cordierite monolith catalysts

cordierite monolith catalysts

Ceramics International (xxxx) xxxx–xxxx Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/c...

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Ceramics International (xxxx) xxxx–xxxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Preparation and characterization of CuO-CeO2-ZrO2/cordierite monolith catalysts Shuaishuai Lua,b, Jingde Zhanga,b, a b

⁎,1

, Yuanyuan Suna, Huichao Liua

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Jinan 250100, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Impregnation reduction method CuO-CeO2-ZrO2/cordierite catalysts Ultrasonic-assisted CO oxidation

CuO-CeO2-ZrO2/cordierite catalysts have been successfully prepared by the impregnation reduction method. The effects of processing parameters such as the condition of reduction process, calcination temperature, the addition of surfactant on the size and shape of catalyst particles were studied by X-ray diffraction (XRD), BET surface area measurements, and scanning electron microscopy (SEM). The catalytic performance of the asprepared catalysts for CO oxidation were evaluated by using a microreactor-GC system. The results indicate that the processing parameters influence greatly the size and shape of catalyst particles. Fragmental shape catalysts were formed when the impregnated samples were reduced in the thermostat water bath. However, spherical-like particles were obtained by ultrasonic-assisted in the reduction process. Tiny flower-like catalyst particles were found when the surfactant were used. When the calcination temperature decreased, the grain size of catalysts has reduced, and the catalytic activity for CO oxidation has been improved. The introduction of ultrasonic and surfactant resulted in larger specific surface area and better catalytic performance.

1. Introduction In the supported metal oxide catalysts, copper oxide (CuO) catalysts have been of great interest due to its potential applications in many important fields of science and technology such as the catalytic oxidation of hydrocarbons [1,2], purification of vehicle exhausts [3] and catalytic elimination of NOx [4,5]. CuO/CeO2-ZrO2 catalysts, which have the strong interaction among CuO, CeO2 and ZrO2, are promising materials that have attracted extensive attention during the last decade due to a series of advantages including good oxygen-storage capacity [6,7], thermostability [8,9] and catalytic activity [10]. Cordierite (2MgO·2Al2O3·5SiO2) monolith is widely used as catalyst supports due to its low thermal expansivity [11], superior thermal stability [12] and good mechanical strength [13,14]. In the traditional preparation process of the cordierite monolith catalysts, γ-alumina coatings were coated on the honeycomb cordierite ceramics [15], in order to increase the specific surface area for meeting the requirements of the load of active components [16]. However, little work has been undertaken on the active component supported directly on the porous cordierite ceramics. Compared with conventional honeycomb cordierite ceramics, porous cordierite ceramics prepared by gel-casting method have high porosity (about 50~60%) that provide higher surface



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area, which might satisfy the requirements of supporting the active component directly. Hence, a new loading method is explored. In addition, it has been proved that catalyst particle size [17], shape/ morphology [18], and dispersion [19] are critical factors for improving catalytic activity and stability. On the basis of this, the efforts were focused on the particle size and shape of the obtained catalyst particles. In this work, the porous cordierite ceramics supported CuO-CeO2-ZrO2 catalysts were prepared via co-impregnation followed by chemical reduction method with assistance from ultrasonic, using porous cordierite ceramics as carrier, hydrazine hydrate as reductive agent, nitrate solution containing the active metals as precursor. The effect of the processing parameters including the condition of reduction process, calcination temperature, the addition of surfactants on the morphology and the catalytic performance of catalysts were mainly investigated. 2. Experimental 2.1. Materials Porous cordierite ceramics prepared by gel-casting methodq were cut into cuboid-shaped samples with the dimensions of 30 mm×10 mm×5 mm, which were used as catalyst carriers.

Corresponding author at: Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China. E-mail address: [email protected] (J. Zhang). Present/permanent address. Shandong University, No. 17923, Jingshi Road, Jinan, Shandong Province, P. R.China.

http://dx.doi.org/10.1016/j.ceramint.2017.01.118 Received 18 November 2016; Received in revised form 12 January 2017; Accepted 23 January 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Lu, S., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.01.118

