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Electrochemical oxidation of_methanol_on_pt-v2_o5-c_composite_catalysts
 

Electrochemical oxidation of_methanol_on_pt-v2_o5-c_composite_catalysts

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    Electrochemical oxidation of_methanol_on_pt-v2_o5-c_composite_catalysts Electrochemical oxidation of_methanol_on_pt-v2_o5-c_composite_catalysts Document Transcript

    • Electrochemical oxidation of methanol on Pt/V2O5–C composite catalysts T. Maiyalagan *, F. Nawaz Khan Department of Chemistry, School of Science and Humanities, VIT University, Vellore 632 014, India a r t i c l e i n f o Article history: Received 5 June 2008 Received in revised form 26 September 2008 Accepted 2 October 2008 Available online 22 October 2008 Keywords: Pt nanoparticles Methanol oxidation DMFC Electro-catalyst a b s t r a c t Platinum nanoparticles have been supported on V2O5–C composite through the reduction of chloroplat- inic acid with formaldehyde. The catalyst was characterized by X-ray diffraction and transmission elec- tron microscopy. Catalytic activity and stability for the oxidation of methanol were studied by using cyclic voltammetry and chronoamperometry. Pt/V2O5–C composite anode catalyst on glassy carbon elec- trode show higher electro-catalytic activity for the oxidation of methanol. High electro-catalytic activities and good stabilities could be attributed to the synergistic effect between Pt and V2O5, avoiding the elec- trodes being poisoned. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Since the last decade, fuel cells have been receiving an increased attention due to the depletion of fossil fuels and rising environmen- tal pollution. Fuel cells have been demonstrated as interesting and very promising alternatives to solve the problem of clean electric power generation with high efficiency. Among the different types of fuel cells, direct methanol fuel cells (DMFCs) are excellent power sources for portable applications owing to its high energy density, ease of handling liquid fuel, low operating temperatures (60 À100 °C) and quick start up [1,2]. Furthermore, methanol fuel cell seems to be highly promising for large-scale commercialization in contrast to hydrogen-fed cells, especially in transportation [3]. The limitation of methanol fuel cell system is due to low catalytic activity of the electrodes, especially the anodes and at present, there is no practical alternative to Pt based catalysts. High noble metal loadings on the electrode [4,5] and the use of perfluorosulf- onic acid membranes significantly contribute to the cost of the de- vices. An efficient way to decrease the loadings of precious platinum metal catalysts and higher utilization of Pt particles is by better dispersion of the desired metal on the suitable support [6]. In order to reduce the amount of Pt loading on the electrodes, there have been considerable efforts to increase the dispersion of the metal on the support. Pt nanoparticles have been dispersed on a wide variety of substrates such as carbon nanomaterials [7,8] poly- mers nanotubules, [9] polymer-oxide nanocomposites [10], three dimensional organic matrices [11], and oxide matrices [12–22]. Most often the catalyst is dispersed on a conventional carbon support and the support material influences the catalytic activity through metal support interaction. Dispersion of Pt particles on an oxide matrix can lead, depending mainly on the nature of sup- port, to Pt supported oxide system that shows better behaviour than pure Pt. On the other hand, if the oxide is not involved in the electrochemical reactions taking place on the Pt sites, it might just provide a convenient matrix to produce a high surface area catalyst [23,24]. Recently metal oxides like CeO2 [25], ZrO2 [26], MgO [17], TiO2 [18] and WO3 [27] were used as electro-catalysts for direct oxida- tion of