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Maiyalagan,Electrochemical oxidation of methanol on pt v2 o5–c composite catalysts


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Platinum nanoparticles have been supported on V2O5–C composite through the reduction of chloroplatinic
acid with formaldehyde. The catalyst was characterized by X-ray diffraction and transmission electron
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 electrode
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 electrodes
being poisoned.

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Maiyalagan,Electrochemical oxidation of methanol on pt v2 o5–c composite catalysts

  1. 1. Catalysis Communications 10 (2009) 433–436 Contents lists available at ScienceDirect Catalysis Communications journal homepage: oxidation of methanol on Pt/V2O5–C composite catalystsT. Maiyalagan *, F. Nawaz KhanDepartment of Chemistry, School of Science and Humanities, VIT University, Vellore 632 014, Indiaa r t i c l e i n f o a b s t r a c tArticle history: Platinum nanoparticles have been supported on V2O5–C composite through the reduction of chloroplat-Received 5 June 2008 inic acid with formaldehyde. The catalyst was characterized by X-ray diffraction and transmission elec-Received in revised form 26 September tron microscopy. Catalytic activity and stability for the oxidation of methanol were studied by using2008 cyclic voltammetry and chronoamperometry. Pt/V2O5–C composite anode catalyst on glassy carbon elec-Accepted 2 October 2008Available online 22 October 2008 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.Keywords:Pt nanoparticles Ó 2008 Elsevier B.V. All rights reserved.Methanol oxidationDMFCElectro-catalyst1. Introduction Most often the catalyst is dispersed on a conventional carbon support and the support material influences the catalytic activity Since the last decade, fuel cells have been receiving an increased through metal support interaction. Dispersion of Pt particles onattention due to the depletion of fossil fuels and rising environmen- an oxide matrix can lead, depending mainly on the nature of sup-tal pollution. Fuel cells have been demonstrated as interesting and port, to Pt supported oxide system that shows better behaviourvery promising alternatives to solve the problem of clean electric than pure Pt. On the other hand, if the oxide is not involved inpower generation with high efficiency. Among the different types the electrochemical reactions taking place on the Pt sites, it mightof fuel cells, direct methanol fuel cells (DMFCs) are excellent power just provide a convenient matrix to produce a high surface areasources for portable applications owing to its high energy density, catalyst [23,24].ease of handling liquid fuel, low operating temperatures (60 Recently metal oxides like CeO2 [25], ZrO2 [26], MgO [17], TiO2À100 °C) and quick start up [1,2]. Furthermore, methanol fuel cell [18] and WO3 [27] were used as electro-catalysts for direct oxida-seems to be highly promising for large-scale commercialization in tion of alcohol which significantly improve the electrode perfor-contrast to hydrogen-fed cells, especially in transportation [3]. mance for alcohols oxidation, in terms of the enhanced reactionThe limitation of methanol fuel cell system is due to low catalytic activity and the poisoning resistance.activity of the electrodes, especially the anodes and at present, V2O5 has been extensively used as cathode in lithium ion bat-there is no practical alternative to Pt based catalysts. High noble teries [28]. Vanadium (IV)/vanadium (III) redox couple has beenmetal loadings on the electrode [4,5] and the use of perfluorosulf- used to construct a redox type of fuel cell [29]. V2O5 has beenonic acid membranes significantly contribute to the cost of the de- tested as anode for electro-oxidation of toluene [30]. Furthermore,vices. An efficient way to decrease the loadings of precious V2O5 is a strong oxidant, V2O5 acts as a good oxidation catalyst forplatinum metal catalysts and higher utilization of Pt particles is methanol [31,32].by better dispersion of the desired metal on the suitable support [6]. The present report focuses on the efforts undertaken to develop