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Microwave-assisted synthesis and characterization of bimetallic PtRu alloy nanoparticles supported on carbon nanotubes
Journal of Alloys and Compounds 649 (2015) 1323e1328
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microwave-assisted synthesis and characterization of bimetallic PtRu alloy nanoparticles supported on carbon nanotubes Mansour Rahsepar a, *, Hasuck Kim b, c, ** a
Department of Materials Science and Engineering, School of Engineering, Shiraz University, Zand Boulevard, Shiraz, 7134851154, Iran Department of Chemistry, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-747, Republic of Korea c Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology, Daegu, 711-873, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2015 Received in revised form 21 July 2015 Accepted 24 July 2015 Available online 26 July 2015
Multiwalled carbon nanotube (MWCNT) supported PtRu nanoparticles were synthesized by using a microwave-assisted improved impregnation technique. X-ray diffraction, transmission electron microscopy and X-ray photo electron spectroscopy were used to characterize the prepared PtRu/MWCNT nanoparticles. The PtRu nanoparticles with a satisfactory dispersion were formed on the external surface of MWCNTs. The CO stripping experiment was performed to evaluate the poisoning resistance of the prepared PtRu/MWCNT nanoparticles. Results of electrochemical measurements indicate that the prepared PtRu/MWCNTs shows an enhanced performance toward CO poisoning. The results of characterization revealed that microwave-assisted improved impregnation technique have a high yield of alloy phase formation and could be effectively used as a simple, quick and efficient technique for preparation of bimetallic PtRu/MWCNT nanoparticles. © 2015 Elsevier B.V. All rights reserved.
Keywords: Microwave synthesis Carbon nanotube PtRu alloy
1. Introduction Platinum-based alloys have attracted considerable attentions due to their superior performance in many areas such as catalysis, electrocatalysis, energy conversion, sensing and biosensing [1e3]. Among these, PtRu has been proposed as one of the most promising materials due to its unique performance and selectivity for various catalytic reactions and high tolerance to CO poisoning [4e9]. However, performance of the PtRu alloys for these applications significantly depends on factors such as morphology, uniform dispersion and degree of alloy phase formation. Recent studies on PtRu catalysts have revealed that uniform distribution of small size nanoparticles on high surface area supporting materials can strongly improve their performance [4]. Recently, carbon nanomaterials have attracted increasing attentions as effective support in areas of electrocatalysis and biosensor due to their unique properties, such as high current carrying capability, chemical stability, thermal conductivity and mechanical strength [4,10,11]. Among these, carbon nanotubes (CNT) has received considerable
* Corresponding author. ** Corresponding author. Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology, Daegu, 711-873, Republic of Korea. E-mail addresses: [email protected], [email protected] (M. Rahsepar), [email protected] (H. Kim). http://dx.doi.org/10.1016/j.jallcom.2015.07.224 0925-8388/© 2015 Elsevier B.V. All rights reserved.
attentions and has been proposed as one of the most promising supporting materials [12e15]. However, since the basal plane of CNT is chemically inert, the optimum utilization of the PtRu particles on the CNT substrate depends on a reasonable selection of the preparation technique [3,16]. Several investigations have been conducted to study the properties of platinum-based alloys decorated on the outside surface of CNT. A variety of techniques have been developed and utilized to prepare the Pt-based nanoparticles on the surface of carbonaceous supporting materials. Impregnation [17e20], colloidal [21e33] and electrodeposition [34] are the most widely applied techniques for this purpose. While in colloidal method, a stabilizing agent is used to prevent the agglomeration and growth of the particles, impregnation presents a low level of control over the particle size and distribution [35]. However colloidal method is a complicated process and it is necessary to remove the remaining organic impurities which could dramatically block the active sites and cause detrimental effect on the activity of the materials for catalytic and sensing applications [36]. On the other hand, almost extremely low loadings of Pt [37,38] and binary alloys [39] have been reported with electrodeposition techniques. Some efforts have been made to improve the performance of the impregnation technique to enhance the control over particle size and distribution without the use of a stabilizing agent [3,40e42]. Recently, we have developed a microwave-assisted improved impregnation technique to prepare a high loading carbon-
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supported Pt catalyst for fuel cell applications [36,37]. This technique is based on the basic principles of improved impregnation. The uniform and volumetric nature of microwave irradiation could provide several advantages for synthesis of carbon supported nanomaterials [43]. More uniform heat transfer to substrate and consequently a more homogeneous nucleation and shorted crystallization time, less unavoidable temperature gradients, remarkable energy and time efficiency are several advantages of microwave-assisted irradiation technique compared to conventional heating methods. In addition, since microwave heating generates heat directly within CNT supporting materials, the CNTs act as preferred nucleation sites which consequently leading to a satisfactory utilization and uniform dispersion of the nanoparticles [36]. The results of our investigation revealed that microwave heating could be an effective technique and provide adequate control over uniform dispersion of Pt particles [13,36]. However, since the bimetallic PtRu is characterized by a high degree of alloying, in this work, we investigate the ability of a microwaveassisted improved impregnation technique as a quick, simple and efficient method for preparation of a homogeneous multiwalled carbon nanotube (MWCNT) supported PtRu with a high degree of alloy phase formation without utilization of any stabilizing agent. 2. Experimental 2.1. Fabrication of PtRu/MWCNT nanoparticles The PtRu/MWCNTs were fabricated by using a microwaveassisted improved impregnation technique which were introduced elsewhere for fabrication of Pt/MWCNT nanoparticles [36]. All the chemicals used in this study were of analytical grade. Platinum (IV) chloride (Aldrich) and ruthenium chloride (III) hydrate (Aldrich) were used as metal precursors. MWCTs with a nanotube diameters of 10e15 nm and length of ~20 mm were used as supporting material. Initially, the MWCNTs were pretreated by refluxing in a mixture of H2SO4/HNO3 (1:1) solution and then washed with copious amounts of distilled deionized water. For synthesis of a 45 wt.% PtRu/MWCNT, required amounts of metal precursors were mixed with pretreated CNTs in an aqueous solution. Then, NaOH aqueous solution was gradually added to adjust the pH of the solution to ~12.5. Formaldehyde with the mole ratio of HCHO:metal precursor ¼ 20:1 was added to the mixture as a reducing agent. The mixture was put in an open glass vial and then heated in a household microwave oven (Magic MWO-20M8, 800 W, 2.45 GHz) with a heating procedure of 5 s on and 120 s pause. This cycle was done ten times. Finally, after re-acidification, the resulting solids were rinsed thoroughly with distillededeionized water to neutral and then dried at 80 C for 12 h. In order to remove impurities and to assure the complete reduction of the PtRu particles, the nanoparticles was further heat-treated at 300 C for 3 h under a reducing (10% H2 in N2) atmosphere. In order to investigate the effect of microwave heating, PtRu/MWCNT nanoparticles were also prepared by using an improved aqueous impregnation technique by means of conventional heating. In the case of conventional heating, the nanoparticles were formed under refluxing using a mantle heater at 90 C for 3 h. The loading of PtRu on PtRu/MWCNT was 40 wt.%, which was stoichiometrically calculated and checked by ICP measurements with a Varian 730-ES ICP optical emission spectrometer.
