A facile solvothermal method to synthesize aluminum-doped ceria-zirconia (Ce0.5Zr0.5-xAlxO2-x/2, x = 0.1 to 0.4) solid solutions was carried out using Ce(NH4)2(NO3)6, Zr(NO3)3·2H2O Al(NO3)3·9H2O, and NH4OH as the starting materials at 200°C for 24 h. The obtained solid solutions from the solvothermal reaction were calcined at 1,000°C for 20 h in air atmosphere to evaluate the thermal stability. The synthesized Ce0.5Zr0.3Al0.2O1.9 particle was characterized for the oxygen storage capacity (OSC) in automotive catalysis. For the characterization, X-ray diffraction, transmission electron microscopy, and the Brunauer-Emmet-Teller (BET) technique were employed. The OSC values of all samples were measured at 600°C using thermogravimetric-differential thermal analysis. Ce0.5Zr0.3Al0.2O1.9 solid solutions calcined at 1,000°C for 20 h with a BET surface area of 18 m2 g−1 exhibited a considerably high OSC of 427 μmol-O g−1 and good OSC performance stability. The same synthesis route was employed for the preparation of the CeO2 and Ce0.5Zr0.5O2. The incorporation of aluminum ion in the lattice of ceria-based catalyst greatly enhanced the thermal stability and OSC.
Ceria (CeO2)-based materials have attracted considerable interest for more than half a century due to their far-ranging applications in catalysts, fuel cells, cosmetics, gas sensors, and solid-state electrolytes and especially their crucial application as promoters of three-way catalysts (TWCs), which are commonly used to reduce the emissions of CO, NOx, and hydrocarbons from automobile exhausts, because of their excellent oxygen storage capacity (OSC) [1–8]. Since 1990s, CeO2-ZrO2 solid solutions have gradually replaced pure CeO2 as OSC materials in the TWCs to reduce the emission of toxic pollutants (CO, NOx, hydrocarbons, etc.) from automobile exhaust and because of their enhanced OSC performance and improved thermal stability at elevated temperatures [9–13].
The redox property of CeO2 can be greatly enhanced by the incorporation of zirconium ions (Zr4+) into the lattice to form a solid solution [14–16]. Nagai et al. have suggested that enhancing the homogeneity of Ce and Zr atoms in the CeO2-ZrO2 solid solution can improve the OSC performance . The detailed structure and property of CeO2-ZrO2 solid solutions were reported in a review article by Monte and Kaspar . This review included the results of reducing performance for a series of samples with gradually elevated Ce contents, and a possible mechanism of structural changes in the reducing process was proposed. Fornasiero et al. have reported that an optimum composition like Ce0.5Zr0.5O2 (molar ratio of Ce:Zr = 1:1) can exist as a cubic phase, which can have a considerably high redox property . Using density functional theory, Wang et al. found that in a series of Ce1-xZrxO2 solutions with a content of 50%, ZrO2 possesses the lowest formation energy of the O vacancy; therefore, Ce0.5Zr0.5O2 exhibits the best OSC performance . Recently, many researchers have paid much attention to prepare the Ce0.5Zr0.5O2 solutions with the homogeneity of the composition, good dispersion of particles, narrow particle size distribution, better crystallinity, and high surface area in order to improve OSC and redox property due to their catalytic applications [20–25].
Although Ce0.5Zr0.5O2 solid solutions have been studied extensively, there are few reports on the preparation of Ce0.5Zr0.5-xMxO2-x/2 in the literature [26, 27]. Considering the smaller cation radius of Al3+ (0.059 nm) compared to those of Zr4+ (0.084 nm) and Ce4+ (0.097 nm), the incorporation of Al3+ into Ce-Zr solid solutions may enhance the oxygen release reaction to form larger Ce3+. In the present work, for the first time, we describe the preparation and characterization of Ce0.5Zr0.3Al0.2O1.9 solid solutions with high surface area via a facile solvothermal route. The further experiment results show that the introduction of aluminum ion enhances the thermal stability and OSC even after calcination at a very strict condition of 1,000°C for 20 h. The OSC of CeO2, Ce0.5Zr0.5O2, and the composites which consisted of different aluminum amounts were also prepared via the same method and compared.
All chemicals used were of analytical grade and were purchased from Kanto Chemical Co. Inc., Tokyo, Japan (purity 99.999%). The chemicals were used without further purification.
