Electrocatalytic activity of multi-walled carbon nanotubes ...composites.utk.edu/papers in pdf/1-s2.0-S0013468613003757...Electrocatalytic activity of multi-walled carbon nanotubes-supported

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Electrochimica Acta 100 (2013) 147– 156

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

lectrocatalytic activity of multi-walled carbon nanotubes-supportedtxPdy catalysts prepared by a pyrolysis process toward ethanolxidation reaction

eqiang Dinga,∗, Yahui Wanga, Hongwei Yanga, Chunbao Zhenga, YanliCaoa, Huige Weib,iran Wangb, Zhanhu Guob,∗∗

College of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050024, PR ChinaIntegrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, TX 77710, USA

a r t i c l e i n f o

rticle history:eceived 2 December 2012eceived in revised form 27 February 2013ccepted 27 February 2013vailable online 6 April 2013

eywords:txPdy nanoparticles

a b s t r a c t

Multi-walled carbon nanotubes (MWCNTs)-supported Pt–Pd bimetallic nanoparticles with various feedmolar ratios (denoted as PtxPdy/MWCNTs) are successfully prepared for the first time by a simple methodof pyrolysis without any additional reducing agent. The obtained PtxPdy/MWCNTs catalysts are char-acterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM). Results show that PtxPdy particles with an average particle size of ∼4.0 nm are dis-persed quite uniformly on the surface of MWCNTs. The electrocatalytic activity of the PtxPdy/MWCNTscatalysts toward ethanol oxidation reaction (EOR) is examined by cyclic voltammetry (CV) in basic

yrolysislectro-catalystthanol oxidation reaction

solution. Surprisingly, significantly enhanced EOR peak currents are observed under the catalysis ofPt3Pd3/MWCNTs, where the feed molar ratio of Pt to Pd is 3:3. The onset potential is found to be ∼200 mVlower and the peak current is 10 times higher for the ethanol oxidation with Pt3Pd3/MWCNTs catalystswhen compared to the ethanol oxidation with Pt/MWCNTs catalysts. The existence of PdO is considered asthe possible reason for the remarkably enhanced peak currents of EOR in the presence of Pt3Pd3/MWCNTscatalysts.

. Introduction

Among the fuels fed to direct liquid fuel cells (DLFCs), ethanols regarded as one of the most promising fuels mainly due tots low toxicity, abundant availability, low permeability (but notegligible) across proton exchange membrane and higher energyensity (8030 Wh kg−1, methanol, 6100 Wh kg−1) [1]. Thus, devel-ping novel catalysts for ethanol oxidation reaction (EOR) hasecome an important issue in the field of electrochemistry.

Although platinum has been recognized to be the most activeatalyst for ethanol oxidation [2], recent work reveals that Pt can beasily poisoned by by-products of oxidation of organic molecules,uch as carbon monoxide besides high cost and limited supply [3].ccording to the bi-functional mechanism proposed previously [4],i-metallic catalysts containing Pt are supposed to possess higher

lectocatalytic activity than Pt alone. Among the bi-metallic cata-ysts, Pt–Ru has been widely investigated due to the fact that theoisoning species, i.e., CO species formed on Pt, can be oxidized into

∗ Corresponding author. Tel.: +86 311 86268311; fax: +86 311 86269217.∗∗ Corresponding author. Tel.: +409 880 7654/7195; fax: +409 880 2197.

E-mail addresses: [email protected] (K. Ding), [email protected] (Z. Guo).

013-4686/$ – see front matter © 2013 Published by Elsevier Ltd.ttp://dx.doi.org/10.1016/j.electacta.2013.02.130

© 2013 Published by Elsevier Ltd.

CO2 by active oxygen atoms formed on Ru [5]. Unfortunately, Pt–Rucatalysts exhibit poor stability owing to the facile electrochemicaldissolution of Ru at high potentials in acid medium [5]. Thus, manyother types of Pt-based binary catalysts such as Pt–Ni [6] and Pt–Co[7] have been developed.

The catalysts of Pt–Pd particles have been also fabricated mainlydue to the reasons (1) Pd has very similar properties to those of Ptin terms of crystal structure (identical – fcc) and atomic size (only3% of difference), and is capable of forming alloys with Pt with anyatomic ratio [8]; (2) Pd is more easily accessible (at least 50 timesmore abundant than Pt on the earth) [9]; (3) The Pt–Pd bimetallicsystem with an appropriate atomic ratio can exhibit higher resis-tance against CO poisoning for the oxidation of some small organicmolecules such as formic acid [10].

To date, two typical methods are used for the preparation ofPt–Pd nanoparticles. (i) Chemical reduction reaction. For example,Antolini et al. [11] reported the synthesis of a carbon sup-ported Pt–Pd catalyst with a Pt:Pd atomic ratio of 77:23, inwhich H2PtCl6·6H2O and PdCl2·2H2O were used as the start-

ing materials, and formic acid the reductant. Zhang et al. [12]described a microwave-assisted synthesis process for prepar-ing graphene-supported Pd1Pt3, where K2PtCl4 and PdCl2 wereused as the precursors and ascorbic acid as a reducing agent.

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ii) Electrochemical reduction reaction. Xu et al. [13] addressedis work on the nano-structured PdxPt1−x composite catalysts sup-orted on Ti substrate fabricated by an electrodeposition methodsing cyclic voltammetry (CV) from 0 V to −0.8 V at a scan rate of

mV s−1. To the best of our knowledge, the preparation of PtxPdy

omposite or alloy nanoparticles by a method of pyrolysis usingistilled water as the solvent, was rarely reported, though we haveuccessfully anchored nanoparticles of Pt on the surface of MWC-Ts via the method of pyrolysis [14].

