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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Particuology 9 (2011) Contents lists available at ScienceDirect Particuology j o ur nal homep ag e: Synthesis and characterization of Cu Pt/MWCNTs for use in electrocatalytic applications Yuh-Jing Chiou a, Hang-Jui Hsu a, Hong-Ming Lin a,, Wei-Jen Liou a, Huey-Wen Liou a, She-Huang Wu a, Shu-Hua Chien b a Department of Materials Engineering, Tatung University, 40, Chungshan N. Rd., 3rd Sec, 104 Taipei, Taiwan, China b Institute of Chemistry, Academia Sinica, 128 Academia Road, Sec. 2, Nankang, 115 Taipei, Taiwan, China a r t i c l e i n f o Article history: Received 15 November 2010 Received in revised form 3 January 2011 Accepted 11 January 2011 Keywords: Electrocatalysis Methanol Multi-walled carbon nanotubes Nanoparticles Platinum Copper a b s t r a c t Improving catalyst performance by studying the preparation parameters, material structures and analysis methods is important work for catalyst researchers. In this study, Cu Pt nanocomposites with different metal ratios were synthesized on MWCNTs using a polyol method. The structures and morphologies of the nano hybrid catalysts were analyzed by XRD and TEM. The XRD results indicate that Pt and Cu formed a bimetallic solid solution. The average diameters of the Pt and Cu Pt nanoparticles on MWCNTs were between 3 10 nm and 5 15 nm, respectively. The electrocatalytic behavior of methanol oxidation reactions was investigated by cyclic voltammetry (CV). Cu(20%)/[Pt/MWCNTs(1:4)], which contained an optimal degree of solid solution, enhanced the methanol oxidation to the greatest degree Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction The direct methanol fuel cell (DMFC) is a good candidate for a power source in portable electronic devices because of its high energy density, ease of handling the liquid fuel and low operating temperatures (Ha, Zhu, & Masel, 2004; Jung, Han, & Ha, 2007; Larsen, Ha, Zakzeski, & Masel, 2006; Miesse et al., 2006; Yu & Pickup, 2008; Zhang, Lu, Bao, Tang, & Li, 2006). Because of its excellent catalytic activity, platinum has been shown to be a good catalyst for the low temperature electrooxidation of methanol, which makes this metal a suitable electrocatalyst for use in DMFC anodes. Alloy catalysts have significantly better catalytic activities for oxygen reduction. The mechanisms responsible for the enhanced activity of platinum alloy catalysts for oxygen reduction have been studied extensively in recent years (Baldauf & Kolb, 1996; Chen, Kim, Sun, & Chen, 2007; Chi, Chan, & Phillips, 2001; Obradovic, Tripkovic, & Gojkovic, 2009; Page, Johnson, Hormes, Noding, & Rambabu, 2000; Pletcher & Solis, 1982; Tripkovic, Popovic, Stevanovic, Socha, & Kowal, 2006). Methanol oxidation is a reaction of great technological relevance because of methanol s potential use as a fuel in DMFCs. In the search for efficient electrocatalysts that would remedy Pt-based anode poisoning by a carbonaceous Corresponding author. addresses: hmlin@ttu.edu.tw, hmlin99@gmail.com (H.-M. Lin). intermediate (most likely CO), many polymetallic systems have been studied in the form of atom surface layers on both Pt and binary Pt alloys. The rate of methanol oxidation for Cu Pt binary systems (Page et al., 2000; Stassi et al., 2006) depends, in general, on the content of the second metal, the degree of its alloying with Pt, the electrode pretreatment, and the type of substrate. A guide to the design of efficient binary catalysts of methanol oxidation is provided in an excellent review by Ishikawa, Liao, and Cabrera (2004). They presented theoretical predictions for the effects of the second metal on the three key steps of the reaction (namely, methanol dissociative chemisorption, CO poison adsorption, and CO removal via its oxidation by adsorbed OH) for Cu-Pt systems. These effects are due to the ligand effect modification of Pt s electronic properties by the second metal and to a synergistic effect bifunctional