16558 Phys. Chem. Chem. Phys., 2012, 14, 16558–16565 This journal is c the Owner Societies 2012

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 16558–16565

Site-dependent catalytic activity of graphene oxides towards oxidative

dehydrogenation of propanewzShaobin Tang

aand Zexing Cao*

b

Received 27th April 2012, Accepted 19th June 2012

DOI: 10.1039/c2cp41343d

Graphene oxides (GOs) may offer extraordinary potential in the design of novel catalytic systems

due to the presence of various oxygen functional groups and their unique electronic and

structural properties. Using first-principles calculations, we explore the plausible mechanisms for

the oxidative dehydrogenation (ODH) of propane to propene by GOs and the diffusion of the

surface oxygen-containing groups under an external electric field. The present results show that

GOs with modified oxygen-containing groups may afford high catalytic activity for the ODH of

propane to propene. The presence of hydroxyl groups around the active sites provided by

epoxides can remarkably enhance the C–H bond activation of propane and the activity

enhancement exhibits strong site dependence. The sites of oxygen functional groups on the GO

surface can be easily tuned by the diffusion of these groups under an external electric field, which

increases the reactivity of GOs towards ODH of propane. The chemically modified GOs are thus

quite promising in the design of metal-free catalysis.

1. Introduction

Graphene and other low-dimensional sp2-hybridized carbon

nanomaterials have attracted considerable attention owing to

their outstanding structural and electronic properties and

potential applications in nanoscale electronics.1–3 These nano-

carbon materials and carbon-based composites, as metal-free

catalysts, also show novel activity in facilitating synthetic

transformations.4–9 For example, carbon nanotubes4 (CNTs)

and fullerenes5 have recently been shown to exhibit fascinating

catalytic activity towards the oxidative dehydrogenation (ODH)

of n-butane and hydrogenation of nitrobenzene, respectively.

There is a continued interest in searching for catalysts consisting

of inexpensive materials, but retaining high activity, in metal-

free catalysts.10,11

The structural and chemical modification of nanocarbon

materials as catalysts, such as introduction of defects and

chemical functional groups, can offer diverse active sites and

enhance catalytic activity and selectivity. Graphene oxides (GOs),

the derivatives of graphene modified by oxygen-containing

functional groups, have emerged as a new class of carbon-

based nanoscale materials with broad application.12–17 Our

recent density functional calculations showed that the presence

of oxygen groups in GOs can improve the interactions of

nitrogen oxides and ammonia with graphene due to the

formation of hydrogen bonds and covalent bonds between

molecules and the surface.16,17 The recent experimental studies

by Bielawski and co-workers18 revealed that GOs can serve as

a high performance catalyst (called a carbocatalyst) for oxidation

of alcohols and hydration of alkynes in the absence of metals. It

was assumed that the plausible catalytic mechanisms18,19 distinctly

differ from those in Suzuki–Miyaura coupling reactions20 and

methanol electro-oxidation,21,22 induced by GOs impregnated

by palladium nanoparticles, because the active centers are

carbon atoms for GOs, but the latter is a supported metal.

The oxidative dehydrogenation of alkanes, which is an

exothermic reaction overall, is an attractive alternative to the

conventional dehydrogenation. However, many current ODH

catalysts have limited activity and/or poor selectivity.23 On the

contrary, the high reactivity of GOs towards ODH of alkane

could be expected due to the presence of the various oxygen-

containing active sites. Despite considerable research for GOs,

the chemical structure of active sites and catalytic mechanisms

of GOs as metal-free catalysts for C–H bond activation are

still unclear, both experimentally and theoretically. Herein the

efficient catalytic activity of GOs for the ODH of propane to

propene, the effect of surrounding oxygen groups on catalysis,

and the detailed mechanisms were explored using density

functional theory (DFT).

a Key Laboratory of Organo-Pharmaceutical Chemistry of JiangxiProvince, Gannan Normal University, Ganzhou 341000, China

b State Key Laboratory of Physical Chemistry of Solid Surfaces andFujian Provincial Key Laboratory of Theoretical and ComputationalChemistry, College of Chemistry and Chemical Engineering, XiamenUniversity, Xiamen 361005, China. E-mail: zxcao@xmu.edu.cn

w This article was submitted as part of a collection on ComputationalCatalysis and Materials for Energy Production, Storage andUtilization.z Electronic supplementary information (ESI) available: optimizedstructures, spin densities, and relative energy profiles for formationof C3H7 on other GO models. See DOI: 10.1039/c2cp41343d