nanoparticles. The morphology and mean particle size of the PtRu/ MWCNT nanoparticles were examined by using high resolution transmission electron microscopy (HR-TEM). Transmission electron microscopy investigations were carried out by using a JEM-3000F JEOL electron microscope operating at 300 kV. X-ray photoelectron spectroscopy (XPS) was used to investigate the valence state and surface composition of the PtRu/MWCNT nanoparticles. The XPS experiments was carried out using an UHV multipurpose surface analysis system (SIGMA PROBE, Thermo, UK) operating at a base pressure of <1010 mbar. The photoelectron spectra were excited by an Al Ka (1486.60 eV) anode operating at a constant power of 100 W (15 KV and 6.7 mA). During the spectra acquisition process, the constant analyzer energy (CAE) mode was employed at a pass energy level of 30.00 eV with 0.10 eV steps. Deconvolution and curve-fitting of the spectra were done using the Avantage software. 2.3. CO stripping The CO stripping experiments were carried out with a threeelectrode cell system consisting of an Ag/AgCl (3.5 M KCl) reference electrode and a platinum wire as the counter electrode. The working electrode was a thin layer of Nafion-bonded PtRu/MWCNT nanoparticle ink cast on a glassy carbon (3 mm in diameter) electrode. For preparation of the ink, a mixture of the PtRu/MWCNT nanoparticle, Nafion solution and isopropyl alcohol were ultrasonically mixed to form a homogeneous ink. Finally, a measured volume of this mixture was dropped on the surface of a glassy carbon electrode. The total Pt loading on the electrode was ~0.226 mg cm2. The CO stripping experiments were performed by using an Autolab potentiostat/galvanostat (Model, PGSTAT-302N) in a 0.5 M H2SO4 solution. CO adsorption was carried out with continuous CO bubbling for 20 min while the anode electrode potential was controlled at 0.14 V vs. Ag/AgCl (3.5 M KCl). The solution was then purged with 99.9% pure nitrogen gas for 30 min to remove the dissolved CO before the stripping test. 3. Results and discussion 3.1. Physicochemical characterization of PtRu/MWCNT nanoparticles The XRD analysis was performed to identify the crystal structure of the fabricated PtRu/MWCNT nanoparticles. The X-ray diffraction
2.2. Characterization of PtRu/MWCNT nanoparticles An X-ray diffractometer D/MAX-2500/PC (Rigaku Co., Japan), using the Cu Ka1 (l ¼ 1.540 56 Å) as the radiation source was used for identification of the crystal structure of the PtRu/MWCNT
Fig. 1. X-ray diffraction patterns of the prepared PtRu/MWCNT, Pt/MWCNT and commercial PtRu/C and Pt/C samples.
M. Rahsepar, H. Kim / Journal of Alloys and Compounds 649 (2015) 1323e1328
pattern of the PtRu/MWCNT is shown in Fig. 1. Also, the crystal structure of commercially available P/C and PtRu/C (E-TEK) samples were analyzed under the same condition and the resulting diffraction patterns are presented for comparison. As shown in Fig. 1, the XRD patterns of both synthesized PtRu/MWCNT and commercial PtRu/C samples are assigned to the XRD patterns of platinum with a faced centered cubic (fcc) crystalline structure. Diffraction peak at the angle (2q) of 26.0 is assigned to the hexagonal graphite crystallographic planes (002) in carbon nanotube. Also, the diffraction peaks at angles (2q) of 39.82, 46.30, 67.52, 81.32, and 85.86 correspond to (111), (200), (220), (311), and (222) diffraction peaks of platinum structure, respectively. In addition, no diffraction pattern associated with the Ru hcp crystalline structure was detected. The results of XRD analysis indicate that all Pt and Ru species are present as a single phase with the characteristic features of the platinum crystal structure. In the other word, it can be concluded that all Ru species were incorporated in the crystal structure of the Pt base metal. However, replacement of Pt with the smaller Ru species accompanying with a reduction of lattice parameters which results in a positive shift of the Pt diffraction signals. As shown in Fig. 1, the diffraction peaks of the PtRu/MWCNT slightly shift towards larger diffraction angles compared with that of Pt/MWCNT which reveals solid solution alloy formation between Pt and Ru. Also, it is revealed that the positive shift of diffraction peaks increases at higher Bragg angles. Consequently, it could be concluded that microwave-assisted improved impregnation technique has a high efficiency of alloy phase formation between Pt and Ru. Fig. 2 displays the HR-TEM micrographs of the microwavesynthesized PtRu/MWCNT nanoparticles. As shown, the synthesized nanoparticles are formed and well dispersed on the external surface of the MWCNT particles. Also, Fig. 3 shows the HR-TEM micrographs of a PtRu/MWCNT that was prepared by using the improved aqueous impregnation technique. It can be observed that microwave irradiation enhances the dispersion and uniformity of the CNT supported nanoparticles. Typical HR-TEM micrographs were used to analyze the mean particle size and particle size distribution of the PtRu nanoparticles. The particle size analysis was performed by measuring the size of more than 300 particles in the HR-TEM micrographs. The histogram of particle size distribution of microwave-synthesized PtRu/MWCNT nanoparticles is presented in Fig. 4. As shown, most of the nanoparticles have a particle size between 1.5 and 3.5 nm. Also the mean size of the PtRu particles was 2.7 nm. The XPS was used to characterize the surface composition and valance state of the Pt and Ru species in the PtRu/MWCNT nanoparticles. Fig. 5 displays the XPS survey scan spectrum of the microwave-synthesized PtRu/MWCNT nanoparticles. As shown, the XPS peaks of Pt 4f, Ru 3p, C 1s, and O 1s were detected in the XPS survey scan spectrum. High-resolution XPS spectra of the Pt 4f and Ru 3p core-level regions of the PtRu/MWCNT nanoparticles are shown in Fig. 6. Since Ru 3d3/2 and Ru 3d5/2 core level region signals overlap with the C 1s peak, the Ru 3p region was used to investigate the valence state of the Ru species. According to Fig. 6a, the XPS spectra of the Pt 4f consisting of a doublet peak with the binding energies of 71.4 (Pt 4f7/2) and 74.7 eV (Pt 4f5/2). As shown, the Pt 4f spectra can be deconvoluted to the corresponding XPS signals of Pt (0), Pt (II) and Pt (IV) species. The results of the XPS analysis reveal that Pt present in the form of Pt, PtO and PtO2 species. Also, as shown in Fig. 6b, the XPS signals of the Ru 3p can be deconvoluted to corresponding XPS signals of the metallic Ru, anhydrous RuO2 and hydrous amorphous RuO2.xH2O. The amount of different species of Pt and Ru were calculated from the relative intensities of the corresponding deconvoluted signals in the high-resolution XPS spectra. The results of XPS analysis are summarized in Table 1. Also,
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Fig. 2. HR-TEM micrographs of PtRu/MWCNTs prepared by microwave-assisted improved impregnation technique.
the XPS analysis of a typical PtRu/MWCNT nanoparticles that was prepared by using an improved aqueous impregnation technique by means of conventional heating is presented in Table 1 for comparison [4]. According to XPS analysis, the surface composition of the PtRu/MWCNT nanoparticles is very close to the theoretically calculated composition (50:50). In addition, while a large content of Pt and Ru are present in the form of metallic species, XPS analysis indicate a partial oxidation of the Pt and Ru in the surface of PtRu/ MWCNT nanoparticles. However, according to the relative intensities of the XPS signals of the Pt and Ru, the proportion of surface oxide species in microwave-synthesized PtRu/MWCNT nanoparticles is less compared to that of conventional heating. 3.2. CO stripping of PtRu/MWCNT nanoparticles Currently, bimetallic PtRu is one of the most effective catalyst
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Fig. 5. XPS survey scan spectrum of the microwave-synthesized PtRu/MWCNT.
Fig. 3. HR-TEM micrographs of PtRu/MWCNTs prepared by conventional improved impregnation technique.
