The popular of nanoscience has exploded within the last two decades, mainly because new nano-synthesis processes have led to a wealth of novel nanostructures with adjustable size and shape parameters, leaving researchers with extraordinary control over their chemical properties. Metal-oxide hybrid nanoparticles are quite interesting and popular among novel nanostructures   . The materials systems are expanded from single-component nanoparticles to hybrid multicomponent heteronanostructures which integrate discrete domains of different compositions within one hybrid nanostructured entity    . Metal-oxide hybrid nanoparticles represent an important class of multicomponent nano-systems that may exhibit not only a combination of properties from the individual components but also further enhanced property and even new synergistic properties which arise essentially from the nanoscale interactions between the disparate metal and oxide components  . Two fundamental explanations of interaction between metal-oxide are proposed, so-called interfacial charge redistribution-electronic interaction   and interfacial atom transport-chemical interaction   . Furthermore, such nanoparticles defined by a diameter of 1 - 10 nm, are creating a new category of materials, which is different either from conventional bulk materials or from atoms, the smallest units of matter   . Thus, oxide supporting with ultrafine metal nanoparticles have tremendous prospects.
Tauster et al.  proposed the strong metal-support interaction (SMSI) firstly in 1978 to explain the suppression of H2 and CO chemisorption capacity of metal cluster on TiO2 chemically reduced at high temperatures. Since then, a large number of researches in catalysis have been reported, and they have extensively demonstrated that the SMSI in supported group VIII metal plays a prominent role in catalytic performances   . The SMSI states are mainly influenced by two factors: geometric and electronic. The geometric factor is due to encapsulation, which results from a physical covering of metal particles by a thin layer of reduced oxide support (e.g. TiO2, V2O5, SiO2 and Al2O3)  . The electronic factor is determined by a perturbation (for example, a charge transfer between the metal and the oxide) of the electronic of the metal catalyst  . It is reported that the metals with high surface energy such as Pt and Pd are more likely to be encapsulated than those with low surface energy (e.g. Au and Cu)  .
The selective oxidation of cyclohexane to cyclohexanone and cyclohexanol is one of the most compelling researches in the field of catalytic chemical research, for the oxidation products (cyclohexanone and cyclohexanol) are very important chemical raw materials and intermediates, mainly for the production of adipic acid and caprolactam, which is the monomer of nylon-6 and nylon-66, respectively   . Metal-oxide hybrid nanoparticles have been applied to cyclohexane oxidation in the literature. Zhu et al.    prepared Au/Al2O3, Au/Al2O3/SiO2 and Au/TiO2/SiO2 catalysts by deposition precipitation method, and the good results of cyclohexane oxidation suggested that gold supported on the oxides might be superior catalysts. However, turnover frequency (TOF) of these catalysts is relatively low and reaction time is long.
Herein, we report a novel complex nano-structured Au@TiO2 catalyst prepared by photocatalytic reduction method and deposition precipitation method. In particular, Au nanoparticles with a diameter around 7 nm are well-decorated on the TiO2/MCM-22 surface. The as-synthesized Au@TiO2/MCM-22 nanocomposites are used for cyclohexane oxidation. For the synergistic effects of Au and TiO2 in the nano-Au@TiO2 hybrid structure, Au@TiO2/MCM-22 has enhanced catalytic activity remarkably for oxidation of cyclohexane. The recyclability tests display that the as-synthesized catalyst has strongly chemical and mechanical stability. Supported noble metal nanoparticles catalysts (Pd, Pt, Ru, etc.) with nano-Me@TiO2 complex structure could be also prepared and applied into different reaction system.
2.1. Catalyst Preparation
2.1.1. Preparation of TiO2/MCM-22 by Sol-Gel
The MCM-22, referring to Wang et al.  using hexamethyleneimine (HMI) as template, was synthesized. A certain amount of MCM-22 (1.00 g) and 5 mL of tetrabutylammonium titanate (TBOT) were dissolved together in ethanol (15 mL), and the solution was stirred at 300 rpm at 293 K for 12 h. Then a small amount of HNO3 was added under stirring. After aging for 24 h at ambient temperature and drying for 24 h at 363 K, the resulting mixture was calcined for 3 h at 823 K thermostatically. According to principles of the spontaneous monolayer distribution, metal oxide would disperse on the support with a state of the spontaneous monolayer distribution, when the content of metal oxide is less than a certain threshold. Thus, the TiO2 loaded on the MCM-22 could form a monolayer or multilayers, which could be achieved by adjusting the loading of TiO2.
