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 + O 2 f OOCO f CO 2 +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 CO 2 ) 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, O 2 , and the Au atom, particularly, among the d states of the Au atom and the antibonding 2π* states of CO and O 2 . 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 elec- tronic 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 Au- embedded 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 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 DMol 3 package. 10,11 The spin-unrestricted DFT in the generalized gradient approximation with the Perdew- Burke-Ernzerhof (PBE) functional 12 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 Dmol 3 , all-electron calcula- tions 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 per- formed with 3 × 3 × 1 k-point sampling. For the calculation of electronic properties, Monkhorst-Pack 6 × 6 × 1 k-point sampling was used 13 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 * To whom correspondence should be addressed. E-mail: phyfyp@ nus.edu.sg. † Department of Physics. ‡ Department of Chemistry. 20156 10.1021/jp908829m CCC: $40.75  2009 American Chemical Society Published on Web 10/30/2009 2009, 113, 20156–20160