mechanism (Watanabe & Motoo, 1975), whereby the second metal disrupts the continuity of the Pt lattice, provides sites for OH adsorption, and improves both the activity and stability of the original single-metal catalyst. Carbon nanotubes (CNTs), a new type of carbon material, have been used in numerous applications in catalysis. In recent years, more attention has been paid to their use as the support for catalysts in the development of fuel cells because of their high surface area, excellent electronic properties, special chemical stability, and hollow geometry (Moraes, Matsubara, & Rosolen, 2008; Guo & Li, 2004). Furthermore, the purification of CNTs can create more surface binding sites and anchoring groups for precursor /$ see front matter 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. doi: /j.partic

3 Y.-J. Chiou et al. / Particuology 9 (2011) metal ions or metal nanoparticles (Wildgoose, Banks, & Compton, 2006). Thus, purified multi-walled carbon nanotubes (MWCNTs)- supported Cu Pt catalysts could catalyze the oxidation reaction of methanol more effectively. The purpose of this study was to synthesize hybrid systems based on MWCNTs. Pt/MWCNTs catalysts and a series of bimetallic Cu Pt/MWCNTs catalysts were synthesized using a polyol process. The electrooxidation of methanol by the hybrid Cu Pt catalysts was investigated and compared with that of the hybrid Pt catalyst. The effects of Cu on the activity and stability of Pt for the oxidation of methanol were determined. 2. Methods and materials As-prepared multi-walled carbon nanotubes (CNT Co., Ltd., Korea; about 93% purity) were refluxed in nitric acid for 6 h to remove impurities and catalyst particles. The purified MWC- NTs were well dispersed in ethylene glycol by ultrasound for 1 h. Then, the solution was stirred and heated to 170 C, aqueous H 2 PtCl 6 6H 2 O (Wako) and/or Cu(NO 3 ) 2 3H 2 O (Nihon Shiyaku) precursor solutions were added, and the ph was adjusted to approximately 7 by adding aqueous KOH solution. The solution was kept at 170 C for 20 min. The product powders were collected by centrifugation, followed by drying on a hot plate at 100 C. Then, the powders were annealed under a 5% H 2 /Ar atmosphere at 200 C for 1 h. An X-ray diffractometer with a maximum power of 21 kw (MAC Science M21X, Brucker-AXS) at the Precise Instrument Center of Tatung University was employed for phase identification and structural determination using a Cu K line ( = nm) and a scan angle range from 10 to 90. The electrochemical reactivity of the catalysts was characterized by CV measurement using a three-electrode cell and an EG&G Instrument Model 283 potentiostat/galvanostat instrument located at Tatung University. In this characterization process, 5 mg catalyst and 5 mg carbon black (Vulcan xc-72) were dispersed in 0.05 ml Nafion (DuPont Fluoroproducts, DE-2021, 5 wt%) and 5 ml ethanol (Showa Chemical Co., Ltd., 99.5 vol%) for 1 h. Then, the solution was deposited on a glassy carbon disc (Toyo Tanso, IG- 45, 9.0 m) by injection at 100 C, and the disc was dried at 200 C. The working electrode consisted of ink-covered glassy carbon that was held in a Teflon holder. The diameter of exposed area was 7 mm. Platinum gauze and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. All measured potentials are quoted versus the Ag/AgCl reference electrode. The CV measurement was conducted at a rotating electrode velocity of 1600 rpm and a scan rate of 10 mv/s, with a scanning potential range from 0.2 to 0.9 V in an electrolyte solution of 1 M CH 3 OH with 2 M H 2 SO 4 at 25 C. During each CV experiment, oxygen was removed from the solution, and the whole test system was protected by purging with nitrogen gas. Table 1 Formulas of Cu Pt/MWCNTs. Cu Pt/MWCNTs Cu (wt%) Pt (wt%) MWCNTs (wt%) 5%Cu/[Pt/MWCNTs(1:4)] %Cu/[Pt/MWCNTs(1:4)] %Cu/[Pt/MWCNTs(1:4)] %Cu/[Pt/MWCNTs(1:4)] %Cu/[Pt/MWCNTs(1:4)] %Cu/[Pt/MWCNTs(1:4)] the graphite (0 0 2) plane of the MWCNTs. In curve A, the other four peaks are characteristic of