Wednesday, 13 March 2019

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