Pathways to Soot Oxidation: Reaction of OH with Phenanthrene

Article pubs.acs.org/JPCA

Pathways to Soot Oxidation: Reaction of OH with Phenanthrene Radicals David E. Edwards,†,‡ Dmitry Yu. Zubarev,§,|| William A. Lester, Jr.,§,⊥ and Michael Frenklach*,†,‡ †

Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720-1740, United States Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California at Berkeley, Berkeley, California 94720-1460, United States ⊥ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: Energetics and kinetics of the oxidation of possible soot surface sites by hydroxyl radicals were investigated theoretically. Energetics were calculated by employing density functional theory. Three candidate reactions were selected as suitable prototypes of soot oxidation by OH. The first two, OH + benzene and OH + benzene−phenol complex, did not produce pathways that lead to substantial CO expulsion. The third reaction, OH attack on the phenanthrene radical, had multiple pathways leading to CO elimination. The kinetics of the latter reaction system were determined by solving the master equations with the MultiWell suite of codes. The barrierless reaction rates of this system were computed using the VariFlex program. The computations were carried out over the ranges 1500−2500 K and 0.01−10 atm. At higher temperatures, above 2000 K, the oxidation of phenanthrene radicals by OH followed a chemically activated path. At temperatures lower than 2000 K, chemical activation was not sufficient to drive the reaction to products; reaction progress was impeded by intermediate adducts rapidly deenergizing before reaching products. In such cases, the reaction system was modeled by treating the accumulating adducts as distinct chemical species and computing their kinetics via thermal decomposition. The overall rate coefficient of phenanthrene radical oxidation by OH forming CO was found to be insensitive to pressure and temperature and is approximately 1 × 1014 cm3 mol−1 s−1. The oxidation of phenanthrene radicals by OH is shown to be controlled by two main processes: H atom migration/ elimination and oxyradical decomposition. H atom migration and elimination made possible relatively rapid rearrangement of the aromatic edge to form oxyradicals with favorable decomposition rates. The reaction then continues down the fastest oxyradical pathways, eliminating CO.

1. INTRODUCTION

atomistically resolved pathways leading to the removal of carbon by OH have been identified. Current models of soot formation typically describe oxidation of soot by OH as an outcome of a collision between gaseous OH and unspecified site of the soot−particle surface. The collision efficiency for the oxidation (i.e., expulsion of gaseous CO), ηOH,11,12 is assigned a value of 0.13, the lower bound determined in the experimental study of Neoh et al.13 In their experiments, Neoh et al. examined the oxidation of soot in a two-stage atmospheric pressure premixed system. They reported ηOH values in the range of 0.13−0.28. Roth and co-workers performed shock tube experiments to study soot oxidation. ηOH = 0.2 was determined as a fitting parameter to account for the observed CO formation.14 A study of Santoro and co-workers15,16 reported lower bounds for ηOH,

Soot is one of the major pollutants produced during combustion of fossil fuels. It is usually formed as a cloud of carbonaceous nanoparticles. Structurally, soot particles are aggregates of polycyclic aromatic hydrocarbons (PAH).1 Their formation comprises a number of sequential steps, including soot precursor formation, particle nucleation, and particle coagulation.2,3 Parallel to these steps are reactions taking place at the edges of PAHs and soot.3,4 These reactions include both the addition and removal of carbon and are an active area of research.5 In the present study we explore surface oxidation reactions, specifically oxidation by OH. Experimentally, OH has been identified to be the major oxidizer of soot.6 In our prior studies7,8 we examined theoretically the decomposition of PAH oxyradicals, presumed intermediates in the oxidation pathways. In pursuit of this objective, we determined reaction rates for the thermal decomposition of oxyradicals located on the PAH zigzag7 and armchair8 edge sites. Detailed pathways of the oxidation of aromatic radicals by O2 have also been studied.9,10 To our knowledge, no © 2014 American Chemical Society

