Density Functional Study of the Chemisorption of O2 Across Two

J. Phys. Chem. C 2007, 111, 5465-5473

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Density Functional Study of the Chemisorption of O2 Across Two Rings of the Armchair Surface of Graphite Karina Sendt* and Brian S. Haynes School of Chemical and Biomolecular Engineering, UniVersity of Sydney, NSW, Australia ReceiVed: NoVember 7, 2006; In Final Form: January 4, 2007

The addition of molecular oxygen across two rings of the armchair surface of a model graphite has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. Chemisorption, desorption, rearrangement, and surface migration pathways were characterized and kinetic parameters were computed in order to provide a mechanistic understanding of the processes occurring during carbon gasification. The initial step of the chemisorption reaction is identical to the addition across one ring of the armchair surface and has a low barrier, 18 kJ mol-1. The barriers of subsequent steps leading to the formation of two ketone groups on separate rings are lower in energy than the initial step, and the overall reaction is 420 kJ mol-1 exothermic. Surface migration reactions of the ketone groups were found to occur with a barrier of ∼140 kJ mol-1 and could form either well-separated ketone groups or a stable furan structure. Rearrangement reactions to form stable lactone and ketene species were found to have overall barriers of 104 kJ mol-1, and these species are expected to be the major products of any molecular O2 chemisorbing across two adjacent rings of the armchair surface. Desorption processes from these species were found to have barriers of 222 and 278 kJ mol-1 (formation of CO) or 297 kJ mol-1 (formation of CO2). Loss of the second CO was found to require ∼200 kJ mol-1, while surface migration reactions following the initial CO desorption have barriers of 131 and 206 kJ mol-1.

Introduction Because of the importance of solid carbonaceous fuels in worldwide energy production, much attention has been given to understanding the underlying combustion mechanism. A goal of both experimental, and, more recently, theoretical studies has been to account for the wide range of reactivities of different carbons. Despite a number of surface oxide species, including ketones, carbonyls, lactones, anhydrides, alcohols, phenols, ethers, and carboxylic acids, having been observed on carbon substrates, their role in carbon gasification has not been well understood. However, a stochastic description of the formation, reaction, and decomposition of surface oxide complexes has been successful in reproducing the experimentally observed properties of carbon oxidation in the form of a turnover model.1 It therefore appears that a deeper understanding of these surface oxides will provide a mechanistic understanding of the carbon oxidation process. Surface oxides are formed readily on clean carbon surfaces. The overall stoichiometry of the formation process varies from Type A behavior (in which no carbon is gasified when O2 is chemisorbed) to Type B behavior (in which one carbon atom is gasified as CO for each oxygen atom that forms a stable surface complex).2 The transition from Type A to Type B behavior occurs at a lower temperature for more disordered carbons. Temperature-programmed desorption (TPD) studies have shown that carbon surface oxides desorb as CO and CO2 over a wide range of activation energies (160-440 kJ mol-1), with CO2 typically comprising a small peak at the lower energy range for desorption. The high-temperature end of the TPD spectra has been found to be insensitive to the temperature at which the oxide species are formed.3 * Corresponding author. E-mail: [email protected].

Quantum chemical methods, which allow the computational characterization of geometries, energies, and vibrational frequencies of species such as intermediates and transition states, are invaluable for obtaining information that cannot be detected experimentally and for providing mechanistic detail. Several groups have used quantum chemical methods in order to understand and predict the behavior of carbon surface oxides, and a recent review by Zhu et al.4 summarizes many of their findings, in particular those of Yang and co-workers. Yang and co-workers have investigated the interaction of O2, CO2, and H2O with the zigzag surface of graphite5,6 and proposed that the rate of gasification of surface oxides is enhanced by the presence of O2, which binds to the surface in an out-of-plane configuration. However, they have based this suggestion only on thermodynamic grounds and they have not provided any kinetic evidence. Montoya et al. have studied a number of interactions between O2, CO2, H2O, and CO with the armchair and zigzag surfaces of a model graphite,7-9 focusing primarily on possible modes of chemisorption7 and the effects of local environment on energetics of desorption processes.9 They were the first to consider the kinetics of CO desorption,10 although their kinetic treatment was disputed by Frankcombe et al.11 and further refined by us.12 Radovic et al.13 have also investigated the modes of chemisorption of CO2 on the zigzag surface, giving particular attention to the nature of the bonding at the zigzag edge. Backreedy et al.14 proposed a mechanism for the gasification of carbon on the zigzag surface; however, they did not present any energetic information and their conclusions were based entirely on bond length calculations. Our approach has been to characterize the highly exothermic chemisorption reactions of molecular oxygen with a fresh carbon

