the effect of ceo2 on pt/ceo2/cnt catalyst for co electrooxidation

The effect of CeO2 on Pt/CeO2/CNTcatalyst for CO electrooxidationH. Yuan1,2, D. Guo1, X. Li1, L. Yuan1, W. Zhu1, L. Chen1,2, and X. Qiu1,2*1 Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University,Beijing 100084, China2 Laboratory of Advanced Power Sources, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

Received September 21, 2008; accepted January 13, 2009

1 Introduction

Carbon monoxide tolerance catalysts are important for thedevelopment of the anode for polymer electrolyte membranefuel cells (PEMFCs) [1, 2]. The bifunctional mechanism hasbeen widely accepted for explaining the process of CO elec-trooxidation [3, 4]. Based on this mechanism, a second metalelement, such as Ru, which can dissociate H2O under lowerpotentials than Pt, has been used to promote the CO electro-oxidation. Other Pt-based binary catalysts (PtMo [5, 6], PtSn[7, 8], etc.) and ternary catalysts (PtRuMo [9], PtRuNi [10],etc.) have also been reported to show good reactivity.Recently, another strategy of using metal oxides, MxOy

(M = Ti [11], W [12], Sn [13], etc.), as supports for Pt catalystshas been developed. These catalysts effectively enhance pro-motion for the electrooxidation of methanol and ethanol andperfect tolerance to CO poisoning.

Ceria, with a good capacity for storing and releasing oxy-gen, has been reported to be used in fuel cells. Xu et al. [14,15] studied the effect of adding CeO2 to Pt/C catalysts for theelectrooxidation of alcohols (methanol, ethanol, glycerol and

ethylene glycol) in the alkaline solution, and found that CeO2

significantly improved the performance for alcohol oxidation.They contributed the high activity to the synergistic effect ofCeO2. Guo et al. [16] and Huang et al. [17] prepared CeO2

modified PtRu/C catalysts with preferable activity for metha-nol oxidation through different methods. Guo et al.[16] sug-gested that CeO2 in an amorphous form with a mixed oxida-tion state (Ce3+-Ce4+) induced Ru to form Ru(OH)x species,which provided an optimal configuration and the active oxy-gen atoms for the methanol oxidation reaction. However,Huang et al. [17] considered that the promotion was attribut-ed to an increased dispersion of PtRu alloy particles by co-deposited CeO2. Explanations for the effect of CeO2 are muchdifferent for the catalysts with various structures.

Due to the predominant characters of CeO2, our groupsynthesised a series of composite platinum and ceria catalysts[18–20]. CeO2 nanoparticles were first deposited on carbon

–[*] Corresponding author, [email protected]

AbstractPt/CeO2/CNT catalysts were prepared by adsorbing Ptnanoparticles on the supports of CNTs coated with CeO2.The electrocatalytic performances in respect to the electro-oxidation of chemisorbed CO were tested using potentialstep and stripping voltammetry methods under variablesweep rate and temperature conditions. At 10 mV s–1, theCO stripping voltammogram exhibited the peak splittingphenomenon. The oxidation charge and the peak potentialof the two voltammetric peaks changed regularly with thenumber of Pt and CeO2 neighbours, the sweep rate, and thetemperature. We considered that the low potential peak ori-ginated from the reaction of COads with hydroxyl groups onCeO2 adjacent to Pt sites, while the high potential peak came

from the reaction of COads with hydroxyl groups producedon pure Pt. Furthermore, the experimental results of thepeak potential against the logarithm of the sweep rate andthe logarithm of the current maximum time against the steppotential were plotted and intersecting lines with differentslopes in high and low potential regions in the plot wereobserved. The lines intersected at lower potentials on thePt/CeO2/CNT electrode than on the Pt/CNT electrode,which was attributed to the contribution of hydroxyl groupson CeO2.

