催化学报  2016, Vol. 37 Issue (10): 1787-1793   PDF    
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Chen Haijun
Liu Chao
Wang Min
Zhang Chaofeng
Li Gao
Wang Feng
Thermally robust silica-enclosed Au25 nanocluster and its catalysis
Chen Haijuna,c, Liu Chaob, Wang Mina, Zhang Chaofenga,c, Li Gaob, Wang Fenga     
a. State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China ;
b. Gold Catalysis Research Centre, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China ;
c. Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Foundation Item: This work was supported by the National Natural Science Foundation of China (21273231, 21422308), Dalian Excellent Youth Foundation (2014J11JH126) (FW), and the Starting Funds of “Thousand Youth Talents Plan” (GL)
* Corresponding author. Gao Li, Tel/Fax: +86-411-82463017; E-mail: gaoli@dicp.ac.cn Feng Wang, Tel/Fax: +86-411-84379762; E-mail: wangfeng@dicp.ac.cn
Abstract: Well-defined gold nanoclusters with average size less than 2 nm have emerged as a new and novel catalyst. The gold nanocluster loaded on the oxide surface usually aggregates to larger particles at high temperature (>300℃), which is caused by the removal of the surface ligands. We herein present a novel method to prepare Au25 cluster catalyst (~1.3 nm) with high thermal stability (up to 400℃). Au25@SiO2 is synthesized via a co-hydrolyzing reaction of Au25[SC3H6Si(OCH3)3]18 and tetraethyl orthosilicate, and then it is treated at different temperature (e.g., 200, 300, 400℃) in air to remove the organic ligands. Au25@SiO2 is well characterized by transmission electron microscopy, ultraviolet-visible spectroscopy and diffuse reflectance UV-vis spectroscopy. Further, the Au25@SiO2 catalysts are investigated in the hydrogenation of p-nitrophenol into p-aminophenol.
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Key words: Gold nanocluster     Au25     Thermal stability     Silica     Hydrogenation    
高热稳定性二氧化硅包覆的Au25纳米簇的制备和催化应用
陈海军a,c, 刘超b, 王敏a, 张超锋a,c, 李杲b, 王峰a     
a. 中国科学院大连化学物理研究所催化基础国家重点实验室, 洁净能源国家实验室(筹), 辽宁大连 116023 ;
b. 中国科学院大连化学物理研究所催化基础国家重点实验室金催化研究中心, 辽宁大连 116023 ;
c. 中国科学院大学, 北京 100049
摘要:具有独特的电子和几何结构,原子精确控制的金纳米簇( < 2 nm)成为一种新的具有广泛研究和应用前景的纳米催化剂.负载在氧化物表面的金纳米簇通常会在高于300℃时聚集或长大.人们已经通过多种方法成功制备了对于非原子精确控制的热稳定性的金纳米颗粒.主要包括利用金属与载体强相互作用,用可还原的金属氧化物来稳定金纳米颗粒;利用物理阻隔作用使用高比表面积的载体或制备核壳、纳米粒子镶嵌在载体中来稳定金纳米颗粒.对于原子精确控制的金纳米簇,由于其外边包覆着一层配体,将其负载到载体上时要保证配体不被破坏才能保证金纳米簇的结构完整性,负载后通常要除去配体才能使催化活性位曝露出来.目前,高热稳定性(>300℃)的金纳米簇的制备方法还较少. 由于金与SiO2相互作用较弱,将超小( < 2 nm)的金纳米粒子包覆于其中非常困难.因此,本文首先制备了1.3 nm的含有硅酯键的巯基配体(3-巯丙基三甲氧基硅烷)保护的Au25[SC3H6Si(OCH33]18,然后将其在刚成核的SiO2表面与正硅酸四乙酯共水解,得到了既保留了Au25的完整结构,又避免了Au25之间相互水解的Au25(SC3H6SiO3)18@SiO2纳米材料.漫反射固体紫外-可见光谱证明了Au25在包覆完成后结构的完整性.透射电镜结果表明,Au25纳米簇焙烧至400℃未发生明显聚集长大.对硝基苯酚还原实验结果表明,不同温度处理后的Au25@SiO2配体在200℃开始脱除,温度高于传统的负载型Au25催化剂,表明Au25是在SiO2内部而不是在表面,从而使配体不易离去.400℃处理后的Au25@SiO2对4-硝基苯酚还原表现出最高的反应活性,表明该纳米簇在400℃处理后没有发生明显聚集长大.
关键词金原子簇     Au25     热稳定性     二氧化硅     氢化    

