Au Bimetallic Nanoparticles on Highly Cross

Feb 9, 2010 - One-Step Synthesis of Thermosensitive Nanogels Based on Highly Cross-Linked Poly(ionic liquid)s. Yubing Xiong , Jingjiang Liu , Yujiao ...
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Seeding Growth of Pd/Au Bimetallic Nanoparticles on Highly Cross-Linked Polymer Microspheres with Ionic Liquid and Solvent-Free Hydrogenation Baoji Hu, Tianbin Wu, Kunlun Ding, Xiaosi Zhou, Tao Jiang, and Buxing Han* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ReceiVed: October 27, 2009; ReVised Manuscript ReceiVed: January 22, 2010

We developed a method to immobilize Pd/Au bimetallic nanoparticles (NPs) on highly cross-linked polymer microspheres. In this route, the cross-linker divinylbenzene and ionic liquid (IL) 1-vinyl-3-butyl imidazolium chloride ([VBIM]Cl) were first copolymerized in ethanol to prepare the cross-linked polymer microspheres with the IL. The PdCl42- ions were immobilized on the polymer support, and the Pd NPs immobilized on the support were formed after the PdCl42- ions were reduced. The supported bimetallic Pd/Au NPs were obtained after seeding growth process. The polymer support and metallic NPs/support composites were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray, inductively coupled plasma analysis, X-ray photoelectron spectroscopy, and elemental analysis. It was demonstrated that the immobilized metallic NPs were nearly monodispersed and the Pd and Au in the composites showed excellent synergy to catalyze the hydrogenation of cyclohexene. Introduction Metal nanoparticles (NPs) have long been used as catalysts. Generally, homogeneous catalysts exhibit high activity and selectivity. However, their practical applications are limited due to their instability and difficulty in products/catalysts separation. Immobilization of NPs can solve the problems effectively, and different inorganic and organic supports have been used.1 Polymers are very attractive supports for metal NPs due to some unusual advantages.2 However, surface prefunctionalization is usually required in order to load the NPs. In addition, common polymer supports have weak structure stability in organic solvents. Therefore, highly cross-linked polymer supports with functional groups are very attractive. Ionic liquids (ILs), which are room temperature molten salts, have some unique properties, such as nonvolatility, nonflammability, and excellent miscibility with different kinds of compounds, and they can be functionalized by designing different cations and anions.3 Recently, some works have been carried out on the immobilization of ILs on different supports, such as silica4a and organic polymers.4b-f These methods can reduce the amount of IL used and/or functionalize the polymers. Bimetallic NPs have attracted considerable attention in recent years, and various bimetallic NPs have been fabricated.5 Immobilization of bimetallic NPs on different supports has also been investigated by many researchers. For example, Crooks and co-workers6 synthesized dendrimer-encapsulated Pd/Au catalysts. Chung and Rhee fabricated Pd/Pt and Pd/Rh dendrimer-encapsulated catalysts.7 Peng and Yang5e prepared Pton-Pd NPs in solution and then immobilized NPs on the carbon supports. Alayoglu and Eichhorn5g synthesized Rh/Pt bimetallic NPs and then loaded on alumina supports. These bimetallic NPs showed superior activities than monometallic NPs in catalysis. Herein, we show that Pd/Au bimetallic NPs can be immobilized onto the highly cross-linked polymer microspheres with IL via in situ seeding growth on the polymer support. This * To whom correspondence should be addressed. Tel/Fax: +86 10 62562821. E-mail: [email protected].