Although many novel kinds of carbon such as carbon nanofibers15,16] and graphene [17] have been developed recently, carbonanotubes (CNTs) still attract a great deal of attention due to itsnique properties such as high specific surface area, electrical con-uctivity, and good thermal and chemical stability [17]. Thesedvantages, coupled with lower cost compared to graphene, makeNTs a good catalyst support candidate for fuel cells [18]. Thus,

mmobilizing metal nanoparticles on CNTs has turned into an inter-sting field because of the key roles of CNTs and metal nanoparticlesn the fields of electrocatalysis, biosensors and so on [19,20]. Tohe best of our knowledge, no paper reporting the immobilizationf PtxPdy composite nanoparticles onto carbon nanotubes (CNTs)y a facile method of pyrolysis was published, though many kindsf metal nanoparticles have been immobilized on the surface ofWCNTs via a chemical reaction or electrochemical method.Although lots of papers concerning Pt–Pd catalyst have been

ublished, the exact catalysis mechanism toward the oxidation ofmall organic molecules remains unclear. For instance, Kadirgant al. [9] attributed the catalysis of Pt–Pd toward methanol oxida-ion reaction (MOR) to the electronic effects from Pd, namely, theresence of Pd involved a change in the electronic density of statef platinum, which can lead to a weakening of the CO Pt bond, as

result, an enhancement of the overall reaction rate by decreasinghe electrode poisoning is observed. While Nogami and co-workers21] thought that the higher value of electrochemical surface areaECSA) of Pt–Pd core–shell nanocatalysts should be mainly respon-ible for its excellent catalysis toward MOR when compared tohe Pt-nanocatalysts. Thus, the discrepancy on the electrocataly-is mechanism of PtxPdy catalysts toward MOR and EOR intrigueds to probe the catalyst of PtxPdy further.

In this work, PtxPdy/MWCNTs with different feed molar ratiosf Pt to Pd of 3:1, 3:2, 3:3 and 3:4.5, respectively, were prepared by

facile method of pyrolysis using distilled water as the solvent andWCNTs as the reducing agent. The physicochemical properties of

he obtained samples were studied, revealing that the Pt3Pd3 com-osite nanoparticles showed a cluster shape and other catalystspherical particles. The electrochemical activities of the obtainedanoparticles for ethanol oxidation reaction (EOR) were investi-ated, showing that the catalyst of Pt3Pd3/MWCNTs exhibited theest electrochemical performance among all the samples based onhe results from cyclic voltammetry (CV) and current-time curves.astly, the reasons for the significantly enhanced performance oft3Pd3/MWCNTs catalyst toward EOR were also discussed, indi-ating that the newly formed PdO can greatly lower the hydrogenvolution potential, which may account for the enhancement oflectrochemical performance partly, as is reported for the first time.

. Experimental

.1. Reagents and materials

MWCNTs (purity >95%) of 10–20 nm diameter were purchased

rom Shenzhen nanotech port Co., Ltd. (China). All electrodes wereupplied by Tianjin Aida Co., Ltd. (China). All chemicals were ana-ytical grade and were used without further purification. Deionized

ater was used to prepare aqueous solutions.

ta 100 (2013) 147– 156

2.2. Preparation of PtxPdy nanoparticles onto MWCNTs

First, 2 ml of H2PtCl6·6H2O containing certain amount of PdCl2(the feed atomic ratios of Pt to Pd are 3:1, 3:2, 3:3 and 3:4.5, respec-tively) was added in 4 ml of double-distilled water, and then 10 mgMWCNTs were introduced into the solution above to give a sus-pension solution. Secondly, the suspension solution was placed ina home-made autoclave after ultrasonicated for 30 min and wellsealed. The autoclave was then transferred to a box-type furnaceand heated. The temperature of the box-type furnace was increasedfrom room temperature to 200 ◦C within 20 min, and held at thattemperature for 2 h to complete the pyrolysis process. After cool-ing down to room temperature, the solution was filtered and thenanoparticles were thoroughly rinsed with distilled water, anddried under ambient conditions to obtain the MWCNTs supportedPtxPdy catalysts (denoted as PtxPdy/MWCNTs) in the end. Both ofPt/MWCNTs and Pd/MWCNTs catalysts were prepared in the samemethod.

2.3. Preparation of PtxPdy/MWCNTs modified electrode

One milligram of PtxPdy/MWCNTs catalysts was dispersed withthe aid of ultrasonic agitation in 1 ml of 0.1 wt.% Nafion ethanolsolution for 20 min. The glassy carbon (GC) electrode with a geo-metric area of 0.07 cm2 was polished with 0.05 �m alumina slurryand washed with distilled water. The GC electrode was coated bycasting ∼15 �l of suspension of the catalyst and slowly dried in airto give PtxPdy/MWCNTs modified electrodes.

