Transcript Photoionization Mass Spectrometry Studies of Combustion Chemistry Craig A. Taatjes, David L.

Photoionization Mass Spectrometry
Studies of Combustion Chemistry
Craig A. Taatjes, David L. Osborn, Leonid Sheps, Nils Hansen
Combustion Research Facility
Sandia National Laboratories
Livermore California USA
Combustion is a Complicated Mix of
Chemistry and Fluid Dynamics
c7h15o2-1=c7h14ooh1-2 2.000e+11 0.000 26850.0 !12-I 5s c7h15o2-1=c7h14ooh1-3 2.500e+10 0.000
20850.0 !12-I 6s c7h15o2-1=c7h14ooh1-4 3.125e+09 0.000 19050.0 !12-I 7s c7h15o2-1=c7h14ooh1-5
3.912e+08 0.000 22050.0 !12-I 8s c7h15o2-2=c7h14ooh2-1 3.000e+11 0.000 29400.0 !12-I 5p c7h15o22=c7h14ooh2-3 2.000e+11 0.000 26850.0 !12-I 5s c7h15o2-2=c7h14ooh2-4 2.500e+10 0.000 20850.0 !12-I 6s
c7h15o2-2=c7h14ooh2-5 3.125e+09 0.000 19050.0 !12-I 7s c7h15o2-2=c7h14ooh2-6 3.912e+08 0.000
22050.0 !12-I 8s c7h15o2-3=c7h14ooh3-1 3.750e+10 0.000 24400.0 !12-I 6p c7h15o2-3=c7h14ooh3-2
2.000e+11 0.000 26850.0 !12-I 5s c7h15o2-3=c7h14ooh3-4 2.000e+11 0.000 26850.0 !12-I 5s c7h15o23=c7h14ooh3-5 2.500e+10 0.000 20850.0 !12-I 6s c7h15o2-3=c7h14ooh3-6 3.125e+09 0.000 19050.0 !12-I 7s
c7h15o2-3=c7h14ooh3-7 5.860e+08 0.000 25550.0 !12-I 8p c7h15o2-4=c7h14ooh4-1 9.376e+09 0.000
22350.0 !12-I 7p c7h15o2-4=c7h14ooh4-2 5.000e+10 0.000 20850.0 !12-I 6s c7h15o2-4=c7h14ooh4-3
4.000e+11 0.000 26850.0 !12-I 5s ! c6h13o2-1=c6h12ooh1-2 2.000e+11 0.000 26850.0 !12-I 5s c6h13o21=c6h12ooh1-3 2.500e+10 0.000 20850.0 !12-I 6s c6h13o2-1=c6h12ooh1-4 3.125e+09 0.000 19050.0 !12-I 7s
c6h13o2-1=c6h12ooh1-5 3.912e+08 0.000 22050.0 !12-I 8s c6h13o2-2=c6h12ooh2-1 3.000e+11 0.000
29400.0 !12-I 5p c6h13o2-2=c6h12ooh2-3 2.000e+11 0.000 26850.0 !12-I 5s c6h13o2-2=c6h12ooh2-4
2.500e+10 0.000 20850.0 !12-I 6s c6h13o2-2=c6h12ooh2-5 3.125e+09 0.000 19050.0 !12-I 7s c6h13o22=c6h12ooh2-6 5.860e+08 0.000 25550.0 !12-I 8p c6h13o2-3=c6h12ooh3-1 3.750e+10 0.000 24400.0 !12-I
6p c6h13o2-3=c6h12ooh3-2 2.000e+11 0.000 26850.0 !12-I 5s c6h13o2-3=c6h12ooh3-4 2.000e+11 0.000
26850.0 !12-I 5s c6h13o2-3=c6h12ooh3-5 2.500e+10 0.000 20850.0 !12-I 6s c6h13o2-3=c6h12ooh3-6
4.688e+09 0.000 22350.0 !12-I 7p ! c5h11o2-1=c5h10ooh1-2 2.000e+11 0.000 26850.0 !12-I 5s c5h11o21=c5h10ooh1-3 2.500e+10 0.000 20850.0 !12-I 6s c5h11o2-1=c5h10ooh1-4 3.125e+09 0.000 19050.0 !12-I 7s
c5h11o2-1=c5h10ooh1-5 5.860e+08 0.000 25550.0 !12-I 8p c5h11o2-2=c5h10ooh2-1 3.000e+11 0.000
29400.0 !12-I 5p c5h11o2-2=c5h10ooh2-3 2.000e+11 0.000 26850.0 !12-I 5s c5h11o2-2=c5h10ooh2-4
2.500e+10 0.000 20850.0 !12-I 6s c5h11o2-2=c5h10ooh2-5 4.688e+09 0.000 22350.0 !12-I 7p c5h11o23=c5h10ooh3-1 7.500e+10 0.000 24400.0 !12-I 6p c5h11o2-3=c5h10ooh3-2 4.000e+11 0.000 26850.0 !12-I 5s
! !pc4h9o2=c4h8ooh1-2 2.000e+11 0.000 26850.0 !12-I 5s !pc4h9o2=c4h8ooh1-3 2.500e+10 0.000 20850.0
!12-I 6s !pc4h9o2=c4h8ooh1-4 4.688e+09 0.000 22350.0 !12-I 7p !sc4h9o2=c4h8ooh2-1 3.000e+11 0.000
29400.0 !12-I 5p !sc4h9o2=c4h8ooh2-3 2.000e+11 0.000 26850.0 !12-I 5s !sc4h9o2=c4h8ooh2-4 3.750e+10
0.000 24400.0 !12-I 6p !
