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Electrochimica Acta 111 (2013) 339– 343

Contents lists available at ScienceDirect

Electrochimica Acta

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

nfluence of the preparation method and the support on H2O2lectrogeneration using cerium oxide nanoparticles

.H.M.T. Assumpç ãoa, A. Moraesa, R.F.B. De Souzaa, M.L. Calegarob, M.R.V. Lanzab,.R. Leitec, M.A.L. Cordeiroc, P. Hammerd, M.C. Santosa,∗

LEMN – Laboratório de Eletroquímica e Materiais Nanoestruturados, CCNH – Centro de Ciências Naturais e Humanas, UFABC – Universidade Federal doBC, CEP 09.210-170, Rua Santa Adélia 166, Bairro Bangu, Santo André, SP, BrazilIQSC – Instituto de Química de São Carlos, USP – Universidade de São Paulo, Caixa Postal 780, CEP 13.566-590, São Carlos, SP, BrazilCMDMC, LIEC, Departamento de Química, Universidade Federal de São Carlos, CEP 13.565-905 São Carlos, SP, BrazilInstituto de Química, UNESP, Universidade Estadual Paulista, CEP 14.800-060 Araraquara, SP, Brazil

r t i c l e i n f o

rticle history:eceived 11 June 2013eceived in revised form 24 July 2013ccepted 25 July 2013vailable online 11 August 2013

eywords:xygen reduction reactionlectrogeneration of hydrogen peroxideerium oxide nanoparticles

a b s t r a c t

This work describes the influence of the preparation method and the carbon support using a low contentof cerium oxide nanoparticles (CeO2/C 4%) on H2O2 electrogeneration via the oxygen reduction reac-tion (ORR). For this purpose, the polymeric precursor (PPM) and sol–gel (SGM) methods with Vulcan XC72R (V) and Printex L6 (P) supports were employed. The materials were characterized by X-ray diffrac-tion (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). TheXRD analysis identified two phases comprising CeO2 and CeO2−x. The smallest mean crystallite size wasexhibited for the 4% CeO2/C PPM P material, which was estimated using the Debye–Scherrer equation tobe 6 nm and 4 nm for the CeO2 and the CeO2−x phases, respectively, and was determined by TEM to be5.9 nm. XPS analysis was utilized to compare the oxygen content of the 4% CeO2/C PPM P to Printex L6.The electrochemical analysis was accomplished using a rotating ring-disk electrode. The results showed

that the 4% CeO2/C specimen, prepared by PPM and supported on Printex L6, was the best electrocatalystfor H2O2 production in 1 mol L−1 NaOH. This material showed the highest ring current, producing 88%H2O2 and transferring 2.2 electrons per O2 molecule via the ORR at the lowest onset potential. Addition-ally, the ring-current of the 4% CeO2/C PPM P material was higher than that of Vulcan XC 72R and PrintexL6, the reference materials for H2O2 production, indicating the highest electrocatalytic activity for the 4%CeO2/C PPM P material.

. Introduction

In the last few decades, there has been a fast evolution ofesearch activity devoted to environmental protection, which hasccurred as a consequence of the special attention being paido environmental issues [1]. Consequently, there has been anmprovement in new technologies for toxic pollutant eliminationimed at the mineralization of contaminants to carbon dioxide andater or to other harmless products.

Among the most used techniques for water treatment, thedvanced oxidation processes (AOPs) are known to achieve highevels of water purity, even in the presence of compounds that are

eported to be highly refractory with respect to more conventionalreatments [2]. AOPs have proven to be effective in degrad-ng organic constituents in industrial wastewaters, including

∗ Corresponding author. Tel.: +55 11 4996 0163; fax: +55 11 4996 0090.E-mail addresses: mauro.santos@ufabc.edu.br, drmcsa@gmail.com (M.C. Santos).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.07.187

© 2013 Elsevier Ltd. All rights reserved.

persistent organic contaminants, which have a high chemical sta-bility and are therefore not readily biodegradable. Moreover, AOPscan serve as an alternative treatment to accelerate the biodegrad-ability of pollutants [3,4].

AOPs use strong oxidizing agents such as O3, H2O2 and/orcatalysts, including Fe and TiO2, to generate the most powerful oxi-dizing agent in water, the hydroxyl radical, for the degradation ofhazardous substances [5–10]. The higher the oxidation potentialof the reagent used in the AOP, the more efficient the oxidationprocess. The oxidizing ability of the hydroxyl radical arises fromits high standard electrode potential of +2.7 V in acid solution, soit can react with most organic compounds through abstraction oraddition reactions at diffusion-controlled rates [1,11].