in situ approach is convenient and can effectively avoid the aggregation of the bimetallic NPs in the loading process. Moreover, the immobilized bimetallic NPs are much more active to catalyze the hydrogenation of cyclohexene than monometallic Pd or Au. Experimental Section Reagents. 1-Vinylimidazole and divinylbenzene(DVB) were purchased from Fluka. All other chemicals (A. R. grade) used were purchased from Sinopharm Chemical Reagent Co., Ltd. Azobis(isobutyronitrile) (AIBN) was purified via recrystallization. Synthesis of 1-Vinyl-3-butyl-imidazolium Chloride. The procedures to prepare 1-vinyl-3-butyl imidazolium chloride ([VBIM]Cl) were similar to that reported by other authors.8 In the experiment, 18.50 g (200 mmol) of 1-chlorobutane (BuCl) and 5.50 g (58.3 mmol) of 1-vinyl imidazole were added into a 25 mL two-necked flask equipped with a magnetic stirrer. The mixture was refluxed for 50 h in an oil bath of 70 °C under the protection of nitrogen with stirring. Then the reaction mixture was cooled down to room temperature. The top phase was poured out and the bottom phase was purified by antisolvent recrystallization, in which ethanol were used as the solvent and ethyl acetate as the antisolvent. Then the solid product was dried at 50 °C for 12 h under vacuum. Synthesis of the Highly Cross-Linked Polymer with [VBIM]Cl. The method to synthesize the cross-linked polymer support with the IL [VBIM]Cl (denoted as PDVB-IL hereafter) was similar to that reported previously, which used chloroform as the solvent.4c The main difference was that ethanol or ethanol/ chloroform solution was used as the solvent in this work, and regular microspheres were obtained. In a typical precipitation polymerization, 80 mg of [VBIM]Cl, 0.7 g of DVB, and 20 mL of ethanol or ethanol/chloroform solution were added into a 50 mL three-necked round flask and heated to 70 °C. The flask was purged with nitrogen and then 40 mg of AIBN was added under magnetic stirring. The polymerization was allowed to proceed under a N2 atmosphere for 24 h before cooling to

10.1021/jp910285s  2010 American Chemical Society Published on Web 02/09/2010

Seeding Growth of Pd/Au Bimetallic Nanoparticles room temperature. After the polymerization process, the product was separated by centrifugation and washed thoroughly with ethanol and water. Synthesis of Pd or Au NPs Immobilized onto PDVB-IL Microspheres. The PDVB-IL microspheres prepared in neat ethanol were dispersed in 150 mL of water via sonication, and 6 mL of PdCl2 · 2HCl aqueous solution (containing 21 mg of PdCl2) or HAuCl4 · 4H2O aqueous solution (containing 60 mg of HAuCl4 · 4H2O) was added dropwise with stirring, and the solution was stirred for 30 min. The suspension was centrifuged and the obtained sediments were washed three times with water (3 × 150 mL) and then dispersed in 150 mL of water. Finally, 12 mL of fresh NaBH4 aqueous solution (10 mg/mL) was added and the solution was stirred for 1 h. The product was centrifuged and washed with water (3 × 150 mL) and 150 mL of ethanol at room temperature. The product was dried under vacuum for 24 h at 40 °C. The products were assigned as PDVB-IL-Pd or PDVB-IL-Au. Seeding Growth Method to Synthesize Supported Bimetallic NPs. The principle to prepare the Pd/Au bimetallic NPs supported on the PDVB-IL was similar to the seeding growth method in solutions.5b,9 The main difference was that in this work the growth of the bimetallic NPs occurred on the polymer support with Pd NPs and the supported Pd/Au bimetallic NPs were obtained. In the experiment, 10 mL of aqueous solution containing HAuCl4 (0.06 mg/mL), cetyltrimethylammonium bromide (CTAB, 20 mg/mL) and ascorbic acid (AA, 4 mg/ mL) was added into the 10 mL of PDVB-IL-Pd dispersion (containing 100 mg of PDVB-IL-Pd prepared above). The mixture was kept at 40 °C for 8 h. The product was centrifuged and washed with water (2 × 20 mL) and ethanol (2 × 20 mL). The product was dried under vacuum at 50 °C for 24 h and it was denoted as PDVB-IL-Pd/Au-1. To prepare the supported bimetallic Pd/Au NPs with lower Pd/Au weight ratio, 30 mL of aqueous solution containing HAuCl4 (0.06 mg/mL), CTAB (20 mg/mL) and AA (4 mg/mL) was added into 10 mL of PDVB-IL-Pd dispersion (containing 100 mg of PDVB-IL-Pd prepared above). Other procedures were the same as that described above, and the obtained product was assigned as PDVB-IL-Pd/Au-2. Characterization. The scanning electron microscopy (SEM) observation was conducted on a Hitachi-s4300 electron microscope operated at 15 kV. The samples were spray-coated with a thin layer of platinum before SEM observation. Samples were dispersed in ethanol and dropped on amorphous carbon coated copper grids for the transmission electron microscopy (TEM) analysis. The images were obtained on a JEM 1011 (100 kV). HRTEM image was obtained on a JEM 2011 (200 kV). Single particle energy dispersive X-ray (EDX) was carried out under high-angle annular dark field scanning TEM (HAADF-STEM) on a Tecnai G2 F20 U-TWIN. The zeta-potential of latex was determined by Zetasizer 3000HS. The sample was dispersed in deionized water, and then the pH was adjusted to 7.0. The solution was injected into the Zetasizer to perform the electrophoresis measurements. The FTIR spectra of the products were collected from a Bruker Tensor 27 spectrometer in KBr pellet form. Inductively coupled plasma (ICP) analysis was performed on ICP-AES (Vista-MPX). X-ray photoelectron spectroscopy (XPS) measurement of the composites was performed on an ESCALab220i-XL spectrometer operated at 15 kV and 20 mA at a pressure of about 3 × 10-9 mbar using Al KR as the exciting source (hν ) 1486.6 eV). Elemental analysis was performed on Flash EA 1112. The size distributions of the NPs were obtained by counting 200 particles.