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tration of sodium hydroxide solution were prepared to adjust the pH values. All the chemicals were of analytical grade, purchased from Aladdin Biological Technology Co., Ltd, Shanghai, China, and were used without further purification. 2.2. Catalyst preparation The cuboid-shaped samples were pretreated for 30 min with 10% (mass fraction) nitric acid solution assisting by ultrasonic, washed with deionized water until the pH of the samples was neutral (pH=7), and then dried at 100 °C for 2 h. At room temperature, 24.2 g Cu(NO3)2· 3H2O, 4.34 g Ce(NO3)3·6H2O and 4.29 g Zr(NO3)4·5H2O were dissolved into 100 mL distilled water under vigorous stirring. Subsequently, the pretreated samples was dipped in the above solution containing the active phase under ultrasound irradiation for 6 h, then dried at 100 °C for 2 h. Sodium hydroxide solution was gradually added into the diluted hydrazine hydrate solution until the pH value was 11– 12. The impregnated samples were put into the above solution in some condition. After reacting for 2 h, the samples were dried at 100 °C for 2 h and calcined in air flow at certain temperature for 3 h in a muffle furnace at a heating rate of 4 °C min−1 to obtain the desired CuO-CeO2ZrO2/cordierite catalysts.

Fig. 1. XRD patterns of supported catalysts prepared at different conditions.

Concentrated nitric acid (HNO 3 , 65~68%) was diluted into 10% (mass fraction) nitric acid solution with deionized water for the later use. Cu(NO 3 ) 2·3H 2O, Ce(NO 3) 3 ·6H 2O, Zr(NO 3) 4 ·5H 2 O (molar ratio=1:0.1:0.1) were used as active metal source. The concentration of hydrazine hydrate solution used as reductant is 10% (mass fraction). The mixture of cetyltrimethylammonium bromide (CTAB) and PEG-1500 was used as surfactant. A certain concen-

2.3. Catalyst characterization X-ray diffraction (XRD) analysis was performed on a Rigaku Dmax2500PC diffractometer to determine the crystalline phases. The average

Fig. 2. SEM micrographs of CuO-CeO2-ZrO2/cordierite catalysts with different reduction conditions: (a)heating in the thermostat water bath, (b)assisting by ultrasonic, (c)non-loaded.

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Fig. 3. EDS energy spectrum of the CuO-CeO2-ZrO2/cordierite catalysts.

sprayed with gold or carbon to increase the electrical conductivity. Nitrogen adsorption was used to characterize specific surface areas of samples with a Quantachrome NOVA 2200e instrument.

Table 1 The loading rate and specific surface area of catalysts prepared using different processing parameters. Sample

w/%

SBET/(m2·g−1)

H−600 U−600 S-H−600

2.92 3.00 2.03

17.58 23.75 20.50

2.4. Catalytic activity test for CO oxidation The catalytic activity measurements for CO oxidation were measured in a plug flow reactor using 80 mg of monolith catalyst in a gas mixture of 1%CO/20%O2/N2 (99.997% purity), at a flow rate of 67 mL min−1, corresponding to a space velocity of 80,000 mL h−1 gcat−1. Before the measurement, the catalysts were pretreated in synthetic air (20%O2/N2) at 300 °C for 1 h for activation. After that, the reactor was cooled down under a flow of pure N2 gas for 30 min. The catalytic tests were carried out in the reactant atmosphere by ramping the catalyst temperature to 300 °C. The outlet gas compositions of CO and CO2 were quantified online by non-dispersive infrared spectroscopy (Gasboard-3500, Wuhan Sifang Company, Wuhan, China). (CO conversion = CO2 output/(CO2 output + COoutput). 3. Results and discussion 3.1. Effect of the reduction condition The reduction condition has an important influence on final catalyst samples. The impregnated samples were put into the reducing agent solution heating by the thermostat water bath at 60 °C and assisting by ultrasonic, respectively. The calcination temperature was 600 °C. The obtained catalyst samples are denoted as H-600 and U-600, respectively. The XRD patterns of the obtained catalyst samples are shown in Fig. 1. It can be seen that the characteristic peaks of cordierite ceramics at 2θ=10.42°, 18.12°, 21.69°, 26.33°, 28.41°, 29.44° and 33.87° appeared for all catalyst samples. Compared with the XRD patterns of non-loaded cordierite, the characteristic peaks of CuO at approximately 2θ = 35.5° [7,8,18] have been detected in all supported catalysts, which indicates that CuO crystallites have been loaded on the cordierite substrates. In addition, the peak intensity of CuO increase slightly in the catalyst samples obtained by heating thermostatically in the reduction process. This illustrates that the smaller particles can be loaded on the cordierite substrates assisting by intense ultrasonic irradiation. Fig. 2 shows the SEM micrographs of CuO-CeO2-ZrO2/cordierite catalysts prepared by different reduction conditions and the image of cordierite substrate after the pretreatment. It is found that the

Fig. 4. Catalytic activity of CuO-CeO2-ZrO2/cordierite catalysts prepared at different conditions for CO oxidation. Table 2 Comparison of catalytic activity for CO oxidation of different catalysts. Sample