The stoichiometric amounts of (NH4)2Ce(NO3)6 (6 mmol), ZrO(NO3)2 (3.6 mmol), and Al(NO3)3·9H2O (2.4 mmol) were dissolved in 60 ml of distilled water. NH4OH solution was slowly dropped into the above mixed solution, and the pH value was maintained at 9. The yellow mixed solution was introduced in a 100-ml Teflon®-lined autoclave (SAN-AI Science, Co. Ltd, Nagoya, Japan), which was maintained at 200°C for 24 h, then cooled to room temperature naturally. The obtained products were washed with distilled water three times and dried in air at 100°C for 12 h to form the as-prepared fresh samples. Finally, the fresh samples were calcined at 1,000°C for 20 h in air atmosphere to evaluate the thermal stability. The same synthesis route was employed for the preparation of the CeO2 and Ce0.5Zr0.5O2.
The OSC of the samples calcined at 1,000°C for 20 h was determined by thermogravimetric-differential thermal analysis (TG-DTA; Rigaku TAS-200, Rigaku Corporation, Tokyo, Japan) at 600°C. Before the measurements, the samples were held in flowing air at 600°C for 30 min to remove residual water and other volatile gases. The mixed gas of CO-N2 (100 cm3 min−1) and air (100 cm3 min−1) was flowed alternately at 600°C. Finally, OSC was analyzed after getting the TGA profile.
The phase composition of the sample was determined by X-ray diffraction analysis (XRD; Bruker D2 Phaser, Bruker Optik GmbH, Ettlingen, Germany) using graphite-monochromized CuKα radiation. The morphology and size of the samples were determined by transmission electron microscopy (TEM; JEOL JEM-2010, JEOL Ltd., Akishima, Tokyo, Japan). The specific surface area was measured using a BET (NOVA 4200e, Quantachrome GmbH and Co. KG, Odelzhausen, Germany) surface area and pore size analyzer.
Results and discussion
All products of (a) CeO2, (b) Ce0.5Zr0.5O2, and (c) Ce0.5Zr0.3Al0.2O1.9 consisted of a single phase of fluorite structure (Figure 1 (a) to (c)). All the diffraction patterns exhibited broad peaks, suggesting that the fresh samples were nanocrystalline materials. The calcined samples had a slight shift in diffraction peaks when compared to the pure CeO2 XRD pattern, indicating the formation of corresponding solid solutions. The calculated lattice parameters of the calcined samples of Ce0.5Zr0.5O2 (a = 0.5384 nm) and Ce0.5Zr0.3Al0.2O1.9 (a = 0.5299 nm) are smaller than that of CeO2 (a = 0.5413 nm). The shrinkage of lattice cells may be due to the substitution of the smaller cation radius of Zr4+ (0.084 nm) and Al3+ (0.0059 nm) with Ce4+ (0.097 nm). No phase separation was noticed even at such high calcination temperatures at 1,000°C for 20 h, except the increase of particle size (Figure 1 (a') to (c')). The crystal sizes of the fresh CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 calculated by Scherer's formula were 9, 5, and 3 nm, while those of the calcined CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 were 35, 10, and 8 nm, respectively.
The morphology and size of the fresh and calcined samples (1,000°C for 20 h) were observed by TEM as shown in Figure 2. For the fresh samples, the particles seem to be partly dispersed and formed small agglomerates (Figure 2 (a) to (c)), and the single particle exhibited a spherical-like morphology with the diameters of 9 to 12 nm, 5 to 8 nm, and 3 to 5 nm for CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9, respectively, which are in agreement with the crystallite size calculated from Scherer's formula. The particle size increased after calcination at 1,000°C for 20 h because of aggregation, and the particle sizes were found to increase to 90 to 100 nm, 50 to 55 nm, and 30 to 35 nm for the CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 samples as shown in Figure 2 (a') to (c'), respectively.
BET nitrogen adsorption-desorption analysis was undertaken to measure the specific surface area of all samples. As a result, the fresh sample of Ce0.5Zr0.3Al0.2O1.9 showed a much higher surface area (232 m2 g−1) than those of CeO2 (119 m2 g−1) and Ce0.5Zr0.5O2 (168 m2 g−1, Figure 3 (a) to (c)). After calcinations at 1,000°C for 20 h in air, the specific surface areas of CeO2 (3 m2 g−1) and Ce0.5Zr0.5O2 (8 m2 g−1) decreased to less than 10 m2 g−1, but the sample of Ce0.5Zr0.3Al0.2O1.9 exhibited a relatively higher BET specific surface area of 18 m2 g−1 (Figure 3 (a') to (c')).
The OSC values of the calcined samples were determined at 600°C with a continuous flow of CO-N2 gas and air alternately. Figure 4 shows the typical TG profiles of the CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 samples. The TG profile shows the oxygen release/storage performance of the CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 samples at 600°C with time. As a result, Ce0.5Zr0.3Al0.2O1.9 exhibited a higher OSC of 427 μmol-O g−1, when compared to those of the CeO2 (25 μmol-O g−1) and Ce0.5Zr0.5O2 (350 μmol-O g−1) samples (Table 1). It is accepted that the OSC is dependent on the specific surface area; it is obvious that Ce0.5Zr0.3Al0.2O1.9 exhibited the highest specific surface area and highest OSC values even after calcination at such high temperature as 1,000°C for 20 h. In order to examine OSC performance stability, oxygen release/storage cycle measurement was tested, and Ce0.5Zr0.3Al0.2O1.9 retained the same OSC even after 22 cycles (Figure 5). The result indicates that Ce0.5Zr0.3Al0.2O1.9 has good OSC performance stability.
