- Email: [email protected]
SiC
Electrochemistry Communications 13 (2011) 1309–1312
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Preparation, characterization and catalytic performance of a novel Pt/SiC Li Fang a,⁎, Xiao-Ping Huang a, Francisco J. Vidal-Iglesias b, Yue-Peng Liu a, Xiao-Li Wang a a b
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China Instituto de Electroquímica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain
a r t i c l e
i n f o
Article history: Received 19 June 2011 Received in revised form 20 July 2011 Accepted 26 July 2011 Available online 2 August 2011 Keywords: Pt/SiC catalyst Cyclic voltammetry Ethanol electrooxidation
a b s t r a c t A novel Pt/SiC (5%Pt loading) catalyst prepared by an ultrasound promoted impregnation followed by calcination in hydrogen flow is reported. The catalysts reduced at different temperatures were characterized by cyclic voltammetry combined with TEM, XPS and XRD. The electrocatalytic activity was investigated for ethanol electrooxidation. A strong interaction between platinum and silicon carbide was observed, which possibly affects the surface electronic structure of platinum, hence improving the catalytic properties of the catalysts. The Pt/SiC reduced at 723 K shows much higher activity and CO tolerance for ethanol oxidation than that of Pt/C and Pt/SiC reduced at other temperatures. The reported results indicate that SiC is a promising support for platinum-based fuel cell catalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon black or graphite supported platinum and platinum-based bimetallic catalysts have been extensively studied in a fuel cell, especially in DMFC [1,2]. However, carbon supports are liable to be oxidized during the working process of a fuel cell due to a strong acidic environment, high temperature, high oxygen concentration and high potentials, which may inevitably cause carbon loss and then active component loss together with a possible structural collapse of the electrode [3,4]. Thus, it is of interest to find new supports resistant to the above limitations. Silicon carbide (SiC) has several advantages compared with carbon, including higher thermal conductivity and stability, higher chemical inertness, higher resistance towards oxidation and high mechanical strength, all of which enable the use of this material as an excellent catalyst support [5–8], especially at the extreme fuel cell working conditions [3,4]. Viswanathan's group investigated the possibility of using nano-SiC prepared in thermal plasma as a support for Pt and examined the electrochemical performance of the Pt/SiC catalyst. The authors concluded that the as-prepared Pt/SiC exhibited an ORR activity comparable with that of a commercial Pt/C (E-TEK) catalyst [3]. Niu and Wang found that macroscopic SiC nanowires (Pt/SiCNWs) (50% Pt loading) displayed a high catalytic performance for oxygen reduction and good chemical stability as well, showing the potential of SiC as catalyst support for fuel cells with long lifetimes [4]. Moreover they also showed some preliminary results for methanol oxidation concluding that the activity was high for that catalyst. Nevertheless, none of the above research concerned a systematic investigation of the electrocatalytic activity for Pt/SiC as an anode catalyst, not to mention that for lower platinum loadings. ⁎ Corresponding author. Tel.: + 86 351 7010588x806; fax: + 86 351 7011688. E-mail address: [email protected] (L. Fang). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.07.023
Ethanol is an attractive liquid fuel due to the fact that this alcohol and its oxidation products are less toxic than methanol. Moreover an ethanol crossover is less important than methanol's and it is a renewable fuel, as it can be obtained by the fermentation of agricultural products and biomass. For these reasons, ethanol electrooxidation has been chosen as the test reaction to compare the electrocatalytic activity of the prepared catalysts. The aim of the present paper is to report our findings of the electrochemical performance of Pt/SiC with 5% Pt loading and its remarkable catalytic properties for ethanol electrooxidation. 2. Experimental SiC (16 m 2/g, Hefei Kaier Nano-Power Technology Inc., China) nanoparticles were activated before use. Briefly, a certain amount of SiC was immersed completely in 1 M HCl solution for 24 h. The slurry was then washed with distilled water and vacuum filtered until no Cl− presence. The obtained SiC was calcined in air at 700 K for 4 h after being dried at room temperature and then reduced in hydrogen at 423 K. To prepare 5 wt.% Pt/SiC catalysts, 1.0 g of activated SiC was dispersed in a H2PtCl6 solution with ultrasound irradiation for 2 h and static impregnation for another 3 h until the solution became colorless [3]. The slurry was dried at 333 K and then reduced in hydrogen at 573, 673, 723 and 773 K for 2 h. The obtained samples were accordingly labeled Pt/SiC-573 (15.8 ± 6.3 nm), Pt/SiC-673 (17.8 ± 10.2 nm), Pt/SiC723 (15.1 ± 7.6 nm) and Pt/SiC-773 (16.2 ± 8.2 nm). TEM images of the catalysts were recorded on a JEM-2100 (JEOL) microscope working at 200 keV. The X-ray diffraction (XRD) analysis was performed with a D/max-RB diffractometer (RIGAKU) with graphite crystal monochromized Cu Kα radiation. The X-ray diffractograms were obtained with a scan rate of 5°/min. The X-ray photoelectron spectroscopy (XPS) analysis was carried out with an ESCALAB 250 machine (ThermoFisher Scientific, USA).