alcohol which significantly improve the electrode perfor- mance for alcohols oxidation, in terms of the enhanced reaction activity and the poisoning resistance. V2O5 has been extensively used as cathode in lithium ion bat- teries [28]. Vanadium (IV)/vanadium (III) redox couple has been used to construct a redox type of fuel cell [29]. V2O5 has been tested as anode for electro-oxidation of toluene [30]. Furthermore, V2O5 is a strong oxidant, V2O5 acts as a good oxidation catalyst for methanol [31,32]. The present report focuses on the efforts undertaken to develop metal oxide supports based platinum catalysts for methanol oxida- tion. In this communication the preparation of highly dispersed plat- inum supported on V2O5–carbon composites, the evaluation of the activity for the methanol oxidation of these electrodes and compar- ison with the activity of conventional 20% Pt/C electrodes are re- ported. These materials are characterized and studied, using XRD, TEM and cyclic voltammetry. The electrochemical properties of the compositeelectrodewerecomparedtothoseofthecommercialelec- trode,usingcyclicvoltammetry.ThePtSupportedV2O5–Ccomposite 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.10.011 * Corresponding author. Tel.: +91 0416 2202465; fax: +91 0416 2243092. E-mail address: maiyalagan@gmail.com (T. Maiyalagan). Catalysis Communications 10 (2009) 433–436 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
    • electrode exhibited excellent catalytic activity and stability com- pared to the 20 wt% Pt supported on the Vulcan XC-72R carbon. 2. Experimental 2.1. Materials All the chemicals used were of analytical grade. V2O5 obtained from Merck was used. Hexachloroplatinic acid was obtained from Aldrich. Vulcan XC-72 carbon black was purchased from Cabot Inc., Methanol and sulphuric acid were obtained from Fischer chemicals. Nafion 5 wt% solution was obtained from Dupont and was used as received. 2.2. Preparation of electro-catalysts The V2O5/C composite used in this study was prepared by a so- lid-state reaction under the microwave irradiation. The aqueous solution of V2O5 was well dispersed with carbon black (Vulcan XC-72R, Cabot Corp., USA) and precipitate was dried in oven at 100 °C. The mixture was then introduced into a microwave oven and heated 10 s and paused 40 s for ten times alternately. Pt nanoparticles supported on V2O5–C composite was prepared through the reduction of chloroplatinic acid with formaldehyde. The V2O5/C composite powder (ca. 100 mg) was ground gently with a mortar and pestle then suspended in about 20 ml H2O. H2PtCl6 solution was used (Aldrich) for deposition of Pt was then added in an amount slightly greater than the desired loading. The suspension was stirred at around 80 °C for 30 min to allow dis- persion and aqueous formaldehyde (BDH, 37%) was added fol- lowed by heating at reflux for 1 h. The composite catalyst were collected by filtration, washed thoroughly with water, and then dried under vacuum (25–50 °C). The same procedure as the above was repeated for the prepara- tion of Pt/C catalyst. The same procedure and conditions were used to make a comparison between the Pt/C and Pt/V2O5–C system. 2.3. Preparation of working electrode Glassy carbon (GC) (Bas electrode, 0.07 cm2 ) was polished to a mirror finish with 0.05 lm alumina suspensions before each experiment and served as an underlying substrate of the working electrode. In order to prepare the composite electrode, the cata- lysts were dispersed ultrasonically in water at a concentration of 1 mg mlÀ1 and 20 ll aliquot was transferred on to a polished glassy carbon substrate. After the evaporation of water, the resulting thin catalyst film was covered with 5 wt% Nafion solution. Then the electrode was dried at 353 K and used as the working electrode. 