Formation mechanism of polycyclic aromatic hydrocarbons and fullerenes in premixed benzene flames

Formation Mechanism of Polycyclic Aromatic Hydrocarbons and Fullerenes in Premixed Benzene Flames HENNING RICHTER, WILLIAM J. GRIECO, and JACK B. HOWARD*

Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA A better understanding of the formation of polycyclic aromatic hydrocarbons (PAH) and fullerenes is of practical interest due to the apparent environmental health effects of many PAH and potential industrial applications of fullerenes. In the present work, a kinetic model describing the growth of PAH up to coronene (C24H12) and of C60 and C70 fullerenes is developed. Comparison of the model predictions with concentration profiles in a nearly sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio ␾ ⫽ 1.8, pressure ⫽ 2.67 kPa) measured by Bittner using a molecular beam system coupled to mass spectrometry shows reasonably good predictive capability for stable and radical intermediates and growth species up to C16H10 isomers. Cyclopentadienyl is found to be a key species for naphthalene formation. The further growth process is based on H abstraction and acetylene addition but also the contribution of small PAH is considered. Good to fair agreement between model predictions and experimental data for larger PAH including the different C16H10 isomers obtained by gas chromatography coupled to mass spectrometry and high performance liquid chromatography could be achieved for PAH in a sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (␾ ⫽ 2.4, 5.33 kPa). C60 and C70 fullerenes are underpredicted, and possible reasons such as uncertainties in rate coefficients or the existence of other formation pathways are discussed. PAH depletion in the burnt gas is not reproduced by the model and is believed to involve supplementary sinks such as reactions involving PAH and growing soot particles. © 1999 by The Combustion Institute

NOMENCLATURE Abbreviations Used in the Mechanism C10H7S C10H7O C10H7P C7H7 C10H7OH C10H7CH3 C10H8 A2R5 A2R5CH3 C12H10 A3 A2YNEP A2YNEP*S A3S*1 A2YNEP*P

secondary naphthyl (1naphthyl) 1-naphthoxy, 2-naphthoxy primary naphthyl (2-naphthyl) benzyl 1-naphthol, 2-naphthol methylnaphthalene naphthalene acenaphthalene methylacenaphthalene biphenyl phenanthrene primary naphthylacetylene (2naphthylacetylene) secondary A2YNEP radical (H-abstraction at the position 2 of naphthalene) 1-, 8-, 9- and 10-phenanthryl primary A2YNEP radical (Habstraction at the position 3 of naphthalene)

*Corresponding author: E-mail: [email protected] COMBUSTION AND FLAME 119:1–22 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.

A3L A3L*S A3L*P A3LYNE A3LYNE*P A4L A4L*S A3*P A3YNE A3YNE*S CHRYSEN CHRYSEN*S C6H6 C7H7 BENZYLB C6H5 FLUORENE BENZNAP BENZNAP*P C17H12 FLTHN ACEPHA FLTHN⫺

anthracene secondary anthracyl primary anthracyl anthracylacetylene primary anthracylacetylene radical benz[a]anthracene secondary benz[a]anthracyl primary phenanthryl phenanthrylacetylene secondary phenanthrylacetylene radical chrysene secondary chrysene radical benzene benzyl (C6H5CH2) benzylbenzene phenyl fluorene benzylnaphthalene primary benzylnaphthalene radical benzo[a]fluorene fluoranthene acephenanthrylene fluoranthene radical (fluoranthyl) 0010-2180/99/$–see front matter PII S0010-2180(99)00032-2

2 FLTHNCH3 BKFLUOR A3*S2 A3CH3 A3CH2R PYRENE*P PYRENECH3 PYRENE*S1 PYRENE*S2 BBFLUOR PYRYNEP PYRYNEP*S BAPYR BAPYR*S ANTHAN ANTHAN*S CPCDPYR CPCDPYR*S DCPP PYRYNE PYRYNE*S BEPYREN BEPYREN*S INPYR BGHIPER BGHIPE*S1 CPBPER BGHIPE*S2 CORONEN FLTHNC2H BGHIF BGHIF⫺ BGHIFCH3 BGHIFC2H FLTHNR FLTHNR*S BGHIFR COR CORCH3

H. RICHTER ET AL. methylfluoranthene benzo[k]fluoranthene 4- and 5-phenanthryl methylphenanthrene cyclopenta[def]phenanthrene primary pyrene radical methylpyrene 4- and 9-pyrenyl 5- and 10-pyrenyl benzo[b]fluoranthene primary pyrenylacetylene secondary radical of primary pyrenylacetylene benzo[a]pyrene secondary benzo[a]pyrenyl anthanthracene secondary anthanthracyl cyclopenta[cd]pyrene secondary cyclopenta[cd]pyrenyl dicylcopenta[cd]pyrene (all isomers) secondary pyrenylacetylene secondary radical of PYRYNE benzo[e]pyrene secondary benzo[e]pyrenyl indeno[1,2,3cd]pyrene benzo[ghi]perylene 1-, 2-, 3-, 8-, 9-, 10-, 11- and 12-benzo[ghi]perylenyl cylcopentabenzo[ghi]perylene (all isomers) 5- and 6-benzo[ghi]perylenyl coronene fluoranthylacetylene (all isomers) benzo[ghi]fluoranthene benzo[ghi]fluoranthyl methylbenzo[ghi]fluoranthene (all isomers) benzo[ghi]fluoranthylacetylene (all isomers) cyclopenta[cd]fluoranthene secondary cyclopenta[cd]fluoranthyl cyclopenta[cd]benzo[ghi] fluoranthene corannulene methylcorannulene (all isomers)

BGHIFR*S COR⫺ COR1 C5H5 C8H6 A1YNE*

secondary cyclopenta[cd]benzo [ghi]fluoranthyl corannulene radical cylcopenta[cd]corannulene cyclopentadienyl phenylacetylene phenylacetylene radical (Habstraction at phenyl)

Intermediates leading to C60 and C70: see Pope et al. [40]

INTRODUCTION The investigation of the chemical mechanism of particle growth in flames is motivated by a growing body of data revealing health effects of combustion generated compounds and particles. Many of the polycyclic aromatic hydrocarbons (PAH) found to be mutagenic or tumorigenic [1– 6] are present in atmospheric aerosols [7]. An association between air pollution and mortality was found in a study conducted in six U.S. cities [8]. The development of reaction mechanisms for complex combustion systems usually involves the comparison of experimental data such as concentration profiles with model predictions using an initial reaction network which can then be improved based on observed deficiencies. After the investigation of simple systems such as H2/O2 or H2/F2 [9] the increase of both computational power and experimental data allowed the description of larger and larger systems [10]. Data suitable for testing chemical models pertinent to PAH formation in flames are available from measurements in a nearly sooting premixed low-pressure benzene flame using molecular beam sampling coupled to mass spectrometry (MBMS) [11]. The data consist of profiles of temperature and concentration of stable species up to 202 amu and radicals up to 91 amu. The data on radicals have been extended to 201 amu species using nozzle beam sampling followed by radical scavenging and subsequent analysis by gas chromatography coupled to mass spectrometry (GC-MS) [12, 13]. Concentration profiles for stable species up to coronene (C24H12) in premixed propane, acety-

FORMATION OF PAH AND FULLERENES lene, and benzene flames at reduced pressure [14], and up to pyrene (C16H10) in premixed methane, ethane, and propane flames at atmospheric pressure [15] were obtained by probe sampling and GC-MS. PAH concentrations have been measured also for the high-temperature pyrolysis of toluene in shock tubes [16]. Global soot yields in shock tubes were determined under oxidative [17] and nonoxidative conditions [18 –20] following the pioneering work of Graham et al. [21] who studied soot formation investigating the pyrolysis of a large range of aromatic but also nonaromatic species. Detailed kinetic models for PAH formation have been developed, first for the growth of fused-ring species in acetylene pyrolysis [22] and later for growth up to benzo[ghi]fluoranthene and cyclopenta[cd]pyrene (C18H10) in premixed flames of methane [23], ethane [23], acetylene [24], and ethylene [24, 25], as well as up to pyrene in toluene pyrolysis [16]. The formation of fullerenes in flames also involves PAH intermediates and therefore is another source of interest in PAH formation. Fullerenes are a new form of carbon with considerable potential for industrial applications. Charged fullerenes were observed in premixed low-pressure acetylene and benzene flames [26, 27] and macroscopic quantities of C60 and C70 were isolated from flame-generated condensable material [28 –32]. Also other fullerene-related molecules such as oxygen- and hydrogen-containing compounds [33, 34] and fullerenic nanostructures [35–37] have been identified in flame samples. Fullerene formation mechanisms have been discussed qualitatively [27, 38, 39] and a kinetic model for C60 and C70 has been tested in a simplified system [40, 41]. The objective of the present work is to extend and assess the modeling of PAH and fullerene formation. To that end a network of chemical reactions describing the formation of PAH up to coronene (C24H12) and of C60 and C70 fullerenes was developed and critically tested against experimental flame data [11, 42]. APPROACH The growth of PAH and fullerenes being the main focus of the present work, the developed

