[The definitive version is available at http://onlinelibrary.wiley.com/doi/10.1002/celc.201402015/abstract].

Pure and alloyed copper-based nanoparticles supported on activated carbon: synthesis and electrocatalytic application in the reduction of nitrobenzeneXia Sheng,§[a], Benny Wouters,§[b] Tom Breugelmans,[b] Annick Hubin,[b] Ivo F.J. Vankelecom,[a] Paolo P. Pescarmona*[a]

§ These authors contributed equally.

A series of Cu/CuxO, Pt-Cu alloy and Ptnanoparticles supported on inexpensive activatedcarbon with large surface area were developed aselectrocatalysts for the reduction ofnitrobenzene. This reaction can produce usefulchemicals with the potential to cogenerateelectricity if applied in a fuel cell. Both thenature of the metal employed and of the carbonmaterial used as support play an important role indetermining the performance of theelectrocatalysts: Cu-based materials display lower

onset potentials for the reduction of nitrobenzenewhile the features of the activated carboninfluence the stability of the electrocatalysts.The most promising electrocatalyst consisted ofhighly dispersed and very small Cu/CuxOnanoparticles (~4 nm) supported on Norit activatedcarbon, which were prepared using H2 as reductant.A chronoamperometric test in acidic ethanolicmedium gave good conversion of nitrobenzene overthis electrocatalyst, with azoxybenzene as themajor product.

[a] Dr. X. Sheng, Prof. I. F.J. Vankelecom, Prof. P. P. PescarmonaCentre for Surface Chemistry and CatalysisUniversity of Leuven (KU Leuven)Kasteelpark Arenberg 23, PO Box 2461, 3001 Heverlee, Belgium.E-mail: Paolo.Pescarmona@biw.kuleuven.be

[b] B. Wouters, Prof. T. Breugelmans, Prof. A. Hubin Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel Pleinlaan 2, 1050 Brussels, Belgium.

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/celc.20xxxxxxx.

1. Introduction

The development of active, cheap, selectiveand stable electrocatalysts is a crucialelement in the growing field of researchrelated to fuel cells. The features of fuelcells are attractive for the conversion ofchemical energy into electricity, offeringhigher efficiencies and significantly loweremissions compared to conventionaltechnologies, together with a quiet operationmode and a modular construction that is easilyscalable. Commonly, fuel cells are regarded aselectrochemical energy conversion devices,with maximum generation of power as mainfocus. The products of the electrochemicalprocess are typically not employed further, asin the widely studied H2/O2 fuel cell anddirect methanol fuel cell, which generatewater and carbon dioxide as end products,respectively. Nevertheless, fuel cellscould also be designed to cogenerateelectricity and useful chemical compounds,provided that the selected redox reactionis thermodynamically favourable. In thisapproach, the target would be to couple theelectrochemical synthesis of valuableproducts and electricity output, leading tomore sustainable and efficient productionprocesses. Our study aims at developingsuitable electrocatalysts for theelectrochemical reduction of nitrobenzene(NB). This reaction was chosen because of itspromising features in the context ofcogeneration of chemicals and electricity: ithas high standard reduction potentials (1.35 Vfor the reduction of NB to aniline) andgenerates products with a broad range ofapplications (Scheme 1).1 For example, anilineis a precursor for polyurethanes2 and azocompounds are important chemicals for organicsyntheses.3

Scheme 1. Nitrobenzene reduction

The complex reaction scheme for thereduction of nitrobenzene can involvecompetitive electrochemical and chemicalsteps. The 6-electron reduction to anilineproceeds through 2-electron transfer stepswith nitrosobenzene (NSB) andphenylhydroxylamine (PHA) as intermediates(Scheme 1, path 1).1 The reduction potentialfor the second 2-electron reduction is muchhigher than the first. Therefore,nitrosobenzene is not expected to accumulate.4

PHA is a very unstable compound, and can reactfurther both through a chemical and anelectrochemical path.5, 6 For example, if theelectrochemical reduction of PHA to aniline isnot achieved, the main product of the reactioncould be azoxybenzene (AOB), which is obtainedthrough partial oxidation of PHA with oxygen(present in solution or by contact with air)followed by condensation (Scheme 1, path 2),and AOB can be further reduced to azobenzene(AZB). When the reaction is carried out in anethanolic solution with high concentration ofan acid, a Bamberger rearrangement of PHA canalso happen to form p-ethoxyaniline (PEA)(Scheme 1, path 3).7

In this work, we present the synthesis,full characterization and electrochemicaltesting of electrocatalysts for the reductionof nitrobenzene in a three electrode cell. Inthe perspective of an application in a fuelcell, it was chosen to evaluate the activityof the electrocatalysts in neutral or acidicorganic medium.8 In previous reports, Pt and Pdhave been employed for the electrochemicalreduction of nitrobenzene,9, 10 but these noblemetals are very expensive for industrialapplication. Cu is an affordable alternative11-

13 and its relatively high hydrogenoverpotential implies that the reduction of NBis achieved through the direct electrochemicalpathway and not through chemical reduction ofNB by H2 that evolved at the electrode. In allthese reports, the electrodes consist uniquelyof metals. An attractive alternative thatallows increasing the efficiency of the costlymetal species consists in preparing them inthe form of nanoparticles supported on anelectron-conductive material. Nanoparticleshave larger surface-to-volume ratio and higherdensity of coordinatively unsaturated sitescompared to bulk materials. These features arebeneficial for electrocatalytic applicationsbecause the electrochemical reactions occurthrough the interaction between the metal

surface and the reagent.13 The catalyticperformance of the nanoparticles is typicallyenhanced if the particles have small anduniform size and are homogenously dispersed onthe support. These features depend on thenature of the metal(s), on the method used todeposit the nanoparticles and on theproperties of the support. Here, wedemonstrate that Cu/CuxO nanoparticles withparticularly small size (~4 nm) can beprepared on an inexpensive support asactivated carbon (AC), leading to enhancedelectrocatalytic performance in the titlereaction. This result was obtained byinvestigating a series of electrocatalystsconsisting of nanoparticles of Cu, Pt andvarious Pt-Cu alloys supported on two types ofcommercially available activated carbon (fromSigma-Aldrich and Norit) and preparedaccording to different methods. Activatedcarbon was selected as support for thenanoparticles because it is electricallyconductive, inexpensive and displays thehighest specific surface area among carbon-based materials, with typical BET valuesaround 1000 cm2 g-1.14 The magnitude of the porevolume, the distribution of pore size and thechemical functional groups on the surface ofactivated carbon depend on the carbonprecursor and the conditions used for itspreparation,15 and can differ significantlybetween various types of activated carbon.These features can have a relevant impact onthe properties of the final electrocatalyst.Indeed, the results presented here underlinenot only the promising electrocatalyticactivity of the supported nanoparticles, butalso the relevant role of the activated carbonsupport on the electrocatalytic performance inthe reduction of nitrobenzene.