2.1.2. Preparation of Au@TiO2/MCM-22 by Photocatalytic Reduction Method
Typically, 0.5% Au@TiO2/MCM-22 (PR) catalyst were prepared as follow: the prepared TiO2/MCM-22 samples (1.00 g) were dispersed in 100 mL distilled water and treated by ultrasonic agitation for 10 min. The slurry was transferred into the reactor, and distilled water was added to dilute the mixture to 90 mL and stirred for 1 h. Subsequently, HAuCl4・4H2O (0.1050 g) were added. Two drops of HNO3 solution were added to adjust the pH to 3; the suspension was irradiated with a 15 W UV lamp for 4 h. Finally, the samples were separated by filtration, washed several times by distilled water, dried for 4 h at 355 K. The obtained material was denoted by Au@TiO2/MCM-22 (PR).
2.1.3. Preparation of Au@TiO2/MCM-22 by Deposition Precipitation Method
Typically, 0.5% Au@TiO2/MCM-22 (DP) catalyst were prepared as follow: HAuCl4・4H2O (0.1050 g) were added into the mixer of TiO2/MCM-22 (1.00 g) and distilled water (90 mL), which was mixed uniformly by ultrasonic. After adjusting pH to 9.0 with NaOH solution, the solution was stirred for 4 h in dark. The suspension was filtrated, washed by distilled water, dried at 255 K, and calcined at 523 K for 6 h. The obtained material was denoted by Au@TiO2/MCM- 22 (DP).
2.2. Catalyst Characterization
The nitrogen cryogenic adsorption isotherms were obtained on an AUTOSORB- 1-MP-type physical adsorption instrument. Specific surface area measured by BET method, pore size distribution and pore volume calculated by the BJH method. The chemical compositions of the samples were determined by ICP-AES (IRIS Intrepid II XSP). Fourier transform infrared (FT-IR) spectroscopy measurement was recorded on Nicolet 380 equipment, using KBr pellet method. X-ray Diffraction (XRD) patterns were collected on a D/max-3c diffract meter operated at 100 mA and 40 kV. The step size was 0.02 degree and the step time was one min. Transmission electron microscopy (TEM) images were obtained on a JEM-2010 at 160 kv. UV-Vis spectroscopy measurement was recorded on Japan UV-2550 equipment.
2.3. Catalytic Reaction Experiments
In a typical oxidation reaction, 100 mL cyclohexane, 0.08 g catalyst and 0.4 g tert-butyl hydrogen peroxide (TBHP) as initiator were added in a 300 mL autoclave reactor. The reactor was heated to 423 K and the O2 pressure was adjusted to 1.0 MPa. After 1.0 h of reaction, the liquid was extracted from the autoclave and the catalysts were separated. The product of cyclohexanol and cyclohexane were analyzed by gas chromatograph (Agilent 6890N) with internal standard method using chlorobenzene as the standard substance. The concentration of cyclohexyl hydroperoxide (CHHP) was analyzed by iodometric, and the acids and esters were determined by acid-base titration.
3. Results and Discussion
3.1.1. N2 Adsorption-Desorption Isotherms
Table 1 shows the N2 adsorption-desorption isotherms of the as-prepared samples. A dramatic decrease of the specific surface area can be discerned after doping of TiO2, while there are only mild decreases in pore volume. This can be the reason of that TiO2 is layered dispersion in the bore wall of the MCM-22 hole ra- ther than a cluster state. Moreover, after loading gold, the pore size of TiO2/MCM- 22 is decreased slightly. This is mainly because a part of pores of TiO2/MCM-22 may be blocked by the depositing of gold onto the substrate   .
3.1.2. ICP-AES Analysis
The Au loading efficiency of prepared samples is revealed by ICP-AES in Table 2. It is evident that modifying MCM-22 with TiO2 is useful to improve the loading of Au indeed (Table 2, Entry 1 and 2), and this is consistent with Xu et al.  . Entry 2 and 5 in Table 2 show that photocatalytic reduction method can
Table 1. Textural properties obtained from N2 physisorption.
aCalculated from the adsorption branch of the nitrogen adsorption and desorption isotherms. Au: 0.5 wt%.
