A Size-Controllable Precipitation Method to Prepare CeO2

Apr 19, 2017 - A Size-Controllable Precipitation Method to Prepare CeO2 Nanoparticles in a Membrane Dispersion Microreactor. Hongbao Yao†, Yujun Wan...
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A Size-Controllable Precipitation Method to Prepare CeO2 Nanoparticles in a Membrane Dispersion Microreactor Hongbao Yao,† Yujun Wang,*,† and Guangsheng Luo† †

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: A membrane dispersion-based microreactor was used to prepare ceria nanoparticles using cerium nitrate and ammonia−water as the raw reagents. The traditional precipitation process makes it difficult to control crystal size in a stirred tank reactor; therefore, a new membrane dispersion microreactor was used, which was able to easily control the crystal size. Most importantly, the size of ceria nanoparticles was significantly reduced because of the enhanced mixing performance of the membrane dispersion microreactor. The crystal size of the prepared ceria in this new procedure reached 8.2 nm in comparison to ∼16.7 nm from the traditional precipitation process under identical conditions. The effects of supersaturation, pH value, mixing intensity, and reaction temperatures were also investigated in detail. Therefore, ceria nanoparticles with average sizes of 7− 12 nm can be controllably obtained. Furthermore, the reaction time to reach 90% AO7 degradation rates was improved by ∼89.2% using the ceria prepared in the membrane dispersion microreactor compared to that in a stirred tank reactor with an initial AO7 concentration of 60 ppm and H2O2 concentration of 80 mmol/L. To conclude, this study provides a size-controllable preparation method for ceria nanoparticles, particularly with smaller particle sizes and superior catalytic activity. scale-up.5 However, this process is not easily controlled because of the poor mixing performance of a conventional stirring tank reactor, which causes the formation of nanoparticles with a broad particle size distribution. Microstructured reactors have been gradually used to prepare nanoparticles considering its high transfer efficiency, instantaneous adjustment ability, simple continuous operation, and small scale-up effect. This study was accordingly performed to solve the aforementioned issues. In our previous work,21−24 a membrane dispersion microreactor was employed to control the preparation of different nanoparticles with specific properties such as SiO221 nanoparticles with specific surface area values as high as 360 m2/g, Al2O322 nanoparticles with pore volume values reaching up to 2.22 mL/g, and ZnO23 nanoparticles with pure hexagonal structure. In addition, the coprecipitation of indium ion and tin ion at the same time in the microreactor was also achieved to prepare ITO24 nanoparticles, a typical kind of bimetal oxides. However, membrane dispersion microreactors have not been applied in the preparation of ceria materials, especially considering crystal size control issues of CeO2 nanoparticles with an improved mixing performance. Therefore, a new preparation technology of CeO2 nanoparticles using a membrane dispersion microreactor was developed in this work. Cerium nitrate and ammonia−water

1. INTRODUCTION Nanoparticles have been widely studied in recent decades because of their remarkable and interesting properties due to their small sizes, large surface areas with free dangling bonds, and reactivity higher than that of their bulk cousins.1,2 Specifically, nanoceria (CeO2), one of the most important rare earth metal oxides, has been widely used in tremendous redox catalytic applications such as automotive three-way converters, water−gas shift (WGS) reactions, wet catalytic oxidation, and so on. Undoubtedly, the useful properties of ceria are amplified in the nanoparticle form.3−5 For example, Kaspar and co-workers6 reported that nanoscale ceria exhibited a much higher performance as three-way catalysts for automobile exhaust. Accordingly, various techniques have been developed to prepare ceria nanoparticles such as precipitation,7−9 hydrothermal synthesis,10−12 sol−gel,13−15 and microemulsion methods.16−18 For example, Ji et. al.19 obtained CeO 2 nanoparticles with an average crystal size of ∼15.8 nm using a precipitation method by adding ammonia−water into a cerium nitrate precursor. Fu et. al.20 reported a new microwaveinduced combustion method to prepare CeO2 particles with crystal sizes of 20−50 nm. Bondioli et al.9 prepared nanograde cerium oxide using a new flux method by adding cerium ammonium nitrate to a eutectic mixture of molten salts. The product had particle sizes of 10−20 nm. The precipitation method using cerate precursor and alkaline agents as raw materials is the simplest method and has the advantages of low cost, mild synthesis conditions, and easy © 2017 American Chemical Society

