J. Phys. Chem. C 2010, 114, 22619–22623
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Reactivity Differences of Nanocrystals and Continuous Films of r-Fe2O3 on Au(111) Studied with In Situ X-ray Photoelectron Spectroscopy Xingyi Deng,*,†,‡ Junseok Lee,†,‡ Congjun Wang,†,‡ Christopher Matranga,† Funda Aksoy,§,| and Zhi Liu§ National Energy Technology Laboratory, United States Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236, United States, URS, P.O. Box 618, South Park, PennsylVania 15129, United States, AdVanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States, and C¸ukuroVa UniVersity, Physics Department, 01330 Adana, Turkey ReceiVed: September 8, 2010; ReVised Manuscript ReceiVed: October 28, 2010
The interaction of CO with nanocrystals and continuous films of R-Fe2O3 grown on Au(111) was investigated using in situ X-ray photoelectron spectroscopy (XPS) at near ambient pressure (200 mTorr) and scanning tunneling microscopy (STM). Adsorbed CO was detected by XPS when R-Fe2O3 nanocrystals (6-7 nm) grown on Au(111) were exposed to 200 mTorr of the gas at room temperature. Under a low H2O background, surface bound hydroxyl groups (adsorbed OH) were also noted on these R-Fe2O3 nanocrystals as a result of H2O dissociation on the edges of the particles. Adsorbed formate (HCOO-) was detected during heating to 373 K and believed to originate from the reaction of adsorbed CO with the OH groups. The adsorbed formate desorbed or decomposed above 473 K. Continuous R-Fe2O3 thin films on Au(111) were inert under the same conditions studied for nanocrystalline R-Fe2O3. Specifically, neither adsorbed CO nor OH groups were observed for the continuous films of R-Fe2O3. This reactivity difference can be explained by the presence of R-Fe2O3 crystal edges and the interface which exists between the R-Fe2O3 nanocrystals and the Au(111) substrate. These edges and interfaces are present for the nanocrystalline R-Fe2O3/Au(111) system but are not present in significant amounts for the continuous films of R-Fe2O3. The implications of these experimental results for the water-gas shift reaction will be also discussed. Introduction R-Fe2O3 (hematite) is an important metal oxide that has been utilized as a catalytic material in a variety of applications. R-Fe2O3 with promoters and additives such as Cu and K serves as the starting catalytic material used in the Fischer-Tropsch (F-T) process, a critical technology which converts coal-derived syngas (CO and H2) into liquid hydrocarbons and fuels.1-3 Both unpromoted and K-promoted R-Fe2O3 are active catalysts for the dehydrogenation of ethylbenzene to styrene in the presence of steam.4-6 R-Fe2O3 is also used as a support in catalytic applications. For example, nanosized gold particles supported on R-Fe2O3 have been shown to be active for the lowtemperature water gas shift (WGS) reaction.7-9 The complexity of the heterogeneous catalysts used in most industrial processes makes it challenging to obtain fundamental insight into the reactivity and selectivity of these systems. Traditionally, single crystals are used as model systems to elucidate mechanistic information about the active sites, kinetics, and reaction barriers of these catalysts due to the simplicity and the intrinsically two-dimensional geometry. In recent years, it has become more common to use surface grown nanoparticles as model catalyst systems since these particles accurately reproduce the structural defects, step densities, and edge sites which tend to govern the reactivity of real catalysts.10 Using a combination of in situ analytical instrumentation (such as X-ray * To whom correspondence should be addressed. E-mail: Xingyi.Deng@ netl.doe.gov. † National Energy Technology Laboratory. ‡ URS. § Lawrence Berkeley National Laboratory. | C¸ukurova University.
photoelectron spectroscopy, XPS) and well-controlled surface grown nanoparticles, we can further narrow the pressure and material gaps which exist between fundamental surface science studies and real world catalysis.10,11 Recently, we have successfully grown R-Fe2O3 nanocrystals on a Au(111) substrate.12 Detailed scanning tunneling microscopy (STM) studies show that these R-Fe2O3 nanocrystals contain missing atom defects, steps, and edges, which closely reproduce the structural and compositional characteristics of realworld Fe oxide catalysts. Continuous films of R-Fe2O3 on Au(111) have also been prepared on the Au(111) substrate with the same O-terminated (0001) surface structure as that of the nanocrystals. One major structural difference between the nanocrystals and films of R-Fe2O3 is the nanoparticle system has a high density of particle edges that have a significant interfacial region along the Au(111) substrate. These edges and the interface with Au(111) are not present at any appreciable density in the continuous films. These two structural analogues (nanocrystals and continuous films) thus provide an ideal platform to investigate the impact of structural defects, such as steps and edges, on the activity of a material. In this work, we present a comparative study of the reactivity of nanocrystals and continuous films of R-Fe2O3 grown on Au(111). This system was studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) which allows for the in situ investigation of catalytic adsorption and reactions in the milliTorr to Torr range.11,13 We find that continuous films of R-Fe2O3 on Au(111) are inert in the presence of 200 mTorr of CO and temperatures up to 466 K, whereas R-Fe2O3 nanocrystals on Au(111) are reactive under the same conditions. In fact, the R-Fe2O3 nanocrystals are reactive enough that the trace amounts
10.1021/jp1085697 2010 American Chemical Society Published on Web 12/06/2010
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J. Phys. Chem. C, Vol. 114, No. 51, 2010
Figure 1. Room temperature STM images of R-Fe2O3 on Au(111) prepared from oxidation of Fe using NO2 at an elevated temperature: (a) 0.5 MLE of R-Fe2O3 (I ) 0.5 nA, V ) 1.0 V, image size 100 × 100 nm2). (b) An individual R-Fe2O3 particle showing stepped edges as demonstrated in the STM linescan across the particle edge (I ) 0.5 nA, V ) 0.5 V, image size: 10 × 10 nm2). (c) 2 MLE of R-Fe2O3 (I ) 0.5 nA, V ) 1.0 V, image size 100 × 100 nm2).
