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RESEARCHARTICLE

Copyright copy 2006 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 6 2067ndash2071 2006

Electro-Oxidation of Methanol on TiO2Nanotube Supported Platinum Electrodes

T Maiyalagan B Viswanathanlowast and U V VaradarajuDepartment of Chemistry Indian Institute of Technology Madras Chennai 600036 India

TiO2 nanotubes have been synthesized using anodic alumina membrane as template Highly dis-persed platinum nanoparticles have been supported on the TiO2 nanotube The supported systemhas been characterized by electron microscopy and electrochemical analysis SEM image showsthat the nanotubes are well aligned and the TEM image shows that the Pt particles are uniformlydistributed over the TiO2 nanotube support A homogeneous structure in the composite nanomate-rials is indicated by XRD analysis The electrocatalytic activity of the platinum catalyst supported onTiO2 nanotubes for methanol oxidation is found to be better than that of the standard commercialE-TEK catalyst

Keywords TiO2 Nanotubes Template Synthesis Catalyst Support Methanol Oxidation

1 INTRODUCTION

Fuel cells operating by the electrochemical oxidation ofhydrogen or methanol as fuels at the anode and reductionof oxygen at the cathode are attractive power sources dueto their high conversion efficiencies low pollution lightweight and high power density While methanol offersthe advantage of easy storage and transportation in com-parison to hydrogen oxygen fuel cell its energy density(sim2000 Whkg) and operating cell voltage (04 V) arelower than the theoretical energy density (sim6000 Whkg)and the thermodynamic potential (sim12 V)12 Howeverthe fuel cells could not reach the stage of commercializa-tion due to the high cost which are mainly associated withthe noble metal loaded electrodes as well as the membraneIn order to reduce the amount of Pt loading on the elec-trodes there have been considerable efforts to increase thedispersion of the metal on the support Pt nanoparticleshave been dispersed on a wide variety of substratessuch as carbon nanomaterials34 Nafion membranes56

polymers78 polymer-oxide nanocomposites9 three-dimensional organic matrices10 and oxide matrices11ndash18

Most often the catalyst is dispersed on a conventionalcarbon support and the support material influences thecatalytic activity through metal support interaction Disper-sion of Pt particles on an oxide matrix can lead dependingmainly on the nature of support to Pt supported oxide sys-tem that shows better behaviour than pure Pt On the otherhand if the oxide is not involved in the electrochemical

lowastAuthor to whom correspondence should be addressed

reactions taking place on the Pt sites it might just pro-vide a convenient matrix to produce a high surface areacatalyst1718

Titanium dioxide is an attractive system for electro-catalysis since if used as the support for metallic cat-alysts or electrocatalysts it may enhance their catalyticactivity on the basis of strong metal support interaction(SMSI)2324 TiO2 is an effective photocatalysts for oxi-dation of methanol19 PtTiO2 is stable in acidic or alka-line medium which has higher active surface area than Ptand shows high activity for oxygen reduction152021 Thereare several articles which deal with the methanol oxi-dation reaction on TiO2 supported Platinum catalyst1718

Titanium mesh supported electrodes showed high activ-ity on the methanol oxidation therefore appears to bea promising alternative to carbon-supported catalysts22

More important in the present case PdTiO2 nanotube hasbeen recently shown to act as a good catalyst for the oxi-dation of methanol23

The present report focuses on the efforts undertaken todevelop unconventional supports based on platinum cat-alysts for methanol oxidation The catalyst supported onmetal oxide nanotubes yields a better dispersion and showsbetter catalytic activity TiO2 nanotubes of the anataseform have been synthesized by sol gel method usinganodic aluminium oxide (AAO) as the template TiO2

nanotubes were used to disperse the platinum particleseffectively without sintering and to increase the catalyticactivity for methanol oxidation The tubular morphologyand the oxide nature of the support have influence on thedispersion as well as the catalytic activity of the electrodeTitanium dioxide is also known to have strong metal

J Nanosci Nanotechnol 2006 Vol 6 No 7 1533-4880200662067005 doi101166jnn2006324 2067

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes Maiyalagan et al

support interaction with Pt particles The present commu-nication deals with the preparation of highly dispersedplatinum supported on TiO2 nanotubes the evaluation ofthe catalytic activity for the methanol oxidation of theelectrodes and a comparison with the catalytic activity ofconventional electrodes

2 EXPERIMENTAL DETAILS

21 Materials

All the chemicals used were of analytical grade Tita-nium isopropoxide (Aldrich) and 2-propanol (Merck) wereused as received Hexachloroplatinic acid was obtainedfrom Aldrich 20 wt PtVulcan carbons were pro-cured from E-TEK Methanol and sulphuric acid wereobtained from Fischer chemicals The alumina templatemembranes (Anodisc 47) with 200 nm diameter poreswere obtained from Whatman Corp Nafion 5 wt solutionwas obtained from Dupont and was used as received

22 Synthesis of PtTiO2 Nanotubes

Titanium isopropoxide (5 mL) was added to 25 mL of2-propanol (mole ratio [Ti4+][2-propanol] = 120) Thesolution was stirred for 3 h at room temperature (298 K)The alumina template membrane was dipped into this solu-tion for 2 min After removal from the solution vacuumwas applied to the bottom of the membrane until the entirevolume of the solution was pulled through the membraneThe membrane was then air-dried for 60 min at 303 Kand then placed in a furnace (in air) with a temperatureramp of 2 C minminus1 to 873 K for 2 h The tempera-ture was then decreased at a ramp rate of 2 C minminus1 toroom temperature (303 K)24 The TiO2alumina compos-ite obtained (before the dissolution of template membrane)was immersed in 73 mM H2PtCl6 (aq) for 12 h Afterimmersion the membrane was dried in air and the ionswere reduced to the corresponding metal(s) by exposure toflowing H2 gas at 823 K for 3 h The resulting compositewas immersed into 3 M aqueous NaOH for several min-utes to dissolve the alumina template membrane This pro-cedure resulted in the formation of Pt nanocluster loadedTiO2 nanotubes

