Article pubs.acs.org/JPCC
Exploring the Structural and Electronic Properties of Pt/CeriaModified TiO2 and Its Photocatalytic Activity for Water Splitting under Visible Light Shankhamala Kundu, † Jim Ciston,§ Sanjaya D. Senanayake, † Dario A. Arena,‡ Etsuko Fujita, † Dario Stacchiola, † Laura Barrio,⊥ Rufino M. Navarro,⊥ Jose L. G. Fierro,⊥ and José A. Rodriguez*,
†
†
Chemistry Department, and ‡National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, United States § National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Instituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, 28049 Madrid, Spain ABSTRACT: In the past few years there has been intensive research focused on doping TiO2 with other metal oxides in order to synthesize a mixed metal oxide based semiconductor with modified band gap which can split water efficiently in visible light. In this study we modified titania−anatase powders through deposition of ceria nanoparticles on them. The formation of a mixed metal oxide at the CeOx/TiO2 interface was observed using transmission electron microscopy and near-edge X-ray absorption spectroscopy. UV−vis spectra show that this mixed metal oxide can absorb photons in the visible region due to the presence of stabilized Ce3+ in the mixed oxide phase. The addition of 0.5 wt % Pt imposes a significant chemical change on the ceria nanoparticles by substantially enhancing the concentration of Ce3+ and makes possible photocatalytic activity by providing an electron trap. When irradiated with visible light, 0.5 wt % Pt loaded on CeO2-modified TiO2 generates oxygen almost seven times more efficiently than a standard WO3 catalyst. A correlation was found between the concentration of Ce3+ in Pt/CeOx/TiO2 and its photocatalytic activity.
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INTRODUCTION The conversion of solar energy into hydrogen via the watersplitting process assisted by semiconductor photocatalysts is one of the more interesting ways of achieving clean and renewable energy systems. Many types of semiconductors, with over 130 materials including oxides, nitrides, sulfides, carbides, and phosphides, have been reported to act as efficient photocatalysts for water splitting.1−3 Water splitting was first demonstrated using a TiO2 photoelectrode with some external bias;4 however, powdered TiO2 photocatalyst does not efficiently split water under visible light. This is mainly due to the wide band gap of TiO2 (3.2 eV) which allows us to use only a small fraction of the solar spectrum4 (UV fraction, 3−4% of total solar spectrum). Intensive studies have been carried out to improve the visible light sensitivity of Ti oxide-based catalysts. Among the various approaches undertaken in the search for more efficient and active TiO2 photocatalysts, the following can be mentioned:1,5,6 (i) nanodesign to control the size, morphology, and defects of photocatalysts, (ii) modifying the band gap energy by substitutional doping (cations or anions), (iii) surface modification of TiO2 by deposition of cocatalysts to reduce the activation energy for gas evolution and/or decrease the electron−hole recombination, (iv) developing multicomponent photocatalyst by forming solid solutions, and (v) sensitization. © 2012 American Chemical Society
In principle, the mixing of two different metal oxides could improve the performance of the involved oxides by producing different physical and chemical properties with respect to the individual components.7−9 Despite that bulk ceria and titania do not adopt similar crystal structures,7,10−12 cerium ions are particularly interesting to mix with TiO2 because the Ti4+ ions can be replaced by cerium ions modifying the physicochemical properties (redox capability, thermal sintering, etc.) of TiO2.12−14 However, the characterization of the interfacial and surface structures of this type of system, i.e., CeO2/TiO2 or TiO2/CeO2, remains as a challenge because of the complexity of the structures that can be generated.8 Recently there have been studies focused on the properties of CeO2−TiO2 powders with the aim of using this mixed metal oxide in catalysis and photocatalysis.15−18 Lopez et al. studied the structure and morphology of the CeO2−TiO2 mixed oxide (10 wt % CeO2−90 wt % TiO2) calcined at different temperatures and found that the presence of cerium conferred an increased thermal stability to the TiO2 materials against particle sintering and pore collapse.15 Furthermore, it has been reported that ceria-modified TiO2 shows activity for the Received: May 8, 2012 Revised: June 7, 2012 Published: June 12, 2012 14062