Fig. 4. Histogram of PtRu nanoparticle size distribution for the microwave-synthesized PtRu/MWCNT.
materials for energy conversion, sensing and biosensing applications. One of the main requirements of catalysts for these applications, is superior performance against poising which is caused by adsorption of impurities or intermediate products of the corresponding reactions. In the present study, the CO stripping experiment was performed to evaluate the poisoning resistance of the prepared PtRu/MWCNT nanoparticles. Fig. 7 displays the corresponding voltammograms of CO stripping in a 0.5 M H2SO4 solution. As shown, PtRu/MWCNT nanoparticles exhibited lower overpotential towards CO oxidation as compared with the commercial PtRu/C samples. According to CO stripping analysis, the microwave-synthesized PtRu/MWCNT catalyst exhibits a less positive peak potential (0.322 V vs. Ag/AgCl, 3.5 M KCl) with respect to PtRu/C catalyst (0.418 V vs. Ag/AgCl, 3.5 M KCl). Also, it can be observed that microwave-synthesized PtRu/MWCNT samples exhibited a slightly lower onset potential toward CO oxidation
Fig. 6. High-resolution (a) Pt 4f and (b) Ru 3p XPS spectra of the microwavesynthesized PtRu/MWCNT.
M. Rahsepar, H. Kim / Journal of Alloys and Compounds 649 (2015) 1323e1328
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Table 1 Binding energies and relative intensities of XPS analysis. Sample PtRu/MWCNT
Species [a]
Pt 4f7/2
Ru 3p3/2
PtRu/MWCNT
[b]
Pt 4f7/2
Ru 3p3/2
a b
Pt metal PtO PtO2 Ru metal RuO2 RuO2.xH2O Pt metal PtO PtO2 Ru metal RuO2 RuO2.xH2O
Binding energy (eV)
Relative intensity (%)
PtRu (at %)
71.7 72.8 74.3 462.11 463.7 466.51 71.7 72.79 74.33 462.1 463.7 466.51
72.15 18.9 8.95 68.07 16.71 15.21 73.8 16.2 10 64.33 12.56 23.12
53:47
53:47
Prepared by microwave-assisted improved impregnation method. Prepared by improved impregnation method [4].
4. Conclusion The efficiency of microwave-assisted improved impregnation method for synthesis of bimetallic PtRu alloy nanoparticles on MWCNT supporting materials was investigated. It was revealed that this technique could be effectively applied for synthesis of a high loading MWCNT supported PtRu nanoparticles. Also, it was found that PtRu nanoparticles with a high degree of alloy phase formation were uniformly dispersed on the supporting materials and exhibited an enhanced resistance against CO poisoning. According to results of the present study, it is concluded that microwave-assisted improved impregnation method could be recommended as a quick, simple and efficient method for preparation of homogeneous MWCNT supported PtRu nanoparticles. References
Fig. 7. CO stripping voltammograms in a 0.5 M H2SO4 aqueous solution; scan rate: 50 mV s1.
compare with that prepared under conventional heating by using a mantle heater. According to literature, the effective electrocatalytic activity of PtRu toward CO oxidation is due to a bifunctional mechanism between Pt and oxygen-containing species. Based on the bifunctional mechanism, the presence of Ru provides an oxygen source for CO oxidation at lower overpotentials [22]. It has been reported that bimetallic PtRu with Pt:Ru atomic ratio of 1:1 shows the superior performance for CO oxidation [44]. Also, the electrocatalytic activity of PtRu alloys for CO oxidation is directly affected by surface oxidation states [38]. According to literature, presence of a large content of the surface oxide species (especially for Ru) would decrease the OH nucleation rate and the COad oxidation reaction rate and thus results in positive shift of the COad stripping peak potential [14,45]. The results of XRD and XPS characterization of the microwave-synthesized PtRu/MWCNT nanoparticles reveals a high degree of alloy phase formation and a relatively low content of surface oxide species. This is in good agreements with the results of CO stripping experiment. Results of physicochemical characterization and CO stripping indicate that microwave-assisted improved impregnation method could be purposed as a fast and energy efficient technique for synthesis of PtRu/MWCNT nanoparticles which exhibit a superior performance against catalyst poisoning. A high degree of alloy phase formation and a satisfactory dispersion of bimetallic nanoparticles without using any organic stabilizing agent are the main advantages of this technique.
[1] L.H. Li, W.D. Zhang, J.S. Ye, Electrocatalytic oxidation of glucose at carbon nanotubes supported PtRu nanoparticles and its detection, Electroanalysis 20 (2008) 2212e2216. [2] F. Xiao, F. Zhao, D. Mei, Z. Mo, B. Zeng, Nonenzymatic glucose sensor based on ultrasonic-electrodeposition of bimetallic PtM (M¼ Ru, Pd and Au) nanoparticles on carbon nanotubeseionic liquid composite film, Biosens. Bioelectron. 24 (2009) 3481e3486. [3] M. Rahsepar, M. Pakshir, Y. Piao, H. Kim, Preparation of highly active 40 wt.% Pt on multiwalled carbon nanotube by improved impregnation method for fuel cell applications, Fuel Cells 12 (2012) 827e834. [4] M. Rahsepar, M. Pakshir, Y. Piao, H. Kim, Synthesis and electrocatalytic performance of high loading active PtRu multiwalled carbon nanotube catalyst for methanol oxidation, Electrochim. Acta 71 (2012) 246e251. [5] C.-T. Hsieh, Y.-W. Chou, W.-Y. Chen, Fabrication and electrochemical activity of carbon nanotubes decorated with PtRu nanoparticles in acid solution, J. Alloys Compd. 466 (2008) 233e240. [6] Y.-C. Hsieh, L.-C. Chang, Y.-C. Tseng, P.-W. Wu, J.-F. Lee, Structural characterizations of PtRu nanoparticles by galvanostatic pulse electrodeposition, J. Alloys Compd. 583 (2014) 170e175. [7] N. Kakati, J. Maiti, S.H. Jee, S.H. Lee, Y.S. Yoon, Hydrothermal synthesis of PtRu on CNT/SnO2 composite as anode catalyst for methanol oxidation fuel cell, J. Alloys Compd. 509 (2011) 5617e5622. [8] Y.-J. Song, S.-B. Han, J.-M. Lee, K.-W. Park, PtRu alloy nanostructure electrodes for methanol electrooxidation, J. Alloys Compd. 473 (2009) 516e520. [9] S. Tungatarova, K. Dossumov, T. Baizhumanova, N. Popova, Nanostructured supported Pt-, Ru-and Pt-Ru catalysts for oxidation of methane into synthesisgas, J. Alloys Compd. 504 (2010) S349eS352. [10] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Appl. Catal. A General 253 (2003) 337e358. [11] X. Wang, N. Li, J.A. Webb, L.D. Pfefferle, G.L. Haller, Effect of surface oxygen containing groups on the catalytic activity of multi-walled carbon nanotube supported Pt catalyst, Appl. Catal. B Environ. 101 (2010) 21e30. s, A. Sepúlveda-Escribano, J. Silvestre[12] E. Jardim, M. Goncalves, S. Rico-France Albero, Superior performance of multi-wall carbon nanotubes as support of Pt-based catalysts for the preferential CO oxidation: effect of ceria addition, Appl. Catal. B Environ. 113 (2012) 72e78. [13] M. Rahsepar, M. Pakshir, P. Nikolaev, Y. Piao, H. Kim, A combined physicochemical and electrocatalytic study of microwave synthesized tungsten mono-carbide nanoparticles on multiwalled carbon nanotubes as a co-catalyst for a proton-exchange membrane fuel cell, Int. J. Hydrogen Energy 39 (2014)
1328
M. Rahsepar, H. Kim / Journal of Alloys and Compounds 649 (2015) 1323e1328