Comprehensive
Kinetic
Mechanism
Turbulent,
multiphase flows
interact with the
chemistry
Autoignition
Detailed chemistry of
single elementary fuel
may have thousands of
reactions and
hundreds of species
R + O2
reactions
In Some Key Areas the Details of the
Chemistry Are Very Important
Pollutant Formation:
– Detailed combustion
chemistry determines nature
and amount of pollutants
– Soot is initiated by reactions
of small unsaturated
hydrocarbon radicals
H. Bockhorn, editor. Soot formation
in combustion: mechanisms and
models. Berlin: Springer, 1994.
Recombination of Propargyl Radicals
Occurs on a Complicated C6H6 Potential
Linear isomers are
relatively benign
Ring isomers are
soot precursors
J. A. Miller and S. J. Klippenstein
J. Phys. Chem. A, 2003, 107, 7783
In Some Key Areas the Details of the
Chemistry Are Very Important
Pollutant Formation:
– Detailed combustion
chemistry determines nature
and amount of pollutants
– Soot is initiated by reactions
of small unsaturated
hydrocarbon radicals
Ignition Chemistry:
– Chain-branching pathways
are a “nonlinear feedback”
for autoignition
– Alkyl + O2 and “QOOH”
reactions are central to lowtemperature chain branching
H. Bockhorn, editor. Soot formation
in combustion: mechanisms and
models. Berlin: Springer, 1994.
Advanced Engines Rely on Autoignition
Chemistry to an Unprecedented Degree
Full Characterization of These Processes
Requires Isomer-Specific Kinetics
• Isomer-resolved product distributions are sensitive probes
of reaction mechanisms.
• Different isomers may have vastly different reactivity,
steering downstream chemistry in different directions.
c
C3H5 + O2  products
H
H
isomerization
H
H
H
H
H
H
cyclopropyl
+O2
allyl
fast
reaction
+O2
isomerization
H
H
H
H H
H
H
methylvinyl
slow
reaction
+O2
fast
reaction
Distinguishing Isomers Is Possible by
Photoionization Mass Spectrometry
C=C=C
H
+
H
IE=9.692 eV
C=C=C
H
+ e-
(l = 127.9 nm)
H
H
C3H4
H
Allene
DHf = +47.4 kcal/mol
+
H
C
C C H
+ e-
H
IE=10.36 eV
(l = 119.7 nm)
H
H
H
H
Potential Energy (eV)
Each isomer of a chemical usually has a distinct ionization energy,
and a characteristic shape of its photoionization curve (Franck-Condon).
H
c
C
C C H
H
Propyne
DHf = +44.32 kcal/mol
Photoionization Efficiency Spectra Can
Give Quantitative Isomer Ratios
From PIE curves
we can extract the
proportion of each
isomer present
S ( E )   i ( E )  ni
i
Allene
H
H
H
H
C=C=C
H
H IE = 9.692 eV
H
Propyne
IE = 10.36 eV
C
C C H
Sandia Combustion Work at ALS Uses
Tunable Synchrotron Photoionization
Collaboration
between Sandia
CRF (David
Osborn, C.A.T.)
and LBNL (Musa
Ahmed, Kevin
Wilson, Steve
Leone)
Osborn et al., Rev. Sci.
Instrum. 79, 104103 (2008)
Taatjes et al., Phys. Chem. Chem. Phys. 10, 20 (2008).