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The cyclic voltammograms (CVs) of the catalysts were obtained in a three-electrode cell at a sweep rate of 10 mV s − 1 by pressing a small quantity of the catalyst (approximately 0.002 g) onto a platinum mesh to form the working electrode [9]. The working solution (either 0.1 M H2SO4 or 0.1 M ethanol + 0.1 M H2SO4) was deaerated by bubbling N2 (99.999%) through the solution for at least 30 min before running CV. All the experiments were performed at room temperature and potentials were measured against a standard calomel reference electrode (SCE).
3. Results TEM images of pure SiC and Pt/SiC reduced in flowing hydrogen at 673 K are presented in Fig. 1. Relatively homogeneous and dispersed platinum nanoparticles on the SiC surface are clearly observed. The use of β-SiC has been reported to give low dispersion in particle size in comparison with α-SiC due to the possible presence of a higher amount of oxygen surface groups, i.e. SiOxCy and SiO2 on β-SiC due to the presence of trace oxygen during its synthesis [10]. The existence of SiO2/SiOxCy on the surface of the support is confirmed by the XPS Si 2p spectrum of Pt/SiC shown in Fig. 2. XPS analysis of the catalyst reveals that silica and/or silicon oxycarbide (SiOxCy) were already present (Fig. 2A) on the SiC surface [10]. This may lead to a partial platinum nanoparticle deposit on the topmost SiO2/SiOxCy surface instead of SiC directly and thus affect the catalytic activity. Moreover, the XPS of Pt 4f (Fig. 2B) shows no apparent platinum oxide formation on the support. The comparison of XRD patterns of pure SiC, Pt/C (5% Pt loading, calcined in air at 673 K) and a series of Pt/SiC reduced in hydrogen at different temperatures is shown in Fig. 3. From the XRD pattern of SiC the diffraction peaks of SiC(111), (200), (220) and (311) at 35.76, 41.49, 60.10 and 71.90° is observed, respectively, indicating the very good crystallinity of β-SiC [3,4,8]. In comparison with the diffraction peaks of Pt(111), (200) and (220) of Pt/C at 39.75, 46.19 and 67.45°,
Fig. 1. TEM images of pure SiC and Pt/SiC after reduction in flowing hydrogen at 673 K.
Fig. 2. (A) Si 2p and (B) Pt 4f XPS spectra of Pt/SiC reduced in flowing hydrogen at 673 K.
those of Pt/SiC reduced at different temperatures shift to higher degrees, that is, 40.15, 46.60 and 67.82°, respectively. The results imply the stronger interaction between Pt and SiC or the topmost SiO2/SiOxCy. The diffraction intensity of Pt increases with an increase of the reduction temperature showing better Pt crystallinity at a higher temperature. However, the peak of Pt(111) decreases for Pt/SiC-773 which we can ascribe to the inter-diffusion between Pt and SiC [11], or even to the formation of platinum silicides Pt3Si [6,12]. Thus, the catalyst with the best crystallinity avoiding this possible formation of platinum silicides is Pt/SiC reduced at 723 K. Fig. 4A shows the cyclic voltammograms for pure SiC, Pt/C and Pt/SiC reduced at different temperatures in 0.1 M H2SO4. For SiC supported Pt catalysts, the two pairs of hydrogen adsorption/desorption (HA/HD) peaks of Pt are observed symmetrically at −0.15 V and −0.03 V for each
Fig. 3. XRD patterns of pure SiC, Pt/C and Pt/SiC reduced at different temperatures.