10.1021/jp067363r CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007

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Figure 1. Model char molecule.

surface and to identify subsequent desorption, rearrangement, and migration reactions that could occur because of the large release of energy from the exothermic chemisorption. The identification of transition states, and hence the computation of high-pressure rate constants, has been a major goal of this work. The reaction on both the zigzag edge15 and on a single ring of the armchair edge16 have revealed a number of energetically accessible desorption, rearrangement, and migration pathways, with the migration pathways having the lowest activation energies. This has implications for the scrambling of oxides on the surface because the type and position of the oxides that form are not necessarily the origin of the CO and CO2 measured by TPD spectra. A study of the reaction of a single ketone on the zigzag and armchair surfaces also confirms the relatively low barrier (∼100-200 kJ mol-1) for migration reactions.12 The characterization of the individual ketone groups provides a foundation for understanding the steric interactions of adjacent oxides. In this paper, the addition of molecular oxygen across two adjacent rings on the armchair surface, and possible subsequent reactions, is discussed. The armchair edge presents a benzynelike C-C triple bond that must be broken in order for molecular oxygen to add to the edge. If an adjacent ring is present on the armchair edge, then it is conceivable that the second oxygen atom will add to the adjacent ring, forming two ketone groups on adjacent rings. The chemical behavior of this product is expected to be qualitatively different from the chemisorption of O2 on the same ring of the armchair surface, not least because each chemisorption product has entirely different possible migration pathways. This paper, which concludes our characterization of the chemisorption and subsequent reaction pathways of O2 with the fresh surface of a single-layer model char, will also present a discussion of the behavior of O2 on the zigzag, single ring of the armchair surface and adjacent rings of the armchair surface. Computational Methods Choice of Model Char. The model char used in this work, with four vacant sites, is illustrated in Figure 1. On the armchair surface, the bonding pattern presented by the edge atoms is a series of triple bonds, for example the pairs of atoms C2/C3 and C6/C7. The model char used in this work was chosen because it satisfied the following properties: (i) Of a large enough size that edge effects were minimized. Molecules with six aromatic rings with edge carbon atoms terminated by hydrogen atoms have been shown to reproduce experimental data for polyaromatics,17 and predicted molecular properties were found to be relatively size-independent above this size.17 (ii) Of a suitable size to represent actual char fragments. Several experimental studies into the domain size of char species are relevant to considering the size of the model char systems. First, lignite and a bituminous coal were prepared by pyrolysis over the temperature range of 900-1250 K, and a polynuclear aromatic domain size of the order of 16-24 atoms (4-7 rings)

Sendt and Haynes was measured by 13C NMR spectroscopy.18 Second, in a reverse Monte Carlo simulation19 the average number of aromatic rings per plate of carbon micropores was calculated to be 11.6 (standard deviation of 7.9), the equivalent of about 38 carbon atoms. (iii) Computationally tractable in a reasonable time frame. In addition, previous work12 has shown that a model char of this size is suitable for representing the semi-infinite graphite limit for many oxide structures. Choice of Method. Density functional theory (B3LYP) has been used throughout this work with the 6-31G(d) basis set. This has been used successfully to obtain accurate geometries and energies of graphene structures17 and has been shown to have little spin contamination in these systems.20 The choice of this method to describe low-spin biradical systems has been shown previously to be suitable for structures with unpaired electrons on different atoms.21 Geometries, energies, and vibrational frequencies were computed at the B3LYP/6-31G(d) level of theory for all stationary points. Several multiplicities for each species were considered, and where electronic states were close in energy, contributions from both electronic states to the electronic partition function were included, with the higher spin-state geometry, energy, and vibrational frequencies used to calculate the remaining partition functions. Where the triplet state was found to be the ground state, with well-separated unpaired electrons, the singlet state was assumed to be degenerate in energy with the triplet state and both singlet and triplet states were included in the calculation of the partition function. Geometries were optimized using redundant internal coordinates, which has been shown to be preferable for polycyclic systems.22 Transition states were identified by the presence of a single imaginary frequency and the intrinsic reaction coordinate was followed if there was any ambiguity about the nature of the transition state. Transition state theory was used to calculate high-pressure rate constants in the temperature range of 3002000 K and fitted to a two-parameter Arrhenius form in this temperature range. Where the calculated barrier of reaction was close in energy to the endothermicity of the reaction, variational transition state theory was used. In practice, the rate constant was calculated as a function of the breaking bond length (partial geometry optimization in ∼0.1 Å increments), and the rate constant for each temperature was taken to be the minimum. In this case, the reverse barriers are small or negative and a threeparameter Arrhenius form including a T n term was used to describe the rate constant. Where intermediates and transition states were found to be slightly nonplanar, they were generally treated as planar for the purpose of allocating a symmetry number, σ, and the number of optical isomers, m. Because of symmetry in some of the oxide structures, the rate constants reported here sometimes include multiple pathways (e.g., migration both to the left and to the right) and this is highlighted when the situation arises. The quantum chemical calculations were carried out using NWChem4.023 and Gaussian03.24 Results and Discussion Selected geometrical parameters for each of the species discussed are presented in Figure 2. Figures 3-7 contain schematics for the potential surfaces for chemisorption, rearrangement, desorption, and migration reactions. Relative energies for species included in the kinetic analysis are presented in Tables 1-5, and kinetic parameters calculated for each of the reactions are presented in Table 6. Reactions of the Armchair Surface with Molecular Oxygen. Chemisorption. The chemisorption of O2 across two rings

