Journal of Electroanalytical Chemistry 649 (2010) 238–247

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Activation of carbon-supported platinum nanoparticles by zeolite-type cesiumsalts of polyoxometallates of molybdenum and tungsten towards moreefficient electrocatalytic oxidation of methanol and ethanol

Artur Zurowski a, Aneta Kolary-Zurowska a, Sonia Dsoke b, Piotr J. Barczuk a, Roberto Marassi b,*,Pawel J. Kulesza a,**

a Department of Chemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Polandb Department of Chemistry, University of Camerino, S. Agostino 1, I-62032 Camerino, Italy

a r t i c l e i n f o

Article history:Received 6 January 2010Received in revised form 22 April 2010Accepted 26 April 2010Available online 4 May 2010

Dedicated to Professor Jacek Lipkowski onoccasion of his 65th Birthday

Keywords:Methanol oxidationEthanol oxidationCesium saltsHeteropolytungstatesHeteropolymolybdatesPlatinum nanoparticles

1572-6657/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jelechem.2010.04.021

* Corresponding author. Tel.: +48 22 822 0211.** Corresponding author.

E-mail addresses: roberto.marassi@unicam.it (R. Medu.pl (P.J. Kulesza).

a b s t r a c t

The activity of Vulcan-supported Pt nanoparticles (Pt40%/Vulcan XC-72) towards oxidation of methanoland ethanol is increased by admixing them with zeolite-type cesium salts of heteropolymolybdic andheteropolytungstic acids: Cs2.5H0.5PMo12O40, Cs2.5H1.5SiMo12O40, Cs2.5H0.5PW12O40, and Cs2.5H1.5-SiW12O40. It is apparent from IR measurements that these salts remain the Keggin-type structure. Theyare electroactive and undergo reversible redox transitions, as well as they are hydrated and containmobile protons. As evidenced from cyclic voltammetric, stair-case voltammetric and chronoamperomet-ric diagnostic experiments, the electrocatalytic enhancement effect has been most pronounced uponapplication of phosphododecamolybdate and phosphododecatungstate salts. The overall activation effectmay also reflect the micro and mesoporous (zeolite-type) high surface area morphology of polyoxomet-allate cesium salts. The presence of well-defined polyoxometallate clusters in the vicinity of Pt isexpected increase population of reactive oxo groups at the electrocatalytic interface.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

To meet world-increasing energy needs and to deal with theserious environmental problems, one has to find alternative energyresources and utilization methods. Direct alcohol fuel cells, that arebased on methanol (DMFC) and ethanol (DEFC) as fuels, are verypromising power-utilization approaches with respect to applica-tion in portable devices and vehicles due to their high theoreticalenergy densities (6.09 kWh kg�1 and 8.01 kWh kg�1 respectively),relatively quick start-up and low operating temperatures [1]. Thestorage and refilling of the liquid fuels are easier in comparisonto gas fuels; and no reformer system is required [2]. Althoughhighly toxic, methanol is typically considered as the most promis-ing fuel because it is more readily oxidized than other alcohols. Onthe other hand, DMFCs suffer from the slow kinetics of methanoloxidation and from the problem of the alcohol crossover through

ll rights reserved.

arassi), pkulesza@chem.uw.

membrane during operation of a fuel cell. Ethanol appears to bean attractive alternative due to its low toxicity and its ready avail-ability from biomass. However, its complete oxidation to CO2 ismore difficult than in a case of methanol because of difficultiesin cleavage of the CAC bond in the ethanol molecule.

Electrocatalytic oxidation of alcohols over various Pt-based cat-alysts is a subject of intense research [3,4]. Platinum has been rec-ognized as the most active catalytic metal towards oxidation ofalcohols at low and moderate temperatures. But Pt anodes arereadily poisoned by the strongly adsorbed intermediates, namelyby CO-type species, requiring fairly high overpotentials for their re-moval. To enhance activity of Pt catalysts towards methanol andethanol oxidation, a secondary metal, such as Ru, Sn, Mo, W orRh, is usually introduced as the alloying component [5–10]. Thesemetals tend to enhance catalytic activity of Pt (during oxidation ofalcohols) because they are characterized by lower potentialsnecessary to drive dissociation of water and to provide interfacialOHads species capable of more facile oxidation of the carbonaceousadsorbate on adjacent Pt sites. More recently it has beendemonstrated that catalytic activity of platinum-based nanoparti-cles towards electrooxidation of ethanol has been significantly

A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247 239

enhanced through interfacial modification with ultra-thin mono-layer-type films of polyoxometallates, particularly heteropolymo-lybdates [11].

