Recoverable Platinum Nanocatalysts Immobilized ... - ACS Publications

Article pubs.acs.org/IECR

Recoverable Platinum Nanocatalysts Immobilized on Magnetic Spherical Polyelectrolyte Brushes Shuang Wu,†,‡,§ Julian Kaiser,‡,§ Xuhong Guo,*,† Li Li,† Yan Lu,*,‡,§ and Matthias Ballauff‡,§ †

State-Key Laboratory of Chemical Engineering, East China University of Science and Technology, 200237 Shanghai, China Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin, Germany § Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: Recoverable platinum nanocatalysts immobilized on magnetic spherical polyelectrolyte brushes (MSPB) were synthesized and characterized by high resolution transmission electron microscope (HRTEM), thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). High catalytic activity was found by photometrically monitoring the reduction of 4-nitrophenol by NaBH4 in the presence of MSPB-Pt composites. An excellent stability and recyclability of catalyst was observed after consecutive eight runs following by external magnetic-separation and redispersion. This novel approach provides an excited potential application in preparation of recyclable metal nanocatalysts with high activity.

■

(ATRP),34−36 as well as cover MNP clusters by in situ polymerization.37−39 In our previous work, novel magnetic spherical polyelectrolyte brushes (MSPB) with magnetic nanoparticles in core were reported.40,41 This nanocomposite particle provides a promising system to immobilize metal nanoparticles onto MSPB behaving as catalyst. Their catalytic activity and recyclable capability after catalytic reaction due to their attraction to magnetic field will be discussed in this paper. As shown in Figure 1, magnetic spherical polyelectrolyte brushes (MSPB) consist of a core of poly(styrene-co-divinylbenzene) particle embedded with magnetite nanoparticles and a welldefined shell of positively charged poly([2-(methacryloyloxy) ethyl] trimethyl ammonium chloride) (PMATAC) brush. MSPB-Pt composites with nanosized platinum particles immobilized into MSPB are synthesized by reduction of PtCl62− ions which are confined as counterions in the brush system in presence of BH4−. Finally, we used the reduction of 4-nitrophenol as a benchmark reaction to investigate the catalytic ability and recyclability of platinum nanoparticle.

INTRODUCTION In recent years, much attention has been focused on metal nanoparticles such as gold, platinum, and palladium because of their high activity and selectively in catalytic application, which markedly differ from the ones of the respective bulk metals.1−3 In principle, their high surface area per volume makes metal nanoparticles ideal candidates for catalysis.4−6 The formation of metal nanoparticles is usually carried out by reduction of metal ions in the presence of a stabilizer like polymers,7,8 dendrimers,9 microgels,10,11 surfactants,12 and colloids,13 which can prevent the nanoparticles from aggregating and serve as carriers. A number of approaches have been reported which focused on catalysts with both high activity and selectivity, such as homogeneous catalysts,14,15 biphasic systems,16 and catalysts immobilized in polymer or membrane.17 Among these techniques mentioned above, spherical polyelectrolyte brushes (SPB) have been proved to be excellent model systems for the generation and immobilization of metal nanoparticles,18−20 in which the generation of nanoparticles take place mainly within the brush layer instead of outside. Even if nanocatalysts enjoy several advantages over conventional catalyst systems, however, the isolation and recovery of these nanosized catalysts from reaction mixture is still a challenge due to their extremely small size and low stability. Therefore, the design of efficient and recoverable nanocatalysts becomes an important issue for the reasons of economic and environmental impact.21 Noticeably, more and more superparamagnetic nanoparticles (MNPs)22−24 and their hybrids25−27 are used as catalyst carriers recently, because they offer a promising option that can meet the requirement of high accessibility with improved reusability by separation of external magnet.28−30 Additionally, in order to produce redispersibility of the magnetic-based catalyst, surface-functionalized MNPs31,32 need to be employed into synthesis. Here, polymers can be encapsulated onto a single MNP by physical absorption,33 surface-initiated atom transfer radical polymerization © 2012 American Chemical Society

