Preparation of Nickel Nanoparticles in Spherical Polyelectrolyte Brush

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Preparation of Nickel Nanoparticles in Spherical Polyelectrolyte Brush Nanoreactor and Their Catalytic Activity Zhongming Zhu, Xuhong Guo,* Shuang Wu, Rui Zhang, Jie Wang, and Li Li State-Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Nickel nanoparticles (Ni-NPs) with controlled size and narrow size distribution were successfully prepared in spherical poly (acrylic acid) (PAA) brushes (SPBs). The brushes act as nanoreactors for generating the Ni-NPs via adsorption of nickel ions in the brushes and subsequent reduction by addition of NaBH4. The size of Ni-NPs can be well-controlled in several nanometers and with narrow size-distribution. Most interestingly, the size can be tuned by the reaction temperature: the average size increased from 3 to 7 nm upon increasing the reaction temperature from 273 to 303 K. The catalytic activity of these Ni-NPs trapped in SPBs were evaluated using a model reaction based on the reduction of 4-nitrophenol to 4-aminophenol by NaBH4. The kinetic data were observed to follow the pseudofirst-order rate law, and the apparent rate constant was linearly dependent on the total surface area of the Ni-NPs and the reaction temperatures. This work opens a new way to prepare well-controlled Ni-NPs which should be ideal candidates for catalysts with high performance.

’ INTRODUCTION In recent years, metal nanoparticles have received increasing attention due to their fascinating chemical and physical characteristics and potential technological applications.14 Because of their high surface-to-volume ratio, metal nanoparticles exhibited excellent catalytic performance in hydrogenation, oxidation, and reduction reactions compared to their bulk materials.57 Nickel particles were reported to be excellent catalysts for hydrogenation of nitrobenzene8,9 and nitrophenol,1014 oxygen reduction,15 and oxidation of olefins.16 Recently, it was reported that spherical polymer brushes (SPBs) can be employed to prepare metal nanoparticles such as Pt,17,18 Au,19,20 Ag,2123 Rh,24 Pd,2527 and AuPt nanoalloys.28 The scaffold of spherical polymer brushes prevents the aggregation of metal particles. The advantage of this method is that the size of metal nanoparticles can be controlled below 10 nm without addition of any stabilizing agent. However, to the best of our knowledge, no report on the preparation of nickel nanoparticles (Ni-NPs) in SPBs can be found in the literature. In this paper, we demonstrated the anionic SPBs were ideal nanoreactors for generating Ni-NPs with controlled size below 10 nm. The SPBs on the surface of colloidal polystyrene (PS) core were synthesized by photoemulsion polymerization (Figure 1).29 As shown in Figure 2, Ni-NPs below 10 nm with narrow size distribution were prepared using SPBs as nanoreactors. In a first step, the carboxylic acid groups in the poly (acrylic acid) (PAA) chains were neutralized by NaOH. In the next step, the Na+ ions were subsequently replaced by Ni2+ ions via ion exchange to form a polychelate.3032 In the last step, Ni-NPs were produced and immobilized in the SPBs after the addition of the NaBH4 as the reductant. To evaluate the catalytic activity of the composite particles after immobilizing Ni-NPs in SPBs, the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 was employed. 4-Nitrophenol is one of the most refractory pollutants often occurring in the industrial wastewater,33 while 4-aminophenol is an important intermediate r 2011 American Chemical Society

in the preparation of analgesic and antipyretic drugs.34,35 This reaction is also very beneficial for us to investigate the catalytic activity of Ni-NPs composite brushes because the reaction can be easily monitored by UVvis spectroscopy for the strong absorption peak at 400 nm of 4-aminophenol at high pH.3641 The extremely small size of Ni-NPs in SPBs makes it possible for high catalytic activity for the reduction of 4-nitrophenol in the chemical industry.