Keywords: CO Oxidation, Cyclic Voltammetry, Electro-catalyst, Fuel Cell Electrode, Pt Based Electrocatalysts

FUEL CELLS 09, 2009, No. 2, 121–127 © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 121

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DOI: 10.1002/fuce.200800100

Yuan et al.: The effect of CeO2 on Pt/CeO2/CNT catalyst for CO electrooxidation

supports, and then Pt nanoparticles were loaded on them. Itwas found that Pt nanoparticles preferentially adsorbed onthe sites adjacent to CeO2 but not on bare carbon supportsand contacted well with CeO2 particles [19]. These catalystsenhanced the catalytic activity for the oxidation of methanoland CO with no apparent decrease in electrochemicalactive surface areas. We initially anticipated that thepromotion effect is from the oxygen-containing species onCeO2. However, the oxidation process and the kinetics infor-mation are not much clear. Herein, we will further investigatethe CO electrooxidation on Pt/CeO2/CNT electrodes com-pared with that on Pt/CNT electrodes using stripping vol-tammetry and potential step methods. The results allow us tounderstand in depth the effect of CeO2 for CO electrooxida-tion.

2 Experimental

2.1 Catalyst Preparation

The synthesis of Pt/CeO2/CNT catalysts was based on themethod reported previously [19]. Briefly, CeO2/carbon nano-tube (CNT) substrates were prepared by depositing CeO2

onto the surface of functionalised CNTs by adding dropwisethe ammonia solution into a solution of Ce(NO3)3 containingCNTs. Pt nanoparticles were prepared by microwave heatingof an ethylene glycol solution containing 0.06 M NaOH and0.2 M H2PtCl6�H2O for 60 s. Then this solution was added tothe aqueous suspension of CeO2/CNTs and stirred for 4 h.After 4 h of adsorption of Pt nanoparticles, the suspensionwas filtered, washed with deionised water, and dried under80 °C. Pt/CNT catalysts were prepared as reference samplesby loading platinum nanoparticles directly onto the functio-nalised CNTs by the same method. The theoretical mass con-tent of Pt was 20 wt.-% for all the samples. The atomic ratiosof Pt and Ce were 1:1.5, 1:2.5 and 1:4.5. The morphology ofthe catalysts was observed by HRTEM (Tecnai G2 F20S-Twin) at 200 kV and analysed by X-ray diffraction with CuKa source. The XPS spectra were acquired using monochro-matic Al Ka X-ray source operated at 250 W. Charging effectswere corrected by adjusting the binding energy of C 1s to284.8 eV in the XPS spectra.

2.2 Electrochemical Measurements

A solution of 25 lL Nafion solution (20% Nafion and 80%ethylene glycol) and 75 lL H2O containing the same mass ofcatalysts (1 mg) each time was ultrasonically dispersed for30 min. Then the whole suspension was cast onto the surfaceof a polished Au electrode (1 cm2) and dried at 80 °C for45 min. Electrochemical measurements were carried out on aPARSTAT 2273 workstation in 1 M HClO4 solution. A con-ventional three-electrode cell, with Pt foil as the counter elec-trode, and with a saturated calomel electrode (SCE) as thereference electrode, was used. The temperature was con-trolled by circulating constant temperature water out of the

cell. Before each measurement, the electrolyte was purgedwith N2 gas for 20 min to remove the dissolved oxygen, andthen the electrode was performed with an electrochemicalcleaning by scanning the potential between –0.2 and 1.0 Vversus SCE at a sweep rate of 50 mV s–1. In the CO strippingexperiment, the pre-adsorption of CO was achieved bybubbling CO gas into the solution and holding the potentialat 0.1 V versus SCE for 20 min. After that, the solution waspurged with N2 gas for another 20 min at the above potentialto remove the dissolved CO. The real surface area of Pt wascalculated with CO stripping voltammetry assuming420 lC cm–2 per CO monolayer [21]. Current density wasrelative to a square centimetre of Pt.

