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Photoionization Mass Spectrometry Studies of Combustion Chemistry

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.

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Photoionization Mass Spectrometry Studies of Combustion Chemistry

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  1. 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

  2. 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 ! Detailed chemistry of single elementary fuel may have thousands of reactions and hundreds of species Comprehensive Kinetic Mechanism Turbulent, multiphase flows interact with the chemistry R + O2 reactions Autoignition

  3. H. Bockhorn, editor. Soot formation in combustion: mechanisms and models. Berlin: Springer, 1994. 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

  4. 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

  5. H. Bockhorn, editor. Soot formation in combustion: mechanisms and models. Berlin: Springer, 1994. 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

  6. Advanced Engines Rely on Autoignition Chemistry to an Unprecedented Degree

  7. H H isomerization isomerization H H H H H H H H H H H H H allyl methylvinyl cyclopropyl +O2 +O2 fast reaction fast reaction +O2 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 slow reaction

  8. + H + H + e- H C C C H H + e- C = C = C H H H Potential Energy (eV) (l = 119.7 nm) (l = 127.9 nm) IE=10.36 eV IE=9.692 eV H H H C = C = C H C C C H H H H Allene DHf = +47.4 kcal/mol Propyne DHf = +44.32 kcal/mol Distinguishing Isomers Is Possible by Photoionization Mass Spectrometry c Each isomer of a chemical usually has a distinct ionization energy,and a characteristic shape of its photoionization curve (Franck-Condon). C3H4

  9. H H C C C H H H H C = C = C H IE = 10.36 eV H Photoionization Efficiency Spectra Can Give Quantitative Isomer Ratios From PIE curveswe can extract theproportion of eachisomer present Allene IE = 9.692 eV Propyne

  10. 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).

  11. 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)

  12. 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).

  13. 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

  14. 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

  15. 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

  16. 4 Franck-Condon factor of c-C3H2 3 Photoionization efficiency 2 1 0 8.5 9.0 9.5 10.0 10.5 Photon energy (eV) Products of CH + Acetylene Appeared to Conflict with Theoretical Predictions CH + C2H2 Cyclo-addition Insertion Expected to be a minor channel [propargyl] ? + H HCCCH + H Cycloaddition appears to dominate? Main observed isomer Main isomer Predicted by Vereecken and Peeters JPC A103 5523 (1999)

  17. 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)

  18. 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

  19. 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

  20. 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.

  21. 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

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