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Risultati finali
Positive and negative hydrocarbon ions as possible intermediates in the production of soot in flames
It has been confirmed that ions are produced early in a flame by chemi-ionisation, in e.g., CH + C2H2 → C3H3+ + e-. Equally some aromatic ions (e.g. C6H5CH2+ and C6H5OH2+) occur extremely soon (before combustion occurs) in flames at atmospheric and reduced pressures, when using simple hydrocarbon like CH4 and C2H2 as fuel. The question thus arises as to whether the origin of these aromatic ions is something like:
C3H3+ + C2H2 → C5H5+; C5H5+ + C2H2 → C6H5.CH2+, etc.
or whether the first benzene ring originates from uncharged hydrocarbon radicals reacting with e.g., C2H2+. If the latter were true, the ion C6H5.CH2+ probably would have to originate from C6H5.CH3. The pursuit of these problems will be very much assisted by the construction of a new time of flight mass spectrometer (in Karaganda) capable of studying ions with masses above 200 amu. The complete task of simultaneously measuring radical concentrations (H, OH and O), as well as the concentrations of hydrocarbon radicals, ions and stable moleucles (including polyaromatic hydrocarbons, fullerenes, etc,etc) along a well-defines flame about to soot is forbiddingly huge.
The measurement of concentration profiles for positive and negative ions along flames is continuing in Cambridge. It was found that the addition of hydrogen to replace some of the argonin a flame of C2H2 + O2 + Ar (the temperature was carefully held constant) actually promoted soot. Also, the addition of hydrogen changes the ion profiles, which are still under analysis. More work on ions in a variety of fuel-rich flames is about to start in Darmstadt, to identify any relationship to soot, polyaromatic hydrocarbons, etc.
Other work at cambridge was succesful in collecting three types of soot particle (positively charged, negatively charged and uncharged) and then examining them in an electron-microscope to see if any systematic differences exist between them. Although analysis of the many samples continues, as yet no strikingly simple differences have been found.
The overall outcome of this project is that it is not easy to dismiss totally a role of some sort for ions in the production of soot. This role could be to provide the nucleus of to facilitate the formation of aromatic rings or accelerate growth or to affect the coagulation of particles. Equally, it is hard to demonstrate any such role for ions in the production of soot and PAHs.
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