PCCP Dynamic Article Links

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2. Models and methods

The density functional theory (DFT) calculations were per-

formed with the DMol3 package24 using the Perdew, Burke,

and Ernzerhof (PBE) exchange–correlation functional.25 The

double numerical plus polarization function (DNP) basis set

and a real-space cutoff of 4.5 A were used. The whole atomic

configuration was allowed to relax until all of the force

components on any atom were less than 10�3 au. The total

energy was converged to 10�5 au with spin-polarized calcula-

tion. The periodic boundary conditions with a supercell of

4 � 4 graphene unit cells composed of 32 carbon atoms were

employed in the calculations. A vacuum region of 12 A was

considered to separate the layer and its images in the direction

perpendicular to graphene plane. The 2D Brillouin zone was

sampled by 7 � 7 � 1 k-points within the Monkhorst–Pack

scheme. We used the linear/quadratic synchronous transit

(LST/QST)26 methods to determine the reaction pathways

and barriers.

The structural features of GOs as promising functional materials

have been extensively investigated, both experimentally27–33

and theoretically.34–44 It is widely accepted that hydroxyl and

epoxy groups are the two major functional groups on the GO

surface, although some new oxidation species, such as ketone,

carbonyl, ether, and other groups, have been reported. Based

on theoretical calculations,34–44 various structural models of

GO were proposed. It was found that the oxygen functional

groups prefer to aggregate together. The energetically favor-

able atomic configuration of GO has also been supported

by X-ray photoelectron spectroscopy (XPS)39 and nuclear

magnetic resonance (NMR) simulations40 as well as molecular

dynamics (MD)41,42 simulations. However, the complete struc-

ture of GO remains elusive due to the random distribution of

hydroxyl and epoxy groups, as well as different preparation

conditions. Accordingly, only the hydroxyl and epoxide func-

tional groups were considered in our computational models

for GOs here.

3. Results and discussion

3.1 ODH of propane on GOs with only epoxides

The oxidative dehydrogenation of propane on GOs with

only epoxides is first investigated. Based on the previous

studies,36–38 the oxygen groups of GOs energetically prefer

to aggregate on the graphene plane. Fig. 1a shows that two

nearest-neighbor epoxy groups at the same side are adsorbed

on graphene, named GO1. There are other possible binding

sites of two epoxy groups for the initial GO candidates apart

from GO1, such as (ac, ed), (ed, bf) and (ed, fg) shown in

Fig. 1a. The total energy calculations show that the structure

of GO1 is energetically more favorable than other configura-

tions with two epoxides located at ac and ed sites (or ed and fg).

For the ed and bf sites, the atomic arrangement of two epoxy

groups is similar to GO1. However, epoxides with active

centres close to the periodic boundary may affect the inter-

action of GO with C3H8, and here only GO1 is chosen as our

initial structure accordingly.

Adsorption of propane on GO1 is slightly exothermic by

1.3 kcal mol�1 (Fig. 1a). Such weak physical adsorption may be

attributed to the electrostatic attraction between H (H1) in a

methylene group and O atoms with the distance of 2.71 A;

both atoms have net Mulliken atomic charges of 0.14 and

�0.36 |e|, respectively. The epoxide species in GO1 with high

electron density may provide the active sites for the C–H bond

activation. The predicted reaction mechanisms for ODH of

propane to propene on GO1, as well as corresponding struc-

tures, are shown in Fig. 2a. The C–H bond in CH2 was broken

through the H atom abstraction by the epoxy group at site 1

with the energy barrier of 22.5 kcal mol�1, leading to a

hydroxyl group and a C3H7 species (Fig. 1b and 2a). The

formation of intermediate (GO1–C3H7-1) via a transition state

(TS1) is predicted to be endothermic by 16.3 kcal mol�1 with

respect to the initial state. The C–H bond cleavage barrier is

comparable to the value4 (21.2 kcal mol�1) for ODH of

n-butane to butene on the surface-modified CNTs, although

different C–H bond activation mechanisms are proposed.

Fig. 1 Top and side views of optimized structures (distances in A)

for ODH of propane to propene on GO with two nearest-neighbor

epoxy groups at the same side (GO1). (a) Adsorption of propane with

sites 1 and 2 of epoxy groups and a–f of hydroxyl groups indicated,

and formation of (b) C3H7 and (c) propene on GO. The spin densities

of GO1 with the formed C3H7 are also shown in the top view of

(b) (blue for spin up and yellow for spin down, and the isosurface is

0.03 e A�3).

