pubs.acs.org/Langmuir © 2010 American Chemical Society
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Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption† ‡
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Zili Wu,*,‡,§ Meijun Li,‡ Jane Howe, Harry M. Meyer III, and Steven H. Overbury*,‡,§ Chemical Science Division, §Center for Nanophase Materials Sciences, and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received April 29, 2010. Revised Manuscript Received June 13, 2010 Defect sites play an essential role in ceria catalysis. In this study, ceria nanocrystals with well-defined surface planes have been synthesized and utilized for studying defect sites with both Raman spectroscopy and O2 adsorption. Ceria nanorods ({110} þ {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amounts (based on surface area) of such defect sites are produced on the two surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallographic surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen species. Superoxide on one-electron defect sites and peroxide on two-electron defect sites with different clustering degree are identified on the ceria nanocrystals depending on their morphology. Furthermore, the stability and reactivity of these oxygen species are also found to be surface-dependent, which is of significance for ceria-catalyzed oxidation reactions.
1. Introduction There has been strong interest in studying the defect sites on oxide materials because these sites are coordinatively unsaturated and actively participate in the activation and conversion of reactants in many catalytic reactions.1-6 Among oxide catalysts, ceria and CeO2-based materials have been intensively studied for their utilization in three-way catalysts, in environmental catalysis, and as catalyst supports.1,7,8 These applications generally take advantage of the excellent redox property and high oxygen storage capacity (OSC) of ceria. It is accepted that the OSC of ceria is well associated with the presence of oxygen vacancies. More O vacancies were found to lead to increased OSC and enhanced redox properties of ceria such as in the case of rare-earth † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail:
[email protected] (Z.W.);
[email protected] (S.H.O.).
(1) Trovarelli, A. Catalytic properties of ceria and CeO2-containing materials. Catal. Rev. - Sci. Eng. 1996, 38 (4), 439-520. (2) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, J. P.; Goodman, D. W. The role of F-centers in catalysis by Au supported on MgO. J. Am. Chem. Soc. 2005, 127 (6), 1604-1605. (3) Murugan, B.; Ramaswamy, A. V. Defect-site promoted surface reorganization in nanocrystalline ceria for the low-temperature activation of ethylbenzene. J. Am. Chem. Soc. 2007, 129 (11), 3062-3063. (4) Liu, X. W.; Zhou, K. B.; Wang, L.; Wang, B. Y.; Li, Y. D. Oxygen vacancy clusters promoting reducibility and activity of ceria nanorods. J. Am. Chem. Soc. 2009, 131 (9), 3140-3041. (5) Ganduglia-Pirovano, M. V.; Hofmann, A.; Sauer, J. Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges. Surf. Sci. Rep. 2007, 62 (6), 219-270. (6) Guzman, J.; Carrettin, S.; Corma, A. Spectroscopic evidence for the supply of reactive oxygen during CO oxidation catalyzed by gold supported on nanocrystalline CeO2. J. Am. Chem. Soc. 2005, 127 (10), 3286-3287. (7) Binet, C.; Daturi, M.; Lavalley, J. C., IR study of polycrystalline ceria properties in oxidised and reduced states. Catal. Today 1999, 50 (2), 207-225. (8) Weckhuysen, B. M.; Rosynek, M. P.; Lunsford, J. H. Destructive adsorption of carbon tetrachloride on lanthanum and cerium oxides. Phys. Chem. Chem. Phys. 1999, 1 (13), 3157-3162. (9) Mamontov, E.; Egami, T.; Brezny, R.; Koranne, M.; Tyagi, S. Lattice defects and oxygen storage capacity of nanocrystalline ceria and ceria-zirconia. J. Phys. Chem. B 2000, 104 (47), 11110-11116.
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oxide doped ceria.9-12 The redox property of ceria is proposed to be controlled by the nature of the oxygen vacancies because oxygen diffusion depends on the type, size, and concentration of oxygen vacancies.3,4,13,14 The concentration of Ce3þ-O-Ce4þ defect sites was also found to influence the catalytic activity of ceria.3 Clearly, defect sites play an essential role in ceria catalysis. Therefore, numerous investigations have been devoted to the study of defect sites, Ce3þ and O vacancy, on ceria using variety of techniques including X-ray photoelectron spectroscopy (XPS),15,16 electron paramagnetic resonance (EPR),17-19 scanning tunneling (10) Daturi, M.; Finocchio, E.; Binet, C.; Lavalley, J. C.; Fally, F.; Perrichon, V.; Vidal, H.; Hickey, N.; Kaspar, J. Reduction of high surface area CeO2-ZrO2 mixed oxides. J. Phys. Chem. B 2000, 104 (39), 9186-9194. (11) Madier, Y.; Descorme, C.; Le Govic, A. M.; Duprez, D. Oxygen mobility in CeO2 and CexZr(1-x)O2 compounds: Study by CO transient oxidation and O-18/ O-16 isotopic exchange. J. Phys. Chem. B 1999, 103 (50), 10999-11006. (12) He, H.; Dai, H.; Au, C. T. Defective structure, oxygen mobility, oxygen storage capacity, and redox properties of RE-based (RE = Ce, Pr) solid solutions. Catal. Today 2004, 90 (3-4), 245-254. (13) Dutta, P.; Pal, S.; Seehra, M. S.; Shi, Y.; Eyring, E. M.; Ernst, R. D. Concentration of Ce3þ and oxygen vacancies in cerium oxide nanoparticles. Chem. Mater. 2006, 18 (21), 5144-5146. (14) Esch, F.; Fabris, S.; Zhou, L.; Montini, T.; Africh, C.; Fornasiero, P.; Comelli, G.; Rosei, R. Electron localization determines defect formation on ceria substrates. Science 2005, 309 (5735), 752-755. (15) Qiu, L. M.; Liu, F.; Zhao, L. Z.; Ma, Y.; Yao, J. N. Comparative XPS study of surface reduction for nanocrystalline and microcrystalline ceria powder. Appl. Surf. Sci. 2006, 252 (14), 4931-4935. (16) Reddy, B. M.; Khan, A.; Yamada, Y.; Kobayashi, T.; Loridant, S.; Volta, J. C. Structural characterization of CeO2-MO2 (M = Si4þ, Ti4þ, and Zr4þ) mixed oxides by Raman spectroscopy, X-ray photoelectron spectroscopy, and other techniques. J. Phys. Chem. B 2003, 107 (41), 11475-11484. (17) Martinez-Arias, A.; Fernandez-Garcia, M.; Belver, C.; Conesa, J. C.; Soria, J. EPR study on oxygen handling properties of ceria, zirconia and Zr-Ce (1:1) mixed oxide samples. Catal. Lett. 2000, 65 (4), 197-204. (18) Martinez-Arias, A.; Conesa, J. C.; Soria, J. O-2-probe EPR as a method for characterization of surface oxygen vacancies in ceria-based catalysts. Res. Chem. Intermed. 2007, 33 (8-9), 775-791. (19) Soria, J.; Coronado, J. M.; Conesa, J. C. Spectroscopic study of oxygen adsorption on CeO2/γ-Al2O3 catalyst supports. J. Chem. Soc., Faraday Trans. 1996, 92 (9), 1619-1626. (20) Norenberg, H.; Briggs, G. A. D. Defect formation on CeO2(111) surfaces after annealing studied by STM. Surf. Sci. 1999, 424 (2-3), L352-L355.
