Metal-Embedded Graphene: A Possible Catalyst with High Activity

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2009, 113, 20156–20160 Published on Web 10/30/2009

Metal-Embedded Graphene: A Possible Catalyst with High Activity Yun-Hao Lu,† Miao Zhou,† Chun Zhang,†,‡ and Yuan-Ping Feng*,† Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore, 117542, and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore, 117543 ReceiVed: September 12, 2009

The catalytic activity of Au-embedded graphene is investigated by the first-principle method using the CO oxidation as a benchmark probe. The first step of CO oxidation catalyzed by the Au-embedded graphene is most likely to proceed with the Langmuir-Hinshelwood reaction (CO + O2 f OOCO f CO2 +O), and the energy barrier is as low as 0.31 eV. The second step of the oxidation would be the Eley-Rideal reaction (CO + O f CO2) with a much smaller energy barrier (0.18 eV). The partially filled d states of Au are localized around the Fermi level due to the interactions between Au and the neighboring carbon atoms. The high activity of Au-embedded graphene may be attributed to the electronic resonance among electronic states of CO, O2, and the Au atom, particularly, among the d states of the Au atom and the antibonding 2π* states of CO and O2. This opens a new avenue to fabricate low cost and high activity carbon-based catalyst. Introduction Graphene, a one-atom-thick carbon sheet with unique electronic and geometric properties, has been regarded as one of the most promising candidates for the next generation of electronic materials.1,2 Perfect graphene is stable in normal circumstances and chemically inactive. However, nanostructured carbon materials with graphene structure such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are good substrate materials for transition-metal catalysts mainly due to their high surface area and have been studied extensively.3,4 Recently, it was reported that metal subnanoclusters, including only a few atoms, on a graphene sheet exhibit an unusually high activity for oxidation reactions.5 Strong interactions between the metal cluster and graphene were observed. Carbon vacancies in the graphene sheet or dangling bonds of carbon atoms are supposed to modulate the electronic structures of supported metal clusters.6 Krasheninnikov et al.7 investigated the transition-metal-atom-embedded graphene using density functional theory and found that the bonding between the transition-metal atom and neighboring carbon atoms determines the magnetic and electronic structures of the system. Therefore, the inert graphene may be transformed to a very active catalyst through the interactions between the carbon vacancies and metal clusters or even a single atom. The metal-atom-embedded graphene structure has been fabricated recently, and the diffusion of metal atoms in the graphene plane can be controlled.8 It opens a new avenue to design advanced catalysts based on graphene. In this paper, we investigate the catalytic activity of Auembedded graphene using the CO oxidation as a benchmark probe. We are particularly interested in Au, because Au is the noblest metal and has not been considered as a good catalyst * To whom correspondence should be addressed. E-mail: phyfyp@ nus.edu.sg. † Department of Physics. ‡ Department of Chemistry.

10.1021/jp908829m CCC: $40.75

until recently.9 Our calculations suggest that Au-embedded graphene is a good candidate for highly efficient catalysts with low cost. Models and Methods The density functional theory (DFT) calculations were carried out using the DMol3 package.10,11 The spin-unrestricted DFT in the generalized gradient approximation with the PerdewBurke-Ernzerhof (PBE) functional12 was used to obtain all of the results presented in the next section. DFT semicore pseudopotentials (DSPPs) and a double numerical basis set including a d-polarization function (DND) were selected. Within the DSPP scheme implemented in Dmol3, all-electron calculations were performed for C and O atoms, and relativistic effects were included for Au. A hexagonal graphene supercell (4 × 4 graphene unit cells) containing 32 atoms was introduced to model a system with one carbon atom substituted by a Au atom, approaching the isolated impurity limit. Test calculations using a 72-atom supercell (6 × 6 graphene unit cells) gave essentially the same formation energy and structures. The minimum distance between the graphene sheet and its mirror images is greater than 20 Å which is sufficiently large to avoid the interactions between them. For geometric optimization and the search for the transition state (TS), the Brillouin zone integration was performed with 3 × 3 × 1 k-point sampling. For the calculation of electronic properties, Monkhorst-Pack 6 × 6 × 1 k-point sampling was used13 and the real-space global orbital cutoff radius was set to 6 Å. The minimum-energy pathway for elementary reaction steps was computed using the nudged elastic band (NEB) method.14 Results and Discussion Every kind of catalyst has its unique electronic and geometric properties resulting in high activity in chemical reaction. Figure 1 shows the electronic and geometric structures of Au-embedded  2009 American Chemical Society