3 Results and Discussion

Representative TEM images and XRD patterns of Pt/CNTand Pt/CeO2/CNT catalysts are shown in Figure 1. Diffrac-tion peaks of Pt [(111), (200), (220)] and CeO2 [(111), (200),(311)] are observed in XRD patterns, which means that Pt iswith the fcc structure and CeO2 is with the cubic fluoritestructure. According to TEM images, Pt nanoparticles homo-geneously distributed on CNT and CeO2/CNT substrates.The particle size is around 2–3 nm, which accords well with2.3 nm calculated by XRD data. Particle sizes of CeO2 accord-ing to XRD patterns are 4.1, 4.7 and 7.1 nm corresponding toatomic ratios of Pt and Ce of 1:1.5, 1:2.5 and 1:4.5, respec-tively. CeO2 particles grow up gradually with the increase inits content. To see the effect of CeO2 on the electronic struc-ture of Pt atoms, we compared the XPS spectra of Pt/CeO2/CNT and Pt/CNT catalysts, as shown in Figure 2. Bindingenergies of Pt 4f for both samples have the same values of71.3 and 74.6 eV, indicating the electronic structure of Pt isnot affected by CeO2.

CO stripping experiments were performed firstly on theCeO2/CNT and CNT substrates to investigate the catalyticactivity of CeO2 and CNTs for CO electrooxidation. The typi-cal voltammograms are presented in Figure 3. As observed inthis figure, the CeO2/CNT and CNT substrates had little cata-lytic activity for CO electrooxidation. The very low CO oxida-tion current might be produced by the reaction of COads withoxygen-containing species on the surface of CNTs. Thereversible oxidation and reduction peaks at 0.35 V versus SCEcorresponded to the oxygen adsorption and desorption onCNTs in HClO4 [22]. The coated CeO2, which is a semicon-ductor, made the capacitance decrease a little.

Figure 4 shows typical CO stripping voltammograms of Pt/CeO2/CNT (solid line) and Pt/CNT (dashed line) electrodes in1 M HClO4 at a sweep rate of 10 mV s–1. The onset potential ofCO electrooxidation on the Pt/CeO2/CNT electrode had a nega-tive shift of about 0.1 V versus SCE compared with that on thePt/CNT electrode. The promotion effect of CeO2 is evident. Asecond obvious change of CO stripping voltammogram was thesplit of the peak, instead of one voltammetric peak. The doublepeaks were assigned to P1 (E = 0.47 V vs. SCE) and P2

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(E = 0.51 V vs. SCE) in this figure. Data in Figure 5 show thechange of the split peaks of P1 and P2 with the content of CeO2.The current density of peak P1 became higher for larger CeO2

content. The calculated percentages of peak P1 to the whole COstripping peak are 37.1, 45.3 and 48.8% for the atomic ratios of Pt

and Ce of 1:1.5, 1:2.5 and 1:4.5, respectively. The contribution ofpeak P1 to the stripping charge increases with the number of Ptand CeO2 neighbouring sites.

As reported in previous publications, the peak splittinghas been usually found on Ru-modified Pt or Pt single crystalelectrodes [4, 23, 24]. On PtRu electrodes, the two peak vol-tammetric behaviour was considered to reflect the existenceof two kinds of CO species: CO species adsorbed at either Rusites or at Pt sites neighbouring in Ru sites and CO speciesadsorbed at Pt sites far away from Ru sites. CO oxidation atthe former sites takes place under lower potential than at thelater sites [4]. The slow CO diffusion between these two kindsof sites results in a two peak behaviour. The peak splitting isimportant for understanding the mechanism of CO oxidationon the PtRu surface. It has been proved in our experimentthat CeO2 cannot adsorb and oxidise CO (Figure 3). How-ever, there are abundant hydroxyl groups on the surface ofCeO2 in aqueous solutions. It was reported that the density ofhydroxyl groups on CeO2 was 4.3 hydroxyl molecules per100 (Å)2 [25]. The hydroxyl groups coordinated with one, twoor three cerium ions [26]. Other results showed that therewere two layers, assuming a fully hydroxylated surface cov-

c)

a) b)

d)

Fig. 1 TEM images of Pt/CNTs (a), and Pt/CeO2/CNTs (b) and (c), andXRD patterns (d).