23 Preparation of Working Electrode

Glassy Carbon (GC) (Bas Electrode 007 cm2) was pol-ished to a mirror finish with 005 m alumina suspensionsbefore each experiment and served as an underlying sub-strate of the working electrode In order to prepare thecomposite electrode the nanotubes were dispersed ultra-sonically in water at a concentration of 1 mg mlminus1 and20 l aliquot was transferred on to a polished glassy car-bon substrate After the evaporation of water the resultingthin catalyst film was covered with 5 wt Nafion solution

Then the electrode was dried at 353 K and used as theworking electrode

24 Characterization Methods

The scanning electron micrographs were obtained usingJEOL JSM-840 model working at 15 keV after theremoval of alumina template For transmission electronmicroscopic studies the nanotubes dispersed in ethanolwere placed on the copper grid and the images wereobtained using Phillips 420 model operating at 120 keVThe X-ray diffraction patterns were obtained on a PhilipsPW 1820 diffractometer with Cu K (154178 Aring)radiation

25 Electrochemical Measurements

The catalyst was electrochemically characterized by cyclicvoltammetry (CV) using an electrochemical analyzer (Bio-analytical Sciences BAS 100) A common three-electrodeelectrochemical cell was used for the measurements Thecounter and reference electrodes were a platinum plate(5 cm2) and a saturated AgAgCl electrode respectivelyThe CV experiments were performed using 1 M H2SO4

solution in the presence of 1 M CH3OH at a scan rateof 50 mVs All the solutions were prepared by usingultra pure water (Millipore 18 M) The electrolyteswere degassed with nitrogen before the electrochemicalmeasurements

3 RESULTS AND DISCUSSION

The scanning electron microscopic (SEM) image of theTiO2 nanotubes obtained after dissolving the 200 nm alu-mina template membrane is shown in Figure 1 It can be

Fig 1 SEM image of TiO2 nanotubes obtained by sol gel method cal-cined at 650 C for 2 h

2068 J Nanosci Nanotechnol 6 2067ndash2071 2006

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Maiyalagan et al Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes

Fig 2 TEM images of (a) TiO2 nanotubes obtained by sol gel methodcalcined at 650 C for 2 h (b) Pt filled TiO2 nanotubes

seen from the figure that an ordered array of nanotubeswith uniform diameter and length is formed The open endand the hollow nature of the TiO2 nanotubes is also con-firmed by transmission electron microscopy (TEM) imageas shown in Figure 2a The outer diameter of the nanotubesis ca 200 nm retaining the size and near cylindrical shapeof the pores of the aluminium oxide template membraneThe TEM image of a PtTiO2 nanotube electrode is shownin Figure 2b which shows that the Pt particles are highlydispersed on the TiO2 nanotube support The Pt particlesize was found to be around 3ndash4 nm while their crystalstructure is confirmed by the XRD method The optimal Ptparticle size for reactions in the H2O2 fuel cell is around3 nm25 The importance of the Pt particle size on the activ-ity for methanol oxidation is due to the structure sensitivenature of the reaction and the fact that particles with dif-ferent sizes will have different dominant crystal planes andhence the different intercrystallite distances which mightinfluence methanol adsorption The commercial PtC has ahigh specific surface area but contributed mostly by micro-pores less than 1 nm and are therefore more difficult to befully accessible It has been reported that the mean valueof particle size for 20 PtVulcan (E-TEK) catalyst wasaround 26 nm26 The TiO2 nanotube matrix of anataseform can provide hydroxide ions to remove CO poisoningMethanol oxidation studies on the prepared electrode havebeen carried out using cyclic voltammetry

The XRD patterns of the PtTiO2 nanotubes as well asP-25 are shown in Figure 3 Rutile and anatase were seenby XRD in P25 titania but rutile was not seen in theTiO2 nanotubes The diffractograms of the synthesizedTiO2 nanotubes mainly belong to the crystalline structureof anatase TiO2 XRD pattern of the TiO2 nanotubes evi-denced the presence of anatase as the main phase AfterPt deposition the colour of the TiO2 nanotubes changedto dark gray and during reduction of Pt oxide reductiontakes place and new diffraction peaks are formed Thepresence of Pt could be observed at diffraction angle of398 indexed to (111) plane of metallic Pt However thepeak intensity is relatively weak presumably due to thecombination of its low content and small particle size

20 30 40 50 60

R(211)

R(220)

A(112)

A(103)

R(203)

R(101)

A(211)A(105)

Pt(c)

(a)

(b)A(200)

A(004)A(103)

R(110)

(a) TiO2 Nanotube(b) TiO2- Degussa(c) Pt TiO2 Nanotube

A(101)

Inte

nsity

( a

u )

2 theta

Fig 3 X-ray diffraction patterns of (a) Degussa TiO2 as a reference(b) TiO2 nanotube and (c) PtTiO2 nanotube

In order to evaluate the electrocatalytic activity of thePtTiO2 nanotube electrodes for the oxidation of methanolcyclic voltammetric studies were carried out in 05 MH2SO4 and 1 M CH3OH During the anodic scan the cur-rent increases due to dehydrogenation of methanol fol-lowed by the oxidation of absorbed methanol residuesand reaches a maximum in the potential range between08 and 10 V versus AgAgCl In the cathodic scan there-oxidation of the residues is observed On the whole thebehaviour of the PtTiO2 nanotube electrodes was found tobe similar to that of Pt This suggests that the electrooxida-tion reaction takes place on the Pt nanoparticles dispersedon the TiO2 nanotube involves basically the same reactionmechanism

