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Catalysis Communications 10 (2009) 433–436

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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 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. 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 environmental 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 (60 100 °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 perfluorosulfonic acid membranes significantly contribute to the cost of the devices. 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] polymers nanotubules, [9] polymer-oxide nanocomposites [10], three dimensional organic matrices [11], and oxide matrices [12–22]. * Corresponding author. Tel.: +91 0416 2202465; fax: +91 0416 2243092. E-mail address: maiyalagan@gmail.com (T. Maiyalagan). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.10.011

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 support, 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 oxidation of alcohol which significantly improve the electrode performance for alcohols oxidation, in terms of the enhanced reaction activity and the poisoning resistance. V2O5 has been extensively used as cathode in lithium ion batteries [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 oxidation. In this communication the preparation of highly dispersed platinum supported on V2O5–carbon composites, the evaluation of the activity for the methanol oxidation of these electrodes and comparison with the activity of conventional 20% Pt/C electrodes are reported. These materials are characterized and studied, using XRD, TEM and cyclic voltammetry. The electrochemical properties of the composite electrode were compared to those of the commercial electrode, using cyclic voltammetry. The Pt Supported V2O5–C composite


T. Maiyalagan, F.N. Khan / Catalysis Communications 10 (2009) 433–436

2.2. Preparation of electro-catalysts The V2O5/C composite used in this study was prepared by a solid-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 dispersion and aqueous formaldehyde (BDH, 37%) was added followed 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 preparation 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 catalysts were dispersed ultrasonically in water at a concentration of 1 mg ml1 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 characterized by X-ray diffraction (XRD) patterns employing Shimadzu XD-D1 diffractometer using Cu Ka radiation (k = 1.5418 Å) operating at 40 kV and 48 mA. XRD samples were obtained by depositing carbon-supported nanoparticles on a glass slide and drying the later 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-

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 attributed to the hexagonal graphite structure (002) of Vulcan carbon. 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 structure. The diffraction peak at 2h = 39.8° for Pt (1 1 1) corresponds 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 standard powder diffraction file of Pt (JCPDS number 1-1311). From the isolated Pt (2 2 0) peak, the mean particle size was about 3.1 nm and 2.8 nm for the Pt/C and Pt/V2O5–C catalysts samples respectively, 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 indicated the presence of smaller Pt nanoparticles. But the diffraction peaks of Pt–V2O5/C are slightly shifted to lower values when compared 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 associated 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 catalysts. 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 results from XRD. The electro-catalytic activities for methanol oxidation of Pt/C and Pt/V2O5–C electro-catalysts were analyzed by cyclic voltam-

(c)

(a) Vulcan XC-72 (b) 20% Pt/C (c) 20% Pt/V2O5- C

Pt (220)

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.

Pt (200)

2.1. Materials

Pt (111)

2. Experimental

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 performed 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 prepared by using ultra pure water (Millipore, 18 MX). The electrolytes were degassed with nitrogen gas before the electrochemical measurements.

C (002)

electrode exhibited excellent catalytic activity and stability compared to the 20 wt% Pt supported on the Vulcan XC-72R carbon.

Intensity (a.u)

434

(b)

(a)

20

30

40

50

60

70

80

2θ (degrees) Fig. 1. XRD spectra of (a) Vulcan XC-72 (b) Pt/Vulcan XC-72 and (c) Pt–V2O5/Vulcan XC-72.


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(a)

(a) 20%Pt/V2O5- C

Current density (mA/cm2 )

15

(b) 20% Pt/C

(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.

Fig. 2. TEM images of (a) Pt/C and (b) Pt/V2O5–C electro-catalysts.

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 compared 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 tolerance 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 shorttime 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 electrodes 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 vanadium 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 subsequently 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

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

1 2

Pt/C (J.M.) Pt–V2O5/C a

0.31 0.21

Reverse sweep

E (V)

I (mA cm2)

E (V)

I (mA cm2)

0.76 0.811

12.25 17.4

0.62 0.63

13.49 16.52

Activity evaluated from cyclic voltammogram run in 1 M H2SO4/1 M CH3OH.

0.9 1.06


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T. Maiyalagan, F.N. Khan / Catalysis Communications 10 (2009) 433–436

60

(a) 20% Pt/V2O5- C (b) 20% Pt/C

2

Current density (mA/cm )

50

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 electrochemical reaction. References

40

[1] [2] [3] [4] [5] [6]

30

20

(a)

[7]

10

[8] [9] [10]

(b)

0 [11]

0

500

1000

1500

Time (Sec) 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.

[12] [13] [14] [15]

[16]

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 necessary 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,

CH3 OHad ! COad þ 4Hþ þ 4e V2 O5 þ 2Hþ ! 2VOþ2 þ H2 O 4VOþ2 þ 4Hþ ! 4VO2þ þ O2 þ 2H2 O VO2þ þ H2 O ! VOOHþ þ Hþ COad þ VOOHþ ! CO2 þ VO2þ þ Hþ þ e 4. Conclusion Highly dispersed nanosized Pt particles on V2O5–C composite have been prepared by formaldehyde reduction.Pt/V2O5–C composite catalyst exhibits higher catalytic activity for the methanol oxidation reaction than Pt/C, which is attributed to the synergetic effects due to formation of an interface between the platinum and V2O5, and by spillover due to diffusion of the CO

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Electrochemical oxidation of methanol on pt v2o5 c composite catalysts