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**MECHANISM OF SOOT FORMATION: OXIDATION**

Michael Frenklach UC Berkeley & LBNL MACCCR Fuel Summit September 17, 2012 extra: aromatic-edge size

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**Homogeneous Nucleation**

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

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**SURFACE REACTIONS Frenklach 1989; Frenklach & Wang 1991:**

assumed analogous to gaseous aromatics assumed armchair sites H-abstraction H-addition O2 oxidation OH C2H2 •

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Graphene Edges zigzag armchair

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**Detailed Kinetic Monte-Carlo Model**

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

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**Developed Morphologies**

2500 K 1500 K 2000 K

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**Homogeneous Nucleation**

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

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**Oxidation of Aromatics**

(Carstensen and Dean, IJCK 2012) (Lin and Lin, JPC 1986) (Zhou, Kislov, Mebel, JPC A 2012) Oxyradicals are likely key intermediates Generally we look at the oxidation of benzene. It forms an oxyradical which decomposes to form a 5-member ring and CO. How does that translate to a larger PAH? We have been examining the decomposition.

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**Computational Details**

Quantum chemistry calculations DFT - B3LYP/6-311G(d,p) Reaction kinetics K, atm using MultiWell 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

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**B3LYP, M05-2X and M06-2X 1 atm + CO + CO Basis set: 6-311G(d,p) (0)**

47.8 (45.4) [43.2] 36.5 (34.7) [33.0] 24.7 (22.9) [21.9] + CO (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]: Basis set: 6-311G(d,p)

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**Multiwell -Argon was the bath gas collider**

0.0001 0.0002 10 -3 -2 -1 Species Fraction Time (sec) 5000 10000 15000 20000 25000 30000 35000 40000 45000 Average Vibrational Energy (cm ) 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 -technique explained well by David Golden -Argon was the bath gas collider

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Edown Edown = 260 cm-1 Edown = cm-1 Lin and Lin (1986) Temperature dependent Edown expression from Hippler, Troe, Wendelken, JCP 1983.

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Oxyradicals zigzag armchair

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**Correlation with Aromaticity**

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

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**Thermodynamic Stability**

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**Energy Correlation with Aromaticity**

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

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Larger Oxyradicals

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**Combining All Oxyradical**

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

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**Decomposition of Zigzag Oxyradicals**

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**Thermal Decomposition of Oxyradicals: Outer-ring Zigzag Edges**

kcal/mol

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**Thermal Decomposition of Oxyradicals: Outer-ring Zigzag Edges**

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**Thermal Decomposition of Oxyradicals: Inner-ring Zigzag Edges**

kcal/mol

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**inner rings of zigzag edges do not oxidize fast**

-note that these results are done (You, Zubarev, Lester, Frenklach, JPC A 2011)

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**Decomposition of Armchair Oxyradicals**

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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 10 atm 1 atm**

-Make sure to emphasizes that the pressure dependence is new. Not seen in past. 1 atm 1 atm 0.1 atm 0.1 atm 0.01 atm 0.01 atm

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**Armchair Decomposition**

“Free Edges” 2 Free Edge Free Edge Non-Free Edge 1 Free Edge 1 atm Ask question about blue line. 1 Free Edge kcal/mol

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**Substrate Size 1 atm -Affect is larger then expected**

-More work needs to be done in understand the assumption from molecular to edge reaction.

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

Armchair I ZZ I ZZ II AC II AC I 1 atm Armchair II ZZ I ZZ II AC II AC I Zigzag I -emphasize two things. Zigzag and armchair are the same. All at same time and same order of magnitude Zigzag II

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**Oxyradical Decomposition**

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**Kinetics Correlation with HOMA?**

HOMAoxyradical ring kT = 2000 K, P = ∞ (s-1) Barrier height (kcal/mol)

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**> >> Conclusions Proliferation of zigzag-edges in flames**

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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**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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