2.4. Characterization

XRD analysis of the catalyst was carried out on a Bruker D8ADVANCE X-ray diffractometer with a Cu K� source (� = 0.154 nm)at 40 kV and 30 mA. Data were obtained at a scan rate of 5◦ min−1

with 1◦ step size in the 2� range of 10–90◦. The morphology wasobserved by scanning electron microscopy (HITACHI, SEM S-570)and transmission electron microscopy (HITACHI, TEM H-7650).Energy Dispersive X-Ray Spectroscopy (EDX) spectrum analysiswas performed on an X-ray energy instrument (EDAX, PV-9900,USA). UV–vis spectra were obtained on a spectrophotometer V-500 (JASCO, Japan). The pyrolysis process was implemented in aSRJX-8-13 box-type furnace equipped with a KSY 12-16 furnacetemperature controller.

Electrochemical measurements including cyclic voltammetry(CV) and electrochemical impedance spectroscopy (EIS) were car-ried out on a CHI 660B electrochemical workstation (ShanghaiChenhua Apparatus, China) connected to a personal computer. EISwas performed in the frequency range of 0.1–105 Hz with an ampli-tude of 5 mV.

A conventional three-electrode system was employed, in whichPtxPdy/MWCNTs modified glassy carbon electrode, a platinumwire, and a saturated calomel electrode (SCE) served as the workingelectrode, counter electrode, and reference electrode, respectively.All potentials in this paper are reported with respect to SCE. Asolution of 2 M ethanol in 1 M KOH was used to study the electro-catalytic activity of the PtxPdy/MWCNTs catalysts toward ethanoloxidation reaction (EOR). Prior to each electrochemical test, theelectrolyte was bubbled with high purity nitrogen gas for 30 min.All the experiments were carried out at room temperature.

3. Results and discussion

3.1. Preparation of PtxPdy nanoparticles onto MWCNTs

Fig. 1A shows the color change of the solution before andafter pyrolysis. For the preparation of Pt/MWCNTs or Pd/MWCNTs

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ig. 1. (A) Photos of the solutions before (a)–(c) and after (a )–(c ) the pyrolysis prohoto (c) Pt3Pd3/MWCNTs. (B) UV–vis absorption spectra for the solutions beforet/MWCNTs; lines (b) and (b′): Pd/MWCNTs; lines (c) and (c′): Pt3Pd3/MWCNTs.

atalysts, the aqueous solution containing H2PtCl6 or PdCl2 with-ut MWCNTs turned from light yellow (photo (a)) or bright yellowphoto (b)), to colorless (photos (a′) and (b′)) after pyrolysis andltered, indicating that Pt4+ or Pd2+ were reduced to be Pt or Pd,

sed for preparing different samples. Photo (a) Pt/MWCNTs; photo (b) Pd/MWCNTs;fter the pyrolysis process used for preparing different samples. Lines (a) and (a′)

respectively. Similar phenomena were observed for the prepara-tion of PtxPdy/MWCNTs catalysts, where the solution turned fromdark-brown to light yellow color, as exemplified by the preparationPt3Pd3/MWCNTs catalyst, photos (c) and (c′).

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Fig. 2. (A) XRD patterns of typical as-prepared samples. Patterns (a)–(d) correspond

the pyrolysis. In other words, a reducing environment was cre-ated by the system containing MWCNTs and water. Consequently,only elementary Pt and Pd were generated. Thus, it can be con-cluded that under proper conditions bimetallic particles can be

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Ultraviolet–visible (UV–vis) absorption spectra of the solutionsefore (curves of a–c) and after pyrolysis process (curves (a′)–(c′))ere recorded in Fig. 1B to further investigate the reactions takinglace in the pyrolysis process. As shown by curve (a), an absorptioneak located at around 254 nm can be observed for the solutionontaining H2PtCl6 [22], which disappeared after the pyrolysis. Itas reported that the spectra in the far-UV region (200–250 nm)

orrespond to the peptide n → �* electronic transition [23]. It iseported that as a result of selfoxidation the carbon surface issually decorated with oxygen containing functional groups rep-esenting compounds such as carboxylic acids, phenols, lactones,arboxylic anhydrates, ketones, ethers, quinones or pyrones [24].robably, at a high temperature of 200 ◦C, Pt ions were reduced toure metal by the reducing groups such as OH and COOH attachedo the surface of the MWCNTs [24]. Similar reaction occurred for thease of Pd. After the pyrolysis process, the peak at 210 nm disap-eared and a smaller absorption peak at around 230 nm appeared,

ndicating of a new complex produced [25]. Interestingly, for theolution having both of the ions Pt and Pd, after the pyrolysisrocess, except for the decreased absorption peak, the peak posi-ion was blue-direction shifted for around 20 nm, which can bexplained by the higher ligand field associated with the Pd metalenter [26]. That is to say, the complex formed in the pyrolysisrocess has been altered to a new one after the pyrolysis process.lso, in all above UV–vis curves, after pyrolysis, the intensities of allbsorption bands decreased to some extent, indicating that all thenions were consumed in some degree in the course of pyrolysis27].

.2. Physical characterization of MWCNTs-supported PtxPdy

atalysts

Fig. 2A shows the XRD patterns of the as-prepared samples.or pure MWCNTs, Fig. 2A(a), a main diffraction peak centeringround 26◦ is assigned to the facet of (0 0 2) of MWCNTs [14]. Fort/MWCNTs and Pd/MWCNTs catalysts, Fig. 2A(b) and (c), exceptor the main diffraction peak of MWCNTs, four characteristic peaksorresponding to the facet of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of theace-centered cubic (fcc) crystalline Pt and Pd can also be observed.hese results are well consistent with the values reported previ-usly for Pt [28] and Pd [29] nanoparticles. Interestingly, as showny the dashed pink line located at around 68◦, the diffraction peak2 2 0) for the Pt3Pd2/MWCNTs catalyst is found to be in betweenhat of Pt/MWCNTs and Pd/MWCNTs, indicative of the alloy forma-ion between Pt and Pd [30], which is caused by the incorporationf Pd in the fcc structure of Pt. No evident peaks of Pt or Pd oxidesere observed in the XRD patterns of the catalysts, probably due to

he small particle size or amorphous form of these oxides existingn the catalysts.