2. Results and Discussion

2.1 Synthesis and characterization of the Ptand Pt-Cu electrocatalystsThe performance of electrocatalysts basedon supported metal nanoparticles isexpected to reach its optimum when theparticles are small and uniform in size andare well dispersed on the support surface(i.e. with no aggregate of particles). In thiswork, we used different synthetic approachesin order to prepare the desired metalnanoparticles of Pt, Cu and Pt-Cu alloys

supported on activated carbon (seeExperimental Section). Cu was selected onthe basis of our previous work in which itwas shown that Cu/CuxO electrocatalysts areparticularly suitable for the reduction ofnitro groups.8 Pt is a widely employed typeof supported nanoparticle electrocatalyst,and was used as a reference. The variousPt-Cu alloys were selected aselectrocatalyst candidates because thepresence of Pt in an alloy with Cu isexpected to help generating smallerparticles compared to those typicallyobtained with Cu alone.16 Additionally, itcan help preventing oxidation of Cu tooxidic species and may lead to a synergybetween the two metals for theelectrocatalytic activity. Anotherimportant parameter determining thefeatures of the nanoparticles and thebehavior of the electrocatalyst is thenature of the support. In this work,activated carbon was chosen as aninexpensive support with very high specificsurface area, which is beneficial toachieve an efficient dispersion of thenanoparticles. Two types of commerciallyavailable activated carbon were employed,supplied by Sigma-Aldrich and Norit AC(denoted as AC(S) and AC(N), respectively).N2-physisorption indicates that the twomaterials have rather different texturalproperties (Table 1 and Figure S1 and S2 inthe SI), with AC(S) displaying largerspecific surface area (SBET) and pore volumecompared to AC(N). Both materials display abroad pore size distribution, with a morerelevant contribution of larger mesoporesin AC(N) (Figure S1). The FT-IR spectra ofthe two activated carbon materials (FigureS3 in the SI) show that more carboxylicgroups are present on AC(S) than on AC(N).This is in line with the higher number ofwater molecules adsorbed per surface unitin AC(S), which is an indication of ahigher hydrophilicity of this material(Table 1). These characterization dataunderline the relevant physicochemicaldifferences between the two selectedactivated carbon supports.

Table 1: Physicochemical properties of the employedactivated carbons.

SBET[m2

g−1]

Porevolume [cm3

g−1]

nH2Onm-2 a

C content[%] b

AC(S) 1300 0.48 5.1 90Cu/AC(S)-H2 580 0.25 - -

AC(N) 878 0.31 1.6 98Cu/AC(N)-H2 468 0.19 - -

a Estimated by TGA (Figure S4 in the SI). b Measured

by EDX.

Supported Pt nanoparticles have been widelystudied, and suitable methods for preparing

nanoparticles of this metal with the desiredsize and dispersion are available.17 Thesynthetic approach adopted in this work forpreparing Pt nanoparticles is inspired by apreviously reported procedure and employs EG

Table 2. Metal loading, atomic ratio of Pt to Cu in the alloys, and average particle size of Pt and Pt-Cuelectrocatalysts.

Electrocatalysta Metal loading [wt%] (EDX)

Pt:Cu molarratio (EDX)

Cu content[wt%]

Particle size [nm]TEM XRDb

Pt/AC(S)-EG 29 - - 2.9 ± 0.1 3PtCu/AC(S)-EG 29 1:1 8 2.6 ± 0.3 3PtCu3/AC(S)-EG 31 1:3 15 2.8 ± 0.5 3PtCu9/AC(S)-EG 30 1:8 22 2.7 ± 0.4 3PtCu/AC(S)-NaBH4 22 1:1 5 2.7 ± 0.4 3PtCu3/AC(S)-NaBH4 25 1:3 12 2.8 ± 0.3 3PtCu9/AC(S)-NaBH4 25 1:9 18 2.9 ± 0.5 3

a The theoretical metal loading of all the measured electrocatalysts is 20 wt%; AC(S): Sigma-Aldrichactivated carbon; -EG: the reductant was EG; -NaBH4: the reductant was NaBH4. b The particle size determinedfrom XRD analysis is only an indicative value, due to the relatively low resolution of the XRDmeasurements.

as reductant and sodium acetate asstabilizer.18 TEM images of the Ptnanoparticles supported on AC(S) show that thedesired high dispersion of small metalnanoparticles with uniform size was achieved,with an average particle size around 3 nm(Figure 1 and Table 2). Powder XRD analysis ofPt/AC(S)-EG shows the characteristicdiffraction pattern of face-centered cubic(fcc) metallic Pt, with peaks at 2θ = 39.4o,46.8o, 67.8o and 81.3o corresponding to the(111), (200), (220) and (311) planes (Figure2).19 The broad diffractions peaks confirm thesmall size of the Pt particles in Pt/AC(S)-EG,which was estimated to be around 3 nm usingthe Scherrer’s formula (Table 2), in excellentagreement with the TEM data. The lowcrystallinity of the AC(S) support wasdemonstrated by the absence of the peak ataround 2θ = 25o, which is associated with (002)graphitic planes.20

(a) (b)

(c)

Figure 1. TEM images of: (a) Pt/AC(S)-EG, (b) PtCu/AC(S)-EG, and (c) PtCu/AC(S)-NaBH4.

20 30 40 50 60 70 802-Theta(°)

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unts

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[pt_39degree800sec6b.raw] 8.257 7226

39.4

45.8

67.3 81.3

2Θ (o)

(111)

(200)

(220) (331)

Figure 2. XRD pattern of Pt/AC(S)-EG.

Pt-Cu alloyed nanoparticles have beenstudied as electrocatalysts for reductionreactions, such as the oxygen reductionreaction,17 and can be considered promising

candidates for the nitrate reduction.9

According to the Pt-Cu thermodynamic phasediagram, a single, stable Pt-Cu alloy phaseexists at room temperature when the molarratio between Pt and Cu is 1:1 or 1:3.21

Therefore, these two molar compositions whereinvestigated in this work. Moreover, thecomposition of Pt : Cu = 1:9 was also studiedwith the aim of achieving an alloy with higherCu content, though this ratio corresponds tothe metastable phases range for the Cu-Ptsystem.21 The PtCuy/AC electrocatalysts weresynthesized by using either ethylene glycol orsodium borohydride (NaBH4) as reductant. Thisstrategy was inspired by the relativelystraightforward literature methods used toprepare materials with highly dispersed Ptnanoparticles using these reductants.22 Thesynthesis method with EG as reductantcorresponds to that used for preparingPt/AC(S)-EG (vide supra) and was carried out athigh temperature (170°C), with the acetateanion as stabilizer of the nanoparticles. Onthe other hand, in the synthesis with NaBH4 asthe reductant, which was performed at roomtemperature, EG was employed as stabilizer. EGis not an efficient reductant at roomtemperature, but it forms the complexNaB(OCH2CH2OH)4 with NaBH4,22 which can play asimilar role to that of sodium acetate in theother synthesis method. TEM images of theprepared materials demonstrate that bothmethods are suitable for generating Pt-Cunanoparticles with the desired small size(about 3 nm, see Table 2, Figure 1 and FigureS5 in the SI) and high dispersion, regardlessof the variation of the Pt to Cu molar ratioin the range 1:1 to 1:9. The Pt to Cu ratio inthe series of PtCuy/AC(S)-EG and PtCuy/AC(S)-NaBH4 electrocatalysts was estimated by meansof EDX analysis (Table 2). For most materialsthe molar ratio in the synthesis mixture andin the prepared electrocatalysts are inexcellent agreement (Table 2). The onlydeviation is observed for PtCu9/AC(S)-EG, whichhas a slightly higher Pt content (1:8) thanthe theoretical value. This suggests that notall copper precursor was reduced with thismethod, and was removed in the washing step atthe end of the synthesis. The XRD patterns(Figure 3) of PtCuy/AC with different Pt to Curatio are consistent with the predictionsbased on the phase diagram: Pt and Cu form abimetallic alloy for any composition in theabove-mentioned region, regardless of the typeof reductant. For the samples with molar ratioof Pt to Cu at 1:1 and 1:3 (Figure 3A.a and band B.a and b), only the expected reflectionsof a Pt-Cu alloy were observed. These XRD

peaks are assigned to the (111), (200) and(220) planes of a disordered face-centeredcubic (fcc) structure in which Cu and Pt atomsare distributed randomly.23 The position ofthese reflections shifts as a function of therelative content of Pt and Cu, in agreementwith Vegard’s law.24 No diffraction lines forpure metallic Cu, pure metallic Pt metal ortheir oxides were detected, confirming thatthe sample contains only bimetallicnanoparticles of a Pt-Cu alloy phase ratherthan a mixed phase of monometallic Cu and Ptnanoparticles. The XRD patterns for Pt-Cualloy nanoparticles exhibit broad peaks owingto the small particle size, which wasestimated to be around 3 nm by using theScherrer’s equation (Table 2), nicely inagreement with the particle sizes determinedby TEM. These characterization datademonstrate that the desired bimetallic Pt-Cualloy nanoparticles with uniform, small sizeand good dispersion25 were successfullyprepared with both methods (with ethyleneglycol and sodium borohydride as reductants).

