Synthesis of a hollow CeO₂/Au/C hierarchical nanostructure for high catalytic activity and recyclability

Abstract

We report uniform hollow CeO₂/Au/C nanocatalysts with hierarchical structures fabricated successfully via an etching process. The whole preparation method involves the synthesis of SiO₂ spheres, a sequential deposition of CeO₂, Au NPs of 2–5 nm and then C layers through hydrothermal processes, crystallization of C by calcination and finally etching of the inner silica spheres to construct the hollow structures. The as-obtained nanostructures are characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Vis spectroscopy and energy dispersion X-ray spectroscopy (EDX). The CeO₂/Au/C composite shows a multilayer structure including the hollow ceria spheres and the C layers, where Au NPs are located between the two layers. In this work, the reduction of 4-NP is employed as a model reaction to test the catalytic performance. The results indicate that the hollow CeO₂/Au/C nanospheres exhibit a higher catalytic performance, comparing with the hollow CeO₂/Au and Au/C nanospheres. The presences of the C shells and CeO₂ layers can improve the catalytic activity. In addition, the hollow CeO₂/Au/C structures can be easily recycled without an obvious decrease of the catalytic activities in the reaction.

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1. Introduction

In recent years, noble metal nanoparticles in heterogeneous catalysis have received considerable attention for their specific catalytic properties.^1,2 Pt,^3–5 Au,^6,7 and Ag^8,9 are the main noble metals used in various chemical reactions such as oxidation of CO,¹⁰ oxidation of alcohols,¹¹ hydrogenation of acetophenone¹² and propane dehydrogenation,^13,14 etc. Compared with other noble metal NPs (such as Pt and Ag), Au NPs are less prone to over-oxidation, self-poisoning, and metal leaching and thus have high stability and activity.¹⁵ However, there are also many challenges in partial catalytic process. One of the main limitations is that Au NPs tend to grow into larger particles during pretreatment and the catalytic process. It is generally accepted that the catalytic activity of Au NPs would change inevitably with the agglomeration of Au NPs in reactions.^16,17 Therefore, fabricating the Au nanocatalysts with the high stability and activity is very significant.

Recently, metallic oxides, such as TiO₂, CeO₂, ZnO and ZrO₂, have been widely used as supporting materials in synthesis of catalysts.^18–22 The applicable supports not only improve the reactivity, but also enhance the catalytic stability.^23–25 Thus, the fabrication of core–shell structure emerges which Au NPs could be encapsulated into the core–shell structure. This kind of core–shell structure could promote the catalytic reactivity. Uniform Au@CeO₂ core–shell microspheres, in which Au nanoparticle core is coated with the shell composed of CeO₂ nanoparticles, are successfully synthesized for catalytic oxidation of CO to CO₂.²⁶ Yu has prepared Au@SnO₂ core/shell supported catalysts with a high catalytic activity.²⁷ Besides, hollow nanostructures have attracted extensive attention because of their unique chemical and physical properties.²⁸ Yolk–shell microspheres possess a unique structure of a hollow shell and an encapsulated, single-nanoparticle core. Yolk–shell microspheres are different from core–shell ones, in which there exists interstitial space between the shell and the nanoparticle core and, therefore, the nanoparticle core is freely movable within the hollow shell.²⁹ The new Au@TiO₂ yolk@shell nanostructure with a high reactivity at low temperatures is prepared to prevent the gold nanoparticles from sintering, making the new catalysts more stable.¹⁷ In fact, the success of the use of Au@ZrO₂ yolk–shell nanostructures for CO oxidation have already been demonstrated by Robert.³⁰ Lee has synthesized Au@SiO₂ yolk/shell structures for the reduction of p-nitrophenol.³¹ As one of the most important functional rare-earth metal oxides, CeO₂ has aroused much interest in catalysts for their unique properties.^32,33 Linen Wu and co-workers³⁴ prepare the novel Ag@CeO₂ core–shell nanostructures with well-controlled shape and shell thickness successfully. The research indicates the nanoscale Ag@CeO₂ core–shell catalysts have significantly enhanced catalytic activity. Zhou et al.³⁵ has fabricated the yolk–shell Au@CeO₂ structures using glucose and metal salt precursors via a one-pot hydrothermal process following by precipitation and calcination. Furthermore research of Bui and his coworkers showed that CeO₂ particles could enhance the catalytic activity and stability.^36,37 The novel hollow mesoporous@M/CeO₂ (M = Au, Pd, and Au–Pd) nanospheres are synthesized for reduction of 4-nitrophenol (4-NP).³⁸ The prepared catalysts (M loading on the inner surface) leave noble metal nanoparticles residing inside the hollow CeO₂ nanospheres. Xu and co-workers¹⁹ have prepared the Au/CeO₂ catalysts for the reduction of 4-nitrophenol (4-NP). They find that the synthesized hollow Au/CeO₂ nanospheres exhibit significantly enhanced catalytic activity.