Having in mind a wide spectrum of powerful catalytic activitiesof zeolites, it is reasonable to expect that application of certain ion-ically/electronically conducting zeolite-type matrices shall lead toenhancement of the overall reactivity of anode electrocatalysisand make them more tolerant or resistive to poisoning withCO-adsorbates. Among potential candidate materials for rigid andactive matrices in electrocatalysis, there are cesium salts of Keg-gin-type heteropolyacids of molybdenum and tungsten. These saltsare insoluble in water and organic solvents, and they posses microand mesopores of high surface area (often exceeding 100 m2 g�1)[12]. They are essentially cationic conductors having the sametemperature dependence typically displayed by solid ionic conduc-tors (higher mobility of ions at higher temperature). Contrary to al-kali-exchanged zeolites, the analogous heteropolyacid salts retaintheir strong acidity. The latter feature would improve mobilityand availability of protons at the catalytic interface; the latter phe-nomenon may result in the enhancement of acid catalyzed electro-chemical processes or their reaction steps (e.g. removal ofinhibiting adsorbates). Polyoxometallates were used in various cat-alyzed reactions such as alkylation, acylation, ester decomposition,Diels–Alder reaction [13–15]. More recently, upon addition to cat-alytic layers at anodes and cathodes, simple heteropolyacids havebeen demonstrated to improve performance of low temperaturefuel cells [16–18]. It has also been reported [19] that Cs2.5H0.5-PW12O40 can be used as a composite component of self-humidify-ing membranes for PEMFC. An important feature of such hetero-polymolybdate or polytungstate based zeolite-type matrix forpractical applications in DAFCs is its insolubility in water andaqueous electrolytes.

In the present work, we describe preparation and physico-chemical (including electrochemical) properties of cesium salts ofselected Keggin-type heteropolyacids of molybdenum and tung-sten, as well as address their influence on catalytic activity of Vucl-can-supported Pt nanoparticles (Pt40%/Vulcan XC-72) duringelectrooxidation of methanol and ethanol. We consider here sim-ple platinum, rather than alloyed systems, because it can beviewed as a model electrocatalyst which utilization would simplifymechanistic consideration on nature of the activation effect origi-nating from the polyoxometallate cesium salts. We have found thatthe overall enhancement effect has been the most pronouncedupon admixing Pt nanoparticles with phosphododecamolybdateand phosphododecatungstate cesium salts.

2. Experimental

2.1. Materials

All chemicals were commercially available materials of analyt-ical grade purity and were used as received without further purifi-cation. Phosphotungstic acid (H3PW12O40), phosphomolybdic acid(H3PMo12O40), silicotungstic acid (H4SiW12O40) and silicomolybdicacid (H4SiMo12O40) were obtained from Sigma–Aldrich. Nafion(5 wt.% solution) was obtained from Ion Power (USA), Cesium ni-tride (99%) and sulfuric acid (99,999%) from Aldrich. Methanoland ethanol (>99.9%) were procured from Baker. Solutions wereprepared using doubly-distilled and subsequently de-ionized (Mil-lipore Milli-Q) water.

2.2. Preparation of zeolitic cesium salts of heteropolyacids

The following cesium salts of Keggin-type heteropolyacidswere considered here: Cs2.5H0.5PMo12O40, Cs2.5H1.5SiMo12O40,

Cs2.5H0.5PW12O40, and Cs2.5H1.5SiW12O40. They were prepared asdescribed elsewhere [20–23] by adding and mixing stoichiometricamounts of aqueous solutions of 0.25 mol dm�3 CsNO3 and0.1 mol dm�3 of a desired heteropolyacid to meet the molar ratioof Cs to phosphotungstate or silicotungstate units equal to 2.5:1in the mixture for precipitation. The suspension was then stirredfor 24 h. The precipitates were washed four times with waterand, subsequently, separated from the liquid phase by centrifuga-tion and subjected to freeze-drying at 75 �C. Later, all cesium het-eropolysalts were dried at 100 �C for 2 h to obtain the hexahydratecrystallites [7]. The data of elemental analysis were consistent withthe proposed stoichiometric formulas in which ratio of Cs to P or Siin the resulting salts was equal as 2.5 (±0.1) to 1.

2.3. Preparation of catalytic electrodes

Glassy carbon (GC) electrode (geometric area, 0.071 cm2) wasfrom CH Instruments. Before each experiment, the substrate waspolished on a cloth wetted with aqueous alumina suspensions ofparticle sizes varying from 5 to 0.05 lm. To prepare electrocata-lytic inks, the calculated amounts of commercial Pt40%/VulcanXC-72 carbon nanoparticles (Pt40%/C) (e.g. 0.0118 g) and of theappropriate cesium salt of Keggin-type heteropolyacid matrix(e.g. 0.0177 g) were mixed in the ethanol–water (1:1) solutionand stirred for 12 h. A measured volume (e.g. 67 ll) of Nafion(5% alcoholic solution) was then added, and the stirring continuedfor additional 6 h to obtain a homogenous dispersion. The mass ra-tio of Pt40%/C nanoparticles to the heteropolyacid salts and to pureNafion solution was 1 to 1.5 to 0.25, respectively. A total volume ofcatalytic suspension was equal to 1 ml. A drop of the resulting ink(ca. 1.5 ll) was introduced using a micropipette onto the surface ofa glassy carbon disk electrode. Consequently, a Pt loading of100 lg cm�2 was achieved. The suspension was air-dried at roomtemperature (20 ± 2 �C) for 30 min. For comparison, the hetero-polyacid salt-free inks having the same loadings of Pt40%/VulcanXC-72 carbon and Nafion were also prepared.