■

EXPERIMENTAL SECTION Materials. Ferric chloride hexahydrate 99% (FeCl3·6H2O), ferrous sulfate heptahydrate 99% (FeSO4·7H2O), anhydrous ethyl alcohol, ammonium hydroxide (25 wt %), acetone, hydrochloride acid (36 wt %), cetyltrimethylammonium bromide 98% (CTAB), octane 99%, styrene 99%, divinylbenzene 99% (DVB), methacryloyl chloride (MC), [2-(methacryloyloxy) ethyl] trimethyl ammonium chloride 80 wt % (MATAC), p-nitrophenol 99%, and sodium borohydride 99% Received: Revised: Accepted: Published: 5608

November 3, 2011 March 15, 2012 March 28, 2012 March 28, 2012 dx.doi.org/10.1021/ie2025147 | Ind. Eng. Chem. Res. 2012, 51, 5608−5614

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic representation of magnetically recoverable Pt nanocatalysts immobilized on magnetic spherical polyelectrolyte brushes (MSPB).

h. The obtained MSPB latex was purified exhaustively by dialysis against water until the conductance keeps constant. Synthesis of the Magnetic Spherical Polyelectrolyte Brushes-Platinum (MSPB-Pt) Composite Nanoparticles. To 0.1 g of MSPB latex particles (calculated by solid content of MSPB latex), 10 mL 1.75 mM H2PtCl6 aqueous solution was added dropwise in 30 min. Afterward, the mixture was stirred for 30 min under nitrogen to remove the oxygen and then the metal ions were reduced by a 10-fold excess NaBH4 in ice-cold condition. The onset of reaction could be seen from a slight discolorization of the suspension, which went from brownish to a gray-brownish. After the last addition, the latex were stirred for another 1 h and then carefully washed against water in a dialysis cell. Catalytic Reduction of 4-Nitrophenol. A total of 0.5 mL sodium borohydride solution (0.1 M) was added to 4.5 mL of 4-nitrophenol solution (0.11 mM) contained in a glass vessel. After that, a given amount of the MSPB-Pt composite particles was added. Immediately after the addition of the composite particles, UV spectra of the sample were taken every minute in the range of 250−550 nm. The rate constant of the reaction was determined by measuring the intensity change at peak of 400 nm with time. For calculation the total surface of Pt nanoparticles, which is a decisive parameter and used in kinetics analysis, the size of the platinum nanoparticles is determined by TEM micrographs (d = 3.5 ± 0.5 nm). The amount of Pt in the composites is 9.57 wt % determined by thermogravimetric measurement (TGA). The bulk density of Pt (ρ = 21.45 g/ cm3) have been used for the density of Pt nanoparticles. For recovery of MSPB-Pt catalyst, Neo-Delta-Magnet (NdFeB, 1.2 T) is employed to provide magnetic field to separate nanocatalysts from catalyst reaction system. Characterization. The UV−visible spectra were measured with Lambda 650 spectrometer supplied by Perkin-Elmer with a temperature-controlled sample holder with an accuracy of ±0.1 °C. Magnetic spherical polyelectrolyte brushes-platinum (MSPB-Pt) nanocomposites were characterized by transmission electron microscopy (TEM) to determine the morphology and monodispersity. TEM images were obtained with CM30 Philips microscope operating at an acceleration voltage of 300 kV. The Pt and Fe3O4 amount in MSPB-Pt composites was determined by TGA using a Mettler Toledo STARe system. The composite particles were heated to 800 °C with a heating rate of 10 °C/min and holding temperature at 800 °C for 30 min. X-ray diffraction (XRD) was carried out on

