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Electrochemical oxidation of methanol on Pt/V 2O5C composite catalysts

T. Maiyalagan * , F. Nawaz KhanDepartment 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 2008Received in revised form 26 September2008Accepted 2 October 2008Available online 22 October 2008

Keywords:Pt nanoparticlesMethanol oxidationDMFCElectro-catalyst

a b s t r a c t

Platinum nanoparticles have been supported on V 2O5C 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 usingcyclic voltammetry and chronoamperometry. Pt/V 2O5C composite anode catalyst on glassy carbon elec-trode showhigher electro-catalyticactivity for the oxidationof methanol. High electro-catalytic activitiesand good stabilities could be attributed to the synergistic effect between Pt and V 2O5, avoiding the elec-trodes being poisoned.

2008 Elsevier B.V. All rights reserved.

1. Introduction

Since thelast decade, fuel cells have been receivingan increasedattention due to thedepletionof fossil fuelsand rising environmen-tal pollution. Fuel cells have been demonstrated as interesting andvery promising alternatives to solve the problem of clean electricpower generation with high efciency. Among the different typesof fuel cells, direct methanol fuel cells (DMFCs) are excellent powersources for portable applications owing to its high energy density,ease of handling liquid fuel, low operating temperatures (60100 C) and quick start up [1,2] . Furthermore, methanol fuel cellseems to be highly promising for large-scale commercialization incontrast to hydrogen-fed cells, especially in transportation [3].The limitation of methanol fuel cell system is due to low catalyticactivity of the electrodes, especially the anodes and at present,there is no practical alternative to Pt based catalysts. High noblemetal loadings on the electrode [4,5] and the use of peruorosulf-onic acid membranes signicantly contribute to the cost of the de-vices. An efcient way to decrease the loadings of preciousplatinum metal catalysts and higher utilization of Pt particles isbybetter dispersionof the desiredmetalon thesuitable 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 onawidevarietyofsubstrates suchas carbonnanomaterials [7,8] poly-mers nanotubules, [9] polymer-oxide nanocomposites [10] , threedimensional organic matrices [11] , and oxide matrices [1222] .

Most often the catalyst is dispersed on a conventional carbonsupport and the support material inuences the catalytic activitythrough metal support interaction. Dispersion of Pt particles onan oxide matrix can lead, depending mainly on the nature of sup-port, to Pt supported oxide system that shows better behaviourthan pure Pt. On the other hand, if the oxide is not involved inthe electrochemical reactions taking place on the Pt sites, it might just provide a convenient matrix to produce a high surface areacatalyst [23,24] .

Recently metal oxides like CeO 2 [25] , ZrO2 [26] , MgO [17] , TiO2[18] and WO 3 [27] were used as electro-catalysts for direct oxida-tion of alcohol which signicantly improve the electrode perfor-mance for alcohols oxidation, in terms of the enhanced reactionactivity and the poisoning resistance.

V2O5 has been extensively used as cathode in lithium ion bat-teries [28] . Vanadium (IV)/vanadium (III) redox couple has beenused to construct a redox type of fuel cell [29] . V2O5 has beentested as anode for electro-oxidation of toluene [30] . Furthermore,V2O5 is a strong oxidant, V 2O5 acts as a good oxidation catalyst formethanol [31,32] .

The present report focuses on the efforts undertaken to developmetal oxide supports based platinum catalysts for methanol oxida-tion. Inthiscommunicationthepreparationof highlydispersedplat-inum supported on V 2O5carbon composites, the evaluation of theactivity for the methanol oxidation of these electrodes andcompar-ison with the activity of conventional 20% Pt/C electrodes are re-ported. These materials are characterized and studied, using XRD,TEMandcyclic voltammetry. The electrochemical properties of thecompositeelectrodewerecomparedto thoseofthe commercialelec-trode,usingcyclicvoltammetry.ThePtSupportedV 2O5Ccomposite

1566-7367/$ - see front matter 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) 433436

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electrode exhibited excellent catalytic activity and stability com-pared to the20 wt%Pt supported on theVulcan XC-72R carbon.

2. Experimental

2.1. Materials

All the chemicals used were of analytical grade. V 2O5 obtainedfrom Merck was used. Hexachloroplatinic acid was obtained fromAldrich. Vulcan XC-72 carbon black was purchased from CabotInc., Methanol and sulphuric acid were obtained from Fischerchemicals. Naon 5 wt% solution was obtained from Dupont andwas 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 aqueoussolution of V 2O5 was well dispersed with carbon black (VulcanXC-72R, Cabot Corp., USA) and precipitate was dried in oven at100 C. The mixture was then introduced into a microwave ovenand heated 10 s and paused 40 s for ten times alternately.