Fig. 2B displays the XRD patterns of PtxPdy/MWCNTs catalystsith different feed molar ratios of Pt to Pd. As expected, the inten-

ities of all diffraction peaks alter correspondingly with the changef atomic ratios of Pt to Pd. Generally, stronger intensity of theiffraction peak corresponds to higher crystallinity [31]. Therefore,ased on the intensity of the diffraction peak at 47◦, the crys-allinity of the samples can be interpreted as Pt3Pd1/MWCNTs >t3Pd4.5/MWCNTs > Pt3Pd3/MWCNTs > Pt3Pd2/MWCNTs. Morenterestingly, for Pt3Pd3/MWCNTs catalyst, an evident new diffrac-ion peak located around 34◦ is clearly observed, which can bessigned to the facet of (1 1 1) of PdO. That is to say, a new phasexists in Pt3Pd3/MWCNTs catalyst, which is rather different fromther catalysts.

To determine the composition of the as-prepared samples, a typ-cal spectrum of EDX for the catalyst of Pt3Pd2/MWCNTs is shownn Fig. 3. Except for the C element, only the peaks corresponding tohe elements of Pt and Pd were observed. This not only strongly

to MWCNTs, pure Pt/MWCNTs, Pd/MWCNTs and Pt3Pd2/MWCNTs. (B) XRD patternsof PdxNiy/MWCNTs catalysts with different atomic ratios of Pt to Pd. Patterns (a)–(d)correspond to Pt3Pd1, Pt3Pd2, Pt3Pd3 and Pt3Pd4.5.

indicated that pure Pt and Pd can be fabricated by this novelmethod, but also verified that the resultant particles seen in image(c) of Fig. 4A are the alloy particles of Pt3Pd2 rather than other sub-stances. Why only the elementary Pt and Pd instead of their oxideswere generated by this pyrolysis process? Probably, in the pres-ence of distilled water and MWCNTs, a redutive environment wasproduced by the some reactions between water and carbon such asC + H2O = CO + H2 [32]. That is to say, the produced CO and H2 areall reducing agents, which can reduce the oxides formed during

Energy/keV

Fig. 3. EDX spectra for three typical catalysts. Patterns (a)–(c) correspond toPt/MWCNTs, Pd/MWCNTs and Pt3Pd2/MWCNTs.

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ig. 4. (A) SEM images of four typical samples prepared in this work. Image (a) Pt/MB) TEM images for the prepared samples. Image (a) Pt3Pd1/MWCNTs; image (b) Pt3

abricated by this simple pyrolysis process, which is expected to

e very beneficial to the industrial production of nanoparticles dueo its simplicity in manufacture.

Fig. 4A shows the SEM images of four typical catalysts oft/MWCNTs, Pd/MWCNTs, Pt3Pd2/MWCNTs, and Pt3Pd3/MWCNTs.

Ts; image (b) Pd/MWCNTs; image (c) Pt3Pd2/MWCNTs; image (d) Pt3Pd3/MWCNTs.WCNTs; image (c) Pt3Pd3/MWCNTs; image (d) Pt3Pd4.5/MWCNTs.

The white dots on the surface of MWCNTs, Fig. 4A(a), are evi-

dent of Pt particles generated during the pyrolysis process, whichis consistent with our previous report [14]. Some small parti-cles can also be observed on the surface of MWCNTs for both ofPd/MWCNTs (Fig. 4A(b)) and Pt3Pd2/MWCNTs (Fig. 4A(c)) catalysts.

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Fig. 5. (A) CVs obtained on the glassy carbon electrode in 1 M KOH + 2 M C2H5OHat the scan rate of 20 mV s−1. Curves (a)–(c) were obtained on the Pt/MWCNTs,Pd/MWCNTs and Pt3Pd2/MWCNTs modified GC electrode. (B) CVs obtained onthe PtxPdy/MWCNTs coated glassy carbon electrode in 1 M KOH + 2 M C2H5OH atthe scan rate of 20 mV s−1. Curves (a)–(c) were obtained on the Pt3Pd1/MWCNTs,Pt3Pd2/MWCNTs and Pt3Pd4.5/MWCNTs modified GC electrode. (C) CVs obtainedon the PtxPdy/MWCNTs coated glassy carbon electrode in 1 M KOH + 2 M C2H5OHat the scan rate of 20 mV s−1. The red and black curves were obtained on thePt3Pd2/MWCNTs and Pt3Pd3/MWCNTs modified GC electrode. (For interpretationof the references to color in figure legend, the reader is referred to the web version

52 K. Ding et al. / Electrochim

nterestingly, for the catalyst of Pt3Pd3/MWCNTs, more particlesere observed to be immobilized onto the surface of the MWC-Ts relative to Pt3Pd2/MWCNTs, which strongly indicated that thetomic ratio of Pt to Pd can greatly affect the amount of particlesroduced in the pyrolysis process.

