Influence of the Preparation Route of Bimetallic Ptâ Au Nanoparticle

J. Phys. Chem. C 2007, 111, 2913-2923

2913

Influence of the Preparation Route of Bimetallic Pt-Au Nanoparticle Electrocatalysts for the Oxygen Reduction Reaction P. Herna´ ndez-Ferna´ ndez,†,‡ S. Rojas,*,‡ P. Oco´ n,† J. L. Go´ mez de la Fuente,‡ J. San Fabia´ n,† J. Sanza,‡ M. A. Pen˜ a,‡ F. J. Garcıá-Garcıá,§ P. Terreros,‡ and J. L. G. Fierro*,‡ Departamento de Quı´mica-Fı´sica Aplicada C-II, Campus UAM, 28049 Madrid, Spain, Instituto de Cata´ lisis y Petroleoquı´mica (CSIC), C/Marie Curie 2, 28049 Madrid, Spain, and Lehrstuhl fu¨r Festko¨rperchemie, Institut fu¨r Physiks, UniVersita¨t Augsburg, UniVersita¨tstrasse 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 thermal treatments are required that to provide aggregation phenomena. However, it is possible to synthesize bimetallic PtAu nanoparticles provided the proper synthesis route is employed. When they are prepared from waterin-oil microemulsions or with the impregnation technique, it is possible to obtain nanosized bimetallic PtAu particles. In contrast, other colloidal routes have been seen to be adequate for the synthesis of other bimetallic Pt-based particles, affording segregated samples with Pt- or Au-enriched zones. When alloyed, bimetallic PtAu nanoparticles display unique physicochemical properties that are different from those of monometallic and nonalloyed solids. Thus, the performance of alloyed PtAu samples as electrocatalysts for the oxygen reduction reaction is superior to that of the PtAu-segregated samples. In fact, the ability of carbon-supported bimetallic PtAu samples in the oxygen reduction reactions equals or even surpasses that of archetypal Pt/C electrocatalysts.

1. Introduction The synthesis and characterization of so-called nanomaterials (solid-state compounds with nanosized particles) is an area of increasing impact due to the unique properties expected for these solids. Among other applications, nanomaterials can be used as sensors, conducting/isolating materials, and catalysts. However, accurate understanding of the underlying physical and chemical properties associated with decreases in particle size remains a challenge. Within these frameworks, the preparation of nanosized Au nanoparticles in general and bimetallic PtAu in particular is currently attracting considerable attention. According to the binary phase diagram, there is a miscibility gap between Pt and Au, and hence, phase segregation can be expected.1 In fact, to obtain homogeneous PtAu samples, severe thermal treatments (beyond 1000 °C, depending on the nominal composition), generally yielding aggregated particles, are required. It has recently been shown that this issue can be overcome if bimetallic PtAu samples are prepared as nanostructured materials.2,3 In those reports, the preparation of PtAu bimetallic particles was achieved by following complicated synthetic routes, involving the use of dendrimer complexes in the former or phase-transfer agents and encapsulating agents together with thermal treatments in the latter. Other approaches for the preparation of PtAu alloys consist in the use of organometallic or coordination complex precursors.4-6 Pt to Au atomic stoichiometry is thus imposed from the available PtAu precursors. PtAu nanostructured samples are of outstanding interest in many respects. The catalytic performance of silica-supported * Corresponding authors. E-mail: [email protected] (S.R.); [email protected] (J.L.G.F.). † Campus UAM. ‡ Instituto de Cata ´ lisis y Petroleoquı´mica. § Universita ¨ t Augsburg.

PtAu nanoparticles has been studied intensively over the last decades, and in most cases phase segregation was observed.4,7-9 Nevertheless, it was the description of the high activity of nanosized gold particles toward CO oxidation that triggered interest in the preparation of Au-based catalysts.10,11 In fact, Au-based solids have been studied as potential electrocatalysts for fuel cell applications, in the oxidation of hydrogen and CO12-15 and that of alcohols,16,17 or even as electrocatalysts for the oxygen reduction reaction (ORR).18,19 Recently, it has been reported that PtAu samples prepared by impregnation afford performances similar to those of pure Pt samples.4 However, when Pt is deposited over Au, electrocatalysts displaying better activities have been described.20 Among the different synthesis routes employed for the preparation of electrocatalysts,21 the microemulsion technique has emerged as a promising route for the preparation of nanosized bimetallic particles, avoiding heat treatments.22-25 Recently, the preparation of Au nanoparticles and their performance as electrocatalysts on the oxygen reduction reaction have been reported.26 In this context the preparation of catalyst materials based on nanosized metallic particles is of special interest. For instance, both monometallic and/or bimetallic carbon-supported Pt-based solids have been studied in depth as electrocatalysts in fuel cell applications.27 Nonetheless, a serious drawback for the implementation 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 have been proposed as a promising alternative. Since the kinetics of the oxygen reduction reaction (ORR) is much slower than that of the anodic reaction,28 a reduction in the content of Pt and even the complete replacement of Pt in the cathode29,30,31 are fields of great interest in the area of materials development. PtAu samples have been tested as electrocatalysts for the oxygen

