Hollow CeO₂ dodecahedrons: one-step template synthesis and enhanced catalytic performance

Abstract

Novel hollow CeO₂ dodecahedrons were synthesized using ZIF-67 nanocrystals as templates via a one-step liquid phase reaction for the first time. The structure, composition, morphology, surface chemical states, and band gap of the as-prepared hollow CeO₂ dodecahedrons were thoroughly investigated with XRD, SEM, TEM, HR-TEM, XPS, Raman, UV-Vis and ICP-AES. The hollow structures, with a specific BET surface area as high as 128 m² g⁻¹, are composed of small CeO₂ nanocrystallites less than 5 nm. The formation mechanism of hollow CeO₂ dodecahedrons was proposed, and the relative rate between Ce³⁺ hydrolysis and template dissolution was found to be the key to the successful formation of well-defined hollow dodecahedrons. The CO oxidation catalytic activities of hollow CeO₂ dodecahedrons revealed an excellent performance with a complete CO conversion at 170 °C and a superior catalytic stability, which was attributed to the synergistic effect of large surface area, small crystallites, hollow structure, large amount of oxygen vacancies, and the containing Co species. This work provides a rapid and cost-efficient approach to synthesize a new dodecahedral CeO₂ hollow structure, which broadens the way to synthesize crystalline metal oxide with controllable morphologies in mild solution conditions.

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1. Introduction

Hollow nanostructures are promising materials for catalytic, electronic and biologic applications due to their large specific surface area, pore volume, low density and large void space.^1–3 In particular, non-spherical hollow nanostructures have attracted considerable attention because of their special morphology and anisotropic texture, and thereby, may display enhanced electrochemical performance and improved catalytic activity compared to their solid counterpart.^4,5

Metal–organic frameworks (MOFs) are a new class of porous solids, composed of metal ions or clusters coordinated to organic linkers, and have appealed intensive interest owing to their large surface areas and high porosity. The diverse morphologies and controllable sizes make them excellent template candidate for synthesizing hollow nanostructures, especially for non-spherical hollow metal oxide nanostructures.^6–10 Hollow metal oxides are generally obtained by calcinating the MOFs templates. For example, Co₃O₄ hollow dodecahedrons were synthesized by calcinating ZIF-67 nanocrystals.¹¹ Fe₂O₃@NiCo₂O₄ porous nanocages were prepared by annealing core–shell Co₃[Fe(CN)₆]₂@Ni₃[Co(CN)₆]₂ nanocubes.¹² However, this calcinating treatment is prone to result in the fragmentation of the hollow structures. Therefore, perfect hollow polyhedrons are difficult to be obtained through this strategy. Liquid phase reaction is usually implemented in mild conditions and is easy to be controlled. Lou et al. synthesized Fe(OH)₃ and Fe(OH)₃/MO_x·yH₂O microboxes by ion exchange reaction between Prussian Blue cubes and alkaline precursor, and Fe₂O₃ and Fe₂O₃/MO_x microboxes were obtained after a subsequent thermal treatment.¹³ Our research group has also developed a method to synthesize hollow structures of layered double hydroxides and metal sulfides through the hydrolysis of metal nitrate in ethanol using MOFs as templates.^14–16 However, there are few reports on the direct synthesis of metal oxide hollow structures in solution using MOFs as templates, mainly due to the conflict between the relatively harsh formation environment of metal oxide and poor stability of MOFs. To obtain hollow metal oxide configurations, calcinations and other post treatment is often needed, but this will lead to the growth of component nanocrystals, and even damage of the hollow structure. As a result, it is still challenging for the direct synthesis of metal oxide hollow structures in liquid phase.

Ceria, an important semiconductor material, possesses excellent catalytic oxidation properties, which can be ascribed to its rich oxygen vacancy defects, large oxygen storage capacity and reversible transformation between III and IV oxidation states.¹⁷ These advantages make it promising in three-way catalysts, supercapacitors, fuel cells, phosphors, and gates for metal-oxide semiconductor devices.¹⁸ One of the most studied properties of ceria is catalytic oxidation of CO, which is of significance to chemical processing, exhaust emission control, and mechanism study of heterogeneous catalysis.¹⁹ CeO₂ with various morphologies, including nanorods,²⁰ nanoarrays,²¹ nanosheets,²² dendrites,²³ nanocubes,¹⁸ polyhedrons,²⁴ and hollow spheres²⁵ have been synthesized. However, the morphology of the hollow CeO₂ structures is mainly spheres,^26–28 and CeO₂ hollow polyhedrons have not been reported as far as we know. This is primarily due to the difficulty in controlling uniform coating on high curvature surface.¹⁴ Since non-spherical hollow structures have the potential of enhancing catalytic activity, the preparation of hollow polyhedral CeO₂ using polyhedral MOFs as templates via liquid phase reaction is considered to be a promising route.