2.4. Electrochemical measurements

The electrochemical experiments were carried out in a three-electrode configuration with CH Instruments (Austin, TX, USA)Model 660B workstation. While glassy carbon served as the work-ing electrode, and a platinum flag was used as the counter elec-trode, the reference system was as follows: mercury/mercurysulfate electrode, Hg/Hg2SO4 (656 mV versus reversible hydrogenelectrode, RHE), placed in a separated compartment and connectedto the main cell using Luggin capillary. Normalization of currents isdone per geometric surface area. All potentials are expressedagainst the RHE.

The CO-stripping measurements were performed in 0.5mol dm�3 H2SO4 electrolyte using the glassy carbon electrode ontowhich surface the appropriate catalytic ink was introduced. Theelectrolyte was first de-oxygenated by purging argon for 30 min.Subsequently, five consecutive voltammetric scans (at 20 mV s�1)were recorded in the potential range from 0.025 to 1.125 V. To sat-urate the solution with CO gas, pure CO (from Air Liquide) wasbubbled through the electrolyte for 10 min. The actual CO-adsorp-tion step (on the surface of catalytic Pt nanocenters) was achievedupon application of the potential of 0.1 V for 4 min. Later, CO waseliminated from the electrolyte by purging argon for 35 min underopen-circuit conditions. The latter step also resulted in eliminationof CO reversibly adsorbed on the catalytic surfaces. As a rule fivecyclic voltammetric scans (at 20 mV s�1) were recorded in the po-tential range from 0.025 to 1.125 V. The first anodic sweep aimedat electrooxidation of the irreversibly adsorbed CO (on Pt sites),

240 A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247

whereas and the subsequent cycles were measured to verify thecompleteness of the CO-oxidation.

The electrocatalytic and blank measurements were conductedusing 0.5 M H2SO4 in the presence and absence of 0.5 mol dm�3

CH3OH or 0.5 mol dm�3 C2H5OH, respectively. Diagnosticexperiments included cyclic voltammetric (CV), square-wave vol-tammetric (SV), and chronoamperometric (CA) approaches. Allsolutions were degassed with argon prior to each measurement.They were carried out about 22 �C. As a rule the catalytic filmswere activated by performing 200 full voltammetric potentialcycles (at 100 mV s–1) in deaerated solutions in the potential rangefrom 0 to 1 V until steady-state voltammetric responses wereobtained.

Infrared spectra in the range from 1800 to 500 cm�1 were re-corded using Perkin Elmer System 2000 FTIR instrument. Cesiumsalts as well as their Keggin-type heteropolyacids were used aspowders for spectroscopic examination.

Morphology of catalysts was monitored using JEM-2100F trans-mission electron microscope (TEM) operating at 200 kV.

3. Results and discussion

3.1. IR characterization of cesium salts relative to parentpolyoxometallates

Cesium salts, such as Cs2.5H0.5[PMo12O40], Cs2.5H1.5[SiMo12O40],Cs2.5H0.5[PW12O40] and Cs2.5H1.5[SiW12O40], as well as for compar-ison, their parent Keggin-type heteropolyacids were subjected tospectroscopic (IR) examination (Fig. 1). In all cases, the appearanceof four bands within the 700–1100 cm�1 region, that correspond tothe Keggin-unit structural vibrations [24–32], clearly suggest thatthe framework of the primary Keggin structure remains unalteredin the cesium salts. There are no significant differences betweenthe spectra of respective heteropolyacids and their salts. In partic-ular, bands appearing at ca. 730–760 and 850–880 cm�1 are typi-cally assigned to the stretching of tungsten–oxygen–tungsten ormolybdenum–oxygen–molybdenum chains involving corner andedge oxygens, respectively. All other bands appearing around1060–1080 cm�1 (asymmetric P–O vibrations), 900–910 cm�1

(SiAO stretching), and 960–970 cm�1 (terminal W@O or Mo@Ovibrations) are also in agreement with those reported in literature[26–31]. Absorption band at ca. 1600–1700 cm�1 is indicative ofthe presence of the protonated water clusters, probably of the pro-ton-type, H5O2

+ [33,34], and it is assigned to d (H2O) vibration[21,27]. The proton substitution with Cs+ ion in the salts [21]causes some decrease of intensities of the d (H2O) peaks at1697 cm�1 (Fig. 1A) and 1700 cm�1 (Fig. 1C). The same commentapplies to the absorption at 1610 cm�1 which is related to the pres-ence of water molecules [21] in phosphomolybdate and silicomo-lybdate systems (Fig. 1B and D). Thus the degree of hydration ofthe polyoxometallate cesium salts is expected to be lower in com-parison to their parent heteropolyacids. A shift in the frequencycharacteristic of the edge oxygen Mo–O–Mo stretching mode from875 cm�1 (for H3PMo12O40) to 865 cm�1 for Cs2.5H0.5PMo12O40

most likely originates from the presence of positively charged cat-ion (Cs+) in the salt.