’ EXPERIMENTAL SECTION Materials. Styrene and acrylic acid were purchased from Shanghai reagent company (SRC), distilled under a reduced pressure, and stored in refrigerator at 4 °C before use. Potassium persulfate (KPS) (from SRC) was recrystallized by water. Sodium hydrogen sulfite (SHS) (from SRC), sodium lauryl sulfate (SDS) (from SRC), and nickel(II) chloride hexahydrate (NiCl2 3 6H2O) (from SRC) were used without further purification. Sodium borohydride and sodium hydroxide were bought from Lingfeng Chemical Reagent Co., Ltd. and used as received. The water used in all experiments was purified by reverse osmosis (Shanghai RO Micro Q). Synthesis of SPBs. Synthesis of SPBs can be divided into three steps (Figure 1). In the first step, the PS core latexes were synthesized using a conventional emulsion polymerization in which 10.0 g of styrene was polymerized in 250 mL of H2O with 0.24 g of SDS as surfactant, 0.74 g of KPS, and 0.15 g of SHS as initiator. In the second step, a thin shell of photoinitiator 2[p-(2hydroxy-2-methylpropiophenone)]-ethylene glycol-methacrylate (HMEM) was generated by adding 1.0 g of HMEM dissolved in 7.0 g of acetone. To achieve well-defined coreshell Received: August 3, 2011 Accepted: October 27, 2011 Revised: October 26, 2011 Published: October 27, 2011 13848

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Figure 3. Dependence of brush thickness L on concentration of added salt for SPBs. Symbols denote: (O) NiCl2, (0) NaCl. Figure 1. Schematic representation of the synthesis of SPBs. The thickness L of the SPBs is measured by dynamic light scattering (DLS).

Figure 2. Schematic representation of the formation of the Ni-NPs inside the SPBs.

morphology, HMEM was added very slowly under starved condition. The prepared core latex was purified by extensive serum replacement against pure water. The PS core latexes modified by a thin shell of HMEM and a defined amount of acrylic acid (AA) were added into a UV-reactor in the last step and diluted to 1 wt % with water until the total weight reached 250 g. The whole reactor degassed by repeated evacuation and subsequent addition of nitrogen more than three times. Photoemulsion polymerization was accomplished with UV radiation at room temperature with vigorous stirring in 1.5 h. The obtained latex was purified by serum replacement against pure water until the conductance of the eluate did not change anymore. Preparation of the Ni-NPs in Brushes. Using the synthesized SPBs as nanoreactors, Ni-NPs were prepared by the reduction of Ni2+ with NaBH4 (Figure 2). In a typical run, 0.012 g of NaOH was added to 50 mL of the brush latex to neutralize carboxylic groups. After stirring for 10 h, the mixture was washed with a 3 L NiCl2 solution of 0.001 mol/L to replace Na+ by Ni2+ in the ultrafiltration cell. To remove the uncoordinated Ni2+ ions, large amounts of pure water were employed to wash the latex until the conductivity of the washed water became constant. The volume of the brushes increased to 450 mL. The Ni-NPs were prepared when 0.03 g of NaBH4 dissolved in 7.5 g of water was added into 50 mL of brush latex (0.15 wt %) at different temperatures under the atmosphere of nitrogen with vigorous stirring. After the addition of NaBH4 in 30 min, the lattices were stirred for another 1 h.

Catalyzed Reduction of 4-Nitrophenol by Ni-NPs. To test the catalytic activity of the Ni-NPs, the reduction of 4-nitrophenol by NaBH4 as the model reaction in water was chosen. At first, the pH value of the 4-nitrophenol solution was adjusted to 10 with NaOH, and then, 1.2 mg of NaBH4 was added to 3 mL of aqueous 4-nitrophenol solution (0.042 mg) in a quartz cuvette. After mixing them with shaking, a given amount of the Ni-NPs was added in to start the reaction, and the UV spectrometry was employed to monitor the reduction of 4-nitrophenol by measuring the absorbance of the solution at 400 nm as a function of time. Characterization. Dynamic light scattering (DLS) was used to determine the hydrodynamic size of particles and performed by a particle sizing system of NICOMP 380 ZLS at a fixed scattering angle of 90°. The high resolution transmission electron microscopy (HRTEM) was performed using a JEOL-2100 electron microscope operating at 200 kV. Inductively coupled plasma-atomic emission spectrometry (ICP-AES, Varian 710 ES) was employed to determine the content of nickel in composite brushes. UVvis absorption spectra of samples were recorded on a UV-2550 UVvis spectrophotometer.

