Surfaceand CoatingsTechnology,27(1986) 359 - 369 359

METHANOL ELECTRO-OXIDATION AND SURFACECHARACTERISTICS OF AMORPHOUS Pt-Zr ALLOYS DOPED WITHTIN OR RUTHENIUM

K. MACHIDA, M. ENYO and I. TOYOSHIMA

ResearchInstitutefor Catalysis,HokkaidoUniversity,Sapporo060 (Japan)

Y. TODA* and T. MASUMOTO

The ResearchInstitute for Iron, Steeland OtherMetals, Tohoku University,Sendai980(Japan)

(ReceivedAugust 30, 1985)

Summary

Methanol electro-oxidationon electrodesof amorphousPt—Zr alloysdopedwith tin or rutheniumwas studiedin 0.5 M H2S04by meansof poten-tiostatic polarizationandthe resultswere correlatedwith surfacecharacteris-tics determinedby surfaceareameasurements,scanningelectronmicroscopyobservationsand X-ray photoelectronspectroscopy(XPS) analyses.Theelectrocatalyticactivity was considerablyenhancedby brief treatmentwithaqueousHF, which yielded a poroussurfacelayeron the alloy electrodes.The layerwas effectively in a higher stateof dispersionthanordinaryplati-num black. The XPS analysesindicated that Raney-typeporousplatinumlayers are formed in the surfaceregion,andthata suitableconcentrationoftin or ruthenium dopant is essentialin realizing a high electrocatalyticactivity.

1. Introduction

A numberof investigationson the electro-oxidationof methanolhavebeencarried out with the aim of constructingafuel cell system.It seemsestablishednow that platinum is in practicethe only electrocatalystwhichmaybe usedin this reaction.The activity of platinum is very labile, however,andbecomesvery poor afteralongperiodof polarization[1]. Somecocata-lysts such as tin, rhenium,rutheniumandosmiumarethereforeemployedinpracticeto maintainthe electrocatalyticactivity [2, 3].

Rapidly quenchedamorphousalloys are of interestfor useas electrodematerialsnot only becauseof their good mechanicalandcorrosion-resistant

*present address: Riken Co., 13 - 5 Kudankita 1-chome, Chiyoda-ku, Tokyo 102,

Japan.

0257-8972/86/$3.50 © Elsevier Sequoia/Printedin The Netherlands

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properties [4] but also becauseof the possibility of preparingmaterialswithwider compositionranges,including multicomponentsystems.Someof thepresent authors have reported previously the cathodepropertiesof amor-phousPd—Zr alloys [5], Ni—Ti andNi—Zr alloys [6] in water electrolysisandthe anodic formaldehydeelectro-oxidationon amorphousCu—Ti and Cu—Zralloys [7]. Throughinvestigationson both electrochemicalandsurfacechar-acterizations,somepeculiaritiesof amorphousalloyshavebeenindicated,inparticular the ability to offer a very largeeffective surfacearea.Thisreportis devotedto the electro-oxidationof methanolon amorphousPt—Zralloysdoped with tin or ruthenium, a systemalreadyknown to havegood electro-catalyticactivity towardsthis reaction.

2. Experimentaldetails

Amorphous Pt—Zr alloy ribbons doped with tin or ruthenium orundoped(Pt24Sn5Zr71,Pt23Sn-,Zr70, Pt22Sn9Zr69,Pt10Ru10Zr80and Pt21Zr79)were preparedby a single-rollerquenchingapparatus.The ribbons were 0.5mm wide and about20 j~tmthick. Recrystallizationof someof the alloyswascarried out by annealingthe amorphousalloys at about1100 K for 30 mmin uacuo. Both the amorphousand the crystalline alloys were treatedwith1 M HF solution for about 10 s at room temperatureto leachout the zirco-nium. The alloy ribbons (about5 mm long) were spotweldedto tantalumwires which were then sealed in Pyrex glass tubing and used as the testelectrodes.

The electrochemicalmeasurementswere carriedout potentiostaticallyin a three-compartmentPyrex glass electrolysiscell undera hydrogenorargon atmosphere.A reversible hydrogenelectrode(RHE) was usedas thereferenceelectrode.

