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Michael Frenklach UC Berkeley & LBNL MECHANISM OF SOOT FORMATION: OXIDATION MACCCR Fuel Summit September 17, 2012 extra: aromatic-edge size

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Coagulation + Agglomeration + Precursor Chemistry fuel + O 2 Homogeneous Nucleation Particle Dynamics Surface Reactions C 2 H 2, … Growth

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H-abstraction H-addition O2O2 oxidation OH C2H2C2H2 Frenklach 1989; Frenklach & Wang 1991: assumed analogous to gaseous aromatics assumed armchair sites

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zigzag armchair

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Whitesides & Frenklach, JPC A 2010 rate coefficients: Schuetz, Whitesides, You, Frenklach, Kollias, Domin, Zubarev, Lester, …

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1500 K 2000 K 2500 K

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Coagulation + Agglomeration + Precursor Chemistry fuel + O 2 Homogeneous Nucleation Particle Dynamics Surface Reactions C 2 H 2, … Growth O 2, OH, … Oxidation

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Oxidation of Aromatics Oxyradicals are likely key intermediates (Lin and Lin, JPC 1986) (Carstensen and Dean, IJCK 2012) (Zhou, Kislov, Mebel, JPC A 2012)

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Computational Details Quantum chemistry calculations DFT - B3LYP/6-311G(d,p) Reaction kinetics K, atm using MultiWell Comparison to experiments Computational results: You, Zubarev, Lester, Frenklach, JPC A 2011 Lin and Lin (1986) 0.4 – 0.9 atm Frank et al. (1994) 1.3 – 2.5 atm

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B3LYP, M05-2X and M06-2X 47.8 (45.4) [43.2] 36.5 (34.7) [33.0] 24.7 (22.9) [21.9] + CO 0 (0) [0] 50.8 (50.5) [49.3] 55.3 (54.6) [52.3] 39.7 (38.6) [36.5] 70.2 (73.4) [69.2] 63.0 (67.6) [63.3] 73.8 (77.3) [74.9] B3LYP: (M05-2X): [M06-2X]: + CO 1 atm Basis set: 6-311G(d,p)

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Species Fraction Time (sec) Average Vibrational Energy (cm ) Multiwell Solves time-dependent 1-D energy transfer master equations Solved stochastically using the Gillespie algorithm -Argon was the bath gas collider average vibrational energy of reactant k T,P = slope k T,P

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Temperature dependent E down expression from Hippler, Troe, Wendelken, JCP E down E down = 260 cm -1 E down = cm -1 Lin and Lin (1986)

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Correlation with Aromaticity Harmonic Oscillator Measure of Aromaticity HOMA = 1 (aromatic form of benzene) HOMA = 0 (Kekulè form of benzene) HOMA = HOMA limiting values

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kcal/mol Energy Correlation with Aromaticity Zubarev, Robertson, Domin, McClean, Wang, Lester, Whitesides, You, Frenklach, JPC C 2010

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Zubarev, You, Domin, McClean, Lester, Frenklach, J Mater Chem 2011

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kcal/mol

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inner rings of zigzag edges do not oxidize fast (You, Zubarev, Lester, Frenklach, JPC A 2011)

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Potential Energy Surfaces kcal/mol

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Decomposition Rate 10 atm 1 atm 0.1 atm 0.01 atm 10 atm 1 atm 0.1 atm 0.01 atm 10 atm 1 atm 0.1 atm 0.01 atm

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Armchair Decomposition kcal/mol 1 atm Free Edge Non-Free Edge Free Edges 2 Free Edge 1 Free Edge

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Substrate Size 1 atm

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Zigzag vs Armchair Armchair I Armchair II Zigzag I Zigzag II 1 atm ZZ I ZZ II AC II AC I ZZ I ZZ II AC II AC I

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Oxyradical Decomposition

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HOMA HOMA oxyradical ring Barrier height (kcal/mol) k T = 2000 K, P = (s -1 ) Kinetics Correlation with HOMA?

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Conclusions The decomposition of oxyradicals: temperature, pressure, substrate-size, site dependent armchair rates are close to those of a zigzag-edge correlated with aromaticity The following generalization can be made: 2 free edges free edge 1 free edge free edge no free edges > >> Proliferation of zigzag-edges in flames

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DOE-BES Prof. William A. Lester, Jr. Dr. Dmitry Zubarev (moving to Harvard U.) Dr. Russell Whitesides (now at LLNL) Prof. Xiaoqing You (now at Tsinghua U.) David Edwards

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