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

2914 J. Phys. Chem. C, Vol. 111, No. 7, 2007 reduction reaction, an enhancement in their activity over Auenriched samples being reported.32 A series of carbon-supported PtAu samples have been prepared with several techniques such as microemulsion, impregnation, and colloidal routes. The carbon-supported solids have been characterized by several techniques, affording the conclusion that for some preparations (microemulsion and impregnation) PtAu alloys have been obtained, whereas the colloidal route yields Pt and Au as separate phases. The development of such alloyed PtAu phases affords materials of performance superior to that of archetypal Pt/C electrocatalysts for the ORR. 2. Experimental Section 2.1. Preparation of Bimetallic Samples. The preparation of Pt and Pt-Au particles was carried out using different approaches such as microemulsion, impregnation, and a colloidal method typically employed for the preparation of PtRu particles.33 H4PtCl6 and HAuCl4 (both purchased from Alfa-Aesar) were used as metal precursors. For the microemulsion preparations, Mem1-PtAu/C and Mem2-PtAu/C, a water solution (5 w%) containing the appropriate amounts of the metal precursor 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 this point onward, two methodologies were employed: (i) For the preparation of the Mem1-PtAu/C sample, the microemulsion containing the metal precursors was added dropwise over a dispersion 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 %) containing hydrazine in excess was added. The mixture was stirred overnight. (ii) For the preparation of the Mem2-PtAu/C sample, a microemulsion (similar to the above one) containing hydrazine was added dropwise to the microemulsion containing the metal precursors. Once the reaction has been completed, the mixture was added dropwise over a dispersion 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 %). The mixture was stirred overnight. As from this step, the solids were recovered in a similar way. THF was added, and the mixture was allowed to decant overnight. The solid was recovered by filtration and centrifugation, washed thoroughly with a water/ethanol mixture, and dried at 100 °C for 12 h. For the impregnation method a water/2-propanol solution of the metal precursors was set to pH ) 4 with Na2CO3 (0.6 M). The mixture was added dropwise to a dispersion of the carbon support in water under reflux. Then, CH2O was added to the mixture and stirred for 3 h. The solid was recovered, washed with thrice-distilled water several times, and dried overnight at 100 °C. The solid thus obtained was designated Imp-Pt-Au/C A further Pt-Au sample was prepared as follows. Sodium bisulfite was added to a water solution of the Pt precursor. The mixture was set to pH ) 5 by the addition of NaCO3 (0.6 M). H2O2 was added to the mixture, and the final pH ()5) was implemented by NaOH addition. A water solution of the Au precursor was added under stirring. Then, carbon was added and the mixture was stirred for 1 h. Finally, H2 was bubbled through the mixture for 2 h. The solid was recovered, thoroughly washed with thrice-distilled water, and dried at 100 °C. The sample was designated Col-Pt-Au/C. The nomenclature, preparation details, and selected physical properties of the samples are shown in Table 1.

Hernańdez-Fernańdez et al. TABLE 1: Sample Labeling, Preparation Route, and Characterization Details of PtAu/C Samples sample

prep route

Mem1-PtAu/C Mem2-PtAu/C Imp-PtAu/C Col-PtAu/

microemulsiona microemulsiona impregnation colloidal

cell param (Å)

metal-metal dist (Å)

3.9733(14) 3.9629(23) 3.9860(17) 3.9084(25)b 4.0542(28)c

2.8096 2.8022 2.8185 2.7637b 2.8668c

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

2.2. Characterization. The Mem1-PtAu/C; Mem2-PtAu/C, Imp-PtAu/C, and Col-PtAu/C (20 wt % metal basis; Pt/Au, 2/1 nominal atomic ratio) samples were tested as electrocatalysts for the ORR. Their performance was compared to that of a commercial (Johnson Matthey) 40 wt % Pt/C electrocatalyst, henceforth denoted as Pt/C. Electrochemical testing was conducted in a conventional three-compartment electrochemical glass cell. The working electrode 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 reference electrodes, respectively. All potentials are quoted with respect to the reversible hydrogen electrode (RHE). During the measurements, a gentle flow of nitrogen or oxygen was maintained over the electrolyte surface. The samples under study were deposited onto the working electrode (glassy carbon disk electrode) by means of an ink. Typically, 5 mg of the solid, 30 µL of 5 wt % of Nafion solution (Aldrich), and 700 µL of Milli-Q water were dispersed in an ultrasonic bath for 45 min, obtaining a homogeneous ink. Before the deposition of the sample, the glassy carbon electrode was polished (0.05 µm alumina) to a mirror finish and rinsed with thrice-distilled water in an ultrasonic bath. A 30 µL volume of the ink (added in 3 consecutive steps involving 10 µL each) was dropped onto the electrode and dried at 75 °C for 5 min, resulting in a homogeneous coating. The final loading of Pt on each electrode was ca. 82 µg for the commercial sample and 27 µg of Pt for the bimetallic PtAu/C electrodes. A 0.5 M H2SO4 (Merck) solution was used as the electrolyte. All solutions were prepared with Milli-Q (Millipore) water. High-purity oxygen and nitrogen (Air Liquid) were used for solution saturation and deaeration, respectively. Cyclic and linear voltammetry and rotation disk polarization measurements were performed with a EG&G 273A potentiostat/ galvanostat controlled by a computer. The electrode rotation speeds were controlled by a Metrohm 628-10 unit. The rotation rate was varied from 500 to 2500 rpm, and the scan rate was 1 mV/s. Before the RDE study, the porous electrodes were cycled at 100 mV between 0.05 and 1.2 V until reproducible cyclic voltammograms were obtained. No marked changes in the shape and size of the voltammograms were observed. The Pt real surface areas of all catalysts were determined by CO stripping voltammetry. Typically, CO was flowed under stirring while the electrode was set at a constant potential of 20 mV for 15 min. CO was purged out of the electrolyte solution by bubbling Ar through it for 45 min. Then, three consecutive potentials scans between 0 and 1 V at a scan 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 as mA cm-2 mg-1metal. X-ray diffraction patterns were collected on a Seifert 3000 powder diffractometer, using Cu KR radiation. The display and

Bimetallic Pt-Au Nanoparticle Electrocatalysts

J. Phys. Chem. C, Vol. 111, No. 7, 2007 2915

handling of the diffraction patterns as well as unit cell refinements were carried out using the PowderCel software.34 Specimens for TEM analyses were prepared by dispersing the powder samples in butanol. One drop of the resulting suspension was placed on a holey carbon film supported by a copper grid. They were studied on a JEM 2100F device equipped with an EDAX detector for X-ray energy dispersive spectroscopy analysis. X-ray photoelectron spectra (XPS) of the samples were acquired with a VG ESCALAB 200R spectrometer fitted with a Mg KR (hν ) 1253.6 eV) 120 W X-ray source. Samples were pressed into small stainless-steel cylinders and then mounted on a sample rod, placed in a pretreatment chamber, and degassed at 25 °C and 10-5 bar for 5 h prior to transfer to the analysis chamber. Residual pressure was maintained below 3 × 10-9 bar. The 50 eV energy regions of the photoelectrons of interest were scanned a number of times to obtain an acceptable signalto-noise ratio. Intensities were estimated by calculating the integral of each peak, determined by subtraction of the Shirley type background and fitting of the experimental curve to a combination of Lorentzian and Gaussian lines of variable proportions. Accurate binding energies ((0.2 eV) were determined by referencing to the C 1s peak at 284.6 eV. 3. Results 3.1. Structural Characterization. The X-ray powder diffraction patterns of all samples are shown in Figure 1 a. For the sake of comparison, the range between 36 and 50° is enlarged 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 Mem1PtAu/C, Mem2-PtAu/C, and Imp-PtA/C samples, all reflections expected for an fcc lattice, corresponding to the structure of the pure bulk metals, are observed. In contrast, the diffractogram of 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. The refined unit cell parameters of all samples are depicted in Table 1. A careful comparison of the refined unit cell parameters with the 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 a single crystalline phase is observed and (ii) in the Col-PtAu/C sample phase separation occurs. To underline this in Figure 1b, the positions of the 111 reflections are projected onto the horizontal axis. The reflections for the Col-PtAu/C sample appear at positions fairly close to those expected for the bulk metals: 39.764° for Pt; 38.188° for Au. Accordingly, the two different phases in this sample seem to correspond to a situation where the metallic constituents condense independently without alloying. This result was further confirmed by the EDS experiments in the electron microscope; see below. The position of the diffraction peaks of the Mem1-PtAu/C, Mem2-PtAu/C, and Imp-PtAu/C samples indicates that the Pt-Au alloy follows a Vergard-type law. Taking into account that the nominal composition of the samples is Pt/Au ) 2/1, the 111 reflection should appear centered at 2θ ≈ 38.700°, which is in very good agreement with the experimental X-ray power diffraction patterns. Low-resolution TEM and high magnification images of the particles are presented in Figures 2 and 3, respectively. The metallic particles were found to be homogeneously dispersed in the supporting material, and no segregation was observed.

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

The histograms of particle size are shown in Figure 4. They were calculated by crystallographic image processing from images such as those shown in Figure 2. For each sample, between 180 and 200 particles were analyzed. As depicted in Figure 2a,b, the Mem2-PtAu/C and Imp-PtAu/C samples display similar characteristics. Additionally, their particle size distribution as depicted in Figure 4a,b are similar. Both samples display a sharp peak centered at ca. 3 nm. Moreover, the Mem2-PtAu/C sample shows a broad band (25-nm width) centered at ca. 30 nm. The Imp-PtAu/C sample displays a similar pattern; only the broad band extends to higher particle size values. For those samples, 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. These crystallographic planes seem to be slightly favored in the crystal growth process. The particle size distribution of the Mem1PtAu/C sample, Figure 4d, shows a maximum centered at 3 nm. Interestingly, the metallic particles had developed a rodlike shape 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 sample Col-PtAu/C. The presence of large spherical particles constitutes

2916 J. Phys. Chem. C, Vol. 111, No. 7, 2007

Hernańdez-Fernańdez et al.

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

the major difference with respect to the other samples; see Figure 2c. Nanosized metallic particles were also formed, as seen in Figure 3. In fact, the smallest particles among all the samples studied were observed for the Col-PtAu/C sample. Their shape is fairly irregular, but a tendency to grow in a direction parallel to {111} may also be inferred. The transmission electron microscope used in the present investigation is equipped with a field emission gun unit. It is therefore possible to carry out compositional analyses with an electron beam of nominally just a few angstroms in diameter. The results are consistent with the formation of Pt-Au alloy in the Mem1-PtAu/C, Mem2PtAu/C, and Imp-PtAu/C samples, but this was not the case in the Col-PtAu/C sample. In this sample, Au and Pt were detected as separate phases, Au only being detected in the largest spherical particles; see Figure 2. However, not all of such large particles were composed by Au; some of them were composed of Pt alone. The XPS spectra of the Pt 4f and Au 4f core-level regions are depicted in Figure 5. The relative abundances of the different species detected by the XPS analysis and the surface atomic ratios are shown in Table 2. It may be observed that the surface composition depends on the samples, which in some cases are very different from the expected value, i.e., Ptat/Auat ) 2. To a lesser extent, the same applies to the bulk value obtained from ICP analysis (results shown in Table 2). For instance the ICP analysis for Col-PtAu/C samples afforded a Ptat/Auat ) 2, close to the theoretical value. However, Au was scarcely detected from the XPS, revealing either a Pt-enriched surface or a rather heterogeneous composition of the sample. To elucidate which explanation 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 traces of Au were detected, confirming that Au is not present in the inner layers of the solid. Instead, the sample was quite heterogeneous, displaying Pt- and Au-enriched zones. 3.2. Electrochemical Characterization. Steady-state cyclic voltammograms of the Pt/C and the bimetallic PtAu/C samples are shown in Figure 6. In general, the hydrogen adsorption/ desorption charge for the Pt-Au/C samples is lower than that of the Pt/C sample (results not shown). Owing to the porous nature of the supporting material, the charge due to the doublelayer charging is rather high. This feature is very important for the Col-PtAu/C sample. Furthermore, it overlaps that of the adsorption of hydrogen in acid solution.37 Therefore, the specific surface area (active surface area/mass of Pt on the electrode) was evaluated from the CO stripping analysis; the results are shown in Table 3. Figure 7 depicts the first and the second cycle recorded during the CO stripping analysis of the PtAu samples. Note that the features of the hydrogen adsorption are clearly observed in the second voltammetry cycle in good agreement with total CO oxidation during the first cycle. 3.3. Oxygen Reduction Reaction (ORR). Polarization curves for the oxygen reduction reaction (ORR) were obtained in 0.5 M H2SO4 on a thin porous coating rotating disk electrode prepared with Vulcan XC-72 carbon. The curves were recorded in the cathodic sweep direction at 1 mV/s from 1.15 to 0.0 V over a range of rotation (500-2500 rpm). As expected, the Vulcan XC-72 electrode lacked any activity in the ORR, only around 0.2 V a certain cathodic current being observed. The ORR is diffusion-controlled at potentials that are more negative than 0.5 V; a mixed diffusion-kinetic control is expected in the potential region between 0.5 and 0.8 V. At more positive



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} direction is indicated.

potentials, kinetic control dominates. Figure 8 shows the results obtained for the Imp-PtAu/C sample at selected rotation rates. For all samples, the current densities (the current normalized to the geometric area of the electrode) are proportional to ω1/2, 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. Under the same conditions, the onset of the ORR for the Pt/C is sample located at 0.987 V. That is, the Imp-PtAu/C sample displays an overpotential of 40 mV. The thermodynamic value of the ORR in acid medium is 1.185V vs RHE.38 All electrocatalysts showed increasing diffusion-limiting currents with electrode rotation; however, at a given rotation rate, all samples reached a similar limiting current, as depicted in Figure 9. 4. Discussion From the characterization results, it may be inferred that a single PtAu bimetallic alloyed phase was obtained from the microemulsion and impregnation preparations, whereas the colloidal method afforded separate Pt and Au phases. To rationalize this result, an in-depth discussion of the different preparations is needed. In a microemulsion, water droplets are stabilized within an organic medium by the presence of a surfactant.22,39 Although stable at macroscopic level, droplet coalescence phenomena take place continuously. During such coalescence, water droplets interact with others, interchanging their content, i.e., the metal precursor. After equilibrium has been reached, all the droplets of the microemulsion will contain

similar amounts and ratios of both the Pt and Au metal precursors. The addition of hydrazine triggers particle nucleation and growth. It has been well described that the nucleation and growth of particles is hindered by the presence of the surfactant within microemulsions, actually favoring the formation of a larger number of practically identical nuclei containing both Pt and Au. In light of these results, it seems that particle formation occurs preferentially within such a homogeneous and stabilized (due to the presence of the surfactant) environment as a microemulsion. Whereas particle nucleation is somehow delayed within microemulsions, the opposite scenario might be expected from particle growth in homogeneous media. However, the preparation by impregnation afforded completely alloyed PtAu particles. This result is somewhat surprising since in a similar preparation for silica-supported PtAu samples both alloyed and segregated Pt and Au phases were detected.4 Nevertheless, the synthetic procedure reported in here is slightly different. First, different metal precursors were employed, and more importantly, the pH of the solution was controlled accurately. In contrast, the preparation route of the Col-PtAu/C sample led to the formation of isolated Pt and Au phases. Although a similar method has been proposed for the preparation of PtRu bimetallic phases,33 this route seems to afford individual metal phases with no interaction between each other, at least not in the case of Pt Au. In this synthetic approach, Pt nanoparticles are stabilized because a bisulfite core is formed before Au is added. In alkaline media Au reduction proceeds very fast, possibly even before it enters into contact with the Pt particle



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) ImpPtAu/C; (b) Mem1-PtAu/C; (c) Mem2-PtAu/C; (d) Col-PtAu/C.

precursor, which is still present as an ion, thereby preventing the simultaneous nucleation (both in time and place) of the Pt and Au particles. We also observed certain morphological differences between the samples: see Figures 2 and 3. The Mem1-PtAu/C sample consists 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-shaped particles. As stated before, during the preparation of particles from reversed micelles water droplets surrounded by an organic phase and stabilized by the presence of a surfactant are formed. Therefore, a similar scenario could be expected for preparation of the Mem2-PtAu/C sample, thus yielding similarly shaped particles. However, prior to the nucleation of the Mem1-PtAu/C particles, the microemulsion enters into contact with the dispersion containing the carbon support. It is not unlikely that the shape of the micelles would change due to the change in the relative composition of the different phases. That is, a transition would occur from the spherical micelles expected within a microemulsion to the large anisotropic micelles expected in liquid crystals.40 Thus, once hydrazine had been added and metallic particles had actually been formed, their shape resembled that of micelles containing Pt and Au salts, as rodlike micelles. The preparation of sample Mem2-PtAu/C must be somewhat different since particles were actually formed within the original microemulsion environment prior to entering into contact with the dispersion containing the support. Therefore, the change in the relative abundance of the different liquid phases would not affect the shape of the metallic particles. Nevertheless, these changes only seemed to affect particle shape and not the particle composition. In both cases Pt-Au bimetallic particles were obtained. However, a certain peak broadening is observed from the diffractogram of sample Mem2-PtAu/C. This feature might indicate that even if the majority of the particles have developed a bimetallic nature, the extension of the alloying process might not be complete. Again, the Col-PtAu/C sample behaved in a way different from the rest of the series. Large Au particles together with nanosized Pt particles coexisted within the solid, as may be observed from Figure 2. This feature is in good agreement with



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) 72.80(36) 74.73(14) 71.21(55) 72.49(26) 73.94(19) 71.47(43) 72.73(35) 74.48(22) 71.22(57) 72.65(29) 74.69(14)

83.88(65) 85.33(35)

1.6

0.0098

0.0047

2.7

83.88

8.2

0.0043

0.0002

3.2

Mem2-PtAu/C Col-PtAu/C Imp-PtAu/C

0.0187 83.92(70) 85.34(30)

the XPS and XRD analyses. Au was detected as a segregated phase 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 CO stripping analysis. Besides obtaining the surface area of Pt, it can provide information about the nature of the nanoparticles. The CO oxidation profile (stripping analysis) was not altered by the presence of Au as compared to Pt/C. Neither did it seem to affect whether actual PtAu-alloyed bimetallic particles or whether Pt and Au segregated phases were formed instead. However, greater differences were found when the voltammograms of the sample were recorded in H2SO4, as depicted in Figure 6. All samples displayed the features of the hydrogen adsorption-desorption region between 0 and 0.35 V, followed by the “double-layer” potential region. At potentials of >0.75 V OH adsorption followed by oxide formation was observed. The contribution of the double-layer charging was considerably larger for the Col-PtAu/C sample than for the other samples. Careful comparison of the oxide reduction region revealed a shift of ca. 30 mV toward more negative potentials for the microemulsion-prepared samples than for the Imp-PtAu/C sample. The peak for the Col-PtAu/C sample was centered at

2.2

0.0151

2 0.0058

2.2

the most negative potential of the series. This shift can be ascribed to the oxophilicity of the samples.41 The increase in oxophilicity favors the formation of OH species, which block the sites responsible for O2 adsorption. This indicates that Pt oxide reduction is slightly favored in the Imp-PtAu/C samples as compared to the microemulsion method (Mem2-PtAu/C > Mem1-PtAu/C) based samples. Except for the Col-PtAu/C sample, the rest of the PtAu/C bimetallic samples exhibited a second oxide reduction peak. This feature has already been reported for PtAu alloys.42 The position of this second peak, ca. 1.0 V vs RHE, is consistent with PtAu-alloyed samples displaying Pt/Au > 1 atomic stoichiometry. The potential region where this peak appears is magnified in the inset to Figure 6. From the XPS analysis, a slight electronic interaction can be deduced. The Pt 4f core-level binding energy of the Pt0 species was shifted to a lower binding energy for the PtAu bimetallic samples as compared to position displayed by the Pt0 species of Col-PtAu/C. The magnitude of the shift is ca. 0.3 eV, and the trend is observed throughout the series. Furthermore, the position of the peaks is consistent with the results of work already published in the literature; i.e., displaying the Pt 4f corelevel of the Pt0 species in Pt/C samples at 71.5 eV.43 However, the opposite trend has been reported, Pt being shifted to higher binding energies44 (ca. 0.4-0.5 eV). Nevertheless, the latter result refers to Au overlayers over Pt(100) rather than to alloyed PtAu nanoparticles. In fact, it is known that whereas Au in the bulk form is the most electronegative of all metals, the electronegativity of nanosized Au clusters might be different.45 The surface of the Mem2-PtAu/C sample was enriched in Pt as may be deduced from the XPS analyses, the relative atomic ratio of the metal phase being Ptat/Auat ) 8.4. The Mem1PtAu/C and Imp-PtAu/C showed samples a Ptat/Auat surface ratio of 1.6 and 2.2, respectively, close to the expected value. The performance of the samples as electrocatalysts in the ORR was evaluated by means of polarization techniques. The limiting currents are depicted in Figure 8. An important parameter for characterizing the performance of samples is the TABLE 3: Kinetic Parameters (ORR) of PtAu Bimetallic Samples Tafel slope

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

sample

EAAa (m2/gPt)

nb

low current

high current

Mem1-PtAu/C Mem2-PtAu/C Col-PtAu/C Imp-PtAu/C

11.5 8.0 19.1 22.3

3.5 3.7 4.0 4.0

59 61 67 65

142 122 126 120

a Electrocatalytically active area as determined from the CO stripping analysis recorded in 0.5 M HClO4. b Number of electrons involved in the ORR (see eqs 5 and 6). The EAA of the Pt/C sample is 32.3 m2/gPt



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

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

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

iD ) 0.62nFACD2/3ν-1/6ω1/2

(1)

or in its short form

iD ) Bω1/2

(2)

where n is the number of electrons transferred for each O2 molecule, F is Faraday’s constant, A is the geometric electrode surface, C is the concentration of O2 dissolved in the solution, D is the diffusion coefficient of oxygen, υ is the kinematic viscosity of the solution, and ω is the angular rotation rate of RDE. Under mass transfer-limiting conditions, B, the Levich slope, is constant. However when the cathodic limiting current densities where plotted vs ω1/2, a slightly curved line was obtained, presumably due to a slight mass transport limitation in the Nafion matrix. The inverse Levich plots, i.e., i-1 vs ω-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:

1 1 1 ) + ) f(ω-1/2)E i ik Bω1/2

(3)

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

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



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

covering the electrocatalyst sites. This situation would be analogous to a smooth polymer film-covered electrode. The relationship between current and the rotation rate for a filmcovered electrode is as follows:

1 1 1 1 ) + + i ik if iD

(4)

where if is the limiting diffusion current in the film. Thus, the intercept of i-1 vs ω-1/2 plot cannot be zero even if the kinetic current is extremely high. For the PtAu samples reported here, the kinetic current (ik) is low in comparison with the values in the mixed kinetic-diffusion control region; therefore its contribution to the total current (i) can be neglected. The plot of the inverse of the current with the potential in the mixed kinetic-diffusion control region afforded parallel lines, as depicted in Figure 10. The number of electrons transferred was constant within the potential range. The actual number of electrons transferred by each O2 molecule was calculated by assuming 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, and normalizing the number of electrons to a value of 4 for the Pt/C sample. A first insight into the efficiency of the electrocatalysts can be obtained from the value of n. Thus, values of n close to 4 were obtained for all the PtAu/C bimetallic samples. The results are shown in Table 3. These values are higher than those reported for Au/C and Au-Pt/C (3:1) ratios in acid solution of 2.2 and 2.9, respectively.47 The ORR may proceed either partially, in a two-electron-transfer process, or completely, in a four-electron-transfer process, yielding H2O2 and H2O, respectively.

O2 + 2H+ + 2e- f H2O2

(5)

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

O2 + 4H+ + 4e- f H2O

(6)

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

Figure 11. Linear sweep (1 mV/s) recorded in oxygen saturated in 0.5 M H2SO4 at 2500 rpm for samples Pt/C (- - -), Imp-PtAu/C (- · -), Col-PtAu/C (···), Mem1-PtAu/C (bold s), and Mem2-PtAu/C (light s ). The inset to the figure is the current density recorded at different overpotentials.

reaction, the higher the electrocatalytic activities expected. Furthermore, the linearity of the plots in Figure 10 implies a first-order dependence of the O2 kinetics on the PtAu/C electrodes. Each straight line intercept corresponds to the kinetic current ik. The intercept gives the order or absolute kinetic activity of the bimetallic surface for the ORR. We corrected the mass transport effect for each polarization curve by using eq 7 and then plotting the curves in Tafel form.

ik )

iDi iD - i

(7)

From the Tafel plots depicted in the inset to Figure 9, it can be observed that the mixed control appears at potentials of ca. -0.759 V, indicating that the kinetic evaluation of mass transport phenomena should be considered. In general, all bimetallic electrodes present two linear Tafel regions, with slopes of 64 and 127 mV decade-1 for the low and high overpotential regions, respectively; see Table 3. 4.1. Correlation between the Nature of the Samples and Their Performance in ORR. The ORR of the bimetallic PtAu/C samples exhibits two linear Tafel regions, both located in the range of potential where the Pt-Au surface is covered by an oxide layer. This result indicates that the different performance of the bimetallic samples compared to the Pt/C one is not due to changes in the oxygen coverage of the electrodes. From the Tafel slope values it may be concluded that the ORR mechanism is not modified by the presence of Au in the samples. For polycrystalline Pt, the existence of the two Tafel slopes of ca. 60 and 120 mV decade-1 for ORR can be explained in terms of the coverage of the electrode surface by adsorbed oxygen, following a Temkin isotherm at low overpotential and a Langmuir isotherm at high overpotential. The slope at low overpotential corresponds to the oxide-covered Pt region, whereas within the high overpotential region, the Pt surface is free of oxide species.48

2922 J. Phys. Chem. C, Vol. 111, No. 7, 2007 The performance of the bimetallic catalysts and the Pt/C electrode in the ORR recorded at 2500 rpm is depicted in Figure 11. The current density has been normalized to the geometric area 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 ImpAu-Pt/C sample shows the highest mass activity for the ORR of the series in all polarization potentials studied. The inset to Figure 11 depicts the current densities obtained at two selected potentials (both adequate for PEMFC applications) for the different catalysts studied. Clearly, the performance of the ImpPtAu/C sample is superior to that of the rest of the series. In fact, the performance of the other PtAu/C samples is comparable to that of the commercial sample at E ) 0.7 V. Furthermore, when the current densities obtained were normalized to the actual metal content of the electrodes, the onset of the ORR of the Imp-PtAu/C sample compared well with that of the Pt/C electrode, as depicted in Figure 11. In acid media, the Au(111) surface is scarcely active as an electrocatalyst for the ORR;48 indeed, polycrystalline Au is not capable of providing adsorption sites for the nucleation of OHads species. Such species, generated from the dissociation of water at the Pt surface in acidic electrolytes, can be considered as poisoning species in the ORR, since their presence reduces the number of active sites for the activation of oxygen via dissociative chemisorption, or splitting of the O-O bond.50 It has also been well documented that activity in the ORR depends on the electrolyte and on the crystal phase51 and on particle size.52 Particle size effects cannot explain the trend toward oxophilicity of the samples, since all samples studied displayed similar particles sizes and distributions (see Figure 4). Neither can the crystal phase be responsible, since the {111} direction was slightly favored in all samples. Another factor to be considered is the interparticle bond distances. It has been proposed that alloying Pt with metals such as Cr, Co, or Ni reduces the Pt-Pt bond distance. It seems as though there could be an optimum Pt-Pt distance for the ORR at ca. 2.73 Å.53 For instance, the Pd-Pd distance on Pd3Fe/C nanoparticles is ca. 2.73 Å, and hence, an increase in the ORR over such samples is seen. However, results pointing otherwise have also been described. Despite this, to the best of our knowledge there are no reports concerning the effect of expanding the intermetallic distance. We observed that the maximum in activity corresponded to a metal-metal distance of 2.8185 Å, actually the highest value of the PtAu/C bimetallic samples, as depicted in Table 1. However, the trend was not fulfilled since the metalmetal distance of the less active sample among the alloyed ones, Mem1-PtAu/C, was larger than that of the Mem2-PtAu/C sample, which was more active. Nevertheless, such comparisons should be taken with caution since the particle sizes and distributions for both microemulsion-prepared samples are rather different. Thus, the superior activity of the Imp-PtAu/C bimetallic sample can be rationalized by taking two contributions into account. On the one hand, this sample is the least oxophilic of the series. Thus, oxygen-free Pt sites might become available at more positive potentials, thereby promoting O2 interactions with the active sites. On the other hand, as deduced from the XPS data, the Pt content at the surface of the solid is the highest of the alloyed samples. Furthermore, the relative abundance of Pt0 species is augmented over this sample. Recently, an enhancement of the use of Pt in electrocatalysts by using Au nanoparticles has been reported.20 Such behavior is due to the appropriate dispersion of the Pt phase on Au rather than to a

Hernańdez-Fernańdez et al. tuning of the properties of Pt through interaction with the gold particles. Nevertheless, the picture depicted here is different. Thus, we propose that the superior ability of the samples as electrocatalysts would be due to the formation of PtAu alloys that can tune the performance of the samples as electrocatalysts. 5. Conclusions Even though Pt and Au are not miscible throughout the concentration range, bimetallic PtAu-alloyed nanosized particles can be prepared if the proper methodology is employed. Furthermore, alloyed particles can be prepared without applying severe thermal treatments, thus avoiding particle agglomeration. A key factor is that the nucleation of both Pt and Au ions occurs concomitantly. Preparation from a microemulsion affords PtAualloyed particles. Preparation by impregnation can also afford bimetallic PtAu particles, provided the appropriate route is employed. However, colloidal techniques affording a preferential encapsulation of one of the metal phases, Pt for the case reported here, would generate separate metal phases. The formation of the PtAu alloy yields materials with properties different from those of materials consisting of Pt and Au segregated phases. Thus, the nanosized PtAu-alloyed samples display unique properties in the ORR probably due to a decrease in the oxophilicity of the samples due to the alloying. This result is very important for the preparation of cathode catalysts for fuel cell applications. Acknowledgment. S.R. acknowledges the Ramon y Cajal program of the Ministerio de Ciencia y Tecnologıá de Espanã for financial support. The HIVELIO program, of the Ministerio de Ciencia y Tecnologıá de Espanã (Project ENE2004 07345 c03 01/A) is also acknowledged for financial support. F.J.G.G. acknowledges the Deutsche Forschungsgemeinchaft, via the Sonderforschungsbereich 484, and the BMBF, via VDI/EKM, for economical support. P.H. acknowledges the FPI program of the Ministerio de Educacioń y Ciencia de Espanã for financial support. References and Notes (1) Villars, P.; Calvert, L. D. Pearson’s Handbook of Crystallographic Data for Intermetallic Phases; ASM: Materials Park, OH, 1991. (2) Lang, H.; Maldonado, S.; Stevenson, K. J.; Chandler, B. D. J. Am. Chem. Soc. 2004, 126, 12949. (3) Luo, J.; Maye, M. M.; Petkov, V.; Kariuki, N. N.; Wang, L.; Njoki, P.; Mott, D.; Lin, Y.; Zhong, Ch-J. Chem. Mater. 2005, 17, 3086. (4) Mihut, C.; Descrome, C.; Duprez, D.; Amiridis, M. D. J. Catal. 2003, 212, 125. (5) Chandler, B. D.; Schabel, A. B.; Blanford, C. F.; Pignolet, L. H. J. Catal. 1999, 367. (6) Chandler, B. D.; Rubenstein, L. I.; Pignolet, L. H. J. Mol. Catal., A 1998, 133, 267. (7) Balakrishnan, K.; Sachdev, A.; Schwank, J. J. Catal. 1990, 121, 441. (8) Sachdev, A.; Schwank, J. J. Catal. 1989, 120, 353. (9) Shen, J.; Hill, M. R.; Watwe, M.; Podkolzin, S. G.; Dumesic, J. A. Catal. Lett. 1999, 60, 1. (10) Haruta, M. Catal. Today 1997, 36, 153. (11) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (12) Schmidt, T. J.; Stamenkovic, V.; Markovic, N. M.; Ross, P. N., Jr. Electrochim. Acta 2003, 48, 3823. (13) Friedrich, K. A.; Henglein, F.; Stimming, U.; Unkauf, W. Electrochim. Acta 2000, 45, 3283. (14) Schmidt, T. J.; Jusys, Z.; Gasteiger, H. A.; Behm, R. J.; Endruschat, U.; Bonnemann, H. J. Electroanal. Chem. 2001, 501, 132. (15) Blizanac, B. B.; Arenz, M.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2004, 126, 10130. (16) Tremiliosi-Filho, G.; Gonzalez, E. R.; Motheo, A. J.; Belgsir, E. M.; Le´ger, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 31. (17) Borkowska, Z.; Tymosiak-Zielinska, A.; Shul, G. Electrochim. Acta 2004, 49, 1209.

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