustavo and Angiras for developing the KMC simulations). Of the molecules that were recently imaged using HR-AFM, over half contained one rim-based pentagon and roughly a quarter had two rim-based pentagons suggesting that the formation of multiradicals in flames is likely.

These results suggest a new mechanism for soot formation where molecules with two or more localised π-radicals can polymerise (a rapid chain reaction) - what we called the aromatic rim-linked hydrocarbon mechanism (ARLH).

There is a nice historical connection with New Zealander Prof. John Abrahamson, Canterbury University. During a sabbatical in the '70s at the Chemical Engineering department at the University of Cambridge (where I did my PhD) he wrote a paper proposing that the partial saturation of aromatic platelets forms soot (see structure below). I spoke with him recently during the lockdown in New Zealand about the HR-AFM results that show partial saturation of aromatic platelets and our results showing the localised π-radicals and he was happy to hear about the recent insights and how close he got in 1977.

Abrahamson, John. "Saturated platelets are new intermediates in hydrocarbon pyrolysis and carbon formation." Nature 266.24 (1977): 323-327.

The Chemical History of a Candle and Structure of an Ember - Webinar

A webinar presented on the 6th of May 2020 for the Churchill College MCR, University of Cambridge, by Jacob W. Martin.

Abstract: Grab a hot chocolate and get cosy for a fireside chat about a candle's flame and the embers left behind. Following the lead of Michael Faraday in his 1848 Royal Institution Christmas lectures, I will provide an updated chemical history of a candle - including some experiments for you to try at home. The fun doesn't end there - after a flame is extinguished, there's a wealth of discoveries to be made in the embers. Understanding the structure of these carbon materials begins with Rosalind Franklin, best-known for her work on DNA, and continues with simulations on a supercomputer. While there are still many unanswered questions around flames and carbon materials, these recent insights are enlightening and important for cleaning up our planet.

Finding the links between reactive molecules involved in soot formation

tl:dr
We still don't know how soot forms and this is stopping us from eliminating it from internal combustion engines and furnaces. Recently, the molecules present, just before soot formation, were directly imaged. For the first time, many of the reactive edges could be seen. In this work, we computationally screened these reactive edges. We then considered all possible crosslinks between these edges. We discovered a new crosslink that allows the molecules to be stabilised by physically stacking on top of each other and then becoming bonded at their rim. This could help explain the rapid growth of soot particles in the flames and lead to new ways to clean up combusiton.

We have just published a new paper in the Journal of Physical Chemistry C. Here is the infographic/abstract figure.

Reactive molecules involved in soot formation

At the 37^th International Symposium on Combustion, an extraordinary paper was presented directly imaging the molecules present just prior to soot formation. In the case of most of these aromatic molecules it is the edge that is the most reactive and over the years many suggestions have been made but never directly observed. So here they are.

Here are some of the most exciting findings. 

Firstly, some were found to be crosslinked suggesting reactions between radicals and molecules during soot formation. This contradicted a commonly held view that only physical interactions and not chemical reactions were involved. 

Secondly, there were lots and lots of pentagonal rings. Out of the 49 molecules (above 4 rings) imaged 28 contained at least one pentagonal ring and 12 contained two pentagonal rings on their rim. Previously only six-membered rings were thought to be stable at flame temperature.

Thirdly, species very close to curvature integration were found. While curved 3D were unable to be imaged using this technique at present, the presence of the almost curved molecules was encouraging for our suggestion of curved aromatic molecules being important in soot formation as I have previously discussed in this blog. 

Finally, some of these pentagonal rings were found to have hydrogen added to them. This forms a completely new radical type (–CH=CH– + H → –C^•HCH₂–  which we found formed a localised π-radical).

Given the wide range of interesting new molecules that were found we considered how their reactivities compared.

We made use of computational chemistry to compute the energy needed to remove an electron from a particular spot on the "surface" of the molecular surface (average local ionisation energy). This told us how likely it was to form a bond with another molecule and therefore allowed us to compare their reactivities.

One significant surprise was the reactivity of pentagonal rings and a new localised π-radical on pentagonal rings B).

Many reactions are important in the flame

Now that the reactive sites were characterised we considered which crosslinks between them could be important in the flame. Below is a figure of the crosslink energies. The green indicates bonds that are strong enough to persist at the high temperatures within a flame.

Most crosslinks are well-known mechanisms, however, the reactions with the localised π-radicals B) were completely novel.