Catalysis starting Temperature (°C)

T50 (°C)

The highest CO conversion (%)

H−600 S-H−600 U−600 U−500 U−400 U−300

225 210 170 170 165 140

270 248 260 252 228 220

82.24 96.26 85.98 91.59 98.13 99.07

crystallite size was calculated from the peak width using the Scherrer's equation. SEM images were obtained using a JEOL JSM-6700F with an accelerating voltage of 10 kV. Before scanning, all samples were

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Fig. 5. SEM micrographs of CuO-CeO2-ZrO2/cordierite catalysts with different calcination temperatures: (a)300 °C, (b)400 °C, (c)500 °C, (d)600 °C.

area. Compared with the reduction process in the thermostat water bath, intense ultrasonic waves produced the acoustic cavitation, which provided enough energy for the formation of small particles by generating the environment of high temperature and high pressure [20]. Acoustic cavitation improved the generation rate of crystal nucleus and decreased the particles size. Besides, a large number of bubbles produced by acoustic cavitation [21] significantly reduced the specific surface free energy of the crystal nucleus, which controlled the aggregation and growth. Hence, the small catalyst particles were obtained. It caused the increase of specific surface area and the improvement of catalytic activity.

reduction condition has an important influence on the shape of catalyst particles. Irregular and fragmental shape catalyst particles were formed when the impregnated samples were reduced in the thermostat water bath. However, the catalyst particles obtained by the ultrasonic reduction were similar to spherical shape with average diameters of 400 nm, accumulated on the surface of substrate. In order to determine the surface composition, EDS energy spectrum was conducted, the results were shown in Fig. 3. As seen, the Mg, Al, Si, O which form the cordierite carrier accounted for the vast majority proportion. The Cu, Ce, Zr used as active components accounted for only a few proportion. Besides, Ce or Zr species characteristic peak [7] have not been detected in the XRD patterns due to the low content or containing within the lattice [10]. The loading rate and BET surface area (SBET) of different catalysts are listed in Table 1. It is very obvious that the introduction of ultrasonic in the reduction process increases the specific surface area (about 23.75%) and the loading rate (about 3.00%) of catalyst particles. Fig. 4 shows the catalytic activity of different catalysts for CO oxidation. Detailed data of catalysis starting temperature, T50 and the highest CO conversion for the catalysts which cannot reach complete conversion in the present condition is presented in Table 2. T50 is the reaction temperature for 50% CO conversion. It is found that the starting temperature and T50 of U-600 sample is lower than that of H-600 sample. The highest conversion rate of U-600 sample increases slightly comparing to the H-600 sample. It illustrates that the catalysts obtained by ultrasonic irradiation have better catalytic activities, which result from the smaller particles size and the larger specific surface

3.2. Effect of the calcination temperature The calcination temperature is also an important factor on the morphology of the catalysts. The samples were calcined at 300 °C, 400 °C, 500 °C, 600 °C, respectively, under the reduction condition of ultrasonic-assisted. The catalyst samples are named U-300, U-400, U-500, U-600, respectively. The X-ray diffraction patterns of catalysts calcined at different temperatures are shown in Fig. 1. The broadening of the peaks indicates the small crystallite size of the catalysts. It is very clear that the width of CuO characteristic peaks broadens with the reduction of calcination temperature, which indicates the formation of smaller CuO grains. Actually, the average diameter of CuO grains, which calcined at 300 °C, 400 °C, 500 °C, 600 °C, are estimated to be 14.4 nm, 18.7 nm, 21.3 nm and 24.6 nm, respectively, according to the Scherrer equation. This can be 4

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Fig. 6. SEM images of CuO-CeO2-ZrO2/cordierite catalysts prepared by adding surfactant.

3.3. Effect of the addition of surfactant

attributed to a better crystal shape integrity as the increasing of calcination temperature. When the calcination temperature is 300 °C, the wide peak shape and weak peak strength of the CuO diffraction peak indicate the tiny CuO grains and incomplete crystal growth. With the increasing of calcination temperature, the peak shape of the CuO diffraction peak become narrow, the peak strength become strong. It illustrates that crystallization gradually tend to be intact and the grain size is larger. The SEM micrographs of CuO-CeO2-ZrO2 catalysts with different calcination temperatures are shown in Fig. 5. During calcination, high temperature caused the severe aggregation due to high surface energy of nanoparticles, the migration and growth occurred. When calcination temperature was 300 °C, the shape of catalysts was resembling a flat, circular plate with burr (Fig. 5(a)). Bean-shaped catalysts particles with diameters of 200–900 nm were formed at calcination temperature of 400 °C (Fig. 5(b)). When the samples were calcined at 500 °C, the larger bean-shaped particles were prepared (Fig. 5(c)). Interestingly, the size of particles decreased sharply when the samples were calcined at 600 °C. Well-separated and spherical-like particles with diameters of 150 nm can be seen (Fig. 5(d)). There is not a regular trend on the morphology of catalyst particles. The migration and aggregation of nanoparticles may be connected with the interaction between the cordierite substrate and catalyst particles. The results of catalytic activity test show that calcination temperature affect the catalytic activity of as-prepared catalysts greatly. As shown in Fig. 4 and Table 2, when calcination temperature decreases, catalysis starting temperature and T50 presents a tendency of decreasing, CO conversion rate has improved. The highest conversion rate on U-300 catalysts can reach 99.07%.