face-centered cubic (fcc) crystalline Pt (JCPDS, Card No ), corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes at 2 of 40.2, 46, 67 and 81, respectively. In curve B, the other four peaks are characteristic of face-centered cubic (fcc) crystalline Cu (JCPDS, Card No ), corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes at 2 of 43.5, 50.5, 74 and 90, respectively. It was found that the peaks of the Cu Pt/MWCNTs in the (1 1 1) plane (curves C, D, E and F) were located between the peaks of the respective (1 1 1) planes in Pt(20%)/MWCNTs (curve A) and Cu(20%)/MWCNTs (curve B), indicating that Cu and Pt may have formed a solid solution structure. For catalysts containing greater amounts of Cu, the peaks shifted to a higher angle (near Cu). For the curves for Cu contents greater than 20 wt%, curves G and H, these solid solution peaks were less intense, and pure Cu peaks appeared. This result indicates that the addition of more Cu promotes the formation of pure Cu particles. This hypothesis will be tested by morphology analysis. High-resolution transmission electron microscopy (HRTEM) images of Pt/MWCNTs, Cu/MWCNTs, and Cu Pt/MWCNTs are shown in Fig. 2. For Pt(20%)/MWCNTs, Pt nanoparticles were individually separated on the surface of MWCNTs via a polyol process, and their sizes were approximately 3 10 nm in diameter. For Cu(20%)/MWCNTs, Cu nanoparticles were dispersed on MWCNTs, and the sizes of Cu nanoparticles were also approximately 3 10 nm. The other images of catalysts with various Cu contents show that the mean size of the bimetallic Cu Pt particles was approximately 5 10 nm. In Fig. 3, the higher magnification image for Cu(20%)/[Pt/MWCNTs(1:4)] reveals the detail of the nanoparticles. 3. Results and discussion To facilitate the comparison of the characteristics of the Cu Pt/MWCNTs catalysts, the ratio of Pt to MWCNTs was kept at 1 4. The formulas shown in Table 1 are the weight ratios of Cu, Pt and MWCNTs of the Cu Pt/MWCNTs nanomaterials. Asprepared Pt/MWCNTs and those including Cu were analyzed by thermal gravimetric analysis (TGA), and the residual weight percentages were consistent with the nominal compositions. Fig. 1 shows the XRD patterns of the synthesized catalysts after annealing under an H 2 /Ar atmosphere for 1 h at different Cu Pt compositions. The XRD peaks at 2 of 26 in curves A to H correspond to Fig. 1. XRD patterns of Pt/MWCNTs, Cu/MWCNTs and Cu Pt/MWCNTs after annealing at 200 C in an H 2/Ar atmosphere for 1 h.

4 524 Y.-J. Chiou et al. / Particuology 9 (2011) Fig. 2. High-resolution TEM images of (a) Pt(20%)/MWCNTs; (b) Cu(20%)/MWCNTs; (c) Cu(5%)/[Pt/MWCNTs(1:4)]; (d) Cu(10%)/[Pt/MWCNTs(1:4)]; (e) Cu(15%)/[Pt/MWCNTs(1:4)]; (f) Cu(20%)/[Pt/MWCNTs(1:4)]; (g) Cu(25%)/[Pt/MWCNTs(1:4)]; and (h) Cu(30%)/[Pt/MWCNTs(1:4)]. For the catalysts with Cu contents of less than 20%, the d-spaces of each particle could be measured and the corresponding 2 values were calculated. For those catalysts containing more Cu, the d-spaces were difficult to recognize. Fig. 4 shows the calculated lattice constant of the (1 1 1) plane of the synthesized catalysts with different compositions. The theoretical line was established by using the mixing rule and the pure metal lattice constants, Å and Å for Pt(1 1 1) and Cu(1 1 1), respectively. The lattice constants of synthesized Cu Pt/MWCNTs were slightly higher than that of the theoretical value. Fig. 5 shows the methanol electrooxidation activities of Cu Pt/MWCNTs with different weight ratios of Cu compared to Pt(20%)/MWCNTs by cyclic voltammetry in 1 M CH 3 OH + 2 M H 2 SO 4 with (a) forward scanning and (b) backward scanning for 20 cycles. According to the results in this figure, Cu(20%)/MWCNTs had no redox activity. However, synthesized Cu Pt/MWCNTs catalysts had higher electrooxidation activities than Pt(20%)/MWCNTs catalyst, and the activities increased with the amount of Cu in Cu Pt/MWCNTs. Moreover, the highest current density of Cu(20%)/[Pt/MWCNTs(1:4)] (320 ma/mg Pt) was almost double that of Pt(20%)/MWCNTs (130 ma/mg Pt) for the redox