3 mechanism was applied to a sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio ␾ ⫽ 2.4, 10% argon, gas velocity at burner at 298 K ⫽ 25 cm s⫺1, pressure ⫽ 5.33 kPa) first studied by McKinnon [43] and known to produce substantial yields of fullerenes [29, 31]. Soot volume fractions, the temperature profile and mole fraction profiles for H2, O2, CO, CO2, CH4, H2O, Ar, C2H2, and C6H6 measured by mass spectrometry are reported for this flame [43]. Recently, concentration profiles for PAH up to ovalene (C32H14) and fullerenes in this flame were measured by Grieco [42] using GC-MS and high-performance liquid chromatography (HPLC) analysis of flame samples. In the following text, this flame will be called Flame I. In order to ensure the correct description of the flame propagation chemistry and of the first growth steps, the model was also tested against MBMS data of Bittner and Howard [11] from a nearly sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (␾ ⫽ 1.8, 30% argon, 50 cm s⫺1, 2.67 kPa), here called Flame II. Modeling the growth of higher PAH requires reasonably good predictions for smaller intermediates and for key radical species such as H and OH. Such capability was achieved as shown in Figs. 1a and 1b by model predictions for H, OH, C5H5, phenyl, acetylene, and phenylacetylene in Flame II compared to experimental data. Experimental data [43] and predictions obtained for H2, H2O, C2H2, and CO in Flame I are shown in Fig. 2. The successful testing of the model for smaller stable and radical species is essential for the confident application of the model to larger species and the assessment of potential errors and uncertainties. Kinetic models describing the high-temperature oxidation or combustion of aromatic compounds include the model of Emdee et al. [44] for toluene which was tested against flow reactor data and the models of Lindstedt and Skevis [45] and Zhang and McKinnon [46] for benzene which were tested against Bittner’s Flame II data [11]. The present development began with the benzene destruction chemistry of Zhang and McKinnon [46] with improvements by Shandross et al. [47]. Rate coefficients were chosen after a careful check of the literature

4

H. RICHTER ET AL.

Fig. 2. Comparison between experimental mole fraction profiles [43] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25⬚C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. Temperature: (experimental). H2: F (experiment, left scale), (prediction, left scale). H2O: } (experiment, left scale), ---- (prediction, left scale). C2H2: ■ (experiment, left scale), 䡠䡠䡠䡠 (prediction, left scale). CO: Œ (experiment, right scale), 䡠-䡠-䡠 (prediction, right scale).

Fig. 1. (a) Comparison between experimental mole fraction profiles [11] and model predictions in a nearly sooting benzene/oxygen flame (␾ ⫽ 1.8, 30% argon, v 25⬚C ⫽ 50 cm s⫺1, 2.67 kPa); Flame II. H: F (experiment, left scale), (prediction, left scale). OH: } (experiment, left scale), ---(prediction, left scale). C5H5: ■ (experiment, right scale), 䡠䡠䡠䡠 (prediction, right scale). (b) Comparison between experimental mole fraction profiles [11] and model predictions in a nearly sooting benzene/oxygen flame (␾ ⫽ 1.8, 30% argon, v 25⬚C ⫽ 50 cm s⫺1, 2.67 kPa); Flame II. Temperature: (experimental). C6H5: F (experiment, left scale), (prediction, left scale). C2H2: } (experiment, right scale), ---(prediction, right scale). Phenylacetylene: ■ (experiment, left scale), 䡠䡠䡠䡠 (prediction, left scale).

and experimental data measured at high temperature were used whenever available. Two types of reactions, H abstraction from aromatic species and acetylene addition to their radicals are crucial for PAH growth as was found in the early computer modeling of this process by Frenklach et al. [22], and has been widely supported in many subsequent studies. Published experimentally determined rate coefficient expressions for H abstraction from benzene [48, 49] and acetylene addition to phenyl [50] were used without modification. No unambiguous trend in the reactivity of PAH and PAH-radicals with an increasing number of rings was seen in the few available studies [51–53], so the rate coefficients were approximated as being independent of molecular

weight over the whole growth process. The computations were performed with PREMIX [54] using experimental temperature profiles shown in Fig. 1b [11] and 2 [43]. The uncertainty of the temperature measurements is estimated to be ⫾50 K. Thermochemical and transport parameters were taken from the literature for benzene destruction [47] and fullerene growth [55] or were calculated using group additivity [56]. All reactions are treated as reversible. The PAH and fullerene growth model developed in this work and the testing of the model against experimental results are described below. Some aspects of the benzene destruction chemistry important for the growth process are discussed briefly. A complete listing of the reactions including the rate coefficients starting with naphthalene formation and some minor changes relative to the Shandross mechanism [47] are given in Table 1. Abbreviations used for certain species are given in the text in brackets together with the molecular formula and the species name. The nomenclature of radicals follows a convention different from that normally used in organic chemistry and distinguishes between primary and secondary radicals, e.g., 1-naphthyl, a secondary radical, is called C10H7S while 2-naphthyl, the corresponding primary radical, is designated C10H7P. An example for a tertiary radical—not used in the present model—is 9-anthryl, ob-

FORMATION OF PAH AND FULLERENES

5

TABLE 1. Growth Process of PAH and Fullerenes n

k ⫽ AT exp (⫺E a /RT)

A: cm mole⫺1 s⫺1 3

Formation of indene 1. C10H7S ⫹ O2 ⫽ C10H7O ⫹ O 2. C10H7S ⫹ OH ⫽ C10H7O ⫹ H 3. C10H7P ⫹ O2 ⫽ C10H7O ⫹ O 4. C10H7P ⫹ OH ⫽ C10H7O ⫹ H 5. C10H7O ⫽ INDENYL ⫹ CO 6. INDENE ⫹ H ⫽ INDENYL ⫹ H2 7. INDENE ⫹ OH ⫽ INDENYL ⫹ H2O 8. INDENE ⫹ O ⫽ INDENYL ⫹ OH 9. INDENYL ⫹ H ⫽ INDENE 10. C7H7 ⫹ C2H2 ⫽ INDENE ⫹ H Formation of naphthol 11. C10H7O ⫹ H ⫽ C10H7OH 12. C10H7OH ⫹ H ⫽ C10H7O ⫹ H2 13. C10H7OH ⫹ H ⫽ C10H8 ⫹ OH 14. C10H7OH ⫹ OH ⫽ C10H7O ⫹ H2O Formation of methylnaphthalene 15. C10H7S ⫹ CH3 ⫽ C10H7CH2 ⫹ H 16. C10H7P ⫹ CH3 ⫽ C10H7CH2 ⫹ H 17. C10H7CH2 ⫹ H ⫽ C10H7CH3 18. C10H7CH3 ⫹ H ⫽ C10H8 ⫹ CH3 Formation of acenaphthalene 19. C10H7S ⫹ C2H2 ⫽ A2R5 ⫹ H Formation of methylacenaphthalene 20. A2R5 ⫹ H ⫽ A2R5*S ⫹ H2 21. A2R5 ⫹ OH ⫽ A2R5*S ⫹ H2O 22. A2R5*S ⫹ CH3 ⫽ A2R5CH2 ⫹ H 23. A2R5CH2 ⫹ H ⫽ A2R5CH3 24. A2R5CH3 ⫹ H ⫽ A2R5 ⫹ CH3 Formation of phenanthrene 25. C12H10 ⫹ H ⫽ C12H9 ⫹ H2 26. C12H10 ⫹ OH ⫽ C12H9 ⫹ H2O 27. C12H9 ⫹ C2H2 ⫽ A3 ⫹ H 28. A2YNEP ⫹ H ⫽ A2YNEP*S ⫹ H2 29. A2YNEP ⫹ OH ⫽ A2YNEP*S ⫹ H2O 30. A2YNEP*S ⫹ C2H2 ⫽ A3*S1 31. A3*S1 ⫹ H ⫽ A3 Formation of anthracene 32. C10H7P ⫹ C2H2 ⫽ A2YNEP ⫹ H 33. A2YNEP ⫹ H ⫽ A2YNEP*P ⫹ H2 34. A2YNEP ⫹ OH ⫽ A2YNEP*P ⫹ H2O 35. A2YNEP*P ⫹ C2H2 ⫽ A3L*S 36. A3L*S ⫹ H ⫽ A3L 37. A3L ⫹ H ⫽ A3L*S ⫹ H2 38. A3L ⫹ OH ⫽ A3L*S ⫹ H2O 39. A3L ⫹ H ⫽ A3L*P ⫹ H2 40. A3L ⫹ OH ⫽ A3L*P ⫹ H2O 41. A3L*P ⫹ H ⫽ A3L 42. A3L ⫽ A3 Benz[a]anthracene formation 43. A3L*P ⫹ C2H2 ⫽ A3LYNE ⫹ H 44. A3LYNE ⫹ H ⫽ A3LYNE*P ⫹ H2 45. A3LYNE ⫹ OH ⫽ A3LYNE*P ⫹ H2O 46. A3LYNE*P ⫹ C2H2 ⫽ A4L*S 47. A4L*S ⫹ H ⫽ A4L

E a : cal

A

n

Ea

1.00E13 5.00E13 1.00E13 5.00E13 7.40E11 2.19E08 3.43E09 1.81E13 2.00E14 3.20E11

0.0 0.0 0.0 0.0 0.0 1.77 1.18 0.0 0.0 0.0

0.0 0.0 0.0 0.0 43850.0 3000.0 ⫺447.0 3080.0 0.0 7000.0

Marinov et al. [23] " " " " " " " " "

2.53E14 1.15E14 2.23E13 6.00E12

0.0 0.0 0.0 0.0

0.0 12400.0 7929.0 0.0

Baulch et al. [58] " " He et al. [59]

5.00E13 5.00E13 1.00E14 1.20E13

0.0 0.0 0.0 0.0

0.0 0.0 0.0 5148.0

Marinov et al. [23] " " "

3.98E13

0.0

10100.0

Fahr and Stein [50]

2.50E14 2.10E13 5.00E13 1.00E14 1.20E13

0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 0.0 0.0 5148.0

Kiefer et al. [48] Madronich and Felder [49] Rxn. 16. Rxn. 17. Rxn. 18.

2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 5.00E13

0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 0.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] estimate, this work

3.98E13 2.50E14 2.10E13 3.98E13 5.00E13 2.50E14 2.10E13 2.50E14 2.10E13 5.00E13 8.00E12

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

10100.0 16000.0 4600.0 10100.0 0.0 16000.0 4600.0 16000.0 4600.0 0.0 65000.0

Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] estimate, this work Kiefer et al. [48] Madronich and Felder [49] Kiefer et al. [48] Madronich and Felder [49] estimate, this work Colket and Seery [16]

3.98E13 2.50E14 2.10E13 3.98E13 5.00E13

0.0 0.0 0.0 0.0 0.0

10100.0 16000.0 4600.0 10100.0 0.0

Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] estimate, this work

Ref.

