Mesoscale Organization of Nearly Monodisperse Flowerlike Ceria

Guijun Zhang, Zhurui Shen, Mi Liu, Chenghua Guo, Pingchuan Sun, Zhongyong Yuan, ..... Yanyan Liu , Yongfu Tang , Zhaohui Ma , Manish Singh , Yunjuan H...
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J. Phys. Chem. B 2006, 110, 13445-13452

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Mesoscale Organization of Nearly Monodisperse Flowerlike Ceria Microspheres Chunwen Sun,† Jie Sun,‡,§ Guoliang Xiao,† Huairuo Zhang,† Xinping Qiu,‡ Hong Li,*,† and Liquan Chen† Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China, and Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed: April 7, 2006; In Final Form: May 16, 2006

Nearly monodisperse flowerlike CeO2 microspheres were synthesized via a simultaneous polymerizationprecipitate reaction, metamorphic reconstruction, and mineralization under hydrothermal condition as well as subsequent calcination. The obtained CeO2 microsphere consists of 20-30 nm thick nanosheets as petals. It has an open three-dimensional (3D) porous and hollow structure and possesses high surface area, large pore volume, and marked hydrothermal stability. It can be doped easily after synthesis, and the initial 3D texture is maintained. The controlling factors and a possible formation mechanism are discussed in detail. This novel material can be used as a support for catalysts with various purposes. With CuO loaded on flowerlike CeO2, the catalytic activities and hydrothermal stability of Cu/CeO2 for ethanol stream reforming were examined. At 300 °C, the H2 selectivity reached a maximum value of 74.9 mol %, while CO was not detected within the precision of the gas chromatogram. It produced a hydrogen-rich gas mixture in the wide temperature range (300-500 °C) and showed excellent hydrothermal stability at high temperature (550 °C), which is a good choice for ethanol processors for hydrogen fuel cell applications.

1. Introduction Hierarchical self-assemblies of nanoscale building blocks with specific morphology are of great interest for scientists due to their novel physical and chemical properties.1,2 In recent years, many methods have been used to prepare complex threedimensional (3D) structures, including the thermal reduction process,2a thermal oxidation process,2b oriented aggregation,2c self-assembly of building blocks through hydrophobic interactions,2d and template-assisted synthesis.2e However, it still remains a challenge to achieve controlled organization from nanoscale units. Ceria (CeO2) has earned intensive interest in the past decade because it plays a vital role in emerging technologies for environmental and energy-related applications. It is widely used as a promotor in three-way catalysts (TWC) for the elimination of toxic auto-exhaust gases,3 low-temperature water-gas shift reaction,4 fuel cells,5 oxygen sensors,6 and oxygen permeation membrane systems.7 Nanaocrystalline CeO2 has showed improved and size-dependent properties.8 In particular, nanocrystalline CeO2 with high surface area and open mesoporous structure is desired for above applications in view of their potential kinetic advantages. Actually, mesoporous ceria has shown great potential as versatile catalysts and catalyst supports due to its high surface area and increased dispersion of active secondary components.9 However, a major problem is its poor thermal stability, which is often caused by structure collapse during surfactant removal at elevated temperature.10 Despite recent advances in synthesizing mesoporous ceria particles with improved thermal stability,11 to the best of our knowledge, there * Corresponding author. E-mail address: [email protected]. Tel: +8610-82649047, +86-62556598. † Institute of Physics, Chinese Academy of Sciences. ‡ Tsinghua University. § Present address: Institute of Chemical Defense of P.L.A., Beijing 102205, China.