J. Phys. Chem. C, Vol. 114, No. 8, 2010 3397 SCHEME 1: Route to Prepare the Cross-Linked Polymer Supported Bimetallic Pd/Au NPs

Hydrogenation Reaction. Hydrogenation of cyclohexene was carried out in a 6 mL high-pressure stainless steel reactor with a magnetic stirrer, which was similar to that used in our previous work.4c In the experiment, 4 mmol of substrate and the desired amount of catalysts were introduced into the reactor and sealed. Then, H2 was charged to 6 MPa. The hydrogenation reaction was carried out for a certain time under stirring at 40 °C. The quantitative analysis of the reaction mixture was conducted using a GC (Agilent 6820) equipped with a flame ionization detector (FID) and a PEG-20 M capillary column (0.25 mm in diameter, 30 m in length). Results and Discussion Synthesis and Characterization of PDVB-IL-Pd/Au Composites. The route to prepare the PDVB-IL-Pd/Au composites is schematically shown in Scheme 1. The PDVB-IL was first synthesized by polymerization of DVB and [VBIM]Cl. The PdCl42- was immobilized on the support PDVB-IL, which was then reduced by NaBH4 to form PDVB-IL-Pd. The supported bimetallic NPs (PDVB-IL-Pd/Au) were obtained by seeding growth method. The morphology of the PDVB-IL support was examined by SEM and TEM observations. As known from the SEM images (Figure 1) and the TEM images (Figure S1, Supporting Information), the morphologies of the copolymer depended strongly on the property of the solvents. Dense microspheres with the size ranging from 200 to 500 nm were obtained in neat ethanol (Figure 1b). With increasing the proportion of chloroform in the solvent, porous microspheres and irregular porous microparticles were formed (Figure 1c-e). This can be explained by the competition between the polymer chain growth rate and the phase separation rate. Ethanol is a much poorer solvent for the polymer than chloroform. Therefore, once the copolymers were formed in neat ethanol, the phase separation occurred quickly, and the copolymer shrunk and grew into the dense microsphere. Coagulate particles were obtained from the polymerization of DVB in neat ethanol without [VBIM]Cl (Figure 1a), indicating that the poly-[VBIM]Cl segments in the copolymers protected the coagulation, which might result from the electrostatic repulsion between the particles. Chloroform is a much better solvent than ethanol for the polymers, and therefore the phase separation rate decreased with increasing concentration of chloroform in the solvent. Swelling gel particles were formed at the early stage of the polymerization, and new monomers were absorbed in the particles for further chain propagation. Therefore, higher concentration of chloroform in the solvent led to irregular porous structure. The zeta potential value of the PDVB-IL support measured was 42.9 mV (Figure 2), indicating the positive nature of the PDVB-IL microparticles, which resulted from the existence of

