Article pubs.acs.org/JPCC
Edges of FeO/Pt(111) Interface: A First-Principle Theoretical Study Yun Wang,† Haimin Zhang,† Xiangdong Yao,‡ and Huijun Zhao*,† †
Centre for Clean Environment and Energy, and Griffith School of Environment, Griffith University, Gold Coast, QLD 4222, Australia ‡ Queensland Micro- and Nanotechnology Centre, Nathan Campus, Griffith University, QLD 4111, Australia S Supporting Information *
ABSTRACT: An understanding of the reaction mechanisms of oxide/metal bicatalysts is important for their design to achieve better catalytic performance. Using the density functional theory calculations based on the GGA+U approach, the ferrous oxide (FeO) clusters on Pt(111) were systematically investigated as a model of oxide/metal bicatalyst since they showed high catalytic capacity on the preferential oxidation of carbon monoxide. Our calculations showed that the role of the coordinatively unsaturated ferrous (CUF) atoms at the edges of the FeO/ Pt(111) interface was to help the dissociative adsorption of oxygen molecules. The oxygen atoms at the edges in the intermediate were more chemically active according to the analysis of their electronic properties. They can selectively attract the carbon monoxide molecules to oxide them. After the desorption of carbon dioxide molecules, the CUF atoms at the edges can be reproduced. The high efficiency and selectivity of FeO/Pt(111) bicatalysts were, therefore, explained using our theoretical results. domains.11−15 However, the preference of adsorption sites for the Fe atoms at the edge of the FeO nanoislands is unidentified. Without information regarding the detail of such interface structures, the electronic properties determining the catalysis efficiency of CO PROX reaction will not be fully disclosed. The dI/dV spectra acquired from the bare Pt(111), inside of the FeO nanoislands and the FeO/Pt interface, revealed a distinct electronic state at 0.65 eV at the interface.10 So far, the nature of this electronic state is still not clarified. To explain the abovementioned experimental observations, a first-principle theoretical study was performed to probe the properties of the edges of the FeO/Pt(111) interface in this study. The catalytic mechanisms of FeO/Pt(111) bicatalysts in the CO PROX reactions were, then, discussed based on our theoretical results.
1. INTRODUCTION Oxide/metal bicatalysts are a special class of materials, widely used in the chemical industry, with high activity and selectivity. The interface is the most important part of a catalyst where the reactions occur.1−7 The understanding of the role of oxide/ metal interfaces at the atomic-level is essential for purposely designing better catalysts. Recently, two-dimensional ferrous oxide (FeO) nanoislands supported on Pt(111), which is termed as FeO/Pt(111) in this paper, were employed for the preferential oxidation (PROX) of carbon monoxide (CO).8 The CO PROX is very important in minimizing CO content in water-gas-shift produced hydrogen for polymer electrolyte fuel cells (PEFCs) since Pt anode catalysts in PEFCs can be inhibited even with very low concentration of CO.9 The FeO/ Pt(111) bicatalysts were found to be extraordinarily stable and active for CO PROX under the operating conditions of PEFCs.8 Therefore, the FeO/Pt(111) system was employed as a model of bicatalysts to explore the mechanisms of the catalytic oxidations in this study. On the basis of the previous experiments, several operational stages, that determine the overall catalytic efficiency of CO PROX reactions, occur at the edges of the FeO/Pt(111) interfaces.8 These stages include dissociative oxygen adsorption, CO adsorption, CO oxidation, and CO2 desorption. As a result, the knowledge of the edges of the FeO/Pt(111) interfaces is essential to reveal the foundation of CO PROX reactions. Because of the lack of direct experimental measurements, the atomic structure of the edges of the FeO/Pt(111) interface is still by and large a mystery.10 For example, when the FeO monolayer lays on Pt(111), Fe atoms may adsorb at the fcc, hcp and top sites of the surface to form three various © 2013 American Chemical Society
2. METHODS The FeO/Pt(111) systems were theoretically investigated using the density functional theory (DFT) calculations. All computations were performed using the VASP program, based on the all-electron projected augmented wave (PAW) method.16−18 We have used the softest pseudopotentials for carbon and oxygen atoms, which default cutoff energies are less than 282 eV. To ensure the energetic and electronic properties of the systems in our calculations are good enough, a cutoff energy as 353 eV, which is 25% higher than the default values, has been employed. We have compared the bond lengths of Received: August 14, 2012 Revised: January 6, 2013 Published: January 7, 2013 1672
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top two surface layers and the FeO nanoislands were allowed to relax, while the lower two layers were fixed at the ideal bulk-like position. The ionic positions were optimized until all forces were smaller than 0.01 eV/Å. To avoid the interaction between the FeO clusters in the neighboring surface cells, a (5 × 3√2) surface cell was employed, which included 30 Pt atoms in each layer. The Brillouin-zone was sampled with the Gamma-point only. We compared the adsorption energy of CO on the Pt(111) (5 × 3√2) surface with gamma-point only k-point mesh at the fcc site with the previous theoretical data. Our adsorption energy is −1.68 eV, which is very close to that previous theoretical data with denser k-point meshes, which are −1.64 to −1.66 eV.24,25 It demonstrated that the calculations using Gamma-point only mesh can give reliable results in this study.