Pt nanoparticles supported on V 2O5C composite was preparedthrough the reduction of chloroplatinic acid with formaldehyde.The V2O5/C composite powder (ca. 100 mg) was ground gentlywith a mortar and pestle then suspended in about 20 ml H 2O.H2PtCl6 solution was used (Aldrich) for deposition of Pt was thenadded in an amount slightly greater than the desired loading.The suspension was stirred at around 80 C for 30min to allow dis-persion and aqueous formaldehyde (BDH, 37%) was added fol-lowed by heating at reux for 1 h. The composite catalyst werecollected by ltration, washed thoroughly with water, and thendried under vacuum (2550 C).

The same procedure as the above was repeated for the prepara-tion of Pt/C catalyst. The same procedureand conditions were used

to make a comparison between the Pt/C and Pt/V 2O5C system.

2.3. Preparation of working electrode

Glassy carbon (GC) (Bas electrode, 0.07 cm 2) was polished to amirror nish with 0.05 l m alumina suspensions before eachexperiment 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 1 mg ml 1 and 20 l l aliquot was transferredon to a polished glassycarbon substrate. After the evaporation of water, the resulting thincatalyst lm was covered with 5 wt% Naon solution. Then theelectrode 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 ShimadzuXD-D1 diffractometer using Cu K a radiation ( k = 1.5418 ) 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 microscopicstudies, the composite dispersed in ethanol were placed on thecopper grid and the images were obtained using JEOL JEM-3010model, operating at 300 keV.

2.5. Electrochemical measurements

All electrochemical studies were carried out using a BAS 100electrochemical analyzer. A conventional three-electrode cell con-

sisting of the GC (0.07 cm 2) working electrode, Pt plate (5 cm 2) ascounter electrode and Ag/AgCl reference electrode were used forthecyclic voltammetry (CV) studies. The CVexperimentswere per-formed using 1 M H 2SO4 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 M X ). The electro-lytes were degassed with nitrogen gas before the electrochemicalmeasurements.

3. Results and discussion

The Pt/V2O5C composite catalysts were characterized by XRD.The XRD pattern of as-synthesized Pt/C and Pt/V 2O5C catalysts isgiven in Fig. 1. The diffraction peak at 2427 observed is attrib-uted to the hexagonal graphite structure (002) of Vulcan carbon.The peaks can be indexed at 2 h = 39.8 (111), 46.6 (200) and67.9 (220) reections of a Pt face-centered cubic (FCC) crystalstructure. The diffraction peak at 2 h = 39.8 for Pt (111) corre-sponds well to the inter-planer spacing of d111 = 0.226 nm andthe lattice constant of 3.924 . The facts agree well with the stan-dard powder diffractionle of Pt (JCPDS number 1-1311). From theisolated Pt (220) peak, the mean particle size was about 3.1 nmand 2.8 nm for the Pt/C and Pt/V 2O5C catalysts samples respec-tively, calculatedwiththe Scherrer formula [33] . This suggests thatvery small Pt nanoparticles dispersed on the Pt/V 2O5C composite.The formation of broad peaks in V 2O5-modied Pt/C catalysts indi-cated the presence of smaller Pt nanoparticles. But the diffractionpeaks of PtV 2O5/C are slightly shifted to lower values when com-pared to Pt/C. This is an indication that an alloy between Pt andV2O5 is being formed on the PtV 2O5/C catalysts. Moreover, inthe XRD patterns of the V 2O5-modied Pt catalysts, the peaks asso-ciated with pure V 2O5 did not appear prominently. This might bedue to the presence of very small amount of V 2O5 in catalysts.

However, XRD measurements cannot supply exact informationof crystallite size when it is less than 3.0 nm, for this reason, thegures obtained by the above equation will be slightly smallerthan true ones. Fig. 2 shows TEM images of Pt/C and Pt/V 2O5C cat-alysts. The mean size was estimated to be 2.9nm for Pt/C and3.4nm for Pt/V 2O5C, which was in good agreement with the re-sults from XRD.

The electro-catalytic activities for methanol oxidation of Pt/Cand Pt/V 2O5C electro-catalysts were analyzed by cyclic voltam-

20 30 40 50 60 70 80

(a)

(b)

(c)

I n t e n s

i t y

( a . u

)

(a) Vulcan XC-72

(b) 20% Pt/C

(c) 20% Pt/V 2O5- C

P t ( 2 0 0 ) P

t ( 1 1 1 )

2 (degrees)

P t ( 2 2 0 )

C ( 0 0 2 )

Fig. 1. XRD spectra of (a)Vulcan XC-72(b) Pt/Vulcan XC-72and (c) PtV 2O5/VulcanXC-72.

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metry in an electrolyte of 1 M H 2SO4 and 1 M CH3OH at 50 mV/s.The cyclic voltammograms of Pt/C and Pt based V 2O5 compositeelectrodes are shown in Fig. 3, respectively.The dataobtainedfromthe cyclic voltammograms of the composite electrodes were com-pared in Table 1 .