Special Issue: A. W. Castleman, Jr. Festschrift Received: April 3, 2014 Revised: April 24, 2014 Published: April 24, 2014 8606

dx.doi.org/10.1021/jp5033178 | J. Phys. Chem. A 2014, 118, 8606−8613

The Journal of Physical Chemistry A

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0.04 for methane and 0.05 for methane/butane flames. They also noted a trend of increasing ηOH with time. A series of experimental studies by Faeth and co-workers17−19 examined the oxidation rate of soot over the ranges of 0.1−8 atm and 1400−2350 K in diffusion and premixed flames burning a variety of fuels. They concluded that, over all of their experimental conditions, ηOH was roughly constant at 0.12, with an uncertainty (95% confidence) of ±0.03.18 Most of their data fall within this range, but some measurements were as low as 0.02 and as high as 0.4. The purpose of the present study is to explore reactions of OH with different types of PAH edge sites with the goal of gaining mechanistic insights into soot oxidation by OH. In our theoretical exploration we were guided by the principle that OH oxidation pathways must be commensurate with the fast oxidation rates observed experimentally. To this end we computed potential energy surfaces (PESs) of OH reacting with three types of PAH edge sites to explore possible pathways leading to CO expulsion. The kinetics of the most promising reaction were then calculated by solving energy-transfer master equations. The rate coefficients obtained at the employed level of theory are within an order-of-magnitude accuracy, appropriate for the exploratory nature of the present investigation, similar to prior experience.7,8,20

from an empirical correlation.36 The exact count, with an energy grain size of 10 cm−1 for the first segment of the double array, and a maximum energy of 500 000 cm−1, was employed to determine the density of states. For each set of initial conditions, the number of trials was varied from 1 × 104 to 8 × 107 to keep statistical error below 5%. Rate coefficients of barrierless reactions were calculated using the variable reaction coordinate (VRC) approach, as implemented in VariFlex.37 Briefly, the VRC approach separates the transition state’s degrees of freedom into “conserved” and “transitional” modes. The conserved modes have little change over the reaction process and are treated as rigid rotors or harmonic oscillators by directly computing the corresponding sums of states. The transitional modes are treated classically through a phase-space integral. The partition function of the transition state is then calculated by a convolution of these two types of modes. The bond length associated with the barrierless reaction is varied, and the contribution of the transitional modes to the partition function recalculated until a minimum reaction rate is found.38 The microcanonical rate constants calculated in this manner were entered into MultiWell. The VariFlex inputs were similar to those of MultiWell, with identical energy grain sizes and maximum energy. A Morse39 or Varshni40,41 potential was fit to a PES scan performed at the B3LYP/6-311G(d,p) level for each barrierless reaction, documented in section 1 of the Supporting Information. The chemical-activated rate coefficients were derived from the accumulated species fractions of the products, as we have done previously.21 The accumulated species fractions were found by running a MultiWell simulation started with a chemical-activated initial energy distribution. The species fractions were found after the average energy of the initial adduct became constant, indicating the end of the chemically activated simulation. The product formation rates were calculated by multiplying the species fraction of the products by the high-pressure reaction rate of the adduct formation reaction. The thermal decomposition rate coefficients, calculated using MultiWell, were derived from the exponential decay of the reactant molecule after a period of initial relaxation.8,42 A MultiWell simulation was started with a thermally activated initial energy distribution. After a period of initial relaxation, the average value of the internal energy approached a constant value, and a fraction of the reactant molecule began to decompose exponentially. The slope of the decay on a semilog plot yielded the rate coefficient of interest. When the system contained multiple products, the rate of formation of each product was calculated by multiplying the accumulated product species fraction by the decomposition rate of the reactant.

