Synthesis and characterization of carbon nanotubes supported Au nanoparticles encapsulated in various oxide shells

Abstract

A novel supported catalyst with various oxide shells (SiO₂, TiO₂, ZnO) assembled on Au nanoparticles with carbon nanotubes as support has been successfully fabricated. This process involves preparation of modified MWCNTs, sequential deposition of Au and then oxide shells, and finally calcination at high temperature to remove the organics. The obtained samples were characterized by several techniques, including N₂ adsorption–desorption isotherms, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), UV-Vis spectra, X-ray diffraction and thermogravimetric analysis (TGA). The results established that all the oxide shells could serve as effective barriers to prevent the migration and aggregation of Au NPs during calcination. Moreover, different oxide layers have an obvious influence on the distribution of Au nanoparticles. Additionally, the prepared catalyst exhibited a mesoporous structure because of the preservation of carbon nanotubes. In our experiments, the catalytic activities of MO_x/Au/CNTs were investigated by photo-metrically monitoring the reduction of p-nitrophenol (p-NPh) by an excess of NaBH₄. It was found that the prepared TiO₂/Au/CNTs catalyst revealed excellent catalytic activity and the sample could be easily recycled without a decrease of the catalytic activity in the reaction.

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1. Introduction

Gold has always been considered as a very stable and inert metal. However, the inert gold has been paid much attention until Haruta et al. observed the extraordinary activity of gold for the oxidation of CO at low temperature.¹ From then on, interest in gold catalysis has grown rapidly and it has been successfully applied in a variety of reactions such as hydrogenation of unsaturated substrates, epoxidation of propene, oxidation of alcohols and aldehydes, among others.^2–5 It was suggested that the catalytic properties of a supported system are strongly dependented on the types of metal and supporting material, a strong interaction with the support resulting in a high catalytic activity.^6–8 One important factor was believed to convert the “inert gold” into a highly active catalyst: a strong interaction with the support.^9–11 The unique morphology and structure of carbon nanotubes (CNTs) mean they exhibit relatively high surface areas, excellent electronic conductivity, and high chemical stability,^12,13 and are suitable to be used as a catalyst support material. Especially, multi-walled carbon nanotubes (MWCNTs) are regarded as metallic conductors that have more remarkable properties. Furthermore, it was also reported that Au/CNT composites could provide the best combination of selectivity and conversion in comparison with the gold catalysts supported on several other carriers like graphite, active carbon and Al₂O₃.¹⁴

Another key factor to convert inert gold into active metal was the reduced diameter of the metal particles (<5 nm).¹⁰ Therefore, much research work has been done to seek a method for keeping metal nanoparticles small sized. In fact, nanocomposite catalysts with tunable particle size could be prepared by the design of the catalyst structure and the use of new synthetic nanotechnologies.^15–17 However, it should be noted that Au NPs was easy to aggregate in high temperature, typically above 300 °C, resulting in a deactivation of the catalytic activity. In this regard, the improved reaction stability and anti-sinter properties for Au NPs is in high demanded. Recently, many reported works have focused on the stabilization of metal NPs by encapsulating with the metal oxide layers.^18–22 Whereas, the key effect of these oxide shells was only limited on the protection of Au nanoparticles, the study of how oxide shells facilitated or passivated the activity of Au NPs was ignored. Moreover, various oxide shells have different properties, the relationship between Au NPs and oxide shells was also not reported so far.

Herein, in this work, we designed the configuration of MWCNTs deposited with Au NPs, then covered with various oxide shells, such as SiO₂, TiO₂, ZnO. Afterwards, the resulting hierarchical nano-composite was calcined at the desired temperature to remove the organics and formed the hollow tubes with mesoporous structure. The detailed structure and synthetic procedure was depicted in Fig. 1. As these oxide layers all can withstand high temperature. In this point, the Au NPs could be prevented to aggregate and maintained at a small size, which can offer a high catalytic performance in high temperature. The catalytic of resulted samples with different oxide shells have been evaluated through the reduction of p-NPh to p-APh to compare the activities. At last, the interaction between Au NPs and various oxide shells was demonstrated and the possible mechanism was also discussed.

Fig. 1 Schematic illustration of multiwall carbon tubes, Au loaded and oxide shells protected calcination procedure for the fabrication of mesoporous hollow tube shells.