Surfaceanalysesof the amorphousalloyswereperformedmainly usingan electron spectroscopyfor chemical analysis (ESCA) apparatus(VG-III)with Al Ka X-ray radiation. The binding energyvaluesrecordedwere cali-brated againstthe 4f712 signal from a gold plate which was employedas aninternal standard.Surface sputtering by Ar~ion bombardmentwas carriedout undervarious conditions: 1.0 keV, 4.4 ~uAcm

2 1.5 keV, 6.3 jiA cm22.0 keV, 8.8 jiA cm2. The sputtereddepthD~wasestimatedusingthe equa-tion given in ref. 8. The sputteringyields at a given Ar~ion energy for thealloy elements were evaluatedby interpolation of the data reported byKanayaet al. [9]. Scanningelectronmicroscopy(SEM) observationsof theamorphousalloys before and after the treatmentwith aqueousHF wereperformedwith an electron accelerationvoltage of 25 keV. The roughnessfactor R of the electrodeswas evaluatedby coulometricmeasurementof theunderpotentialdepositionof hydrogen. The surfaceareaof the sampleswasmeasured occasionally after thorough treatment with aqueousHF byobservingthe amountof nitrogenadsorptionat about80 K accordingto theconventionalBrunauer—Emmett—Teller(BET) method.

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3. Resultsanddiscussion

3.1. Methanolelectro-oxidationThe electrocatalytic activity for methanolelectro-oxidation on amor-

phousPt—Zr alloys with tin or rutheniumdopantwassignificantly enhancedby treatmentwith aqueousHF. At the sametime, a significant increaseinthe surfaceareaof the alloyswasobserved.Typical cyclic voltammogramsofthe HF-treatedamorphousPt21Zr79and Pt24Sn5Zr71alloys in a solution of1.0 M CH3OH and0.5 M H2S04are shown in Fig. 1. The oxidation on theundopedPt—Zr alloy startedat potentialsaround0.5 V(RHE) andthen thecurrent increasedsteeplyabove this potential (curve a). On the tin-dopedPt—Zr alloy, the activity was found to be improved(curveb), particularly inthe low potential region, comparedwith that of the undopedsystem. (Theactivity of the alloys containingan amount of tin abovea given threshold,e.g. Pt22Sn9Zr69,was very poor, however.) Repeatedpotential sweepsup to1.5 V, however,gaverise to a lossof the higheractivity in the low potentialregionand the resulting voltammogram(curve c) becamesimilar to that forthe undopedalloy electrode.

‘~la_ ~

E (V,RHE)

Fig. 1. Cyclic voltammograms for amorphousalloy electrodesin 0.5 M H2S04 containing1.0 M CH3OH at 303 K: (a) Pt21Zr79 (R = 710); (b) Pt24Sn5Zr71(R 500); (c) theelec-trode of (b) after severalpotential sweepsover 005 - 1.5 V. The potential sweeprate is10 mVs’.

In Fig. 2, Tafel plots of the methanol electro-oxidationcurrent foramorphousPt—Zr and Pt—Sn—Zr alloys are shown,togetherwith those forthe crystalline counterparts.It was seenthat the activity in the steadystatecondition was higher on the amorphousalloys than on the correspondingcrystallinespecimens.Similar trendswere observedfor the ruthenium-dopedamorphousand for the crystalline Pt—Zr alloys but no decreasein the activ-ity in the low potential regioncausedby potential sweepsup to 1.5 V was

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a-’

0’2 0~06 0.8

E (V, RHE)

Fig. 2. Anodic polarization behaviour of amorphous(full symbols) and of crystalline(open symbols) alloy electrodesin 0.5 M H

2504containing1.0 M CH3OHat 333 K: O,•,

Pt21Zr79 (R = 710); 0, •, Pt24Sn5Zr71(R 500).

TABLE 1

Methanol electro-oxidationcurrent on electrodesof amorphousand of crystalline Pt—Zralloys dopedwith tin or rutheniumin a solutionof 1.0 M CH3OH + 0.5 M H2S04

Alloy RF i(apparent) i(true)(Acm

2) (Acm2)

a-Pt21Zr79 710 2.3 3.4

a-Pt24Sn5Zr71 ~500 23.8 ~48a-Pt10Ru10Zr50 = 500 20.1 40

c-Pt21Zr79 150 0.09 0.6c-Pt~Sn5Zr71 ~150 5.9 =39c-Pt10Ru10Zr80 150 5.3 35

Potential,0.40 V(RHE);temperature, 333 K.a-, amorphous;c-, crystalline.

observedand the activity was very stable,in contrastwith the amorphousPt—Sn—Zrternaryalloys. Theseresultsaresummarizedin Table1.