15706e15717. [14] Y. Liang, J. Li, Q.-C. Xu, R.-Z. Hu, J.-D. Lin, D.-W. Liao, Characterization of composite carbon supported PtRu catalyst and its catalytic performance for methanol oxidation, J. Alloys Compd. 465 (2008) 296e304. [15] C. Yang, D. Wang, X. Hu, C. Dai, L. Zhang, Preparation and characterization of multi-walled carbon nanotube (MWCNTs)-supported Pt-Ru catalyst for methanol electrooxidation, J. Alloys Compd. 448 (2008) 109e115. [16] S. Wang, X. Wang, S.P. Jiang, PtRu nanoparticles supported on 1aminopyrene-functionalized multiwalled carbon nanotubes and their electrocatalytic activity for methanol oxidation, Langmuir 24 (2008) 10505e10512. [17] T. Maiyalagan, Synthesis and electro-catalytic activity of methanol oxidation on nitrogen containing carbon nanotubes supported Pt electrodes, Appl. Catal. B Environ. 80 (2008) 286e295. [18] M.S. Saha, R. Li, X. Sun, S. Ye, 3-D composite electrodes for high performance PEM fuel cells composed of Pt supported on nitrogen-doped carbon nanotubes grown on carbon paper, Electrochem. Commun. 11 (2009) 438e441. [19] M.S. Saha, R. Li, X. Sun, High loading and monodispersed Pt nanoparticles on multiwalled carbon nanotubes for high performance proton exchange membrane fuel cells, J. Power Sources 177 (2008) 314e322. [20] N. Rajalakshmi, H. Ryu, M. Shaijumon, S. Ramaprabhu, Performance of polymer electrolyte membrane fuel cells with carbon nanotubes as oxygen reduction catalyst support material, J. Power Sources 140 (2005) 250e257. [21] X. Wang, I.-M. Hsing, Surfactant stabilized Pt and Pt alloy electrocatalyst for polymer electrolyte fuel cells, Electrochim. Acta 47 (2002) 2981e2987. [22] V. Radmilovic, H. Gasteiger, P. Ross, Structure and chemical composition of a supported Pt-Ru electrocatalyst for methanol oxidation, J. Catal. 154 (1995) 98e106. [23] Z. Tang, D. Geng, G. Lu, Size-controlled synthesis of colloidal platinum nanoparticles and their activity for the electrocatalytic oxidation of carbon monoxide, J. Colloid Interface Sci. 287 (2005) 159e166. [24] F. Bonet, V. Delmas, S. Grugeon, R.H. Urbina, P. Silvert, K. Tekaia-Elhsissen, Synthesis of monodisperse Au, Pt, Pd, Ru and Ir nanoparticles in ethylene glycol, NanoStructured Mater. 11 (1999) 1277e1284. [25] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Shapecontrolled synthesis of colloidal platinum nanoparticles, Science (1996) 1924e1925. New York Then Washington. [26] T. Schmidt, H. Gasteiger, R. Behm, Methanol electrooxidation on a colloidal PtRu-alloy fuel-cell catalyst, Electrochem. Commun. 1 (1999) 1e4. [27] N. Toshima, Y. Wang, Preparation and catalysis of novel colloidal dispersions of copper/noble metal bimetallic clusters, Langmuir 10 (1994) 4574e4580. [28] Z. Liu, L.M. Gan, L. Hong, W. Chen, J.Y. Lee, Carbon-supported Pt nanoparticles as catalysts for proton exchange membrane fuel cells, J. Power Sources 139 (2005) 73e78. [29] X. Li, S. Ge, C. Hui, I.-M. Hsing, Well-dispersed multiwalled carbon nanotubes supported platinum nanocatalysts for oxygen reduction, Electrochem. SolidState Lett. 7 (2004) A286eA289. [30] Y. Xing, Synthesis and electrochemical characterization of uniformlydispersed high loading Pt nanoparticles on sonochemically-treated carbon
nanotubes, J. Phys. Chem. B 108 (2004) 19255e19259. [31] C.-L. Lee, Y.-C. Ju, P.-T. Chou, Y.-C. Huang, L.-C. Kuo, J.-C. Oung, Preparation of Pt nanoparticles on carbon nanotubes and graphite nanofibers via selfregulated reduction of surfactants and their application as electrochemical catalyst, Electrochem. Commun. 7 (2005) 453e458. [32] Z. Liu, J.Y. Lee, W. Chen, M. Han, L.M. Gan, Physical and electrochemical characterizations of microwave-assisted polyol preparation of carbonsupported PtRu nanoparticles, Langmuir 20 (2004) 181e187. [33] X. Zhao, W. Li, L. Jiang, W. Zhou, Q. Xin, B. Yi, G. Sun, Multi-wall carbon nanotube supported PteSn nanoparticles as an anode catalyst for the direct ethanol fuel cell, Carbon 42 (2004) 3263e3265. [34] H. Kim, N.P. Subramanian, B.N. Popov, Preparation of PEM fuel cell electrodes using pulse electrodeposition, J. Power Sources 138 (2004) 14e24. [35] A.M.C. Luna, G.A. Camara, V.A. Paganin, E.A. Ticianelli, E.R. Gonzalez, Effect of thermal treatment on the performance of CO-tolerant anodes for polymer electrolyte fuel cells, Electrochem. Commun. 2 (2000) 222e225. [36] M. Rahsepar, M. Pakshir, H. Kim, Synthesis of multiwall carbon nanotubes with a high loading of Pt by a microwave-assisted impregnation method for use in the oxygen reduction reaction, Electrochim. Acta 108 (2013) 769e775. [37] M. Rahsepar, M. Pakshir, P. Nikolaev, A. Safavi, K. Palanisamy, H. Kim, Tungsten carbide on directly grown multiwalled carbon nanotube as a co-catalyst for methanol oxidation, Appl. Catal. B Environ. 127 (2012) 265e272. [38] J. Lee, J. Seo, K. Han, H. Kim, Preparation of low Pt loading electrodes on Nafion (Naþ)-bonded carbon layer with galvanostatic pulses for PEMFC application, J. Power Sources 163 (2006) 349e356. [39] S. Woo, I. Kim, J.K. Lee, S. Bong, J. Lee, H. Kim, Preparation of cost-effective PteCo electrodes by pulse electrodeposition for PEMFC electrocatalysts, Electrochim. Acta 56 (2011) 3036e3041. [40] Z. Zhou, S. Wang, W. Zhou, L. Jiang, G. Wang, G. Sun, B. Zhou, Q. Xin, Preparation of highly active Pt/C cathode electrocatalysts for DMFCs by an improved aqueous impregnation method, Phys. Chem. Chem. Phys. 5 (2003) 5485e5488. [41] Z. Zhou, W. Zhou, S. Wang, G. Wang, L. Jiang, H. Li, G. Sun, Q. Xin, Preparation of highly active 40wt.% Pt/C cathode electrocatalysts for DMFC via different routes, Catal. Today 93 (2004) 523e528. [42] I. Kim, S. Bong, S. Woo, R.K. Mahajan, H. Kim, Highly active 40 wt.% PtRu/C anode electrocatalysts for PEMFCs prepared by an improved impregnation method, Int. J. Hydrogen Energy 36 (2011) 1803e1812. [43] M. Oghbaei, O. Mirzaee, Microwave versus conventional sintering: a review of fundamentals, advantages and applications, J. Alloys Compd. 494 (2010) 175e189. [44] R.J. Wiltshire, C.R. King, A. Rose, P.P. Wells, H. Davies, M.P. Hogarth, D. Thompsett, B. Theobald, F.W. Mosselmans, M. Roberts, Effects of composition on structure and activity of PtRu/C catalysts, Phys. Chem. Chem. Phys. 11 (2009) 2305e2313. [45] C. Bock, M.-A. Blakely, B. MacDougall, Characteristics of adsorbed CO and CH 3 OH oxidation reactions for complex Pt/Ru catalyst systems, Electrochim. Acta 50 (2005) 2401e2414.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microwave-assisted synthesis and characterization of bimetallic PtRu alloy nanoparticles supported on carbon nanotubes Mansour Rahsepar a, *, Hasuck Kim b, c, ** a
Department of Materials Science and Engineering, School of Engineering, Shiraz University, Zand Boulevard, Shiraz, 7134851154, Iran Department of Chemistry, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-747, Republic of Korea c Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology, Daegu, 711-873, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2015 Received in revised form 21 July 2015 Accepted 24 July 2015 Available online 26 July 2015
Multiwalled carbon nanotube (MWCNT) supported PtRu nanoparticles were synthesized by using a microwave-assisted improved impregnation technique. X-ray diffraction, transmission electron microscopy and X-ray photo electron spectroscopy were used to characterize the prepared PtRu/MWCNT nanoparticles. The PtRu nanoparticles with a satisfactory dispersion were formed on the external surface of MWCNTs. The CO stripping experiment was performed to evaluate the poisoning resistance of the prepared PtRu/MWCNT nanoparticles. Results of electrochemical measurements indicate that the prepared PtRu/MWCNTs shows an enhanced performance toward CO poisoning. The results of characterization revealed that microwave-assisted improved impregnation technique have a high yield of alloy phase formation and could be effectively used as a simple, quick and efficient technique for preparation of bimetallic PtRu/MWCNT nanoparticles. © 2015 Elsevier B.V. All rights reserved.
