Controllable Preparation and Catalytic Performance of Magnetic

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Controllable Preparation and Catalytic Performance of Magnetic Fe3O4@CeO2-Polysulfone Nano-composites with Core-shell Structure Juan Wei, Hongbao Yao, Yujun Wang, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03191 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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Industrial & Engineering Chemistry Research

Controllable Preparation and Catalytic Performance of Magnetic Fe3O4@CeO2-Polysulfone Nano-composites with Core-shell Structure

Juan Wei, Hongbao Yao, Yujun Wang*, and Guangsheng Luo

State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

* Corresponding author*: Tel: 86-10-62798447, Fax: 86-10-62770304 E-mail: [email protected].

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Abstract In this research, a brand-new magnetic CeO2 nano-composite with core-shell structure was controllably synthesized. Specifically, Fe3O4 was used as the magnetic core to separate active components; polysulfone (PSF) was used as a binder to protect the Fe3O4 from leaching, thus enhancing the stability of whole catalyst. Hydrothermal method was applied to coat Fe3O4 nanocores with CeO2, and the PSF layer was added by mixing two emulsion systems. The optimum nano-composite contains approximately 66 wt% Fe3O4, 23 wt% CeO2 and 11 wt% PSF with a mean diameter of 350 nm. The degradation experiments showed that 90% AO7 was removed in 3 h and the catalyst could be separated easily by external magnetic field. Notably, little CeO2 loss and iron leaching was achieved in six cyclic degradation loops indicating excellent catalytic stability. The synthetic strategy provides a new methodology for the preparation of stable catalysts.

Keywords nano-ceria; polysulfone; core-shell; magnetic

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1. Introduction Ceria is a kind of rare earth oxide with high reactivity and bright application prospect in oxygen storage, photo catalysis and tail gas treatment, owing to its unique fluorite type structure and oxygen vacancy properties1-5. The oxygen vacancy defects on the surface of ceria can rapidly formed and eliminated, which brings about the redox loop of Ce3+/Ce4+ in ceria6-8. For example, ceria has a wide application in azo dyes wastewater treatment by catalyzing the H2O2 to generate the HO·, which indicates that ceria can be used as a Fenton-like catalyst for the degradation of organic pollutants. Ji et al.6 found that acid orange 7 (AO7) could be degraded efficiently in CeO2/H2O2 system under visible light irradiation. However, the high dispersity of nanomaterial such as pure ceria leads to super difficulty to recycle, resulting in a great loss in industrial production. Adding magnetism to the nano-particles has been a widely employed method to solve the recycling problems and reduce the high cost of filtration steps913.

The most commonly used magnetic material is Fe3O4 considering its easy preparation and excellent

magnetically responsive properties14. However, the corresponding Fe3O4 components could easily result in the release of Fe3+ and Fe2+ during the degradation reaction13, 15, resulting in secondary pollution and magnetic weakening when used as magnetic component. On the other hand, there are little literatures available about the examination of magnetic ceria. Xu et al.13 synthesized magnetic nanoscaled Fe3O4/CeO2 composite as a catalyst for degradation of 4chlorophenol. The Fe3O4 and CeO2 were completely mixed in the composite and the Fe3O4 could also react with the solution, leading to catalyst dissolution and secondary pollution. In 2 hours of reaction, about 2% of total iron had dissolved in the solution. The activity of catalyst decreased gradually during successive runs due to the leaching of iron. In comparison with the fully hybrid structure, core-shell structure can achieve higher catalytic efficiency and stability8-12, 16. The promoted catalytic activity was suggested to be associated with the special structure and large surface area which could provide more active centers to the reactants17. Proper choices of core and shell materials could improve resistance to corrosion, induce the absorption of the reactants, even inhibit the generation of byproducts17-19. Wang et al.11 synthesized dual-heterostructured Fe3O4@CeO2/Pt catalysts for reduction of 4-nitrophenol and oxidation of methanol in alkaline solution. 3