has been no report on hollow spherical CeO2-based mesoporous materials with controlled morphologies and high hydrothermal stability. Herein, we report a novel hydrothermal approach to synthesize flowerlike mesoporous CeO2 microspheres consisting of nanosheets with an open 3D porous microstructure. A new in situ template formation strategy is adopted, using a graft copolymerization reaction between acrylamide and glucose initiated by ceric ions. By a controlled calcination procedure, the obtained CeO2 microspheres possess flowerlike morphology, open porous structure, large pore volumes, and high surface areas. When used in steam reforming of ethanol for producing H2, these CeO2 microspheres show excellent catalytic properties and marked hydrothermal stability. 2. Experimental Section 2.1. Synthesis of Flowerlike CeOHCO3 Microspheres. All chemicals were purchased from Beijing Chemical Reagents Company and used without further purification. In a typical experiment, glucose (0.01 mol) was dissolved into deionized water (80 mL) with magnetic stirring, which was followed by addition of acrylamide (0.015 mol) and hydrated cerium(III) nitrate (0.005 mol) to form a transparent solution. After that, ammonia solution (3.2 mL, 25 wt %) was added to the solution dropwise with stirring. Upon the addition of ammonia solution, the solution became stiff gel. The color of the gelatinous mixture finally became deep brown with continuous stirring. The pH value of the resultant gelatinous mixture is about 10. This solution mixture was stirred for 5 h before being transferred into a Teflon-lined autoclave (100 mL capacity). Then the autoclave was sealed and kept at 180 °C for 72 h in an electric oven. After that, the autoclave was cooled to room temperature naturally. The orange suspension and precipitate were separated by centrifugation, and the suspension was washed with water

10.1021/jp062179r CCC: $33.50 © 2006 American Chemical Society Published on Web 06/16/2006

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Figure 1. XRD patterns of (a) the products synthesized at 180 °C for 72 h (mainly hexagonal CeOHCO3 (JCPDS file no. 32-0189), 4 orthorhombic CeOHCO3 (JCPDS file no.41-0013)) and (b) the calcined products (CeO2, JCPDS file no. 34-0394).

and alcohol three times and then dried at 80 °C for more than 10 h. The flowerlike CeOHCO3 microspheres were finally obtained. 2.2. Synthesis of Flowerlike CeO2 Microspheres. The flowerlike CeO2 microspheres were obtained from the asprepared CeOHCO3 microspheres via a two-step calcination procedure. First, the as-prepared products were calcined under Ar with a flow rate of 10 mL min-1 in a tube furnace at 600 °C for 6 h. Then, the obtained products were calcined in air in a tube furnace at 400 °C for 4 h. 2.3. Catalyst Preparation. Ceria-supported CuO catalyst was prepared by impregnating 1.5 g of CeO2 microsphere powder with an aqueous solution containing the requisite amount copper nitrate. The suspension was stirred for 2 h at room temperature to guarantee the solution soaking sufficiently into the CeO2 powder. Then it was stirred on a hot plate with magnetic stirring at 90 °C until the water was completely evaporated. Subsequently, it was dried at 100 °C for 4 h and then calcined at 450 °C in air for 3 h. If the concentration of copper salt is higher, the obtained CuO particles tend to congregate during drying and calcination. Therefore, for the CuO particles uniformly dispersed, the desired load amount of CuO was achieved separately by three times; the above processes were repeatedly done every time. The total amount of CuO loaded was about 15 wt %. The powder catalysts were pelletized, crushed, and sieved to 20-40 mesh. Before the reforming reactions, the catalysts were reduced in situ in hydrogen at 400 °C for 2 h. So the resultant active catalyst is CeO2-Cu. 2.4. Characterization of Materials. The phases and purity of the products were examined by X-ray powder diffraction (XRD) performed on Rigaku X-ray diffractometer with Cu KR radiation (Japan, Rigaku, D/max-RB). The morphology of the products was observed by a scanning electron microscope (SEM, XL30s-FEG, 10 kV) and a transmission electron microscope (TEM, JEOL JEM-2010). The nitrogen adsorption and desorption isotherms at 77 K were measured using a Quantachrome Instruments NOVA4000. Fourier transformed infrared (FTIR) spectra were recorded on a FTS-60V spectrometer between 4000 and 400 cm-1. 2.5. Characterization of Catalysts for Ethanol Steam Reforming. The catalyst’s activity and selectivity of steam reforming of ethanol were conducted with a fixed-bed reactor with inner diameter of 19 mm fitted in a programmable oven with three heat sections, with operating ranges of up to 300, 700, and 1000 °C, respectively. The mole ratio of H2O to ethanol is 3:1. The flow rate of ethanol-water was controlled at 0.05 mL min-1. The components of the effluent passed through a condenser and drying agent and were then introduced to a gas

