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An activecatalystsystemfor protonreductioncomposedof a bipyridyl platinumcomplexandapolymermembrane

ToshiyukiAbe1,3, KumikoTakahashi1, Yukihide Shiraishi2, NaokiToshima2, MasaoKaneko*1,4

1 Facultyof Science,IbarakiUniversity, 2-1-1Bunkyo,Mito 310-8512,Japan2 Departmentof MaterialsScienceandEngineering,ScienceUniversityof Tokyo in Yamaguchi,Onoda-shi,Yamaguchi756-0884,Japan

3 Presentaddress:Facultyof ScienceandTechnology, HirosakiUniversity, Aomori 036-8561,Japan4 Visiting seniorresearcherof TheInstituteof PhysicalandChemicalResearch(RIKEN), 2-1 Hirosawa,Wako,Saitama351-0198,Japan

(Received:March16,1999;revised:May 25,1999)

Intr oductionWater photolysis to obtain O2 and H2 is a promisingmodel for constructing an artificial photosyntheticsys-tem. However, therehasbeenso far almost no exampleof such an artificial photosynthetic system.The waterphotolysis into O2 andH2 needsto takeplaceby a multi-electron transfer, so that it is of importancetoward thisgoal thathighly activecatalysts aredevelopedto produceO2 and H2 at the oxidation/reduction sites. Up to nowthereare no highly active oxidation/reduction sitescap-able of efficient O2 and H2 formation. We have beenstudying efficient catalyst systemscomposedof a func-tional polymer membrane and a metal complexsuchasammine ruthenium complexes1), metal porphyrins andphthalocyanines2b,2c), which shouldbe coupled later witha photoexcitation center. It has beenfound that such aheterogeneouspolymer catalyst systemcan show muchhigher activity than a neat complex in the oxidation/reduction catalyses3). Suchpolymer catalyst systemscanbring aboutspecific catalysis by cooperative interactionbetween the catalysts1b,2b,2c), concentration effect of thesubstratein a matrix1), etc., andmoreover, the incorpora-tion of the catalyst into a polymermatrix can often sup-press its deactivation1). A molecule-basedcatalystis also

a promising candidate to construct a photochemicalenergy conversionsystembecausethe reaction compo-nents canbe arranged in a molecular level in the matrixasseenin nature.

Proton (H+) reductionto produce H2 is a fundamentalredox reaction, but it hasbeensolely known that Pt col-loids4) or its particles5) show an active catalysis to formH2. In some cases,stabilization of active Pt colloids byan aqueouspolymer solution as well as a solid polymerhas been attempted6). Recently, we have reported thatpolynuclear iron-cyanide complex2a), and metallo-por-phines (metal; Co, Fe and Mn) dispersedin a polymermembrane2b,2c) canwork asefficient molecule-basedcat-alysts to reduceH+, andthat their activity exceedsthatofa conventional Pt catalyst.Pt complexmaybeaninterest-ing material asa molecule-basedcatalyst in reducing H+,but the catalysis featureof Pt complexhasnot beenwelldocumentedexcept for thepolychloroplatinatecomplex7).Structural and voltammetric studieson the reduction ofthe Pt(bpy)22+ (bpy; 2,29-bipyridine) solution have beendone8), but the electrocatalysis for the H+ reduction hasnot beenstudied.In the present work, electrocatalytic H+

reduction by bis(2,29-bipyridine)platinum(II) nitrate(Pt(bpy)22+) incorporatedinto a Nafion membranecoated

Macromol.Chem.Phys.201, No. 1 i WILEY-VCH Verlag GmbH,D-69451 Weinheim 2000 1022-1352/2000/0101–0102$17.50+.50/0

Electrocatalysisfor the H+ reductionwith bis(2,29-bipyri-dyl) Pt complex (Pt(bpy)22+) incorporatedin a polymermatrix wasstudied.Whenthecyclic voltammogram(CV)was measuredat a basal-planepyrolytic graphite(BPG)electrodecoatedwith a Nafionm (Nf) membraneincorpor-ating Pt(bpy)22+ (denotedasBPG/Nf[Pt(bpy)22+]), a remark-able growth of the cathodiccurrentdue to H+ reductionwas observedbelow –0.4 V (vs. Ag/AgCl) in a repeatedscanning.A muchhigheramountof H2 wasobtainedwith

the systemBPG/Nf[Pt(bpy)22+] than by the conventionalPt-black.The XPS spectrumof Nf[Pt(bpy)22+] showedtheformationof zero-valentPt after the electrochemicalpro-cess,indicatingthatH+ reductionbeginsto takeplaceafterformation of the catalytically active species.This workshowsthata highly activecatalystsystemfor H+ reductioncanbefabricatedfrom a noblemetalcomplexanda poly-mermatrix.

