Aluminum-doped ceria-zirconia solid solutions with enhanced thermal stability and high oxygen storage capacity
© Dong et al.; licensee Springer. 2012
Received: 23 July 2012
Accepted: 17 September 2012
Published: 1 October 2012
A facile solvothermal method to synthesize aluminum-doped ceria-zirconia (Ce0.5Zr0.5-xAl x O2-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, NO x , 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, NO x , 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-xZr x O2 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-xM x O2-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
OSC at 600 ° C of the CeO 2 , Ce0 .5Zr0 .5O 2 , and Ce0 .5Zr0 .3Al0 .2O1 .9calcined at 1 ,000 ° C for 20 h
OSC (μmol-O g− 1)
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).
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