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Figure 1. Electronic and geometric structures of Au-embedded graphene. (a) Spin-polarized local density of states projected on 5d (red curve) and 6s (black curve) orbitals of Au as well as 2p (blue curve) orbitals of neighboring carbon atoms. The Fermi level is set to zero. (b,c) side view and top view of the geometric structure of Au-embedded graphene with spin-density isosurface. The spin density is defined as the difference between spin-up and spin-down electron densities, Fv - FV. The big, yellow (small, gray) ball represents the Au atom (carbon atoms).

graphene. The bond length between the Au atom and each neighboring carbon atom is 2.09 Å, and the height of the Au atom above the graphene base plane is 1.85 Å both in good agreement with those of previous calculations based on the plane-wave basis.7 The spin density mainly accumulates on the Au atom and the three neighboring carbon atoms (Figure 1b and c). The isolated Au atom has a filled d-shell and half-filled 6s orbital. When a carbon atom in graphene is replaced by Au, the 6s orbital and spin-down component of the top 5d orbital are empty due to the formation of C-Au bonds and charge transfer from the Au atom to the carbon atoms, as shown in Figure 1a. The spin-up components of the top 5d orbital of Au and 2p orbital of neighboring carbon atoms are partially filled and dominate the magnetic moment of the system. As a result, the high density of spin-polarized states is localized around the Fermi level. These localized d states are important to activate reactants to lower the reaction barriers, which will be discussed later. Unlike the Au particle supported on an appropriate oxide film, where the activity is mainly due to the excess electronic charge coming from substrate at the Au cluster/oxide interface,15 the Au atom in this system is positively charged. Moreover, the Au-embedded graphene system is a monolayer catalyst with a much higher surface area, ensuring the catalytic efficiency. Before investigating the CO oxidation reaction, we first study the adsorption properties of O2, CO, CO2, and O on the Auembedded graphene. We examined many adsorption sites in order to find out the most stable configuration for each adsorbate. It was found that the CO and O2 molecules are only physisorbed on the graphene plane while they interact strongly with the Au atom embeded in graphene (Ead(CO) ) -1.53 eV and Ead(O2) ) -1.34 eV, see Table 1), indicating both of them can be readily adsorbed onto the embeded-Au atom at room temperature. Both CO and O2 are about 2.0 Å away from the Au atom. The C-O bond length is almost the same as that of the isolated CO molecule after adsorption, while the O-O bond of the adsorbed O2 molecule is elongated about 0.16 Å due to the electronic charge transfer (about 0.52e) from the embedded Au atom to the 2π* orbital of O2. However, from the energetic point of view, adsorption of the CO molecule on Au-embedded graphene is preferred rather than the O2 molecule (see Table 1). The coadsorption of CO and O2 molecules on the same embedded Au atom is also an exothermic process with Ead(CO+O2) ) -1.82 eV, larger than Ead(CO) and Ead(O2). For the CO2 molecule, we studied various possible adsorption sites and found the most stable adsorption site locates on the Au atom with

TABLE 1: Adsorption Energy (Ead) of Various Adsorbates on Au-Embedded Graphene and Shortest Distance (d) between the Adsorbate and Au Atoma adsorbate

Ead

d (Å)

CO O2 CO2 O CO + O2

-1.53 -1.34 -0.13 -4.80 -1.82

1.97 2.06 2.24 1.84 2.23 (CO)/2.17 (O2)

a Adsorption energy is defined as Ead ) Etotal[Au-graphene + adsorbate] - (Etotal[Au-graphene] + Etotal[adsorbate]), where Etotal is the total energy of the system per supercell.

