ARTICLE pubs.acs.org/JPCC
Fe-Anchored Graphene Oxide: A Low-Cost and Easily Accessible Catalyst for Low-Temperature CO Oxidation Fengyu Li,† Jijun Zhao,‡ and Zhongfang Chen*,† † ‡
Department of Physics, Department of Chemistry, University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico 00931 Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, College of Advanced Science and Technology, Dalian University of Technology, Dalian 116024, China
bS Supporting Information ABSTRACT: By means of first-principles computations, we investigated the catalytic capability of the Fe-anchored graphene oxide (FeGO) for CO oxidation with O2. The high-energy barrier of Fe atom diffusion on GO and the strong binding strength of Fe anchored on GO exclude the metal clustering problem and enhance the stability of the FeGO system. The Fe-anchored GO exhibits good catalytic activity for CO oxidation via the favorable Eley Rideal (ER) mechanism with a two-step route, while the Langmuir Hinshelwood (LH) mechanism is not kinetically favorable. The low-cost Fe-anchored GO system can be easily synthesized and serves as a promising green catalyst for low-temperature CO oxidation.
1. INTRODUCTION Low-temperature oxidation of CO is one of the most extensively studied reactions of heterogeneous catalysis,1,2 due to the impending demand of lowering emissions from automobiles, industrial processes, etc. Noble metal nanocatalysts exhibit high activity for CO oxidation even in the presence of moisture; however, typically such catalysts are only efficient at high temperature (>100 °C).3 A shining exception is the gold nanoparticles supported on transition-metal oxides, which were reported to be highly active at low temperature from both experimental49 and theoretical aspects.1013 Though the noble metal (Pd,3,14 Au,413,15 Pt,16 Rh,17 etc.) based catalysts are efficient for the CO oxidation, the cost and limited amount in nature impede their general use in large-scale production. With dwindling supplies of the noble metals, replacing these metals used in catalysts with inexpensive abundant metals is extremely important. Thus, it is a grand challenge to develop low-cost and high-activity non-noble metal based catalysts for low-temperature CO oxidation under ambient atmospheres, and rather encouraging results have been achieved, such as the iron clusters18 and the oxide clusters of iron1921 and cobalt.2224 Substrates may enhance the stabilities and improve the catalytic properties of nanocatalysts. In this regard, graphene has been chosen as a substrate for noble metals (Pt, Au), and such systems demonstrated high activity for CO oxidation at room temperature.2528 For non-noble metal based catalysts, Li et al.29 recently performed first-principles computations to explore the catalytic performance of graphene embedded with Fe (where one carbon atom of the graphene hexagonal lattice is substituted by an Fe atom) for CO oxidation, and they found that such low-cost r 2011 American Chemical Society
Fe-embedded graphene shows good catalytic activity for the CO oxidation via the more favorable EleyRideal (ER) mechanism with a two-step route. Similarly, Song et al. theoretically designed the Cu-embedded graphene, which shows comparable catalytic behavior.30 However, synthesizing these Fe/Cu embedded graphenes is not easy in experiments, although in principle they can be prepared, for example, by trapping the metal atoms in the defective graphenes.31 Graphene oxide (GO), the single-layered graphite oxide, serves as the alternative substrate. Besides many applications of GO and reduced GO (RGO) materials in the fields of memory devices, optical micrograph devices, electrodes for solar cells, etc.,3244 the RGO was used as substrate for catalytic systems,45,46 and GO was predicted as an ideal substrate for hydrogen storage.47,48 Since GO possesses ample O sites on the surface, which are the key to anchoring metal atoms and enhancing the metalGO binding to avoid metal clustering, GO can serve as the potential substrate to covalently anchor metal atoms. In this work, we performed first-principles computations to explore the catalytic performance of the Fe-anchored graphene oxide system, aiming to search a low-cost and green catalyst for CO oxidation to relieve the growing environmental problems caused by CO emission. Our computations show that Fe atoms bind strongly to the oxygen sites with the binding energy as high as 7.09 eV, which excludes metal clustering. Adsorption of O2 on FeGO is preferred over the CO molecule by 0.55 eV, and the Received: October 4, 2011 Revised: December 21, 2011 Published: December 22, 2011 2507
dx.doi.org/10.1021/jp209572d | J. Phys. Chem. C 2012, 116, 2507–2514
The Journal of Physical Chemistry C reaction of CO with O2 is followed by the EleyRideal (ER) mechanism on Fe-anchored graphene oxide. The low reaction barriers indicate that the CO oxidation process can occur at relatively low temperature.
2. COMPUTATIONAL DETAILS Although GO was first produced over 150 years ago, and its physical/chemical properties and applications were extensively studied, the detailed atomic structures of GO are still not clear.4956 Experimentally, the structures of GO have been characterized by nuclear magnetic resonance (NMR),4958 X-ray photoelectron spectroscopy (XPS),51,5862 scanning tunneling microscopy (STM),63,64 and Raman spectroscopy.51,52,60,62 It was found that hydroxyl and epoxy groups are the two major functional groups on the GO surface, along with small amount of ketone, carbonyl, phenol, and other groups. NMR measurements suggested that the hydroxyl and epoxy groups prefer to attach to the adjacent carbon atoms.49 However, the detailed GO structures depend on the specific synthesis method and the degree of oxidation. Theoretically, various structural models of GO were proposed,47,54,6570 and the NMR54 and XPS71 spectra were also simulated to assist experimental characterization. Yan et al.66,69 and Wang et al.47,67 independently identified the energetically favorable atomic configuration of GO, which contains epoxy and hydroxyl groups in close proximity with each other, and found that these functional groups prefer to aggregate together. Very recently, Lu et al. studied the thermodynamics and kinetics of GO and revealed that the kinetic effects during the GO synthesis play an important role in the GO structure.72 According to the simulated NMR spectra by Yang and co-workers,54 the structural model proposed by Wang et al.47 agrees best with the NMR experiments of GO. Thus, we chose Wang’s GO model as our simulation supercell, which is periodically repeated in a (3 2)
Figure 1. Possible sites (14) depicted by the dashed lines for anchoring Fe atom on graphene oxide, where the Fe atom is bonded to the oxygen atoms. Color scheme: C, gray; H, green; O, red.
ARTICLE
unit cell (containing 72 C, 24 H, and 36 O, in total 132 atoms) separated by ∼15 Å of vacuum space (see Figure 1). Our spin-polarized density functional theory (DFT) computations were based on the generalized gradient approximation (GGA) with the PerdewWang exchange-correlation functional (PW91).73 The frozen-core all-electron projector augmented wave (PAW) method74 was used as implemented in the Vienna ab initio simulation package (VASP) code.75 The Monkhorst Pack (MP) scheme76 was applied to generate k points to sample the reciprocal space. According to our test calculations, it is sufficient to choose a kinetic energy cutoff of 500 eV for the plane-wave basis set and a 2 2 1 k points mesh, respectively (increasing the kinetic energy cutoff to 550 eV or larger only lowers the energy of the GO supercell by
