After more than 150 years of study, combustion seems to be well
understood in terms of average energy output, high-concentration
intermediates, and major products. However, for improving combustion
efficiency and controlling pollution, it is necessary to understand
flame chemistry at the parts-per-million level while simultaneously
facing the turbulent fluid dynamics of a "real" flame.
As a result, many important rate constants have never been measured
directly, nor have all the species included in mathematical flame
models been directly observed. To this end, a joint collaboration
between Sandia National Laboratories, Cornell University, the University
of Massachusetts (Amherst), and Berkeley Lab has developed a low-pressure-flame
photoionization mass spectrometer that allows experimenters to isolate
the chemistry.
A schematic of the flat-flame burner and molecular
beam sampling assembly at ALS Chemical Dynamics Beamline 9.0.2.
In the apparatus, premixed reagent gases enter the flame chamber
through the porous flat face of a burner that translates horizontally
relative to a fixed quartz sampling cone and nickel skimmer, which
allows the temperature profiles and concentration profiles to be
mapped to very high precision. A well-collimated molecular beam
from the skimmer enters a differentially pumped (10-6
Torr) chamber, where it is photoionized by a crossed tunable VUV
beam. Photoions are mass-analyzed using a time-of-flight (TOF) mass
spectrometer (MS).
Among enols, the simplest is ethenol (vinyl alcohol). It is thermodynamically
unstable relative to its isomer acetaldehyde and has only recently
been observed as an intermediate in an ethene flame [T.A. Cool et
al., J. Chem. Phys. 119, 8356 (2003)].
The team launched a systematic search for enols among 24 different
flames of 14 prototypical single fuels found in modern fuel blends;
they also studied commercial gasoline. Experiments were conducted
with similar flame chambers operating at a branch of ALS Chemical
Dynamics Beamline 9.0.2 and at the National Synchrotron Radiation
Laboratory (Hefei, China).
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The luminous zone of the flame shown here
(just to left of the glowing red sampling cone) has the typical
blue-violet or blue-green color associated with chemiluminescence
from electronically excited CH and C2. |
Photoionization efficiency curves taken for m/z = 44 ions sampled
from four representative flames showed that ethenol is present in
all. The 0.9-eV difference between the photoionization thresholds
for ethenol and its isomer acetaldehyde made them easily distinguishable. In
addition to ethenol, larger enols also occur in flames of both simple
fuels and commercial gasoline. Photoionization measurements of m/z
= 58 ions show the presence of propenols, while butenols occur in
measurements for m/z = 72 ions sampled from a gasoline flame.
Enols are widespread intermediates in flames.
Left: Photoionization efficiency curves for m/z = 44 ions sampled
from four flames burning representative fuel compounds show that
ethenol is present in all. Right: Larger enols also occur in flames.
Photoionization measurements of m/z = 58 ions show the presence
of propenols, while butenols occur in measurements of m/z = 72
ions.
In the case of ethenol, not only are the concentrations far too
high to be explained as the isomerization of acetaldehyde, but the
data suggest that ethenol kinetics in flames are distinct. Markedly
differing distributions of ethenol and acetaldehyde with distance
from the flame burner suggest either separate formation mechanisms
or differential removal of ethenol as it diffuses toward the burner.
And the increasing fraction of ethenol relative to acetaldehyde
with distance from the burner for two flames suggests that the chemical
fates of the two are not at all the same.
Chemistries of ethenol and acetaldehyde in the
flame are not the same. Left: Differing distributions of the two
isomers with distance from the flame burner. Right: Increasing
fraction of ethenol relative to acetaldehyde with distance from
the burner.
While the practical impact of these findings on combustion remains
speculative for the moment, understanding the fundamental chemistry
of enols, important not only in combustion but also in other forms
of hydrocarbon oxidation important in such widely varied settings
as fuel cells, planetary atmospheres, and interstellar space, clearly
requires much more theoretical and experimental study.
Research conducted by C.A. Taatjes (Sandia National Laboratories
and JILA); N. Hansen, A. McIlroy, J.A. Miller, J.P. Senosiain, and
S.J. Klippenstein (Sandia National Laboratories); F. Qi (Sandia
National Laboratorires and National Synchrotron Radiation Laboratory,
China); L. Sheng and Y. Zhang (National Synchrotron Radiation Laboratory,
China); T.A. Cool and J. Wang (Cornell University); P.R. Westmoreland
and M.E. Law (University of Massachusetts, Amherst); T. Kasper and
K. Kohse-Höinghaus (Universität Bielefeld, Germany).
Research funding: U.S. Department of Energy, Office of Basic Energy
Sciences (BES) and National Nuclear Security Administration; the
U.S. Army Research Office; the Chinese Academy of Sciences; the
National Natural Science Foundation of China; and the Deutsche Forschungsgemeinschaft.
Operation of the ALS is supported by BES.
Publication about this research: C.A. Taatjes, N. Hansen, A. McIlroy,
J.A. Miller, J.P. Senosiain, S.J. Klippenstein, F. Qi, L. Sheng,
Y. Zhang, T.A. Cool, J.Wang, P.R. Westmoreland, M.E. Law, T. Kasper,
and K. Kohse-Höinghaus, "Enols are common intermediates
in hydrocarbon oxidation," Science 308,
1887 (2005).
ALSNews
Vol. 255, July 27, 2005
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