C Catalyst for the

J. Phys. Chem. C 2007, 111, 5605-5617

5605

Design and Preparation of Highly Active Pt-Pd/C Catalyst for the Oxygen Reduction Reaction Huanqiao Li,†,‡ Gongquan Sun,*,†,§ Na Li,| Shiguo Sun,† Dangsheng Su,⊥ and Qin Xin†,# Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, Graduate School of the Chinese Academy of Sciences, Beijing 100039, China, Laboratory of Fuel Cell, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, College of Chemistry and Chemical Engineering, Liaoning Normal UniVersity, Dalian 116029, China, Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, ELCASS, European Laboratory of Catalysis and Surface Science, Faradayweg 4-6, D-14195 Berlin, Germany, and State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ReceiVed: NoVember 22, 2006; In Final Form: January 31, 2007

ORR (Oxygen Reduction Reaction) has been studied on Pt-Pd/C catalyst both experimentally and theoretically to gain insight into the promotion effect of PtPd nanoclusters to ORR and to provide guidelines for the design of an improved ORR catalyst. First, Pt3Pd1/C, Pt1Pd1/C, and Pt/C catalysts were prepared by the polyol process in ethylene glycol solution and characterized by XRD (X-ray diffraction), TEM (transmission electron microscope), and CV (cyclic voltammetry) techniques. XRD patterns reveal that all the catalysts have disordered face-centered cubic structures similar to the commercial Pt/C catalyst. Low-resolution TEM images indicate that the dispersion of the metal nanoparticles on the carbon support is uniform and in a narrow particle size range for Pt3Pd1/C and Pt/C catalysts, while for Pt1Pd1/C catalyst, the dispersion of the metal nanoparticles on the carbon support is uneven with a little agglomeration. Point resolved EDS (energy dispersive X-ray spectroscopy) of the individual particles have shown that both Pt and Pd are represented in the single nanoparticle for the Pt-Pd/C catalysts. CV tests show that Pd-containing catalysts of Pt3Pd1/C and Pt1Pd1/C have different features in the hydrogen adsorption-desorption region from pure Pt/C catalyst. The catalytic activity of Pt3Pd1/C for ORR is a little improved compared with Pt/C or Pt1Pd1/C catalyst in RDE (rotating disk electrode) measurement. Detailed work reveals that the activity of ORR could be further improved on Pt-Pd/C catalyst with Pt rich on the metal nanoparticles surface. This point is consistent with the theoretical calculation results. DFT (Density Function Theory) studies on the adsorption and dissociation of O2 on PtPd cluster indicate that the presence of Pd atoms facilitates the dissociation of O2 on Pt sites.

Introduction The electroreduction of oxygen has been extensively studied owing to its important role as the rate-determining reaction in corrosion processes and electrochemical energy conversion devices such as fuel cells.1-3 One of the challenging problems involving the oxygen reduction reaction (ORR) is to find out an effective electrocatalyst to accelerate this reaction with low over-potential at low temperature. Numerous studies in the literature over the past few years have demonstrated that Ptbased binary or multimetallic alloy catalysts exhibit enhanced activity to ORR by a factor of 1.5 to 3 in comparison with pure Pt in acidic solution.4-10 The improvement has been ascribed to several factors such as electronic and structural effects. Jalan and Taylor suggested that the enhanced electrocatalytic activity * Address correspondence to this author. E-mail: [email protected]. Phone/fax: +86-411-84379063. † Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics. ‡ Graduate School of the Chinese Academy of Sciences. § Laboratory of Fuel Cell, Dalian Institute of Chemical Physics. | College of Chemistry and Chemical Engineering, Liaoning Normal University. ⊥ Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society. # State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics.

was due to the contraction of the Pt-Pt interatomic distance by adding a smaller transition metal atom.4 Ross and Paffet commented that the roughening of Pt surface due to the addition of the transition metal could account for the ORR improvement.5,6 Mukerjee and Min explained this improvement by the increment in the Pt d-band vacancy and a favorable Pt-Pt mean interatomic distance.7,8 Wantanabe showed that the leaching out of the base metal from Pt alloy during operation in acidic electrolytes, while leaving a thin Pt layer of about 1 nm on the surface, was responsible for the activity enhancement for ORR.9 Tseung ascribed the significant improvement in the performance of Pt-Fe/C for ORR to the higher H2O2 decomposition activity in the presence of the redox couple of the transition metal leaching out from the catalyst in acidic solution.10 Self-consistent periodic density functional theory (DFT) calculations on the adsorption and dissociation of O2 on Pt, Pt-Co, and Pt-Fe alloy demonstrated that the presence of Co (or Fe) atoms facilitated the dissociation of O2 on the Pt(111) face.11 Recently, it has been reported that monolayer Pt deposited on Pd(111) could enhance ORR activity greatly with very low Pt loading.12,13 Compared with Pt-M (M: base metals) catalysts, the stability of the Pt-Pd catalyst could be improved due to the stability of Pd in an acidic environmental. Another advantage of Pt-Pd catalyst over Pt-M is its high selectivity to ORR in the presence

10.1021/jp067755y CCC: $37.00 © 2007 American Chemical Society Published on Web 03/24/2007

