Selective Growth of Fe2O3 Nanoparticles and Islands on Au(111

May 28, 2009 - Selective growth of well-defined α-Fe2O3 structures was achieved on a Au(111) surface and characterized using X-ray photoelectron ...
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Selective Growth of Fe2O3 Nanoparticles and Islands on Au(111) Xingyi Deng*,†,‡ and Christopher Matranga† National Energy Technology Laboratory (NETL), United States Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236, and Parsons Project SerVices, Inc., P.O. Box 618, South Park, PennsylVania 15129 ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: April 29, 2009

Selective growth of well-defined R-Fe2O3 structures was achieved on a Au(111) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Although oxidation of Fe particles on Au(111) with molecular O2 at room temperature forms FeO, Fe2O3 is prepared by oxidation of Fe particles on Au(111) with NO2 at an elevated temperature, as verified by XPS, based on the binding energy (BE) value of the Fe 2p3/2 (710.9 eV) peak and atomic ratio of O and Fe ∼1.5:1. STM images reveal that Fe2O3 adopted ordered three-dimensional structures on Au(111). Although the general morphology of the Fe2O3 structures on Au(111) depends on the coverage, varying from nanoparticles at low coverage to islands at high coverage, all of these Fe2O3 structures have nearly identical heights of 5-6 Å at all coverages. The surface structures of the Fe2O3 are all consistent with an O-terminated R-Fe2O3(0001), showing a hexagonal unit cell with a lattice constant of ∼3 Å in atomically resolved STM images. Introduction Iron and iron-oxide based catalysts play an important role in the Fischer-Tropsch reaction, gas-sensing applications, ammonia synthesis, styrene synthesis, and many other important industrial processes.1 The complexity of the heterogeneous catalysts used in these processes makes it difficult to elucidate critical information about kinetics, reaction barriers, active sites, and the role of promoters. Some of the complexity of these systems can be simplified by using Fe-based single-crystals as model substrates; however, these single crystals lack the unique structural defects, step densities, and edge sites inherent to realistic catalysts. These features tend to dominate reactivity, particularly for nanocrystalline size regimes. The coupling of scanning tunneling microscopy (STM) capabilities with traditional ultrahigh vacuum surface science techniques gives researchers the ability to grow model catalyst particles on well-defined substrates, fully characterize these model catalysts, and study their reactivity. These model catalysts mimic, in many ways, the size, shape, and structural defects present in real heterogeneous catalyst systems. More importantly, STM and other surface techniques can be used to characterize the structure and reactivity of these model systems in a manner that cannot be achieved with traditional heterogeneous catalysts. In this regard, small Au clusters grown on single-crystal TiO2 surfaces have been used as a model gold catalyst for low-temperature CO oxidation studies.2 This work illustrated that a strong correlation exists between the size of the Au clusters and their reactivity. TiOx and CeOx nanoparticles grown on Au(111) have also been used as model catalysts and have been shown to have activity for the water-gas shift (WGS) reaction.3 Using this approach, we have chosen to focus on Fe-based catalysts grown on Au(111). These Fe-based catalysts were chosen because they are used to convert the CO and H2 from * To whom correspondence should be addressed. E-mail address: [email protected]. † National Energy Technology Laboratory (NETL). ‡ Parsons Project Services, Inc.