CO and O2 using harder pseudopotentials with the cutoff energies as 500 eV. The bond length difference is about 0.05 Å, which can be accepted within the chemistry accuracy. To reduce the computation cost, we, therefore, used the softest pseudopotentials for oxygen and carbon atoms here. For the electron−electron exchange and correlation interactions, the functional of Perdew, Burke and Ernzerhof (PBE),19 a form of the general gradient approximation (GGA), was used throughout. With respect to the previous studies on the FeO film,11 different magnetic states of FeO clusters were considered in an attempt to find the right magnetic structures of the FeO nanoislands on Pt(111). The energies with nonspinpolarization, ferromagnetic (FM) spin-polarization and antiferromagnetic (AFM) spin-polarization were calculated. In the AFM calculations, two magnetic structures, nearest neighbor (NN) and row-wise (RW), as shown in Figure 1, were
3. RESULTS AND DISCUSSION While some theoretical studies have addressed the FeO monolayer or slabs on the Pt(111) surface,8,11,12,15,26 such models have difficulty reproducing the realistic details of the interface between the FeO nanoislands and Pt(111). In this study, the interactions between the small FeO clusters and Pt(111) were theoretically investigated. Since the studies using the FeO clusters as large as the realistic ones in the experiments are computationally formidable, three rather small FeO clusters (Fe10O6, Fe7O6, and Fe7O12) were employed on Pt(111) (see Figure 1). The three clusters were selected to model the FeO nanoislands with the coordinatively unsaturated ferrous (CUF) atoms before (Fe10O6, Fe7O6) or after (Fe7O12) oxygen dissociative adsorption (the details will be discussed below). Previous studies have demonstrated that the adsorption of the FeO monolayers on Pt(111) forms three domains: top, fcc and hcp (the name of the domains is in terms of the adsorption sites of Fe atoms on Pt(111)).11−14 As a result, the three clusters with the different adsorption sites of Fe atoms were theoretically explored. The stabilities of the FeO clusters at various locations were compared through the adsorption energies (ΔE) of the FeO clusters at various adsorption sites with different electronic states. The structural and energetic properties are listed in Table 1. The adsorption energies (ΔE) were calculated based on the equation: ΔE = Etot − Esurf − xEFe − (y/2)Eo2 . Here, the Etot or Esurf represents the total energy of system with or without FeO clusters, respectively. EFe is the average energy of each Fe atom in Fe bulk. And Eo2 is the energy of an isolated oxygen molecule in a 15 Å × 15 Å × 15 Å supercell. The calculations on Fe bulk and O2 molecules were performed with the consideration of spin-polarization. The value of x or y is the number of Fe or O atoms in the FeO cluster, respectively. Our
Figure 1. Optimized atomic structures and adsorption energies (ΔE) of three clusters: Fe10O6, Fe7O6, and Fe7O12 on Pt(111) with the nearing neighboring (NN) or row-wise (RW) magnetic structures. Key: gray, Pt; red, O; brown or purple, Fe with opposite magnetization.
