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Quantum Chemical Prediction of Reaction Pathways and Rate Constants for the Reactions of O-x (x=1 and 2) with Pristine and Defective Graphite (0001) Surfaces

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Published: December 07, 2011

ARTICLE

pubs.acs.org/JPCC

Quantum Chemical Prediction of Reaction Pathways and Rate

Constants for the Reactions of O

x

(

x = 1 and 2) with Pristine and

Defective Graphite (0001) Surfaces

S. C. Xu,

†,||

Hui-Lung Chen,

and M. C. Lin*

,†,§

Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322,

United States

Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei, 111, Taiwan

§Institute of Molecular Science, Department of Applied Chemistry, National Chiao Tung University, Hsichu, Taiwan 300

b

S Supporting Information

ABSTRACT:

We present reaction pathways for adsorption reactions of the O atom and O2molecule in the pristine and monovacancy defective

graphite (0001) based on quantum chemical potential energy surfaces (PESs) obtained by the dispersion-augmented density-functional tight-binding (DFTB-D) method. We use a dicircumcoronene C96H24(L0D) graphene slab as the pristine graphite

(0001) model and dicircumcoronene C95H24 (LIV) as the graphite (0001) monovacancy defect model. We found that the

adsorption reactions of O and O2on the L0D surface can produce defects on the graphite surface. O can yield CO, while O2can yield

both CO and CO2molecules. The adsorption reactions of the O and O2on the LIV surface can produce a 2-C defective graphite

surface and CO, and CO and CO2, respectively. The O and O2more readily oxidize the defected surface, LIV, than the defect-free

surface, L0D. On the basis of the computed reaction pathways, we predict reaction rate constants in the temperature range between 300 and 3000 K using RiceRamspergerKasselMarcus (RRKM) theory. High-temperature quantum chemical molecular dynamics simulations at 3000 K based on on-the-fly DFTB-D energies and gradients support the results of our PES studies.

1. INTRODUCTION

Graphite is an important surface lining material for systems operating under high temperature and high pressure and is being tested as surface material for rocket nozzles1and for plasma divertors in nuclear fusion technology.2Despite the im-portance of these technologies, not much is known about the high-temperature, high-pressure (high-T,P) processes causing graphite erosion due to reactions with oxidizing agents from fuel combustion, most importantly H2O and CO2.1Recently,

numer-ical simulations have given insight into the dynamics of reactive turbulent combustions that stress the importance of OxHy(x =

1,2, y = 02) species.3

With most modern fuels, not only OxHy

but also COxand NOx(x = 1,2) are exhaust species that are the

most likely candidates for inducing graphite erosion. In addition, an experimental investigation for the nitrogen adsorption on

defects of carbon black surfaces at 77 K by surface spectroscopy (SIMS)4was reported and compared with theoretical results obtained by the grand canonical Monte Carlo simulations.4 It was found that nitrogen adsorption was sensitive to the de-fects and was enhanced at low temperature and high pressure. Recently, the chemical functionalization of semiconducting graphene nanoribbons (GNRs) with StoneWales (SW) de-fects by carboxyl (COOH) groups has been investigated using density functional theory.5It was found that the geometrical structures and electronic properties of the GNRs changed signi-ficantly, and the electrical conductivity of the system could Received: July 20, 2011

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The Journal of Physical Chemistry C ARTICLE

be considerably enhanced by monoadsorption and double adsorption of COOH, depending sensitively upon the axial concentration of SW defects COOH pairs (SWDCPs). How graphite defect formation occurs under the high-T,P con-ditions is still not completely understood. Knowledge of these processes is a prerequisite for the improvement of both graphite-based nozzle lining material and the chemical com-position of the fuel.

Some time ago, we developed a methodology that allows us to systematically investigate a priori high-T,P dissociative adsorption processes on the basal graphite surface.6This meth-odology uses for the description of the graphite (0001) surface the dicircumcoronene C96H24finite-size graphene flake,

op-tionally stacked up to trilayers, and employs density functional theory (DFT) as well as density functional tight binding7,8 aug-mented with London dispersion (DFTB-D)9,10quantum chemical Figure 1. Local structures for the dissociative adsorption of Ox(x = 1 and 2) on the pristine graphite L0D model calculated at the DFTB-D level.