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Nevertheless, the process of adding ceria nanopolyhedra onto the Fe3O4 cores was rather complicated, and the Fe2+/Fe3+ were also involved in the catalytic reaction. A facile synthesis method of core-shell Fe3O4@CeO2 particles with stable structure and complete ceria crystal shape is needed, and the problem of iron leaching should be improved. Adding an additional protective shell may fulfil the requirements. Polysulfone (PSF) is a kind of stable, cheap functional material with biological compatibility, which is widely used for making ultrafiltration membranes and microcapsules20-23. To our knowledge, the PSF has been applied as a shell in many micro-scaled core-shell materials but has not applied in Fe3O4@CeO2 system yet. Gao et al.20 immobilized ionic liquid with PSF shell by spraying suspension dispersion to obtain uniform spherical microcapsules with an average diameter of 70μm. Wang et al.24 prepared PSF/glass bead microspheres with good mechanical strength by a facile two-step phase-inversion method. Besides, PSF microspheres with uniform size could also be prepared by mixing two emulsion systems and showed a good absorption to many organics25. For the first time, PSF has been coated onto Fe3O4@CeO2 particles to prepare a catalyst with cycle stability and excellent catalytic activity. The accurate speed control of phase-inversion process is critical for the shaping and thickness of a monodisperse and nano-scaled PSF shell. In this research, we aim to synthesize the brand-new ceria particles with both excellent catalytic stability and convenient recycling. By using Fe3O4 as magnetic core and PSF as a binder, we can obtain a stable core-shell nano-catalyst which can be easily retrieved in an external magnetic field.

2. Experimental 2.1 Materials and chemicals Ferric chloride (FeCl3, CP grade), trisodium citrate (Na3Cit·2H2O, AR grade), sodium acetate (NaAc, AR grade), ethylene glycol (EG, AR grade), Ether (AR grade), SPAN 85 (C60H108O8, CP grade), AO7, and cetyltrimethyl ammonium bromide (CTAB, AR grade) were purchased from Sinopharm Chemical ReagentCo., Ltd. Cerium nitrate (Ce(NO3)3·6H2O, AR grade), N,N-Dimethylformamide

(DMF,

C3H7NO, AR grade), n-Octane (AR grade), Hydrogen peroxide (H2O2, 30w%, AR grade) were purchased from Beijing Tong Guang Fine Chemicals Company. The PSF was purchased from America 4


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Sigma Aldrich, Ltd. 2.2 Catalyst preparation There are three steps to synthesize the magnetically recyclable core-shell type Fe3O4@CeO2-PSF nano-composite, as shown in the Figure 1.

Figure 1. Schematic diagram of Fe3O4@CeO2-PSF preparation procedure 2.2.1 Preparation of Fe3O4 seeds The Fe3O4 nanospheres were synthesized following a modified solvothermal method14. The NaAc and Na3Cit were used as alkali source and electrostatic stabilizer, respectively. Typically, a certain amount of FeCl3, Na3Cit·2H2O and NaAc with the mass ratio of 3.25:1:6 were dissolved in 20ml EG under vigorous stirring into a 50 ml Teflon bottle and the mixture was kept at a reaction temperature of 200 °C for 10 h. After cooling to ambient temperature, the precipitate was washed with deionized water and ethanol three times and dried at 60°C overnight in air, ready for further use. 2.2.2 Preparation of Fe3O4@CeO2 and CeO2 samples A hydrothermal method was employed to load CeO2 active components onto the surface of Fe3O4 seeds. Firstly, some abovementioned Fe3O4 nanospheres were mixed with Ce(NO3)3·6H2O, CTAB, NaAc with a mass ratio of 1:10:10:7 and the mixture was dissolved in 20ml deionized water in a 50 ml Teflon bottle. For better dispersion of Fe3O4 in the solution, the mixture was kept mechanically stirring for 60 min in ultrasonic, resulting in the formation of brown slurries. Afterwards, the bottle was sealed in a stainless steel autoclave tightly and transferred into an electric oven. By changing the hydrothermal time (4 h, 6 h, 8 h), reaction temperature (160 °C, 180 °C, 200 °C), concentration of Ce3+ and CTAB in the reactant, Fe3O4@CeO2 samples with different morphologies were eventually synthesized after the further ethanol-washing and drying process at 60 °C overnight. For comparison, pure CeO2 was also 5