Herein, we report for the first time the one-step synthesis of hollow CeO₂ dodecahedrons by using ZIF-67 nanocrystals as the sacrificial template in solution. Zeolitic imidazole frameworks (ZIFs) are a sub-class of MOFs, which have isomorphic topology of zeolite, and usually exhibit a rhombic dodecahedral morphology.^29,30 ZIF-67, an analogue of ZIF-8, is more easily dissolved in acidic condition.³¹ In our approach, ZIF-67 dodecahedrons with a submicron size synthesized at room temperature were used for the deposition of CeO₂ nanoshell through the hydrolysis of cerium nitrate. During this process, the ZIF-67 template was dissolved simultaneously as H⁺ ions were released from hydrolysis. Upon a complete removal of the template, hollow CeO₂ structures that replicate the dodecahedron shape were obtained. We found that the control over the relative rate between the formation of CeO₂ shell and the dissolution of the template is crucial for the successful formation of well-defined hollow nanostructures. The hollow CeO₂ dodecahedrons were characterized by large surface areas, oxygen vacancies, and containing Co species, all of which were beneficial to their excellent performance in catalyzing CO oxidation.

2. Experiments

2.1. Chemicals and materials

Co(NO₃)₂·6H₂O, Ce(NO₃)₃·6H₂O, CeCl₃·7H₂O, methanol and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. Commercial CeO₂ powder was obtained from Tianjin Kermel Co. Ltd. 2-Methyl imidazole was purchased from Sigma-Aldrich Co. LLC. All reagents were of analytical grade and were used without further purification.

2.2. Synthesis of ZIF-67 template

ZIF-67 dodecahedrons with a submicron size were synthesized following a reported method.¹⁵ 0.291 g of Co(NO₃)₂·6H₂O and 0.328 g of 2-methyl imidazole were dissolved in 25 mL methanol, respectively. The two solutions were mixed under magnetic stirring and then left undisturbed at room temperature for 24 h. The precipitation was collected by centrifugation and washed with ethanol twice.

2.3. Synthesis of hollow CeO₂ dodecahedrons

Typically, 0.07 g of Ce(NO₃)₃·6H₂O was dissolved in 25 mL ethanol. 0.04 g ZIF-67 templates were dispersed in the solution under ultrasonification. Hollow structures were obtained by refluxing the mixture in a 50 mL round bottom flask under magnetic stirring for 2 h. The product was collected by centrifugation and washed with ethanol twice before dried at room temperature.

2.4. Characterization

Powder X-ray diffraction (XRD) results were gathered on a Rigaku D/Max 2200PC diffractometer with a graphite monochromator and Cu Kα radiation (λ = 0.15418 nm). Morphology and microstructure of the products were characterized using a transmission electron microscope (TEM, JEOL JEM-1011) with an accelerating voltage of 100 kV, a field emission-scanning electron microscope (FE-SEM, ZEISS SUPRATM 55), and a high resolution transmission electron microscope (HR-TEM, JEOL JEM-2100) with an accelerating voltage of 200 kV. The X-ray photoelectron spectrum (XPS) was recorded on a PHI-5300 ESCA spectrometer (Perkin Elmer) with its energy analyzer working in the pass energy mode at 35.75 eV, and the Al Kα line was used as the excitation source. The binding energy reference was taken at 284.7 eV for the C 1s peak arising from surface hydrocarbons. The composition of the sample was determined by inductively coupled plasma atomic emission spectrometer (ICP-AES). Specifically, 10 mg sample was dissolved in a mixture solution of 1 : 1 HNO₃ (20 mL) and H₂O₂ (6 mL). Then the mixture was concentrated to about 1 mL by evaporation, after which the volume of the solution was fixed at 100 mL before measurement.³² Raman spectra were recorded with a micro-Raman LabRAM HR800 spectroscopy. Thermal gravimetric analysis (TGA) was carried out with a Mettler Toledo TGA/SDTA 851E analyzer at a heating rate of 10 °C min⁻¹ from room temperature to 1000 °C under air atmosphere. Nitrogen adsorption–desorption data were recorded on Quadrasorb SI apparatus at liquid nitrogen temperature (T = 77 K).