Keywords: Microwave synthesis Carbon nanotube PtRu alloy
1. Introduction Platinum-based alloys have attracted considerable attentions due to their superior performance in many areas such as catalysis, electrocatalysis, energy conversion, sensing and biosensing [1e3]. Among these, PtRu has been proposed as one of the most promising materials due to its unique performance and selectivity for various catalytic reactions and high tolerance to CO poisoning [4e9]. However, performance of the PtRu alloys for these applications significantly depends on factors such as morphology, uniform dispersion and degree of alloy phase formation. Recent studies on PtRu catalysts have revealed that uniform distribution of small size nanoparticles on high surface area supporting materials can strongly improve their performance [4]. Recently, carbon nanomaterials have attracted increasing attentions as effective support in areas of electrocatalysis and biosensor due to their unique properties, such as high current carrying capability, chemical stability, thermal conductivity and mechanical strength [4,10,11]. Among these, carbon nanotubes (CNT) has received considerable
* Corresponding author. ** Corresponding author. Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology, Daegu, 711-873, Republic of Korea. E-mail addresses: [email protected], [email protected] (M. Rahsepar), [email protected] (H. Kim). http://dx.doi.org/10.1016/j.jallcom.2015.07.224 0925-8388/© 2015 Elsevier B.V. All rights reserved.
attentions and has been proposed as one of the most promising supporting materials [12e15]. However, since the basal plane of CNT is chemically inert, the optimum utilization of the PtRu particles on the CNT substrate depends on a reasonable selection of the preparation technique [3,16]. Several investigations have been conducted to study the properties of platinum-based alloys decorated on the outside surface of CNT. A variety of techniques have been developed and utilized to prepare the Pt-based nanoparticles on the surface of carbonaceous supporting materials. Impregnation [17e20], colloidal [21e33] and electrodeposition [34] are the most widely applied techniques for this purpose. While in colloidal method, a stabilizing agent is used to prevent the agglomeration and growth of the particles, impregnation presents a low level of control over the particle size and distribution [35]. However colloidal method is a complicated process and it is necessary to remove the remaining organic impurities which could dramatically block the active sites and cause detrimental effect on the activity of the materials for catalytic and sensing applications [36]. On the other hand, almost extremely low loadings of Pt [37,38] and binary alloys [39] have been reported with electrodeposition techniques. Some efforts have been made to improve the performance of the impregnation technique to enhance the control over particle size and distribution without the use of a stabilizing agent [3,40e42]. Recently, we have developed a microwave-assisted improved impregnation technique to prepare a high loading carbon-
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supported Pt catalyst for fuel cell applications [36,37]. This technique is based on the basic principles of improved impregnation. The uniform and volumetric nature of microwave irradiation could provide several advantages for synthesis of carbon supported nanomaterials [43]. More uniform heat transfer to substrate and consequently a more homogeneous nucleation and shorted crystallization time, less unavoidable temperature gradients, remarkable energy and time efficiency are several advantages of microwave-assisted irradiation technique compared to conventional heating methods. In addition, since microwave heating generates heat directly within CNT supporting materials, the CNTs act as preferred nucleation sites which consequently leading to a satisfactory utilization and uniform dispersion of the nanoparticles [36]. The results of our investigation revealed that microwave heating could be an effective technique and provide adequate control over uniform dispersion of Pt particles [13,36]. However, since the bimetallic PtRu is characterized by a high degree of alloying, in this work, we investigate the ability of a microwaveassisted improved impregnation technique as a quick, simple and efficient method for preparation of a homogeneous multiwalled carbon nanotube (MWCNT) supported PtRu with a high degree of alloy phase formation without utilization of any stabilizing agent. 2. Experimental 2.1. Fabrication of PtRu/MWCNT nanoparticles The PtRu/MWCNTs were fabricated by using a microwaveassisted improved impregnation technique which were introduced elsewhere for fabrication of Pt/MWCNT nanoparticles [36]. All the chemicals used in this study were of analytical grade. Platinum (IV) chloride (Aldrich) and ruthenium chloride (III) hydrate (Aldrich) were used as metal precursors. MWCTs with a nanotube diameters of 10e15 nm and length of ~20 mm were used as supporting material. Initially, the MWCNTs were pretreated by refluxing in a mixture of H2SO4/HNO3 (1:1) solution and then washed with copious amounts of distilled deionized water. For synthesis of a 45 wt.% PtRu/MWCNT, required amounts of metal precursors were mixed with pretreated CNTs in an aqueous solution. Then, NaOH aqueous solution was gradually added to adjust the pH of the solution to ~12.5. Formaldehyde with the mole ratio of HCHO:metal precursor ¼ 20:1 was added to the mixture as a reducing agent. The mixture was put in an open glass vial and then heated in a household microwave oven (Magic MWO-20M8, 800 W, 2.45 GHz) with a heating procedure of 5 s on and 120 s pause. This cycle was done ten times. Finally, after re-acidification, the resulting solids were rinsed thoroughly with distillededeionized water to neutral and then dried at 80 C for 12 h. In order to remove impurities and to assure the complete reduction of the PtRu particles, the nanoparticles was further heat-treated at 300 C for 3 h under a reducing (10% H2 in N2) atmosphere. In order to investigate the effect of microwave heating, PtRu/MWCNT nanoparticles were also prepared by using an improved aqueous impregnation technique by means of conventional heating. In the case of conventional heating, the nanoparticles were formed under refluxing using a mantle heater at 90 C for 3 h. The loading of PtRu on PtRu/MWCNT was 40 wt.%, which was stoichiometrically calculated and checked by ICP measurements with a Varian 730-ES ICP optical emission spectrometer.
