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Michael Frenklach UC Berkeley & LBNL

MECHANISM OF SOOT FORMATION: OXIDATION. Michael Frenklach UC Berkeley & LBNL. MACCCR Fuel Summit September 17, 2012. extra: aromatic-edge size. +. +. Precursor Chemistry. fuel + O 2. Homogeneous Nucleation. Coagulation. Particle Dynamics. Agglomeration. C 2 H 2 , …. Growth.

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Michael Frenklach UC Berkeley & LBNL

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

  2. + + Precursor Chemistry fuel + O2 Homogeneous Nucleation Coagulation Particle Dynamics Agglomeration C2H2, … Growth Surface Reactions

  3. SURFACE REACTIONS H-abstraction • C2H2 H-addition OH O2 oxidation oxidation • Frenklach 1989; Frenklach & Wang 1991: • assumed analogous to gaseous aromatics • assumed armchair sites

  4. GrapheneEdges zigzag armchair

  5. Detailed Kinetic Monte-Carlo Model Whitesides & Frenklach, JPC A 2010 rate coefficients:Schuetz, Whitesides, You, Frenklach, Kollias, Domin, Zubarev, Lester, … 2005-10

  6. Developed Morphologies 2500 K 1500 K 2000 K

  7. + + Precursor Chemistry fuel + O2 Homogeneous Nucleation Coagulation Particle Dynamics Agglomeration C2H2, … Growth Surface Reactions O2, OH, … Oxidation

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

  9. Computational Details • Quantum chemistry calculations • DFT - B3LYP/6-311G(d,p) • Reaction kinetics • 1500-2500 K, 0.01- atm using MultiWell 2011.3 • Comparison to experiments Frank et al. (1994) 1.3 – 2.5 atm Lin and Lin (1986) 0.4 – 0.9 atm Computational results: You, Zubarev, Lester, Frenklach, JPC A 2011

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

  11. Multiwell 0 10 45000 ) 40000 -1 35000 -1 10 30000 25000 Species Fraction Average Vibrational Energy (cm 20000 -2 10 15000 10000 5000 -3 10 0 0 0.0001 0.0002 Time (sec) • Solves time-dependent 1-D energy transfer master equations • Solved stochastically using the Gillespie algorithm average vibrational energy of reactant kT,P= slope kT,P -Argon was the bath gas collider

  12. Edown Lin and Lin (1986) Edown = 439-545 cm-1 Edown = 260 cm-1 Temperature dependent Edownexpression from Hippler, Troe, Wendelken, JCP 1983.

  13. Oxyradicals zigzag armchair

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

  15. Thermodynamic Stability

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

  17. Larger Oxyradicals

  18. Combining All Oxyradical Zubarev, You, Domin, McClean, Lester,Frenklach, J Mater Chem2011

  19. Decomposition of Zigzag Oxyradicals

  20. Thermal Decomposition of Oxyradicals: Outer-ring Zigzag Edges kcal/mol

  21. Thermal Decomposition of Oxyradicals: Outer-ring Zigzag Edges

  22. Thermal Decomposition of Oxyradicals: Inner-ring Zigzag Edges kcal/mol

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

  24. Decomposition of Armchair Oxyradicals

  25. Potential Energy Surfaces kcal/mol

  26. Decomposition Rate 10 atm 1 atm 10 atm 0.1 atm 10 atm 1 atm 1 atm 0.1 atm 0.01 atm 0.1 atm 0.01 atm 0.01 atm

  27. Armchair Decomposition 1 Free Edge “Free Edges” 2 Free Edge 1 Free Edge Free Edge Non-Free Edge 1 atm kcal/mol

  28. Substrate Size 1 atm

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

  30. Oxyradical Decomposition

  31. Kinetics Correlation with HOMA? kT = 2000 K, P = ∞ (s-1) Barrier height (kcal/mol) HOMAoxyradical ring ∆HOMA

  32. 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: > >> free edge free edge 2 free edges no free edges 1 free edge free edge Proliferation of zigzag-edges in flames

  33. Acknowledgements 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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