Fig. 2 XPS spectra of Pt 4fdoublets of Pt/CNTs (a) and Pt/CeO2/CNTs (b).

Fig. 3 CO stripping voltammograms of CeO2/CNTs (solid line) and CNTs(dashed line) in 1 M HClO4 at 20 °C; v = 10 mV s–1.

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ered by 4.6 H2O molecules per 100 (Å)2 on the CeO2 surface[27]. Therefore, we may explain the occurrence of the splitpeaks by the participation of hydroxyl groups on CeO2, simi-lar with the two peak voltammetric behaviour on PtRu elec-trodes.

The hypothetic CO oxidation process of the Pt/CeO2/CNTelectrode is shown in Scheme 1. At low potentials, OHads neces-sary for CO electrooxidation are hard to form on the Pt surface.The oxygen-containing species are mainly afforded by CeO2

adjacent to Pt sites. The reaction equations may be written as

Pt�CeO2�H2O→Pt�CeO2�OHads�H��e� (1)

Pt � COads�Pt�CeO2�OHads→CO2�H��e��Pt � Pt�CeO2

�2�With the progress of reactions (1) and (2), COads at the

adjacent sites of Pt and CeO2 particles are gradually con-sumed. COads far away have to diffuse to these sites to reactwith CeO2-OHads continuously, which forms the peak P1. Athigh potentials, OHads can be largely produced by the waterdissociation on Pt. Reactions (3) and (4) take place at the sametime until COads are consumed.

Pt � H2O→Pt � OHads�H��e� (3)

Pt � COads�Pt � OHads→CO2�H��e��2Pt (4)

Because of the slow diffusion of COads, reactions (3) and(4) play an important role in high potential regions. This pro-cess yields the peak P2. The COads coverage on Pt decreasesdue to reactions (1) and (2) occurred before. More sites for theadsorption and dissociation of water on Pt are released. ThusCOads are much easier to be oxidised, and the potential ofpeak P2 is more negative than the peak potential of CO elec-trooxidation on the Pt/CNT electrode.

Based on this assumed process, with increase in thecontent of CeO2, Pt particles will contact with more CeO2

particles, and Pt sites neighbouring in CeO2 sites increase.Hence the adsorbed CO are preferable to be oxidised throughreactions (1) and (2) at low potentials and the contribution ofpeak P1 to the stripping charge increases, which is consistent

with the results in Figure 5.To prove the existence of reactions (1) and (2),

we performed the stripping voltammetry after along time oxidation step at Eox on the Pt/CeO2/CNT electrode. First, we applied an oxidation stepat 0.3 V for 1800 s, and then the CO stripping wasmeasured. The data compared with the originalstripping voltammogram are shown in Figure 6.The stripping charge has a decrease in about 53%.At 0.3 V, the reaction rate of water dissociation andthe resulting CO electrooxidation on Pt was veryslow. Hydroxyl groups for CO oxidation weremainly afforded by CeO2. With the consumption ofthe adsorbed CO, water dissociation on Pt wasinduced. Accordingly, the peak potential moved to0.48 V. In addition, the peak splitting disappearedbecause COads in close proximity to CeO2 weremostly consumed and the diffusion rate was veryslow.

Fig. 4 CO stripping voltammograms of Pt/CeO2/CNTs (1:2.5) (solid line)and Pt/CNTs (dashed line) in 1 M HClO4 at 20 °C; v = 10 mV s–1.

Fig. 5 CO stripping voltammograms of different CeO2 contents.

