Article pubs.acs.org/JPCC
Magnetic Recyclable Nanocomposite Catalysts with Good Dispersibility and High Catalytic Activity Mingyue An, Jiabin Cui, and Leyu Wang* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, School of Science, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *
ABSTRACT: Nanocomposite catalysts containing both magnetite (Fe3O4) and palladium (Pd) nanoparticles with magnetic separation and recyclability were successfully fabricated via polymer encapsulation and then silica coating and applied for catalytic hydrogenation of 4-nitrophenol. Fe3O4 nanoparticles were used as not only the prerequisite of magnetic separation but also the supports to prevent the aggregation of Pd nanoparticles at high temperature. Moreover, the surfactants and polymer supports on the particle surface were removed by calcination, and thus the catalysis centers (Pd nanoparticles) were totally exposed to the reactants, which is preferable for a good catalysis efficacy. The results of catalytic hydrogenation of 4-nitrophenol demonstrated that the catalytic activity of these as-prepared nanocomposite catalysts were well maintained even after 10 repeated cycles. Unlike the noble metal nanoparticle decorated large magnetic nanosphere with residual magnetism, these as-prepared nanocomposites based on superparamagnetic nanoparticles possess many advantages including high catalytic activity, convenient magnetic separation, good dispersibility, high water stability, and excellent recyclability.
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oble metal nanoparticles based catalysts1−4 have attracted considerable interest due to their extensive applications in the field of catalysis.5−14 Generally, as catalytic centers, the noble metal nanoparticles should be small in size and welldispersed on the catalyst support.15−18 However, due to the high surface energy, small particle size usually leads to serious aggregation of noble metal nanoparticles especially at high reaction temperature.19−21 Recently, with the development of nanotechnology, this purpose may be achieved via immobilizing metal nanoparticles onto inorganic supports or stabilizing them by stabilizers including surfactants and polymers.22−26 Ideally, the surfactants or stabilizers should have moderate affinity toward noble metal nanoparticles, so that the aggregation of noble metal nanoparticles can be prevented while the high catalytic activities are well retained. Unfortunately, the surfactants or stabilizers immobilized onto the noble metal nanoparticles often result in partial loss of catalytic activity.21,27 On the other hand, the separation and recyclability of the catalysts was another challenge. To overcome this problem, magnetic nanomaterials were introduced in the fabrication of nanocomposite catalysts because these nanocomposites can be easily separated and recovered in a reaction mixture by using an external magnet.28−34 Up to now, much attention has been focused on the design of these efficient and recyclable nanocomposite catalysts.35−39 To the best of our knowledge, magnetic Pt−Fe40 (Pt−Co,41 Pt−Ni42) alloy nanoparticles and Au−Fe3O443 dumbbell nanocomposites have been fabricated. Although their small size and shape are desirably controlled, it is still a challenge to maintain their small size and thus good catalytic activity at high reaction temperature after the removal © XXXX American Chemical Society
of surfactants coated on the nanoparticles. In order to render the catalysts magnetically recyclable, the small noble metal nanoparticles were sometimes coated on the large magnetic nanospheres.44,45 However, these nanocomposites often suffer from bad dispersibility and serious aggregation because of the residual magnetism of the large magnetic core. Therefore, it is still a challenging target to get magnetic composite nanocatalysts with excellent catalytic activity, good dispersibility, and facile magnetic recyclability. Herein, we developed a facile strategy for the fabrication of Pd−Fe3O4@SiO2 nanocomposite catalysts. The magnetite nanoparticles were not only regarded as the foundation of magnetic recoverability but also the supports to prevent the aggregation of the Pd nanoparticles at high temperature. Because lots of magnetite nanoparticles were encapsulated in one composite nanosphere, the separation of the nanocomposites was easy to be achieved via an assistant magnet. On the other hand, these magnetic nanoparticles are superparamagnetic, so no residual magnetism can be found, which makes these composite nanocatalysts well dispersible without aggregation. As illustrated in Scheme 1A, oleic acid (OA)capped Pd and Fe3O4 nanoparticles were first embedded in the polymer matrix via the in situ emulsion polymerization46 to form the polystyrene (PS)-coated nanocomposites (Pd− Fe3O4@PS) before a thin layer of SiO2 was coated on the Received: September 16, 2013 Revised: December 23, 2013
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Scheme 1. Schematic Diagram for the Fabrication (A) and Catalytic Activity Evaluation (B) of Pd−Fe3O4@SiO2 Nanocomposite Catalystsa
a
NPs: nanoparticles; 4-NP: 4-nitrophenol; 4-AP: 4-aminophenol. Figure 1. TEM images of the hydrophobic Pd (a) and Fe3O4 (b) nanoparticles, Pd−Fe3O4@PS nanocomposites (c), Pd−Fe3O4@PS@ SiO2 nanocomposites before (d) and after calcination (e), and XRD pattern of the Pd−Fe3O4@SiO2 nanocomposites (f). PS: polystyrene.