Ead(CO2) ) -0.13 eV, suggesting that a CO2 molecule is weakly adsorbed on Au-embedded graphene and can easily desorb from the reaction site at room temperature. As expected, the atomic O strongly binds to the embedded Au atom with Ead(O) ) -4.80 eV, consistent with a previous study.16 We investigated both Eley-Rideal (ER) and LangmuirHinshelwood (LH) mechanisms of CO oxidation catalyzed by Au-embedded graphene. For the ER mechanism, in which the gas-phase CO molecules directly react with activated O2, i.e., adsorbed atomic O, the activation of the O2 molecule is the rate-limiting step, and for the LH mechanism, the coadsorbed CO and O2 molecules react to form a peroxo-type complex intermediate, which is the rate-limiting step. First, we explored the ER mechanism as the starting point for CO oxidation with the O2 molecule and found the reaction barrier is more than 0.5 eV (see the Supporting Information), much larger than that of the LH mechanism (Figure 2). Additionally, the adsorption energy of an individual CO molecule on Au-embedded graphene is larger than that of O2 (Table 1). Thus, the LH mechanism is the more favorable mechanism in energy rather than the ER mechanism for CO oxidation with O2 on Au-embedded graphene. On the basis of these reasons, we considered the LH reaction CO + O2 f OOCO f CO2 + O as a starting point, followed by the ER reaction CO + O f CO2. To search for the minimumenergy pathway (MEP) for the CO oxidation, we selected the most stable coadsorption configuration as an initial state (IS), where the adsorbed CO molecule is tilted to the graphene plane while the O2 molecule is parallel to the graphene plane (Figure 3). The final state (FS) consists of a CO2 molecule physisorbed on the graphene with a chemisorbed atomic O nearby. To achieve sufficient accuracy in MEP calculation, 30 image structures were inserted between the IS and FS. The MEP profile

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Letters TABLE 2: Structural Parameters for the Intermediate States along the MEP for the CO Oxidation on the Au-Embedded Graphene: (a) CO + O2 f OOCO f CO2 + O, IS, TS, MS, and FS Are Displayed in Figure 3; (b) CO + O f CO2, IS, TS, and FS Are Displayed in Figure 4a

Figure 2. Schematic energy profile corresponding to local configurations shown in Figure 3 along the minimum-energy pathway via the CO + O2 f OOCO f CO2 + O route. All energies are given with respect to the reference energy, i.e., the sum of energies of the Auembedded graphene and individual CO and O2 molecules.

(a)

IS

TS

MS

FS

d(C-O) d(C-Au) d(C-O1) d(O1-O2) d(O2-Au) ∠(O-C-O1)

1.139 2.231 2.755 1.388 2.175 117.2

1.172 2.147 1.694 1.449 2.144 121.0

1.207 2.148 1.355 1.501 2.081 122.8

1.178 4.063 1.173 3.374 1.849 180.0

(b)

IS

TS

FS

d(C-O) d(C-Au) d(C-O2) d(O2-Au) ∠(O-C-O2)

1.142 3.887 3.190 1.847 96.6

1.145 2.360 2.475 1.902 107.2

1.192 2.246 1.253 2.244 148.5

a The units of the bond distances and angles are angstroms and degrees, respectively.

Figure 3. Local configurations of the adsorbates on the Au-embedded graphene at various intermediate states, including the initial state (IS), transition state (TS), metastable state (MS), and final state (FS) along the minimum-energy pathway via the CO + O2 f OOCO f CO2 + O route. Both side (upper panel) and top (lower panel) views are displayed, as well as the energy change (in eV) between neighboring states. Red, gray, and yellow balls represent oxygen, carbon, and gold atoms, respectively.

is summarized in Figure 2. The energies are schematically plotted with respect to the reference energy, which is the sum of the energies of the Au-embedded graphene and individual CO and O2 molecules, assuming CO and O2 are far apart. The local configurations of the adsorbates on the Au-embedded graphene at each state along the MEP are displayed in Figure 3, and the corresponding structural parameters are listed in Table 2a. Once CO and O2 are coadsorbed on the Au-embedded graphene, one of the oxygen atoms (O1) in the O2 molecule starts to approach the carbon atom of CO to reach the TS. The O-O bond length of O2(dO1-O2) is elongated to 1.45 Å, while the distance between the Au atom and the carbon atom of CO decreases about 0.1 Å in this endothermic process. The energy barrier along the reaction pathway is estimated to be 0.31 eV. Meanwhile, a peroxo-type O2-O1-C-O complex is formed over the Au atom. Passing over the TS, the peroxo-type complex is still maintained until a metastable configuration (MS) is reached and the O-O bond length (dO1-O2) in the O2-O1-C-O complex is continually elongated from 1.45 to 1.50 Å. Passing over the MS without an energy barrier, a CO2 molecule is formed, leaving an atomic O adsorbed on the Au atom embedded in graphene. The CO2 molecule can be easily desorbed at room temperature due to the weak interaction between CO2 and the Au-embedded graphene. We also checked whether the CO oxidation with atomic O (ER mechanism) is conceivable after CO2 is formed via the LH mechanism. A configuration of the CO molecule more than 3.0 Å away from

Figure 4. Local configurations of the adsorbates on the Au-embedded graphene at various intermediate states, including the initial state (IS), transition state (TS), and final state (FS), along the minimum-energy pathway via the CO + O f CO2 route. Both side (upper panel) and top (lower panel) views are displayed, as well as the energy change (in eV) between neighboring states. Red, gray, and yellow balls represent oxygen, carbon, and gold atoms, respectively.

