ARTICLE pubs.acs.org/JPCC
Facile Low-Temperature Synthesis of Ceria and Samarium-Doped Ceria Nanoparticles and Catalytic Allylic Oxidation of Cyclohexene Narottam Sutradhar,† Apurba Sinhamahapatra,† Sandip Pahari,† Muthirulandi Jayachandran,‡ Balasubramanian Subramanian,‡ Hari C. Bajaj,† and Asit Baran Panda*,† †
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (Council of Scientific and Industrial Research), G.B. Marg, Bhavnagar-364021, Gujarat, India ‡ Electrochemical Materials Science Division, Central Electrochemical Research Institute (CSIR), Karaikudi, India ABSTRACT: CeO2 as one of the most promising oxidation catalysts has attracted much attention because of its superior performance. The extent of oxidation properties is controlled by the ratio of Ce3þ/Ce4þ, particle size, and surface area. Here, a facile low-temperature aqueous solution-based chemical route for the synthesis of CeO2 and samarium-doped CeO2 (SmCeO2) nanoparticle aggregates, with high content of Ce3þ and surface area, using aqueous solution of ammonium carbonate complex of cerium is presented. The morphologies and structures of the prepared CeO2 nanoparticle were characterized by X-ray diffraction (XRD), thermal analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and hydrogentemperature programmed desorption. The synthesized CeO2 and SmCeO2 nanoparticle aggregate showed significantly improved catalytic activity toward allylic oxidation to corresponding en-ones compared to bulk CeO2. The present method provides a new and facile strategy toward the synthesis of CeO2 and SmCeO2 nanoparticle aggregates with extensive applications.
’ INTRODUCTION Ceria (CeO2) has attracted significant interest in the present frontier research for their wide application as oxygen ion conductor in solid oxide fuel cells (SOFCs),1 oxygen pumps, and amperometric oxygen monitors;2 gas sensor,3 solar cell,4 polishing materials,5 phosphors,6 oxidation catalyst in automotive three-way catalysts (TWCs),7 and fine chemical synthesis.811 It is also used as an efficient UV absorber/blocker, shielding material, and in sunscreen cosmetics due to its strong light absorption in the UV range.12,13 Even still, its inherent chemical properties, high oxygen storage capacity, and high oxygen mobility originating from facile Ce3þ/Ce4þ redox cycle dominate its use, its functional performance is influenced by its size, shape, that is, morphology and structure, and surface area.14 The particle size of the ceria also plays an important role toward its catalytic activity and other properties. The ceria with average particle size in the range of 310 nm are typically used as catalyst and as precursor for SOFC applications15,16 as it shows significant enhancement in cell parameters due to the presence of oxygen vacancies (due to the presence of Ce(III))17 and a typical quantum confinement effect.18 Several chemical protocols such as solgel,19,20 micelle,12,18 hydrothermal,21,22 surfactantassisted synthesis,14,23 use of carbonate intermediate,2426 sonochemical and microwave,27 and supercritical solvent8,9 have been employed to prepare ceria nanoparticles. However, precise and uniform control of the particle size still remains a major challenge for the synthesis of ceria nanoparticles in aqueous solution. Here, r 2011 American Chemical Society
it should be mentioned that most aqueous solution-basedmethods result in intermediate products like Ce(OH)4 or Ce(OH)CO3 yielding nanocrystalline ceria after subsequent drying or calcination with increased particle size.2426,28 For the synthesis of very small particles ( 0.2 suggests a certain contribution from both type I and type IV isotherms; whereas, for the CeO2 synthesized at higher temperature, the isotherms are apparently of type IV, typical of mesoporous materials. The isotherm of these samples showed two adsorption steps at P/P0 of about 0.3 and 0.8, respectively. The pore size distribution calculated using BJH model of CeO2 synthesized at reflux conditions (insert, Figure 3) depicted one narrow peak centered at 4.2 nm and another broad peak in the range of 1220 nm, showing a dual pore distribution. The samples synthesized in hydrothermal conditions showed peak centered in the range of 34 nm. The low surface area of (42 mm2/g) CeO2 synthesized at room temperature most probably due to the presence of amorphous cerium carbonate/hydroxide, as observed in TGA. Surface area of the CeO2 nanoparticles synthesized in reflux conditions is reasonably high (368 m2/g), which decreased gradually with the increase of the synthesis as well as calcination temperature. The decrease in the surface area is attributed to the crystal growth (larger crystallite) at the higher temperature.
