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(DFT), we studied the catalytic activity of small Au and Pt clusters supported on pristine (defect-free) or defective graphene. Our investigation showed that the ...

THE JOURNAL OF CHEMICAL PHYSICS 132, 194704 共2010兲

Greatly enhanced adsorption and catalytic activity of Au and Pt clusters on defective graphene Miao Zhou,1 Aihua Zhang,1 Zhenxiang Dai,1 Chun Zhang,1,2,a兲 and Yuan Ping Feng1,b兲 1

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543

2

共Received 7 January 2010; accepted 15 April 2010; published online 19 May 2010兲 We report an investigation on CO oxidation catalyzed by Au8 or Pt4 clusters on defective graphene using first-principles approach based on density functional theory. The simplest single-carbon-vacancy defect on graphene was found to play an essential role in the catalyzed chemical reaction of CO oxidation. When supported on a defect-free graphene sheet, the reaction barrier of CO oxidation catalyzed by Au8 共Pt4兲 clusters was estimated to be around 3.0 eV 共0.5 eV兲, and when adsorbed on defective graphene, the reaction barrier was greatly reduced to around 0.2 eV 共0.13 eV兲. © 2010 American Institute of Physics. 关doi:10.1063/1.3427246兴 Graphene, a one-atom thick carbon sheet, has been regarded as one of the most promising future electronic materials due to its unique mechanical and electronic properties originating from the two-dimensional 共2D兲 honeycomb atomic structure.1–3 Potential applications of graphene on electronic and photonic devices have been extensively explored since the discovery of this fascinating 2D material.4–6 Recently, a new direction of nanocatalysis, graphenesupported transition metal catalysis, has attracted considerable interests.7,8 Unusually high catalytic activity of small Pt clusters supported on graphene has been observed in experiments.7 However, the origin of such high reactivity of graphene-supported Pt clusters is still unclear. In this paper, via first-principles method based on density functional theory 共DFT兲, we studied the catalytic activity of small Au and Pt clusters supported on pristine 共defect-free兲 or defective graphene. Our investigation showed that the single-carbonvacancy defect in graphene could greatly enhance the catalytic activity of small Au and Pt clusters. Our first-principles electronic structure calculations are based on DFT using VASP package.9 The reaction barrier of catalyzed CO oxidation was calculated by incorporating the constrained energy minimization method10,11 into VASP. DFT calculations were performed with a plane wave basis 共30 Ry for the kinetic energy cutoff兲, a 6 ⫻ 6 K-points sampling of Brillouin zone, and a scalar relativistic ultrasoft pseudopotential for Au and Pt.12 In all calculations, the generalized gradient approximation in Perdew–Burke–Ernzerhof format13 was included. A supercell that includes 6 ⫻ 6 unit cells of graphene in graphene plane, and a vacuum region of 20 Å in the direction normal to graphene was employed. In optimizing atomic structure, the force convergence criterion was set to 0.01 eV/Å. In this work, Au8 and Pt4 clusters supported on graphene were chosen to be model catalysts for the chemical reaction of CO oxidation. In literature, Au8 and Pt4 clusters were a兲

Electronic mail: [email protected] Electronic mail: [email protected]

b兲

0021-9606/2010/132共19兲/194704/3/$30.00

often used as examples of Au or Pt nanocatalysts.14–17 Three configurations of Au8 clusters, two planar islands 共P1 and P2兲 and one three-dimensional 共3D兲 structure 关as shown in Figs. 1共a兲–1共c兲兴 were considered. These three configurations are most stable isomers of Au8 clusters in gas phase.18 Previous studies15,19–21 showed that when adsorbed on metal oxide materials, Au8 clusters always take the form of the 3D structure, as shown in Fig. 1共c兲. In contrast, when adsorbed on pristine 共defect-free兲 graphene, our calculations found that two planar structures, as shown in Fig. 1 are nearly degenerate, and slightly more stable than the 3D one by about 0.4 eV. At room temperature, they may coexist. We therefore considered all these three Au8 clusters as catalysts for CO oxidation. For Pt4 clusters, here we considered the commonly used 3D tetrahedral structure. When the underlying graphene sheet is defect-free, our calculations showed that the O2 molecule does not bind to the Au8 cluster adsorbed on the graphene regardless of its geometry 共planar or 3D兲, indicating that Au8 clusters are not catalytically active for the chemical reaction of CO oxidation. Therefore, the reaction barrier of CO oxidation in this case is the gas-phase value 3.1 eV 共our DFT calculation for the first step of the CO oxidation 1 / 2O2 + CO→ CO2兲. For the case of Pt4, the O2 molecule binds to the Pt4 cluster adsorbed on defect-free graphene with a binding energy of