Fig 4 Cyclic voltammogram of (a) pure Pt (b) PtC and (c) PtTiO2

nanotube in 05 M H2SO41 M CH3OH run at 50 mVs (area of theelectrode = 007 cm2)

J Nanosci Nanotechnol 6 2067ndash2071 2006 2069

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RESEARCHARTICLE

Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes Maiyalagan et al

Table I Electrocatalytic activity of various catalysts for methanoloxidation

Specific activity Mass activityElectrocatalyst (mA cmminus2) (mA mgminus1 Pt)

Bulk Pt 0167 mdashPtC 13 325PtTiO2 nanotube 132 33

The results of the voltammetric curves for the oxidationof methanol obtained with the PtTiO2 nanotube Pt andPtC (E-TEK) electrodes are shown in Figure 3 ThePtTiO2 nanotube shows a higher current density of132 mAcm2 compared to PtC (E-TEK) electrodes(123 mAcm2) The specific activity and the mass activityfor the different electrodes are given in Table I The resultsshow that the high electrocatalytic activity for methanoloxidation for PtTiO2 nanotube electrode It is evidentthat the mass activity observed with the Pt supportedTiO2 nanotubes shows around ten-fold increase in currentthan PtC (E-TEK) electrode The PtTiO2 nanotube cat-alyst had a better electrocatalytic activity for methanoloxidation when compared with that of bulk Pt and PtC(E-TEK) catalysts This higher catalytic activity can bemainly attributed to remarkably platinum active reactionsites on the nanotube oxide matrix and the role of the TiO2

nanotube facilitates as a path for methanol (CH3OH) as afuel and Protons (H+) produced during an electrochemicalreaction

It is possible that TiO2 nanotube functions in the sameway as Ru does in Pt-RuC catalysts because hydroxideion species could easily form on the surface of the TiO2

nanotubes The formation of hydroxide ion species on thesurface of the TiO2 nanotubes transforms CO like poi-soning species on Pt to CO2 leaving the active sites onPt for further electrochemical reaction has been shown inFigure 52728 The participation of the TiO2 nanotube sup-port the high dispersion of Pt particles on TiO2 nanotubeelectrode OH groups generated near the Pt-oxide interfacepromote CO removal and strong metal support interaction(SMSI) could be a reason for enhanced electrocatalyticactivity of methanol oxidation2930

Fig 5 A possible mechanism for the removal of CO poisoning inter-mediates during methanol oxidation over TiO2 nanotube supported Ptcatalysts

4 CONCLUSIONS

The Pt was deposited on TiO2 nanotubes in order to studythe effect of the properties of the support for methanol oxi-dation reaction The PtTiO2 nanotube catalyst exhibits ahigh electrocatalytic activity for methanol oxidation com-pared to the commercial E-TEK catalysts Overall therelative activities are of the order PtTiO2 nanotubes gtE-TEKgt pure Pt The observed improved catalytic activityof PtTiO2 nanotube catalysts can be due to oxidationof CO to CO2 by the surface hydroxyl groups of TiO2

nanotube support which otherwise poison the active Ptsites The electronic interaction between TiO2 support andthe Pt particles could also be another factor contributing tothe observed higher activity Further study on the detailedmechanism and stability of the TiO2 nanotube supportedcatalysts are now under progress

Acknowledgment We thank the Council of Scien-tific and Industrial Research (CSIR) India for a seniorresearch fellowship to one of the authors T Maiyalagan

References and Notes

1 B D McNicol D A J Rand and K R Williams J Power Sources83 47 (2001)

2 L Carrette K A Friedrich and U Stimming Fuel Cells 1 5(2001)

3 C L Lee Y C Ju P T Chou Y C Huang L C Kuo and J COung Electrochem Commun 7 453 (2005)

4 T Maiyalagan B Viswanathan and U V Varadaraju ElectrochemCommun 7 905 (2005)

5 M Watanabe H Uchida and M Emori J Phys Chem B 102 3129(1998)

6 H Uchida Y Mizuno and M Watanabe J Electrochem Soc 149A682 (2002)

7 W T Napporn H Laborde J M Leger and C Lamy J Elec-troanal Chem 404 153 (1996)

8 M T Giacomini E A Ticianelli J McBreen andM Balasubramanian J Electrochem Soc 148 A323 (2001)

9 B Rajesh K R Thampi J M Bonard N Xanthapolous H JMathieu and B Viswanathan Electrochem Solid-State Lett 5 E71(2002)

10 H Bonnemann N Waldofner H G Haubold and T Vad ChemMater 14 1115 (2002)

11 V Raghuveer and B Viswanathan Fuel 81 2191 (2002)12 L F DrsquoElia L Rincoacuten and R Ortiacutez Electrochim Acta 49 4197

(2004)13 M I Rojas M J Esplandiu L B Avalle E P M Leiva and V A

Macagno Electrochim Acta 43 1785 (1998)14 M J Esplandiu L B Avalle and V A Macagno Electrochim Acta

40 2587 (1995)15 V B Baez and D Pletcher J Electroanal Chem 382 59

(1995)16 A Hamnett P S Stevens and R D Wingate J Appl Electrochem

21 982 (1991)17 T Ioroi Z Siroma N Fujiwara S Yamazaki and K Yasuda Elec-

trochem Commun 7 183 (2001)18 B E Hayden and D V Malevich Electrochem Commun 3 395

(2001)19 P A Mandelbaum A E Regazzoni M A Blesa and S A Bilmes

J Phys Chem B 103 5505 (1999)

2070 J Nanosci Nanotechnol 6 2067ndash2071 2006

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes Maiyalagan et al

support interaction with Pt particles The present commu-nication deals with the preparation of highly dispersedplatinum supported on TiO2 nanotubes the evaluation ofthe catalytic activity for the methanol oxidation of theelectrodes and a comparison with the catalytic activity ofconventional electrodes