Chemisorption of O2

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Figure 2. Geometrical parameters for stable species and transition states. Bond lengths in angstroms.

on the armchair surface takes place via several low-barrier exothermic steps as shown in Figure 3. The formation of a linear oxide complex (II) via TS1 is the first step in the chemisorption of O2 either across one ring16 or two, and it occurs with a barrier of 18 kJ mol-1 and is 85 kJ mol-1 exothermic. Rotation about the C3-O bond via TS2 has a barrier of 23 kJ mol-1 and is ∼190 kJ mol-1 exothermic, producing a six-membered cyclic peroxide structure (III). Cleavage of the O-O bond via TS3 readily occurs with a barrier of just 15 kJ mol-1 to produce a diketone structure (IV). The overall reaction of (I) + O2 f (IV)

is ∼420 kJ mol-1 exothermic, slightly less than twice the value of 218 kJ mol-1 for the formation of a single ketone.12 This ∼420 kJ mol-1 is available for further reaction via rearrangement, desorption, and migration processes, and a number of processes with overall barriers less than 420 kJ mol-1 are presented in Figures 4-6 with (IV) as the reference energy. Although the difference between 420 kJ mol-1 and 2 × 218 ) 436 kJ mol-1 is clearly within the expected error (∼30 kJ mol-1) of the method, it would be incorrect to conclude that the two ketone groups could be considered to have no

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Sendt and Haynes

Figure 3. Potential energy surface for O2 chemisorption across two armchair rings. 2 and 4 indicate distortions above and below the plane, respectively.

Figure 4. Potential energy surface for oxide rearrangement.

interaction, because there is clearly steric hindrance, as demonstrated by the deviation of the O atoms from the plane in (IV). The chemisorption of O2 across two rings of the armchair surface is substantially less exothermic than that to a single ring of the armchair surface (∼580 kJ mol-1) because in the former two triple bonds are broken, leaving two well-separated unpaired spins, while in the latter only a single triple bond is broken. Because the first step in the chemisorption of O2 on one or two rings of the armchair surface is common to both processes, the nature of the subsequent steps will play a role in determining the final position of the chemisorbed oxygen atoms, if a second adjacent ring is available. The linear complex (II) has unpaired electrons located on C2 and the end oxygen atom. The barrier to C3-O rotation, which results in chemisorption across two adjacent rings, is 23 kJ mol-1, while the barrier to C2-O bond formation, which results in chemisorption on one ring, is reported as ∼65 kJ mol-1.16 Both of these possible second steps are lower in energy than the first step for the addition of O2 (TS1). The latter barrier was previously based on an estimate of the energy at the point where singlet-triplet surface crossing occurs. We now believe that this estimate was too high because a restricted form of the singlet wavefunction was used. The barrier to the surface crossing is expected to be negligible because no stable singlet state for (II) could be located. Because there is therefore sufficient energy for both the C3-O bond rotation and C2-O bond formation to occur, the distribution of