Morphology. Electron microscopy was utilized to study the microstructure of as-synthesized as well as calcined CeO2 samples synthesized under different conditions. The samples synthesized at atmospheric pressure, that is, CeRT and Ce100, showed irregular shapes similar to teased cotton (Figure 4). The samples synthesized in hydrothermal conditions gave spherical morphology with a smooth surface. The synthesized spheres are monodispersed and their size increased with the increase in the hydrothermal temperature. The shape and size of the particles are almost unchanged after calcination. TEM analysis clearly revealed the microstructures of CeO2 nanoparticles (Figure 5). The shapes of the CeO2 nanoparticles (as shown in SEM) constitute of very small porous particles and confirm the 3D hierarchical structures of small particles. The particle sizes calculated from TEM analysis are depicted in Table 1. Distinct lattice planes in HRTEM images of all the CeO2 nanoparticles synthesized in varying conditions demonstrate their high degree of crystallinity, and the identified {111} and {200} facets with interplanar distance of 0.32 and 0.27 nm respectively correspond to the fluorite cubic structure of CeO2 and supports the XRD results. The formation of amorphous ceria can easily be seen in the HRTEM image of CeRT (CeO2 particles synthesized at room temperature). XPS Analysis. The wide survey XPS spectra of the CeO2 samples reveal the prominent presence of cerium and oxygen (part a of Figure 6) and the valence state of cerium in the synthesized CeO2. Part b of Figure 6 illustrates the 3d core level 7632
dx.doi.org/10.1021/jp200645q |J. Phys. Chem. C 2011, 115, 7628–7637
The Journal of Physical Chemistry C
ARTICLE
Figure 5. TEM and corresponding HR-TEM images of CeO2 nanoparticles at room temperature (a, b), reflux conditions (c, d), and hydrothermal conditions at 130 °C (e, f).
spectra of CeO2 in the region of 880925 eV. The deconvoluted peaks (using Gaussian fit) consist of two series of distinguishable peaks of the Ce4þ and Ce3þ ionic states (3d5/2 and 3d3/2). Indexing of the deconvoluted spectra revealed the presence of mixed valence cerium in all the samples, even in the calcined samples. The peaks labeled v and v00 are assigned to a mixing of Ce 3d9 4f2 Ln-2 and Ce 3d9 4f1 Ln-1 Ce(IV) final state, and v000 assigned to Ce 3d9 4f0 Ln Ce(IV) final state. More over v0 and v0 are assigned to the Ce 3d9 4f2 Ln-1 and Ce 3d9 4f1 Ln states respectively, the states of Ce(III). Similarly the u, u00 , and u000 are the states of Ce4þ and u0, u0 are the states of Ce3þ. The ratio of valence states of ceria present in the synthesized CeO2 was semiquantitatively analyzed using the integrated peak area of the
respective valence state using the following equation:40 ½Ce3þ % ¼
Avo þ Av0 þ Auo þ Au0 100% Av þ Av00 þ Av000 þ Au þ Au00 þ Au000
Ai is the area of the corresponding peaks. The calculated concentration of Ce3þ of as-synthesized CeO2 synthesized at reflux conditions (Ce100) was ∼47% and its concentration gradually decreases with the increase in the formation temperature. It was ∼27% for the sample synthesized at 190 °C in hydrothermal conditions (Ce190). Again, on increasing the calcination temperature the extent of Ce3þ 7633
dx.doi.org/10.1021/jp200645q |J. Phys. Chem. C 2011, 115, 7628–7637
The Journal of Physical Chemistry C
ARTICLE
Figure 7. H2-TPR curves of some selected synthesized ceria samples.