2 EXPERIMENTAL DETAILS

21 Materials

All the chemicals used were of analytical grade Tita-nium isopropoxide (Aldrich) and 2-propanol (Merck) wereused as received Hexachloroplatinic acid was obtainedfrom Aldrich 20 wt PtVulcan carbons were pro-cured from E-TEK Methanol and sulphuric acid wereobtained from Fischer chemicals The alumina templatemembranes (Anodisc 47) with 200 nm diameter poreswere obtained from Whatman Corp Nafion 5 wt solutionwas obtained from Dupont and was used as received

22 Synthesis of PtTiO2 Nanotubes

Titanium isopropoxide (5 mL) was added to 25 mL of2-propanol (mole ratio [Ti4+][2-propanol] = 120) Thesolution was stirred for 3 h at room temperature (298 K)The alumina template membrane was dipped into this solu-tion for 2 min After removal from the solution vacuumwas applied to the bottom of the membrane until the entirevolume of the solution was pulled through the membraneThe membrane was then air-dried for 60 min at 303 Kand then placed in a furnace (in air) with a temperatureramp of 2 C minminus1 to 873 K for 2 h The tempera-ture was then decreased at a ramp rate of 2 C minminus1 toroom temperature (303 K)24 The TiO2alumina compos-ite obtained (before the dissolution of template membrane)was immersed in 73 mM H2PtCl6 (aq) for 12 h Afterimmersion the membrane was dried in air and the ionswere reduced to the corresponding metal(s) by exposure toflowing H2 gas at 823 K for 3 h The resulting compositewas immersed into 3 M aqueous NaOH for several min-utes to dissolve the alumina template membrane This pro-cedure resulted in the formation of Pt nanocluster loadedTiO2 nanotubes

23 Preparation of Working Electrode

Glassy Carbon (GC) (Bas Electrode 007 cm2) was pol-ished to a mirror finish with 005 m alumina suspensionsbefore each experiment and served as an underlying sub-strate of the working electrode In order to prepare thecomposite electrode the nanotubes were dispersed ultra-sonically in water at a concentration of 1 mg mlminus1 and20 l aliquot was transferred on to a polished glassy car-bon substrate After the evaporation of water the resultingthin catalyst film was covered with 5 wt Nafion solution

Then the electrode was dried at 353 K and used as theworking electrode

24 Characterization Methods

The scanning electron micrographs were obtained usingJEOL JSM-840 model working at 15 keV after theremoval of alumina template For transmission electronmicroscopic studies the nanotubes dispersed in ethanolwere placed on the copper grid and the images wereobtained using Phillips 420 model operating at 120 keVThe X-ray diffraction patterns were obtained on a PhilipsPW 1820 diffractometer with Cu K (154178 Aring)radiation

25 Electrochemical Measurements

The catalyst was electrochemically characterized by cyclicvoltammetry (CV) using an electrochemical analyzer (Bio-analytical Sciences BAS 100) A common three-electrodeelectrochemical cell was used for the measurements Thecounter and reference electrodes were a platinum plate(5 cm2) and a saturated AgAgCl electrode respectivelyThe CV experiments were performed using 1 M H2SO4

solution in the presence of 1 M CH3OH at a scan rateof 50 mVs All the solutions were prepared by usingultra pure water (Millipore 18 M) The electrolyteswere degassed with nitrogen before the electrochemicalmeasurements

3 RESULTS AND DISCUSSION

The scanning electron microscopic (SEM) image of theTiO2 nanotubes obtained after dissolving the 200 nm alu-mina template membrane is shown in Figure 1 It can be

Fig 1 SEM image of TiO2 nanotubes obtained by sol gel method cal-cined at 650 C for 2 h

2068 J Nanosci Nanotechnol 6 2067ndash2071 2006

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Maiyalagan et al Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes

Fig 2 TEM images of (a) TiO2 nanotubes obtained by sol gel methodcalcined at 650 C for 2 h (b) Pt filled TiO2 nanotubes

seen from the figure that an ordered array of nanotubeswith uniform diameter and length is formed The open endand the hollow nature of the TiO2 nanotubes is also con-firmed by transmission electron microscopy (TEM) imageas shown in Figure 2a The outer diameter of the nanotubesis ca 200 nm retaining the size and near cylindrical shapeof the pores of the aluminium oxide template membraneThe TEM image of a PtTiO2 nanotube electrode is shownin Figure 2b which shows that the Pt particles are highlydispersed on the TiO2 nanotube support The Pt particlesize was found to be around 3ndash4 nm while their crystalstructure is confirmed by the XRD method The optimal Ptparticle size for reactions in the H2O2 fuel cell is around3 nm25 The importance of the Pt particle size on the activ-ity for methanol oxidation is due to the structure sensitivenature of the reaction and the fact that particles with dif-ferent sizes will have different dominant crystal planes andhence the different intercrystallite distances which mightinfluence methanol adsorption The commercial PtC has ahigh specific surface area but contributed mostly by micro-pores less than 1 nm and are therefore more difficult to befully accessible It has been reported that the mean valueof particle size for 20 PtVulcan (E-TEK) catalyst wasaround 26 nm26 The TiO2 nanotube matrix of anataseform can provide hydroxide ions to remove CO poisoningMethanol oxidation studies on the prepared electrode havebeen carried out using cyclic voltammetry