polymer shell of Pd−Fe3O4@PS with the sol−gel method. The SiO2-coated nanocomposites were then calcinated at 400 °C for 5 h to remove the surfactants and polymer, and thus the surface of noble metal nanoparticles was exposed. Meanwhile, the gas produced by the combustion of surfactants and polymers acted as the porogenic agent, and the newly generated pores in Pd− Fe3O4@SiO2 nanocomposites can allow the chemical molecules to reach the surface of the Pd core. The thin SiO2 shell sustained the structure and morphology of nanocomposites and prevented them from collapsing after calcination in air atmosphere. The results in the case of catalytic hydrogenation of 4-nitrophenol suggested that these porous Pd−Fe3O4@SiO2 nanocomposite catalysts have many advantages including high catalytic activity, convenient magnetic separation, good dispersibility, high water stability, and excellent recyclability (Scheme 1B)
embedded in the amorphous polymer matrixes. In spite of coating a thin layer of silica shell through hydrolysis of tetraethyl orthosilicate (TEOS), the shape and size of the nanocomposites (Pd−Fe3O4@PS@SiO2) have no obvious change (Figure 1d). After calcination at 400 °C in the air to remove the polymer and surfactants, the amorphous polymer matrixes were invisible; however, the shape and size of the Pd− Fe3O4@SiO2 nanocomposites were well retained, and no aggregation of Pd nanoparticles was observed (Figure 1e). The compositions of the final nanocomposite catalysts (Pd− Fe3O4@SiO2) were further investigated by means of the powder X-ray diffraction (XRD). As shown in Figure 1f, the broad peak around 25° was attributed to the synergetic effect of amorphous silica. The peak at 40.15° can be assigned to the (111) reflection of Pd (JCPDS card no. 46-1043), and all other peak positions and relative intensities were in good agreement with those of Fe3O4 (JCPDS card no. 75-0449), suggesting the successful encapsulation of both Pd and Fe3O4 nanoparticles in the nanocomposites, which has been further confirmed by energy dispersive spectrometer (EDS) characterization (Figure S1 of the Supporting Information). As shown in the high resolution transmission electron microscopy (HRTEM) image (Figure S2 of the Supporting Information), the 0.224 nm of spacing between two adjacent lattice planes fits well with the data of the (111) plane of Pd. Meanwhile, the spacing of 0.200
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RESULTS AND DISCUSSION The morphology and size distribution of the building blocks (Pd and magnetite nanoparticles) and the nanocomposites were characterized via the transmission electron microscope (TEM). As shown in Figure 1, the as-prepared Pd (Figure 1a) and Fe3O4 (Figure 1b) are small nanoparticles with an average diameter of 4.68 ± 1.3 and 5.96 ± 1.35 nm, respectively. Both the Pd and Fe3O4 nanoparticles were encapsulated in the polystyrene (PS) matrixes via in situ polymerization to get Pd− Fe3O4@PS nanocomposites with an average size of 70 ± 8.10 nm (Figure 1c). As shown in the TEM image (Figure 1c), the inorganic nanoparticles with higher contrast were well B
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exhibits the FTIR spectra of oleic acid (OA)-capped Pd nanoparticles (Figure 2a), OA-capped Fe3O4 nanoparticles (Figure 2b), polystyrene (PS)-coated nanocomposites (Pd− Fe3O4@PS) (Figure 2c), Pd−Fe3O4@PS@SiO2 nanocomposites (Figure 2d), and Pd−Fe3O4@SiO2 nanocomposites (Figure 2e). The absorptions in the range of 3420−3444 cm−1 can be attributed to the O−H stretching vibration arising from the coated polymer (spectra c and d) and the adsorbed water (spectra a, b, and e). Meanwhile, the vibration peaks around 2920 and 2850 cm−1 are ascribed to the C−H asymmetric (υas) and symmetric (υs) stretching vibration of methylene (CH2) groups in the alkyl chain in oleic acid (spectrum a and b) or polymer (spectrum c), respectively. The bands at 1635, 1636, 1626 cm−1 and 1419, 1425, and 1454 cm−1 (spectra a, b, and c) can be assigned to the asymmetric (υas) and symmetric (υs) stretching vibration of the carboxylate group (−COO−), respectively. The results confirm the existence of oleic acids on the surface of hydrophobic Fe3O4 and Pd nanoparticles. Compared with Figure 2a and 2b, the obvious difference of Figure 2c and 2d is the peak at 1728 cm−1 arising from the CO stretching vibration of polymer shell resulting from 2-hydroxyethyl methacrylate (HEMA). The new band at 1094 cm−1 (spectra d and e) can be assigned to the moieties of Si−O single bond vibration, suggesting that SiO2 shell has been immobilized on the surface of nanocomposites (Pd−Fe3O4@PS). It should be noted that the absorption peaks at 1728 and 1636 cm−1 of the oleic acids on the surface of nanoparticles and the polymer disappeared after the calcination, indicating the organic compounds were removed. The removal of surfactants and polymers would be beneficial to the improvement of the catalytic activity by releasing the active sites on metal nanoparticles.