coal-derived syngas into liquid hydrocarbons and waxes through the Fischer-Tropsch process. These liquids can then be utilized as fuels or as hydrogen carriers suitable for transport through existing infrastructure.4 Currently, there is little atomistic level experimental data on how specific surface sites, crystallite size, and particle structure affect the reactivity of these FischerTropsch catalysts. Recent in situ X-ray studies have observed higher Fischer-Tropsch reaction rates from smaller Fe carbide crystallites suggesting that size variations or the associated density of active sites plays a critical role in the Fischer-Tropsch process.5 The mechanistic role of Cu and K promoters in the activation steps and associated mechanisms are also not fully understood. The Au(111) surface was chosen as a growth substrate for Fe-based materials because of its inertness in comparison to other metals. Previous studies have evaluated the growth of TiO2,6-8 MoO3,9-13 and CeO214,15 nanostructures on Au(111). The morphology of these oxide nanostructures depends on the preparation method, which provides a route to grow a variety of structures in a controlled fashion. For example, a one-step oxidation of Mo clusters by NO2 results in ramified MoO3 nanoclusters;10 whereas iterative Mo deposition and NO2 oxidation yields novel, crystalline, MoO3 nanostructures.11 In addition, reduction of these MoO3 structures by annealing in an ultrahigh vacuum creates one-dimensional oxygen vacancies associated with a Mo5+ state.13 Previous researchers have studied the growth of Fe oxide films on Pt(111)16-22 and Ru(0001).23 This work provides critical information to guide our current Fe oxide growth efforts using the Au(111) substrate. On Pt(111), Fe oxides grow in a Stranski-Krastanov mode forming well-ordered films. Depending on the growth conditions, one can prepare FeO(111), Fe3O4(111), and R-Fe2O3(0001) films.1 In spite of the extensive body of literature on epitaxial Fe oxide films, the growth of Fe oxide nanoparticles on single crystals has not been investigated at the same level of detail. These nanoparticles preserve many of the edges and defects inherent to realistic catalysts and serve as a system for bridging the “material gap” that exists between

10.1021/jp9021954 CCC: $40.75  2009 American Chemical Society Published on Web 05/28/2009

Selective Growth of Fe2O3 Nanoparticles on Au(111)

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the single-crystal substrates used in traditional ultrahigh vacuum surface science work and heterogeneous materials used in catalysis studies. In this work, we demonstrate that it is possible to selectively grow R-Fe2O3 nanoparticles on Au(111) using a strong oxidizer such as NO2 for converting elemental Fe nanoparticles into an oxide phase. The ability to selectively grow metal oxides with controlled oxidation states is important because the oxidation states dictate the structure and catalytic properties of the material. One example of this is illustrated by the inertness of Oterminated FeO(111) toward the chemisorption of ethylbenzene and styrene in comparison to Fe3O4(111) and R-Fe2O3(0001), which are capable of chemically adsorbing both molecules.1 We have characterized the R-Fe2O3 nanoparticles grown in our studies using X-ray photoelectron spectroscopy (XPS) and STM and compared the results to our previous work on Fe and FeO grown on Au(111).24 The atomic structure of the R-Fe2O3 nanoparticles were revealed by STM and compared to previous work on R-Fe2O3(0001) films grown on Pt(111).19 Experimental Section All experiments were performed in a commercial UHV chamber from Omicron Nanotechnology GmbH as described in detail elsewhere.24 Briefly, the chamber is equipped with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and low-energy ion scattering (LEIS). The base pressure was maintained at ∼3 × 10-10 mbar. The Au(111) sample (Princeton Scientific Corp.) was cleaned by cycles of Ar+ sputtering (P ) 1 × 10-6 mbar, 1.5 keV) at 300 K, followed by annealing at 700 K for 30 min. The procedure was repeated until no contaminants, especially C and O, were detected using XPS. After this procedure, the herringbone pattern, a characteristic of the Au(111) reconstruction,25,26 was observed in STM images. Iron was deposited on Au(111) at 300 K using an electron beam-assisted evaporator (Omicron EFM3T) from a rod material (Goodfellow, 2.0 mm, 99.99%) at 920 eV and 7.5 mA (emission). Fe coverage was controlled by varying the evaporation time, typically 2-20 min, while maintaining the same evaporation flux (∼2.7 nA). We estimated that evaporation for 20 min under these conditions results in Fe coverage of about 1 ML, which was determined using the following relations

Fe coverage )

NFe )

NAu )

NFe NAu

IFe2p FFe2p

IAu4f7/4 FAu4f7/4

(1 - e-d/λ)

(1)

(2)

(3)

Figure 1. Fe 2p and O 1s XP spectra of (a) 0.25 ML of elemental Fe, (b) 0.25 MLE of FeO, and (c) 0.25 MLE of Fe2O3 on Au(111). Fe was deposited on Au(111) at 300 K using an electron beam-assisted evaporator from a rod material. FeO was prepared from oxidation of elemental Fe at 323 K by backfilling the chamber with O2 (3 × 10-7 mbar) for 500 s followed by annealing at 700 K in UHV for 10 min. Fe2O3 was prepared from the oxidation of elemental Fe by heating the sample in NO2 (directional dosing at 1 × 10-8 mbar with an enhancement factor of 30-50) from 327 to 450 K, followed by annealing in UHV at 700 K for 10 min.

