CeO₂ hollow nanospheres synthesized by a one pot template-free hydrothermal method and their application as catalyst support

Abstract

Uniform ceria hollow nanospheres composed of ceria nanocrystals have been synthesized via a simple one-step hydrothermal method without using any template. Afterwards, these hollow materials were used as support to prepare the Au/CeO₂ catalyst for the reduction of 4-nitrophenol (4-NP). It was found that the obtained porous CeO₂ hollow nanospheres were morphologically uniform, with an average diameter of 210 nm and high specific surface area of 167 m² g⁻¹. According to the basis of a time-dependent experiment, a self-assembly process coupled with an Ostwald ripening mechanism was proposed to explain the evolution of CeO₂ hollow nanospheres. In comparison with the commercial CeO₂ powder supported sample, the synthesized hollow Au/CeO₂ nanospheres catalyst exhibited significantly enhanced catalytic activity. In addition, the results of cyclic stability of the catalyst indicated that similar catalytic performance without visible reduction could be found after 7 repeated cycles. As for this catalyst system, the unique porosity structures of the support, uniform distribution of metallic particles together with the high thermal stability of Au NPs were all responsible for the improved reaction properties.

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1. Introduction

Hollow colloidal nanoparticles have attracted special attention because of their unique structures and promising applications in multitudinous fields ranging from chemical reactors, sensors, drug delivery, photonic crystals, to light weight filler materials.^1–5 In recently years, some hollow structures with a single crystalline shell wall were synthesized, such as single crystalline TiO₂, Cu₂O, ZnO and hollow mesoporous aluminosilica spheres (designated as HMAS).^6–9 Among them, the most common synthetic protocols for hollow nanostructures are template-based ones, which includes soft template methods using organic surfactants and hard template methods by the solid templates.^9,10 As for the template methods, soft or hard templates play an important role because they can direct the formation of hollow spheres by adsorption or chemical reaction on their surfaces.^11,12 However, many of template materials have high solubility or react with reagent and cannot survive under reaction conditions.^13,14 Moreover, templates need to be removed by calcination or strong acid/base erosion during the process of preparation, which brings many inconveniences. Nowadays, many reported works have focused on the facile template-free method for the synthesis of hollow structure.¹⁵ For instance, Shang and co-workers¹⁶ fabricated the submicrometer-sized anatase TiO₂ hollow spheres through a template-free solvothermal route using TiCl₄ as a raw material and a mixture of alcohols–acetone as solvent. Another example can be found in the work of Ma et al.,¹⁷ who synthesized the double-wall Cu₂O hollow spheres by a facile hydrothermal process in a ternary solvent system including water, ethanol and glycerol. As one of the most important functional rare-earth metal oxides, CeO₂ has excellent application potentials in many fields, such as catalysis, fuel cells, gas sensors, and ferromagnetism due to its oxygen storage capacity, oxygen deficiency and electronic conductivity.^18–22 Notably, ceria is the key component and a useful promoter in three-way catalysts mainly because of its unique properties.^23–25 In addition, it is well-known that the performance of CeO₂ strongly depends on its morphology, size and arrangement.^26,27 As a result, CeO₂ nanostructures with various morphologies, such as nanoparticles, nanotubes, nanorods, and hollow structures, have been prepared successfully through different methods.^28–32 In particular, CeO₂ hollow colloids have been of increasing interest in catalyst due to their unique structural properties.^33–36 It is suggested that the hollow structure can effectively enhance the spatial dispersion, thus increasing the high specific surface area and the pore channels.^37–39 Strandwitz et al.¹⁴ reported a synthetic strategy for the formation of hollow ceria spheres templated by colloidal silica and the SiO₂ cores could be chemically etched with NaOH. Following Wang et al.,³⁸ CeO₂ hollow spheres with porous shell were synthesized via the electric-current assisted dense polyacrylamide hydrogel template method, and the CeO₂ precursors should be calcined at 600 °C to remove the template. Liu's group⁴⁰ fabricated CeO₂ hollow spheres by the one spot way with the direction agents of HCl and PVP. But HCl is the dangerous agent and PVP is not easy detached only by washing way. Although much progress has been made in the fabrication of hollow spheres, it is still a challenge to seek a simple and template-free way for synthesizing hollow colloids. In the present work, we report a template-free and benign one-pot hydrothermal method to prepare CeO₂ hollow spheres with a single crystalline mesoporous shell. The formation of hollow structures is unusual because no template is intentionally used during the process and the raw materials readily available and environmentally. Through the TEM monitoring of the samples obtained with different reaction times, a self-assembly process coupled with an Ostwald ripening mechanism for the hollow structures formation was proposed. Under the typical conditions, the ceria hollow nanospheres have a uniform size and are composed of CeO₂ nanocrystals. Furthermore, it is found that ceria hollow nanospheres are excellent supports for gold nanoparticles. In our experiments, the prepared hollow Au/CeO₂ nanosphere catalyst shows remarkable activity in the reduction of 4-NP when comparing with the catalyst that supported on the commercial bulk CeO₂. This makes us believe that the CeO₂ hollow nanospheres merit additional attention and may be proved to be very useful in some applications.