2. Experimental section

2.1 Materials

MWCNTs (main range of diameter: 20–40 nm, length: 5–15 μm, special surface area: 90–120 m² g⁻¹) were purchased from Shenzhen Nanotech Port Co., Ltd. (China). Tetrabutyl titanate (TBOT), Zn(NO₃)₂·6H₂O, ammonium peroxydisulfate, polyaniline (PANI), ethanol, isopropanol, concentrated sulfuric acid (95–98%), concentrated nitric acid (65–68%), concentrated hydrochloric acid (36–38%), trisodium citrate, sodium borohydride (NaBH₄), γ-aminopropyl-triethoxysilane (APTES), PVP K30 (M_w ≈ 38 000), HAuCl₄ (10 mg mL⁻¹), ammonia solution (25–28%), and tetraethyl orthosilicate (TEOS, 98%) was obtained from Shanghai in China, p-nitrophenol (p-NPh), deionized (DI) water was used throughout all the experiments. All reagents were used without further purification.

2.2 Synthesis

Synthesis of Au/CNTs colloids. Before loaded of Au on the CNTs, the MWCNTs were pre-treated successively by mixed acid, APTES and PANI. The detail information was shown in our previous work.²² Then Au/CNTs were obtained by the reduction of HAuCl₄ with NaBH₄ under the protection of trisodium citrate (TSC). Typically, 0.2 g of CNTs, 0.72 mL of HAuCl₄ and 1.2 mL of TSC (40 mg mL⁻¹) dispersed in 150 mL of DI water. Then 9 mL of fresh NaBH₄ was immediately injected to the solution under vigorous stirring at RT for 2 h, the product was collected by dried at 50 °C for 12 h.

Synthesis of MO_x/Au/CNTs. The different oxide precursors were deposited on Au/CNTs, and keep MO_x at the constant molar ratio.
(a) Synthesis of SiO₂/Au/CNTs. SiO₂/Au/CNTs were prepared through a modified Stöber method.²³ The as-prepared Au/CNTs were mixed with 0.5 g of PVP before adding 66 mL of ethanol, 8.4 mL of DI water and 1.2 mL of ammonia solution under vigorous stirring. Then, 1.72 mL of TEOS that dispersed in 20 mL of ethanol was added drop by drop. The resulting samples were collected after 12 h and washed three times with ethanol and DI water, respectively, and dried in air at 50 °C for 12 h.
(b) Synthesis of TiO₂/Au/CNTs. The TiO₂ layer was prepared using a method developed by Yin et al.²⁴ The PVP modified Au/CNTs were mixed with 65 mL of ethanol and 0.5 mL of DI water. Then, a stock solution containing 1.32 mL of TBOT and 20 mL of ethanol was injected into the mixture for at least 10 min. After stirring at 40 °C under the refluxing conditions for 12 h, the TiO₂/Au/CNTs colloids were collected by centrifugal separation, washed three times with ethanol and dried in air at 50 °C for 12 h.
(c) Synthesis of ZnO/Au/CNTs. The ZnO layer was deposited by the hydrolyzation of Zn(NO₃)₂·6H₂O. Typically, the modified Au/CNTs were injected with 1.5 mL of TSC and 80 mL of DI water under stirring for 1 h. Afterwards, determined amount of NH₃·H₂O was added to the solution to keep the pH = 12, then 1.15 g Zn(NO₃)₂·6H₂O was injected to the solution for refluxing 12 h at 90 °C.

Calcination and obtained oxide shells protection samples. The obtained product was calcined in air at 400 °C for 4 h to remove the organics.

2.3 Characterization

The morphology of the obtained products was characterized using a JEM-2010 transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and FEI Inspect F50 field emission scanning electron microscopy (FESEM). The TEM samples were prepared by transferring one drop of sample dispersion in ethanol onto a carbon-coated copper grid and then dried in air. The nitrogen adsorption–desorption isotherms were measured at −196 °C on an ASAP 2020 (Micromertics USA). The specific surface area was determined from the linear part of the BET equation (P/P₀ = 0.05–0.25). The pore size distribution was derived from the desorption branch of the N₂ isotherm using the Barrett–Joyner–Halenda (BJH) method. Thermo Gravimetric Analysis (TGA) of the samples was performed with Rigaku Thermo Plus TG8120 system from RT to 750 °C at the rate of 10 °C min⁻¹ under air. UV-Vis spectra analysis was performed on a Shimadzu UV 3600 spectrometer.

