Synthesis of carbon nanotube supported Pt-Sn nanoparticles by replacement reaction and their electrocatalytic properties for ethanol oxidation

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1 Indian Journal of Chemistry Vol. 48A, October 2009, pp Synthesis of carbon nanotube supported Pt-Sn nanoparticles by replacement reaction and their electrocatalytic properties for ethanol oxidation B Zhang, Y J Kuang, H L Pang, B Liu, J H Chen* & X H Zhang* State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering Hunan University, Changsha, , PR China chenjinhua@hnu.cn/guoxin512@tom.com Received 5 June 2009; revised and accepted 1 September 2009 Carbon nanotube supported Pt-Sn nanoparticles have been prepared by using a replacement reaction method. The micrography, elemental composition and structure analysis of Pt-Sn catalysts have been studied by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffractograms, respectively. The electrocatalytic activity of Pt-Sn/CNTs for ethanol electrooxidation has been investigated by cyclic voltammetry in 1.0 M CH 3 CH 2 OH M H 2 SO 4 solution. The results show that the obtained Pt-Sn/CNT nanoparticles exhibit higher electrocatalytic activity and better longterm stability for ethanol oxidation as compared to the PtSn/CNT nanoparticles prepared by the conventional coreduction method. Keywords: Fuel cells, Catalysts, Carbon nanotubes, Nanomaterials, Electrocatalysis, Pt-Sn nanoparticles, Replacement reactions, Oxidations, Ethanol oxidation, Electrooxidations IPC Code: Int. Cl. 9 B01J21/18C; C25B3/02; H01M4/00 Electrooxidation of alcohols including methanol and ethanol is attracting more and more extensive studies in view of the important applications in fuel cells. Ethanol is considered a more attractive alternative renewable fuel due to some of its advantages such as low toxicity, good stability, low permeability across proton exchange membranes 1, and higher energy density ( KJ mol -1 ) at room temperature as compared to methanol 2 ( KJ mol -1 ). Additionally, ethanol can be produced indirectly in large quantities by fermentation of biomass. However, the complete eletrooxidation of ethanol, which involves a transfer of 12 electrons per ethanol molecule and the cleavage of C-C bond 3, is more difficult and complicated than that of methanol. Searching or designing effective catalytic systems, hence, are particularly important for direct ethanol fuel cells (DEFC). Recently, with the development of fuel cells, various metallic electrocatalysts have been reported extensively in literatures 4,5. With respect to the electrocatalytic efficiency, unsupported or supported Pt catalysts are considered indispensable and are the most effective catalysts for alcohol oxidation 6. The practical application of Pt catalysts in DEFC, however, is still impeded by catalyst poisoning due to CO-like intermediates or other byproducts produced during the process of ethanol electrooxidation and also by their prohibitive cost 7. Platinum is often alloyed or combined with a second precious or non-precious metal (like Ru, Pb, Sb, Rh, Sn, etc. 5,8-11, which are referred to as bimetallic catalysts) to enhance its electrocatalytic activity and tolerance to poisoning reaction intermediates 12 by the so-called bifunctional or ligand mechanisms. According to the bifunctional mechanism 13, usually an oxygenated surface is created by dissociating water on the secondary metallic sites at potentials lower than that of Pt. This promotes the oxidation of adsorbed CO to CO 2 leading to a decrease in the CO poisoning, thus improving the CO tolerance. Contrary to this, Tillmann et al. 14,15 have reported the ligand mechanism wherein the secondary metal changes the chemical properties of Pt at the surface such that the Pt CO bond strength is reduced, and hence minimizing CO poisoning. Among these Pt-based binary catalysts, to date, PtRu catalysts are considered as most suitable for methanol oxidation 16-20, whereas in the case of ethanol electrooxidation, PtSn nanoparticles have been demonstrated to be a more attractive materials 12,