the atomic O preadsorbed on the embedded Au atom was chosen as the IS (Figure 4). The FS is simply set to the configuration of CO2 adsorbed on the Au-embedded graphene. The carbon atom of CO approaches the adsorbed oxygen atom and pushes it away from the Au atom to reach a TS. The distance between the carbon atom and the Au atom is decreased to 2.36 Å and the O2-Au bond length (dO2-Au) is elongated by about 0.05 Å in this endothermic process. It was found that a small energy barrier (0.18 eV), about half of the barrier of the LH mechanism for the CO + O2 reaction, separates the IS and FS along the MEP. Note that the adsorption energy of CO2 on the Auembedded graphene is only -0.13 eV and it is also easily desorbed from the site at room temperature. On the basis of the above discussions, we conclude that the CO oxidization on the Au-embedded graphene may be characterized as a two-step process: the LH reaction initiates the CO oxidation followed by the ER reaction. Next, we investigate the electronic structures in these reaction progresses to gain more insight into the origin of the high activity of the Au-embedded graphene system. As we discussed above, the partially occupied d-orbital of Au is localized around the Fermi level due to the interactions between the graphene and Au atom. These states are very crucial for the activity. Figure 5 illustrates the spin-polarized local density of states (LDOS) projected onto the C-O (left panel in Figure 5) and

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Figure 5. Spin-polarized local density of states (LDOS) projected onto C-O (left panel) and O1-O2 (right panel) on the Au-embedded graphene (Figure 3), together with the d-projected LDOS of the Au atom in the IS, TS, and MS. Black dashed curve, gas-phase CO or O2; black solid curve, C-O or O1-O2 on Au-embedded graphene; red curve, d-projected LDOS of the Au atom. The Fermi level is set to zero. The orbital notations are assigned only to the gas-phase molecule CO2 and O2 and roughly indicated by a, b, c, and d in reaction processes.

O1-O2 (right panel in Figure 5) bonds, as well as the d-projected LDOS of the Au atom in the IS, TS, and MS of the LH step, respectively (see Figure 3 for the geometrical structures). The s- and p-projected LDOS of the Au atom and p-projected LDOS of the carbon atoms have no significant change in the reaction progresses and are not shown here. The highest d-orbital of Au is partially occupied before CO and O2 coadsorption. Up on the CO and O2 coadsorption, this d-orbital shifts upward and becomes completely empty in the IS configuration due to the charge transfer between Au and adsorbates. The antibonding orbital 2π* of gas O2 is half-filled. After coadsorption with CO on the Au atom, the initial empty spin-up component becomes partially occupied, inducing the elongation of the O1-O2 bond in O2 (see Table 2a). The 1π and 5σ orbitals are also broadened because of the interaction with the d-orbital of Au atom. The 1π and 2π* orbitals are more broadened and involved with the d-orbital of Au to weaken the O1-O2 bond in the TS. From the TS to MS, the occupied antibonding orbital 2π* is elevated above the Fermi level due to the interaction between the O1 and CO molecule mediated by the Au atom, as indicated by the decrease of C-O1 bond length, even shorter than the O1-O2 bond length (Table 2a). As expected, the antibonding 4σ* state is not involved in the reaction due to its position far below the Fermi level. For the C-O species on the Au-embeded graphene, the top occupied 5σ orbital, located on the Fermi level in the gas CO molecule, is shifted far below the Fermi level and strongly hybridized with the d-orbital of Au at the IS. The empty antibonding orbital 2π* of the gas-phase CO is also pulled close to the Fermi level at IS, and it is partially populated due to the electronic resonance between the antibonding 2π* state of C-O and the d states of the Au atom at the TS and MS. The partial occupation of this antibond leads to the slight increase of C-O bond length and finally reduces the C-O bond length close to the value of dC-O