6

H. RICHTER ET AL. TABLE 1 continued n

k ⫽ AT exp (⫺E a /RT)

3

⫺1

A: cm mole

⫺1

s

Chrysene formation 48. A3 ⫹ H ⫽ A3*P ⫹ H2 49. A3 ⫹ OH ⫽ A3*P ⫹ H2O 50. A3*P ⫹ C2H2 ⫽ A3YNE ⫹ H 51. A3YNE ⫹ H ⫽ A3YNE*S ⫹ H2 52. A3YNE ⫹ OH ⫽ A3YNE*S ⫹ H2O 53. A3YNE*S ⫹ C2H2 ⫽ CHRYSEN*S 54. CHRYSEN*S ⫹ H ⫽ CHRYSEN Formation of fluorene 55. C6H6 ⫹ C7H7 ⫽ BENZYLB ⫹ H 56. C6H5 ⫹ C7H7 ⫽ BENZYLB 57. BENZYLB ⫹ H ⫽ BENZYLB* ⫹ H2 58. BENZYLB ⫹ OH ⫽ BENZYLB* ⫹ H2O 59. BENZYLB* ⫽ FLUORENE ⫹ H Formation of benzo[a]fluorene 60. C10H8 ⫹ C7H7 ⫽ BENZNAP ⫹ H 61. C10H7P ⫹ C7H7 ⫽ BENZNAP 62. BENZNAP ⫹ H ⫽ BENZNAP*P ⫹ H2 63. BENZNAP ⫹ OH ⫽ BENZNAP*P ⫹ H2O 64. BENZNAP*P ⫽ C17H12 ⫹ H Formation of fluoranthene 65. C10H7S ⫹ C6H5 ⫽ FLTHN ⫹ H ⫹ H 66. C10H7S ⫹ C6H6 ⫽ FLTHN ⫹ H2 ⫹ H 67. ACEPHA ⫽ FLTHN Methylation of fluoranthene 68. FLTHN⫺ ⫹ CH3 ⫽ FLTHNCH2 ⫹ H 69. FLTHNCH2 ⫹ H ⫽ FLTHNCH3 70. FLTHNCH3 ⫹ H ⫽ FLTHN ⫹ CH3 Formation of benzo[k]fluoranthene 71. C10H7P ⫹ C10H7S ⫽ BKFLUOR ⫹ H ⫹ H 72. C10H8 ⫹ C10H7S ⫽ BKFLUOR ⫹ H2 ⫹ H 73. C10H8 ⫹ C10H7P ⫽ BKFLUOR ⫹ H2 ⫹ H Formation of cyclopenta[def]phenanthrene 74. A3 ⫹ H ⫽ A3*S2 ⫹ H2 75. A3 ⫹ OH ⫽ A3*S2 ⫹ H2O 76. A3*S2 ⫹ CH3 ⫽ A3CH2 ⫹ H 77. A3CH2 ⫹ H ⫽ A3CH3 78. A3CH3 ⫹ H ⫽ A3CH2 ⫹ H2 79. A3CH3 ⫹ H ⫽ A3 ⫹ CH3 80. A3CH3 ⫹ OH ⫽ A3CH2 ⫹ H2O 81. A3CH2 ⫽ A3CH2R ⫹ H Formation of acephenanthrylene 82. A3 ⫹ H ⫽ A3*S1 ⫹ H2 83. A3 ⫹ OH ⫽ A3*S1 ⫹ H2O 84. A3*S1 ⫹ C2H2 ⫽ ACEPHA ⫹ H Formation of pyrene 85. A3*S2 ⫹ C2H2 ⫽ PYRENE ⫹ H Formation of methylpyrene 86. PYRENE ⫹ H ⫽ PYRENE*P ⫹ H2 87. PYRENE ⫹ OH ⫽ PYRENE*P ⫹ H2O 88. PYRENE*P ⫹ CH3 ⫽ PYRENECH2 ⫹ H 89. PYRENECH2 ⫹ H ⫽ PYRENECH3 90. PYRENECH3 ⫹ H ⫽ PYRENE ⫹ CH3 Pyrene-oxidation 91. PYRENE ⫹ OH ⫽ A3*S1 ⫹ CH2CO 92. PYRENE ⫹ OH ⫽ A3*S2 ⫹ CH2CO

E a : cal

A

n

Ea

Ref.

2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 2.50E14 5.00E13

0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 16000.0 0.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Kiefer et al. [48] estimate, this work

1.20E12 2.00E22 2.50E14 2.10E13 4.00E11

0.0 ⫺3.045 0.0 0.0 0.0

15940.0 2304.0 16000.0 4600.0 4000.0

estimate based on [60] estimate based on [47] Kiefer et al. [48] Madronich and Felder [49] Rxn. 66.

1.20E12 2.00E22 2.50E14 2.10E13 4.00E11

0.0 ⫺3.045 0.0 0.0 0.0

15940.0 2304.0 16000.0 4600.0 4000.0

Rxn. 55. Rxn. 56. Kiefer et al. [48] Madronich and Felder [49] Rxn. 59.

5.00E12 4.00E11 8.51E12

0.0 0.0 0.0

0.0 4000.0 62860.0

Marinov et al. [23] " Brouwer and Troe [61]

5.00E13 1.00E14 1.20E13

0.0 0.0 0.0

0.0 0.0 5148.0

Rxn. 16. Rxn. 17. Rxn. 18.

5.00E12 4.00E11 4.00E11

0.0 0.0 0.0

0.0 4000.0 4000.0

Rxn. 65. Rxn. 66. Rxn. 66.

2.50E14 2.10E13 5.00E13 1.00E14 1.20E14 1.20E13 1.26E13 1.20E12

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 0.0 0.0 8235.0 5148.0 2583.0 15940.0

Kiefer et al. [48] Madronich and Felder [49] Rxn. 16. Rxn. 17. estimate based on [44] Rxn. 18. estimate based on [44] estimate based on [60]

2.50E14 2.10E13 3.98E13

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

3.98E13

0.0

10100.0

Fahr and Stein [50]

2.50E14 2.10E13 5.00E13 1.00E14 1.20E13

0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 0.0 0.0 5148.0

Kiefer et al. [48] Madronich and Felder [49] Rxn. 16. Rxn. 17. Rxn. 18.

1.30E13 1.30E13

0.0 0.0

10600.0 10600.0

Wang and Frenklach [24] "

FORMATION OF PAH AND FULLERENES

7

TABLE 1 continued n

k ⫽ AT exp (⫺E a /RT)

3

⫺1

A: cm mole

s

⫺1

E a : cal

93. PYRENE ⫹ O ⫽ A3*S1 ⫹ HCCO 94. PYRENE ⫹ O ⫽ A3*S2 ⫹ HCCO 95. PYRENE*S1 ⫹ O2 ⫽ A3*S2 ⫹ 2CO 96. PYRENE*S2 ⫹ O2 ⫽ A3*S2 ⫹ 2CO Formation of benzo[b]fluoranthene 97. A3*S1 ⫹ C6H5 ⫽ BBFLUOR ⫹ H ⫹ H 98. A3*S1 ⫹ C6H6 ⫽ BBFLUOR ⫹ H2 ⫹ H Formation of benzo[a]pyrene 99. PYRENE*P ⫹ C2H2 ⫽ PYRYNEP ⫹ H 100. PYRYNEP ⫹ H ⫽ PYRYNEP*S ⫹ H2 101. PYRYNEP ⫹ OH ⫽ PYRYNEP*S ⫹ H2O 102. PYRYNEP*S ⫹ C2H2 ⫽ BAPYR*S 103. BAPYR*S ⫹ H ⫽ BAPYR Benzo[a]pyrene-oxidation 104. BAPYR ⫹ OH ⫽ PYRYNEP ⫹ CH2CO ⫹ H 105. BAPYR ⫹ O ⫽ PYRYNEP ⫹ CH2CO 106. BAPYR*S ⫹ O2 ⫽ PYRYNEP ⫹ HCO ⫹ CO 107. PYRYNEP ⫹ OH ⫽ PYRENE*P ⫹ CH2CO 108. PYRENEP ⫹ O ⫽ PYRENE*P ⫹ HCCO Formation of anthanthracene 109. BAPYR ⫹ H ⫽ BAPYR*S ⫹ H2 110. BAPYR ⫹ OH ⫽ BAPYR*S ⫹ H2O 111. BAPYR*S ⫹ C2H2 ⫽ ANTHAN ⫹ H Anthanthracene-oxidation 112. ANTHAN ⫹ H ⫽ ANTHAN*S ⫹ H2 113. ANTHAN ⫹ OH ⫽ ANTHAN*S ⫹ H2O 114. ANTHAN ⫹ OH ⫽ BAPYR*S ⫹ CH2CO 115. ANTHAN ⫹ O ⫽ BAPYR*S ⫹ HCCO 116. ANTHAN*S ⫹ O2 ⫽ BAPYR*S ⫹ 2CO Formation of cyclopenta[cd]pyrene 117. PYRENE ⫹ H ⫽ PYRENE*S1 ⫹ H2 118. PYRENE ⫹ OH ⫽ PYRENE*S1 ⫹ H2O 119. PYRENE*S1 ⫹ C2H2 ⫽ CPCDPYR ⫹ H Formation of dicylcopentapyrene 120. CPCDPYR ⫹ H ⫽ CPCDPYR*S ⫹ H2 121. CPCDPYR ⫹ OH ⫽ CPCDPYR*S ⫹ H2O 122. CPCDPYR*S ⫹ C2H2 ⫽ DCPP ⫹ H Formation of benzo[e]pyrene 123. PYRENE ⫹ H ⫽ PYRENE*S2 ⫹ H2 124. PYRENE ⫹ OH ⫽ PYRENE*S2 ⫹ H2O 125. PYRENE*S2 ⫹ C2H2 ⫽ PYRYNE ⫹ H 126. PYRYNE ⫹ H ⫽ PYRYNE*S ⫹ H2 127. PYRYNE ⫹ OH ⫽ PYRYNE*S ⫹ H2O 128. PYRYNE*S ⫹ C2H2 ⫽ BEPYREN*S 129. BEPYREN*S ⫹ H ⫽ BEPYREN Formation of indeno[1,2,3-cd]pyrene 130. PYRENE*S1 ⫹ C6H5 ⫽ INPYR ⫹ H ⫹ H 131. PYRENE*S1 ⫹ C6H6 ⫽ INPYR ⫹ H2 ⫹ H Formation of benzo[ghi]perylene 132. BEPYREN ⫹ H ⫽ BEPYREN*S ⫹ H2 133. BEPYREN ⫹ OH ⫽ BEPYREN*S ⫹ H2O 134. BEPYREN*S ⫹ C2H2 ⫽ BGHIPER ⫹ H Formation of cyclopentabenzo[ghi]perylene 135. BGHIPER ⫹ H ⫽ BGHIPE*S1 ⫹ H2 136. BGHIPER ⫹ OH ⫽ BGHIPE*S1 ⫹ H2O 137. BGHIPE*S1 ⫹ C2H2 ⫽ CPBPER ⫹ H