Synthesis of hollow carbon spheres (HCSs) has attracted considerable attention because of the promising potential in energy storage and conversion and the adsorption-based and catalytic application. There are many unique features, such as spherical morphology, low density, high specific surface areas, large void space fraction, outstanding thermal and chemical stability, etc.^39,40 Hard templating methods have been widely used to synthesize hollow carbon spheres.⁴¹ Typically, the templates, mainly silica spheres with monodisperse size and spherical shape, are firstly coated with a thin layer of a C precursor (glucose^42,43) to form a core–shell structure, followed by carbonization of the shell and removed the core to obtain hollow carbon spheres. Such a C shell can function as barrier to prevent encapsulated nanoparticle from coalescence. The chemical and thermal stability and inherent electrical conductivity of such a carbon coating are especially beneficial for catalytic and electrochemical applications.⁴⁴ For example, Pt@Carbon⁴⁵ or Rh@Carbon⁴⁶ and Sn@Carbon⁴² structures show an excellent performance in catalytic hydrogenation reactions. Choi and co-workers⁴⁷ have prepared a novel nanostructure by synthesis of Au or Ag nanoparticles-embedded hollow carbon nanospheres. Liu⁴⁸ has synthesized a novel Pd@HCSs structure by encapsulating Pd NPs into hollow carbon spheres using a feasible template method. They find that the obtained Pd@HCSs nanocomposites show high catalytic performance in the reduction of 4-nitrophenol to 4-aminophenol, comparing with that of commercial carbon nanotube supported Pd NPs.

Herein, in this paper, we report the preparation of hollow CeO₂/Au/C hierarchical nanostructures with Au NPs embedded between CeO₂ layers and C layers. In this catalytic system, CeO₂ and C layer offered an energy barrier to prevent the migration and agglomeration of individual Au NPs and improved the catalytic activity as well. The synthetic procedure of the hollow CeO₂/Au/C structures involves several sequential steps as following the Fig. 1: (1) synthesis of monodisperse SiO₂ spheres through sol–gel process, (2) sequential coating of the CeO₂ layer, (3) tunable amount of Au NPs absorbed on the surface of SiO₂/CeO₂ cores via colloidal dispersion, (4) synthesis of SiO₂/CeO₂/Au/C and heat-treated at 500 °C for 3 h under N₂ atmosphere to carbonize the carbon precursor shell, (5) removal of the overall silica by etching with NaOH solution. When used in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), the catalysts exhibit an enhanced catalytic activity and recyclability.

Fig. 1 Schematic illustration of the synthetic procedures for hollow CeO₂/Au/C nanocomposites.

2. Experimental section

2.1 Materials

Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.99%), (3-aminopropyl) triethoxysilane (APTES), sodium borohydride (NaBH₄), sodium hydroxide (NaOH), trisodium citrate, and 4-nitrophenol (≥99%) were purchased from Aladdin Chemistry Co. Ltd, ammonia (28 wt%), HAuCl₄ (10 mg mL⁻¹), tetraethyl orthosilicate (TEOS), cetyltrimethyl ammonium bromide (CTAB), hexamethylenetetramine (HMT), ethanol and glucose were purchased from the China National Pharmaceutical Group Corp. All of the reactants were analytical grade and used without further purification. Deionized water and ethanol were used throughout the experiments.