nanoparticles. The morphology and mean particle size of the PtRu/ MWCNT nanoparticles were examined by using high resolution transmission electron microscopy (HR-TEM). Transmission electron microscopy investigations were carried out by using a JEM-3000F JEOL electron microscope operating at 300 kV. X-ray photoelectron spectroscopy (XPS) was used to investigate the valence state and surface composition of the PtRu/MWCNT nanoparticles. The XPS experiments was carried out using an UHV multipurpose surface analysis system (SIGMA PROBE, Thermo, UK) operating at a base pressure of <1010 mbar. The photoelectron spectra were excited by an Al Ka (1486.60 eV) anode operating at a constant power of 100 W (15 KV and 6.7 mA). During the spectra acquisition process, the constant analyzer energy (CAE) mode was employed at a pass energy level of 30.00 eV with 0.10 eV steps. Deconvolution and curve-fitting of the spectra were done using the Avantage software. 2.3. CO stripping The CO stripping experiments were carried out with a threeelectrode cell system consisting of an Ag/AgCl (3.5 M KCl) reference electrode and a platinum wire as the counter electrode. The working electrode was a thin layer of Nafion-bonded PtRu/MWCNT nanoparticle ink cast on a glassy carbon (3 mm in diameter) electrode. For preparation of the ink, a mixture of the PtRu/MWCNT nanoparticle, Nafion solution and isopropyl alcohol were ultrasonically mixed to form a homogeneous ink. Finally, a measured volume of this mixture was dropped on the surface of a glassy carbon electrode. The total Pt loading on the electrode was ~0.226 mg cm2. The CO stripping experiments were performed by using an Autolab potentiostat/galvanostat (Model, PGSTAT-302N) in a 0.5 M H2SO4 solution. CO adsorption was carried out with continuous CO bubbling for 20 min while the anode electrode potential was controlled at 0.14 V vs. Ag/AgCl (3.5 M KCl). The solution was then purged with 99.9% pure nitrogen gas for 30 min to remove the dissolved CO before the stripping test. 3. Results and discussion 3.1. Physicochemical characterization of PtRu/MWCNT nanoparticles The XRD analysis was performed to identify the crystal structure of the fabricated PtRu/MWCNT nanoparticles. The X-ray diffraction
2.2. Characterization of PtRu/MWCNT nanoparticles An X-ray diffractometer D/MAX-2500/PC (Rigaku Co., Japan), using the Cu Ka1 (l ¼ 1.540 56 Å) as the radiation source was used for identification of the crystal structure of the PtRu/MWCNT
Fig. 1. X-ray diffraction patterns of the prepared PtRu/MWCNT, Pt/MWCNT and commercial PtRu/C and Pt/C samples.
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pattern of the PtRu/MWCNT is shown in Fig. 1. Also, the crystal structure of commercially available P/C and PtRu/C (E-TEK) samples were analyzed under the same condition and the resulting diffraction patterns are presented for comparison. As shown in Fig. 1, the XRD patterns of both synthesized PtRu/MWCNT and commercial PtRu/C samples are assigned to the XRD patterns of platinum with a faced centered cubic (fcc) crystalline structure. Diffraction peak at the angle (2q) of 26.0 is assigned to the hexagonal graphite crystallographic planes (002) in carbon nanotube. Also, the diffraction peaks at angles (2q) of 39.82, 46.30, 67.52, 81.32, and 85.86 correspond to (111), (200), (220), (311), and (222) diffraction peaks of platinum structure, respectively. In addition, no diffraction pattern associated with the Ru hcp crystalline structure was detected. The results of XRD analysis indicate that all Pt and Ru species are present as a single phase with the characteristic features of the platinum crystal structure. In the other word, it can be concluded that all Ru species were incorporated in the crystal structure of the Pt base metal. However, replacement of Pt with the smaller Ru species accompanying with a reduction of lattice parameters which results in a positive shift of the Pt diffraction signals. As shown in Fig. 1, the diffraction peaks of the PtRu/MWCNT slightly shift towards larger diffraction angles compared with that of Pt/MWCNT which reveals solid solution alloy formation between Pt and Ru. Also, it is revealed that the positive shift of diffraction peaks increases at higher Bragg angles. Consequently, it could be concluded that microwave-assisted improved impregnation technique has a high efficiency of alloy phase formation between Pt and Ru. Fig. 2 displays the HR-TEM micrographs of the microwavesynthesized PtRu/MWCNT nanoparticles. As shown, the synthesized nanoparticles are formed and well dispersed on the external surface of the MWCNT particles. Also, Fig. 3 shows the HR-TEM micrographs of a PtRu/MWCNT that was prepared by using the improved aqueous impregnation technique. It can be observed that microwave irradiation enhances the dispersion and uniformity of the CNT supported nanoparticles. Typical HR-TEM micrographs were used to analyze the mean particle size and particle size distribution of the PtRu nanoparticles. The particle size analysis was performed by measuring the size of more than 300 particles in the HR-TEM micrographs. The histogram of particle size distribution of microwave-synthesized PtRu/MWCNT nanoparticles is presented in Fig. 4. As shown, most of the nanoparticles have a particle size between 1.5 and 3.5 nm. Also the mean size of the PtRu particles was 2.7 nm. The XPS was used to characterize the surface composition and valance state of the Pt and Ru species in the PtRu/MWCNT nanoparticles. Fig. 5 displays the XPS survey scan spectrum of the microwave-synthesized PtRu/MWCNT nanoparticles. As shown, the XPS peaks of Pt 4f, Ru 3p, C 1s, and O 1s were detected in the XPS survey scan spectrum. High-resolution XPS spectra of the Pt 4f and Ru 3p core-level regions of the PtRu/MWCNT nanoparticles are shown in Fig. 6. Since Ru 3d3/2 and Ru 3d5/2 core level region signals overlap with the C 1s peak, the Ru 3p region was used to investigate the valence state of the Ru species. According to Fig. 6a, the XPS spectra of the Pt 4f consisting of a doublet peak with the binding energies of 71.4 (Pt 4f7/2) and 74.7 eV (Pt 4f5/2). As shown, the Pt 4f spectra can be deconvoluted to the corresponding XPS signals of Pt (0), Pt (II) and Pt (IV) species. The results of the XPS analysis reveal that Pt present in the form of Pt, PtO and PtO2 species. Also, as shown in Fig. 6b, the XPS signals of the Ru 3p can be deconvoluted to corresponding XPS signals of the metallic Ru, anhydrous RuO2 and hydrous amorphous RuO2.xH2O. The amount of different species of Pt and Ru were calculated from the relative intensities of the corresponding deconvoluted signals in the high-resolution XPS spectra. The results of XPS analysis are summarized in Table 1. Also,
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Fig. 2. HR-TEM micrographs of PtRu/MWCNTs prepared by microwave-assisted improved impregnation technique.
the XPS analysis of a typical PtRu/MWCNT nanoparticles that was prepared by using an improved aqueous impregnation technique by means of conventional heating is presented in Table 1 for comparison [4]. According to XPS analysis, the surface composition of the PtRu/MWCNT nanoparticles is very close to the theoretically calculated composition (50:50). In addition, while a large content of Pt and Ru are present in the form of metallic species, XPS analysis indicate a partial oxidation of the Pt and Ru in the surface of PtRu/ MWCNT nanoparticles. However, according to the relative intensities of the XPS signals of the Pt and Ru, the proportion of surface oxide species in microwave-synthesized PtRu/MWCNT nanoparticles is less compared to that of conventional heating. 3.2. CO stripping of PtRu/MWCNT nanoparticles Currently, bimetallic PtRu is one of the most effective catalyst
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Fig. 5. XPS survey scan spectrum of the microwave-synthesized PtRu/MWCNT.
Fig. 3. HR-TEM micrographs of PtRu/MWCNTs prepared by conventional improved impregnation technique.