investigated. Because of the insufficient consideration of the onsite Columbic repulsion between the Fe 3d electrons, DFT may fail to describe the electronic structures of the FeO.20 To overcome this shortcoming, the GGA+U approach was used.21 Following previous studies, we chose U-J as = 3.0 eV for the Fe atoms.8,12 The Pt(111) surface was modeled by a supercell comprising a four-layer slab, separated by a vacuum region of six-layer equivalent thickness. The methods proposed by Neugebauer et al. and Makov et al. were used to correct for the surface dipole moment.22,23 When the atomic geometries were optimized, the
Table 1. Adsorption Energy of Fe10O6, Fe7O6, and Fe7O12 Clusters on Pt(111) Surface and Their Structural Properties with Different Magnetic Statesa Fe10O6 Fe10O6 Fe7O6 Fe7O6 Fe7O12 Fe7O12 a
adsorption site
magnetic state
ΔE (eV)
dFe‑surf (Å)
hcp fcc hcp fcc hcp fcc
RW NN RW RW RW RW
6.91 7.89 2.70 3.19 −5.16 −4.84
2.17 2.22 2.15 2.30 2.22 2.40
dOI‑surf (Å)
1.66 2.00
dOII‑surf (Å)
dOIII‑surf (Å)
2.94 2.88 2.93 3.31
3.11 3.19 2.98 3.18 2.87 2.83
The negative adsorption energy value corresponds to an exothermic adsorption process. 1673
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results demonstrate that Fe atoms can only adsorb at the hcp or fcc sites. As a result, only the properties of clusters at the hcp and fcc domains are shown in this paper. The total energies of the systems with the AFM states are about 10.0 or 1.0 eV lower than those with nonspin-polarization or FM state, respectively. The RW/AFM magnetic structure is the most stable state for all clusters when Fe atoms adsorb both at the hcp and fcc sites except the Fe10O6 cluster when Fe atoms adsorb at fcc sites. The RW/AFM and NN/AFM magnetic structures are shown in Figure 1. The various ΔE values demonstrate that the magnetic states affect the stability of FeO clusters greatly. And the configurations with the hcp adsorption sites of Fe atoms are found to be more stable for all three clusters. From Table 1, the adsorption energies of Fe7O12 are much lower than those of other two clusters. As shown in Figure 1, the edges of the FeO/ Pt(111) interface are oxygen atoms in the Fe7O12 cluster. As a comparison, the edges of the Fe10O6 or Fe7O6 clusters are comprised of the CUF atoms. In the Fe7O6 cluster, the coordination number of the Fe atoms at the edge is two, while in Fe10O6, the coordination number of the Fe atoms, at the corner, is merely one. It indicates that the FeO clusters can be stabilized under the oxygen-rich conditions. Indeed, FeO nanoislands are formed under the oxidizing conditions after depositing Fe on Pt(111) in experiments.10 The Fe7O12 clusters can be regarded as the product of three O2 molecules dissociatively adsorb at the edge of the Fe7O6 clusters through the interaction between O2 molecules and CUF atoms. Our calculations using the improved dimer method27 showed that there is almost no energy barrier for the dissociative adsorption of oxygen molecules at the edge of Fe7O6 clusters with the help from CUF atoms, which is in agreement with the conclusions drawn by the previous theoretical studies.8 As a result, the CUF atoms at the edge of the Fe7O6 cluster can easily attract O2 molecules to form intermediates as Fe7O12 for further CO PROX reactions. In our calculations, the average dissociative adsorption energy of the O2 atoms is −2.62 eV, which is much larger than the adsorption energy of the O2 molecules on the bare Pt(111) surface (−0.71 eV).8 On the other hand, according to the Bader charge analysis,28 each O atom at the edges of the Fe7O12 cluster carries −0.90 to −1.10 charge. The negatively charged O atoms at the edges can prohibit the aggregation of the FeO clusters because of their electrostatic repulsion. Since the Fe7O12 cluster with RW/AFM magnetic states represents the possible intermediate configuration during the catalysis process, its structural, electronic and catalytic properties were analyzed. As shown in Figure 2, there are three types of oxygen atoms, according to their coordination number in the Fe7O12 cluster. The oxygen atoms at the edges of the FeO/ Pt(111) interface (labeled as OI) interact with just one Fe atom. The remaining oxygen atoms are above the Fe layer. Each OII atom bonds with two Fe atoms; and each OIII atom bonds with three Fe atoms. At the same time, there are two types of Fe atoms in the Fe7O12 cluster. Except for the Fe2 atom, which interacts with three OIII atoms, the other Fe1 atoms bond with one OI, one OII and one OIII, simultaneously. To analyze the electronic properties of the FeO/Pt(111) system, the partial density of the states (PDOS) of the chemically distinctive atoms in Fe7O12 were calculated, as shown in Figure 2. It can be found that there are some PDOS peaks of OI and Fe1 ranging between 0.5 and 1.0 eV (highlighted in the gray areas in Figure 2). These states coincide with the electronic state at 0.65 eV found in the dI/dV
Figure 2. Atomic structure (left) and partial density of states (PDOS) of chemically distinctive Fe and O atoms (right) in Fe7O12 cluster on Pt(111) at hcp site. Key: gray, Pt; red, O; brown or purple, Fe with opposite magnetization.