Comparisonof the electrocatalytic activities of the amorphousandcrystalline alloys, after the effectivesurfaceareas,namelythe valuesof R, ofthe specimenshad been taken into account,indicatedthat they are essen-tially of the sameorderof magnitude.Also, the contributionsto the activity

363

of the tin and of the ruthenium were comparable.Nevertheless,undersimilar preparativeconditions and treatmentswith aqueousHF, the amor-phous alloys yielded a surfaceareamuch (three to five times) larger thanthat of the crystalline alloys. The amorphousalloys thusgenerallyattain amuch higherelectrocatalyticactivity on an apparentunit areabasis.

3.2. Surfacecharacterizations3.2.1.Brunauer—Emmett—TellerareaThe BET areavalues of the amorphousand of the crystalline Pt—Zr

alloys undopedand dopedwith tin andwith rutheniumafterdeepacid treat-ments (which yielded avery fragile state)are summarizedin Table 2. Whilethe surface area of the crystalline system was 5 - 15 m2 g1, the valuesobservedon the amorphousalloys werevery large(48 - 85 m2 g’) andevenmuch larger than that of usualplatinum blacks, e.g. 20 m2 g1 [10]. Themeaneffectiveparticle sizesevaluatedfrom the measuredsurfaceareavalueswere 42 - 74 A and 235 - 595 A for the amorphousandthe crystallinespec-imens respectively. These data are in good agreement with the trendsobservedin the electrochemicalstudy.

TABLE 2

Surfaceporosity of theHF-treatedamorphousandcrystalline Pt—Zr alloys dopedwith tinand with ruthenium

Alloy BET area Meanparticle size(m~g’) of Pt (A)

a-Pt21Zr79 61 46

a-Pt24Sn5Zr71 48 74a-Pt10Ru10Zr80 85 42

c-Pt21Zr79 5 560c-Pt24Sn5Zr71 15 235c-Pt10Ruj0Zr80 6 595

a-, amorphous;c-, crystalline.

3.2.2.ScanningelectronmicroscopyobservationsTypical SEM picturesof the amorphousPt—Zr binary alloysbeforeand

after treatmentwith aqueousHF areshownin Fig. 3. In asimilar mannertoamorphousNi—Ti and Ni—Zr alloys [6], the untreatedamorphousPt—Zralloy had many furrows on the surface,running parallelto the direction ofthe cooling roller. After the acid treatment,however, thesefurrows werecompletelyremovedand a porouslayer with dry mud cracksandmany fineporeswasformedon the surface.

3.2.3.X-ray photoelectronspectroscopyobservationsFigure 4 shows the X-ray photoelectronspectraof the Pt 4f andZr 3d

electrons on the amorphousPt—Zr alloy before and after treatmentwith

364

11111(a) (b)

Fig. 3. Scanningelectronmicrographsof theamorphousPt21Zr79alloy (a) beforeand (b)

after treatment with aqueousHF. The scalebar represents50pm.

68 72 76 80 178 82 186 90

E~i1 183.2 185.523eV71 2 745 3d5/2 3d312

(a) I I I I I

71.05 74.35

i 4f7.’2~-~-~~’[4f5/2

l\ l~\PtC 83.0 185.4

film,,,

(b) 68 72 76 80~I78 182 186 90

8lndlng Ener~y(eV)

Fig. 4. X-ray photoelectron spectraof the Pt 41 and Zr 3d electronson the amorphousPt21Zr79alloy (a) beforeand (b) after treatmentwith aqueousHF.