5606 J. Phys. Chem. C, Vol. 111, No. 15, 2007 of methanol, which makes it an improved methanol tolerant cathode catalyst in direct methanol fuel cells (DMFCs).14,15 To explore the promotion effect of Pt-Pd/C catalyst to ORR, several Pt-Pd/C catalysts with different Pt-to-Pd atomic ratios were synthesized and their activities to ORR in comparison with Pt/C catalyst were investigated in the present work. Meanwhile, a hybrid density functional theory (DFT) study on the adsorption and dissociation of O2 on the bimetallic Pt-Pd clusters was also carried out to elucidate this enhancement. Experimental Details Catalyst Preparation. Pt-Pd/C catalysts were prepared by a modified polyol process described in detail in previous papers with the precursors of hexahydrated chloroplatinic acid (H2PtCl6‚6H2O) and palladium(II) chloride (PdCl2).15-17 In a typical process, a Pt-Pd/C catalyst with a nominal atomic Pt/Pd ratio of 3/1, denoted as Pt3Pd1/C, was prepared as follows. Vulcan XC-72R carbon black (500 mg) was pretreated with 5 M HCl and 2 M HNO3 solution before being suspended in 50 mL of ethylene glycol. Appropriate amounts of H2PtCl6‚6H2O (279 mg, 0.54 mmol) and PdCl2 (32 mg, 0.18 mmol) aqueous solution were added dropwise to carbon suspension under ultrasonic stirring. The pH scale of the reaction mixture was adjusted to 12-13, using NaOH aqueous solution (1 mol L-1). The reduction reactions were performed by heating the mixture at 130 °C for 3 h, during which high purity argon gas was passed through the reaction system to remove the dissolved oxygen. Subsequently, the resulting powder was washed with hot distilled water to remove chloride until no chloride anion in the filtrate was detected with AgNO3 solution (1 mol L-1). The obtained catalyst was dried in a vacuum oven at 70 °C overnight. Pt1Pd1/C catalyst with a nominal atomic Pt/Pd ratio of 1/1 was also obtained by co-reducing the Pt and Pd precursors together according to the above procedure. For comparison, the Pt/C catalyst was also synthesized in the same way. The total metal loading (Pt + Pd) was maintained at 20 wt % for all the catalysts. To explore the ORR behavior on Pt-Pd/C catalyst with different microstructures, we obtained three Pt1Pd1/C catalysts with different microstructures by adjusting the catalyst preparation process. Pt1Pd1/C catalyst with Pt (or Pd) rich on the nanoparticle surface was obtained by adopting the successive reduction method. For the preparation of Pt1Pd1/C with Pt rich on the nanoparticles surface, Pd/C catalyst was first prepared in the modified polyol process, and then Pt precursor was added successively and deposited on the preformed Pd nanoparticles at 130 °C for 2 h. By a sequence of filtration, washing, and drying in a vacuum oven, the sample was denoted as Pt1Pd1/ C-Pd(f). Similarly, Pt1Pd1/C catalyst with Pd rich in the nanoparticles surface was prepared by reducing Pt precursor in a polyol process at first, and then Pd was reduced successively on Pt nanoparticles. Pt1Pd1/C-(co) with co-reducing the Pt and Pd precursors together was presented here for comparison. X-ray Diffraction (XRD) Characterization. The characteristics of the crystalline structure of the supported catalysts were determined by using the powder XRD technique. The data were obtained by using a Rigaku X-3000 diffractometer with a Cu KR radiation source and a Ni filter. About 30 mg of catalyst powders was pressed into the quartz block, using a glass slide to obtain a uniform distribution. The 2θ Bragg angles were scanned over a range of 15° to 85° at a rate of 5 degree min-1 with a 0.02 ° angular resolution. The tube current was 100 mA and the tube voltage was 40 kV. Scherrer and Bragg formula were employed to calculate the mean diameter and the lattice parameter of the catalysts.18