TEM images of these four typical samples are displayed in Fig. 4B.t can be seen that for all the catalysts as-prepared particles areniformly distributed on the outer walls of the MWCNTs, and nobvious agglomeration of the nanoparticles was found. The particleizes of Pt3Pd1, Pt3Pd2, Pt3Pd3 and Pt3Pd4.5 were estimated, to beround 3.6, 3.4, 3.3 and 3.7 nm, respectively, from the TEM images.t is evident that cluster particles rather than spherical particles arebserved for the catalyst of Pt3Pd3/MWCNTs. From the informationcquired from XRD results, the difference in the particle shape iselieved to be caused by the new substance of PdO generated inhe pyrolysis process at a feed molar ratio of Pt to Pd of 3:3.

.3. Electrochemical characterization

The cyclic voltammograms (CVs) of EOR on various catalysts-odified GC electrode in 1 M KOH are shown in Fig. 5A. Close

nspection reveals that on all the three catalysts two well-definedeaks, with (f) located at ∼−0.25 V in the anodic sweep and (b)

ocated at ∼−0.42 V in the cathodic sweep, are clearly observed,trongly demonstrating that EOR can occur on all the catalysts oft/MWCNTs, Pd/MWCNTs and Pt3Pd2/MWCNTs in a basic medium,hich is in good accordance with the previous report [10,33,34].

t is generally accepted that the forward oxidation current peakf) is assigned to the oxidation of freshly chemisorbed speciesoming from ethanol adsorption, and the reverse oxidation peakb) is primarily associated with removal of carbonaceous speciesot completely oxidized in the forward scan, rather caused by

reshly chemisorbed species [35]. Obviously, the peak currents off) and (b) of Pt3Pd2/MWCNTs are significantly larger than thosen the Pt/MWCNTs and Pd/MWCNTs, implying that a significantlynhanced catalytic activity can be achieved through the alloy of Ptnd Pd via the facile method of pyrolysis.

To compare the electrocatalytic activities of PtxPdy/MWCNTsith different atomic ratios of Pt to Pd toward EOR, CV curves

btained on the catalysts are present in Fig. 5B and C. The peakurrent of EOR in Pt3Pd3/MWCNTs is much larger than that onther catalysts, thus, CV curves of EOR on these catalysts wereot drawn in a figure. In Fig. 5B, the catalyst of Pt3Pd2/MWCNTshows the largest peak current among all the samples, imply-ng that Pt3Pd2/MWCNTs has the highest electrocatalytic abilityoward EOR as compared to the catalysts of Pt3Pd1/MWCNTs andt3Pd4.5/MWCNTs. Meanwhile, when the atomic ratio of Pt to Pds 3:4.5, the current peaks corresponding to EOR decreased signif-cantly, indicating that excessive amount of Pd is not good for theatalysis of Pt toward EOR. Except for the peak current, the onsetotential of the first oxidation peak in the anodic sweep is also

key parameter reflecting the catalytic activity exhibited by theatalyst [35]. The onset potentials and some other electrochemicalarameters obtained from Fig. 5B are summarized in Table 1. Thenset potential for the catalyst of Pt3Pd2/MWCNTs is around 55 mVnd 33 mV lower compared to the catalyst of Pt3Pd1/MWCNTsnd Pt3Pd4.5/MWCNTs, respectively. Evidently, the peak currentsf peak (f) and peak (b) obtained on the catalyst of Pt3Pd2/MWCNTsre the largest ones among all the peak currents. More interestingly,he peak current ratio of peak (f) to peak (b), as shown in Table 1,aried dramatically with the atomic ratio of Pt to Pd. The incompletexidized carbonaceous species, such as CH3COads, could accumu-

ate on the electrode and poison the electrode. Thus, the ratio of theorward anodic peak current (peak (f)) to the reverse anodic peakurrent (peak (b)), i.e., If/Ib, could be used to evaluate the poison-ng tolerance of catalyst [36]. A larger If/Ib ratio indicates a better

of the article.)

oxidation ability of ethanol during the anodic scan. However, in thiscase, the If/Ib ratio in Pt3Pd2/MWCNTs catalyst is the smallest oneamong all the ratios, and the ratio of Qf to Qb in Pt3Pd2/MWCNTs

is also the smallest one among the ratios obtained. Therefore, thisresult indicated that it is not comprehensive to evaluate the catalyticactivity of catalyst only by using the ratio of the If to Ib.

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Table 1Electrochemical parameters obtianed from Fig.5B.

Catalysts Peak (f) Peak (b) If/Ib Qf/Qb Onset potentials (V)

Current (�A) Potentials (V) Electric quantity (mC) Current (�A) Potentials (V) Electric quantity (mC)

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ties toward EOR, CVs of four typical catalysts, i.e., Pt3Pd1/MWCNTs,Pt3Pd2/MWCNTs, Pt3Pd3/MWCNTs and Pt3Pd4.5/MWCNTs, in 1 MKOH are plotted in Fig. 7A. No evident redox peaks were foundon the catalyst of Pt3Pd1 and Pt3Pd4.5. However, on the other two