FIG. 1. 关共a兲–共c兲兴 Three most stable isomers of Au8 clusters in gas phase 共Au in yellow兲: 共a兲 P1, 共b兲 P2, and 共c兲 3D. 共d兲 Pt4 cluster 共dark blue兲 in gas phase. 关共e兲–共h兲兴 Configurations for Au and Pt clusters adsorbed on defective graphene 共C in gray兲. Superimposed, an isosurface of the excess electronic charge 共red兲 and depleted electronic charge 共blue兲 with an isosurface value of 0.02 e / Å3 was shown. In the inset, we show the optimized atomic structure of a single-carbon-vacancy in graphene.

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0.9 eV and the O–O bond length of 1.39 Å. We then investigated two types of reactions of CO oxidation 共1 / 2O2 + CO→ CO2兲 catalyzed by Pt4 / graphene: the Langmuir– Hinshelwood 共LH兲 type, and the Eley–Rideal 共ER兲 type. The reaction barrier of the catalyzed CO oxidation was estimated by our calculations to be around 0.5 eV for both LH and ER types of reactions. In light of the inactivity of Au8 clusters and the relatively low reactivity 共high barrier兲 of Pt4 clusters on defectfree graphene which is against the experimental observation,7 we introduced a single-carbon-vacancy defect in graphene 共as shown in the inset of Fig. 1兲, and focused our study on effects of this simplest defect on catalytic activity of Au8 and Pt4 clusters adsorbed on top of it. Our calculations showed that the defect can greatly enhance the adsorption of both Au8 and Pt4 clusters on graphene. After the defect is introduced, the adsorption energies of three configurations of Au8 clusters were found to increase from 0.71 eV 共P1兲, 0.65 eV 共P2兲, and 0.52 eV 共3D兲 to 1.56 eV 共P1兲, 1.52 eV 共P2兲, and 1.51 eV 共3D兲, respectively, and the adsorption energy of the Pt4 cluster increases from 1.4 to 7.7 eV. The origin of this greatly enhanced adsorption is the strong interaction between the carbon-vacancy defect in graphene and the adsorbed metal clusters originating from the defect-induced breaking of the SP2 bonding of the graphene, and the hybridization of carbon 2p and Au/Pt 5d orbital around the defect, which can be seen from the isosurface of charge redistribution shown in Figs. 1共e兲–1共h兲. For Au clusters, the significant charge redistribution only happens in the interfacial region between the defect in graphene and the adsorbed clusters, and for the Pt4 cluster, the defect drastically change the electronic structure of the whole cluster, leading to the surprisingly high adsorption energy 共7.7 eV兲. The significant defect-induced changes of electronic structures of Au/Pt clusters are expected to have great effects on the adsorption of the O2 molecule on clusters, and in turn to drastically influence the catalyzed reaction of CO oxidation. After the defect is introduced, the O2 molecule strongly binds to all three types of Au8 clusters with the adsorption energy 1.34 eV for P1, 1.26 eV for P2, and 1.18 eV for 3D. The adsorption energy of O2 on the Pt4 cluster changes from 0.90 to 1.92 eV after the defect is introduced. Note that adsorption energies of O2 presented here are for lowest-energy states we found after testing various adsorption sites. When adsorbed on Au8 共Pt4兲 clusters on defective graphene, the O–O bond of the O2 molecule is significantly elongated compared to its gas-phase value 共1.23 Å兲. In particular, the O–O bond length, d共O–O兲, is around 1.41 Å for all three types of Au8 clusters, and 1.44 Å for the Pt4 cluster. The great elongation of the O–O bond is due to electrons populated to the antibonding 2␲ⴱ orbital of the O2 molecule mainly from the metal cluster for the case of Au8 and from both the metal cluster and the interfacial region for the case of Pt4, as we can see from the Figs. 2共a兲, 3共a兲, and 4共a兲. The Bader charge population analysis22 showed that about 0.7 共0.9兲 electrons are transferred to the O2 molecule for the case of Au8 共Pt4兲 clusters. Next we studied by first-principles calculations both LH and ER types of CO oxidations catalyzed by Au8 or Pt4