A new type of bonding is possible - rim-bonding

Most of the ideas for how the molecules in flames come together to form soot particles have been either stacked physically interacting interactions or chemical bonds in a long polymer that did not stack. However, the localised π-radicals B) allows for stacked and bonded structures that are strongly bound.

This could allow molecules to rapidly condense and then crosslink which could explain the rapid growth of soot. Below is a drawing of how such a cluster could form we are calling an aromatic rim-linked hydrocarbon.

We need to figure out the concentration of this reactive site in the flame. We also need to compare how all of the possible crosslinks contribute to soot formation. Once this is achieved we can consider how to stop particular reactive sites from being made and reduce soot emissions.

Combustion science for Climate Solutions - Pint of Science

I recently gave a presentation at a Pint of Science event in Singapore entitled "Combustion Science for Climate Solutions". Here are the slides with my transcript added into the slides.

Looking inside soot particles

By looking into early soot particles using a high magnification electron microscopes and a "computational microscope" we can start to construct a picture of how the molecules are arranged and better understand how to halt soot formation which is bad for human health and for our planet.

We have recently published two papers that have approached this question from two different directions; Kimberly Bowal leading the charge from the computational side and Dr Maria Botero from the experimental side. 

Together they provide an interesting picture of how mixtures of aromatic molecules are arranged in liquid soot, carbonise as they are heated in the flame and helps us better understand how to stop soot from forming or provides means of destroy them more readily.

Computational microscope

Using accurate mathematical descriptions of the interactions between atoms we can simulate the way molecules self-assemble and see inside a small soot particle. Previously our group has focused only on clusters composed of a single type of molecule. An example of a cluster of coronene is shown below.

Soot is not made up of a single molecule but many hundreds of different sized aromatic molecules. So in this paper, we wanted to study how mixtures of different sized aromatic molecules arrange themselves. There are quite a few challenges simulating such as system;

1. We can only simulate small isolated clusters otherwise the simulations will take years to compute
2. We can only simulate for a short duration of simulation time (nanoseconds) but we want to figure out what the most stable arrangement of molecules is over the timescale of soot formation (milliseconds).

The first challenge is problematic as clusters in the flame will be exchanging molecules with the gas phase. This would be too challenging to attempt with such large clusters. Instead we apply a rubber band to molecules that leave the cluster so they always rebound back to the cluster. 

The second challenge is overcome by simulating hundreds of each cluster in parallel all at slightly different temperatures we then exchange clusters that are more ordered with lower temperature clusters. In this way low energy orientations get cooled and quenched providing the most stable arrangement. This method is called replica exchange molecular dynamics and has been used to simulate transformation of cellulose into coals on geological timescales so it is able to really accelerate the whole simulation to flame timescales.

The low temperature structure is also not influenced by the rubber band (as it is a solid so no evaporation occurs on simulation timescales) and therefore we are able to find out given an initial configuration of different sized aromatic molecules what is the most stable cluster geometry. 

Here are the results from two of the low temperature clusters (below). You can see the large aromatics on the inside and the small aromatics on the outside. We found this pattern in all of our clusters studied. Big molecules in the middle small molecules on the outside. 

This is also consistent with theory which suggests that the molecules with the greatest interactions would reside in the centre. But what do we see experimentally?

Electron microscope

We used an electron microscope which is able to see the aromatic molecules. In order to convert the images into numbers we used a custom program Maria wrote to convert the dark fringes seen in the image below (a) into lines (b) we can then split these lines up and measure how long they are and how curved they are.

Here are some of the many images that Maria collected at each of these heights with the images already converting into lines. Just looking at these fingerprints you can start to see some patterns. The early soot particle contains short fringes (indicating small molecules) while the particles further downstream have long fringes around the outside and smaller fringes inside.

We also found that the fringes in the smallest particles are really quite curved. The fringes suggest that over 60% of the aroamtics at the lowest height above the burner contain pentagonal rings indicating curvature. You can look at a blog post on the impact of these curved aromatic which we published earlier this year.

We also found that at the lowest height you have longer fringes in the middle of the particle compared with the outside. From the computational studies this indicates these early soot cluster are liquid. 

But as you go through the flame this pattern inverts with long fringes around the outside and smaller molecules molecules in the middle. This suggests they are no longer liquid clusters but are now starting to chemically react in the high temperatures (carbonising) and forming longer aromatics when two fuse to become one. This makes sense of recent nanoindentation studies that show that soot that has gone through the entire flame is actually quite hard when you isolate a single soot sphere almost the same hardness as charcoal.