comparing the evaporation times and assuming that the 20 min evaporation equals 1 ML. Following deposition on Au(111), Fe was oxidized using NO2. In the oxidation experiments, the coverage is noted as a monolayer equivalent of Fe (MLE), referring to the initial coverage of evaporated Fe on the Au(111) surface used to form the Fe oxide material under investigation as suggested in previous work.24 Oxidation of Fe using NO2 was carried out by heating the sample in NO2 (directional dosing at 1 × 10-8 mbar with an enhancement factor of 30-50) from 327 to 450 K (heating rate ∼8 K/min) followed by annealing in UHV at 700 K for 10 min. The directional dosing was used here to minimize the impact of NO2 on the base pressure of the chamber. NO2 (research grade) was used as received and without further purification. XPS measurements were collected using an Mg KR X-ray source (1253.6 eV, 300 W) and a hemispherical analyzer with pass energy of 20 eV at room temperature. The binding energy was calibrated with the Au 4f7/2 peak at 83.8 eV for each spectrum. All STM measurements were performed at room temperature in constant current mode using etched W tips purchased from Omicron. The STM was calibrated using the 2.4 Å step height of Au(111).25,28 All STM images were processed using scanning probe imaging processor (SPIP) brand software, using only plane corrections and contrast enhancements (as noted). Results

where IFe2p and IAu4f7/4 are integrated intensities measured from XPS, FFe2p () 3.8) and FAu4f7/4 () 1.9) are the atomic sensitivity factors obtained from the XPS handbook,27 d () 2.4 Å) is the distance between two identical Au(111) planes, and λ () 15.8 Å) is the inelastic mean free path (IMFP) of photoelectrons with kinetic energy (KE) of 1200 eV in gold, based on the NIST database. Fe coverage below 1 ML was then obtained by

Figure 1 shows the XP spectra of the Fe 2p and O 1s regions of 0.25 MLE Fe2O3 grown on the Au(111) surface. For comparison, the XP spectra of elemental Fe (0.25 ML) and FeO (0.25 MLE) are also shown in Figure 1. The binding energy (BE) of the Fe 2p3/2 peak of elemental Fe is located at 706.6 eV, consistent with the literature value (Figure 1a).27 As expected, no peak was detected in the O 1s region. In FeO, the

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TABLE 1: Quantification of XPS Signals of Fe after Oxidation by NO2 at Elevated Temperature evaporation Fe coverage time (min) (ML) IFe2p ASFFe2p IO1s ASFO1s O-to-Fe ratio 2 5 10 15 20