(NaBH4) were purchased from Aldrich and used as received. Hexadecane 99% was purchased from Alfa Aesar, and a,a′azodiisobutyramidine-dihydrochloride 98% (V-50) was purchased from Fluka. The 2-hydroxy-4′-hydroxyethoxy-2-methlpropiopheone (HMP) (Irgacure 2959) was donated by Ciba Specialty Chemical Inc. Milli-Q water with a resistivity higher than 18.2 MΩ was used in all experiments. Synthesis of Magnetic Poly(Styrene/DVB) Latex (MPL). Magnetic nanoparticles (MNPs) were synthesized by coprecipitation method in the presence of oleic acid (OA), FeCl3·6H2O, FeSO4·7H2O, and ammonium hydroxide, which was described in detail in our previous publication.40 For the synthesis of a polymer core containing magnetic nanoparticles, a mini-emulsion polymerization was employed. First, a magnetic fluid contained 0.65 g magnetic nanoparticles and 3 mL octane was dispersed into 200 mL 0.1 wt % positive surfactant CTAB solution followed by stirring for 15 min and ultrasonication for 15 min. The ultrasonication device is Sonorex Digitec DT103H from BANDELIN Co. with highest frequency of 35 kHz and power of 560 W. Second, 4 g styrene, 1 g DVB, and 0.26 mL hexadecane were mixed to form oil phase and added into surfactant solution followed by miniemulsification for 60 min in an ice bath. For MPL with higher magnetic content, 2 g styrene and 0.5 g DVB was used while keeping other reactant constant in polymerization. Miniemulsion polymerization then starts after adding 0.1 g V50 with positive charge as initiator at 80 °C and performed for 2 h under stirring of 300 rpm. At the end of the miniemulsion polymerization, 8 g of acetone solution with 1 g photoinitiator 2-[p-(2-hydroxy-2-methylpropiophenone)]-ethyleneglycol methacrylate (HMEM) synthesized in our group42 was added in 30 min under so-called “starved-condition” at 70 °C. The MPL with a thin layer of HMEM were thus obtained after further 1 h reaction. The obtained MPL was purified in Milli-Q water by dialysis to remove unreacted monomer and surfactant. Synthesis of Cationic Magnetic Spherical Polyelectrolyte Brushes (MSPB). MPL were charged into a UVreactor (wavelength 200−600 nm, power 150 W) and diluted to 0.5 wt % with water to obtained 300 g reaction dispersion. Then 0.799 g cationic monomer MATAC solution (50 mol % of styrene monomer) was added into system. The whole reactor was degassed by repeated evacuation and subsequent addition of nitrogen at least three times. The so-called photoemulsion polymerization43−45 was accomplished with UV radiation at room temperature with vigorous stirring in 1.5 5609

dx.doi.org/10.1021/ie2025147 | Ind. Eng. Chem. Res. 2012, 51, 5608−5614


Article

Table 1. Magnetic Properties (Saturation Magnetization, Ms, Residual Magnetization, Mr, and Coercivity, Hc) of Samples Containing Magnetite

a Bruker D8 Advance X-ray diffractometer with a scan of 0.3°/ min. The magnetic properties of samples were characterized by vibrating sample magnetometer (VSM) of Quantum DesignMPMS-SQUID-VSM system at room temperature.

■

RESULTS AND DISCUSSION 1. Catalyst Preparation and Characterization. Magnetic nanoparticles modified by oleic acid were synthesized by

sample

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

MNP MPL MSPB MSPB-Pt

49.2 47.4 45.1 44.0

1.28 1.78 1.44 1.43

19.0 23.8 20.7 19.5

Figure 2. XRD patterns for MNP-based catalysts: (lower) bare magnetic nanoparticles, (upper) platinum nanoparticles immobilized on magnetic nanoparticles: spherical polyelectrolyte brushes.

coprecipitation route. Here, OA is adsorbed on MNP surface via coordination interaction between carboxylate group in oleic acid and iron atom. As shown in Figure 2, the crystal structure of obtained MNPs was confirmed by XRD. The seven characteristic peaks of magnetite appear at 30.2° (220), 35.6° (311), 43.3° (400), 53.6° (422), 57.1° (511), 62.8° (440), and 74.1° (533), which can be indexed to the face-center-cubic phase of Fe3O4 (JCPDS Card No. 19-629). According to the Scherrer equation,46 the average crystallite size which is calculated based on the XRD pattern (311) is around 11 nm. This agrees well with the average size determined by TEM (10 ± 4 nm), as shown in Figure S1 in the Supporting Information. The pattern of platinum nanoparticles immobilized on MSPB has been determined by XRD (upper curve in Figure 2) as well in order to compare with bare MNPs. Herein, four characteristic peaks of Pt nanoparticles at 39.9° (111), 46.4° (200), 67.7°

Figure 4. TEM and HRTEM images of MSPBs and MSPB-Pt: (a) MSPB (TEM); (b) MSPB (HRTEM); (c) MSPB-Pt (TEM); and (d) single Pt nanoparticle immobilized on MSPB (HRTEM).