’ RESULTS AND DISCUSSION Effect of Ionic Strength on Brushes. Figure 3 shows the thickness L of SPBs as a function of the concentration of added salt (Ca) determined by DLS. The brush thickness decreased upon increasing the concentration of added salt. The addition of divalent Ni2+ ions leads to a much stronger shrinking than the monovalent Na+ ions. No aggregation appeared even at the concentration of 0.02 mol/L NiCl2. Figure 3 also displays that the thickness L and Ca follows the scaling laws LµCam.42 The data in Figure 3 with bilogarithm fit well to a linear line at the range of our experiments, and the exponent m is 0.11 for NaCl and 0.20 for NiCl2. The larger m value means the stronger effect on brush thickness. The salt effect on the brush thickness is due to the influence on the dissociation of carboxylic groups in SPBs.43,44 This observation here should be very helpful for finding the optimized concentration window for the reaction of generating Ni-NPs in SPBs. Size Control of Ni-NPs. Figure 2 summarizes the preparation of Ni-NPs in SPBs in a schematic manner. Due to the Donnan effect, the counterions Ni2+ mainly distribute in the SPBs. The reduction of the Ni2+ ions mainly took place inside the SPBs after thorough ultrafiltration against water. Upon addition of NaBH4, the color of the white solution turned to black immediately which reflected the formation of the Ni-NPs. Figure 4 displays the color change and the shift of brush size and size distribution. The reduction of Ni2+ to Ni-NPs led to the decrease of ionic strength 13849

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Figure 4. Change of size and color of the SPBs during reduction of Ni2+.

Figure 6. TEM images of the Ni-NPs encapsulated in SPBs prepared at 273 K. (a) and (b) Ni-NPs immobilized on the brush. (c) HRTEM image of a single nickel nanoparticle. (d) Size distribution of the Ni-NPs counted from (a) and (b).

Figure 7. UVvis spectra of the reduction of 4-nitrophenol in the presence of Ni-NPs composite brushes at 291 K. The time difference between two neighbor curves is 4 min. The concentration of Ni-NPs is 0.148 mg/L. Figure 5. TEM images of the Ni-NPs encapsulated in SPBs prepared at 303 K. (a), (b), and (c): Ni-NPs immobilized in the brush. (d) Size distribution of the Ni-NPs counted from (a), (b), and (c).

(Ni2+) and thus the expansion of SPBs. Although the size distribution of the brushes becomes slightly broad, the brushes kept stable without aggregation of brushes and precipitation of Ni-NPs. The morphology of the Ni-NPs composite brushes was observed by TEM. Figure 5 shows the TEM pictures of the NiNPs obtained at the temperature of 303 K in SPBs. The nanosized black dots reflect the successful immobilization of Ni-NPs from the reduction of Ni2+ ions in the SPBs. The particle size distribution histogram of Ni-NPs (Figure 5d) evaluated from the images in Figure 5ac showed that the Ni-NPs were in a narrow size distribution with an average diameter of 7 nm. When the reaction temperature was reduced from 303 to 273 K, the generation of Ni-NPs from Ni2+ became obviously slower as shown from the color change. More interestingly, the Ni-NPs size became significantly smaller and the distribution of particle size became narrower as shown in Figure 6. The crystal lattice can

be clearly seen in the HRTEM image of a single nickel nanoparticle (Figure 6c). Compared to the Ni-NPs prepared at 303 K, the average size was reduced from 7 to 3 nm and particles were narrowly dispersed (Figure 6d). Therefore, the size and size distribution of Ni-NPs can be controlled conveniently by the reaction temperature of reduction Ni2+ ions to Ni-NPs. We use the reduction of 4-nitrophenol by NaBH4 to 4-aminphenol as the model reaction to evaluate the catalytic activity of Ni-NPs. For all the experiments, the initial concentrations of 4-nitrophenol and NaBH4 were kept as 0.1 and 10 mmol/L, respectively. The concentration of NaBH4 was so much in excess to that of 4-nitrophenol that the kinetics of this reaction can be treated as a pseudofirst-order reaction. The pH value of 4-nitrophenol was adjusted to 10 by NaOH in order to strengthen the absorption peak at 400 nm from 4-nitrophenolate ions.45 After the Ni-NPs composite brushes were added, the intensity of the absorption peak at 400 nm dropped gradually during the reduction 4-nitrophenol reaction (Figure 7). During the reaction, a new absorption peak at 315 nm from 4-aminophenolate ions appeared.46 13850

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Figure 9. The reaction rate constant (kapp) as a function of the concentration of Ni-NPs (a) and total surface area of the Ni-NPs (b) for the reduction of 4-nitrophenol to 4-aminophenol based on the data in Figure 8d.