Figure 6. (a) Survey, (b) Ce3d, and (c) O1s high resolution XPS spectra of the synthesized CeO2 nanoparticles.
decreased gradually, such as it is ∼31% and ∼19% in Ce100300 and Ce100500, respectively. The XPS data demonstrate the presence of mixed valence cerium in the synthesized CeO2 nanoparticles, moreover extent of ratio of Ce3þ/ Ce4þ can be tuned by varying the synthetic conditions. The amount of Ce3þ in as-synthesized samarium-doped ceria; and ceria without samarium is almost the same. However, the percentage of Ce3þ in the calcined samarium-doped ceria (SmCe) is slightly higher than that of the calcined pure ceria, such as the content of Ce3þ in Ce100 and CeSm100 are ∼47% and ∼46% respectively, whereas in Ce100300 and CeSm100300 they are ∼31% and ∼38%, respectively. This may be due to the presence of trivalent samarium.41 Part c of Figure 6 represents the XPS O1s spectra of the synthesized ceria samples. All spectra are composed of two components. The main peak can be assigned to O2- ions and the second low intense peak toward higher binding energy can be assigned to the OH ions of surface hydroxyl group, whose intensity gradually decreased with an increase in the calcination temperature. With increasing calcination temperature the surface hydroxyl content was reduced and as a consequence the intensity of the peak of surface hydroxyl group also reduced. Incorporation of samarium ion significantly influenced the surrounding chemical environment of oxygen; as a result the peak position was shifted.42 H2-TPR Analysis. The redox ability or oxygen storage/release capability of CeO2 can be accomplished by monitoring the interaction of ceria with H2 at various temperatures by TPD. The reduction profile of bulk CeO2 consists of two steps: the low-temperature step at ∼497 °C and the high temperature step at ∼827 °C.43 The low-temperature step is the surface shell reduction, includes the reduction of the surface Ce from Ce4þ to Ce3þ and the formation of bridging OH groups; and the high temperature step is due to bulk reduction. The difference in these two successive reduction temperatures is due to the different binding energy of oxygen. However, in the synthesized samples the reduction temperature for both surface and bulk decreased noticeably (Figure 7). The reduction temperature of Ce100 and
Figure 8. FTIR spectra of the aqueous solution of (a) cerium ammonium carbonate, (b) ammonium carbonate, and (c) the precipitate.
Ce100300 are 388 and 354 °C, respectively; and 506 and 528 °C for surface and bulk reduction, respectively. On incorporation of the samarium in ceria, there was a decrease in the surface and bulk reduction temperature but amount of consumed H2, which is ∼1.5 times greater than that of pure CeO2 synthesized under identical conditions. The significant decrease in the reduction temperature of both surface and bulk CeO2 in the synthesized samples with respect to bulk ceria may be due to the reduced particle size, specific morphology with exposed active crystal planes and high surface area.24,42,44 Terribile et al.44 stated that the reduction behavior is not directly dependent on surface area; and the low-temperature reduction can be promoted by careful design of synthetic procedure. The gradual increase in the bulk reduction temperature with the increase in the calcination temperature can be attributed to the gradual bulk sintering and crystal growth, thereby decrease in the diffusion efficiency of bulk oxygen. Furthermore, the incorporation of trivalent samarium facilitates the bulk oxygen diffusion to surface, which in turn effect the reduction temperature. Here it should be mentioned that in all the samples the intake of H2 at (>650 °C) due to the loss of lattice oxygen. 7634
dx.doi.org/10.1021/jp200645q |J. Phys. Chem. C 2011, 115, 7628–7637
The Journal of Physical Chemistry C Ceria Nanoparticles Formation Mechanism. The precursor cerium ammonium carbonate solution is analogous to the zirconium ammonium carbonate45 as cerium behaves like Gr IVB elements.46 However, the formed cerium ammonium carbonate is metastable and able to produce CeO2 nanoparticles by hydrolysis, followed by dehydration at low temperature under basic conditions. The formation of such complex was confirmed by FTIR spectra of clear solutions of ammonium carbonate, cerium ammonium carbonate complex, and the precipitates (Figure 8). A significant shifting of carbonate peak toward higher wavenumber and NH peak toward lower wavenumber in cerium ammonium carbonate solutions compared to pure ammonium carbonate solutions clearly indicated the complexation. Li et al.47 reported that the solubility of cerium ion increased with an increase in the concentration of ammonium carbonate through complexation with NH4þ and CO32- ions. The presence of the characteristic XRD diffraction peaks of only CeO2 in the precipitate and absence of any intermediate carbonate phase2426,28 indicate the direct formation of CeO2 from the precursor solution, further augmented by very low weight loss in the TGA/DTA compared to carbonate.47,48 From the above discussion, probable reaction steps can be expressed as follows,