Among all the AOPs, the Fenton reaction (a mixture of hydrogenperoxide and iron (II) salt) and the photo-Fenton reaction are con-

sidered to be the most promising for the remediation of waste watercontaining toxic compounds [12]. Because the industrial produc-tion method of hydrogen peroxide utilizes anthraquinone [13,14]and demands energy and organic chemicals, there has been great

dx.doi.org/10.1016/j.electacta.2013.07.187http://www.sciencedirect.com/science/journal/00134686http://www.elsevier.com/locate/electactahttp://crossmark.crossref.org/dialog/?doi=10.1016/j.electacta.2013.07.187&domain=pdfmailto:mauro.santos@ufabc.edu.brmailto:drmcsa@gmail.comdx.doi.org/10.1016/j.electacta.2013.07.187

3 trochimica Acta 111 (2013) 339– 343

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methods [30,37]. The material that exhibited the smallest meancrystallite size (4–6 nm) was the 4% CeO2/C PPM P catalyst. Themean nanoparticle size for this material was also determined usingTEM by analyzing 300 particles from different regions. The TEM

Table 1Average size of cerium oxide nanoparticles.

Electrocatalyst Means crystallite size (nm)

40 M.H.M.T. Assumpç ão et al. / Elec

nterest in the last few decades in producing hydrogen peroxiden situ within the reaction medium to efficiently promote subse-uent oxidation reactions [15]. Hydrogen peroxide is considered toe a green oxidant because its decomposition only generates waternd oxygen [13] and can be generated by the cathodic reduction ofissolved oxygen in alkaline media [16–18].

Oxygen can be reduced at the cathode via two different path-ays: a two-electron transfer yielding hydrogen peroxide or a

our-electron transfer yielding water [19–21]. It is also known thatarbon electrodes are active for hydrogen peroxide production inlkaline media [15,22,23]. However, recent studies have demon-trated that the use of some metals and metal oxides biases thectivity of the oxygen reduction reaction toward the two-electronransfer in ORR [24–26].

Considering the many electrocatalysts composed of metalxides, ceria has superior properties, such as corrosion resistance,nd it has been found to be a critical component of automotivemission control catalysts and electrodes used in solid oxide fuelells, oxygen pumps and other applications [27,28]. Ceria has alsonother interesting property: it is a fluorite oxide whose cations canwitch between +3 and +4 oxidation states and can be an oxygenuffer, as it can release oxygen reversibly [29]. Additionally, sometudies have proposed that a higher oxygen content on the surfacef the electrocatalyst will lead to an increase in catalytic activity30,31].

Based on the fact that ceria acts as an oxygen buffer andhat increasing the oxygen content on the surface will enhanceydrogen peroxide formation [29,30], this work investigates the

nfluence of the preparation method and the carbon support usinghe 4% CeO2/C electrocatalyst. The use of 4% CeO2/C has previouslyeen determined to be the best ratio [30]. For this study, 4% CeO2/Cas prepared by two methods, PPM and SGM, and was supported

n two different carbons, Printex L6 and Vulcan XC 72R. The elec-rocatalysts were analyzed for ORR in alkaline media.

. Materials and methods

.1. CeO2 synthesis

CeO2 4% (w/w) on carbon black were prepared using PPM asescribed by De Souza et al. [32–35] and using SGM as previ-usly described [36,37]. The materials were supported on Printex6 (Evonik) and Vulcan XC 72R (Cabot Corporation). These sup-ort materials were used to evaluate the effects of the preparationethod and the carbon support on H2O2 electrogeneration.

.2. Characterization of CeO2 materials

.2.1. XRD, TEM and XPS analysisThe cerium electrocatalysts were characterized by XRD using a

8 Focus diffractometer (Bruker AXS) with a CuK� radiation sourceperating in continuous scan mode (2◦ min−1) from 20◦ to 80◦. ForEM analysis, a TECNAI F20 FEI microscopy was used, operating at00 kV.

XPS analysis was carried out at a pressure of less than 10−7 Pasing a commercial spectrometer (UNI-SPECS UHV). The Mg K�

ine was used (h� = 1253.6 eV), and the analyzer pass energy waset to 10 eV. The inelastic background of the Ce 3d, C 1s and O 1slectron core-level spectra was subtracted using Shirley’s method.he composition of the near surface region was determined withn accuracy of about ±5% from the ratio of the relative peak areasorrected by Scofield’s sensitivity factors of the corresponding ele-

ents. The spectra were fitted without placing constraints usingultiple Voigt profiles. The width at half maximum (FWHM) var-

ed between 1.2 eV and 2.1 eV and the accuracy of the peak positionsas ±0.1 eV.