Macromol.Chem.Phys.201, 102–106(2000)

An activecatalystsystemfor protonreduction composedof a bipyridyl platinumcomplex... 103

on an electrode was studied.It hasbeenfound that thissystemshowsa remarkably high catalytic activity for H+

reduction under acidic conditions. The electrochemistryof Pt(bpy)22+ in a heterogeneouspolymer matrix and itselectrocatalysisin theH+ reductionwill bereported.

Experimental partA 5 wt.-% Nafionm (Nf) alcoholic solution was purchasedfrom Aldrich ChemicalCo.Ltd. A basal-planepyrolytic gra-phite (BPG) plateandan indium-tin oxide (ITO) platewerepurchasedfrom Union Carbide Co. Ltd. and KinoeneKogakuCo.Ltd., respectively.

Pt(bpy)2(NO3)2 wassynthesizedfrom PtCl42– via the inter-mediate [PtII(bpy)Cl2] according to the previous proce-dure9,10). Firstly, K2PtCl4 (0.8 g), 2,29-bipyridine (0.32g) and2 N HCl solution(4 mL) weremixedin water(100mL), andthen the mixture was heatedto boiling. The liquid gavefibrousyellow needles.After filtration, theobtainedmaterial([PtII(bpy)Cl2]) was dried under vacuum(258C). The [PtII-(bpy)Cl2] (0.5 g) was suspendedin water (25 mL) with anexcessof 2,29-bipyridine (0.4 g) in ethanol(25 mL) andtheresultingmixture heateduntil a clearyellow-brownsolutionof the Pt(bpy)22+ wasobtained.Excessbpy was removedbyprecipitation on cooling, and the solution was furtherextractedwith dichloromethane.NaNO3 (0.35 g) wasaddedto the aqueoussolutionof Pt(bpy)22+, andthenthenitratesaltof thecomplexwasprecipitatedby partialevaporationof thesolventandsubsequentcooling in an ice bath.After the fil-tration, the salt was washedwith cold water, ethanol,andetherandthendriedundervacuum(258C). Elementalanaly-sisof the obtainedcompoundgavegoodagreementwith thetheoreticalvalues.

The electrodemodification wascarriedout as follows. Amethanolsolutioncontaining1 wt.-% Nf wasprepared.TheNf-coatedBPGwasobtainedby casting5 ll of this solutiononto a BPG electrode(effective area;0.21cm2) andevapor-atingthesolventunderair. Subsequently, theNf-coatedBPGwasdippedin purewaterfor 30 min. The introductionof thePt(bpy)22+ into thecoatedmembranewascarriedout by cationexchange.The Nf-coatedBPG wasimmersedin an aqueoussolutioncontaininga known concentrationof Pt(bpy)22+, andthe adsorbedamountof Pt(bpy)22+ in the membranewasesti-matedby theUV absorptionspectralchangebeforeandaftertheadsorptionof thePt(bpy)22+ (kmax, 323nm; e, 2.36104 M–1

cm–1). The Nf-coatedITO electrode(effective area;1 cm2)adsorbingPt(bpy)22+ (ITO/Nf[Pt(bpy)22+]) was preparedby asimilar procedure as BPG/Nf[Pt(bpy)22+]. The membranethicknesswascalculatedto beca.1 lm in everyexperiment.

In order to comparethe electrocatalysisof BPG/Nf[Pt-(bpy)22+] with aconventionalPtcatalyst,thePt-blackwaselec-trodepositedonto a BPG electrodefrom an aqueousPtCl62–

solutionaccordingto a previousprocedure11). In the presentwork, potentiostaticconditions(appliedpotential,+0.1V (vs.Ag/AgCl); passedcharge, 1.5 mC) insteadof galvanostaticoneswereemployedfor theelectrodepositionof Pt.

An electrochemicalcell was equipped with the BPG/Nf[Pt(bpy)2]2+ working,a spiralPt counteranda silver/silverchloride (Ag/AgCl, in saturatedKCl electrolyte) reference

electrode.Electrochemicalstudy was carried out using apotentiostat(HOKUTO DENKO, HA-301) equippedwith afunction generator(HOKUTO DENKO, HB-104), a cou-lomb meter (HOKUTO DENKO, HF-201) and a recorder(RIKA DENKI, RW-211). All the electrochemicalstudieswere run in a pH 1 aqueousphosphatebuffer solution.TheH2 producedin the electrolysiswasanalyzedby a gaschro-matograph(Shimadzu,GC-4CPT) with a molecular sieve5 A columnandAr carriergas.Faradayicefficiency (%) fortheH2 productionin theelectrolysisexceededalways80%.