nm is in accordance with the distance of the (311) plane of magnetite. From the HRTEM image, it is clear that the Pd nanoparticle is surrounded by the magnetite nanoparticles, which is highly desirable for preventing Pd nanoparticles from aggregating during the high temperature treatment. These results further indicated that both Pd and Fe3O4 nanoparticles were simultaneously encapsulated in the SiO2 matrixes. As a result, these nanocomposites (Pd−Fe3O4@SiO2) were endowed with bifunctionality of catalysis and magnetic collection. The as-prepared nanocomposites were further characterized by Fourier transform infrared (FTIR) spectrometer. Figure 2
Figure 2. FTIR spectra of OA =capped Pd nanoparticles (a), OA -capped Fe3O4 nanoparticles (b), Pd−Fe3O4@PS nanocomposites (c), Pd−Fe3O4@PS@SiO2 nanocomposites (d), and the Pd−Fe3O4@SiO2 nanocomposites (e). PS: polystyrene. OA: oleic acid.
Figure 3. (a) UV−vis spectra of 4-nitrophenol before (black line) and after (blue line) adding NaBH4 and 4-aminophenol (red line) solution, (b) evolution of the absorption spectra of the 4-nitrophenol solution in the presence of Pd−Fe3O4@SiO2 nanocomposites (3.5 μg/mL) and NaBH4 at different times, (c) ln(Ct/C0) against the reaction time for the catalytic hydrogenation of 4-nitrophenol, and (d) the reusability test of Pd−Fe3O4@ SiO2 nanocomposites as catalysts for the reduction of 4-nitrophenol. C
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To evaluate the catalytic activity and recyclability of the asfabricated Pd−Fe3O4@SiO2 nanocomposite catalysts, the catalytic hydrogenation of 4-nitrophenol to 4-aminophenol with NaBH4 as the reductant at room temperature was chosen as the model reaction. This model reaction has been widely employed to evaluate the catalytic activity of noble metal nanoparticles, and the color change of the solution involved in the reduction also provides a simple spectrophotometry method to monitor the catalytic reaction kinetics.47,48 Generally, the absorption peak of 4-nitrophenol is centered at 317 nm (Figure 3a, black spectrum). Upon the addition of the NaBH4 solution, the absorption peak immediately red-shifted to 400 nm (Figure 3a, blue spectrum), which can be attributed to the formation of 4-nitrophenolate ions under the alkaline conditions. However, after the addition of Pd−Fe3O4@SiO2 nanocomposites, the reduction of 4-nitrophenol was initiated and the absorption peak at 400 nm decreased gradually along with the emergence of a new absorption peak at 300 nm (Figure 3a, red spectrum), suggesting the formation of 4aminophenol. The evolution of the absorption spectrum versus the reaction time is shown in Figure 3b. Because the peak at 400 nm was much stronger than that at 300 nm, the progress or kinetics of the reduction reaction was monitored by recording the absorbance at 400 nm. The ratio of Ct/C0 was calculated from the relative absorbance ratio (At/A0) according to the Lambert−Beer’s law. Herein, the Ct is the concentration of 4nitrophenolate ions at time t and C0 is the initial concentration, respectively. Meanwhile, At and A0 represent the absorbance at 400 nm at time t and initial point, respectively. A linear relationship between ln(Ct/C0) and time (t) was observed for Pd−Fe3O4@SiO2 catalysts (Figure 3c), indicating that this catalytic reduction reaction can be considered as a pseudo-firstorder reaction where the conversion is rising by prolonging the reaction time and the rate constant is estimated to be 0.0130 s−1. In addition, the conversion was over 95% when the reaction time was 5 min. As a control, the Fe3O4@SiO2 nanocomposites without Pd nanoparticles or the Pd−Fe3O4@ PS@SiO2 nanocomposites without calcination were utilized as catalysts for the catalytic hydrogenation of 4-nitrophenol. As depicted in Figure 3c, no measurable catalytic activity was observed. These results clearly indicated that the surface coating