Fig. 1. XRD patterns of 4% CeO2/C nanoparticles prepared by different methodolo-gies and supported on different carbons.

2.2.2. Electrochemical characterizationElectrochemical experiments were conducted using an Autolab

PGSTAT 302N potentiostat/galvanostat equipped with a rotatingring-disk electrode system and an EDI101 rotator, including aCTV101 speed control unit. A standard 3-electrode cell contain-ing a counter electrode (platinum wire), a reference electrode(Hg/HgO) and a rotating ring-disk electrode (working electrode,from Pine Instruments) were used. The working electrodes wereconstructed using the configuration proposed by Paulus et al. [38]and Assumpç ão et al. [30,31,37] and supported on the central glassycarbon (area = 0.2475 cm2). NaOH (1 mol L−1) was used as the sup-porting electrolyte. The electrode rotation rate was varied from 100to 3600 rpm, and the experiments were controlled using GeneralPurpose electrochemical system (GPES) software.

During the ORR measurements, the electrolyte was first satu-rated with oxygen for 40 min and the flow was maintained overthe electrolyte during the experiment. The measurements wereperformed at a fixed scan rate of 5 mV s−1 at room temperature.

3. Results and discussion

3.1. X-ray diffraction analysis

Fig. 1 shows the XRD patterns of the CeO2/C catalysts producedby the PPM and SGM while supported on different carbon sub-strates. The XRD patterns show that both methods result in theformation of two different phases, CeO2 and CeO2−x. However,the 4% CeO2/C SGM V showed low crystallinity. The characteris-tic diffraction peaks related to the CeO2 and CeO2−x phases wereattributed according to reference files JCPDF # 65-5923 and JCPDF# 49-1415, respectively.

The average crystallite size of cerium oxide was estimated usingthe Debye–Scherrer equation, shown in Table 1. The obtainedvalues were similar to those reported for the same preparation

CeO2 (2 0 0) CeO2−x (0 4 4)

4% CeO2/C PPM P 6 44% CeO2/C SGM P 11 5

M.H.M.T. Assumpç ão et al. / Electrochimica Acta 111 (2013) 339– 343 341

1 2 3 4 5 6 7 8 9 10 11 12 130

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Table 2Composition of the near surface region of the 4% CeO2/C PPM P electrocatalyst andthe Printex L6 reference.

Element Concentration (at.%)

4% CeO2/C PPM P Printex L6

Carbon (C 1s) 87.1 92.1

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Oxygen (O 1s) 12.3 7.9Cerium (Ce 3d) 0.6 –

mages of 4% CeO2/C PPM P shown in Fig. 2(a) illustrate that theanoparticles are well dispersed on the carbon support. Fig. 2(b)isplays the mean particle diameter distribution for this catalyst,hich shows a relatively straight histogram. The nanoparticlesave an average size of 5.9 nm, which is in good agreement withhe XRD analysis.

The compositions of the near surface region of the 4%eO2/C PPM P electrocatalyst and the Printex L6 reference samplesre summarized in Table 2, which were obtained from XPS quan-itative analysis. The results show that the oxygen content of theatalyst is more than 4% higher than that of the Printex support.aking into consideration the fraction of oxygen bonded to Ce, theatalyst samples contain approximately 3% of additional oxygenpecies, which might affect the catalytic activity of the material.

The local bonding structure of Ce in the 4% CeO2/C PPM P elec-rocatalyst was investigated by the deconvolution of the Ce 3d coreevel spectrum. The Ce 3d spectrum, shown in Fig. 3, was fittedsing 5 spin–orbit doublets in order to obtain the Ce4+ to Ce3+

oncentration ratio of the catalyst. The fitted Voigt components,abeled v and u, are related to 3d5/2 and 3d3/2 spin-orbit states,espectively. The complex but distinct Ce 3d features are related

ig. 3. Deconvoluted XPS Ce 3d spectrum of the 4% CeO2/C PPM P electrocatalyst.