In order to reduce the amount of Pt loading on the electrodes, metal oxide supports based platinum catalysts for methanol oxida-there have been considerable efforts to increase the dispersion of tion. In this communication the preparation of highly dispersed plat-the metal on the support. Pt nanoparticles have been dispersed on inum supported on V2O5–carbon composites, the evaluation of thea wide variety of substrates such as carbon nanomaterials [7,8] poly- activity for the methanol oxidation of these electrodes and compar-mers nanotubules, [9] polymer-oxide nanocomposites [10], three ison with the activity of conventional 20% Pt/C electrodes are re-dimensional organic matrices [11], and oxide matrices [12–22]. ported. These materials are characterized and studied, using XRD, TEM and cyclic voltammetry. The electrochemical properties of the * Corresponding author. Tel.: +91 0416 2202465; fax: +91 0416 2243092. composite electrode were compared to those of the commercial elec- E-mail address: (T. Maiyalagan). trode, using cyclic voltammetry. The Pt Supported V2O5–C composite1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.catcom.2008.10.011
  2. 2. 434 T. Maiyalagan, F.N. Khan / Catalysis Communications 10 (2009) 433–436electrode exhibited excellent catalytic activity and stability com- sisting of the GC (0.07 cm2) working electrode, Pt plate (5 cm2) aspared to the 20 wt% Pt supported on the Vulcan XC-72R carbon. 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 of2. Experimental 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-2.1. Materials lytes were degassed with nitrogen gas before the electrochemical measurements. All the chemicals used were of analytical grade. V2O5 obtainedfrom Merck was used. Hexachloroplatinic acid was obtained from 3. Results and discussionAldrich. Vulcan XC-72 carbon black was purchased from CabotInc., Methanol and sulphuric acid were obtained from Fischer The Pt/V2O5–C composite catalysts were characterized by XRD.chemicals. Nafion 5 wt% solution was obtained from Dupont and The XRD pattern of as-synthesized Pt/C and Pt/V2O5–C catalysts iswas used as received. given in Fig. 1. The diffraction peak at 24–27° observed is attrib- uted to the hexagonal graphite structure (002) of Vulcan carbon.2.2. Preparation of electro-catalysts The peaks can be indexed at 2h = 39.8° (1 1 1), 46.6° (2 0 0) and 67.9° (2 2 0) reflections of a Pt face-centered cubic (FCC) crystal The V2O5/C composite used in this study was prepared by a so- structure. The diffraction peak at 2h = 39.8° for Pt (1 1 1) corre-lid-state reaction under the microwave irradiation. The aqueous sponds well to the inter-planer spacing of d111 = 0.226 nm andsolution of V2O5 was well dispersed with carbon black (Vulcan the lattice constant of 3.924 Å. The facts agree well with the stan-XC-72R, Cabot Corp., USA) and precipitate was dried in oven at dard powder diffraction file of Pt (JCPDS number 1-1311). From the100 °C. The mixture was then introduced into a microwave oven isolated Pt (2 2 0) peak, the mean particle size was about 3.1 nmand heated 10 s and paused 40 s for ten times alternately. and 2.8 nm for the Pt/C and Pt/V2O5–C catalysts samples respec- Pt nanoparticles supported on V2O5–C composite was prepared tively, calculated with the Scherrer formula [33]. This suggests thatthrough the reduction of chloroplatinic acid with formaldehyde. very small Pt nanoparticles dispersed on the Pt/V2O5–C composite.The V2O5/C composite powder (ca. 100 mg) was ground gently The formation of broad peaks in V2O5-modified Pt/C catalysts indi-with a mortar and pestle then suspended in about 20 ml H2O. cated the presence of smaller Pt nanoparticles. But the diffractionH2PtCl6 solution was used (Aldrich) for deposition of Pt was then peaks of Pt–V2O5/C are slightly shifted to lower values when com-added in an amount slightly greater than the desired loading. pared to Pt/C. This is an indication that an alloy between Pt andThe suspension was stirred at around 80 °C for 30 min to allow dis- V2O5 is being formed on the Pt–V2O5/C catalysts. Moreover, inpersion and aqueous formaldehyde (BDH, 37%) was added fol- the XRD patterns of the V2O5-modified Pt catalysts, the peaks asso-lowed by heating