Fig. 2 Relative energy profiles for ODH of propane to propene on

(a) GO1 and (b) GO2 with one added hydroxyl group at the opposite

side. All energies (in kcal mol�1) in (a) and (b) are relative to the

reactants GO1–C3H8 and GO2–C3H8, respectively, and the optimized

configurations (distances in A) of species involved in ODH are shown.

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16560 Phys. Chem. Chem. Phys., 2012, 14, 16558–16565 This journal is c the Owner Societies 2012

In TS1 (Fig. 2a), the C–H bond is elongated from 1.08 to

1.37 A, and the distance between H and O atoms is shortened

to 1.2 A. The spin density populations (see top view of Fig. 1b)

show that the net magnetic moments of 0.86 mB for C3H7 in the

intermediate GO1–C3H7-1 are mainly located at the carbon

atom of CH. Therefore, the electronic and structural features

of TS1 and the newly-formed C3H7 radical indicate that the

initial C–H bond cleavage follows a radical mechanism. As

shown in Fig. 1b (side view), the distances from the carbon

atom of CH to graphene, and to the epoxide of GO1 are 4.13

and 3.6 A, respectively. The effect of steric hindrance may be

responsible for the physisorption of the C3H7 radical on

the GO surface without any covalent bond interaction. The

present radical mechanism for the C–H bond activation on

GO distinctly differs from the previous studies for ODH of

propane on Pt clusters45 and vanadium oxides,46 where the

breaking of C–H bond leads to an adsorbed propoxide

intermediate.

In the subsequent conversion of the propyl radical to

propene, another epoxide accommodates the second H

abstraction of a methyl group in the propyl radical. As

Fig. 2a shows, the consecutive H abstraction is initiated by

the tiny rotation of C3H7 relative to graphene plane, leading to

a slightly more stable intermediate GO1–C3H7-2, where the

separation between H (H2) and O atoms at site 2 in

GO1–C3H7-2 is shortened to 2.74 A, facilitating the H migra-

tion to the neighboring epoxy group. The propene formation

is achieved by overcoming an energy barrier of 11.7 kcal mol�1

relative to the intermediate GO1–C3H7-2. The overall conver-

sion from the propyl radical to propene is found to be

thermodynamically favorable with an exothermicity of

14.9 kcal mol�1 with respect to the initial adsorbed state of

C3H8 on GO1. The predicted relative energies in Fig. 2a

indicate that the first H abstraction may dominate the entire

ODH process.

The equilibrium structures in Fig. 1c show that the propene

is physically adsorbed on the GO surface. Based on previous

studies,41–44 two newly-formed OH groups from the consecu-

tive H atom abstractions can couple together to form H2O and

one epoxy or carbonyl group by overcoming the different

barriers of 5–18 kcal mol�1, depending on the atomic arrange-

ment of other oxygen groups. Therefore, the active sites of GO

are easily recovered from the reaction media by filtration.18,19

3.2 ODH of propane on GOs with both epoxide and

hydroxyl groups

Usually, the GO surface contains both epoxide and hydroxyl

functional groups.27–44 Our calculations show that the

presence of a neighboring hydroxyl group at the opposite side

can improve the activity of the epoxy group towards the

H abstraction of propane. Fig. 2b shows the relative energy

profiles for ODH of propane to propene on GO2 constructed

by adding one nearest-neighbor hydroxyl group on the oppo-

site side of the oxygen group of GO1. In comparison with the

ODH of propane on GO1, the barrier for the first H abstraction

on GO2 is remarkably reduced from 22.5 to 15 kcal mol�1, and

the formation of intermediate (GO2–C3H7) is only less stable in

energy by 4.6 kcal mol�1 than the initial state. Similar to GO1,

the first C–H bond activation on GO2 follows the radical

mechanism. Although the activation energy of the second C–H

bond (10.1 kcal mol�1) is less affected by the added hydroxyl

group, the predicted exothermicity for propene formation on

GO2 relative to the initial state is increased by 12.2 kcal mol�1,

compared to the ODH reaction on GO1 (Fig. 2a and b).

Presumably, the presence of a nearest-neighbor OH on the

opposite side of active sites strikingly increases the reactivity of

GO as the metal-free catalyst.

