Dimension-Manipulated Ceria Nanostructures - American Chemical

20366

J. Phys. Chem. C 2008, 112, 20366–20374

Dimension-Manipulated Ceria Nanostructures (0D Uniform Nanocrystals, 2D Polycrystalline Assembly, and 3D Mesoporous Framework) from Cerium Octylate Precursor in Solution Phases and Their CO Oxidation Activities Huan-Ping Zhou,† Ya-Wen Zhang,*,† Rui Si,† Le-Sheng Zhang,‡ Wei-Guo Song,‡ and Chun-Hua Yan*,† Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, and PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China, and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: August 8, 2008; ReVised Manuscript ReceiVed: September 26, 2008

Dimension-tunable ceria nanostructures including 0D uniform nanocrystals, 2D polycrystalline assembly, and 3D mesoporous framework were selectively synthesized from the same cerium precursor of Ce(OC8H17)4 with the designed solution methods. The thermolysis of Ce(OC8H17)4 in tri-n-octylphosphine oxide and dioctyl ether at 250 °C yielded uniform ceria nanopolyhedra and elongated nanocrystals (around 2.2 nm in size). Through the polyol alcoholysis in ethylene glycol at 195 °C, polycrystalline assembled nanostructures (PANs) composed of small ceria crystallites (around 3-4 nm) were achieved, showing adjustable cross-linkages in the 2D dendritic structure. The raise of Ce(OC8H17)4 concentration and reaction time would increase the cross-linkages of the PANs. By a conventional sol-gel route, mesoporous ceria gels in an ordered 2D hexagonal mesostructure were synthesized at 40 °C through controlling the hydrolysis rate, and the condensation and aggregation of the metal precursor, as well as the critical micelle concentration (cmc) of the block-copolymer surfactants as used (e.g., EO20-PO70-EO20, P123). Catalytic measurements indicated that three CeO2 catalysts prepared from the above different nanostructures were active for CO oxidation at temperatures beyond 250 °C and showed T50 (50% conversion of CO) at 310 °C for the PAN catalysts, 340 °C for the nanopolyhedra catalyst, and 390 °C for the mesoporous catalyst. 1. Introduction In the latest decade, dimension-tunable nanoarchitectures have attracted extensive interest because of their structure-dependent properties and underdeveloped technological applications.1-6 The manipulation of nanomaterials with a particular structure possessing exceptional material properties, via conveniently tuning the synthetic parameters, is one of the challenging issues in multidisciplinary areas including chemistry, physics, and materials science. Many chemists are now exploring efficient ways to obtain such dimension-manipulated nanostructures through ingenious design and precise control. As an important functional rare earth oxide, ceria has drawn considerable attention due to its unique applications in conversion catalysts, three-way catalysts (TWCs), fuel cells, solar cells, gates for metal-oxide semiconductor devices, and phosphors.7-16 For instance, CeO2 can be used as three-way catalyst (TWCs) owing to the relative ease with which it can be reduced (formation of oxygen vacancies) and reoxidized by the emission of the toxic pollutants (CO, NOx, and hydrocarbons, etc.) from automobile exhaust.7-10 CeO2 doped with other rare earth ions has high oxide ion conductivity at a comparable low temperature (about 600 °C) and thus has been applied in solid oxide fuel cells.11-13 CeO2 also has strong absorption in the ultraviolet range and is hence used as ultraviolet (UV) blocking materials.13-15 Furthermore, CeO2 nanoparticles are one kind of the most * Corresponding authors. Fax: +86-10-6275-4179. E-mail: [email protected]; [email protected]. † Peking University. ‡ Institute of Chemistry.