30 40 50 60 70 802-Theta(°)

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[Pt-Cu AC 1-3 1000 sb.raw] 5.258 10959[Pt-Cu AC 1-1 2011 1000Sb.raw] 5.258 13858[2Pt-Cu Ac 14 180S.raw] 5.258 19838

(a)

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41.1 42.3

47.7 50.459.8 74.1

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[Pt-Cu AC 1-1 NaBH4 1000Sb.raw] 5.258 12874[2Pt-Cu AC 15-NaBH4 900S.raw] 5.258 25580

(a)

(b)

(c)

35.042.5

48.672.8

2Θ (o)

B

Figure 3. XRD patterns of Pt-Cu alloys with different Ptto Cu ratio supported on AC(S) and prepared by (A) EGmethod: (a) PtCu/AC(S)-EG, (b) PtCu3/AC(S)-EG, (c)PtCu9/AC(S)-EG; or (B) NaBH4 method: (a) PtCu/AC(S)-NaBH4,(b) PtCu3/AC(S)-NaBH4,(c) PtCu9/AC(S)-NaBH4.

When the molar ratio between platinum andcopper was increased to 1:9, other phases wereformed besides the alloy, both for PtCu9/AC(S)-EG (Figure 3Ac) and PtCu9/AC(S)-NaBH4 (Figure3Bc). For the material in which the Pt and Cu

precursors were reduced by ethylene glycol(PtCu9/AC(S)-EG), the three intense reflectionpeaks at 42.3° (111), 50.4° (200) and 74.1°(220) clearly indicate the presence of anunalloyed pure metallic Cu phase.24 Anadditional broad peak at 2θ = 35.3° is assignedto the (111) plane of Cu2O, showing that notall Cu remains in the metallic state. For thematerial prepared using sodium borohydride asthe reductant (PtCu9/AC(S)-NaBH4), no separatemetallic Cu phase was observed while the broadsignal at 2θ = 35° suggests the presence of aCu2O phase. The (111) diffraction peak of thebimetallic alloy appears at 41.7° forPtCu9/AC(S)-EG and at 42.5° for PtCu9/AC(S)-NaBH4, which suggests that in the latter samplethe composition of the Pt-Cu alloy is richerin Cu. Combining the molar ratio measurementsof Pt and Cu by EDX (Table 2) with the XRDpatterns of these two samples, it can beconcluded that NaBH4 is a better choice asreductant for promoting the formation of thePtCu9 bimetallic alloy.

Figure 4. (A) Pt 4f XPS spectrum of PtCu/AC(S)-EG and(B).Cu 2p XPS spectrum for PtCu/AC(S)-EG.

To further investigate the chemical bondingstates of Pt and Cu, an XPS measurement wasperformed for a selected sample, PtCu/AC(S)-EG. The relative atomic ratio of Pt and Cu inthis sample was found to be 1:1, in line withthe EDX results. The Pt 4f XPS spectrum of theanalyzed catalyst displays two signals, whichwere deconvoluted in two pairs of doublets(Figure 4A). The most intense doublet,centered at 71.1 and 74.4 eV, originates fromthe spin-orbital splitting of 4f7/2 and 4f5/2

states of metallic Pt.26 The bonding energy ofPt is slightly shifted to lower energy,because of the presence of Cu.27 The lowintensity doublet observed at higher bindingenergy (72.5 and 75.8 eV) is ascribed toPt(II), indicating the presence of a smallamount of PtO and/or Pt(OH)2 species on thesurface of metallic Pt particles, which arebelieved to be formed by exposure of thesamples to air.25 The XPS spectrum of the Cu 2pof PtCu/AC(S)-EG (Figure 4B) presents the twopeaks of Cu 2p3/2 and 2p1/2 photoelectrons at932.7 and 952.9 eV, respectively. The Cu 2p3/2

core level was employed to investigate theoxidation state of Cu species on the surfaceof the material. Characteristic XPS signals ofthe Cu 2p3/2 core

Table 3. Metal loading, particle size and phases of the Cu/CuxO electrocatalysts.

Electrocatalysta Cu loading [wt%](EDX)

Crystal phases(XRD)

Cu species (XPS)Particle size

[nm]TEM XRDb

Cu/AC(S)-H2 30 Cu, Cu2O Cu(OH)2, CuO, Cu2O/Cu 9 ± 2 11Cu/AC(S)-N2H4 20 Cu, Cu(OH)2, CuO, Cu2O/Cu 17 ± 7 22

Cu /AC(S)-EG 11 Cu2O n.d aggregates 22

Cu/AC(S)-NaBH4 18 Cu2O Cu(OH)2, CuO, Cu2O/Cu 5.5 ± 1.2 5Cu/AC(N)-H2 17 Cu, CuO CuO 4.1 ± 0.5 7

Cu-CTAB/AC(N)-N2H4 8 Cu, Cu2O Cu(OH)2, CuO, Cu2O/Cu n.d 22a The theoretical metal loading of all the measured electrocatalysts is 20 wt%; AC(S): Sigma-Aldrich activatedcarbon; AC(N): Norit activated carbon; -H2: the reductant was H2; -N2H4: the reductant was hydrazine; -EG: thereductant was EG; -NaBH4: the reductant was NaBH4; -CTAB: CTAB (cetyltrimethylammonium bromide) was used assurfactant in this synthesis method; n.d.: not determined. b The particle size determined from XRD analysis is onlyan indicative value, due to the relatively low resolution of the XRD measurements.

66687072747678

Intensity (a.u.)

Binding energy (eV)

A

960 950 940 930 920

C u 2p3/2

C u 2p1/2In

tensity (a

.u.)

B inding energy (eV )

C u/C u2O(932.7 eV )

C u(O H )2

C u 2+

S atelitte peaksIntensity(a.u.)

Binding energy (eV)

B

level at 932.1±0.2, 933.7±0.2, and 934.6 eVhave been assigned to Cu/Cu2O, CuO and Cu(OH)2,respectively.28 The existence of all the copperspecies in PtCu/AC(S)-EG is indicated by theshape and features of the Cu2p3/2 peaks and bythe presence of the Cu(II) satellite peaks.26

The shift of the position of this peakcompared to the typical value for Cu/Cu2O(932.7 eV vs 932.1 eV) suggests a partialoverlap with a peak due to CuO (933.7 eV). Thepeak of Cu 2p3/2 centered at 932.1 eV, isattributed to either metallic Cu or Cu2O:29

these two species cannot be distinguished inthe Cu 2p3/2 range, because their bindingenergies are very close to each other. Theadditional peak at 934.7 eV in the Cu 2p3/2

region is attributed to the presence ofCu(OH)2. On the other hand, no peaks of Cu(II)species were observed in the XRD pattern ofPtCu/AC(S)-EG (Figure 3A.a): this suggeststhat the Cu(II) species in this material arein the form of an amorphous oxide/hydroxidelayer located on the surface of the metallicalloy nanoparticles,30 similarly to whatproposed for the Pt (vide supra).

