Soot
Soon after the discovery of C60, Sir Harry Kroto and colleagues wrote a paper about the possibility that the mechanism which produced the carbon cage might also be involved in soot formation.
There are many mechanisms which have been proposed for soot formation and we did not, at least in my memory, claim that ours was the only one. However, it did lead to a controversy with some members of the soot community because perhaps they felt we were treading on their toes and did not know what we were talking about.
Sir Harry Kroto
Kroto's paper
Q L Zhang, S C O'Brien, J R Heath, Y Liu, R F Curl, H W Kroto and R E Smalley (1986), 'Reactivity of large carbon clusters: spheroidal carbon shells and their possible relevance to the formation and morphology of soot', J. Phys. Chem. 1986, 90, 4, 525–528.
Controversy
'Ideas on soot formation spark controversy', C&en archives, 1990.
Read the article on soot formation controversy (PDF, 1.46MB).
Tesner article
This extract, from a paper by Tesner, strongly influenced Harry Kroto's thinking on soot formation. The paper pointed out new data on soot structure from electron microscope studies. This showed that previous assumptions about structure of soot were not correct and was at variance with the consensus at the time.
"The soot particles have a very compact structure. The density of their material equals 2.0g/cm3 which is only 10% less than that of three-dimensionally-ordered graphite (2.26g/cm3). Following Biscoe and Warren and on the basis of the results of X-ray investigations, it has been assumed until recently that the soot particles comprised separate, irregularly located crystallites consisting of several approximately parallel graphite layers. Such a structure was termed "turbostratical".
"The application of high-resolution electron microscopes provided proof, but graphite layers, and not crystallites serve as building blocks for soot particles. These layers are bent to conform to the shape of the particle. Thermal soot particles consist of concentric spherical shells. In particles of other types of soot, only the external shells repeat the shape of the outer surface, the internal structure of such particles containing obviously several centres of growth. The distance between the layers has a spread conforming to the general distribution law.
"The compact and largely regular structure of the soot particles should be taken into account when considering the growth mechanism. It is evident, for instance, that one cannot logically imagine the creation of such a structure by consecutive association of more or less large blocks. Most probably, such a structure is the result of a molecular growth process for which the term "chemical crystallisation" would perhaps be the most exact one. Experimental results have actually proved this."
Extract from 'Formation of soot particles' (PDF, 504KB), P A Tesner, 1973.
This led to the discovery of structures which were perfectly in line with the new electron microscope structures and earlier carbon particle structures by Iijima.
Download the article about the structure of the particles, 'The formation of quasi-icosahedral spiral shell carbon particles' (PDF, 513KB), H Kroto and K McKay, Nature, 1988.
Sir Harry Kroto's reflections
Sir Harry Kroto commented, "I should note that I personally used to present our model as only a small part of my overall presentation on our C60 discovery.
"Furthermore, I did go to see Homann in Darmstadt at some stage and point out that the C60 signal that he observed in his soot formation system was almost certainly buckminsterfullerene.
My memory was that he did not believe me. After our structure was confirmed in 1990, Silke Loeffler (one of his students at the time) emailed me to say “Oh you were right after all” which suggests that non of the group believed me.
Sir Harry Kroto
"Later on, Howard showed that C60 could be made by combustion and Mitsubishi built a factory to make it commercially by the combustion of methane. This facility is now owned by Showa Denko.
"I think it is worth considering that whatever mechanism produces C60 may be involved to some extent in soot formation. This is because:
- As far as I can tell no combustion scientists detected C60 in their experiments prior to our discovery.
- No mechanism that the combustion community proposed predicted that C60 would form in combustion.
- Mitsubishi scientists tell me that their combustion system produces a high percentage of C60 and no PAHs from methane!
- C60 formation in combustion systems was only considered after our discovery.
"The upshot of all this is that whatever the mechanism is that produces C60 by hydrocarbon combustion, it should be considered as at least a possible contributor to soot formation. This was not done until after our proposal. (C J Pope, J A Marr and J B Howard, (1993) 'Chemistry of fullerenes C60 and C70 formation in flames', J. Phys. Chem. 1993, 97, 42, 11001–11013.)
"As Socrates said "It is a sign of intelligence to consider arguments with which you do not agree".
"This paper (PDF, 3.86MB) reminds me of the famous quote by Max Planck. Perhaps some sign of intelligence after all, though the origins of the mechanism are unreferenced!"
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.
Max Planck
Scientific Autobiography and Other Papers