0.1 0.25 0.5 0.75 1

1136 3163 6488 9801 12732

3.8 3.8 3.8 3.8 3.8

265 777 1610 2513 3197

0.63 0.63 0.63 0.63 0.63

1.41 1.48 1.50 1.55 1.51

Fe 2p3/2 XP peak shifts +2.7 eV to 709.3 eV due to the Fe2+ state and is accompanied by the appearance of an O 1s XP peak with a BE of 529.8 eV (Figure 1b). The atomic ratio of O and Fe is estimated to be ∼1:1, based on integrated XPS signal intensities after taking the atomic sensitivity factors (ASF) into account, consistent with the FeO stoichiometry. The BE of the Fe 2p3/2 peak of Fe2O3 is located at 710.9 eV (Figure 1c), in a good agreement with the literature value.29,30 A further +1.6 eV shift compared to that of FeO is consistent with the formation of a higher oxidation state, i.e., Fe3+ state. Correspondingly, an O 1s peak is located at 529.8 eV but with a higher intensity than was observed for FeO. The atomic ratio of O and Fe is estimated to be ∼1.5:1, based on XPS measurements (Table 1), consistent with the Fe2O3 stoichiometry. Although Fe2O3 has three phases (R, γ, and ε) and the latter two are metastable, we suggest that Fe2O3 prepared in the present study is R-phase. This has also been verified by our high-resolution STM studies and will be discussed in detail below. The morphologies and structures of elemental Fe and FeO grown on Au(111) have been studied in detail in previous work.24,31,32 Briefly, Fe preferably nucleates and grows at the elbow sites of the herringbone reconstruction on Au(111). After oxidation using molecular O2 at room temperature followed by annealing in UHV at 700 K for 10 min, the resultant FeO forms two-dimensional particles (and continuous films at coverage >0.8 MLE). The atomic structure of these particles suggests the growth of FeO(111) on Au(111). The morphology of the R-Fe2O3 particles prepared from NO2 oxidation differs dramatically from Fe and FeO particles as revealed by STM images (Figure 2). Specifically, R-Fe2O3 tends to exist as smaller particles in comparison to FeO particles formed from an identical initial Fe coverage ∼0.25 ML and annealed under the same conditions (Figure 2a,b). Analysis of the particle size distribution shows that most of the R-Fe2O3 particles reside in the size range of 18-35 nm2 with an average diameter of ∼5 nm (Figure 2c). The R-Fe2O3 particles adopt either a triangular or hexagonal shape and reside mostly on the terraces of the Au(111) surface. Linescans show that all of the Fe2O3 particles are ∼6 Å in height (Figure 2d), which is higher than the ∼2.0 Å for Fe or the ∼1.7 Å for FeO.24 A detailed analysis of over 100 particles shows that the average height of the Fe2O3 particles is 5.8 ( 0.2 Å in our STM studies. Figure 3 shows a detailed image of one of the R-Fe2O3 particles. This particle has an irregular hexagonal shape and shows an ordered surface structure (Figure 3a). This image looks blurry because of the large height difference (∼6 Å) between the particle surface and substrate, which makes it difficult for the STM tip to respond. Nevertheless, a clear atomic image was obtained once we focused on imaging the flatter particle surface as shown in Figure 3b. The atomically resolved image shows a hexagonal unit cell with a lattice constant of ∼3 Å. A defect appearing as a depression is also detected, and a linescan across this defect site is shown in Figure 3c. The corrugation at the regular sites is about 0.1 Å, while at the defect site the

Figure 2. Room temperature STM images of 0.25 MLE R-Fe2O3 on Au(111) prepared from Fe oxidation in NO2 at an elevated temperature, followed by annealing in UHV at 700 K for 10 min: (a) 300 × 300 nm2 (I ) 0.5 nA, V ) 1.0 V), (b) 100 × 100 nm2 (I ) 0.5 nA, V ) 1.0 V), (c) distribution of particle sizes from image in panel b, and (d) linescan of panel b.

Figure 3. Room temperature STM images of an individual R-Fe2O3 nanoparticle: (a) 6 × 6 nm2 (I ) 0.5 nA, V ) 1.0 V), (b) 2.5 × 2.5 nm2 (I ) 0.5 nA, V ) 0.5 V), and (c) linescan.

corrugation is ∼0.3 Å under the tunneling conditions specified in Figure 3b. The XPS results for Fe2O3 prepared using NO2 oxidation with varying initial Fe coverages are shown in Figure 4. With an increasing initial Fe deposition coverage ranging from 0.1 ML to 1 ML, there is no BE shift in the Fe 2p3/2 and O 1s peaks. Specifically, the Fe 2p3/2 peak and O 1s peak appear at 710.9 and 529.8 eV, respectively, at all coverages with increasing intensity corresponding to increasing Fe coverage. At 1 MLE Fe coverage, the shakeup satellite line characteristic for the Fe3+ species33 at ∼719.0 eV starts to become visible. The narrow and symmetric line shape of the Fe 2p3/2 peak at this coverage also suggests that we may rule out the formation of Fe3O4, which typically has a broader Fe 2p peak due to the presence of Fe3+ and Fe2+ species. The atomic ratios derived from the XPS results are listed in Table 1. Atomic ratios of O and Fe are found to be ∼1.5:1 for all initial coverages of Fe. This result is consistent with the stoichiometry of R-Fe2O3. STM images of R-Fe2O3 at coverages of 0.1, 0.5, 0.75, and 1 MLE are displayed in Figure 5. Linescans of the R-Fe2O3 particles (not shown) illustrate that the particles formed from all coverages of Fe adopt a more three-dimensional structure in comparison to those of elemental Fe particles or FeO particles. Although the structure heights vary slightly between 5 and 6 Å for the different initial Fe coverages, they are almost identical within a specific coverage. For example, the average heights for R-Fe2O3 at 0.1 and 0.75 MLE coverages are 5.1 ( 0.1 and 5.7 ( 0.1 Å, respectively. The overall morphology of the

Selective Growth of Fe2O3 Nanoparticles on Au(111)

Figure 4. Fe 2p and O 1s XP spectra of R-Fe2O3 on Au(111) prepared from oxidation of Fe with initial deposition coverage of 0.1, 0.25, 0.5, 0.75, and 1 ML, using NO2 at an elevated temperature, followed by annealing in UHV at 700 K for 10 min.