Li et al. Transmission Electron Microcope (TEM) and Energy Dispersive X-ray Spectroscopy (EDS) Measurements. The morphology and dispersion of the catalysts were examined by TEM on a JEOL JEM-2000EX electron microscope operated at 100 kV with a magnification of 25 0000×. A Philips CM200 FEG electron microscope, operating at 200 kV and equipped with a Gatan GIF100 imaging filter, was used for high-resolution TEM observation (HRTEM). Point resolved EDX analysis was performed by using the same high-resolution microscope with a DX4 analyzer system to obtain the metal composition in the binary catalysts of Pt-Pd/C catalyst. For all the microscopic examinations, about 5 mg of sample was first ultrasonically dispersed in ethanol solution for a few minutes and then one drop of the catalyst ink was deposited on a copper grid covered by carbon film. The particle size distribution of the metal nanoparticles of the supported catalysts was obtained by directly measuring the size of 400 randomly chosen particles in the TEM images. Electrochemistry Tests. A CHI 760B potentistat/galvanostat was used for the electrochemistry measurements in a standard three-compartment electrochemical cell. The working electrode was a glass carbon disk with a diameter of 4 mm held in a Teflon cylinder. A Pt-foil and a saturated calomel reference electrode (SCE) connected to the electrochemical cell by a KNO3 salt bridge were used as the auxiliary and reference electrode, respectively. All potentials in this work are referred to normal hydrogen electrode (NHE). Cyclic voltammetry (CV) experiments were carried out at 25 ( 0.5 °C in 0.5 M HClO4 solution saturated with high-purity nitrogen. A thin porous coating disk electrode design has been described in previous work.19 Five milligrams of catalyst was ultrasonically suspended with 2 mL of ethanol and 50 µL of 5 wt % Nafion solution (DuPont, USA) for 30 min to obtain the catalyst paint, then 10 µL of the catalyst ink was spread on the surface of the clean glass carbon electrode with a micropipet and dried at room temperature to eliminate solvent and obtain a thin active layer with metal loading of 38 µg/cm2. A solution of 0.5 M HClO4 was prepared from guaranteed grade reagent and triply distilled water. Before each measurement, fast potential pulses (100 mV s-1) between 0 and 1400 mV were applied to the electrodes for surface cleaning and for obtaining a reproductive active electrochemical surface. The electrochemical surface area (ECSA) of the Pt/C catalyst was determined by integrating the hydrogen adsorption/desorption areas of the cyclic voltammogram (assuming 210 µC/cm2Pt after double layer correction) obtained in a potential pulse at a scan rate of 50 mV s-1.20 However, for Pd-containing catalyst, the method based on the hydrogen adsorption charge seems less reliable because Pd can absorb large quantities of hydrogen (up to 900 times its own volume). To determine the real ECSA of the Pd-containing PtPd/C catalyst, a CO stripping voltammetry test was carried out.21-23 In a typical CO stripping voltammetry test, CO was adsorbed onto the catalyst coated thin-film electrode at 0.1 V by bubbling CO gas through the 0.5 M HClO4 solution for 15 min. Solution CO was subsequently removed by high-purity N2 gas bubbling for 30 min, maintaining the potential at 0.1 V. The potential was then cycled at 25 mV s-1 starting at 0.1 V for two complete oxidation/reduction cycles. The activity of ORR on these catalysts was evaluated on the thin-film rotating disk electrode (Pine Instruments, USA). For the thin-film RDE measurements, 10 µL of the well-dispersed catalyst ink, which was prepared in the CV test, was spread onto the clean glass carbon disk, as described in CV tests. ORR activity was measured in oxygen-saturated 0.5 M HClO4

Design and Preparation of Highly Active Pt-Pd/C Catalyst

J. Phys. Chem. C, Vol. 111, No. 15, 2007 5607 oxygen flow for about 1 h before each experiment and maintained over the electrolyte during the whole voltammetry measurement.

Figure 1. XRD patterns of Pt-Pd/C and Pt/C catalysts: (a) Pt/C; (b) Pt3Pd1/C; (c) Pt1Pd1/C; and (d) Pt/C-JM.

electrolyte at 25 ( 0.5 °C with a CHI 760B potentistat/ galvanostat. A linear sweep was started from the cathodic direction at a scan rate of 10 mV s-1 and the rotating speed was fixed at 1600 rpm. The electrolyte was bubbled with an

Theoretical Calculations. All the theoretical calculations were performed with the Gaussian 98 package.24 The density functional theory (DFT) method was employed with the Becke’s three-parameter hybrid functional (B3LYP), which consists of the nonlocal exchange functional of Becke’s three-parameter set and the nonlocal correlation functional of Lee, Yang, and Parr.25-27 For oxygen, the polarized basis set 6-311g* was used because the presence of a polarization d shell can ensure a possible charge transfer from the substrate to the adsorbate. For Pt and Pd atoms, the Los Alamos National Laboratory’s (LANL) basis set with effective core potentials (ECP) of double-ζ type was adopted.27-29 Different spin multiplicities were considered in this calculation to find out the most stable spin sate. Frequency calculations were also performed to determine the zero point energy and confirm the local minima states. To give a reasonable and reliable explanation of the charge-transfer phenomenon and reveal the electronic properties of each cluster, the natural bond orbital analysis (NBO) method was also employed in this work.30

TABLE 1: XRD, TEM, and CV Analysis Results of the As-Received and Commercial Catalysts catalyst Pt3Pd1/C Pt/C Pt1Pd1/C-(co) Pt/C-JM bulk Pta bulk Pda Pt1Pd1/C-Pt(f) Pt1Pd1/C-Pd(f) a

particle size d (nm) XRD TEM 2.8 2.2 3.6 2.8

2.9 2.4 3.9 3.0

2.3 2.8

3.1 3.2

lattice parameter afcc (Å)

mPt/mPd (EDS results)

3.916 3.926 3.910 3.930 3.923 3.890 3.922 3.915

3.4:1 1.1:1

ECSA (m2/g) H adsorption CO stripping 67.2 71.7 36.9 51.3

64.0 70.8 45.0 54.5

58.6 54.5

58.8 56.7

Data from JCPDS. Bulk Pt (PDF# 040802) and bulk Pd (PDF# 461043).

Figure 2. Low-resolution TEM photographs and size distributions of the catalysts: (a) Pt3Pd1/C and (b) Pt/C and (c) Pt1Pd1/C and (d) Pt/C-JM.

