Facile Deposition of Pd Nanoparticles on Carbon Nanotube

China, Department of Chemistry and Biochemistry, Southern Illinois UniVersity, MC 4409, Carbondale,. Illinois 62901, and Department of Chemistry, Jili...
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J. Phys. Chem. C 2008, 112, 8172–8176

Facile Deposition of Pd Nanoparticles on Carbon Nanotube Microparticles and Their Catalytic Activity for Suzuki Coupling Reactions Xuecheng Chen,†,‡ Yuqing Hou,§ Hang Wang,†,| Yang Cao,† and Junhui He†,* Functional Nanomaterials Laboratory and Key Laboratory of Organic Optoelectronic Functional Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS), Zhongguancun Beiyitiao 2, Haidianqu, Beijing 100080, China, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100864, China, Department of Chemistry and Biochemistry, Southern Illinois UniVersity, MC 4409, Carbondale, Illinois 62901, and Department of Chemistry, Jilin Normal UniVersity, Siping, Jilin 136000, China ReceiVed: January 22, 2008; ReVised Manuscript ReceiVed: March 19, 2008

Pd nanoparticles were successfully deposited on carbon nanotube (CNT) microparticles by reduction of Na2PdCl4 using ethylene glycol as both solvent and reducing agent in the presence of sodium dodecyl sulfate. The synthesized Pd/CNT particles showed excellent catalytic activity and selectivity for Suzuki cross-coupling reactions. The catalytic activity did not deteriorate, even after repeated applications, and was supposed to result from the small size, narrow size distribution, and high surface area of Pd nanoparticles. The Pd/CNT particles catalyst could be separated and recovered from reaction solutions easily by simple filtration. Introduction Nanoscale palladium particles have drawn particular attention due to their catalytic and electronic properties.1 The use of palladium nanoparticles in catalysis is not only industrially important (e.g., automobile catalytic converters and various hydrogenation reactions)2 but also scientifically interesting, given the sensitive relationship between catalytic activity and nanoparticle size and shape as well as the nature of the surrounding media.3 One of the well-known applications of palladium nanoparticles is as catalysts for Suzuki coupling reactions.4 Unfortunately, aggregation of naked nanoparticles often prohibits tailoring of particle size.5 To overcome this problem, catalytic nanoparticles have been immobilized on solid supports, such as polystyrene spheres,6 carbon,7 titania,8 zeolites,9 alumina,10 silica spheres,11 natural and artificial polymers,12 and montomorillonite.13 Nanometer-sized catalyst supports have recently attracted a great deal of interest because of their high surface area and outstanding stability and activity in liquid phase. Among these promising nanostructured catalyst-supporting materials are carbon nanotubes (CNTs) and carbon nanofibers (CNFs).14 Network-like carbon nanotube aggregates have a low tortuosity and consequently maximize the mass transfer rate. This in turn ensures the same environment for all the catalyst particles, which may be beneficial for catalytic selectivity. However, carbon nanotubes in the powder form suffer from some drawbacks for slurry phase operation. In order to optimize the hydrodynamic properties and allow effective contact with reactants, it is therefore necessary to implement CNTs into larger objects. On the other hand, assembly of carbon nanotubes into a macroscopic support, which is big enough in size for conventional separation techniques, would allow much easier recovery and regeneration than homogeneous catalysts, which in fact have substantial difficulties in recovery and regeneration.15 Recycling of such CNT-supported catalytic nanoparticles * Corresponding author. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. § Southern Illinois University. | Jilin Normal University.