significant abrasive materials for chemical-mechanical planarization of advanced integrated circuits.17 In these studies, CeO2 nanoparticles have been markedly adverted not only with their tunable size and shape but also with respect to their controllable self-organization structures. Over the past years, some methods such as sol-gel process, hydrothermal or solvothermal synthesis, forced hydrolysis, microemulsion, precipitation, etc., have been developed to produce different CeO2 nanostructures.13-35 For example, size-tunable CeO2 nanocrystals obtained from various wet chemical approaches (including modified precipitation, alcohothermal treatment, microemulsion, and sonochemical method) and their sizedependent UV absorption behavior were investigated in order to clarify the confinement effects in CeO2.13-24 Hoffman et al. prepared CeO2 films from a cerium alkoxide precursor through a chemical vapor deposition, which can act as buffer layers in YBCO-based coated conductor applications.25 Chen et al. fabricated polycrystalline CeO2 nanowires via a solution-phase route using sodium bis(2-ethylhexyl) sulfosuccinate as a structuredirecting agent, and observed size-dependent effect on Raman spectra.26 Seal et al. synthesized polycrystalline CeO2 nanorods by a microemulsion method, which showed an enhanced CO conversion activity.27 Corma et al. obtained mesoporous structure ceria from strictly uniform ceria nanoparticles via the template method.28 It is well-known that the most effective and economical way to synthesize materials with different nanostructures is to start from one precursor. However, the reports on different CeO2 nanostructures selectively synthesized through the same precursor are scarce.

10.1021/jp807091n CCC: $40.75  2008 American Chemical Society Published on Web 11/26/2008

Dimension-Manipulated Ceria Nanostructures

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20367

TABLE 1: Synthetic Conditions of 0D CeO2 Nanocrystals precursor

np/mmol

nTOPO/mmol

VDOE/mL

T/°C

t/min

morphology

size/nm

Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4

0.2 0.5 0.2 0.2 0.2

2.4 6 0 2.4 2.4

10 10 10 10 10

280 250 280 280 280

20 20 20 5 60

nanopolyhedra quasi-rods quasi-rods quasi-rods nanopolyhedra

2.23 ( 0.22 1.72 ( 0.17 × 4.53 ( 0.38 3.86 ( 0.51 × 2.22 ( 0.29 3.36 ( 0.53 × 1.14 ( 0.14 2.01 ( 0.19

Due to the reaction temperature dependent structural diversity and special properties of the cerium alkoxide, such as (1) relative low hydrolysis rate which was favored for the hydrolysis, alcoholysis, and sol-gel process and (2) low temperature of thermolysis (240-260 °C),36 we consider Ce(OC8H17)4 as a good precursor for the synthesis of diverse ceria nanostructures by ingeniously designing the appropriate reaction temperature range and selecting suitable reaction solutions. In this article, we report the efficient and selective preparation of three different nanoscale architectures (0D uniform nanocrystals, 2D polycrystalline assembly, and 3D mesoporous framework) for CeO2 via thermolysis approach, polyol method, and sol-gel route, respectively. From the viewpoint of physical chemistry, the

reaction pathways that lead to dimension-controllable ceria nanostructures are forwarded. Furthermore, the CO oxidation activities of the ceria catalysts derived from the above different nanostructures are presented. 2. Experimental Section The synthesis was carried out using standard oxygen-free procedures and commercially available reagents. Ceric ammonium nitrate ((NH4)2Ce(NO3)6, Sigma-Aldrich), dioctyl ether (DOE, Fluka), tri-n-octylphosphine oxide (TOPO, >90%, Acros), P123 (EO20-PO70-EO20) (Sigma-Aldrich), F127 (EO106PO70-EO106) (Sigma-Aldrich), F68 (EO77-PO29-EO77) (Sigma-

Figure 1. (a) XRD patterns of as-synthesized and the calcined CeO2 nanocrystals (calcined at 370 °C for 4 h): 0.5 mmol Ce(OC8H17)4, 6 mmol TOPO, 10 mL DOE, 250 °C, 20 min. TEM and HRTEM images of as-synthesized CeO2 nanocrystals: (b) 0.2 mmol Ce(OC8H17)4, 2.4 mmol TOPO, 10 mL DOE, 280 °C, 20 min; (c) 0.5 mmol Ce(OC8H17)4, 6 mmol TOPO, 10 mL DOE, 250 °C, 20 min; (d) 0.2 mmol Ce(OC8H17)4, 10 mL DOE, without TOPO, 280 °C, 20 min; (e) 0.2 mmol Ce(OC8H17)4, 2.4 mmol TOPO, 10 mL DOE, 280 °C, 5 min; (f) 0.2 mmol Ce(OC8H17)4, 2.4 mmol TOPO, 10 mL DOE, 280 °C, 60 min.