2.4 Catalytic evaluation

To study catalytic properties of the MO_x/Au/CNTs, reduction of p-NPh to p-APh in the presence of NaBH₄ was chosen as a model reaction. The aqueous solution of dispersed catalyst (1 mL, 0.5 g L⁻¹), 0.5 mL of freshly NaBH₄ aqueous solution (0.25 M) and 0.03 mL of p-NPh aqueous solution (0.01 M) were added to the quartz cell. The reaction was continued at room temperature, and a UV-Vis spectrometer was used to monitor the progress of the reaction at regular intervals. In all catalytic runs, the experimental conditions were kept constant. For the recycling experiment, the catalysts were collected, washed with deionized water, calcined at 200 °C and reused in the next cycles.

3. Results and discussion

Since the decompose temperature of MWCNTs was a key parameter, so Thermo Gravimetric Analysis had been first established to obtain the TGA curve for modified MWCNTs by nitration mixture, APTES and PVP. As shown in the left of Fig. 2, it can be seen that the sample obviously started to decompose and released heat at 350 °C, inferred the burning of PVP and other organics. In addition, a large amount heat was released from 550 °C, demonstrating that the CNTs had been decomposed. Moreover, the TGA curve shown that the CNTs at the temperature of 680 °C were decomposed completely. From this experiment, it was reasonable that the modified MWCNTs had a relative high decompose temperature and they couldn't decompose at 400 °C.

Fig. 2 Left: TGA curve for modified MWCNTs by nitration mixture, APTES and PVP. Right: XRD patterns of the different samples (a) Au/CNTs, (b) ZnO/Au/CNTs, (c) TiO₂/Au/CNTs, (d) SiO₂/Au/CNTs calcined at 400 °C for 4 h.

The crystallization of different catalysts was determined by XRD analysis. As exhibited in the right of Fig. 2, significant differences could be found for the samples with different oxide shells. From Fig. 2(a), the Au/CNTs sample showed the typical CNTs peaks at 26.4° (002), 29.3° (113), 31.7° (130) and 42.8° (101).²⁵ While three peaks corresponding to diffraction from 38.2° (111), 44.3° (200), 64.6° (220) planes of face centered cubic (fcc) gold.²⁶ And the sample exhibited fcc structure with d-values matching with that of Au metal (JCPDS no. 4-784). After different oxide shells deposited on Au/CNTs, typical peaks for ZnO, TiO₂ and SiO₂ could be observed in Fig. 3(b)–(d), respectively. It's worth noting that, compared with Fig. 2(a), the synthesized samples with oxide shells deposited only a broad and blurred peak at 26.4° appears for CNTs, the other CNTs peaks were not visible, which indicated that the existence of the outside oxide layer may hinder the carbon peaks to some extent. To explain this, it should be noted that the CNTs in the catalyst system were wrapped by the outside oxide material, thus the carbon peaks was not easy to be detected because of the “protection effect” that originated from the oxide shell.²⁷ A similar phenomenon was also found with Au peaks, which suggested that the existence of the outside oxide layer may weaken the phase transformation of Au to a certain extent.

Fig. 3 Comparison of thermal stability of the (a) ZnO/Au/CNTs, (b) TiO₂/Au/CNTs, (c) SiO₂/Au/CNTs, (d) Au/CNTs calcined at 400 °C in air for 4 h. Insets in each image were the Au NPs size distribution histograms.