2.2 Synthesis

2.2.1 Synthesis of SiO₂ nanospheres. Monodisperse bare SiO₂ spheres were prepared with the procedure originally described by Stöber et al.⁴⁹ In a typical experiment, TEOS (4 mL) was mixed with concentrated ammonia (28 wt%, 1 mL), deionized water (10 mL) and ethanol (50 mL). The mixture was stirred at 25 °C for 6 h and the resulting SiO₂ spheres were separated by centrifugation (8000 rpm and 5 minutes) and washed with ethanol and deionized water for three times, respectively. Then, the products were collected and dried at 60 °C for 12 h.

2.2.2 Synthesis of SiO₂/CeO₂. The SiO₂/CeO₂ was synthesized by homogeneous precipitation process. The obtained silica spheres (0.2 g) were dispersed ultrasonically in mixed solution of water (50 mL) and ethanol (50 mL). Ce(NO₃)₃·6H₂O (0.3 g) and HMT (0.2 g) were added in turn. Then, the temperature of the mixture was increased to 70 °C and kept under reflux for 4 h before being cooled to room temperature. The products were purified by centrifugation (8000 rpm and 5 minutes) and washed with water for three times, then dried at 60 °C.

2.2.3 Synthesis of SiO₂/CeO₂/Au. The obtained SiO₂/CeO₂ was transferred into a mixture of isopropanol (100 mL) and 2 mL APTES, and heated up to 80 °C for 24 hours to functionalize the surface with amino groups. The treated SiO₂/CeO₂ was washed with ethanol twice and dried in vacuum at 60 °C overnight, then re-dispersed in 80 mL of deionized water. SiO₂/CeO₂/Au was obtained by the reduction of HAuCl₄ with NaBH₄ under the protection of trisodium citrate. Typically, 80 mL of above SiO₂/CeO₂ aqueous solution was mixed with HAuCl₄ (0.72 mL) and trisodium citrate (40 mg mL⁻¹, 1.2 mL). After 30 min, NaBH₄ (0.1 M, 9 mL) was immediately injected to the above solution. The color of the solution was changed from light yellow to light red. After stirring at room temperature for 2 h, the Au colloids were completely absorbed, as indicated by the discoloration of the solution. The SiO₂/CeO₂/Au was collected by centrifugation (8000 rpm and 5 minutes), and washed more than three times with water to completely remove the trisodium citrate and then dried at 50 °C for 12 h.

2.2.4 Synthesis of SiO₂/CeO₂/Au/C. SiO₂/CeO₂/Au/C was synthesized by a solvothermal system.⁵⁰ In short, the prepared SiO₂/CeO₂/Au microspheres and cetyltrimethyl ammonium bromide (CTAB, 0.10 g) and glucose (2.00 g) were first dissolved in mixed solution (80 mL) (ethanol/deionized water = 1 : 1). After being stirred for 1 h, the resulting solution was carefully transferred into a Teflon-lined stainless steel autoclave, sealed and heated at 170 °C for 24 h. Then, the products were washed with water and ethanol for three times and dried at 60 °C for 12 h. Finally, the SiO₂/CeO₂/Au/C was heat-treated at 500 °C for 3 h under N₂ atmosphere to carbonize the carbon precursor shells.

2.2.5 Synthesis of hollow CeO₂/Au/C structures. Etching method used to remove SiO₂ core developing a hollow structure.⁵¹ As prepared SiO₂/CeO₂/Au/C microspheres were dispersed in 100 mL sodium hydroxide solution (0.5 M) and heated at 60 °C under stirring for 12 h. Then, the solution was removed and replaced with 100 mL new sodium hydroxide solution, and mechanical stirring for another 24 h. The products were rinsed with DI water five times and dried at 60 °C for 12 h.

