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).
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.
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.
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.
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%.
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.
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.
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].
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.
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.
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 ℃).
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.
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.