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methods for the exploration of the dissociative adsorption poten-tial energy surfaces (PESs). Beginning from stationary points along the reaction pathways, DFTB-D-based molecular dynamics (QM/MD) are then performed at constant temperature to check whether most important reaction pathways have been considered, andfinally reaction rate constants are predicted for a wide range of temperatures, based on the RiceRamspergerKasselMarcus (RRKM) theory.11,12Using this methodology, we predicted high-T,P dissociative adsorption processes and their reaction kinetics for H2O, 6 COx, 13,14 and NOx 14 (x = 1,2) on the defect-free graphite (0001) surface and those of OHx,15COx,16and NOx17

(x = 1,2) species on the basal face of graphite containing mono-vacancy defects. In short, we found that on an intact graphene surface H2O6and even more so the radical species NO and NO2

can cause irreversible oxide defects on the graphite surface,14 with gaseous CO leaving as a thermodynamically highly stable and highly entropic erosion product. Likewise, the OH radical readily leaves a hydrogenated monovacancy defect (model L1H1Vin ref 15) behind. We further noticed a pathway for nitridation in the case of NO attack.14In the investigation of dissociative adsorptions on various defective models, we found that attacks on the monovacancy defect have generally much lower barriers than corresponding reactions on the defect-free graphite surface.1517In the case of reactions of CO and CO2with monovacancy defects, we found that the

CO molecule reacts readily with the monovacancy defects and partially“heals” the carbon hexagon network leading to the formation of a stable epoxide, whereas CO2oxidizes the defect

Figure 2. Schematic drawing of the potential energy surfaces for the dissociative adsorption pathways of O on the pristine graphite L0D model calculated at the DFTB-D level.

Figure 3. Schematic drawing of the potential energy surfaces for the dissociative adsorption pathways of O2on the pristine graphite L0D model

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The Journal of Physical Chemistry C ARTICLE

via a dissociative adsorption pathway following CO elimi-nation.16In the case of reactions of NO and NO2with

mono-vacancy defects, we found that the reactions of NOx on the

monodefective graphite surface were initiated by rapid associa-tion processes with negligible barriers, leading to nitridaassocia-tion and oxidation of the graphite surface and eventually producing gaseous COx, NO, and CN species leaving from an even more

defective graphite surface.17

The interactions and reactions for Ox(x = 1 and 2, atomic or

molecular oxygen) with graphite have brought great interest to scientists because of important applications of graphite since the 1960s. Experimentally, Gleit et al. reported the reaction kinetics of atomic oxygen with graphitic carbon in the temperature range of 215300 °C.18

Blyholder and Eyring reported the kinetics of graphite oxidation using O2in the temperature range of 600

1300°C.19,20Bennett et al. have investigated the vacuum ultra-violet photodissociation of molecular oxygen physisorbed on graphite.21Theoretically, Incze et al. reported thefirst-principles studies of the atomic oxygen adsorption on the (0 0 0 1) graphite

surface and oxidation of graphite by atomic oxygen.22,23 Lamoen et al.24and Sorescu et al.25reported theoretical studies of molecular oxygen adsorption on graphite. Sendt and Haynes reported density functional study of the chemisorption of O2on

the zigzag surface of graphite.26Recently, Paci et al.27reported the theoretical direct dynamics based on density functional theory (DFT) and beam-surface scattering experiments for study-ing the reaction between O(3P) and highly oriented pyrolytic graphite (HOPC). They found that the incoming O atoms can react with the graphite surface and produce CO by direct collision reaction.

In this work, we focus on the adsorption reactions of Ox

(x = 1 and 2) with the single-layer pristine model L0D (C96H24)

and the monovacancy defect model L1 V (C95H24) using the

model systems with the same composition and names as in our previous studies of COxand NOx on the pristine and

mono-vacancy defect graphite surfaces,14,16,17which are the parts of a series of studies for the adsorption reactions of gas species on the graphite surface.