The addition of surfactant could influence the formation of catalyst particles. The surfactant was added into the impregnation liquid. The impregnated samples were reduced in the thermostat water bath, calcined at 600 °C, under otherwise identical conditions. The obtained samples are named S-H-600. As shown in Fig. 6(a)(c), it can be observed that spherical particles with holes, whose average size was in the range of 300–800 nm, were obtained when the surfactant were added into the impregnation liquid. It is found that the larger particles with rugged surface has unconsolidated porous structure. Fig. 6(b) is the figure of a partial enlarged image of Fig. 6(a). It shows that, in addition to these larger particles, tiny flower-like catalyst nanoparticles with diameters of 50 nm has been loaded on the cordierite framework. Fig. 6(d) is the figure of a partial enlarged image of Fig. 6(c). It is clear that a large spherical particle has been assembled by small nanoparticles with the average diameter of 50 nm, and there is no obvious grain boundary. BET analysis shows that the addition of surfactant increases the BET surface area (about 20.50%), whereas the loading rate decreases (about 3.00%), compared with the H-600 catalyst (Table 1). It can also be seen from Fig. 4 and Table 2 that, catalysis starting temperature and T50 of S-H-600 catalyst decrease, the highest CO conversion can reach 96.26%. Clearly, the addition of surfactant can significantly enhance the catalytic activity for CO oxidation. This can be attributed to the modification effects of surfactant on the surface of particles. The generated nanoparticles in the initial stage of reaction were easy to cause the severe aggregation due to the high 5

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surface energy, resulting in a larger size. The addition of surfactant made small nanoparticles highly-dispersed. One possible explanation for this is the fact that surfactant accumulated on the surface of crystallites, reducing the interfacial energy and changing the surface properties of solid particles [22], made the particles dispersed homogeneously. Meanwhile, surfactants were favorable for the increasing of electrostatic repulsion between the nanoparticles, hindering the aggregation of nanoparticles. After fine grains loading on the surface of a layer, excessive grains assembled into a large spherical-shape particle to decrease the Gibbs free energy of system [23]. Thus, the tiny flowerlike nanoparticles and spherical particles with holes were obtained. The rugged surface of large particles and tiny catalyst nanoparticles provided abundant active sites for the CO oxidation reaction, which is responsible for the higher catalytic activity. 4. Conclusions In summary, CuO-CeO2-ZrO2/cordierite monolith catalysts have been successfully prepared by the impregnation reduction method. The size and shape of catalyst particles were significantly influenced by the processing parameters. Fragmental-shaped, bean-shaped, sphericalshaped and flower-like catalyst particles were obtained under different conditions. The grain size of catalysts has reduced and the catalytic activity for CO oxidation has been improved with the calcination temperature decreasing. The highest conversion rate can reach 99.07% when the catalysts are calcined at 300 °C. The introduction of ultrasonic and surfactant result in larger specific surface area and better catalytic activity for CO oxidation. Acknowledgements The work described in this paper is supported by The Major Program of the National Natural Science Foundation of China, China (No.50942024) and Project of Science and Technology Development Plans of Shandong Province (No. 2012GGE27018). References [1] S.C. Kim, The catalytic oxidation of aromatic hydrocarbons over supported metal oxide, J. Hazard Mater. 91 (2002) 285–299. [2] C.H. Wang, S.S. Lin, C.L. Chen, H.S. Weng, Performance of the supported copper oxide catalysts for the catalytic incineration of aromatic hydrocarbons, Chemosphere 64 (2006) 503–509. [3] Y.Q. Zuo, L.N. Han, W.R. Bao, L.P. Chang, J.C. Wang, Effect of CuSAPO-34 catalyst preparation method on NOx removal from diesel vehicle exhausts, Chin. J. Catal. 34 (2013) 1112–1122.

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