prepared by the same hydrothermal method in the same condition. 2.2.3 Preparation of Fe3O4@CeO2-PSF samples PSF was coated onto Fe3O4@CeO2 with octane as the dispersed phase, and SPAN 85 or sodium dodecyl sulfate (SDS) as the emulsifier. The first emulsion system was mixed by 10ml PSF-containing DMF solution, 60ml octane and 4g SPAN 85. Another emulsion system contained the 0.1g Fe3O4@CeO2 composite particles, 10ml deionized water, 50ml octane and 4g SPAN 85. The former emulsion was injected into the latter one by an injection(10ml) under mechanical stirring. After the mixture was mechanical stirred for 30min, the solid particles were separated by an external magnetic field and washed by hot ethanol (70°C) for several times to remove the organics absorbed by the particles. The influence of PSF concentration and emulsifier type of mixing two emulsion systems to the morphology of target particles was investigated. For comparison, the two-step phase-inversion method in previous work24 for PSF coating was also applied. Ether was injected firstly and H2O secondly to a PSF containing DMF solution under mechanical agitation. 2.3 Characterization The morphology and size of different samples were characterized and screened by transmission electron microscope (TEM, JEM-2010, JEOL, Japan), scanning electron microscope (SEM, JSM-7401, JEOL, Japan) and X-ray diffraction (XRD, D8, Bruker, Germany), respectively. The hysteresis loop was tested in a vibrating sample magnetometer (VSM, 730T, Lakeshore, America). The PSF content of the final product was analyzed by thermogravimetry analysis (TGA, STA 409PC, Netzsch, Germany) in N2 circumstance. The concentration of dissolved iron was tested by inductive coupled plasma spectrometer (ICP, X Series, Thermo, America). 2.4 Degradation experiment Batch AO7 degradation experiments were carried out in a beaker under room temperature. Typically, 0.2 g Fe3O4@CeO2-PSF was added into 50 ml AO7 aqueous solution with an initial concentration of 10 ppm. After an-hour stirring to reach absorption equilibrium, 400 μL H2O2 was added to initiate the catalytic oxidation reaction and the instant AO7 concentration was defined as the original point of degradation curve. Samples were taken out at set intervals by a 5mL syringe and separated immediately 6


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by an external magnetic field. The supernate was then analyzed using a UV–vis spectrophotometer at 480 nm. Comparison experiments were also conducted under identical experimental conditions but with the addition of different catalysts, namely, pure CeO2 and Fe3O4@CeO2. The degradation efficiency in this research was defined as the percentage reduction in the AO7 concentration relative to the initial value.

3. Results and discussion 3.1 Characterization of Fe3O4@CeO2 samples It can be clearly seen from Figure S1a-c that Fe3O4 nanoparticles are relatively uniform in shape with average diameters of 190 nm. In addition, both pure Fe3O4 and CeO2 nanoparticles present crystalline structures from the XRD results shown in Figure S1d. Morphology contrast images between Fe3O4 and Fe3O4@CeO2 could be seen in Figure 2a and b. The layer of ceria shell added by hydrothermal method achieves uniform coating and ideal coating thickness. The scanning result in Energy Dispersive X-ray Spectrometer (EDX) of the specific Fe3O4@CeO2 particles (Figure 2c-e) also proves that the CeO2 are uniformly distributed on the surface of Fe3O4 cores.

Figure 2. TEM images of (a) Fe3O4 and (b) Fe3O4@CeO2 particles, (c-e) EDX elemental mapping of 7



Fe3O4@CeO2

Here, different factors on the hydrothermal preparation of Fe3O4@CeO2 nano-composites were also discussed in detail. Figure 3 shows the effect of hydrothermal time on the size and shape of CeO2 shell. It seems that Six hours might be the most proper duration to prepare uniformly coating particles. Shorter time (4 h) could not lead to the formation of CeO2 with good crystallinity, while longer time (8 h) causes thick layers onto the magnetic core as well as serious agglomeration. The XRD patterns (Figure 4d) also suggests that it is much easier to produce ceria crystals with better crystallinity under longer hydrothermal time.