L. Fang et al. / Electrochemistry Communications 13 (2011) 1309–1312
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different temperatures. There are two main oxidation peaks for ethanol oxidation for all supported Pt catalysts illustrated in Fig. 4B. For Pt/C the potential of the first peak takes place at 0.63 V and it is ascribed to the oxidation of ethanol to acetaldehyde, acetic acid and CO2, while the second peak, which takes place at around 1.03 V, is ascribed to the formation of acetic acid [14,15], indicating that the end product of ethanol oxidation is mainly acetic acid. Nevertheless, for the reduced Pt/SiC catalysts, the onset and peak potentials of ethanol oxidation shift 100 mV negatively and current densities remarkably increase, in comparison with those on Pt/C. The current density of ethanol oxidation on Pt/SiC-723 is higher than those on the other Pt/SiC samples and it nearly reaches a twofold increase in comparison with Pt/C (apart from the remarkable negative shift in the potential), highlighting the great enhancement when using SiC as a support on both catalytic activity and CO tolerance of Pt. It is interesting to stress that increasing the reduction temperature has a positive effect in the ethanol oxidation, especially in the contribution between 0.25 and 0.8 V vs SCE. Nevertheless, if the temperature is high enough, platinum silicides are formed [6,12] and the electrocatalytic activity is notably reduced. Interestingly, from Fig. 4A it is observed that, in comparison with the carbon supported catalyst, the platinum surface oxidation seems to begin at lower potentials for the SiC supported catalysts, probably due to a lower OH adsorption potential. In catalysis, it is known that for several Pt-based catalyst systems, stable oxides on the surface promote the activity due to the easier adsorption of OH/Oads [16,17]. In the case of SiC-supported catalysts the presence of a very reactive oxidic layer on the SiC has been reported [18], which could be the cause of the observed catalytic enhancement towards ethanol oxidation reported in the present communication. Thus, for the ethanol oxidation reaction and according to Fig. 4A, in the presence of OH, adsorbed at lower potentials, the weakly adsorbed acetaldehyde and the strongly adsorbed CO fragments coming from the ethanol adsorption, can be oxidized at lower potentials to acetic acid and CO2 respectively [19]. The study of other fuel cell related oxidation reactions which could be enhanced due to this negative shift in the OH adsorption will be addressed in future studies. Fig. 4. Cyclic voltammetric profiles of Pt/C and Pt/SiC reduced at different temperatures in (A) 0.1 M H2SO4 and (B) 0.1 M H2SO4 + 0.1 M EtOH (first positive-going potential sweep shown). Sweep rate: 10 mV s− 1.
sample, showing a good reversibility [4]. The peak at −0.15 V is assigned to weak hydrogen adsorption (w-HA) and the other one at −0.03 V to strong hydrogen adsorption (s-HA), which are related to Pt(110) and (100) orientations, respectively [13]. In addition, both w-HA and s-HA peaks are sharper than those obtained for Pt/SiC catalysts (with 20% and 50% Pt loading) reported by Rao et al. [3] and Niu et al. [4]. This fact may be explained by the differences in particle size, specific surface area of the catalysts or cleanliness of the experimental system. The specific surface area of SiC support used in this paper was 16 m 2/g measured by BET showing very good crystallinity (see also XRD patterns in Fig. 2), whereas those used in previous publications were 35 m 2/g [3] and 1.6 m 2/g (10–30 nm) [4], and showed a higher degree of nanoparticle size dispersion and lower crystallinity. Furthermore, for the Pt/SiC catalysts reduced at different temperatures, the interaction Pt-SiC of the composites changes. From Fig. 4A (inset), it can be seen that in the anodic scan for Pt/SiC-573 and Pt/SiC-673 the peak at −0.15 V has a well-defined shoulder at −0.13 V. With an increase in the reduction temperature, the relative contribution of the main peak with the shoulder weakens. For Pt/SiC-723 the size of both contributions becomes nearly the same and finally for the Pt/SiC773 the previously referred “shoulder” becomes the main peak. This fact may indicate a strong interaction between deposited platinum and SiC, which is affected dramatically by the reduction temperature [12]. Fig. 4B shows the first, positive-going potential sweeps obtained for ethanol electrooxidation on Pt/C and Pt/SiC catalysts reduced at
4. Conclusion Pt/SiC catalysts with 5% platinum loading were firstly prepared by ultrasound promoted impregnation followed by calcination in flowing hydrogen for 2 h at different temperatures. From the XRD and CV profiles a strong interaction between platinum and SiC is observed which possibly affects the surface electronic structure of platinum, hence improves its catalytic properties. Ethanol electrooxidation on Pt/SiC-723 shows the best catalytic activity and CO tolerance probably owing to the best crystalline formation of platinum with a reduction at 723 K. We conclude that SiC is a promising support for platinum-based fuel cell catalysts due to the much higher activity of Pt/SiC towards ethanol oxidation than that of Pt/C. Acknowledgements We wish to thank the financial support from the Shanxi Scholarship Council of China (project no. 200813) and the Shanxi Science and Technology Department (no. 20100321085). References [1] E. Antolini, Mater. Chem. Phys. 78 (2003) 563. [2] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J. Power Sources 155 (2006) 95. [3] Ch.V. Rao, S.K. Singh, B. Viswanathan, Indian J. Chem. A 47 (2008) 1619. [4] J.J. Niu, J.N. Wang, Acta Mater. 57 (2009) 3084. [5] R. Moene, M. Makkee, J.A. Moulijn, Appl. Catal. A-Gen. 167 (1998) 321. [6] Ch. Méthivier, B. Béguin, M. Brun, J. Massardier, J.C. Bertolini, J. Catal. 173 (1998) 374.