Hypotheses to test

What are planning to do next and how are we going to extend this work

• We have been talking with other collaborators on using nanoindentation to study early soot particles to confirm if they are indeed liquid. 
• If we can understand how the carbonisation process occurs we can limit this process. From imaging of soot as they are broken down it was found that the outer shell is the last part to burn away making it important to reduce this can be achieved by lowering the temperature and inhibiting the particles grow too large.
• We have also been considering clusters of curved molecules and seeing if the same trends of partitioning hold. They certainly look quite different (see below).

So through a combination of computational and experimental studies we have been able to understand how different sized aromatics partition inside soot particles with the arrangement of longer aromatic molecules switching from the inside to the outside of the soot spheres indicating that early on liquid clusters give way to carbonised clusters.

Fingerprinting soot: finding curved aromatics in soot

Fingerprints of molecules in soot particles imaged in an electron microscope showing curved species in the early flame. 

Fingerprinting is a unique way of identifying people by reading off the ridges found on a person's fingers. In a new paper, I and some colleagues used an electron microscope to image the fingerprint of aromatic molecules in early soot particles, finding evidence for a large number of curved molecules, which could be important for reducing soot pollution. Here is a link to the paper or to the open access preprint, "Flexoelectricity and the Formation of Carbon Nanoparticles in Flames". Here is a short video explaining the findings.

Soot emissions cause many deaths around the world while also contributing to global warming, but scientists are still at a loss to explain how soot is formed. We have previously suggested a new mechanism where aromatic molecules curve via pentagon integration and become electrically polarised, interacting strongly with charged species produced in the flame. However, as of yet, no one had measured just how curved (and therefore polar) the molecules are in the early soot particles.

To answer this question we imaged the molecules in soot on the nanoscale. To collect some soot we injected a small copper grid covered in amorphous carbon into a flame very similar to a candle. Soot stuck to the amorphous carbon and using an electron microscope we were able to image the molecules in the soot particles sticking to the grid. As you can see in the first image of this post we could convert the dark regions of the image, corresponding to aromatic molecules on their side, into lines. We could determine how many molecules that we imaged indicate pentagonal ring integration and we found this value to be greater than 62.5% of the fringes. This high amount suggests that they are important for soot formation.

We found even in the earliest soot particles sampled at the lowest height, which give us the most insight into soot formation, long fringes 0.9-1.0 nm in length are present (around 15 aromatic rings). We then simulated three curved aromatic molecules of the same length with one, two and three pentagonal rings, providing different amounts of curvature, and computed the tortuosity/curvature. From this analysis, we could conclude that early soot particles (at 10 mm above the flame) have a tortuosity/curvature indicating that two pentagons rings are integrated. Below is a figure of the molecule which corresponds to the average species around 1 nm in width with two pentagonal carbon rings. 

 

Computing the electric polarisation due to the curved structure, we found a large value of 5.32 debye - around three times that of water, which is substantial. Below is a plot of the electric potential around the molecules introduced earlier.

We found this molecule bound very strongly to charged chemi-ions which are produced in abundance in the flame, using computer simulations. 

The strong interactions of these polar aromatic with chemi-ions need to be explored in more detail as it might help explain many of the electrical effects that have been seen, such as the ability of an electric field to stop soot formation in certain circumstances.

37th International Symposium on Combustion

tl;dr >1800 combustion scientists descended on Dublin last week to clean up combustion and pave the way for low carbon fuels. I gave my first talk at an international conference, where we suggested a new mechanism for soot formation based on curved aromatic molecules. 

Over the last week, I have been in Dublin joining with a group of scientists from around the world to better understand combustion. This biannual conference has been going on since 1928 and has a strong community of scientists from universities and industry. 

Why study combustion?

You might be fooled into thinking combustion is a dying field with the world transitioning from fossil fuels to electric transportation. However, this would be too small a view of the field of combustion. In the near future we need to be transitioning our current combustion engines to use low-carbon fuels such as biofuels, hydrogen and perhaps ammonia. This was a large part of the discussion at the conference.  Carbon soot, or particulates from combustion, contribute substantially to global warming and by understanding the formation of soot many hope we will be able to quickly remove this contribution from combustion.