Figure 3. TEM images of Fe3O4@CeO2 particles synthesized in different hydrothermal duration: (a) 4 h, (b) 6 h, (c) 8 h and corresponding (d) XRD patterns. [Hydrothermal temperature 180°C, CTAB concentration 0.05 g/ml and Ce3+ concentration 0.09 mol/L].

The TEM images (Figure 4a-c) revealed that hydrothermal temperatures also evidently influenced the 8


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shape and size of the particles. Under lower temperature (160°C), the CeO2 coating layer was too thin to form complete spherical shell because of the slower reaction rate. Under higher temperature (200°C), the coating layer was very thick and the homogeneous nucleation phenomenon of CeO2 was fairly severe. Besides, the XRD patterns (Figure 4d) illustrated that the characteristic peaks of CeO2 in the composite particles synthesized in 200°C were stronger than lower temperatures. It can be assumed from both the TEM and XRD results that higher temperature accelerates the growth of CeO2 crystals. Consequently, 180°C seems to be the proper hydrothermal temperature to prepare Fe3O4@CeO2 samples under experimental conditions.

Figure 4. TEM images of Fe3O4@CeO2 particles synthesized in different hydrothermal temperature: (a) 160°C, (b) 180°C, (c) 200°C and corresponding (d) XRD patterns. [Hydrothermal duration 6 h, CTAB concentration 0.05 g/ml and Ce3+ concentration 0.09 mol/L].

The dosage of surfactant should also be examined considering both the coating effect and separation difficulty. The surfactant, CTAB, could build a molecular film in the solid liquid interface to prevent 9



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agglomeration and help the CeO2 precipitate uniformly on the magnetic cores to generate regular spheres. Under lower surfactant concentration (Figure 5a-b), the CeO2 assembled by itself.

Although

more surfactant (Figure 5d) contributes to a more uniform morphology, it also causes high viscosity of the solution, severe separation problems and lower yields. What’s more, the XRD patterns (Figure 6) indicates that under lower surfactant concentration the lattice forms are more complete. The surfactant hinders the growth of crystals while preventing solids from agglomerating. So the surfactant concentration of 0.05g/ml was chosen as the optimal dosage under experimental conditions.

Figure 5. TEM images of Fe3O4@CeO2 particles synthesized in different CTAB concentration: (a) 0, (b) 0.025g/ml, (c) 0.05g/ml, (d) 0.1g/ml. [Hydrothermal temperature 180°C, hydrothermal duration 6 h and Ce3+ concentration 0.09 mol/L].

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Figure 6. XRD patterns of Fe3O4@CeO2 particles synthesized in different CTAB concentration

The impact of Ce3+ concentration to the coating particles could be seen in Figure 7 and Figure 8. As the Ce3+ concentration increased from 0.06 to 0.12mol/L, the average diameter of coating particles increased from 225 to 236nm, which indicates that higher Ce3+ concentration lead to thicker CeO2 layer. By changing the Ce3+ concentration in the hydrothermal synthesis, the CeO2 loading on the magnetic cores could be easily controlled.

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Figure 7. TEM images(a-c) and XRD patterns(d) of Fe3O4@CeO2 particles synthesized in different Ce3+ concentration: (a)0.06mol/L, (b)0.09mol/L, (c)0.12mol/L. [Hydrothermal temperature 180°C, hydrothermal duration 6 h and CTAB concentration 0.05 g/mL].

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Figure 8. SEM images and corresponding particle size distribution of Fe3O4@CeO2 particles synthesized in different Ce3+ concentration: (a) 0.06mol/L, (b) 0.09mol/L, (c) 0.12mol/L. [Hydrothermal temperature 180°C, hydrothermal duration 6 h and CTAB concentration 0.05 g/mL].