For comparison, the C–H bond activation of propane by

other GO structures was also investigated. Our calculated

results indicate that when the active site of an epoxy group

is assisted by the adjoining hydroxyl group at the opposite

side, the formation of a propyl radical is generally more

favorable than that of GO without such OH, both thermo-

dynamically and kinetically (see Fig. 3 and 4). As shown in

Fig. 3, the barrier of C–H bond cleavage on GO with only one

Fig. 3 Relative energy profiles for the first H abstraction from CH2

of propane on GO10 and GO20. The red and blue lines represent the

pathways with two different GO structures including one epoxide

group (GO10) and one added neighboring OH at the opposite side with

respect to other oxygen groups on GO10 (GO20), respectively. All

energies (in kcal mol�1) are relative to propane adsorbed on GO, and

the optimized configurations (distances in A) of initial, transition, and

final states are shown.

Fig. 4 Relative energy profiles for the first H abstraction from CH2

of propane on GO10 0 and GO20 0. The red and blue lines represent the

pathways with two different GO structures including two next-nearest

neighbor epoxide groups (GO10 0) and one added nearest-neighbor OH

at the opposite side with respect to oxygen groups of GO10 0 (GO20 0),

respectively. All energies (in kcal mol�1) are relative to propane

adsorbed on GO, and the optimized configurations (distances in A)

of initial, transition, and final states are shown.

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epoxide (named as GO10) is predicted to be 24.5 kcal mol�1.

However, when one OH is bound to the neighboring carbon

site at the opposite side relative to the oxygen group of GO10

(called GO20), the barrier for C–H bond breaking is reduced to

14.7 kcal mol�1, and the formation of C3H7 on GO20 is more

favorable in energy than that of GO10 with endothermicity of

9.3 kcal mol�1 (Fig. 3). Similarly, for the GO with two epoxy

groups separated by two adjacent C–C bonds (GO10 0), in

which the hydroxyl group is located at the opposite side of

GO10 0 (GO20 0) with respect to the epoxy group, the barrier is

reduced from 23.7 to 18.2 kcal mol�1. The presence of such a

hydroxyl group may enhance the reactivity of GO (Fig. 4). The

radical mechanism for the first C–H bond activation on those

GO structures is similar to that on GO1 and GO2 (see Fig. S1

in the ESIw).The OH-induced activity enhancement ofan epoxy group

towards H abstraction may be attributed to the local structural

deformation around the active site and the transition of the

electronic properties of GO from the magnetic to the non-

magnetic phase (see Fig. 5a and b). The structures for propane

and C3H7 adsorbed on GO (Fig. 5a) show that the bond length

between carbon atoms 1 and 2 is increased from 1.46 to 1.5 A

after the H abstraction leading to OH formation. When one

hydroxyl group is bound to the nearest-neighbor carbon site at

the opposite side relative to the existing oxygen groups of

GO1, local structural deformation around the epoxy group is

found (Fig. 5b). The C–C bond length for propane on GO2 is

elongated to 1.52 A, larger than the 1.46 A for GO1, sug-

gesting the sp3-hybridized bonding characteristic. Based on the

spin density calculations (top view of Fig. 5a and b), the GO1

with propane adsorption has a nonmagnetic ground state. The

H abstraction by the epoxy group leads to unpaired electrons

delocalized at these carbon atoms around the newly-formed

OH, destroying the sublattice balance of graphene. In con-

trast, for GO2, the electronic property of GO has been

changed from magnetic to nonmagnetic states after the first

H abstraction because the two carbon atoms connecting the

formed 1,2-hydroxyl group pair belong to two different sub-

lattices. The activity enhancement of GO10 0 and GO20 0 is

relative to the magnetic and structural property change

induced by the adsorbed OH group. Owing to the strong steric

interactions, the formation of C3H7 on GO2 with the

added OH at the same side, is endothermic by 14.8 kcal mol�1

with a larger energy barrier of 28 kcal mol�1 relative to the

initial state, although the same nonmagnetic state of GO

is found.

Owing to the larger space size of propane and presence of

multiple oxygen groups, the 4 � 4 supercell used in our models

might be at the limit for suitable description of the interaction

or the surface reaction of propane with GO structurally. The

coverage and distribution effect of hydroxyl and epoxides on

the reactivity of GO for the first C–H bond cleavage is

discussed by using larger supercells with 5 � 5 and 6 � 6

graphene unit cells. The calculated results for selected struc-

tures of GO1 and GO2 are shown in ESIw (see Fig. S2–S4).