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microscopy (STM),14,20 neutron scattering,9,21 Raman,22-25 probing molecules such as dioxygen,7,18,26-29 and theoretical modeling.5,30-33 It is notable that most studies were conducted on polycrystalline ceria with differently exposed crystal planes. Recent theoretical simulations have indicated that different crystal planes of ceria such as {111}, {110}, and {100} exhibit different properties such as surface stability,34,35 oxygen vacancies formation energy,31,36 and interaction with surface molecules.37 So the knowledge obtained from polycrystalline ceria is an ensemble average of those from individual crystal surfaces, which makes it difficult to correlate to the catalytic behavior and thus prevents us from fundamental understanding of ceria catalysis and further designing of better ceria catalysts. It is highly desirable to study high surface area ceria with controlled surface terminations in analogy to surface science study where single crystals with (21) Mamontov, E.; Egami, T. Structural defects in a nano-scale powder of CeO2 studied by pulsed neutron diffraction. J. Phys. Chem. Solids 2000, 61 (8), 1345-1356. (22) Mcbride, J. R.; Hass, K. C.; Poindexter, B. D.; Weber, W. H. Raman and X-Ray Studies of Ce1-Xrexo2-Y, Where Re = La, Pr, Nd, Eu, Gd, and Tb. J. Appl. Phys. 1994, 76 (4), 2435-2441. (23) Taniguchi, T.; Watanabe, T.; Sugiyama, N.; Subramani, A. K.; Wagata, H.; Matsushita, N.; Yoshimura, M. Identifying Defects in Ceria-Based Nanocrystals by UV Resonance Raman Spectroscopy. J. Phys. Chem. C 2009, 113 (46), 19789-19793. (24) Nakajima, A.; Yoshihara, A.; Ishigame, M. Defect-Induced RamanSpectra in Doped CeO2. Phys. Rev. B 1994, 50 (18), 13297-13307. (25) Luo, M. F.; Yan, Z. L.; Jin, L. Y.; He, M. Raman spectroscopic study on the structure in the surface and the bulk shell of CexPr1-xO2-δ mixed oxides. J. Phys. Chem. B 2006, 110 (26), 13068-13071. (26) Li, C.; Domen, K.; Maruya, K.; Onishi, T. Dioxygen Adsorption on WellOutgassed and Partially Reduced Cerium Oxide Studied by FT-IR. J. Am. Chem. Soc. 1989, 111 (20), 7683-7687. (27) Li, C.; Domen, K.; Maruya, K.; Onishi, T. Oxygen-Exchange Reactions over Cerium Oxide - an FT-IR Study. J. Catal. 1990, 123 (2), 436-442. (28) Pushkarev, V. V.; Kovalchuk, V. I.; d’Itri, J. L. Probing defect sites on the CeO2 surface with dioxygen. J. Phys. Chem. B 2004, 108 (17), 5341-5348. (29) Choi, Y. M.; Abernathy, H.; Chen, H. T.; Lin, M. C.; Liu, M. L. Characterization of O-2-CeO2 interactions using in situ Raman spectroscopy and first-principle calculations. ChemPhysChem 2006, 7 (9), 1957-1963. (30) Ganduglia-Pirovano, M. V.; Da Silva, J. L. F.; Sauer, J. Density-Functional Calculations of the Structure of Near-Surface Oxygen Vacancies and Electron Localization on CeO2(111). Phys. Rev. Lett. 2009, 102 (2), 026101. (31) Nolan, M.; Parker, S. C.; Watson, G. W. The electronic structure of oxygen vacancy defects at the low index surfaces of ceria. Surf. Sci. 2005, 595 (1-3), 223-232. (32) Skorodumova, N. V.; Simak, S. I.; Lundqvist, B. I.; Abrikosov, I. A.; Johansson, B. Quantum origin of the oxygen storage capability of ceria. Phys. Rev. Lett. 2002, 89 (16), 166601. (33) Yang, Z. X.; Woo, T. K.; Baudin, M.; Hermansson, K. Atomic and electronic structure of unreduced and reduced CeO2 surfaces: A first-principles study. J. Chem. Phys. 2004, 120 (16), 7741-7749. (34) Baudin, M.; Wojcik, M.; Hermansson, K. Dynamics, structure and energetics of the (111), (011) and (001) surfaces of ceria. Surf. Sci. 2000, 468 (1-3), 51-61. (35) Nolan, M.; Grigoleit, S.; Sayle, D. C.; Parker, S. C.; Watson, G. W. Density functional theory studies of the structure and electronic structure of pure and defective low index surfaces of ceria. Surf. Sci. 2005, 576 (1-3), 217-229. (36) Nolan, M.; Fearon, J. E.; Watson, G. W. Oxygen vacancy formation and migration in ceria. Solid State Ionics 2006, 177 (35-36), 3069-3074. (37) Nolan, M.; Watson, G. W. The surface dependence of CO adsorption on ceria. J. Phys. Chem. B 2006, 110 (33), 16600-16606. (38) Beste, A.; Mullins, D. R.; Overbury, S. H.; Harrison, R. J. Adsorption and dissociation of methanol on the fully oxidized and partially reduced (111) cerium oxide surface: Dependence on the configuration of the cerium 4f electrons. Surf. Sci. 2008, 602 (1), 162-175. (39) Zhou, K. B.; Wang, X.; Sun, X. M.; Peng, Q.; Li, Y. D. Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes. J. Catal. 2005, 229 (1), 206-212. (40) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C. H. Shape-selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes. J. Phys. Chem. B 2005, 109 (51), 24380-24385. (41) Yan, L.; Yu, R. B.; Chen, J.; Xing, X. R. Template-free hydrothermal synthesis of CeO2 nano-octahedrons and nanorods: Investigation of the morpholog evolution. Cryst. Growth Des. 2008, 8 (5), 1474-1477. (42) Aneggi, E.; Llorca, J.; Boaro, M.; Trovarelli, A. Surface-structure sensitivity of CO oxidation over polycrystalline ceria powders. J. Catal. 2005, 234 (1), 88-95.
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crystallographically defined surfaces are used.38 Recent successful synthesis of ceria nanocrystals with controlled morphology39-42 makes this effort possible and offers a great opportunity for studying surface-dependent properties of ceria. The hydrothermally synthesized ceria nanorods, nanocubes, and nanopolyhedra, exposing crystal planes of {110} þ {100}, {100}, and {111} þ {100}, respectively, have been demonstrated to show different OSC and CO oxidation activity.39,40,42,43 The observations were attributed to the differently exposed surfaces of ceria. However, little is known about the nature of the defect sites which is critical for understanding ceria catalysis. This is the subject of the current paper. Ceria nanorods, nanocubes, and nano-octahedron crystals are synthesized, and their defect sites are characterized by both UV resonance Raman spectroscopy and dioxygen (O2) adsorption. UV resonance Raman has been applied successfully for catalytic material structural characterization44-46 and was recently shown to be very sensitive to the defect formation in ceria and doped ceria materials.23,25 O2 adsorption probed by Raman spectroscopy28,29 is very powerful in identifying the nature of surface oxygen vacancies. Invaluable information is also provided on how oxygen is activated and transformed on ceria, which is vital in understanding the reaction mechanism of oxidation reactions catalyzed by ceria. Our results show that the defect sites on these ceria nanocrystals are surface-dependent both qualitatively and quantitatively. The stability and reactivity of surface oxygen species are also dependent on the crystallographic termination of ceria.
2. Experimental Section 2.1. Material Synthesis. All the materials used were of analytical purity. Single-crystalline CeO2 nano-octahedra, nanorods, and nanocubes were prepared by a hydrothermal process as reported prevously.40,41 For CeO2 nano-octahedra, 0.434 g of Ce(NO 3 )3 3 6H2O (ARCOS, 99.50%) and 0.0038 g of Na3 PO4 3 12H2O (EM Science) were dissolved in 40 mL of distilled water. After being stirred at room temperature for 1 h, the mixed solution was transferred into a Teflon-lined stainless steel autoclave and heated at 443 K for 10 h under autogenous pressure and static conditions in an electric oven. Upon letting the solution cool to room temperature, the precipitates were separated by centrifuging, washed with distilled water and ethanol three times alternatively, and then dried at 333 K for 1 day. To obtain nanorods and nanocubes, 0.868 g of Ce(NO3)3 3 6H2O and 9.6 g of NaOH (BDH) were dissolved in 5 and 35 mL of deionized water, respectively. Then, these two solutions were mixed in a Teflon bottle, and this mixture was kept stirring for 30 min with the formation of a milky slurry. Subsequently, the Teflon bottle with this mixture was held in a stainless steel autoclave and then heated at 373 and 453 K for 24 h to get nanorods and nanocubes, respectively. After the hydrothermal treatment, fresh white precipitates were separated by centrifugation, washed with deionized water and ethanol several times, followed by drying at 333 K in air overnight. Nitrate/phosphate species are detected on these ceria nanocrystals by Raman spectroscopy (see bands denoted by asterisks in Figures 3-7). The Raman bands at ∼1070 cm-1 on nanorods (43) Tana; Zhang, M. L.; Li, J.; Li, H. J.; Li, Y.; Shen, W. J. Morphologydependent redox and catalytic properties of CeO2 nanostructures: Nanowires, nanorods and nanoparticles. Catal. Today 2009, 148 (1-2), 179-183. (44) Wu, Z. L.; Stair, P. C. UV Raman spectroscopic studies of V/θ-Al2O3 catalysts in butane dehydrogenation. J. Catal. 2006, 237 (2), 220-229. (45) Wu, Z. L.; Kim, H. S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. On the structure of vanadium oxide supported on aluminas: UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction studies. J. Phys. Chem. B 2005, 109 (7), 2793-2800. (46) Wu, Z. L.; Dai, S.; Overbury, S. H. Multiwavelength Raman spectroscopic study of silica-supported vanadium oxide catalysts. J. Phys. Chem. C 2010, 114 (1), 412-422.