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ceria for the TiO2 support was calculated taking into account the dispersion capacity of ceria on TiO2 (7.23 Ce4+ ions/nm2 TiO2) published in literature. After cerium impregnation, the powders were calcined under air at 873 K for 8 h to obtain the Ce-modified TiO2 photocatalysts. Below the two sample prepared by this methodology are labeled as 6 wt % CeO2/ TiO2 (in short 6CeTi) and 15 wt % CeO2/TiO2 (in short 15CeTi). The supported-Pt photocatalysts (0.5 wt % Pt) were prepared by impregnating each of the CeO2/TiO2 supports under stirring at 353 K for 2 h using aqueous solutions of H2PtCl6 metal precursor (Johnson−Matthey, 40.78 wt % Pt)). After Pt loading, the samples were dried at 393 K for 12 h, and subsequently they were calcined in air at 773 K to remove the chlorine.14 Pt loaded on two different ceria-modified TiO2 supports are labeled as Pt6CeTi and Pt15CeTi for short. TEM experiments were performed on the spherical and chromatic aberration-corrected TEAM I microscope at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory (http://ncem.lbl.gov/TEAM-project/). Images were obtained in both scanning TEM (STEM) and conventional high-resolution TEM (HREM) geometries. Electron energy loss spectroscopy (EELS) was collected using a Gatan Tridiem energy filter with a probe convergence of 30 mrad, collection angle of 35 mrad, and 0.2 eV/pixel sampling. In order to minimize damage to the beam-sensitive TiO2 phase, an accelerating voltage of 80 kV was used, and STEM probe current was limited to 40 pA. TEM images were simulated using the MacTempas software package (totalresolution.com). XRD patterns for the CeO2/TiO2 and Pt/CeO2/TiO2 samples were collected at beamline X7B of the National Synchrotron Light Source (NSLS) as described in refs 14 and 21. XANES spectra of Ce L3 absorption edge were collected at beamline X19A of the National Synchrotron Light Source (NSLS) in the ‘‘fluorescence-yield mode’’ using a passivated implanted planar silicon (PIPS) detector cooled with circulating water. The XANES data was then analyzed using the Athena program.22 The NEXAFS experiments with soft Xrays were performed at the end station of the U4B beamline at the NSLS. The end station is equipped with a partial electron yield detector and a partial fluorescence yield detector which allowed us to simultaneously measure fluorescence and electron yields. Partial electron yield intensities were measured with a single channeltron with a front-end bias of −250 V to reduce signals from secondary electrons. All NEXAFS data reported here were collected with the photon beam fixed at 55° from the surface normal of the sample. All NEXAFS scans were normalized by the incident beam flux, monitored by an Au mesh located upstream of the sample. The NEXAFS spectra were measured as a function of the incident X-ray photon energy in the vicinity of the titanium L-edge (445−490 eV), the oxygen K-edge (520−590 eV), and the cerium M-edge (870− 930 eV) regions. The energy resolution in the NEXAFS experiments was about 0.2−0.4 eV depending on the edge. UV−vis spectroscopy measurements were performed in diffuse reflection mode using UV−vis/NIR spectrophotometer PerkinElmer Lambda 950 located in the center of functional nanomaterials at Brookhaven National Laboratory. The photocatalytic experiments were carried out in a 30 mL closed quartz reactor. The head space of the reactor was connected to a gastight valve, allowing the measurement of the evolved gas. In the photocatalytic reactions the photocatalyst powder (45 mg) was dispersed in 20 mL of water containing 0.01 M of AgNO3. In the overall water splitting, water becomes reduced to hydrogen
photochemical degradation of methylene blue under visiblelight irradiation.16,17 The authors of this study speculate that the CeO2 species may be confined in the anatase nanocrystal framework. Such interaction between the frameworks of CeO2 and TiO2 could be the key for the enhancement of photoactivity in the visible-light region. Theoretical calculations done by Catlow et al. suggest that the presence of reduced cerium cations is necessary in order to enhance the photocatalytic activity of TiO2.18 However, until now experimental studies have not fully explored the correlation between the photocatalytic activities of CeO2-modified TiO2 with the presence of Ce3+ and a possible mechanism behind it. Recently Primo et al. reported that a small amount of gold supported on cerium nanoparticles generates oxygen from water more efficiently than the standard WO3 under visible light irradiation, indicating