2. COMPUTATIONAL METHODOLOGY Following the methodology of our previous studies,7,8,20−23 density functional theory (DFT) was employed to calculate potential energy surfaces of all stable species and transition states for the oxidation systems. Geometry optimization and vibrational frequency calculations were performed to identify all stationary points on the reaction pathways using the B3LYP hybrid functional24,25 or M06-2X functional26 and a 6311G(d,p) basis set.27 The bond dissociation energy of several barrierless reactions was also calculated using the CBS-QB3 level of theory.28 Zero-point energies (ZPE) and vibrational frequencies were scaled by a factor of 0.967.29 Transition states were confirmed to connect the reactant and product species by inspection of the normal mode for the single imaginary frequency of each saddle point. All the quantum-chemical calculations were carried out using the Gaussian 0330 and 0931 program packages. The rate coefficients of the thermal and chemically activated reaction systems were computed using the MultiWell suite of codes.32−34 MultiWell solves the one-dimensional time-dependent energy-transfer master equations for a multiwell and multichannel unimolecular reaction system using the Monte Carlo stochastic method. Microcanonical rate coefficients for the elementary reactions of these systems were calculated with MultiWell at the Rice−Ramsperger−Kassel−Marcus level of theory. The key inputs to MultiWellreaction barriers, frequencies, and moments of inertiawere determined from DFT calculations. Following Gilbert and Smith, 35 the real frequencies were examined by graphically visualizing the associated normal mode vibrations to identify internal rotational modes. All internal rotors were treated as 1-D hindered rotors. Reaction rates were computed at temperatures ranging from 1500 to 2500 K and pressures from 0.01 to 10 atm. Argon was chosen as the bath gas collider. The exponential-down model with ⟨ΔE⟩down = 260 cm−1 was used to describe the collisional energy transfer.8 Lennard-Jones parameters were estimated

3. RESULTS AND DISCUSSION 3.1. Minimum Energy Paths. Three PAH edge sites were investigated as possible prototypes of soot surface sites for reaction with OH: Csurface−H, Csurface•, where Csurface designates a soot surface carbon atom, and a benzene−phenol complex. For these three systems, pathways leading to CO expulsion were only found for OH attacking a Csurface• site. For this latter case, hydrogen migration and expulsion were key steps that opened up the CO eliminating pathways. For all of the molecular structures discussed below, Table S2 in the Supporting Information contains the zero-point energies, expectation values of the S2 operator, vibrational frequencies, 8607