5 Y.-J. Chiou et al. / Particuology 9 (2011) Fig. 4. Lattice constants of synthesized Cu Pt/MWCNT catalysts in the (1 1 1) plane. Fig. 3. HRTEM image of Cu(20%)/[Pt/MWCNTs(1:4)]. reaction. For Cu(20%)/[Pt/MWCNTs(1:4)], the larger particle size may result in less active surface area, but we propose that the formation of a solid solution of Cu and Pt enhanced the methanol electrooxidation activity. Nevertheless, the redox peaks of Cu(25%)/[Pt/MWCNTs(1:4)] and Cu(30%)/[Pt/MWCNTs(1:4)] were lower than that of Pt(20%)/MWCNTs, which may result from the redundant Cu present around the Cu Pt nanoparticles. These redundant Cu particles could occupy the reactive sites of the catalysts and decrease the activity. Fig. 5. Cyclic voltammograms of methanol oxidation of Pt(20%)/MWCNTs, Cu(20%)/MWCNTs and Cu Pt/MWCNTs with (a) forward scanning and (b) backward scanning at the 20th cycle. The scan rate was 10 mv/s. Fig. 6. Cyclic voltammograms of methanol oxidation of Cu(20%)/[Pt/MWCNTs(1:4)] with (a) forward scanning and (b) backward scanning for various scan cycles. The scan rate was 10 mv/s.

6 526 Y.-J. Chiou et al. / Particuology 9 (2011) Conclusions Fig. 7. CV current density of Pt/MWCNTs(1:4) and Cu(20%)/[Pt/MWCNTs(1:4)] at 0.75 V for forward scanning for various cycle numbers. Fig. 6 shows the cyclic voltammetry analyses of the Cu(20%)/[Pt/MWCNTs(1:4)] catalyst in 1 M CH 3 OH + 2 M H 2 SO 4 with (a) forward scanning and (b) backward scanning for various cycles. The activity of the catalyst decayed as the cycle number increased for both forward and backward scanning. Comparing the performances of Cu(20%)/[Pt/MWCNTs(1:4)] and Pt/MWCNTs(1:4), as shown in Fig. 7, we found that the current density of the former was always much higher than that of the latter, regardless of how many cycles had occurred. As shown in Fig. 8, the CV performance after 20 cycles was the highest for Cu(20%)/[Pt/MWCNTs(1:4)]. This high CV performance is the result of the solid solution having a valence band (for the Cu Pt bimetal) that is lower than that of pure Pt. This lower valence band causes a decrease in the activation energy for bimetallic catalyst. As more Cu is added (25% or more), Cu metal crystals form and separate from the solid solution. The activities of the catalysts with higher Cu contents were lower than that of Cu(20%)/[Pt/MWCNTs(1:4)]. Fig. 8. CV current density of Cu/[Pt/MWCNTs(1:4)] at 0.75 V for forward scanning for different Cu contents after 20 cycles. Pt/MWCNTs, Cu/MWCNTs and Cu Pt/MWCNTs nanomaterials can be synthesized successfully via a polyol process. Moreover, globular Pt, Cu and Cu Pt nanoparticles can be formed individually on the surfaces of MWCNTs with an average diameter of 3 10 nm. This nano size shows that the metal catalysts can be dispersed well on the surface of MWCNTs, resulting in good catalytic ability. The addition of Cu can enhance the methanol oxidation reaction if the Cu weight percentage is between 5% and 20%. In this range, more Cu will result in the formation of more Cu Pt solid solution and will produce larger particles. The increased metal catalyst particle size could cause a decrease in the size of the active area. However, the dominant effect is the bimetallic structure. In this structure, the formation of a solid solution results in a lower valance band energy, which mean that the catalysts have a lower activation energy. When the amount of Cu exceeded 20%, the performance of the hybrid electrocatalysts decreased; they were less active and less stable. With respect to the structure, some crystalline Cu separated from the Cu Pt solid solution and occupied the active sites. We conclude that catalysts with a higher degree of formation of Cu Pt solid solution are more efficient DMFC electrocatalysts. Acknowledgments The authors gratefully acknowledge the financial support of the Taiwan National Science Council. References Baldauf, M., & Kolb, D. M. (1996). 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