Figure 4. Local structures for the dissociative adsorption of Ox(x = 1 and 2) on the defective graphite L1V model calculated at the DFTB-D level. The

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2. COMPUTATIONAL METHODOLOGY

In our previous studies of the reactions of COx16 and

NOx17 (x = 1,2) on the defective model surfaces, we

com-pared the differences in the geometry parameters and ener-gies of the PESs using the B3LYP/6-31+G(d) and DFTB-D methods and those on the different defective graphite S1V, M1V, and L1V models at the DFTB-D level. By compar-ing the data uscompar-ing the DFTB-D method with those uscompar-ing the B3LYP/6-31+G(d) method computed for COx16and NOx17

(x = 1,2) with the S1V model, we found that the average difference between the methods is 6.5 kcal/mol. In this work, the PES exploration of the reaction of Ox on the

single-layer pristine model L0D (C96H24) and monovacancy defect

model L1V (C95H24) was performed using the DFTB-D level

of theory. The location of transition states with the DFTB-D method is presently only possible for a finite molecular system. The single-layer L0D and L1V models are therefore offinite size and are terminated by hydrogen at the edges. For the investigation of small-molecule interactions with graph-ite surfaces, cluster calculations were employed as well, with members of the coronene family as model systems for in-dividual graphite layers.28,29Our previous study6found that the DFTB-D method with thefinite graphite model was able to reproduce the experimental bulk graphite CC bond length, interlayer distances, and binding energies of bulk graphite reasonably well.

Figure 5. Schematic drawing of the potential energy surfaces for the dissociative adsorption pathways of O on the defective graphite L1V model calculated at the DFTB-D level.

Figure 6. Schematic drawing of the potential energy surfaces for the dissociative adsorption pathways of O2on the defective graphite L1V model

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The Journal of Physical Chemistry C ARTICLE

QM/MD simulations at the DFTB-D level were carried out for the dissociation product species of Oxreactions with the L0D

and L1V models, using a Verlet algorithm with a 0.12 fs time step up to 400 steps. This time interval was checked to conserve total energy to within 3 kcal/mol accuracy during microcanonical dynamics with temperature-adjusted initial velocities for 3000 K. In all production trajectories, a target temperature of 3000 K was maintained by using the scaling of velocities approach with 20% overall scaling probability. Although at very high temperatures electronic excitations should be abundant, internal conversion and thereby quench-ing of electronic excitations due to vibrational excitations is very efficient.30

We therefore give an approximation that only the electronic ground state in our QM/MD simulations is considered.

For the quantum chemistry calculations, the GAUSSIAN 03 revision C.131was used, and the“external” keyword was employed to perform DFTB-D with a stand alone code. In addition, the reaction rate constants for the adsorption and dissociation reactions have been determined using the Chem-Rate program.32

3. RESULTS AND DISCUSSION

3.1. Adsorption Reaction of Oxon Pristine Graphite.The

optimized molecular structures and potential energy surfaces (PESs) for the adsorption reactions of Ox(x = 1 and 2) on the

C96H24 (L0D) surface are presented in Figures 1, 2, and 3,

respectively. For the reaction of O + L0D shown in Figure 2, the O atom can react directly with the two carbon atoms on the L0D surface as a stable adduct L0D-O (an epoxide structure), with a binding energy of 78.7 kcal/mol. Then, the CC bond in the epoxide ring cleaves to form the L0D-O-P1 adduct by

over-coming a 7.5 kcal/mol barrier at L0D-O-TS1. Finally, L0D-O-P1

can also dissociate to give a gaseous CO and defective graphite L1V. This step of the reaction is endothermic by 96.9 kcal/mol and occurs with a 103.7 kcal/mol energy barrier at L0D-O-TS2.

