Photoionization Mass Spectrometry Studies of Combustion Chemistry

Craig A. Taatjes, David L. Osborn, Leonid Sheps, Nils Hansen

Combustion Research FacilitySandia 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 + O2 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 C6H6 Potential

J. A. Miller and S. J. KlippensteinJ. 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 + O2 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

fastreaction

fastreaction

HH

HH

H

HH

HH

HHH

HHH

cyclopropyl allyl methylvinyl

+O2+O2+O2

isomerization isomerization

C3H5 + O2 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).

C3H4

C C C H

H

H

H

IE=10.36 eV

C C C H

H

H

H

++ e-

PropyneDHf = +44.32 kcal/mol

(l = 119.7 nm)IE=9.692 eV

Pote

ntia

l Ene

rgy

(eV

) ++ e-

AlleneDHf = +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 curveswe can extract theproportion of eachisomer present

IE = 9.692 eV

C C C H

H

H

H

IE = 10.36 eVC = C = C

H

H

H

H

Allene

Propyne

i

ii nEES )()(

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 O2 produces ethylperoxy radicalsPhotoionization 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 energyDissociative 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 + O2 reactionsDifferent O-heterocycles arise from QOOH of differing reactivityPhotoionization measurements can quantify the production of these

isomers

Butyl + O2 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 reactionsSecondary reactions can complicate interpretation of results

Products of CH + Acetylene Appeared to Conflict with Theoretical Predictions

4

3

2

1

0

Phot

oion

izat

ion

effic

ienc

y

10.510.09.59.08.5

Photon energy (eV)

Franck-Condon factor of c-C3H2CH + C2H2

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 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 reactionsSecondary reactions can complicate interpretation of results

Sensitivity is important for moving to higher pressuresHigh-pressure combustion chemistry has been repeatedly identified

as a priority research area by DOENew 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+ barTemperatures 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