2.2 Syntheses and characterization of theCu/CuxO electrocatalysts

The last group of electrocatalysts proposedin this work consists of Cu/CuxO nanoparticlessupported on activated carbon materials.Copper is prone to oxidation and coppernanoparticles typically consist not only of ametallic Cu phase but also of oxide andhydroxide species: it is thus more appropriateto refer to these materials as Cu/CuxOnanoparticles.8 The presence of small amountsof oxidized copper (I or II), possibly in theform of a thin surface layer surrounding ametallic copper core, has been reported not tohinder the electrocatalytic activity in thereduction of nitrobenzene. Five differentmethods and two types of activated carbon wereused to prepare the Cu-CuxO/ACelectrocatalysts. Two methods employed mildreductants, i.e. EG and NaBH4, and wereanalogous to the protocols used for the Pt andPt-Cu electrocatalysts (vide supra). Twostronger reductants, H2 gas and N2H4, were alsoemployed. The method with H2 as reductant isbased on a procedure that was recentlydeveloped by our groups for the preparation ofCu/CuxO nanoparticles supported on multi-walledcarbon nanotubes.4, 8 In the method with N2H4 asreductant, the use of a surfactant(cetyltrimethyl-ammonium bromide, CTAB) wasalso explored.31

TEM images of Cu/CuxO nanoparticles preparedwith H2 as reductant highlight the crucial role

exerted by the support in determining thefeatures of the nanoparticles (Figure 5). Thismethod generates small Cu/CuxO nanoparticles(<10 nm) with

(a) (b)

(c)

10 nm10 nm

(d)

10 nm10 nm

(e)

Figure 5. TEM images of (a) Cu/AC(S)-H2; (b) Cu/AC(S)-N2H4; (c) Cu/AC(S)-NaBH4; (d) Cu/AC(N)-H2; (e)Cu-CTAB/AC(N)-N2H4.

well-defined dispersion,8 but the averageparticle size is much smaller and more uniformin the material prepared on the Noritactivated carbon (4 ± 0.5 nm, Figure 5d)compared to the Sigma-Aldrich activated carbon(9 ± 2 nm, Figure 5a). These differences inparticles size are ascribed to the differenttextural properties of the two activatedcarbons (Table 1), although the exact natureof this effect cannot be easily clarified dueto the complex micro- and mesoporous structureof these carbon materials. In both cases, theformation of the supported nanoparticles leadsto a dramatic decrease in surface area to porevolume while the pore size is unaffected,indicating that some of the particles blockthe carbon pores (Table 1). Remarkably, theCu/CuxO nanoparticles in Cu/AC(N)-H2 display auniquely small size compared to any known wetchemical method. When using N2H4 as reductant,only large particles with a broad sizedistribution are obtained (17 ± 7 nm, Figure

5b). A surfactant (CTAB) was added to thissynthesis method with the aim of controllingthe size and dispersion of the particles.However, the CTAB surrounding the metalparticles could be removed only by pyrolysis at300 °C, and under these conditions theparticles tend to aggregate (Figure 5e). Themethod employing NaBH4 as reductant leads tosmall and well dispersed Cu/CuxO nanoparticleswith a size of 5 ± 1.2 nm (Figure 5c). On theother hand, the method in which EG acts asreductant leads to the formation of aggregatesof particles (Figure S5 in the SI). Thisunsatisfactory result is in line with previousattempts to prepare Cu/CuxO nanoparticles withthis method but on a different support,8 and isascribed to the employed synthesis conditions(high temperature and aqueous environment).

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[Cu AC NaBH4 37 720s.raw] 6.258 12550[Cu AC Hydrazin 180 S.raw] 5.258 4136

[Cu AC H2 800 Sb 2011.raw] 5.258 11895[Cu AC 900 S.raw] 5.258 44171

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(a)

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Cu Cu2O CuO ∆

∆ ∆

∆ ∆ ∆ ∆ ∆ ∆ ∆

B

Figure 6. XRD patterns of Cu/CuxO nanoparticles preparedusing different synthetic methods and supported on (A)Sigma-Aldrich AC: (a) Cu/AC(S)-N2H4, (b) Cu/AC(S)-H2, (c)Cu/AC(S)-EG, (d) Cu/AC(S)-NaBH4; or on (B) Norit AC: (a)Cu-CTAB/AC(N)-N2H4, (b) Cu/AC(N)-H2.

The analysis of the series of supportedCu/CuxO nanoparticles by powder XRD allowedevaluating the effect of the synthesis methodon the crystalline phases of the preparedmaterials (Figure 6). The XRD pattern ofCu/AC(S)-N2H4, which was prepared with a strong

reductant as hydrazine, displays only thecharacteristic reflections of the face-centered cubic (fcc) structure of metalliccopper at 2θ = 43.3°, 50.4° and 73.4° (Figure6A.a), which correspond to the (111), (200)and (220) planes.19, 32 The sample prepared withthe same reductant but in the presence ofCTAB, Cu-CTAB/AC(N)-N2H4, also displaysmetallic Cu as the main crystalline phase,though a weak reflection at 36.5° attributedto the (111) plane Cu2O is also visible (Figure6B.a). When H2 was used as the reductant(Cu/AC(S)-H2), besides the XRD reflections ofmetallic Cu, broad peaks at 2θ = 36.5° and61.8° were observed (Figure 6A.b), whichindicate the presence of small Cu2Onanoparticles.19, 33 The presence of Cu2O isascribed to the incomplete reduction of thedivalent metal ion in the precursor, or topartial reoxidation of the metallic Cu. Forthe materials prepared by using EG and NaBH4 asreductants, only Cu2O reflections are presentin the diffractogram, namely: (111) at 36.5°,(200) at 42.3°, (220) at 61.8° and (311) at73.2° (Figure 6A.c and d).19 These resultsindicate that, contrarily to what observedwith Pt and Pt-Cu alloys, EG and NaBH4 are toomild reductants for achieving the completereduction of the Cu(II) precursor. The size ofthe Cu and Cu2O particles was estimated byapplying Scherrer’s equation to the broad peakcorresponding to the Cu (111) or Cu2O (111)reflections, and show the same relative trendas those obtained from the TEM images (Table3). The nature of the support does not onlyaffect the particle size (vide supra), but alsothe crystalline Cu-phases. In the XRD patternof Cu/AC(N)-H2, the dominant reflectionsoriginate from the monoclinic CuO phase andco-exist with a smaller amount of metallic Cu(Figure 6B.b). The smaller Cu nanoparticlesobtained with AC(N) can be oxidized morereadily than those supported on AC(S): thiscan explain the relatively lower amount ofmetallic copper in Cu/AC(N)-H2 compared toCu/AC(S)-H2. The average particle size ofCu/AC(N)-H2 was calculated to be around 7 nmfrom the width of the CuO (111, 200) peak at2θ = 38.6°.19, 34

930940950960

C u/A C (N )-H 2

C u-C TA B /A C (N )-N 2H 4

C u/A C (S )-N aB H 4

C u/A C (S )-N 2H 4

C u/A C (S )-H 2

C u(O H )2

C uO

C u2O

B inding E nergy (eV )

C u

Inte

nsity

(a.u

.)

Figure 7. Cu 2p core level XPS spectra for supportedCu/CuxO nanoparticles on activated carbons. The XPSspectrum of Cu/AC(S)-EG (not shown) does not present anysignal, most likely because of the low metal loading.