(220), and 81.6° (311) can be observed from XRD spectrum, whose crystallite can be also indexed to a phase of Pt (JCPDS Card No. 87-640). This indicates that platinum nanoparticles with fine structure were successfully prepared and immobilized into MSPB particles. The magnetic properties of composite nanocatalysts have been investigated using a vibrating sample magnetometer (VSM). Figure 3 shows hysteresis loops of typical MNP, MPL, MSPB, and MSPB-Pt composites measured by sweeping the

Figure 3. Magnetic properties of all MNPs. (a) Magnetization curves of (black) bare MNPs, (red) MPL, (green) MSPB, and (blue) MSPB-Pt observed by vibrating sample magnetometer (VSM) at room temperature. The magnetization value refers to per gram of magnet content. (b) Magnetization curves at low magnetic field to show residual magnetization and coercive force. Symbols denote the following: (square) bare MNPs, (circle) MPL, (triangle) MSPB, (inverse triangle) MSPB-Pt. 5610



Article

4a and b). The MSPB shows a narrow size distribution and well-defined structure although the PMATAC brush is invisible in the dry state. The negatively charged PtCl62− are confined in SPBs and reduced to platinum nanoparticles with darker color and smaller size compared to MNPs encapsulated inside the PS core. By observing size distribution and morphology of platinum nanoparticles on MSPB with HRTEM (Figure 4c), Pt particles that were generated mostly in SPBs have a diameter of 3.5 ± 0.5 nm with a rather narrow size distribution. Obviously, the nucleation of the Pt particles takes place on well-distributed positions on the colloidal particle surface. Figure 4d shows welldefined lattice space 0.226 nm of as-synthesized platinum nanoparticle corresponding to (111) face of Pt nanocrystal by calculating from lattice constant of platinum. Herein, magnetite and platinum content of MSPB-Pt composites was determined by TGA (Figure S3). By calculating from residual amount of each sample, magnetite and Pt content is 9.04% (from MSPB) and 9.57% (after subtracting magnetite content), respectively. MSPB-Pt with higher magnetite content can be obtained by raising MNPs/monomer ratio during miniemulsion polymerization of MPL. Therefore, by the similar route discussed above, MSPB-Pt composites with tunable magnetic properties have been synthesized, whose morphology were determined by TEM (Figure S4). 2. Catalytic Performance of MSPB- Pt Composites. As a benchmark reaction for monitoring the catalytic activity of metal nanoparticles,11,19 we used the reduction of 4-nitrophenol by an excess of NaBH4. It is worth noting that 4nitrophenol is one of the most refractory pollutants in industrial wastewaters, which leads to industrial importance in the reduction of 4-nitrophenol to 4-aminophenol as an intermediate for the manufacture of the analgesic and antipyretic drugs.48 The kinetics of this reaction can be easily monitored by UV−vis spectroscopy as shown in Figure 5. After addition of MSPB-Pt composites, the peak at 400 nm due to the 4nitrophenolate ions decreases gradually with reaction time and a new peak appears at 297 nm which results from product of 4aminophenol. Time dependence of the UV-peak of 4-nitrophenolate ions at 400 nm in the reaction with different Pt concentrations is shown in Figure 6a. The generation of 4-nitrophenolate ions takes place immediately after addition of NaBH4 to the system. The reaction was hypothesized to follow a first-order rate law due to the excess of NaBH4. The conversion of the process can

Figure 5. Absorption spectra of 4-nitrophenol reduced by sodium borohydride. The peak of 4-nitrophenol at 400 nm decreases gradually with reaction time.

external field between −2 and 2 T at room temperature. Magnetization curves displayed typical characteristics of superparamagnetic behavior for all MNP-related composites after normalizing the magnetic content in each sample. The digital image of MSPB and MSPB-Pt and their magnetic separation are displayed in Figure S2 in the Supporting Information. As shown in Table 1, the saturation magnetization of MNPPt, MSPB, and MPL composites exhibits a tiny reduction by comparison with that of bare MNP, which is probably due to the polymer encapsulation. According to magnetization data at low magnetic field in Figure 3b, the residual magnetization Mr and coercive forces Hc of each sample were obtained from magnetization curves and displayed in Table 1 as well. These coercive forces and remanences suggest being within the superparamagnetism range.47 Superparamagnetism is the responsiveness to an applied magnetic field without retaining any magnetism after removal of the applied magnetic field. Therefore, with the superparamagnetic property, in all cases of catalytic application used here, the separation of the catalyst would be achieved by applying an external permanent magnet, making the recovery of the catalyst very straightforward. The size distribution and morphology of as-synthesized MSPB and MSPB-Pt are investigated by HRTEM as shown in Figure 4. Obviously, MNPs are successfully embedded in MSPB and mainly locate inside the poly(St/DVB) core (Figure