would be accomplished by conventional gravitational sedimentation or filtration. Currently known methods of depositing palladium or platinum nanoparticles on CNTs include those of supercritical CO2,16 microemulsion,17 electrochemistry,18 formaldehyde reduction,19 surfactant self-reduction,20 solid-state H2 reduction,21 and ethylene glycol reduction.22 Despite these significant studies regarding carbon nanotube supports, most of them suffer from the need for a sophisticated synthetic procedure for surface deposition and the aggregation of Pd particles, which pose major obstacles to practical applications. Therefore, it remains a challenge to develop easy, reliable synthetic routes to structure-controlled, highly efficient, and reusable catalysts. In the current work, we reported a novel approach to controlled deposition of Pd nanoparticles on micrometer-sized carbon nanotube particles (CNTPs) without tedious surface prefunctionalization. The obtained Pd/CNTP composites were in the form of micrometer-sized particles and thus could be easily separated from reaction solutions by conventional filtration or centrifugation. The catalytic activities of these Pd/CNTP composites for Suzuki coupling reactions were investigated. Experimental Section Materials. Carbon nanotubes were synthesized by our recently reported method.23 Sodium dodecyl sulfate (SDS) was purchased from Nanjing Saike Chemicals. Na2PdCl4 was bought from Lanyi Chemicals. Ethylene glycol (EG) was obtained from Beihua Fine Chemicals. These reagents were used as received. Deposition of Pd Nanoparticles on CNT Spheres. Sodium tetrachloropalladate (Na2PdCl4) was used as the palladium precursor, and ethylene glycol as both solvent and reducing agent. Sodium dodecyl sulfate (SDS) was also added to the reaction system. In a typical procedure, 100 mg of CNTPs were mixed with a solution of SDS (8.02 g) in EG (400 mL). After ultrasonication of the mixture for 20 min, 1.1 mL of aqueous Na2PdCl4 (0.1 M) was added under constant agitation for 1 h. The reaction mixture was then heated to 110 °C in an oil bath and aged at this temperature for 3 h under vigorous agitation to ensure completion of the reaction. The product was centrifuged,

10.1021/jp800610q CCC: $40.75  2008 American Chemical Society Published on Web 05/13/2008

Deposition of Pd Nanoparticles on CNT Particles

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Figure 1. XRD pattern of Pd/CNTPs.

rinsed several times with ethanol, and dried for future use. The Pd loading (given by the ratio of Pt/CNTP) of the products thus prepared was nominally 7 wt %. Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements also yielded a metal loading of 7 wt %. Characterization. Transmission electron microscopy (TEM) measurements were carried out on a JEOL JEM-200K transmission electron microscope operated at an accelerated voltage of 200 kV. The Pd/CNT composite was ground into powder and characterized by X-ray diffractometry (XRD) using a Rigaku Dmax-RB X-ray diffractometer with graphite-monochromatized Cu KR radiation (λ ) 1.5406 Å). The prepared samples were also observed by scanning electron microscopy (SEM) on a Hitachi S-430 field emission scanning electron microscope operated at 10 kV. N2 adsorption and desorption measurements were carried out at -196 °C using a Quantachrome instrument. The sample was degassed at 300 °C in high vacuum before measurements. The pore surface area distribution against pore size was also obtained from nitrogen desorption data by the BJH method.

Figure 2. Low (a) and high (b) magnification SEM images of CNTPs.

Results and Discussion Figure 1 shows the X-ray diffraction (XRD) pattern of Pd/ CNTP. Characteristic diffraction peaks at 26.5°, 43.2°, 44.6°, 54.2°, and 77.7° were observed, which correspond to the (002), (100), (101), (004), and (110) reflections of graphite, respectively.24 Diffraction peaks were also observed at 40.1°, 46.7°, and 68.7°, respectively, which could be well indexed as the (111), (110), and (100) reflections of crystalline Pd(0).25 Thus, metallic Pd was successfully produced in the reaction process. It was noted that the (111) peak had the highest intensity, and therefore the (111) plane was the predominant crystal facet. The structures and morphologies of CNTP and Pd/CNTP were revealed by scanning electron microscopy (SEM). Figure 2a shows a typical SEM image of low magnification. Clearly, a large number of microparticles of 5-20 µm in size had been produced. Their structural details were revealed at higher magnifications. As shown in Figure 2b, each microparticle was in fact assembled by a huge number of carbon nanotubes. Such assemblies were previously found to have large surface areas.23 The deposition of Pd nanoparticles on CNTP was carried out in presence of defined amounts of SDS and H2PdCl4. Densely popular spherical Pd nanoparticles were obtained and homogeneously distributed on the carbon nanotube surface by adding SDS in the reaction mixture (Figure 3a). In fact, when SDS was mixed with carbon nanotubes, part of SDS self-assembled onto the surface of carbon nanotubes by the hydrophobic