that ceria can be an useful material in photocatlysis.19 Considering the importance in developing novel photocatalysts with visible-light activity,20 our objective here is to explore the possibility of using photocatalysts prepared by depositing nanoparticles of CeO2 on TiO2, thus designing a semiconductor with a modified band gap which can perform the challenging goal of splitting water using visible light. As mentioned above, the third way to enhance the activity of TiO2 photocatalyst is through the addition of small amounts of noble metals such as Pt and Rh/Ru.1,5,20 Such enhancement in activity has been explained in terms of a photoelectrochemical mechanism in which the electrons generated by light irradiation of the semiconductor transfer to the loaded metal particles, while the holes remain in the semiconductors, resulting in a decrease in the electron−hole recombination. Additionally, the presence of noble metals on the metal oxide surfaces could impose chemical changes on the cerium and/or titania nanoparticles that could have influence on the optoelectronic properties of the photocatalytic systems. With this background, in the present paper, we have investigated the structural and electronic properties of ceria nanoparticles modified on titania combining transmission electron microscopy (TEM) and different variations of nearedge X-ray absorption fine structure (NEXAFS). Our results demonstrate that the interface between a CeO2 nanoparticle and the TiO2 support is partially disordered and contains Ce3+ cations. UV−vis measurements were conducted to study the photon absorption capability of this mixed metal oxide in the UV and visible region. The presence of the CeOx nanoparticles on the TiO2 surface enhanced the absorption of light in the visible range substantially, but it did not produce an efficient catalyst for the splitting of water. When a very small amount of Pt was loaded on the ceria-modified titania support, complex interactions led to an increase in the amount of Ce3+ present, and we obtained a system that generated oxygen 10 times more efficiently than Pt/TiO2 during the photocatalytic splitting of water at λ ≥ 400 nm under our experimental conditions.
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EXPERIMENTAL SECTION TiO2 powders (titanium(IV) oxide, catalyst support Alfa Aesar) were stabilized by thermal treatment at 773 K for 4 h before being used for cerium deposition.14,21 The Ce-modified TiO2 samples (Ce−TiO2) were prepared by wet impregnation of the thermally stabilized TiO2 powders using an aqueous solution of cerium nitrate (Alfa Aesar, Reacton 99.5%) with two different loading of Ce, 6 and 15 wt %. The ceria loadings used correspond to half and complete theoretical monolayer coverage, respectively. The theoretical monolayer coverage of 14063
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and simultaneously oxidized to oxygen. Of these two semireactions, hydrogen generation is considered to be the simpler step compared to oxygen generation. In principle, to produce hydrogen the protons present in the water accept electrons trapped by noble metals that act as hydrogen evolution centers. In contrast, formation of oxygen from water is conceptually more challenging since it involves several intermediate steps, requiring transfer of four positive holes and the formation of O−O bonds.19,23,24 For that reason in our study we choose the AgNO3 as sacrificial reagent in order to probe the oxygen evolution as a measure of efficiency of the photocatalyst. The suspension was purged with an argon flow for at least 60 min before irradiation in order to remove dissolved air. Then it was irradiated for 4 h using a 150 W xenon lamp with a visible light cutoff filter (λ > 400 nm). The stationary temperature of the reactor was kept at 15 °C by a continuous water circulation. The formation of oxygen was confirmed by injecting 0.5 mL of the reactor head space gas in a gas chromatograph operating at isothermal conditions (40 °C) using argon as the carrier gas.
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RESULTS AND DISCUSSION Structure and Morphology of Ceria Nanoparticles on TiO2. A. Structural Characterization. Table 1 summarizes the samples studied along this work, their chemical composition as determined by TXRF analysis, and their BET surface area, as well as the cell dimension of CeO2 fluorite and TiO2 anatase phases as determined by Rietveld refinement of XRD patterns. Figure 1. (a) HAADF STEM image of Pt/15 wt % CeO2/TiO2 catalyst; (b) HRTEM image of a typical CeO2 nanoparticle deposited on TiO2.