Particle diameter / nm

b) histogram of the mean particle diameter distribution.

to the final-state occupation of the Ce 4f level. The v3/u3 and v1/u1doublets, separated by 18.6 eV, are due to primary photoemissionfrom Ce(IV) and Ce(III) cations, respectively. The v/u and v2/u2 dou-blets are shake down features resulting from the transfer of oneor two electrons from a filled O 2p to an empty Ce 4f orbital dur-ing the Ce(IV) photoemission, and the v0/u0 doublet is related tothe shakedown of Ce(III) cations [39]. The analysis of the spectrumrevealed that the percentage of Ce4+ species was 52.5%, obtainedfrom the Ce4+/(Ce4+ + Ce3+) peak area ratio, indicating that almosthalf of the CeO2 was reduced to Ce(III). Although X-ray inducedreduction of Ce(IV) can occur to some extent, tests using ammo-nium ceric nitrate indicated a Ce(IV) reduction of approximately20%. According to XPS data, the CeO2−x detected in the XRD pat-tern of the 4% CeO2/C PPM P sample is associated with the Ce(III)phase.

Fig. 4 shows the deconvoluted O 1s and C 1s spectra of the4% CeO2/C PPM P electrocatalyst and the Printex L6 support refer-ence. Comparison of spectral features provides information on thespecies introduced by cerium. It can be observed that for PrintexL6 the O 1s spectra are dominated by two components attributedto O C, OH groups at 532.2 eV, and C O, O C O groups locatedat 533.7 eV. In the case of the 4% CeO2/C PPM P catalyst, two addi-tional features can be observed: O Ce bonds at 530.7 eV of the O2−

oxidation state and a high-energy component (334.9 eV), attributedto the presence of molecular water [40] on the sample surface. TheC 1s spectra have very similar peak-form for both samples, con-sisting of five components with the most prominent sub-peaks at284.5 eV and 285.2 eV, related to the aromatic and hydrocarbonphase, respectively, as well as ether/alcohol, carboxyl, hydroxylgroups, and plasmon transitions at 286.1 eV, 287.5 eV, 289.3 eV and291.1 eV, respectively [40].

It is known that the incorporation of ceria into the catalystsmight increase the local oxygen concentration [29,41,42], and theresults in Table 2 show the presence of more oxygenated species onthe cerium material, which could increase H2O2 electrogeneration[30,31]. Hence, the increased oxygen content due to the presence ofwater on the cerium catalyst surface also increases the hydrophiliccharacter, thus most likely conferring more conductivity and thencontributing to H2O2 formation [43,44].

3.2. Electrochemical characterization

To determine which pathway was followed on the ceria-basedcatalysts, the electrocatalytic activity of these materials for oxygenreduction was evaluated using a rotating ring-disk electrode. Fig. 5compares the polarization curves for the oxygen reduction on

342 M.H.M.T. Assumpç ão et al. / Electrochimica Acta 111 (2013) 339– 343

% CeO

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Fig. 4. Deconvoluted XPS C 1s and O 1s spectra of the 4

% CeO2/C PPM P, 4% CeO2/C SGM P, 4% CeO2/C SGM V, VulcanC-72R, Printex L6 and Pt/C E-TEK catalysts. From this figure, itan be verified that the 4% CeO2/C PPM P showed the highest ringurrent, corresponding to a higher amount of generated H2O2ompared to Pt/C, which showed the lowest ring current and wasxpected for the 4-electron pathway of the Pt/C [45]. However,ll of the cerium oxide materials studied catalyzed a 2-electronransfer in ORR, resulting in H2O2 production, as evidenced by theing current in Fig. 5a. The 4% CeO2 PPM P catalyst yielded the bestverage performance. This material presented the highest ringurrent and, consequently, the highest amount of H2O2 producedf all ceria-modified catalysts studied. The ring current producedy the 4% CeO2 PPM P catalyst was also higher than that for VulcanC 72R and Printex L6 carbon reference materials for H2O2 pro-uction, and it was also higher than that of CeO2 PPM V, observed

n our previous work [30]. Furthermore, the onset potential forhe hydrogen peroxide using 4% CeO2 PPM P catalysts was lowerompared to the other materials, resulting in an improvementith respect to electric energy consumption.

ig. 5. Steady state polarization curves for ORR on the 4% CeO2/C materials, Printex6, Vulcan XC-72R and Pt/C E-TEK in oxygen-saturated 1 mol L−1 NaOH at a scan ratef 5 mV s−1. (a) Ring current at Ering = 0.2 V. (b) Disk current at 1600 rpm.

2/C PPM P electrocatalyst and the Printex L6 reference.

The ORR data for all materials, presented in Fig. 5, were analyzedusing the Koutecky–Levich (K–L) equation. Fig. 6 shows the corre-sponding K–L plots for all studied catalysts. The slopes of the curvesare directly proportional to the number of electrons exchangedin the ORR. The slopes obtained for the 4% CeO2 PPM P and 4%CeO2 SGM P materials were 99 and 81 (rpm)1/2 (mA)−1, respec-tively, and were very close to the value obtained for Vulcan XC 72Rcarbon, 112 (rpm)1/2 (mA)−1. Because carbon and Pt/C are referencematerials for the 2- and 4-electron oxygen reduction to hydrogenperoxide and water, respectively, the results indicate that for ceria-modified catalysts the hydrogen peroxide pathway is preferentiallyfollowed, corroborating the results presented in Fig. 5.

The number of electrons transferred (nt) during the ORR andthe H2O (p(H2O)) and H2O2 (p(H2O2)) percentages were calculatedaccording to Demarconnay et al. [46] and Jakobs et al. [47], andresulting values are presented in Table 3. The values were calcu-

lated considering a potential of −0.4 V, which is the potential wherethe ring-current shows significant production of H2O2. From theseresults, we can affirm once more that all ceria materials follow a

Fig. 6. Koutecky–Levich plot for ORR on cerium oxide nanomaterials, Printex L6,Vulcan XC-72R and Pt/C E-TEK.

M.H.M.T. Assumpç ão et al. / Electrochim

Table 3The number of electrons transferred and the H2O and H2O2 percentages during ORR.

Material nt p(H2O) p(H2O2)

CeO2/C 4% PPM P 2.2 12 88CeO2/C 4% SGM P 2.2 12 88CeO2/C 4% SGM V 2.3 17 83

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-electron pathway in the ORR. Both materials, CeO2 PPM P and% CeO2 SGM P, produced the highest H2O2 percentages.

The electrochemical measurements indicate that all ceria mate-ials are promising electrocatalysts for H2O2 electrogeneration; allf them showed a higher amount of H2O2 production during ORRhan that of Vulcan XC 72R and Printex L6, known as reference

aterials for the 2-electrons transference. Nevertheless, the 4%eO2/C PPM P showed the highest ring-current, which is a conse-uence of the highest amount of H2O2 produced and the lowestnset potential of H2O2 production, thus improving the electricnergy consumption.

The structural characterization of 4% CeO2/C PPM P revealedwo phases and the smallest mean crystallite size. It is known fromhe literature [48] that ceria has the ability to switch the oxida-ion states between +3 and +4, and it can release oxygen reversibly29]. In our previous study with different carbon supports [31], webserved that Printex L6 showed more oxygenated acid speciesompared to Vulcan XC 72R. Consequently, using the best ceriaatalytic material [30] with the best carbon support caused aynergic effect, increasing the oxygen concentration, which wasbserved by XPS and H2O2 production because the oxygenated acidpecies contribute to the improvement in H2O2 electrogeneration30,31,37,49]. These properties might explain the superior activityf the 4% CeO2 PPM P for hydrogen peroxide generation.

. Conclusions

All tested 4% CeO2/C electrocatalysts indicate that the num-er of electrons transferred during ORR was close to 2. However,% CeO2 PPM P had the highest ring current, producing 88% H2O2nd transferring 2.2 electrons per O2 molecule at the lowest onsetotential, presenting a promising electrocatalyst for H2O2 electro-eneration. This study confirmed the importance of the preparationethod and the carbon support on H2O2 production. The 4%

eO2/C PPM P electrocatalyst comprises a mixture of oxides, CeO2nd CeO2−x, at similar concentrations, conferring more oxygentoms to the structure, enhancing the H2O2 production. Hence,he best activity toward ORR was obtained by the electrocatalystupported on the most efficient carbon substrate with the high-st number of oxygenated species. This caused a synergic effectnd facilitated the 2-electron transfer, thus improving the H2O2lectrogeneration.

cknowledgments

The authors wish to thank the Brazilian Funding Insti-utions CNPq (577292/2008-0, 473308/2010-0, 150639/2013-), CAPES, FAPESP (2005/59992-6, 2008/58789-0, 2009/09145-, 2010/04539-3, 2010/16511-6, 2007/04759-0, 2011/14314-1,012/03516-5), Instituto Nacional de Ciência e Tecnologia (INCT)e Energia e Meio Ambiente (Process Number 573.783/2008-0) andFABC for their financial support.

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Influence of the preparation method and the support on H2O2 electrogeneration using cerium oxide nanoparticles1 Introduction2 Materials and methods2.1 CeO2 synthesis2.2 Characterization of CeO2 materials2.2.1 XRD, TEM and XPS analysis2.2.2 Electrochemical characterization

3 Results and discussion3.1 X-ray diffraction analysis3.2 Electrochemical characterization

4 ConclusionsAcknowledgmentsReferences