Table 2. Loading efficiency of Au nanoparticles under different conditions.
aAu@TiO2/MCM-22 (PR) (repeated use four times). bLoading efficiency = Actual Au * m (catalyst)/No- minal Au * 1.00 (1.00: theoretical mass of the catalyst).
enhance the loading efficiency of Au significantly, compared with deposition precipitation method. The gold loading on Au@TiO2/MCM-22 (PR) doesn’t alter basically, even after being utilized four times (Table 2, Entry 5 and 9). Nominal Au raises the Au loading but obviously reduces the loading efficiency (Table 2, Entry 3, 4, 5 and 7). This may be the result of deficient illumination time. Perhaps because of the loss of the support during preparation, the loading efficiency exceeds 100% (Table 2, Entry 3 and 4). With the growth of illumination time, the loading efficiency is also improved (Table 2, Entry 6, 7 and 8), which indicates that the ample illumination time could achieve gold load entirely.
3.1.3. XRD Analysis
The X-ray diffraction (XRD) patterns of TiO2/MCM-22, Au@TiO2/MCM-22 (DP) and Au@TiO2/MCM-22 (PR) are shown in Figure 1. It is obvious that all the samples have some similar peaks. The peaks at 2θ = 25.3˚, 38.2˚, 48.1˚, 53.5˚, 55.1˚, 62.7˚, 68.8˚, 70.3˚ and 75.8˚ can be indexed to crystallite planes of (101), (004), (200), (105), (211), (204), (116), (220) and (215), respectively, which are ascribed to the anatase-TiO2 (JCPDS card No. 21-1272)  . The diffraction peaks at 2θ of 38.3˚, 44.4˚, 64.6˚ and 77.6˚ (Figure 1(c) and Figure 1(d)) are corresponding to (111), (200), (220), (311) Au plane, respectively, according to JCPDS card No. 76-1082  . Notably, no typical diffraction peaks belonging to the Au are observed in the Au@TiO2/MCM-22 (DP) nanocomposites. The reason can be
Figure 1. X-ray diffraction pattern of samples. (a) TiO2/MCM-22; (b) Au@TiO2/MCM-22 (DP); (c) Au@TiO2/MCM-22 (PR); (d) Au@TiO2/ MCM-22 (PR) (repeated use four times). Au: 0.5 wt%.
ascribed to the poor weight loading of metal nanoparticles on the TiO2/MCM-22 by deposition precipitation. In particular, after being used for four times, the Au@TiO2/MCM-22 (PR) sample still exhibits the typical signals of Au, which is in accordance with the ICP-AES result.
3.1.4. FT-IR Spectra
Figure 2 shows the FT-IR spectra of the prepared catalysts. It can be seen that there are strong absorption bands at 3440 and 1630 cm−1, which can be assigned to stretching vibration of −OH of adsorbed water and the vibration of Si-O-H bond  , respectively. The stretching vibration of Si-O-Si is the cause of bands position around 460 and 1080 cm−1  . Two additional absorption peaks at 965 and 557 cm−1 are attributed to the vibration of the newly-formed Si-O-Ti bond in the spectrum of TiO2/MCM-22 and Au@TiO2/MCM-22 (PR)   which indicate that the TiO2 has doped on the framework of MCM-22.
3.1.5. TEM Images
TEM images and Au nanoparticles size distribution histograms of prepared gold catalysts are shown in Figure 3. The gold nanoparticles are evident as small dark spots, while the TiO2/MCM-22 substrate is brighter and larger. Apparently, gold assembles together on MCM-22, and the diameter range of gold particles is rather broad (Figure 3(a)). The Au@TiO2/MCM-22 (DP) sample shows that modifying MCM-22 with TiO2 could decrease the loaded gold nanoparticles size (6 - 10 nm) remarkably (Figure 3(b)). Au nanoparticles are successfully immobilized on the TiO2/MCM-22 with a diameter around 5 - 9 nm (Figure 3(c)). After being utilized four times, the Au@TiO2-MCM-22 (PR) still maintains the original
Figure 2. FT-IR spectra of samples. (a) MCM-22; (b) Au/MCM-22; (c) TiO2/ MCM-22; (d) Au@TiO2/MCM-22 (PR). Au: 0.5 wt%.
morphology, which suggests that the gold nanoparticles load on the support stably (Figure 3(d)). Therefore, photocatalytic direct reduction is an effective method to obtain small size, well dispersion and stable structure nanoparticles.