J. Chem. Phys. 132, 194704 共2010兲

FIG. 2. LH type of CO oxidation catalyzed by the P1 isomer of Au8 on the defective graphene. 共a兲 The initial state of the reaction: d共O共1兲-O共2兲兲 = 1.41 Å and d共C – O共2兲兲 = 2.81 Å. The isosurface of excess 共red兲 and depleted 共blue兲 electronic charge is also shown here. 共b兲 The transitional state: d共C – O共2兲兲 = 1.65 Å and d共O共1兲-O共2兲兲 = 1.50 Å. 共c兲 The final state of forming CO2. 共d兲 The energy profile along the reaction coordinate d共C–O共2兲兲.

clusters supported on graphene with a single-carbon-vacancy defect. In all of our calculations, all atoms are allowed to relax. In Fig. 2, the LH mechanism of reaction catalyzed by the planar Au8 cluster 共P1兲 was shown. With the binding of O2 molecule, the coadsorption of CO molecule on the peripheral Au atom, as shown in the figure yielded the adsorption energy of 1.2 eV. The atomic structures of the initial state, transition state, final state of the reaction, and the energy profile along the reaction coordinate were shown in Figs. 2共a兲–2共d兲, respectively. The reaction barrier was estimated to be 0.1 eV. The LH reaction catalyzed by the other planer structure of Au8 共P2兲 was calculated to be 0.18 eV 共not shown in the figure兲. The details of the LH reaction catalyzed by the 3D Au8 clusters were shown in Fig. 3. In this case, the adsorption energy of the CO molecule after the binding of O2 is 1.1 eV, and the reaction barrier was calculated to be 0.2 eV. In Fig. 4, we showed the initial state, transition state, final state, and the energy profile of the LH

FIG. 3. LH type of CO oxidation catalyzed by the 3D isomer of Au8 on the defective graphene. 共a兲 The initial state of the reaction: d共O共1兲-O共2兲兲 = 1.42 Å and d共C – O共2兲兲 = 3.45 Å. The isosurface of excess 共red兲 and depleted 共blue兲 electronic charge is also shown here. 共b兲 The transitional state: d共C – O共2兲兲 = 1.60 Å and d共O共1兲-O共2兲兲 = 1.46 Å. 共c兲 The final state of forming CO2. 共d兲 The energy profile along the reaction coordinate d共C–O共2兲兲.

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This work was supported by NUS Academic Research Fund 共C.Z.兲 共Grant Nos. R-144-000-237-133 and R-144-000255-112兲. 1

FIG. 4. LH type of CO oxidation catalyzed by the Pt4 on the defective graphene. 共a兲 The initial state of the reaction: d共O共1兲-O共2兲兲 = 1.45 Å and d共C – O共2兲兲 = 3.25 Å. The isosurface of excess 共red兲 and depleted 共blue兲 electronic charge is also shown here. 共b兲 The transitional state: d共C – O共2兲兲 = 1.80 Å and d共O共1兲-O共2兲兲 = 1.47 Å. 共c兲 The final state of forming CO2. 共d兲 The energy profile along the reaction coordinate d共C–O共2兲兲.

reaction catalyzed by the Pt4 cluster. In this case, the reaction barrier was estimated to be 0.13 eV. We also considered the ER mechanism of the CO oxidation catalyzed by Au8 or Pt4 clusters on defective graphene. Reaction barriers of ER type of CO oxidations were calculated to be 0.26 eV, 0.23 eV, 0.22 eV, and 0.16 eV, for P1-Au8, P2-Au8, 3D-Au8, and Pt4 clusters, respectively. In summary, we present in this paper a first-principles investigation of effects of the single-carbon-vacancy defect in graphene on the catalytic activity of Au8 and Pt4 clusters supported on graphene. We found that the defect greatly enhances the reactivity of Au8 and Pt4 clusters, and reduces the reaction barrier of catalyzed CO oxidation from around 3.0 eV for the case of Au8 and 0.5 eV for the case of Pt4 to less than 0.2 eV. Since the carbon-vacancy defect is inevitable in the experimental preparation of graphene sheets, results presented in this paper may be useful in explaining recent experiments,7 and helpful for the future design of graphene based nanocatalysts.

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