Combustion research doesn't just focus on combustion for transport and heating - surprisingly, many common materials are produced in flames. Car tires are full of very stable carbon particles which are formed in a furnace. These carbon blacks also hold onto the lithium in our batteries allowing us to store electricity in our laptop or cellphone. Practically all of the white titania (titanium dioxide) powder in paint is formed in a flame. The optical fibres that provide us with high-speed internet are made from very pure silica, which can only be produced in a flame. A large part of the symposium was focused on these materials and making new novel materials in flames. 

What happens if we put the flame in there?

I saw talks from people who have put flames in space, super high pressure, vacuum chambers, synchrotrons, high magnetic fields, high voltage electric fields. In this section, I want to show some of the amazing experimental setups and instruments that were presented in talks and posters that are working on understanding soot formation.

Flame experiments in space

Flame spray burners using biofuels imaged with lasers [Image Creidt: Kaust]

Imaging of single soot molecules using atomic force microscopes.
 [Image credit:IBM Research-Zurich and F. Schulz et al./Proc. Combust. Inst.]

Flame probed with an intense beam of x-rays from a stadium-sized instrument [Image Credit: Soilel]

These new experimental setups are helping us to make leaps and bounds in our understanding of soot formation. Pentagonal rings and radicals were found to be prevalent in aromatic molecules in soot, which were previously thought to contain mainly hexagonal arrangements of carbons and could change the way these molecules self-assemble and react. However, we are still lacking some critical mechanistic understandings of soot formation. 

Soot formation and curved aromatics 

I presented a talk on the mechanism for soot formation and the impact of polar aromatic molecules. I have embedded a copy of the slides below.

Polar curved polycyclic aromatic hydrocarbons in soot formation from Jacob Martin

We received some excellent questions and suggestions from the community on how to extend these preliminary suggestions in order to test this hypothesis.

An exciting presentation from Francesco Carbone showed evidence that positive ions and not negative ions continue to grow into the soot particles (some of the early work is found in a paper last year), which is what you would expect from the soot nucleation mechanism we are suggesting. 

The great fun of this conference was meeting all of the other researchers from around the world and picking each other's brains over meals and pints of Guinness. Below is a picture of the Kraft research group which was the most numerous we have sent to a combustion symposium so far. Hopefully next symposium two years from now we will be closer to understanding how soot forms and whether these curved molecules contribute.

Can curved molecules in flames help reduce soot pollution?

Soot is a serious problem which impacts human and the planet's health. The world health organisation estimates that in 2012 seven million people died or 1/8 total deaths around the world, from air pollution with around half from ambient air pollution and the rest from indoor air pollution from cooking on inefficient stoves (WHO). Soot emissions are also the biggest contributor to global warming after the greenhouse gases. So how do we stop soot from forming? The first step is to understand the mechanism by which soot forms, which is surprisingly still not yet fully understood. We have just published an article considering the impact of curved aromatic molecules on the formation mechanism of soot (here is a link to the preprint).

Credit

So what is currently understood about soot formation? The animation below will aid in illustrating soot formation. It shows the inside of a small lawn mower motor with a transparent plastic engine head allowing for a high-speed camera to capture the combustion inside the engine. 

Credit

A small spark ignites the petrol and air mixture and a blue flame is seen to spread through the cylinder. At the flame front there is enough oxygen and so the fuel burns completely to carbon dioxide and water with the blue colour (the colour is due to excited intermediates C2*, CH* and OH*). However, not all of the flame has enough oxygen to completely combust. Behind the flame front, a build-up of carbon molecules forms soot which glows yellow because it is heated by the combustion. So the yellow colour you see in the figure is soot which is being exhausted into the atmosphere. You can confirm that the yellow colour is from soot at home by placing a metal spoon into the yellow part of the flame and picking up black soot on the spoon.

From the animation inside the engine, you will also have seen the complexity of the flame. To simply the system we often use simple burners to study soot formation which produce very stable candle-like flames (diffusion flame). Below is one of the burners with a schematic to highlight the molecules involved in soot formation. The steps for soot formation are as follows:

1. Precursor pyrolysis - the fuel breaks down and without enough oxygen a build up of carbon molecules - primarily acetylene occurs.
2. PAH formation - The acetylene C2H2 reacts through a radical chain reaction into aromatic fused ring molecules.
3. Inception/nucleation - Small carbon nanoparticle nuclei are formed 
4. Growth - Some of these nuclei grow to become primary particles tens of nanometres in size.
5. Aggregation/agglomeration - Primary particles stick together and form fractal-like stringy aggregates.