OSC at 600 ° C of the CeO2, Ce0 .5Zr0 .5O2, and Ce0 .5Zr0 .3Al0 .2O1 .9calcined at 1 ,000 ° C for 20 h
OSC (μmol-O g− 1)
aIt is accepted that the Ce0.5Zr0.5O2 composition possessed excellent OSC property [10–14].
The amount of incorporated aluminum was also controlled to test its effect on the OSC of the calcined sample as shown in Figure 6 and Table 2. As a result, Ce0.5Zr0.3Al0.2O1.9 exhibited the highest OSC of 427 μmol-O g−1 (Table 1), when compared to those of the Ce0.5Zr0.4Al0.1O1.95 (378 μmol-O g−1), Ce0.5Zr0.2Al0.3O1.85 (389 μmol-O g−1), and Ce0.5Zr0.1Al0.4O1.8 (261 μmol-O g−1) samples (Table 2), Therefore, in Ce0.5Zr0.5-xAlxOy (0.1 <x < 0.5, x is the amount of incorporated aluminum), the most appropriate amount of incorporated aluminum might be around x = 0.2.
OSC at 600 ° C of the Ce0 .5Zr0 .4Al0 .1O1 .95, Ce0 .5Zr0 .2Al0 .3O1 .85, and Ce0 .5Zr0 .1Al0 .4O1 .8calcined at 1 , 000 ° C for 20 h
OSC (μmol-O g− 1)
Ce0.5Zr0.3Al0.2O1.9 solid solutions with high surface area were successfully synthesized via a facile solvothermal method. The structures of the fresh samples and calcined samples were characterized by X-ray diffraction. The lattice parameters of the Ce0.5Zr0.3Al0.2O1.9 solid solution are smaller than those of CeO2 and Ce0.5Zr0.5O2, suggesting the incorporation of the Al3+ into Ce-Zr solid solutions. The fresh particles showed spherical-like morphology with a diameter of 3 to 5 nm determined by TEM. The Ce0.5Zr0.3Al0.2O1.9 solid solutions exhibited a remarkably higher oxygen storage capacity than those of the CeO2 and Ce0.5Zr0.5O2 samples prepared via the same method, even after calcination at 1,000°C for 20 h, indicating the improvement of the OSC and thermal stability due to the incorporation of aluminum. An appropriate amount of incorporated aluminum is also suggested.
QD, SY, CG, and TS are an assistant professor, an associate professor, a Ph.D. candidate, and a full professor, respectively, at the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University.
This work was supported by the Rare Metal Substitute Materials Development Project of New Energy and Industrial Technology Development Organization (NEDO), Japan and the Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports and Science for Technology of Japan (MEXT).
Center for Exploration of New Inorganic Materials (CENIM), Institute of Multidisciplinary Research for Advanced Materials, Tohoku University
Yao HC, Yu YF: Ceria in automotive exhaustcatalysts: I. Oxygen storage. J Catal 1984, 86: 254. 10.1016/0021-9517(84)90371-3View Article
Di Monte R, Kasper J, Bradshaw H, Norman C: A rationale for the development of thermally stable nanostructured CeO2-ZrO2-containing mixed oxides. J Rare Earth 2008, 26: 136. 10.1016/S1002-0721(08)60053-8View Article
Yin S, Minamidate Y, Sato T: Synthesis and morphological control of monodispersed microsized ceria particles. Surf Rev Lett 2010, 17(2):147. 10.1142/S0218625X10013552View Article
Yin S, Minamidate Y, Sato T: Synthesis of monodispersed plate-like CeO2 particles by precipitation process in sodium hydrogen carbonate solution. Adv Sci Technol 2010, 63: 30.View Article
Yin S, Minamidate Y, Tonouchi S, Goto T, Dong Q, Yamane H, Sato T: Solution synthesis of homogeneous plate-like multifunctional CeO2 particles. RSC Adv 2012, 2: 5976. 10.1039/c2ra20280hView Article
Devaraju MK, Yin S, Sato T: Morphology control of cerium oxide particles synthesized via a supercritical solvothermal method. Appl Mater Interfaces 2009, 1(11):2694. 10.1021/am900574mView Article
Kašpar J, Fornasiero P, Graziani M: Use of CeO2-based oxides in the three-way catalysis. Catal Today 1999, 50: 285. 10.1016/S0920-5861(98)00510-0View Article