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that was identified had a nearly 100 kcal/mol barrier, indicating that continuing along that pathway was unlikely. 3.1.3. OH + Csurface•. A phenanthrene radical was selected as the prototype for the Csurface• soot armchair surface site. In making this selection, we were guided by the conclusion of our previous studies that oxidation of PAH moieties should occur primarily at armchair but not zigzag edges.7,8 The phenanthryl− OH adduct and initial wells of the singlet PES for the phenanthrene radical reacting with OH are shown in Figure 3a. The PES shows the formation of five oxyradicals: 8, 7, 5, 6, and 4 associated with channels I−V shown in Figure 3b−f, respectively. The formation of the first four oxyradicals, 8, 7, 5, and 6, is facilitated by H atom migration, whereas the formation of 4 is enabled by H atom elimination. Each of these five oxyradicals, as shown in Figure 3b−f, can continue to isomerize, eventually eliminating CO. The CBS-QB3 level of theory was used to calculate the bond dissociation energy (BDE) for the 2 → 4 and 2 → 1 reactions. This was done due to the importance of these two barrierless reactions to the overall system and because of B3LYP’s known BDE underestimation.45 The B3LYP/6-311G(d,p) dissociation energy for 2 → 1 was calculated to be 102.3 kcal/mol, whereas the CBS-QB3 value was 108.5 kcal/mol. The B3LYP/6311G(d,p) dissociation energy for 2 → 4 was 75.5 kcal/mol whereas the CBS-QB3 value was 82.9 kcal/mol. In addition, 7 → 4 BDE was indirectly modified by the CBS-QB3 calculations. Although the CBS-QB3 calculation was not performed for species 7, by using the BDE of 2 → 4 and the relative energy between 2 and 7, calculated at the B3LYP/6311G(d,p) level of theory, the BDE for 7 → 4 was calculated to be 74.15 kcal/mol, whereas the B3LYP/6-311G(d,p) value was 66.68 kcal/mol. The energies shown in Figure 3 include the CBS-QB3 values for these reactions. Pathways I−V are oxyradical decomposition pathways, similar in general features to each other and to those of the phenanthrene oxyradicals studied previously;8 in fact, pathway V is one of the oxyradical pathways studied in that paper. Each of these pathways, however, has distinctive reactions and varying barrier heights. For instance, the barrier heights for the transition from oxyradical to cyclic intermediate for PES I, III, IV, and V were 15.3, 86.4, 39.2, and 54.7 kcal/mol, respectively. This wide range of barrier heights is related to the location of the hydrogen atom with respect to the chemisorbed oxygen atom. In all of these cases oxygen was located on the same site of the phenanthrene oxyradical; the relative location of the hydrogen atom is the primary cause for the differences between the five oxyradical decomposition pathways. The presence of oxyradicals in OH oxidation highlights the importance of oxyradicals in soot oxidation and supports the notion that oxyradicals are key intermediates in soot oxidation. 3.2. Reaction Rate Coefficients. We first discuss the barrierless reaction channels, followed by the overall kinetics. 3.2.1. Barrierless Reactions: 2 → 1, 2 → 4, 7 → 4, 20 → 21. Reactions 2 → 1 and 20 → 21 were found to be well represented by a Morse potential, whereas reactions 2 → 4 and 7 → 4 were best represented using a Varshni potential. Plots showing the fits for each of the four barrierless reactions are shown in Figures S1−S4 of the Supporting Information. The reaction degeneracy for the barrierless reactions were assigned a value of 2 because the transition structures are optical isomers. The potentials for reactions 2 → 1, 2 → 4, and 7 → 4 were scaled by the CBS-QB3 values.

rotational constants, and the Cartesian coordinates for all optimized structures. 3.1.1. OH + Csurface−H. Benzene was selected as a prototype for the Csurface−H soot surface site. The PES for this system, shown in Figure 1, identified two main products, H2O and

Figure 1. Potential energy surface for the reaction of benzene with OH at the B3LYP/6-311G(d,p) level of theory, including ZPE. Energies are in kcal/mol at 0 K relative to the reactants, 1.

phenol. There were no pathways leading to CO expulsion. Prior theoretical studies of the same reaction43,44 also did not identify any major channels leading to CO expulsion. We conclude, therefore, that OH reacting with a Csurface−H edge site does not lead to CO expulsion at a rate fast enough to explain the observed collision efficiency. 3.1.2. Benzene−Phenol Complex. The influence of interacting PAH layers on oxidation was investigated by a benzene−phenol complex used to model interacting layers of soot, where OH attaching to one layer may react with another layer. The PES of this system was calculated with the M06-2X density functional and 6-311G(d,p) basis set. B3LYP was not used in this case because it is unable to account for mediumrange interactions26 such as those in the benzene−phenol complex. The computed PES is shown in Figure 2. No pathways were identified for the removal of CO. The single transition state

Figure 2. Potential energy surface for the reaction of OH with benzene−phenol complex at the M06-2X/6-311G(d,p) level of theory, including ZPE. Energies are in kcal/mol at 0 K relative to the reactants, 1. 8608



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Figure 3. Potential energy surface for the reaction of OH with phenanthryl. The bond dissociation energies for 2 → 4 and 2 → 1 were calculated at the CBS-QB3 level of theory; the remaining energies were obtained at the B3LYP/6-311G(d,p) level of theory. Energies include ZPE and are in kcal/mol at 0 K relative to the reactants, 1. (a) The initial wells of the reaction. The roman numerals I−V in (a) correspond to the PESs in (b)−(f), respectively.