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[7] N.R. Mathews, Eric L. Miller, P.J. Sebastian, M.M. Hernandez, X. Mathew, S.A. Gamboa, Int. J. Hydrogen Energy 29 (2004) 941. [8] W.-Z. Sun, G.-Q. Jin, X.-Y. Guo, Catal. Commun. 6 (2005) 135. [9] G.A. Attard, J.E. Gillies, C.A. Harris, D.J. Jenkins, P. Johnston, M.A. Price, D.J. Watson, P.B. Wells, Appl. Catal. A 222 (2001) 393. [10] L. Pesant, J. Matta, F. Garin, M.-J. Ledoux, P. Bernhardt, C. Pham, C. Pham-Huu, Appl. Catal. A-Gen. 266 (2004) 281. [11] H.J. Na, J.K. Jeong, M.Y. Um, B.S. Kim, C.S. Hwang, H.J. Kim, Solid-State Electron. 45 (2001) 1565. [12] J.S. Chen, E. Kolawa, M.-A. Nicolet, R.P. Ruiz, L. Baud, C. Jaussaud, R. Madar, J. Mater. Res. 9 (1994) 648.
[13] J. Clavilier, in: M.P. Soriaga (Ed.), Electrochemical Surface Science, ACS Symposium Series, 378, 1988, (Washington DC). [14] Q. Yuan, Z. Zhou, J. Zhuang, X. Wang, Chem. Mater. 22 (2010) 2395. [15] H. Hitmi, E.M. Belgsir, J.M. Leger, C. Lamy, R.O. Lezna, Electrochim. Acta 39 (1994) 407. [16] B. Moreno, E. Chinarro, J.L.G. Fierro, J.R. Jurado, J. Power Sources 169 (2007) 98. [17] L. Jiang, G. Sun, S. Sun, J. Liu, S. Tang, H. Li, B. Zhou, Q. Xin, Electrochim. Acta 50 (2005) 5384. [18] R. Moene, E.P.A.M. Tijsen, M. Makkee, J.A. Moulijn, Appl. Catal. A-Gen. 184 (1999) 127. [19] M.T.M. Koper, S.C.S. Lai, E. Herrero, in: M.T.M. Koper (Ed.), Fuel Cell Catalysis: A Surface Science Approach, John Wiley & Sons, Inc., Hoboken, New Jersey, 2009.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Preparation, characterization and catalytic performance of a novel Pt/SiC Li Fang a,⁎, Xiao-Ping Huang a, Francisco J. Vidal-Iglesias b, Yue-Peng Liu a, Xiao-Li Wang a a b
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China Instituto de Electroquímica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain
a r t i c l e
i n f o
Article history: Received 19 June 2011 Received in revised form 20 July 2011 Accepted 26 July 2011 Available online 2 August 2011 Keywords: Pt/SiC catalyst Cyclic voltammetry Ethanol electrooxidation
a b s t r a c t A novel Pt/SiC (5%Pt loading) catalyst prepared by an ultrasound promoted impregnation followed by calcination in hydrogen flow is reported. The catalysts reduced at different temperatures were characterized by cyclic voltammetry combined with TEM, XPS and XRD. The electrocatalytic activity was investigated for ethanol electrooxidation. A strong interaction between platinum and silicon carbide was observed, which possibly affects the surface electronic structure of platinum, hence improving the catalytic properties of the catalysts. The Pt/SiC reduced at 723 K shows much higher activity and CO tolerance for ethanol oxidation than that of Pt/C and Pt/SiC reduced at other temperatures. The reported results indicate that SiC is a promising support for platinum-based fuel cell catalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon black or graphite supported platinum and platinum-based bimetallic catalysts have been extensively studied in a fuel cell, especially in DMFC [1,2]. However, carbon supports are liable to be oxidized during the working process of a fuel cell due to a strong acidic environment, high temperature, high oxygen concentration and high potentials, which may inevitably cause carbon loss and then active component loss together with a possible structural collapse of the electrode [3,4]. Thus, it is of interest to find new supports resistant to the above limitations. Silicon carbide (SiC) has several advantages compared with carbon, including higher thermal conductivity and stability, higher chemical inertness, higher resistance towards oxidation and high mechanical strength, all of which enable the use of this material as an