2. Experimental

2.1 Materials

Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.99%) was purchased from Aladdin Chemistry Co. Ltd, acetic acid (C₂H₂O₄), glycol (C₂H₆O₂), ammonia (28 wt%), HAuCl₄ (10 mg mL⁻¹), commercial powders CeO₂, and ethanol were purchased from the China National Pharmaceutical Group Corp. All of the reactants were of analytical grade and used without further purification. Deionized water and ethanol were used throughout the experiments.

2.2 Synthesis of porous CeO₂ hollow nanospheres by a one-step hydrothermal method

In the typical synthesis of the CeO₂ hollow spheres with mesoporous shells, 2.0 g of Ce(NO₃)₃·6H₂O was dissolved into 80 mL glycol with ultrasonication. Subsequently, 4 mL of deionized water and 4 mL acetic acid were added with vigorous magnetic stirring for other 30 min to form a homogeneous solution. Then obtained solution was transferred into a Teflon-lined autoclave of 100 mL capacity and heated at 180 °C for 8 h. And then the products were separated from the solution by centrifugation (8000 rpm and 6 minutes). Finally, the products were washed with water and ethanol to remove ions and organic solvents possibly remnant in the products, and dried at 60 °C.

2.3 Preparation of the hollow ceria-supported Au catalyst

The Au-loaded hollow ceria nanoparticles with 3% Au were synthesized by homogeneous deposition precipitation method using urea as the precipitating agent,²⁷ while the ceria mesoporous-shelled hollow nanospheres were used as supports. The required amounts of HAuCl₄·4H₂O and urea (urea/Au = 100, molar ratio) were added to the CeO₂ under mild stirring conditions. The temperature of the reaction mixture was gradually increased up to 80 °C to ensure a slow decomposition of urea. The stirring was continued for 12 h at the same temperature and then, the suspension was cooled to room temperature. The obtained slurry was filtered off and washed with deionized water several times and dried at 60 °C for 12 h. After this, the deposition was calcined at 300 °C for 2 h to obtain the catalysts.

For comparison, the Au-loaded commercial ceria catalysts were also prepared. The preparation procedures were the same as those of the above-mentioned ones, except to use the support of commercial ceria powders instead of the ceria hollow nanospheres.

2.4 Characterization

The morphology and microstructure of the products were characterized using a transmission electron microscope (TEM, JEM-1100) with an accelerating voltage of 80 kV, a high-resolution transmission electron microscope, selected area electron diffraction (HRTEM/SAED, JEM-2100) with an accelerating voltage of 200 kV. SEM images were taken on a LEO-1530 scanning electron microscope. Powder X-ray diffraction (XRD) patterns were obtained on an XRD measurement (Bruker, D8 Advance) at room temperature using Cu Kα radiation (kα = 1.54059 Å). The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated from the desorption branch using the Barrett–Joyner–Halenda (BJH) theory. UV-vis adsorption spectral values were measured on an UV-3600 spectrophotometer.