3.2.2. Master Equation Modeling. The species fractions resulting from the chemical-activation simulations of the system

shown in Figure 3 are given in Table 1. The intermediates and products with species fractions less than 0.02 are assumed to be 8609



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Table 1. Species Fractions Obtained in the Chemical-Activation Simulations of the System Shown in Figure 3a T = 1500 K species

T = 2000 K

0.01 atm

0.1 atm

1 atm

10 atm

2 7 4

0.04 0.01 0.11

0.19 0.07 0.23

0.49 0.15 0.20

0.82 0.09 0.06

1 11 33 total

0.01 0.05 0.78 0.99

0.01 0.03 0.46 0.99

0.01 0.01 0.12 0.99

0.00 0.00 0.01 0.98

0.01 atm

0.1 atm

1 atm

T = 2500 K 10 atm

0.01 atm

0.1 atm

1 atm

10 atm

0.13 0.05 0.13

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.01

0.04 0.02 0.60 0.97

0.11 0.01 0.85 0.97

0.11 0.01 0.85 0.97

0.11 0.01 0.85 0.97

0.11 0.01 0.84 0.97

Intermediate Species Fraction

a

0.00 0.00 0.02 0.00 0.00 0.01 0.00 0.01 0.04 Product Species Fraction 0.04 0.04 0.04 0.02 0.02 0.02 0.91 0.91 0.85 0.98 0.98 0.98

Intermediate species indicate species that were collisionally stabilized and accumulated by the end of chemical activation.

negligible and are not included in the table. The chemicalactivation rate constants were calculated by multiplying the species fraction in Table 1 by the high-pressure rate of OH combining with a phenanthrene radical. Table 2 lists these highTable 2. High-Pressure Rates of the OH−Phenanthryl Reaction T (K)

k∞ (cm3 mol−1 s−1)

1500 2000 2500

9.6 × 1013 1.1 × 1014 1.3 × 1014

pressure values for the three temperatures studied. The main products of the chemically activated system were 11 (CO), 33 (H, CO), and 1 (reactants). In addition, at higher pressures and lower temperatures, species 2, 7, and 4 were collisionally stabilized and accumulated. Under these conditions chemical activation was not sufficient to drive the reaction completely to products. The time required for the chemical-activation simulations to reach thermal equilibrium was roughly from 5 × 10−8 to 5 × 10−5 s, possibly exceeding the time scale of their collisions with surrounding flame species. In this way, hot intermediates could undergo secondary reactions, thereby increasing, for instance, the rate of growth by acetylene or the rate of oxidation by O2 and OH. The accumulating cold intermediates may also undergo secondary reactions. For this work, we assumed that such secondary reactions did not occur. A representative plot of species fractions from MultiWell chemical-activation runs is shown in Figure 4. The high-pressure elementary reaction rates for each reaction of the overall system are reported in Table S1 of the Supporting Information. When chemical activation was not sufficient to drive the reaction to products, the accumulating intermediates were modeled as distinct species undergoing thermal decomposition. These thermal-decomposition rate constants were computed in additional MultiWell thermal-decomposition simulations. The thermal product formation rates for 11, 33, and 1, calculated from the thermal decomposition of species 2, are shown in Table 3. Species 7 rates are not listed in Table 3 because species 7 is in quasi-equilibrium with species 2 and both decomposed at the same rate. Thermal rates were calculated only for conditions that led to accumulation of intermediate species in the chemical-activation simulations. The thermal decomposition of species 4 was taken from our previous work.8,46 Thermal rate coefficients were used in conjunction with the chemical-activation rate coefficients to calculate the overall rate

Figure 4. Major-species fractions from a MultiWell chemically activated simulation at T = 1500 K and P = 0.01 atm, along with the average internal energy of 2 plotted as the thick gray line.