The predicted relative energies and barriers from different

supercell sizes considered here are comparable. The formation

of a propyl radical on GO1 with 5 � 5 and 6 � 6 graphene

supercells is predicted to be endothermic by 17.8 and

15.4 kcal mol�1, respectively. Owing to the adsorbed OH

group on GO2, the C–H bond cleavage of C3H8 on the

5 � 5 supercell (6 � 6 supercell) is only endothermic by

1.9 kcal mol�1 (3 kcal mol�1) relative to the initial state. These

reaction energies of propane on GO1 and GO2 with the larger

supercells are compared to the results (16.3 and 4.6 kcal mol�1)

with the small supercells (Fig. 2), respectively. As shown in

Fig. S4,w the predicted barrier of C–H bond activation on

GO1 with 5 � 5 supercells is 24.8 kcal mol�1, consistent with

the result from the 4 � 4 supercell (22.5 kcal mol�1).

Presumably, the larger coverage of oxygen groups may have

less effect on the interaction of GO with propane.

3.3 Effect of site of hydroxyl groups on activity of epoxides

To study the dependence of the catalytic activity of GOs on

the site of oxygen groups, several selected locations of the OH

group as shown in Fig. 1a were considered. When the OH

group is bound to carbon site a near the active site of GO1,

named GO2-2 (see Fig. 6), the energy barrier for the C–H

bond activation is reduced to 9.2 kcal mol�1, compared to

GO1 and GO2, although the initial structure of GO2-2–C3H8

is higher in energy by 13.7 kcal mol�1 than GO2–C3H8 due to

Fig. 5 Optimized structures (top and side views) and spin densities (top views) for the conversion from propane to C3H7 on (a) GO1, (b) GO2,

(c) GO2-2, and (d) GO2-3. Selected bond lengths (in A) are indicated. The isosurface is 0.03 e A�3 with blue for spin up and yellow for

spin down.

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16562 Phys. Chem. Chem. Phys., 2012, 14, 16558–16565 This journal is c the Owner Societies 2012

the larger surface strain. The formation of a propyl radical is only

endothermic by 1.6 kcal mol�1 relative to the initial state. The

formed 1,4-hydroxyl pair on GO2-2, retaining the sublattice

balance of graphene, is responsible for the higher reactivity of

GO (Fig. 5c). On the contrary, as shown in Fig. 6, no OH-induced

activity enhancement of epoxide group at site b (GO2-3) is found

for the first H abstraction on GO1, because such an additional

group destroys the sublattice balance, leading to the ferro-

magnetic ground state with magnetic moments of 1.52 mB locatedat graphene (Fig. 5d). For other binding sites of OH, similar

tendencies are found (see Fig. S5, S6, and S7 in the ESIw).The energetically favorable structures of GOs are deter-

mined theoretically by considering both the concentration of

epoxy and hydroxyl groups and the atomic arrangement of

these groups on the surface.35–38 The initial local structures

of GOs including several OH groups around the epoxy group

may be more stable in energy than those including only one

such group. Accordingly, now the effect of several OH groups

on the activity of an epoxy group toward C–H bond cleavage

is discussed. Our calculated results (Fig. 7) show that the

reactivity of GO2-3 can be enhanced by adding another OH

at site a (GO2-3a–C3H8). Such an atomic arrangement of these

OH groups, giving rise to the nonmagnetic phase of GO,

can lower the activation barrier from 25.2 to 16.5 kcal mol�1

(see Fig. 6 and 7). In the presence of two OH groups at sites e

and g of GO2, the first H abstraction by the epoxy group is

unfavorable compared to the reaction on GO2 with one OH at

site e (Fig. 8), both thermodynamically and kinetically,

although this initial structure is very close to in energy the

GO2-3a–C3H8 with the enhanced activity. The calculations for

other GO structures containing two OH groups also show the

site dependence of the activity of epoxides (see Fig. S8 in the

ESIw). Therefore, the C–H bond activation by an epoxy group,

leading to the sublattice balance of carbon atoms around the

formed OH, is more favorable energetically, compared to

situation with relatively large magnetic moments located at

GO in the presence of an OH group.