Table 1. Summary of Structural Characterization Results14,21 TXRF
TiO2 phase
sample
% Ce
% Pt
BET (m2/g)
a=b
c
TiO2 6CeTi 15CeTi Pt6CeTi Pt15CeTi
− 6.6 14.9 6.6 14.9
− − − 0.5 0.5
82 100 101 96 93
3.786 3.786 3.786 3.785 3.786
9.512 9.513 9.514 9.511 9.514
CeO2 phase
relationships were confirmed with TEM image simulations. CeO2 nanoparticles formed a highly ordered cubic fluorite structure of about 2−3 nm height above the interface of the CeO2 nanoparticle and TiO2 support. However, at the interface we can observe the presence of a more distorted structure indicating the intermixing of two metal oxides. Previously, using crystal simulation, Lopez et al. have suggested that cerium affected the crystallographic and sintering characteristics of CeO2−TiO2 compounds due to the progressive segregation of cerium on the surface of TiO2 globules as the temperature was raised and by possible substitutions of Ti4+ by Ce4+ species within the TiO2 anatase structure,10 thus supporting our observation of intermixing two metal oxides. However, whether the intermixing of the two oxides induces any change in the oxidation state of cerium has yet to be determined. Electron energy loss spectroscopy (EELS) was employed to measure the Ce3+/Ce4+ ratio with high spatial resolution within and around the ceria nanoparticles. The Ce valence state was quantified using the second derivative method of Fortner et al.25 with Ce +3 reference data taken from Yang et al.26 The most sensitive signals with a clear dependency on cation valence are the Ti L-edges, which probe d-electron occupancy in Ti, and the Ce M-edges, which probe f-electron occupancy in Ce. These edges were measured in a spatially resolved way using EELS line scans. The red line in the image inset of Figure 2 shows the EELS line scan that was performed along with the STEM imaging. Ti L2- and L3-edges were integrated to get the Ti signal, and Ce M4- and M5-edges were used to obtain the Ce signal. The first two points of the Ce valence plot with respect to the probing depth, when there is only Ce signal present, show Ce4+ oxidation state. The first significant change in the
− − 5.415 − 5.413
Diffraction patterns of the CeO2/TiO2 samples were dominated by the expected lines for TiO2−anatase14,21 with very small features for ceria in a fluorite structure. After adding Pt to the CeO2/TiO2 systems, there were no extra diffractions lines. This is due to the low concentration of Pt in the samples, but it could also be a sign of the high dispersion of Pt species (size 400 nm) illumination for 4 h. Right-side axis: amount of Ce3+ present in the Pt/CeOx/TiO2 catalysts calculated from the corresponding Ce M-edge NEXAFS spectra obtained by PEY measurements.
Figure 7. UV−vis measurements in reflection mode obtained from (a) TiO2, (b) 15 wt % CeO2 on TiO2, and (c) 0.5 wt % of Pt on 15 wt % CeO2/ TiO2.
from the interband Ce 4f level to the conduction band leading to the formation of electron and hole pairs. The presence of Pt further enhances the absorption in the visible range which agrees well with the NEXAFS results. As observed from the Ce M-edge data (Figure 5b), the existence of a minute amount of Pt increases the amount of Ce3+ on the surface, further enhancing photon absorption in the visible region associated with the occupation of the 4f band. This result indicates that the photon absorption is related to the amount of Ce3+ present on the catalyst surface. Photocatalytic Water-Splitting Activity. Formation of oxygen is conceptually more challenging compared to hydrogen production from water splitting, since it involves several intermediate steps, requiring transfer of four positive holes and the formation of O−O bonds.19,20,23 Since oxygen evolution is frequently the rate-determining step, in our study we choose to probe the evolved oxygen as a measure of efficiency of the photocatalysts. The photocatalytic activity of Pt dispersed on pure TiO2, and on CeO2-modified (6 and 15 wt %) TiO2 under visible light (using a filter with cutoff λ ≥ 400 nm) was investigated by monitoring O2 evolution from water using 45 mg of catalyst in a 0.01 M Ag(NO)3 aqueous solution (Figure 8). Also in our study we included WO3 as a reference photocatalyst for oxygen evolution from water splitting, as it can be used as a benchmark material for oxygen generation to which other photocatalysts can be compared.19,20,23 Under our experimental condition WO3 generates about 1.9 μmol of oxygen per hour while irradiating with visible light, which is similar to the value reported in the literature.19,23 Stoichiometric TiO2 does not absorb visible light. A very small amount of O2 evolved from the aqueous silver nitrate solution using Pt/TiO2 as a catalyst under our experimental conditions, possibly due to the presence of defects in TiO2, and when using a cutoff filter of λ ≥ 400 nm a small fraction of UV light is always present in the filtered spectrum causing a small photocatalytic activity.40 The presence of 6 wt % CeO2 on titania along with Pt significantly improved the O2 evolution, as shown in Figure 8, due to formation of a large amount of Ce3+. The amount of Ce3+ increased from 4 wt % to 7.4 wt % when the ceria loading was increased to 15 wt %, and this leads to an even higher activity for the oxygen evolution. The Pt/CeOx/ TiO2 system has about seven times higher activity than the standard WO3 catalyst and an activity as good as that of the