Figure 6. (a) Influence of Pt nanoparticle concentration on reduction of 4-nitrophenol at 20 °C. The parameter of different curves is the concentration of Pt nanoparticles in the reaction solution: (square) 0.04; (circle) 0.08; (triangle) 0.12; (inverse triangle) 0.16; (diamond) 0.192; (star) 0.24 mg/L. (b) Apparent rate constant kapp as the function of surface area S of Pt nanoparticles normalized to the unit volume of the system. 5611



Article

Figure 7. (a) Influence of temperature on the kinetic constant measure with Pt concentration of 0.12 mg/L. The parameter of the different curves is temperature T: (diamond) 10; (square) 15; (triangle) 20; (circle) 25; (inverse triangle) 30 °C. (b) Arrhenius plots of the reaction rate constant surface-area normalized rate constant k1 measured in the presence of MSPB-Pt composite particles at different temperatures. The Pt concentration here is 0.12 mg/L.

the system. The catalytic activities which were obtained from kapp by varying catalyst usage have been normalized to surface area of Pt nanoparticles in MSPB-Pt composites according to eq 1. As shown in Figure 6b, kapp is found to strictly linear fitting with the number of catalytic composite particles, concomitantly, k1 = 0.39 L/s·m can be obtained as the slope by linear fitting between kapp and S. Here the total surface area of the Pt nanoparticles has been calculated from TGA and average size of particles determined by TEM as mentioned in experimental part. Figure 7a shows the dependence of the reaction rate on temperature. Again, the pseudo-first-order kinetics is seen at all temperatures T. Figure 7b demonstrates that the reaction constant k1 that obtains from Figure 7a is completely described by a conventional Arrhenius expression. The activation energy EA for normalized rate constant was 49 kJ/mol. This is in good agreement with previous work18 for metal nanoparticles immobilized in SPB where EA was found to be 44 kJ/mol. However, the EA data of metal nanoparticles seem to depend on the structure49 and immobilizer. For example, Mahmoud50 compared Pt nanocubes with Pt nanocubes immobilized on PS microsphere and obtained the EA of 14 and 12 kJ/mol, respectively. 3. Recycling and Recovery of Catalytic MSPB-Pt Composites. The recycling and recovery of used catalyst, as one of the most important properties of magnetic-based catalyst, can be realized in our MSPB-Pt system. As shown in Figure 8a, after each catalytic reaction, MSPB-Pt catalyst was recovered by magnetic-separation from the reaction solution and following by washing away remaining reactant with water. This procedure provides a possibility for our catalyst to be recovered by external magnetic field. After eight runs of cycles, the structure of MSPB and Pt nanocatalysts appears to be stable and no agglomeration has been observed through TEM (Figure 8b). This high colloidal stability and redispersibility might come from the strong electrostatic repulsion and steric hindrance among spherical polyelectrolyte brushes. As shown in Figure 8c, interestingly, after eight cycles of catalytic reaction, apparent rate constant kapp barely decreased less than 5% after comparing the kapp of the first run (4.98 × 10−2 1/s) with that of the eighth run (4.74 × 10−2 1/s), which means the efficiency of the catalyst remains unaltered during the runs. Additionally, under all typical condition used, the conversion of each cycle run maintains above 97% for a reaction time of 30 min. Therefore, the MSPB-Pt nanocatalyst is approved to be an excellent

Figure 8. (a) Photographs of magnetically recovery of MSPB-Pt composite catalyst from catalytic reaction suspension. (b) TEM image of magnetic-recovered MSPB-Pt nanoparticle after eight cycles of catalytic reaction. (c) Rate constant and conversion of catalytic reaction using recovered MSPB-Pt composite as catalyst. After each run, catalysts were separated by magnet and washed with water twice to remove remaining reactants. Reaction parameters: [4-nitrophenol] = 0.1 mM, [NaBH4] = 10 mM, and 0.16 mg/L of Pt catalyst, with T = 20 °C.