The XRD patterns of the PtTiO2 nanotubes as well asP-25 are shown in Figure 3 Rutile and anatase were seenby XRD in P25 titania but rutile was not seen in theTiO2 nanotubes The diffractograms of the synthesizedTiO2 nanotubes mainly belong to the crystalline structureof anatase TiO2 XRD pattern of the TiO2 nanotubes evi-denced the presence of anatase as the main phase AfterPt deposition the colour of the TiO2 nanotubes changedto dark gray and during reduction of Pt oxide reductiontakes place and new diffraction peaks are formed Thepresence of Pt could be observed at diffraction angle of398 indexed to (111) plane of metallic Pt However thepeak intensity is relatively weak presumably due to thecombination of its low content and small particle size

20 30 40 50 60

R(211)

R(220)

A(112)

A(103)

R(203)

R(101)

A(211)A(105)

Pt(c)

(a)

(b)A(200)

A(004)A(103)

R(110)

(a) TiO2 Nanotube(b) TiO2- Degussa(c) Pt TiO2 Nanotube

A(101)

Inte

nsity

( a

u )

2 theta

Fig 3 X-ray diffraction patterns of (a) Degussa TiO2 as a reference(b) TiO2 nanotube and (c) PtTiO2 nanotube

In order to evaluate the electrocatalytic activity of thePtTiO2 nanotube electrodes for the oxidation of methanolcyclic voltammetric studies were carried out in 05 MH2SO4 and 1 M CH3OH During the anodic scan the cur-rent increases due to dehydrogenation of methanol fol-lowed by the oxidation of absorbed methanol residuesand reaches a maximum in the potential range between08 and 10 V versus AgAgCl In the cathodic scan there-oxidation of the residues is observed On the whole thebehaviour of the PtTiO2 nanotube electrodes was found tobe similar to that of Pt This suggests that the electrooxida-tion reaction takes place on the Pt nanoparticles dispersedon the TiO2 nanotube involves basically the same reactionmechanism

Fig 4 Cyclic voltammogram of (a) pure Pt (b) PtC and (c) PtTiO2

nanotube in 05 M H2SO41 M CH3OH run at 50 mVs (area of theelectrode = 007 cm2)

J Nanosci Nanotechnol 6 2067ndash2071 2006 2069

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes Maiyalagan et al

Table I Electrocatalytic activity of various catalysts for methanoloxidation

Specific activity Mass activityElectrocatalyst (mA cmminus2) (mA mgminus1 Pt)

Bulk Pt 0167 mdashPtC 13 325PtTiO2 nanotube 132 33

The results of the voltammetric curves for the oxidationof methanol obtained with the PtTiO2 nanotube Pt andPtC (E-TEK) electrodes are shown in Figure 3 ThePtTiO2 nanotube shows a higher current density of132 mAcm2 compared to PtC (E-TEK) electrodes(123 mAcm2) The specific activity and the mass activityfor the different electrodes are given in Table I The resultsshow that the high electrocatalytic activity for methanoloxidation for PtTiO2 nanotube electrode It is evidentthat the mass activity observed with the Pt supportedTiO2 nanotubes shows around ten-fold increase in currentthan PtC (E-TEK) electrode The PtTiO2 nanotube cat-alyst had a better electrocatalytic activity for methanoloxidation when compared with that of bulk Pt and PtC(E-TEK) catalysts This higher catalytic activity can bemainly attributed to remarkably platinum active reactionsites on the nanotube oxide matrix and the role of the TiO2

nanotube facilitates as a path for methanol (CH3OH) as afuel and Protons (H+) produced during an electrochemicalreaction

It is possible that TiO2 nanotube functions in the sameway as Ru does in Pt-RuC catalysts because hydroxideion species could easily form on the surface of the TiO2

nanotubes The formation of hydroxide ion species on thesurface of the TiO2 nanotubes transforms CO like poi-soning species on Pt to CO2 leaving the active sites onPt for further electrochemical reaction has been shown inFigure 52728 The participation of the TiO2 nanotube sup-port the high dispersion of Pt particles on TiO2 nanotubeelectrode OH groups generated near the Pt-oxide interfacepromote CO removal and strong metal support interaction(SMSI) could be a reason for enhanced electrocatalyticactivity of methanol oxidation2930

Fig 5 A possible mechanism for the removal of CO poisoning inter-mediates during methanol oxidation over TiO2 nanotube supported Ptcatalysts

4 CONCLUSIONS

The Pt was deposited on TiO2 nanotubes in order to studythe effect of the properties of the support for methanol oxi-dation reaction The PtTiO2 nanotube catalyst exhibits ahigh electrocatalytic activity for methanol oxidation com-pared to the commercial E-TEK catalysts Overall therelative activities are of the order PtTiO2 nanotubes gtE-TEKgt pure Pt The observed improved catalytic activityof PtTiO2 nanotube catalysts can be due to oxidationof CO to CO2 by the surface hydroxyl groups of TiO2

nanotube support which otherwise poison the active Ptsites The electronic interaction between TiO2 support andthe Pt particles could also be another factor contributing tothe observed higher activity Further study on the detailedmechanism and stability of the TiO2 nanotube supportedcatalysts are now under progress

Acknowledgment We thank the Council of Scien-tific and Industrial Research (CSIR) India for a seniorresearch fellowship to one of the authors T Maiyalagan

References and Notes

1 B D McNicol D A J Rand and K R Williams J Power Sources83 47 (2001)

2 L Carrette K A Friedrich and U Stimming Fuel Cells 1 5(2001)

3 C L Lee Y C Ju P T Chou Y C Huang L C Kuo and J COung Electrochem Commun 7 453 (2005)

4 T Maiyalagan B Viswanathan and U V Varadaraju ElectrochemCommun 7 905 (2005)

5 M Watanabe H Uchida and M Emori J Phys Chem B 102 3129(1998)

6 H Uchida Y Mizuno and M Watanabe J Electrochem Soc 149A682 (2002)

7 W T Napporn H Laborde J M Leger and C Lamy J Elec-troanal Chem 404 153 (1996)

8 M T Giacomini E A Ticianelli J McBreen andM Balasubramanian J Electrochem Soc 148 A323 (2001)

9 B Rajesh K R Thampi J M Bonard N Xanthapolous H JMathieu and B Viswanathan Electrochem Solid-State Lett 5 E71(2002)

10 H Bonnemann N Waldofner H G Haubold and T Vad ChemMater 14 1115 (2002)

11 V Raghuveer and B Viswanathan Fuel 81 2191 (2002)12 L F DrsquoElia L Rincoacuten and R Ortiacutez Electrochim Acta 49 4197