Ammonium carbonate plays a very crucial role during the synthesis of nanosized CeO2. During the formation of CeO2, large amounts of CO2 and NH3 gases are evolved by the decomposition of cerium ammonium carbonate or unreacted ammonium carbonate, which in turn help to disintegrate the agglomerated particles; and inhibit the sintering of nanosized particles. The crystallite size of the particles can be varied from 315 nm and concentration of Ce3þ can also be varied from 47 to 19% by varying the synthetic conditions. In a controlled experiment, the resultant precipitate at room temperature from 50 mL (0.18 molar) cerium ammonium carbonate solution, after consequent drying at 90 °C for 12 h, gave 3 nm particles. Refluxing 50 mL (0.18 molar) cerium ammonium carbonate solutions for 6 h resulted in 4 nm particles. Again hydrothermal treatment of 33 mL (0.18 molar) solution in 50 mL Teflon lined autoclave at 130, 170, and 190 °C resulted 6, 8, and 15 nm particles, respectively. The particle size increased with the increase in the reaction temperature due to the crystal growth in higher temperature. Moreover when the as-synthesized particles of CeO2 obtained from reflux conditions, and calcined at 300 and 500 °C/6 h in air resulted in 47, 28 and 19% of Ce3þ. On calcination or with as increase of the synthetic temperature, the content of Ce3þ decreased due to gradual oxidation of Ce3þ to Ce4þ. From the above discussion, it is clearly evident that, by varying the synthesis protocol, the particle size and ratio of Ce3þ/ Ce4þ can be controlled. Catalytic Application for Allylic or Benzylic Oxidation to Corresponding En-one. Ceria and doped (rare earth metal or
ARTICLE
Table 2. Allylic Oxidation of Cyclohexene over Different Synthesized CeO2 Catalysta sample
conversion (%)
selectivity (%)
Ce100 Ce100300
76 83
cyclohexenone 67 66
cyclohexenol 33 34
Ce100500
67
60
40
Ce150
72
69
31
Ce150300
56
61
39
Ce150500
73
66
34
CeSm100
77
62
38
CeSm100300
70
60
40
CeSm100500 CeSm150
71 77
55 65
45 35
CeSm150300
83
76
34
CeSm150500
81
73
27
a
Reaction conditions: cyclohexene, 1 mL; 30% (w/v) H2O2, 5 mL; catalyst, 95 mg; time, 24 h.
transition metal) ceria is known as a catalyst or as a catalyst support for a wide range of reaction, especially in oxidation and water gas shift reactions 810,4950. There are many reports document the ceria, doped ceria, and ceria-supported noble metal for the CO and hazardous organic compound oxidation14,23,24,4849,51. However, use of ceria as a catalyst or catalyst support for fine chemical synthesis is scanty. Recently, Miedziak et al.9 have reported the oxidation of alcohols to corresponding ketones using ceria-supported goldpalladium as a catalyst; Juarez et al.10 reported the trans-alkylation of propylene carbonate over gold-supported ceria; Sun et al.8 reported the hydrogenation of nitroarenes using platinumsupported ceria nanowires. Radhika et al.11 described the benzylic oxidation of ethyl benzene to acetophenone over vanadia-supported ceria. Here, we have studied the selective oxidation of allylic or benzylic compounds to their corresponding en-one as model reactions to evaluate the catalytic activity of the synthesized Ce100, Ce150, SmCe100, SmCe150 as-synthesized and calcined at 300 and 500 °C. The selective oxidation of allylic or benzylic compounds to their corresponding en-one has tremendous industrial importance in pharmaceutical, agricultural and natural products, resin, steroids, and fine chemicals synthesis 5253. Catalytic performance was evaluated for the oxidation of cyclohexene using H2O2 as oxident in solvent-free conditions. In the first set of experiments, the allylic oxidation of cyclohexene was performed over all of the selected catalysts under refluxed conditions (24 h) by taking 1 mL cyclohexene, 5 mL H2O2, 95 mg catalyst, and 0.1 mL tetradecane as an internal standard (Table 2). During the reaction, mainly two products (cyclohexanone and cyclohexanol) were observed. The Ce100300 and SmCe150300 showed the best catalytic activity with respect to conversion (of cyclohexene) and selectivity for cyclohexanone (Table 2). The conversion increased with as the amount of catalyst increased from 5 wt % to 10 wt % with respect to the cyclohexene keeping the selectivity same for cyclohexanone. There was no change in the conversion with further increase in the catalyst amount (above 10 wt %). To understand the reaction profile, the oxidation of cyclohexene was performed for different time durations using Ce100 as catalyst (Figure 9). The conversion was increased with time; however, 7635
dx.doi.org/10.1021/jp200645q |J. Phys. Chem. C 2011, 115, 7628–7637
The Journal of Physical Chemistry C
Figure 9. Reaction profile (conversion and selectivity of corresponding products) obtained for the allylic oxidation of cyclohexene over Ce100300 catalyst.