spectra acquired from the edges of the FeO/Pt(111) interface.10 The 0.65 eV interfacial electronic state was once ascribed to the orbitals of CUF atoms at the edge based on the PDOS of the FeO monolayer on Pt(111).11 According to our PDOS analysis, it is also highly possible to ascribe the electronic state at 0.65 eV to the hybrid states between Fe and O atoms at the edges.10 The surface PDOS images can also be used to explain some experimental phenomena observed in the scanning tunneling microscopic (STM) images according to the Tersoff−Hamann theory.29 Figure 2 shows that the electronic states of all the Fe and O atoms at the energy area above EFermi + 2.0 eV are dense. As a comparison, the PDOS of the atoms are much more sparse at the low energy range (EFermi < E < EFermi + 1.0 eV). This explains why the STM images can only show atomic resolution under lower bias voltage, as found in the experiments.10 At the same time, the average height of the OI atoms in the intermediate (the oxygen atoms at the edges of the interface) to the surface is 1.66 Å, which is 0.56 or 1.23 Å lower than the average distance from the surface to the Fe layer and another O layer, respectively (see Table 1). Our recent STM simulation of the alkanethiol self-assembled monolayer on Au(111) has demonstrated that the PDOS of surface atoms is exponentially decayed according to their height.30,31 As a result, the OI atoms in the intermediates may not be observed through the STM experiments. From Figure 2, the PDOS images of the distinctive oxygen atoms are different. In comparison with the PDOS of the OII and OIII atoms, there are more states around the Fermi energy level in the PDOS of OI. Since the denser electronic states around the Femi energy level indicate that the reactivity of atoms are higher, the activity of the OI atoms is expected to be much higher than those of other oxygen atoms. On the basis of our knowledge, there is no theoretical study on the role of OI in CO PROX reactions. The adsorption of CO molecules on the Fe7O12/Pt(111) surface was, therefore, investigated. On the bare Pt(111) surface, the adsorption energy of CO at the fcc site is −1.68 eV, which is 0.29 eV lower than that at the top site. Our results agree with the previous theoretical conclusions.25,32 On the Fe7O12/Pt(111) surface (Figure 3B), the adsorption properties of CO change significantly. The adsorption energy of CO at the top site is the lowest when CO 1674
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for the adsorption of H atoms, such back-donation between the substrate and the H atoms is lack. Moreover, the electrostatic repulsions between the electrons in the O 2p fully occupied orbital and the H atoms will prohibit the formation of the OI-H bonds; and H atoms can only adsorb on the Pt(111) surface, which is far from the OI atoms. This explains the specifically high catalytic selectivity of the FeO nanoislands on Pt(111) for the CO PROX under the practical PEFC working conditions.8 On the basis of our calculations, the desorption energy of CO2 from the edge of Fe7O11/Pt(111) interface is 0.21 eV. The low desorption energy of CO2 suggests that the produced CO2 can desorb at the room temperature. After the produced CO2 molecules desorb from the catalysts, new interfacial CUF atoms will be reproduced (Figure 3A) since the OI atoms at the edges have been lifted. The active CUF atoms can dissociatively adsorb O2 molecules again under the oxidizing conditions since the interaction between O2 molecules and the CUF atoms are much stronger than the adsorption energy of CO and CO2 molecules, as demonstrated above in our calculations. Such a reaction cycle, as illustrated in Figure 3, explains the catalytic mechanism of FeO nanoislands on Pt(111) for CO PROX.