2.4. Characterization methods The phases and lattice parameters of the catalyst were charac- terized by X-ray diffraction (XRD) patterns employing Shimadzu XD-D1 diffractometer using Cu Ka radiation (k = 1.5418 Å) operat- ing at 40 kV and 48 mA. XRD samples were obtained by depositing carbon-supported nanoparticles on a glass slide and drying the la- ter in a vacuum overnight. For transmission electron microscopic studies, the composite dispersed in ethanol were placed on the copper grid and the images were obtained using JEOL JEM-3010 model, operating at 300 keV. 2.5. Electrochemical measurements All electrochemical studies were carried out using a BAS 100 electrochemical analyzer. A conventional three-electrode cell con- sisting of the GC (0.07 cm2 ) working electrode, Pt plate (5 cm2 ) as counter electrode and Ag/AgCl reference electrode were used for the cyclic voltammetry (CV) studies. The CV experiments were per- formed using 1 M H2SO4 solution in the absence and presence of 1 M CH3OH at a scan rate of 50 mV/s. All the solutions were pre- pared by using ultra pure water (Millipore, 18 MX). The electro- lytes were degassed with nitrogen gas before the electrochemical measurements. 3. Results and discussion The Pt/V2O5–C composite catalysts were characterized by XRD. The XRD pattern of as-synthesized Pt/C and Pt/V2O5–C catalysts is given in Fig. 1. The diffraction peak at 24–27° observed is attrib- uted to the hexagonal graphite structure (002) of Vulcan carbon. The peaks can be indexed at 2h = 39.8° (111), 46.6° (200) and 67.9° (220) reflections of a Pt face-centered cubic (FCC) crystal structure. The diffraction peak at 2h = 39.8° for Pt (111) corre- sponds well to the inter-planer spacing of d111 = 0.226 nm and the lattice constant of 3.924 Å. The facts agree well with the stan- dard powder diffraction file of Pt (JCPDS number 1-1311). From the isolated Pt (220) peak, the mean particle size was about 3.1 nm and 2.8 nm for the Pt/C and Pt/V2O5–C catalysts samples respec- tively, calculated with the Scherrer formula [33]. This suggests that very small Pt nanoparticles dispersed on the Pt/V2O5–C composite. The formation of broad peaks in V2O5-modified Pt/C catalysts indi- cated the presence of smaller Pt nanoparticles. But the diffraction peaks of Pt–V2O5/C are slightly shifted to lower values when com- pared to Pt/C. This is an indication that an alloy between Pt and V2O5 is being formed on the Pt–V2O5/C catalysts. Moreover, in the XRD patterns of the V2O5-modified Pt catalysts, the peaks asso- ciated with pure V2O5 did not appear prominently. This might be due to the presence of very small amount of V2O5 in catalysts. However, XRD measurements cannot supply exact information of crystallite size when it is less than 3.0 nm, for this reason, the figures obtained by the above equation will be slightly smaller than true ones. Fig. 2 shows TEM images of Pt/C and Pt/V2O5–C cat- alysts. The mean size was estimated to be 2.9 nm for Pt/C and 3.4 nm for Pt/V2O5–C, which was in good agreement with the re- sults from XRD. The electro-catalytic activities for methanol oxidation of Pt/C and Pt/V2O5–C electro-catalysts were analyzed by cyclic voltam- 20 30 40 50 60 70 80 (a) (b) (c) Intensity(a.u) (a) Vulcan XC-72 (b) 20% Pt/C (c) 20% Pt/V2 O5 - C Pt(200) Pt(111) 2θ (degrees) Pt(220) C(002) Fig. 1. XRD spectra of (a) Vulcan XC-72 (b) Pt/Vulcan XC-72 and (c) Pt–V2O5/Vulcan XC-72. 434 T. Maiyalagan, F.N. Khan / Catalysis Communications 10 (2009) 433–436