Figure 3. (a) TEM image of Pd/CNTPs prepared using 0.07 mol/L of SDS and (b) histogram of Pd nanoparticles. (H2PdCl4, 2.5 × 10-4 mol/ L; CNTP, 5 mg).

interaction between alkyl chains and graphitic surface. During the reduction process, the interaction between SDS and Pd nanoparticles resulted in the attachment of Pd nanoparticles onto the surface of carbon nanotubes.26 Thus, these well-dispersed spherical Pd nanoparticles were anchored tightly onto the external walls of MWNTs by the interaction between Pd nanoparitcles and SDS. The histogram of Pd nanoparticles (Figure 3b) shows that the Pd nanoparticles have a narrow size distribution ranging from 2 to 5 nm with a peak centered at ca. 3 nm. The mean diameter and its standard deviation of Pd nanoparticles were estimated to be ca. 2.7 and ca. 0.7 nm, respectively. We also investigated the effects of H2PdCl4 and SDS concentrations on the formation of Pd nanoparticles supported

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Chen et al.

Figure 6. The Pd 3d region of XPS spectrum of Pd/CNTP composite.

Figure 4. TEM images of Pd/CNTPs obtained (a) at a H2PdCl4 concentration of 3.75 × 10-4 mol/L, (b) in the absence of SDS, and (c and d) in the presence of SDS at 0.1 mol/L (c) and 0.17 mol/L (d). Other conditions were identical to those in Figure 3.

on CNTPs. When an excessive amount of H2PdCl4 was used, conglomeration of Pd nanoparticles was observed (Figure 4a). Thus, the amount of H2PdCl4 added must not be too large. Figure 4b shows a TEM image of the Pd/CNTP composite that was prepared without adding SDS. Large Pd nanoparticles with a wide particle-size distribution (ranging from 3 to 20 nm) were obtained, and they were distributed on the surface of MWNTs inhomogeneously. Clearly, small Pd clusters agglomerated to some extent in the absence of SDS. When excessive amounts of SDS were added into the solution, however, aggregation of Pd nanoparticles was observed. Some of Pd nanoparticles conglomerated into blocks. If more SDS was used, conglomeration of Pd nanoparticles became more serious. Thus, the amount of SDS added into the solution must also be controlled. We performed X-ray energy dispersive spectroscopy (EDS) measurements to characterize the elemental composition of the particles. Figure 5 shows an intense palladium peak at 2.85 keV, indicating that Pd nanopariticles were supported on carbon nanotube microparticles. Moreover, there are no peaks of Cl and S in the EDS spectrum, implying the high purity of the composite. To clarify the chemical state of the palladium particles deposited on CNTPs, we carried out X-ray photoelectron

Figure 7. SEM image of Pd/CNTPs.

spectroscopy (XPS) measurements. The Pd 3d region of the XPS spectrum of the Pd/CNTPs composite is illustrated in Figure 6, which reveals the presence of Pd 3d5/2 and 3d3/2 peaks at binding energies of 335.7 and 341 eV, respectively. These binding energy values are in accordance with those reported for metallic Pd.27 Figure 7 shows a SEM image of Pd/CNT microparticles. The external surfaces of the Pd-loaded CNTPs are rough compared with the original CNTPs, which probably resulted from supersonic treatments applied in the deposition experiments. However, CNTPs did not have significant changes in size after deposition of Pd nanoparticles. The surface areas and mesoporosity of CNTP and the Pd/ CNTP composite were revealed by N2 adsorption-desorption measurements. Obtained isotherms are shown in Figure 8. They are of typical type IV with a hysteresis loop, representative of mesoporosity based on the IUPAC nomenclature. Considering

Figure 5. (a) SEM image of Pd/CNTPs and (b) EDX spectrum of Pd/CNTPs as prepared using 0.07 mol/L SDS, 2.5 × 10-4 mol/L H2PdCl4, and 5 mg of CNTP.