chromatogram (GC) with a sensitivity of 0.01%. The GC was equipped with two packed columns (Porapak, 80-100 mesh, 1.5 m long; TDX-01, 60-80 mesh, 1.5 m long), and one thermal conductivity detector (TCD). Three runs were done in order to obtain a consistent result for ethanol peak area at each temperature point. 2.6. TPR Measurement. Temperature-programmed reduction (TPR) measurements were carried out with a TCD of a gas chromatogram. Typically 40 mg of catalyst was loaded in a quartz tube reactor, and the reactor was heated from room temperature to 700 °C at a rate of 10 °C‚min-1 in a 5% H2-N2 mixture flowing at 40 mL‚min-1. 3. Results and Discussion 3.1. Physical Characterization of Flowerlike CeOHCO3 and CeO2 Microspheres. The phase purity and crystal structure of the products obtained were examined by XRD pattern. Figure 1a shows that the as-prepared product after hydrothermal reaction is a mixture of hexagonal (JCPDS file no. 32-0189) and orthorhombic (JCPDS file no. 41-0013) CeOHCO3, mainly consisting of hexagonal CeOHCO3. The grain size of CeOHCO3 is about 6.7 nm estimated from the Scherrer equation. The pattern of calcined product can be indexed as a facecentered cubic pure phase (space group: Fm3m(225)) of ceria (JCPDS no.34-0394), as shown in Figure 1b. The grain size of the CeO2 product is about 6.2 nm. The size and morphology of the products were examined by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements. Figures 2a and 3a show most of CeOHCO3 and CeO2 particles (>98%) are nearly monodisperse spherical particles with flowerlike texture, respectively. The average diameter of the microspheres is 1-3 µm, and no obvious shrinkage occurred after being calcined at 600 °C (Figures 2b and 3b). A crushed CeOHCO3 microsphere (Figure 2d) demonstrated that the product is a radially grown structure. From parts c and d of Figure 3, it can be seen that these flowerlike microspheres are composed of many nanosheets as the petals with an average thickness of about 20-30 nm and these nanosheets interweave together forming an open porous structure. Figures 4a and 5a show the low-magnification TEM images of CeOHCO3 and CeO2 microspheres, respectively. It can be seen that the microspheres are nearly monodisperse. The microsphere displays an obvious contrast between the dark edge and the light center (Figure 4b). This confirms that the microsphere has a hollow structure. A representative TEM image of an average-sized CeO2 microsphere is shown in Figure 5b.

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Figure 2. Representative SEM images of the CeOHCO3 microspheres: (a, b) overall morphology of the products; (c) high-magnification SEM image of an individual microspheres; (d) a crushed microsphere. Figure 5. TEM images of CeO2 microspheres: (a) overall product morphology; (b) detailed view on an average-sized microsphere; (c) a detailed view on an individual microsphere and SAED patterns (inset) of the CeO2 microsphere; (d) HRTEM image of a typical microsphere taken from the area marked in (b).

Figure 3. Representative SEM images of the CeO2 microspheres: (a, b) overall morphology of the products; (c, d) high-magnification SEM image of an individual microspheres, revealing the constituent details of the microspheres.

Figure 4. TEM images of CeOHCO3 microspheres: (a) overall product morphology; (b) detailed views on an individual microsphere.

The selected area electron diffraction pattern (SEAD, inset in Figure 5c) further indicates that the structure of the microsphere is a face-centered cubic CeO2. Figure 5d is a high-resolution TEM (HRTEM) image of the marked area in Figure 5b. The interplanar spacings of the ordered stripes marked in Figure 4b