    • metry in an electrolyte of 1 M H2SO4 and 1 M CH3OH at 50 mV/s. The cyclic voltammograms of Pt/C and Pt based V2O5 composite electrodes are shown in Fig. 3, respectively. The data obtained from the cyclic voltammograms of the composite electrodes were com- pared in Table 1. The onset for methanol oxidation on Pt/C was found to be 0.31 V, which is 100 mV more positive than Pt/V2O5–C electrode (0.21 V). This gives clear evidence for the superior electro-catalytic activity of Pt/V2O5–C composite electrodes for methanol oxidation. The ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib) can be used to describe the catalyst toler- ance to accumulation of carbonaceous species [34–38]. A higher ratio indicates more effective removal of the poisoning species on the catalyst surface. The If/Ib ratios of Pt/V2O5–C and Pt/C are 1.06 and 0.90, respectively, which are higher than that of Pt/C (0.90), showing better catalyst tolerance of Pt/V2O5–C composites. Chronoamperometric experiments were carried out to observe the stability and possible poisoning of the catalysts under short- time continuous operation. Fig. 4 shows the evaluation of activity of Pt/C and Pt/V2O5–C composite electrodes with respect to time at constant potential of +0.6 V. It is clear from Fig. 4 when the elec- trodes are compared under identical experimental conditions; the Pt/V2O5–C composite electrodes show a comparable stability to the 20% Pt/C electrodes. The higher activity of composite electrodes demonstrates the better utilization of the catalyst. Also the redox potential of vana- dium oxide (VO2+ /V3+ ) is +337 mV (vs. SHE) which lying on the electrode potential of methanol oxidation favours oxidation of methanol. Enhancement in catalytic activity of Pt–Ru compared to pure platinum can be attributed to a bifunctional mechanism: platinum accomplishes the dissociative chemisorption of methanol whereas ruthenium forms a surface oxy-hydroxide which subse- quently oxidizes the carbonaceous adsorbate to CO2 [39,40]. Based on most accepted bifunctional mechanism of Pt–Ru, similar type of mechanism has been interpreted for enhancement in the catalytic activity of Pt–V2O5 [41]. First, methanol is preferred to bind with Pt surface atoms, and dehydrogenated to form CO adsorbed species. The COad intermediates are thought as the main poisoning species during electro-oxidation of methanol. Thus how to oxidize COad intermediates as quickly as possible is very important to methanol oxidation. Due to the higher affinity of vanadium oxides towards Fig. 2. TEM images of (a) Pt/C and (b) Pt/V2O5–C electro-catalysts. -0.2 0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 Currentdensity(mA/cm2 ) Potential (V) vs Ag/AgCl (b) (a) (a) 20%Pt/V2 O5 - C (b) 20% Pt/C Fig. 3. Cyclic voltammograms of (a) Pt/V2O5–C and (b) Pt/C in 1 M H2SO4/1 M CH3OH run at 50 mV/s. Table 1 Comparison of activity of methanol oxidation between Pt/V2O5–C and Pt/C electrodes. S. No. Electrode Onset potential (V) Activitya If/Ib Forward sweep Reverse sweep E (V) I (mA cmÀ2 ) E (V) I (mA cmÀ2 ) 1 Pt/C (J.M.) 0.31 0.76 12.25 0.62 13.49 0.9 2 Pt–V2O5/C 0.21 0.811 17.4 0.63 16.52 1.06 a Activity evaluated from cyclic voltammogram run in 1 M H2SO4/1 M CH3OH. T. Maiyalagan, F.N. Khan / Catalysis Communications 10 (2009) 433–436 435
    • oxygen-containing species, sufficient amounts of OHad to support reasonable CO oxidation rates are formed at lower potential on V2O5 composite sites than on Pt sites. The OHad species are neces- sary for the oxidative removal of COad intermediates. This effect leads to the higher activity and longer lifetime for the overall methanol oxidation process on Pt/V2O5–C composite. Based on the experimental results, to illustrate the enhanced activity of methanol electro-oxidation a similar promotional reaction model is proposed as follows, CH3OHad ! COad þ 4Hþ þ 4eÀ V2O5 þ 2Hþ ! 2VOþ 2 þ H2O 4VOþ 2 þ 4Hþ ! 