best metal oxide catalysts reported in the literature in recent years for water photo-oxidation under visible light.19,23 However, without the presence of Pt, the pure CeO2/TiO2 mixed metal oxide showed a very small activity toward water splitting in visible light. This result indicates that the existence of Ce3+ drastically enhances the formation of electron−hole pairs, but it does not help to suppress their recombination. The presence of a small amount of Pt is needed in order to act as an electron trap and prolong the lifetime of electron−hole pairs. However in this particular system, Pt not only acts as an electron-trapping center, but also increases the amount of Ce3+ in the support itself, thus enhancing the photon absorption efficiency of the catalyst in the visible region. On the basis of the previous results, we can propose a possible mechanism for the photocatalytic activity of the Pt/ CeOx/TiO2 system as shown in Scheme 1: Formation of mixed metal oxides at the interface of the CeO2 nanoparticles and TiO2 support leads to the presence of a significant amount of reduced cerium. The existence of Ce3+ ions is associated with Scheme 1. Proposed Mechanism for the Photocatalytic Evolution of Oxygen from Water upon the Irradiation of Pt/ CeOx/TiO2a
a
The exact band positions were calculated based on the method described by Xu and Schoonen22 and valence photoemission data for CeO2/TiO2.8,41,42. 14068
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Dr. David Grill for his help with the photocatalytic activity measurements. The work at BNL was financed by the U.S. Department of Energy (DOE), Office of Basic Energy Science (BES) under Contract DE-AC02-98CH10086. The authors also acknowledge support of the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, which is supported by the DOE BES under Contract DE-AC0205CH11231. The work at ICP-CSIC was financed by MCIIN (ENE2010-21198-C04-01).
an extra energy state, belonging to the partially occupied 4f level, between the conduction and valence bands of TiO2.13,41,42 This generates a reduced band gap of about 2.2 eV in the mixed metal oxide phase. Visible light now can excite the electrons from the valence band to the intermediate Ce 4f band or from the Ce 4f band to the conduction band, thus forming electron−hole pairs. Electrons then get trapped by Pt nanoparticles and reduce the silver nitrate present in the solution. On the other hand, the holes will oxidize the water to oxygen. Previous experimental and theoretical studies have examined the adsorption of water on CeOx/TiO2(110) surfaces where there is a significant amount of Ce3+ sites, showing the facile dissociation of O−H bonds.41,42 In DFT calculations there is a large reduction in the activation energy for the dissociation of water when going from pure TiO2(110) to CeOx/TiO2(110), with the activation energy for O−H bond cleavage on the mixed metal oxide being only 0.04 eV.41,42 Thus, the Pt/CeOx/TiO2(110) photocatalyst operates in a complex way where the Pt and ceria nanoparticles each have double roles. The Pt nanoparticles help in the formation of Ce3+ centers and slow down the rate of electron−hole recombination. On the other hand, the Ce3+ centers in the ceria nanoparticles dissociate water and facilitate the formation of electron−hole pairs with visible light, with the holes eventually interacting with adsorbed OH or O to form O2 molecules. The coupling of the titania and ceria is essential for stabilizing the Ce3+ sites that enhance the formation of electron−hole pairs with visible light. The behavior of Pt/CeO2/TiO2 illustrates the advantages of using a multifunctional configuration in which the photocatalyst exposes metal nanoparticles and chemically active oxide nanoparticles to the reacting water. Thus, the water molecules can interact with specific sites of the oxide nanoparticles, metal sites, and metal−oxide interfaces. Nanoparticles of VOx, WOx, RuOx, and CeOx in contact with a titania support exhibit unique electronic properties and create electronic states within the titania band gap.42−46
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CONCLUSIONS In summary, the deposition of CeOx nanoparticles on TiO2 proves to be an efficient way to modify the chemical and electronic properties of both metal oxides as observed from NEXAFS and UV−vis measurements. Although Ce2O3 is not a stable oxide, our study shows that it is possible to stabilize Ce3+ cations within a mixed oxide interface, which then can absorb photons in the visible region. Pt/CeOx/TiO2 proved to be excellent for the photocatalytic splitting of water in the visible region being about seven times more active than a WO3 standard. Our findings open up the way to design photocatalysts taking advantage of the unique interactions that occur in a mixed metal oxide at the nanometer level.