initial rotational energy of O2 (relative to the model char) will play an important role in determining, for a given trajectory, whether chemisorption occurs across two adjacent rings or on a single aromatic ring. In any case, both processes are expected to be relevant. Rearrangement. The diketone species (IV), like a single ketone,12 can undergo an exothermic rearrangement reaction by cleaving C3-C4 and forming a bond between C2-C4, as shown in Figure 4. In the case of a single, isolated ketone group a five-membered ring with an attached ketene group is formed. However, for the diketone, the ketene group is not stable and appears as a shoulder on the potential energy surface, with the rearrangement product a lactone (V). The transition state for this reaction, TS4, is ∼105 kJ mol-1 above the diketone and resembles the formation of a ketene from an isolated ketone;12 however, all attempts to optimize a stable ketene resulted in the geometry collapsing onto that of the lactone (V), which is ∼220 kJ mol-1 more stable than the diketone (cf. ketene being ∼140 kJ mol-1 more stable than an isolated ketone12). The nature of this transition state was confirmed with an IRC calculation. Because of symmetry, the A factor reported in Table 6 describes the formation of both possible mirror-image lactones; the A factor should be halved in order to describe the reaction to form just one of the possible species. The lactone (V) was identified previously16 as a stable species formed following

Chemisorption of O2

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Figure 5. Potential energy surface for CO and CO2 desorption. Energies are relative to (IV).

Figure 6. Potential energy surface for oxide migration. 2 indicates distortions above the plane.

Figure 7. Potential energy surface for oxide migration following CO desorption. 2 indicates distortions above the plane.

migration of chemisorbed species on the armchair surface, although no further reactions were reported. Despite the nonexistence of a ketene as a product of TS4, a diketene species (VI) in which both ketone groups were converted to ketene functionality was found to be stable. The transition state linking the lactone (V) to the diketene (VI), TS5, was found to be ∼150 kJ mol-1 above the lactone, with the diketene ∼35 kJ mol-1 more stable than the lactone. Its nature was confirmed by an IRC calculation. Unlike the lactone, the diketene has no unpaired electrons; hence, the triplet state of

(VI) is an excited state. The diketene (VI) is ∼255 kJ mol-1 more stable than the diketone (IV), which is slightly less stable than if the two functional groups were isolated (2 × 140 ) 280 kJ mol-1). This may indicate that each ketene/fivemembered ring pair has a slight destabilizing effect on the other, although this discrepancy is also within the estimated error of the computational method. The formation of a bond between C3 and C6 via TS6 produces an extra ring in the system (VII). This occurs with a barrier of ∼115 kJ mol-1, and the diketone species formed (VII) is a

5470 J. Phys. Chem. C, Vol. 111, No. 14, 2007 TABLE 1: Relative Energies of Species on the Chemisorption Pathway

species (I) + O2 TS1 (II) TS2 (III) TS3 (IV)

singlet state relative energy (kJ mol-1)

TABLE 5: Relative Energies of Species on the Migration Following CO Desorption Pathway

triplet state relative energy (kJ mol-1)

135 50

0 19 -85 -62 -276 -261 -418

-62 -276 -263 -419

Sendt and Haynes

species

singlet state relative energy (kJ mol-1)

triplet state relative energy (kJ mol-1)

(X) TS17 (XVI) TS18 (XVII) TS19 (XVIII) TS20 (XIX)

0 197 174 208 109 131 34 106 -115

1 197 175 206 107 136 47 114 -74

TABLE 2: Relative Energies of Species on the Rearrangement Pathway

species

singlet state relative energy (kJ mol-1)

triplet state relative energy (kJ mol-1)

(IV) TS4 (V) TS5 (VI) TS6 (VI)

-1 108 -221 -73 -255 -138 -260

0 103 -218 -180 -157

TABLE 3: Relative Energies of Species on the Desorption Pathway

species (IV) (V) TS7 (VIII) TS8 (IX) + CO2 TS9 (X) + CO TS10 + CO (XI) + CO (VI) TS11 TS12 + CO (XII) + 2CO a

singlet state relative energy (kJ mol-1)

triplet state relative energy (kJ mol-1)

-1 -221

0 -218 2 3 76 49 58 60 132 -30 -180 -34a 171a 175

76 50 57 59 133 -28 -255 -33a 167a 174

Geometry obtained using Variational TST at 1000 K.

TABLE 4: Relative Energies of Species on the Migration and Subsequent Rearrangement Pathway

species (IV) TS13 (XIII) TS14 (XIV) TS15 (XV) TS16 (V)

singlet state relative energy (kJ mol-1) -1 144 111 148 -163 54 -221

triplet state relative energy (kJ mol-1) 0 138 105 142 58 57 -163 49 -218

closed-shell system that is approximately isoenergetic with (VI). An IRC calculation was used to confirm the connection between TS6 and stable (VI) and (VII). Because ring strain is present in (VI), as evidenced by the long C3-C6 of 1.65 Å, it is expected to be less stable in larger chars, and because of its nonplanarity, it will also be less stable in multilayer systems.