On the whole, the results of Fig. 1 are indicative of the existenceof Keggin-type molecular metal oxide clusters (organized aroundphosphate or silicate heterogroup) within the cesium salts of poly-acids of tungsten or molybdenum. This conclusion is in agreementwith the literature data concerning selected salts of polyoxometal-lates [35,36]. It should be remembered that ultra-thin films of het-eroplyacids were shown to activate Pt-based catalysts towardselectrooxidation of ethanol [11]. But monolayer-type deposits havelimited stability. It cannot be excluded that our cesium salts are

interfacially covered with simple parent heteropolyacids (e.g.H3PW12O40 is epitaxially deposited on the cesium salt crystallites[20–23]). Even then, the bulk material is sparingly soluble, zeoliticand protonated [37], and it is likely to form well-defined robustmetal oxide environment capable of activating dispersed electro-catalytic noble metal nanocenters. Although the IR data of cesiumsalts indicate lower population of structural protons and their low-er degree of hydration, the literature reports [13,37]are consistentwith strong acidity and high proton mobility in such systems.

3.2. Microscopic examination of composite catalytic inks

Fig. 2 illustrates two typical examples of TEM micrographs ofthe inks composed of Vulcan-supported Pt nanoparticles (Pt40%/C): (A) bare and (B) admixed with the representative example ofthe cesium salt, Cs2.5H0.5PW12O40. Dark dots stand for Pt nanopar-ticles. Despite some tendency to form agglomerates, Pt sites aredispersed on Vulcan XC-72 carbon supports (that seem to formsomewhat lighter spots of 20–30 nm diameters). Less regular lightspots originate presumably from the Cs2.5H0.5PW12O40 salt(Fig. 1B). Here it is difficult to distinguish Vulcan carriers (expectedto exist as 20–30 nm well-visible spheres in Fig. 1A) from the lessuniform Cs2.5H0.5PW12O40 nanostructures. Appearance ofPt-agglomerates under conditions of the present TEM microscopicmeasurements does not imply the existence of analogous morphol-ogies during operation of catalysts following deposition on theelectrode surface. On the whole, the presence of the Cs-salt doesnot seem to affect significantly distribution of Pt centers. SimilarTEMs (for simplicity not shown here) were obtained when Pt cat-alysts were deposited together with the other cesium salts consid-ered in the present work.

3.3. Voltammetric characterization of composite catalytic inks

Fig. 3 illustrates cyclic voltammetric responses of Nafion-containing composite catalytic inks (deposits on glassy carbon)containing Vulcan-supported Pt nanoparticles (Pt40%/C) admixedwith cesium salts of Keggin-type heteropolyacids: (a)Cs2.5H0.5PW12O40, (b) Cs2.5H0.5PMo12O40, (c) Cs2.5H1.5SiW12O40,and (d) Cs2.5H1.5SiMo12O40. A blank voltammetric response (Curvee) of bare (salt-free) ink of Pt40%/C nanoparticles is provided aswell. The presence of hydrogen adsorption peaks (within the po-tential range from 0 to 0.3 V) and Pt/PtO redox transitions (atpotentials higher than 0.7 V) characteristic of the platinuminterfacial behavior is evident in the voltammetric responses ofall composite catalytic inks (Curves a–d in Fig. 3). For referencepurposes, we also show cyclic voltammograms of Nafion-sup-ported deposits of the Pt-free cesium salts: (a0) Cs2.5H0.5PW12O40,(b0) Cs2.5H0.5PMo12O40, (c0) Cs2.5H1.5SiW12O40, and (d0) Cs2.5H1.5Si-Mo12O40. These materials unergo partial reversible reduction toproduce heteropolyblue structures containing mixed-valentW(VI,V) or Mo(VI,V) sites [38,39]. The processes are reversible ifthe number of electrons transferred per Keggin-type heteropolyunit is typically not larger than four. The exact nature of the sys-tems’ electrode reactions will be a subject of our separate commu-nication. Here it is noteworthy that the peaks originating fromheteropolytungstate salts appear at potentials more negative than0.2 V (Curves a0 and c0), and they seem only to affect somewhat thecharacteristics of hydrogen adsorption peaks on Pt (Curves a andb). On the other hand, heteropolymolybdate salts are electroactivein broader range of potentials (Curves b0 and d0), and their exis-tence in the vicinity of Pt results in the appearance of additionalpeaks at about 0.4–0.45 V (Curves b and d). It is likely that, by anal-ogy to parent heteropolyacids of molybdenum and tungsten, whichundergo strong adsorption on platinum surfaces [39], the Keggin-type molecular metal oxide clusters existing on or within the

60080010001200140016001800 60080010001200140016001800

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523

525 33

883

Wave number/cm-1 Wave number/cm-1

Wave number/cm-1 Wave number/cm-1

1077971

883

756

1697

975760

592

592

H3PW1W 2O4O 0

Tran

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1077

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961 591760

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529

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727

871905

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H4SiW1W 2O4O 044

C1020

970

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735

CsC 2.5H1.5SiW1W 2O4OO 044

900851

740

740

851901

9641610

H4HH SiMo12O4044

D

1612

959

Tran

smitt

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Cs2.5H1.5SiMo

o

12O4440

40

Fig. 1. IR spectra (recorded at ambient temperature) of (A) H3PW12O40 and Cs2.5H0.5PW12O40; (B) H3PMo12O40 and Cs2.5H0.5PMo12O40; (C) H4SiW12O40 and Cs2.5H1.5SiW12O40,and (D) H4SiMo12O40 and Cs2.5H1.5SiMo12O40.