2.5. Measurement of catalytic activity

The catalytic activity was evaluated by a continuous flow fixed-bed microreactor operating under atmospheric pressure. In a typical experiment, catalyst particles (50 mg) were placed in the reactor. Then, the samples were pretreated in air with a flow rate of 67 mL min⁻¹ and heated at 300 °C for 30 min. After cooling down to room temperature, the gas stream was switched to the reaction atmosphere, i.e., CO oxidation in excess O₂: 1% CO and 20% O₂ balanced with N₂, with a flow rate of 67 mL min⁻¹. The composition of the gas exiting the reactor was analyzed with an online infrared gas analyzer (Gasboard-3121, China Wuhan Cubic Co.), which simultaneously detected CO and CO₂ with a resolution of 10 ppm. The crushed sample was obtained by ultrasonically treating the hollow structures in water for 2 h. The annealed sample was obtained by thermal treatment of the hollow structure at 500 °C in air for 2 h with a ramp rate of 0.5 °C min⁻¹. The acid washed sample was prepared by soaking the hollow structure in an aqueous HNO₃ solution (pH = 3) for 24 h.

3. Results and discussion

3.1. Synthesis and characterizations of hollow CeO₂ dodecahedrons

The hollow CeO₂ polyhedrons were synthesized using ZIF-67 nanocrystals as sacrificial templates via liquid phase reaction. In order to compare the structure before and after reaction, we first characterized the ZIF-67 template. Powder X-ray diffraction (XRD) result of the synthesized template (Fig. S1a, ESI†) fits well with that of the previous reported ZIF-67,¹⁵ which confirms the templates were phase-pure ZIF-67 crystals. The transmission electron microscope (TEM) (Fig. S1b, ESI†) and field emission-scanning electron microscope (FE-SEM) images (Fig. S1c and d, ESI†) clearly demonstrate the rhombic dodecahedral morphology of ZIF-67 nanocrystals with a narrow size distribution and an average edge length of ca. 600 nm.

By simply refluxing the ZIF-67 template in an ethanol solution of Ce(NO₃)₃, hollow polyhedrons were formed (Fig. 1). The XRD result of the hollow polyhedrons was indexed to the fluorite-like cubic-phase CeO₂ (JCPDS card no. 34-0394; Fig. 1a), and no other phase was found. The broaden peaks indicate small crystalline sizes. According to Debye–Scherrer equation, the particle size was calculated from the full width at half maximum of (111) reflection to be about 3.4 nm. The TEM image in Fig. 1b reveals the nanostructures are rhombic dodecahedrons and the contrast between the inner and outer parts demonstrates their hollow intracavity. The SEM image in Fig. 1c shows the CeO₂ nanocages have well replicated the morphology of the pristine ZIF-67 nanocrystals. Some cracked ones further indicate their hollow structure and a shell thickness about 33 nm. The HR-TEM image in Fig. 1d displays the shell is made of small particles with size of 3–4 nm, which is consistent with the result from XRD pattern. The lattice spacings are measured to be 0.312, 0.270, and 0.191 nm, which can be respectively indexed to (111), (200), and (220) planes of cubic-phase CeO₂. The selected area electron diffraction (SAED) on a facet of a hollow dodecahedron reveals that the hollow structure is polycrystalline (inset of Fig. 1b).

Fig. 1 Hollow CeO₂ dodecahedrons synthesized by using ZIF-67 template via a liquid phase reaction: XRD pattern (a), TEM image (b) with inset showing the SAED pattern, SEM image (c), and HRTEM image (d).

To investigate the surface composition and chemical state of the synthesized CeO₂ hollow dodecahedrons, X-ray photoelectron spectroscopy (XPS) is performed (Fig. 2). After curve fitting, the binding energies on Ce 3d spectra correspond to Ce 3d_3/2 (labeled as u) and 3d_5/2 (labeled as v) spin–orbit states. The peak u₃ is the character of Ce(IV). The peaks v₀, v₁, u₀, u₁ are due to Ce(III), while the others v, v₂, v₃, u, u₂, u₃ are ascribed to Ce(IV). The ratio of Ce³⁺/Ce⁴⁺ can be semi-quantified based on the peak area. The amount of Ce³⁺ is calculated to be 16.2% according to the formulas (1) to (3). The presence of Ce³⁺ may cause more oxygen vacancies and defects.³³

Ce(III) = v₀ + v₁ + u₀ + u₁ (1)

Ce(IV) = v + v₂ + v₃ + u + u₂ + u₃ (2)

Ce³⁺ = Ce³⁺/(Ce³⁺ + Ce⁴⁺) (3)

Fig. 2 XPS spectra of hollow CeO₂ dodecahedrons: the survey scan data (a), and high-resolution XPS spectra of Ce 3d (b), Co 2p (c), and O 1s (d).