As expected, when the intermediate GO1–C3H7-2 on GO1 is

formed (Fig. 2a), the reactivity of the second H abstraction in

CH3 by another epoxy group at site 2 can be improved by

adding nearest-neighbor hydroxyl (named GO3) or epoxy

(GO4) groups on the opposite side relative to existing groups,

although the reactivity increase is not so remarkable compared

to that for the first H abstraction. Fig. 9 shows relative energy

profiles and corresponding structures involved in the propene

formation starting from the C3H7 radical on three different

GO models. The added hydroxyl and epoxy groups lower the

barriers of the second C–H bond cleavage to 8.7–7.7 kcal mol�1,

compared to 11.7 kcal mol�1 for GO1 without such additional

oxygen groups. The activity of such an epoxide for the second

C–H bond breaking is comparable to the Pt clusters stabilized on

high-surface-area substrates with a barrier of 8.5 kcal mol�1.45

The corresponding reaction energies with respect to GO with

C3H7 increase by 2.9 and 6.8 kcal mol�1. In Fig. 2 and 9, it is

found that the formation of propene leads to OH� � �O hydrogen

bonds between two newly-formed OH groups with distances of

1.78–1.83 A, which may be useful for triggering water formation

and release.

3.4 Diffusion of oxygen groups under an external electric field

As discussed above, the formations of neighboring oxygen

groups are important for the catalytic C–H bond activation by

Fig. 6 Energy profiles for the first H abstraction from CH2 of

propane on GO. The blue and black lines represent the pathways with

two initial structures GO2-2 and GO2-3 constructed by adding one

OH at carbon sites a and b of GO1, respectively. All energies (in kcal

mol�1) are relative to propane adsorbed on GO2-2, and the optimized

configurations (distances in A) of initial, transition, and final states are

shown. The structure of GO2–C3H8 is shown in Fig. 2b.

Fig. 7 Energy profiles for the first H abstraction from CH2 of

propane on GO2 with two added OH groups at sites a and b. All

energies (in kcal mol�1) are relative to the initial structure, and the

optimized configurations (distances in A) of initial, transition, and

final states are shown.

Fig. 8 Energy profiles for the first H abstraction from CH2 of

propane on GO2 with two adsorbed OH groups at sites e and g. All

energies (in kcal mol�1) are relative to the initial structure, and the

optimized configurations (distances in A) of initial, transition, and

final states are shown. For comparison, the initial structure of

GO2-3a–C3H8 is also shown.

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GOs, and the OH-induced activity enhancement of epoxides

is predicted to have a strong site dependence. The atomic

configurations of GO with aggregation of oxygen functional

groups in our computational models are energetically more

favorable than other atomic configurations.34–38 However,

GO is inherently amorphous due to the randomly decorated

oxygen-containing groups. Therefore, tuning the relative site

of oxygen groups on GOs surface is very important for

improving the catalytic activity of GOs towards ODH of

propane. Herein we investigated the reconstruction of the

surface oxygen species on GOs, and our calculations show

that the sites of oxygen groups on the GOs surface can be

controlled by diffusion of these functional groups. More

importantly, the diffuse barriers are well tuned by the applied

electric field with the direction along graphene - functional

groups. Fig. 10a provides one possible diffusion path of epoxy

groups to form two nearest-neighbor oxygen groups. Fig. 10b

shows relative energy profiles for epoxide diffusion without

and with the applied electric field. The present results show

that the diffusions of one epoxy group along paths a, and then b,

must overcome energy barriers of 17.9 and 14.3 kcal mol�1 to

form the two next-nearest and nearest neighbor epoxide

groups, respectively. The predicted diffusion barriers are

consistent with the previous reports for oxidative unzipping

and cutting of graphene and thermal reduction of GO with

0.73–1.11 eV.47–50

When the diffusion of the epoxy group is subjected to an

applied electric field, the diffusion barriers (Fig. 10b) are

reduced from 17.9 kcal mol�1 for path a to 14.5, 10.8, and

3.3 kcal mol�1 with 0.1, 0.2, and 0.4 V A�1 (from 14.3 kcal mol�1

for path b to 11.6, 9.3, and 3.1 kcal mol�1), respectively. The

exothermicity for the aggregation-state formation of the func-

tional groups is less affected by the electric field. A similar

effect of the applied electric field on the aggregation of

hydroxyl groups towards epoxides of GO has also been found

(see Fig. 11). The calculated results show that the migration of

Fig. 10 (a) A schematic representation of the surface diffusion path of an epoxy group towards aggregation of two oxygen groups (top view) and

this diffusion subjected to an applied electric field in the E direction along graphene - epoxide functional group (side view). (b) Relative energy

profiles for epoxide diffusion along the paths shown in (a). Black, red, green, and blue lines represent the diffusion under the applied electric fields

of 0.0, 0.1, 0.2, and 0.4 (V A�1), respectively. The partial structures of initial (A), intermediate (B), and final states (C) are shown. All energies

(in kcal mol�1) of initial states are set to 0.