2.5 Catalytic evaluation

The catalytic performance of the catalysts was tested by employing the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with NaBH₄ aqueous solution at room temperature as a model reaction. In a typical experiment, the 4-nitrophenol (0.012 M, 0.25 mL) and NaBH₄ aqueous solution (0.5 M, 0.5 mL) were added to quartz cell. Then, 0.5 mL of aqueous dispersion of the catalyst particles (0.5 mg mL⁻¹) was added to the above suspension, and the suspension was maintained at 25 °C. Reaction progress was monitored on-line by the UV-vis absorption spectra of the mixture to evaluate the catalytic activity and stability of the catalysts, as the reactant of 4-nitrophenol has a strong absorption peak at 400 nm, while the product of 4-aminophenol has a medium absorption peak at about 300 nm. To determine the catalytic recycling properties, the catalyst was separated after reaction for 1 h, and washed thoroughly with water and ethanol, followed by drying at 60 °C for 12 h in vacuum oven. Finally, the catalyst was redispersed in a new reaction system for subsequent catalytic experiments under the same reaction conditions.

3. Results and discussion

3.1 Characterization of CeO₂ hollow nanospheres

In this work, porous CeO₂ hollow nanospheres were synthesized through a one-step free-template solvothermal method. The XRD pattern of the as-prepared sample is shown in Fig. 1. As can be noted, the diffraction peaks of (111), (200), (220), (311), (222), (400), (331) and (420) planes can be indexed to the cubic fluorite-type CeO₂ structure, respectively (JCPDS card no. 65-2975). Apparently, the strong diffraction peaks confirm the good crystallinity of the samples. Meanwhile, no additional peaks can be observed, which implies the phase purity of the as-prepared CeO₂ hollow sphere. From Fig. 1, the broad diffraction peaks of the synthesized material can be attributed to the small size of the nanocrystals. After the calculation with the Debye–Scherrer formula for the strongest peak (111), the average grain size of the nanocrystals is about 4.0 nm.

Fig. 1 XRD pattern of the as-synthesized porous CeO₂ hollow nanospheres.

The size and morphology of the obtained products were further investigated by SEM and TEM techniques. It is clear that the size of the as-prepared hollow CeO₂ spheres is about 210 nm and the hollow structures can be identified by several broken ceria precursors, as shown in the inset in Fig. 2a. Besides, the corresponding TEM images (Fig. 2b and c) further identify the hollow structure of the product. The contrast between the dark margins and the pale center verifies the existence of the hollow structures and the wall thickness is about 40 nm. A closer observation from the high-resolution TEM (HRTEM) image (Fig. 2d) reveals that the hollow sphere are composed of numerous nanoparticles with diameters of 3–4 nm, which is identical to the calculated value in Fig. 1. Furthermore, there are obvious voids with the size of 2–3 nm among the small particles, implying the presence of mesoporous structure in the CeO₂ hollow nanospheres. In this case, the obvious lattice fringes in the HRTEM image confirm the high crystallinity of the sample. They are crystalline with an interplanar spacing of 0.312 nm corresponds to the (111) plane of the cubic CeO₂ phase, which is in good agreement with the wide-angle XRD results (Fig. 1).

Fig. 2 (a) SEM image (inset shows a broken nanosphere, scale bar = 200 nm), (b and c) TEM images (d) HRTEM image (inset is SAED pattern).

The porosity and the pore size distribution of the as-prepared CeO₂ hollow nanospheres were determined using N₂ adsorption–desorption isotherm. Fig. 3 displays the N₂ adsorption–desorption isotherm and pore size distribution of the mesoporous hollow spheres. As can be observed, the obtained isotherm (Fig. 3a) can be recognized as a type IV N₂ adsorption/desorption isotherm according to the IUPAC nomenclature with two hysteresis loops in the relative pressure range of 0.4–1.0. In this situation, the special surface area of the porous CeO₂ hollow nanospheres was found to be 167 m² g⁻¹, while the value of commercial CeO₂ was 50 m² g⁻¹ (Fig. S1 and Table S1†). On the other hand, from the results of pore size distribution (Fig. 3b), the porous CeO₂ hollow nanospheres presented a sharp and strong peak at about 5 nm calculated by Barrett–Joyner–Halenda (BJH) analysis. This phenomenon indicates that the synthesized hollow nanospheres have narrow pore size distributions and the building blocks are not tightly adhered to each other. Usually, a large BET surface area and the hierarchical mesoporous architectures are beneficial to enhance the catalytic ability of catalyst.⁶

Fig. 3 (a) N₂ adsorption–desorption isotherms and (b) BJH pore size distribution plots of porous CeO₂ hollow nanospheres.