excellent catalyst support [5–8], especially at the extreme fuel cell working conditions [3,4]. Viswanathan's group investigated the possibility of using nano-SiC prepared in thermal plasma as a support for Pt and examined the electrochemical performance of the Pt/SiC catalyst. The authors concluded that the as-prepared Pt/SiC exhibited an ORR activity comparable with that of a commercial Pt/C (E-TEK) catalyst [3]. Niu and Wang found that macroscopic SiC nanowires (Pt/SiCNWs) (50% Pt loading) displayed a high catalytic performance for oxygen reduction and good chemical stability as well, showing the potential of SiC as catalyst support for fuel cells with long lifetimes [4]. Moreover they also showed some preliminary results for methanol oxidation concluding that the activity was high for that catalyst. Nevertheless, none of the above research concerned a systematic investigation of the electrocatalytic activity for Pt/SiC as an anode catalyst, not to mention that for lower platinum loadings. ⁎ Corresponding author. Tel.: + 86 351 7010588x806; fax: + 86 351 7011688. E-mail address: [email protected] (L. Fang). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.07.023
Ethanol is an attractive liquid fuel due to the fact that this alcohol and its oxidation products are less toxic than methanol. Moreover an ethanol crossover is less important than methanol's and it is a renewable fuel, as it can be obtained by the fermentation of agricultural products and biomass. For these reasons, ethanol electrooxidation has been chosen as the test reaction to compare the electrocatalytic activity of the prepared catalysts. The aim of the present paper is to report our findings of the electrochemical performance of Pt/SiC with 5% Pt loading and its remarkable catalytic properties for ethanol electrooxidation. 2. Experimental SiC (16 m 2/g, Hefei Kaier Nano-Power Technology Inc., China) nanoparticles were activated before use. Briefly, a certain amount of SiC was immersed completely in 1 M HCl solution for 24 h. The slurry was then washed with distilled water and vacuum filtered until no Cl− presence. The obtained SiC was calcined in air at 700 K for 4 h after being dried at room temperature and then reduced in hydrogen at 423 K. To prepare 5 wt.% Pt/SiC catalysts, 1.0 g of activated SiC was dispersed in a H2PtCl6 solution with ultrasound irradiation for 2 h and static impregnation for another 3 h until the solution became colorless [3]. The slurry was dried at 333 K and then reduced in hydrogen at 573, 673, 723 and 773 K for 2 h. The obtained samples were accordingly labeled Pt/SiC-573 (15.8 ± 6.3 nm), Pt/SiC-673 (17.8 ± 10.2 nm), Pt/SiC723 (15.1 ± 7.6 nm) and Pt/SiC-773 (16.2 ± 8.2 nm). TEM images of the catalysts were recorded on a JEM-2100 (JEOL) microscope working at 200 keV. The X-ray diffraction (XRD) analysis was performed with a D/max-RB diffractometer (RIGAKU) with graphite crystal monochromized Cu Kα radiation. The X-ray diffractograms were obtained with a scan rate of 5°/min. The X-ray photoelectron spectroscopy (XPS) analysis was carried out with an ESCALAB 250 machine (ThermoFisher Scientific, USA).
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The cyclic voltammograms (CVs) of the catalysts were obtained in a three-electrode cell at a sweep rate of 10 mV s − 1 by pressing a small quantity of the catalyst (approximately 0.002 g) onto a platinum mesh to form the working electrode [9]. The working solution (either 0.1 M H2SO4 or 0.1 M ethanol + 0.1 M H2SO4) was deaerated by bubbling N2 (99.999%) through the solution for at least 30 min before running CV. All the experiments were performed at room temperature and potentials were measured against a standard calomel reference electrode (SCE).