A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247 241

cesium salts interact similarly and show strong affinity to Ptnanoparticles.

3.4. Electrochemical oxidation of methanol at composite catalysts

Because the slow kinetics, methanol oxidation reaction occursat high oxidation overpotentials, far from the thermodynamic limit(E0 = 0.02 V) [40]. To comment on the activating role of cesiumpolyoxometallate salts on the electrocatalytic oxidation of metha-nol, we performed stair-case voltammetric experiments where

current densities were recorded every 25 mV in the potential rangefrom 0.25 to 1.1 V upon application of 50 s potential steps.

We considered here composite catalytic inks (deposits on glassycarbon) containing Vulcan-supported Pt nanoparticles (Pt40%/C)admixed with cesium salts of Keggin-type heteropolyacids(Cs2.5HPAs): (a) Cs2.5H0.5PW12O40, (b) Cs2.5H0.5PMo12O40, (c)Cs2.5H1.5SiW12O40, and (d) Cs2.5H1.5SiMo12O40. For comparison,electrocatalytic currents were also recorded at bare (salt-free)Pt40%/C catalytic nanoparticles (Curve e). What is the most impor-tant is that, in all cases, addition of Cs2.5HPAs results in a significant

Fig. 2. TEM images of inks composed of Vulcan-supported Pt nanoparticles (Pt40%/C): (A) bare, and (B) admixed with Cs2.5H0.5PW12O40.

242 A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247

increase of electrocatalytic currents for methanol oxidation understair-case voltammetric conditions. This result can be rationalizedin terms of the existence of possible activating interactions be-tween polyoxometallates and Pt sites, and the availability of suffi-cient numbers of Pt centers for efficient oxidation of methanol. Thebest performance (in terms of currents) is exhibited by catalyticlayers containing Cs2.5H0.5PMo12O40 and Cs2.5H0.5PW12O40 salts.To enlarge the possible activating effect at less positive potentials,we present in Inset to Fig. 4 dependencies of stair-case voltammet-ric responses in the potential range from 0.45 V to 0.67 V. Althoughthe difference is not striking, it seems from the data of Fig. 4 is thatthe methanol oxidation voltammetric currents tend to appear atless positive potentials than on bare Pt. The possibility of suchenhancement effect (potential shift and the increased currentdensities) has to be further verified under chronoamperometricconditions.

The actual chronoamperometric current–time measurementswere performed upon application of a constant potential of0.53 V (Fig. 5) where the methanol oxidation had formally startedunder stair-case voltammetric conditions (Fig. 4). Here the sameelectrocatalytic systems were considered as for Fig. 4. It is apparentfrom Fig. 5 that, during the first 500–600 s, the methanol oxidationcurrent densities decayed continuously thus indicating a pro-nounced loss in activity. The factor responsible for such decaysis, apparently, a blockage of the Pt surface by some organic residue,which is slowly formed and can only be oxidized at high anodicpotentials [40]. In all cases, the electrocatalytic current densitiesreached practically stationary state after 700 s. The impregnationof Pt40%/C with Cs2.5H0.5PMo12O40 (Curve b) and Cs2.5H1.5Si-W12O40 (Curve c) led to significant increases of the methanol elec-trooxidation current densities. On the other hand, catalytic layerscontaining Cs2.5H0.5PW12O40 (Curve a) and Cs2.5H1.5Si-Mo12O40 (Curve d) exhibited activities comparable to that charac-teristic of unmodified platinum (Curve e).

Both under stair-case voltammetric (Fig. 4) and chronoampero-metric (Fig. 5) conditions, the highest activity towards electrooxi-dation of methanol was displayed by the system containingCs2.5H0.5PMo12O40. It is reasonable to expect that, the presence ofmolybdates in the catalytic layer may facilitate the electrooxida-tion of intermediate species, such CO, that are adsorbed on Pt sur-faces thus leading to suppression of the poisoning effect on Ptcatalysts by CO or CO-like intermediates [11]. It cannot be ex-cluded that molybdate groups (APMo12O40) in the cesium salt

would act as redox mediators for the oxidation of CO-adsorbateson Pt surfaces. The proposed mechanism can be described usingthe following reaction scheme [18]:

�PMo12O40 þ COþH2O ! CO2 þ�H2PMo12O40 ð1Þ

�H2PMo12O40 ! �PMo12O40 þ 2Hþ þ 2e� ð2Þ

This mechanism is particularly true for the molybdenum-containing heteropolyacids in which the oxidation potential is suf-ficiently high, and it is more positive than that of heteropolytung-states [38,39].