2 1346 INDIAN J CHEM, SEC A, OCTOBER 2009 In practice, the activity of Pt-based catalysts is closely related not only to the chosen second elements but also to the synthetic route. Till date, PtSn electrocatalysts have been prepared with various electrochemical and chemical methods as well as physical methods. For example, Xin et al. 30 have prepared and characterized PtSn/C anode electrocatalysts for direct ethanol fuel cell using a direct heating method. Tanaka et al. 31 have prepared the PtSn binary electrocatalysts to further improve the performance for anodic oxidation of ethanol by employing the co-sputtering process. Recently, Zheng et al. 32 relied on the hydrothermal method to prepare the PtSn bimetallic nanoparticles which show enhanced electrocatalytic activity and lifetime for the electrooxidation of methanol as compared with those of Pt nanoclusters. Amongst the various methods, one of the most popular synthesis procedures is the coreduction of Pt and Sn by appropriate reducing agents 11,32,33, which probably leads to the formation of PtSn alloys or monometallic Pt and Sn mixtures depending on the synthesis procedure. Up to now, mechanism of the promoting effect of Sn for ethanol oxidation is not well understood, and the effects of alloyed or unalloyed PtSn/C catalyst on ethanol electrooxidation reaction still need to be investigated. Nevertheless, it is noteworthy that the procedure followed to prepare PtSn catalysts has a marked influence on the composition and structure of the resulting material and, consequently, on its electrocatalytic activity towards ethanol oxidation. Better ways to prepare more efficient PtSn electrocatalysts for ethanol oxidation should be investigated. A replacement reaction is a simple approach to synthesize a number of nanostructured materials 34. An important feature of this techanique is that metal deposition is a surface controlled reaction, where the total amount of deposited metals is defined by stoichiometry of the redox reaction 35. It has also been found that depositing one metal onto the surface of another metal by replacement reaction can often introduce unique physical and chemical properties due to the electronic and structural interactions at the metal-metal interface 36. The method can be applied to a wide variety of metallic systems. For example, Wang et al. 37 have prepared powder Pt/Ni bimetallic catalysts by replacement reactions, which showed higher hydrogenation activity for C=C and C=O bonds than catalysts with the same Pt loading but prepared by the impregnation method. Sun et al. 38 have demonstrated that the replacement reaction can be used to prepare metal nanostructures with controllable geometric shapes and structures, and thus their physical and chemical properties. However, to the best of our knowledge, there has been no study on the Pt-Sn bimetallic catalysts prepared by replacement reaction for ethanol electrooxidation. In the present work, carbon nanotubes (CNTs) have been used as catalyst support due to the excellent electronic properties of CNTs in DEFC and CNT supported PtSn nanoparticles (Pt-Sn/CNTs) have been prepared by using a chemical replacement reaction method. The electrocatalytic properties of the Pt-Sn/CNTs catalysts for ethanol oxidation have also been investigated by the typical electrochemical methods. Materials and Methods Preparation of Pt-Sn/CNTs nanocatalysts CNTs (dia.: nm, purchased from Shenzhen Nanotech. Port. Co. Ltd., China) was used as the catalyst support. H 2 PtCl 6 6H 2 O and SnCl 2 2H 2 O were used as the precursors of the Pt-Sn catalysts. Unless otherwise stated, all chemicals were of analytical grade and used as received. Doubly distilled water was used throughout. Pt-Sn/CNT nanocatalysts with a 1:2 atomic ratio of Pt : Sn were prepared via the replacement reaction, described as follows. Firstly, to introduce more binding sites and surface anchoring groups like carboxyl 39,40, CNTs were refluxed in 100 ml of 98% H 2 SO 4 and 65% HNO 3 (3:1, v:v) at 100 ºC for 6 h. Secondly, the oxidised CNTs were mixed with a definite amount of SnCl 2 in a rockered flask with ethanol as the impregnant. Then excess sodium borohydride (NaBH 4 ) solution was added dropwise to the mixture as a reducing agent under vigorous ultrasonic stirring for 30 min. After further ultrasonic treatment for 3 h to remove the residual NaBH 4, a solution of H 2 PtCl 6 was added dropwise to the above resulting CNT supported Sn (Sn/CNTs) suspension with vigorous magnetic stirring for 3 h at room temperature. This led to the oxidation of Sn nanoparticles and the reduction of PtCl 2-6, that is to say, the Sn surface atoms were partly substituted by a layer of Pt atoms to produce Pt-Sn nanoparticles. After filteration and washing with doubly distilled water for several times, the final product, labelled as Pt-Sn/CNTs, was dried in vacuum at 60ºC for 8 h. The entire procedure was carried out under nitrogen atmosphere.