2.2.6 Synthesis of hollow Au/C structures. The Au/C catalysts were fabricated by etching SiO₂/Au/C spheres. The prepared method involved three processes. Prepared SiO₂/Au particles, prepared SiO₂/Au/C spheres and etching were essential.
2.2.6.1 Synthesis of SiO₂/Au. The obtained SiO₂ particles were transferred into a mixture of 100 mL isopropanol and 2 mL APTES, and then heated up to 80 °C for 24 hours to functionalize the surface with amino groups. The treated SiO₂ particles were washed with ethanol twice and dried in vacuum at 60 °C overnight, then re-dispersed in 80 mL of deionized water. Typically, 80 mL of above SiO₂ aqueous solution was mixed with HAuCl₄ (0.72 mL) and trisodium citrate (40 mg mL⁻¹, 1.2 mL). After stirring about 30 min, NaBH₄ (0.1 M, 9 mL) solution was immediately injected to the above solution. The color of the solution was changed from light yellow to light red. After stirring at room temperature for 2 h, the Au colloids were completely absorbed, as indicated by the discoloration of the solution. The SiO₂/Au was collected by centrifugation, and washed more than three times with water to completely remove the trisodium citrate and then dried at 50 °C for 12 h.
2.2.6.2 Synthesis of SiO₂/Au/C. In short, the prepared SiO₂/Au spheres and cetyltrimethyl ammonium bromide (CTAB, 0.10 g) and glucose (2.00 g) were first dissolved in mixed solution (80 mL) (ethanol/deionized water = 1 : 1). After being stirred for 1 h, the resulting solution was carefully transferred into a Teflon-lined stainless steel autoclave, sealed and heated at 170 °C for 24 h. Then, the products were washed with water and ethanol for three times and dried at 60 °C for 12 h. Finally, the SiO₂/Au/C was heat-treated at 500 °C for 3 h under N₂ atmosphere to carbonize the carbon precursor shells.
2.2.6.3 Synthesis of hollow Au/C structures. As prepared SiO₂/Au/C spheres were dispersed in 100 mL sodium hydroxide solution (0.5 M) and heated at 60 °C under stirring for 12 h. Then, the solution was removed and replaced with 100 mL new sodium hydroxide solution and mechanical stirring for another 24 h. The products were rinsed with DI water five times and dried at 60 °C for 12 h.

2.2.7 Synthesis of hollow CeO₂/Au structures. For comparing the catalytic activity, the hollow CeO₂/Au catalysts were also prepared. The method was introducing as follows. The as-prepared SiO₂/CeO₂/Au microspheres were dispersed in 100 mL sodium hydroxide solution (0.5 M) and heated at 60 °C under continuous stirring for 12 h. Then, the solutions were removed and replaced with 100 mL new sodium hydroxide solution and mechanical stirring for another 24 h. The products were rinsed with DI water five times and dried at 60 °C for 12 h.

2.3 Characterization

The particle size and shape were analyzed with TEM images using a JEM-1230 transmission electron microscope (TEM) with an accelerating voltage of 100 kV. The samples for the TEM measurements were suspended in ethanol and supported onto a Cu grid. Scanning electron microscope (SEM) was performed on a Hitachi S-3400N scanning electron microscope were conducted on a JEM-1230 microscope operated at 100 kV. The powder X-ray diffraction (XRD) patterns of the products were collected on a Bruker D8 Advance Diffractometer (Germany) with Cu-Kα radiation (γ = 1.5406 Å) at a scanning rate of 0.02 S⁻¹ in the 2θ range from 10° to 90°, with an operation voltage and current maintained at 40 kV and 40 mA. The energy dispersive X-ray spectroscopy (EDX) analysis was recorded on a JEM-1230 microscope operated at 100 kV. 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. The UV-visible diffuse reflectance spectra was measured on a UV-3600 spectrophotometer equipped with a lab sphere diffuse reflectance accessory.