are about 0.31 and 0.27 nm, which are consistent with the (111) and (200) lattice planes of CeO2, respectively. The HRTEM image indicates clearly that the nanosheet is composed of many tiny grains at different orientations (average grain size of ∼6 nm, Figure 5d). Figure 6 show the nitrogen adsorption-desorption isotherms and the corresponding BJH (Barret-Joyner-Halenda) pore size distribution curves of the obtained CeOHCO3 and CeO2 microspheres. Both showed a type IV adsorption-desorption isotherm with H3-type hysteresis (Figure 6a,b), a feature of mesoporous material. The measured Brunauer-Emmett-Teller (BET) areas for CeOHCO3 and CeO2 microspheres are about 75.7 and 92.2 m2 g-1, respectively. The average pore diameters of CeOHCO3 and CeO2 microspheres are 8.38 and 6.99 nm, respectively, calculated from the desorption branch of the nitrogen isotherm with the BJH method. The corresponding BJH desorption cumulative volumes are 0.16 and 0.17 cm3 g-1, respectively. 3.2. Controlling Factors on the Synthesis of Flowerlike CeOHCO3 and CeO2 Microspheres. In this synthesis, it is found that the presence of both acrylamide and glucose plays a crucial role in the formation of flowerlike microspheres. When the synthesis was performed using sucrose or starch instead of glucose, no flowerlike products were observed and only bulk particles were observed. If acrylamide was absent, under the same reaction conditions, aggregated spherelike products were observed without flowerlike textures. The XRD pattern in Figure 7a shows the obtained products consist of hexagonal CeOHCO3. If glucose was absent, even under the same reaction conditions, only bulk particles were observed instead of flowerlike microspheres. The obtained products consist of face-centered cubic CeO2 (Figure 7b). The above results imply that the formation of CeOHCO3 is related to glucose. Chemical reactions of glucose are very complex under hydrothermal conditions.12 It was reported that the major products from hot alkaline degradation of glucose are lactic acid, formate, glycolic acid, acetate, and carbonate.13 In our experiment, the result of chemical

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Figure 6. Nitrogen adsorption-desorption isotherms (a and b) and the corresponding BJH pore size distribution (c and d) for the samples: (a, c) CeOHCO3 microspheres; (b, d) CeO2 microspheres.

Figure 7. XRD patterns of (a) the products synthesized without acrylamide (mainly hexagonal CeOHCO3 (JCPDS file no. 32-0189)) and (b) the products synthesized without glucose (face-centered cubic CeO2, JCPDS file no. 34-0394).

analysis confirms that there are CO32- ions in the final liquid products obtained by centrifugation and the concentration is 0.022 mol‚L-1. This result also explains that CO32- ions originate from glucose. It is also noted that flowerlike microspheres cannot be formed without addition of ammonia solution. If other basic sources such as NaOH or KOH were used, flowerlike microspheres were also observed. Therefore, the basic environment of the reaction system is favorable for the formation of hierarchical architectures. It was found that the atmosphere of calcination can influence the morphology of the resultant CeO2 products. If calcined directly in air at higher temperature, the flowerlike microspheres can partially collapse (Figure S1), because the CeOHCO3

decomposes rapidly under air. All the figures and tables containing S are in the Supporting Information. By using twostep calcination procedures as described in the Experimental Section, we obtain pure phase CeO2 microspheres with perfect flowerlike morphology. 3.3. Formation Mechanism of Flowerlike CeOHCO3 and CeO2 Microspheres. Under basic conditions, the cerium salts precipitate to form gelatinous hydrous cerium hydroxide. The pH of the isoelectric point for hydrous cerium hydroxide is in the range 6.75-8, depending on the environment (concentration, presence of different ions).14 Under the highly basic conditions (pH ≈ 9-10) in our experiments, the hydrous cerium hydroxide species carried net negative charges on their surface.15 It was reported that acrylamide graft copolymerized onto poly(3-O-methacrylogyl D-glucose) (PMG) as a backbone can form highly branched copolymers by the ceric ion initiation.16 We analyzed the components of the liquid products of hydrothermal reaction by gas chromatogram-mass spectrometry (GC-MS), just after the reaction was finished. In this system, lots of highly branched copolymers containing -NH2 groups were formed via acrylamide graft copolymerization onto glucose, which were evidenced by GC-MS results (Figure S2 and Table S1). These copolymer molecules were hydrolyzed and carried positive charges. They can interact with hydrous cerium hydroxide species via electrostatic Coulomb force or hydrogen bonding and thus act as templates for subsequent selfassembly processes. Under hydrothermal conditions, the graft copolymers could undergo decomposition in alkaline solution.13,17 The resultant liquid products mainly consist of C6H8N2, C7H10N2, C7H9N, C7H14O2, C8H12N2, C9H14N2, C9H12N2, as shown in Figure S2 and Table S1. To investigate the growth process of the flowerlike microspheres, a series of reaction-time experiments were carried out.