be directly read off from these curves as the ratio of the concentration Ct of 4-nitrophenol at time t to its value C0 at t0 and, thus, can be directly given by the ratio of the respective absorbance. Moreover, to compare the catalytic activity of composite particles quantitatively, the apparent rate constant kapp was proportional to the surface S of the Pt nanoparticles present in the system as shown in eq 1: −

dCt = kappCt = k1SCt dt

(1)

where Ct is the concentration of 4-nitrophenol at time t and k1 is the apparent rate constant normalized to S, the surface area of Pt nanoparticles which is normalized to the unit volume of 5612



Article

(8) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. Sizespecific catalytic activity of polymer-stabilized gold nanoclusters for aerobic alcohol oxidation in water. J. Am. Chem. Soc. 2005, 127, 9374. (9) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. Synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles. J. Phys. Chem. B 2005, 109, 692. (10) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M. Catalytic activity of palladium nanoparticles encapsulated in spherical polyelectrolyte brushes and core-shell microgels. Chem. Mater. 2007, 19, 1062. (11) Lu, Y.; Yuan, J.; Polzer, F; Drechsler, M.; Preussner, J. In situ growth of catalytic active Au-Pt bimetallic nanorods in thermoresponsive core-shell microgels. ACS Nano 2010, 4, 7078. (12) Bönnemann, W.; Brijoux, A.; Schulze, T.; Siepen, K. Application of heterogeneous colloid catalysts for the preparation of fine chemicals. Top. Catal. 1997, 4, 217. (13) Kidambi, S.; Bruening, M. L. Multilayered polyelectrolyte films containing palladium nanoparticles: synthesis, characterization, and application in selective hydrogenation. Chem. Mater. 2005, 17, 301. (14) Lin, Y. Y.; Tsai, S. C.; Yu, S. J. Highly efficient and recyclable Au nanoparticle-supported palladium(II) interphase catalysts and microwave-assisted alkyne cyclotrimerization reactions in ionic liquids. J. Org. Chem. 2008, 73, 4920. (15) Wittmann, S.; Schätz, A.; Grass, R. N.; Stark, W. J.; Reiser, O. A recyclable nanoparticle-supported palladium catalyst for the hydroxycarbonylation of aryl halides in water. Angew. Chem., Int. Ed. 2010, 49, 1867. (16) Cole-Hamilton, D. J. Homogeneous catalysisnew approaches to catalyst separation, recovery, and recycling. Science 2003, 299, 1702. (17) Leadbeater, N. E.; Marco, M. Preparation of polymer-supported ligands and metal complexes for use in catalysis. Chem. Rev. 2002, 102, 3217. (18) Mei, Y.; Sharma, G.; Lu, Y.; Ballauff, M. High catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes. Langmuir 2005, 21, 12229. (19) Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. Kinetic analysis of catalytic reduction of 4-nitrophenol by metallic nanoparticles immobilized in spherical polyelectrolyte brushes. Phys. Chem. C 2010, 114, 8814. (20) Sharma, G.; Mei, Y.; Lu, Y.; Ballauff, M.; Irrgang, T.; Proch, S.; Kempe., R. Spherical polyelectrolyte brushes as carriers for platinum nanoparticles in heterogeneous hydrogenation reactions. J. Catal. 2007, 246, 10. (21) Polshettiwar, V.; Luque, R.; Fihri, A.; Zhu, H.; Bouhrara, M.; Basset, J. M. Magnetically recoverable nanocatalysts. Chem. Rev. 2011, 111, 3036. (22) Long, W.; Gill, C. S.; Choi, S.; Jones, C. W. Recoverable and recyclable magnetic nanoparticle supported aluminium isopropoxide for ring-opening polymerization of ε-caprolactone. Dalton Trans. 2010, 39, 1470. (23) Polshettiwar, V.; Varma, R. S. Nanoparticle-supported and magnetically recoverable palladium (Pd) catalyst: a selective and sustainable oxidation protocol with high turnover number. Org. Biomol. Chem. 2009, 7, 37. (24) Laska, U.; Frost, C. G.; Price, G. J.; Pulcinski, P. K. Easyseparable magnetic nanoparticle supported Pd catalysts: Kinetics, stability and catalyst re-use. J. Catal. 2009, 268, 318. (25) Schkouhimehr, M.; Piao, Y.; Kim, J.; Jang, Y.; Hyeon, T. A magnetically recyclable nanocomposite catalyst for olefin epoxidation. Angew. Chem., Int. Ed. 2007, 46, 7039. (26) Xuan, S.; Wang, Y. X. J.; Yu, J. C.; Leung, K. C. F. Preparation, characterization, and catalytic activity of core/shell Fe3O4 @ Polyaniline @ Au nanocomposites. Langmuir 2009, 25, 11835. (27) Panella, B.; Vargas, A.; Baiker, A. Magnetically separable Pt catalyst for asymmetric hydrogenation. J. Catal. 2009, 261, 88. (28) Polshettiwar, V.; Varma, R. S. Green chemistry by nanocatalysis. Green Chem. 2010, 12, 743. (29) Shylesh, S.; Schünemann, V.; Thiel, W. R. Magnetically separable nanocatalysts: Bridges between homogeneous and heterogeneous catalysis. Angew. Chem., Int. Ed. 2010, 49, 3428.