A

n

Ea

2.20E13 2.20E13 2.10E12 2.10E12

0.0 0.0 0.0 0.0

4530.0 4530.0 7470.0 7470.0

5.00E12 4.00E11

0.0 0.0

0.0 4000.0

3.98E13 2.50E14 2.10E13 3.98E13 5.00E13

0.0 0.0 0.0 0.0 0.0

10100.0 16000.0 4600.0 10100.0 0.0

6.50E12 1.10E13 2.10E12 2.18E-4 2.04E07

0.0 0.0 0.0 4.5 2.0

10600.0 4530.0 7470.0 ⫺1000.0 1900.0

2.50E14 2.10E13 3.98E13

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

2.50E14 2.10E13 1.30E13 2.20E13 2.10E12

0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10600.0 4530.0 7470.0

Kiefer et al. [48] Madronich and Felder [49] Wang and Frenklach [24] " "

2.50E14 2.10E13 3.98E13

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

2.50E14 2.10E13 3.98E13

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 5.00E13

0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 0.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] estimate, this work

5.00E12 4.00E11

0.0 0.0

0.0 4000.0

2.50E14 2.10E13 3.98E13

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

2.50E14 2.10E13 3.98E13

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

Ref. " " " " Rxn. 65. Rxn. 66. Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] estimate, this work Wang and Frenklach [24] " " " "

Rxn. 65. Rxn. 66.

8

H. RICHTER ET AL. TABLE 1 continued n

k ⫽ AT exp (⫺E a /RT)

3

A: cm mole

⫺1

s

⫺1

E a : cal

A

Formation of coronene 138. BGHIPER ⫹ H ⫽ BGHIPE*S2 ⫹ H2 2.50E14 139. BGHIPER ⫹ OH ⫽ BGHIPE*S2 ⫹ H2O 2.10E13 140. BGHIPE*S2 ⫹ C2H2 ⫽ CORONEN ⫹ H 3.98E13 Formation of ethinylfluoranthene 141. FLTHN⫺ ⫹ C2H2 ⫽ FLTHNC2H ⫹ H 3.98E13 Formation of benzo[ghi]fluoranthene 142. FLTHN ⫹ H ⫽ FLTHN⫺ ⫹ H2 2.50E14 143. FLTHN ⫹ OH ⫽ FLTHN⫺ ⫹ H2O 2.10E13 144. FLTHN⫺ ⫹ C2H2 ⫽ BGHIF ⫹ H 3.98E13 Formation of methylbenzo[ghi]fluoranthene 145. BGHIF⫺ ⫹ CH3 ⫽ BGHIFCH2 ⫹ H 5.00E13 146. BGHIFCH2 ⫹ H ⫽ BGHIFCH3 1.00E14 147. BGHIFCH3 ⫹ H ⫽ BGHIF ⫹ CH3 1.20E13 Formation of ethinylbenzo[ghi]fluoranthene 148. BGHIF⫺ ⫹ C2H2 ⫽ BGHIFC2H ⫹ H 3.98E13 Formation of cyclopentafluoranthene 149. FLTHN⫺ ⫹ C2H2 ⫽ FLTHNR ⫹ H 3.98E13 Formation of cyclopentabenzo[ghi]fluoranthene 150. FLTHNR ⫹ H ⫽ FLTHNR*S ⫹ H2 2.50E14 151. FLTHNR ⫹ OH ⫽ FLTHNR*S ⫹ H2O 2.10E13 152. FLTHNR*S ⫹ C2H2 ⫽ BGHIFR ⫹ H 3.98E13 153. BGHIF ⫹ H ⫽ BGHIF⫺ ⫹ H2 2.50E14 154. BGHIF ⫹ OH ⫽ BGHIF⫺ ⫹ H2O 2.10E13 155. BGHIF⫺ ⫹ C2H2 ⫽ BGHIFR ⫹ H 3.98E13 Formation of corannulene 156. BGHIF⫺ ⫹ C2H2 ⫽ COR ⫹ H 3.98E13 157. CPCDPYR*S ⫹ C2H2 ⫽ COR ⫹ H 3.98E13 Methylation of corannulene 158. COR⫺ ⫹ CH3 ⫽ CORCH2 ⫹ H 5.00E13 159. CORCH2 ⫹ H ⫽ CORCH3 1.00E14 160. CORCH3 ⫹ H ⫽ COR ⫹ CH3 1.20E13 Formation of cyclopentacorannulene 161. COR ⫹ H ⫽ COR⫺ ⫹ H2 2.50E14 162. COR ⫹ OH ⫽ COR⫺ ⫹ H2O 2.10E13 163. COR⫺ ⫹ C2H2 ⫽ COR1 ⫹ H 3.98E13 164. BGHIFR ⫹ H ⫽ BGHIFR*S ⫹ H2 2.50E14 165. BGHIFR ⫹ OH ⫽ BGHIFR*S ⫹ H2O 2.10E13 166. BGHIFR*S ⫹ C2H2 ⫽ COR1 ⫹ H 3.98E13 Formation of C60 and C70 fullerenes (based on Pope and Howard [40, 41] Formation of FB (C50H10) 167. COR1 ⫹ H ⫽ COR1⫺ ⫹ H2 2.50E14 168. COR1 ⫹ OH ⫽ COR1⫺ ⫹ H2O 2.10E13 169. COR1⫺ ⫹ C2H2 ⫽ COR2 ⫹ H 3.98E13 170. COR2 ⫹ H ⫽ COR2⫺ ⫹ H2 2.50E14 171. COR2 ⫹ OH ⫽ COR2⫺ ⫹ H2O 2.10E13 172. COR2⫺ ⫹ C2H2 ⫽ COR3 ⫹ H 3.98E13 173. COR3 ⫹ H ⫽ COR3⫺ ⫹ H2 2.50E14 174. COR3 ⫹ OH ⫽ COR3⫺ ⫹ H2O 2.10E13 175. COR3⫺ ⫹ C2H2 ⫽ COR4 ⫹ H 3.98E13 176. COR4 ⫹ H ⫽ COR4⫺ ⫹ H2 2.50E14 177. COR4 ⫹ OH ⫽ COR4⫺ ⫹ H2O 2.10E13 178. COR4⫺ ⫹ C2H2 ⫽ HB ⫹ H 3.98E13 179. HB ⫹ H ⫽ HB⫺ ⫹ H2 2.50E14 180. HB ⫹ OH ⫽ HB⫺ ⫹ H2O 2.10E13 181. HB⫺ ⫹ C2H2 ⫽ HB1 ⫹ H 3.98E13

n

Ea

Ref.

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

0.0

10100.0

Fahr and Stein [50]

0.0 0.0 0.0

16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

0.0 0.0 0.0

0.0 0.0 5148.0

0.0

10100.0

Fahr and Stein [50]

0.0

10100.0

Fahr and Stein [50]

0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

0.0 0.0

10100.0 10100.0

Fahr and Stein [48] Fahr and Stein [48]

0.0 0.0 0.0

0.0 0.0 5148.0

0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50]

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50]

Rxn. 16. Rxn. 17. Rxn. 18.

Rxn. 16. Rxn. 17. Rxn. 18.