The binding energy at 781.1 eV and 796.6 eV is ascribed to Co 2p_3/2 and Co 2p_1/2, respectively.³⁴ The satellite peak at around 786 eV is the feature of Co(II), revealing the presence of Co(II) in the product.³⁵ The O 1s peak can be deconvolved into three peaks. The binding energy at 529.5 eV is attributed to the lattice oxygen of CeO₂, and the one at 531.6 eV can be assigned to hydroxyl species, defective oxygen and absorbed oxygen.^25,36 The peak at 533 eV is ascribed to absorbed water molecules.³⁷ XPS signals reflect the chemical state of the surface atoms, and the strong peak at 531.6 eV reveals there are lots of surface hydroxyls in the product and the existence of Ce³⁺ would facilitate the increase of absorbed oxygen.³⁸ These results also imply small amounts of Co(OH)₂ and Ce(OH)₃ may exist in the sample.

To determine the elemental composition and especially the cobalt content in the product, the sample was further characterized with inductively coupled plasma atomic emission spectrometer (ICP-AES). According to the respective content of Ce and Co, the molar ratio of Ce to Co is calculated to be 5.29. The distribution of Ce, O and Co in the product was revealed by the energy dispersive spectrometer (EDS) mapping data (Fig. S2, ESI†), from which one can see that these elements are all uniformly distributed in the sample. However, the XRD pattern has shown no other phase except CeO₂. This implies that Co exist as amorphous phase or is highly dispersed in CeO₂.³⁹

Because oxygen vacancies in CeO₂ is very closely related to its catalytic oxidation activities, the oxygen vacancies of the synthesized hollow CeO₂ dodecahedrons were further studied with Raman spectra measurement (Fig. 3). Raman spectra can detect oxygen vacancies and defects via the vibrational structure changes in CeO₂ lattice. The band at 453 cm⁻¹ of the synthesized CeO₂ hollow structure has shifted from that of pure CeO₂, which is at 462 cm⁻¹. This band is attributed to the first order vibrational mode of Ce–O–Ce with F_2g symmetry in a cubic fluorite-type structure.⁴⁰ The shift may be due to the small crystallite size and the asymmetry caused by the existence of Ce(III) and Co. The bands at 603 and 1160 cm⁻¹ can be assigned to the presence of oxygen vacancies and defects in CeO₂ lattice.⁴⁰ The band at 532 cm⁻¹ is ascribed to Co–O stretching mode, which further confirms the presence of cobalt compound.⁴¹

Fig. 3 Raman spectrum (a) and the corresponding deconvolutional Raman spectrum (b) of hollow CeO₂ dodecahedrons.

The band gap of the as-prepared hollow CeO₂ was determined though the UV-Vis diffusion reflectance spectra as compared to the pure-phase bulk material (Fig. 4). The absorption band at 248 and 343 nm can be attributed to the charge transfer from O²⁻ to Ce⁴⁺ and interband transitions, respectively.³⁹ However, the first band moved to 298 nm, became broader and overlapped with the latter one. By plotting (αhν)^1/2 versus photon energy, the band gap of hollow CeO₂ particles and pure-phase bulk CeO₂ are calculated to be 2.15 and 2.70 eV, respectively. The significantly decreased bandgap can be ascribed to the small crystal size, oxygen vacancies, and the containing cobalt species.³⁹

Fig. 4 UV-Vis diffusion reflectance spectra (a) and plots of (αhν)^1/2 versus photon energy (hν) (b) of the hollow and bulk CeO₂.