Received: Revised: Accepted: Published: 4993

January 20, 2017 April 18, 2017 April 19, 2017 April 19, 2017 DOI: 10.1021/acs.iecr.7b00289 Ind. Eng. Chem. Res. 2017, 56, 4993−4999

Article

Industrial & Engineering Chemistry Research

After 2 h, the resulting purple precipitate was collected and washed with water and ethanol. Then, the precipitate was dried at 60 °C for 12 h in vacuum. The final CeO2 nanoparticles were obtained after being calcined at 550 °C for 4 h with a heating rate of 2 °C/min. 2.2. Characterization. The specific surface areas of the prepared samples were measured at 77 K on a Quantachrome Autosorb-1-C chemisorption−physisorption analyzer. The pH during the preparation procedure was recorded using a PHS-3C pH meter (Shsan-xin, Shanghai, China) with a MODEL E-900 pH electrode (RUOSULL, Shanghai, China). Transmission electron microscopy (TEM, JEOL-2010, Japan) was applied to observe the morphology of CeO2 nanoparticles. In addition, the mean primary particle size and its distribution were quantified based on a statistical number-weighted method by surveying more than 300 particles on the TEM images through Digital Micrograph Software. X-ray diffraction (XRD, Model D8 ADVANCE, Bruker) was used to determine the crystal structure of the prepared samples. The Scherrer equation was used to determine the particle size of the crystals as follows:

were selected as the reaction agents. The varied operation parameters and their effect on the crystal size and specific surface area were investigated in detail. Most importantly, ceria nanoparticles successfully prepared with the new method are much smaller than those prepared with traditional precipitation methods. These nanoparticles also exhibited much higher catalytic activity in degrading acid orange 7 (AO7) in the presence of hydrogen peroxide.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Experimental Procedure. Analytical-grade cerium nitrate (Ce(NO3)3·6H2O), ammonia−water (NH3·H2O), acid orange 7 (AO7), hydrogen peroxide (H2O2, 28%), and ethanol were purchased from Beijing Chemical Plant and used without further purification. Figure 1 shows the

D=

0.89λ Bcos θ

(1)

where D is the grain size, λ is the X-ray wavelength, which was maintained at 0.15406 nm in our experiments, and θ is the Bragg angle. 2.3. Measurements of Catalytic Performance. The catalytic performance of the prepared samples was evaluated by degrading an AO7 aqueous solution in the presence of H2O2. Typically, 0.2 g of prepared CeO2 was first added to the 50 mL AO7 solution with a certain concentration. The mixture was vigorously stirred for 2 h to establish an adsorption/desorption equilibrium. Then, hydrogen peroxide was added to initiate the catalytic oxidation reaction. After the reaction, the AO7 solution sample was immediately withdrawn and analyzed using a UV−vis spectrophotometer at 480 nm. The degradation efficiency in this work was defined as the percentage reduction in the AO7 concentration relative to the initial value.

Figure 1. Experimental setup of the CeO2 nanoparticle preparation.

experimental setup and illustration of the microreactor inner structure. The membrane dispersion microreactor consisted of two stainless-steel plates (40 × 40 × 10 mm), where a 5-μm stainless-steel microfiltration membrane (Zhen Yuan Purification Technology Co., Ltd.) was used. Ammonia−water was pressed through the micropores by an advection pump into a crossflow channel to mix with the aqueous Ce(NO3)3 solution. The geometric size of the mixing chamber was 12 mm (length) × 4 mm (wide) × 1 mm (height). The products were withdrawn from the outlet, washed, vacuum-dried at 60 °C for 12 h, and calcined at 550 °C for 4 h with a heating rate of 2 °C/ min. The CeO2 nanoparticles were also prepared with a widely used precipitation method in a stirred tank reactor for comparison.19,25 Specifically, NH4OH (28 wt % NH3) was added to a 0.1 M Ce(NO3)3 solution with vigorous stirring.