Table 3. Schematic Representations and Corresponding Results of the Allylic Oxidations Performed Using Ce100-300 Samplea
a Reaction conditions: substrate, 1 mL; 30% (w/v) H2O2, 5 mL; catalyst, 95 mg; time, 24 h.
the selectivity remained almost constant. From the results, it is evident that, for pure CeO2, surface area, crystalinity, and percentage of Ce3þ present in the sample have a crucial effect on conversion and selectivity. The surface area of as-synthesized Ce100 is higher than that of Ce100500 but the crystalinity of Ce100500 is higher than that of as-synthesized Ce100. The amount of Ce3þ decreased with calcination temperature and is maximum in Ce100. However, both the samples (Ce100 and Ce100500) showed less catalytic activity than that of Ce100300, most probably due to the presence of moderate amounts of Ce3þ, higher crystallinity, and surface area in Ce100300. Again, in samarium-doped samples (SmCe) the crystalinity as well as the amount of Ce3þ are the major controlling factors for catalytic activity. In the SmCe samples, the amount of Ce3þ decreased with calcination temperature, but the rate of reduction of Ce3þ is quite lower than that of pure ceria. As a result, CeSm150300 gave better activity due to its high crystallinity and presence of a moderate amount of Ce3þ. The reaction was also performed using conventional bulk CeO2 as catalyst and without catalyst for comparison in identical conditions. Only 6% conversion was observed when the reaction was run without catalyst, whereas bulk ceria gave 24% conversion
ARTICLE
with 47% selectivity of cyclohexanone, which clearly illustrate that the synthesized ceria-based catalysts are superior to that of conventional ceria. Because of its better catalytic activity for the oxidation of cyclohexene to cyclohexenone, the allylic oxidation reaction was extended toward ethyl benzene and tetrahydronapthalene using Ce100300 in optimized conditions (Table 3). The catalytic activity of the synthesized ceria for ethyl benzene oxidation is significantly higher than that of vanadiasupported ceria.11 Under identical reaction conditions, the conversion in the case of oxidation of tetrahydronapthalene was not so encouraging, and most probably it required further optimization of reaction conditions. The stability of the synthesized catalyst was examined by reusing the recovered catalyst after a simple regeneration step and it remain almost unaffected even after the third cycle.
’ CONCLUSIONS A facile aqueous solution-based chemical protocol for the synthesized CeO2 and samarium-doped CeO2 (SmCeO2) nanoparticles at low temperature, as well as at room temperature, is develop using clear aqueous solution of cerium salt and ammonium carbonate as a precursor solution. The crystalline CeO2 nanocrystals with particle sizes in the range of 315 nm were synthesized without using organic substrate; the particle sizes of synthesized CeO2 can be controlled by varying the reaction parameters. The synthesized CeO2 nanoparticles possess high contents of oxygen vacancies (high concentration Ce3þ) and high specific surface area, which enhances the catalyst activity of CeO2. The synthesized nanoparticles showed excellent catalytic activity toward conversion of allylic compounds to corresponding en-one (allylic oxidation) as compare to bulk CeO2. The catalytic activity is highly dependent on surface area, crystallinity, and extent of Ce3þ present in the catalyst. The simple approach, the use of ammonium carbonate solution, is hoped to extend to prepare the nanostructures of other metal oxides, which can be used in the fields of catalysis. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; fax: þ(91)278-2567760, ext.: 704; phone: þ(91)278-2567562.