2.4 Catalytic test

The catalytic properties were investigated by the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH₄ aqueous solution under ambient temperature. Typically, the 4-NP (0.012 M, 0.030 mL) and NaBH₄ aqueous solution (0.5 M, 0.5 mL) and 2 mL H₂O were added to quartz cell. Then, 0.5 mL of aqueous dispersion of the hollow CeO₂/Au/C particles (0.5 mg mL⁻¹) was injected into the cell with stirring to trigger the action and the suspension was maintained at room temperature. The catalytic activity and stability were measured by the UV-vis absorption spectra of the mixture. The reactant of 4-NP had a strong absorption peak at 400 nm, while the product of 4-AP had a median absorption peak at 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 dispersed in a new reaction system for subsequent catalytic experiments under the same reaction conditions.

3. Results and discussion

3.1 Characterization of hollow CeO₂/Au/C nanocomposites

The work was to synthesize hollow CeO₂/Au/C nanocatalysts with core–shell structure to enhance stability and activity of Au NPs. The catalysts had a sandwich structure consisting of a hollow CeO₂ layer, some Au NPs and the carbonized carbon shells. As observed in Fig. 1, the all synthesis procedures were depicted. At the beginning, the SiO₂ nanospheres were attained from the hydrolysis of TEOS. The TEM of the prepared samples, as shown in Fig. 2a, demonstrated the spherical morphology of the nanoparticles with an average size of 150 nm diameters. All of the prepared silicon dioxide nanoparticles dispersed uniformly. The thin CeO₂ layer was then coated onto the SiO₂ nanospheres by addition of a certain amount of Ce(NO₃)₃·6H₂O. As described in Fig. 2b, a large number CeO₂ particles were closely arranged along the surface of the SiO₂ spheres, with an average thickness of 10 nm. After coated with CeO₂ particles, the nanostructures had a rough superficial morphology due to the lager nano-crystallites of CeO₂ probably. From the TEM results (Fig. 2c), a number of uniform Au NPs (ca. 2–5 nm) that highly dispersed onto the surface of SiO₂/CeO₂ nanospheres could be barely observed, which was attributed to that the CeO₂ particles had a grey black appearance similar to the Au NPs. All the Au NPs possessed a narrow size distribution. In order to clearly distinguish between CeO₂ particles and Au NPs, the high-resolution transmission electron microscopy (HRTEM) of SiO₂/CeO₂ and SiO₂/CeO₂/Au was obtained in Fig. S1.† The EDX spectrum data (Fig. S2†) were also provided to identify the presence of the Au particles on the CeO₂ surface. The picture (Fig. 2d) showed that the particles were coated with the C layer. As observed, the C layer uniformly deposited onto the surface and had the same thickness (ca. 35 nm). The Fig. 2e and f showed two different magnifications of CeO₂/Au/C structures after etching with sodium hydroxide solution. The results showed that the uniform hollow CeO₂/Au/C core–shell nanoparticles with a hierarchical structure had been fabricated successfully. Moreover, the SEM of the prepared nanostructures was showed in the Fig. S3a and the elemental mapping images of hollow CeO₂/Au/C microspheres were indicated in Fig. S3(b–d).† In addition, the hollow particle was containing an inner carbon surface with CeO₂ and Au nanoparticles on the surface and there are not distinct layers.

Fig. 2 TEM images of (a) SiO₂, (b) SiO₂/CeO₂, (c) SiO₂/CeO₂/Au, (d) SiO₂/CeO₂/Au/C, (e and f) hollow CeO₂/Au/C with two different magnifications.