Scheme 1 Schematic representation of the reaction mechanism of CO electrooxida-tion.

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The influence of the sweep rate on CO electrooxidation forthe Pt/CeO2/CNT electrode is presented in Figure 7. Peakpotentials shifted positively as the sweep rate increased.Stripping voltammograms split into two peaks from thesweep rate of 2–20 mV s–1. When the sweep rate becamehigher, the peak splitting was not obvious. As mentionedabove, the peak splitting indicates the slow COads diffusion todifferent reaction sites [4]. With the increase in the sweeprate, the diffusion of COads to Pt sites adjacent to CeO2 wasnot much effective, and the percentage of the low potentialpeak accounting for the total peak decreased. The peak split-ting disappeared gradually. The peak potential, Ep, againstthe logarithm of the sweep rate, v, is shown in Figure 8. Onthe Pt/CeO2/CNT electrode, there are two lines intersectingat 0.50 V versus SCE with slopes of 63 and 190 mV. On thePt/CNT electrode, the two lines intersect at 0.64 V versus SCEwith slopes of 70 and 180 mV. Generally, the plot of Ep versuslog v is observed linearly in the measured potential range on

Pt electrodes. The slope has been reported of 60–80 mV onsingle crystal Pt and Pt nanoparticles [1, 28]. However, Koperet al. [29] found the change of the slope from 40 mV for thelowest sweep rate to 119 mV for the highest sweep rates onplatinum by Monte Carlo simulation, which was similar withour results. Meanwhile, they found the same nonlinear rela-tionship in the plot of the logarithm of the current maximumtime against the step potential. In our experiment, potentialsteps were also measured to prove the existence of this phe-nomenon.

Various step potentials from 0.1 V versus SCE to differentoxidation potentials in the interval from 0.4 to 0.7 V versusSCE were performed. As can be seen from the insert inFigure 9, the current transient curves exhibit an initial steepdecay followed by a current maximum, and then decay witha tailing. The current maximum corresponds to the CO oxida-tion on Pt nanoparticles. The plot of the logarithm of the cur-rent maximum time, tmax, against the step potential, E, isdrawn in Figure 9. Two intersecting lines yielded. The corre-

Fig. 6 CO stripping voltammogram of Pt/CeO2/CNTs (1:4.5) (solid line)after the potentiostatic oxidation for Eox = 0.3 V, tox = 1800 s. Thedashed line is the original CO stripping voltammogram.

Fig. 7 CO stripping voltammograms of Pt/CeO2/CNTs (1:4.5) at differentsweep rates multiplied by the number showed in the brackets. From up todown: 2 mV s–1 (×50), 5 mV s–1 (×20), 10 mV s–1 (×10), 20 mV s–1 (×5),70 mV s–1 (×10/7), 100 mV s–1 (×1), 200 mV s–1 (×1/2).

Fig. 8 Plot of the peak potential against the logarithm of the sweep rate.

Fig. 9 Plot of the logarithm of the current maximum time against the steppotential. Insert is the current transient curves of Pt/CeO2/CNTs (1:2.5)and Pt/CNTs for CO oxidation the after the potential step from 0.1 Vversus SCE to different step potentials.

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sponding lines for the Pt/CeO2/CNT and Pt/CNT electrodesare almost parallel with each other. The absolute values of theslopes are 50 (mV)–1 for low potentials and (210 mV)–1 forhigh potentials. However, the intersection point appears atlower potential on the Pt/CeO2/CNT electrode.

Koper et al. [29] have proved that the change of the slopewas caused by the multistep character of the reaction mecha-nism related to the adsorption and desorption of hydroxylgroups. In detail, at low potentials, the OH desorption is fas-ter than the OH adsorption and the formation of OHads canbe considered in a kind of quasi-equilibrium, while, at highpotentials, the OH adsorption is the fastest process and theOH desorption can be neglected. Hence the changes of theslopes were observed in the plot of Ep versus log v (Figure 8)and the plot of log tmax versus E (Figure 9). Given the abovediscussion, their intersection points both appeared at lowerpotentials on the Pt/CeO2/CNT electrode, which indicatedthat the OH adsorption rate became the fastest process in thelower potential region than that on the Pt/CNT electrode.The kinetics of CO oxidation sped up.