5608 J. Phys. Chem. C, Vol. 111, No. 15, 2007

Li et al.

Figure 3. High-resolution TEM photograph and the corresponding point resolved EDS results of the catalysts: (3a) Pt3Pd1/C and (3b) Pt1Pd1/C.

Results and Discussion Physical Characterizations. The powder XRD patterns of Pt3Pd1/C, Pt1Pd1/C, and Pt/C catalysts are shown in Figure 1 along with a commercial Pt/C catalyst with a 20 wt % Pt metal loading (Pt/C-JM, from Johnson Matthey Corporation) for comparison. It can be seen from Figure 1 that all the exploited catalysts clearly display the main characteristic patterns of Pt face-centered cubic (FCC) crystalline structure. The diffraction

peaks at 26 oC in all the XRD patterns are ascribed to the (002) plane of the hexagonal structure of Vulcan XC-72R carbon support. The corresponding widths of the diffraction peaks of Pt/C are much broader than the commercial one, indicating that Pt/C catalyst obtained from the modified polyol process has much smaller Pt nanoparticle crystalline size than Pt/C-JM. The mean nanoparticle crystalline sizes of these catalysts are calculated based on the X-ray line broad analysis method and


J. Phys. Chem. C, Vol. 111, No. 15, 2007 5609

Figure 6. Current-potential curves for ORR in O2-satuated 0.5 M HClO4 at a rotating rate of 1600 rpm with a glass carbon electrode employing the catalysts at 25 °C on negative sweep from 1000 mV at 10 mV s-1.

Figure 4. (a) Cyclic voltammetry curves of Pt3Pd1/C (s), Pt/C (---), Pt/C-JM (‚‚‚), and Pt1Pd1/C (-‚-). (b) The hydrogen desorption region of these catalysts intercepted from the CV curves in part a.

Figure 5. A typical CO stripping voltammogram recorded at 25 mV s-1 in 0.5 m HClO4 electrolyte for Pt/C catalyst. (s) The first oxidation cycle and (‚‚‚) the second cycle of the CO stripping voltammogram. The ECSA was determined from the shadow region.

summarized in Table 1. The lattice parameter of these catalysts is also obtained from the XRD patterns and displayed in Table 1. It could be found from Table 1 that the addition of Pd to Pt/C catalyst has a slight effect on the crystalline structure of Pt/C because Pt and Pd have almost the same FCC crystalline structure. The corresponding diffraction peak positions of PtPd/C are a little positively shifted with the increment of Pd amount in Pt-Pd/C and result in decreased lattice parameters of Pt3Pd1/C and Pt1Pd1/C to 0.3916 and 0.3910 nm, compared

Figure 7. Comparisons of the mass activities, j mass, for the ORR on the catalysts at 850 and 900 mV (data from Figure 6).

with 0.3926 nm of pure Pt/C. It looks like the addition of Pd element has a strong effect on the crystalline size of metal nanoparticles in Pt-Pd/C catalyst. Just as listed in Table 1, the mean particle sizes of Pt/C and Pt3Pd1/C are only 2.2 and 2.8 nm, while that of Pt1Pd1/C is increased to 3.6 nm. Low-resolution TEM images of these catalysts and the corresponding particle size distribution histograms based on the observation of more than 400 nanoparticles are shown in Figure 2 and the TEM results are also listed in Table 1. Consistent with XRD results, Pt/C has the smallest nanoparticle size of 2.4 nm and Pt1Pd1/C has the biggest one (3.9 nm), while Pt3Pd1/C and Pt/C-JM have almost equal mean nanoparticle sizes (2.9 nm for Pt3Pd1/C and 3.0 nm for Pt/C-JM). In comparison with the Pt/C-JM catalyst, all the homemade catalysts in the polyol process are well-dispersed on carbon support with quite uniform distribution except for the Pt1Pd1/C catalyst, in which a few agglomerations of metal nanoparticles could be clearly observed. This indicates that the addition of the Pd element in the Pt-Pd/C catalyst not only increases the particle size of nanoparticles but also changes the dispersion state of the nanoparticles on the carbon support. To explore the effect of the addition of Pd on the morphology of Pt-Pd/C in a microscale, HRTEM images accompanied with point resolved EDS measurements were employed to characterize Pt3Pd1/C and Pt1Pd1/C catalysts and the corresponding results are displayed in Figure 3. As displayed in the low-magnification images, the distribution of metal nanoclusters on the carbon surface in Pt1Pd1/C is totally different from that in Pt3Pd1/C.


Li et al.

Figure 8. TEM photograph and the corresponding size distribution diagrams of the catalysts: (8a) Pt1Pd1/C-Pt(f) and (8b) Pt1Pd1/C-Pd(f).