Fig. 9 Relative energy profiles for the second H abstraction from

methyl of C3H7 by three GO models. The red, blue, and green lines

represent the reaction pathways with different initial states corre-

sponding to the intermediate state GO1–C3H7-2 shown in Fig. 2a,

and this state by adding one nearest-neighbor epoxy or OH group at

the opposite side relative to the epoxide at site b of GO1 (named as

GO3 and GO4). All energies (in kcal mol�1) are relative to the

associated systems of GO with C3H7. The related structures (distances

in A) are shown for pathways with blue and green lines, and the

structures for red line are shown in Fig. 2a.

Fig. 11 A schematic representation of the diffusion path of a hydroxyl

group from carbon sites a to b at the opposite side (a) and the same side

(c) relative to an epoxy group (top view), and this diffusion subjected to

the applied electric field in the E direction along (a) hydroxyl- epoxide

functional groups and (c) graphene - oxygen groups (side view).

(b) and (d) Relative energy profiles for OH diffusion along the paths

shown in (a) and (c), respectively. Black, red, and blue lines represent

the diffusion under the applied electric fields of 0.0, 0.4 (0.1) and

0.6 (0.2) V A�1 for OH at the opposite side (at the same side),

respectively. The structures of initial (A) and final states (B) omitting

other carbon atoms are shown. All energies (in kcal mol�1) of initial

states are set to 0.

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16564 Phys. Chem. Chem. Phys., 2012, 14, 16558–16565 This journal is c the Owner Societies 2012

a hydroxyl group along the path from carbon site a to b on

the GO surface can be improved by the applied electric

field. The diffusion barrier is reduced from 15.4 to 11.9 and

5.2 kcal mol�1 under the electric field of 0.4 and 0.6 V A�1

(Fig. 11b), respectively. The exothermic energies for the

aggregation of functional groups are less affected by the electric

field.

Our calculated results show that the initial structures of

GOs with high catalytic activity are more easily achieved by

diffusion of oxygen groups under the applied electric field

(Fig. 11c and d). As shown in Fig. 4, the first C–H bond

cleavage barrier for one OH at site b is 36.3 kcal mol�1, but the

barrier is reduced to 9.2 kcal mol�1 when the OH is diffused to

the carbon site a. The diffusion of OH from site b to a is

predicted to become more facile when subjected to an electric

field, as shown in Fig. 11d. The diffusion barriers are reduced

from 8.4 kcal mol�1 to 6.9 and 4.6 kcal mol�1 under the

electric fields of 0.1 and 0.2 V A�1, respectively. As Fig. 11b

and d show, the GOs with different OH configurations have a

similar dependence of the improved migration of OH group on

the electric field, although the diffusion barrier depends on the

atomic arrangement of all oxygen groups. Therefore, the sites

of oxygen functional groups on the GO surface can be easily

tuned by the diffusion of these groups under the electric field.

These results are also important for the thermal reduction and

the oxidative unzipping of graphene.47–50

4. Conclusions

To summarize, we first report the chemical structures of the

active sites and catalytic mechanisms for ODH of propane on

GO by DFT calculations. The epoxy groups on the GO

surface provide active sites for the C–H bond activation.

The first C–H bond breaking of propane through the H

abstraction by epoxide leads to the formation of a propyl

radical, which is the rate-determining step for the conversion

from propane to propene. The presence of OH groups around

the active site can remarkably improve the activity of the

epoxy group and facilitate the H abstraction, and the activity

enhancement exhibits strong site dependence. The sites of

oxygen functional groups on the GO surface are well con-

trolled by diffusion of these groups under the applied electric

field, which increases the reactivity of GO towards ODH of

propane. Thus, the surface modification of graphene with the

oxygen functional groups opens an alternative avenue for the

use of graphene-based materials the metal-free catalysts.

Acknowledgements

This work was supported by the National Science Foundation

of China (21103026 and 21133007) and the Ministry of Science

and Technology (2011CB808504 and 2012CB214900).

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