Fig. 4. Histogram of PtRu nanoparticle size distribution for the microwave-synthesized PtRu/MWCNT.
materials for energy conversion, sensing and biosensing applications. One of the main requirements of catalysts for these applications, is superior performance against poising which is caused by adsorption of impurities or intermediate products of the corresponding reactions. In the present study, the CO stripping experiment was performed to evaluate the poisoning resistance of the prepared PtRu/MWCNT nanoparticles. Fig. 7 displays the corresponding voltammograms of CO stripping in a 0.5 M H2SO4 solution. As shown, PtRu/MWCNT nanoparticles exhibited lower overpotential towards CO oxidation as compared with the commercial PtRu/C samples. According to CO stripping analysis, the microwave-synthesized PtRu/MWCNT catalyst exhibits a less positive peak potential (0.322 V vs. Ag/AgCl, 3.5 M KCl) with respect to PtRu/C catalyst (0.418 V vs. Ag/AgCl, 3.5 M KCl). Also, it can be observed that microwave-synthesized PtRu/MWCNT samples exhibited a slightly lower onset potential toward CO oxidation
Fig. 6. High-resolution (a) Pt 4f and (b) Ru 3p XPS spectra of the microwavesynthesized PtRu/MWCNT.
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Table 1 Binding energies and relative intensities of XPS analysis. Sample PtRu/MWCNT
Species [a]
Pt 4f7/2
Ru 3p3/2
PtRu/MWCNT
[b]
Pt 4f7/2
Ru 3p3/2
a b
Pt metal PtO PtO2 Ru metal RuO2 RuO2.xH2O Pt metal PtO PtO2 Ru metal RuO2 RuO2.xH2O
Binding energy (eV)
Relative intensity (%)
PtRu (at %)
71.7 72.8 74.3 462.11 463.7 466.51 71.7 72.79 74.33 462.1 463.7 466.51
72.15 18.9 8.95 68.07 16.71 15.21 73.8 16.2 10 64.33 12.56 23.12
53:47
53:47
Prepared by microwave-assisted improved impregnation method. Prepared by improved impregnation method [4].
4. Conclusion The efficiency of microwave-assisted improved impregnation method for synthesis of bimetallic PtRu alloy nanoparticles on MWCNT supporting materials was investigated. It was revealed that this technique could be effectively applied for synthesis of a high loading MWCNT supported PtRu nanoparticles. Also, it was found that PtRu nanoparticles with a high degree of alloy phase formation were uniformly dispersed on the supporting materials and exhibited an enhanced resistance against CO poisoning. According to results of the present study, it is concluded that microwave-assisted improved impregnation method could be recommended as a quick, simple and efficient method for preparation of homogeneous MWCNT supported PtRu nanoparticles. References
Fig. 7. CO stripping voltammograms in a 0.5 M H2SO4 aqueous solution; scan rate: 50 mV s1.
compare with that prepared under conventional heating by using a mantle heater. According to literature, the effective electrocatalytic activity of PtRu toward CO oxidation is due to a bifunctional mechanism between Pt and oxygen-containing species. Based on the bifunctional mechanism, the presence of Ru provides an oxygen source for CO oxidation at lower overpotentials [22]. It has been reported that bimetallic PtRu with Pt:Ru atomic ratio of 1:1 shows the superior performance for CO oxidation [44]. Also, the electrocatalytic activity of PtRu alloys for CO oxidation is directly affected by surface oxidation states [38]. According to literature, presence of a large content of the surface oxide species (especially for Ru) would decrease the OH nucleation rate and the COad oxidation reaction rate and thus results in positive shift of the COad stripping peak potential [14,45]. The results of XRD and XPS characterization of the microwave-synthesized PtRu/MWCNT nanoparticles reveals a high degree of alloy phase formation and a relatively low content of surface oxide species. This is in good agreements with the results of CO stripping experiment. Results of physicochemical characterization and CO stripping indicate that microwave-assisted improved impregnation method could be purposed as a fast and energy efficient technique for synthesis of PtRu/MWCNT nanoparticles which exhibit a superior performance against catalyst poisoning. A high degree of alloy phase formation and a satisfactory dispersion of bimetallic nanoparticles without using any organic stabilizing agent are the main advantages of this technique.
[1] L.H. Li, W.D. Zhang, J.S. Ye, Electrocatalytic oxidation of glucose at carbon nanotubes supported PtRu nanoparticles and its detection, Electroanalysis 20 (2008) 2212e2216. [2] F. Xiao, F. Zhao, D. Mei, Z. Mo, B. Zeng, Nonenzymatic glucose sensor based on ultrasonic-electrodeposition of bimetallic PtM (M¼ Ru, Pd and Au) nanoparticles on carbon nanotubeseionic liquid composite film, Biosens. Bioelectron. 24 (2009) 3481e3486. [3] M. Rahsepar, M. Pakshir, Y. Piao, H. Kim, Preparation of highly active 40 wt.% Pt on multiwalled carbon nanotube by improved impregnation method for fuel cell applications, Fuel Cells 12 (2012) 827e834. [4] M. Rahsepar, M. Pakshir, Y. Piao, H. Kim, Synthesis and electrocatalytic performance of high loading active PtRu multiwalled carbon nanotube catalyst for methanol oxidation, Electrochim. Acta 71 (2012) 246e251. [5] C.-T. Hsieh, Y.-W. Chou, W.-Y. Chen, Fabrication and electrochemical activity of carbon nanotubes decorated with PtRu nanoparticles in acid solution, J. Alloys Compd. 466 (2008) 233e240. [6] Y.-C. Hsieh, L.-C. Chang, Y.-C. Tseng, P.-W. Wu, J.-F. Lee, Structural characterizations of PtRu nanoparticles by galvanostatic pulse electrodeposition, J. Alloys Compd. 583 (2014) 170e175. [7] N. Kakati, J. Maiti, S.H. Jee, S.H. Lee, Y.S. Yoon, Hydrothermal synthesis of PtRu on CNT/SnO2 composite as anode catalyst for methanol oxidation fuel cell, J. Alloys Compd. 509 (2011) 5617e5622. [8] Y.-J. Song, S.-B. Han, J.-M. Lee, K.-W. Park, PtRu alloy nanostructure electrodes for methanol electrooxidation, J. Alloys Compd. 473 (2009) 516e520. [9] S. Tungatarova, K. Dossumov, T. Baizhumanova, N. Popova, Nanostructured supported Pt-, Ru-and Pt-Ru catalysts for oxidation of methane into synthesisgas, J. Alloys Compd. 504 (2010) S349eS352. [10] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Appl. Catal. A General 253 (2003) 337e358. [11] X. Wang, N. Li, J.A. Webb, L.D. Pfefferle, G.L. Haller, Effect of surface oxygen containing groups on the catalytic activity of multi-walled carbon nanotube supported Pt catalyst, Appl. Catal. B Environ. 101 (2010) 21e30. s, A. Sepúlveda-Escribano, J. Silvestre[12] E. Jardim, M. Goncalves, S. Rico-France Albero, Superior performance of multi-wall carbon nanotubes as support of Pt-based catalysts for the preferential CO oxidation: effect of ceria addition, Appl. Catal. B Environ. 113 (2012) 72e78. [13] M. Rahsepar, M. Pakshir, P. Nikolaev, Y. Piao, H. Kim, A combined physicochemical and electrocatalytic study of microwave synthesized tungsten mono-carbide nanoparticles on multiwalled carbon nanotubes as a co-catalyst for a proton-exchange membrane fuel cell, Int. J. Hydrogen Energy 39 (2014)