The HR-TEM image of the Pt3Pd1/C catalyst indicates that most of the metal nanoparticles are well-dispersed on the carbon support surface with bigger contact area between the metal nanoparticles and carbon support, which is consistent with pure Pt/C catalyst.31 However, for Pt1Pd1/C catalyst, a portion of the metal nanoclusters looks like only mixing with the carbon support with less contact area. The different dispersion morphology of the metal nanoparticles on the carbon support is mainly due to their different nucleus-growth mechanism of metal nanoparticles in Pt3Pd1/C and Pt1Pd1/C catalysts. A representative EDX spectrum from many individual particles has shown that both Pt and Pd are represented in the single nanoparticle and the concentration ratio is consistent with that from many particles. This indicates that the metal nanoparticle in Pt3Pd1/C and Pt1Pd1/C is the Pt-Pd alloy, not the physical mixture of the Pt and Pd nanoparticles. Because Pt takes the majority amount in the Pt3Pd1/C catalyst, the nucleus-growth mechanism of metal nanoparticles mainly follows the formation model of Pt/C catalyst. The formation of Pt nanoparticles on carbon support in a polyol process has been proved result from the reduction of Pt(OH)x colloids by ethylene glycol at elevated temperature in basic solution.16 The quick reduction reaction could result in a large amount of product with a very tiny nucleus in a short time and then Pt(OH)x in the bulk solution could be successively reduced on the preformed Pt catalyst surface and result in the well-dispersed nanoparticles. However, for Pt1Pd1/C catalyst with increased amount of Pd, the metal

nanoparticles formation mechanism is changed. Our previous research work displayed that Pd had a strong tendency to be deposited as Pd(OH)x precipitation in the ethylene glycol of alkaline during the Pd/C catalyst preparation process.32 Thus the reduction reaction of Pt(OH)x compound on carbon support will be accompanied by a large amount of Pd(OH)x precipitation during the Pt1Pd1/C catalyst preparation. With prolonged time, the precipitation of Pd(OH)x could also be reduced by the ethylene glycol. The precipitation-reduction process of Pd(OH)x could result in the uneven dispersion of nanoparticles on the carbon support. CVs of Pt-Pd/C and Pt/C in 0.5 M HClO4 solutions at a scan rate of 50 mV s-1 between 0 and 1200 mV at 25 ( 0.5 °C are presented in Figure 4a. It can be clearly observed that the voltammograms of Pt/C (whether the homemade or the commercial one) catalyst represent the typical characteristic features of a polycrystalline Pt electrode in the hydrogen and oxygen adsorption-desorption regions whether in the anodic or the cathodic sweeps. However, Pd-containing Pt3Pd1/C and Pt1Pd1/C catalyst has a different electrochemical response in the hydrogen desorption region (0-400 mV) as displayed by the curves in Figure 4b, which are intercepted from the full CV curves in Figure 4a. As shown in Figure 4b, the typical three resolved peaks in the hydrogen adsorption-desorption region, which are associated with weakly and strongly bonded hydrogen species on different crystal faces of Pt metal, disappear and new features with one big broad peak are observed. The ECSA of these



Figure 11. Comparisons of the mass activities, j mass, for the ORR on the catalysts at 850 and 900 mV (data from Figure 10).

Figure 9. (a) Cyclic voltammetry curves of Pt1Pd1/C-(co) (s), Pt1Pd1/C-Pd(f) (---), and Pt1Pd1/C-Pt(f) (‚‚‚). (b) The hydrogen desorption region of these catalyst intercepted from the CV curves in part a.

Figure 10. Current-potential curves for ORR in O2-satuated 0.5 M HClO4 at a rotating rate of 1600 rpm with a glass carbon electrode employing Pt1Pd1/C catalysts with different preparation process at 25 °C on negative sweep from 1000 mV at 10 mV s-1.

catalysts are determined by integrating the hydrogen adsorption/ desorption areas of the cyclic voltammogram (assuming 210 µC/cm2Pt after double-layer correction) obtained in a potential pulse and the values are also listed in Table 1. It would be cautious to adopt the ECSA values of Pd-containing Pt3Pd1/C and Pt1Pd1/C catalyst because of the absorbed hydrogen species in Pd metal. To define the ECSA values of Pd-containing PtPd/C catalysts, COads stripping voltammetry is employed to estimate the surface area of the catalyst surfaces. The ECSA of the catalysts are determined by adopting a conversion factor of 420 µC cm-2Pt according to the literatures.21-23 Figure 5 shows a typical COads stripping voltammogram for Pt/C catalyst. The