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Pt3Pd1 153 −0.30 2.50 23

Pt3Pd2 908 −0.24 150 1818

Pt3Pd4.5 233 −0.24 2.70 260

To one’s surprise, when the atomic ratio of Pt to Pd is 3:3, ashown by the black curve in Fig. 5C, significantly enhanced peaksf EOR are seen clearly. It can be seen that the peak currents foreak (f) and peak (b) on the catalyst of Pt3Pd3/MWCNTs are about

and 2.4 times larger than that on the Pt3Pd2/MWCNTs, respec-ively. Also, the onset potentials of peak (f) and peak (b) in theatalyst of Pt3Pd3/MWCNTs are, respectively, negatively shifted for50 mV and positively shifted for 60 mV as compared to those oft3Pd2/MWCNTs. Along with the results shown by XRD (Fig. 2B)nd TEM (Fig. 4B), it is believed that the enhanced peak currenthown on the Pt3Pd3/MWCNTs was not simply resulted from thencreased amount of prepared particles. That is to say, the mecha-ism of EOR on Pt3Pd3/MWCNTs differed from that on the catalystf Pt3Pd2/MWCNTs, probably due to the existence of the new sub-tance of PdO as verified by pattern (c) in Fig. 2B. In fact, it has beeneported that introducing some metal oxides, such as CeO2 [37],nO2 [38], RuO2 [39] and ZrO2 [40], into the catalyst of Pt can greatlyiminish the CO poisoning effect, leading to an increased electro-hemical performance. Very recently, Xi’s group [41] found thatiO2 nanoparticles can promote the catalytic activity of Pt towardOR, and thought that when there is a certain amount of TiO2anoparticles of proper size (e.g. 10 nm) added evenly, a high Pttilization and good electron conductivity can be achieved, leadingo an improvement of more than 50% in terms of ethanol oxidationeak current. To the best of our knowledge, till present, no papereporting the promoting effect of PdO on the catalysis of Pt towardOR was published.

To probe the tolerance of the MWCNTs-supported PtxPdy

atalyst in the EOR, Chronoamperometric measurements were per-ormed in 1 M KOH solution containing 2 M ethanol for a durationf 1000 s. Fig. 6A shows the current decay at a fixed potential of0.3 V of GC electrodes modified with Pt, Pd, and Pt3Pd2 in 1 M KOH

ontaining 2 M ethanol. The polarization currents at the Pt and Pdatalysts decayed rapidly during the initial period. However, at theC electrode modified with Pt3Pd2, the current decayed slowly.he decrease in current is due to the surface poisoning inducedy the intermediate CO species. The lower current decay rate oft3Pd2 may be due to the effective cleaning of the electrode sur-ace, which is consistent with the CV curves described in Fig. 5A.urthermore, it was observed that the current for EOR on the elec-rocatalyst of Pt3Pd2 remained much higher than the other catalystsven at 1000 s, which probably was resulted from the alloying ofd with Pt as compared to that of the pure Pt and Pd catalysts.

To compare the electrochemical stability of all the catalysts forOR, the chronoamperometry curves for the electrooxidation ofthanol at −0.5 V on various catalysts are illustrated in Fig. 6B.lthough the currents are decayed for all the catalysts, the limitingurrent of Pt3Pd3/MWCNTs catalyst keeps the highest value whenompared to the other catalysts throughout all the range under theame experimental conditions. To one’s surprise, the ploarizationurrents of EOR displayed on the catalysts of Pt3Pd4.5 and Pt3Pd1re close to zero compared to those presented on the catalysts oft3Pd3 and Pt3Pd2. This further confirms that a proper amount of Pd

lloyed with Pt can effectively increase the catalysis of Pt. The elec-rochemical performances toward EOR of catalysts of PtxPdy, basedn Fig. 6B, are in the following decreasing order: Pt3Pd3, Pt3Pd2,t3Pd4.5, Pt3Pd1. This result is consistent with that obtained from

−0.36 0.50 6.70 4.7 −0.67−0.37 92 0.50 1.6 −0.72−0.41 0.85 0.90 3.1 −0.69

CV curves in Fig. 5A–C very well. It should be mentioned that theelectrochemical performance order displayed by the catalysts is notconsistent with the crystallinity order deduced from the XRD pat-terns shown in Fig. 2A and B. Is it right to evaluate the catalysis ofa catalyst toward EOR only by the crystallinity and particle sizes?More works should be done to clarify the relationship between thecatalysis and the employed catalysts.

3.4. Analyzing mechanism

To investigate the reasons why PtxPdy catalysts with variousfeed molar ratios of Pt to Pd exhibit different electrocatalytic activi-

Chronoamperometry curves at −0.5 V of as-prepared samples-coated GC electrodein 1 M KOH + 2 M C2H5OH. These colored curves correspond to Pt3Pd1/MWCNTs,Pt3Pd2/MWCNTs, Pt3Pd3/MWCNTsand Pt3Pd4.5/MWCNTs modified GC electrode.(For interpretation of the references to color in figure legend, the reader is referredto the web version of the article.)