1328
M. Rahsepar, H. Kim / Journal of Alloys and Compounds 649 (2015) 1323e1328
15706e15717. [14] Y. Liang, J. Li, Q.-C. Xu, R.-Z. Hu, J.-D. Lin, D.-W. Liao, Characterization of composite carbon supported PtRu catalyst and its catalytic performance for methanol oxidation, J. Alloys Compd. 465 (2008) 296e304. [15] C. Yang, D. Wang, X. Hu, C. Dai, L. Zhang, Preparation and characterization of multi-walled carbon nanotube (MWCNTs)-supported Pt-Ru catalyst for methanol electrooxidation, J. Alloys Compd. 448 (2008) 109e115. [16] S. Wang, X. Wang, S.P. Jiang, PtRu nanoparticles supported on 1aminopyrene-functionalized multiwalled carbon nanotubes and their electrocatalytic activity for methanol oxidation, Langmuir 24 (2008) 10505e10512. [17] T. Maiyalagan, Synthesis and electro-catalytic activity of methanol oxidation on nitrogen containing carbon nanotubes supported Pt electrodes, Appl. Catal. B Environ. 80 (2008) 286e295. [18] M.S. Saha, R. Li, X. Sun, S. Ye, 3-D composite electrodes for high performance PEM fuel cells composed of Pt supported on nitrogen-doped carbon nanotubes grown on carbon paper, Electrochem. Commun. 11 (2009) 438e441. [19] M.S. Saha, R. Li, X. Sun, High loading and monodispersed Pt nanoparticles on multiwalled carbon nanotubes for high performance proton exchange membrane fuel cells, J. Power Sources 177 (2008) 314e322. [20] N. Rajalakshmi, H. Ryu, M. Shaijumon, S. Ramaprabhu, Performance of polymer electrolyte membrane fuel cells with carbon nanotubes as oxygen reduction catalyst support material, J. Power Sources 140 (2005) 250e257. [21] X. Wang, I.-M. Hsing, Surfactant stabilized Pt and Pt alloy electrocatalyst for polymer electrolyte fuel cells, Electrochim. Acta 47 (2002) 2981e2987. [22] V. Radmilovic, H. Gasteiger, P. Ross, Structure and chemical composition of a supported Pt-Ru electrocatalyst for methanol oxidation, J. Catal. 154 (1995) 98e106. [23] Z. Tang, D. Geng, G. Lu, Size-controlled synthesis of colloidal platinum nanoparticles and their activity for the electrocatalytic oxidation of carbon monoxide, J. Colloid Interface Sci. 287 (2005) 159e166. [24] F. Bonet, V. Delmas, S. Grugeon, R.H. Urbina, P. Silvert, K. Tekaia-Elhsissen, Synthesis of monodisperse Au, Pt, Pd, Ru and Ir nanoparticles in ethylene glycol, NanoStructured Mater. 11 (1999) 1277e1284. [25] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Shapecontrolled synthesis of colloidal platinum nanoparticles, Science (1996) 1924e1925. New York Then Washington. [26] T. Schmidt, H. Gasteiger, R. Behm, Methanol electrooxidation on a colloidal PtRu-alloy fuel-cell catalyst, Electrochem. Commun. 1 (1999) 1e4. [27] N. Toshima, Y. Wang, Preparation and catalysis of novel colloidal dispersions of copper/noble metal bimetallic clusters, Langmuir 10 (1994) 4574e4580. [28] Z. Liu, L.M. Gan, L. Hong, W. Chen, J.Y. Lee, Carbon-supported Pt nanoparticles as catalysts for proton exchange membrane fuel cells, J. Power Sources 139 (2005) 73e78. [29] X. Li, S. Ge, C. Hui, I.-M. Hsing, Well-dispersed multiwalled carbon nanotubes supported platinum nanocatalysts for oxygen reduction, Electrochem. SolidState Lett. 7 (2004) A286eA289. [30] Y. Xing, Synthesis and electrochemical characterization of uniformlydispersed high loading Pt nanoparticles on sonochemically-treated carbon
nanotubes, J. Phys. Chem. B 108 (2004) 19255e19259. [31] C.-L. Lee, Y.-C. Ju, P.-T. Chou, Y.-C. Huang, L.-C. Kuo, J.-C. Oung, Preparation of Pt nanoparticles on carbon nanotubes and graphite nanofibers via selfregulated reduction of surfactants and their application as electrochemical catalyst, Electrochem. Commun. 7 (2005) 453e458. [32] Z. Liu, J.Y. Lee, W. Chen, M. Han, L.M. Gan, Physical and electrochemical characterizations of microwave-assisted polyol preparation of carbonsupported PtRu nanoparticles, Langmuir 20 (2004) 181e187. [33] X. Zhao, W. Li, L. Jiang, W. Zhou, Q. Xin, B. Yi, G. Sun, Multi-wall carbon nanotube supported PteSn nanoparticles as an anode catalyst for the direct ethanol fuel cell, Carbon 42 (2004) 3263e3265. [34] H. Kim, N.P. Subramanian, B.N. Popov, Preparation of PEM fuel cell electrodes using pulse electrodeposition, J. Power Sources 138 (2004) 14e24. [35] A.M.C. Luna, G.A. Camara, V.A. Paganin, E.A. Ticianelli, E.R. Gonzalez, Effect of thermal treatment on the performance of CO-tolerant anodes for polymer electrolyte fuel cells, Electrochem. Commun. 2 (2000) 222e225. [36] M. Rahsepar, M. Pakshir, H. Kim, Synthesis of multiwall carbon nanotubes with a high loading of Pt by a microwave-assisted impregnation method for use in the oxygen reduction reaction, Electrochim. Acta 108 (2013) 769e775. [37] M. Rahsepar, M. Pakshir, P. Nikolaev, A. Safavi, K. Palanisamy, H. Kim, Tungsten carbide on directly grown multiwalled carbon nanotube as a co-catalyst for methanol oxidation, Appl. Catal. B Environ. 127 (2012) 265e272. [38] J. Lee, J. Seo, K. Han, H. Kim, Preparation of low Pt loading electrodes on Nafion (Naþ)-bonded carbon layer with galvanostatic pulses for PEMFC application, J. Power Sources 163 (2006) 349e356. [39] S. Woo, I. Kim, J.K. Lee, S. Bong, J. Lee, H. Kim, Preparation of cost-effective PteCo electrodes by pulse electrodeposition for PEMFC electrocatalysts, Electrochim. Acta 56 (2011) 3036e3041. [40] Z. Zhou, S. Wang, W. Zhou, L. Jiang, G. Wang, G. Sun, B. Zhou, Q. Xin, Preparation of highly active Pt/C cathode electrocatalysts for DMFCs by an improved aqueous impregnation method, Phys. Chem. Chem. Phys. 5 (2003) 5485e5488. [41] Z. Zhou, W. Zhou, S. Wang, G. Wang, L. Jiang, H. Li, G. Sun, Q. Xin, Preparation of highly active 40wt.% Pt/C cathode electrocatalysts for DMFC via different routes, Catal. Today 93 (2004) 523e528. [42] I. Kim, S. Bong, S. Woo, R.K. Mahajan, H. Kim, Highly active 40 wt.% PtRu/C anode electrocatalysts for PEMFCs prepared by an improved impregnation method, Int. J. Hydrogen Energy 36 (2011) 1803e1812. [43] M. Oghbaei, O. Mirzaee, Microwave versus conventional sintering: a review of fundamentals, advantages and applications, J. Alloys Compd. 494 (2010) 175e189. [44] R.J. Wiltshire, C.R. King, A. Rose, P.P. Wells, H. Davies, M.P. Hogarth, D. Thompsett, B. Theobald, F.W. Mosselmans, M. Roberts, Effects of composition on structure and activity of PtRu/C catalysts, Phys. Chem. Chem. Phys. 11 (2009) 2305e2313. [45] C. Bock, M.-A. Blakely, B. MacDougall, Characteristics of adsorbed CO and CH 3 OH oxidation reactions for complex Pt/Ru catalyst systems, Electrochim. Acta 50 (2005) 2401e2414.