3.2 Characterization of the Fe3O4@CeO2–PSF particles The production of two-step phase-inversion method showed unfavorable coating results as seen in Figure S2. PSF could not solidify uniformly on every Fe3O4@CeO2 particle. When ether entered the mixture of PSF-containing DMF solution and Fe3O4@CeO2 particles, the initial solidification happened 13



randomly in the solution but not on the surface of particles. In contrast, by mixing two emulsion systems, the morphology of the final particles was much more uniform. In the W/O emulsion system stabilized by the emulsifier, the Fe3O4@CeO2 particles were covered by water membrane. By mixing the two emulsion systems, the dissolved PSF contact the water membrane and solidify on the surface of Fe3O4@CeO2 particles. When using SDS as emulsifier to replace SPAN 85, the PSF solidified in irregular block rather than spherical shell (Figure S3). The molecular structure differences between SPAN 85 and SDS determine that they are respectively suitable for W/O and O/W systems. Therefore, SPAN 85 benefit the formation of a water membrane on the surface of Fe3O4@CeO2 particles. By increasing the PSF concentration in DMF from 0.1w% to 0.5w%, the shape and size of the final particles changed dramatically (Figure 9d-f). Under lower concentration (0.1w%), the PSF could wrap the Fe3O4@CeO2 particles into regular spheres, a little CeO2 were inlayed outside the particles. As the concentration increased, the PSF layer became thicker and finally attached upon each other to form PSF plane with the Fe3O4@CeO2 particles beset in it.

Figure 9. SEM images of Fe3O4@CeO2-PSF particles synthesized by mixing two emulsion system in different PSF (DMF) concentration: (a) 0.1w%, (b) 0.25w%, (c) 0.5w%.

The TGA curves and the PSF content of final coating particles were shown in Figure 10 and Table 1, respectively. The thermogravimetric curve of Fe3O4@CeO2 showed that the particles lost weight since the beginning of heating due to the volatilization of H2O. And the weight dropped around 300°C and stabilized around 500°C, which were attributed to thermal decomposition of residual CTAB. By analyzing the thermogravimetric curve of pure PSF, it could be inferred that PSF began to lose weight in about 500°C and only 34% of the quality was left in 700°C due to the Benzene-supported structure 14


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of the PSF. Particles synthesized in higher PSF concentration solution had greater weight loss in TGA, which suggests that higher PSF concentration in reactant solution produces higher PSF content in the final particles.

Figure 10. TGA curves of (a)Fe3O4@CeO2 particles and pure PSF, (b)Fe3O4@CeO2-PSF synthesized in different PSF concentration (0.5%, 0.25%,0.1%)

Table 1 PSF content of Fe3O4@CeO2-PSF synthesized in different PSF concentration PSF concentration

PSF percentage of Fe3O4@CeO2-PSF particles

0.1%

11.1%

0.25%

23.0%

0.5%

37.2%

The room temperature magnetization curves of these three samples (Fe3O4, Fe3O4@CeO2 and Fe3O4@CeO2-PSF) are shown in Figure 11. The saturation magnetization (Ms) of the particles gradually declined as the coating layers increased, which is mainly attributed to the existence of CeO2 and PSF. The Ms value of Fe3O4@CeO2-PSF particles was 15 emu/g，indicating that the particles could be easily separated and reused from solution by applying an external magnetic field, which had been proved by magnetic recycling experiments(Figure 11b-c). Figure 11d-f showed the obvious color changes from black to brown to yellow after the CeO2 and PSF loading, indicating that the CeO2 and PSF were successfully coated on the Fe3O4 magnetic cores. 15