3 ZHANG et al.: SYNTHESIS OF CNT SUPPORTED PT-NANOPARTICLES BY REPLACEMENT REACTION 1347 Pt and Sn contents and the elemental composition in the Pt-Sn/CNTs catalysts were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and energy dispersive X-ray spectroscopy (EDS, INCA), respectively. The morphology and structure of the Pt-Sn/CNT catalysts were investigated by scanning electron microscopy (SEM, JSM-6700F) and X-ray diffractograms (XRD), respectively. Electrochemical properties of Pt-Sn/CNT nanocatalysts The electrochemical performance of the Pt-Sn/CNTs was investigated in 1.0 M CH 3 CH 2 OH M H 2 SO 4 aqueous solutions by typical electrochemical methods, on a CHI 660A electrochemical working station (CH Instrument, Inc.) at room temperature. A standard three-electrode system was employed with a platinum foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The graphite electrode loaded with a definite mass of Pt-Sn/CNT catalyst with an exposure area of 0.72 cm 2 was used as the working electrode. For preparing the Pt-Sn/CNTs/graphite electrode, 5 mg of Pt-Sn/CNTs was first sonicated for 30 min in 5ml fresh doubly distilled water, and then 200 L of the above solution was transferred onto the surface of the polished graphite electrode (0.72 cm 2 ) by a microsyringe. The working electrode was finally dried under an infrared lamp. The quantity of Pt in the Pt-Sn/CNT catalyst loaded on the graphite electrode was 10 g. All potential values given below are referred to SCE. For comparision, control samples of CNTsupported PtSn bimetallic nanoparticle catalysts, denoted as PtSn/CNTs, were fabricated via conventional coreduction of H 2 PtCl 6 and SnCl 2 with freshly prepared NaBH 4 solution as the reductant. The atomic ratio of Pt : Sn for the PtSn/CNT catalyst, determined by ICP-AES, is 1:1.98. Besides, Pt/CNT catalyst was also prepared following a similar process mentioned above with only H 2 PtCl 6 as the precursor. The electrochemical performances of the PtSn/CNTs/graphite and Pt/CNTs/graphite electrodes (the quantity of Pt in the above catalysts is the same as that in the Pt-Sn/CNT catalyst) were also studied. Results and Discussion Characterization of the Pt-Sn/CNT nanocatalysts SEM micrographs of all samples were recorded to study the catalyst morphology, however, only the most representative examples are presented in Fig. 1. It can be seen clearly that Pt-Sn/CNT nanoparticles (Fig. 1a) have a smaller particle size and more uniform distribution than PtSn/CNT (Fig. 1b) and Pt/CNT (Fig. 1c) nanoparticles. (The mean particle size of Pt-Sn/CNT, PtSn/CNT, and Pt/CNT is about 2.9, 3.9, and 4.8 nm, respectively). To identify the elemental composition of the Pt-Sn/CNT nanoparticles, EDS investigation was carried out. The corresponding result is shown in Fig. 1d. It can be seen that Pt, Sn, Cu and C are the major elements in the Pt-Sn/CNT nanoparticles. Cu comes from the Cu foil, while the atomic Pt-Sn ratio is 1:2, which is close to the result obtained from ICP-AES (1:1.94). The structure of the Pt-Sn/CNT nanoparticles was further investigated by XRD (Fig. 2). XRD diffraction patterns were collected by using a fixed Cu K radiation. For comparison, the XRD patterns of the PtSn/CNT, the Sn/CNT and the Pt/CNT nanoparticles are also presented in Fig. 2. The diffraction peaks observed on Pt-Sn/CNT (Fig. 2a), PtSn/CNT(Fig. 2b) and Pt/CNT(Fig. 2d) catalysts at around 25 correspond to the (002) plane of the hexagonal structure of CNT. The diffraction peaks at around 39, 46 and 68 correspond to Pt (111), (200) and (220) planes, respectively, which represent the typical character of a crystalline face centered cubic (fcc) phase of Pt. For Sn/CNTs (Fig. 2c), three high diffraction peaks of SnO 2, (110), (101) and (211) planes are observed at around 26, 34 and 52, implying that Sn can be oxidized to SnO 2 in the synthesis procedure of the catalyst. From Fig. 2a and Fig. 2b, it can be clearly observed that the diffraction peaks indexed to SnO 2 (110), (101) and (211) planes are consistent with those on Sn/CNTs and the diffraction peak positions of Pt (111) and (200) planes are shifted to a slightly higher angle than that of Pt/CNT catalyst. These results demonstrate that Pt and Sn are present mainly in the mixture form in the synthesized Pt-Sn/CNT and PtSn/CNT nanoparticles. However, alloy formation between Pt and Sn cannot be discarded because these may be present in small amounts. Besides, as compared with those on Pt/CNT catalysts, the diffraction peaks on Pt- Sn/CNT and PtSn/CNT nanoparticles are broadened, probably indicating the presence of smaller Pt nanoparticles in the binary metallic catalysts. The diffraction peak of the Pt (220) plane is usually selected to calculate the crystallite size D of nanoparticles as follows 41 : D = 0.9λ / βcos θ, where,

4 1348 INDIAN J CHEM, SEC A, OCTOBER 2009 Fig. 1 SEM images of Pt-Sn catalysts. [a, Pt-Sn/CNTs; b, PtSn/CNTs; c, Pt/CNTs]. d, EDS pattern of Pt-Sn/CNTs ]. Fig. 2 XRD patterns of the Pt-Sn catalysts. [a, Pt-Sn/CNTs; b, PtSn/CNTs; c, Pt/CNTs; d, Sn/CNTs nanoparticles]. and are the X-ray wavelength ( nm), Bragg diffraction angle, and the full width at halfmaximum of the diffraction peak, respectively. According to Fig. 2 and D, the mean particle size of Pt-Sn/CNT and PtSn/CNT catalysts calculated from the XRD patterns is 2.5 nm and 4.5 nm, respectively, which agrees with the results observed in the SEM image. The results of XRD measurements show that the size of Pt particles in the Pt-Sn/CNT and PtSn/CNT sample is smaller than that in the Pt/CNT sample (5.8 nm), and the smallest Pt particles can be obtained in the Pt-Sn/CNT sample prepared by the replacement method. This is because the Pt nanoparticles, prepared by the strong reducing agent NaBH 4, usually have a relatively greater particle size 12. However, when the replacement method is used to prepare Pt nanoparticles, metallic Sn nanoparticles act as the weaker reducing agent, producing more active sites where charge transfer arises among metallic ions and Sn atoms, thus attributing to producing Pt nanoparticles with small particle size.