’ ACKNOWLEDGMENT Authors acknowledge Department of Science and Technology (DST), India, (SR/SI/IC-11/2008) and network project of CSIR (NWP 0010) for financial support. Authors also acknowledge analytical discipline of CSMCRI for materials characterization. ’ REFERENCES (1) Park, S. D.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265. (2) Hirano, M.; Kota, E. J. Am. Ceram. Soc. 1996, 79, 777. (3) Izu, N.; Shin, W.; Murayama, N.; Kanzaki, S. Sens. Actuators, B 2002, 87, 95. (4) Corma, A.; Atienzar, P.; Garcia, H.; Chane-Ching, J. Y. Nat. Mater. 2004, 3, 394. (5) Feng, X.; Sayle, D. C.; Wang, Z. L.; Paras, M. S.; Santora, B.; Sutorik, A. C.; Sayle, T. X. T; Yang, Y.; Ding, Y.; Wang, X.; Her, Y. S. Science 2006, 312, 1504. 7636
dx.doi.org/10.1021/jp200645q |J. Phys. Chem. C 2011, 115, 7628–7637
The Journal of Physical Chemistry C (6) Xijuan, Y.; Pingbo, X.; Qingde, S. Phys. Chem. Chem. Phys. 2001, 3, 5266. (7) Deluga, G. A.; Salge, R. J.; Schmidt, L. D.; Verykios, X. E. Science 2004, 303, 993. (8) Sun, Z.; Zhang, H.; An, G.; Yang, G.; Liu, Z. J. Mater. Chem. 2010, 20, 1947. (9) Miedziak, P. J.; Tang, Z.; Davies, T. E.; Enache, D. I.; Bartley, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Taylor, S. H.; Hutchings, G. J. J. Mater. Chem. 2009, 19, 8619. (10) Juarez, R.; Corma, A.; Garcia, H. Green Chem. 2009, 11, 949. (11) Radhika, T.; Sugunan, S. Catal. Commun. 2007, 8, 150. (12) Masui, T.; Fujiwara, K.; Machida, K. I.; Adachi, G. Y. Chem. Mater. 1997, 9, 2197. (13) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256. (14) Ho, C.; Yu, C. J.; Kwong, T.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514. (15) Schermanz, K. Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; World Scientific: London, 2002; ch. 2. (16) Lu, C.; Worrell, W. L.; Wang, C.; Park, S.; Kim, H.; Vohs, J. M.; Gorte, R. J. Solid State Ionics 2002, 152, 393. (17) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Phys. Rev. Lett. 2000, 85, 3440. (18) Hernandez-Alonso, M. D.; Hungria, A. B.; Martinez-Arias, A.; Coronado, J. M.; Conesa, J. C.; Soria, J.; Fernandez-Garcia, M. Phys. Chem. Chem. Phys. 2004, 6, 3524. (19) Laberty-Robert, C.; Long, J. W.; Lucas, E. M.; Pettigrew, K. A.; Stroud, R. M.; Doescher, M. S.; Rolison, D. R. Chem. Mater. 2006, 18, 50. (20) Xiao, H.; Ai, Z.; Zhang, L. J. Phys. Chem. C 2009, 113, 16625. (21) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C. H. J. Phys. Chem B 2005, 109, 24380. (22) Hirano, M.; Inagaki, M. J. Mater. Chem. 2000, 10, 473. (23) Sun, C.; Chen, L. Eur. J. Inorg. Chem. 2009, 3883. (24) Li, H.; Lu, G.; Dai, Q.; Wang, Y.; Guo, Y.; Guo, Y. ACS Appl. Mater. Interface 2010, 2, 838. (25) Cho, M. Y.; Roh, K. C.; Park, S. M.; Choi, H. J.; Lee, J. W. Mater. Lett. 2010, 64, 323. (26) Sun, C.; Sun, J.; Xiao, G.; Zhang, H.; Qiu, X.; Li, H.; Chen, L. J. Phys. Chem. B 