Kašpar J, Fornasiero P: Nanostructured materials for advanced automotive de-pollution catalysts. J Solid State Chem 2003, 171: 19. 10.1016/S0022-4596(02)00141-XView Article
Di Monte R, Kašpar J: Heterogeneous environmental catalysis-a gentle art: CeO2-ZrO2 mixed oxides as a case history. Catal Today 2005, 100: 27. 10.1016/j.cattod.2004.11.005View Article
Di Monte R, Kašpar J: Nanostructured CeO2-ZrO2mixed oxides. J Mater Chem 2005, 15: 633. 10.1039/b414244fView Article
Fornasiero P, Balducci G, Di Monte R, Kašpar J, Sergo V, Gubitosa G, Ferrero A, Graziani M: Modification of the redox behaviour of CeO2induced by structural doping with ZrO2. J Catal 1996, 164: 173. 10.1006/jcat.1996.0373View Article
Yao MH, Baird RJ, Kunz FW, Hoost TE: An XRD and TEM investigation of the structure of alumina-supported ceria-zirconia. J Catal 1997, 166: 67. 10.1006/jcat.1997.1504View Article
Kenevey K, Valdivieso F, Soustelle M, Pijolat M: Thermal stability of Pd or Pt loaded Ce0.68Zr0.32O2and Ce0.50Zr0.50 O2catalyst materials under oxidizing conditions. Appl Catal B: Environ 2001, 29: 93. 10.1016/S0926-3373(00)00196-XView Article
Zhang F, Chen CH, Hanson JC, Robinson RD, Herman IP, Chan SW: Phases in ceria-zirconia binary oxide (1-x)CeO2-xZrO2 nanoparticles: the effect of particle size. J Am Ceram Soc 2006, 89: 1028. 10.1111/j.1551-2916.2005.00788.xView Article
Nagai T, Nonaka T, Suda A, Sugiura M: Structure analysis of CeO2-ZrO2mixed oxides as oxygen storage promoters in automotive catalysts. R&D Rev Toyota CRDL 2002, 37: 20.
Fornasiero P, Di Monte R, Rao GR, Kašpar J, Meriani S, Trovarelli A, Graziani M: Rh-loaded CeO2-ZrO2solid solutions as highly efficient oxygen exchangers: dependence of the reduction behavior and the oxygen storage capacity on the structural properties. J Catal 1995, 151: 168. 10.1006/jcat.1995.1019View Article
Wang HF, Gong XQ, Guo YL, Guo Y, Lu GZ, Hu P: A model to understand the oxygen vacancy formation in Zr-doped CeO2: electrostatic interaction and structural relaxation. J Phys Chem C 2009, 113: 10229. 10.1021/jp900942aView Article
Devaraju MK, Liu XW, Yusuke K, Yin S, Sato T: A rapid hydrothermal synthesis of rare earth oxide activated Y(OH)3and Y2O3nanotubes. Nanotechnology 2009, 20: 405606. 10.1088/0957-4484/20/40/405606View Article
Sanchez-Domingueza M, Liotta LF, Carlod GD, Pantaleob G, Veneziab AM, Solansa C, Boutonnet M: Synthesis of CeO2, ZrO2, Ce0.5Zr0.5O2, and TiO2nanoparticles by a novel oil-in-water microemulsion reaction method and their use as catalyst support for CO oxidation. Catal Today 2010, 158: 35. 10.1016/j.cattod.2010.05.026View Article
Fuentes RO, Baker RT: Synthesis of nanocrystalline CeO2-ZrO2solid solutions by a citrate complexation route: a thermochemical and structural study. J Phys Chem C 2009, 113: 914. 10.1021/jp808825cView Article
Yang JO, Yang HM: Investigation of the oxygen exchange property and oxygen storage capacity of CexZr1-xO2nanocrystals. J Phys Chem C 2009, 113: 6921. 10.1021/jp808075tView Article
Teng ML, Luo LT, Yang XM: Synthesis of mesoporous Ce1-xZrxO2(x = 0.2–0.5) and catalytic properties of CuO based catalysts. Micropor Mesopor Mat 2009, 119: 158. 10.1016/j.micromeso.2008.10.019View Article
Dong Q, Yin S, Guo CS, Sato T: A new oxygen storage capacity material of tin doped ceria-zirconia supported paradium-alumina catalyst with high CO oxidation activity. Chem Lett in press in press
Dong Q, Yin S, Guo CS, Sato T: Ce0.5Zr0.4Sn0.1O2/Al2O3catalysts with enhanced oxygen storage capacity and high CO oxidation activity. Catal Sci Technol 2012. 10.1039/C2CY20425H
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