aqueousHF. The Pt 4f signalsof the untreatedsamplewerevery weakcom-paredwith those derivedfrom the Zr 3d electrons.They consistof two pairsof 4f bandsat 71.2 - 74.5 eV andat about73.7 - 77.0eV, which areassignedto metallic platinum and PtO respectively. The Zr 3d signals (183.2 and185.5 eV) are very strong andare assignedto Zr02.Therefore,the low elec-trocatalytic activity of the untreatedamorphousalloysmust be causedbythe Zr02 layer with which the surfacesare effectively covered.In contrast,the HF-treated specimensgave strong Pt 4f and weak Zr 3d signals, verydifferent from the X-ray photoelectronspectroscopy(XPS) intensity profileson the untreatedspecimen:metallic platinum only is likely to be responsiblefor the Pt 4f bandsbut the Zr 3d signalsarefrom the Zr02, asbefore.Theseresults show that the porous layer formed on the amorphous alloys bythe acid treatmentconsistsmainly of platinum, forming a kind of Raney

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485 490 495 ~, 530 535

530.93d

512 3d312 IS

4868 495.3 533.0

3d512 3d312 1111]486.8 495.3 530.2 IS

4848 —8 5eV —~-y lOx del 532.3 —

(b) 485 490 49i

1’ 530 535

Blndln9 Energy lxv)

Fig. 5. X-ray photoelectron spectraof the Sn 3d and 0 is electronson the amorphousPt

24Sn5Zr71 alloy (a) before and (b) after treatment with aqueous HF. The values inparenthesesrepresentthe sputtereddepths.

platinum. Analogousresultswere obtainedon the amorphousPt—Sn—Zr andPt—Ru—Zrternaryalloys.

The XPS results for tin on the ternary alloy surfacesare shown inFig. 5. Before the acid treatment, the X-ray photoelectronspectraof the Sn3d electrons,with peaksat 486.8 and 495.3 eV, correspondto thoseof tinoxide (SnO or Sn02). The spectrumof oxygen consistsof two bandswhichmay be derivedfrom the is electronsof the oxide and 0H groups(530.9and 533.0 eV). However, the Ar~ion bombardmentcausedthe appearanceof signalsfrom metallic tin with peakpositionsat 484.8 and493.3eV and,at the sametime, the disappearanceof the Sn 3d (tin oxide) and 0 is (0Hgroup)signals.

On the HF-treatedalloy two setsof Sn 3d signalswereobservedsimul-taneously.Thesewere at 484.8 - 493.3eV andat 486.8 - 495.3eV andwereassignedto the metallic stateand to the oxide state respectively,but thelatter was removedfrom the surfaceby the Ar~ion bombardment.Thus,tinwas present in the platinum-rich porous layer even after acid treatment.Also, the XPS signalsobtainedshowedthat the tin at the surfaceof the alloyboth before and afteracid treatmentwas mostly or at leastpartly oxidized.In aqueouselectrolytes such as H2S04, however, the tin is likely to bereduced to the metallic state according to the underpotentialdeposition(UPD) data [11]. It appearsto be possible,therefore,that the platinumonthe alloy containing tin with a concentrationabovea certain level may be

366

covered with a monolayer of tin. This view would explain the above-mentionedfact that the activity towardsthe methanolelectro-oxidationwasvery low on Pt22Sn9Zr69but high on the alloys containingasmalleramountof tin. However, the loss of the onceimproved catalytic activity in the lowpotential region, causedby potential sweepsup to 1.2 V(RHE), is probablydue to the dissolutionof tin from the platinum matrix. The dissolutionoftin was indeed confirmed by the XPS measurementsand this might havecauseda small degreeof activity enhancementin the high potential region,possibly owing to afurther increasein the effectivesurfacearea.

The XPS patternsof the Ru 3d and 0 is electronson the amorphousPt10Ru10Zr80alloy are shown in Fig. 6. Sincethe Ru 3d312 signaloverlappedwith that of the (impurity) C is (284.7 eV), the Ru 3d bands on theuntreatedsamplecould not be obtainedclearly.After the Ar~ion bombard-ment two bands derived from the 3d512 and 3d3,2 electrons were clearlyobservedat 279.8 eV and284.0eV respectively,andwereassignedto metal-lic ruthenium. Similarly, the HF-treatedalloy gaveXPS signalsfrom the 3delectrons of metallic ruthenium(280.3and284.5 eV). The 0 is spectracon-sisted of bandsfrom the oxygensof Zr02 and the 0H group at 53i.2 eVand 533.4 eV respectively.Therefore,the rutheniumon the alloy mustexistmainly in the metallic state even in aqueoussolutions and is scarcelydis-solved from the alloy matrix. This conclusionis probably reasonablefrom

280 285 290 530 535I I I

531 2

IS3d,,2 3d312

279~8 2840 533 4

(a)___

3ds,2 3d3,2280.3 2845 53L0 IS

(b) 280 285 2901 530 535

Binding Energy leVI

Fig. 6. X-ray photoelectron spectraof the Ru 3d and 0 is electronson the amorphousPt10Ru10Zr80alloy (a) before and (b) after treatment with aqueousHF. The valuesinparenthesesrepresentthesputtereddepths.