3. Results TEM images of pure SiC and Pt/SiC reduced in flowing hydrogen at 673 K are presented in Fig. 1. Relatively homogeneous and dispersed platinum nanoparticles on the SiC surface are clearly observed. The use of β-SiC has been reported to give low dispersion in particle size in comparison with α-SiC due to the possible presence of a higher amount of oxygen surface groups, i.e. SiOxCy and SiO2 on β-SiC due to the presence of trace oxygen during its synthesis [10]. The existence of SiO2/SiOxCy on the surface of the support is confirmed by the XPS Si 2p spectrum of Pt/SiC shown in Fig. 2. XPS analysis of the catalyst reveals that silica and/or silicon oxycarbide (SiOxCy) were already present (Fig. 2A) on the SiC surface [10]. This may lead to a partial platinum nanoparticle deposit on the topmost SiO2/SiOxCy surface instead of SiC directly and thus affect the catalytic activity. Moreover, the XPS of Pt 4f (Fig. 2B) shows no apparent platinum oxide formation on the support. The comparison of XRD patterns of pure SiC, Pt/C (5% Pt loading, calcined in air at 673 K) and a series of Pt/SiC reduced in hydrogen at different temperatures is shown in Fig. 3. From the XRD pattern of SiC the diffraction peaks of SiC(111), (200), (220) and (311) at 35.76, 41.49, 60.10 and 71.90° is observed, respectively, indicating the very good crystallinity of β-SiC [3,4,8]. In comparison with the diffraction peaks of Pt(111), (200) and (220) of Pt/C at 39.75, 46.19 and 67.45°,
Fig. 1. TEM images of pure SiC and Pt/SiC after reduction in flowing hydrogen at 673 K.
Fig. 2. (A) Si 2p and (B) Pt 4f XPS spectra of Pt/SiC reduced in flowing hydrogen at 673 K.
those of Pt/SiC reduced at different temperatures shift to higher degrees, that is, 40.15, 46.60 and 67.82°, respectively. The results imply the stronger interaction between Pt and SiC or the topmost SiO2/SiOxCy. The diffraction intensity of Pt increases with an increase of the reduction temperature showing better Pt crystallinity at a higher temperature. However, the peak of Pt(111) decreases for Pt/SiC-773 which we can ascribe to the inter-diffusion between Pt and SiC [11], or even to the formation of platinum silicides Pt3Si [6,12]. Thus, the catalyst with the best crystallinity avoiding this possible formation of platinum silicides is Pt/SiC reduced at 723 K. Fig. 4A shows the cyclic voltammograms for pure SiC, Pt/C and Pt/SiC reduced at different temperatures in 0.1 M H2SO4. For SiC supported Pt catalysts, the two pairs of hydrogen adsorption/desorption (HA/HD) peaks of Pt are observed symmetrically at −0.15 V and −0.03 V for each
Fig. 3. XRD patterns of pure SiC, Pt/C and Pt/SiC reduced at different temperatures.