Table 3. Product Formation Rates from Thermal Decomposition of Species 2 k (s−1) T (K)

P (atm)

1500 1500 1500 1500 2000 2000

0.01 0.1 1 10 1 10

1

11

1.8 × 103 1.0 × 104

68 120 160 190 1.5 × 104 3.6 × 104

33 560 1.2 × 1.9 × 2.1 × 2.7 × 8.3 ×

103 103 103 105 105

of CO expulsion from the reaction of OH with phenanthrene radical. The reaction system used to calculate the overall reaction rate coefficients for the combined system of thermal and chemical-activation rates is shown in Figure 5. In this figure, the reactions starting from 1 are chemical activation, and those starting from 2, 7, and 4 are thermal decompositions. Based on the reaction system in Figure 5, the overall formation rate of CO is given by 8610



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Figure 5. Combined chemical-activation and thermal-decomposition pathways of the overall oxidation of phenanthrene radicals by OH.

Figure 6. Representative plot of kCO. The ramp-up time, defined as the time for kCO to reach 90% of its final value, is shown.

d[CO] d[11] d[33] = + dt dt dt

Table 4. Ramp-Up Times (ms) of kCO, Defined as the Time for kCO To Reach 90% of Its Final Value

= (k11 + k 33)[OH][phenanthryl] = k CO[OH][phenanthryl]

(1)

The reaction rate coefficients k11 and k33 were derived by solving the set of differential equations defined by the kinetic system shown in Figure 5, k11(t ) = k1 → 11 +

0.01 atm

0.1 atm

1 atm

10 atm

1500 2000 2500

1.8

1.2

1.0 0.005

1.0 0.002

k k k1 → 2k 2 → 11 (1 − e−k 2t ) + 1 → 7 7 → 11 k2 k7

(1 − e−k 7t ) k 33(t ) = k1 → 33 +

T (K)

(2)

k1 → 2k 2 → 33 k k (1 − e−k 2t ) + 1 → 7 7 → 33 k2 k7

(1 − e−k 7t ) + k1 → 4(1 − e−k4→ 33t )

(3)

k 2 = k 2 → 1 + k 2 → 11 + k 2 → 33

(4)

k 7 = k 7 → 1 + k 7 → 11 + k 7 → 33

(5)

When chemical activation carries the system to completion, i.e., at high temperatures and low pressures, kCO was calculated as the sum of k1→11 and k1→33. Under these conditions, k1→2, k1→7, and k1→4 equal zero, and eqs 2 and 3 reduce to timeindependent k1→11 and k1→33, respectively. Conversely, when the accumulation of 2, 7, or 4 was significant, kCO became time-dependent. This is seen in the time dependence of eqs 2 and 3. Figure 6 shows a representative plot of kCO versus time for such a case. The initial value of kCO (k0CO) is the sum of the chemical-activation rate constants, k1→11 + k1→33. kCO then grows from this initial value and eventually reaches the steady-state value, k∞ CO. We defined a “ramp-up” time as the time for kCO to reach 90% of k∞ CO. Table 4 lists the ramp-up times for the conditions studied. Strong temperature dependence, caused by the temperature dependence of the thermal rates, is observed from these results. The contributions of chemical activation and thermal decomposition to the total oxidation rate of phenanthrene radicals at 10 atm are shown in Figure 7. At 1500 K the oxidation occurred exclusively through thermal decomposition. Under these conditions collisional deactivation of the reacting species was faster than chemical activation, which led to the

Figure 7. Representative plot showing contributions of the chemical activation and thermal decomposition to the overall oxidation rate.

accumulation of intermediates. These intermediates then decomposed thermally. At 2500 K, the opposite occurred. Chemical activation was sufficiently fast and all of the species reacted to products before any accumulation occurred. Similar trends were obtained for other pressures studied with thermal decomposition contributing less at lower pressures. ∞ The computed values of kCO , the overall steady-state oxidation rate of phenanthrene radicals by OH, were independent of pressure and only slightly temperature dependent. For instance, the k∞ CO values at 1500, 2000, and 2500 K were 0.94, 1.05, and 1.09 × 1014 cm3 mol−1 s−1, respectively. This insensitivity to pressure and temperature arises mainly due to two factors. First, the high-pressure rates for the initial elementary step of OH + phenanthrene radical, 1 → 2 in Figure 3a, change very little with temperature (Table 2) 8611