To investigate the formation process of the hollow nanosphere structures, samples prepared at different reaction times are collected. Fig. 4 represents the morphologies of the products (TEM) at different solvothermal times. As can be seen, when the solvothermal reaction is conducted for 2 h (Fig. 4a), irregular CeO₂ nano-clusters are formed. The XRD pattern demonstrates that the product is poor-crystalline cubic fluorite CeO₂ (Fig. 5). In contrast, when the reaction time increases to 4 h, homogeneous solid nanospheres with a relative smooth surface and the average diameter of 155 nm can be found (Fig. 4b). With the increasing time of solvothermal reaction (6 h), the tendency for the formation of the hollow structures becomes more apparent (Fig. 4c). In this circumstance, the pristine dense structure becomes loose with a rough surface, which is formed by composing of small nanoparticles. Finally, the hollow sphere is formed when the solvothermal time increases to 8 h (Fig. 4d) and the crystallinity of the CeO₂ significantly increased from the XRD pattern. Interestingly, the size of the samples becomes large slightly, which may be related with the mechanism for the formation of mesoporous-shelled hollow CeO₂ nanospheres. This will be discussed in the following section. Summary, the evolution of the CeO₂ hollow spheres goes through the formation of solid spheres and the transformation of the solid spheres to hollow ones and Ostwald ripening mechanism happened in the hollowing process.

Fig. 4 TEM images of the particles obtained at 180 °C at different reaction times: (a) 2 h, (b) 4 h, (c) 6 h, (d) 8 h.

Fig. 5 XRD patterns of the samples at different reaction times.

To gain an insight into the formation process of the mesoporous-shelled hollow sphere structure, condition-dependent experiments have been carried out by altering the reaction environment, including the amounts of the acetic acid and the types of the solvent. To validate the role of propanoic acid, a series of comparative experiments with different amounts of acetic acid (C₂H₂O₄) such as 0 mL, 2 mL, 4 mL and 6 mL were conducted, while the other conditions were kept constant. X-Ray diffraction analysis (Fig. 7) shows that all the final samples with different amounts of the acetic acid are cubic fluorite CeO₂. From the TEM images of the as-obtained products, when the reaction is carried out with the absence of acid, some kind of nanospheres with diameters around 80 nm and also the severe aggregation of the nanospheres can be found (Fig. 6a). Comparing with is, with the increasing amount of acid (2 mL, Fig. 6b), the hollow nanospheres can be obtained and the diameter increases to about 100 nm. With the continuous addition of acid (4 mL), uniform mesoporous hollow CeO₂ nanospheres assembled from the small nanoparticles have been synthesized, which can be identified from the clear contrast between the dark edges and the pale center (Fig. 6c). Additionally, the size of the nanospheres further increases to about 210 nm. However, with the excessive amount of acid (6 mL), the product presents a wide size distribution ranging from 150 nm to 230 nm, and the hollow structure cannot be further observed from the TEM image (Fig. 6d).

Fig. 6 The TEM images of the products when the dosage of the acetic acid is 0 mL (a), 2 mL (b), 4 mL (c) and 6 mL (d).

Fig. 7 XRD patterns of the samples at different dosage of the acetic acid.