Figure 8. Extinction of UVvis absorption intensity at 400 nm and the concentration reduction of 4-nitrophenol during the reaction. (a) Extinction of absorbance at various temperatures with Ni-NPs concentration of 0.118 mg/L. (b) Extinction of absorbance at different Ni-NPs concentrations at 288 K. (c) Decrease of 4-nitrophenol concentration at various temperatures with Ni-NPs concentration of 0.118 mg/L. (d) Decrease of 4-nitrophenol concentration at different Ni-NPs concentrations at 288 K.

Choosing the change of absorbance at 400 nm to represent the consumption of 4-nitrophenol, the reduction reaction of 4nitrophenol was monitored by the UVvis spectra in situ (Figure 8). Without addition of Ni-NPs composite brushes, no reaction at all was detected. The rate of decrease of absorbance at 400 nm increased at higher reaction temperature and at higher concentration of the added Ni-NPs (Figure 8a,b). It also can be found that there existed an induction time for the reduction of 4-nitrophenol upon addition of Ni-NPs composite brushes as catalyst, especially at low temperature and low Ni-NPs concentration. This induction time is a typical phenomenon of heterogeneous catalysis which was related to the time required for the activation of the catalyst in the reaction mixtures.47 As shown in Figure 8, the induction time decreased with the increase of reaction temperature and with the increase in the concentration of nickel particles. Since that the concentration of NaBH4 was in great excess compared to 4-nitrophenol in our experiments, its concentration can be considered as constant during the reaction. When we plotted the 4-nitrophenol concentration ln(c/c0) versus t, excellent linear fittings with R2 > 0.999 were obtained (Figure 8c,d). Therefore, pseudofirst-order kinetics (eq 1):

Figure 10. Plot of ln kapp versus 1000/T for the reduction of 4-nitrophenol at different temperatures based on the data in Figure 8c.

ð1Þ

kapp and both the concentration of and surface areas of Ni-NPs. Here, the estimation of total surface area of Ni-NPs was based on ICP-AES analysis, the average size determined by TEM and the assumption of their spherical structure. The reaction rate constants (kapp) were around 103 s1 with 0.14 mg/L Ni-NPs (Figure 9), which reflects the high catalytic activity of Ni-NPs composite brushes for the reduction reaction of 4-nitrophenol to 4-aminophenol. Figure 10 shows a plot of ln kapp versus 1000/T for the reduction of 4-nitrophenol. On the basis of the linear fitting by the Arrhenius equation, the apparent activation energy (Ea), which shows the temperature dependency of the reaction rate constant, was obtained. From the slope in Figure 10, the apparent activation energy Ea was calculated to be 41.7 kJ/mol or 9.9 kcal/ mol. This result demonstrates that the reduction of 4-nitrophenol in the presence of Ni-NPs in our research work occurs via surface catalysis, since the activation energy for the surface catalysis reactions was always in the range of 210 kcal/mol.49 Moreover, the pre-exponential factor A = 1.56  104 s1 was calculated from the intercept of the linear dependence of ln kapp versus 1000/T, and the entropy of activation ΔS = 80.3 J/(mol.K) was obtained from the equation ln A = ΔS/R.50

can be applied to fit our kinetic data, where ct is the concentration of 4-nitrophenol at reaction time t, kapp is the apparent rate constant, and k1 is the rate constant normalized to S, the surface area of Ni-NPs normalized to the unit volume of the system. Starting from the same initial concentration of 4-nitrophenol and NaBH4, the reaction rate constants were obtained from the slope of the kinetic curves (Figure 8d). In case of heterogeneous or microheterogeneous catalysis, the reaction rate constant is always propotional to catalyst concentration.48 Indeed, Figure 9 shows the linear correlation between the reaction rate constant