Thermostability is of great importance for catalysts. To evaluate the thermal robustness of the synthesized hollow CeO₂ dodecahedrons, thermal gravity analysis (TGA) was carried out in air at a heating rate of 10 °C min⁻¹ (Fig. 5a). The weight loss can be divided into three steps. The first one below 180 °C accounting for 4.1% weight loss is assigned to the removal of the absorbed water. The second one between 180 and 300 °C accounting for 5.0% weight loss may be due to the removal of surface hydroxyl species and the decomposition of absorbed organics. The last step from 300 to 510 °C accounting for 2.7% weight loss corresponds to the dehydration of cobalt and cerium hydroxide. From 500 to 800 °C, a slight weight gain of 0.5% is observed, which relates to the oxidation of Ce³⁺ and Co²⁺, as well as the reduction of oxygen vacancies.

Fig. 5 TGA curve (a) and N₂ adsorption–desorption isotherm (b) of the hollow CeO₂ dodecahedrons.

The porosity and specific surface area of the samples, which can affect catalytic performances, were carefully examined by nitrogen adsorption–desorption isotherm measurements (Fig. 5b). The N₂ adsorption–desorption isotherm exhibits a type II adsorption branch reflecting a nonporous or macroporous character, which is consistent with the observations from electronic microscopy. Meanwhile, the calculated BET surface area is 128 m² g⁻¹. Such a large specific surface area is mainly attributed to the small particle sizes. Together with the hollow interiors, the CeO₂ hollow dodecahedrons are very favorable for surface-related applications, such as adsorption and heterogeneous catalysis.

3.2. Formation mechanism of hollow CeO₂ dodecahedrons

To investigate the formation mechanism of CeO₂ hollow dodecahedrons by using the ZIF-67 template, several sets of control experiments were conducted. Primarily, the relative rate between Ce³⁺ hydrolysis and template dissolution is found to be the key factor for the successful formation of well-defined hollow CeO₂ dodecahedrons. On the one hand, increasing the concentration of Ce(NO₃)₃·6H₂O will promote the hydrolysis of Ce³⁺. On the other hand, its increase will decrease the pH value of the solution due to the hydrolysis of Ce(NO₃)₃·6H₂O, which will inhibit the hydrolysis of Ce³⁺ and accelerate the dissolution of the template. Therefore, a proper concentration of Ce(NO₃)₃·6H₂O should be selected in order to obtain well-defined hollow structures. The appropriate amount range of Ce(NO₃)₃·6H₂O was found to be 0.07–0.5 g in the reaction system based on our experiments. A lower amount of Ce(NO₃)₃·6H₂O than this range led to the formation of ZIF-67@CeO₂ yolk–shell structures as a result of insufficient H⁺ to dissolve the template (Fig. S3a, ESI†). A higher amount of Ce(NO₃)₃·6H₂O than this range produced a more acidic condition that resulted in a rapid dissolution of the template and the formation of only cracked plates (Fig. S3b, ESI†).

The solvent used in the synthesis process is also crucial because it affects the hydrolysis rate of Ce³⁺. If water is used as the solvent, irregular nanoparticle aggregates instead of hollow CeO₂ shells were formed as a result of accelerated hydrolysis (Fig. S4, ESI†). To control the rate of Ce³⁺ hydrolysis and template dissolution, ethanol was selected as the suitable solvent to prepare well-defined CeO₂ hollow dodecahedrons.

Besides, the cerium source plays an important role in the formation of hollow structures. When CeCl₃·7H₂O is used as cerium source instead of Ce(NO₃)₃·6H₂O, only spindle-like nanostructures composed of nanoparticles were obtained (Fig. S5, ESI†). This is mainly due to different acidity of the two cerium salts. The pH value of equimolar CeCl₃·7H₂O and Ce(NO₃)₃·6H₂O in ethanol solvent was measured to be 3.63 and 4.95, respectively. Lower pH value of the CeCl₃·7H₂O solution resulted in the rapid dissolution of the template, thus only spindle-like aggregates instead of hollow structures were formed.

To prove the function of ZIF-67 template in the formation of CeO₂ hollow dodecahedron, we conducted the experiment under the same condition, but in the absence of the template. The solution was still clear after reaction and no CeO₂ was found to be formed. This implies that the formation of CeO₂ nanocrystals was driven by a heterogeneous nucleation process induced by the template. The template may act as a crystal seed that lowers the barrier of CeO₂ crystallization.

