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Highly dispersed Pd nanoparticles on covalent functional MWNT surfaces for methanol oxidation in alkaline solution
Electrochemistry Communications 11 (2009) 557–561
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Highly dispersed Pd nanoparticles on covalent functional MWNT surfaces for methanol oxidation in alkaline solution Zhi-Peng Sun a, Xiao-Gang Zhang a, Yan-Yu Liang a,b,*, Hu-Lin Li a,c,* a
College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany c College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China b
a r t i c l e
i n f o
Article history: Received 17 November 2008 Received in revised form 14 December 2008 Accepted 15 December 2008 Available online 24 December 2008 Keywords: 4-Aminobenzoic acid MWNTs Pd nanoparticle Methanol oxidation Alkaline solution
a b s t r a c t 4-Aminobenzoic acid (4-ABA) was covalently grafted on to the surface of multiwalled carbon nanotubes (MWNTs) via a novel amine-cation induced radical reaction. Then, Pd nanoparticles were deposited on functional 4-ABA group-grafted MWNTs (F-MWNTs) by NaBH4 reduction method. The structure and nature of the resulting product were characterized by Fourier transform infrared (FT-IR) spectrometry, transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. The electrocatalytic properties of the Pd/F-MWNTs catalyst for methanol oxidation have been investigated by cyclic voltammetry, linear sweep voltammetry and chronomperometry methods. In contrast to the unfunctionalized counterpart, the electrochemical results demonstrate that Pd/F-MWNTs exhibit better electrocatalytic activities and stability, mainly due to the uniform dispersion and small particle size of Pd nanoparticles on the F-MWNT supports. The results imply that the Pd/F-MWNTs catalyst shows the better electrocatalytic performances and has a promising application potential in fuel cells. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs), as a new form of carbon, have attracted considerable attention due to their unique structure, electrical and mechanical properties [1–4]. In recent years, considerable efforts have been devoted to anchor noble metal particles onto the framework of CNTs for their potential applications in the area of catalysis. Some metals and their alloys, such as Pt, Pd, Ag, Au, Ni, have been successfully deposited onto the surface of CNTs [5–10]. CNTs were used as a potential support material for heterogeneous catalysts. However, realistic applications have been hindered by difficulties associated with processing. In principle, metal nanoparticles are spontaneously formed at the defect sites on the surface not on sidewall of CNTs. Therefore to obtain the homogeneously dispersed nanoparticles, the surface of CNTs must be modified via a proper functionalization [11]. Generally speaking, this may be done either by covalent [12,13] or by noncovalent interactions [14,15]. Recently, Dai et al. [16] proposed a novel solvent-free method in which the diazonium compound was in situ generated and reacted with the carbon surface. Moreover, considering the recent interest in the use of CNTs from both a chemical * Corresponding authors. Address: College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China. Tel./ fax: +86 25 52112626 (Y.-Y. Liang). E-mail addresses: [email protected] (Y.-Y. Liang), [email protected] (H.-L. Li). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.12.049
modified point of view and as metal nanoparticle catalyst supports, we can find some examples of dispersed metal nanoparticle deposition on modified CNTs with organic molecules [11–13]. However, the preparation of CNTs with 4-ABA group is rarely reported, especially as the supports applied for direct methanol fuel cells. So this research work will be of great significance. In this paper, a facile approach has been described for the preparation of Pd nanoparticles through covalent functionalization of MWNTs. 4-ABA group was first grafted onto MWNT surfaces [16], then, Pd2+ was uniformly adsorbed onto the grafted MWNT surface via electrostatic interaction. Finally, Pd nanoparticles could be obtained through NaBH4 reduction method. The electrocatalytic properties of the product for methanol oxidation in alkaline solution were also investigated in detail. The procedure for preparation of Pd-modified MWNTs composite was shown in Fig. 1a. This strategy could increase the number of surface nucleation sites for the nanoparticles and provide sufficient adhesion to prevent their diffusion along the MWNT surfaces [11]. The functional 4-ABA group is modified on the surface of MWNTs via a CAN covalent bond which is strong and propitious to anchor Pd nanoparticles onto them. Specially, this method is generally facile to implement and is thus an attractive choice for large-scale synthesis. 2. Experimental MWNTs (20–40 nm in diameter) used were produced via chemical vapor deposition method and purified (denoted P-MWNTs)
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Z.-P. Sun et al. / Electrochemistry Communications 11 (2009) 557–561
a NH2
COOH
isoamyl nitrite
P-MWNTs
NH
COOH
NH
COOH
NH
COOH
F-MWNTs
Pd2+ NaBH4
NH
COO- Pd2+
NH
COO- Pd2+
NH
COO- Pd2+
Pd/F-MWNTs
Pd/F-MWNTs
Fig. 1. (a) Schematic illustration of the synthesis procedure of the composite. (b) FT-IR spectrum of the MWNTs functionalized with 4-ABA group.