of H2O displaced from the chamber walls during high-pressure CO dosing can dissociate on the nanocrystals forming OH groups on the particle edges. These OH groups seem to react with the CO at an elevated temperature producing formate and other carbonaceous species. We will discuss the enhanced activity of R-Fe2O3 nanocrystals with respect to continuous films of R-Fe2O3 as well as the active sites responsible for different adsorbed species and reactions. The implications of our results for the low temperature WGS reaction will also be discussed. Experimental Section Experiments were performed at beamline 9.3.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory. The end station is equipped with a specially designed photoemission spectrometer that can operate at near-ambient pressures (up to 2 Torr).14 The growth and characterization of R-Fe2O3 on Au(111) has been described in detail previously.12 Briefly, metallic Fe was deposited on Au(111) at room temperature using an electronbeam assisted evaporator (Omicron EFM3T) from an iron rod (Goodfellow, 2.0 mm, 99.99%), followed by oxidation in NO2 while ramping the sample temperature up to 700 K. An ultrahigh vacuum leak valve was used to control the NO2 pressure. Prior to R-Fe2O3 growth, the Au(111) surface was cleaned by cycles of Ar+ sputtering (1 × 10-5 Torr, 1.5 keV) at room temperature, followed by annealing at 700 K for 10 min. In this work, we evaluated R-Fe2O3 samples with two distinct morphologies. At a coverage of 0.5 MLE (monolayer equivalent of Fe, referring to the atomic ratio of Fe and Au used to make the oxide15), R-Fe2O3 made by our method exists as ordered nanocrystals on the Au(111) surface with an average size of 6-7 nm and with stepped edges being visible around the periphery of the particle (parts a and b of Figure 1). Note that the STM line profile shown in Figure 1b does not necessarily represent the step heights in topography because of the tunneling conditions used to acquire the image. Previous publications from our group12 using tunneling conditions better suited for topographic imaging have shown that the particle height is ∼5.8 Å. The tunneling conditions in Figure 1b were chosen to better illustrate the presence of step edges in these particles. At a coverage of 2
Deng et al. MLE, R-Fe2O3 forms continuous films on Au(111). The STM images show domain boundaries and in some areas small regions of the Au(111) surface can be seen from missing patches of R-Fe2O3 in the film (Figure 1c). The XPS spectra of C 1s, O1s, and Fe 2p were recorded at photon energies of 405, 650, and 830 eV, respectively, corresponding to a constant photoelectron kinetic energy of 120 eV to ensure the same probe depth for all spectral regions. The binding energy was calibrated using the bulk Au 4f7/4 peak (BE ) 84.0 eV) recorded at the same photon energy. The XPS data analysis involved subtracting the nonlinear background (Shirley) and fitting the XP spectral peaks using mixed Gaussian-Lorentzian functions. The sample was heated by a ceramic button heater and the temperature was measured with a type K thermal couple placed in direct contact with the Au crystal. CO gas (Ultrahigh Purity) was purified to remove possible nickel carbonyl contamination by passing the gas through a nickel carbonyl trap (Cu pellets heated at T > 533 K) prior to introduction into the AP-XPS system through a UHV leak value. No nickel was detected on the surface after high pressure of CO exposure indicating the absence of nickel carbonyl. NO2 (research grade) was used without further purification. Results and Discussion The XP spectra of as-prepared R-Fe2O3 thin films (2 MLE) on Au(111) are shown in Figure 2. The binding energy (BE) of the Fe 2p3/2 peak of R-Fe2O3 is located at 710.7 eV. The shakeup satellite peak at ∼719 eV (marked as an arrow), a characteristic for the Fe3+ species,16 is also clearly seen. In the O 1s region, a peak at 529.8 eV was observed and attributed to the lattice oxygen in the R-Fe2O3 film. Trace amounts (