Therefore, the dissociative adsorption of the O atom on the L0D surface can defect the graphite surface and produce CO. For the reaction of O2+ L0D shown in Figure 3, O2first absorbs

physi-cally on the L0D surface as a van der Waals complex L0D-O2

with a 6.3 kcal/mol binding energy. Then, two O atoms combine with the two closest carbon atoms on the L0D surface as a stable adduct L0D-O2-P1. This step of the reaction is endothermic by

22.9 kcal/mol and occurs with a 41.0 kcal/mol barrier at L0D-O2-TS1. One of O-C-a’s in L0D-O2-P1migrates and combines

with another O-a to form a-O-C-O-a (“a” is defined as car-bon atom site) as L0D-O2-P2. This process is endothermic by

7.4 kcal/mol and occurs with a 10.5 kcal/mol barrier at L0D-O2

-TS2. The CO in L0D-O2-P2can dissociate to give off a gaseous

CO by overcoming a barrier of 31.0 kcal/mol at L0D-O2-TS9

and finally form a defective site LIV-O. At the same time, the OCO in L0D-O2-P2can also dissociate to give off a gaseous

CO2by overcoming a barrier of 39.1 kcal/mol at L0D-O2-TS3

and finally form a defective site LIV. Furthermore, the O-a in L0D-O2-P1 can shift from one site to a nearby site by

isomerization reactions. For example, L0D-O2-P1 can

iso-merize to L0D-O2-P3with a barrier of 30.8 kcal/mol at

L0D-O2-TS4 or to L0D-O2-P4 with a barrier of 29.9 kcal/mol

at L0D-O2-TS5. Further, L0D-O2-P3can isomerize to

L0D-O2-P2 with a barrier of 12.6 kcal/mol at L0D-O2-TS6, to

L0D-O2-P4 with a barrier of 19.3 kcal/mol at L0D-O2-TS7,

or to L0D-O2-P5with a barrier of 29.0 kcal/mol at L0D-O2-TS8.

In summary, the dissociative adsorption of O2 on the L0D

surface can create a bigger defected ring on the graphite surface and produce a CO2molecule.

3.2. Adsorption Reaction of Oxon Defective Graphite.The optimized molecular structures and potential energy surfaces (PESs) for the adsorption reactions of Ox(x = 1 and 2) on the

C95H24defective model (L1V) surface are presented in Figures 4,

5, and 6, respectively. For the O atom on the L1V surface shown in Figure 5, it reacts directly with defective graphite L1V to form a stable adduct L1V-O with a binding energy of 172.6 kcal/mol. Then, L1V-O isomerizes to adduct L1V-O-P1with an

endother-micity of 32.7 kcal/mol. This isomerization occurs by a 47.9 kcal/mol barrier at L1V-O-TS1. Finally, L1V-O-P1can dissociate to give

off a gaseous CO and 2-C defective graphite L2V by overcoming a 18.4 kcal/mol barrier at L1V-O-TS2. This step of the reaction is

exothermic by 27.7 kcal/mol. Therefore, the dissociative adsorp-tion of the O atom on the LIV surface can create a 2-C defective graphite surface more easily than that on the L0D and pro-duce CO. For the O2on the L1V surface shown in Figure 6,

the O2 molecule reacts directly with the defective graphite

L1Vto form a stable adduct L1V-O2with a binding energy of

78.8 kcal/mol. Then, one of the O-C-a’s in L1V-O2migrates and

combines with the other O-a to form a-O-C-O-a (“a” is defined as a carbon atom site) as L1V-O2-P1. This process is exothermic by

33.9 kcal/mol and occurs with a 14.2 kcal/mol barrier at L1V-O2-TS1. The OCO in L1V-O2-P1can dissociate to give off a

gaseous CO2by overcoming a barrier of 8.8 kcal/mol at L1V-O2

-TS2 and form the 2C-defective graphite L2V. This process is

exothermic by 33.2 kcal/mol. At the same time, L1V-O2-P1can

isomerize to L1V-O2-P2with a barrier of 34.2 kcal/mol at

L1V-O2-TS3. This process is exothermic by 86.8 kcal/mol. Finally,

The CO in L1V-O2-P2 can dissociate to give off a gaseous

CO by overcoming a barrier of 81.9 kcal/mol at L1V-O2-TS4and

form the defective graphite L2V-O. This process is endothermic by 51.1 kcal/mol. Therefore, the dissociative adsorption of O2

on the L1V surface can create defective graphite surface L2V or L2V-Omore easily than that on the L0D and produce CO2

and CO.