Step three (inception) is currently the most difficult to understand. Formation of small carbon nanoparticles from aromatic molecules in the flame happens too quickly to be a chemical reaction and must include some physical sticking or clustering to explain the speed of formation. My research group has previously determined that flat PAH (planar aromatics with only hexagonal rings) have been found to not be sticky enough to overcome the high temperatures where soot forms in a flame (~1000-1250°C). This thermal energy shakes the clusters apart.

So what about these curved PAH molecules? It is well known that they are present in soot, corannulene (a simple curved PAH with a pentagon ring surrounded by hexagonal rings) has been extracted from soot and if you decrease the pressure around the flame completely enclosed spherical cages of carbon (fullerenes) can be produced.

           

curved PAH corannulene (left and below click and drag to move the grey model around and zoom with your mouse wheel) and closed cage fullerene (right)

Energy: -2339647.6573446 Avogadro 30 35 0 0 0 0 0 0 0 0999 V2000 -1.4200 3.2390 0.0390 C 0 0 0 0 0 0 0 0 0 0 0 0 -2.6190 2.3660 0.0420 C 0 0 0 0 0 0 0 0 0 0 0 0 -2.5240 0.9410 -0.3280 C 0 0 0 0 0 0 0 0 0 0 0 0 -1.2840 0.5370 -1.0010 C 0 0 0 0 0 0 0 0 0 0 0 0 -0.0930 1.4030 -1.0030 C 0 0 0 0 0 0 0 0 0 0 0 0 -0.0940 2.7080 -0.3330 C 0 0 0 0 0 0 0 0 0 0 0 0 1.2310 3.2400 0.0400 C 0 0 0 0 0 0 0 0 0 0 0 0 2.4310 2.3690 0.0430 C 0 0 0 0 0 0 0 0 0 0 0 0 2.3370 0.9430 -0.3270 C 0 0 0 0 0 0 0 0 0 0 0 0 1.0980 0.5380 -1.0000 C 0 0 0 0 0 0 0 0 0 0 0 0 -0.8290 -0.8630 -0.9960 C 0 0 0 0 0 0 0 0 0 0 0 0 0.6440 -0.8630 -0.9960 C 0 0 0 0 0 0 0 0 0 0 0 0 -1.5940 -1.9150 -0.3190 C 0 0 0 0 0 0 0 0 0 0 0 0 -2.9780 -1.5650 0.0530 C 0 0 0 0 0 0 0 0 0 0 0 0 -3.4370 -0.1550 0.0490 C 0 0 0 0 0 0 0 0 0 0 0 0 1.4100 -1.9140 -0.3190 C 0 0 0 0 0 0 0 0 0 0 0 0 2.7930 -1.5620 0.0550 C 0 0 0 0 0 0 0 0 0 0 0 0 3.2510 -0.1520 0.0500 C 0 0 0 0 0 0 0 0 0 0 0 0 -0.8330 -3.1220 0.0590 C 0 0 0 0 0 0 0 0 0 0 0 0 -3.5830 -2.2990 0.5730 H 0 0 0 0 0 0 0 0 0 0 0 0 -4.3590 0.0860 0.5650 H 0 0 0 0 0 0 0 0 0 0 0 0 -3.5060 2.7160 0.5580 H 0 0 0 0 0 0 0 0 0 0 0 0 -1.4770 4.1910 0.5540 H 0 0 0 0 0 0 0 0 0 0 0 0 1.2870 4.1920 0.5550 H 0 0 0 0 0 0 0 0 0 0 0 0 3.3170 2.7190 0.5590 H 0 0 0 0 0 0 0 0 0 0 0 0 4.1730 0.0900 0.5670 H 0 0 0 0 0 0 0 0 0 0 0 0 3.3990 -2.2960 0.5740 H 0 0 0 0 0 0 0 0 0 0 0 0 0.6500 -3.1210 0.0590 C 0 0 0 0 0 0 0 0 0 0 0 0 -1.3450 -3.9230 0.5790 H 0 0 0 0 0 0 0 0 0 0 0 0 1.1630 -3.9220 0.5800 H 0 0 0 0 0 0 0 0 0 0 0 0 1 2 1 0 0 0 0 1 6 1 0 0 0 0 1 23 1 0 0 0 0 2 3 1 0 0 0 0 2 22 1 0 0 0 0 3 4 1 0 0 0 0 3 15 1 0 0 0 0 4 5 1 0 0 0 0 4 11 1 0 0 0 0 5 6 1 0 0 0 0 5 10 1 0 0 0 0 6 7 1 0 0 0 0 7 8 1 0 0 0 0 7 24 1 0 0 0 0 8 9 1 0 0 0 0 8 25 1 0 0 0 0 9 10 1 0 0 0 0 9 18 1 0 0 0 0 10 12 1 0 0 0 0 11 12 1 0 0 0 0 11 13 1 0 0 0 0 12 16 1 0 0 0 0 13 14 1 0 0 0 0 13 19 1 0 0 0 0 14 15 1 0 0 0 0 14 20 1 0 0 0 0 15 21 1 0 0 0 0 16 17 1 0 0 0 0 16 28 1 0 0 0 0 17 18 1 0 0 0 0 17 27 1 0 0 0 0 18 26 1 0 0 0 0 19 28 1 0 0 0 0 19 29 1 0 0 0 0 28 30 1 0 0 0 0 M END