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Figure 6. (a) Top view of the R-Fe2O3(0001) surface. Red balls represent O atoms, and blue and green balls are Fe atoms from two different layers. Unit cells of the O (0001) layer and Fe (0001) layer are highlighted. (b) Side view of the R-Fe2O3(0001) surface with O-termination. (c) Room-temperature STM image of a R-Fe2O3 particle showing layered edges consisting of three visible steps. Image size is 10 × 10 nm2 (I ) 0.5 nA, V ) 0.5 V).

structure, with a lattice constant of ∼3 Å, was observed by STM and is highlighted in the inset of Figure 5b. Further increasing the R-Fe2O3 coverage to 0.75 MLE causes the formation of even larger Fe2O3 particles, which start to coalesce (Figure 5c). As the coverages of R-Fe2O3 are increased to 1 MLE, the resulting R-Fe2O3 particles coalesce to form large irregular shape islands (Figure 5d). These R-Fe2O3 islands cover only ∼80% of the Au(111) surface at 1 MLE coverage, consistent with their three-dimensional character. Nevertheless, they have the same surface structure and a similar height (∼5 Å) in comparison to those of the R-Fe2O3 particles observed at low coverage. Discussion

Figure 5. Room temperature STM images of R-Fe2O3 on Au(111) prepared from oxidation of Fe using NO2 at an elevated temperature, followed by annealing in UHV at 700 K for 10 min: (a) 0.1 MLE (I ) 0.5 nA, V ) 1.0 V), (b) 0.5 MLE (I ) 0.5 nA, V ) 1.0 V), (c) 0.75 MLE (I ) 0.5 nA, V ) 1.0 V), and (d) 1 MLE (I ) 0.05 nA, V ) 1.0 V). Image sizes are 100 × 100 m2. Inset in panel a shows clear Au(111) reconstruction after enhancing the image contrast (image size, 30 × 30 nm2). Inset in panel b shows the atomic structure of an individual R-Fe2O3 nanoparticle (image size, 2 × 2 nm2, I ) 0.9 nA, V ) 0.1 V).

R-Fe2O3 structures also depends on the initial coverage of Fe used to form the oxide. At low coverage (0.1 MLE), R-Fe2O3 forms small particles, adopting either a triangular or hexagonal shape. The average diameter of these particles is ∼4 nm, which is slightly smaller than that seen for the R-Fe2O3 particles at 0.25 MLE coverage. After enhancing the image contrast, the reconstruction of Au(111) is clearly visible at this coverage as shown in the inset of Figure 5a. Increasing R-Fe2O3 coverage to 0.5 MLE causes the formation of larger R-Fe2O3 particles (Figure 5b). Most of the particles shown in Figure 5b have an irregular hexagonal shape with a few having more elongated shapes. The average size of the particles in Figure 5b is ∼6-7 nm. In some cases, 2-3 particles coalesce to form larger particles, with boundaries showing contrast differences in the STM image. A hexagonal surface

