Wednesday, 10 June 2020

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.

Wednesday, 20 November 2019

Finding the links between reactive molecules involved in soot formation

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.

Figure 1

Reactive molecules involved in soot formation

At the 37th 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 → –CHCH2–  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.

Wednesday, 28 August 2019

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!