UV-visible spectraof the Pt complexin both solutionandmembranewere measuredby a Hitachi 265 spectrophoto-meter. X-ray photoelectronspectrumwasobtainedwith Kra-tos AXIS-HS (KRATOS)usingMg-Ka radiationasthe exci-tationsource.

Resultsand discussion

Electrochemistryandcatalysis

Fig. 1 shows the cyclic voltammogram (CV) at BPG/Nf[Pt(bpy)2]2+ (a) compared with that at a bareBPG (b).TheCV showedthatthecathodic currentdueto H+ reduc-tion at the BPG/Nf[Pt(bpy)2

2+] starts to increasein morepositivepotential regionsthanat a bareBPG,andthat thecurrent below–0.4V (vs.Ag/AgCl) increases remarkablywith repeatedscanning. When the repeated scans werecarriedout at a bareBPGplate,no growthof thecathodic

Fig. 1. Typical CVs at both BPG/Nf[Pt(bpy)22+] (a) and a bareBPGplate(b) in repeatedscan. Scan rate,20mV N s–1

104 T. Abe,K. Takahashi,Y. Shiraishi,N. Toshima, M. Kaneko

currentwasobserved.If theNf[Pt(bpy)2]2+ (or its reducedspecies)workedasa catalyst (molecular catalyst) withoutchangein thepresentH+ reduction,a steady statevoltam-mogramshould be obtained. The potentialat which thecatalytic current dueto theH+ reduction starts to increasewasca.500mV morepositivethantheredoxpotential ofa Pt(bpy)22+/+ couple (–0.80 V vs. SCE8b)), showing thatthe Pt complex at first changedto some other complexspeciesunder cathodicbiasandthenworksasa H+ reduc-tion catalyst; zero-valent Pt0 rather than Pt(bpy)2

+ mightbe formed directly from Pt(bpy)22+. It hasbeenreportedthat the bisbipyridyl Pd complex is not so stable,espe-cially in highly acidic condition suchas the hydrophiliccolumnof the Nafion12). Thegeneration of the active cat-alyst from the Nf[Pt(bpy)2

2+] in spiteof the morepositiveapplied potential (–0.6 V) than the redox potential ofPt(bpy)22+/+ in a aqueous solution (–0.80 V) might beexplainedby the following reasons. (i) The redox poten-tial of Pt(bpy)22+ is shifted to more positive potentials intheSO3

– environmentin Nafion capable of stronginterac-tion with thePt complex thanin anaqueoussolution. (ii)A two-electronreduction of Pt(bpy)22+ to the zero-valentPt could take placeat more positive potentials than thatof one electron reduction in the specific concentratedconditionsin the Nafion. Sucha changeof the Pt com-plex in the reduction is discussedlater by both UV/Visspectroscopy andXPS.

Potentiostatic electrolysis at ITO/Nf[Pt(bpy)22+] wascarriedout at the applied potential of –0.40 V (vs. Ag/AgCl). Fig. 2 shows the time-course of the current (I-tcurve). As expected from Fig. 1, the cathodic currentsincreasedremarkably with the reaction time. After anelectrolysisfor 10 min, ca. 2 ml H2 wasproduced(turn-over number of the Pt(bpy)22+ to produceH2, 100 min–1).Almost no H2 productionwasobservedwhena bareITOplateor a Nf-coated ITO platewasusedfor theelectroly-sis under the sameconditions, showing evidently thathighly activeelectrocatalysisby Nf[Pt(bpy)22+] takesplaceto reduceH+. Fig. 3 shows UV/Vis absorption spectralchangesof Nf[Pt(bpy)2

2+] before(a) andafter (b) theelec-trolysis for H+ reduction.After the electrolysis, the bandof Pt(bpy)22+ at 323nm (dueto p-p* transition) decreasedandthe absorptionin the visible region from 400 to 800nm increased. It wasalsonoted that the shoulderpeakat350 nm, which hasbeenassignedto MLCT transition13),almost disappeared after the electrolysis. The UV/Visabsorptionof the Pt(bpy)22+ was never recoveredevenafter theanodicpolarization,andmoreover, the spectrumis not in agreementwith theoriginal Pt(bpy)22+, especiallyin the UV region. These spectral data as well as theremarkablegrowth of the cathodic currentsduring elec-trolysis (Fig. 1 and 2), showthat irreversible changesofthePt complex,mostprobablyto anactivezero-valent Ptspeciesand/or to someothercomplex,occurundercatho-dic conditions,resultingin a catalytic H+ reduction.