3.3. Reaction Rate Constants Predicted by the RRKM Theory.The rate constants for these gassurface reactions have been computed with the RRKM theory using the ChemRate code.32The predicted rate constants of the Ox(x = 1 and 2)

adsorption reactions on the pristine graphite surfaces are as follows

O þ L0D f L0D-O ð1Þ

f CO þ L1V ð2Þ

f CO þ L1V-O ð4Þ

f CO2 þ L1V ð5Þ

The predicted rate constants for the reactions in the temperature range from 300 to 3000 K can be represented by the expressions

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The Journal of Physical Chemistry C ARTICLE in units of cm3/s k1¼ 5:29  107 T1:20expð  236=TÞ at T ¼ 300  1100 K, k1¼ 1:59  10198 T58:4expð  78700=TÞ at T ¼ 1100  3000 K, k2¼ 7:10  1063 T15:4expð  16300=TÞ at T ¼ 300  1100 K, k2¼ 2:40  10240 T71:7expð  111500=TÞ at T ¼ 1100  3000 K, k3¼ 4:29  1014expð  19700=TÞ, k4¼ 3:03  1012expð  30700=TÞ, k5¼ 1:27  1011expð  34800=TÞ

The predicted rate constants of the Ox (x = 1 and 2)

ad-sorption reactions on the defective graphite surfaces are

as follows

O þ L1V f CO þ L2V ð6Þ

O2 þ L1V f L1V-O2 ð7Þ

f CO2 þ L2V ð8Þ

f CO þ L2V-O ð9Þ

The predicted rate constants for the reactions in the temperature range from 300 to 3000 K can be represented by the expressions Figure 7. Dynamics structures for the QM/MD simulations of dissociation products of Oxon the L0D and L1V models at 3000 K using the DFTB-D

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The Journal of Physical Chemistry C ARTICLE in units of cm3/s k6¼ 4:36  1032 T15:3expð  9218=TÞ at T ¼ 300  1100 K, k6¼ 1:20  10221 T70:3expð  62900=TÞ at T ¼ 1100  3000 K, k7¼ 6:51  1063 T0:11expð340=TÞ at T ¼ 300  1100 K, k7¼ 7:30  10178 T53expð  65000=TÞ at T ¼ 1100  3000 K, k8¼ 9:13  1083 T21:9expð  6800=TÞ at T ¼ 300  1100 K, k8¼ 2:00  10230 T68:0expð  100900=TÞ at T ¼ 1100  3000 K, k9¼ 3:03  1083 T21:9expð  18400=TÞ at T ¼ 300  1100 K, k9¼ 2:08  10229 T68:4expð  112400=TÞ at T ¼ 1100  3000 K

The rate constants (ki) for the adsorption reactions of Oxon

graphite are defined by33

d½Xsurf=dt ¼ kiðθ=AsÞ½Xg

which has the unit of a flux, molecule/(cm2 s). In the rate equation,θ represents the fraction of available surface sites; Asis

the surface area; and [X]gis the gas phase concentration of Oxin

molecules/cm3.

In comparison, the rate constant for the adsorption of the O on the monodefective graphite surface for creating CO at 1000 K is 5.75  105 times faster than that on the defect-free graphite surface but 9.16 106times slower than that of the NO on the monodefective graphite surface.17The rate constant of the O2on

the monodefective graphite surface for creating CO2at 1000 K

is 5.21  106 times faster than on the defect-free graphite surface but is 3.00 1016times slower than that of the NO2on

the monodefective graphite surface.17Therefore, by predicting rate constants of Ox on the graphite surface, we can learn

quantitatively how fast the reactions of Oxon the graphite surface

are, providing us a guideline for related experimental investigations. 3.4. QM/MD Simulations of OxDissociative Products on the Pristine and Defective Graphite Surfaces. QM/MD constant-temperature simulations of the dissociative products of Oxon the surfaces of the L0D and L1V models as described

above were carried out using the DFTB-D quantum chemical potential at a temperature of 3000 K. When the temperature is higher than 3000 K, the enthalpy and heat capacity of graphite increase slowly,34so 3000 K is a standard temperature for simu-lating the high-temperature property of graphite, In the follow-ing, we describe the details of these simulations.

