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 2 3 4 5 6 7 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 10 11 12 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 15 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 17 . The detailed structure and property of CeO₂-ZrO₂ solid solutions were reported in a review article by Monte and Kaspar 12 . 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 Ce_0.5Zr_0.5O₂ (molar ratio of Ce:Zr = 1:1) can exist as a cubic phase, which can have a considerably high redox property 18 . Using density functional theory, Wang et al. found that in a series of Ce_1-x Zr _x O₂ solutions with a content of 50%, ZrO₂ possesses the lowest formation energy of the O vacancy; therefore, Ce_0.5Zr_0.5O₂ exhibits the best OSC performance 19 . Recently, many researchers have paid much attention to prepare the Ce_0.5Zr_0.5O₂ 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 21 22 23 24 25 .

Although Ce_0.5Zr_0.5O₂ solid solutions have been studied extensively, there are few reports on the preparation of Ce_0.5Zr_0.5-x M _x O_2-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 Ce_0.5Zr_0.3Al_0.2O_1.9 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, Ce_0.5Zr_0.5O₂, 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.

All products of (a) CeO₂, (b) Ce_0.5Zr_0.5O₂, and (c) Ce_0.5Zr_0.3Al_0.2O_1.9 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 Ce_0.5Zr_0.5O₂ (a = 0.5384 nm) and Ce_0.5Zr_0.3Al_0.2O_1.9 (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, Ce_0.5Zr_0.5O₂, and Ce_0.5Zr_0.3Al_0.2O_1.9 calculated by Scherer's formula were 9, 5, and 3 nm, while those of the calcined CeO₂, Ce_0.5Zr_0.5O₂, and Ce_0.5Zr_0.3Al_0.2O_1.9 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, Ce_0.5Zr_0.5O₂, and Ce_0.5Zr_0.3Al_0.2O_1.9, 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, Ce_0.5Zr_0.5O₂, and Ce_0.5Zr_0.3Al_0.2O_1.9 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 Ce_0.5Zr_0.3Al_0.2O_1.9 showed a much higher surface area (232 m² g⁻¹) than those of CeO₂ (119 m² g⁻¹) and Ce_0.5Zr_0.5O₂ (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 Ce_0.5Zr_0.5O₂ (8 m² g⁻¹) decreased to less than 10 m² g⁻¹, but the sample of Ce_0.5Zr_0.3Al_0.2O_1.9 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, Ce_0.5Zr_0.5O₂, and Ce_0.5Zr_0.3Al_0.2O_1.9 samples. The TG profile shows the oxygen release/storage performance of the CeO₂, Ce_0.5Zr_0.5O₂, and Ce_0.5Zr_0.3Al_0.2O_1.9 samples at 600°C with time. As a result, Ce_0.5Zr_0.3Al_0.2O_1.9 exhibited a higher OSC of 427 μmol-O g⁻¹, when compared to those of the CeO₂ (25 μmol-O g⁻¹) and Ce_0.5Zr_0.5O₂ (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 Ce_0.5Zr_0.3Al_0.2O_1.9 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 Ce_0.5Zr_0.3Al_0.2O_1.9 retained the same OSC even after 22 cycles (Figure 5). The result indicates that Ce_0.5Zr_0.3Al_0.2O_1.9 has good OSC performance stability.

^aIt is accepted that the Ce_0.5Zr_0.5O₂ composition possessed excellent OSC property 10 11 12 13 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, Ce_0.5Zr_0.3Al_0.2O_1.9 exhibited the highest OSC of 427 μmol-O g⁻¹ (Table 1), when compared to those of the Ce_0.5Zr_0.4Al_0.1O_1.95 (378 μmol-O g⁻¹), Ce_0.5Zr_0.2Al_0.3O_1.85 (389 μmol-O g⁻¹), and Ce_0.5Zr_0.1Al_0.4O_1.8 (261 μmol-O g⁻¹) samples (Table 2), Therefore, in Ce_0.5Zr_0.5-x Al _x O _y (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.

Ce_0.5Zr_0.3Al_0.2O_1.9 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 Ce_0.5Zr_0.3Al_0.2O_1.9 solid solution are smaller than those of CeO₂ and Ce_0.5Zr_0.5O₂, 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 Ce_0.5Zr_0.3Al_0.2O_1.9 solid solutions exhibited a remarkably higher oxygen storage capacity than those of the CeO₂ and Ce_0.5Zr_0.5O₂ 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.

The authors declare that they have no competing interests.

QD participated in the design of the study, carried out the total experiments, and performed the result analysis as well as drafted the manuscript. SY participated in the design of the study, gave the theoretical and experimental guidance, and made the corrections of manuscript. CG mainly helped in the experiments and measurements. TS gave the theoretical and experimental guidance and helped to amend the manuscript. All authors read and approved the final manuscript.

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.