(2004)13 M I Rojas M J Esplandiu L B Avalle E P M Leiva and V A

Macagno Electrochim Acta 43 1785 (1998)14 M J Esplandiu L B Avalle and V A Macagno Electrochim Acta

40 2587 (1995)15 V B Baez and D Pletcher J Electroanal Chem 382 59

(1995)16 A Hamnett P S Stevens and R D Wingate J Appl Electrochem

21 982 (1991)17 T Ioroi Z Siroma N Fujiwara S Yamazaki and K Yasuda Elec-

trochem Commun 7 183 (2001)18 B E Hayden and D V Malevich Electrochem Commun 3 395

(2001)19 P A Mandelbaum A E Regazzoni M A Blesa and S A Bilmes

J Phys Chem B 103 5505 (1999)

2070 J Nanosci Nanotechnol 6 2067ndash2071 2006

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Maiyalagan et al Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes

Fig 2 TEM images of (a) TiO2 nanotubes obtained by sol gel methodcalcined at 650 C for 2 h (b) Pt filled TiO2 nanotubes

seen from the figure that an ordered array of nanotubeswith uniform diameter and length is formed The open endand the hollow nature of the TiO2 nanotubes is also con-firmed by transmission electron microscopy (TEM) imageas shown in Figure 2a The outer diameter of the nanotubesis ca 200 nm retaining the size and near cylindrical shapeof the pores of the aluminium oxide template membraneThe TEM image of a PtTiO2 nanotube electrode is shownin Figure 2b which shows that the Pt particles are highlydispersed on the TiO2 nanotube support The Pt particlesize was found to be around 3ndash4 nm while their crystalstructure is confirmed by the XRD method The optimal Ptparticle size for reactions in the H2O2 fuel cell is around3 nm25 The importance of the Pt particle size on the activ-ity for methanol oxidation is due to the structure sensitivenature of the reaction and the fact that particles with dif-ferent sizes will have different dominant crystal planes andhence the different intercrystallite distances which mightinfluence methanol adsorption The commercial PtC has ahigh specific surface area but contributed mostly by micro-pores less than 1 nm and are therefore more difficult to befully accessible It has been reported that the mean valueof particle size for 20 PtVulcan (E-TEK) catalyst wasaround 26 nm26 The TiO2 nanotube matrix of anataseform can provide hydroxide ions to remove CO poisoningMethanol oxidation studies on the prepared electrode havebeen carried out using cyclic voltammetry

The XRD patterns of the PtTiO2 nanotubes as well asP-25 are shown in Figure 3 Rutile and anatase were seenby XRD in P25 titania but rutile was not seen in theTiO2 nanotubes The diffractograms of the synthesizedTiO2 nanotubes mainly belong to the crystalline structureof anatase TiO2 XRD pattern of the TiO2 nanotubes evi-denced the presence of anatase as the main phase AfterPt deposition the colour of the TiO2 nanotubes changedto dark gray and during reduction of Pt oxide reductiontakes place and new diffraction peaks are formed Thepresence of Pt could be observed at diffraction angle of398 indexed to (111) plane of metallic Pt However thepeak intensity is relatively weak presumably due to thecombination of its low content and small particle size

20 30 40 50 60

R(211)

R(220)

A(112)

A(103)

R(203)

R(101)

A(211)A(105)

Pt(c)

(a)

(b)A(200)

A(004)A(103)

R(110)

(a) TiO2 Nanotube(b) TiO2- Degussa(c) Pt TiO2 Nanotube

A(101)

Inte

nsity

( a

u )

2 theta

Fig 3 X-ray diffraction patterns of (a) Degussa TiO2 as a reference(b) TiO2 nanotube and (c) PtTiO2 nanotube

In order to evaluate the electrocatalytic activity of thePtTiO2 nanotube electrodes for the oxidation of methanolcyclic voltammetric studies were carried out in 05 MH2SO4 and 1 M CH3OH During the anodic scan the cur-rent increases due to dehydrogenation of methanol fol-lowed by the oxidation of absorbed methanol residuesand reaches a maximum in the potential range between08 and 10 V versus AgAgCl In the cathodic scan there-oxidation of the residues is observed On the whole thebehaviour of the PtTiO2 nanotube electrodes was found tobe similar to that of Pt This suggests that the electrooxida-tion reaction takes place on the Pt nanoparticles dispersedon the TiO2 nanotube involves basically the same reactionmechanism

Fig 4 Cyclic voltammogram of (a) pure Pt (b) PtC and (c) PtTiO2

nanotube in 05 M H2SO41 M CH3OH run at 50 mVs (area of theelectrode = 007 cm2)

J Nanosci Nanotechnol 6 2067ndash2071 2006 2069

Delivered by Ingenta toUniversity College Cork

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RESEARCHARTICLE

Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes Maiyalagan et al

Table I Electrocatalytic activity of various catalysts for methanoloxidation

Specific activity Mass activityElectrocatalyst (mA cmminus2) (mA mgminus1 Pt)