at reflux for 1 h. The composite catalyst were ciated with pure V2O5 did not appear prominently. This might becollected by filtration, washed thoroughly with water, and then due to the presence of very small amount of V2O5 in catalysts.dried under vacuum (25–50 °C). However, XRD measurements cannot supply exact information The same procedure as the above was repeated for the prepara- of crystallite size when it is less than 3.0 nm, for this reason, thetion of Pt/C catalyst. The same procedure and conditions were used figures obtained by the above equation will be slightly smallerto make a comparison between the Pt/C and Pt/V2O5–C system. 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 and2.3. Preparation of working electrode 3.4 nm for Pt/V2O5–C, which was in good agreement with the re- sults from XRD. Glassy carbon (GC) (Bas electrode, 0.07 cm2) was polished to a The electro-catalytic activities for methanol oxidation of Pt/Cmirror finish with 0.05 lm alumina suspensions before each and Pt/V2O5–C electro-catalysts were analyzed by cyclic voltam-experiment and served as an underlying substrate of the workingelectrode. In order to prepare the composite electrode, the cata-lysts were dispersed ultrasonically in water at a concentration of Pt (111)1 mg mlÀ1 and 20 ll aliquot was transferred on to a polished glassy (a) Vulcan XC-72carbon substrate. After the evaporation of water, the resulting thin (b) 20% Pt/Ccatalyst film was covered with 5 wt% Nafion solution. Then the Pt (200) (c) 20% Pt/V2O5- C C (002)electrode was dried at 353 K and used as the working electrode. Pt (220) (c) Intensity (a.u)2.4. Characterization methods The phases and lattice parameters of the catalyst were charac-terized by X-ray diffraction (XRD) patterns employing Shimadzu (b)XD-D1 diffractometer using Cu Ka radiation (k = 1.5418 Å) operat-ing at 40 kV and 48 mA. XRD samples were obtained by depositingcarbon-supported nanoparticles on a glass slide and drying the la-ter in a vacuum overnight. For transmission electron microscopic (a)studies, the composite dispersed in ethanol were placed on thecopper grid and the images were obtained using JEOL JEM-3010model, operating at 300 keV. 20 30 40 50 60 70 802.5. Electrochemical measurements 2θ (degrees) All electrochemical studies were carried out using a BAS 100 Fig. 1. XRD spectra of (a) Vulcan XC-72 (b) Pt/Vulcan XC-72 and (c) Pt–V2O5/Vulcanelectrochemical analyzer. A conventional three-electrode cell con- XC-72.
  3. 3. T. Maiyalagan, F.N. Khan / Catalysis Communications 10 (2009) 433–436 435 (a) (a) 20%Pt/V O5- C 2 15 (b) 20% Pt/C Current density (mA/cm2 ) (b) 10 5 0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Potential (V) vs Ag/AgCl 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. 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 Fig. 2. TEM images of (a) Pt/C and (b) Pt/V2O5–C electro-catalysts. 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-metry in an electrolyte of 1 M H2SO4 and 1 M CH3OH at 50 mV/s. quently oxidizes the carbonaceous adsorbate to CO2 [39,40]. BasedThe cyclic voltammograms of Pt/C and Pt based V2O5 composite on most accepted bifunctional mechanism of Pt–Ru, similar type ofelectrodes are shown in Fig. 3, respectively. The data obtained from mechanism has been interpreted for enhancement in the catalyticthe cyclic voltammograms of the composite electrodes were com- activity of Pt–V2O5 [41]. First, methanol is preferred to bind with Ptpared in Table 1. surface atoms, and dehydrogenated to form CO adsorbed species. The onset for methanol oxidation on Pt/C was found to be The COad intermediates are thought as the main poisoning species0.31 V, which is 100 mV more positive than Pt/V2O5–C electrode during electro-oxidation of methanol. Thus how to oxidize COad(0.21 V). This gives clear evidence for the superior electro-catalytic intermediates as quickly as possible is very important to methanolactivity of Pt/V2O5–C composite electrodes for methanol oxidation. oxidation. Due to the higher affinity of vanadium oxides towardsTable 1Comparison 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 0.92 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.
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