20368 J. Phys. Chem. C, Vol. 112, No. 51, 2008

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Figure 2. TEM and HRTEM (inset) images of as-synthesized CeO2 polycrystalline assembled nanostructures: (a) precipitates settling down on the bottom of the flask; (b) precipitates separated from the colloidal suspension in the flask: 1 mmol Ce(OC8H17)4, 10 mL EG, 195 °C, 20 min; (c) 1 mmol Ce(OC8H17)4, 10 mL EG, 195 °C, 1 h. (d) XRD patterns of a typical PAN sample, as prepared and calcined at 370 °C for 4 h.

TABLE 2: Synthetic Conditions of 2D CeO2 Polycrystalline Assembled Nanostructures (PANs) precursor

np/mmol VEG/mL T/°C t/min

Ce(OC8H17)4

1

10

195

20

Ce(OC8H17)4 Ce(OC8H17)4

1 3

10 20

195 195

60 20

Ce(OC8H17)4

3

20

195

60

Ce(OC8H17)4

3

20

195

180

Ce(OC8H17)4

10

20

195

180

morphology lowly cross-linked PANs cross-linked PANs highly cross-linked PANs highly cross-linked PANs highly cross-linked PANs highly cross-linked PANs

Aldrich), ethylene glycol (HOCH2CH2OH, EG, Beijing Chemical Plant), were used as received. Methanol (dehydrated by 4 Å zeolite), sodium, octanol (C8H17OH), isopropanol, ethanol, HCl, and cyclohexane were purchased from Beijing Chemical Reagents Co. or Beijing Chemical Plant. Synthesis of Ce(OC8H17)4. An improved method (see literature36) was developed to synthesize Ce(OC8H17)4 precursor by us. In a typical synthesis, 24.0 g of anhydrous methanol dehydrated by 4 Å zeolite reacted with sodium in a flask (100 mL) to form CH3ONa. After that, 40.0 g of dehydrated methanol (water was removed by sodium) was mixed with 11.0 g of dry (NH4)2Ce(NO3)6, 10.4 g of octanol, and fresh CH3ONa in turn, and was kept stirring for 8 h at room temperature to form a light yellow mixture. Then, the mixture was concentrated to remove methanol at 45 °C by a rotary evaporator. Hexane was added to the concentrated solution, followed by shaking, weighing, and filtering, and a brilliant yellow solution was thus obtained. The solution was continuously concentrated at 45 °C by a rotary evaporator until a transparent orange-red viscous liquid (the Ce(OC8H17)4 product) was formed. The 1H NMR spectrum of the precursor with the peaks d 0.90 (t, 3 H), 1.27 (s, 12 H), 3.56 (t, 2 H) was identified as belonging to

Ce(OC8H17)4.36 Comparing with the literature report, our synthesis process not only improved the purity and yield of the product but also saved time.36 The yield of Ce(OC8H17)4 was higher than 80% and 12.5 g of product can be synthesized in one batch. In the synthesis, to obtain high-quality Ce(OC8H17)4, the key factor is assuring the whole process free of any water. Synthesis of Monodisperse CeO2 Nanocrystals. To obtain monodisperse CeO2 nanocrystals, quantitative DOE and TOPO were added in a three-necked flask at room temperature. Ar was bubbled through the solution for 20 min and then the solvents were evacuated at about 100 °C for 30 min. The flask was then heated to the desired temperature (280 or 250 °C) and then quantitative Ce(OC8H17)4 was injected into the solution. The mixture was aged at this temperature for 20 min to give a brown colloidal solution. When the reaction was complete, an excess amount of ethanol was poured into the solution at about 70 °C. The resultant mixture was separated by centrifugation and the precipitates were collected. The as-precipitated nanocrystals were washed several times with ethanol and then dried in vacuo at 70 °C overnight. The CeO2 nanocrystals are dispersible in nonpolar organic solvents (e.g., cyclohexane). Synthesis of CeO2 Polycrystalline Assembled Nanostructures (PANs). The synthetic procedure was the same as that used to synthesize the monodisperse CeO2 nanocrystals, except that quantitative ethylene glycol (bubbled by Ar) and Ce(OC8H17)4 were added in the flask at the beginning and aged for 20 min to 3 h under 195 °C. Synthesis of CeO2 Mesoporous Gel (Table 3). Typically, quantitative block-copolymer P123 was dissolved in 10 mL isopropanol in a jar. The pH value was adjusted by the addition of HCl, then a certain amount of Ce(OC8H17)4 was added in this solution quickly and kept stirred for 0.5 h at room temperature to give an orange or yellow sol. The as-prepared sol was transferred into a culture dish and aged under a desired temperature and humidity for a certain time. The product was



