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Journal of Nanomaterials
Volume 2011 (2011), Article ID 657516, 7 pages
http://dx.doi.org/10.1155/2011/657516
Research Article

Synthesis and Characterization of Nanometer Ce0.75Zr0.25O2 Powders by Solid-State Chemical Reaction Method

1College of Light and Chemical Engineering, Guangdong University of Technology, Guangzhou, 510006, China
2College of Physics and Optoelectronics Engineering, Guangdong University of Technology, Guangzhou, 510006, China
3School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, 510631, China

Received 8 May 2010; Revised 8 July 2010; Accepted 24 August 2010

Academic Editor: Bo Zou

Copyright © 2011 Yuying Zheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The nanostructure Ce0.75Zr0.25O2 powders have been successfully synthesized by using the mechanically activated solid-state chemical reaction method for the first time, with Ce2(CO)3, ZrOCl2·8H2O, and H2C2O4 ·2H2O as raw materials. The structure and morphology of Ce0.75Zr0.25O2 powders were characterized to be in the single cubic phase and spherical shape via X-ray powder diffraction, transmission electron microscopy, X-ray Photoelectron Spectrometer, and BET surface area testing technique. The average size of particles was measured to be less than 20 nm, and the specific surface area was 85.4 m2/g. The TG-DTA investigation was used to reveal the possibly chemical reaction mechanism during the synthesis process. The activity of Ce0.75Zr0.25O2 oxide solution in three-way catalysts was also valued in this work.

1. Introduction

The nanostructure,Ce1−x ZrxO2 materials have drawn extraordinary research interests in recent years due not only to its good mechanical and electrical properties [14], but also to the potentials of applications in various fields, such as the catalysts [57], solid oxide electrolyte materials [810], and advanced ceramics [1113], There have been many methods to prepare nano-Ce1−x ZrxO2 solid solution, such as coprecipitation [14, 15], sol-gel [16, 17], high-temperature calcinations [18], high-energy mechanical milling, and hydrothermal method [19]. However, the procedure of the above approaches is so complicated that they are inevitably limited to apply to industrial and commercial situations.

Although substantial efforts have been exerted to the development of new synthetic methodologies for both efficiently making nano-Ce1−x ZrxO2 and greatly lowering the synthesis costs, big difficulties still exist in the industrialization and productivity of each method. So far, the mechanically activated solid state chemical reaction method has gradually become a novel synthesis way that it exhibited high efficiency and lower cost of water and energy [20]. By comparing with other methods, the mechanically activated solid state chemical reaction method was considered to be a more simple industrial art without needing of plenty of solvents. In this work, the nanostructured Ce0.75Zr0.25O2 was prepared by the mechanically activated solid state chemical reaction method for the first time. The characterization and properties of nano-Ce0.75Zr0.25O2 were investigated and determined. Their catalysis efficiency was also checked at the aspect of potential applications.

2. Experimental

2.1. Preparation
2.1.1. Synthesis of Nano-Ce1−x ZrxO2 Powders

The particles of nano-Ce0.75Zr0.25O2 were prepared by mechanically activated solid-state chemical reaction method for the first time. At first, precursors were prepared by mixing of ZrOCl2·8H2O, Ce2(CO3)3 and H2C2O4·2H2O at the molar ratio of 𝑛(ZrOCl2·8H2O): 𝑛(Ce2(CO3)3): 𝑛(C2H2O4·2H2O) = 0.25: 0.375: 0.375, then, the precursors were placed into the ball mill (model No. XQM-4L), operating at the initial angular speed 150 r/min for 2 hour. The amount of grinding media was kept a ratio of 10 : 1 to precursors. After one and a half hours grinding, the surfactant (5% total weight) was doped into the ball milling pot. At last the white resultant composition was calcined for 3 hours at the temperature of 600°C, and the yellow nanoparticles of Ce0.75Zr0.25O2 were obtained.