The various morphologies were shown in Fig. 3 after the samples were calcined at 400 °C for 4 h. It was found that a large quantity of CNTs was still maintained owing to the temperature was not enough to fully decompose of the CNTs. As analyzed before (Fig. 2) the initial decomposition temperature of CNTs was above 550 °C. Besides, the calcined samples with different oxide shells were shown in Fig. 3(a)–(c). It can be clearly noticed that with the existence of oxide shells, the size of Au NPs can be maintained from 4.8 nm to 7.6 nm, which can keep Au NPs at a small average size. Among these catalysts, the encapsulation of SiO₂ shells exhibits the best thermal stability for preventing the Au NPs. Possibly, this behavior can be explained in the terms of its excellent chemical stability. However, in a sharp contrast, the Au NPs were observed to aggregate severely in the absence of oxide (Fig. 3(d)) shells during the calcination. It is clear that the Au nanoparticles agglomerated extensively to form particles larger than 50 nm in size. All of the above results demonstrated that the oxide shells could serve as an effective barrier to prevent the migration and aggregate of Au NPs during the calcination, thus enabling the Au NPs to become anti-sintering and the catalyst exhibits a high thermal stability. In addition, the HRTEM and FESEM for the samples was shown in Fig. S2 and S3.† The results of HRTEM demonstrated that the lattice fringe d = 0.23, 0.26 nm and 0.35 nm was matched that of (1 1 1) crystallographic plane of Au phase, (002) planes of hexagonal wurtzite ZnO and the anatase phase, respectively. It was suggested that the Au/CNTs catalysts with different oxide shells were successful obtained. From the FESEM characterization, it can be clearly seen that the carbon nanotubes was cut off to many small segments. It was mainly because in the process of pre-treatment, the MWCNTs was treated by mixed acid (concentrated sulfuric acid and concentrated nitric acid with the volume ratio is 1 : 1).

The low-temperature nitrogen adsorption–desorption isotherms were commonly used to evaluate the pore structure parameters of materials. The left of Fig. 4 were typical nitrogen adsorption–desorption isotherms of the prepared MO_x/Au/CNTs with different oxide shells. The BJH pore size distribution curve calculated from the analysis of the desorption branch of the isotherms (right of Fig. 4). As revealed in Fig. 4, all the prepared materials had mesoporous structure, while the distribution of mesoporous was different with the variation of oxide shells. As can be seen, the ZnO/Au/CNTs and SiO₂/Au/CNTs catalysts exhibited the hysteresis loops in the relative pressure range of 0.9–1.0. In a contrast, the TiO₂/Au/CNTs sample exhibited the hysteresis loops in the relative pressure range of 0.4–0.8, which suggested that the TiO₂/Au/CNTs had the well-developed mesoporous characteristics. Furthermore, TiO₂/Au/CNTs catalyst possessed a large value of Brunauer–Emmett–Teller surface and small pore diameter (Table 1), which maybe the main reasons for the improved catalytic activity, this was discussed in later.

Fig. 4 Left: nitrogen adsorption–desorption isotherm (a) ZnO/Au/CNTs, (b) TiO₂/Au/CNTs, (c) SiO₂/Au/CNTs calcined at 400 °C for 4 h. Right: corresponding pore size distribution curve.

Table 1 Characterization data of the different samples

Catalysts	BET surface area (m^2 g^−1)	Pore diameter (nm)	Vtotal (cm^3 g^−1)	k/10^3 (s^−1)
ZnO/Au/CNTs	49.50	23.97	0.28	2.81
TiO2/Au/CNTs	52.97	8.85	0.17	17.22
SiO2/Au/CNTs	159.32	38.5	0.74	13.74

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From the pore size distribution curve, it could be found that the ZnO/Au/CNTs and SiO₂/Au/CNTs catalysts had wide pore size distribution, which was in the range of 20–40 nm. Clearly, this finding was consistent with the diameter of carbon nanotubes. By contrast, the TiO₂/Au/CNTs catalyst exhibited narrow pore size distribution in the range of 2–8 nm and 20–40 nm, which suggested it may be formed from the accumulation of crystal TiO₂ shells and carbon nanotubes.

The reduction of p-NPh to p-APh by an excess amount of NaBH₄ was chosen as a probe reaction to evaluate the catalytic activity. It was well documented that this reaction had become one of the model reactions for testing the catalytic activity of various noble metal nanoparticles. Moreover, the yellow fading and eventual bleaching involved in the reduction also provided a simple way to monitor the reaction kinetics by using UV-Vis spectroscopy.²⁸ Under the acidic or neutral condition, aqueous p-NPh showed a peak centered at 317 nm. Upon the addition of NaBH₄, the alkalinity of the solution increased and p-nitrophenolate ions would become the dominating species, together with a spectral shift to 400 nm of the absorption peak.²⁹ At the same time, the color of solution then changed from pale yellow to yellow. Without the presence of the catalyst, the maximum absorption peak stayed unaltered, and the mixture remained yellow, suggesting the p-NPh was inert to NaBH₄ and the reduction would not proceed. After Au-based catalyst was added, the Au NPs acted as an electron relay system and the absorption peak at 400 nm gradually dropped in intensity. The photograph of Fig. 5 displayed the typical evolution of the UV-Vis spectra of the reduction.