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REFERENCES
(1) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253−278. (2) Osterloh, F. E. Chem. Mater. 2008, 20, 35−54. (3) Kudo, A. Catal. Surv. Asia 2003, 7, 31−38. (4) Fujishima, A.; Honda, K. Nature 1972, 238, 37−38. (5) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891−2959. (6) Navarro, R. M.; Á lvarez-Galván, M. C.; del Valle, F.; Villoria de la Mano, J. A.; Fierro, J. L. G. ChemSusChem 2009, 2, 471−485. (7) Garcıa, M. F.; Arias, A. M.; Hanson, J. C.; Rodriguez, J. A. Chem. Rev. 2004, 104, 4063−4104. (8) Rodriguez, J. A.; Stacchiola, D. Phys. Chem. Chem. Phys. 2010, 12, 9557−9565. (9) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935−938. (10) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735−758. (11) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269−272. (12) Trovarelli, A. Catal. Rev. Sci. Eng. 1996, 38, 439−520. (13) Park, J. B.; Graciani, J.; Evans, J.; Stacchiola, D.; Ma, S.; Liu, P.; Nambu, A.; Sanz, J. F.; Hrbek, J.; Rodriguez, J. A. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 4975−4980. (14) Gonzalez, I. D.; Navarro, R. M.; Wenb, W.; Marinkovic, N.; Rodriguez, J. A.; Rosa, F. J.; Fierro, L. G. Catal. Today 2010, 149, 372−379. (15) Lopez, T.; Rojas, F.; Alexander-Katz, R.; Galindo, F.; Balankin, A.; Buljan, A. J. Solid State Chem. 2004, 177, 1873−1881. (16) Li, G.; Zhang, D.; Yu, J. C. Phys. Chem. Chem. Phys. 2009, 11, 3775−3782. (17) Xiea, J.; Jianga, D.; Chena, M.; Lia, D.; Zhua, J.; Lua, X.; Yan, X. Colloids Surf., A 2010, 372, 107−114. (18) Catlow, C. R. A.; Guo, Z. X.; Miskufova, M.; Shevlin, S. A.; Smith, A. G. H.; Sokol, A. A.; Walsh, A.; Wilson, D. J.; Woodley, S. M. Philos. Trans. R. Soc. A 2010, 368, 3379−3456. (19) Primo, A.; Marino, T.; Corma, A.; Molinari, R.; García, H. J. Am. Chem. Soc. 2011, 13, 6930−6933. (20) Kubacka, A.; Fernández-García, M.; Colon, G. Chem. Rev. 2012, 12, 1555−1614. (21) Barrio, L.; Zhou, G.; Gonzalez, I. D.; Estrella, M.; Hanson, J.; Rodriguez, J. A.; Navarro, R. M.; Fierro, J. L. G. Phys. Chem. Chem. Phys. 2012, in press. (22) Xu, Y.; Schoonen, M. A. Am. Mineral. 2000, 85, 543−556. (23) Silva, C. G.; Bouizi, Y.; Fornes, V.; Garcia, H. J. Am. Chem. Soc. 2009, 131, 13833−13839. (24) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537. (25) Fortner, J. A.; Buck, E. C.; Ellison, A. J. G.; Bates, J. K. Ultramicroscopy 1997, 67, 77−81. (26) Yang, G.; Mobus, G.; Hand, R. J. Micron 2006, 37, 433−441. (27) Mamontova, E.; Egamia, T.; Dmowskia, W.; Kao, C. C. J. Phys. Chem. Solids 2001, 62, 819−823. (28) Zhang, J.; Ju, X.; Wu, Z. Y.; Liiu, T.; Hu, T. D.; Xie, Y. N. Z.; Zhang, L. Chem. Mater. 2001, 13, 4192−4197. (29) Soldatov, A. V.; Ivanchenko, T. S.; Della Longa, S.; Kotani, A.; Iwamoto, Y.; Bianconi, A. Phys. Rev. B 1994, 50, 5074. (30) Kucheyev, S. O.; Clapsaddle, B. J.; Wang, Y. M.; van Buuren, T.; Hamza, A. V. Phys. Rev. B 2007, 76, 235420−5.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.B. acknowledges support from E.U. FP7-People-2007-4-IOF219674. Dr. G. Zhou is gratefully acknowledged for providing the Ce0.8Ti0.2O2 mixed oxide sample. The authors also thank 14069