The present H+ reduction beginsto take place after thechangeof Pt(bpy)22+ undercathodic polarization. A pre-treatment of theBPG/Nf[Pt(bpy)2]2+ undercathodic polar-ization (applied potential, –1.5V; passedcharge, 3 C)was carriedout. The CV of BPG/Nf[Pt(bpy)22+] (denotedas BPG/Nf[Pt(bpy)22+(–1.5V)]) was measured,and isshown in Fig. 4. Steady state voltammograms wereobtainedat this pre-treatedBPG/Nf[Pt(bpy)22+(–1.5V)] ina repeatedscanning. This showsthat the electrogenera-tion of activePt speciestakes placeundercathodicpolar-

Fig. 2. Time-courseof currentduring potentiostaticelectroly-sisat ITO/Nf[Pt(bpy)2

2+] for 10min. Applied potential, –0.4V

Fig. 3. Absorptionspectrum changesbefore (a) and after (b)electrolysisat ITO/Nf[Pt(bpy)2

2+]. After theelectrolysis,theITO/Nf[Pt(bpy)22+] platewaspolarizedonceunderanodicconditionsprior to spectrum measurementin order to reoxidizeany possi-ble reversibly reducedPt complex (appliedpotentials, +0.7 V;passedcharge,0.2C)

An activecatalystsystemfor protonreduction composedof a bipyridyl platinumcomplex... 105

izationat anapplied potential of –1.5 V, andthenits elec-trocatalysis takes placeto reduceH+. Potentiostaticelec-trolysis wasstudiedusing BPG/Nf[Pt(bpy)22+(–1.5V)] andcomparedwith the BPG/Nf[Pt(bpy)2

2+] (Tab.1). All theseelectrolyseswerecarried out with almostsimilar amountsof thecatalyst. Much higherH2 production takes placebyBPG/Nf[Pt(bpy)22+(–1.5V)] than by BPG/Nf[Pt(bpy)2

2+].The differenceof the catalytic activity could be ascribedto thedegreeof activespeciesformationfor theH+ reduc-tion. The electrocatalytic activity of the BPG/Nf[Pt(bpy)2

2+(–1.5V)] system was also comparedwithconventional BPG/Pt-black underthe sameconditionsofthe catalyst amount. It was noted that much higher H2

production takes placeat BPG/Nf[Pt(bpy)22+(–1.5V)] thanat BPG/Nf[Pt(bpy)22+] and BPG/Pt-black, and that theactive Pt species formed from Pt(bpy)22+ works asa veryefficient catalystin the H+ reduction. Although free bpyligands shouldbe releasedwith the decomposition of thePt complex, it wasconfirmed that bpy shows only negli-gible H+ reductioncatalysis.

XPSstudy

The valencestateof the Pt complex after the electrolysisfor H2 formationwasinvestigatedby X-ray photoelectronspectroscopy(XPS). Fig. 5(a) showsthe XPS spectrum

of Nf[Pt(bpy)22+] after electrolysis at the appliedpotentialof –0.6 V. In this case, no cathodicpolarization wascar-ried out prior to the H+ reduction. The XPS spectrumshows that morethantwo kinds of Pt species arepresentin themembrane. Theshoulderpeaksareobserved firstlyat both 76.9 eV (Pt 4f5/2) and 73.6eV (Pt 4f7/2) corre-sponding to Pt(bpy)22+, ashavebeenreportedby Franketal.10) in a crystal state.It was also noted that the peaksarising from the zero-valent Pt are present at both74.5 eV (Pt 4f5/2) and71.2eV (Pt 4f7/2), which is in goodagreement with the Pt0 foil 14), showing that the zero-valent Pt0 was formed from the Pt complex under the

Fig. 4. Cyclic voltammogramsat BPG/Nf[Pt] in repeatedscan-ning.Scanrate,20mV N s–1

Tab.1. Results of potentiostatic electrolysis at an appliedpotential of –0.6V (vs.Ag/AgCl) in pH 1 aqueoussolution(1 h)

System H2 produced/ml TN/h–1

BareBPG L0 –BPG/Nf L0 –BPG/Nf[Pt(bpy)22+]a) 0.67 6.06103

BPG/Nf[Pt(bpy)22+(–1.5 V)] b) 2.19 2.36104

BPG/Pt-blackc) 0.48 5.06103

a) Catalyst amount, 4.6610–9 mol.b) Catalyst amount,3.9610–9 mol; Nf[Pt(bpy)22+(–1.5V)] was

preparedat –1.5V prior to theH+ reduction.c) Catalyst amount, 3.9610–9mol.