[49]

[49]

[49]

[49]

[49]

FORMATION OF PAH AND FULLERENES

9

TABLE 1 continued n

k ⫽ AT exp (⫺E a /RT)

3

A: cm mole

⫺1

⫺1

s

182. HB1 ⫹ H ⫽ HB1⫺ ⫹ H2 183. HB1 ⫹ OH ⫽ HB1⫺ ⫹ H2O 184. HB1⫺ ⫹ C2H2 ⫽ HB2 ⫹ H 185. HB2 ⫹ H ⫽ HB2⫺ ⫹ H2 186. HB2 ⫹ OH ⫽ HB2⫺ ⫹ H2O 187. HB2⫺ ⫹ C2H2 ⫽ HB3 ⫹ H 188. HB3 ⫹ H ⫽ HB3⫺ ⫹ H2 189. HB3 ⫹ OH ⫽ HB3⫺ ⫹ H2O 190. HB3⫺ ⫹ C2H2 ⫽ HB4 ⫹ H 191. HB4 ⫹ H ⫽ HB4⫺ ⫹ H2 192. HB4 ⫹ OH ⫽ HB4⫺ ⫹ H2O 193. HB4⫺ ⫹ C2H2 ⫽ TB ⫹ H 194. TB ⫹ H ⫽ TB⫺ ⫹ H2 195. TB ⫹ OH ⫽ TB⫺ ⫹ H2O 196. TB⫺ ⫹ C2H2 ⫽ TB1 ⫹ H 197. TB1 ⫹ H ⫽ TB1⫺ ⫹ H2 198. TB1 ⫹ OH ⫽ TB1⫺ ⫹ H2O 199. TB1⫺ ⫹ C2H2 ⫽ TB2 ⫹ H 200. TB2 ⫹ H ⫽ TB2⫺ ⫹ H2 201. TB2 ⫹ OH ⫽ TB2⫺ ⫹ H2O 202. TB2⫺ ⫹ C2H2 ⫽ TB3 ⫹ H 203. TB3 ⫹ H ⫽ TB3⫺ ⫹ H2 204. TB3 ⫹ OH ⫽ TB3⫺ ⫹ H2O 205. TB3⫺ ⫹ C2H2 ⫽ TB4 ⫹ H 206. TB4 ⫹ H ⫽ TB4⫺ ⫹ H2 207. TB4 ⫹ OH ⫽ TB4⫺ ⫹ H2O 208. TB4⫺ ⫹ C2H2 ⫽ FB ⫹ H Formation of C60 209. FB ⫹ H ⫽ FB⫺ ⫹ H2 210. FB ⫹ OH ⫽ FB⫺ ⫹ H2O 211. FB⫺ ⫹ C2H2 ⫽ FB1 ⫹ H 212. FB1 ⫹ H ⫽ FB1⫺ ⫹ H2 213. FB1 ⫹ OH ⫽ FB1⫺ ⫹ H2O 214. FB1⫺ ⫹ C2H2 ⫽ FB2Q ⫹ H 215. FB2Q ⫽ FB2QR 216. FB2QR ⫹ H ⫽ FB2QR⫺ ⫹ H2 217. FB2QR ⫹ OH ⫽ FB2QR⫺ ⫹ H2O 218. FB2QR⫺ ⫽ FB2QRD ⫹ H 219. FB2QRD ⫹ H ⫽ FB2QRD⫺ ⫹ H2 220. FB2QRD ⫹ OH ⫽ FB2QRD⫺ ⫹ H2O 221. FB2QRD⫺ ⫹ C2H2 ⫽ FB3Q ⫹ H 222. FB3Q ⫹ H ⫽ FB3Q⫺ ⫹ H2 223. FB3Q ⫹ OH ⫽ FB3Q⫺ ⫹ H2O 224. FB3Q⫺ ⫽ FB3QD ⫹ H 225. FB3QD ⫹ H ⫽ FB3QD⫺ ⫹ H2 226. FB3QD ⫹ OH ⫽ FB3QD⫺ ⫹ H2O 227. FB3QD⫺ ⫹ C2H2 ⫽ FB4Q ⫹ H 228. FB4Q ⫹ H ⫽ FB4Q⫺ ⫹ H2 229. FB4Q ⫹ OH ⫽ FB4Q⫺ ⫹ H2O 230. FB4Q⫺ ⫽ FB4QD ⫹ H 231. FB4QD ⫹ H ⫽ FB4QD⫺ ⫹ H2 232. FB4QD ⫹ OH ⫽ FB4QD⫺ ⫹ H2O 233. FB4QD⫺ ⫹ C2H2 ⫽ FB5Q ⫹ H 234. FB5Q ⫹ H ⫽ FB5Q⫺ ⫹ H2 235. FB5Q ⫹ OH ⫽ FB5Q⫺ ⫹ H2O 236. FB5Q⫺ ⫽ FB5QD ⫹ H

E a : cal

A

n

Ea

2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0

Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder Fahr and Stein [50]

2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 8.51E12 2.50E14 2.10E13 1.00E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 1.00E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 1.00E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 1.00E13

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 62860.0 16000.0 4600.0 0.0 16000.0 4600.0 10100.0 16000.0 4600.0 0.0 16000.0 4600.0 10100.0 16000.0 4600.0 0.0 16000.0 4600.0 10100.0 16000.0 4600.0 0.0

Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Brouwer and Troe [61] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22]

Ref. [49]

[49]

[49]

[49]

[49]

[49]

[49]

[49]

[49]

10

H. RICHTER ET AL. TABLE 1 continued n

k ⫽ AT exp (⫺E a /RT)

3

A: cm mole

⫺1

s

⫺1

E a : cal

237. FB5QD ⫹ H ⫽ FB5QD⫺ ⫹ H2 238. FB5QD ⫹ OH ⫽ FB5QD⫺ ⫹ H2O 239. FB5QD⫺ ⫽ C60A ⫹ H Formation of C70 240. FB1⫺ ⫹ C2H2 ⫽ FB2 ⫹ H 241. FB2 ⫹ H ⫽ FB2⫺ ⫹ H2 242. FB2 ⫹ OH ⫽ FB2⫺ ⫹ H2O 243. FB2⫺ ⫹ C2H2 ⫽ FB3 ⫹ H 244. FB3 ⫹ H ⫽ FB3⫺ ⫹ H2 245. FB3 ⫹ OH ⫽ FB3⫺ ⫹ H2O 246. FB3⫺ ⫹ C2H2 ⫽ FB4 ⫹ H 247. FB4 ⫹ H ⫽ FB4⫺ ⫹ H2 248. FB4 ⫹ OH ⫽ FB4⫺ ⫹ H2O 249. FB4⫺ ⫹ C2H2 ⫽ XB ⫹ H 250. XB ⫹ H ⫽ XB⫺ ⫹ H2 251. XB ⫹ OH ⫽ XB⫺ ⫹ H2O 252. XB⫺ ⫹ C2H2 ⫽ XB1 ⫹ H 253. XB1 ⫹ H ⫽ XB1⫺ ⫹ H2 254. XB1 ⫹ OH ⫽ XB1⫺ ⫹ H2O 255. XB1⫺ ⫹ C2H2 ⫽ XB2Q ⫹ H 256. XB2Q ⫽ XB2QR 257. XB2QR ⫹ H ⫽ XB2QR⫺ ⫹ H2 258. XB2QR ⫹ OH ⫽ XB2QR⫺ ⫹ H2O 259. XB2QR⫺ ⫽ XB2QRD ⫹ H 260. XB2QRD ⫹ H ⫽ XB2QRD⫺ ⫹ H2 261. XB2QRD ⫹ OH ⫽ XB2QRD⫺ ⫹ H2O 262. XB2QRD⫺ ⫹ C2H2 ⫽ XB3Q ⫹ H 263. XB3Q ⫹ H ⫽ XB3Q⫺ ⫹ H2 264. XB3Q ⫹ OH ⫽ XB3Q⫺ ⫹ H2O 265. XB3Q⫺ ⫽ XB3QD ⫹ H 266. XB3QD ⫹ H ⫽ XB3QD⫺ ⫹ H2 267. XB3QD ⫹ OH ⫽ XB3QD⫺ ⫹ H2O 268. XB3QD⫺ ⫹ C2H2 ⫽ XB4Q ⫹ H 269. XB4Q ⫹ H ⫽ XB4Q⫺ ⫹ H2 270. XB4Q ⫹ OH ⫽ XB4Q⫺ ⫹ H2O 271. XB4Q⫺ ⫽ XB4QD ⫹ H 272. XB4QD ⫹ H ⫽ XB4QD⫺ ⫹ H2 273. XB4QD ⫹ OH ⫽ XB4QD⫺ ⫹ H2O 274. XB4QD⫺ ⫹ C2H2 ⫽ XB5Q ⫹ H 275. XB5Q ⫹ H ⫽ XB5Q⫺ ⫹ H2 276. XB5Q ⫹ OH ⫽ XB5Q⫺ ⫹ H2O 277. XB5Q⫺ ⫽ XB5QD ⫹ H 278. XB5QD ⫹ H ⫽ XB5QD⫺ ⫹ H2 279. XB5QD⫺ ⫹ OH ⫽ XB5QD⫺ ⫹ H2O 280. XB5QD⫺ ⫽ C70A ⫹ H Changes in this work relative to R. A. Shandross [47] Formation of cyclopentadienyl 281. C6H5 ⫹ O ⫽ C5H5 ⫹ CO Formation of naphthalene 282. C6H5 ⫹ C2H2 ⫽ C8H6 ⫹ H 283. C8H6 ⫹ H ⫽ A1YNE* ⫹ H2 284. C8H6 ⫹ OH ⫽ A1YNE* ⫹ H2O 285. C8H6 ⫹ CH3 ⫽ A1YNE* ⫹ CH4 286. A1YNE* ⫹ C2H2 ⫽ C10H7S 287. C10H7S ⫹ H ⫽ C10H8 288. C10H7P ⫹ H ⫽ C10H8

A

n

Ea

Ref.

