Photoionization Mass Spectrometry Studies of Combustion Chemistry

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22 Φεβ 2014 (πριν από 3 χρόνια και 5 μήνες)

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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 c7h15o2
-
2=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 c7h15o2
-
3=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 c6h13o2
-
1=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 c6h13o2
-
2=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 c5h11o2
-
1=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 c5h11o2
-
3=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 !

Autoignition

Comprehensive
Kinetic
Mechanism

R + O
2

reactions

Turbulent,
multiphase flows
interact with the
chemistry

Detailed chemistry of
single elementary fuel
may have thousands of
reactions and
hundreds of species

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 C
6
H
6

Potential

J. A. Miller and S. J. Klippenstein

J. Phys. Chem. A,

2003,

107,

7783

Linear isomers are
relatively benign

Ring isomers are
soot precursors

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 + O
2

and “QOOH”
reactions are central to low
-
temperature 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

c

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.

slow

reaction

fast

reaction


fast

reaction


H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

cyclopropyl

allyl

methylvinyl

+O
2

+O
2

+O
2

isomerization


isomerization


C
3
H
5

+ O
2



products

c

Distinguishing Isomers Is Possible by
Photoionization Mass Spectrometry

Each isomer of a chemical usually has a distinct
ionization energy
,

and a
characteristic shape

of its photoionization curve (Franck
-
Condon).

C
3
H
4

C C C H

H

H

H

IE=10.36 eV

C C C H

H

H

H

+

+
e
-

Propyne

D
H
f

= +44.32 kcal/mol

(
l

= 119.7 nm)

IE=9.692 eV

Potential Energy (eV)

+

+
e
-

Allene

D
H
f

= +47.4 kcal/mol

(
l

= 127.9 nm)

C = C = C

H

H

H

H

C = C = C

H

H

H

H

Photoionization Efficiency Spectra Can
Give Quantitative Isomer Ratios

From PIE curves

we can extract the

proportion of each

isomer present

IE = 9.692 eV

C C
C

H

H

H

H

IE = 10.36 eV

C = C = C

H

H

H

H

Allene

Propyne




i
i
i
n
E
E
S
)
(
)
(

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

3
-
D dataset can be “sliced” along different axes to
probe different aspects of the reaction

Taatjes et al., Phys. Chem. Chem. Phys. 10, 20 (2008).

Time Resolution Permits Kinetic
Discrimination of Ionization Processes

Reaction of ethyl with O
2

produces ethylperoxy radicals

Photoionization of C
2
H
5
OO is dissociative to form C
2
H
5
+

+ O
2

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

Autoignition is sensitive to the product branching in R + O
2

reactions

Different O
-
heterocycles arise from QOOH of differing reactivity

Photoionization measurements can
quantify

the production of these
isomers

Butyl + O
2

reactions

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

4

3

2

1

0

Photoionization efficiency

10.5

10.0

9.5

9.0

8.5

Photon energy (eV)

Franck
-
Condon factor of c
-
C
3
H
2

CH + C
2
H
2

C

H
H
H

[propargyl]

HCCCH + H

C

H
H
+ H

Main isomer Predicted by
Vereecken and Peeters

JPC A

103

5523 (1999)


Main observed isomer

?

Expected to be a
minor channel

Cyclo
-
addition

Insertion

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
C
3
H
2



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!


C
2
H
3

+ O
2



CH
2
O + HCO (in great excess of helium)


Rate =
-
d
/
dt

[C
2
H
3
] =
k
[C
2
H
3
][O
2
]


0.01 atm


100 atm increased dilution by
10
4
.


Best solution is increase of VUV photon flux by 10
4
.

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 (> 10
13

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 (10
17

photons / s at 0.1% bandwidth)


Tunability from 6.0


16 eV