Figure 3. TEM and HRTEM images of as-synthesized CeO2 polycrystalline assembled nanostructures: (a) 3 mmol Ce(OC8H17)4, 20 mL EG, 195 °C, 20 min; (b) 3 mmol Ce(OC8H17)4, 20 mL EG, 195 °C, 1 h; (c) 3 mmol Ce(OC8H17)4, 20 mL EG, 195 °C, 3 h; (d) 10 mmol Ce(OC8H17)4, 20 mL EG, 195 °C, 3 h.

TABLE 3: Synthetic Conditions of the As-Prepared Ceria Mesoporous Gel

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21 M22 M23 M24 M25 M26 a

precursor

M (mmol)

VHCl(mL)

template

M (g)

Visopropanol (mL)

T (°C)

RHa (%)

T (h)

structure

Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC8H17)4 Ce(OC6H13)4 Ce(OC10H21)4 Ce(OC12H25)4 Ce(OC6H13)4 Ce(OC10H21)4 Ce(OC12H25)4

5 5 5 10 2.5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

2.5 0.083 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83

P123 P123 P123 P123 P123 P123 P123 P123 P123 P123 P123 F127 F68 P123 P123 P123 P123 P123 P123 P123 P123 P123 P123 P123 P123 P123

1 1 1 1 1 1 1 1 1 1 1 1 1 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1 1 1 1 1 1

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

40 40 40 40 40 50 80 50 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 50 50 50

50 50 50 50 50 40 15 15 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 40 40 40

48 48 48 48 48 48 48 48 8 16 168 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48

disordered disordered ordered disordered disordered disordered disordered disordered disordered disordered ordered disordered disordered disordered disordered disordered ordered ordered disordered disordered ordered disordered disordered disordered ordered disordered

RH: relative humidity.

a transparent yellow gel followed by drying in an oven at 100 °C overnight. Synthesis of Bulk CeO2 Powder. Bulk CeO2 powder was obtained by calcining Ce(OH)4 precipitate (prepared by the precipitation reaction between (NH4)2Ce(NO3)6 and ammonia in H2O) at 1000 °C for 4 h. Characterization. The powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku D/MAX-2000 diffractometer

(Japan) with a slit of 1/2° at a scanning rate of 4° min-1, using Cu KR radiation (λ ) 1.5406 Å). Particle sizes and shapes were examined by transmission electron microscopy (TEM, 200CX, JEOL, Japan) operated at 160 kV. High-resolution TEM (HRTEM) characterization was performed with a Philips Tecnai F30 FEG-TEM (USA) operated at 300 kV. The combined TGDTA runs were performed with a Universal V2.60 TA instrument at a heating rate of 5 °C min-1 from room temperature to


Zhou et al.

Figure 4. (a) SAXRD pattern of as-synthesized mesoporous CeO2 gel: 5 mmol Ce(OC8H17)4, P123 ) 1 g, Visopropanol ) 10 mL, VHCl ) 0.83 mL, 40 °C, 50% humidity, 48 h. (b) TEM and HRTEM (inset) images of as-synthesized mesoporous CeO2 gel. (c) XRD patterns of the mesoporous ceria: as prepared and calcined at 370 °C for 4 h.