Bulk Pt 0167 mdashPtC 13 325PtTiO2 nanotube 132 33

The results of the voltammetric curves for the oxidationof methanol obtained with the PtTiO2 nanotube Pt andPtC (E-TEK) electrodes are shown in Figure 3 ThePtTiO2 nanotube shows a higher current density of132 mAcm2 compared to PtC (E-TEK) electrodes(123 mAcm2) The specific activity and the mass activityfor the different electrodes are given in Table I The resultsshow that the high electrocatalytic activity for methanoloxidation for PtTiO2 nanotube electrode It is evidentthat the mass activity observed with the Pt supportedTiO2 nanotubes shows around ten-fold increase in currentthan PtC (E-TEK) electrode The PtTiO2 nanotube cat-alyst had a better electrocatalytic activity for methanoloxidation when compared with that of bulk Pt and PtC(E-TEK) catalysts This higher catalytic activity can bemainly attributed to remarkably platinum active reactionsites on the nanotube oxide matrix and the role of the TiO2

nanotube facilitates as a path for methanol (CH3OH) as afuel and Protons (H+) produced during an electrochemicalreaction

It is possible that TiO2 nanotube functions in the sameway as Ru does in Pt-RuC catalysts because hydroxideion species could easily form on the surface of the TiO2

nanotubes The formation of hydroxide ion species on thesurface of the TiO2 nanotubes transforms CO like poi-soning species on Pt to CO2 leaving the active sites onPt for further electrochemical reaction has been shown inFigure 52728 The participation of the TiO2 nanotube sup-port the high dispersion of Pt particles on TiO2 nanotubeelectrode OH groups generated near the Pt-oxide interfacepromote CO removal and strong metal support interaction(SMSI) could be a reason for enhanced electrocatalyticactivity of methanol oxidation2930

Fig 5 A possible mechanism for the removal of CO poisoning inter-mediates during methanol oxidation over TiO2 nanotube supported Ptcatalysts

4 CONCLUSIONS

The Pt was deposited on TiO2 nanotubes in order to studythe effect of the properties of the support for methanol oxi-dation reaction The PtTiO2 nanotube catalyst exhibits ahigh electrocatalytic activity for methanol oxidation com-pared to the commercial E-TEK catalysts Overall therelative activities are of the order PtTiO2 nanotubes gtE-TEKgt pure Pt The observed improved catalytic activityof PtTiO2 nanotube catalysts can be due to oxidationof CO to CO2 by the surface hydroxyl groups of TiO2

nanotube support which otherwise poison the active Ptsites The electronic interaction between TiO2 support andthe Pt particles could also be another factor contributing tothe observed higher activity Further study on the detailedmechanism and stability of the TiO2 nanotube supportedcatalysts are now under progress

Acknowledgment We thank the Council of Scien-tific and Industrial Research (CSIR) India for a seniorresearch fellowship to one of the authors T Maiyalagan

References and Notes

1 B D McNicol D A J Rand and K R Williams J Power Sources83 47 (2001)

2 L Carrette K A Friedrich and U Stimming Fuel Cells 1 5(2001)

3 C L Lee Y C Ju P T Chou Y C Huang L C Kuo and J COung Electrochem Commun 7 453 (2005)

4 T Maiyalagan B Viswanathan and U V Varadaraju ElectrochemCommun 7 905 (2005)

5 M Watanabe H Uchida and M Emori J Phys Chem B 102 3129(1998)

6 H Uchida Y Mizuno and M Watanabe J Electrochem Soc 149A682 (2002)

7 W T Napporn H Laborde J M Leger and C Lamy J Elec-troanal Chem 404 153 (1996)

8 M T Giacomini E A Ticianelli J McBreen andM Balasubramanian J Electrochem Soc 148 A323 (2001)

9 B Rajesh K R Thampi J M Bonard N Xanthapolous H JMathieu and B Viswanathan Electrochem Solid-State Lett 5 E71(2002)

10 H Bonnemann N Waldofner H G Haubold and T Vad ChemMater 14 1115 (2002)

11 V Raghuveer and B Viswanathan Fuel 81 2191 (2002)12 L F DrsquoElia L Rincoacuten and R Ortiacutez Electrochim Acta 49 4197

(2004)13 M I Rojas M J Esplandiu L B Avalle E P M Leiva and V A

Macagno Electrochim Acta 43 1785 (1998)14 M J Esplandiu L B Avalle and V A Macagno Electrochim Acta

40 2587 (1995)15 V B Baez and D Pletcher J Electroanal Chem 382 59

(1995)16 A Hamnett P S Stevens and R D Wingate J Appl Electrochem

21 982 (1991)17 T Ioroi Z Siroma N Fujiwara S Yamazaki and K Yasuda Elec-

trochem Commun 7 183 (2001)18 B E Hayden and D V Malevich Electrochem Commun 3 395

(2001)19 P A Mandelbaum A E Regazzoni M A Blesa and S A Bilmes

J Phys Chem B 103 5505 (1999)

2070 J Nanosci Nanotechnol 6 2067ndash2071 2006

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes Maiyalagan et al

Table I Electrocatalytic activity of various catalysts for methanoloxidation

Specific activity Mass activityElectrocatalyst (mA cmminus2) (mA mgminus1 Pt)

Bulk Pt 0167 mdashPtC 13 325PtTiO2 nanotube 132 33

The results of the voltammetric curves for the oxidationof methanol obtained with the PtTiO2 nanotube Pt andPtC (E-TEK) electrodes are shown in Figure 3 ThePtTiO2 nanotube shows a higher current density of132 mAcm2 compared to PtC (E-TEK) electrodes(123 mAcm2) The specific activity and the mass activityfor the different electrodes are given in Table I The resultsshow that the high electrocatalytic activity for methanoloxidation for PtTiO2 nanotube electrode It is evidentthat the mass activity observed with the Pt supportedTiO2 nanotubes shows around ten-fold increase in currentthan PtC (E-TEK) electrode The PtTiO2 nanotube cat-alyst had a better electrocatalytic activity for methanoloxidation when compared with that of bulk Pt and PtC(E-TEK) catalysts This higher catalytic activity can bemainly attributed to remarkably platinum active reactionsites on the nanotube oxide matrix and the role of the TiO2

nanotube facilitates as a path for methanol (CH3OH) as afuel and Protons (H+) produced during an electrochemicalreaction