TABLE 6: Kinetic Parameters for Reactions reaction

forward A factora

forward Ea b

reverse A factora

reverse Ea b

(I) + O2 T (II) (II) T (III) (III) T (IV) (IV) T (V) (V) T (VI) (VI) T (VII) (V) T (VIII) (VIII) T (IX) + CO2 (V) T (X) + CO (X) T (XI) (VI) T (XI) + CO (XI) T (XII) + CO (IV) T (XIII) (XIII) T (XIV) (XIV) T (XV) (XV) T (V) (X) T (XVI) (XVI) T (XVII) (X) T (XVIII) (XVIII) T (XIX)

5.7 × 1012 d 3.5 × 1012 1.3 × 1014 6.3 × 1013 d 9.3 × 1013 3.9 × 1012 1.4 × 1014 3.9 × 1014 1.1 × 1017 5.4 × 1013 4.5 × 1015 d 4.7 × 1014 4.1 × 1013 d 1.1 × 1013 2.8 × 1012 2.3 × 1014 e 1.9 × 1013 5.1 × 1012 1.6 × 1012 e 3.8 × 1013 e

30 23 19 108 154 119 227 79 289 136 227 205 142 39 0.2 220e 200 33 136e 72e

4.8 × 1014 3.6 × 1014 d 5.1 × 1012 4.9 × 1014 5.0 × 1012 2.6 × 1014 1.4 × 1012 3.9 × 1012 2.5 × 107 × T1.636 c 1.7 × 1013 2.2 × 108 × T1.155 c 2.6 × 108 × T1.188 c 1.5 × 1013 6.0 × 1012 1.1 × 1014 3.3 × 1014 f 3.6 × 1012 9.8 × 1012 4.6 × 1013 e 5.3 × 1012 e

110 219 157 330 185 129 -0.1 40 0.9 225 -0.9 -2.7 36 86 227 274 f 23 102 93e 231e

a Units: s-1 for unimolecular reactions; cm3 mol-1 s-1 for bimolecular reactions. b Ea in kJ mol-1. c This reaction required a threeparameter fit due to variational TST. d This A factor contains a factor of 2 because of symmetry, accounting for reaction in both directions. The A factor should be halved if a single process is to be considered. e Values from ref 16. f Corrected values from ref 16.

Desorption. Although direct CO loss from (IV) does not occur, desorption of CO and CO2 from (V) and of CO from (VI) is possible with barriers well under 420 kJ mol-1, as shown in Figure 5. Desorption of CO from (VII) will occur with (VI) as an intermediate. Desorption of CO2 from (V) is possible because C3 is bound to two oxygen atoms. Initially, the C6 bond is cleaved in TS7 with a barrier of 220 kJ mol-1 to form a chemisorbed CO2 on a five-membered ring (VIII). The chemisorbed CO2 then desorbs via TS8 with a further barrier of ∼75 kJ mol-1, leaving behind a carbon substrate with a defect consisting of a five-membered ring (IX). The overall CO2 desorption reaction is ∼265 kJ mol-1 endothermic. Because the barrier to reforming the lactone (V) from (VIII) is negligible, (VIII) will not be stabilized, and the reaction (V) f (IX) + CO2 will appear as a single step. For modeling purposes, it is appropriate to use the A factor for (V) f (VIII) from Table 5 with the overall activation energy of 300 kJ mol-1 to describe the kinetics of the single step. Loss of CO from (V) via TS9 involves cleavage of both the C3-O and C2-C3 bonds, with a barrier of 278 kJ mol-1. This transition state was a maximum in the PES, and the zero-point vibrational energy correction causes the barrier to be slightly lower than the endothermicity of the reaction. The char fragment