Laser Photolysis Reactor is Coupled to
Time-of-Flight Mass Spectrometer
Multiplexed photoionization mass spectrometry (MPIMS)
Universal detection (mass spectrometry)
High sensitivity (synchrotron radiation + single ion counting)
Simultaneous detection (multiplexed mass spectrometry)
Isomer-resolved detection (tunable VUV, ALS synchrotron)
Kinetic Data is Acquired as a Function of
Time, Mass, and Photoionization Energy
Taatjes et al., Phys. Chem. Chem. Phys. 10, 20 (2008).
3-D dataset can be “sliced” along different axes to
probe different aspects of the reaction
Time Resolution Permits Kinetic
Discrimination of Ionization Processes
Reaction of ethyl with O2 produces ethylperoxy radicals
Photoionization of C2H5OO is dissociative to form C2H5+ + O2
Ethyl cation signal as a function of ionization energy shows:
Direct ionization of ethyl radical at low photon energy
Dissociative ionization of ethylperoxy emerging at higher photon energy
Distinct Photoionization Spectra Reveal
Isomeric Branching in Key Reactions
Butyl + O2 reactions
Autoignition is sensitive to the product branching in R + O2 reactions
Different O-heterocycles arise from QOOH of differing reactivity
Photoionization measurements can quantify the production of these
isomers
So What’s the Problem? Sensitivity!
Sensitivity limits ability to isolate individual chemical reactions
Radical + stable molecule reactions always in competition with
radical-radical reactions
Secondary reactions can complicate interpretation of results
Products of CH + Acetylene Appeared to
Conflict with Theoretical Predictions
CH + C2H2
Insertion
H
C
H
Expected to be a
minor channel
[propargyl]
H
+H
Main observed isomer
?
Franck-Condon factor of c-C3H2
3
2
1
0
8.5
C
H
H
Photoionization efficiency
Cyclo-addition
4
9.0
9.5
10.0
10.5
Photon energy (eV)
HCCCH + H
Main isomer Predicted by
Vereecken and Peeters
JPC A 103 5523 (1999)
Cycloaddition appears to dominate?
Photoionization Spectrum Changes with
Time, Indicating Secondary Reaction
• Early time signal has a
threshold near IE of triplet
propargylene
• Later signal looks more
like cyclopropenylidene
• Isomerization or faster
reaction of propargylene?
• In fact it is secondary
reaction of H atom with
C3H2 – could reduce if
sensitivity were better!
Goulay et al., JACS 131,
993–1005 (2009)
So What’s the Problem? Sensitivity!
Sensitivity limits ability to isolate individual chemical reactions
Radical + stable molecule reactions always in competition with
radical-radical reactions
Secondary reactions can complicate interpretation of results
Sensitivity is important for moving to higher pressures
High-pressure combustion chemistry has been repeatedly identified
as a priority research area by DOE
New engines will operate at higher boost and higher peak pressures
to increase power density while downsizing
What Happens to Autoignition Chemistry
at In-Cylinder Pressures!?
•
•
•
•
Collisional energy transfer will change
the product branching fractions
Previous experiments were at < 10 Torr –
in-cylinder this chemistry is at > 20 bar!
Isn’t everything just in the high-pressure
limit in an engine?
Optical measurements of autoignition
reactions at high pressure show – NO!
Predicting autoignition in advanced
engines requires understanding of
chemistry at:
Pressures 15 – 150+ bar
Temperatures 600 – 1100+ K
High Pressure Mass Spectrometry
Measurements Bring Many Challenges
• Extrapolation to these regimes is not reliable – We require
new and rigorous measurements
• For understanding fundamental chemical reactions the
timescale of the production needs to be resolved
• In sampling systems like our mass spectrometry
experiment, transit limits time resolution
• Time resolution limits reactant concentrations = signal!
– C2H3 + O2  CH2O + HCO (in great excess of helium)
– Rate = -d/dt [C2H3] = k[C2H3][O2]
– 0.01 atm  100 atm
increased dilution by104.
• Best solution is increase of VUV photon flux by 104.
The Right Light Source Could Help
Overcome Many of These Challenges
• Light-Source Needs (e.g., undulator radiation from ALS)
–
–
–
–
–
–
Repetition Rate 50 kHz or greater
High average power (> 1013 photons / s at 0.1% bandwidth)
Continuous, rapid tunability (7.3 – 16 eV)
Light with no higher harmonics (at most 10-4 of desired beam)
High brightness (optimum spot size ~ 1 x 1 mm)
Only moderate peak power (to avoid multiphoton processes)
• Light-Source Wants – Breakthrough Capabilities (FEL?)
– Much higher average power (1017 photons / s at 0.1% bandwidth)
– Tunability from 6.0 – 16 eV