600 °C, using R-Al2O3 as a reference. The phosphorus content in the nanocrystals was determined by a Leeman Laboratories Profile spec (USA) inductively coupled plasma atomic emission spectrometer (ICP-AES). The BET specific surface area (SBET) was measured by nitrogen adsorption at 78.3 K, using an ASAP 2010 analyzer (Micromeritics Co. Ltd.), and measurements were performed after outgassing the sample at 423 K for 4 h under a vacuum, down to a residual pressure better than 10-3 Torr. CO Oxidization Test. A homemade flow reactor system including a quartz reaction tube (8 mm × 42 mm) was used for the catalytic test. In a typical CO oxidation experiment, 50 mg of as-calcined CeO2 nanomaterials and 500 mg of sea sand were mixed as a catalyst, and the experiment was carried out under the flow of the reactant gas mixture (0.5% CO, 10% O2, balanced with N2) with a rate of 50 mL min-1. 3. Results and Discussion 3.1. 0D Uniform CeO2 Nanocrystals. 3.1.1. Characterization. The Bragg reflections shown in the PXRD patterns of Figure 1a can be attributed to CeO2 with a cubic fluorite structure, with the calculated lattice constants of a ) 5.4344 Å for the CeO2 elongated nanocrystals, (JCPDS: 34-394). The PXRD peaks seem quite broad, revealing the rather small sizes of the CeO2 nanocrystals. Panels b and c of Figure 2 show that we obtained monodisperse 2.23 ( 0.22 nm CeO2 nanopolyhedra, and 1.72 ( 0.17 nm × 4.53 ( 0.38 nm elongated CeO2 nanocrystals, respectively, under different reaction conditions. The HRTEM images inserted in Figure 1b,c show the lattice fringes of the (111) plane for the CeO2 nanopolyhedra and the elongated nanocrystals. 3.1.2. Formation Pathway. Metal precursors, surfactant ligands, and solvents are three basic elements in constructing a typical synthesis system for monodisperse inorganic nanocrystals. Upon heating a reaction medium to a sufficiently high temperature, the precursors chemically transform into active atomic or molecular species (monomers). Subsequently, the

monomers evolve into nanocrystals with a certain size and shape. The nanocrystal growth is greatly affected by the thermolytic behavior of the precursor and the coordinating ability of the surfactant molecules, and is largely dependent upon the following several factors: type and amount of metal precursor and surfactant ligand, reaction temperature and time, and feeding mode of the metal precursor.6,37 During our synthesis, monodisperse ceria nanocrystals can be obtained through the thermolysis of Ce(OC8H17)4 at 250-280 °C (as suggested by the TG-DTA analysis shown in Figure S1 in Supporting Information) in a combined solvent of TOPO and DOE. To ensure the instant thermolysis of the Ce(OC8H17)4 precursor, it is injected into the solution at the reaction temperature. Without the capping agent of TOPO in the solution, 2-D dendritic aggregates of polydisperse CeO2 nanocrystals were formed at a relatively high temperature (280 °C) in pure DOE (3.86 ( 0.51 nm × 2.22 ( 0.29 nm; Figure 1d and Table 1). As TOPO was added in the reaction solution, 1-D aggregated wormlike CeO2 nanocrystals were obtained under TOPO: Ce(OC8H17)4 ) 12 at 5 min (3.36 ( 0.53 nm × 1.14 ( 0.14 nm; Figure 1e and Table 1). As the reaction time was extended to 20 min, uniform 2.2 nm CeO2 nanocrystals were obtained (2.23 ( 0.22 nm; Figure 1b and Table 1). These results strongly suggest that the addition of TOPO in the present synthesis can prevent as-formed CeO2 nanocrystals from aggregating, as the reaction time was long enough. With further extension of the reaction time from 20 to 60 min, the size and shape of the ceria nanocrystals changed little (2.01 ( 0.19 nm; Figure 1f and Table 1). To sum up, monodisperse CeO2 nanopolyhedra and elongated CeO2 nanocrystals can be obtained selectively via manipulating the parameters of the reaction temperature and the amount of the Ce(OC8H17)4 precursor. CeO2 nanopolyhedra were often formed under a relative high reaction temperature as well as a relatively low precursor concentration due to a relatively quick release and subsequent consumption of the monomers under



Figure 5. SAXRD patterns of the as-synthesized ceria mesoporous gel: (a) P123 ) 1 g, Visopropanol ) 10 mL, VHCl ) 0.83 mL, 40 °C, 50% humidity, 48 h, with 5 mmol corresponding precursor; (b) P123 ) 1 g, Visopropanol ) 10 mL, VHCl ) 0.83 mL, 50 °C, 40% humidity, 48 h with 5 mmol, with 5 mmol corresponding precursor; (c) 5 mmol Ce(OC8H17)4, P123 ) 0.2-1.6 g, Visopropanol ) 10 mL, VHCl ) 0.83 mL, 40 °C, 50% humidity, 48 h.