[17]. The functional approach for MWNTs was based on the in situ radical polymerization of isoamyl nitrite and 4-aminobenzoic acid [16]. Specifically, some isoamyl nitrite was stirred with a proper amount of MWNTs and 4-aminobenzoic acid for 1 h at room temperature. Then the reaction temperature was gradually raised to 70 °C, and kept for 3 h. The resulting product was repeatedly washed with dimethylformamide and hot chloroform 3 times, and then dried at 70 °C overnight in a vacuum oven. The obtained sample was denoted as F-MWNTs. 2ml (6 mg ml 1) PdCl2 solution was dissolved in deionized water, and then 60 mg F-MWNTs were added to this solution. The NaBH4 solution (NaBH4/metal molar ratio = 8) was slowly dropped into this mixture and vigorously stirred for 12 h. The resulting slurry was filtered, washed thoroughly with deionized water and then dried in a vacuum oven. For comparison, Pd nanoparticles supported on P-MWNTs were prepared under the same preparation conditions. Five milligram of catalysts, 50 ul of Nafion solution (5 wt%, Aldrich) and 1.0 ml of alcohol were mixed. A measured volume (ca. 25 ul) of this mixture was transferred via a syringe onto a glassy carbon electrode and heated under an infrared lamp to remove the solvent. X-ray diffraction (XRD) analysis was carried out on Bruker D8ADVANCE diffractometer with Cu Ka radiation of wavelength k = 0.15418 nm. Transmission electron microscopy (TEM, FEI Tecnai G2 20 S-TWIN) operating at 200 kV was applied to characterize the morphology and particle size distribution. Infrared spectra were recorded with a model 360 Nicolet AVATAR FT-IR spectrophotometer. A conventional cell with a three-electrode configuration was used throughout this work. The working electrode was modified glassy carbon electrode. A platinum sheet and a saturated calomel electrode were used as counter and reference electrode, respec-
tively. All electrochemical measurements were performed at room temperature using a CHI660C electrochemical working station (Shanghai, China). 3. Results and discussion Here we use FT-IR analysis of F-MWNTs to confirm covalent interactions, for the in situ radical polymerization of 4-ABA with isoamyl nitrite, is shown in Fig. 1b. The sample synthesized with 4-ABA group shows characteristic peaks at 1596, 1499 cm 1 (aromatic C@C), 1042 and 1123 cm 1 (@CH), 3419 cm 1 (AOH), 1123 cm 1 (AC@O), and 810 cm 1 (CAH para-aromatic out of plane vibration) [16]. These representative absorption peaks indicate that 4-ABA group is successfully modified on the MWNT surfaces. It also proves that the functionalization process of FMWNTs is in good accordance with the synthetic route shown in Fig. 1a. Fig. 2a–d shows the TEM images and particle size distribution histograms of the Pd/P-MWNTs and Pd/F-MWNTs catalysts. As shown in Fig. 2a, Pd/P-MWNTs seemed to present some agglomerates and large Pd clusters on MWNT surfaces with the calculated mean particle size is about 5.3 nm. In the case of Pd/F-MWNTs uniform and well-dispersed Pd particles on MWNT surface is observed. The average size of the Pd particles is 5.0 nm, which is smaller than that of Pd/P-MWNTs. Previous attempts to prepare Pd nanoparticles on MWNT surfaces have not often obtained size-similar and highly dispersed Pd nanoparticles. One of the main reasons is that Pd nanoparticles are spontaneously preferred absorption at the defects on the MWNT surfaces. In our study, grafting functional 4-ABA group on MWNT surfaces in advance provides a uniformly negative-charged sites for absorption of Pd2+, in this way preventing the randomly electroless deposition of Pd nanoparticles. Therefore, Pd nanopa-
Z.-P. Sun et al. / Electrochemistry Communications 11 (2009) 557–561
559
Fig. 2. (a–d) TEM images and particle size distribution histograms of Pd/P-MWNTs (a and c) and Pd/F-MWNTs catalysts (b and d). (e) XRD patterns of Pd/P-MWNTs and Pd/FMWNTs catalysts.