Fig. 5 Successive UV-visible absorption spectra of p-NPh solution reduced by NaBH₄ in the presence of (a) ZnO/Au/CNTs (b) TiO₂/Au/CNTs, (c) SiO₂/Au/CNTs calcined at 400 °C for 4 h, (d) comparison of rate constants of the catalytic reduction reaction of p-NPh catalyzed by the samples.

The reaction rate of a chemical reaction was affected by the concentration of the reacting materials, the temperature and the surface area of the catalyst.³⁰ To compare the catalytic properties of the prepared MO_x/Au/CNTs with different oxide shells, we studied the efficiency of these catalysts in catalyzing the above reduction reaction. Taking into account that the concentration of NaBH₄ largely exceeded the concentration of p-NPh, the reduction rate could be assumed to be independent of NaBH₄ concentration. At this point, a pseudo-first-order rate kinetics was regarded to the p-nitrophenolate.³¹ In all runs discussed here, linear relation of ln(C₀/C) versus reaction time was observed (Fig. 5(d)). The reaction rate was calculated from the decrease in the concentration of p-NPh from the UV-Vis spectra. In all catalytic runs, the experimental conditions were kept constant at molar ratio Au: p-NPh : NaBH₄ of 1 : 4 : 1600. The reaction rate constants (Table 1) were estimated from the slopes of the straight line.

As can be seen from Fig. 5, the sample with TiO₂ shells and ZnO shells exhibit the highest and lowest catalytic performance, respectively. The rate constant was shown in Table 1. Furthermore, the reaction rate constant of sample with SiO₂ shells lower slightly than TiO₂. Apparently, this behavior can be attributed to the following reasons. Firstly, TiO₂ was active material and has a strong synergistic effect with Au cores, While, the SiO₂ was a kind of inert material for its weak metal–support interaction.³² Secondly, TiO₂/Au/CNTs catalyst possessed larger values of Brunauer–Emmett–Teller surface than ZnO/Au/CNTs (Table 1). It was well established that catalyst with higher BET surface area and the more exposed atom in the internal surface of tube were beneficial to improve the reaction activity.³³ Thirdly, from the nitrogen adsorption–desorption isotherms, it can be seen TiO₂/Au/CNTs catalyst exhibited two kinds of hysteresis loops in the relative pressure range of 0.4–0.8 and 0.9–1.0, respectively, revealed that two kinds of pore was existed in this sample. This implies that the mesoporous structure not only offered by the carbon nanotubes but also offered by the accumulation of crystal TiO₂ shells, this facility the transfer of the reactants to the Au active sites, resulting in an enhanced catalytic activity. The properties of pore were also demonstrated from the pore distribution from the right of Fig. 4. Finally, the catalytic efficiency of metal nanoparticles for the electron-transfer process greatly depended on their size redox properties.³⁴ Based on Plieth's study, the redox potential lowered with decreasing size of small metal nanoparticles.^35,36 Compared with the sample without the protection of oxide shells, the calcined TiO₂/Au/CNTs sample owned the smaller nanoparticle size (Fig. 3). Accordingly, the potential barrier height at the interface between the Au NPs and p-NPh was lower. Therefore, the TiO₂/Au/CNTs catalyst would exhibit a faster electron transfer rate, resulting in higher catalytic activities. It should be underlined that the encapsulation of gold nanoparticles with an oxide layer was quite effective in preserving the size and distribution of the Au nanoparticles. Therefore, the catalytic capacity to resist sintering might have an important influence on the reaction performance. In other words, the aggregation of the nanoparticles in the process of pretreatment might lead to the loss of catalytic activity.

To confirmed the advantage of oxide shells, the evaluate catalyst of Au/CNTs was also carried out. The successive UV-visible absorption spectra of p-NPh solution reduced by NaBH₄ were shown in Fig. S1.† As expected, from Fig. 5(d), it can be clearly seen that the rate constant of TiO₂/Au/CNTs and SiO₂/Au/CNTs catalysts was higher than Au/CNTs catalyst (3.37 × 10⁻³ s⁻¹) obviously. From this point, the oxide shells play an important role in enhancing the catalytic activities. However, the rate constant of ZnO/Au/CNTs catalyst was lower than Au/CNTs catalyst. Apparently, this phenomenon can be attributed to the transferred electron from the Au to ZnO, thus decreasing the catalytic activities, this will be discussed in detail later.