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Figure 8. The phase transformation of XRD patterns of the as-prepared products synthesized at different reaction times (0-72 h) at 180 °C. The peaks indexed are assigned to hexagonal CeOHCO3 (JCPDS file no. 32-0189), and the peaks marked with ∆ are assigned to orthorhombic CeOHCO3 (JCPDS file no.41-0013).

Figure 9. FTIR spectra of the as-prepared products synthesized at different reaction times (0-72 h) at 180 °C: (a) 0 h; (b) 3 h; (c) 6 h; (d) 9 h; (e) 12 h; (f) 18 h; (g) 24 h; (h) 48 h; (i) 72 h.

Figure 8 shows XRD patterns of the products synthesized at different reaction times (0-72 h) at 180 °C. It can be seen clearly that the CeOHCO3 phase is formed mainly after 12 h. For further insight into the change of the chemical composition of colloid particles, the FTIR spectra of the as-prepared products synthesized at different reaction times at 180 °C were also recorded (Figure 9) and assigned (Table S2). The bands of hydroxyl functional groups, CdO stretch of carbonate, CO32stretch, and Ce-O stretching appear when reaction time is beyond 12 h. These spectral variations indicate further that the colloid particles gradually convert into CeOHCO3 phase, especially after 12 h. The products synthesized at different reaction times at 180 °C were also investigated by SEM. Figure 10 shows the SEM images of the evolution of the flowerlike CeOHCO3 microspheres with time. Before hydrothermal treatment, we examined the solid products obtained by centrifugation, which showed loosely aggregated bulk particles (Figure 10a). In the early stage of hydrothermal treatment (less than 6 h), dense and large particles formed gradually (Figure 10b). However, as shown in parts c and d of Figure 10, subsequent partial dissolution should occur and the particles evolved into networks due to some side reactions (such as the degradation decomposition of the copolymers) under hydrothermal conditions (less than 9 h). Simultaneously, the partially dissolved particles may selfassemble further into hierarchically flowerlike microspheres

Figure 10. Morphology evolution of the flowerlike CeOHCO3 microspheres with reaction time. SEM images of the products obtained at 180 °C after (a) 0 h, (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, (f) 18 h, and (g) 24 h.

(more than 12 h), shown in Figure 10e. The driving force for the colloid particle reorganization corresponds to a reduction in surface free energy. Thus the overall shape of the resultant products, though variable, is typically spherical. With a longer reaction time, the flowerlike morphology gradually became perfect (Figure 10f,g). According to above experimental results, one possible formation mechanism of the flowerlike CeO2 microspheres is proposed as shown schematically in Figure 11. The major steps involved in the formation process of the flowerlike CeO2 microspheres mainly consist of polymerization-precipitation reaction, metamorphic reconstruction, mineralization, and controlled calcinations. The perfect preservation of the shape of individual nanosheets as well as the morphology of an entire 3D flowerlike microsphere upon calcinations suggest that this novel synthesis strategy is a promising route for fabricating other 3D porous flowerlike nanostructures. In fact, flowerlike La2O3 and several doped CeO2 microspheres have been prepared, shown in Figure 12; the details will be published elsewhere. 3.4. Catalytic Properties of Flowerlike CeO2-Cu Microsphere Catalysts. The use of biomass-derived ethanol for the production of hydrogen has significant interest for clean energy supply and environmental protection.18 The catalytic activities of flowerlike CeO2-Cu microspheres and flowelike CeO2 have been investigated for the reactions of steam reforming of ethanol for producing H2. Catalyst activity is evaluated in terms of ethanol conversion and the selectivity of products. The catalyst

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Figure 11. Schematic illustration of the evolution of flowerlike CeO2 microspheres.

Figure 12. Representative SEM images of the flowerlike microspheres: (a) La2O3; (b) Ce0.9Mn0.1O2; (c) Ce0.9Gd0.1O1.95; (d) Ce0.9Eu0.1O1.95.