ricles show no tendency to aggregate at steps on the MWNT surfaces. Fig. 2e shows the XRD patterns of Pd/P-MWNTs and Pd/FMWNTs catalysts. The main diffraction peaks of Pd nanoparticels are observed. All peaks can be indexed as Pd cubic crystallite, corresponding to the planes (1 1 1), (2 0 0) and (2 2 0) [5]. The crystalline peaks of Pd/F-MWNTs are broader than those of Pd/PMWNTs, which implies that the average crystallite size of Pd/FMWNTs is smaller than that of Pd/P-MWNTs. It can be calculated
from the Scherrer equation based on the Pd (2 0 0) peak [18,19], the averaged size of Pd nanoparticles on Pd/P-MWNTs and Pd/FMWNTs is 5.3 nm and 5.0 nm, respectively, well consistent with the TEM results. The catalytic properties of Pd/F-MWNTs and Pd/P-MWNTs catalysts for the methanol oxidation reaction have also been characterized by cyclic voltammetry (Fig. 3a). The cyclic voltammogram features are in good agreement with the literature [20]. There is an oxidation peak at about 0.05 V in the forward scan,
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which corresponds to the oxidation of methanol, while in the reverse scan, another oxidation peak is observed at around 0.32 V, which should be primarily attributed to the removal of the residual carbon species formed in the forward scan [21]. Furthermore, it is obvious that the oxidation current density observed with Pd/F-MWNTs catalyst is considerably higher than that of Pd/ P-MWNTs, which may be attributed to the high dispersion of Pd nanoparticles and effective functionalization of MWNTs. Fig. 3b shows the linear sweep voltammograms of Pd/F-MWNTs catalyst in 0.2 M KOH + 0.5 M CH3OH solution at different scan rates. It can be seen that the oxidation potential and peak current density for methanol oxidation become more prominent with increasing the scan rates. Insert is a dependence curve of the peak currents on the square root of scan rates. The peak current densities are linearly proportional to the square root of scan rates, suggesting the electrocatalytic oxidation methanol on Pd/F-MWNTs catalyst could be controlled by a diffusion process [22]. The above results demonstrate several important points. Firstly, grafting functional the 4-ABA group can greatly enhance the nucleation of nanocrystalline metals onto CNTs by providing large numbers of hydrophilic molecular group on the surface [11]. Secondly, 4-ABA group does not necessarily act as blocking layers for electron transport, with the result that electron-transfer reactions to redox species in solution and the adsorbed nanoparticles are facile. Finally, the specific electrostatic interactions between the 4-ABA group grafted, negative charged MWNTs and positive-charged Pd2+ could lead to the deposited Pd nanoparticles in a narrower size distributions than in the case of random nucleation [4,11]. To test the stability of the as-prepared catalysts, the chronoamperometry curves were recorded at the oxidation potential of 0.10 V for 500 s as shown in Fig. 3c. The decay tendency of current density is in the similar model for two catalysts, however, the current densities for CH3OH oxidation on Pd/F-MWNTs catalyst is found to higher than that of Pd/P-MWNTs in the whole process. The result shows that Pd/F-MWNTs catalyst is much more efficient than Pd/P-MWNTs, which is consistent with the cyclic voltammogram results. 4. Conclusion In summary, functional 4-ABA group grafted onto the surface of MWCNTs derived from in situ radical polymerization reactions was employed as the carbon support for Pd catalysts. The uniform and highly dispersed Pd nanoparticles supported on the novel carbon support were synthesized by a facile NaBH4 assisted chemical reduction. The Pd/F-MWNTs catalyst shows better electrocatalytic activity and stability for methanol oxidation than that of unmodified counterpart, which may be attributed to the high dispersion of Pd nanoparticles and special frame and properties of F-MWNTs. This achievement opens up enormous opportunities to use the unique carbon nanostructure for CNTs related technological applications such as composite materials, micromechanical resonators, transistors and energy storage devices. Acknowledgments
Fig. 3. (a) Cyclic voltammograms of Pd/F-MWNTs and Pd/P-MWNTs catalysts in 0.2 M KOH + 0.5 M CH3OH solution at a scan rate of 50 mVs 1. (b) The linear sweep voltammograms of 0.2 M KOH + 0.5 M CH3OH solution at Pd/F-MWNTs catalyst at different scan rates. Insert: the plot of peak current density vs. square root of sweep rates. CCH3 OH = 0.5 mol l 1. (c) Chronoamperometry curves for Pd/F-MWNTs and Pd/ P-MWNTs catalysts in 0.2 M KOH + 0.5 M CH3OH solution of 0.10 V.