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(31) Pompa, M.; Flank, A. M.; Lagarde, P.; Rife, J. C.; Stekhin, L.; Stekhin, I.; Nakazawa, M.; Ogasawara, H.; Kotani, A. Phys. Rev. B 1997, 56, 2267−2272. (32) Turchini, S.; Delaunay, R.; Lagarde, P.; Vogel, J.; Sacchi, S. J. Electron Spectrosc. 1995, 71, 31−37. (33) Lusvardi, V. S.; Barteau, M. A.; Chen, J. G.; Fruhberger, J.; B.; Teplyakov, A. Surf. Sci. 1998, 397, 237−250. (34) Brydson, R.; Sauer, H.; Engel, W.; Thomas, J. M.; Zeitler, E.; Kosugi, N.; Kuroda, H. J. Phys.: Condens. Matter 1989, 1, 797−812. (35) Zhou, G.; Shah, P. R.; Kim, T.; Fornasiero, P.; Gorte, R. J. Catal. Today 2007, 123, 86. (36) (a) Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Satudt, T.; Petrova, G. P.; Tsud, N.; Skala, T.; Bruix, A.; Illas, F.; Prince, K. C.; Matolin, V.; Neyman, K. M.; Libuda, J. Nat. Mater. 2011, 10, 310−315. (b) Bruix, A.; Rodriguez, J. A.; Ramirez, P. J.; Senanayake, S. D.; Evans, J.; Park, J. B.; Stacchiola, D.; Liu, P.; Hrbek, J.; Illas, F. J. Am. Chem. Soc. 2012, 134, 8968−8974. (37) de Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. Phys. Rev. B 1990, 41, 928−937. (38) Skorodumova, N. V.; Ahuja, R.; Simak, S. I.; Abrikosov, I. A.; Johansson, B.; Lundqvist, B. I. Phys. Rev. B 2001, 64, 115108−9. (39) Andersson, D. A.; Simak, L.; Johansson, B.; Abrikosov, I. A.; Skorodumova, N. V. Phys. Rev. B 2007, 75, 035109−035113. (40) Kitano, M.; Masato, T.; Masaya, M.; Thomas, J. M.; Anpo, M. Catal. Today 2007, 120, 133−138. (41) Park, J. B.; Graciani, J.; Evans, J.; Stacchiola, D.; Senanayake, S. D.; Barrio, L.; Liu, P.; Sanz, J. F.; Hrbek, J.; Rodriguez, J. A. J. Am. Chem. Soc. 2010, 132, 356−363. (42) Graciani, J.; Plata, J. J.; Sanz, J. F.; Liu, P.; Rodriguez, J. A. J. Chem. Phys. 2010, 132, 104703. (43) Lee, S.; Zajac, G. W.; Goodman, D. W. Top. Catal. 2006, 38, 127−131. (44) Zhang, Z.; Henrich, V. E. Surf. Sci. 1992, 277, 263−272. (45) Yang, F.; Kundu, S.; Vidal, A. B.; Graciani, J.; Ramirez, P. J.; Senanayake, S. D.; Stacchiola, D.; Evans, J.; Liu, P.; Sanz, J. F.; Rodriguez, J. A. Angew. Chem., Int. Ed. 2011, 50, 10198−10202. (46) Kundu, S.; Vidal, A. B.; Yang, F.; Ramirez, P. J.; Senanayake, S. D.; Stacchiola, D.; Evans, J.; Liu, P.; Rodriguez, J. A. J. Phys. Chem. C 2012, 116, 4767−4773.
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