these conditions (Figure 1b and Table 1); while, elongated CeO2 nanocrystals were obtained under the reverse conditions (Figure 1c and Table 1). 3.2. 2D CeO2 Polycrystalline Assembled Nanostructures. 3.2.1. Characterization. Panels a and b of Figure 2 depict the TEM images of lowly cross-linked rodlike polycrystalline assembled nanostructures (PANs) of ceria acquired. The interplanar distances of 0.32 and 0.27 nm are attributable to the (111) and (200) facets of small ceria crystallites (around 3 nm) incorporated in the as-prepared PANs, respectively (inset in Figure 2b), while Figure 2c shows the TEM image of moderately cross-linked PANs, composed of small CeO2 crystallites in the size around 4 nm. The PXRD patterns of the as-prepared PANs and the as-calcined PANs at 370 °C for 4 h is shown in Figure 3d, indicating the typical features of a fluorite-structured ceria. After the calcinations, the PANs product showed greatly improved crystallinity. 3.2.2. Formation Pathway. In the present synthesis, CeO2 PANs in different cross-linkages can be obtained selectively via controlling the synthetic parameters including reaction time and amount of metal precursor. When the reaction time is short (20 min) as well as the amount of precursor is low (0.1 mol L-1) in the solution, lowly cross-linked rodlike PANs were obtained (Figure 2a,b and Table 2). On the other hand, longer reaction time (1 h, 0.1 mol L-1) (Figure 2c and Table 2) or higher concentration of metal precursor (20 min, 0.15 mol L-1) (Figure 3a and Table 2) contributes to the formation of highly cross-linked PANs. Further extending the reaction time under such a high precursor concentration (0.15 mol L-1) resulted in PANs with more complicated networks (Figure 3b,c and Table 2). Figure 3d revealed that the cross-linkages

of polycrystalline networks originated from 0.5 mol L-1 Ce(OC8H17)4 were significantly denser than those originated from 0.1 mol L-1 (Figure 2) and 0.15 mol L-1 (Figure 3a,b,c) Ce(OC8H17)4 (Table 2). As demonstrated above, the CeO2 PANs could be obtained via the polyol method from the Ce(OC8H17)4 precursor in ethylene glycol. Plausible reaction pathways that led to the PANs were proposed to proceed according to the following equations:38-40

Ce(OC8H17)4 + HOCH2CH2OH f Ce(OCH2CH2O)(OC8H17)2 Ce(OCH2CH2O)(OC8H17)2 + HOCH2CH2OH f Ce(OCH2CH2O)2 ∆

Ce(OCH2CH2O)2 98 CeO2 It seemed that the key step in the above processes was the oligomerization of cerium glycolates.35 During refluxing at 195 °C, ethylene glycol not only retarded the hydrolysis rate of cerium alkoxide but also replaced the OC8H17 groups of Ce(OC8H17)4 in turn, forming ceria glycolates ultimately. Due to the fact that ceria glycolates appear a cross-linking structure,38-40 PANs with different cross-linkages can be selectively obtained from the retarded hydrolysis and the concurrent oligomerization of cerium glycolates. A relatively high Ce(OC8H17)4 concentration and a prolonged reaction time were demonstrated to increase the cross-linkage of the ceria PANs (Figures 2a-c and 3, and Table 2).


Zhou et al.

Figure 6. (a) N2 adsorption-desorption isotherms, pore distribution calculated from the desorption branch of isotherm using the BJH method of the as-calcined CeO2 catalysts at 370 °C for 4 h in still air: monodisperse CeO2 nanopolyhrdra (0.5 mmol Ce(OC8H17)4, 6 mmol, TOPO, 10 mL DOE, 250 °C, 20 min, CeO2 PANs (1 mmol Ce(OC8H17)4, 10 mL EG, 195 °C, 20 min), and CeO2 mesoporous gel (MG) (5 mmol Ce(OC8H17)4, P123 ) 1 g, Visopropanol ) 10 mL, VHCl ) 0.83 mL, 40 °C, 50% humidity, 48 h). (b) CO conversion percent vs reaction temperature for the ascalcined typical CeO2 catalysts and the bulk CeO2.