1 Introduction

Well-defined gold nanoclusters with average particle size less than 2 nm have emerged as a novel catalyst in the selective oxidation and hydrogenation and carbon-carbon coupling reactions [1-5]. In recent decades, thiolate-stabilized Au25(SR)18 nanoclusters are well investigated (where SR denotes thiolate). Late studies found that Au25(SR)18cluster was catalytically active in styrene epoxidation [6-8], cyclohexane oxidation [9], CO oxidation [10], hydrogenation of aldehydes and ketones [11], 2-nitrobenzonitrile reduction [12] and C-C bond formation reactions [13, 14]. Some of these studies attributed the catalysis of Au25(SR)18 nanocluster to its unique core-shell structure consisting of electron-rich Au13 core and electron-deficient Au12 shell. However, poor thermal stability of gold nanocluster remains a challenge for its catalysis application. When the thiolate ligands are removed at high temperature ( > 300 ℃), either supported or unsupported gold nanocluster tends to aggregate into large particles [15]. On the other hand, many efforts have been taken to stabilize gold nanoparticles. One method is to stabilize gold nanoparticles on the reducible metal oxide via their interaction. Reducible metal oxides, such as CuO [16] and CoOx[17] were added into silica to enhance the stability of gold nanoparticles. The other method is to confine the gold nanoparticles in the matrix of support, such as the core-shell structure, or in the partially-embedded support [18-20]. Up to now, the selection of support is believed to enhance the stability of nanoparticles [21-23].

Silica is a frequently used support for catalyst. Apart from its high specific surface area and diverse pore structures, its inert nature renders it an attractive support reflecting the intrinsic catalysis of gold species. Confinement of Au nanoparticles in silica is an efficient way to retard the growth of gold nanoparticles. Thus, some endeavors were taken to prepare Au@SiO2 core-shell structure [18, 24, 25]. Above all, the above efforts to maintain the size of gold are for larger gold nanoparticles (e.g., > 2 nm) or polydispersed ones.

As for gold nanocluster, heterostructured mesoporous support [26] and porous carbon [27] can maintain the size of Au25 nanocluster separately after thermal treatment to remove the thiolate ligands. It is still imperative to stabilize these ultrasmall atomic precise clusters to resist sintering and deactivation. Herein, we present a simple method to prepare gold cluster with uniform size enclosed by silica (Au25@SiO2). Firstly, the parent nanocluster Au25[SC3H6Si(OCH3)3]18 was prepared by “size-focusing” of polydispersed Aun(PPh3)mClz nanoclusters in the presence of (3-mercaptopropyl) trimethoxysilane (MPTS). Further, MPTS ligands on the surface of Au25 nanoclusters were hydrolyzed with tetraethyl orthosilicate (TEOS) on the periphery of silica core to facilitate the coating of silica to prepare the Au25@SiO2 catalyst. Finally it showed that the Au25@SiO2 was stable at 400 ℃ in air and exhibited good catalytic activity in the reduction of 4-nitrophenol (TOF reach up to 343 h-1).

2 Experimental
2.1 Materials

All chemicals were of analytical grade and used as purchased without further purification. HAuCl4·3H2O was purchased from Sigma-Aldrich. Ammonia water (25%-28%) was purchased from Kermel Chemical Reagent Co., Ltd. PPh3 and NaBH4 were purchased from Sinopharm Chemical Reagent Co., Ltd. MPTS was purchased from WD Silicone Co., Ltd. TEOS and 4-nitrophenol (4-NP) were from Shanghai Aladdin Bio-Chem Technology Co., Ltd.

2.2 Synthesis of Au cluster precursor Aun(PPh3)mClz

HAuCl4·3H2O (1.418 g, 3.6 mmol) was dissolved in 50 mL ethanol. Then triphenylphosphine (2 equiv., 1.904 g) was gradually added in the above ethanol solution with constant stirring. The system became white suspension in ultrasonic for 2 min. The dark yellow solution turned to faint yellow in several minutes and white precipitate emerged, followed by agitated stirring for 2 h. The obtained white product Aun(PPh3)mClz was purified by washing with water for three times and ethanol for two times. The as-obtained white product was dried under vacuum for 5 h at room temperature.