ECSA of these catalysts are determined from CO stripping curves and are also listed in Table 1. It could be found that the difference between the ECSA values for pure Pt/C or Pt3Pd1/C obtained by using two methods of hydrogen adsorption and CO stripping is within 5%. However, for Pt1Pd1/C catalyst, the difference is enlarged to 18% between these two methods. The experimental results regarding ORR in 0.5 M HClO4 are summarized in Figure 6. To facilitate the activity comparisons, all the currents are normalized to the geometric area of the electrode, that is, to normalize the ORR activity to the total metal loading. Among all these catalysts, Pt3Pd1/C catalyst shows higher activity in the kinetic controlled region for the ORR than the pure Pt/C catalysts (both the homemade and the commercial ones), while Pt1Pd1/C gives the worst ORR performance as shown in Figure 7, in which the ORR currents at different electrode potentials on these catalysts are compared. Similar mass transport corrected tafel plots for all the explored catalysts indicate the same ORR mechanism on these catalysts. The low performance of Pt1Pd1/C to ORR is partially attributed to its comparably bigger nanoparticle size and worse dispersion of metal nanoparticles on the carbon support. On the other hand, the inferior ORR may result from too much metal oxide in the Pt1Pd1/C catalyst. As displayed by the CV curves in Figure 7, the value of the double layer current of Pt1Pd1/C is higher than that of the other catalysts, indicating the presence of some metal oxide. X-ray photoelectron spectroscopy characterization shows that about 67% of Pd is in the zero-valence state and the remaining 33% is in the Pd(II) species, assuming the oxide to be PdO, while for Pt3Pd1/C the ratio of the Pd(0) species is about 75%. Too much metal oxide in the catalyst not only decreases the catalyst performance, but also reduces its stability after the long-time run. To study the promotion effect of Pt-Pd/C to ORR, three Pt1Pd1/C catalysts with different microstructures are designed by adjusting the catalyst preparation process and characterized by XRD, TEM, and CV techniques. The corresponding XRD results are also listed in Table 1. In comparison with Pt1Pd1/C-(co) catalyst, the average particle size of the catalysts obtained by the successive reduction method is small, especially for Pt1Pd1/ C-Pt(f) catalyst. The TEM images of Pt1Pd1/C-Pt(f) and Pt1Pd1/C-Pd(f) are displayed in Figure 8. The dispersion of metal nanoparticles on the carbon support in the two catalysts is much better than Pt1Pd1/C-(co) catalyst. Point resolved EDX spectra from many individual particles have shown that both Pt and Pd are represented in the single nanoparticles. This indicates that the metal nanoparticle in Pt1Pd1/C-Pt(f) and Pt1Pd1/C-Pd(f) is the Pt-Pd solid solution, not the physical mixture of the Pt and Pd nanoparticles. A CV voltammogram was employed to


Li et al.

TABLE 2: Optimized Geometry, Multiplicity (M), Equilibrium Bond Lengths and Angles, Zero Point Energy (ZPE), Total Energy (E) (Includes the Zero Point Energy), Atomization Energy (ΣD0), and Mulliken Charges of Metallic Clusters



TABLE 2: (Continued)

study the electrochemical behavior of the catalyst. As displayed by the CV curves in Figure 9, Pt1Pd1/C-Pt(f) has an almost identical electrochemistry response to Pt1Pd1/C-(co) in the hydrogen adsorption-desorption region, except for its bigger integrating area due to the smaller nanoparticle size, while for Pt1Pd1/C-Pd(f) catalyst, the characteristics in this region are a

little similar to those of pure Pt/C catalyst except for a characteristic sharp peak between 15 and 50 mV, which is ascribed to hydrogen mainly absorbed as the β phase, analogously to Pd metal.33 The ECSA values of Pt1Pd1/C-Pt(f) and Pt1Pd1/C-Pd(f) are obtained both from the hydrogen adsorption method and CO stripping and the results are also listed in Table

5614 J. Phys. Chem. C, Vol. 111, No. 15, 2007 1. In comparison with Pt1Pd1/C-(co) catalyst, there is a small difference between the ECSA values of Pt1Pd1/C-Pt(f) and Pt1Pd1/C-Pd(f) obtained by using these two methods. Pt1Pd1/CPt(f) gives the biggest ECSA value among these three Pt1Pd1/C samples due to its smallest nanoparticles size, which is consistent with XRD and TEM results. This is possible because large amounts of very small Pt nanoparticles are formed first during the catalyst preparation of Pt1Pd1/C-Pt(f), and then subsequently reductive deposition of Pd on the Pt nanoparticle surface could facilitate the formation of smaller nanoparticles. It is interesting to note that the CV characteristic of Pt1Pd1/C-Pt(f) resembles that of Pt1Pd1/C-(co) to some extent, indicating a similar distribution of Pt and Pd elements in both catalysts. The activities of ORR on these three catalysts in a RDE measurement are displayed in Figure 10. Although Pt1Pd1/C-Pd(f) possesses a little smaller ECSA than Pt1Pd1/C-Pt(f), it represents the best ORR activity among all these Pt1Pd1/C catalysts and it is even a little better than the Pt3Pd1/C catalyst, as displayed in Figures 10 and 11. This indicates that Pt-Pd nanoclusters with Pt rich on the shell of the metal nanoparticles could enhance ORR activity with a smaller amount of Pt. The improvement may be ascribed to the Pt-Pd electronic interaction in the Pt-Pd/C catalyst, in which the Pt atom is rich in electron density due to the presence of the Pd atoms so as to facilitate the dissolution of molecular oxygen.34 To explain this enhancement of ORR on Pt1Pd1/C-Pd(f) catalyst, theoretical studies on the bimetallic Pt-Pd clusters with respect to oxygen adsorption and dissociation are also carried out in this work. At first, the optimized geometries, multiplicity (M), equilibrium bond lengths and angles, zero point energy, total energy (E), atomization energy (ΣD0), and Mulliken charges of pure Pt, Pd, and Pt-Pd clusters are calculated and listed in Table 2. For dimers, it can be seen that the ground states are all triplets and Pt2 is the most stable cluster with the biggest atomization energy of 2.50 eV. For the trimers, Pt3 is the most stable cluster and the Pd3 is the lowest, whereas Pt2Pd is more stable than Pd2Pt according to the atomization energy. Whether in dimers or trimer or even more atoms in the cluster, it can be found in Table 2 that the stability of the Pt-Pd clusters is increased with the number of Pt atoms in the clusters. In PtPd clusters, it is found that the Pt atom bears negative charge while Pd is positively charged. This mainly results from the different valence electron structures of Pt and Pd (Pt: 5d96s1 and Pd: 4d10). It is interesting to note that whether in pure Pt5 or Pd5 clusters, the apex atoms (M1, M5) of Pt5 or Pd5 bear negative charges that are transferred from the middle atoms (M2, M3, M4), which indicates that the apex sites are prone to bear negative charges in trigonal-bipyramid structure. Further Mulliken population analysis of s, p, and d orbitals (Table 3) of PtmPdn dimers or trimers reveals that the s orbital population of Pt atoms is increased, while the d orbital population for the Pd atoms is decreased in comparison with their population in the pure Pt or Pd clusters. This indicates that the charges are mainly transferred from the d orbital of Pd to the s and p orbital of Pt. With respect to the O2 adsorption and dissociation on the Pt-Pd cluster, the corresponding calculated results are listed in Tables 4 and 5. Table 4 shows the spin multiplicity, optimized bond lengths and angles, stretching vibration frequencies of the O-O bond, total energies, atomization energies, and Mulliken charges of the optimized PtnPdm-O2 clusters. The bond length of O2 in our calculation is 1.206 Å, in agreement with the value of the free oxygen in the gas phase of 1.208 Å.35 The calculated