3.3. 3D CeO2 Mesoporous Framework. 3.3.1. Characterization. The as-prepared mesoporous ceria gel was transparent yellow (Figure S2 in Supporting Information). Small-angle PXRD pattern of the as-prepared gel shows a very strong diffraction peak around 0.74° and one weak peak around 1.4° (Figure 4a). Further assisted by TEM observation, the mesophase can be indexed to a p6mm hexagonal symmetry (Figure 4b). The powder XRD patterns for the as-prepared gel and the calcined gel at 370 °C for 4 h are shown in Figure 4c, indicating the typical features of a fluorite-structured ceria. However, before the calcinations, the as-prepared gel was composed of lowly crystallized CeO2 nanoparticles. 3.3.2. Formation Pathway. Mesoporous structures are typically produced via evaporation of solutions containing an inorganic precursor, an organic supramolecular template, some additives, and the volatile solvent. This process is called “evaporation-induced self-assembly” (EISA).41 During this process, ordered mesoporous materials are formed through a mechanism that combines block copolymer self-assembly with complexation of the inorganic species. In fact, it is welldocumented that alkylene oxide segments can form crown-ethertype complexes with many metal ions, which associate preferentially with the hydrophilic poly(ethylene oxide) (PEO) moieties.42 The resultant complexes then self-assemble via crosslinking and polymerization, so as to form the mesoscopically ordered inorganic/block-copolymer composites. In the present synthesis, initially, P123 was dissolved in isopropanol and formed micelles at an appropriate concentration. Then, ceria monomers hydrolyzed from the Ce(OC8H17)4 precursor were chelated by the alkylene oxide segments from the P123, forming the so call “crown-ether-type” complexes. Finally, these complexes would assembly via the well-known EISA pathway (condensation and aggregation) to form the ordered ceria mesoporous structure.41,42 In this work, we found that the control of hydrolysis rate of the metal precursor and the selection of appropriate surfactant and the associated critical micelle concentration (cmc) were key factors in governing the formation of mesoporous ceria gel at low temperatures. On one hand, the formation of ordered mesoporous structure largely depended on a proper hydrolysis rate of the metal precursor. To confirm this suggestion, several precursors, the homologues of Ce(OC8H17)4 such as Ce(OC6H13)4, Ce(OC10H21)4, and Ce(OC12H25)4, were served to form the ordered mesoporous structure. It turned out the

hydrolysis rates of these precursors decreased with increasing the carbon-chain length. SAXRD patterns (Figure 5, a and b) showed that when using Ce(OC6H13)4 as precursor, ordered mesostructure was obtained at 40 °C. Nevertheless, as the precursor was Ce(OC10H21)4 or Ce(OC12H25)4, which showed lower hydrolysis rates, ordered mesostructures would only be formed under a higher temperature (50 °C or even higher), because the acceleration of their hydrolysis rates was necessary. On the other hand, the choice of block copolymer surfactant was crucial if considering the self-assembled aggregation to form the mesoporous structure. Generally, the surfactants with different EO/PO ratios and different molecular weights will influence the surface aggregation of the micelle complexes, resulting in different structures and pore sizes of the mesoporous materials. In our study, F68 and F127 were detrimental to the formation of ordered structure when served as surfactants, since the hydrophilic groups of F68 and F127 are larger than that of the P123, and tended to occupy more space around the Ce4+ ions. In this case, the formation of ion-surfactant complexes was hindered. As a result, mesoporous structure could not form because of the reduced trend for ordered condensation. Furthermore, no ordered mesostructures could form when the amount of surfactant was far beyond the optimized condition (either too high or too low) (Figure 5c). This result might indicate that neither micellization nor condensation could march ahead smoothly under too low surfactant concentrations, while, the condensation process might be hindered under too high surfactant concentrations. To summarize, the optimized conditions for the formation of ordered mesoporous ceria in this work were demonstrated as below: Visopropanol ) 10 mL, VHCl ) 0.83 mL, 5 mmol precursor, a reaction temperature at 40 °C, a relative humidity at 50%, and 1.0 g P123. Moreover, the ideal dry gel could not be acquired until 48 h aging (6-12 h (M9), 16 h (M10), Table 3). As the aging time exceeded 48 h, the samples showed welldefined peaks in the SAXRD pattern (M11, Figure S3 in Supporting Information). 3.4. CO Oxidization Activities of Various CeO2 Nanostructures. Three CeO2 catalysts, 1.72 nm × 4.53 nm monodisperse CeO2 nanopolyhrdra (Table 1), rodlike CeO2 PANs (Table 2), and CeO2 mesoporous gel (M3 in Table 3) were selected to evaluate their catalytic activities for CO oxidation after calcining at 370 °C for 4 h in still air. BET nitrogen adsorption-desorption isotherms of the three catalysts are shown