It is easy found that when the acid is added, the size of the CeO₂ spheres becomes bigger and the morphology is more stable. It is apparent that the presence of the acid can not only stabilize and disperse the CeO₂ solid nanospheres, but also control the size of the nanospheres and the formation of the mesoporous-shelled hollow nanospheres. Clearly, these behaviors may be explained in terms of the effect of acetic acid, which can react with glycol by esterification, thus enlarging the surface modification on the surface state. On the other hand, it should be pointed out that the different amount of acid may exhibit different esterification reaction rates in the system, which in consequence affects the formation and aggregation of primary CeO₂ nanocrystals, as well as the Ostwald ripening process of the solid spheres. Meantime, the reaction medium can also have a great effect on the morphology of the product. Experiments involving change glycol (C₂H₆O₂) to propane-1,2,3-triol while keeping the other parameters constant are exhibited in Fig. S2.† The result indicates that the glycol is not only as reaction media but also play an important role in the formation of CeO₂ hollow sphere.

Based on the above obtained results, we propose a new mechanism for the formation of mesoporous-shelled hollow CeO₂ nanospheres, which has been schematically illustrated in Fig. 8. As evidenced by the experiment of different reaction time (Fig. 4), at the first stage (step A), CeO₂ nanoparticles are formed initially through the hydrolysis and oxidation of Ce³⁺ under solvothermal conditions. At the same time, acetic acid reacted with glycol by esterification and adsorbed onto the surface of the CeO₂ nanoparticles. Once the primary CeO₂ nanocrystals are formed, they initially lead to the formation of dense solid spheres driven by the minimization of the total energy of the system (step B). And ester functions control the growth rate and self-assembly process as the role of the structure-directing agents. The hollowing process starts in step C by Ostwald ripening with the further increase in the time of the hydrothermal reaction. In this case, the initial dense structure became loose nanospheres during the self-assembly process. As a result, the structure of mesoporous sphere can be reserved, which is composed of the numerous primary nanocrystals. Moreover, the size of the nanospheres increases to about 180 nm by the outward expansion of nanoparticles. In step D, with this Ostwald ripening,^42–44 nanoparticles continue to expand outwards and inner nanocrystals are disappeared and attach to the shell to accomplish the whole process for the hollow structure. At the same time, the size of the sample further increases to 210 nm.

Fig. 8 A schematic illustration of the formation of the CeO₂ hollow spheres in the whole synthetic process.

3.2 Catalytic property

Ceria is a great support in catalyst area,⁴⁴ since CeO₂ has abundant oxygen vacancy defects, a high oxygen storage capacity and relatively easily shuttles between Ce³⁺ and Ce⁴⁺, thus giving rise to the enhanced rates of the oxidation reaction.⁴¹ To address the potential application of the mesoporous-shelled ceria hollow spheres in catalysis, we deposited the gold nanoparticles onto the ceria hollow spheres. For comparison, the Au-loaded commercial ceria catalysts were also prepared. Fig. 9 presents the representative TEM images with different magnifications of the as-prepared hollow Au/CeO₂ composite after thermal treatment at 300 °C for 2 h. As expected, the Au NPs are successfully deposited on the CeO₂ material and the morphologies of the porous CeO₂ hollow nanospheres are well maintained (Fig. 9a). A closer observation (Fig. 9b) indicates that Au nanoparticles are uniformly deposited on the CeO₂ support and the mean diameter of the Au nanoparticles is about 3 nm. At this moment, it is worth noting that the prepared catalyst possesses high thermal stability since no obvious changes in the size of Au NPs are observed after the calcined treatment at 300 °C. Taking into account that the catalyst is prepared by homogeneous deposition precipitation method, it is reasonable that the dispersible and encapsulation of Au NPs with the mesoporous CeO₂ supports is relatively uniform and full. That is to say, the existence of hollow CeO₂ nanospheres can act as “physical barrier” to prevent the Au NPs from aggregating effectively. To get further insight into the elemental distribution in the as-prepared hollow Au/CeO₂, EDX mapping analysis of the Au, Ce and O by field emission scanning electron microscopy (FESEM) were performed (Fig. S3†). It should be noted that the Au map is not as dense as that of Ce, which can be ascribed to the very low content of Au in the hollow Au/CeO₂ composite. Actually, the total Au loading of the Au/CeO₂ composite is 2.95 wt%, as determined by ICP-MS.

Fig. 9 (a and b) TEM and HRTEM images of as-prepared Au/CeO₂ composite, the inset in (b) is the SAED pattern; (c) UV-vis diffuse reflectance spectra of (1) CeO₂ hollow nanospheres, (2) Au/CeO₂ composite; (d) XRD patterns of Au/CeO₂ composite.