3. RESULTS AND DISCUSSION 3.1. pH Values and Crystal Structure. CeO2 particles with a uniform size distribution, which were prepared in the hydrolysis of cerium salts, were reported in both acidic26−28 and basic environments.5,29−31 Therefore, the effect of pH on the preparation procedure was first studied. For our continuous microreaction system, different pH can be easily achieved by

Figure 2. Effect of the pH value on the (a) CeO2 crystal size and (b) XRD pattern [reaction temperature: 20 °C; continuous phase: Ce(NO3)3; concentration: 0.2 mol/L; flow rate: 10 mL/min; dispersed phase: NH3·H2O; concentration: 0.2 mol/L; phase ratio: 3, 4, 5 and 6; aging time: 2 h]. 4994

DOI: 10.1021/acs.iecr.7b00289 Ind. Eng. Chem. Res. 2017, 56, 4993−4999

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structure, JCPDS 34-0349), and the XRD results are shown in the Supporting Information (Figures S1−S4). To study the effect of aging time, the experiment was conducted with different aging times (2, 4, 6, and 8 h), a fixed continuous phase flow rate of 10 mL/min, and a dispersed phase flow rate of 10 mL/min. The results show that the average diameter of the CeO2 nanoparticles is approximately 8.2 nm (Figure S5); thus, the aging time hardly affects the grain size for our procedure. Here, we mainly discuss the other three key factors. Supersaturation. Figure 4 shows the plot of the logarithm of supersaturation and the resulting grain size of CeO2 nano-

adjusting the phase ratio. The result is shown in Figure 2a. Specifically, all of the dispersed phase volume flow rates were maintained above 30 mL/min to ensure sufficient mixing and avoid possible interference from inadequate mass transfer. All of the obtained values of the logarithm of S are in the range of 34.1 ± 0.1. In other words, the degree of supersaturation should hardly affect the particle size in this circumstance (S is supersaturation and is discussed in Section 3.2). The pH increased (5.32, 7.6, 8.82, and 9.05) when the phase ratio varied from 3 to 6. The corresponding crystal sizes of the prepared CeO2 nanoparticles were 7.7, 8.0, 8.5, and 9.1 nm, respectively. Apparently, the grain size of CeO2 nanoparticles increases with increasing pH. The XRD patterns of CeO2 nanoparticles prepared at different pH values are also presented in Figure 2b. All samples display noteworthy characteristic peaks at 28.6, 33.1, 47.5, and 56.3°, which are assigned to the (111), (200), (220), and (311) planes, respectively. Thus, the prepared CeO2 particles have a single cubic crystal structure (fluorite structure, JCPDS 34-0349). The high resolution transmission electron microscopy (HRTEM) images in Figure 3a show that the primary particle

Figure 4. Effect of supersaturation on the CeO2 crystal size [reaction temperature: 20 °C; continuous phase: Ce(NO3)3; flow rate: 10 mL/ min; dispersed phase: NH3·H2O; flow rate: 5, 10, 30, and 40 mL/min; concentration ratio: 1:1, 0.03−0.2 mol/L; aging time: 2 h].

particles versus different volume flow rates and reactant concentrations. S can be described by eq S3 in our system. The crystal size of prepared CeO2 nanoparticles significantly decreases with the increase in S. Specifically, the average diameter of CeO2 nanoparticles changes from 11.7 to 8.2 nm, which is a ∼29.6% decrease, when the logarithm of the S value changes from 27.9 to 35.5 (the concentration of raw reactants accordingly varies from 0.03 to 0.2 M). At the given concentration, the crystal size also decreases with an increasing dispersed-phase volume flow rate, which appears to result from the increasing S value and improved mixing intensity. However, the crystal size begins to increase when the dispersed phase volume flow rate is larger than 30 mL/min, as observed from the group of 0.2 M. The main reason may be the significant improvement of pH, as previously discussed. Reaction Temperature. In general, the reaction temperature affects the reagent solubility, diffusion coefficients, degree of supersaturation, and so on. Therefore, the reaction temperature appears to play an important role in nanoparticle synthesis and corresponding particle size control. On the basis of eq S2, the crystal size should decrease with the increase in reaction temperatures. In addition, higher temperatures can induce higher diffusion coefficients, which improve the mass transfer and cause the formation of small particles. Figure 5 presents the experimental results of the effect of the reaction temperature on the crystal size of the prepared CeO2 nanoparticles. The result shows that the crystal size decreases from 10 to 7.9 nm, which is a ∼21% decrease, when the reaction temperature increases from 20 to 80 °C, which validates the above assumption. However, higher temperatures can also incur higher costs. Mixing Intensity. Figure 6 shows the effect of different dispersed-phase volume flow rates on the average crystal size of CeO2 nanoparticles. An increase in the dispersed-phase flow