Fe and O form a number of phases with different stoichiometries and crystal structures, including R-Fe2O3 presented in this study. Bulk R-Fe2O3 (hematite) crystallizes in the corundum structure with a hexagonal unit cell. The O2- anions in hematite form a hcp sublattice with ABAB stacking, and the Fe3+ cations are located in the interstitials arranged as distorted octahedra, which form two hexagonal sublayers. We will discuss the surface structures of Fe2O3 prepared in this work, based on its corresponding bulk phase. R-Fe2O3 favors a more three-dimensional structure with smaller-sized particles on the Au(111) surface compared to those in the previous report of FeO on Au(111).24 At initial Fe coverages of 0.25 ML, where FeO forms large two-dimensional particles with sizes of 10-25 nm, R-Fe2O3 remains comparable in size to that of the originally grown Fe particles (∼ 5 nm). As the free surface energy of R-Fe2O3 (0.754-1.153 J m-2)34 is lower than that of gold (1.62 J m-2),35 formation of large R-Fe2O3 particles seems to be kinetically hindered. This may be associated with the limited mobility of the three-dimensional structures or due to a stronger interaction between R-Fe2O3 and the Au substrate. At coverages >0.5 MLE, the R-Fe2O3 particles coalesce as a result of increasing particle density. The atomic structure obtained from high-resolution STM experiments suggests that the surfaces of the R-Fe2O3 nanoparticles and islands are O-terminated (0001). The top view and side view of R-Fe2O3 with an O-terminated (0001) surface are schematically shown in panels a and b of Figure 6. Briefly, the O2- anions form a slightly deviated hexagonal unit cell with

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an average O-O interatomic distance of 2.91 Å, corresponding to the lattice constant of this O-terminated (0001) surface. However, the Fe3+ cations form two hexagonal sublayers (labeled as blue and green in Figure 6a, respectively). If the (0001) surface is terminated with Fe, the lattice constant should be 5.03 Å, corresponding to the Fe-Fe interatomic distance within a Fe layer. The lattice constant of the R-Fe2O3 structures in this work was measured to be ∼3 Å, consistent with that of an O-terminated (0001) surface. Accordingly, the defects observed on the R-Fe2O3 particle surface most likely correspond to oxygen vacancies (Figure 3), which could be formed during annealing. The atomic structure of the R-Fe2O3(0001) films grown on Pt(111) has also been characterized using high-resolution STM.1 Although the surface was determined to be O-terminated on the basis of dynamical LEED analysis and ion scattering spectroscopy (ISS) measurements,36 the atomic structure in STM showed a hexagonal unit cell with a lattice constant of ∼5 Å, corresponding to the periodicity of the Fe layer(s) between the O layers. The study believes that the Fe sublayer(s) were imaged under their STM conditions because of a strong electronic contribution from the Fe sublayer(s), where the Fe 3d-derived states are formed and represent the only occupied and unoccupied states near the Fermi edge.37,38 In the present study, however, the atomic structure revealed by our STM analysis only reflects the periodicity of the topmost O layer on the R-Fe2O3(0001) surface. While R-Fe2O3(0001) grown on Pt(111) forms large films with a mesoscopic surface roughness of ∼50 Å over a 1 µm length, R-Fe2O3 on Au(111) has a completely different morphology, showing instead individual particles of a few nanometers in size or coalesced islands formed from these particles. In comparison to the R-Fe2O3 films grown on Pt(111), it is possible that the electronic properties of these R-Fe2O3 structures on Au(111) are modified because of their reduced dimensions or as a result of the underlying Au substrate itself. This electronic effect could account for why previous authors have seen the periodicity of the Fe sublayers in STM images of O-terminated R-Fe2O3 films grown on Pt(111), whereas our images of O-terminated R-Fe2O3 structures on Au(111) only see the periodicity of the top O surface layer. The observation that all the R-Fe2O3 structures in our study have a nearly identical height at every coverage examined is a particularly interesting result. As shown in Figure 6b, the distance between two equivalent O (0001) planes in hematite is ∼2.29 Å. The observation of a uniform height of ∼5-6 Å thus implies that these R-Fe2O3 structures grown on Au(111) consist of two or three layers of O (0001) planes with Fe positioned at the interstitial sites between these planes. This is directly supported by our high-resolution STM images. Specifically, the edges of a R-Fe2O3 particle were carefully imaged, showing a layered structure consisting of three visible steps as indicated by arrows in Figure 6c. Growth of taller R-Fe2O3 structures seems to be kinetically hindered under our preparation conditions. The R-Fe2O3 structures grown on the Au(111) surface provides a model system for studying Fischer-Tropsch-based reactions. Traditional Fischer-Tropsch synthesis starts with R-Fe2O3, which is activated to a mixture of Fe, Fe oxide, and Fe carbide phases, before CO and H2 from coal-derived syngas are converted into liquid hydrocarbon fuels and waxes.5,39-44 Previous reports noting a linear relation between the Fe carbide concentration and Fischer-Tropsch synthesis rate during catalytic reactions suggests that Fe carbide is the “true” active phase for this highly heterogeneous system. However, there is still