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and hence the overall rate changes very little with temperature. Second, the barrier of the reverse reaction, 2 → 1, is quite high, 108 kcal/mol. Because of this high backward barrier and multiple lower energy pathways from 2 to the release of CO, the rate of 2 → 1 is very low compared to the rate of 2 forming CO, even as pressure changes. This, in turn, leads to pressure insensitivity and contributes to temperature insensitivity. Note that the thermal rates, the ramp-up time, and the competition between the thermal and chemical-activation pathways were temperature and pressure dependent. The insensitivity of the overall computed rate to temperature and pressure is in agreement with the experimental findings of Faeth and coworkers.18 The PES and rates reported here were calculated on the singlet surface. The system of OH reacting with a phenanthrene radical on the triplet surface was also calculated47 and the rates were about a factor of 6 slower than the rates computed on the singlet surface. Applying collision theory48 to the computed overall reaction rate, a ηOH value of 0.17 was obtained for the range of pressures and temperatures covered by the present study, which is close to the results of both Neoh et al.13 and Faeth and co-workers.18 However, this comparison presumes that the creation of surface radical sites is not rate controlling. Such an assumption is reasonable, given the largely oxidative experimental environment created for the study of soot oxidation.13 A fitting comparison requires modeling the entire system, including formation of Csurface• sites and their disappearance due to various reactions. The detailed reaction kinetics obtained here is a step toward such modeling, as the computed pathways are promisingly fast.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4 contain the potentials and fits for the four barrierless reactions. Table S1 contains the high-pressure rate coefficients of the elementary reactions in the OH + phenanthryl reaction system. Table S2 contains the following data for the molecular structures of the present study: total energies, zero-point energies, expectation values of S2, vibrational frequencies, rotational constants, unsymmetrical hindered rotor MultiWell parameters, and Cartesian coordinate geometries. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. Frenklach: tel, 0015106431676; e-mail, frenklach@ berkeley.edu. Present Address

|| Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, 02138, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.E.E., W.A.L., and M.F. were supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division of the U.S. Department of Energy, under Contract No. DE-AC0376F00098. M.F. was supported by the US Army Corps of Engineers, Humphreys Engineering Center Support Activity, under Contract No. W912HQ-11-C-0035. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC0205CH11231.

4. CONCLUSIONS The energetics of three different PAH edge sites reacting with OH were computed to explore pathways leading to CO expulsion. Two reaction systems, OH + benzene and OH + benzene−phenol complex, did not show viable pathways to rapid CO expulsion. The third one, OH reacting with a phenanthrene radical, is the most promising of the three PAH edge sites studied, containing multiple pathways for CO elimination. The kinetics of OH + phenanthrene radical reaction were studied by calculating both the chemical-activation and thermaldecomposition reaction rates. Chemical activation, at lower temperatures and higher pressures, was not sufficient to drive the reaction completely to products. The reaction progress was impeded by rapid de-energizing of intermediate species. In such cases, the rates were calculated by chemical activation followed by thermal decomposition of the de-energized intermediate species. This led to time dependence of the overall rate coefficients for CO formation. After an initial ramp-up time, the rate coefficient reached its steady-state value. The computed overall oxidation rate of phenanthrene radicals by OH was found to be insensitive to pressure and temperature and was approximately 1 × 1014 cm3 mol−1 s−1. The two principal processes involved in phenanthrene radical oxidation by OH were H atom migration/elimination followed by oxyradical decomposition. H atom migration/elimination made possible the relatively rapid rearrangement of the PAH edge, forming kinetically favorable oxyradicals, which then decomposed. We would expect these same two processes to be present in soot surface oxidation by OH.

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REFERENCES

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Pathways to Soot Oxidation: Reaction of OH with Phenanthrene

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