2.1.2. Preparation of Catalysts

Catalyst A was mainly composed of the two noble metals totally taking a fraction 0.40% of weight in the three-way catalyst (TWC) [21, 22]. Pd and Rh took 0.25% and 0.15%, respectively. Here the preparation was briefly outlined Firstly, Ce0.75 Zr0.25 O2 were blended with γ-Al2O3 under mechanical mixing, which only takes 30% of total weight. Secondly, the mixture of water solutions of PdCl2 and RhCl3 was prepared at the mass ratio of 𝑚(Pd2+) and 𝑚(Rh3+)=53; then the mixture of water solutions of Pd- and Rh-contained salts was loaded into the mixture of Ce0.75Zr0.25O2 and γ-Al2O3 for necessary reactions by incipient wetness method. Finally, the composite was calcined at 500°C for 2 hours and subsequently reduced at 500°C under atmosphere of H2 for 2 hours.

Preparation of catalyst B. The preparation process was almost the same as catalyst A. The difference was that nano-Ce0.75 Zr0.25O2 powders were prepared by coprecipitation method, within which, Ce(NO3)3·8H2O and ZrOCl2·8H2O were firstly dissolved inwater, respectively, to form water solutions both in 0.2 mol/L, and then they were mixed together. Subsequently, aqueous ammonia in 1 mol/L was poured into the solutions mixtures until the pH=9.8 under continuously stirring. Finally, precipitate was washed and dried, then calcined at 600°C for 3 hours.

2.2. Characterization

TG-DTA analysis was performed based on SPA409PC analytic apparatus operated at the 30 mL/min airflow rate and the 5°C/min temperature rise rate. X-ray powder diffraction (XRD) measurements were made with a Rigaku D/Max-3B diffractometer employing Cu-Kα radiation. Transmission electron microscopy (TEM) (JEM-100CXII) was used to study the crystallization property and the size of the particles. Particle size distribution histograms were obtained on the basis of measurements from about 300 particles. X-ray photoelectron spectra (XPS) were recorded on an Scanning ESCA Microprobe (Quantum-2000, PHI) photoelectron spectrometer using Al-Kα radiation under a vacuum environment of 1×106Pa. The binding energy of C1s (284.6 eV) was used for the calibration of all binding energies. The specific surface area (SBET) of the products was checked by measuring the adsorptive capacity of N2 at −196°C by using full-automatic adsorption instrument ASAP2010M.

2.3. Catalysis

As for the activity of catalysts, it was evaluated by monitoring the contents of five components such as NO, CH, CO, O2, and CO2 in automobile exhaust gas by using gas chromatograph (GC 9790) and the automobile exhaust analytic system(FGA-4100), with the reference exhaust composed of 𝜑(NO)=0.1%,𝜑{CH[V(C3H8)V(C3H6)=11]}=0.1%,𝜑(CO)=1.5%, and𝜑(O2)=1.37% in volume fraction. The instruments were run with He as buffer gas at S.V.=40000h1 and air/fuel (A/F) ratio of 14.6. The temperature rise rate was set at 10°C per minute. The rate of conversion could be calculated as following:𝜎𝑋=0𝜎1𝜎0×100%,(1) where 𝑋 denotes the rate of conversion, 𝜎0 and 𝜎1 stand by the starting volume fraction and final volume fraction of gases, respectively.

3. Results and Discussion

3.1. Determination of Solid Solution Manner

Figure 1 and Table 1 displayed that XRD patterns of the final products prepared from the different ratio of raw materials, and the corresponding crystal volumes, respectively. As can be seen, the crystal volume decreases gradually with the increment of the concentration of Zr4+ ions. In general, there are two ways that Zr4+ ions could be doped into the crystal lattice of CeO2, one is that Zr4+ is filled into the crystal lattice in the form of gap ions, the existence of Zr4+  ion thus causes a vacancy of Ce4+, leading to enlargement of the volume of Ce1−x ZrxO2 crystal cell. The other is that Zr4+ fully substituted for Ce4+ leading to the compactness of the crystal lattice as the ionic radius of Zr4+ (0.079 nm) is less than that of Ce4+ (0.092 nm). The experimental results obviously showed that the lattice constant (𝑑XRD) declined with the increasing amount of Zr4+, in accordance with the second doping manner of Zr ions, namely, Ce4+ ions in CeO2 were partially replaced by Zr4+ in the Ce1−x ZrxO2 solid solution, resulting in the reduction of the crystal volume.

tab1
Table 1: Different molar ratios of raw materials correspond to different products and particle size.
657516.fig.001
Figure 1: XRD spectra of CexZr1−x O2 solid solutions.