Deng and Matranga little atomic level experimental data regarding the activation process for these catalysts. For example, the active site(s) present in the starting R-Fe2O3 material, which lead to the formation of the active Fe carbide phase, have not been identified. Neither have the mechanistic steps involved when Cu and K promoters facilitate the reduction of Fe2O3 to Fe3O4 and the carburization of the system to form Fe carbide. In addition, the atomic structure of the active Fe carbide phase formed during FT synthesis has not yet been determined experimentally, although DFT calculations have suggested a few Fe carbide structures.45-47 The success of growing R-Fe2O3 particles in a controlled fashion would be a significant first step toward addressing these questions. We are currently pursuing these studies in our group. Conclusions Well-ordered R-Fe2O3 structures have been selectively grown on a Au(111) surface via deposition of metallic Fe followed by oxidation using NO2, based on XPS and STM characterizations. R-Fe2O3 adopts a more three-dimensional structure with an identical height of 5-6 Å. At low coverage (up to 0.25 MLE), the three-dimensional R-Fe2O3 forms small crystalline particles with average sizes of about 4-5 nm. Atomically resolved STM images show that the surface of these R-Fe2O3 structures consists of a hexagonal unit cell with a lattice constant of ∼3 Å, in agreement with an O-terminated R-Fe2O3(0001) surface. With increasing coverage, some particles coalesce to form larger R-Fe2O3 structures. At 1 MLE, R-Fe2O3 islands with irregular shapes are formed. Acknowledgment. We thank Dr. Junseok Lee for his experimental assistance and discussions regarding our results. This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research in FischerTropsch catalysts for the Hydrogen from Coal Program under the RDS Contract DE-AC26-04NT41817. Reference in this work to any specific commercial product is to facilitate understanding and does not necessarily imply endorsement by the U.S. Department of Energy. References and Notes (1) Weiss, W.; Ranke, W. Prog. Surf. Sci. 2002, 70, 1. (2) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (3) Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Perez, M. Science 2007, 318, 1757. (4) Hydrogen from Coal Program: Research, DeVelopment, and Demonstration Plan for the Period 2008 through 2016; U.S. Department of Energy, National Energy Technology Laboratory: Pittsburgh, PA, September, 2008. (5) Li, S. Z.; Meitzner, G. D.; Iglesia, E. J. Phys. Chem. B 2001, 105, 5743. (6) Biener, J.; Farfan-Arribas, E.; Biener, M.; Friend, C. M.; Madix, R. J. J. Chem. Phys. 2005, 123. (7) Song, D.; Hrbek, J.; Osgood, R. Nano Lett. 2005, 5, 1327. (8) Potapenko, D. V.; Hrbek, J.; Osgood, R. M. ACS Nano 2008, 2, 1353. (9) Chang, Z. P.; Song, Z.; Liu, G.; Rodriguez, J. A.; Hrbek, J. Surf. Sci. 2002, 512, L353. (10) Song, Z.; Cai, T. H.; Chang, Z. P.; Liu, G.; Rodriguez, J. A.; Hrbek, J. J. Am. Chem. Soc. 2003, 125, 8059. (11) Biener, M. M.; Friend, C. M. Surf. Sci. 2004, 559, L173. (12) Biener, M. M.; Biener, J.; Schalek, R.; Friend, C. M. J. Chem. Phys. 2004, 121, 12010. (13) Deng, X.; Quek, S. Y.; Biener, M. M.; Biener, J.; Kang, D. H.; Schalek, R.; Kaxiras, E.; Friend, C. M. Surf. Sci. 2008, 602, 1166. (14) Zhao, X. E.; Ma, S. G.; Hrbek, J.; Rodriguez, J. A. Surf. Sci. 2007, 601, 2445. (15) Ma, S. G.; Rodriguez, J.; Hrbek, J. Surf. Sci. 2008, 602, 3272. (16) Galloway, H. C.; Benitez, J. J.; Salmeron, M. Surf. Sci. 1993, 298, 127. (17) SchedelNiedrig, T.; Weiss, W.; Schlogl, R. Phys. ReV. B 1995, 52, 17449.

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