In order to further discuss the station of Au NPs and core–shell structure, high-resolution transmission electron microscopy (HRTEM) was obtained. As can be noted, the image in Fig. 3a suggested that Au NPs were completely covered with the C shells and closely contacted with inner wall of C shells standing at external surface of the CeO₂ layers. The cores were composed of uniform CeO₂ nanoparticles that were ∼5 nm in size and loosely bound together. Furthermore, the obvious lattice fringes in the HRTEM images (Fig. 3(b and c)) confirmed the high crystallinity of the samples. The HRTEM image (Fig. 3b) represented the lattice fringes measured with a spacing of 0.235 nm and the SAED pattern (Fig. 3d) described a dim ring corresponding to the (111) plane of face centered cubic golden, which demonstrated the load of Au NPs. From the Fig. 3c, we can find an interplanar spacing of 0.312 nm corresponding to the (111) plane of the cubic CeO₂ phase (Fig. 3d) which was in good agreement with the SEAD images and the wide-angle XRD results. To ensure that the hollow CeO₂/Au/C microspheres were prepared successfully, the EDX spectroscopy was applied to comprehend the composition of the obtained microspheres. After coating and calcination procedures, the EDX analysis of SiO₂/CeO₂/Au/C structures (Fig. 4a) showed the existence of Si, Ce, Au, C, and O elements, which further proved the coating of carbon layer onto the SiO₂/CeO₂/Au nanospheres. The EDX results (Fig. 4b) indicated the presence of Ce, Au, C, O and a small number of Si elements, and this phenomenon may be due to the silicon dioxide existence with the uncompleted etching processes. In addition, the peaks of Cu element existing in the EDX images were contributed to the copper mesh. Meanwhile, we could find the content of gold was about 2.8 wt% in the obtained hollow CeO₂/Au/C nanospheres.

Fig. 3 (a–c) HRTEM images of the obtained hollow CeO₂/Au/C, (d) the SEAD pattern obtained from the hollow CeO₂/Au/C. The auxiliary chart in figure b is derived from magnifying the partial images.

Fig. 4 The EDX spectrum data for the obtained (a) SiO₂/CeO₂/Au/C, (b) hollow CeO₂/Au/C microspheres.

The crystal structure and element composition of different samples were characterized by power X-ray diffraction (XRD). As can be seen, Fig. 5a represented a broad peak at 2θ of 22–26° corresponding to the amorphous peak of SiO₂.⁵² However, after coated with CeO₂ layer, the intensity of the amorphous silicon dioxide became weaker, which suggested that the existence of the outside ceria oxide layer may prevent the diffraction of SiO₂ to a certain extent.^22,53 In Fig. 5b, after the deposition of CeO₂ layer, typical diffraction peaks for CeO₂ particles could be observed at 2θ = 28.55°, 33.08°, 47.48°, 56.09°, 69.42°, 76.70° and 88.43° corresponding to the characteristic (111), (200), (220), (311), (400), (331) and (422) reflections of fluorite-phase CeO₂, respectively (JCPDS no. 34-0394).⁵⁴ In contrast, after the loading of Au NPs on the SiO₂/CeO₂ (SiO₂/CeO₂/Au), another three peak at 38.18°, 44.39° and 64.58° can be found, which were corresponded to the (111), (200) and (220) plane of the face central cubic gold.²⁰ These results were consistent with the former analyses which suggests that the Au NPs were formed the CeO₂ layers successfully. After coating with the carbon, there were no obvious diffraction peaks of carbon indicating that the carbon coatings were amorphous.⁵⁵ From the XRD patterns, we can guest that the hollow CeO₂/Au/C nanospheres were prepared successfully.

Fig. 5 XRD patterns of (a) SiO₂, (b) SiO₂/CeO₂, (c) SiO₂/CeO₂/Au, (d) SiO₂/CeO₂/Au/C, (e) hollow CeO₂/Au/C.