The persistence of the electrocatalytic effect was tested by per-forming repetitively cyclic voltammetric experiments for long per-iod of time in 0.5 mol dm�3 CH3OH solution of 0.5 mol dm�3 H2SO4

under argon atmosphere (Fig. 6). A typical cyclic voltammetric re-sponse for oxidation of methanol is shown in Inset to Fig. 6 (forsimplicity, only behavior at the representative Cs2.5H0.5PMo12O40-containing film is illustrated here). Current densities where ob-tained from the forward methanol-oxidation peak currents charac-teristic of the last (representative) voltammetric cycle measured(9th cycle). The gap between measurements was 20 min. It isapparent from the data of Fig. 6 that the electrocatalytic durabilityof the systems containing Cs2.5H1.5SiW12O40 and Cs2.5H1.5Si-Mo12O40 salts are fairly good thus indicating that the catalyticlayers are stable at given conditions. Despite some initial decreasesin electrocatalytic responses, two other systems that containCs2.5H0.5PW12O40 and Cs2.5H0.5PMo12O40 are quite reproducibleduring long-term investigations. Some reduction of currents densi-ties found during long-term stability tests may result not only fromaccumulation of poisonous species (such as COads) on the surface ofPt particles but also from the methanol consumption during thesuccessive scans. Finally, to comment on prolonged stability ofthe electrocatalytic systems mentioned above under chronoamp-erometric conditions in the presence of methanol, the long-termchronoamperometric measurements (as for Fig. 5 except that for1 h) have also been performed (for simplicity not shown here). Un-der such conditions, fairly stable steady-state current – potentialresponses have been observed (current decreases are lower than5%).

On the whole, it is important to note that for all systems utiliz-ing cesium salts, the observed current densities related to metha-nol oxidation are higher than those recorded for a bare catalyticmaterial containing only Nafion-treated Pt40%/C.

E / V vs. RHE

-0.2 0.0 0.2 0.4 0.6 0.8

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Fig. 3. Cyclic voltammetric responses of Nafion-containing composite catalytic films (deposits on glassy carbon electrode) containing Vulcan-supported Pt nanoparticles(Pt40%/C) admixed with (a) Cs2.5H0.5PW12O40, (b) Cs2.5H0.5PMo12O40, (c) Cs2.5H1.5SiW12O40, (d) Cs2.5H1.5SiMo12O40. While e stands for bare (salt–free) Pt40%/C, curves a0–d0

refer to responses of deposits of the following salts Cs2.5H0.5PW12O40, Cs2.5H0.5PMo12O40, Cs2.5H1.5SiW12O40, and Cs2.5H1.5SiMo12O40, respectively. Electrolyte: argon saturated0.5 mol dm�3 H2SO4. Scan rate, 10 mV s�1. Temperature, 24 �C. Loadings, 375 and 750 lg cm�2 of the appropriate cesium salts in the case of curves a–d and curves a0–d0 ,respectively.

A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247 243

3.5. Electrooxidation of ethanol

Ethanol electrooxidation is a complex (more inert relative tomethanol) redox process, and it does require very efficient catalyticsystems. In the present report, we limit our considerations to twosystems that have occurred to be the most efficient in electrocata-lytic oxidation of methanol. Fig. 7 illustrates steady-state cyclicvoltammetric responses recorded during electrooxidation of etha-nol in the argon-saturated 0.5 mol dm�3 C2H5OH in 0.5 mol dm�3

H2SO4 solution using catalysts fabricated from Nafion-treatedPt40%/C admixed with Cs2.5H0.5PW12O40 (Fig. 7a) and withCs2.5H0.5PMo12O40 (Fig. 7b) relative to the bare Pt40%/C system(Fig. 7c). By analogy to the previous reports [41,42], the ethanoloxidation process has produced a well-defined anodic peak (A)around 0.89 V in the forward scan. In the reverse scan, anotheranodic peak (B) has been detected at around 0.79 V. This peakmay be due to the oxidation of all adsorbed carbonaceous species

(e.g. PtAOCH2CH3, PtACHOHACH3, (Pt)2@COHACH3, PtACOCH3

and PtAC„O) [41–43]. It is apparent from Fig. 7 that the systemcontaining Cs2.5H0.5PMo12O40 has exhibited the highest peak cur-rent density during positive voltammetric scan during ethanolelectrooxidation. The backward-scan peak–current density hasalso been relatively the highest.

Fig. 8 illustrates stair-case voltammetric responses (step period,50 s; currents taken every 25 mV) for ethanol electrooxidation re-corded using catalytic layers containing Nafion-treated Pt40%/Cand cesium salts of Keggin-type heteropolyacids: Cs2.5H0.5PW12O40

(Fig. 8a) and with Cs2.5H0.5PMo12O40 (Fig. 8b). Comparison hasbeen made to behavior of the bare commercial electrocatalyst(Fig. 8c). It is noteworthy that the presence of Cs2.5H0.5PMo12O40

salt in the system has resulted in the most profound increase ofthe electrocatalytic currents for ethanol oxidation (Fig. 8, Curveb). When we have calculated mass activities (in mA mg�1) definedby ratios of peak current densities per mass unit of the catalyst (Pt)

0,4 0,6 0,8 1,0

0

5

10

15

20

25

0,5 0,6

0

5

10

d

c

b

aj /

mA

cm-2

E / V vs. RHE

eE / V vs. RHE

j / m

A cm

-2

Fig. 4. Stair-case voltammetric–current densities for the methanol (0.5 mol dm�3) oxidation recorded every 25 mV (between 0.25 and 1.0 V) following application of 50 spotential steps at the Nafion-containing layer of (a) Pt40%/C admixed with Cs2.5H0.5PW12O40, (b) Pt40%/C admixed with Cs2.5H0.5PMo12O40, (c) Pt40%/C admixed withCs2.5H1.5SiW12O40, (d) Pt40%/C admixed with Cs2.5H1.5SiMo12O40. Curve e refers to bare Pt40%/C electrode. Electrolyte: argon saturated 0.5 mol dm�3 H2SO4.. Temperature,24 �C.