Dimension-Manipulated Ceria Nanostructures in Figure 6a. For the mesoporous and PAN catalysts, the isotherms can be ascribed to type IV hysteresis loop, indicating the presence of mesopores (around 3.2 and 7.6 nm, respectively) in the catalysts,43 while the nanopolyhedra catalyst shows type I hysteresis loop,44 suggesting the presence of micropores in the catalyst. The BET specific surface areas of the mesoporous catalyst, the PAN catalyst, and the nanopolyhedra catalyst were determined as 106, 80 and 104 m2 g-1, respectively, which are much larger than that (determined as 0.62 m2 g-1) of bulk ceria. Figure 6b shows the plot of CO conversion percent as a function of the reaction temperature for the three catalysts between 50 and 500 °C. It can be found, mainly because of the great difference of the surface area between the bulk and the nanostructured ceria, that the three typical nanostructured ceria showed superior CO conversion activity over bulk ceria, for which the CO conversion was less than 50% when the temperature reached 500 °C. While all the three catalysts showed similar conversion percentages either at low temperatures below 250 °C, or at high temperatures beyond 450 °C. The T50 (50% conversion of CO) was 310 °C for the PAN catalyst, 340 °C for the nanopolyhedra catalyst, and 390 °C for the mesoporous catalyst. As is known, several factors such as surface area, surface impurities, particle dispersity, and exposed crystal facets affect catalytic activity of inorganic nanocrystals.45-47 In the present case, if we consider that the specific surface areas of the three CeO2 nanocatalysts are comparable, the exhibition of lower T50 for both the PAN catalyst and the nanopolyhedra catalyst in comparison with the mesoporous catalyst might be due to the higher particle dispersity of the former two than that of the latter, as revealed by the HRTEM measurements (see Figure S4 in Supporting Information). Further, the nanopolyhedra catalyst showed higher T50 than the PAN catalyst might be caused by the presence of poisonous P impurities (originated from the thermolysis of TOPO surfactant during the calcining process) on the surface of the nanocrystals.48 As determined with the ICP method, the ratio of the remnant P to Ce is 0.11:1 for the as-calcined monodispersed ceria nanocrystals. 4. Conclusions From the same cerium precursor of Ce(OC8H17)4 via various solution approaches, we prepared dimension-manipulated ceria nanostructures (0D uniform nanocrystals, 2D polycrystalline assembly, and 3D mesoporous framework). Monodisperse ceria nanopolyhedra and elongated nanocrystals in the size around 2.2 nm were obtained by the thermolysis route at 250 °C in a combined solvent of TOPO and DOE. Ceria polycrystalline assembled nanostructures (PANs) with adjustable cross-linkages were achieved at 195 °C via the polyol method in ethylene glycol. The cross-linkages of the PANs tended to be enhanced by increasing Ce(OC8H17)4 concentration and reaction time. Mesoporous ceria gel possessing an ordered 2D hexagonal mesostructure was synthesized at 40 °C by the conventional sol-gel route, through controlling the hydrolysis rate, and the condensation and aggregation of the metal precursor, as well as the cmc of the block-copolymer surfactants. Catalytic tests demonstrated that the ceria catalysts prepared from the above three different nanostructures were active for CO oxidation at temperatures beyond 250 °C, and showed T50 (50% conversion of CO) at 310 °C for the PAN catalysts, 340 °C for the nanopolyhedra catalyst, and 390 °C for the mesoporous catalyst. Acknowledgment. We gratefully acknowledge financial aids from the MOST of China (Grant No. 2006CB601104), the

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