recoverable and reproducible catalyst with high conversion in reduction of 4-nitrophenol.

■

CONCLUSION In summary, novel magnetically recoverable platinum nanocatalysts which composed of magnetic spherical polyelectrolyte brushes (MSPB) and platinum nanoparticles (average size: 3.5 nm) immobilized have been introduced. High catalytic activity was found when photometrically monitoring the reduction of 4nitrophenol by NaBH4 in the presence of MSPB-Pt nanocomposites. The analysis of kinetic data showed that the reaction is pseudo-first-order with regard to p-nitrophenol. The additional advantage of being superparamagnetic facilitates the separation, recovery, and efficient reuse of MSPB-Pt catalyst with almost complete retention of activity and conversion. This synthesis approach opens a new way to prepare recyclable metal nanocatalysts with high activity.

■

ASSOCIATED CONTENT

* Supporting Information S

TEM and digital images of magnetic nanoparticles and magnetic spherical polyelectrolyte brushes. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 21 6425 3491. E-mail: [email protected]. cn (X.G.); [email protected] (Y.L.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are thankful for the financial support of the National Natural Science Foundation of China (Grant No. 20774028), the Fundamental Research Funds for the Central Universities, the Key Basic Research Project of Shanghai Science and Technology Commission (10JC1403800), the Scientific and Technological Project of Shanghai Science and Technology Commission (10111100103), and the Chinese Scholarship Council.

■

REFERENCES

(1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chemistry and properties of nanocrystals of different shapes. Chem. Rev. 2005, 105, 1025. (2) Hutchings, G. J; Brust, M.; Schmidbaur, H. Gold- an introductory perspective. Chem. Soc. Rev. 2008, 37, 1759. (3) Schrinner, M.; Ballauff, M.; Talmon, Y.; Kauffmann, Y.; Thun, J.; Möller, M.; Breu, J. Single nanocrystals of platinum prepared by partial dissolution of Au-Pt nanoalloys. Science 2009, 323, 617. (4) Campbell, C. T.; Parker, S. C.; Starr, D. E. The effect of sizedependent nanoparticle energetics on catalyst sintering. Science 2002, 298, 811. (5) Hashmi, A. S.K.; Hutchlings, G. J. Gold catalysis. Angew. Chem., Int. Ed. 2006, 45, 7896. (6) Wei, Y.; Liu, J.; Zhao, Z.; Chen, Y.; Xu, C.; Duan, A.; Jiang, G.; He, H. Highly active catalysts of gold nanoparticles supported on three-dimentionally ordered macroporous LaFeO3 for soot oxidation. Angew. Chem., Int. Ed. 2011, 50, 2326. (7) Esumi, K.; Isono, R.; Yoshimura, T. Preparation of PAMAM- and PPI- Metal (silver, platinum and plladium) nanocomposities and their catalytic activities for reduction of 4-nitrophenol. Langmuir 2004, 20, 237. 5613