Li et al. TABLE 3: Mulliken Population Analysis for Ground State Metallic Dimers and Trimers atomic orbital population cluster

atom

s

p

d

Pt2

Pt1 Pt2 Pd1 Pd2 Pd1 Pt2 Pt1 Pt2 Pt3 Pd1 Pd2 Pd3 Pt1 Pt2 Pd3 Pd1 Pd2 Pt3

3.02 3.02 2.15 2.15 2.43 3.49 2.77 2.77 2.77 2.20 2.26 2.20 2.99 3.04 2.27 2.40 2.40 3.19

6.08 6.08 6.05 6.05 6.02 6.06 6.12 6.12 6.12 6.09 6.10 6.09 6.11 6.12 6.08 6.08 6.08 6.11

8.90 8.90 9.80 9.80 9.19 8.81 9.19 9.19 9.19 9.69 9.69 9.69 9.07 9.02 9.30 9.36 9.36 9.02

Pd2 PtPd Pt3 Pd3 Pt2Pd Pd2Pt

value of O-O vibration frequency is 1640 cm-1, in comparison with the experimental value of 1580 cm-1.36 Two optimized geometries are adopted for the adsorption of O2 on monomer Pt (seen in Table 4): the Pauling model37 where O2 is connected with a Pt atom thorough a single bond, and the Griffiths model38 where both oxygen atoms of O2 are bonded with the same Pt atom. For the Pauling model, the bond length of O-O is 0.05 Å longer than the value in the gas phase, while for the Griffiths model, the bond length is enlarged to 1.372 Å, which is 0.166 Å longer than the O-O bond length. In comparison with the O-O stretching frequency in the gas phase, the corresponding O-O stretching frequency in the Pauling model is V ) 1297 cm-1, while that of the Griffiths model is V ) 1050 cm-1, indicating that the O-O bond strength in both models is weakened. The adsorption of the O-O bond in the Griffiths model facilities the dissociation of O2 more than in the Pauling model due to the longer O-O bond length and weakened O-O bond strength in the Griffiths model. The adsorption model of Pd-O2 is similar to that on Pt metal. Whether in the Pauling model or the Griffiths model, the charges are all transferred from the metal atoms to the oxygen atoms for both Pt-O2 and Pd-O2. However, the corresponding enlarged O-O bond value and the decreased vibration frequencies in Pd-O2 are lower than the values in Pt-O2, indicating that although both Pt and Pd have catalytic activity for the dissociation of molecular oxygen, pure Pt is more active than pure Pd metal. For the dimers of Pt2 and Pd2 and Pt-Pd clusters, the Yeager model,1 where O2 is adsorbed on a bridge site, is found to be more suitable for the O2 adsorption on the dimers. The Pt-Pt bridge in the Pt2 cluster is the most active site for the dissociation of O2. The charges are transferred from the metal atoms to oxygen atoms, in which each of the oxygen atoms bears a negative charge of -0.28 au, and each of the metal atoms bears a positive charge of 0.28 au accordingly. The orbital distribution of Pt2O2, Pd2O2, and PtPdO2 is shown in Table 5. The orbital populations of O2 and corresponding metal clusters are also listed for comparison. For Pt2O2 and PtPdO2, all the s orbital populations of metal atoms are decreased, while the p orbital populations of oxygen atoms are increased accordingly. The p and d orbital populations of metal atoms in them are slightly higher than that in the corresponding pure metal clusters, and the s-shell populations of oxygen atoms are slightly bigger than those of the corresponding free oxygen atoms. Charges mainly transfer from the s orbitals of metal atoms to the s and p orbitals of adsorbed oxygen atoms. The



TABLE 4: Optimized Geometry, Multiplicity (M), Equilibrium Bond Lengths and Angles, Stretching Vibration Frequencies of the O-O Bond (WO-O), Total Energy (E) (Includes the Zero Point Energy), Atomization Energy (ΣD0), and Mulliken Charges of the Adsorbed System


Li et al.

