Presentation on theme: "Photoionization Mass Spectrometry Studies of Combustion Chemistry Craig A. Taatjes, David L. Osborn, Leonid Sheps, Nils Hansen Combustion Research Facility."— Presentation transcript:
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
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 allylmethylvinyl +O 2 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). C3H4C3H4 C C C H H H H IE=10.36 eV C C C H H H H + + e - Propyne H f = +44.32 kcal/mol ( = 119.7 nm) IE=9.692 eV Potential Energy (eV) + + e - Allene H f = +47.4 kcal/mol ( = 127.9 nm) C = C = C H H H H 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
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 CH + C 2 H 2 [propargyl] HCCCH + 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 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 by10 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