of catalysts has great effects on the catalytic activity. On the other hand, the stability and recyclability are of great importance for the practical applications of catalysts. Herein, the cyclic stability of the as-prepared Pd−Fe3 O 4@SiO2 nanocomposite catalysts was also evaluated by monitoring the catalytic activity during successive cycles of the reduction reaction. The Pd−Fe3O4@SiO2 nanocomposite catalysts can be separated and reused from the catalytic reaction solution with an assistant magnet (Figure S3 of the Supporting Information). As shown in Figure 3d, Pd−Fe3O4@SiO2 nanocomposites were still highly active with a conversion over 95% after 5 min reduction even after 10 successive cycles. The first-order rate constant (K) decreased slightly from 0.0137 to 0.0089 s−1 after 10 successive cycles, which may partly be due to the surface passivation of the nanocatalysts. The TEM image (Figure S4 of the Supporting Information) of the reclaimed catalysts revealed that the structure of the nanocomposites was well retained after 10 catalytic cycles. These results indicated that the as-developed strategy was feasible for the design of active, stable, and recyclable nanocatalysts. In order to investigate the reason that the catalytic activity of Pd−Fe3O4@SiO2 is higher than that of Pd−Fe3O4@PS@SiO2
nanospheres, the porosity of the obtained nanocomposites (Pd−Fe3O4@PS@SiO2 and Pd−Fe3O4@SiO2) was investigated by nitrogen adsorption−desorption measurements. Both Pd−Fe3O4@SiO2 and Pd−Fe3O4@PS@SiO2 exhibit a type IV isotherm with a type H4 hysteresis loop (Figure 4).49,50 In the
Figure 4. Nitrogen sorption isotherms of Pd−Fe3O4@SiO2 and Pd− Fe3O4@PS@SiO2 at 77 K, respectively. Inset: pore size distribution of the nanocomposites before (Pd−Fe3O4@PS@SiO2) and after (Pd− Fe3O4@SiO2) calcination.
relative pressure (P/P0) range of 0.9−1, a steep increase occurred and a desorption hysteresis loop appeared in the sorption isotherms of both Pd−Fe3O4@SiO2 and Pd−Fe3O4@ PS@SiO2, which demonstrated a large pore with an average pore size around 25.2 and 21.5 nm (inset of Figure 4), respectively. This pore should be attributed to the internanosphere spaces, which has also been confirmed by other researchers.49,50 The BET surface areas and the pore volumes were calculated to be 122.3 m2/g, 71.2 m2/g, 0.75 cm3/g, and 0.38 cm3/g for Pd−Fe3O4@SiO2 and Pd−Fe3O4@PS@SiO2, respectively. It was obviously observed that the BET surface areas and the pore volumes were dramatically enlarged after calcination in the air due to the removal of polymer matrix and surfactants. It is noteworthy that different from the Pd− Fe3O4@PS@SiO2, the Pd−Fe3O4@SiO2 nanospheres have another small size pore (∼2.1 nm) (inset of Figure 4), which results from the combustion of surfactants and polymers. This pore size is in accordance with the space between the hydrophobic nanoparticles coated with oleic acids.51 So the catalysis centers (Pd nanoparticles) were totally exposed to the reactants, which is preferable for a good catalysis efficacy. The catalytic activity was further investigated by tuning the dosage of the Pd−Fe3O4@SiO2 nanocomposites while fixing the concentration of NaBH4 and 4-nitrophenol. As shown in Figure 5, with the increase of the Pd−Fe3O4@SiO2 nanocomposite concentration from 0.5, 1.5, 3.5, to 5.0 μg/mL, the first-order rate constant increased from 0.0054, 0.0070, 0.0130, to 0.0201 s−1. It is notable that the balance conversion was up to 95% for all cases in spite of different concentration of catalysts. In addition, the effects of Pd loading in the Pd− Fe3O4@SiO2 nanocomposites on the catalytic activity were also evaluated. From Figure 6, it can be seen that the reaction was largely accelerated with the increase of Pd dosage in the nanocomposite. Therefore, it is flexible to optimize the catalytic activity via tuning the loading of noble metal nanoparticles in the Pd−Fe3O4@SiO2 nanocomposites during the fabrication of nanocomposites. As depicted in Figure 6, by means of the analysis with inductively coupled plasma mass spectrometry D
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on superparamagnetic nanoparticles demonstrate many advantages such as convenient magnetic separation, high catalytic activity, high water stability, good dispersibility, and excellent recyclability. This facile and efficient fabrication strategy presented a new paradigm for the fabrication of multifunctional structured catalysts.