The UV-vis spectra and XRD pattern of Au/CeO₂ catalyst are shown in Fig. 9c and d. With respect to the UV-vis spectra, a fundamental absorption sharp edge around 300 nm appears over the CeO₂ hollow nanospheres and the Au/CeO₂ composite (Fig. 9c), which is a typical spectrum of CeO₂.¹³ Additionally, as shown in the second curve, an absorption band at about 550 nm is also found, which can be assigned to the characteristic of nanocrystalline Au particle.⁴⁵ Apparently, this finding makes us believe that the Au NPs become exposed after the deposition on the CeO₂ hollow nanospheres, which is favorable for the chemical reactants to contact with the Au nanoparticles. Moreover, from Fig. 9d, an almost identical XRD pattern can be observed when comparing with the XRD patterns of the CeO₂ sample (Fig. 1), proving again that the crystal structure of the porous CeO₂ hollow nanospheres is well maintained after the loading of Au NPs. Besides, the weak diffraction peak for Au (2θ = 38.2)^46,48 (Fig. 9d) can be found carefully as for the prepared Au/CeO₂ composite, suggesting that crystalline phases of gold can be detected, even though the content of Au is low.

To address the catalytic performance and the potential application of the mesoporous-shelled ceria hollow spheres in catalysis, we chose reduction of 4-nitrophenol to 4-aminophenol as a model catalytic reaction. It is well-known that the reduction of aromatic nitro compounds to amines is a very important process in synthetic organic chemistry and industrial production of many industrially important chemicals.^47,49 The difference between the absorption bands of the reactant p-nitrophenol (centered at 400 nm) and the product p-aminophenol (centered at 300 nm) allowed the catalytic process to be monitored recording UV-vis absorption spectra. Generally, the absorption peak of 4-nitrophenol is centered at 317 nm and after addition of NaBH₄, the absorption peak of 4-NP under goes a red shift from 317 nm to 400 nm due to the formation of 4-nitrophenolate (Fig. 10a). However, after addition of a trace amount of catalysts into the system, the intensity of the characteristic absorption peak at 400 nm corresponding to 4-nitrophenol quickly decreased and the characteristic absorption of 4-aminophenol at 300 nm appeared accordingly, which indicates the reduction of 4-NP and formation of 4-AP (Fig. 10b). Furthermore, the progress or kinetics of the reduction reaction was monitored by recording the absorbance at 400 nm, because the peak at 400 nm was much stronger than that at 300 nm.⁴⁶

Fig. 10 (a) UV-vis spectra of 4-NP before (black line) and after (blue line) adding NaBH₄ and 4-AP (red line) solution, (b) evolution of the absorption spectra of the 4-NP solution in the presence of hollow Au/CeO₂ nanocomposites at different times, (c) relationship of ln(C_t/C₀) and reaction time for the reduction of 4-NP catalyzed by different samples: (1) pure CeO₂ (2) Au/CeO₂ (commercial powders), (3) Au/CeO₂ (hollow nanospheres), (d) catalytic stability tests of Au/CeO₂ hollow nanospheres.