2.3 Synthesis of Au25[SC3H6Si(OCH3)3]18 nanoclusters

Aun(PPh3)mClz (44 mg) was dissolved in 2.5 mL dichloromethane and 5 mL ethanol under ultrasonic. Another 2.5 mL dichloromethane was added to get a homogeneous system, followed by stirring for 30 min. Then, MPTS (49 μL) was added to the solution under constant stirring. After 30 min, NaBH4 (3.4 mg) dispersed in 2 mL ethanol was added dropwise and the color of the solution turned to orange, gradually to brown, finally to dark. Another amount of MPTS (24 μL) was injected after 30 min. The reaction was terminated when the absorption features of Au25 clusters appeared and no longer stronger in the ultraviolet-visible (UV-vis) analysis. The solvent was evaporated by rotary evaporation and the obtained black products were extracted by acetonitrile and then centrifuged to remove insolubles. Evaporate the solvent and the obtained Au25[SC3H6Si(OCH3)3]18 (Au25(SR)18, hereafter) nanocluster was re-dispersed in 5 mL ethanol.

2.4 Preparation of Au25 enclosed in SiO2

About 2.7 mL aqueous ammonia was dissolved in 98 mL ethanol and 31 mL water (solution A). TEOS (3.45 mL) was dissolved in 65 mL ethanol (solution B). Firstly, 17 mL solution B was added to solution A dropwise. Then, 4.1 mL Au25 solution and the remaining solution B were mixed together to add into solution A drop by drop. After stirring for 12 h, the obtained Au25 enclosed in SiO2 (Au25(SR)18@SiO2) was washed with ethanol and acetone four times by centrifugation. The solid was dried in the oven at 60 ℃, then the material was calcined at different temperatures (e.g., 150, 200, 300, 400 and 600℃) in air at a heating rate of 2 ℃/min for 1 h, named as Au25@ SiO2-150, Au25@SiO2-200, Au25@SiO2-300, Au25@SiO2-400 and Au25@SiO2-600, respectively. Of note, the nominal gold content of these catalysts is 1.4 wt%.

2.5 Characterization

The transmission electron microscopy (TEM) was performed on a JEM-2100 microscope at an accelerating voltage of 200 kV. The X-ray energy dispersive (EDX) results were obtained by JEM-2100 energy dispersive spectrometer. The UV-vis spectra were acquired on a Hewlett-Packard Agilent 8453 diode array spectrophotometer at room temperature. Diffuse reflectance UV-vis spectra were recorded by Perkin Elmer Lamada 750 UV/VIS/NIR Spectrometer.

2.6 Catalytic hydrogenation of 4-nitrophenol

A fresh aqueous solution of NaBH4 (6.3×10-2 mol/L, 1.0 mL) was mixed with an aqueous solution of 4-nitrophenol (4-NP) (2.0 × 10-3 mol/L, 1.0 mL) in a plastic tube. The catalysts (2 mg) dispersed in 0.8 mL of H2O was added to the solution. The absorption spectra of the mixed system were recorded by UV-vis spectrophotometer. During the reaction, 0.02 mL of the mixed system was diluted with 0.8 mL H2O at certain time and then measured by UV-vis spectrophotometer. The reaction was conducted at room temperature (25 ℃). The turnover frequency (TOF) was calculated based on the nominal molar content of gold.

3 Results and discussion
3.1 Synthesis and characterization of catalysts

The preparation procedure of Au25 nanocluster is shown in Scheme 1. The structure of Au25[SC3H6Si(OCH3)3]18 is confirmed by UV-vis spectrum which is deemed as the “fingerprint” of the atomically precise gold clusters. As shown in Fig. 1, the distinct absorption peak appeared at 692 nm, in consistent with the previous literatures [1, 28]. The blue shift of the absorption peak may be caused by the intact silicon ester bond due to the anhydrous environment in our system, which also is found in case of the Au25(SC2H4Ph)18 [10, 28]. The 416 nm absorption peak arised from an interband transition within icosahedral Au13 units, and the 450 nm band comprised of mixed intraband and interband transitions. Further, TEM analysis indicates that the Au25 nanocluster shows a uniform size distribution of about 1.3 nm as indicated by Fig. 2, which matches well with the size of Au25 clusters [1, 28].