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different temperatures. There are two main oxidation peaks for ethanol oxidation for all supported Pt catalysts illustrated in Fig. 4B. For Pt/C the potential of the first peak takes place at 0.63 V and it is ascribed to the oxidation of ethanol to acetaldehyde, acetic acid and CO2, while the second peak, which takes place at around 1.03 V, is ascribed to the formation of acetic acid [14,15], indicating that the end product of ethanol oxidation is mainly acetic acid. Nevertheless, for the reduced Pt/SiC catalysts, the onset and peak potentials of ethanol oxidation shift 100 mV negatively and current densities remarkably increase, in comparison with those on Pt/C. The current density of ethanol oxidation on Pt/SiC-723 is higher than those on the other Pt/SiC samples and it nearly reaches a twofold increase in comparison with Pt/C (apart from the remarkable negative shift in the potential), highlighting the great enhancement when using SiC as a support on both catalytic activity and CO tolerance of Pt. It is interesting to stress that increasing the reduction temperature has a positive effect in the ethanol oxidation, especially in the contribution between 0.25 and 0.8 V vs SCE. Nevertheless, if the temperature is high enough, platinum silicides are formed [6,12] and the electrocatalytic activity is notably reduced. Interestingly, from Fig. 4A it is observed that, in comparison with the carbon supported catalyst, the platinum surface oxidation seems to begin at lower potentials for the SiC supported catalysts, probably due to a lower OH adsorption potential. In catalysis, it is known that for several Pt-based catalyst systems, stable oxides on the surface promote the activity due to the easier adsorption of OH/Oads [16,17]. In the case of SiC-supported catalysts the presence of a very reactive oxidic layer on the SiC has been reported [18], which could be the cause of the observed catalytic enhancement towards ethanol oxidation reported in the present communication. Thus, for the ethanol oxidation reaction and according to Fig. 4A, in the presence of OH, adsorbed at lower potentials, the weakly adsorbed acetaldehyde and the strongly adsorbed CO fragments coming from the ethanol adsorption, can be oxidized at lower potentials to acetic acid and CO2 respectively [19]. The study of other fuel cell related oxidation reactions which could be enhanced due to this negative shift in the OH adsorption will be addressed in future studies. Fig. 4. Cyclic voltammetric profiles of Pt/C and Pt/SiC reduced at different temperatures in (A) 0.1 M H2SO4 and (B) 0.1 M H2SO4 + 0.1 M EtOH (first positive-going potential sweep shown). Sweep rate: 10 mV s− 1.
sample, showing a good reversibility [4]. The peak at −0.15 V is assigned to weak hydrogen adsorption (w-HA) and the other one at −0.03 V to strong hydrogen adsorption (s-HA), which are related to Pt(110) and (100) orientations, respectively [13]. In addition, both w-HA and s-HA peaks are sharper than those obtained for Pt/SiC catalysts (with 20% and 50% Pt loading) reported by Rao et al. [3] and Niu et al. [4]. This fact may be explained by the differences in particle size, specific surface area of the catalysts or cleanliness of the experimental system. The specific surface area of SiC support used in this paper was 16 m 2/g measured by BET showing very good crystallinity (see also XRD patterns in Fig. 2), whereas those used in previous publications were 35 m 2/g [3] and 1.6 m 2/g (10–30 nm) [4], and showed a higher degree of nanoparticle size dispersion and lower crystallinity. Furthermore, for the Pt/SiC catalysts reduced at different temperatures, the interaction Pt-SiC of the composites changes. From Fig. 4A (inset), it can be seen that in the anodic scan for Pt/SiC-573 and Pt/SiC-673 the peak at −0.15 V has a well-defined shoulder at −0.13 V. With an increase in the reduction temperature, the relative contribution of the main peak with the shoulder weakens. For Pt/SiC-723 the size of both contributions becomes nearly the same and finally for the Pt/SiC773 the previously referred “shoulder” becomes the main peak. This fact may indicate a strong interaction between deposited platinum and SiC, which is affected dramatically by the reduction temperature [12]. Fig. 4B shows the first, positive-going potential sweeps obtained for ethanol electrooxidation on Pt/C and Pt/SiC catalysts reduced at
4. Conclusion Pt/SiC catalysts with 5% platinum loading were firstly prepared by ultrasound promoted impregnation followed by calcination in flowing hydrogen for 2 h at different temperatures. From the XRD and CV profiles a strong interaction between platinum and SiC is observed which possibly affects the surface electronic structure of platinum, hence improves its catalytic properties. Ethanol electrooxidation on Pt/SiC-723 shows the best catalytic activity and CO tolerance probably owing to the best crystalline formation of platinum with a reduction at 723 K. We conclude that SiC is a promising support for platinum-based fuel cell catalysts due to the much higher activity of Pt/SiC towards ethanol oxidation than that of Pt/C. Acknowledgements We wish to thank the financial support from the Shanxi Scholarship Council of China (project no. 200813) and the Shanxi Science and Technology Department (no. 20100321085). References [1] E. Antolini, Mater. Chem. Phys. 78 (2003) 563. [2] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J. Power Sources 155 (2006) 95. [3] Ch.V. Rao, S.K. Singh, B. Viswanathan, Indian J. Chem. A 47 (2008) 1619. [4] J.J. Niu, J.N. Wang, Acta Mater. 57 (2009) 3084. [5] R. Moene, M. Makkee, J.A. Moulijn, Appl. Catal. A-Gen. 167 (1998) 321. [6] Ch. Méthivier, B. Béguin, M. Brun, J. Massardier, J.C. Bertolini, J. Catal. 173 (1998) 374.
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