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Figure 1. XRD patterns of as-synthesized ceria nanorods (a), nanocubes (b), and nano-octahedra (c). and nanocubes and around 904, 950, 980, and 1050 cm-1 on nanooctahedra are due to residue nitrate and phosphate species left from the synthesis process, respectively. These bands are denoted with asterisks in the figures and are not due to adsorbed oxygen species for two reasons. First, they are very thermally and chemically stable. They are still observed even after 873 K treatment in hydrogen, while adsorbed oxygen species on ceria are usually thermally stable at most up to 573 K. Second, when isotopically labeled O2 is used for adsorption, these bands are still at the same position. So these Raman bands will not be mentioned in following context. The amount of nitrogen and phosphor on the ceria surface was determined by XPS to be negligible. 2.2. Material Characterizations. X-ray diffraction (XRD) data of the as-synthesized ceria samples were collected on a PANalytical powder diffractometer using Cu KR radiation. High-resolution transmission electron microscopic (HR-TEM) images of the as-synthesized ceria were taken on a HF-3300 field emission transmission electron microscope that operated at 300 kV. The TEM images of the ceria nanocrystals pretreated at different temperatures were recorded on a Hitachi HD2000 scanning transmission electron microscope (STEM) system. The Brunauer-Emmett-Teller (BET) surface area of the as-synthesized samples was measured via nitrogen adsorption at 77 K by using a Micromeritic Gemini 275 system. XPS measurements were performed using a Thermo Fisher Scientific KR XPS instrument using monochromatic Al KR X-rays. Ceria samples were pressed into a 1 mm hole in a stainless steel sample stage for analysis. Sample charging was minimized using a charge compensator system. 2.3. Raman Spectroscopy. The CeO2 samples were dehydrated in situ in a Raman catalytic reactor (Linkam CCR1000) before Raman spectral collection at room temperature. The dehydration was done by heating the sample (ca. 20 mg) in flowing 5% O2/He (60 mL/min) from room temperature to 673 K (ramping rate 10 K/min) and holding there for an additional 1 h. The sample was then cooled down to room temperature in O2/He, and Raman spectra were collected with different laser excitations (244, 325, and 532). The Raman measurements were performed on a newly built multiwavelength Raman system at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences.46 Raman scattering was collected via a customized ellipsoidal mirror and directed by a fiber optics bundle to the spectrograph stage of a triple Raman spectrometer (Princeton Instruments Acton Trivista 555). Edge filters (Semrock) were used in front of the UV-vis fiber optic bundle (Princeton Instruments) to block the laser irradiation. The 244 nm laser excitation (∼2 mW Langmuir 2010, 26(21), 16595–16606
Article at sample) is obtained via second-harmonic generation (SHG) from an Ar ion laser (Coherent, MotorFred I300C) with fundamental at 488 nm. The 325 nm excitation (5 mW at sample) is from a HeCd laser (Melles Griot). The 532 nm excitation (20 mW at sample) is emitted from a solid state laser (Princeton Scientific, MSL 532-50). A UV-enhanced liquid N2-cooled CCD detector (Princeton Instrument) was employed for signal detection. The Raman reactor sits on an XY stage (Prior Scientific, OptiScan XY system) and translates in raster mode while collecting the spectrum. The fast translation has shown been able to eliminate/ minimize any laser damage of the samples. Cyclohexane is used as a standard for the calibration of the Raman shifts. 2.4. Raman Spectroscopy of O2 Adsorption. O2 adsorption coupled with in situ Raman spectroscopy was done on the same Raman system as mentioned above. For room temperature O2 adsorption, the experiments were done in the Linkam CCR1000 reactor. The ceria sample was prereduced prior to O2 adsorption. The as-synthesized CeO2 was calcined in 5% O2/He at 673 K for 1 h before reduction in 2% H2/Ar/He (60 mL/min) at different temperatures (673, 773, and 873 K) for 2 h. The sample was cooled down in 2% H2/Ar/He and switched to He (60 mL/min) at room temperature before exposure to O2 adsorption (5% O2/He with flow rate of 30 mL/min). The adsorption time is usually 30 min, and the sample is then purged with He (60 mL/min) and ramped to 473 K for desorption study. Raman spectra were collected at different temperature intervals. In oxygen isotope studies, 2% 18O2/He at a flow rate of 15 mL/min was used. For O2 adsorption at liquid nitrogen (LN2) temperature (80 K), a Linkam THMS600 stage was used instead and the experimental procedure is the same as that for O2 adsorption at room temperature.
2.5. O2 Chemisorption and Temperature Programmed Desorption (TPD-QMS). To quantify the amount of O2 adsorbed on ceria samples after reduction, O2 pulse chemisorption was carried out on a plug-flow microreactor system (Altamira AMI 200) with subambient temperature capability down to 173 K and a downstream quadruple mass spectrometer (QMS) (OmniStar GSD-301 O2, Pfeiffer Vacuum). Approximately 25 mg of sample was loaded in a U-shaped microreactor and was calcined in 2% O2/He (30 mL/min) at 673 K for 1 h before reduction in 2% H2/He (30 mL/min) at the same temperature for 1 h. The sample was cooled down to 173 K in He (30 mL/min) with a cooling rate of 30 K/min. After being stabilized at 173 K, 2%O2/He was pulsed multiple times onto the sample in He flow (30 mL/min) until there was no change in the O2 peak intensity monitored by QMS. The O2 peak intensity was calibrated by pulsing 2%O2/He through the reactor bypass. After the oxygen pulse chemisorption, the sample was purged with He (30 mL/min) at 173 K for 15 min before TPD in He or in 1% 18O2/He (30 mL/min). All TPD experiments were run up to 623 K at a ramping rate of 10 K/min. Evolution of different oxygen isotopes (m/e = 32 for 16O2, 34 for 16 18 O O, and 36 for 18O2) were monitored by QMS during the desorption process.
3. Results 3.1. XRD, TEM, and BET Analysis of Ceria Nanocrystals. Figure 1 shows the XRD patterns of the three ceria polymorphs. They all can be indexed to the pure fluorite cubic structures (space group Fm3m (225), JCPDS 34-0394). The nanocubes and nano-octahedra exhibit relatively higher crystallinity than the nanorods, and the particle size, determined from the Scherrer equation, is 11 nm for nanorods, 95 nm for nanocubes, and 117 nm for nano-octahedra. Figure 2 shows the TEM images of the ceria nanocrystals. The nanorods (Figure 2a, b) are about 10 nm across and 50-200 nm long. According to previous studies,39-41 the synthesized rods grow along the [110] direction and prefer to expose {110} and {100} surfaces. The black spots in the image suggest the presence of DOI: 10.1021/la101723w
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Figure 2. TEM and HRTEM images of as-synthesized ceria nanorods (a, b), nanocubes (c, d), and nano-octahedra (e, f).