“selectivity” of products was defined as the mole fraction of each product.19 Figure 13a shows the typical experimental results of flowerlike CeO2-Cu catalyst, in which the selectivity of each product and the conversion of ethanol are shown as a function of reaction temperature in the temperature range of 200-650 °C. The conversion of ethanol reached 100% in the range of 300-650 °C. It produces a hydrogen-rich gas mixture in a wide temperature range (300-500 °C). No liquid products were detected, such as acetone or acetaldehyde. At 300 °C, typical steam reforming of ethanol and water-gas shift (WGS) reactions occurred obviously, the H2 selectivity reached maximum value of 74.9 mol %, while CO was not detected within the precision of GC. Compared with our previously reported data using Ni/Y2O3, Ni/La2O3, and Ni/Al2O3 as catalysts,19 the catalytic properties are obviously improved in view of the H2

selectivity and the selectivity for undesired byproducts, such as CO. In addition, our results are better than those recently reported by other groups using Ni-Rh-CeO2 catalysts and Rh/ CeO2-ZrO2 catalysts.20 For comparison, the catalytic activity of flowerlike CeO2 was also tested (Figure 13b), which is not as good as that of flowerlike CeO2-Cu catalyst. Below 450 °C, the conversion ratio of ethanol is lower. However, the CO selectivity remained below 4 mol % in the whole tested temperature range. At 450 °C, the H2 selectivity reached a maximum value of 70.1 mol %, the conversion of ethanol reached 88.3%, and the CO selectivity is 0.451 mol %. Therefore, it can be concluded that the flowerlike CeO2 microsphere is a good support for catalysts used in steam reforming reactions of ethanol. To explain the high catalytic activity of the flowerlike CeO2Cu compared to that of flowerlike CeO2 catalysts, the redox behaviors of the flowerlike CeO2 and CuO loaded CeO2 microspheres have been studied by temperature-programmed reduction (TPR) (Figure 14). For the flowerlike CeO2 microsphere support, the reduction proceeds in two steps (Figure 14a). The first reduction peak started at 350 °C and showed a maximum at about 492 °C. This is mainly attributed to the reduction of surface-capping oxygen of ceria.21a The second peak increased gradually above 600 °C, which is associated with further reduction of the bulk material. 21b,21c It should be pointed out that the second peak did not completely occur due to the upper limit temperature of 700 °C. It can be seen that the loading of CuO onto flowerlike CeO2 support modifies its TPR profile. For the CuO loaded catalyst, two new low-temperature reduction peaks centered at about 202 and 230 °C occurred (Figure 14b). It could be interpreted as a stepwise reduction of the surface-dispersed CuO,22 corresponding to the reduction from Cu2+ to Cu+ (202 °C) and from Cu+ to Cu0 (230 °C), respectively. In addition, compared with that

Figure 13. Effect of reaction temperature (200-650 °C) on conversion of ethanol (CEtOH) and on selectivity of hydrogen (SH2), carbon monoxide (SCO), carbon dioxide (SCO2), and methane (SCH4), obtained over (a) the flowerlike CeO2-Cu catalyst and (b) flowerlike CeO2 samples. Experimental conditions: mass of the catalyst, 1.37 g; particle size, 0.45-0.9 mm; H2O/EtOH mol ratio, 3:1; liquid flow rate, 0.05 cm3 min-1; P ) 1 atm.

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Figure 14. Hydrogen consumption profiles during temperatureprogrammed reduction of CeO2 samples: (a) flowerlike CeO2 microsphere; (b) flowerlike CeO2 microsphere loaded with CuO (10 wt %).