2.50E14 2.10E13 1.00E13

0.0 0.0 0.0

16000.0 4600.0 0.0

Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22]

3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 3.98E13 8.51E12 2.50E14 2.10E13 1.00E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 1.00E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 1.00E13 2.50E14 2.10E13 3.98E13 2.50E14 2.10E13 1.00E13 2.50E14 2.10E13 1.00E13

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 16000.0 4600.0 10100.0 62860.0 16000.0 4600.0 0.0 16000.0 4600.0 10100.0 16000.0 4600.0 0.0 16000.0 4600.0 10100.0 16000.0 4600.0 0.0 16000.0 4600.0 10100.0 16000.0 4600.0 0.0 16000.0 4600.0 0.0

Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Brouwer and Troe [61] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22] Kiefer et al. [48] Madronich and Felder [49] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22] Kiefer et al. [48] Madronich and Felder [49] Frenklach et al. [22]

9.00E13

0.0

0.0

3.98E13 2.50E14 2.10E13 1.67E12 3.98E13 1.00E14 1.00E14

0.0 0.0 0.0 0.0 0.0 0.0 0.0

10100.0 16000.0 4600.0 15057.0 10100.0 0.0 0.0

Tan and Frank [62] Fahr and Stein [50] Kiefer et al. [48] Madronich and Felder [49] Marinov et al. [23] Fahr and Stein [50] Marinov et al. [23] "

FORMATION OF PAH AND FULLERENES

11

TABLE 1 continued n

k ⫽ AT exp (⫺E a /RT)

3

⫺1

A: cm mole

s

⫺1

E a : cal

289. C10H8 ⫹ H ⫽ C10H7P ⫹ H2 290. C10H8 ⫹ OH ⫽ C10H7P ⫹ H2O 291. C10H8 ⫹ C2H3 ⫽ C10H7P ⫹ C2H4 292. C10H8 ⫹ C2H ⫽ C10H7P ⫹ C2H2 293. C10H8 ⫹ H ⫽ C10H7S ⫹ H2 294. C10H8 ⫹ OH ⫽ C10H7S ⫹ H2O 295. C10H8 ⫹ C2H3 ⫽ C10H7S ⫹ C2H4 296. C10H8 ⫹ C2H ⫽ C10H7S ⫹ C2H2 297. 2C5H5 ⫽ C10H8 ⫹ H ⫹ H Toluene formation 298. C6H5 ⫹ CH3 ⫽ C7H7 ⫹ H

tained after H abstraction from the center ring of anthracene. The model consists of 246 species and 834 reactions, and a listing of the complete interpreter-output file as well as the thermodynamic and transport properties are provided elsewhere [57]. The PAH included in the present mechanism are shown in Fig. 3.

A

n

Ea

Ref.

2.50E14 2.10E13 5.00E13 5.00E13 2.50E14 2.10E13 5.00E13 5.00E13 2.00E12

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

16000.0 4600.0 16000.0 16000.0 16000.0 4600.0 16000.0 16000.0 4000.0

Kiefer et al. [48] Madronich and Felder [49] Harris et al. [63]

5.00E13

0.0

0.0

Kiefer et al. [48] Madronich and Felder [49] Harris et al. [63] " this work estim. based on [23]

mole fraction predicted in Flame II despite the satisfactory agreement for phenylacetylene (Fig. 1b). Cyclopentadienyl is mainly formed by the oxidation of C6H5 followed by the degradation of C6H5O: C 6H 6 ⫹ O N C 6H 5O ⫹ H C 6H 5 ⫹ O 2 N C 6H 5O ⫹ O

FORMATION OF NAPHTHALENE, PHENANTHRENE, AND RELATED SPECIES Naphthalene Naphthalene (C10H8) formation is the first step in the growth process to larger and larger molecules. There are two contributing pathways, two consecutive H-abstraction/C2H2-additions via phenylacetylene and the reaction between two cyclopentadienyl radicals 2C5H5 N naphthalene ⫹ 2H initially suggested by Dean [64] but with H2 as product and by Marinov et al. [23] using a rate coefficient about 100-fold larger than that of Dean [64]. In the present work, the Marinov et al. rate coefficient was reduced arbitrarily by a factor 10 in order to ensure a reasonably good agreement between predicted and experimental naphthalene profiles in Flame II (Fig. 4), despite an overprediction of cyclopentadienyl radicals by the model as shown in Fig. 1a reflecting the remaining uncertainties on benzene oxidation chemistry. The cyclopentadienyl pathway is found to be predominant for naphthalene formation, its removal leading to a 50-fold reduction of the peak

C6H5O N C5H5 ⫹ CO C6H5 ⫹ O N C5H5 ⫹ CO The formation of benzoquinone by the reaction C6H5 ⫹ O2 N C6H4O2 ⫹ H [62, 65] which leads to a significant reduction of phenoxy and consequently of cylcopentadienyl radical concentrations was tested in the nearly sooting benzene flame [11] and ruled out due to a predicted peak mole fraction of about 100-fold higher than the experimental value at 108 amu commonly attributed to the different cresol isomers, species with the same molecular mass as benzoquinone. Methylnaphthalene and Acenaphthalene Naphthalene reacts to 1- and 2-naphthyl radicals. The peak values predicted for both naphthyl radicals in Flame II are about 3– 4 times smaller than the experimental data obtained via scavenging reaction by Hausmann et al. [12], a still acceptable agreement considering potential uncertainties on rate coefficients and experiments. The reaction of 1-naphthyl with acetylene leads to acenaphthalene (A2R5, C12H8).

12

H. RICHTER ET AL.

Fig. 3. PAH contributing to the growth process.

Methylnaphthalene is formed via reaction of both naphthyl radicals with CH3 while a similar reaction sequence also gives methylacenaphthalene. As shown in Fig. 4, the model predicts the right location for the peak value of acenaphtha-

lene in Flame II but underpredicts the magnitude by nearly 20-fold, possibly indicating the existence of another yet unknown formation pathway already suggested by Hausmann et al. [12]. A pathway via methylnaphthalene [23] was

FORMATION OF PAH AND FULLERENES

13

Fig. 4. Comparison between experimental mole fraction profiles [11] and model predictions in a nearly sooting benzene/oxygen flame (␾ ⫽ 1.8, 30% argon, v 25⬚C ⫽ 50 cm s⫺1, 2.67 kPa); Flame II. Naphthalene: F (experiment), (prediction). Acenaphthalene: } (experiment), ---(prediction).

Fig. 5. Comparison between the experimental mole fraction profile of C14H10 [11] and model prediction for the corresponding species in a nearly sooting benzene/oxygen flame (␾ ⫽ 1.8, 30% argon, v 25⬚C ⫽ 50 cm s⫺1, 2.67 kPa); Flame II. C14H10: F (experiment). Phenanthrene: (prediction). Anthracene: ---- (prediction).

tested and found to be negligible. Similar to phenyl, both naphthyl radicals are also oxidized, the resulting naphthoxy radicals are stabilized via the formation of naphthol (C10H7OH) or decay to indenyl via CO loss. The comparison of the present model predictions to the experimental data of Hausmann et al. for Flame II [12] reveals good agreement for indene formation but the lack of total depletion in the postflame zone indicates supplementary pathways that are not considered in the present model. A significant overprediction was observed for indenyl (60-fold) and naphthol (20-fold) which reflects poor knowledge of naphthalene oxidation, indene formation, and the corresponding rate coefficients. The lack of complete indenyl depletion in the postflame-zone shows again evidence of further consumption pathways. The contribution of indene and indenyl to PAH and particle growth cannot be excluded.

guish between phenanthrene and anthracene (also C14H10), so only an overall profile including both species is available and is compared to the model predictions in Fig. 5. The predicted higher abundance of phenanthrene than of anthracene is consistent with the measurements in Flame I [42], although partial sublimation of C14H10 species prevented a comparison of absolute quantities to the prediction. Phenanthrene formation by two consecutive H-abstraction/acetylene-additions is included in the mechanism but is found to be negligible under the present conditions. Use of this pathway alone underpredicts by 650-fold the peak concentration in Flame II.

Phenanthrene Phenanthrene (A3, C14H10), a key species for further PAH growth, is mainly formed by the reaction of C2H2 with biphenyl radicals [24]. A slight overprediction is observed for C14H10 species in Flame II and can be attributed to an overprediction of phenyl radicals, shown in Fig. 1b, and related to uncertainties in the benzene destruction chemistry. The MBMS technique used in the measurements [11] could not distin-

Anthracene and Benzo[a]anthracene Anthracene (A3L, C14H10) is formed corresponding the hydrogen-abstraction/acetyleneaddition pathway [22]. Two consecutive H-abstraction/C2H2-additions begin with 2-naphthyl and lead to an anthracene peak concentration about 100-fold smaller than that of phenanthrene in Flame I. This result is consistent with the experimental observation [42] of only traces of anthracene in Flame I, but a partial sublimation of anthracene during sample preparation cannot be ruled out. Using only consecutive H-abstraction/C2H2addition for anthracene formation and a similar pathway for benzo[a]anthracene (A4L, C18H12)

14

Fig. 6. Comparison between experimental mole fraction profiles [42] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25⬚C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. Benzo[a]anthracene: F (experiment), (prediction). Chrysene: } (experiment), ---(prediction).

underpredicts the measured peak concentration of the latter [42] by more than 100-fold. The benzo[a]anthracene underprediction is reduced to less than 10-fold (Fig. 6) by using the isomerization of phenanthrene to anthracene suggested by Colket and Seery [16] and used by Marinov et al. [23]. Nevertheless, the importance of this reaction remains questionable considering the pyrolysis results of Scott and Roelofs [66] showing the reverse reaction to phenanthrene to be of little or no significance.

H. RICHTER ET AL.

Fig. 7. Comparison between experimental mole fraction profiles [42] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25⬚C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. Fluorene: F (experiment, left scale), (prediction, left scale). Fluoranthene: } (experiment, right scale), ---- (prediction, right scale).

CHRYSEN*S ⫹ H N CHRYSEN The chrysene concentration profile for Flame I compared against experimental data [42] reveals encouraging agreement concerning shape and peak location but a 3- to 5-fold underprediction of the peak value (Fig. 6). This discrepancy could be explained by uncertainties in the rate coefficients for H-abstraction/C2H2-abstraction but also by a contribution of the reaction between two indenyl radicals, similar to the main pathway of naphthalene formation.