defects in the nanorods. The nanocubes (Figure 2c, d) show less uniform size distribution ranging from 20 to 100 nm, but all in perfect cubic shape, exposing the {100} surface. The nanooctahedra (Figure 2e, f) are very uniform in size distribution and shaped as almost perfect octahedra with little truncation, so they are dominated by {111} surface termination. The TEM observations are in good accord with previous studies of ceria nanomaterials,39-41 indicating successful synthesis of the desired morphology of ceria crystals in our study. The BET areas of the nanorods, nanocubes, and nano-octahedra are 79, 24, and 13 m2/g, respectively. Morphology changes of these ceria samples under thermal treatments were followed by TEM measurements (see Figure S1 in the Supporting Information). The shapes of the three ceria polymorphs are well kept at temperatures below 773 K. Both nanorods and nanocubes start to undergo morphology changes at 773 K. Rounded particles are observed after 873 K treatment, especially for the ceria nanocubes. Nano-octahedra are thermally stable and do not show evident shape change even after 873 K treatment. The thermal stability of the three ceria polymorphs is related to their surface terminations as predicted by theoretical calculations.34,35 In order to study the morphology effect of ceria on the surface defects, most of our following experiments were carried out at 673 K so that the shape of the ceria nanocrystals can be kept intact. 3.2. Raman Spectra of Ceria Nanorods, Nanocubes, and Nano-Octahedra. Figure 3 compares the visible (532 nm) and UV (325 nm) Raman spectra of ceria nanorods, nanocubes, and nano-octahedra calcined at 673 K. The visible Raman spectra are dominated by the strong F2g mode of the CeO2 fluorite phase at 462 cm-1 with weak bands at 258, 595, and 1179 cm-1, due to second-order transverse acoustic (2TA) mode, defect-induced (D) 16598 DOI: 10.1021/la101723w
Figure 3. Visible (λex = 532 nm) (A) and UV (λex =325 nm) (B) Raman spectra of ceria nanorods (a), nanocubes (b), and nanooctahedra (c). Asterisks (/) represent Raman bands due to trace nitrate/phosphate species in ceria.
mode, and second-order longitudinal optical (2LO) mode, respectively.22-24,47,48 The nanorods give a much broader 462 cm-1 peak than nanocubes and nano-octahedra, which is a size-dependent phenomenon observed on ceria nanoparticles and can be explained by the inhomogeneous strain broadening associated with dispersion in particle size and by phonon confinement.47 This is consistent with the XRD and TEM measurements where nanorods have the smallest size. For all ceria samples, the UV Raman spectra features are quite different from those in visible Raman in terms of relative bands intensity. First, the 2LO band at 1179 cm-1 is as strong as the F2g mode at 462 cm-1. The third overtone band (3LO) is also clearly observed at 1760 cm-1. The strong enhancement of the LO overtones under UV excitation is due to the multiphonon excitation by resonance Raman effect.23,49 Second, the D band at around 592 cm-1 has comparable intensity as the F2g mode at 462 cm-1. (47) Spanier, J. E.; Robinson, R. D.; Zheng, F.; Chan, S. W.; Herman, I. P. Sizedependent properties of CeO2-y nanoparticles as studied by Raman scattering. Phys. Rev. B 2001, 64 (24), 245407. (48) Weber, W. H.; Hass, K. C.; Mcbride, J. R. Raman-study of CeO2 - 2ndorder scattering, lattice-dynamics, and particle-size effects. Phys. Rev. B 1993, 48 (1), 178-185. (49) Livneh, T.; Sterer, E. Effect of pressure on the resonant multiphonon Raman scattering in UO2. Phys. Rev. B 2006, 73 (8), 085118.
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Figure 4. UV (λex =325 nm) Raman spectra of ceria nanorods reduced by hydrogen at different temperatures: (a) 673 K calcined; (b) 673 K reduced; (c) 773 K reduced; (d) 873 K reduced; and (e) room temperature O2 adsorption after 873 K reduction. All spectra were collected at room temperature and normalized to collection time.
In the case of nanorods, the D band is even stronger than the F2g band. Evidently, UV Raman is more sensitive to defect sites in ceria than visible Raman. This is due to the resonance Raman effect,23,25 since ceria strongly absorbs in the UV region. Extending the excitation wavelength to the deep UV (see Figure S2 in the Supporting Information where 244 nm excitation is used) leads to noisy spectra (due to stronger absorption) and less sensitivity to the defect sites (D band is much weaker than the F2g band). It appears that the electronic absorption band associated with the defect sites in ceria is in the middle to near UV range because both our 325 nm excitation and the 363.8 nm excitation used in Taniguchi’s work23 result in resonance enhancement of the D band. The relative intensity ratio of ID/IF2g inserted in the plots in Figure 3B follows the sequence nanorods > nanocubes > nanooctahedra, indicating that the nanorods have the most intrinsic defect sites while nano-octahedra have the least. The Raman spectra of the calcined ceria in Figure 3 give information on the intrinsic defect sites on ceria. It is also interesting to investigate the defect sites created by reduction because this is an important step in the redox cycle of ceria catalysis. Figure 4 compares the UV Raman spectra of ceria nanorods reduced by hydrogen at different temperatures. There is no obvious change in the two Raman bands at 592 and 462 cm-1 (both bandwidth and relative intensity) after reduction at 673 K. The 592 cm-1 band shifts to lower Raman shift with a shoulder band at ∼560 cm-1 after 773 K reduction. Meanwhile, the 462 cm-1 band decreases in bandwidth, due to an increase in particle size as shown in the TEM images (Supporting Information Figure S1). Also, the ID/IF2g ratio decreases considerably. The trend is more evident after reduction at 873 K. Interestingly, upon O2 adsorption at room temperature, the shoulder at 560 cm-1 disappears instantly and the D band now shifts back to 592 cm-1, the same position as that of the calcined sample. Previous Raman spectra of doped ceria also showed a D band at around 550 cm-1 which was assigned to O vacancy.23,24 However, such a low energy D band has not been reported for pure CeO2. The facile oxygen annihilation of the defect sites associated with the 560 cm-1 band clearly indicate that this D band is due to the presence of oxygen vacancies created during reduction of ceria. Langmuir 2010, 26(21), 16595–16606
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This is the first time for direct Raman detection of O vacancy sites on a reduced ceria surface. UV Raman spectra of ceria nanocubes (Supporting Information Figure S3A) reduced at different temperatures are similar to those of nanocubes: the growth of a shoulder band at 560 cm-1 and decrease of bandwidth of the F2g mode are observed as the reduction temperature increases, indicating the creation of oxygen vacancies by reduction and growth of particle size upon thermal annealing. For ceria nanooctahedra (Supporting Information Figure S3B), there is essentially no change in the spectra features even when the sample was reduced at 873 K. This is likely because the nano-octahedra are barely reducible up to 873 K. 3.3. Raman Study of O2 Adsorption. O2 adsorption was conducted at room temperature on both calcined and reduced ceria samples. On all calcined ceria samples, no Raman band due to adsorbed O2 was observed, implying that the oxidized ceria surface does not adsorb oxygen. This is also confirmed by O2 chemisorption measurements (described below) where no measurable oxygen adsorption is found on oxidized ceria samples. Raman bands due to adsorbed O2 species are readily observed when the ceria samples were reduced by hydrogen, suggesting that oxygen vacancies generated from the reduction with H2 play a significant role in the formation of adsorbed species. Figure 5 exhibits the visible Raman spectra of O2 adsorption on CeO2 nanorods (Figure 5A), nanocubes (Figure 5B), and nano-octahedra (Figure 5C) reduced in hydrogen at different temperatures. On ceria nanorods (Figure 5A), O2 adsorption gives three bands at 1139, 862 (shoulder), and 830 cm-1 after 673 K reduction, suggesting the formation of three surface adsorbed oxygen species. For O2 adsorption on the nanorods after 773 K reduction, the 1139 cm-1 band is barely observable above the strong 2LO band, while the shoulder band at 862 cm-1 gains more intensity as compared with the main band at 830 cm-1. Surprisingly, no Raman band is observed for O2 adsorption on 873 K reduced ceria nanorods. To verify that the observed new bands originate from oxygen-containing species, 18O2 instead of 16O2 was used for adsorption on the 673 K reduced ceria nanorods. As depicted in Figure 5A, the bands at 830 and 862 cm-1 from 16O2 adsorption now shift to 782 and 813 cm-1, an isotopic shift ratio of 1.060 that agrees with the theoretical ratio of 1.061. The counterpart for the 1139 cm-1 band is expected to be around 1074 cm-1 which is likely overlapping with the impurity nitrate peaks. On nanocubes reduced at three different temperatures (Figure 5B), O2 adsorption results in an asymmetric band at 833 cm-1 with tailing at the high Raman shift side, suggesting the dominance of one type of oxygen species on reduced ceria nanocube surfaces. On ceria nano-octahedra (Figure 5C), two very weak Raman bands at 832 and 851 cm-1 are observed upon O2 adsorption on the reduced surface. The band at 851 cm-1 increases in intensity relative to the one at 832 cm-1 for the higher reduction temperatures. It appears that two types of adsorbed oxygen species are formed on reduced nano-octahedron surfaces and they change in relative population as a function of reduction degree. The Raman bands of surface oxygen species are weak on the nano-octahedra, mostly because the amount of adsorbed oxygen is very small as demonstrated by O2 chemisorption measurements (section 3.4). O2 adsorption on the ceria samples was also carried out at liquid nitrogen temperature (80 K). Figure 6 shows the Raman spectra following adsorption of oxygen at 80 K on ceria nanocrystals that were prereduced at 773 K, and compares it with the corresponding adsorption at 295 K. There is no apparent difference in the Raman spectra of adsorbed oxygen on nano-octahedra at the two different temperatures. For nanorods, Raman DOI: 10.1021/la101723w
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Figure 6. Comparison of Raman spectra from O2 adsorption at 80 K and room temperature on 773 K reduced nanorods, nanocubes, and nano-octahedra. Asterisks (/) represent Raman bands due to trace nitrate/phosphate species in ceria.