Chemisorption of O2 remaining (X) has a ketone group that cannot desorb a further CO directly, but can undergo rearrangement via TS10 to produce a ketene group (XI), in an analogous reaction to an isolated ketone.12 The barrier for this rearrangement is ∼75 kJ mol-1, and the rearrangement is ∼90 kJ mol-1 exothermic. Desorption of CO from the diketene (VI) also produces (XI). The transition state for this reaction, TS11, was located using variational TST because there was no barrier to the reverse reaction. Desorption of CO from (VI) is ∼225 kJ mol-1 endothermic, which is significantly less than the 296 kJ mol-1 required to desorb CO from a single five-membered ring/ketene pair.12 The A factor in Table 6 describes the desorption of either CO molecule. Further loss of CO from (XI) via TS12 was also studied using variational TST. This reaction is ∼205 kJ mol-1 endothermic and produces (XII), a char fragment with two vacant sites located on five-membered rings. It is expected that these vacant sites, as well as one each on (IX), (X), and (XI) would behave like a zigzag site with respect to further chemisorption reactions; that is, the addition of O2 would be barrierless. Although the reactions (VI) f (XI) + CO and (XI) f (XII) + CO are similar processes, the A factors reported in Table 6 differ by an order of magnitude. Most of this discrepancy (a factor of 8) arises because of the difference in (a) the electronic degeneracies and (b) the rotational symmetry numbers of (VI) and (XI). No direct pathway for CO desorption from (VII) was observed. Given the relatively low barrier to rearrangement to (VI), it is expected that loss of CO from (VII) will occur via the sequence (VII) f (VI) f (XI) + CO. Gasification of both C3 and C6, either by (V) f (X) + CO f (XI) + CO f (XII) + 2CO or (VI) f (XI) + CO f (XII) + 2CO, requires substantially less energy than the energy released during chemisorption and hence would be energetically accessible processes. Migration. Because unpaired electrons are located on both C2 and C7 of (IV), migration of an oxygen atom along the armchair surface can proceed without breaking a triple bond, as shown in Figure 6. The barrier to formation of a bond between C2 and the migrating oxygen atom, via TS13 (confirmed with and IRC calculation), is ∼140 kJ mol-1, only slightly higher than the equivalent barrier of ∼130 kJ mol-1 for an isolated ketone 12. The A factor reported in Table 6 describes the migration of both possible O atoms because of symmetry. The epoxide intermediate formed, (XIII), is unlikely to be stabilized because it is just ∼25 kJ mol-1 below either TS13 (returning to (IV)) or TS14 (completion of the oxide migration). The product of oxide migration, (XIV), is also a diketone with oxygen atoms bound to C2 and C6 and is ∼60 kJ mol-1 less stable than the diketone (IV). The diketone (XIV) can also be formed from the chemisorption of O2 on a single ring of the armchair surface, and has been described earlier.16 Although further migration along the armchair surface is possible (either to another adjacent ring or migration of the second O atom to C7), it is most likely that a bond will form between the second O atom and C3 (TS15) because of the negligible barrier to this reaction. The transition state for this reaction (TS15) is just 1 kJ mol-1 higher than (XIV) on the electronic PES with a small increase in the C6-O bond length and a small decrease in the C3-O bond length when compared with (XIV). The furan formed, (XV), and its subsequent reactions, including its conversion via TS16 to the lactone (V), has been described previously16 and is included here for completeness.

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5471 Migration Following CO Desorption. Following CO desorption from the lactone (V), the oxygen atom on C6 of (X) can migrate to either C2 or C7 (Figure 7). Migration to C7 occurs via an epoxide intermediate (XVI). The formation of a bond between C7 and the O atom via TS17 has a barrier of ∼195 kJ mol-1, while the barrier to cleaving the C6-O bond (TS18) is just ∼10 kJ mol-1 higher. Because the intermediate (XVI) is ∼175 kJ mol-1 above (X), it is unlikely to be stabilized. The ketone formed (XVII) is ∼105 kJ mol-1 less stable than (X). Alternatively, the migration of the oxygen atom to C2 occurs via a furan intermediate (XVIII) to produce a ketone group on a five-membered ring (XIX). Overall, this pathway is 115 kJ mol-1 exothermic. However, given the heights of the two barriers TS19 and TS20 on this pathway, the intermediate furan (XVIII) could be stabilized. The species in this reaction were identified earlier16 and are included here for a comparison between the two possible migration pathways. Fate of Oxygen Following Chemisorption on the Armchair Surface. The chemisorption of O2 on the armchair surface has a computed activation energy of 30 kJ mol-1, regardless of whether it adds to a single ring16 or across two aromatic rings. Experimental studies of the kinetics of oxygen chemisorption suggest that the first oxide formation occurs with an activation energy comparable with this. Bansal et al.25 found the activation energy for chemisorption of O2 on Graphon (graphitized carbon black) to be in the range of 15-50 kJ mol-1; Waters et al.26 obtained 30-50 kJ mol-1; Brown et al.2 reported ∼20 kJ mol-1 for Spherocarb; Feng and Bhatia27 obtained values