3.1.6. UV-Vis Spectra
UV-visible spectroscopy is an effective method for characterizing the composition and structure of catalysts. Figure 4 shows the UV-vis spectra of MCM/22, Au/MCM-22, Au@TiO2/MCM-22 (DP) and Au@TiO2/MCM-22 (PR). Generally, a broad absorption band in the visible region around 520 nm is typical for the surface plasma resonance (SPR) of nano-sized gold  . Samples MCM-22 and TiO2/MCM-22 exhibit featureless spectrum because of the absence of gold. It was reported that with the increase of Au particle size, the SPR peak shifts to longer wavelength or intensifies at longer wavelength   , that is, the original peak became broader due to the coupling of the individual surface Plasmon of nanoparticles in the aggregated structure   . Thus, it is suggestion that the loading of the gold nanoparticles is Au@TiO2/MCM-22 (PR) > Au/TiO2/ MCM-22 (DP) > Au/MCM-22, perfectly matching the ICP-AES result. An obvious absorption peak at 330 nm can be found in the spectrum of samples c, d and e, which is attributed to the extra-framework TiO2  and indicates that TiO2 has doped on the support successfully.
3.2. Catalytic Oxidation of Cyclohexane
The catalytic performances of the above catalysts were investigated for cyclohexane oxidation. The blank experiment was carried out under the same reac-
Figure 3. TEM images of catalysts. (a) Au/MCM-22 (DP); (b) Au@TiO2/MCM- 22 (DP); (c) Au@TiO2/MCM-22 (PR); (d) Au@TiO2/MCM-22 (PR) (repeated use four times). Au: 0.5 wt%.
Figure 4. UV-visible spectra of samples. (a) MCM-22; (b) Au/MCM-22; (c) TiO2/ MCM-22; (d) Au@TiO2/MCM-22 (DP); (e) Au@TiO2/MCM-22 (PR). Au: 0.5 wt%.
Table 3. The results of cyclohexane oxidation over different catalystsa.
aReaction conditions: cyclohexane (100 mL), catalyst (0.08 g), TBHP (0.4 g), pressure of O2 (1.0 MPa), temperature (423 K), reaction time (1.0 h). bK: cyclohexanone; A: cyclohexanol; CHHP: cyclohexyl hydroperoxide; Others: acid and ester. Au: 0.5 wt%.
tion conditions. All the results are listed in Table 3. It is obvious that the reaction proceeds spontaneously (Table 3, Entry 1) and MCM-22 increases the conversion slightly at the cost of selectivity (Table 3, Entry 2). After TiO2 was deposited on MCM-22, the catalytic activity of TiO2/MCM-22 was improved because of the subsidiarity of TiO2 (Table 3, Entry 2 and 3). The prepared gold catalysts remarkably promote conversion and selectivity of cyclohexane oxidation simultaneously (Table 3, Entry 4 to 6), which confirms that the Au nanoparticles play an essential role in cyclohexane oxidation. Among all the gold catalysts, the Au@TiO2/MCM-22 (PR) exhibits the best catalytic performance for the oxidation of cyclohexane, in respect of conversion and selectivity (Table 3, Entry 6). Remarkable differences can be observed in TOF of cyclohexanol oxidation catalyzed by different gold catalysts (Table 4). The nano-structured Au@TiO2/ MCM-22 (PR) exhibits extremely higher TOF than the reported gold catalysts    . Experiment results also show that the TOF become higher with a decrease in the amount of Au (Table 4). There are three main factors which can explain the excellent catalytic performance. Firstly, the synergistic effect between nano Au and nano TiO2 plays a vital role. The interaction between the gold and the TiO2 leads to the gold becoming electron-rich  . With the help of TiO2, gold could activate oxygen more effectively. The outstanding oxygen storage capacity of TiO2 will be beneficial to the catalytic performance of nano-Au@TiO2 catalyst for cyclohexane oxidation. Secondly, the small particle size and even size distribution is another major reason for the prominent catalytic results. Thirdly, molecular sieve provides nano-Au@TiO2 with high specific surface area place. Additionally, TiO2 is a kind of reducible metal oxide that is able to store and release oxygen under oxygen rich and lean conditions, respectively  . It is indicated that the complex nano-structured catalyst Au@TiO2/MCM-22 exhibits extremely higher TOF than the reported gold catalysts, although the catalytic performance of Au@TiO2/MCM-22 prepared by photocatalytic reduction method is slightly superior to that of prepared by deposition precipitation method.