on different temperature reduced ceria (A) nanorods, (B) nanocubes, and (C) nano-octahedra. Asterisks (/) represent Raman bands due to trace nitrate/phosphate species in ceria. All spectral normalized to collection time.
contribution from the higher Raman shift end in the 860900 cm-1 range than those in spectra collected at room temperature. The spectral difference between two temperatures indicates that some of the adsorption sites probed by O2 at 80 K may be annihilated at room temperature by oxygen adsorption and thus are not observable. A similar spectral difference is made on nanocubes, but less evident as on nanorods. The different Raman bands resulting from O2 adsorption on ceria samples can be assigned based on previous spectroscopic studies.6,26-29 The band at 1139 cm-1 is due to O-O stretching of adsorbed superoxide species (O2-) while the bands at 890, 862, and 833 (840) cm-1 can be assigned to O-O stretching of adsorbed peroxide species (O22-) with different degrees of defect aggregation. For O22- species, the higher the Raman shift, the more aggregated are the defect sites, due to the dipole-dipole coupling effect between adsorbed peroxide species. The band at around 830 cm-1 was attributed to peroxide species adsorbed on isolated two-electron defect sites, while the ones at around 860 and 890 cm-1 to peroxide species on clustered two-electron defect sites with increasing aggregation degree.28 The stability of the different surface oxygen species was studied via thermal desorption (TPD) in helium flow and followed by Raman spectroscopy (Raman-TPD). Because the nano-octahedra give very weak Raman signals of adsorbed oxygen species, the following Raman-TPD experiments were conducted on nanorods and nanocubes. Figure 7A shows the Raman spectra from O2-TPD on ceria nanorods in flowing He. Superoxide is stable only up to 348 K as evident by the disappearance of the 1139 cm-1 band above this temperature. The peroxide species are more stable. The two peaks at 862 and 830 cm-1 gradually decrease in intensity relative to the ceria 2LO band at 1179 cm-1 and vanish at temperatures above 473 K. The band at 862 cm-1 decreases faster relative to the band at 830 cm-1. For ceria nanocubes (Supporting Information Figure S4A), the peroxide species are found to be less stable than the ones on nanorods: the band at 833 cm-1 disappears at temperature of 393 K, about 75 K lower than the one on nanorods. Similar TPD experiments were also made in previous studies of O2 adsorption on polycrystalline CeO2.26-28 The decrease of the Raman/IR bands due to adsorbed oxygen species can be ascribed to the reoxidation of ceria via following steps:26,28
spectra collected at 80 K show a broad band ranging from 840 to 865 cm-1 with a weak one at around 890 cm-1, exhibiting more
O2 - ads f O2 2 - ads f 2O - ads f 2O2 - lattice
Figure 5. Raman spectra of O2 adsorption at room temperature
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Figure 7. Raman spectra collected during temperature programmed desorption of oxygen on nanorods in (A) He, (B) 2% 18O2/He, and (C) 1% CO/He. Asterisks (/) represent Raman bands due to trace nitrate/phosphate species in ceria. All spectral normalized to collection time.
Superoxide is the least stable, followed by peroxide on clustered defect sites and then peroxide on isolated defect sites.28 The conversion of these adsorbed oxygen species into lattice oxygen was evident by the relative increase of the ceria 2LO mode at 1179 cm-1 (also the one at 462 cm-1, not shown) and the gradual color change of the sample (gray to light yellow). During the thermal desorption process, a small portion of the oxygen species will desorb as detected in the TPD-QMS experiments (see following section). Langmuir 2010, 26(21), 16595–16606
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The TPD experiments were also done with different carrier gases: isotopically labeled oxygen or carbon monoxide. Figure 7B and C exhibits the changes of Raman spectra of adsorbed oxygen species on nanorods in 2% 18O2/He and 1% CO/He as a function of temperature, respectively. When 18O2 is introduced to the nanorods surfaces with preadsorbed 16O species, an immediate change is the concomitant disappearance of the 1139 cm-1 band and appearance of a shoulder band at around 1075 cm-1, indicating a facile isotopic exchange between surface superoxide species and gas phase oxygen at room temperature. This process is more clearly demonstrated when 16O2 is introduced to the nanorod surfaces preadsorbed with 18O2 species (Supporting Information Figure S5), and the 1139 cm-1 due to 16O2- is instantly observed. The isotopic exchange between peroxide species and gas phase oxygen is also observed at room temperature but at much slower pace. As shown in Figure 7B, a weak band at 780 cm-1 is observed due to 18O22- after 45 min contact with gaseous 18O2 at room temperature. The isotopic exchange involving peroxide species is facilitated at higher temperature as seen in Figure 7B. The bands at 862 and 830 cm-1 continuously decrease in intensity with temperature, due to on the one hand conversion of these species into ceria lattice oxygen, and on the other hand isotopic exchange with 18O2 to form 18O22- species because the 780 cm-1 band increases in intensity relative to the one at 830 cm-1 during the thermal desorption process. Both bands are still observable after heating up to 478 K. The superoxide species are stable up to 348 K as evident in Supporting Information Figure S5 where the band at 1139 cm-1 disappears at temperatures of 348 K and above. Similar results (see Supporting Information Figure S4B) are obtained on ceria nanocubes where the isotopic exchange between surface peroxide species and gas phase oxygen also starts at room temperature and accelerates at higher temperatures. Again, the two peroxide species (18O22- and 16 O22-) on nanocubes are less stable (up to 448 K) than those on nanorods (up to 473 K). When the desorption carrier gas is 1% CO/He (Figure 7C) for nanorods preadsorbed with oxygen species, the band at 1139 cm-1 gradually disappears at room temperature while the bands at 862 and 830 cm-1 are stable up to 398 K. Compared to the desorption in inert (He) or oxidizing atmosphere (18O2/He), both superoxide and peroxide species are consumed at much lower temperature in the reducing atmosphere (CO/He), obviously due to their reaction with CO. The surface superoxide species are more reactive toward CO oxidation than peroxide species. Similarly, the peroxide species on nanocubes (Supporting Information Figure S4C) are less stable in CO/He than in He. 3.4. O2 Chemisorption and TPD-QMS Results. Table 1 compares the amount of O2 adsorption from pulse chemisorption at 173 K on the different ceria nanocrystals pretreated in different atmospheres. In all cases, there is no measurable adsorbed O2 on the precalcined ceria nanocrystals. No O2 adsorption is observed on either the precalcined or prereduced ceria nano-octahedra, suggesting that the surface of the nano-octahedra is barely reducible up to 773 K. Both nanorods and nanocubes adsorb O2 after 673 K reduction, corresponding to ca. 0.21 monolayer of oxygen atoms in both cases. Around 20% of the adsorbed O2 is desorbed during the TPD process on both nanocrystals. The similar numbers indicate that the nanorods and nanocubes are reduced to a similar extent at 673 K by hydrogen. The reduction of nanocubes is further increased when the temperature increases to 773 K as ca. 0.23 monolayer of oxygen is adsorbed. Surprisingly, the O2 adsorption amount on nanorods decreases significantly when the reduction temperature increases to 773 K, and only ca. 0.03 monolayer of oxygen is adsorbed. This is likely DOI: 10.1021/la101723w
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Table 1. Comparison of O2 Chemisorption at 173 K on Different Ceria Nanocrystals Either Precalcined at 673 K or Prereduced at 673 and 773 K 673 K calcined
673 K reduced a
adsorbed (μmol O2 /g)
adsorbed (μmol O2 /g)
a
773 K reduced b
adsorbed (μmol O2 /g)a
desorbed (μmol O2 /g)
desorbed (μmol O2 /g)b
nano-octahedra 0 0 0 0 0 12 70 (0.23c) 6 nanocubes 0 66 (0.21c) 34 18 (0.02c) 0 nanorods 0 163 (0.21c) a b Total amount of adsorbed O2 based on weight from pulse chemisorption at 173 K. Total amount of desorbed O2 based on weight from TPD-QMS. c In monolayer based on surface area, calculated according to the theoretical number of surface oxygen atoms on {111}, {110}, and {100} surfaces in the literature,11 which are 26.2, 16.1, and 22.7 μmol O atom/m2, respectively. For nanorods, the number is an average of half {110} and half {100}.