Based on the above discussion, the formation mechanism of hollow CeO₂ dodecahedrons is suggested as follows (Scheme 1). Ce(NO₃)₃·6H₂O was used as the cerium source, in which cerium is of a trivalent chemical state. During the formation of CeO₂ hollow structures, Ce(III) is oxidized to Ce(IV) by NO₃⁻ and environmental oxygen accompanying its hydrolysis to form the final CeO₂ product. The hydrolysis and accompanying oxidation of Ce³⁺ formed CeO₂ nanoparticles on the outside of the ZIF-67 template. At the same time, the template dissolved as H⁺ releasing from the hydrolysis. Finally, the template was completely consumed, forming CeO₂ hollow dodecahedron. Part of dissolved Co was blended in the shells probably in the form of amorphous Co(OH)₂. Due to the relative weak oxidizing ability of these species, Ce(III) is also easily retained in the product, which results in more oxygen vacancies that affect the chemical and physical property of the sample. The relative rate between Ce³⁺ hydrolysis and template dissolution determines the product structure. When hydrolysis is faster than dissolution, a ZIF-67@CeO₂ yolk–shell structure is formed. When hydrolysis is slower than dissolution, only cracked pieces is obtained. Therefore, a proper relative rate between the two processes is the key to the formation of well-defined hollow structure.

Scheme 1 Illustration of the formation mechanism of hollow CeO₂ dodecahedrons.

The morphology of hollow structures can be further controlled. First, the yolk size of the yolk–shell structure can be tuned by reaction time or amount of Ce(NO₃)₃ added (Fig. S6a, ESI†). Second, because the shell can resist dilute acid while the ZIF-67 yolk can gradually dissolve in it, hollow dodecahedrons with thinner wall thickness (24 nm) were obtained by etching the yolk–shell structure (Fig. S6b, ESI†).

3.3. CO catalytic oxidation activities of hollow CeO₂ dodecahedrons

The as-prepared hollow CeO₂ dodecahedrons possess not only high specific surface area and hollow structures, but also a high Ce³⁺ content, oxygen vacancies and Co species. All these features favor its application in heterogeneous catalysis. Therefore, the CO catalytic oxidation performance of the synthesized hollow CeO₂ structure was investigated (Fig. 6).

Fig. 6 CO oxidation catalytic performance of hollow CeO₂ dodecahedrons as compared to a commercial CeO₂ powder, the crushed sample, the annealed sample, and the acid washed sample (a). Catalytic stability of hollow CeO₂ dodecahedrons at 130 °C (b). Cycling test of hollow CeO₂ dodecahedron catalyst (c).

When hollow CeO₂ dodecahedrons was used as the catalyst, the conversion of CO to CO₂ started at room temperature, and 100% conversion was reached at a temperature as low as 170 °C. To compare the performance, a commercial CeO₂ powder with crystal sizes in the range of 50–150 nm and a specific BET surface area of 4.0 m² g⁻¹ was used as the reference (Fig. S7, ESI†). Under the same catalytic conditions, however, the conversion starts at about 185 °C and could only reached 65% conversion at 300 °C for the commercial CeO₂. The much higher catalytic performance of CeO₂ hollow dodecahedrons than the commercial one can be probably ascribed to its large surface area, small nanocrystallites, hollow structures, as well as the containing Co(II) species. This result shows the potential of CeO₂ hollow dodecahedrons as highly efficient catalyst for CO oxidation at relative low temperatures.

To shed light on the effects of hollow structure, oxygen vacancy, and Co species on the catalytic performance, CeO₂ hollow dodecahedrons were crushed, annealed at 500 °C and washed in dilute acid to eliminate the hollow structure, oxygen vacancy and containing Co species, respectively (Fig. S8, ESI†), and their catalytic performance was characterized for comparison. Using the cracked nanostructure as catalyst, a complete CO oxidation was acquired at about 195 °C, and it increased to 205 °C and 224 °C for the annealed and acid washed sample. Both the starting and completing temperature for CO oxidation are much higher than that of the original hollow CeO₂ dodecahedrons.

A higher catalytic activity of the hollow dodecahedron than the crushed one indicates the advantage of the hollow structure in catalysis due to the faster oxygen diffusion rate. As for the annealed sample, the calcinations at 500 °C caused not only the collapse and aggregation of hollow structures, but also the decrease of the surface area and oxygen vacancies, which led to decrease in catalytic activity. By acid washing, the Co content in the sample was significantly reduced to Ce : Co = 13.3 : 1 as determined by ICP-AES while the hollow structure still maintained. There are reports on cobalt compounds facilitate the catalytic activity of CeO₂ by enhancing the reducibility and affinity between CO molecules and the CeO₂ surface.^42–45 Table 1 compares the catalytic activity of our work with some typical CeO₂-containing hollow structures in the literature, confirming the superior catalytic activity of the present sample.