We have previously shown that these molecules contain a large electric polarisation which might be important. So how do these polarised cPAH effect soot formation? In this paper we considered three questions:

1. What causes lead to curvature and how small do the molecules need to become curved?
2. Can curved PAH cluster together in the flame where soot forms (~1000-1250°C)?
3. What other interactions could cPAH have with other precursors (e.g. chemi-ions)?

What causes the curvature and how small do the molecules need to be to curve? Using computational chemistry we could calculate the geometry of lots of different curved aromatic molecules. We found the minimum size is 6 rings to curve a PAH with at least one ring being pentagonal.

It turns out the pentagon containing PAH are planar for larger molecules than you would expect from considering a perfect net of pentagons and hexagons. We found this is due to the bonding of electrons on the top and bottom of the molecule favouring planar geometries which are only overcome when the in-plane bonds are strong enough to pull the molecule into a curved configuration.

Can curved PAH cluster together in the flame where soot forms (~1000-1250°C)? Using another calculation method we worked out how sticky the cPAH are compared with their planar cousins (more negative binding energies means more strongly bound) shown below. Triangles indicate flat PAH and squares indicate cPAH. We found that for one or two pentagons, cPAH have similar binding energies to flat PAH. With three or greater number of pentagons the cPAH bound with less energy than the same sized flat PAH.

We have previously shown that for the size of flat PAH in soot it is unlikely to be able to stablise the small clusters in the soot inception zone. So for cPAH with up to two pentagons, we expect the situation to be similar and with three or more the nucleation is definitely not possible. 

What other interactions could cPAH have with other precursors? In the schematic near the top of the article, we also highlighted some ionic species called chemi-ions (coloured blue). There is a surprising amount of these charges in flames which form from reactions between an excited C-H molecule (which is one of the intermediates that glows blue). When excited CH  reacts with oxygen or acetylene it ejects an electron and becomes positively charged. The impact of these charges can be quite dramatic if you apply an electric field. At a given voltage you can also see the flame conduct electricity with arcs going through the flame. 

Soot formation can also be halted with a strong electric field. The picture below shows with a counterflow burner with fuel from below and air from above. With no electric field applied a yellow sooting flame is found. Applying a strong electric field removes the soot from the flame leaving a blue clean burning flame. This has been explained as charged nuclei and carbon being rapidly sucked out of the flame by the strong electric field.

Top - counterflow diffusion flame without an electric field applied. Bottom - with a strong electric field applied
Credit: Lawton and Weinberg 1969

Curved aromatics are significantly polar and might explain these electrical aspects of soot formation.  The interaction between charge and a dipolar molecule is long range and substantial. The plot below shows a range of curved PAH (covering the range of sizes we see in the flame 10-20 rings) with their binding energy to a charge. To give you a sense of what sort of energies are needed to hold something together at flame temperatures around 165 kJ/mol is usually quoted.

There is quite a lot more work needed to consider this ionic interaction and what impact soot formation here are some of the questions that need answering:

• How many cPAH are present in early soot nanoparticles?
• What is the polarity of the cPAH in early soot?
• Can a cluster of cPAH be stabilised at high temperatures in a flame with a chemi-ion?
• Is flexoelectricity involved in any other way in soot formation?

So the question Can curved molecules help reduce soot pollution? is yet to be fully answered. We will be presenting these results at the next combustion symposium in Dublin at the end of next month and have more results on the way.