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Figure 1. SEM images and FTIR spectra of the products obtained in different solvent mixtures. (a) neat ethanol without [VBIM]Cl, (b) neat ethanol with [VBIM]Cl, (c) ethanol/chloroform (75/25 v/v) with [VBIM]Cl, (d) ethanol/chloroform (50/50 v/v) with [VBIM]Cl, (e) ethanol/chloroform (25/75 v/v) with [VBIM]Cl, and (f) FTIR for the sample shown in a-e (up down).

Figure 2. Zeta potential of the PDVB-IL prepared in neat ethanol.

the cations of IL [VBIM]Cl in the copolymer. Therefore, PdCl42- could be immobilized on the support easily by ionic interaction. So, there should be a maximum loading amount of metal NPs because the positive charges on the polymer support could be saturated. FTIR and elemental analysis were also carried out to confirm the existence of poly-[VBIM]Cl segments in the product. As shown in Figure 1f, the main differences between these spectra were that the absorption band at 1165 cm-1, which belongs to poly-[VBIM]Cl structure, became stronger with increasing amount of chloroform in the solvent. The contents of chloride in the copolymers synthesized in the solvents with ethanol/ chloroform ratios of 100/0, 75/25, 50/50, and 25/75 were 0.44%, 1.1%, 1.4%, and 1.5% (wt %), respectively, as were determined by elemental analysis. The corresponding contents of poly[VBIM]Cl segments in the products were 2.3%, 5.6%, 7.5%, and 8.1% (wt %). This shows that the higher proportion of chloroform in the solvent led to higher content of poly[VBIM]Cl structure. This could be attributed to the slower phase separation at higher chloroform concentration in the synthesis process, which facilitated the diffusion of [VBIM]Cl in the polymer matrix and enhanced the probability of the copolymerization of [VBIM]Cl and DVB. In this work the polymer spheres prepared in neat ethanol were used as the support, which contained 2.3 wt % poly[VBIM]Cl segments, as discussed above. The TEM images of the PDVB-IL-Pd and PDVB-IL-Au microparticles and the size distribution diagrams of the metal NPs are given in Figure 3.

The average sizes of Pd NPs (Figure 3a,b) and Au NPs (Figure 3c,d) were about 5 and 4 nm, respectively, and the size distribution of the NPs was very narrow. In addition, most of the metal NPs were loaded on the polymer microspheres homogeneously without obvious aggregation. Pd and Au contents in the PDVB-IL-Pd and PDVB-IL-Au microparticles determined by the ICP method were 0.25 wt % (metal/ imidazolium mole ratio ) 1:5.3) and 0.38 wt % (metal/ imidazolium mole ratio ) 1:6.4), respectively. The above PDVB-IL-Pd microparticles were prepared by adding 21 mg of PdCl2 in the experiment and the loading of Pd was 0.25 wt % (metal/imidazolium mole ratio ) 1:5.3), as discussed above. We also prepared the PDVB-IL-Pd microparticles by adding 2.1 mg, 10.5 mg and 42 mg of PdCl2 at the same experimental condition, and the contents of Pd were 0.094 wt % (metal/imidazolium mole ratio ) 1:14), 0.13 wt % (metal/ imidazolium mole ratio ) 1:10) and 0.24 wt % (metal/ imidazolium mole ratio ) 1:5.5), respectively. The results show that the content of Pd in the microparticles increased with the amount of the metal precursor added when the loading was not saturated. However, the content of Pd was independent of the amount of the PdCl2 after enough metal precursor was added, indicating that the maximum exchange between the Cl- in the polymer support and the PdCl42- was reached. If all the Clions in the polymer support were replaced by PdCl42-, the mole ratio of Pd and imidazolium in the PDVB-IL-Pd microparticles should be 1:2. However, the maximum mole ratio in the prepared PDVB-IL-Pd microparticles was 1:5.3, which was smaller than 1:2. The main reason may be that some of the imidazolium groups in the polymer support were embedded and could not contribute to the loading of the metal. The Pd/Au weight ratios of PDVB-IL-Pd/Au-1 and PDVBIL-Pd/Au-2 obtained from ICP method were 1:0.5 and 1:1.2, respectively. This indicates that using this method the Pd/Au weight ratio can be easily controlled by the dosage of the Au precursor in the solution during the seeding growth process. The sizes and size distributions of Pd/Au bimetallic NPs in PDVB-IL-Pd/Au-1 (Pd/Au weight ratio ) 1:0.5) and PDVBIL-Pd/Au-2 (Pd/Au weight ratio ) 1:1.2) are given in Figure 3e-h. As expected, the size of the metal NPs increased after

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Figure 4. XPS spectra of (a) Pd in PDVB-IL-Pd, (b) Au in PDVBIL-Au, (c) Pd in PDVB-IL-Pd/Au-1 (Pd/Au weight ratio of 1:0.5), (d) Au in PDVB-IL-Pd/Au-1 (Pd/Au weight ratio of 1:0.5), (e) Pd in PDVB-IL-Pd/Au-2 (Pd/Au weight ratio of 1:1.2), (f) Au in PDVB-ILPd/Au-2 (Pd/Au weight ratio of 1:1.2).

Figure 3. (a) TEM image of PDVB-IL-Pd, (b) the size distribution of Pd NPs in PDVB-IL-Pd, (c) TEM image of PDVB-IL-Au, (d) the size distribution of Au NPs in PDVB-IL-Au, (e) TEM image of PDVBIL-Pd/Au-1(Pd/Au weight ratio ) 1:0.5), (f) the size distribution of Pd/Au bimetallic NPs in PDVB-IL-Pd/Au-1(Pd/Au weight ratio ) 1:0.5), (g) TEM image of PDVB-IL-Pd/Au-2(Pd/Au weight ratio ) 1:1.2), (h) the size distribution of Pd/Au bimetallic NPs in PDVB-ILPd/Au-2(Pd/Au weight ratio ) 1:1.2); (i) HAADF-STEM image of PDVB-IL-Pd/Au-1; (j) EDX spectrum of the NP as marked in (i).

the seeding growth process, as can be known by comparing Figure 3, panels a, e, and g. In addition, the size of the metal NPs in PDVB-IL-Pd/Au-2 was bigger than that in PDVB-ILPd/Au-1. All these indicated that bimetallic NPs were formed in the seeding growth process. Single particle energy dispersive X-ray (EDX) characterization of PDVB-IL-Pd/Au-1 was carried out under high-angle annular dark field scanning TEM (HAADFSTEM). Corresponding results were given in Figure 3, panels i and j, which clearly showed that the metal NPs were consisted by Pd and Au. Controlled experiment was carried out to study the significance of Pd seeds, in which the PDVB-IL microspheres without

Pd NPs seeds were added in the Au growth solution. The TEM images showed that large individual Au particles were formed, and no Au NPs were loaded on the polymer microspheres. These results indicated that the Pd NPs seeds played a very important role in the growth of Au NPs. The reason may be that the already formed Pd particles helped the Au to overcome the energy barrier for nucleation, leading to the seeding growth of Au on the Pd seeds.5a,9 The XPS spectra of Pd in PDVB-IL-Pd, Au in PDVB-ILAu, Pd in PDVB-IL-Pd/Au-1, Au in PDVB-IL-Pd/Au-1, PDVBIL-Pd/Au-2, and Au in PDVB-IL-Pd/Au-2 are shown in Figure 4a-f, respectively. The XPS spectra of Pd in PDVB-IL-Pd (Figure 4a), PDVB-IL-Pd/Au-1 (Figure 4c), and PDVB-IL-Pd/ Au-1 (Figure 4e) showed that the Pd 3d XPS spectrum can be deconvoluted into two sets of doublet components. The relatively lower binding energy set of doublet is attributed to Pd(0) species, and the other set of doublet belongs to Pd(II) species,10 which may be attributed to the partial oxidation of the metal after exposing in the air. As shown in Figure 4, panels b, d, and f, the binding energy peaks of Au0 in PDVB-IL-Pd/Au-1 and PDVB-IL-Pd/Au-2 shifted 0.4 and 0.6 eV, respectively, relative to that of the Au0 in PDVB-IL-Au, while keeping the inter peak distance identical. This shift indicated the different chemical environment of Au, which may result from the combination of Au with Pd in PDVBIL-Pd/Au composites. This further supports the argument that Pd/Au bimetallic NPs were formed in PDVB-IL-Pd/Au-1 and PDVB-IL-Pd/Au-2. It is difficult to figure out whether there were considerable metal NPs inside the support or not. However, we believe that the density of the metal NPs on the surface of the support was much larger than that inside support because of two reasons. First, the polymer support was highly cross-linked and had high