Figure 11. (a) Magnetization curves in room temperature of Fe3O4, Fe3O4@CeO2 and Fe3O4@CeO2PSF; Fe3O4@CeO2-PSF particles in deionized water (b) before and (c) after adding an external magnetic field; color change of (d) Fe3O4, (e) Fe3O4@CeO2 and (f) Fe3O4@CeO2-PSF 3.3 Catalytic activity and recycling performance Degradation experiments were conducted to compare the removal efficiencies of AO7 by different catalysts with the same initial AO7 concentration of 10ppm. In the first catalytic round, the composite catalyst showed almost the same catalytic activity comparing with pure CeO2 and Fe3O4@CeO2 particles, catalyzing the degradation of over 90% AO7 in 3 h (Figure 12a). The catalytic activity of Fe3O4@CeO2-PSF in six degradation loops can be seen in Figure 12b. The activity declined a little bit after the first loop because part of AO7 was absorbed in the PSF layer during the first reaction, which was hard to be washed off and reduced the absorption capacity of the regenerative catalyst. However, it should be noted that Fe3O4@CeO2-PSF still maintain relatively high catalytic activity in the following recycle experiments, suggesting excellent stability. On the other hand, without the PSF layer, the catalytic efficiency of Fe3O4@CeO2 particles (Figure 12c) showed a sharp downward trend due to the great loss of CeO2 during the reaction and separation process. In comparison, the Fe3O4@CeO2 particles was so unstable that the released CeO2 could even be seen after 3-hour degradation experiment (Figure 12d). Moreover, the concentration of dissolved iron tested by ICP suggests that the after 3 h of reaction, 16


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the iron concentration in the solution achieved 0.562 mg/L with Fe3O4@CeO2 as catalysts. After coating by PSF, the iron leaching decreased greatly to about 0.0124 mg/L, which suggests that the PSF layer helps to immobilize the CeO2 and protect the Fe3O4 cores from leaching.

Figure 12. (a) Catalytic performance comparison of different catalyst, (b) recycling catalytic performance of Fe3O4@CeO2-PSF in six degradation loops, (c) recycling catalytic performance of Fe3O4@CeO2 in three degradation loops, (d) magnetic recovery contrast between Fe3O4@CeO2-PSF and Fe3O4@CeO2 after 3-hour degradation experiment in the first catalytic round

4. Conclusion The research aims to synthesize recyclable and stable core-shell ceria nanoparticles by adding a magnetic Fe3O4 core and a PSF outermost shell. Three steps were taken in this process. First of all, Fe3O4 cores with 190nm diameter were prepared by a solvothermal method at 200°C for 10h. Secondly, CeO2 was added uniformly on Fe3O4 cores by hydrothermal method. Ce(NO3)3, NaAc and CTAB were respectively used as cerium source, alkali source and surfactant. Hydrothermal temperature, time and 17



the concentration of CTAB and Ce3+ had obvious influence on the shape and size of the core-shell particles. By changing the concentration of Ce3+ and CTAB, particles with different coating thicknesses and crystal forms could be produced. The optimal hydrothermal conditions are respectively 180℃, 6 hours, 0.1g/L and 0.09mol/L. Finally, the former particles were coated in PSF by mixing two emulsion systems. The coating thickness increases when the concentration of PSF solution raises. The final coating particles contained 23%CeO2, 66%Fe3O4 and 11%PSF. The synthesized catalyst has achieved stable and efficient catalytic activity in six degradation loops to degrade over 90% AO7 in 3 h and effectively reduced the amount of iron leaching from 0.5624mg/L to 0.0126g/L. By adding magnetic cores and organic binder to ceria, the target product achieved prospective characteristics: excellent retrievability, stability and catalytic activity. The structure and synthetic method of the catalyst provides a new way to build recyclable and stable nano-particles for future research. However, the synthetic cost of the product is still too high to accomplish industrialization. Further research is needed to lower the production costs.

ASSOCIATED CONTENT Supporting Information Available TEM, SEM images and XRD patterns of Fe3O4 cores and pure CeO2, SEM images of Fe3O4@CeO2-PSF composites synthesized by different methods.

Conflicts of interest There are no conflicts to declare.

Acknowledgements We gratefully acknowledge the support of the National Basic Research Foundation of China (Grant No. 2013CB733600), the National Natural Science Foundation of China (Grant Nos. 21276140 and 21036002).

Reference (1) Chen, Y.; Liu, T. M.; Chen, C. L.; Guo, W. W.; Sun, R.; Lv, S. H.; Saito, M.; Tsukimoto, S.; Wang, Z. C., Hydrothermal synthesis of ceria hybrid architectures of nano-rods and nano-octahedrons. Mater. Lett. 2013, 96, 210-213. (2) 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. Journal of Physical Chemistry B 2005, 109 (51), 24380-24385. 18


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Controllable Preparation and Catalytic Performance of Magnetic

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