The onset for methanol oxidation on Pt/C was found to be0.31V, which is 100 mV more positive than Pt/V 2O5C electrode(0.21 V). This gives clear evidence for the superior electro-catalyticactivity of Pt/V 2O5C composite electrodes for methanol oxidation.

The ratio of the forward anodic peak current ( I f ) to the reverseanodic peak current ( I b) can be used to describe the catalyst toler-ance to accumulation of carbonaceous species [3438] . A higherratio indicates more effective removal of the poisoning specieson the catalyst surface. The I f /I b ratios of Pt/V 2O5C and Pt/C are1.06 and 0.90, respectively, which are higher than that of Pt/C(0.90), showing better catalyst tolerance of Pt/V 2O5C composites.

Chronoamperometric experiments were carried out to observethe stability and possible poisoning of the catalysts under short-time continuous operation. Fig. 4 shows the evaluation of activityof Pt/C and Pt/V 2O5C composite electrodes with respect to timeat constant potential of +0.6 V. It is clear from Fig. 4 when the elec-trodes are compared under identical experimental conditions; thePt/V2O5C composite electrodes show a comparable stability to the20% Pt/C electrodes.

The higher activity of composite electrodes demonstrates thebetter utilization of the catalyst. Also the redox potential of vana-dium oxide (VO 2+/V3+) is +337 mV (vs. SHE) which lying on theelectrode potential of methanol oxidation favours oxidation of methanol. Enhancement in catalytic activity of PtRu comparedto pure platinum can be attributed to a bifunctional mechanism:platinum accomplishes the dissociative chemisorptionof methanolwhereas ruthenium forms a surface oxy-hydroxide which subse-quently oxidizes the carbonaceous adsorbate to CO 2 [39,40] . Basedon most acceptedbifunctional mechanism of PtRu, similar typeof mechanism has been interpreted for enhancement in the catalyticactivity of PtV 2O5 [41] . First , methanol is preferred to bind with Ptsurface atoms, and dehydrogenated to form CO adsorbed species.The COad intermediates are thought as the main poisoning speciesduring electro -oxidation of methanol. Thus how to oxidize CO adintermediates as quickly as possible is very important to methanoloxidation. Due to the higher afnity of vanadium oxides towards

Fig. 2. TEM images of (a) Pt/C and (b) Pt/V 2O5C electro-catalysts.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

C u r r e n

t d e n s i t y

( m A / c m

2 )

Potential (V) vs Ag/AgCl

(b)

(a)(a) 20%Pt/V 2O5- C

(b) 20% Pt/C

Fig. 3. Cyclic voltammograms of (a) Pt/V 2O5C and (b) Pt/C in 1 M H 2SO4/1 MCH3OH run at 50 mV/s.

Table 1

Comparison of activity of methanol oxidation between Pt/V 2O5 C and Pt/C electrodes.

S. No. Electrode Onset potential (V) Activity a I f /I b

Forward sweep Reverse sweep

E (V) I (mA cm 2) E (V) I (mA cm 2)

1 Pt/C (J.M.) 0.31 0.76 12.25 0.62 13.49 0.92 PtV2O5/C 0.21 0.811 17.4 0.63 16.52 1.06

a Activity evaluated from cyclic voltammogram run in 1 M H 2SO4/1M CH3OH.

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oxygen-containing species, sufcient amounts of OH ad to supportreasonable CO oxidation rates are formed at lower potential onV2O5 composite sites than on Pt sites. The OH ad species are neces-sary for the oxidative removal of CO ad intermediates. This effectleads to the higher activity and longer lifetime for the overallmethanol oxidation process on Pt/V 2O5C composite. Based onthe experimental results, to illustrate the enhanced activity of methanol electro-oxidation a similar promotional reaction modelis proposed as follows,

CH 3 OH ad ! CO ad 4H

4e

V 2 O 5 2H

! 2VO

2 H 2 O4VO

2 4H

! 4VO2

O 2 2H 2 O

VO 2 H 2 O ! VOOH

H

CO ad VOOH

! CO 2 VO2

H e

4. Conclusion

Highly dispersed nanosized Pt particles on V 2O5C compositehave been prepared by formaldehyde reduction.Pt/V 2O5C com-posite catalyst exhibits higher catalytic activity for the methanoloxidation reaction than Pt/C, which is attributed to the syner-getic effects due to formation of an interface between the plati-

num and V 2O5, and by spillover due to diffusion of the CO

intermediates. Easier formation of the oxygen-containing specieson the surface of V 2O5 favours the oxidation of CO intermediatesto CO2 and releasing the active sites on Pt for further electro-chemical reaction.

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(2008) 2191.

0 500 1000 1500

0

10

20

30

40

50

60

Time (Sec)

C u r r e n

t d e n s

i t y ( m A / c m

2 )

(b)

(a)

(a) 20% Pt/V 2O5- C(b) 20% Pt/C

Fig. 4. Current density vs. time curves at (a) Pt/V 2O5C (b) Pt/C measured in 1 M

H2SO4 + 1M 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) 433436