0 200 400 600 800 10000,0

0,2

0,4

0,6

aj / m

A cm

-2

t / s

b

ce d

Fig. 5. Chronoamperometric curves recorded upon application of 0.53 V for themethanol oxidation at the Nafion-containing layer of (a) Pt40%/C admixed withCs2.5H0.5PW12O40, (b) Pt40%/C admixed with Cs2.5H0.5PMo12O40, (c) Pt40%/Cadmixed with Cs2.5H1.5SiW12O40, (d) Pt40%/C admixed with Cs2.5H1.5SiMo12O40,and (e) at bare Pt40%/C. Electrolyte: argon saturated 0.5 mol dm�3 H2SO4. Temper-ature, 24 �C.

244 A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247

loading [44], the highest mass activity value has also been obtainedfor the catalytic layer utilizing Cs2.5H0.5PMo12O40.

As in the case of methanol electrooxidation (Fig. 6), wealso tested persistence of the electrocatalytic effect as well asreproducibility of the ethanol oxidation voltammetric responsesby performing repetitively cyclic voltammetric experiments (at50 mV s�1) for long periods of time in 0.5 mol dm�3 C2H5OH inthe 0.5 mol dm�3 H2SO4 solution under argon atmosphere(Fig. 9). A typical cyclic voltammetric response for oxidation of eth-anol is shown in Inset to Fig. 9 (for simplicity, only behavior with

use of the representative Cs2.5H0.5PMo12O40-containing film isillustrated here). Current densities were obtained from the forwardethanol-oxidation peak currents characteristic of the last (repre-sentative) voltammetric cycle measured (9th cycle). For simplicitywe show only a voltammogram for the system utilizingCs2.5H0.5PW12O40 (Inset to Fig. 9). The time-gap between measure-ments was 20 min. The stabilities of electrocatalytic responses ofthe systems containing Cs2.5H0.5PMo12O40 and Cs2.5H0.5PW12O40

salts quite good over the period of almost 12 h. In addition, thepeak current densities related to the oxidation of ethanol (Fig. 9,Curves a and b) were always higher in comparison to the bare cat-alytic material containing only Nafion-treated Pt40%/C (Curve c).

We also carried out diagnostic chronoamperometric experi-ments (at 0.4 V) to get more information about the activity of theproposed electrocatalytic systems during oxidation of ethanol(Fig. 10). Comparison has been made to bare (salt-free) Pt40%/Ccatalyst (Curve c). Following a few hundred seconds, fairly stablesteady-state current – potential responses have been observed.Some drop in the observed currents has been observed after250–300 s during the measurement with the system utilizingCs2.5H0.5PMo12O40 (Curve b), but not Cs2.5H0.5PMo12O40(Curve a), salt. This effect is unclear and may have origin ingenerally lower stability of phosphomolybdates (in comparisonto phospotungstates); partial decomposition or hydrolysis ofmolybdates at the catalytic interface with Pt cannot be excluded.Further research is necessary to clarify the effect. Nevertheless,the superior electrocatalytic activities of the salt-containing sys-tems (in comparison to the bare catalyst) have always beenretained.

Obviously, Vulcan-supported Pt nanoparticles are primarilyresponsible for the catalytic reactivity of the investigated materi-als. The activating role of heteropolymolybdate and heteropoly-tungstate cesium salts on Pt catalytic sites may have physical(morphological) and/or chemical nature. It should be rememberedthat these salts have zeolitic character, i.e. they have special porous

0 200 400 600 8000

5

10

15

20

25

30

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30 e

dc

a

b

j p / m

A cm

-2

t / min

E / V vs. RHE

j / m

A cm

-2

Fig. 6. Long-term voltammetric testing of the Nafion-containing (a) Pt40%/C admixed with Cs2.5H0.5PW12O40, (b) Pt40%/C admixed with Cs2.5H0.5PMo12O40, (c) Pt40%/Cadmixed with Cs2.5H1.5SiW12O40, (d) Pt40%/C admixed with Cs2.5H1.5SiMo12O40, and (e) bare (salt-free) Pt40%/C electrodes in argon saturated 0.5 mol dm�3 H2SO4 containing0.5 mol dm�3 CH3OH solution at 50 mV s�1. Temperature, 24 �C. Pt loading for all cases, LPt = 100 lg cm�2. Inset illustrates a typical volatmmetric response for the methanoloxidation recorded at the Cs2.5H0.5PMo12O40-containing film.