Chrysene The formation of chrysene (CHRYSEN, C18H12) is described by means of a H-abstraction/C2H2-addition sequence beginning with primary phenanthryl radicals (2- and 7-phenanthryl, A3*P) and forming the corresponding phenanthryl-acetylene as intermediate. In the next steps the hydrogen atom adjacent to the acetylene group is abstracted and then the ring closure achieved by means of another acetyleneaddition. The resulting secondary chrysene radical is stabilized by reaction with atomic hydrogen. A3 ⫹ H, OH N A3*P ⫹ H2, H2O A3*P ⫹ C2H2 N A3YNE ⫹ H A3YNE ⫹ H, OH N A3YNE*S ⫹ H2, H2O A3YNE*S ⫹ C2H2 N CHRYSEN*S

FORMATION OF FLUORENE AND BENZO[A]FLUORENE The formation of fluorene (FLUOREN, C13H10) starts with the reaction of benzene or phenyl with benzyl radical (C7H7, C6H5CH2) and, after H-abstraction, is achieved by the closure of a new five-membered ring: C6H6 ⫹ C7H7 N Benzylbenzene ⫹ H C6H5 ⫹ C7H7 N Benzylbenzene Benzylbenzene ⫹ H,OH N Benzylbenzeneradical ⫹ H2, H2O Benzylbenzene-radical N Fluorene ⫹ H Comparison against experimental data (Fig. 7) shows the predicted consumption in the burnt

FORMATION OF PAH AND FULLERENES

15

gas to be too slow, possibly attributable to missing reactions. Benzo[a]fluorene (C17H12) is formed by a similar mechanism but with naphthalene and 2-naphthyl as reactants. Its predicted peak concentration in Flame I is about 60 times smaller than that of fluorene, consistent with being barely detectable by HPLC in flame samples [42]. FORMATION OF FLUORANTHENE AND BENZO[K]FLUORANTHENE Similar to acenaphthalene, the formation of fluoranthene (FLTHN, C16H10), a key species for fullerene formation as discussed later, starts with 1-naphthyl reacting with benzene or phenyl [23]: 1-naphthyl ⫹ C6H5 N fluoranthene ⫹ 2H 1-naphthyl ⫹ C6H6 N fluoranthene ⫹ H2 ⫹ H A supplementary significant pathway is isomerization of acephenanthrylene (see below). Fluoranthene is consumed by reactions forming the isomers benzo[ghi]fluoranthene (BGHIF, C18H10) and cyclopenta[cd]fluoranthene (FLTHNR, C18H10), described below, and by methylation and ethynylation. Figure 7 shows the comparison of the prediction with the experiment for Flame I while for Flame II only an experimental profile including all C16H10 isomers is available (Fig. 8). The striking underprediction of consumption in the burnt gas is observed for nearly all PAH as discussed below. Benzo[k]fluoranthene (BKFLUOR, C20H12) is formed in a similar way but with naphthalene and 1- or 2-naphthyl as reactants. The prediction of a peak mole fraction of 5 ⫻ 10⫺8 in Flame I is consistent with the measured concentration being close to the detection limit [42]. In contrast to the experimental data, the predicted mole fraction is constant through the burnt gas due to the lack of consumption reactions. FURTHER GROWTH OF PHENANTHRENE Besides primary phenanthryl radicals leading to the above described formation of chrysene, two

Fig. 8. Comparison between the experimental mole fraction profile of C16H10 [11] and model prediction for the corresponding species in a nearly sooting benzene/oxygen flame (␾ ⫽ 1.8, 30% argon, v 25⬚C ⫽ 50 cm s⫺1, 2.67 kPa); Flame II. C16H10: F (experiment). Fluoranthene: (prediction). Pyrene: ---- (prediction). Acephenanthrylene: 䡠䡠䡠䡠 (prediction).

different secondary phenanthryl radicals are involved in the present model. The first one (A3S*1), assigned to 1-, 8-, 9- and 10-phenanthryl gives, after acetylene addition, acephenanthrylene (ACEPHA, C16H10) while its reaction with phenyl or benzene leads to benzo[b]fluoranthene (BBFLUOR, C20H12). The other secondary phenanthryl radical represents 4- and 5-phenanthryl which via acetylene addition gives pyrene, another C16H10 isomer, or via methylation and with methylphenanthrene (A3CH3, C15H12) as intermediate, cyclopenta[def]phenanthrene (A3CH2R, C15H10). Evidence for the thermal interconversion between acephenanthrylene and fluoranthene has been shown by Scott and Roelofs [66]. Based on their results, the isomerization acephenanthrylene N fluoranthene was included in the mechanism and leads to about 50% increase of the fluoranthene concentration and to a 4-fold decrease of the acephenanthrylene peak concentration in Flame I. This behavior is consistent with experimental results [42] showing only small quantities of acephenanthrylene close to the detection limit. A similar situation is predicted in Flame II where the acephenanthrylene peak mole fraction represents less than 10% of the fluoranthene one (Fig. 8). The prediction of benzo[b]fluoranthene formation (Fig. 9) in the flame front of Flame I is 2.5 to 5 times lower than the experimental data [42] and its con-

16

Fig. 9. Comparison between experimental mole fraction profiles [42] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25⬚C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. Benzo[b]fluoranthene: F (experiment), (prediction). Pyrene: } (experiment), ---(prediction).

sumption in the postflame zone cannot be reproduced because no further reactions are included in the mechanism. Methylphenanthrene and cyclopenta[def]phenanthrene were detected in Flame I by Grieco [42] but very low concentrations did not allow quantifications. The validation for pyrene in Flame I (Fig. 9) shows a good agreement of its formation but it must be concluded that its consumption by further growth reactions, methylation, and oxidation [24] as considered in the present model is not sufficient to explain its depletion in the burnt gases as observed experimentally [42]. Oxidation was tested and found to account for about 35% reduction of the mole fraction in the burnt gas while barely affecting the peak value. In the following pyrene growth reactions, distinction is drawn between primary 2-pyrenyl radicals and two types of secondary pyrenyl radicals, all of them formed by H-abstraction from pyrene with H and OH. FORMATION OF BENZO[A]PYRENE AND ANTHANTHRACENE Two subsequent H-abstraction/acetylene-addition sequences beginning with 2-pyrenyl lead to benzo[a]pyrene (BAPYR, C20H12). Benzo[a]pyrene is oxidized and reacts via H-abstraction/acetylene-addition to anthanthracene (ANTHAN, C22H12). Anthanthracene

H. RICHTER ET AL.

Fig. 10. Comparison between experimental mole fraction profiles [42] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25°C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. Benzo[a]pyrene: F (experiment), (prediction). Anthanthracene (with oxidation): ---(prediction). Anthanthracene (without oxidation): 䡠䡠䡠䡠 (prediction).

decays by further oxidation partially to benzo[a]pyrene. Benzo[a]pyrene mole fractions predicted for Flame I (Fig. 10) show a peak concentration close to the scatter of the experimental data considering a relative error of ⫾13% for PAH concentrations obtained by GC/MS analysis as quoted by Grieco et al. [42] and an additional error due to sample collection and flame reproducibility. A further increase of concentration in the burnt gas after the consumption following the peak reflects missing sinks for PAH in the postflame zone. The impact of oxidation of anthanthracene on its mole fraction profile was tested for Flame I (Fig. 10). Removal of all oxidation reactions leads to slight shift of the profile towards the burnt gases and to the disappearance of the local maximum at about 0.4 cm. Both predictions, with and without oxidation show a similar increase in the postflame zone so that additional sinks responsible for PAH depletion must exist. Anthanthracene was found to be present in Flame I [42] but could not be quantified due to difficulties associated with high molecular species [67] leading to an increase of the detection limits. A predicted local maximum of 2 ⫻ 10⫺8 for the anthanthracene mole fraction profile (with oxidation) is in agreement with the experimental findings.

FORMATION OF PAH AND FULLERENES

Fig. 11. Comparison between experimental mole fraction profiles [42] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25°C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. Cyclopenta[cd]pyrene: F (experiment), (prediction). Indeno[1,2,3-cd]pyrene: } (experiment), ---- (prediction).

FORMATION OF CYCLOPENTA [CD]PYRENE, BENZO[E]PYRENE AND INDENO[1,2,3-CD]PYRENE Two pyrene radicals, 4- and 9-pyrenyl, are represented in the model by one secondary radical (PYRENE*S1), which reacts with acetylene to form cyclopenta[cd]pyrene (CPCDPYR, C18H10) and with benzene and phenyl to form indeno[1,2,3-cd]pyrene (INPYR, C22H12). Similarly, 5- and 10-pyrenyl is represented by the other secondary radical (PYRENE*S2) which undergoes two H-abstraction/acetylene-addition steps to form benzo[e]pyrene (BEPYREN, C20H12). The formation of cylcopenta[cd]pyrene is predicted well (Fig. 11) but not the consumption, as it can be seen by the concentration in the burnt gas to be greatly overpredicted. An efficient sink for PAH in the burnt gas is missing from the model. Similarly, the prediction of indeno[1,2,3-cd]pyrene formation in Flame I agrees with experiment [42] but the depletion in the burnt gas is underpredicted reflecting again the lack of adequate consumption reactions (Fig. 11). A striking phenomenon is the absence of dicylopentapyrenes (DCPP, C20H10) in fullereneforming flames as described by Lafleur et al. [68] and confirmed by Flame I data [42]. In the present work a direct pathway by acetylene addition from cylcopenta[cd]pyrene-radicals to corannulene, in competition with the formation of the different

17

Fig. 12. Comparison between experimental mole fraction profiles [42] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25°C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. Benzo[ghi]perylene: F (experiment, left scale), (prediction, left scale). Benzo[ghi]fluoranthene: } (experiment, right scale), ---- (prediction, right scale).