This hierarchical structure was further confirmed by the nitrogen sorption measurements. Fig. S4† displayed the N₂ adsorption–desorption isotherm and pore size distribution of the porous hollow spheres. As observed, the obtained isotherm (Fig. S4a†) was recognized as a type IV N₂ adsorption–desorption isotherm with two hysteresis loops in the relative pressure range of 0.4–1.0. The results indicated that bimodal pore-size distributed in the mesoporous and macroporous region. This bimodal pore-size distribution was further confirmed by its corresponding pore-size distribution curve (Fig. S4b†) calculated from the desorption branch of the nitrogen isotherm by the BJH method. The special surface area of the hollow CeO₂/Au/C nanospheres was found to be 310.8 m² g⁻¹. The powder contained small mesoporous (2.4 nm) and lager mesopores with average pore diameter of ca. 12.8 nm. The results indicated that the synthesized hollow nanospheres had narrow pore size distributions corresponding to the results of the TEM. Usually, a large BET surface area and the hierarchical microporous architectures were beneficial to enhance the catalytic ability of catalysts.⁵⁶

3.2 Catalytic reduction of 4-nitrophenol

To explore the advantages of novel CeO₂/Au/C nanostructures as catalysts, we chose the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 at room temperature as a model reaction, which was demonstrated to be useful for the analysis of the catalytic activity of Au nanocatalysts.^19,25,57 Under the neutral or acidic condition, aqueous 4-NP showed a peak centered at 317 nm. After the addition of NaBH₄, the absorption maximum of 4-NP at 317 nm shifted to 400 nm due to the formation of 4-nitrophenolate ion under the alkaline conditions.⁵⁸ In addition, 4-NP was inert to NaBH₄ and the reaction would not proceed without catalysts. However, after the addition of the catalysts, the absorption peak at 400 nm decreased gradually with increasing time and the characteristic absorption of 4-aminophenol at 300 nm appeared accordingly, implying that the formation of 4-AP taken place. In our experiment, the progress 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. Besides, in all catalytic reactions, the dosage of every catalyst kept constant.

Fig. 6a showed the evolution of the absorption spectrum and the reaction time after addition of the hollow CeO₂/Au/C nanocatalysts. It was clear that the conversion of 4-NP was more than 50% within the initial 240 s and reduction of 4-NP into 4-AP was completely finished in 12 min with the color change of bright yellow to colorless. For comparison, the reduction reactions using unprotected CeO₂/Au (Fig. 6b) and Au/C (Fig. 6c) nanoparticles were also carried out under the same conditions. The images of Fig. 6b and c showed the reduction reaction of the 4-NP solution with respective CeO₂/Au and Au/C nanocatalysts. In addition, the result that the baseline in Fig. 6b was not smooth as that of in Fig. 6a may attribute to the ultraviolet absorption of the CeO₂/Au catalysts. The UV-vis spectra of prepared CeO₂/Au, Au/C and hollow CeO₂/Au/C nanostructures were indicated in Fig. S5.† It can be seen that the reaction using CeO₂/Au was completely finished about 15 min. Meanwhile, for the Au/C, the absorbance of 4-nitrophenolate ions only decreased by half after 10 min, indicating that the presences of CeO₂ and C layer were better for catalytic reaction. These comparative results indicated that the as-prepared CeO₂/Au/C catalysts exhibited higher catalytic activity than Au/C and traditional noble metal supported CeO₂/Au. Fig. 6d showed the linear relationships between ln(C₀/C_t) and reaction time in the reaction catalyzed by different samples (C_t is ordinate values of the absorption peak at 400 nm), which confirmed the first-order reaction kinetics.^22,53 According to the linear relationship, the reaction rate constant k was calculated from the slopes of the straight lines. The apparent rate constant k of the Au/C samples was 0.1837 min⁻¹, the reaction rate constant k of CeO₂/Au particles was 0.3106 min⁻¹. However, in sharp contrast, the samples of CeO₂/Au/C showed the highest catalytic activity (0.5088 min⁻¹) in this work which was roughly 2.8 times higher than that of the Au/C and was 1.7 times higher than that of the Au/C. The results of reaction rate indicated that the presences of CeO₂ and C layer can help to enhance the catalysis efficiency. This might be attributed to the presences of synergistic effect of ceric oxide and the Au NPs could be well fixed in the presence of the C shells.