Fig. 10c shows the linear relationships between ln(C₀/C_t) and reaction time in the reaction catalyzed by different samples. Here, the ratio of C_t and C₀, where C_t and C₀ are p-nitrophenol concentrations at time t and 0, respectively, is measured from the relative intensity of the respective absorbances. The linear relations of ln(C_t/C₀) versus time can be observed for all the catalysts, indicating that the reactions followed first-order kinetics. According to the linear relationship, it can be calculated the reaction rate constant (k) from the slope of the straight lines. The rate constant k of the as-prepared Au/CeO₂ (commercial powders) is 0.249 min⁻¹, while for the Au/CeO₂ (hollow nanospheres), the value of k increases to 0.361 min⁻¹, meaning that the use of hollow nanospheres improves the catalytic activity significantly. As a control, CeO₂ nanocomposites without Au nanoparticles are utilized as catalysts for the catalytic hydrogenation of 4-nitrophenol. As depicted in Fig. 10c, no measurable catalytic activity can be observed. On the other hand, the stability and recyclability are of great importance for the practical applications of catalysts. In our experiments, the cyclic stability of the as-prepared Au/CeO₂ (hollow nanospheres) nanocomposite catalysts was also evaluated by monitoring the catalytic activity during successive cycles of the reduction reaction. As shown in Fig. 10d, it is obvious that the hollow Au/CeO₂ nanospheres catalyst presents similar catalytic performance without visible reduction in the conversion for the same reaction time (10 min) even after running for more than 7 cycles. Considering that the synthesized CeO₂ hollow nanospheres were used as the support to prepare the Au/CeO₂ nanocomposite catalysts, the physicochemical properties of the support and the configuration of the catalyst can affect the reaction performances significantly. In this contribution, it is suggested that the use of CeO₂ hollow nanospheres works in the following three ways. First, the synthesized CeO₂ hollow nanospheres by a one pot template-free hydrothermal method possess the relatively high surface area and narrow pore size distribution. As can be observed in Table S1,† the surface area value of the Au–CeO₂ (hollow nanospheres) is much higher than that of the commercial one. In addition, the BJH analysis indicates that the distribution of pore size of the sample is mainly concentrated at about 5 nm. Thus, the formation of mesoporous CeO₂ shells and hollow structure of the support is favorable for the fast diffusion of reactants and products from the active sites, which is believed to be responsible for the increased catalytic efficiency. The second effect of the catalyst configuration is related with the dispersive status of Au NPs over the support. Combined the HRTEM results with the XRD patterns, it can be deduced that Au NPs deposit uniformly on the CeO₂ support and the crystal structure of the porous CeO₂ hollow nanospheres can also be well maintained. In this circumstance, the Au nanoparticles can be exposed after the loading on the support. This means that the Au NPs become available to the chemical reactants due to the unique mesoporous structure of the support, which is favorable for the reactants to contact with the active sites. Third, in terms of this catalyst system, the outside CeO₂ porous shell can isolate the contiguous Au nanoparticles and prevent them from coming together even after the calcination at 300 °C. Just as the Fig S4† shows, the size of gold nanoparticles in commercial ceria is bigger than hollow CeO₂ spheres. In other words, the Au NPs are effectively from sintering, thus enhancing the synergistic effect between the Au NPs and the CeO₂ shells. In this way, during the reduction process of 4-NP to 4-AP, the rate of charge transfer will speed up, which is beneficial to accelerate the reaction conversion. The excellent reaction performances of the Au/CeO₂ nanocomposite catalyst makes us believe that the synthesis and use of CeO₂ nanospheres with unique mesoporous structure have a good potential application. At present, relevant work is still in progress to investigate its application.

4. Conclusions

In summary, well-dispersed CeO₂ hollow nanospheres were synthesized using a simple template-free solvothermal method. The results of XRD, N₂ adsorption and TEM establish that the obtained nanospheres show unique structural features, such as high crystallinity, mesoporous CeO₂ shells and large specific surface area. It is suggested that the formation of the CeO₂ hollow spheres can be contributed to a self-assembly and Ostwald ripening processes. Acetic acid and glycol play important roles in the determining morphology and the formation of the hollow structure. Furthermore, compared with the gold catalyst that supported on commercial bulk CeO₂, the use of these CeO₂ hollow nanospheres as support exhibited excellent catalytic properties for the reduction of 4-nitrophenol because of their unique porosity structures and high thermal stability of Au NPs. Under our experimental conditions, the as-prepared Au/CeO₂ nanocomposite catalyst could be recycled seven times without any visible loss in the activity. These facts make us believe that those hollow nanospheres materials can be great candidates for a catalyst support to construct catalytically active sites and stable nanoreactors, showing a good potential application.

Acknowledgements

The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21376051, 21106017 and 21306023), Natural Science Foundation of Jiangsu Province of China (Grant No. BK20131288), China Scholarship Council (no. 201308320238), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2011086) and Instrumental Analysis Fund of Southeast University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08124f

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