Scheme1. The flowchart of preparation procedure of Au25[SC3H6Si(OCH3)3]18 nanocluster.

Fig. 1. UV-vis spectrum of Au25[SC3H6Si(OCH3)3]18 (dispersed in ethanol).

Fig. 2. (a) TEM image of Au25[SC3H6Si(OCH3)3]18nanoclusters and (b) size distribution histogram of the Au25[SC3H6Si(OCH3)3]18.

Then, the gold nanoclusters were hydrolyzed at the periphery of the silica core (Scheme 2). A portion of TEOS was first added to a water and ethanol mixed solution (volume ratio 98:31) which contained 2.7 mL ammonia water, followed by addition of the ethanol solution of Au25 and TEOS. This process allowed the silica first to nucleate and Au25(SR)18 nanoclusters were enclosed in silica by the subsequent co-hydrolysis with the added TEOS. Therefore, the gold nanoclusters were avoided to be cross-hydrolyzed and aggregated easily. The as-obtained Au25 enclosed in silica was firstly characterized by diffuse reflectance UV-vis spectrum. The distinct absorption peaks of Au25 were reserved as indicated in Fig. 3(1). It indicates the Au25 nanoclusters were intact and well enclosed in silica. The broadening of the absorption peak around 439 nm probably caused by the hydrolyzing of the silicon ester bound to Au25.

Scheme2. Preparation of Au25[SC3H6SiO3]18@SiO2.

Fig. 3. Diffuse reflectance UV-vis spectra of Au25@SiO2 samples calcined at different temperatures. (1) As prepared sample; (2) 150 ℃; (3) 200 ℃; (4) 300 ℃; (5) 400 ℃. The spectra are shifted in order to distinguish them.

3.2 Thermal treatment

We next investigated the thermal stability of silica enclosed gold clusters. The annealed temperature was conducted from 150 to 600 ℃. At 150 and 200 ℃, the remained distinct absorption peaks of Au25 indicate little or no sintering of Au25 occurred (Fig. 3). The small shift of the peak may be caused by partial rupture of ligands during the annealing course. Previous studies pointed out that the ligands of Au25 would be ruptured at 200 ℃ [29, 30]. The naked ultrasmall gold nanoclusters (core size < 1.5 nm) would be aggregated at above 150 ℃ [31]. Our Au25@SiO2 material can still be stable at 200 ℃ as demonstrated by the partial distinct absorption characterization of Au25. When the temperature exceeded 200 ℃, the disappeared absorption peak at 692 nm indicates the removal of the -SR moieties and destruction of the Au13structure, Fig. 3. Meanwhile, plasmon resonance absorption peak at around 520 nm was observed in all the samples (Fig. 3), which is probably due to partial aggregation of gold clusters.

No apparent cluster sintering was demonstrated by TEM as exhibited in Fig. 4. It is worthwhile to note that all the size of gold nanoparticle in Au25@SiO2, Au25@SiO2-150, Au25@ SiO2-250, Au25@SiO2-300, and Au25@SiO2-400 samples is ca. 1.7-2 nm. And it is slightly larger than ligand-protected Au25 nanoclusters (core size is ca. 1.3-1.4 nm), which is mainly due to the gold atoms of Au25 nanosphere would extend and be structure reconstruction of Au25 after the removal of the protecting ligands. The cluster size remained 2 nm even when elevated temperature up to 400 ℃ was employed. EDX results indicated that the gold content was 1.2 wt%. We also plotted the molar ratio of S to Au versus treated temperature for the Au25@SiO2 catalyst from the EDX results, as shown in Fig. 5. This indicated that the thiolate ligands were gradually departed from the surface of gold with the increasing temperatures.

Fig. 4. TEM images and size distributions of Au25@SiO2 (a, b), Au25@ SiO2-150 (c, d), Au25@SiO2-250 (e, f), Au25@SiO2-300 (g, h), Au25@ SiO2-400 (i, j), and Au25@SiO2-600 (k, l).