Table 1 CO catalytic activity of various CeO₂ hollow structures

Catalyst	100% conversion temperature (°C)	Reference
CeO2 core–shell spheres	317	26
CeO2–ZnO hollow spheres	300	27
Double-shell MnO2/CeO2–MnO2 spheres	206	28
Ce–Mn binary oxide nanotubes	205	46
Co3O4@CeO2 core–shell cubes	190	47
CeO2 hollow dodecahedrons	170	This work

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These results indicate that the enhanced catalytic activity of hollow CeO₂ dodecahedrons for CO oxidation is due to a synergistic effect of hollow structure, large surface area, abundant oxygen vacancies, and containing Co species. During the synthesis, the incomplete oxidation of Ce³⁺ led to a certain amount of Ce³⁺ in the product, the presence of which could generate oxygen vacancies due to the charge imbalance. The oxygen vacancies play an important role in the activation and transportation of active oxygen species in crystal lattices. During CO oxidation, the absorbed CO reacts with the lattice oxygen and forms oxygen vacancies that are filled by O₂ adsorption.^42,43,48 On the basis of the above mechanism and according to the previous study,⁴⁹ the Ce³⁺ sites can be considered as Lewis acid sites and the lattice oxygen can be assigned to the Lewis base site. The large surface area provides large numbers of active sites. The containing Co(II) species facilitate the adsorption of CO molecules and react with the adjacent lattice oxygen forming oxygen vacancies that are refilled with gaseous oxygen afterwards.⁵⁰ In addition, the possible interaction between Ce–O–Co makes a weaker Ce–O bond and more active lattice oxygen.⁴²

The catalytic stability of CeO₂ hollow dodecahedrons was tested at 130 °C for 12 h (Fig. 6b). The initial CO conversion is ca. 65%, and for the first 5 hours the CO conversion exhibited a slight decrease. As the reaction time further prolonged, the level of CO conversion gradually stabilized. At last, the CO conversion remained at around 60%. This result shows the excellent stability of CeO₂ hollow dodecahedrons in catalyzing CO oxidation. Further experiment reveals that the hollow structure can be well retained after catalyzing CO oxidation (Fig. S9, ESI†), which contributes to their high catalytic stability. The recyclability of the catalyst was demonstrated by the cycling test (Fig. 6c). After 3 successive cycles from 50 to 200 °C, the complete CO conversion temperature can be maintained at 170 °C, which shows the good recyclability of the catalyst.

4. Conclusions

In summary, we have synthesized hollow CeO₂ dodecahedrons via a facile one-step liquid phase method using ZIF-67 nanocrystals as template. The hollow structures are composed of small CeO₂ crystallites less than 5 nm and have specific BET surface area as high as 128 m² g⁻¹. CeO₂ nanoshell was formed by the hydrolysis of cerium nitrate and simultaneous deposition of CeO₂ on the template, and in the meantime the template was removed by dissolving under acidic conditions induced by the hydrolysis. The relative rate between Ce³⁺ hydrolysis and template dissolution is found to be the key factor for the formation of well-defined hollow dodecahedrons. Besides fluorite-like cubic-phase CeO₂, small amounts of amorphous Ce(III) and Co(II) species were also found in the product due to incomplete oxidation and Co released from the template. The hollow CeO₂ dodecahedron exhibited a higher CO oxidation catalytic performance than the commercial CeO₂ powder, the crushed sample, the annealed sample, and the acid washed sample, which can be attributed to its large surface area, small crystallites, hollow structure, large amount of oxygen vacancies, as well as the containing Co species. This work provides a rapid and cost-efficient approach to synthesize a new dodecahedral CeO₂ hollow structure, which broadens the way to synthesize crystalline metal oxide with controllable morphologies in mild solution conditions.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (NSFC Grant No. 21271118, 21303095), and the Taishan Scholars Climbing Program of Shandong Province (Grant No. tspd20150201). Z. L. thanks Dr Zhen Jiang for valuable discussions and instructions. The authors acknowledge Prof. Dr Chunjiang Jia for generous help in catalytic measurement. Mr Zhiming Han is thanked for chart drawing.

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Footnote

† Electronic supplementary information (ESI) available: XRD, TEM, SEM, EDS mapping. See DOI: 10.1039/c6ra11268d

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