We acknowledge the financial supports of National Natural Science Foundation of China (50701023), Post-Doctoral Science Foundation of China (20060400283), Post-Doctoral Science Foundation of Jiangsu Province of China (2006277) and Graduated Student Innovation Foundation of Jiangsu Province (CX07B_088z). References [1] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [2] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971.
Z.-P. Sun et al. / Electrochemistry Communications 11 (2009) 557–561 [3] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Top. Appl. Phys. 80 (2001) 1. [4] Z.P. Guo, D.M. Han, D. Wexler, R. Zeng, H.K. Liu, Electrochim. Acta 53 (2008) 6410. [5] G.G. Wildgoose, C.E. Banks, R.G. Compton, Small 2 (2006) 182. [6] X.H. Chen, J.T. Xia, J.C. Peng, W.Z. Li, S.S. Xie, Compos. Sci. Technol. 60 (2000) 301. [7] B. Yoon, C.M. Wai, J. Am. Chem. Soc. 127 (2005) 17174. [8] L.M. Ang, T.S.A. Hor, G.Q. Xu, C.H. Tung, S.P. Zhao, J.L.S. Wang, Carbon 38 (2000) 363. [9] F.Z. Kong, X.B. Zhang, W.Q. Xiong, E. Liu, W.Z. Huang, Y.L. Sun, J.P. Tu, X.W. Chen, Surf. Coat. Technol. 155 (2002) 33. [10] X. Hu, T. Wang, X. Qu, S. Dong, J. Phys. Chem. B 110 (2006) 853. [11] D.J. Guo, H.L. Li, Carbon 1259 (2005) 43. [12] F. Peng, L. Zhang, H.J. Wang, P. Lv, H. Yu, Carbon 43 (2005) 2397.
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[13] Y.T. Kim, T. Mitani, J. Catal. 238 (2006) 394. [14] W. Yang, X.L. Wang, F. Yang, C. Yang, X.R. Yang, Adv. Mater. 9999 (2008) 1. [15] A. Star, J.F. Stoddart, D. Steuerman, M. Diehl, A. Boukai, E.W. Wong, X. Yang, S.W. Chung, H. Choi, J.R. Heath, Angew. Chem. 113 (2001) 1721. [16] Z.J. Li, S. Dai, Chem. Mater. 17 (2005) 1717. [17] J.L. Bahr, J.M. Tour, Chem. Mater. 13 (2001) 3823. [18] Z. Liu, X.Y. Ling, X. Su, J.Y. Lee, J. Phys. Chem. 108 (2004) 8234. [19] W.M. Wang, D. Zheng, C. Du, Z.Q. Zou, X.G. Zhang, B.J. Xia, H. Yang, D.L. Akins, J. Power Sources 167 (2007) 243. [20] C.W. Xu, L.Q. Cheng, P.K. Shen, Y.L. Liu, Electrochem. Commun. 9 (2007) 997. [21] H.Q. Li, G.Q. Sun, Q. Jiang, M.Y. Zhu, S.G. Sun, Q. Xin, J. Power Sources 172 (2007) 641. [22] K. Honda, M. Yoshimura, T.N. Rao, D.A. Tryk, A. Fujishima, J. Electroanal. Chem. 514 (2001) 35.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Highly dispersed Pd nanoparticles on covalent functional MWNT surfaces for methanol oxidation in alkaline solution Zhi-Peng Sun a, Xiao-Gang Zhang a, Yan-Yu Liang a,b,*, Hu-Lin Li a,c,* a
College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany c College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China b
a r t i c l e
i n f o
Article history: Received 17 November 2008 Received in revised form 14 December 2008 Accepted 15 December 2008 Available online 24 December 2008 Keywords: 4-Aminobenzoic acid MWNTs Pd nanoparticle Methanol oxidation Alkaline solution
a b s t r a c t 4-Aminobenzoic acid (4-ABA) was covalently grafted on to the surface of multiwalled carbon nanotubes (MWNTs) via a novel amine-cation induced radical reaction. Then, Pd nanoparticles were deposited on functional 4-ABA group-grafted MWNTs (F-MWNTs) by NaBH4 reduction method. The structure and nature of the resulting product were characterized by Fourier transform infrared (FT-IR) spectrometry, transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. The electrocatalytic properties of the Pd/F-MWNTs catalyst for methanol oxidation have been investigated by cyclic voltammetry, linear sweep voltammetry and chronomperometry methods. In contrast to the unfunctionalized counterpart, the electrochemical results demonstrate that Pd/F-MWNTs exhibit better electrocatalytic activities and stability, mainly due to the uniform dispersion and small particle size of Pd nanoparticles on the F-MWNT supports. The results imply that the Pd/F-MWNTs catalyst shows the better electrocatalytic performances and has a promising application potential in fuel cells. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs), as a new form of carbon, have attracted considerable attention due to their unique structure, electrical and mechanical properties [1–4]. In recent years, considerable efforts have been devoted to anchor noble metal particles onto the framework of CNTs for their potential applications in the area of catalysis. Some metals and their alloys, such as Pt, Pd, Ag, Au, Ni, have been successfully deposited onto the surface of CNTs [5–10]. CNTs were used as a potential support material for heterogeneous catalysts. However, realistic applications have been hindered by difficulties associated with processing. In principle, metal nanoparticles are spontaneously formed at the defect sites on the surface not on sidewall of CNTs. Therefore to obtain the homogeneously dispersed nanoparticles, the surface of CNTs must be modified via a proper functionalization [11]. Generally speaking, this may be done either by covalent [12,13] or by noncovalent interactions [14,15]. Recently, Dai et al. [16] proposed a novel solvent-free method in which the diazonium compound was in situ generated and reacted with the carbon surface. Moreover, considering the recent interest in the use of CNTs from both a chemical * Corresponding authors. Address: College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China. Tel./ fax: +86 25 52112626 (Y.-Y. Liang). E-mail addresses: [email protected] (Y.-Y. Liang), [email protected] (H.-L. Li). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.12.049
modified point of view and as metal nanoparticle catalyst supports, we can find some examples of dispersed metal nanoparticle deposition on modified CNTs with organic molecules [11–13]. However, the preparation of CNTs with 4-ABA group is rarely reported, especially as the supports applied for direct methanol fuel cells. So this research work will be of great significance. In this paper, a facile approach has been described for the preparation of Pd nanoparticles through covalent functionalization of MWNTs. 4-ABA group was first grafted onto MWNT surfaces [16], then, Pd2+ was uniformly adsorbed onto the grafted MWNT surface via electrostatic interaction. Finally, Pd nanoparticles could be obtained through NaBH4 reduction method. The electrocatalytic properties of the product for methanol oxidation in alkaline solution were also investigated in detail. The procedure for preparation of Pd-modified MWNTs composite was shown in Fig. 1a. This strategy could increase the number of surface nucleation sites for the nanoparticles and provide sufficient adhesion to prevent their diffusion along the MWNT surfaces [11]. The functional 4-ABA group is modified on the surface of MWNTs via a CAN covalent bond which is strong and propitious to anchor Pd nanoparticles onto them. Specially, this method is generally facile to implement and is thus an attractive choice for large-scale synthesis. 2. Experimental MWNTs (20–40 nm in diameter) used were produced via chemical vapor deposition method and purified (denoted P-MWNTs)
558
Z.-P. Sun et al. / Electrochemistry Communications 11 (2009) 557–561
a NH2
COOH
isoamyl nitrite
P-MWNTs
NH
COOH
NH
COOH
NH
COOH
F-MWNTs
Pd2+ NaBH4
NH
COO- Pd2+
NH
COO- Pd2+
NH
COO- Pd2+
Pd/F-MWNTs
Pd/F-MWNTs
Fig. 1. (a) Schematic illustration of the synthesis procedure of the composite. (b) FT-IR spectrum of the MWNTs functionalized with 4-ABA group.