dicylcopentapyrenes is assumed. This reaction requires an isomerization leading to a five-membered ring which could occur at an energized state on the potential surface which would allow a substantial reduction of the activation barrier. A more detailed investigation will be necessary in order to assess the importance of this reaction. The prediction of a dicyclopentapyrene mole fraction of about 1 ⫻ 10⫺6 is inconsistent with its not being detectable in Flame I [42] and could be explained by the preponderance of the corannulene pathway in fullerene forming flames. The predicted and experimental values of corannulene concentration are discussed below. A peak mole fraction of about 6 ⫻ 10⫺8 for benzo[e]pyrene in Flame I agrees with the experimental value close to the detection limit [42]. A significant increase of the predicted concentration in the postflame gas reveals a considerable lack of consumption reactions in this zone of the flame. Benzo[ghi]perylene (BGHIPER, C22H12) is formed in the next H-abstraction/acetylene-addition sequence yielding about a 10-fold underprediction of the peak concentration but a significant overprediction in the burnt gas (Fig. 12), again reflecting a missing sink in this flame region. In the following step two different benzo[ghi]perylene-radicals (BGHIPE*S1 and BGHIPE*S2), formed by H-abstraction, react via acetylene-addition to cyclopentabenzo[ghi]perylene (CPBPER, C24H12) and coronene (CORONEN, C24H12). As in the case of dicyclopentapyrene, the different cyclo-

18 pentabenzo[ghi]perylene isomers are represented as only one species in the model since their thermodynamic properties are indistinguishable using group additivity. Use of sufficiently high level calculation of thermodynamic properties to differentiate between those isomers as well as to assess the energies of different radical sites beyond the primary, secondary, and tertiary distinctions of group additivity would be interesting for future work. Two cyclopentabenzo[ghi]perylene isomers and coronene were found in Flame I [42] but could not be quantified. The predicted cyclopentabenzo[ghi]perylene peak mole fraction is about 6 ⫻ 10⫺9 in Flame I which is reasonable, whereas the concentration in the burnt gas is overpredicted, again indicating missing consumption reactions. The predicted peak mole fraction of coronene in the reaction zone of Flame I is about 3 ⫻ 10⫺8 consistent with coronene detected but not being quantifiable [42]. FORMATION OF C60 AND C70 FULLERENES Homann et al. [27, 38, 39] suggest that fullerenes could form by a so-called zipper mechanism, the reaction of two large PAH followed by hydrogen loss and bond formation; Frenklach and Ebert [69] suggest the formation could occur by a sequence of alternating hydrogen abstractions and acetylene additions. Direct growth to C60 and C70 by sequential C2H2 addition combined with internal rearrangement and hydrogen loss was formulated by Pope et al. [40, 41]. Also fullerene formation via reactive coagulation of C30H10 units with subsequent hydrogen loss and bond formation was taken into account but less than 3 ⫻ 10⫺7 of the C60 ⫹ C70 came from the coagulation pathway [40]. A preliminary kinetics test [40, 41] of the Pope et al. mechanism using a plug flow simulator and experimental flame species concentrations as input gave peak C60 and C70 mole fractions close to experimental flame measurements [29 – 31]. Thermodynamic constraints in the direct growth mechanism have been considered and no insuperable thermodynamic barriers were revealed [70]. In the present work the above described PAH growth mechanism was combined with the di-

H. RICHTER ET AL. rect fullerene growth mechanism [40, 41], with H abstraction by OH as well as H taken into account. OH is even more important than H up to a temperature of 1400 K which is reached at about 3 mm above the burner. Preliminary tests to include reactive coagulation showed a negligible contribution, 105- to 108-fold smaller than that by the direct pathway. The fullerene growth begins with fluoranthene which forms benzo[ghi]fluoranthene (BGHIF, C18H10) via H-abstraction/C2H2-addition. The predicted concentration of benzo[ghi]fluoranthene compared with experimental data [42] shows good agreement close to the burner but the peak value is 3-fold overpredicted and displaced downstream, and there is insufficient depletion in the postflame zone (Fig. 12). The overprediction of the peak value could be explained by an inaccurate assessment of the branching between benzo[ghi]fluoranthene (BGHIF, C18H10) and cyclopenta[cd]fluoranthene (FLTHNR, C18H10), another product of acetylene addition to fluoranthene radicals. Benzo[ghi]fluoranthene is methylated and ethynylated and forms in the next H-abstraction/ acetylene-addition sequence corannulene (COR, C20H10), the smallest bowl-shaped PAH, which has a curvature and a carbon framework similar to those of fullerenes [71]. Corannulene formation from cyclopenta[cd]pyrene (see above) leads to an approximately 2-fold increase of its peak mole fraction, and in the postflame zone an increase that is even larger but not significant due to the missing PAH sink in that region. The methylation of corannulene is included in the mechanism. The presence of corannulene in Flame I was confirmed experimentally but quantification was not possible because of low concentration [42], consistent with a predicted mole fraction of 5.6 ⫻ 10⫺7. Cylcopenta[cd]corannulene (COR1, C22H10) is the product of the next H-abstraction/ acetylene-addition growth step but is also formed in an additional pathway. Beginning with fluoranthene three H-abstraction/C2H2addition sequences via cyclopenta[cd]fluoranthene (FLTHNR, C18H10) and cylcopenta[cd]benzo[ghi]fluoranthene (BGHIFR, C20H10), which is also formed from benzo[ghi]fluoranthene, lead to cylcopenta[cd]corannulene. Cyclopenta[cd]fluoranthene was synthesized [72]

FORMATION OF PAH AND FULLERENES

Fig. 13. Comparison between experimental mole fraction profiles [42] and model predictions in a sooting premixed benzene/oxygen flame (␾ ⫽ 2.4, 10% argon, v 25°C ⫽ 25 cm s⫺1, 5.33 kPa); Flame I. C60: F (experiment, left scale), (prediction, right scale). C70: } (experiment, left scale), ---(prediction, right scale).

and found in ethylene flames [73], and will be searched for in the present flames. The formation of C60 and C70 fullerene structures is completed by sequential C2H2-addition combined with internal rearrangement and hydrogen loss as suggested by Pope et al. [40, 41] taking into account H-abstraction by OH as described above. The molecular structure and the nomenclature of the intermediates leading finally to C60 and C70 are given in reference [40]. The comparison of C60 and C70 fullerene formation in Flame I with experimental data [29 –31] reveals a 50- to 100-fold underprediction of the peak values (Fig. 13). Also, the first of two maxima in the C60 and C70 experimental concentration profiles, at about 1 cm above the burner, was not reproduced by the model. This first maximum could be attributed to fullerene oxidation, not included in the present mechanism, considering a similar shape of the anthanthracene profile when oxidation is taken into account (Fig. 10). C60O and C70O, detected in flame samples [29 –31] could be the oxidation products but also adsorption on and reaction with growing soot particles should be considered [42]. The sensitivity of the predicted C60 and C70 mole fractions to uncertainties in the rate coefficients of H-abstraction and C2H2addition reactions with larger PAH was tested. A 2-fold increase of the rate coefficient for H-abstraction by hydrogen radicals beginning with corannulene led to a 3-fold increase of the

19 final C60 and C70 concentrations; the same operation performed for C2H2-addition showed a 2.3-fold increase for C60 and a 4.2-fold increase for C70. Also an assumed ⫾100 K uncertainty in the experimental temperature profile [43] gave a 2-fold uncertainty in the predicted peak concentrations of C60 and C70 fullerenes (and PAH). This result added to an uncertainty of ⫾15% [42] in fullerene analysis by HPLC and even an additional error related to the reproductivity of the experimental flame conditions gives a total error that is less than the observed discrepancy between model predictions and experimental results. Consistent with the increasing underprediction of peak concentrations for larger and larger PAH as seen for benzo[ghi]perylene, the underprediction of C60 and C70 fullerenes may be due to uncertainties in the rate coefficients for Habstraction and C2H2-addition with more and more steps of this sequence being involved. Nevertheless, the existence of other fullerene formation mechanisms [27, 38, 39] cannot be excluded. Unambiguous identification of larger intermediates would be helpful, but is difficult because of limitations in chemical analysis. GC-MS reaches its limits at about 300 amu due to the sublimation temperature increasing with molecular mass; state-of-the-art HPLC can be used up to about 450 amu [67]. The synthesis of potential intermediates of the suggested fullerene formation mechanism by means of methods of organic chemistry such as hightemperature gas phase cyclization is of great interest [74]. CONCLUSIONS The model developed here was found to give encouraging predictive capability for the growth of PAH up to coronene in a sooting premixed benzene flame. The availability of MBMS data for smaller species and radicals in a nearly sooting benzene flame allowed the model to be tested for smaller and unstable species, and its reliability was confirmed. C60 and C70 fullerenes formed by sequential C2H2-addition combined with internal rearrangement and hydrogen loss was included in the model but no definitive answer concerning the importance of

20

H. RICHTER ET AL.

additional fullerene forming pathways could be given. The concentrations of nearly all PAH were significantly overpredicted in the postflame zone where a rapid depletion observed experimentally was not reproduced by the model. This discrepancy may reflect the absence of PAH sinks such as reactions with smaller radicals (methyl, ethyl, vinyl, . . .), single-ring aromatics, other PAH, soot, etc. In this context it should be mentioned that some experimental evidence of reactive coagulation, e.g. the dimerization of C14H10 to C28H14 has been shown in laminar diffusion flames by Siegmann et al. [75]. The contribution of PAH to soot formation, studied recently by Benish et al. [76] receives additional support by the coincidence of fast soot formation and PAH depletion as shown by Grieco et al. [42] indicating the importance of the reaction of PAH with growing soot particles. Those to be considered PAH-soot interactions could lead to the immediate formation of chemical bonds or—taking into account the increasing molecular mass and van der Waals interaction—to physical adsorption. Stronger, chemical bonds could be established later, during the soot aging process. Definitive explanation of PAH consumption and fullerene formation will require the identification of larger intermediates of the growth process. The study of PAH formation mechanisms was funded by the Chemical Sciences Division, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy under Grant DE-FGO2-84ER13282. Analytical chemistry support was funded by the National Institute of Environmental Health Sciences Center Grant NIH-5P30-ES02109. The testing of the fullerene formation model was funded by the National Aeronautics and Space Administration under Grant NAG3-1879. REFERENCES 1. 2.

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Received 26 August 1998; revised 16 February 1999; accepted 25 February 1999