From industrial point of view, reusability was important for good catalyst, since it contributed significantly to lowering the operational cost in the catalytic process and wastewater treatment. In order to investigate the advantage of MO_x/Au/CNTs nano-composites and their applicability, reuse cycles of catalysts were tested for the reduction of p-NPh (Fig. 6). Experiment was performed by recovering and reusing the catalyst MO_x/Au/CNTs and keeping all other parameters constant. The results revealed that the MO_x/Au/CNTs series catalysts showed good catalytic activity for five reaction cycles activity without any significant decrease in the p-NPh conversion except for the ZnO shells. In addition, the sample with TiO₂ shells exhibited excellent activity, the conversion yield of p-NPh was still as high as 95.4% even after five runs. This finding made us believe that this catalyst might merit additional attention and have a good potential for practical applications.

Fig. 6 Comparison of conversion of the MO_x/Au calcined at 400 °C with repeated usage.

In accordance with the above experimental results and the theory analysis, a possible catalytic mechanism was illustrated in Fig. 7. According to traditional theory about the catalytic reduction of p-NPh by Au NPs, electron transfer takes place from BH₄⁻ to p-NPh through adsorption of the reactant molecules onto the Au catalyst surface, the catalytic efficiency is highly dependent on the large surface areas of Au NPs.³⁷ In our work, the CNTs treated by mixed acid offered a large number of active sites, which were advantageous for uniform growth and distribution of Au nanoparticles. As a result, the large surface areas of Au NPs and a number of Au/CNTs interfaces would form, which were beneficial to the improvement of catalytic activity. It is known that Fermi level alignment occurs whenever a metal and semiconductor are placed in contact, resulting in charge redistribution and the formation of a depletion layer surrounding the metal.³⁸ Since Au (5.1 eV) has a higher work function than CNTs (4.28 eV), electrons leave the CNTs from a thus depleted region near an Au/CNTs interface into the Au, which ends up with an electron-enriched region. In a similar way, the charge redistribution was occurred between Au and oxide shells. When the oxide shells have a lower work function than Au, the electrons will leave MO_x/Au near an Au interface into the Au. As we know, TiO₂ (4.2) and SiO₂ (3.03) has a lower work function, the electrons will leave MO_x near a MO_x/Au interface into the Au, resulting in the electrons gathered into the Au. However, when the oxide shells have a higher work function than Au, the electrons will leave Au near an MO_x/Au interface into the MO_x/Au. ZnO (5.2) has a higher work function, so the electrons gathered into the ZnO. When the existence of the surplus electrons added on the Au nanoparticles facilitates the uptake of electrons by p-NPh molecules that happen to be close to these regions. The more interfaces there are, the more such regions with surplus electrons exist. This in turn increases the chances for random of absorbed p-NPh to happen to be on top of such regions. Furthermore, lower work function results in a lower potential barrier.³⁹ Therefore, the samples of TiO₂/Au/CNTs and SiO₂/Au/CNTs allow larger electronic density into the Au creating interfaces. But ZnO/Au/CNTs catalyst resulted from a larger electronic density into the ZnO creating interfaces. In another words, the reaction would occur in the inert site, which in consequence decrease the reactive constant.

Fig. 7 Postulate mechanism of the catalytic reduction of p-NPh with the MO_x/Au/CNTs catalyst.

4. Conclusion

Various oxide shells nanotubes catalyst with mesoporous structure has been successfully fabricated. This material was obtained by successively deposited Au nanoparticles, MO_x shells on modified CNTs, and then calcined at high temperature to remove organics and produce mesoporous structure. The results demonstrated that all the oxide shells could serve as an effective barrier to prevent the migration and aggregate of Au NPs during the calcination, especially for the SiO₂ shells, which exhibited the most excellent sintering resistance and maintained the Au NPs at 4.8 nm. In our experiments, the synthesized MO_x/Au/CNTs catalyst was evaluated by the reduction of p-NPh and showed good activity and reusability. Particularly, the sample with TiO₂ shells exhibited excellent performance and the conversion yield of p-NPh to p-APh was still maintained at 95.4% even after five cycles.

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). Fund Project for China Scholarship Council (no. 201308320238) and Instrumental Analysis Fund of Southeast University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07236g

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