XRD patterns of Ce0.75 Zr0.25O2 solid solution in Figure 1 indicated that Ce0.75Zr0.25O2 solid solution has a crystal structure in a cubic phase. From the analysis of XRD pattern, we could calculate the 𝑑XRD and the degree of crystallization (𝑋c). The 𝑋c was calculated according to𝑋𝑐=𝐼𝑐𝐼𝑐+𝐼𝑎×100%,(2) where 𝐼𝑐 denotes the area of crystallinity phase, and 𝐼𝑎 stands for the area of amorphous phase. As a result, the 𝑑XRD and the degree of crystallization (𝑋𝑐) were obtained to be 14.46 nm and 95.3%, respectively. No peak of impurity was found in the XRD patterns referenced by the standard card 28-271 in Powder Diffraction File (PDF).

3.2. TG-DTA Analysis

The TG-DTA data were plotted in Figure 2. The TG curve displayed three main stages through the whole process. Loss of free water from precursors surface firstly took place at about 116°C. A further loss of weight followed, as seen from the exothermic peak on the DTA curve at 170°C. Continuous loss of water from oxalate and heat decomposition of excessive oxalic acid resulted in the reduction of weight. The loss of weight was up to about 22% after this step. Then the heat decomposition of oxalate began and lasted from 240°C to 600°C, resulting in the reduction of the precursors weight by 26.5%, which was basically consistent with the theoretical the loss of weight of 27.7%. According to the TG-DTA analysis, the following chemical reactions occurred within the process of heat decomposition:ZrOC2O42H2O+O2ZrO2+2CO2+2H2O,(3)2Ce2CO33+O24CeO2+6CO2,(4)(1𝑥)CeO2+𝑥ZrO2Ce1𝑥Zr𝑥O2,(5) Equations (3) and (4) indicated the heat decomposition reactions, whilst (5) showed the stoichiometric reaction without the loss of weight. As shown in Figure 1, Ce2 (CO3) was not involved in the reaction with H2C2O4·2H2O, only ZrOCl2·8H2O reacted with H2C2O4·2H2O during the process of ball milling.

657516.fig.002
Figure 2: TG-DTA curves of the precursors.
3.3. Influence of Surfactant

In order to avoid the aggregation, the influence of surface active agents onto the product was also investigated. TEM images of the nanosized samples, prepared under the introduction of different surface active agents to the precursors, were shown in Figure 3, indicating the effect of different surfactants onto the crystallization property and the size of the particles. The cationic surfactant of hexadecyl trimethyl ammonium bromide (CTAB) showed no distinct effect. The anionic surfactant of neopelex made the particles size rise up to 37.4 nm. On the contrary, the neutral surfactants PEG-400 and Tween-60 enabled the particles to maintain relatively small sizes of less than 25 nm and more homogeneous distributions. With tween-60 as surfactant, the property of the particles was superior to those prepared under other surface active agents. The experimental results demonstrated that the particles were in more uniformly spherical shape with homogeneous dispersion. The average size of particles was obtained to be less than 20 nm, and the SBET was 85.4 m2/g, obviously larger than the SBET of 60.2 m2/g, 76.9 m2/g, and 69.7 m2/g, corresponding to surfactants (b), (c), and (d) in Figure 3.

fig3
Figure 3: TEM images of Ce0.75 Zr0.25 O2 produced under surfactants (a) Tween-60, (b) neopelex, (c) PEG-400, and (d) CTAB.