Figure 3. (a) HRTEM images of CeO2 nanoparticles. (b) SEAD pattern of the prepared sample.

size of CeO2 is approximately 8 nm with clear lattice fringes. The corresponding selected area electron diffraction (SAED) pattern in Figure 3b further confirms that the prepared ceria nanoparticles have a fluorite structure with high crystallinity. 3.2. Effect of Several Other Key Factors and the Corresponding Crystal Size Control. To controllably prepare CeO2 nanoparticles, a series of experiments under different operating conditions was investigated in detail, including the reaction temperatures, degree of supersaturation, mixing intensity, and aging time. All samples in our system present the typical XRD pattern of cubic CeO2 (fluorite 4995

DOI: 10.1021/acs.iecr.7b00289 Ind. Eng. Chem. Res. 2017, 56, 4993−4999

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Industrial & Engineering Chemistry Research

stirred tank reactor under identical experimental conditions. The two samples are hereafter denoted as M-CeO2 and SCeO2, respectively. The mean diameter, which was calculated from the XRD results for M-CeO2 is 8.2 nm, which is nearly 2 times smaller than that of S-CeO2 (16.7 nm). Ji and coworkers19 also prepared CeO2 nanoparticles in a stirred tank reactor using the conventional precipitation method, and the resulting average diameter is approximately 15.8 nm. Figure 7 shows the corresponding TEM images and primary particle size distribution of two types of samples. The statistical average diameter from the TEM results of S-CeO2 nanoparticles is 15.3 ± 1.2 nm, whereas that of M-CeO2 prepared in the membrane dispersion microreactor is only 7.5 ± 0.7 nm, which is consistent with the grain size data calculated from XRD results. Furthermore, the CeO2 nanoparticles prepared in the stirred tank reactor are severely agglomerated and present a much broader particle size distribution. Table 1 summarizes different preparation methods for CeO2 nanoparticles in this work and other studies. We conclude that the precipitation method coupled with the membrane dispersion microreactor is superior. In the stirred tank reactor, the reactant mixing process requires a long period of time to obtain a uniform concentration in the system. Therefore, the uneven concentration distribution will cause the formation of large particle size and a wide particle size distribution. Notably, efficient mixing and fast mass transfer can be easily achieved within a short time in the membrane dispersion microreactor. Accordingly, small nanoparticles with a narrow particle size distribution can be prepared. 3.4. Catalytic Performance for AO7 Degradation. The catalytic performance of the two types of samples was examined by mineralizing acid orange 7, a typical azo dye, in the presence of H 2O2. The experiment was conducted following a preadsorbed mode.25 First, the CeO2 samples were mixed with the AO7 solution with vigorous stirring for 2 h to establish an adsorption/desorption equilibrium. Then, H2O2 was added to initiate the catalytic oxidation reaction. The result is shown in Figure 8. The zero point on the x-axis is based on the adsorption/desorption equilibrium. AO7 was significantly adsorbed on the surface of two types of CeO2 samples. Moreover, the introduction of H2O2 induced a quick desorption of AO7 because of a competitive adsorption with AO7 on the surface of CeO2. A similar phenomenon was reported in previous work.25,38,39 As shown in Figure 8, the AO7 aqueous solution is notably stable and can hardly decompose even in the presence of H2O2 at room temperature. However, it degraded with the addition of the prepared CeO2 samples. Importantly, the degradation efficiency approached 84.1% for M-CeO2 within 0.5 h, which is ∼40.6% better than that of S-CeO 2 under identical experimental conditions. Furthermore, the reaction time for the 90% degradation efficiency was 1.02 h for M-CeO2 and 1.93 h for S-CeO2, which is an ∼89.2% savings. Chen et al.25 conducted deep kinetics investigation to degradation of AO7 in the CeO2/H2O2 system and obtained AO7 degradation rate constant (ka) of 0.035 min−1 under the condition of H2O2 concentration of 80 mmol/L. The calculated ka value under the identical conditions for M-CeO2/H2O2 could reach 0.0602 min−1, improving by ∼71.4%, as shown in Figure S6. An explanation is that higher surface areas tend to induce many more active sites on the surface of CeO2 nanoparticles, which results in higher catalytic activity. The BET experiments

Figure 5. Effect of the reaction temperature on the CeO2 crystal size [continuous phase: Ce(NO3)3; concentration: 0.1 mol/L; flow rate: 10 mL/min; dispersed phase: NH3·H2O; concentration: 0.1 mol/L; flow rate: 10 mL/min; aging time: 2 h].