Figure 16. Conversion of ethanol and selectivity of products as function of time-on-stream, obtained over the flowerlike CeO2-Cu catalyst sample reduced by hydrogen for 2 h at 400 °C. Experimental conditions: T ) 550 °C, others are the same as those given in Figure 13.

of the selectivity of each product and conversion of ethanol with time-on-stream. It can be seen that the catalyst keeps excellent stability for steam reforming of ethanol to hydrogen production at high temperature. 4. Conclusions

Figure 15. Representative SEM images of the flowerlike CeO2 loaded with CuO.

of the flowerlike CeO2 microspheres support, the TPR profiles indicating the characteristic peaks of CeO2 corresponding to the surface reduction process shift to lower temperature after CuO addition, which located at 253 °C. These significant shifts in the surface ceria reduction after CuO addition can be related to the presence of hydrogen spillover processes from the metal Cu surface.23 Simultaneously, the reduced Cu is beneficial to electron transfer from CeO2, which facilitates the H2 consumption. Here, we briefly address three reasons to explain the enhanced catalytic activity of the flowerlike CeO2-Cu catalyst for ethanol steam reforming. First, this flowerlike CeO2 microsphere is beneficial for gas transport due to its open 3D porous structure. Second, compared with other oxide supports (such as Al2O3 or La2O3), redox support CeO2 is known to play an important role in water-gas shift reaction (WGS) reactions to convert CO into CO2.24 It is believed that the excellent catalytic activity of CeO2based materials stemmed from the reversible CeO2-Ce2O3 transition in CeO2 support associated with oxygen-vacancy formation and migration.25 In addition, in this system, the CO2 or H2O is a potential oxidant able to fill in oxygen vacancies of ceria via reactions 1 and 2.24a Third, the catalysts containing Cu favor the dehydrogenation reaction.24b,26 In our case, CuO nanoparticles are well-dispersed in flowerlike CeO2 support (Figure 15), which is favorable for related reactions in view of kinetics.

H2(g) + 2Ce4+ + O2- f H2O(g) + 2Ce3+ + VO¨

(1)

CO(g) + 2Ce4+ + O2- f CO2(g) + 2Ce3+ + VO¨ (2) In eqs 1 and 2, VO.. denotes oxygen vacancies. To examine the high-temperature hydrothermal stability of this novel structure catalyst, we chose 550 °C as the temperature for a long-term test, although the selectivity of H2 is not the highest at this temperature. Figure 16 shows the relationships

In summary, we have developed a novel hydrothermal method to synthesize nearly monodisperse flowerlike CeO2 microspheres. It has open 3D hollow porous structure and possesses high surface area, large pore volume, as well as marked hydrothermal stability. As a catalyst for ethanol steam reforming, the flowerlike CeO2-Cu microspheres show excellent catalytic properties and marked hydrothermal stability. At 300 °C, it showed the highest selectivity to H2. It is a good choice for ethanol processors for hydrogen production. Furthermore, the synthetic strategy can be generalized to other oxides for the formation of hollow and porous structures. This novel method can be extended to obtain various monodisperse porous materials for use in fields such as catalysis, energy storage, and conversation. Acknowledgment. The authors are grateful for the financial support from National Key Basic Research Program (2002CB211802). The authors acknowledge Dr. Run Xu and Professor Yadong Li in Tsinghua University for help in TPR measurements. Supporting Information Available: SEM images of the CeO2 microspheres obtained by direct calcined in air at 600 °C, results of GC-LS analysis of liquid product, assignment of the bands of FTIR spectra of the as-prepared products synthesized at different reaction times at 180 °C, and supplemental experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Cao, M. H.; Liu, T. F.; Gao, S.; Sun, G. B.; Wu, X. L.; Hu, C. W.; Wang, Z. L. Angew. Chem., Int. Ed. 2005, 44, 4197-4201. (b) Cao, A. M.; Hu, J. S.; Liang, H. P.; Wan, L. J. Angew. Chem., Int. Ed. 2005, 44, 4391-4395. (2) (a) Li, Y. B.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2003, 82, 1962-1964. (b) Chen, A.; Peng, X.; Koczkur, K.; Miller, B. Chem. Commun. 2004, 1964-1965. (c) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124-8125. (d) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348-351. (e) Zhu, L.; Xie, Y.; Zheng, X.; Liu, X.; Zhou, G. J. J. Cryst. Growth 2004, 260, 494-499. (3) Trovarelli, A. Catal. ReV. Sci. Eng. 1996, 38, 439-520. (4) Carrettin, S.; Concepcion, P.; Corma, A.; Nieto, J. M. L.; Puntes, V. F. Angew. Chem., Int. Ed. 2004, 43, 2538-2540.

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