The experimental results also indicated that the neutral surfactant could effectively prevent the aggregation of particles such that the particle size could be well controlled. The nonionic surfactant was hardly affected by the inorganic ions during the grind and could facilitate the formation of hydrogen bond, thus a protectively hydrophilic film was easily formed on the surface of the powder. The hydrophilic film enabled the powders to have spatially steric hindrance and electrostatic effects, preventing product particles from aggregation into large size. However, the cationic surfactant could easily react with the H2C2O4·2H2O due to the alkalinity, so that no obvious effect was brought onto the particles. The anionic surfactant could cause the rising of the particles sizes.

3.4. XPS Analysis

The XPS spectra of Zr 3d, Ce 3d, and O 1s in Ce0.75Zr0.25O2 oxides were displayed in Figure 4. The binding energies of Zr 3d, Ce 3d, and O 1s in oxides were listed in Table 2 for reference and comparison [23]. Based on the fitting of XPS spectra with a Gaussian shape, we obtained the binding energies (𝐸𝑏) of Ce 3d5/2 and Ce3d3/2 to be 882.42 eV and 900.85 eV, respectively, comparatively close to the reference 𝐸𝑏(Ce3d5/2)=884.98 eV and 𝐸𝑏(Ce3d3/2)=903.68 eV measured in solid solution of oxides CeO2/(CeOx+Y2O3+ZrO2) and Ce2O3/(CeOx+Y2O3+ZrO2), respectively. So it was reasonable to believe that both Ce3+ and Ce4+ exist in the products. 𝐸𝑏 (Zr 3d) was measured to be 182.6 eV, equal to the binding energy of Zr 3d in ZrO2/(CeOx+Y2O3+ZrO2). As for the 𝐸𝑏 (O 1s), two binding energies, 529.98 eV and 532.03 eV, were obtained, corresponding to 530.0 and 532.0 eV in solid solution of CeO2/(CeOx+Y2O3+ZrO2) and Ce2O3/(CeOx+Y2O3+ZrO2), respectively. These facts proved that the products prepared by mechanically activated solid state chemical reaction to be in the state of solid solution of oxides, which was favored by the XRD results.

tab2
Table 2: Referenced binding energy of Zr 3d, Ce 3d, and O 1s in oxides [23].
fig4
Figure 4: XPS spectra of Zr 3d, Ce 3d and O 1s in Ce0.75Zr0.25O2.
3.5. Evaluation of Catalyst

Catalyst A and B were used to decontaminate the automotive exhaust. The catalysis efficiency was determined by monitoring the contents of greenhouse gas, such as NO, CH, and CO, through the conversion rate. The conversion rates in percentage of NO, CH and CO have been calculated according to (1) and shown in Figure 5. The light-off temperatures (𝑇50) of the above gases were listed in Table 3. As seen from Figure 5 and Table 3, no obvious difference could be found between the two catalysts, except for the fluctuation of 12C at the light-off temperature. Although the two catalysts, prepared by different methods, they almost had the same catalysis function. The mechanically activated solid state chemical reaction method still possessed obvious advantages of easily manufacturing, energy saving, and less solvent, indicating its promisingly commercial and industrial potentials.

tab3
Table 3: Light-off temperature of catalysts A and B.
fig5
Figure 5: The catalysis activity of catalyst (a) and (b).

4. Conclusion

In summary, Nano-Ce0.75Zr0.25O2 powders were prepared by using the solid-solid chemical reaction method under the action of mechanical power. The products were determined to be in the single-cubic-phase. The particles were found to be in more spherical shape with the average size less than 20 nm, the better specific surface area of 85.4 m2/g, and more uniform dispersion.

The effects of surfactants employed in the synthesis process of the products have also been investigated. The facts proved that Tween-60 and PEG-400 had better effects on the control of particle sizes.

The mechanically activated solid state chemical reaction method possessed obvious advantages in manufacturing, energy-saving, and less solvent.

Acknowledgments

This work was supported financially by Government of Guangdong Province for 211 Subject Construction. The authors are also grateful to the financial support from National Natural Science Foundation of China (NSFC) under Grant no. 60977029, 10774031, and 60878063, and from High Technology Research Program 2007AA021806. Y. Zhang and Z. Hu equally contributed to this work.

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