Finally, we studied the influence of the temperature on COelectrooxidation. As shown in Figure 10, the split peaks ofboth P1 and P2 shift negatively as the temperature increasesfrom 273 to 323 K. This indicated that the catalytic activitywas enhanced. The shape of the peak also changed gradually.First, the peak split into two distinct peaks at low tempera-tures, and then the split peaks overlapped as the temperatureincreased. To exhibit the split peaks more clearly, we did thepeak fitting for CO stripping voltammograms at each temper-ature. Peak potentials of the split peaks of the Pt/CeO2/CNTelectrode and the single peak of the Pt/CNT electrode allchanged linearly with the temperature (Figure 11). The slopeof the potential of peak P1 was the smallest, indicating reac-tions (1) and (2) were much less influenced by the tempera-ture. Herrero et al. [30] have studied the influence of the tem-perature on COads electrooxidation on Pt (111), Pt (100), andPt (110) electrodes. They also found the linear relationship forthe plot of the peak potential against the temperature, andused the intercept to calculate the activation energy for COoxidation. It is proved that the intercept of this plot yieldedthe activation energy of CO electrooxidation minus the stan-dard potential of adsorbed CO electrooxidation which wastaken to be 0.0 V versus SHE. In Figure 11 the intercepts forthe Pt/CeO2/CNT electrode are 0.98 V versus SCE (peak P1)and 1.57 V versus SCE (peak P2), both lower than that of1.72 V versus SCE for the Pt/CNT electrode. Therefore, theprocess of OHads forming on CeO2 and reacting with COads

on the adjacent Pt sites has much lower activation energythan the CO electrooxidation on pure Pt. At the same time,there were also some decreases in the activation energy ofpeak P2 owing to the induced water dissociation by the con-sumption of the adsorbed CO, as has been discussed above.

4 Conclusion

In this paper, electrochemical performances of CO electro-oxidation on Pt/CeO2/CNT electrodes were studied usingstripping voltammetry and potential step methods. We foundthe changes of the slopes in the plot of the logarithm of thecurrent maximum time against the step potential and thepeak potential against the logarithm of the sweep rate, whichwere accordant with the simulated results reported pre-viously [29]. Compared with the Pt/CNT electrode, changesof the slopes appeared at lower potentials on the Pt/CeO2/CNT electrode, which was attributed to the contribution ofhydroxyl groups on the surface of CeO2. In addition, the peaksplitting in the voltammogram of CO electrooxidationoccurred at 0.47 and 0.51 V versus SCE under the sweep rateof 10 mV s–1. The increase in the number of Pt and CeO2

neighbouring sites increased the proportion of the charge ofthe lower potential peak. We thought that the peak at thelower potential corresponded to the reaction between Pt-COads and CeO2-OHads at the interfacial surface, and the peakat the higher potential originated from the reaction betweenPt-COads and Pt-OHads on Pt itself. In the experiment of tem-

Fig. 10 CO stripping voltammograms of Pt/CeO2/CNTs (1:4.5) at differ-ent temperatures in 1 M HClO4; v = 10 mV s–1.

Fig. 11 Fitting peaks of CO stripping voltammograms of Pt/CeO2/CNTs(solid squares and circles) and Pt/CNTs (hollow squares) at differenttemperatures.

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perature variation, we found that the reaction correspondingto the lower potential peak had much lower activationenergy, which can accelerate the kinetics of CO electrooxida-tion significantly.

Acknowledgements

This work was supported by the State Key Basic ResearchProgram of PRC (2009CB220105) and Beijing National ScienceFoundation (2071001).

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