TABLE 4: (Continued)

transferred charges occupy the antibonding πg orbital of molecular oxygen, resulting in the weakening of the O-O bond.

So the more charges are transferred to the molecular oxygen, the weaker are the O-O bonds.

Design and Preparation of Highly Active Pt-Pd/C Catalyst TABLE 5: Mulliken Orbital Populations Analysis for Pt3, Pt2Pd Metal Clusters and O2 Adsorbed Pt3O2, Pt2PdO2 Clusters species Pt3 Pt3O2 Pt2Pd Pt2PdO2

s p d s p d s p d s p d

Pt1

Pt2

Pt3

2.77 6.12 9.19 2.74 6.16 8.83 2.99 6.11 9.07 2.97 6.17 8.78

2.77 6.12 9.19 2.74 6.16 8.83 3.04 6.12 9.02 3.01 6.17 8.75

2.77 6.12 9.19 2.75 6.09 9.17 2.27 6.08 9.29 2.22 6.11 9.32

O4

O5

3.95 4.29 0.02

3.95 4.28 0.02

3.95 4.27 0.02

3.96 4.27 0.02

On the basis of the above discussions, it is found that O2 is more readily adsorbed and easily dissociated on the Pd-modified Pt surface. This result mainly originates from the weakening of the O-O bond on Pd-modified Pt clusters. In our calculation, the Pt-Pt bridge site modified by the underlying Pd atoms is considered to be the most active site for the dissociation of O2 and thus accelerates the following ORR, which is very consistent with the above experiment results. High cost and decreased performance after long-time runs of Pt-based electrocatalysts are the main factors that hinder the commercialization of proton exchange membrane fuel cells (PEMFCs). Developing a high performance and more stable binary noble metal catalyst with low Pt amounts is crucial for commercial applications of low-temperature PEMFCs. Additional work on optimizing the Pt-Pd/C catalyst system is currently in progress. Conclusions Pt3Pd1/C, Pt1Pd1/C, and Pt/C catalysts with the same total metal loading have been prepared by a simple modified polyol process method without any surfactants and characterized by XRD, TEM, and CV techniques. The modified polyol process is proved to be a good method to prepare high-activity Pt/C and Pt rich Pt3Pd1/C catalysts with uniform particle size distribution. From TEM observations, it is found that the addition of Pd to Pt/C not only increases the mean particle size, but also changes the dispersion property of metal nanoparticles on the carbon support. CV tests show that Pd-containing catalysts of Pt3Pd1/C and Pt1Pd1/C have a different feature in the hydrogen adsorption-desorption region compared with pure Pt/C catalyst. The activity to ORR is characterized by the RDE method and it is found that the catalytic activity of Pt3Pd1/C for ORR is improved compared with that for Pt/C or Pt1Pd1/C catalyst. Detailed work reveals that the activity of ORR could be further improved on Pt-Pd/C catalyst with Pt rich on the metal nanoparticle surface. This enhancement has been proved by theoretical calculations. DFT calculation shows that the Pt atom bears a negative charge while Pd is positively charged in PtmPdn clusters and the negatively charged Pt atoms in PtmPdn facilitate the dissociation of O2 on the Pt surface, thus enhancing the ORR activity. Acknowledgment. This work was financially supported by Innovation Foundation of Chinese Academy of Science (K2006D5), Hi-Tech Research and Development Program of China (2006AA05Z137′2006AA05Z139), and National Natural Science Foundation of China (Grant No. 50676093).

J. Phys. Chem. C, Vol. 111, No. 15, 2007 5617 References and Notes (1) Yeager, E. Electrochim. Acta 1984, 29, 1527. (2) Anderson, A. B.; Roques, J.; Mukerjee, S.; Murthi, S. J. Phys. Chem. B 2005, 109, 1198. (3) Chen, S. L.; Kucernak, A. J. Phys. Chem. B 2004, 108, 3262. (4) Jalan, V.; Taylor, E. J. J. Electrochem. Soc. 1983, 130, 2299. (5) Beard, B. C.; Ross, P. N. J. Electrochem. Soc. 1990, 137, 3368. (6) Paffet, M. T.; Beery, G. J.; Gottesfeld, S. J. Electrochem. Soc. 1988, 135, 1431. (7) Mukerjee, S.; Srinvasan, S.; Soriaga, M. P. J. Electrochem. Soc. 1995, 142, 1409. (8) Min, M. K.; Cho, J. H.; Cho, K. W.; Kim, H. Electrochim. Acta 2000, 45, 4211. (9) Watanabe, M.; Tsurumi, K.; Mizukami, T.; Nakamura, T.; Stonehart, P. J. Electrochem. Soc. 1994, 141, 2659. (10) Sun, Z. A.; Tseung, C. C. Electrochem. Solid State Lett. 2000, 3, 413. (11) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 4717. (12) Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Klie, R.; Sasaki, K.; Adzic, R. R. J. Phys. Chem. B 2004, 108, 10955. (13) