Influence of the Preparation Route of Bimetallic Pt-Au Nanoparticle Electrocatalysts for the Oxygen...

Influence of the Preparation Route of Bimetallic Pt-Au Nanoparticle Electrocatalysts forthe Oxygen Reduction Reaction

P. Hernandez-Fernandez,†,‡ S. Rojas,*,‡ P. Ocon,† J. L. Gomez de la Fuente,‡ J. San Fabian,†J. Sanza,‡ M. A. Pena,‡ F. J. Garcıa-Garcıa,§ P. Terreros,‡ and J. L. G. Fierro* ,‡

Departamento de Quı´mica-Fısica Aplicada C-II, Campus UAM, 28049 Madrid, Spain, Instituto de Cata´ lisis yPetroleoquı´mica (CSIC), C/Marie Curie 2, 28049 Madrid, Spain, and Lehrstuhl fu¨r Festkorperchemie, Institutfur Physiks, UniVersitat Augsburg, UniVersitatstrasse 1, D-86159 Augsburg, Germany

ReceiVed: October 17, 2006; In Final Form: December 5, 2006

Pt and Au are not miscible within a whole range of concentrations. To obtain PtAu alloys, severe thermaltreatments are required that to provide aggregation phenomena. However, it is possible to synthesize bimetallicPtAu nanoparticles provided the proper synthesis route is employed. When they are prepared from water-in-oil microemulsions or with the impregnation technique, it is possible to obtain nanosized bimetallic PtAuparticles. In contrast, other colloidal routes have been seen to be adequate for the synthesis of other bimetallicPt-based particles, affording segregated samples with Pt- or Au-enriched zones. When alloyed, bimetallicPtAu nanoparticles display unique physicochemical properties that are different from those of monometallicand nonalloyed solids. Thus, the performance of alloyed PtAu samples as electrocatalysts for the oxygenreduction reaction is superior to that of the PtAu-segregated samples. In fact, the ability of carbon-supportedbimetallic PtAu samples in the oxygen reduction reactions equals or even surpasses that of archetypal Pt/Celectrocatalysts.

1. Introduction

The synthesis and characterization of so-called nanomaterials(solid-state compounds with nanosized particles) is an area ofincreasing impact due to the unique properties expected for thesesolids. Among other applications, nanomaterials can be usedas sensors, conducting/isolating materials, and catalysts. How-ever, accurate understanding of the underlying physical andchemical properties associated with decreases in particle sizeremains a challenge. Within these frameworks, the preparationof nanosized Au nanoparticles in general and bimetallic PtAuin particular is currently attracting considerable attention.According to the binary phase diagram, there is a miscibilitygap between Pt and Au, and hence, phase segregation can beexpected.1 In fact, to obtain homogeneous PtAu samples, severethermal treatments (beyond 1000°C, depending on the nominalcomposition), generally yielding aggregated particles, arerequired. It has recently been shown that this issue can beovercome if bimetallic PtAu samples are prepared as nano-structured materials.2,3 In those reports, the preparation of PtAubimetallic particles was achieved by following complicatedsynthetic routes, involving the use of dendrimer complexes inthe former or phase-transfer agents and encapsulating agentstogether with thermal treatments in the latter. Other approachesfor the preparation of PtAu alloys consist in the use oforganometallic or coordination complex precursors.4-6 Pt to Auatomic stoichiometry is thus imposed from the available PtAuprecursors.

PtAu nanostructured samples are of outstanding interest inmany respects. The catalytic performance of silica-supported

PtAu nanoparticles has been studied intensively over the lastdecades, and in most cases phase segregation was observed.4,7-9

Nevertheless, it was the description of the high activity ofnanosized gold particles toward CO oxidation that triggeredinterest in the preparation of Au-based catalysts.10,11 In fact,Au-based solids have been studied as potential electrocatalystsfor fuel cell applications, in the oxidation of hydrogen andCO12-15 and that of alcohols,16,17or even as electrocatalysts forthe oxygen reduction reaction (ORR).18,19Recently, it has beenreported that PtAu samples prepared by impregnation affordperformances similar to those of pure Pt samples.4 However,when Pt is deposited over Au, electrocatalysts displaying betteractivities have been described.20

Among the different synthesis routes employed for thepreparation of electrocatalysts,21 the microemulsion techniquehas emerged as a promising route for the preparation ofnanosized bimetallic particles, avoiding heat treatments.22-25

Recently, the preparation of Au nanoparticles and their perfor-mance as electrocatalysts on the oxygen reduction reaction havebeen reported.26

In this context the preparation of catalyst materials based onnanosized metallic particles is of special interest. For instance,both monometallic and/or bimetallic carbon-supported Pt-basedsolids have been studied in depth as electrocatalysts in fuel cellapplications.27 Nonetheless, a serious drawback for the imple-mentation of fuel cells is the vast quantity of metal required,probably beyond the whole of the Earth’s content of Pt. Thus,electrocatalysts based upon bimetallic alloyed nanoparticles havebeen proposed as a promising alternative. Since the kinetics ofthe oxygen reduction reaction (ORR) is much slower than thatof the anodic reaction,28 a reduction in the content of Pt andeven the complete replacement of Pt in the cathode29,30,31arefields of great interest in the area of materials development.PtAu samples have been tested as electrocatalysts for the oxygen

* Corresponding authors. E-mail: [email protected] (S.R.); [email protected](J.L.G.F.).

† Campus UAM.‡ Instituto de Cata´lisis y Petroleoquı´mica.§ Universitat Augsburg.

2913J. Phys. Chem. C2007,111,2913-2923

10.1021/jp066812k CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 01/26/2007

reduction reaction, an enhancement in their activity over Au-enriched samples being reported.32

A series of carbon-supported PtAu samples have beenprepared with several techniques such as microemulsion,impregnation, and colloidal routes. The carbon-supported solidshave been characterized by several techniques, affording theconclusion that for some preparations (microemulsion andimpregnation) PtAu alloys have been obtained, whereas thecolloidal route yields Pt and Au as separate phases. Thedevelopment of such alloyed PtAu phases affords materials ofperformance superior to that of archetypal Pt/C electrocatalystsfor the ORR.

2. Experimental Section

2.1. Preparation of Bimetallic Samples.The preparation ofPt and Pt-Au particles was carried out using different ap-proaches such as microemulsion, impregnation, and a colloidalmethod typically employed for the preparation of PtRu par-ticles.33 H4PtCl6 and HAuCl4 (both purchased from Alfa-Aesar)were used as metal precursors. For the microemulsion prepara-tions, Mem1-PtAu/C and Mem2-PtAu/C, a water solution(5 w%) containing the appropriate amounts of the metalprecursor was added to a mixture of isooctane (75 wt %)(Aldrich) and Tergitol 15-S-5 surfactant (20 wt %) (Aldrich).After stirring, a transparent mixture was obtained. From thispoint onward, two methodologies were employed: (i) For thepreparation of the Mem1-PtAu/C sample, the microemulsioncontaining the metal precursors was added dropwise over adispersion of the carbon support (Carbon Vulcan 72 XR,supplied by Cabot) in a mixture of isooctane/tergitol 15-S-5/water (75/20/5 wt %). Simultaneously, a similar microemulsion(isooctane/tergitol 15-S-5/water (75/20/5 wt %) containinghydrazine in excess was added. The mixture was stirredovernight. (ii) For the preparation of the Mem2-PtAu/C sample,a microemulsion (similar to the above one) containing hydrazinewas added dropwise to the microemulsion containing the metalprecursors. Once the reaction has been completed, the mixturewas added dropwise over a dispersion of the carbon support(Carbon Vulcan 72 XR, supplied by Cabot) in a mixture ofisooctane/tergitol 15-S-5/water (75/20/5 wt %). The mixture wasstirred overnight.

As from this step, the solids were recovered in a similar way.THF was added, and the mixture was allowed to decantovernight. The solid was recovered by filtration and centrifuga-tion, washed thoroughly with a water/ethanol mixture, and driedat 100°C for 12 h.

For the impregnation method a water/2-propanol solution ofthe metal precursors was set to pH) 4 with Na2CO3 (0.6 M).The mixture was added dropwise to a dispersion of the carbonsupport in water under reflux. Then, CH2O was added to themixture and stirred for 3 h. The solid was recovered, washedwith thrice-distilled water several times, and dried overnight at100°C. The solid thus obtained was designated Imp-Pt-Au/C

A further Pt-Au sample was prepared as follows. Sodiumbisulfite was added to a water solution of the Pt precursor. Themixture was set to pH) 5 by the addition of NaCO3 (0.6 M).H2O2 was added to the mixture, and the final pH ()5) wasimplemented by NaOH addition. A water solution of the Auprecursor was added under stirring. Then, carbon was addedand the mixture was stirred for 1 h. Finally, H2 was bubbledthrough the mixture for 2 h. The solid was recovered, thoroughlywashed with thrice-distilled water, and dried at 100°C. Thesample was designated Col-Pt-Au/C.

The nomenclature, preparation details, and selected physicalproperties of the samples are shown in Table 1.

2.2. Characterization.The Mem1-PtAu/C; Mem2-PtAu/C,Imp-PtAu/C, and Col-PtAu/C (20 wt % metal basis; Pt/Au, 2/1nominal atomic ratio) samples were tested as electrocatalystsfor the ORR. Their performance was compared to that of acommercial (Johnson Matthey) 40 wt % Pt/C electrocatalyst,henceforth denoted as Pt/C.

Electrochemical testing was conducted in a conventionalthree-compartment electrochemical glass cell. The workingelectrode was rotating disk electrode (RDE) with a glassy carbon(GC-Typ zu628) 0.07 cm2 area. An Au plate and a mercury/mercury sulfate electrode were used as the counter and referenceelectrodes, respectively. All potentials are quoted with respectto the reversible hydrogen electrode (RHE). During the mea-surements, a gentle flow of nitrogen or oxygen was maintainedover the electrolyte surface.

The samples under study were deposited onto the workingelectrode (glassy carbon disk electrode) by means of an ink.Typically, 5 mg of the solid, 30µL of 5 wt % of Nafion solution(Aldrich), and 700µL of Milli-Q water were dispersed in anultrasonic bath for 45 min, obtaining a homogeneous ink. Beforethe deposition of the sample, the glassy carbon electrode waspolished (0.05µm alumina) to a mirror finish and rinsed withthrice-distilled water in an ultrasonic bath. A 30µL volume ofthe ink (added in 3 consecutive steps involving 10µL each)was dropped onto the electrode and dried at 75°C for 5 min,resulting in a homogeneous coating. The final loading of Pt oneach electrode was ca. 82µg for the commercial sample and27 µg of Pt for the bimetallic PtAu/C electrodes. A 0.5 M H2-SO4 (Merck) solution was used as the electrolyte. All solutionswere prepared with Milli-Q (Millipore) water. High-purityoxygen and nitrogen (Air Liquid) were used for solutionsaturation and deaeration, respectively.

Cyclic and linear voltammetry and rotation disk polarizationmeasurements were performed with a EG&G 273A potentiostat/galvanostat controlled by a computer. The electrode rotationspeeds were controlled by a Metrohm 628-10 unit. The rotationrate was varied from 500 to 2500 rpm, and the scan rate was 1mV/s. Before the RDE study, the porous electrodes were cycledat 100 mV between 0.05 and 1.2 V until reproducible cyclicvoltammograms were obtained. No marked changes in the shapeand size of the voltammograms were observed. The Pt realsurface areas of all catalysts were determined by CO strippingvoltammetry. Typically, CO was flowed under stirring whilethe electrode was set at a constant potential of 20 mV for 15min. CO was purged out of the electrolyte solution by bubblingAr through it for 45 min. Then, three consecutive potentialsscans between 0 and 1 V at ascan rate 10 mV s-1 were recorded.Normalized currents are given in terms of either geometric(mA cm-2) or mass-specific current densities as reported asmA cm-2 mg-1

metal.X-ray diffraction patterns were collected on a Seifert 3000

powder diffractometer, using Cu KR radiation. The display and

TABLE 1: Sample Labeling, Preparation Route, andCharacterization Details of PtAu/C Samples

sample prep routecell param

(Å)metal-metal dist

(Å)

Mem1-PtAu/C microemulsiona 3.9733(14) 2.8096Mem2-PtAu/C microemulsiona 3.9629(23) 2.8022Imp-PtAu/C impregnation 3.9860(17) 2.8185Col-PtAu/ colloidal 3.9084(25)b 2.7637b

4.0542(28)c 2.8668c

a The difference between both microemulsion based methods is thenucleation and growth environment of the metal particles. See the textfor further details.b Parameters for the Pt phase.c Parameters for theAu phase.

2914 J. Phys. Chem. C, Vol. 111, No. 7, 2007 Hernandez-Ferna´ndez et al.

handling of the diffraction patterns as well as unit cellrefinements were carried out using the PowderCel software.34

Specimens for TEM analyses were prepared by dispersing thepowder samples in butanol. One drop of the resulting suspensionwas placed on a holey carbon film supported by a copper grid.They were studied on a JEM 2100F device equipped with anEDAX detector for X-ray energy dispersive spectroscopyanalysis.

X-ray photoelectron spectra (XPS) of the samples wereacquired with a VG ESCALAB 200R spectrometer fitted witha Mg KR (hν ) 1253.6 eV) 120 W X-ray source. Samples werepressed into small stainless-steel cylinders and then mountedon a sample rod, placed in a pretreatment chamber, and degassedat 25°C and 10-5 bar for 5 h prior to transfer to the analysischamber. Residual pressure was maintained below 3× 10-9

bar. The 50 eV energy regions of the photoelectrons of interestwere scanned a number of times to obtain an acceptable signal-to-noise ratio. Intensities were estimated by calculating theintegral of each peak, determined by subtraction of the Shirleytype background and fitting of the experimental curve to acombination of Lorentzian and Gaussian lines of variableproportions. Accurate binding energies ((0.2 eV) were deter-mined by referencing to the C 1s peak at 284.6 eV.

3. Results

3.1. Structural Characterization. The X-ray powder dif-fraction patterns of all samples are shown in Figure 1 a. Forthe sake of comparison, the range between 36 and 50° isenlarged in Figure 1b. Note that the broad peak centered at ca.25° corresponds to the carbon used as the supporting matrix;therefore, it is invariably observed in all patterns. For the Mem1-PtAu/C, Mem2-PtAu/C, and Imp-PtA/C samples, all reflectionsexpected for an fcc lattice, corresponding to the structure ofthe pure bulk metals, are observed. In contrast, the diffractogramof the Col-PtAu/C sample displays two sets of such reflections.This denotes the condensation of two different crystalline phases,although both correspond to the same crystal structure type. Therefined unit cell parameters of all samples are depicted in Table1.

A careful comparison of the refined unit cell parameters withthe values reported for pure Pt,a ) 3.923 Å,35 and Au,a )4.078 Å,36 suggest that (i) in the Mem1-PtAu/C, Mem2-PtAu/C, and Imp-PtA/C samples Pt and Au are alloyed and hence asingle crystalline phase is observed and (ii) in the Col-PtAu/Csample phase separation occurs. To underline this in Figure 1b,the positions of the 111 reflections are projected onto thehorizontal axis. The reflections for the Col-PtAu/C sampleappear at positions fairly close to those expected for the bulkmetals: 39.764° for Pt; 38.188° for Au. Accordingly, the twodifferent phases in this sample seem to correspond to a situationwhere the metallic constituents condense independently withoutalloying. This result was further confirmed by the EDSexperiments in the electron microscope; see below. The positionof the diffraction peaks of the Mem1-PtAu/C, Mem2-PtAu/C,and Imp-PtAu/C samples indicates that the Pt-Au alloy followsa Vergard-type law. Taking into account that the nominalcomposition of the samples is Pt/Au) 2/1, the 111 reflectionshould appear centered at 2θ ≈ 38.700°, which is in very goodagreement with the experimental X-ray power diffractionpatterns.

Low-resolution TEM and high magnification images of theparticles are presented in Figures 2 and 3, respectively. Themetallic particles were found to be homogeneously dispersedin the supporting material, and no segregation was observed.

The histograms of particle size are shown in Figure 4. Theywere calculated by crystallographic image processing fromimages such as those shown in Figure 2. For each sample,between 180 and 200 particles were analyzed. As depicted inFigure 2a,b, the Mem2-PtAu/C and Imp-PtAu/C samples displaysimilar characteristics. Additionally, their particle size distribu-tion as depicted in Figure 4a,b are similar. Both samples displaya sharp peak centered at ca. 3 nm. Moreover, the Mem2-PtAu/Csample shows a broad band (25-nm width) centered at ca. 30nm. The Imp-PtAu/C sample displays a similar pattern; onlythe broad band extends to higher particle size values. For thosesamples, the high-resolution images, depicted in Figure 3a,b,show spherically shaped particles with no structural defects. The{111} direction is indicated in some of the crystals. Thesecrystallographic planes seem to be slightly favored in the crystalgrowth process. The particle size distribution of the Mem1-PtAu/C sample, Figure 4d, shows a maximum centered at 3nm. Interestingly, the metallic particles had developed a rodlikeshape and the axis of the rods was seen to be parallel to{111},as shown in Figure 3d.

The most striking peculiarities are observed in the sampleCol-PtAu/C. The presence of large spherical particles constitutes

Figure 1. Diffractograms of carbon-supported PtAu bimetallic samples.The crystallographic planes are marked in (a). The projection of theAu and Pt phases are shown in (b).

Bimetallic Pt-Au Nanoparticle Electrocatalysts J. Phys. Chem. C, Vol. 111, No. 7, 20072915

the major difference with respect to the other samples; see Figure2c. Nanosized metallic particles were also formed, as seen inFigure 3. In fact, the smallest particles among all the samplesstudied were observed for the Col-PtAu/C sample. Their shapeis fairly irregular, but a tendency to grow in a direction parallelto {111} may also be inferred. The transmission electronmicroscope used in the present investigation is equipped witha field emission gun unit. It is therefore possible to carry outcompositional analyses with an electron beam of nominally justa few angstroms in diameter. The results are consistent withthe formation of Pt-Au alloy in the Mem1-PtAu/C, Mem2-PtAu/C, and Imp-PtAu/C samples, but this was not the case inthe Col-PtAu/C sample. In this sample, Au and Pt were detectedas separate phases, Au only being detected in the largestspherical particles; see Figure 2. However, not all of such largeparticles were composed by Au; some of them were composedof Pt alone.

The XPS spectra of the Pt 4f and Au 4f core-level regionsare depicted in Figure 5. The relative abundances of the differentspecies detected by the XPS analysis and the surface atomicratios are shown in Table 2. It may be observed that the surfacecomposition depends on the samples, which in some cases arevery different from the expected value, i.e., Ptat/Auat ) 2. To alesser extent, the same applies to the bulk value obtained fromICP analysis (results shown in Table 2). For instance the ICPanalysis for Col-PtAu/C samples afforded a Ptat/Auat ) 2, closeto the theoretical value. However, Au was scarcely detected fromthe XPS, revealing either a Pt-enriched surface or a ratherheterogeneous composition of the sample. To elucidate whichexplanation best described the nature of the Col-PtAu/C sample,the solid was subjected to Ar sputtering for 2 and 4 h within

the XPS treatment chamber. Even after sputtering, only tracesof Au were detected, confirming that Au is not present in theinner layers of the solid. Instead, the sample was quiteheterogeneous, displaying Pt- and Au-enriched zones.

3.2. Electrochemical Characterization.Steady-state cyclicvoltammograms of the Pt/C and the bimetallic PtAu/C samplesare shown in Figure 6. In general, the hydrogen adsorption/desorption charge for the Pt-Au/C samples is lower than thatof the Pt/C sample (results not shown). Owing to the porousnature of the supporting material, the charge due to the double-layer charging is rather high. This feature is very important forthe Col-PtAu/C sample. Furthermore, it overlaps that of theadsorption of hydrogen in acid solution.37 Therefore, the specificsurface area (active surface area/mass of Pt on the electrode)was evaluated from the CO stripping analysis; the results areshown in Table 3. Figure 7 depicts the first and the second cyclerecorded during the CO stripping analysis of the PtAu samples.Note that the features of the hydrogen adsorption are clearlyobserved in the second voltammetry cycle in good agreementwith total CO oxidation during the first cycle.

3.3. Oxygen Reduction Reaction (ORR).Polarization curvesfor the oxygen reduction reaction (ORR) were obtained in 0.5M H2SO4 on a thin porous coating rotating disk electrodeprepared with Vulcan XC-72 carbon. The curves were recordedin the cathodic sweep direction at 1 mV/s from 1.15 to 0.0 Vover a range of rotation (500-2500 rpm). As expected, theVulcan XC-72 electrode lacked any activity in the ORR, onlyaround 0.2 V a certain cathodic current being observed. TheORR is diffusion-controlled at potentials that are more negativethan 0.5 V; a mixed diffusion-kinetic control is expected inthe potential region between 0.5 and 0.8 V. At more positive

Figure 2. TEM micrographs of bimetallic samples: (a) Mem2-PtAu/C; (b) Imp-PtAu/C; (c) Col-PtAu/C; (d) Mem1-PtAu/C.


potentials, kinetic control dominates. Figure 8 shows the resultsobtained for the Imp-PtAu/C sample at selected rotation rates.For all samples, the current densities (the current normalizedto the geometric area of the electrode) are proportional toω1/2,confirming that the ORR is limited by the diffusion of oxygen.The onset of the oxygen reduction wave is centered at ca. 0.947,0.824, 0.843, and 0.773 V for the Imp-PtAu/C, Col-PtAu/C,Mem2-PtAu/C, and Mem1-PtAu/C samples, respectively. Underthe same conditions, the onset of the ORR for the Pt/C is samplelocated at 0.987 V. That is, the Imp-PtAu/C sample displaysan overpotential of 40 mV. The thermodynamic value of theORR in acid medium is 1.185V vs RHE.38 All electrocatalystsshowed increasing diffusion-limiting currents with electroderotation; however, at a given rotation rate, all samples reacheda similar limiting current, as depicted in Figure 9.

4. Discussion

From the characterization results, it may be inferred that asingle PtAu bimetallic alloyed phase was obtained from themicroemulsion and impregnation preparations, whereas thecolloidal method afforded separate Pt and Au phases. Torationalize this result, an in-depth discussion of the differentpreparations is needed. In a microemulsion, water droplets arestabilized within an organic medium by the presence of asurfactant.22,39 Although stable at macroscopic level, dropletcoalescence phenomena take place continuously. During suchcoalescence, water droplets interact with others, interchangingtheir content, i.e., the metal precursor. After equilibrium hasbeen reached, all the droplets of the microemulsion will contain

similar amounts and ratios of both the Pt and Au metalprecursors. The addition of hydrazine triggers particle nucleationand growth. It has been well described that the nucleation andgrowth of particles is hindered by the presence of the surfactantwithin microemulsions, actually favoring the formation of alarger number of practically identical nuclei containing both Ptand Au. In light of these results, it seems that particle formationoccurs preferentially within such a homogeneous and stabilized(due to the presence of the surfactant) environment as amicroemulsion.

Whereas particle nucleation is somehow delayed withinmicroemulsions, the opposite scenario might be expected fromparticle growth in homogeneous media. However, the prepara-tion by impregnation afforded completely alloyed PtAu particles.This result is somewhat surprising since in a similar preparationfor silica-supported PtAu samples both alloyed and segregatedPt and Au phases were detected.4 Nevertheless, the syntheticprocedure reported in here is slightly different. First, differentmetal precursors were employed, and more importantly, the pHof the solution was controlled accurately.

In contrast, the preparation route of the Col-PtAu/C sampleled to the formation of isolated Pt and Au phases. Although asimilar method has been proposed for the preparation of PtRubimetallic phases,33 this route seems to afford individual metalphases with no interaction between each other, at least not inthe case of Pt Au. In this synthetic approach, Pt nanoparticlesare stabilized because a bisulfite core is formed before Au isadded. In alkaline media Au reduction proceeds very fast,possibly even before it enters into contact with the Pt particle

Figure 3. HRTEM micrographs of PtAu/C samples: (a) Mem2-PtAu/C; (b) Imp-PtAu/C; (c) Col-PtAu/C; (d) Mem1-PtAu/C. The{111} directionis indicated.


precursor, which is still present as an ion, thereby preventingthe simultaneous nucleation (both in time and place) of the Ptand Au particles.

We also observed certain morphological differences betweenthe samples: see Figures 2 and 3. The Mem1-PtAu/C sampleconsists of well dispersed rod-shaped particles. Actually, this

was the only sample for which such a morphology was found,the rest of the solids displaying roughly spherical-shapedparticles. As stated before, during the preparation of particlesfrom reversed micelles water droplets surrounded by an organicphase and stabilized by the presence of a surfactant are formed.Therefore, a similar scenario could be expected for preparationof the Mem2-PtAu/C sample, thus yielding similarly shapedparticles. However, prior to the nucleation of the Mem1-PtAu/Cparticles, the microemulsion enters into contact with thedispersion containing the carbon support. It is not unlikely thatthe shape of the micelles would change due to the change inthe relative composition of the different phases. That is, atransition would occur from the spherical micelles expectedwithin a microemulsion to the large anisotropic micellesexpected in liquid crystals.40 Thus, once hydrazine had beenadded and metallic particles had actually been formed, theirshape resembled that of micelles containing Pt and Au salts, asrodlike micelles. The preparation of sample Mem2-PtAu/C mustbe somewhat different since particles were actually formedwithin the original microemulsion environment prior to enteringinto contact with the dispersion containing the support. There-fore, the change in the relative abundance of the different liquidphases would not affect the shape of the metallic particles.Nevertheless, these changes only seemed to affect particle shapeand not the particle composition. In both cases Pt-Au bimetallicparticles were obtained. However, a certain peak broadening isobserved from the diffractogram of sample Mem2-PtAu/C. Thisfeature might indicate that even if the majority of the particleshave developed a bimetallic nature, the extension of the alloyingprocess might not be complete.

Again, the Col-PtAu/C sample behaved in a way differentfrom the rest of the series. Large Au particles together withnanosized Pt particles coexisted within the solid, as may beobserved from Figure 2. This feature is in good agreement with

Figure 4. Histograms of samples: (a) Mem2-PtAu/C; (b) Imp-PtAu/C; (c) Col-PtAu/C; (d) Mem1-PtAu/C.

Figure 5. Au 4f and Pt 4f core-level regions of samples: (a) Imp-PtAu/C; (b) Mem1-PtAu/C; (c) Mem2-PtAu/C; (d) Col-PtAu/C.


the XPS and XRD analyses. Au was detected as a segregatedphase from the XRD analysis; however, it was not observed(or only as traces) in the XPS analysis, even after Ar sputtering.

The samples were also subjected to electrochemical analysis.A strong tool for elucidating the nature of particles is COstripping analysis. Besides obtaining the surface area of Pt, itcan provide information about the nature of the nanoparticles.The CO oxidation profile (stripping analysis) was not alteredby the presence of Au as compared to Pt/C. Neither did it seemto affect whether actual PtAu-alloyed bimetallic particles orwhether Pt and Au segregated phases were formed instead.However, greater differences were found when the voltammo-grams of the sample were recorded in H2SO4, as depicted inFigure 6. All samples displayed the features of the hydrogenadsorption-desorption region between 0 and 0.35 V, followedby the “double-layer” potential region. At potentials of>0.75V OH adsorption followed by oxide formation was observed.The contribution of the double-layer charging was considerablylarger for the Col-PtAu/C sample than for the other samples.Careful comparison of the oxide reduction region revealed ashift of ca. 30 mV toward more negative potentials for themicroemulsion-prepared samples than for the Imp-PtAu/Csample. The peak for the Col-PtAu/C sample was centered at

the most negative potential of the series. This shift can beascribed to the oxophilicity of the samples.41 The increase inoxophilicity favors the formation of OH species, which blockthe sites responsible for O2 adsorption. This indicates that Ptoxide reduction is slightly favored in the Imp-PtAu/C samplesas compared to the microemulsion method (Mem2-PtAu/C>Mem1-PtAu/C) based samples. Except for the Col-PtAu/Csample, the rest of the PtAu/C bimetallic samples exhibited asecond oxide reduction peak. This feature has already beenreported for PtAu alloys.42 The position of this secondpeak, ca. 1.0 V vs RHE, is consistent with PtAu-alloyedsamples displaying Pt/Au> 1 atomic stoichiometry. Thepotential region where this peak appears is magnified in theinset to Figure 6.

From the XPS analysis, a slight electronic interaction can bededuced. The Pt 4f core-level binding energy of the Pt0 specieswas shifted to a lower binding energy for the PtAu bimetallicsamples as compared to position displayed by the Pt0 speciesof Col-PtAu/C. The magnitude of the shift is ca. 0.3 eV, andthe trend is observed throughout the series. Furthermore, theposition of the peaks is consistent with the results of workalready published in the literature; i.e., displaying the Pt 4f core-level of the Pt0 species in Pt/C samples at 71.5 eV.43 However,the opposite trend has been reported, Pt being shifted to higherbinding energies44 (ca. 0.4-0.5 eV). Nevertheless, the latterresult refers to Au overlayers over Pt(100) rather than to alloyedPtAu nanoparticles. In fact, it is known that whereas Au in thebulk form is the most electronegative of all metals, theelectronegativity of nanosized Au clusters might be different.45

The surface of the Mem2-PtAu/C sample was enriched in Pt asmay be deduced from the XPS analyses, the relative atomicratio of the metal phase being Ptat/Auat ) 8.4. The Mem1-PtAu/C and Imp-PtAu/C showed samples a Ptat/Auat surface ratioof 1.6 and 2.2, respectively, close to the expected value.

The performance of the samples as electrocatalysts in theORR was evaluated by means of polarization techniques. Thelimiting currents are depicted in Figure 8. An importantparameter for characterizing the performance of samples is the

TABLE 2: Pt 4f and Au 4f Core Level Binding Energies (eV), Surface Atomic Ratios (by XPS), and Bulk Ratios (from ICP)

sample Pt Au Pt/Au Pt/C Au/C Pt/AuICP

Mem1-PtAu/C 71.15(50) 83.88(65) 1.6 0.0098 0.0047 2.772.80(36) 85.33(35)74.73(14)

Mem2-PtAu/C 71.21(55) 83.88 8.2 0.0043 0.0002 3.272.49(26)73.94(19)

Col-PtAu/C 71.47(43) 0.0187 272.73(35)74.48(22)

Imp-PtAu/C 71.22(57) 83.92(70) 2.2 0.0151 0.0058 2.272.65(29) 85.34(30)74.69(14)

Figure 6. (a) Cyclic voltammetry (10 mV/s, H2SO4) of samples Imp-PtAu/C (black line), Mem1-PtAu/C (gray line), Mem2-PtAu/C (dashedgray line), and Col-PtAu/C (dotted black line). The 1.0-1.2 V regionis magnified as an inset to the figure.

TABLE 3: Kinetic Parameters (ORR) of PtAu BimetallicSamples

Tafel slope

sample EAAa (m2/gPt) nb low current high current

Mem1-PtAu/C 11.5 3.5 59 142Mem2-PtAu/C 8.0 3.7 61 122Col-PtAu/C 19.1 4.0 67 126Imp-PtAu/C 22.3 4.0 65 120

a Electrocatalytically active area as determined from the COstripping analysis recorded in 0.5 M HClO4. b Number of electronsinvolved in the ORR (see eqs 5 and 6). The EAA of the Pt/C sampleis 32.3 m2/gPt


evaluation of the number of electrons transferred during thereaction. This feature can be evaluated from the Levich equation

or in its short form

where n is the number of electrons transferred for each O2

molecule,F is Faraday’s constant,A is the geometric electrodesurface,C is the concentration of O2 dissolved in the solution,D is the diffusion coefficient of oxygen,υ is the kinematicviscosity of the solution, andω is the angular rotation rate ofRDE. Under mass transfer-limiting conditions,B, the Levichslope, is constant. However when the cathodic limiting currentdensities where plotted vsω1/2, a slightly curved line wasobtained, presumably due to a slight mass transport limitationin the Nafion matrix. The inverse Levich plots, i.e.,i-1 vsω-1/2,afforded a straight line with a nonzero intercept (Figure 10).This plot is known as Koutecky-Levich expression (K-L),based on eq 3:

From the well-defined limiting-current plateau observed inFigure 8, it is difficult to justify the nonzero intercept for thelimiting currents in the K-L plots due to mere kinetic controlof the reaction. More likely, the problem would be due to thepresence of a Nafion layer surrounding the Pt particles, thus

Figure 7. First (black straight line) and second (dotted gray line) cyclesrecorded during the CO stripping analysis of samples (A) Mem1-PtAu/C, (B) Mem2-PtAu/C, (C) Imp-PtAu/C, and (D) Col-PtAu/C. Volta-mmograms were recorded in HClO4 (0.5 M) at 10 mV/s.

iD ) 0.62nFACD2/3ν-1/6ω1/2 (1)

iD ) Bω1/2 (2)

1i

) 1ik

+ 1

Bω1/2) f(ω-1/2)E (3)

Figure 8. Linear sweep (1 mV/s) recorded in oxygen saturated in 0.5M H2SO4 at different rotation rates for the sample Imp-PtAu/C.

Figure 9. Tafel plot of the different samples Col-PtAu/C (0), Imp-PtAu/C (9), Pt/C (O), Mem1-PtAu/C (b), and Mem2-PtAu/C (2). Inthe inset to the figure, the (a) Tafel and (b) corrected Tafel plots forsample Mem2-PtAu/C are depicted. Two slopes-122 mV/decade(dashed line) and-64 mV/decade (straight line) are found.


covering the electrocatalyst sites. This situation would beanalogous to a smooth polymer film-covered electrode. Therelationship between current and the rotation rate for a film-covered electrode is as follows:

whereif is the limiting diffusion current in the film. Thus, theintercept ofi-1 vs ω-1/2 plot cannot be zero even if the kineticcurrent is extremely high. For the PtAu samples reported here,the kinetic current (ik) is low in comparison with the values inthe mixed kinetic-diffusion control region; therefore its contri-bution to the total current (i) can be neglected.

The plot of the inverse of the current with the potential inthe mixed kinetic-diffusion control region afforded parallel lines,as depicted in Figure 10. The number of electrons transferredwas constant within the potential range. The actual number ofelectrons transferred by each O2 molecule was calculated byassuming CO2 ) 1.1 × 10-3 M, DO2 ) 1.9 × 10-5 cm2 s-1,and υ ) 1.009 × 10-2 cm2 s-1, as reported in ref 46, andnormalizing the number of electrons to a value of 4 for the Pt/Csample. A first insight into the efficiency of the electrocatalystscan be obtained from the value of n. Thus, values of n close to4 were obtained for all the PtAu/C bimetallic samples. Theresults are shown in Table 3. These values are higher than thosereported for Au/C and Au-Pt/C (3:1) ratios in acid solution of2.2 and 2.9, respectively.47 The ORR may proceed eitherpartially, in a two-electron-transfer process, or completely, in afour-electron-transfer process, yielding H2O2 and H2O, respec-tively.

The partial reaction (eq 5) is a result of inefficient electro-

catalysts, originating voltage and current losses in the PEMFC.The PtAu/C samples reported here promoted the 4-electron-transfer process (eq 6); that is, the more complete the reduction

reaction, the higher the electrocatalytic activities expected.Furthermore, the linearity of the plots in Figure 10 implies afirst-order dependence of the O2 kinetics on the PtAu/Celectrodes.

Each straight line intercept corresponds to the kinetic currentik. The intercept gives the order or absolute kinetic activity ofthe bimetallic surface for the ORR. We corrected the masstransport effect for each polarization curve by using eq 7 andthen plotting the curves in Tafel form.

From the Tafel plots depicted in the inset to Figure 9, it can beobserved that the mixed control appears at potentials of ca.-0.759 V, indicating that the kinetic evaluation of masstransport phenomena should be considered. In general, allbimetallic electrodes present two linear Tafel regions, withslopes of 64 and 127 mV decade-1 for the low and highoverpotential regions, respectively; see Table 3.

4.1. Correlation between the Nature of the Samples andTheir Performance in ORR. The ORR of the bimetallicPtAu/C samples exhibits two linear Tafel regions, both locatedin the range of potential where the Pt-Au surface is coveredby an oxide layer. This result indicates that the differentperformance of the bimetallic samples compared to the Pt/Cone is not due to changes in the oxygen coverage of theelectrodes. From the Tafel slope values it may be concludedthat the ORR mechanism is not modified by the presence ofAu in the samples. For polycrystalline Pt, the existence of thetwo Tafel slopes of ca. 60 and 120 mV decade-1 for ORR canbe explained in terms of the coverage of the electrode surfaceby adsorbed oxygen, following a Temkin isotherm at lowoverpotential and a Langmuir isotherm at high overpotential.The slope at low overpotential corresponds to the oxide-coveredPt region, whereas within the high overpotential region, the Ptsurface is free of oxide species.48

Figure 10. Levic-Koutechky plots for the Imp-PtAu/C sample: 0.7V (0); 0.65 V (9); 0.6 V (O); 0.55 V (b); 0.5 V (2).

1i

) 1ik

+ 1i f

+ 1iD

(4)

O2 + 2H+ + 2e- f H2O2 (5)

O2 + 4H+ + 4e- f H2O (6)

Figure 11. Linear sweep (1 mV/s) recorded in oxygen saturated in0.5 M H2SO4 at 2500 rpm for samples Pt/C (- - -), Imp-PtAu/C (-· -),Col-PtAu/C (···), Mem1-PtAu/C (bolds), and Mem2-PtAu/C (lights). The inset to the figure is the current density recorded at differentoverpotentials.

ik )iDi

iD - i(7)


The performance of the bimetallic catalysts and the Pt/Celectrode in the ORR recorded at 2500 rpm is depicted in Figure11. The current density has been normalized to the geometricarea of the electrode and the actual mass of the metal phases:Au plus Pt. The mass activities obtained expressed as mA cm-2

mg-1 are similar to those reported by other authors.49 The Imp-Au-Pt/C sample shows the highest mass activity for the ORRof the series in all polarization potentials studied. The inset toFigure 11 depicts the current densities obtained at two selectedpotentials (both adequate for PEMFC applications) for thedifferent catalysts studied. Clearly, the performance of the Imp-PtAu/C sample is superior to that of the rest of the series. Infact, the performance of the other PtAu/C samples is comparableto that of the commercial sample atE ) 0.7 V. Furthermore,when the current densities obtained were normalized to theactual metal content of the electrodes, the onset of the ORR ofthe Imp-PtAu/C sample compared well with that of the Pt/Celectrode, as depicted in Figure 11.

In acid media, the Au(111) surface is scarcely active as anelectrocatalyst for the ORR;48 indeed, polycrystalline Au is notcapable of providing adsorption sites for the nucleation of OHads

species. Such species, generated from the dissociation of waterat the Pt surface in acidic electrolytes, can be considered aspoisoning species in the ORR, since their presence reduces thenumber of active sites for the activation of oxygen viadissociative chemisorption, or splitting of the O-O bond.50 Ithas also been well documented that activity in the ORR dependson the electrolyte and on the crystal phase51 and on particlesize.52 Particle size effects cannot explain the trend towardoxophilicity of the samples, since all samples studied displayedsimilar particles sizes and distributions (see Figure 4). Neithercan the crystal phase be responsible, since the{111} directionwas slightly favored in all samples. Another factor to beconsidered is the interparticle bond distances. It has beenproposed that alloying Pt with metals such as Cr, Co, or Nireduces the Pt-Pt bond distance. It seems as though there couldbe an optimum Pt-Pt distance for the ORR at ca. 2.73 Å.53

For instance, the Pd-Pd distance on Pd3Fe/C nanoparticles isca. 2.73 Å, and hence, an increase in the ORR over such samplesis seen. However, results pointing otherwise have also beendescribed. Despite this, to the best of our knowledge there areno reports concerning the effect of expanding the intermetallicdistance. We observed that the maximum in activity cor-responded to a metal-metal distance of 2.8185 Å, actually thehighest value of the PtAu/C bimetallic samples, as depicted inTable 1. However, the trend was not fulfilled since the metal-metal distance of the less active sample among the alloyed ones,Mem1-PtAu/C, was larger than that of the Mem2-PtAu/Csample, which was more active. Nevertheless, such comparisonsshould be taken with caution since the particle sizes anddistributions for both microemulsion-prepared samples are ratherdifferent.

Thus, the superior activity of the Imp-PtAu/C bimetallicsample can be rationalized by taking two contributions intoaccount. On the one hand, this sample is the least oxophilic ofthe series. Thus, oxygen-free Pt sites might become availableat more positive potentials, thereby promoting O2 interactionswith the active sites. On the other hand, as deduced from theXPS data, the Pt content at the surface of the solid is the highestof the alloyed samples. Furthermore, the relative abundance ofPt0 species is augmented over this sample. Recently, anenhancement of the use of Pt in electrocatalysts by using Aunanoparticles has been reported.20 Such behavior is due to theappropriate dispersion of the Pt phase on Au rather than to a

tuning of the properties of Pt through interaction with the goldparticles. Nevertheless, the picture depicted here is different.Thus, we propose that the superior ability of the samples aselectrocatalysts would be due to the formation of PtAu alloysthat can tune the performance of the samples as electrocatalysts.

5. Conclusions

Even though Pt and Au are not miscible throughout theconcentration range, bimetallic PtAu-alloyed nanosized particlescan be prepared if the proper methodology is employed.Furthermore, alloyed particles can be prepared without applyingsevere thermal treatments, thus avoiding particle agglomeration.A key factor is that the nucleation of both Pt and Au ions occursconcomitantly. Preparation from a microemulsion affords PtAu-alloyed particles. Preparation by impregnation can also affordbimetallic PtAu particles, provided the appropriate route isemployed. However, colloidal techniques affording a preferentialencapsulation of one of the metal phases, Pt for the case reportedhere, would generate separate metal phases. The formation ofthe PtAu alloy yields materials with properties different fromthose of materials consisting of Pt and Au segregated phases.Thus, the nanosized PtAu-alloyed samples display uniqueproperties in the ORR probably due to a decrease in theoxophilicity of the samples due to the alloying. This result isvery important for the preparation of cathode catalysts for fuelcell applications.

Acknowledgment. S.R. acknowledges the Ramon y Cajalprogram of the Ministerio de Ciencia y Tecnologıá de Espanãfor financial support. The HIVELIO program, of the Ministeriode Ciencia y Tecnologıá de Espanã (Project ENE2004 07345c03 01/A) is also acknowledged for financial support. F.J.G.-G. acknowledges the Deutsche Forschungsgemeinchaft, via theSonderforschungsbereich 484, and the BMBF, via VDI/EKM,for economical support. P.H. acknowledges the FPI programof the Ministerio de Educacioń y Ciencia de Espanã for financialsupport.

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