5 ZHANG et al.: SYNTHESIS OF CNT SUPPORTED PT-NANOPARTICLES BY REPLACEMENT REACTION 1349 Electrochemical properties of Pt-Sn/CNT catalysts for ethanol electrooxidation The electrochemical properties of the Pt-Sn/CNT/graphite electrodes were investigated by cyclic voltammetry (scan rate, 50 mv s -1 ) in 1.0 M CH 3 CH 2 OH M H 2 SO 4 aqueous solution and the corresponding cyclic voltammograms are shown in Fig. 3. For comparison, the cyclic voltammograms at the PtSn/CNT/graphite and Pt/CNT/graphit electrodes determined at the same condition are also shown in Fig. 3. Figure 3 shows the typical cyclic voltammograms of ethanol oxidation with three oxidation peaks, in good agreement with previous literature 42. The current densities of all three peaks at the Pt-Sn/CNT/graphite electrode are higher than those at the PtSn/CNT/graphite and Pt/CNT electrodes. The current density of peak Ⅱ at the Pt-Sn/CNT/graphite electrode (Fig. 3a) is 9.7 ma cm -2, while the current densities of peak Ⅱ at the PtSn/CNT/graphite (Fig. 3b) and the Pt/CNT/graphite (Fig. 3c) electrodes are 5.6 ma cm -2 and 3.0 ma cm -2, respectively, under the same mass loading of Pt and experimental conditions. This indicates that the preparation procedure of binary PtSn catalysts indeed has a marked influence on their electrocatalytic activities towards ethanol oxidation; the activities of the Pt-Sn/CNT catalysts are better than those of the PtSn/CNT and Pt/CNT catalysts. Moreover, it should be noted in Fig. 3 that the peakⅡonset potentials of ethanol oxidation at both Pt-Sn/CNT/graphite and PtSn/CNT/graphite electrodes are about 0.38V (vs SCE) and lower than that at Pt/CNT/graphite electrode (0.52V vs SCE). This indicates that the introduction of Sn obviously enhances the electrocatalytic activities of the Pt/CNT catalysts for ethanol oxidation. It is deduced that surface Sn and/or its oxides in the catalysts can provide oxygen species at much lower potentials compared to Pt, which can promote the oxidation of CO and intermediates in ethanol oxidation. To further clarify the effect of the preparation procedure of PtSn binary catalysts on the catalytic activity, the specific electrochemically active surface area (EAS) of Pt-Sn/CNT, PtSn/CNT and Pt/CNT catalysts were determined by cyclic voltammetry based on the charge associated with the hydrogen adsorption and desorption processes 43,44 in 0.5 M H 2 SO 4 solution at a scan rate of 50 mv/s in the potential range of -0.2 to 1.0 V (vs SCE). The corresponding cyclic voltammograms are presented in Fig. 4. The electrochemical active surface (EAS) of the catalysts nanoparticles 45 was calculated as EAS = Q ads /Q ref [Pt], where Q ads is the integrated hydrogen absorption charge, Q ref the hydrogen adsorption charge on a smooth platinum electrode (0.21 mc/cm 2 ), and [Pt] is Pt loading (mg/cm 2 ) on the electrode. The calculated EAS value of the Pt-Sn/CNTs is about m 2 g -1 and obviously higher than that of the PtSn/CNTs (111.4 m 2 g -1 ) and Pt/CNTs (40.9 m 2 g -1 ). This result explains why the Pt-Sn/CNT catalysts exhibit higher catalytic activity for ethanol electrooxidation than the PtSn/CNTs. In Fig. 3 Cyclic voltammograms of various electrodes at 50 mv s -1 in 1.0 M CH 3 CH 2 OH M H 2 SO 4 aqueous solutions. [a, Pt-Sn/CNTs/graphite; b, PtSn/CNTs/graphite; c, Pt/CNTs/ graphite; loading mass of Pt: 10 g]. Fig. 4 Cyclic voltammograms of various electrodes at 50 mv s -1 in 0.5 M H 2 SO 4 aqueous solutions. [a, Pt-Sn/CNTs/ graphite; b, PtSn/CNTs /graphite; c, Pt/CNTs/graphite; loading mass of Pt:10 g].