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Figure 10. Photo of the filtration kit for separation of Pd/CNT microparticles from the reaction products. Figure 8. N2 adsorption-desorption isotherms of pure CNT (filled circle) and Pd/CNTP (filled square).

TABLE 1: Suzuki Cross-Coupling Reactions Using Pd/CNTPs as the Catalyst entry 1a 2a 3a 4a 5c

cycles yield (%)b

substrates p-iodoacetophenone p-iodoacetophenone p-iodoacetophenone p-iodoacetophenone p-iodoacetophenone

+ + + + +

phenylboronic phenylboronic phenylboronic phenylboronic phenylboronic

acid acid acid acid acid

1 2 3 4 5

98.6 99.5 100 98.6 98

a p-Iodoacetophenone (15 mmol), phenylboronic acid (20 mmol), anhydrated K3PO4 (40 mmol), and absolute ethanol (50 mL); reflux for 30 min; 30 mg of catalyst. b Yield of isolated product. c p-Iodoacetophenone (15 mmol), phenylboronic acid (20 mmol), anhydrated K3PO4 (40 mmol), and absolute ethanol (50 mL); reflux for 40 min, 2.7 mg of catalyst.

Figure 9. Thermal analyses of CNTPs and Pd/CNTPs.

the geometry of MWNTs, MWNT samples are assumed to contain cylindrical pores and no pore networks, thus satisfying the two main requirements of the BJH method. The above results suggest that the CNT microparticles contain abundant mesopores. From Figure 8, the total surface area and pore volume were calculated to be 250 m2/g and 0.5877 cm3/g, respectively. When the Pd nanoparticles were deposited on CNTPs, the total surface area and pore volume of Pd/CNTPs decreased and were estimated to be 128.8 m2/g and 0.4311 cm3/g. Clearly, some of the nanopores of CNTPs might have been occupied by Pd nanoparticles. In order to compare the thermal stability of obtained Pd/CNTP with CNT, thermal analysis was carried out by TGA measurements. As shown in Figure 9, the Pd/CNTP composite began to decompose at 564 °C. When the temperature was further increased, the weight loss increased rapidly until all the CNTs were exhausted at 900 °C. The residue at 900 °C was 20 wt %. Ideal graphite starts to be oxidized above 600 °C, while the purified CNTs would be oxidized above 500 °C. Figure 9 indicates that the purified CNTs had a similar TGA behavior though they began to decompose at 585 °C in air and were burnt in the temperature range of 500-640 °C. The weight (ca. 20 wt %) of the residue obtained from Pd/CNTPs is larger than that (ca. 3.3 wt %) obtained from the purified CNTs. This difference was largely attributed to the deposited Pd nanoparitlces. It is very interesting that the thermal stability of CNTP was lowered by depositing Pd nanoparticles. Clearly, Pd nanoparticles could accelerate the decomposition of CNTs. One of the possible advantages of using the above Pd/CNTP composite as catalyst is that CNTPs have large sizes and might be separated and recovered simply by conventional centrifuga-

tion or filtering. This is in fact the case. Figure 10 shows the result of simple filtration of Pd/CNTP from the reaction solution. The catalyst was completely reserved on the filter paper, and a clear solution with no black catalyst particles was obtained in the beaker. This advantage was also taken repeatedly in the following catalytic experiments. To investigate the catalytic activities of Pd/CNTP, the Suzuki coupling reaction of ethyl p-iodobenzoate (or p-iodoacetophenone) and phenylboronic acid was used as a test reaction. After completion of the coupling reaction, Pd/CNTP could be readily separated and recovered from the reaction mixture simply by filtration. This allowed an easy investigation of the stability of Pd/CNTP during the catalytic reactions. Thus, the catalyst was used, recovered, and reused. This constitutes one cycle of application of the catalyst, and multicycles were performed. The reaction was carried out by using Pd/CNTP (30 mg, 7 wt % Pd) catalyst. After 40 min of reaction time, the conversion reached a yield of 98%, indicating that the Pd/CNTP catalyst was quite active for this reaction. This catalytic activity is much higher than that of hollow palladium spheres under similar reaction conditions.28 In the latter case, more than 9 times the amount of Pd catalyst and 4.5 times the reaction time are needed to achieve similar conversion. Unlike other heterogeneous catalysts that often suffer from extensive leaching of active metal species during reactions and eventual loss of their catalytic activities, the yield of phenylacetophenone (98%) of the fifth cycle remained as high as that of the first cycle (98.6%), indicating that no deterioration had occurred after a total of four cycles of application of the catalyst (Table 1). Recently, Hyeon et al. reported that the yield of the catalytic coupling reactions using Pd nanoparticles assembled on silica spheres gradually decreased as recycling proceeded, which was attributed to the significant loss of Pd nanoparticles.11a The TEM image of the Pd/CNTP catalyst after five cycles of application of the catalyst showed that the Pd nanoparticles remained highly dispersed on