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25

c

b

j / m

A cm

-2

E / V vs. RHE

a

Fig. 7. Cyclic voltammetric responses during ethanol oxidation at Nafion-contain-ing films (deposited on glassy carbon electrode) of (a) Pt40%/C admixed withCs2.5H0.5PW12O40, (b) Pt40%/C admixed with Cs2.5H0.5PMo12O40, and (c) barePt40%/C electrode. Electrolyte: argon saturated 0.5 mol dm�3 C2H5OH + 0.5 moldm�3 H2SO4. Scan rate, 10 mV s�1. Temperature, 24 �C.

0,4 0,6 0,8 1,0

0

2

4

6

8

10

12

ab

c

j / m

A cm

-2

E / V vs. RHE

Fig. 8. Stair-case voltammetric–current densities for the ethanol (0.5 mol dm�3)oxidation recorded every 25 mV (between 0.25 and 1.0 V) following applicationof 50 s potential steps at the Nafion-containing layer of (a) Pt40%/C admixedwith Cs2.5H0.5PW12O40, (b) Pt40%/C admixed with Cs2.5H0.5PMo12O40 and (c) barePt40%/C. Electrolyte: argon saturated 0.5 mol dm�3 H2SO4. Temperature, 24 �C.

A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247 245

structure of relatively high permeability and ability to providegood micromedia for oxidation of alcohols [45]. Polyoxometallatethemselves adsorb strongly on metallic platinum but they do notblock its surface during oxidation of ethanol or methanol. This phe-nomenon may affect interfacial electronic properties of platinumas well as relative strengths of adsorption of ethanol or methanol(initial reactants), and reaction intermediates CO-adsorbates oracetic acid or acetic aldehyde (in the case of ethanol oxidation).In the case of phosphomolybdate-based salt, the activating rolemay originate from the system’s electroactivity (electron transfermediating capabilities) at potentials up to ca. 0.5 V as well as fromthe possibility of donation of oxo groups by molybdates [46]. Be-fore it was also reported that the presence of molybdenum could

increase catalytic activity of platinum towards electrooxidationof ethanol [47].

4. Conclusions

Admixing of Pt nanoparticles with cesium salts of Keggin-typeheteropoly molybdates and tungstates results in the enhancementof their electrocatalytic properties towards electrooxidation ofmethanol and ethanol, as demonstrated in terms of increases ofthe respective voltammetric and amperometric catalytic currents.The IR spectra clearly show that the primary Keggin structures ofthe poloxometallate salts are unaltered even when large portionof protons form the parental heteropolyacids are substituted by

0,0 0,2 0,4 0,6 0,8 1,0

0

10

20

0 200 400 600 8000

5

10

15

20

25

30

j / m

A cm

-2

E / V vs. RHE

c

ab

j p / m

A cm

-2

t / min

Fig. 9. Long-term voltammetric testing of the Nafion-containing (a) Pt40%/C admixed with Cs2.5H0.5PW12O40, (b) Pt40%/C admixed with Cs2.5H0.5PMo12O40 and (c) bare Pt40%/C electrodes in argon saturated 0.5 mol dm�3 H2SO4 containing 0.5 mol dm�3 C2H5OH solution at 50 mV s�1. Temperature, 24 �C. Same Pt loadings mounted in all cases(LPt = 100 lg cm�2). Inset illustrates a typical volatmmetric response for the ethanol oxidation recorded at the Cs2.5H0.5PMo12O40-containing film.

0 500 1000 1500 2000 25000,0

0,1

0,2

0,3

0,4

ba

t / s

j / m

A cm

-2

c

Fig. 10. Chronoamperometric curves recorded for ethanol oxidation at the Nafion-containing layer of (a) Pt40%/C admixed with Cs2.5H0.5PW12O40, (b) Pt40%/Cadmixed with Cs2.5H0.5PMo12O40, and (c) bare Pt40%/C upon application of 0.4 V.Electrolyte: argon saturated 0.5 mol dm�3 H2SO4. Temperature, 24 �C.

246 A. Zurowski et al. / Journal of Electroanalytical Chemistry 649 (2010) 238–247

cesium cations. The activation by polyoxometallate salts is themost effective in a case of utilization of cesium phosphododecamo-lybdate and cesium phosphododecatungstate. These salts are elec-troactive themselves, and they may act as redox mediators ofinterfacial electron transfers during electrooxidation of alcohols.Molybdates (by analogy to ruthenium species) may contribute tothe bifunctional mechanism and, thus, provide oxo groups capableof removing the poisoning species (e.g. CO) from platinum surface.When compared to simple monolayer-type coatings of polyoxo-metallates (which tend to desorb from the electrocatalytic sur-faces) [11], the proposed cesium salt catalysts exhibit muchlonger stability.

When it comes to ethanol oxidation, our results indicate thatthe best performance with respect to the persistence of the electro-catalytic effect is exhibited by the catalytic layer modified by the

tungstate salt, rather than molybdate salt. On the other hand, thehighest current densities have been obtained under voltammetricconditions with use of Cs2.5H0.5PMo12O40 system.

Acknowledgments

This work was supported by Ministry of Science and HigherEducation (Poland) under the Singapore/112/2007 collaborativeGrant. P. J. Kulesza and A. Zurowski appreciate support from Foun-dation for Polish Science (FNP) under ‘‘Mistrz” project.

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