Fig. 6 UV-vis spectra of catalytic reduction of 4-NP to 4-AP: (a) CeO₂/Au/C, (b) CeO₂/Au, (c) Au/C, (d) relationships of ln(C₀/C_t) and reaction time for the reduction of 4-NP catalyzed by different samples: (1) CeO₂/Au/C (2) CeO₂/Au (3) Au/C.

On the other side, the stability and recyclability were great importance for the practical applications of catalysts.⁵⁹ Herein, the cyclic stability of the prepared CeO₂/Au/C and CeO₂/Au catalysts was evaluated by monitoring the catalytic activity during successive cycles of the reduction reaction. As shown in Fig. 7, it was obvious that the hollow CeO₂/Au/C nanospheres exhibited relatively stable catalytic performance without visible reduction in the conversion for the same reaction time even after running for more than 8 cycles. TEM images (Fig. 8a) showed that Au NPs remained small particle size about 5 nm after 8 successive cycles, which owned to the existence of C shells. In the reaction, the layer prevented the Au NPs from detaching of the supports and connecting with each other during the plenty of repeating catalytic processes. By comparison, as for the CeO₂/Au, the conversion of 4-NP reduced to 88% after four cycles and to 62% after eight cycles. The low stability and reusability might be attributed to the agglomeration of Au NPs and leaching of the Au NPs from the surface of CeO₂. As shown in Fig. 8b, the Au NPs of CeO₂/Au sample changed to 20 nm in diameter during the repeating operations of reactions. The bigger Au NPs would lose most of active sites and lead to rapid decay of catalytic activity.⁶⁰ All results demonstrated that the structural feature of C shells encapsulation was feasible for the design of active, stable, and recyclable nanocatalysts.

Fig. 7 The reusability test of CeO₂/Au/C and CeO₂/Au catalysts.

Fig. 8 TEM of catalysts used for eight cycle tests (a) CeO₂/Au/C, (b) CeO₂/Au.

To explain the above experimental results, a possible catalytic mechanism was illustrated vividly in Fig. 9. When a metal and semiconductor were placed in together, the famous Fermi level alignment may occur normally with the charge redistribution and the formation of a depletion layer surrounding the metal.⁶¹ As Au (5.1 eV) had a higher work function than CeO₂ (2.75 eV), the electrons could leave the CeO₂ into the near Au nanoparticles resulting in an electro-enriched region. The C shells could also provide the Au NPs with electrons.^62–64 Thus, BH₄⁻¹ might be easily absorbed on the surface of CeO₂ and C shells. The electrons transfer might take place from BH₄⁻¹ to 4-NP through the adsorption of reactant molecules mostly onto the surface of Au NPs and contact surfaces with CeO₂ and C shells.

Fig. 9 Speculated mechanism of the catalytic reduction of 4-NP with the CeO₂/Au/C catalysts.

4. Conclusion

A novel type of hollow CeO₂/Au/C hierarchical structure has been prepared successfully. The as-prepared nanocatalysts have core–shell structures with good dispersion of gold nanoparticles, inner mesoporous CeO₂ layers and outer C shells. The catalysts show high catalytic activity to convert from 4-NP to the 4-AP. The two layers can immobilize the Au NPs and prevent possible migration in reaction procedures, leading to a higher catalytic stability. Besides, the C shells and CeO₂ layers can also improve the catalytic activity. Furthermore, even after 8 cycles of reuse, the catalytic activity of the catalysts is well retained. Future work will research the varying of protective layers, such as TiO₂ or ZrO₂, to obtain unique hollow core–shell structures with higher reactivity and stability.

Acknowledgements

The authors are grateful to the financial supports of the National Natural Science Foundation of China (Grant No. 21376051, 21676056, ​51673040 21106017, and 21306023), Natural Science Foundation of Jiangsu Province of China (Grant No. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA​2016105), Prospective Joint Research Project of Jiangsu Province (Grant No. BY2016076-01), Qing Lan Project of Jiangsu Province, the Fundamental Research Funds for the Central Universities (No. 3207045421, 3207046302 and 3207046409), Instrumental Analysis Fund of Southeast University and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) (1107047002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23521b

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