8176 J. Phys. Chem. C, Vol. 112, No. 22, 2008 TABLE 2: Suzuki Coupling Reactions for Various Substrates Catalyzed by Pd/CNTPs

Chen et al. The authors are grateful to Mr. H. Chen and Ms. X. Liu for helpful discussions. Supporting Information Available: The experimental details of the Suzuki coupling reactions. This material is available free of charge via the Internet at http://pubs.acs.org.

a

Ethyl p-Iodoacetophenone (15 mmol), phenylboronic acid (30 mmol), anhydrated K3PO4 (60 mmol), and absolute ethanol (50 mL) under reflux for 10 min, with 30 mg of Pd/CNTP catalyst. b p-Iodoacetophenone (15 mmol), phenylboronic acid (20 mmol), anhydrated K3PO4 (40 mmol), and absolute ethanol (50 mL) under reflux for 40 min, with 30 mg of Pd/CNTP catalyst. c Yield of isolated product.

the surface of the carbon nanotubes. The composition of solution after removal of Pd/CNTP was analyzed by ICP, and the results indicated that no Pd species were leached from Pd/CNTP into the reaction solution. No significant detachment or aggregation of Pd nanoparticles during the catalytic reaction was observed. This allows the Pd/CNTP catalyst to preserve its excellent catalytic activity during multiple cycles of applications. Table 2 shows the catalytic activity and selectivity of Pd/ CNTPs for the Suzuki coupling reactions of two kinds of phenyl iodide with phenylboronic acid. The yields were approximately 90 and 98.6%, respectively. Clearly, high catalytic activity and selectivity were achieved when the substituent of phenyl iodide was different. This was probably attributed to the high available surface areas of the Pd nanoparticles in the current case, where edge and vertex sites were abundant.4 Pan et al. 29 also reported carbon nanotube-supported Pd nanoparticles for Suzuki coupling reactions. In that paper, the activity of Pd/MWCNT was tested by the reaction of 4-iodoacetophenone and phenylboronic acid. The highest yield was 96%, and TOF was 473/h. Comparing the catalytic activity of Pd/CNTPs with that of Pd/ MWCNT reported in the literature, Pd/CNTPs were found to have several advantages. First, the catalytic activity is relatively higher than the reported one, the TOF of the Suzuki coupling reaction was 1520/h, larger than the reported one (475/h). Second, the lowest yield in the five cycles of reaction was 98% in the current work, but the highest yield in the six cycles of reaction was 94% in the literature. Thus, Pd/CNTPs have high potentials as heterogeneous hydrogenation catalysts. In conclusion, we have successfully fabricated Pd/CNTP by reduction of sodium tetrachloropalladate to Pd(0) on the surface of CNTs using the easy method. The synthesized Pd/CNTP showed excellent catalytic activity and selectivity for the Suzuki cross-coupling reactions. Such high catalytic activities are supposed to result from the small size, narrow size distribution, and high surface area of the Pd nanoparticles. The Pd/CNTP catalyst could be separated and recovered from reaction solutions easily by simple filtration. Its catalytic activity did not deteriorate even after repeated applications. Therefore, these materials would be ideal heterogeneous catalysts for the Suzuki crosscoupling reactions. Acknowledgment. This work was supported by the “Hundred Talents Program” of CAS, the National Natural Science Foundation of China (Grant No.20471065), and the National Basic Research Program of China (Grant No. 2006CB933000).

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