It is possible that TiO2 nanotube functions in the sameway as Ru does in Pt-RuC catalysts because hydroxideion species could easily form on the surface of the TiO2

nanotubes The formation of hydroxide ion species on thesurface of the TiO2 nanotubes transforms CO like poi-soning species on Pt to CO2 leaving the active sites onPt for further electrochemical reaction has been shown inFigure 52728 The participation of the TiO2 nanotube sup-port the high dispersion of Pt particles on TiO2 nanotubeelectrode OH groups generated near the Pt-oxide interfacepromote CO removal and strong metal support interaction(SMSI) could be a reason for enhanced electrocatalyticactivity of methanol oxidation2930

Fig 5 A possible mechanism for the removal of CO poisoning inter-mediates during methanol oxidation over TiO2 nanotube supported Ptcatalysts

4 CONCLUSIONS

The Pt was deposited on TiO2 nanotubes in order to studythe effect of the properties of the support for methanol oxi-dation reaction The PtTiO2 nanotube catalyst exhibits ahigh electrocatalytic activity for methanol oxidation com-pared to the commercial E-TEK catalysts Overall therelative activities are of the order PtTiO2 nanotubes gtE-TEKgt pure Pt The observed improved catalytic activityof PtTiO2 nanotube catalysts can be due to oxidationof CO to CO2 by the surface hydroxyl groups of TiO2

nanotube support which otherwise poison the active Ptsites The electronic interaction between TiO2 support andthe Pt particles could also be another factor contributing tothe observed higher activity Further study on the detailedmechanism and stability of the TiO2 nanotube supportedcatalysts are now under progress

Acknowledgment We thank the Council of Scien-tific and Industrial Research (CSIR) India for a seniorresearch fellowship to one of the authors T Maiyalagan

References and Notes

1 B D McNicol D A J Rand and K R Williams J Power Sources83 47 (2001)

2 L Carrette K A Friedrich and U Stimming Fuel Cells 1 5(2001)

3 C L Lee Y C Ju P T Chou Y C Huang L C Kuo and J COung Electrochem Commun 7 453 (2005)

4 T Maiyalagan B Viswanathan and U V Varadaraju ElectrochemCommun 7 905 (2005)

5 M Watanabe H Uchida and M Emori J Phys Chem B 102 3129(1998)

6 H Uchida Y Mizuno and M Watanabe J Electrochem Soc 149A682 (2002)

7 W T Napporn H Laborde J M Leger and C Lamy J Elec-troanal Chem 404 153 (1996)

8 M T Giacomini E A Ticianelli J McBreen andM Balasubramanian J Electrochem Soc 148 A323 (2001)

9 B Rajesh K R Thampi J M Bonard N Xanthapolous H JMathieu and B Viswanathan Electrochem Solid-State Lett 5 E71(2002)

10 H Bonnemann N Waldofner H G Haubold and T Vad ChemMater 14 1115 (2002)

11 V Raghuveer and B Viswanathan Fuel 81 2191 (2002)12 L F DrsquoElia L Rincoacuten and R Ortiacutez Electrochim Acta 49 4197

(2004)13 M I Rojas M J Esplandiu L B Avalle E P M Leiva and V A

Macagno Electrochim Acta 43 1785 (1998)14 M J Esplandiu L B Avalle and V A Macagno Electrochim Acta

40 2587 (1995)15 V B Baez and D Pletcher J Electroanal Chem 382 59

(1995)16 A Hamnett P S Stevens and R D Wingate J Appl Electrochem

21 982 (1991)17 T Ioroi Z Siroma N Fujiwara S Yamazaki and K Yasuda Elec-

trochem Commun 7 183 (2001)18 B E Hayden and D V Malevich Electrochem Commun 3 395

(2001)19 P A Mandelbaum A E Regazzoni M A Blesa and S A Bilmes

J Phys Chem B 103 5505 (1999)

2070 J Nanosci Nanotechnol 6 2067ndash2071 2006

Delivered by Ingenta toUniversity College Cork

IP 1432396556Sat 29 Jul 2006 162005

RESEARCHARTICLE

Maiyalagan et al Electro-Oxidation of Methanol on TiO2 Nanotube Supported Platinum Electrodes

20 L Xiong and A Manthiram Electrochim Acta 49 4163(2004)

21 J Shim C-R Lee H-K Lee J-S Lee and E J Cairns J PowerSources 102 172 (2001)

22 E H Yu and K Scott J Electrochem Commun 6 361(2004)

23 M Wang D J Guo and H L Li J Solid State Chem 178 1996(2005)

24 S Lee C Jeon and Y Park Chem Mater 16 4292 (2004)25 K Kinoshita J Electrochem Soc 137 845 (1990)

26 E Antolini L Giorgi F Cardellini and E Passalacqua J SolidState Electrochem 5 131 (2001)

27 L Huaxin J Mol Catal A Chem 144 189 (1999)28 M Takeuchi K Sakamoto G Martra S Coluccia and M Anpo

J Phys Chem B 109 15422 (2005)29 S J Tauster S C Fung and R L Garten J Am Chem Soc 100

170 (1978)30 S G Neophytides S Zafeiratos G D Papakonstantinou J M

Jaksic F E Paloukis and M M Jaksic Int J Hyd Energy 30 393(2005)

Received 23 September 2005 RevisedAccepted 2 February 2006

J Nanosci Nanotechnol 6 2067ndash2071 2006 2071