Figure 6. Effect of the dispersed-phase volume flow rate on the CeO2 crystal size [reaction temperature: 20 °C; continuous phase: Ce(NO3)3; concentration: 0.2 mol/L; flow rate: 10 mL/min; dispersed phase: NH3·H2O; concentration: 0.2 mol/L; flow rate: 5, 10, 20, and 30 mL/min; aging time: 2 h].

rate will accordingly result in an enhanced mixing intensity.32 Experimentally, the dispersed phase flow rate was varied from 5 to 30 mL/min with a fixed continuous phase flow rate of 5 mL/ min and a raw-agent concentration of 0.2 mol/L. Importantly, the pH value and logarithm of S of the system remained almost at approximately 5.2 and 34.1, respectively, in this circumstance. Thus, the effect of these two factors on the crystal size variation can be neglected. The results show that crystal size of the final CeO2 nanoparticles decreases with enhanced mixing intensity. The resulting grain size of CeO2 nanoparticles changed from 8.3 to 7.6 nm, which is a ∼10% decrease, when the dispersed phase flow rate changed from 5 to 30 mL/min. The main reason is assumed to be the easy achievement of sufficient mixing performance in the membrane dispersion microreactor.33 The increased phase flow rate provides a strong cross-flow drag force, which can reduce the droplet sizes and provide large mass transfer areas between two reactants. Therefore, the nucleation process costs a large amount of reactants to generate many small nuclei and decrease the crystal size.34,35 3.3. Comparison with Particles Prepared in the Microreactor and a Stirred Tank Reactor. To quantitatively compare the qualities of the particles and evaluate the superiority of the microreactor for CeO2 preparation, CeO2 nanoparticles were prepared using the microreactor and a 4996

DOI: 10.1021/acs.iecr.7b00289 Ind. Eng. Chem. Res. 2017, 56, 4993−4999

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Figure 7. TEM micrographs and primary particle size distribution of the prepared CeO2 particles: (a and b) in the membrane dispersion microreactor and (c and d) in the stirred tank reactor.

were accordingly examined, and the corresponding specific surface areas of S-CeO2 and M-CeO2 are 45.3 and 73.6 m2/g, respectively, as listed in Table 1. Undoubtedly, the decrease in nanoparticle size of the CeO2 samples results in better properties.

Table 1. Summary of Different Preparation Methods and the Corresponding Particle Sizes preparation method precipitation coupled with microreactor conventional precipitation method conventional precipitation method microwave-induced combustion alcohothermal method

dXRD (nm)

dTEM (nm)

specific surface area (m2/g)

8.2

7.5 ± 0.7

73.6

16.7

15.3 ± 1.2

45.3 42

∼19.25

43.22

20

∼10

79

36,37

15.8 25.31

ref this work this work 19

4. CONCLUSIONS In this work, a chemical precipitation procedure coupled with a membrane dispersion microreactor was first developed to prepare CeO 2 nanoparticles. Several key factors were investigated in detail: the mixing intensity, supersaturation, reaction temperature, and pH value. Therefore, CeO2 nanoparticles with average diameters of 8−10 nm can be controllably synthesized. Most importantly, this procedure presents many advantages over the traditional precipitation method, particularly considering the feasibility for the formation of small CeO2 nanoparticles and superior catalytic performance in the presence of H2O2. Future work can focus on the preparation of nanocomposites and the corresponding potential applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Catalytic performance comparison of M-CeO2 and S-CeO2 for the AO7 degradation [AO7 concentration:60 ppm; AO7 volume: 25 mL; CeO2 concentration: 1 g/L; H2O2 concentration: 80 mmol/L; 25 °C].

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00289. XRD patterns of prepared ceria nanoparticles, discussion of supersaturation equation, and calculation of AO7 degradation rate constant (PDF) 4997

DOI: 10.1021/acs.iecr.7b00289 Ind. Eng. Chem. Res. 2017, 56, 4993−4999

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-10-62798447; Fax: 86-10-62770304; E-mail: [email protected]. ORCID

Yujun Wang: 0000-0002-8495-9811 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Basic Research Program of China (Grant 2013CB733600) and the National Natural Science Foundation of China (Grants 21276140, 20976069, and 21036002).

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