[17]. The functional approach for MWNTs was based on the in situ radical polymerization of isoamyl nitrite and 4-aminobenzoic acid [16]. Specifically, some isoamyl nitrite was stirred with a proper amount of MWNTs and 4-aminobenzoic acid for 1 h at room temperature. Then the reaction temperature was gradually raised to 70 °C, and kept for 3 h. The resulting product was repeatedly washed with dimethylformamide and hot chloroform 3 times, and then dried at 70 °C overnight in a vacuum oven. The obtained sample was denoted as F-MWNTs. 2ml (6 mg ml 1) PdCl2 solution was dissolved in deionized water, and then 60 mg F-MWNTs were added to this solution. The NaBH4 solution (NaBH4/metal molar ratio = 8) was slowly dropped into this mixture and vigorously stirred for 12 h. The resulting slurry was filtered, washed thoroughly with deionized water and then dried in a vacuum oven. For comparison, Pd nanoparticles supported on P-MWNTs were prepared under the same preparation conditions. Five milligram of catalysts, 50 ul of Nafion solution (5 wt%, Aldrich) and 1.0 ml of alcohol were mixed. A measured volume (ca. 25 ul) of this mixture was transferred via a syringe onto a glassy carbon electrode and heated under an infrared lamp to remove the solvent. X-ray diffraction (XRD) analysis was carried out on Bruker D8ADVANCE diffractometer with Cu Ka radiation of wavelength k = 0.15418 nm. Transmission electron microscopy (TEM, FEI Tecnai G2 20 S-TWIN) operating at 200 kV was applied to characterize the morphology and particle size distribution. Infrared spectra were recorded with a model 360 Nicolet AVATAR FT-IR spectrophotometer. A conventional cell with a three-electrode configuration was used throughout this work. The working electrode was modified glassy carbon electrode. A platinum sheet and a saturated calomel electrode were used as counter and reference electrode, respec-
tively. All electrochemical measurements were performed at room temperature using a CHI660C electrochemical working station (Shanghai, China). 3. Results and discussion Here we use FT-IR analysis of F-MWNTs to confirm covalent interactions, for the in situ radical polymerization of 4-ABA with isoamyl nitrite, is shown in Fig. 1b. The sample synthesized with 4-ABA group shows characteristic peaks at 1596, 1499 cm 1 (aromatic C@C), 1042 and 1123 cm 1 (@CH), 3419 cm 1 (AOH), 1123 cm 1 (AC@O), and 810 cm 1 (CAH para-aromatic out of plane vibration) [16]. These representative absorption peaks indicate that 4-ABA group is successfully modified on the MWNT surfaces. It also proves that the functionalization process of FMWNTs is in good accordance with the synthetic route shown in Fig. 1a. Fig. 2a–d shows the TEM images and particle size distribution histograms of the Pd/P-MWNTs and Pd/F-MWNTs catalysts. As shown in Fig. 2a, Pd/P-MWNTs seemed to present some agglomerates and large Pd clusters on MWNT surfaces with the calculated mean particle size is about 5.3 nm. In the case of Pd/F-MWNTs uniform and well-dispersed Pd particles on MWNT surface is observed. The average size of the Pd particles is 5.0 nm, which is smaller than that of Pd/P-MWNTs. Previous attempts to prepare Pd nanoparticles on MWNT surfaces have not often obtained size-similar and highly dispersed Pd nanoparticles. One of the main reasons is that Pd nanoparticles are spontaneously preferred absorption at the defects on the MWNT surfaces. In our study, grafting functional 4-ABA group on MWNT surfaces in advance provides a uniformly negative-charged sites for absorption of Pd2+, in this way preventing the randomly electroless deposition of Pd nanoparticles. Therefore, Pd nanopa-
Z.-P. Sun et al. / Electrochemistry Communications 11 (2009) 557–561
559
Fig. 2. (a–d) TEM images and particle size distribution histograms of Pd/P-MWNTs (a and c) and Pd/F-MWNTs catalysts (b and d). (e) XRD patterns of Pd/P-MWNTs and Pd/FMWNTs catalysts.