Figure 3. Illustration of the catalytic mechanisms of FeO nanoislands on Pt(111) for CO PROX reactions. Key: gray, Pt; red, O; brown or purple, Fe with opposite magnetization; blue, C.
is close to the OI atom at the edge. The adsorption configuration is shown in Figure 3C. The adsorption strength of CO is 0.47 eV stronger than that on the bare Pt(111) surface. This strengthened interaction can be ascribed to the direct interaction between CO and OI. After the structural optimization, the OI−C bond length is 1.37 Å, just 0.13 Å longer than that in the adsorbed CO molecule. The OI−CO interaction is also confirmed to be covalent by the electron localization function (ELF) analysis,33 as shown in the inset in Figure.3C (ELF is a chemically intuitive tool to estimate the probability of finding electron pairs in space). The adsorption energy of CO at the fcc site, without the direct OI−CO interaction, is 0.28 eV weaker than that with the configuration shown in Figure 3C. Meanwhile, using the improved dimer method,27 the energy barrier of CO adsorption was calculated to be less than 0.1 eV, which suggests that the adsorption of CO can occur with small dynamics barrier at the room temperature. Consequently, the oxygen atoms at the edges of the oxide/metal interface can attract CO molecules on Pt(111) preferentially, which is crucial for the CO PROX reactions. Since the purpose of the CO PROX is to remove CO from H2 for the PEFC, whether the OI atoms can oxidize dissociatively adsorbed H2 is important for the selectivity of this bicatalyst. In this end, the systems with adsorbed H atoms on Pt(111) in vicinity of Fe7O12 cluster were investigated. After the atomic structure optimization, it is found that the shortest O−H distance is 2.77 Å, which is much farther than the normal O−H bond length (∼1.0 Å). If the OI atom bonds with the H atom factitiously in the calculations, the formation energy is about +0.5 eV, which suggests that the formation of OI-H bond is energetically forbidden. This is in contrast to the attractive adsorption of CO close to the OI atoms. The differentia is ascribed to the electronic structures of adsorbates. For the Fe7O12/Pt(111) system, the unpaired electrons in the OI 2p orbitals are hybridized by the electrons in the d orbitals of the surrounding metallic atoms. Consequently, only OI lone-pair 2p electrons can be used for the further interaction with the CO or H. The previous studies have demonstrated that there are two main interactions of the CO binding with the Pt(111) surface: electron-donation from the CO HOMO 5σ orbital to the substrate 5d orbital, and back-donation from the substrate to the CO LUMO 2π* orbital when the CO molecules adsorb on Pt(111).25,34 When the CO molecules adsorb on FeO/Pt(111) surface, the back-donation can be from the lone-pair electrons in OI 2p orbital to the CO 2π* orbital. This is the reason why the CO molecules can be attracted by the OI atoms. However,
4. CONCLUSIONS In summary, the DFT calculations were performed to investigate the properties of the edges of the FeO/Pt(111). The interactions between the three FeO clusters and Pt(111) were systematically calculated. It was found that the adsorption energies of the FeO clusters are strongly related to their magnetic states and the adsorption sites of the Fe atoms. Our calculations demonstrate that the atomic structure comprised of Fe atoms adsorbed at the hcp sites with the RW/AFM magnetic state is the most stable configuration for the edges of the FeO/Pt(111) interfaces. The oxygen molecules can dissociatively adsorb through the help from the CUF atoms in FeO clusters to form oxygen atoms at the edges under the oxidizing conditions. According to our PDOS analysis, the oxygen atoms at the edges are more chemically active, compared with other oxygen atoms above the Fe layers. As a result, the oxygen atoms, confined at the edges, can contribute to the selective adsorption of CO molecules in their vicinities to form CO2. After the desorption of CO2, the active CUF atoms will be reproduced at the edge of the FeO/Pt(111) interface. Our catalytic mechanisms can explain the experimental observation very well, which offers the theoretical base for the molecular design of novel bicatalysts in the future.
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ASSOCIATED CONTENT
S Supporting Information *
Corresponding atomic coordinates. This material is available free of charge via the Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
*E-mail: h.zhao@griffith.edu.au. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Australian Research Council (ARC) for funding. This research was undertaken on the National Computational Infrastructure (NCI) in Canberra, Australia. 1675
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