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MATERIALS AND METHODS Reagents and Chemicals. Ethyleneglycol dimethacrylate (EGDMA), PdCl2, 1-dodecanethiol, oleic acid, and oleylamine were purchased from Aldrich. Styrene (St), 2-hydroxyethyl methacrylate (HEMA), sodium dodecyl sulfate (SDS), 2,2azobisisobutyronitrile (AIBN), tetraethyl orthosilicate (TEOS), cyclohexane, ethanol, and chloroform were obtained from Beijing Chemical Reagent Co. Deionized (DI) water was used throughout the experiments. Styrene and AIBN were purified before use. All the other reagents were analytical grade and used without further purification. Characterization. A Shimadzu Model XRD-7000 X-ray diffractometer with a Cu Kα radiation (λ = 1.5418 Å) was used to record the X-ray diffraction (XRD) pattern. The morphology and size characterization of the as-prepared nanomaterials were carried out on a Hitachi H-800 transmission electron microscope with a tungsten filament at an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra were obtained on a Nicolet 670 Fourier transform infrared spectrophotometer. UV−vis absorption spectra were recorded using a UV-3600 UV−vis spectrophotometer (Shimadzu). Adsorption and desorption isotherms for nitrogen were obtained at 77 K using a Micrometrics ASAP 2020 system. The samples were outgassed at 70 °C for 12 h before measurements were performed. Specific surface area values and the pore size distribution were calculated by the Brunauer− Emmett− Teller (BET) and the Barret−Joyner−Halenda (BJH) method, respectively. Preparation of the Fe3O4 Nanoparticles. Hydrophobic magnetite nanoparticles were prepared according to the reported method with some alteration.52 In brief, into a Teflon-lined autoclave, 1 g of NaOH was dissolved in 10 mL of deionized water. Thereafter, 10 mL of ethanol and 10 mL of oleic acid were added before the addition of 784 mg of Fe(NH4)2(SO4)2·6H2O (dissolved in 10 mL of deionized water). The Teflon-lined autoclave was treated at 180 °C for 10 h. After the autoclave was cooled to room temperature, the Fe3O4 nanoparticles were collected. Synthesis of Pd Nanoparticles. Pd nanoparticles were prepared via the LSS strategy.53 Briefly, 0.2 g of NaOH, 5 mL of deionized water, 15 mL of ethanol, 3 mL of oleic acid, 2 mL of oleylamine, and 3 mL of cyclohexane were mixed to form a clear solution. Fifteen milliliters of ethanol containing 15 mg of PdCl2 was then added under magnetic stirring followed by the addition of 200 μL of cyclohexane solution containing 1dodecanethiol (20 μL). This solution was transferred into a Teflon-lined autoclave and heated at 100 °C for 5 h. The products were collected from the bottom of the autoclave via centrifugation. Fabrication of Nanocomposites. The fabrication of the nanocomposites was carried out according to our previously developed method with some modification.46 First, the desired amount of oleic acid stabilized Pd nanoparticles and 100 mg of Fe3O4 nanoparticles were dispersed in chloroform (1 mL). Then this solution was transferred into an aqueous solution containing 0.05 g of SDS. After ultrasonication for 5 min in an
Figure 5. Effects of Pd−Fe3O4@SiO2 dosage on the reduction rate and conversion of 4-nitrophenol. The star represents the conversion of 4nitrophenol in the presence of Pd−Fe3O4@SiO2 with different concentration (0.5, 1.5, 3.5, and 5.0 μg/mL). The amount of Pd in the nanocomposites was 1.0 wt %.
Figure 6. Effects of Pd loading in the nanocomposites on the reaction rate. The concentration of Pd−Fe3O4@SiO2 nanocomposites was 5.0 μg/mL.
(ICP-MS), the Pd concentrations in the Pd−Fe3O4@SiO2 nanocomposites were 2.64 and 1.17 wt % for the black and red plot (Figure 6), respectively.
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CONCLUSIONS In summary, we demonstrated a facile strategy to fabricate the Pd−Fe3O4@SiO2 nanocomposite catalysts with high catalytic activity, excellent dispersibility, and good recyclability. Via the calcination, the surfactants and polymers were burned out and lots of mesopores were left in the nanocomposites, which gives rise to a good exposure to reactants of the catalytic activity center (noble metal nanoparticles). In addition, the noble metal nanoparticles were sustained by the magnetite nanoparticles, and consequently no aggregation of the noble metal nanoparticles was observed, which is highly desirable for the satisfying catalytic activity especially at high temperature. On the other hand, these composite nanocatalysts were easily reclaimed via magnetic separation owing to the magnetite nanoparticles. The results of the catalytic hydrogenation of 4nitrophenol at room temperature indicated that these Pd− Fe3O4@SiO2 nanocomposites are highly active, stable, and reusable. Even after 10 cycles of reuse, the catalytic activity and the structure (shape and size) of the nanocomposite catalysts were well retained. These as-prepared nanocomposites based E
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ice−water bath, and followed by the rotary evaporation of chloroform by heating at 60 °C for 10 min, a homogeneous oilin-water (O/W) emulsion was obtained. Meanwhile, the emulsion consisting of styrene (St), 2-hydroxyethyl methacrylate (HEMA), and cross-linking agent ethylene glycol dimethacrylate (EGDMA) was prepared as follows. In brief, 0.01 g of SDS was dissolved in deionized water, and styrene (200 μL), EGDMA (50 μL), as well as HEMA (100 μL) were then added and treated with ultrasonication for 5 min. Subsequently, these two as-prepared emulsions were mixed together in a 50 mL flask followed by the addition of AIBN (10 mg) as the initiator to trigger the polymerization. The flask was then heated at 80 °C for 10 h under magnetic stirring. The polymer-coated Pd−Fe3O4@PS nanocomposites were then obtained. Second, the as-prepared Pd−Fe3O4@PS nanocomposites were coated with a thin layer of silica. Briefly, into deionized water (25 mL), Tween-20 (0.125 g), the obtained Pd−Fe3O4@PS nanocomposites, isopropyl alcohol (75 mL), NH3·H2O (1 mL), and TEOS (500 μL) were added subsequently, and the mixture was reacted for 12 h under magnetic stirring. Finally, the silica-coated Pd−Fe3O4@PS@ SiO2 nanocomposites were calcinated at 400 °C for 5 h to remove the oleic acid and the polymer supports, and then the Pd−Fe3O4@SiO2 nanocomposite catalysts were obtained. Catalytic Activity Evaluation. The desired amount of nanocomposite catalyst was added into a NaBH4 aqueous solution (1 mL, 0.4 M), and the mixture was stirred for 30 min at room temperature. Thereafter, 4-nitrophenol aqueous solution (1 mL, 1.4 mM) was added, and the reaction volume was fixed to 5 mL with deionized water. The reaction progress was monitored by UV−vis absorption spectrophotometry in the range of 250−500 nm until the solution became colorless from yellow at 20 °C.
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ASSOCIATED CONTENT
S Supporting Information *
EDS characterization, HRTEM images, photos of magnetic separation, and TEM image of Pd−Fe3O4@SiO2 (Figures S1− S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported in part by the National Natural Science Foundation of China (Grant No. 21275015), the State Key Project of Fundamental Research of China (Grants No. 2011CBA00503 and 2011CB932403), the Program for New Century Excellent Talents in University of China (No. NCET100213), the Science Foundation of Xinjiang Uygur Autonomous Region (201191170), the Fundational Research Funds for the Central University (ZZ1321), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
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