ricles show no tendency to aggregate at steps on the MWNT surfaces. Fig. 2e shows the XRD patterns of Pd/P-MWNTs and Pd/FMWNTs catalysts. The main diffraction peaks of Pd nanoparticels are observed. All peaks can be indexed as Pd cubic crystallite, corresponding to the planes (1 1 1), (2 0 0) and (2 2 0) [5]. The crystalline peaks of Pd/F-MWNTs are broader than those of Pd/PMWNTs, which implies that the average crystallite size of Pd/FMWNTs is smaller than that of Pd/P-MWNTs. It can be calculated
from the Scherrer equation based on the Pd (2 0 0) peak [18,19], the averaged size of Pd nanoparticles on Pd/P-MWNTs and Pd/FMWNTs is 5.3 nm and 5.0 nm, respectively, well consistent with the TEM results. The catalytic properties of Pd/F-MWNTs and Pd/P-MWNTs catalysts for the methanol oxidation reaction have also been characterized by cyclic voltammetry (Fig. 3a). The cyclic voltammogram features are in good agreement with the literature [20]. There is an oxidation peak at about 0.05 V in the forward scan,
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Z.-P. Sun et al. / Electrochemistry Communications 11 (2009) 557–561
which corresponds to the oxidation of methanol, while in the reverse scan, another oxidation peak is observed at around 0.32 V, which should be primarily attributed to the removal of the residual carbon species formed in the forward scan [21]. Furthermore, it is obvious that the oxidation current density observed with Pd/F-MWNTs catalyst is considerably higher than that of Pd/ P-MWNTs, which may be attributed to the high dispersion of Pd nanoparticles and effective functionalization of MWNTs. Fig. 3b shows the linear sweep voltammograms of Pd/F-MWNTs catalyst in 0.2 M KOH + 0.5 M CH3OH solution at different scan rates. It can be seen that the oxidation potential and peak current density for methanol oxidation become more prominent with increasing the scan rates. Insert is a dependence curve of the peak currents on the square root of scan rates. The peak current densities are linearly proportional to the square root of scan rates, suggesting the electrocatalytic oxidation methanol on Pd/F-MWNTs catalyst could be controlled by a diffusion process [22]. The above results demonstrate several important points. Firstly, grafting functional the 4-ABA group can greatly enhance the nucleation of nanocrystalline metals onto CNTs by providing large numbers of hydrophilic molecular group on the surface [11]. Secondly, 4-ABA group does not necessarily act as blocking layers for electron transport, with the result that electron-transfer reactions to redox species in solution and the adsorbed nanoparticles are facile. Finally, the specific electrostatic interactions between the 4-ABA group grafted, negative charged MWNTs and positive-charged Pd2+ could lead to the deposited Pd nanoparticles in a narrower size distributions than in the case of random nucleation [4,11]. To test the stability of the as-prepared catalysts, the chronoamperometry curves were recorded at the oxidation potential of 0.10 V for 500 s as shown in Fig. 3c. The decay tendency of current density is in the similar model for two catalysts, however, the current densities for CH3OH oxidation on Pd/F-MWNTs catalyst is found to higher than that of Pd/P-MWNTs in the whole process. The result shows that Pd/F-MWNTs catalyst is much more efficient than Pd/P-MWNTs, which is consistent with the cyclic voltammogram results. 4. Conclusion In summary, functional 4-ABA group grafted onto the surface of MWCNTs derived from in situ radical polymerization reactions was employed as the carbon support for Pd catalysts. The uniform and highly dispersed Pd nanoparticles supported on the novel carbon support were synthesized by a facile NaBH4 assisted chemical reduction. The Pd/F-MWNTs catalyst shows better electrocatalytic activity and stability for methanol oxidation than that of unmodified counterpart, which may be attributed to the high dispersion of Pd nanoparticles and special frame and properties of F-MWNTs. This achievement opens up enormous opportunities to use the unique carbon nanostructure for CNTs related technological applications such as composite materials, micromechanical resonators, transistors and energy storage devices. Acknowledgments
Fig. 3. (a) Cyclic voltammograms of Pd/F-MWNTs and Pd/P-MWNTs catalysts in 0.2 M KOH + 0.5 M CH3OH solution at a scan rate of 50 mVs 1. (b) The linear sweep voltammograms of 0.2 M KOH + 0.5 M CH3OH solution at Pd/F-MWNTs catalyst at different scan rates. Insert: the plot of peak current density vs. square root of sweep rates. CCH3 OH = 0.5 mol l 1. (c) Chronoamperometry curves for Pd/F-MWNTs and Pd/ P-MWNTs catalysts in 0.2 M KOH + 0.5 M CH3OH solution of 0.10 V.
We acknowledge the financial supports of National Natural Science Foundation of China (50701023), Post-Doctoral Science Foundation of China (20060400283), Post-Doctoral Science Foundation of Jiangsu Province of China (2006277) and Graduated Student Innovation Foundation of Jiangsu Province (CX07B_088z). References [1] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [2] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971.
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