Article

(30) Lu, A. H.; Salabas, E. L.; Schuth, F. Magnetic nanoparticles: Synthesis, preparation, functinalization and application. Angew. Chem.; Int. Ed. 2007, 46, 1222. (31) Gao, J.; Gu, H.; Xu, B. Multifunctional magnetic nanoparticles: Design, synthesis, and biomedical applications. Acc. Chem. Res. 2009, 42, 1097. (32) Miguel-Sancho, N.; Bomatí-Miguel, O.; Colom, G.; Salvador, J. P.; Marco, M. P.; Santamaría, J. Development of stable, waterdispersible, and biofunctionalizable superparamagnetic iron oxide nanoparticles. Chem. Mater. 2011, 23, 2795. (33) Wu, C. K.; Hultman, K. L.; O’Brien, S.; Koberstein, J. T. Functional oligomers for the control and fixation of spatial organization in nanoparticle assemblies. J. Am. Chem. Soc. 2008, 130, 3516. (34) Gelbrich, T.; Feyen, M.; Schmidt, A. M. Magnetic thermoresponsive core-shell nanoparticles. Macromolecules 2006, 39, 3469. (35) Huang, J.; Han, B.; Yue, W.; Yan, H. Magnetic polymer microspheres with polymer brushes and the immobilization of protein on the brushes. J. Mater. Chem. 2007, 17, 3812. (36) García, I.; Tercjak, A.; Rueda, L.; Mondragon, I. Self-assembled nanomaterials using magnetic nanoparticles modified with polystyrene brushes and poly(styrene-b-butadiene-b-styrene). Macromolecules 2008, 41, 9295. (37) Xu, H.; Cui, L. L.; Tong, N. H.; Gu, H. Development of high magnetization Fe3O4 /polystyrene /silica nanospheres via combined miniemulsion/emulsion polymerization. J. Am. Chem. Soc. 2006, 128, 15582. (38) Cui, L. L.; Xu, H.; He, P.; Sumitomo, K.; Yamaguchi., Y.; Gu., H. Developing a hybrid emulsion polymerization system to synthesize Fe3O4 /polystyrene latexes with narrow size distribution and high magnetite content. J. Polym. Sci., Part A: Polymer Chem. 2007, 45, 5285. (39) Gong, T.; Yang, D.; Hu, J. H.; Yang, W. L.; Wang, C. C.; Lu, J. Q. Preparation of monodispersed hybrid nanospheres with high magnetite content from uniform Fe3O4 clusters. Colloid. Surface A 2009, 37, 217. (40) Chen, K.; Zhu, Y.; Li, L.; Lu, Y; Guo, X. Recyclable spherical polyelectrolyte brushes containing magnetic nanoparticles in core. Macromol. Rapid Commun. 2010, 31, 1440. (41) Chen, K.; Zhu, Y.; Zhang, Y.; Li, L.; Lu, Y.; Guo, X. Synthesis of magnetic spherical polyelectrolyte brushes. Macromolecules 2011, 44, 632. (42) Guo, X.; Weiss, A.; Ballauff, M. Synthesis of spherical polyelectrolyte brushes by photoemulsion polymerization. Macromolecules 1999, 32, 6043. (43) Guo, X.; Ballauff, M. Spatial dimensions of colloidal polyelectrolyte brushes as determined by dynamic light scattering. Langmuir 2000, 16, 8719. (44) Guo, X.; Ballauff, M. Spherical polyelectrolyte brushes: Comparison between annealed and quenched brushes. Phys. Rev. 2001, 64, 051406. (45) Wang, X.; Wu, S.; Li, L.; Zhang, R.; Zhu, Y.; Ballauff, M.; Lu, Y.; Guo, X. Synthesis of spherical polyelectrolyte brushes by photoemulsion polymerization with different photo-initiators. Ind. Eng. Chem. Res. 2011, 50, 3564. (46) Klug, H. P.; Alexander, L. E. X-ray Difraction Procedures for Polycrystalline and Amorphous Materials; Wiley: New York, 1962; pp 491−538. (47) Li, L.; Yang, Y.; Ding, J.; Xue, J. Synthesis of magnetite nanooctahedra and their magnetic field-induced two-/three-dimensional superstructure. Chem. Mater. 2010, 22, 3183. (48) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Thermosensitive core-shell particles as carrier system for metallic nanoparticles. J. Phys. Chem. B 2006, 110, 3930. (49) Panigrahi, S.; Basu, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, A.; Gosh, S. K.; Pal, T. Synthesis and size-selective catalysis by supported gold nanoparticles: study on heterogeneous and homogeneous catalytic process. J. Phys. Chem. C 2007, 111, 4596.

(50) Mahmoud, M. A.; Snyder, B.; El-Sayed, M. A. Polystyrene microsphere: Inactive supporting material for recycling and recovering colloidal nanocatalysts in solution. J. Phys. Chem. Lett. 2010, 1, 28.

5614


Recoverable Platinum Nanocatalysts Immobilized ... - ACS Publications

Recommend Documents