The surface properties of the supported Cu/CuxOnanoparticles were analyzed by XPS (Figure 7).The dominant peak in the Cu 2p3/2 signal ofCu/AC(S)-N2H4 corresponds to either Cu or Cu2Ospecies and is assigned to metallic copperbased on the XRD data of these samples. On theother hand, the center of the Cu/Cu2O peak inCu/AC(S)-H2 and Cu/AC(S)-NaBH4 is shifted tohigher binding energy, indicating the presenceof copper in oxidized state, in agreement withthe XRD data (Table 3). The XPS spectra ofCu/AC(S)-H2, Cu/AC(S)-N2H4, Cu/AC(S)-NaBH4 andCu-CTAB/AC(N)-N2H4 also display signals due toCu(II) species. These are attributed toamorphous (and thus XRD silent) CuO/Cu(OH)2

layers at the surface of the nanoparticles, ascopper can be easily oxidized in the presenceof air and moisture, leading to the formationof oxides and hydroxides. The intensity of theCu(II) signals is higher in Cu/AC(S)-H2 andCu/AC(S)-NaBH4, in line with the smaller sizeof their particles having a larger fraction ofexposed copper that can undergo oxidation. TheXPS spectrum of Cu/AC(N)-H2 displays a dominantsignal centered at 933.7 eV corresponding to aCuO phase, which is consistent with its XRDpattern. This XPS spectrum is markedlydifferent from that Cu/AC(S)-H2, which wasprepared by the same method, underlining thecrucial effect of the support on the featuresof the nanoparticles. Particularly, theformation of Cu(OH)2 in Cu/AC(S)-H2 is ascribedto the more hydrophilic nature of AC(S)compared to AC(N).

In summary, the influences of differentmethods and supports for preparing Cu/CuxOelectrocatalysts were studied (Table 3). N2H4

is the strongest reductant, and pure metallicCu crystals could be achieved, but only largesize particles were obtained. The addition of

a surfactant (CTAB) in this method did nothelp, as the particles tended to aggregateupon removal of the surfactant at hightemperature. The method employing H2 asreductant provided the most suitable featuresin term of particles size and dispersion,particularly when AC(N) was the support. Thesupported nanoparticles obtained with thismethod consist of a mixture of metallic Cu anCu(II) phases. Mild reductants, i.e. NaBH4 andEG, can only reduce Cu(II) to Cu(I). Small-sizeCu2O particles can be prepared by NaBH4, whileonly aggregates are obtained with the EGmethod.

All the supported metal nanoparticlesreported in this work were prepared with aconstant total metal loading of 20 wt%.However, the metal loadings measured by EDXfor most of the materials supported on AC(S)are higher than the theoretical value (Table 2and 3). This is ascribed to the features ofthe AC(S) employed as support, which containsadsorbed water and impurities (Table 1) thatcan get removed during the preparation of thesupported nanoparticles. This hypothesis issupported by the much higher loading of Cu inCu/AC(S)-H2 compared to Cu/AC(N)-H2, in linewith the lower hydrophilicity and higherpurity of AC(N) (Table 1). The content of Cuin Cu/CuxO electrocatalysts prepared using amild reductant (EG or NaBH4) is lower than thetheoretical value (20 wt%), indicating thatsome unreduced Cu precursor was removedtogether with the solvent in the washing stepat the end of the synthesis.

2.3 Electrocatalytic performance in thereduction of nitrobenzene of Pt, Pt-Cu andCu/CuxO nanoparticles supported on AC

A voltammetric study of the reduction ofnitrobenzene over the preparedelectrocatalysts was carried out by linearsweep voltammetry (LSV) with a rotating diskelectrode in an electrochemical cell dividedinto two compartments (working electrode andcounter electrode compartment), separated by aZirfon© membrane. The reactions were performedin 0.2 M LiClO4 ethanolic medium containing 5mM nitrobenzene by varying the potential ofthe working electrode between -0.3 till -2.4 Vvs Fc/Fc+. The results have been corrected forthe substantial electrolyte resistance (196Ω). The voltamogramms obtained at differentrotation speeds of the electrode withCu/AC(N)-H2 and Cu-CTAB/AC(N)-N2H4 show theexpected increase in the diffusion-limitedcurrent upon increase of the rotation speed(Figure 8A and B).31 The current densitiesmeasured at -1.3 V vs Fc/Fc+ and at different

rotation speeds for these two electrocatalystsallowed to estimate the number of electronstransferred per nitrobenzene molecule and thekinetic current density by means of theKoutécky-Levich (K-L) equation.35, 36

Figure 8. Linear sweep voltammetry (LSV) of Cu/AC(N)-H2(A) and Cu-CTAB/AC(N)-N2H4 (B) for the reduction ofnitrobenzene (5 mM) in 0.2 M LiClO4 ethanolic solutionwith various rotation speeds. Scan rate was 5 mV s−1. Theblank was measured in the 0.2 M LiClO4 ethanolic solutionwithout nitrobenzene. (C) K−L plots of Cu/AC(N)-H2, Cu-

CTAB/AC(N)-N2H4 and AC(N) for the determination of thenumber of electrons transferred at -1.3 V vs Fc/Fc+.

The slopes of the K-L plots (Figure 8C)correspond to a 4-electron exchange process,which is identified as the reduction of NB toPHA (Scheme 1).4 The same number of electronsper molecule of nitrobenzene is also beingexchanged at this potential if the AC(N)support alone is employed as electrocatalyst(Figure 8C and Figure S6 in the SI). Thisindicates that the same reaction occurs on thesupported nanoparticles and on the supportalone (tough the activity of the latter ismuch lower, vide infra). The kinetic currentdensity for the Cu/AC(N)-H2 and Cu-CTAB/AC(N)-N2H4 electrocatalysts, determined from thevalue of the K-L curve intercept, are 55.5 mAcm-2

(Cu/AC(N)-H2) and 23.8 mA cm-2

(Cu-CTAB/AC(N)-N2H4). These current densitiesare reported using the geometrical surfacearea of the porous electrode. Contrarily tothe results with nanoparticles supported onAC(N), the voltammograms over electrocatalystssupported on AC(S) were not reproducible whenrepeating the measurement at the same rotationspeed. A similar behavior has been observedbefore for Cu and Pt nanoparticles supportedon functionalized multi-walled carbonnanotubes and has been ascribed to instabilityof the functional groups on the surface of thesupport during the electrocatalytic test.8

AC(S) is richer in functional groups comparedto AC(N) (vide supra) and these species on thesurface of AC(S) may get reduced or interactwith NB under the applied conditions.Therefore, the number of electrons exchangedand the kinetic current cannot be estimatedfor the electrocatalysts prepared using AC(S)as support. The instability of the AC(S)affects the reliability of the measuredcurrent, but does not influence notably theonset potential, i.e. the potential at which thereduction of nitrobenzene starts.37 Therefore,the onset potential was chosen as parameter torank the activity of the electrocatalysts(Figure 9): a less negative onset potential isassociated to a higher electrocatalyticactivity.38 In all cases, the supportednanoparticles displayed less negative onsetpotential compared to the pure supports, AC(S)and AC(N), which indicates the role of themetal nanoparticles as active electrocatalyticspecies. The order of activity is stronglydetermined by the chemical composition of thenanoparticles, with Cu being much moresuitable than Pt for catalyzing theelectrochemical reduction of nitrobenzene.4,8

This trend is nicely evidenced by the gradualshift of onset potential to less negative

