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

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

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

4. Graphene Edges
zigzag
armchair

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

6. Developed Morphologies
2500 K
1500 K
2000 K

7. Homogeneous Nucleation
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
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.

9. 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

10. 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)

11. 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

12. Edown
Edown = 260 cm-1
Edown = cm-1
Lin and Lin (1986)
Temperature dependent Edown expression 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 Chem 2011

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
-note that these results are done
(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 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

27. 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

28. Substrate Size 1 atm -Affect is larger then expected
-More work needs to be done in understand the assumption from molecular to edge reaction.

29. 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

30. Oxyradical Decomposition

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

32. > >> 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

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