Fig. 5. The molar ratio of S to Au of the Au25@SiO2 catalysts at the elevated temperatures (based on the EDX results).

3.3 Catalytic tests for reduction of 4-NP

Catalytic reduction is one of the most important applications of gold nanoparticles [32-34]. We conducted the reduction of 4-NP to 4-aminophenol (4-AP) using Au25@SiO2 calcined at different temperature as the catalyst. As shown in Fig. 6, the absorption peak at 400 nm appeared when a fresh aqueous of NaBH4 was added to the 4-NP solution, which suggests that 4-NP (NO2PhOH) is transferred to 4-nitrophenolate (NO2PhO-). Au25@SiO2 and Au25@SiO2-150 shows no catalytic activity in the reactions as the 400 nm absorption peak remained unchange in 60 min, indicating the Au25 cluster was enclosed by silica rather than on the silica surface. On the other hand, the absorption peak of 4-nitrophenolate diminished gradually after the addition of the catalysts which was annealed at 200 ℃, indicating the Au25@SiO2-200 showed some catalytic activity. Two new peaks appear at 230 and 300 nm in the absorption spectra signified the formation of 4-AP. The results show that the catalytic activity of Au25 nanoclusters are largely improved after the ligand removal, in consistent with the reported literatures [35-37]. The catalytic reaction ceased after 15 min when it is catalyzed by Au25@SiO2-200, which is caused by the only partial thiolate ligand was detached (Fig. 6(a)) and a few active sites are exposed to the reactants. Meanwhile, the absorption peak of 4-nitrophenolate ion diminished gradually and almost disappeared with 10 min in the case of the Au25[SC3H6SiO3]18@SiO2 treated above 200 ℃ (e.g., 300 and 400 ℃).

Fig. 6. Time-dependent UV-vis spectra for the reduction of 4-NP using Au25@SiO2 samples after different annealing temperatures. (a) 200 ℃; (b) 300 ℃; (c) 400 ℃; (d) 600 ℃.

The kinetics of 4-NP reduction were showed in Fig. 7. The kinetic reaction rate constants for Au25@SiO2-200, Au25@SiO2-300, Au25@SiO2-400, and Au25@SiO2-600 were calculated to be 0.067, 0.163, 0.180, and 0.156 min-1. There are two possibilities for the increasing activity after high-temperature annealing. One is that the Au 25 clusters sinter and grow into large nanoparticles, and the large particles are better catalyst for this reaction. The other one is that more ligands are removed from the Au25 clusters at high temperatures. For Au25@SiO2-600, a small fraction of nanoparticles ( > 4 nm) appears while the catalytic constant rate (0.156 min-1) for the reduction of 4-NP decreases (0.180 min-1 for Au25@SiO2-400). The fractions with the diameter > 3 nm are 3.4% of statistical 236 nanoparticles for Au25@SiO2-400 and 8.9% of statistical 226 nanoparticles for Au25@SiO2-600 respectively (from the TEM results, Fig. 4). These results indicate that large nanoparticles give rise to lower catalytic activity. The increasing activity of catalysts with high-temperature annealing (e.g., > 200 ℃) is caused by ligands removal. Au25@SiO2-400 catalyst shows the best catalytic activity, which was caused by the ultrasmall size of the nanoclusters (ca. 2 nm). The TOF of Au25@SiO2-400 is 343 h-1. It is noteworthy that the reaction constant of Au25@SiO2-400 catalyst was about equal to previous work [33], while the concentration of NaBH4 was only half amount.

Fig. 7. Plots of ln(Ct/C0) vs. time using Au25@SiO2 samples after different annealing temperatures.

4 Conclusions

In summary, we prepare uniform Au25 nanocluster enclosed in silica with high thermal stability (up to 400 ℃ in air atmosphere). The structure of Au25@SiO2 was confirmed by diffusion reflectance UV-vis spectrum with the comparison with that of the parent cluster―Au25[SC3H6Si(OCH3)3]18. The Au25@SiO2 material can be stable even at 400 ℃ and can effectively catalyze reduction of 4-nitrophenol to 4-aminophenol. Au25 with atomically precise control was very vital to reveal the origin of gold catalysis. We believe this work paves the way to design and fabricate uniform robust nanoclusters for catalysis and other applications.

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