C

A

B

values upon increase of the Cu content of theelectrocatalysts passing from particlescontaining only Pt to alloyed Pt-Cu particlesand finally to particles containing only Cu(Figure 9A). The same trend is observed withthe two synthesis methods employed, i.e. witheither EG or NaBH4 as reductant. The slightlymore negative onset potential of Cu/AC(S)-EGcompared to PtCu9/AC(S)-EG is ascribed to thelower content of Cu of the former (Table 2 and3). These results demonstrate that thecomposition is a more relevant factor than theparticles size in determining theelectrocatalytic activity in the reduction ofnitrobenzene, because the Pt and Pt-Cu alloyelectrocatalysts all display smaller, well-dispersed particles compared to theircounterparts containing exclusively Cu (Table2 and 3). It can also be concluded that nosynergy occurs between Cu and Pt in thealloyed nanoparticles. The performance of the supported Cu/CuxOnanoparticles was optimized by varying theirsynthesis method and the type of activatedcarbon used as support. The nature of theactivated carbon used as support plays animportant role in determining the activity ofthe electrocatalyst, despite the fact thatAC(S) and AC(N) alone display similar, lowactivity in the reduction of nitrobenzene(Figure 9). All four Cu/CuxO electrocatalystssupported on AC(S) are less active than theAC(N)-supported Cu/CuxO electrocatalysts. Thistrend is particularly clear when comparingCu/AC(N)-H2 and Cu/AC(S)-H2, which wereprepared by the same method but on differentsupports and show very different activity withthe former being much superior based on theonset potential values (Figure 9B). AlthoughCu/AC(S)-H2, Cu/AC(S)-N2H4, Cu/AC(S)-EG,Cu/AC(S)-NaBH4 exhibit different physiochemicalproperties, their electrocatalyticperformances in the reduction of nitrobenzeneare not significantly different (Figure 9B).This suggests that on this support theelectrocatalytic performance of the copper-based materials is not controlled by thefeatures of the nanoparticles but is ratherlimited by the nature of the support, whichmight experience a high degree of poreblockage upon deposition of the nanoparticles(as suggested by the comparison of thedecrease in surface area after deposition ofthe nanoparticles in Cu/AC(N)-H2 and Cu/AC(S)-H2, see Table 1). On the other hand, thedifferent performance of the Cu/CuxOelectrocatalysts on AC(N) can be correlatedwell with their physiochemical features. Thehigher activity of Cu/AC(N)-H2 compared to Cu-

CTAB/AC(N)-N2H4, both in terms of a lessnegative onset potential (Figure 9B) and ofhigher kinetic current density (vide supra), isattributed to the smaller particle size (~4nm) and higher copper loading of the former(Table 3).

-1.15

-1.10

-1.05

-1.00

-0.95

-0.90

-0.85

-0.80

-0.75

-0.70

-0.65

Onset potential vs

Fc/Fc+ /V

PtCu

9/AC(S)-EG

PtCu

/AC(S)-NaBH

4

PtCu

3/AC(S)-NaBH

4

PtCu

9/AC(S)-NaBH

4

Cu/AC(S)

-NaBH

4

Cu/AC(S)

-EG

PtCu

3/AC(S)-EG

PtCu

/AC(S)-E

G

Pt/AC(S)

-EG

AC(S)

(A)

-1.15

-1.10

-1.05

-1.00

-0.95

-0.90

-0.85

-0.80

-0.75

-0.70

-0.65

Onset potential vs Fc/Fc

+/ V

Cu/AC(S)-H

2

Cu/AC(N)

-H2

Cu/AC(S)-N

2H4

Cu/AC(S)-E

G

Cu/AC(S)-N

aBH

4

Cu-CTAB/AC

(N)-N

2H4

AC(N)

(B)

Figure 9. Ranking of the activity of the Pt, Pt-Cu andCu/CuxO electrocatalysts based on their onset potential(defined as the potential at which the slope of thevoltammogram exceeds 0.1 mA V-1). The tests wereperformed in neutral medium (5 mM nitrobenzene). (A) Pt,Pt-Cu and Cu/CuxO electrocatalysts prepared with AC(S) assupport and EG and NaBH4 as reductant. (B) Cu/CuxOelectrocatalysts prepared using different reactants andsupports.

These results underline the relevant roleexerted by the support on the performance ofthe electrocatalyst, with AC(N) being a muchmore suitable support than AC(S). The use ofAC(N) as support to prepare Cu/CuOx

nanoparticles with a method employing H2 asreductant led to homogenously dispersedparticles with exceptionally small size(Cu/AC(N)-H2), which displayed the bestelectrocatalytic performance among the set ofmaterials presented in this work.

Cyclic voltammetry (CV) was employed tomonitor the long-term stability of the best

electrocatalyst, Cu/AC(N)-H2 (Figure 10).Cu/AC(N)-H2 gives very similar voltammograms inthe first and after 1000 cycles, indicatingthat the electrocatalyst does not deteriorateover time under these testing conditions. Theslightly different current is most likely dueto the set-up sealing not being completelytight, leading either to diffusion of oxygeninto the solution, or to solvent evaporation.

-1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1st S can 1000th S can

j/mA

E /V vs Fc/Fc+

Figure 10. Cyclic voltammograms (CV) of Cu/AC(N)-H2. Thepotential was cycled from -0.3 to -1.7 V vs Fc/Fc+. Thefirst and last scan are shown. The solution was stirredbefore the last cycle: this procedure allows increasingthe comparability of the starting and final reactionconditions. The voltammogram does not show a closed loopbecause of stopping and restarting the measurement.

2.4 Chronoamperometric test for Cu/AC(N)-H2

Cu/AC(N)-H2 was identified as the mostpromising electrocatalyst for the reduction ofnitrobenzene on the basis of its onsetpotential, number of exchanged electrons andstability. This catalyst was studied in moredetail by means of a chronoamperometric testof the reduction of nitrobenzene (15 mM) inneutral and acidic medium at a constantpotential. The potential for each test waschosen in the region where the K-L slope isconstant. After running the chronoamperometricexperiment for 52 h, the products werecollected and analyzed by HPLC. The conversionof nitrobenzene, the yield of the variousproducts and their selectivity are summarizedin Table 4.

Table 4. Chronoamperometric test of Cu/AC(N)-H2 inthe reduction of NB. The reaction products were

analyzed by HPLC.Reactionconditions

Conversion

Product Yield[%]

Selectivity[%]

-1.15V vsFc/Fc+ (0.2 M

LiClO4)Neutralmedium

19 AOBaniline

154.6

7623

-0.62V vsFc/Fc+ (0.3 M

HClO4)Acidicmedium

51 AOBaniline

PEANSBAZB

404.23.61.21.7

77872

0.3

Since the reduction of nitrobenzene involvesprotons in each reduction step (Scheme 1), thereaction in neutral medium is slower than inacidic conditions.4 In neutral medium, theconversion of NB is 19% and only two products,AOB and aniline, are obtained with selectivityof 76% and 23%, respectively (Table 4). Inacidic solution, the conversion of NBincreased to 51% (Table 4). Remarkably, thisvalue is higher than that reached with thepreviously reported optimum electrocatalystbased on Cu/CuOx nanoparticles supported onmulti-walled carbon nanotubes, having asimilar copper loading but larger particlesize compared to Cu/AC(N)-H2.8 The observedconversions of NB are similar yet slightlylower compared to the expected conversionscalculated according to Faraday's law36 on thebasis of the total charge transferred duringthe measurement and assuming a 4-electrontransfer (20% for the reaction in neutralmedium and 56% for the reaction in acidicmedium). This result suggests that thereaction proceeds mainly through a 4-electrontransfer, in line with the n value calculatedfrom the K-L plots (vide supra), though a 6-electron transfer could also take place to alesser extent. This analysis is in agreementwith the high selectivity towards azoxybenzene(76-77%), which is obtained from PHA throughoxidation of this very unstable compound toNSB, followed by condensation of one PHAmolecule with one NSB molecule (Scheme 1). PHAis the product of the 4-electron reduction ofNB, while the 6-electron reduction generatesaniline, which was indeed the major side-product of the reaction (Table 4). However, itshould be taken into account that aniline canalso be obtained through disproportionation ofPHA (Scheme 1). This complex reaction networkis completed by the chemical reaction of PHAwith ethanol in acidic environment, whichleads to the formation of p-ethoxyaniline(PEA) as the other major side-product (Table4). The chemical conversion of the unstablePHA can occur due to residual oxygen even ifthe sample was stored under nitrogenatmosphere, both during the chronoamperometrictest and in the period between the end of thetest and the HPLC analysis, which wastypically performed more than 24 h after theend of the electrochemical experiment.4 Thebehavior of PHA as intermediate, unstableproduct was confirmed by performing the HPLCanalysis of the products shortly after the end

the reaction: under these conditions 20% PHAwas detected in the sample (Table S1 in theSI). However, the extreme instability of PHA,not only in the presence of residual oxygenbut also if the sample is neutralized byaddition of a base or of a buffer, impliesthat a more reliable analysis is obtained ifthe PHA is allowed to convert to AOB incontact with oxygen.