observed at 393 and 468 K for nanorods, and 343 and 468 K for nanocubes. More low-temperature O2 desorption is observed on nanocubes, while most O2 desorbs at higher temperature on nanorods. This is consistent with the Raman observations where surface oxygen species are more stable on nanorods than on nanocubes. The observation of gas phase O2 during the TPD process is likely due to the disproportionation reaction of surface oxygen species: 16
16
O2
O2
-
2-
ads þ
ads þ
16
16
O2
O2
-
2-
ads
f
ads
f 216 O 2 - lattice þ 16 O 2
2-
16
O2
ads þ
16
O2
ð1Þ ð2Þ
Reaction 1 can take place at low temperature while reaction 2 is likely to happen at higher temperature as evident from the Raman-TPD spectra (see Figure 6) where superoxide species disappear at much lower temperature than peroxide species. When desorbing in 1% 18O2/He (Figure 8B) after 16O2 adsorption, both 16O2 and 16O18O are observed in the desorption products. The 16 O2 desorption profiles are mostly similar to those from desorption in He for both ceria nanorods and nanocubes, namely most 16 O2 is produced at lower temperature on nanocubes while more 16 O2 is produced at higher temperature on nanorods. However, the total amount of 16O2 is increased compared to desorption in He. This is because 16O2 desorption results not only from the disproportionation reaction of surface oxygen species as in reactions 1 and 2 but also from site exchange between surface oxygen species and gas phase 18O2 as in reactions 3 and 4: 16
16
Figure 8. QMS profiles from O2-TPD in (A) He and (B) 1% 18O2/ He after oxygen chemisorption on 673 K reduced ceria nanorods and nanocubes. In (B), 16O2 produced on (black) nanocubes and (red) nanorods; 16O18O produced on (red) nanorods and (black) nanocubes.
due to the reorganization of the nanorod surface which is oxidized by bulk oxygen and will be further elaborated in the Discussion section. After O2 chemisorption, TPD experiments were performed in either He or 18O2/He and the oxygen products were followed by online QMS. Figure 8A compares the O2 desorption profiles in He after O2 adsorption at 173 K on ceria nanorods and nanocubes. On both ceria nanocrystals, O2 desorption starts at around 263 K and extends to 573 K with two desorption maxima 16602 DOI: 10.1021/la101723w
O2
O2
-
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18
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þ 18 O 2 f
18
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-
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2-
16
ads þ
O2
16
O2
ð3Þ ð4Þ
The presence of both 18O2- and 18O22- is supported by the Raman observations in Figure 7B when 18O2 is introduced to the nanorod surface with preadsorbed 16O species. Nanorods produce much more 16O18O than nanocubes, although both show similar 16O18O desorption profiles: a major desorption at low temperature (∼295 K) and a minor one at high temperature (463 K). The production of 16 18 O O is due to the isotopic exchange (a) between surface oxygen species and gas phase 18O2 and (b) between surface oxygen species as in reactions 5-14: 16
16
16
O2
ads
þ 18 O 2 f
16
O 18 O - ads þ 16 O 18 O
O 18 O - ads þ 18 O 2 f
O2
2ads
þ 18 O 2 f
16
18
O2
ads
þ 16 O 18 O
O 18 O 2 - ads þ 16 O 18 O
ð5Þ ð6Þ ð7Þ
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16
16
16
O 18 O 2 - ads þ 18 O 2 f
O2
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O2
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2ads
f
16
18
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O2
2-
ads þ
16
ð8Þ
O 18 O
O 18 O 2 - ads þ 16 O 18 O
ð9Þ
O 18 O 2 - ads þ 16 O 18 O
ð10Þ
16
O 2 - lattice þ 18 O 2 - lattice þ 16 O 18 O ð11Þ
16
O 18 O 2 - ads þ 16 O 18 O 2 - ads f
16
O 2 - lattice þ 18 O 2 - lattice
þ 16 O 18 O 16
18
O2
O2
2-
2-
ads þ
16
ads þ
16
ð12Þ
O 18 O 2 - ads f 216 O 2 - lattice þ 16 O 18 O
ð13Þ
O 18 O 2 - ads f 218 O 2 - lattice þ 16 O 18 O
ð14Þ
16
18
Figure 9. Ce3d XPS spectra of ceria nanorods, nanocubes, and nano-octahedra. Dashed lines indicate the BE for Ce3þ 3d.
-
The presence of superoxide O O is supported by the observation of a Raman band at around 1104 cm-1 when 16O2 is coadsorbed with 18O2 at room temperature on ceria nanorods (see Supporting Information Figure S6). A Raman band anticipated for peroxide 16O18O2- at 807 cm-1 is likely overwhelmed by the strong Raman band of 16O22- at 830 cm-1. Considering the thermal stability of the surface oxygen species, reactions 5, 6, 9, and 10 involving superoxide reactants would proceed at lower temperature while the other reactions involving peroxide reactants would contribute mostly to higher temperature 16O18O production.