’ CONCLUSIONS Anionic SPBs were synthesized by photoemulsion polymerization. Using them as nanoreactors, the Ni2+ ions were concentrated as counterions within the SPBs layer and reduced by NaBH4 to narrowly dispersed Ni-NPs. When the reaction temperature of the reduction decreased from 303 to 273 K, the average diameter of the obtained Ni-NPs reduced from 7 to 3 nm. The size of the prepared Ni-NPs can be well controlled by the reaction temperature.



dct ¼ kapp ct ¼ k1 Sct dt

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Industrial & Engineering Chemistry Research The Ni-NPs immobilized on SPBs showed high catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 as monitored by the UVvis spectrophotometer. From the kinetics data, we obtained the activation energy of 41.7 kJ/mol, the pre-exponential factor of 1.56  104 s1, and the entropy of activation of 80.3J/(mol.K). All the data showed that the anionic spherical PAA brush is an ideal nanoreactor for the preparation of Ni-NPs with controlled size and size distribution, and the obtained Ni-NPs composite brushes are ideal candidates for novel catalysts with the high catalytic activity.

’ AUTHOR INFORMATION Corresponding Author

*Tel/Fax: +86 21 6425 3491. E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the National Natural Science Foundation of China (No. 20774028 and 21004021), the Fundamental Research Funds for the Central Universities, the Key Basic Research Project of Shanghai Science and Technology Commission (10JC1403800), and the Scientific and Technological Project of Shanghai Science and Technology Commission (10111100103) for support of this work. ’ REFERENCES (1) Rosi, N. L.; Mirkin, C. A. Nanostructures in Biodiagnostics. Chem. Rev. 2005, 105, 1547. (2) Kamat, P. V. Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles. J. Phys. Chem. B 2002, 106, 7729. (3) Lu, Y.; Mei, Y.; Schrinner, M.; Ballauff, M.; Moller, M. W.; Breu, J. In Situ Formation of Ag Nanoparticles in Spherical Polyacrylic Acid Brushes by UV Irradiation. J. Phys. Chem. C 2007, 111, 7676. (4) Signori, A. M.; Santos, K. O.; Eising, R.; Albuquerque, B. L.; Giacomelli, F. C.; Domingos, J. B. Formation of Catalytic Silver Nanoparticles Supported on Branched Polyethyleneimine Derivatives. Langmuir 2010, 26, 17772. (5) Tian, N.; Zhou, Z.; Sun, S.; Ding, Y.; Wang, Z. Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity. Science 2007, 316, 732. (6) Nolte, P.; Stierle, A.; Jin-Phillipp, N. Y.; Kasper, N.; Schulli, T. U.; Dosch, H. Shape changes of supported Rh nanoparticles during oxidation and reduction cycles. Science 2008, 321, 1654. (7) Wang, A.; Yin, H.; Lu, H.; Xue, J.; Ren, M.; Jiang, T. Catalytic activity of nickel nanoparticles in hydrogenation of p-nitrophenol to p-aminophenol. Catal. Commun. 2009, 10, 2060. (8) Holy, N.; Shalvoy, R. Hydrogenation with Anthranilic Acid Anchored, Polymer-Bound Nickel Catalysts. J. Org. Chem. 1980, 45, 1418. (9) Lu, P.; Teranishi, T.; Asakura, K.; Miyake, M.; Toshima, N. Polymer-Protected Ni/Pd Bimetallic Nano-Clusters: Preparation, Characterization and Catalysis for Hydrogenation of Nitrobenzene. J. Phys. Chem. B 1999, 103, 9673. (10) Du, Y.; Chen, H.; Chen, R.; Xu, N. Synthesis of p-aminophenol from p-nitrophenol over nano-sized nickel catalysts. Appl. Catal., A: Gen. 2004, 277, 259. (11) Wang, A.; Yin, H.; Lu, H.; Xue, J.; Ren, M.; Jiang, T. Effect of Organic Modifiers on the Structure of Nickel Nanoparticles and Catalytic Activity in the Hydrogenation of p-Nitrophenol to p-Aminophenol. Langmuir 2009, 25, 12736. (12) Liu, H.; Deng, J.; Li, W. Synthesis of Nickel Nanoparticles Supported on Boehmite for Selective Hydrogenation of p-Nitrophenol and p-Chloronitrobenzene. Catal. Lett. 2010, 137, 261.

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