3. Conclusions

Three classes of electrocatalysts based onsupported nanoparticles of Pt, Pt-Cu alloysand Cu/CuxO were successfully synthesized,fully characterized, and tested in theelectrochemical reduction of NB. Differentmethods were used to prepare the nanoparticlessupported on two types of activated carbon,which is an attractive cheap support materialwith very high specific surface area.Interestingly, the electrochemical tests bylinear sweep voltammetry revealed that theprepared supported Cu/CuxO nanoparticles showedthe best electrocatalytic activity, inparticular when Norit AC was used as supportand H2 was employed as reductant in thesynthesis. With this method, the obtainedCu/CuxO nanoparticles have a very small anduniform size (~4 nm) and are highly dispersed.This promising and highly affordableelectrocatalyst outperforms previouslyreported electrocatalysts based on Cu/CuxOnanoparticles in terms of onset potential,kinetic current density and conversion ofnitrobenzene (51%, with 77% selectivitytowards azoxybenzene according to a 4-electronreduction process). Moreover, it displays agood electrochemical stability as determinedafter 1000 cycles in a cyclic voltammetrytest.

Experimental Section

Preparation of Pt, Pt-Cu and Cu/CuxOnanoparticles supported on activated carbon

All chemical reagents were of analytical gradeand used as received without purification.Activated carbon supports were obtained fromtwo suppliers, Sigma-Aldrich and Norit, andwere denoted as AC(S) and AC(N), respectively.The specific surface area and pore volume ofthe two types of activated carbon weredetermined by N2 adsorption at 77 K (t-method)on a Micromeritics Tristar 3000, and evaluatedwith the application of the Brunauer-Emmett-

Teller (BET) and Dubinin-Radushkevich (DR)equations. The pore size distributions werecalculated using the Barrett-Joyner-Halenda(BJH) method.39 The hydrophilicity of thematerials was estimated from the number ofwater molecules adsorbed per nm2 of catalystsurface of carbon material.40 This number wasobtained from the water loss between 25 and200 °C as measured by thermal gravimetricanalysis (TGA) in O2 at a heating rate of 10°C/min, using a TA instruments Q500thermogravimetric analyzer. Pt nanoparticles supported on AC(S) with ametal loading of 20 wt.% were prepared bymeans of a previously reported method,17, 18 inwhich ethylene glycol (EG) is employed asreductant and sodium acetate as stabilizer ofthe metal particles. The metal precursor(H2PtCl6·6H2O, 0.1 M) and sodium acetate (1.0M, acetate: Pt = 7:1 molar) were added to asuspension of 80 mg of carbon support in 40 mlof EG/water solution (1:1 in vol.) understirring for 4 h under refluxing conditions inAr flow. The resulting solid was separated byfiltration, washed with water and acetone anddried overnight at 100 °C in a vacuum oven.This electrocatalyst was labeled Pt/AC(S)-EG.

Two methods were used to synthesize Pt-Cualloy nanoparticles supported on AC(S).Aqueous solutions of H2PtCl6·6H2O (0.1 M) andCuSO4·5H2O (0.05 M) were used as precursors.Different Pt:Cu molar ratios, i.e. 1:1, 1:3 and1:9, were employed in the synthesis, leadingto samples denoted as PtCu, PtCu3 and PtCu9,respectively. In all cases, the theoreticaltotal metal loading was kept at 20 wt%. Thefirst method is equal to that described abovefor the preparation of Pt/C electrocatalyst,with EG as reductant (PtCuy/AC(S)-EG, with y =1, 3, 9). The second method used sodiumborohydride, NaBH4, as reductant, while EGplayed the role of stabilizer. The precursorswere added together to a stirred suspension ofthe activated carbon in 20 ml of EG. The pH ofthe mixture was adjusted to 10 by graduallyadding 1 M aqueous NaOH. After ultrasoundtreatment for 1 h, a freshly made aqueoussolution of NaBH4 (20 ml, 0.5 M) was addeddropwise. The suspension was stirred for 3 hunder N2, and then the solid was filtered anddried at 100 °C overnight in a vacuum oven.The materials obtained by this method aredenoted as PtCuy/AC(S)-NaBH4 (with y = 1, 3,9).

Cu/CuxO nanoparticles supported on AC(S)with a theoretical Cu loading of 20 wt% wereprepared according to five different methods.The first two methods are similar to thoseemployed for the preparation of the PtCuy/AC

electrocatalysts described above, with a 0.05M aqueous solution of CuSO4 as precursor andwith AC(S) as support. The obtainedelectrocatalysts were named Cu/AC(S)-EG, andCu/AC(S)-NaBH4, respectively. The third methodis based on wet impregnation.8 An aqueoussolution of Cu(NO3)2·3H2O (0.05 M, 6.3 ml) wasadded to a suspension of AC (80 mg) in 30 mlH2O. The resulting suspension was stirred for24 h. Next, the solvent was removed by using arotary evaporator. The obtained solid wastransferred into a quartz U-tube, treated at200 °C under N2 flow (1 cm3 s-1) for 2 h andreduced by H2 at 100 °C for 1 h. Two types ofactivated carbon (AC(S) and AC(N)) wereemployed as support in this method, and theobtained electrocatalysts are denoted asCu/AC(S)-H2 and Cu/AC(N)-H2, respectively. Inthe fourth method, the reductant was hydrazine(N2H4). An aqueous solution of CuCl2·2H2O (0.05M, 6.3 ml) was mixed with 80 mg of activatedcarbon previously suspended in 20 ml distilledwater, and the pH was adjusted to 10 by addingan aqueous ammonia solution (25%), in order toform the dark blue Cu(NH3)2

2+ cation. Thesuspension was then homogenized by ultrasound.20 ml of an 8 M aqueous solution of hydrazinewas added to the mixture and the sample waskept under stirring for 3 h under N2.31

Finally, the solid was separated by filtrationand dried in a vacuum oven at 100 ºCovernight. The obtained electrocatalyst isnamed Cu/AC(S)-N2H4. The last method is similarto the fourth apart from the addition of thesurfactant cetyltrimethylammonium bromide(CTAB) to favor the formation of metallic Cunanoparticles. An aqueous solution (15 ml)containing CTAB (0.1 M) and CuCl2·2H2O (0.02 M)was mixed to a second aqueous solution (15 ml)containing CTAB (0.1 M) and hydrazine (8 M).Then, the pH of the solution was adjusted to10 by adding an aqueous solution of ammonia(25%). After mixing the solution for 2 h, 80mg of AC(N) were added. The sample was keptunder stirring for 24 h under N2. The solid wasseparated by filtration and washed repeatedlywith distilled water. Next, the solid waspyrolyzed under N2 at 300 oC for 3 h todecompose the surfactant. The temperature ofthis pyrolysis step was selected on the basisof a prior thermal gravimetric analysis (TGA)of the material under N2 with a heating rate of10 oC min-1. These electrocatalyst is referredto as Cu-CTAB/AC(N)-N2H4. The goodreproducibility of these synthetic methods wasproved by the very similar physicochemicalfeatures of different batches of selectedmaterials.