6 1350 INDIAN J CHEM, SEC A, OCTOBER 2009 Fig. 5 Chronoamperograms of Pt-Sn catalysts. [a, Pt-Sn/CNTs/ graphite electrodes; b, PtSn/CNTs/graphite electrodes; c, Pt/CNTs/graphite electrodes at 0.5 V in 1.0 M CH 3 CH 2 OH M H 2 SO 4 aqueous solution; loading mass of Pt: 10 g]. other words, compared to the coreduction method, the replacement reaction method detailed in the experimental section leads to PtSn nanoparticles with smaller particle size, higher dispersion and electrochemically active surface area, thus resulting in higher electrocatalytic activities of the Pt-Sn/CNT catalysts. Stability of Pt-Sn/CNT/graphite for ethanol oxidation Chronoamperometric curves were recorded to check the stability of the catalysts. Figure 5 shows the chronoamperograms of the Pt-Sn/CNT/graphite, PtSn/CNT/graphite and Pt/CNT/graphite electrodes at 0.5 V in 1.0 M CH 3 CH 2 OH M H 2 SO 4 aqueous solution. As shown in Fig. 5, a similar gradual decay of current density with time could be observed on all the electrodes. The decay of current in the initiative regions may be attributed to a rapid increase of the surface coverage with partially oxidized species that results in poisoning of the Pt surface and diminution of its ability to oxidize ethanol 31,41. In the case of Pt/CNTs, this results in a very low steady-state current density. However, during the entire experiment, the Pt-Sn/CNT/graphite electrode displays the maximum current density. This confirms that the Pt-Sn/CNT/graphite electrode exhibits higher electrocatalytic stability than that of the PtSn/CNT/graphite and the Pt/CNT/graphite electrodes. From a practical viewpoint, long term cycle stability is important for the anode catalyst activity. The long term cycle catalytic stability of the three electrodes for ethanol electrooxidation has been Fig. 6 Long-term stability of the Pt-Sn catalysts. [a, Pt-Sn/CNTs/ graphite electrode; b, PtSn/CNTs/graphite electrode; c, Pt/CNTs/graphite electrode at 50 mv s -1 in 1.0 M CH 3 CH 2 OH M H 2 SO 4 aqueous solution]. investigated in 1.0 M CH 3 CH 2 OH M H 2 SO 4 aqueous solutions by cyclic voltammetry (Fig. 6). As seen from Fig. 6a, the peak current density (i P ) of the Pt-Sn/CNT/graphite electrode decreases gradually with the increase in the cycle number. When the potential was cycled continuously for 300 cycles, 25.9% loss of the current density at peak Ⅱ of ethanol oxidation was observed at the Pt-Sn/CNT/graphite electrode. Figure 6 (b & c) shows the larger loss of i p at peak Ⅱ of ethanol oxidation on the PtSn/CNT/graphite (56.8%) and the Pt/CNT/graphite electrodes (52.1%) after 300 cycles as compared to that on the Pt-Sn/CNT/graphite electrode. These results imply that the Pt-Sn/CNT catalysts show better long term stability of the catalysts during the ethanol electrooxidation. However, the long-term stability of Pt-Sn/CNT catalysts still needs to be improved. Conclusions Pt-Sn/CNT nanoparticles have been prepared by replacement reaction method and the electrochemical properties of the Pt-Sn/CNT/graphite electrode have been investigated by cyclic voltammetry and chronoamperometry. Compared with the PtSn/CNT bimetallic nanocatalysts prepared by the conventional coreduction method, the Pt-Sn/CNT/graphite electrode shows better electrochemical performances (higher electrocatalytic activity and better long-term cycle stability) under the same mass loading of the catalysts and experimental conditions due to smaller particle size and higher electrochemically active surface area of the Pt-Sn/CNT catalysts.

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