4. Discussion 4.1. Defect Sites on Oxidized Ceria Nanocrystals. Raman spectroscopy is among the most powerful techniques in direct characterization (without probing molecules) of defect sites in ceria, Ce3þ, and oxygen vacancies. This is especially the case when using UV Raman, which due to the resonance Raman effect is very sensitive to the defect sites formed in ceria and doped ceria samples.23,25 The UV Raman spectra (see Figure 3) of oxidized ceria nanocrystals show a pronounced D band associated with defect sites, and the ID/IF2g can be an indicator for the amount of defect sites in ceria samples. Although the nanocubes and nanooctahedra have roughly similar particle size as determined from Raman (bandwidth of 462 cm-1), XRD, and TEM, the ID/IF2g is quite different, which indicates that the crystal size is not a major factor affecting this Raman spectral feature. It was also suggested by Taniguchi et al.23 that local disorders in the lattice instead of the grain size have a major influence on ID/IF2g. UV Raman is not a surface technique, and thus, the D-band in the UV Raman spectra is supposedly associated with defect sites not only on the outmost surface but also in the subsurface/bulk. Therefore, the amount of defect sites in the subsurface/bulk of ceria nanocrystals must be affected by their surface terminations: nanorods with exposed {110} and {100} surfaces have the most defect sites, followed by nanocubes ({100}) and nano-octahedra ({111}). This is consistent with the theoretical prediction31 that the vacancies formation energy follows the order of {110} < {100} < {111}. The Raman results are well supported by the HRTEM images of the ceria nanocrystals where the nanorods show dark spots due to the presence of defect sites. Langmuir 2010, 26(21), 16595–16606
The nature of the defect sites associated with the D band at 592 cm-1 was previously suggested to be due to the presence of Ce3þ in the octahedral site (MO8)23 by comparison with the Raman spectra of doped ceria where a similar D band was observed when M4þ or M3þ elements were doped into ceria.24 However, the presence of Ce3þ in the MO8 site in pure ceria is questionable because Ce3þ defect sites are supposed to be paired with O-vacancies since the excess electrons left behind by removal of oxygen would localize on adjacent Ce cations.14 Here we propose an alternative assignment of the D band at 592 cm-1, to the vacancy-interstitial (Frenkel-type) oxygen defects in ceria.9,21 According to neutron scattering studies, some oxygen anions in ceria may relocate themselves from the tetrahedral sites to the octahedral sites, leaving vacancies in the tetrahedral sites and resulting in the formation of the Frenkel-type anion defects. This assignment can well explain the observed thermal stability of this kind of defect site and their oxygen adsorption property. Mamontov et al.9,21 showed that the recombination of interstitial oxygen ions with O-vacancies takes place in ceria at temperatures around 1033 K. This is in line with our Raman result (Figure 3) which shows that these defect sites can be sustained at elevated temperatures even when the ceria nanocrystals are heated in oxygen atmosphere at 673 K. It is also reasonable that the attempt to probe the defect sites via O2 adsorption on oxidized ceria nanocrystals is not successful in either Raman or chemisorption experiments. This is because the O-vacancies are charge-balanced by the interstitial oxygen in the octahedral sites so that they are not able to adsorb extra oxygen. The presence/absence of Ce3þ in the oxidized ceria nanocrystals may be used to distinguish the two different assignments of the D band at 592 cm-1. For this purpose, XPS spectra of the Ce 3d region (Figure 9) were measured on the three ceria samples. The three ceria polymorphs show very similar spectral profiles and approximately match that for a fully oxidized CeO2 surface,50 indicating that even if there is Ce3þ present, the amount is very small and similar on the three different ceria polymorphs. This is in clear contrast to the big difference in ID/IF2g as observed in Raman for the three samples, indicating that the variation of ID/IF2g is not directly related to the amount of Ce3þ. Considering all (50) Mullins, D. R.; Overbury, S. H.; Huntley, D. R. Electron spectroscopy of single crystal and polycrystalline cerium oxide surfaces. Surf. Sci. 1998, 409 (2), 307-319.
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our experimental observations, namely, the high temperature stability of the D band on oxidized ceria compared to the rapid oxidation of vacancies on reduced ceria, the inability for these sites to adsorb O2, and the nearly complete absence of Ce3þ in the XPS of all three polymorphs, it appears more likely that the D band at 592 cm-1 is due to the Frenkel-type oxygen vacancies rather than Ce3þ trapped in MO8 sites in ceria. The Frenkel-type oxygen vacancies presumably start to form on the surface/subsurface and progress into the bulk of ceria. So their amount is affected by the surface termination of ceria, which explains the most abundant defects on nanorods and least on nano-octahedra. These intrinsic defect sites are supposed to provide oxygen mobility in ceria catalysis.9,21 4.2. Defect Sites on Reduced Ceria Nanocrystals. The defect sites of the three ceria polymorphs after reduction were characterized by both direct UV Raman spectroscopy and Raman of O2 adsorption. In the direct UV Raman measurements, a shoulder band at around 560 cm-1 is observed when the ceria nanorods and nanocubes are reduced at high temperatures, indicating the generation of O-vacancy sites. The assignment of the 560 cm-1 band to O-vacancies is directly proved by the observation that this band disappears immediately upon O2 adsorption. These generated O-vacancies are different from the intrinsic defect sites on the oxidized ceria because they are observed to be active in the adsorption and transformation of gas O2. Raman of O2 adsorption provides information on the nature of the defect sites on the reduced ceria nanocrystals. Different adsorbed oxygen species are formed on the three nanocrystals. On ceria nanorods, both superoxide and peroxide species are formed after 673 K reduction, indicating the presence of both oneelectron and two-electron defect sites on nanorods. The observation of two bands at 830 and 862 cm-1 implies the existence of both isolated and clustered two-electron defects. No superoxide, that is, one-electron defect site, is observed on reduced nanocubes and nano-octahedra. The peroxide species on nanocubes are characterized by a main band at 833 cm-1 with tailing at 860 cm-1, suggesting that nanocubes are dominated by isolated twoelectron defect sites. On the nano-octahedra, the peroxide species give weak bands at 832 and 851 cm-1, evidence for small amounts of both isolated and clustered two-electron defect sites. The amount of these defect sites can be estimated from both the Raman band intensity ratio and the amount of chemisorbed oxygen. The relative intensity of Raman bands of peroxide species to that of the 2LO band of ceria (Figure 5) follows the trend: nanorods > nanocubes . nano-octahedra. Raman probes the same volume for each ceria sample, and thus, the signal intensity reflects the total amount of adsorbed oxygen on a volume (weight) basis. This is consistent with the chemisorption results (Table 1) where the total amount of adsorbed oxygen (μmol O2/g CeO2) is the largest on nanorods, followed by nanocubes, and negligible on nano-octahedra. The immeasurable amount of adsorbed oxygen on nano-octahedra is due to the difficult reduction of the {111} surface at the temperatures employed. This is supported by a separate H2-TPR experiment which shows that the reduction of nano-octahedra does not occur below 773 K. It is noted that O2 adsorption on polycrystalline ceria in previous studies gives vibrational bands due to variety of superoxide and peroxide species,26-28 indicating that the surface of polycrystalline ceria is a mixture of different crystal planes. If normalized to surface area, the amount of adsorbed oxygen is very similar on nanorods and nanocubes after 673 K reduction, both at around 0.21 monolayer. Considering the surface O-atom density on different ceria planes, the O-atom is most dense on 16604 DOI: 10.1021/la101723w
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the {111} surface (15.8 O-atom/nm2), then the {100} surface (13.7 O-atom/nm2), and the least on the {110} surface (9.7 O-atom/nm2).11 So by removing a similar number of O-atoms from the surfaces, it is expected that more clustered defect sites are on the {110} surface than on {100}. This well explains the observation of more clustered two-electron defect sites on nanorods ({110} þ {100}) than on nanocubes ({100}). For nanooctahedra dominated by the {111} plane, a slight reduction should generate mostly isolated defect sites. However, the Raman of O2 adsorption shows the presence of both clustered and isolated defects. The contradiction seems indicate that the observed oxygen species on clustered defect sites may be related to the octahedron edge sites that are reducible by hydrogen at the temperature used here. The desorption of surface oxygen species can be well affected by the mobility of defect sites on ceria nanocrystals. The fate of adsorbed oxygen species can be either desorbed as gas phase O2 or converted into lattice oxygen. As shown in Table 1, ca. 20% of adsorbed oxygen is desorbed in the TPD process on nanorods and nanocubes, so the majority of surface dioxygen species is converted into lattice oxygen. According to reactions 1 and 2, majority oxygen species would be desorbed into gas phase via the disproportionation reaction of superoxide and peroxide species. So another route for converting adsorbed oxygen species into lattice oxygen should play a major role: annihilation of O-vacancies (VO••) by the adsorbed oxygen species: 2O2 - ads þ 3VO •• f 4O2 - lattice
ð15Þ
O2 2 - ads þ VO •• f 2O2 - lattice
ð16Þ
The similar amount of oxygen (ca. 80%) being converted into lattice oxygen on both nanorods and nanocubes again suggests that the amount of O-vacancy sites (per surface area) created by hydrogen reduction is similar on the two nanocrystals when reduced by hydrogen at 673 K. The mobility of the vacancies would affect the easiness of reactions 15 and 16. A comparison of the Raman spectra (Figure 6) of oxygen adsorption at 80 K and room temperature shows that some oxygen species (mainly those on clustered two-electron defect sites) have been consumed between the two temperatures. While there is no appreciable desorption of oxygen in the QMS-TPD profiles below room temperature, it is likely that the consumed oxygen species are converted to lattice oxygen by reaction with the O-vacancy sites. It appears that some vacancies on both nanocrystals are quite mobile even below room temperature, in agreement with the finding of Namai et al.51 The larger change in Raman spectral profile of adsorbed oxygen between the two temperatures on nanorods than on nanocubes is likely an indication that the hydrogen-created vacancies are more mobile on nanorods than on nanocubes. This is possibly promoted by the larger amount of intrinsic defect sites available on nanorods as determined by the UV Raman spectroscopy. One interesting observation of oxygen adsorption on nanorods is that the amount of adsorbed oxygen decreases dramatically after 773 K reduction (Table 1) and the Raman signal due to adsorbed oxygen species is hardly seen after reduction at 873 K (Figure 5A). The difference in temperature in the two sets of (51) Namai, Y.; Fukui, K.; Iwasawa, Y. Atom-resolved noncontact atomic force microscopic observations of CeO2(111) surfaces with different oxidation states: Surface structure and behavior of surface oxygen atoms. J. Phys. Chem. B 2003, 107 (42), 11666-11673.