Fig. 5. XPS spectrumof the Pt(bpy)22+ dispersedin a Nafion

membraneafterapplyingpotentialsof –0.6 V (a) and–1.5V (b).After the electrolysis, no polarization of Nf[Pt(bpy)22+] wasapplied prior to spectrummeasurement

106 T. Abe,K. Takahashi,Y. Shiraishi,N. Toshima, M. Kaneko

cathodic polarization. No peakby Pt(bpy)2+ wasobtained.This may support that the zero-valentPt0 forms directlyfrom thePt(bpy)2

2+.TheseXPS resultsshowthat the remarkableH+ reduc-

tion could be inducedmost probablyby electroformationof Pt0 particles.However, intensepeaksthat can not beassigned to elemental Pt0 or to Pt(bpy)22+ were alsoobserved at both 77.6 eV and 74.0 eV. Thesepeaksareassignableto somechangedPt complexwith higherbind-ing energy than Pt(bpy)22+. Although the details of thestructural changecannotbedetermined,it is mostprobablethat the intense peaksmay be causedby the intermediatePtcomplex to form thecatalytically activespecies,result-ing in acomplicatedcomplexformation with higherbind-ing energy. It shouldbehereaddedthatmere reductionofthe aqueoussolutionof [Pt(bpy)2

2+] without Nf coating atthesameelectrodedid notproducesuchactivecatalyst8b).

The XPS was also studied for BPG/Nf[Pt(bpy)22+-(–1.5V)] preparedby applying the potentials of –1.5 V,andtheresultis shown in Fig. 5(b). Althoughin this pro-cesscompletion of the Pt0 formation wasexpectedto beachieved,theXPSstudygavealsoacomplexspectrum.Inaddition to the zero-valentPt0 and Pt(bpy)22+, new peakswereconfirmed at both76.2eV (Pt 4f5/2) and72.9eV (Pt4f7/2). Thesebinding energiesarein good agreement withthe mono-valent Pt(bpy)2+ crystal electrodeposited fromPt(bpy)2

2+ solution,ashasbeenshownin theearlierlitera-ture8b). Theintensepeaks,whicharealsoseenin Fig. 5(a),shiftedto higherenergy regionsthanthoseobtainedunderthemorepositivepotential conditions(–0.6V). This indi-catesprobably that the intermediate changesto stablestructures.The Pt(bpy)2+ should be unstable, especially inan aprotic solvent, which might result in an irreversibleformation of sometriangleform [Pt2(bpy)3]2+ and/orPt-Ptdimerwith bpybridgingligand,following thelossof abpyligand as reportedearlier8a). Nafion membranehasthreedifferent kinds of region, i. e., asa hydrophilic, a hydro-phobic, and an interfacial one. Therefore, thoseintensepeaksmight beascribedto the formation of sucha dimeror amorecomplexstructurein ahydrophobicand/orinter-facial region. The species having high binding energymight alsobeinvolved in thepresentH+ reductioncataly-sis. It should also be noted that part of the Pt(bpy)2

2+

remains unchangedin spiteof applying a very low poten-tial of –1.5V, asshownin Fig. 5(b). This shows thatpartof thecomplexis isolated in themembrane.Electrontrans-fer in a molecular assembly containing dispersedredoxcenter molecules usually takes place by electron hop-ping15), physical diffusion16), or by combinationsof both17).If theelectron transferoccurs by a physicaldiffusion thatis independentof the redox centerconcentration, almostall the complexescanbe reduced to the products16). Thepresent incompletereduction of the Pt(bpy)22+ to form Pt0

orotherspeciesshowsthattheelectrontransfertakesplaceby electronhoppingbetweenthecomplexes. Thepresence

of theunreactedPt(bpy)22+ is ascribedto isolatedclustersofthe complexthat makeit difficult to acceptelectronsbyhopping.

In conclusion, theelectrocatalytic H+ reductionwasstu-died by using a molecular assembly composed of aPt(bpy)22+ and Nafion membrane in order to develop ahighly active catalyst system.It wasindicatedthatelectro-deposition of thezero-valentPt from its complexinducestheH+ reduction.Thepresentwork enabledformation of anovel catalyst from its complexin apolymermembranetoobtain anefficient catalystsystemfor H+ reduction.

Acknowledgement:The authors acknowledge the Grant-in-Aid (No. 475/10126207) from Ministry of Education, Science,Sportsand Culture. T. A. hasbeengrantedby JSPS ResearchFellowshipsfor YoungScientists.

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