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154 K. Ding et al. / Electrochimica Acta 100 (2013) 147– 156

Fig. 7. (A) CVs obtained on the glassy carbon electrode in 1 M KOH at the scan rate of 20 mV s−1. Colored curves were obtained on the Pt3Pd1/MWCNTs, Pt3Pd2/MWCNTsPt3Pd3/MWCNTs and Pt3Pd4.5/MWCNTs modified GC electrode. (B) Open circuit potentials of various catalyst modified GC electrodes in 1 M KOH. (C) Nyquist plots for thecatalysts coated GC electrode in 1 M KOH + 2 M C2H5OH aqueous solution, in which the catalysts are various. It should be mentioned that these experiments were conducteda ) vs. la from Fr

chcPtr−eie−ebEt

E

wow4Ucmovcct

t the open circuit potentials. (D) Bode plots, showing the plots of (−phase angle (◦

queous solution. It should be mentioned that these data used in this figure were

eferred to the web version of the article.)

atalysts, i.e., Pt3Pd2/MWCNTs and Pt3Pd3/MWCNTs, the peaks ofydrogen evolution at around −1.4 V are clearly observed, andorrespondingly, the oxidations peak of hydrogen on Pt3Pd2 andt3Pd3 at about −0.76 and −0.62 V, respectively, are present. Also,wo reduction peaks in the catalysts of Pt3Pd2 and Pt3Pd3, cor-esponding to the reduction of PdO [42], are seen at −0.42 V and0.4 V, respectively. Besides the properties of the catalyst, thelectrochemically active surface area (EASA) of the electrode alsonfluences the reaction rate [43], and thus the value of EASA wasstimated first. As addressed above, the reduction peak at around0.4 V corresponds to the reduction of PdO to Pd, from which thelectrochemically active surface area (EASA) of the electrodes cane estimated. Based on the previously published work [43], theASA of the electrodes can be estimated using the following equa-ion:

ASA = Q

SI(2)

here Q is the coulombic charge for the reduction of palladiumxide [35]. ‘S’ is the proportionality constant used to relate chargeith area and ‘I’ is the catalyst loading in ‘g’. A charge value of

05 mC/cm2 is assumed for the reduction of PdO monolayer [35].nfortunately, the exact amounts of PtxPdy/MWCNTs compositesannot be weighed by our present technique. Thus, we can only esti-ate the values of EASA approximately assuming that the loading

f two catalysts are identical. Based on CV curves in Fig. 7A, the

alues of EASA for Pt3Pd2/MWCNTs, Pt3Pd3/MWCNTs were cal-ulated to be 0.028, 0.034 mm2, respectively. That is to say, theatalyst of Pt3Pd3/MWCNTs has the larger value of EASA relativeo Pt3Pd2/MWCNTs. As verified by the XRD pattern in Fig. 2B, when

og(f/Hz)), obtained for the catalysts coated GC electrode in 1 M KOH + 2 M C2H5OHig. 7C. (For interpretation of the references to color in figure legend, the reader is

the feed molar ratio of Pt to Pd is 3:3, a new phase of PdO wasformed in the catalyst, which may account for the larger valueof EASA in Pt3Pd3/MWCNTs partly. Also, from Fig. 7A, it can beseen that the hydrogen evolution peak potential on the catalyst ofPt3Pd3 is positively shifted for about 100 mV compared with that onPt3Pd2. Correspondingly, the hydrogen desorption peak potentialon catalyst of Pt3Pd3 is positively shifted for about 130 mV com-pared to that of Pt3Pd2. That is to say, compared to the catalyst ofPt3Pd2, the hydrogen evolution on the catalyst of Pt3Pd3 gets eas-ier and desorption of hydrogen becomes difficult. Thus, more ionsof OH− were generated in the vicinity of the catalyst of Pt3Pd3 incomparison with that of Pt3Pd2. Although it is accepted that theprocess of EOR involves many intermediate products (such as lin-early adsorbed CO and CO2, CHx,ads species [44]) and final products(such as CO2, acetaldehyde and acetic acid), and its mechanismremains suspended, the simplified mechanistic steps can be rep-resented recently as follows [45]

CH3CH2OH ↔ (CH3CH2OH)ads (3)

(CH3CH2OH)ads + 3OH− → CH3COads + 3H2O + 3e− (4)

OH− ↔ OHads + e− (5)

CH3COads + OHads → CH3COOH (6)

CH3COOH + OH− → CH3COO− + H2O (7)

Thus, according to above steps, more created OH− can yielda larger amount of OHads, which can greatly accelerate the steps

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4)–(6). As a result, the process of EOR was significantly facil-tated, leading to an enhanced peak current of EOR. To theest of our knowledge, this is the first time to report that theddition of a proper amount of PdO can catalyze the EOR, thoughiang’ group [46] has found that the proper addition of oxides likeeO2, NiO, Co3O4 and Mn3O4 can significantly promote catalyticctivity and stability of the Pd/C electrocatalysts for the alcohollectrooxidation. Recent work has revealed that hydrous RuO2 is aore active catalyst for alcohol oxidation than that of Ru0 as part

f bimetallic Pt–Ru alloy, and it was interpreted that hydrous RuO2RuO2−ı(OH)ı) can play the role of donor of the oxygen-containingpecies that promote the CO to CO2 oxidation. And it was reported47] that RuO2 is rapidly and reversibly oxidized and reduced bylectrochemical protonation.

uO2 + ıH+ + ıe− ↔ RuO2-ı(OH)ı 0 ≤ ı ≤ 2 (8)

Similarly, the hydrous PdO may behave like the oxide of RuO2ince the presence of PdO can facilitate the hydrogen evolution ashown in Fig. 7A.