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experiments is likely due to the use of different reactors. The data suggest that very few defect sites are created on the surface after reduction at the higher temperature. However, the UV Raman spectrum (Figure 4) of nanorods reduced at 873 K still shows the presence of the 560 cm-1 band due to O-vacancies. It seems that a surface reorganization has occurred so that subsurface/bulk oxygen migrates to the surface to reoxidize the surface vacancies while the subsurface/bulk is being reduced. This kind of surface reorganization is not uncommon in ceria, and was shown to be promoted by the presence of defect sites in ceria.3,52 So it is not surprising that the nanorods show such a surface reorganization phenomenon since the nanorods have the most intrinsic defect sites and thus the oxygen mobility is high. Thus, it appears both the intrinsic defect sites and the reduction-induced defects play a significant role in the redox property of ceria. The reduction step is promoted by the intrinsic defect sites (promoting oxygen mobility), while the reoxidation step is facilitated by the reduction-generated defect sites (promoting O-vacancies mobility). Furthermore, defect sites on different ceria surface planes show different reactivity and mobility and thus affect the overall redox property of ceria. 4.3. Stability and Reactivity of Surface Oxygen Species. The thermal stability of adsorbed oxygen is surface dependent as evident from the O2-TPD results. The temperature programmed Raman spectra collected in He (Figure 7A) show that the peroxide species are more stable on nanorods than on nanocubes (Supporting Information Figure S4A), in good agreement with the TPD-QMS profiles (Figure 8A) where most oxygen desorption occurs at lower temperature on nanocubes than on nanorods. According to the Raman spectra, the main O2 desorption peak at 343 K on nanocubes can be due to the disproportionation reaction of peroxide species on isolated defect sites while the two desorption peaks at 393 and 468 K for nanorods are due to the disproportionation reaction of peroxide species on clustered and isolated two-electron defect sites, respectively. The different thermal stability is likely related to the surface terminations of nanorods and nanocubes. Ce cations are more exposed on the {110} surface than on the oxygen terminated facets of the {100} surface, so the electrostatic interaction between adsorbed anionic dioxygen and cationic Ce is expected to be stronger on nanorods than on nanocubes. This is analogous to the DFT calculations53 which show that the binding energy of superoxide on the ceria {110} surface is much larger than that on the {111} surface (Ce cations are less exposed on the {111} surface). The reactivity of surface oxygen species to gas phase oxygen can be inferred from the Raman and TPD-QMS study using labeled oxygen. The superoxide species observed on nanorods can exchange completely with gas phase 18O2 at room temperature, similar to previous IR studies26,27 on polycrystalline ceria samples. The exchange between superoxide and gas phase 18O2 can occur via two different mechanisms as observed in the Raman study: site exchange (without O-O bond breaking) leading to adsorbed 18O2- species and gas phase 16O2 as in reaction 3 (Figure 7B); isotopic exchange (with O-O bond breaking and making) leading to adsorbed 16O18O- and gas phase 16O18O as in reactions 5 and 6 (Supporting Information Figure S6). Similarly, the peroxide species on both nanorods and nanocubes can (52) Binet, C.; Badri, A.; Lavalley, J. C. A Spectroscopic characterization of the reduction of ceria from electronic-transitions of intrinsic point-defects. J. Phys. Chem. 1994, 98 (25), 6392-6398. (53) Huang, M.; Fabris, S. Role of surface peroxo and superoxo species in the low-temperature oxygen buffering of ceria: Density functional theory calculations. Phys. Rev. B 2007, 75 (8), 081404-1-081404-4.
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undergo exchange with gas phase 18O2 at room temperature but at a much slower pace than superoxide species do, likely due to the difficulty in transferring two electrons for peroxide species. It is reasonable that the exchange can be accelerated at elevated temperature as evident in the Raman results (Figure 7B). The TPD-QMS profiles of 16O2 desorption in He and 18O2/He are quite similar for both nanorods and nanocubes. However, about four times more 16O18O is produced on nanorods than on nanocubes. Considering that the major 16O18O desorption happens at low temperature (∼295 K) and that exchange between superoxide species and gas phase 18O2 is also facile, we can deduce that the presence of superoxide species on nanorods may play a significant role in the production of 16O18O at such low temperature as in reactions 5, 6, 9, and 10. Only at higher temperature do the peroxide species contribute to the production of 16O18O. So the surface oxygen speciation affects greatly the reactivity of ceria nanocrystals toward gas phase oxygen, which has great implications for ceria-catalyzed oxidation reactions at a temperature range where surface oxygen species participate in oxidation reaction. The reactivity of surface dioxygen species to a reductant is reflected in the Raman and TPD studies in CO/He atmosphere. The results on nanorods indicate that superoxide species is more reactive to CO than peroxide species, since the Raman spectra (Figure 7C) show that superoxide is consumed preferably even at room temperature. This is consistent with previous Raman observations.28 The high reactivity of superoxide species can be explained by its radical nature. Peroxide species participate in CO oxidation at higher temperatures. No evident difference is observed in the reactivity to CO either between different peroxide species (isolated vs clustered) or and between peroxide species adsorbed on nanorods or nanocubes (Supporting Information Figure S4C). It appears that different surface dioxygen species play a catalytic role in CO oxidation in different temperature regimes. Thus, due to the presence of different surface oxygen species, nanorods may display both low temperature and high temperature activity for CO oxidation while nanocubes may only show high temperature activity.
5. Conclusions Two types of defect sites on both oxidized and reduced ceria nanocrystals with different morphologies have been characterized by both Raman spectroscopy and O2 adsorption. It is shown that both their quantity and quality are surface dependent on ceria. The intrinsic defect sites on oxidized ceria, most likely the Frenkel-type oxygen defects, are the most abundant on nanorods and the least on nano-octahedra with nanocubes in the middle. The reduction-induced defect sites, O-vacancies and Ce3þ defects, are similar in surface density on nanorods and nanocubes, while they are negligible on nano-octahedra. These O-vacancy sites are more clustered on nanorods than on nanocubes, in line with their surface terminations. The difference in defect sites results in different surface adsorbed oxygen species on the nanocrystals. The stability and reactivity of these surface oxygen species are also dependent on the surface terminations. The results presented in this study imply that the catalytic properties of ceria could be controlled and tuned by controlling its shape, which points to a strategy for both the improvement of current heterogeneous catalysts and the design of highly efficient catalysts without the change of the catalyst composition. Acknowledgment. Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic DOI: 10.1021/la101723w
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Energy Sciences, U.S. Department of Energy. Raman and part of the TEM measurements were conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory, by the Division of Scientific User Facilities, U. S. Department of Energy. XPS and HRTEM measurements were conducted using the SHaRE facilities, which is sponsored at Oak Ridge National Laboratory, by the Division of Scientific User Facilities, U. S. Department of Energy. The research was
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supported in part by the appointment for M.J.L. to the ORNL Postdoctoral Research Associates Program, administered jointly by ORNL and the Oak Ridge Associated Universities. Supporting Information Available: Additional TEM images (Figure S1) and Raman spectra (Figures S2-S6). This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(21), 16595–16606