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20159 in the isolated CO2 molecule (Table 2a). A net charge transfer occurs from the Au atom to the adsorbates in these reaction progresses, and no spin-polarized states are accumulated on Au due to the empty top d orbital of the Au atom. As the hybridization between the Au atom and neighboring carbon atoms of graphene, the p states of the neighboring carbon atoms are also lifted over the Fermi level and not spin-polarized. Overall, the formation of the unstable peroxo-type O2-O1-C-O complex results in a redistribution of the LDOS and an orbital shift for both C-O and O1-O2 species. From IS to TS to MS, the 2π* states of C-O and O1-O2 are expanded and involved with the Au d states. Moreover, mediated by the Au atom, the states of C-O and O1-O2 interact with each other, strengthening (forming) the C-O1 bond while weakening (breaking) the O1-O2 bond, as indicated by the superposition of the C-O and O1-O2 states at the MS (Figure 5). In addition, the d states (red curve in Figure 5) rather than the s states of the Au atom or p states of the carbon atom dominate the interaction between CO and O2 on Au-embedded graphene. For the ER reaction, the Au atom in IS and TS is a little positively charged due to the large electron negativity of atomic oxygen (O2) which is prebonded with Au. The electronic states on Au atom have no significant change from IS to TS, and the CO molecule approaches O2 directly before the formation of the C-O2 bond. In general, the CO oxidation with a reaction barrier of less than 0.5 eV is expected to occur at room temperature. The reaction barrier of the first step of CO oxidation catalyzed by the Au-embedded graphene is as low as 0.31 eV and both reaction steps are likely to proceed rapidly at room temperature because of the low activation barriers involved. Moreover, the catalytic activity of this graphene-based catalyst may be tuned or enhanced by the curvature of a sheet17 due to its flexible character. Another important property of Au-embedded graphene is its plane structure with high surface area, which ensures its high catalytic efficiency and activity in reaction progresses. Conclusion On the basis of the first-principle calculation, we investigate the reaction mechanism of CO oxidation catalyzed by the Auembedded graphene, as well as structural and electronic properties of adsorbates and adsorbents. We find that the Au-embedded graphene exhibits high catalytic activity for the CO oxidation. The CO oxidation is most likely to proceed with the LH reaction at the starting point with a low activation barrier of 0.31 eV, followed by the ER reaction with a much smaller energy barrier (0.18 eV). The CO oxidation on Au-embedded graphene is predicted to occur at room temperature. The high catalytic activity of Au-embedded graphene may be attributed to the partially occupied d orbital localized in the vicinity of the Fermi level because of the interactions between the Au atom and graphene. It can be expected that the metal atom is not limited to Au and other transition-metal-atom-embedded graphene is also highly active, such as Ti-embedded graphene. The proposed method transforms the inert graphene to a highly active material for catalysis. This opens a new avenue to fabricate low cost and highly efficient catalysts based on carbon. Acknowledgment. This work is supported by a National Research Foundation (Singapore) Competitive Research Program (Grant No. NRF-G-CRP 2007-05). Part of the work was carried out using the computing facility at the Computer Center of National University of Singapore.

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Supporting Information Available: Local configurations and energy change of the adsorbates on the Au-embedded graphene at various intermediate states along the minimumenergy pathway via the ER mechanism (CO + O f CO2) at the starting point. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (3) Prabhuram, J.; Zhao, T. S.; Tang, Z. K.; Chen, R.; Liang, Z. X. J. Phys. Chem. B 2006, 110, 5245. (4) Liao, S.; Holmes, K. A.; Tsaprailis, H.; Briss, V. I. J. Am. Chem. Soc. 2006, 128, 3504.

Letters (5) Yoo, E. J.; Okata, T.; Akita, T.; Kohyama, M.; Nakamura, J.; Honma, I. Nano Lett. 2009, 9, 2255. (6) Okamoto, Y. Chem. Phys. Lett. 2006, 420, 382. (7) Krasheninnikov, A. V.; Lehtinen, P. O.; Foster, A. S.; Pyykko, P.; Nieminen, R. M. Phys. ReV. Lett. 2009, 102, 126807. (8) Gan, Y.; Sun, L.; Banhart, F. Small 2008, 4, 587. (9) Yoon, B.; Hakkinen, H.; Landman, U.; Worz, A. S.; Antonietti, J. M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403. (10) Delley, B. J. Chem. Phys. 1990, 92, 508. (11) Delley, B. J. Chem. Phys. 2003, 113, 7756. (12) Delley, B. Phys. ReV. Lett. 1996, 77, 3865. (13) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (14) Henkelman, G.; Jonsson, H. J. Chem. Phys. 2000, 113, 9978. (15) Zhang, C.; Yoon, B.; Landman, U. J. Am. Chem. Soc. 2007, 129, 2228. (16) Liu, Z. P.; Hu, P. J. Am. Chem. Soc. 2002, 124, 14770. (17) Boukhvalov, D. W.; Katsnelson, M. I. J. Phys. Chem. C 2009, 113, 14176–14178.

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