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Hu et al. synergy for catalyzing hydrogenation of cyclohexene. We believe that this in situ seeding growth method of bimetallic NPs on the support can also be used to prepare some other bimetallic NPs/polymer support composites. Acknowledgment. We thank the financial support from National Natural Science Foundation of China (20973177, 20773144), Ministry of Science and Technology of China (2009CB930802), and Chinese Academy of Sciences (KJCX2.YW.H16).

Figure 5. Dependence of conversion of cyclohexene on reaction time for different catalysts at 40 °C. (a) PDVB-IL-Au(7.7 × 10-5 mmol Au), (b) PDVB-IL-Pd(9.4 × 10-5 mmol Pd), (c) PDVB-IL-Pd/Au-1 (Pd/Au weight ratio of 1:0.5; 9.4 × 10-5 mmol Pd), and (d) PDVBIL-Pd/Au-2 (Pd/Au weight ratio of 1:1.2; 9.4 × 10-5 mmol Pd). The amount of cyclohexene was 4 mmol for all of the experiments.

density, and therefore it was difficult for the metal precursor to diffuse into the support. Second, the density of the IL on the surface of the support should be higher than that inside the support because the polar solvent of ethanol was used when synthesizing the support, which tended to induce the orientation of the IL toward the surface of the polymer spheres. Hydrogenation of Cyclohexene Catalyzed by PDVB-ILPd/Au. Hydrogenation of cyclohexene to produce cyclohexane was carried out to test the catalytic activity of the PDVB-ILPd, PDVB-IL-Au, PDVB-IL-Pd/Au-1, and PDVB-IL-Pd/Au-2 at 40 °C. The dependence of the conversion of cyclohexene on reaction time is shown in Figure 5. In this work, no byproduct was detected. As expected, PDVB-IL-Pd was an effective catalyst because Pd(0) NPs are very active for the hydrogenation of olefins, while PDVB-IL-Au could not catalyze the hydrogenation reaction. However, the PDVB-IL-Pd/Au-1 and PDVBIL-Pd/Au-2 were much more active than the PDVB-IL-Pd, indicating excellent synergy of Pd and Au for catalyzing the reaction. The time of the reaction catalyzed by the PDVB-ILPd/Au-2 was only 1/4 of that catalyzed by PDVB-IL-Pd for the completion of the reaction. On one hand, this indicates that the bimetallic Pd/Au NPs (PDVB-IL-Pd/Au) are very effective catalyst. On the other hand, this provides further evidence that the bimetallic Pd/Au NPs were formed on the support. It is very difficult to give the structure of the Pd/Au NPs because only a very small lattice mismatch existed between Pd and Au metals. However, we believe that the Pd seeds on the polymer microspheres were covered or partial covered by Au shell. Conclusions In summary, Pd metallic NPs can be immobilized on PDVBIL without further surface modification of the polymer support. The loaded Pd NPs are nearly monodispersed and have high catalytic activity for cyclohexene hydrogenation reaction. Furthermore, the immobilized Pd NPs can be used as the seeds to prepare PDVB-IL-Pd/Au composites by seeding growth method. The Pd and Au in the composites have excellent

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