Meanwhile, Mann et al. studied the direct ethanol fuel cellsased on the PtSnO and PtSnInO catalysts [48]. And he explainedhat the enhanced performance of PtSnO and PtSnInO electrocata-ysts as compared to pure Pt catalyst was due to the fact that thelectron affinity of tin oxide on a Pt surface can sufficiently pull theethyl group off the �-carbon of a surface-bound ethanol, facilitat-

ng the complete oxidation of ethanol. Probably PdO formed in thisork can play the same role as tin oxide does in PtSnO. Also, it is

ddressed that for the alcohol electrooxidation on Pt–Ru/C catalyst,t acts as the main active site for catalyzing the dehydrogenationf alcohol during the oxidation reaction and oxygen-containingpecies (OHad) can form on the Ru surface at lower potentials, andhese oxygen-containing species react with CO-like intermediatepecies on the Pt surface to produce CO2 and release the active sites.imilarly, when the content of oxide is low, there are not enoughxide sites to effectively assist the releasing of adsorbed CO-likeoisoning specie and the catalysis effect is not most evident. Withhe increasing of the oxide content (for the catalyst of Pt3Pd3 rel-tive to Pt3Pd2), the sites for the formation of oxygen-containingpecies, OHad, increase, leading to the quick recovery of the activeites on the Pt and thus high oxidation current for the reaction.

Fig. 7B shows the curve of open circuit potential (Eocp) of theorking electrode against time. It can be seen for that except for

he catalyst of Pt3Pd4.5, in the duration the values of Eocp for thether catalysts changed accordingly, namely, a significant increasef Eocp is observed in the first immersion stage and a steady statealue is established after a certain time of about 100 s. Generally, its though that the initial electrode surface was relatively active andt became passive during corrosion because of the development of

protective layer [49]. Thus, at the beginning stage of immersion,etal ions may be released due to the dissolution of metal parti-

les in basic medium, leading to an enhanced electrode potential.nd then a steady state value was achieved due to the equilibriumetween the dissolution of metal and the sedimentation of metal

ons [49]. Evidently, the values of Eocp for the catalysts at 400 s are inhe following declining order: Pt3Pd3, Pt3Pd2, Pt3Pd4.5 and Pt3Pd1.his order accords with the electrochemical response in CVs veryell (Fig. 5B and C).

As one typical curve in EIS measurement, Nyquist plot is of vitalmportance for evaluating the electrochemical performance of the

orking electrode. Based on the previous report [50], the semicir-le appearing at the high frequency region corresponds to a circuitaving a resistance element parallel to a capacitance element, and

semicircle with a larger diameter corresponds to a larger chargeransfer resistance (Rct). It can be seen from Fig. 7C that for the cat-lysts of Pt3Pd1, Pt3Pd2 and Pt3Pd4.5, a semicircle appearing in theigher frequency region is followed by a declined line appearing in

ta 100 (2013) 147– 156 155

the lower frequency region, which is similar to the previous reporton the CNTs-supported Pd nanoparticles [51]. The catalyst of Pt3Pd1shows the largest semicircle, indicative of the biggest value of Rct.This result is consistent with that showed in the CVs of Fig. 5B.Interestingly, for the catalyst of Pt3Pd3, the semicircle is too smallto be seen as shown in Fig. 7C. Therefore, EOR can take place mucheasier on the catalyst of Pt3Pd3 than on the other samples prepared,which accords with the results obtained from the CVs in Fig. 5C verywell.

To acquire more useful information on the electrical propertiesof the catalysts, Bode plots for all the samples are illustrated inFig. 7D. It is evident that all the four catalysts coated GC electrodesshowed a symmetric peak, which may correspond to the relax-ation process of the electrode|solution interface [52]. The lowerphase angle at 0.02 Hz of the Pt3Pd3/MWCNTs coated GC electrode(54.0◦) compared to other samples (62.4◦ for Pt3Pd1/MWCNTs,62.2◦ for the Pt3Pd2/MWCNTs, 73.4◦ for the Pt3Pd4.5/MWCNTs) isindicative of less capacitive behavior since ideal capacitive systemsusually give a phase angle of ca. −90◦ [53]. It should be noticed thatfor the Pt3Pd3/MWCNTs modified GC electrode, a flat plateau isexhibited in the frequency region from 0.1 Hz to 10 Hz. This curvemay correspond to a new process occurring on the interface ofelectrode|solution. This phenomenon probably was resulted fromthe new phase of PdO that has been detected by XRD patterns.While for the catalyst of Pt3Pd1, a second time constant locatedat the frequency from 100 Hz to 10 kHz is clearly displayed, indica-tive of another interfacial process occurring at the metal substrate.According to results shown in Fig. 7C and D, it can be concludedthat the surface structure of the Pt3Pd3/MWCNTs modified GC elec-trodes is different from the other catalysts.

4. Conclusion

For the first time, MWCNTs-supported PtxPdy compositenanoparticles with a diameter close to 4 nm were prepared by afacile method of pyrolysis. XRD result reveals an alloy of Pt3Pd2coexisting with PdO at a feed molar ratio of Pt to Pd of 3:3. Theresults from electrochemical measurement showed a significantlyincreased peak current of EOR on the Pt3Pd3/MWCNTs modifiedelectrode. The presence of PdO is inferred to be responsible forthe greatly enhanced peak current of EOR. The facile preparationof PtxPdy composite nanoparticles supported by MWCNTs withenhanced catalytic activities is promising for catalysts manufac-turing at a large scale for ethanol oxidation reaction.

Acknowledgements

This work was financially supported by the National Natu-ral Science Foundation of China (No. 21173066), Natural ScienceFoundation of Hebei Province of China (No. B2011205014). Z. Guoacknowledges the support from US National Science Foundation(EAGER:CBET 11-37441) managed by Dr. Rosemarie D. Wesson.

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