2006, 110, 13445. (27) Wang, H.; Zhu, J. J.; Zhu, J. M.; Liao, X. H.; Shu, X.; Ding, T.; Chen, H. Y. Phys. Chem. Chem. Phys. 2002, 4, 3794. (28) Ahniyaz, A.; Sakamoto, Y.; Bergstrom, L. Cryst. Growth Design 2008, 8, 1798. (29) Li, X.; Li, J. G.; Huo, D.; Xiu, Z.; Sun, X. J. Phys. Chem. C 2009, 113, 1806. (30) Kar, S.; Patel, C.; Santra, S. J. Phys. Chem. C 2009, 113, 4862. (31) Pan, C.; Zhang, D.; Shi, L.; Fang, J. Eur. J. Inorg. Chem. 2008, 2429. (32) Pan, C.; Zhang, D.; Shi, L. J. Solid State Chem. 2008, 181, 1298. (33) Zhang, D.; Huang, L.; Zhang, J.; Shi, L. J. Mater. Sci. 2008, 43, 5647. (34) Zhang, D.; Niu, F.; Li, H.; Shi, L.; Fang, J. J. Powder Technol. 2011, 207, 35. (35) Zhang, D.; Yan, T.; Pan, C.; Shi, L.; Zhang, J. Mater. Chem. Phys. 2009, 113, 527. (36) Klug, P.; Alexander, L. E. X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.; Wiley: New York, 1974. (37) Leonim, M.; Maggio, R. D; Polizzi, S.; Scardi, P. J. Am. Ceram. Soc. 2004, 87, 113. (38) Scardy, P. A New Whole Powder Pattern Approach. In X-ray Powder Diffraction Analysis of Real Structure of Materials, International Union of Cryatallography Series; Bunge, H. J., Fiala, J., Snyder, R. L., Eds.; Oxford University press: U.K., 1999; p 570. (39) Maensiri, S.; Masingboon, C.; Laokul, P.; Jareonboon, W.; Promarak, V.; Anderson, P. L.; Seraphin, S. Cryst. Growth Des. 2007, 7, 950.
ARTICLE
(40) Korsvik, C.; Patil, S.; Seal, S.; Self, W. T. Chem. Commun. 2007, 1056. (41) Hungria, A. B.; Arias, A. M.; Garcia, M. F.; Juez, A. I.; Ruiz, A. G.; Calvino, J. J.; Conesa, J. C.; Soria, J. Chem. Mater. 2003, 15, 4309. (42) Reddy, B. M.; Katta, L.; Thrimurthulu, G. Chem. Mater. 2010, 22, 467. (43) Trovarelli, A. Comments Inorg. Chem. 1999, 20, 263. (44) Terribile, D.; Trovarelli, A.; de Leitenburg, C.; Dolcetti, G.; Llorca, J. Chem. Mater. 1997, 9, 2676. (45) Sinhamahapatra, A.; Sutradhar, N.; Roy, B.; Tarafdar, A.; Bajaj, H C.; Panda, A. B. Appl. Catal. A: Gen. 2010, 385, 22. (46) Sidgwick, N. V. The Chemical Elements and Their Compounds; Vol. 1, Oxford University Press: Amen House, London, 1950. (47) Li, J. G.; Ikegami, T.; Wang, Y.; Mori, T. J. Am. Ceram. Soc. 2002, 85, 2376. (48) Zhai, Y.; Zhang, S.; Pang, H. Mater. Lett. 2007, 61, 1863. (49) Carrettin, S.; Concepcion, P.; Corma, A.; Nieto, J. M. L.; Puntes, V. F. Angew. Chem., Int. Ed. 2004, 43, 2538. (50) Fu, Q.; Saltsburg, H.; Stephanopoulos, M. F. Science 2003, 301, 935. (51) Hernandez, W. Y.; Centeno, M. A.; Sarria, F. R.; Odriozola, J. A. J. Phys. Chem. C 2009, 113, 5629. (52) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidation of Organic Compounds; Academic Press: New York, 1981; p 318. (53) Shing, T. K. M.; Yeung, Y. Y.; Su, P. L. Org. Lett. 2006, 8, 3149.
7637
dx.doi.org/10.1021/jp200645q |J. Phys. Chem. C 2011, 115, 7628–7637