4VO2þ þ O2 þ 2H2O VO2þ þ H2O ! VOOHþ þ Hþ COad þ VOOHþ ! CO2 þ VO2þ þ Hþ þ eÀ 4. Conclusion Highly dispersed nanosized Pt particles on V2O5–C composite have been prepared by formaldehyde reduction.Pt/V2O5–C com- posite catalyst exhibits higher catalytic activity for the methanol oxidation reaction than Pt/C, which is attributed to the syner- getic effects due to formation of an interface between the plati- num and V2O5, and by spillover due to diffusion of the CO intermediates. Easier formation of the oxygen-containing species on the surface of V2O5 favours the oxidation of CO intermediates to CO2 and releasing the active sites on Pt for further electro- chemical reaction. References [1] M.P. Hogarth, G.A. Hards, Platinum Met. Rev. 40 (1996) 150. [2] T.R. Ralph, Platinum Met. Rev. 41 (1997) 102. [3] B.D. McNicol, D.A.J. Rand, K.R. Williams, J. Power Sources 83 (2001) 47. [4] A. Hamnett, Catal. Today 38 (1997) 445. [5] S. Wasmus, A. Kuver, J. Electroanal. Chem. 461 (1999) 14. [6] T. Matsumoto, T. Komatsu, K. Arai, T. Yamazaki, M. Kijima, H. Shimizu, Y. Takasawa, J. Nakamura, Chem. Commun. 7 (2004) 840. [7] T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, Electrochem. Commun. 7 (2005) 905. [8] T. Maiyalagan, Appl. Catal. B: Environ. 89 (2008) 286. [9] T. Maiyalagan, J. Power Sources 179 (2008) 443. [10] B. Rajesh, K.R. Thampi, J.M. Bonard, N. Xanthapolous, H.J. Mathieu, B. Viswanathan, Electrochem. Solid-State Lett. 5 (2002) E71. [11] H. Bonnemann, N. Waldofner, H.G. Haubold, T. Vad, Chem. Mater. 14 (2002) 1115. [12] T. Maiyalagan, B. Viswanathan, J. Power Sources 175 (2008) 789. [13] T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, J. Nanosci. Nanotech. 6 (2006) 2067. [14] K. Sasaki, R.R. Adzic, J. Electrochem. Soc. 155 (2008) B180. [15] J.M. Macak, P.J. Barczuk, H. Tsuchiya, M.Z. Nowakowska, A. Ghicov, M. Chojak, S. Bauer, S. Virtanen, P.J. Kulesza, P. Schmuki, Electrochem. Commun. 7 (2005) 1417. [16] M.I. Rojas, M.J. Esplandiu, L.B. Avalle, E.P.M. Leiva, V.A. Macagno, Electrochim. Acta 43 (1998) 1785. [17] C. Xu, P.K. Shen, X. Ji, R. Zeng, Y. Liu, Electrochem. Commun. 7 (2005) 1305. [18] M. Hepel, I. Kumarihamy, C.J. Zhong, Electrochem. Commun. 8 (2006) 1439. [19] Y. Bai, J. Wu, J. Xi, J. Wang, W. Zhu, L. Chen, X. Qiu, Electrochem. Commun. 7 (2005) 1087. [20] A.L. Santos, D. Profeti, P. Olivi, Electrochim. Acta 50 (2005) 615. [21] V.B. Baez, D. Pletcher, J. Electroanal. Chem. 382 (1995) 59. [22] P.K. Shen, K.Y. Chen, A.C.C. Tseung, J. Electrochem. Soc. 142 (1995) L85. [23] T. Ioroi, Z. Siroma, N. Fujiwara, S. Yamazaki, K. Yasuda, Electrochem. Commun. 7 (2001) 183. [24] B.E. Hayden, D.V. Malevich, Electrochem. Commun. 3 (2001) 395. [25] C. Xu, P.K. Shen, Chem. Commun. 19 (2004) 2238. [26] Y. Bai, J. Wu, J. Xi, J. Wang, W. Zhu, L. Chen, X. Qiu, Electrochem. Commun. 7 (2005) 1087. [27] S. Jayaraman, Thomas F. Jaramillo, Sung-Hyeon Baeck, Eric W. McFarland, J. Phys. Chem. B 109 (2005) 2958. [28] Y. Wang, G.Z. Cao, Adv. Mater. 20 (2008) 2251. [29] R. Larsson, B. Folkesson, Inorg. Chim. Acta 162 (1) (1989) 75. [30] Luis F. D’Elia, L. Rincon, R. Ortız, Electrochim. Acta 50 (2004) 217. [31] B. Folkesson, R. Larsson, J. Zander, J. Electroanal. Chem. 267 (1–2) (1989) 149. [32] K.F. Zhang, D.J. Guo, X. Liu, J. Li, H.L. Li, Z.H. Su, J. Power Sources 162 (2) (2006) 1077. [33] S. Trasatti, O.A. Petrii, Pure Appl. Chem. 63 (1991) 711. [34] Z. Liu, J.Y. Lee, W. Chen, M. Han, L.M. Gan, Langmuir 20 (2004) 181. [35] Y. Mu, H. Liang, J. Hu, L. Jiang, L. Wan, J. Phys. Chem. B 109 (2005) 22212. [36] R. Manoharan, J.B. Goodenough, J. Mater. Chem. 2 (1992) 875. [37] Z. Liu, X.Y. Ling, X. Su, J.Y. Lee, J. Phys. Chem. B 108 (2004) 8234. [38] T.C. Deivaraj, J.Y. Lee, J. Power Sources 142 (2005) 43. [39] K. Wang, H.A. Gasteiger, N.M. Markovic, P.N. Ross, Electrochim. Acta 41 (1996) 2587. [40] E. Ticanelli, J.G. Beery, M.T. Paffett, S. Gottesfeld, J. Electroanal. Chem. 258 (1989) 61. [41] C. Roth, N. Benker, R. Theissmann, R.J. Nichols, D.J. Schiffrin, Langmuir 24 (2008) 2191. 0 500 1000 1500 0 10 20 30 40 50 60 Time (Sec) Currentdensity(mA/cm 2 ) (b) (a) (a) 20% Pt/V2 O5 - C (b) 20% Pt/C Fig. 4. Current density vs. time curves at (a) Pt/V2O5–C (b) Pt/C measured in 1 M H2SO4 + 1 M CH3OH. The potential was stepped from the rest potential to 0.6 V vs. Ag/AgCl. 436 T. Maiyalagan, F.N. Khan / Catalysis Communications 10 (2009) 433–436