Table 4. TOF of different gold catalysts.
areaction conditions: cyclohexane 100 mL, catalyst 0.08 g, 423 K, 1.0 Mpa O2, 1 h. breaction conditions: cyclohexane 100 mL, catalyst 0.08 g, 423 K, 1.0 MPa O2, 1 h. creaction conditions: cyclohexane 100 mL, catalyst 0.02 g, 423 K, 1.0 Mpa O2, 2.5 h. dreaction conditions: cyclohexane 2 mL, catalyst 0.005 g, 423 K, 1.0 MPa O2, 3 h. ereaction conditions: cyclohexane 20 mL, catalyst 0.005 g, 423 K, 1.5 MPa O2, 3 h. freaction conditions: cyclohexane 2 mL, catalyst 0.005 g, 423 K, 1.0 MPa O2, 6 h. greaction conditions: cyclohexane 100 mL, catalyst 0.08 g, 423 K, 1.0 Mpa O2, 1 h. hreaction conditions: cyclohexane 100 mL, catalyst 0.08 g, 423 K, 1.0 MPa O2, 1 h. ireaction conditions: cyclohexane 20 mL, catalyst 0.05 g, 423 K, 1.5 Mpa O2, 3 h. jreaction conditions: cyclohexane 20 mL, catalyst 0.05 g, 423 K, 1.5 MPa O2, 1 h. kcalculated by moles of cyclohexane converted per mole of Au per hour.
3.3. Recyclability Tests of the Prepared Catalyst
The recyclability tests were performed to evaluate the reused ability and stability of Au@TiO2/MCM-22 (PR) in cyclohexane oxidation. At completion of each stage, the products were analyzed and the catalyst was recovered by filtration, thoroughly washed in ethanol, dried at 255 K, and then reused for a new set of cyclohexane oxidation experiment. The results in Figure 5 show that there is little fluctuation of conversion or selectivity, namely, Au@TiO2/MCM-22 (PR) performs good recyclability. We believe that the SMSI between Au and TiO2 largely prevented the agglomeration of active sites during the oxidation reaction  .
In summary, a novel gold nano-catalyst with nano-structured Au@TiO2 has been successfully prepared. Nano-gold is immobilized on the nano-TiO2 layer which is spread on the microporous molecular sieve MCM-22. Characterized results show that gold nanoparticles are immobilized on the substrate with small size (5 - 9 nm) and uniform dispersion. The complex nano-structured Au@TiO2/ MCM-22 exhibits enhanced catalytic activity for cyclohexane oxidation. The catalytic performance of Au@TiO2/MCM-22 prepared by photocatalytic reduction method is slightly superior to that of prepared catalyst by deposition precipitation. The outstanding catalytic performance could attribute to the synergistic effects of Au and TiO2 in the complex nano-structured Au@TiO2/MCM-22. The as-synthesized catalyst is chemically and mechanically robust and can be easily separated and reused. Supported noble metal nanoparticles catalysts (Pd, Pt, Ru, etc.) with the complex nano-structured Me@TiO2
Figure 5. Effect of regeneration time on selective oxidation of cyclohexane.
could also be high efficient and be applied to different reaction systems. Recently, our group finds that nano-Pd@TiO2 exhibits remarkably catalytic performance for hydrogenation of phenol under mild condition. We will also implement research on the nano-struc-ture Me@MOx (MOx: metal oxides, such as CeO2, ZnO, ZrO2, etc.). The relative results will be revealed subsequently.
We gratefully acknowledge the National Natural Science Foundation of China (No. 21676227) and the Project supported by Hunan Provincial Natural Science Foundation of China (No. 09JJ3021) for the financial supports to the present work.
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