367

the standardelectrodepotential (Ru!Ru2~,0.45 V (standardhydrogenelec-trode) [12]) and, in this case,suchloweringof activity by the dissolutienofdopantduring potential cycling, as observedon the alloys containing tin, isunlikely.

3.2.4.DepthprofilesThe depth profiles of the elements,platinum, tin, rutheniumandzirco-

nium, of untreatedand of HF-treatedamorphousPt—Zr alloys dopedwithtin or ruthenium are shown in Figs. 7(a) - 7(c). The atomic fraction of eachelement in the amorphous Pt

21Zr79, Pt24Sn5Zr71 and Pt10Ru10Zr80alloyspecimenswas evaluatedon the basisof the integral peak intensities of thePt 4f7,2, Sn 3d5,2, Ru 3d5,2 and Zr 3d5,2 signalsaccordingto the equationgiven in the literature [8]. The following atomic sensitivity factors S forX-rayswere used[13]: S~,= 1.75, ~ = 3.2, SRU = 1.55and

5Zr = 0.87.The growth to a considerablethicknessof aplatinum-enrichedlayeron

HF treatmentof the amorphousPt—Zr alloy was demonstratedby the com-parisonof depth profiles of the platinum fraction on the untreatedandHF-treatedamorphousPt—Zr binary alloys, aspresentedin Fig. 7(a). Exceptfor

_.-_ S80% . 80

S ___._!_.

— iS — S- — -

C- C-)

40 - 40

O ~ — -

O 100 200 300 0 100 200 300

(a) Ds (A) (b) D

0 IA)

80

~4O

0 100 200 300

(c) D~ (A)Fig. 7. Depth profiles of the atomic fraction of platinum, tin and ruthenium on amor-phousalloys of (a)Pt21Zr7g,(b) Pt24Sn5Zr71and(c) Pt10Ru10Zr80before(opensymbols)and after (full symbols)treatment with aqueousHF: 0, •, platinum; A, A, tin; 0, 5, ruthe-nium.

368

the thin surfaceregion, the platinum fraction is about 20 at.% (closeto thebulk concentration)for the untreatedspecimen,whereasit becomes85 at.%and showedno tendencyto decreaseevenat adepthof severalhundredâng-strömsfor the HF-treatedalloys.

In the Pt—Sn—Zr ternary system, the characteristicsof the depthpro-files of platinum and tin were found to differ. In the untreatedalloy, thefraction of tin was constantwhile that of platinumgradually increasedwithdepth.The HF-treatedalloy gaveprofiles indicating that the tin fraction atthe surface(D8 30 A) was high comparedwith the bulk fraction. The highsurfaceconcentrationof tin on the HF-treatedalloy specimensmay be dueto the UPD effect as mentionedabove. Presumably,the tin atomsthatweredissolvedduring the acid treatmentimmediatelyredepositedon the platinumatomsat the alloy surface,giving rise to a surfacetin compositiondifferingfrom that of the bulk for the HF-treatedalloys. Careful control of the sur-facecompositionappearsto be necessaryfor the preparationof Raney-typeplatinumelectrocatalystscontainingtin.

For the amorphousPt—Ru—Zrternaryalloy, similardepth profiles wereobtained with both platinum and ruthenium on the untreated and HF-treatedspecimens(Fig. 7(c)). The atomic fractionsof platinumor rutheniumincreasedmonotonically with depth and,particularly in the untreatedspec-imen, gradually becameclose to the valuesfor the bulk fraction, althoughthe value for ruthenium wasusually somewhatlow comparedwith that forplatinum.

Acknowledgment

The presentwork is partly supported by Grants-in-Aid for ScientificResearchunderGrant 59045007 from the Ministry of Education,ScienceandCulture.

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