QM/MD simulations starting from the dissociative products of L0D-O-P1and L0D-O2-P2for Oxon the pristine graphite

surface were carried out, and some snapshots of trajectories are shown in Figure 7. As can be seen from Figure 7, for L0D-O-P1,

the O atom has formed strong bridge bonds with graphitic car-bons. After 10 fs, the OC bridge bonds change a little, and after 20 fs, the O atom moved up on the surface and still sticks with surface carbons. The local graphite recovered to 66 pristine rings. In the whole simulation, we did not see CO fragmenting out because the dissociation barrier for producing CO is too high. For L0D-O2-P2, the CO bond in OCO was broken after 10 fs,

and after 20 fs CO moved up to the surface giving the gaseous CO product.

The QM/MD simulations for the dissociative products of L1V-O-P1and L1V-O2-P1for Oxon the defective graphite

sur-face are also shown in Figure 7. For L1V-O-P1, CO in L1V-O-P1

dissociated away from the surface and produced gaseous CO after 5 fs. This result is in good agreement with the previous study by the DFT dynamics simulation,27which has shown that the O atoms on the single-atom vacancy graphene surface can produce CO. For L1V-O2-P1, CO2moved up to the surface after 10 fs,

and after 20 fs, CO2 dissociated away from the surface and

produced gaseous CO2. In summary, our results from the

QM/MD simulations of Oxon the pristine and defective graphite

as well as graphene surfaces have indicated that reaction paths in the simulations easily followed the minimum energy paths shown in the dissociative adsorption PESs.

4. CONCLUSIONS

The reactions of the O atom and O2molecule on the pristine

and monovacancy defective graphite surface were calculated using the DFTB-D method. The PESs for the adsorption reac-tions of Ox(x = 1 and 2) on the pristine and defective graphite

were computed, and intermediate structures, transition states, and low-lying product structures were characterized. For the adsorption of O and O2on the pristine graphite, we found that

Oxcan defect the graphite surface. O can yield CO, while O2can

yield CO and CO2molecules. For the adsorption of the O and O2

on the LIV surface, we found that they can produce a 2-C defec-tive graphite surface and CO and CO and CO2, respectively. The

O and O2more readily oxidize the defected surface, LIV, than the

defect-free surface, L0D. On the basis of the TS structures, fre-quencies, and energetics, we also predict the rate constants of the adsorption reactions of Oxon the pristine and defective graphite

using RRKM. Quantum chemical molecular dynamics (QM/MD) simulations at the DFTB-D level of theory were also carried out for reaction products to elucidate reverse adsorption path-ways and to further explore the PESs accessible at T = 3000 K. The QM/MD simulations of Oxon the pristine and defective

graphite surfaces are consistent with the PES results. Overall, we conclude that Oxcan oxidize much more easily the monovacancy

defective graphite (0001) surface than the defect-free graphite and produce gaseous CO and CO2species. Chain reactions of

these product species may occur afterward, leading to further erosion of the graphite surface.

’ ASSOCIATED CONTENT

b

S Supporting Information. Table S1 lists names, the DFTB-D total energies, and imaginary frequencies for all structures of the reaction pathways, and Table S2 lists corresponding Carte-sian coordinates. Figure S1 shows the models S1V, M1V, and L1V. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author *E-mail: chemmcl@emory.edu. Present Addresses )

School of Chemistry and Biochemistry, Georgia Institute of Technology. E-mail: sxu65@mail.gatech.edu.

’ ACKNOWLEDGMENT

We gratefully acknowledgefinancial support from the Office of Naval Research under a MURI grant. MCL acknowledges

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The Journal of Physical Chemistry C ARTICLE

support from the Taiwan Semiconductor Manufacturing Co. (TSMC) and the National Science Council of Taiwan for a Distinguished Visiting Professorship at the National Chiao Tung University in Hsinchu, Taiwan. We thank the Cherry L. Emerson Center for Scientific Computation at Emory University for valu-able computer time.

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數據

Figure 3. Schematic drawing of the potential energy surfaces for the dissociative adsorption pathways of O 2 on the pristine graphite L0D model
Figure 4. Local structures for the dissociative adsorption of O x (x = 1 and 2) on the defective graphite L1V model calculated at the DFTB-D level
Figure 6. Schematic drawing of the potential energy surfaces for the dissociative adsorption pathways of O 2 on the defective graphite L1V model

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