Characterization of the materials

The metal loading of the supportednanoparticles was measured by EnergyDispersion X-ray (EDX) spectroscopy on a NORANSystem SIX X-ray spectrometer (ThermoScientific, US). The reported values are theaverage of the analysis of at least four zonesfor each sample. Transmission electronmicroscopy (TEM) images of the supportednanoparticles were recorded on a Philips FEGCM200 operating at 200 kV. The samples weredispersed in ethanol and deposited on 50 nm300 mesh carbon-coated Cu grids (Pacific Grid-Tech, USA). 100 particles were manuallycounted to calculate the average size, and thestandard deviation was calculated covering 90%of the particles. Powder X-ray diffraction(XRD) analysis of supported nanoparticles wascarried out on a STOE Stadi P instrument withCu Kα radiation (λKα = 0.154 nm). A glasscapillary (0.5 mm of diameter) was used as thesample holder. A rough estimate of the meansize of the metal particles could becalculated using Scherrer’s formula: L =0.9λKα /(β2θ cos θ), where L is the mean size of thenanoparticles, θ is the angle corresponding tothe selected peak and B2θ describes the half-peak width for the peak in radians.16 The (220)peaks were used to calculate the particle sizeof Pt and Pt-Cu alloys, while for the Cu/CuxOnanoparticles the most intense peak in eachdiffraction pattern was employed. X-rayphotoelectron spectroscopy (XPS) measurementswere used to study the surface structure ofPt-Cu and Cu/CuxO nanoparticles on AC supports.The spectra were recorded with a PhysicalElectronics PHI 1600 multi-technique systemusing Al Kα (1486.6 eV) monochromatic X-raysource, which was operated at 15 kV and 150Wat a basis pressure of 2×10−9 Torr. Thegraphitic C 1s band at 284.6 eV was taken asinternal standard, in order to correctpossible deviations caused by electric chargeof the samples. The XPS signal of Pt (4f) wasdeconvoluted by using the Multipack software.

Electrochemical testsThe performance of the supported nanoparticlesas electrocatalysts for the reduction of NBwas evaluated by means of linear sweepvoltammetry (LSV) analysis. All themeasurements were carried out following ourpreviously reported procedure.8 An Autolab 302Npotentiostat or a BioLogic VMP3 multichannelpotentiostat were used. The tests wereperformed in a three-electrode cell withglassy carbon porous rotating disk electrodesas working electrode; a Pt grid as the counterelectrode; Ag/AgCl with saturated LiCl as thereference electrode. The electrocatalysts weredeposited on the surface of a glassy carbon

rotating disk electrode with an area of 0.28cm². For each measurement, an ink was preparedusing 8.0 mg of electrocatalyst and 300 µL ofa 1% polystyrene solution in toluene. A ~5 µLdrop of this ink was deposited on theelectrode, after which the toluene wasevaporated at 50 °C. This procedure resultedin electrodes with an average loading of 0.45mg cm-² of electrocatalyst. The cell was purgedwith N2 gas for 15 min before starting thetest. A solution containing NB (5 mM) andLiClO4 (0.2 M) in absolute ethanol (50 ml) wasused as electrolyte. The applied potential wasvaried in the range between -0.3 V and -2.4 Vvs Fc/Fc+, with a scan rate of 5 mV s−1. Everymeasurement was repeated at least three times.The measured resistance of the electrolytebetween the reference electrode and theworking electrode was 196 Ω. In this work, thereported potentials are referred to theferrocene/ferrocenium (Fc/Fc+, -0.64 V vsS.H.E.) reference couple. A Ag/AgCl (sat.LiCl) electrode was used as referenceelectrode. The exact potential of thiselectrode was measured by cyclic voltammetry(0.142 V vs. S.H.E). The onset potential ofeach electrocatalyst was determined as thepotential at which the slope of thevoltammogram exceeded 0.1 mA cm-2 V−1. Thenumber of electrons transferred was calculatedby means of the Koutécky-Levich equation at -1.3 V vs Fc/Fc+ .4 Stability measurements were carried out bycyclic voltammetry (CV). During thesemeasurements, the potential was cycled between-0.3 V and -1.7 V vs Fc/Fc+, with a scan rateof 100 mV s-1. The solution was stirred againbefore the 1000th scan in order to homogenizethe concentration of NB, thus increasing thecomparability of the analysis in the startingand final cycle.Chronoamperometric experiments were carriedout for 52 h, in order to study the reactionof NB (15 mM) over the most promisingelectrocatalyst. The tests were performed in atwo-compartment cell, separated by a Zirfon®

membrane (VITO, Belgium).4 Two types ofexperiments were performed: with the workingelectrode placed in neutral ethanolic mediumwith 0.2M LiClO4 as support electrolyte (at -1.15 V vs Fc/Fc+); and with the workingelectrode placed in acidic ethanolic mediumcontaining 0.3M HClO4 (70%, VWR) (-0.62 V vsFc/Fc+). Both experiments were performed withthe working electrode operating at a rotatingspeed of 500 rpm. The concentration of NB andof the reaction products at the end of thetest were measured by High-performance liquidchromatography HPLC (Shimadzu). Each analysis

was done in triplicate, and the average of thequantification results was reported. Theacidic solution obtained from eachchronoamperometric experiment needed to beneutralized before the analysis: the pH wasadjusted to a value between 4 and 5 by addinga 1 M aqueous solution of KOH, neutralizedwith a phosphate buffer solution (pH = 7,Fisher), and filtered to remove the insolubleKClO4.

Acknowledgements

The authors acknowledge sponsoring from the Flemish Agencyfor Innovation by Science and Technology (I.W.T.) in the frameof an S.B.O. project (OCPEC), and are very grateful for supportby VITO research center and the IAP-PAI research programs.We thank Kitty Baert and Oscar Steenhaut for their help withSEM-EDX and XPS measurements, Dr. Chalida Klaysom for TEMoperation, Prof. Christine Kirschhock and Dr. Elena Gobechiyafor the assistance in XRD analysis.

Keywords: Cu/CuxO nanoparticles • electrocatalyst • activated carbon • nitrobenzene • azoxybenzene

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Received: ((will be filled in by the editorial staff))Published online: ((will be filled in by the editorial staff))

ARTICLESVery small and uniform copper nanoparticles (~4 nm)supported on activated carbon showed remarkably enhanced electrocatalytic performance in the reduction of nitrobenzene comparedto supported Pt and PtCu alloy

Xia Sheng, Benny Wouters, Tom Breugelmans, Annick Hubin, Ivo F.J. Vankelecom, Paolo P. Pescarmona*

[The definitive version is available athttp://onlinelibrary.wiley.com/doi/10.1002/celc.201402015/abstract].

Title Pure and alloyed copper-based nanoparticles supported on activated carbon: synthesis and electrocatalytic application in the

10 nm10 nm

(d)

NO O

NH O

HNO

2H+ + 2e-

H2O

Nitrobenzene Nitrosobenzene Phenylhydroxylamine

2H+ + 2e-

Cu/CuxO nanoparticles on AC

Cu-Cu xO

Pt

Ons

et poten

tial

Pt-C

u

N

N

O

azoxybenzene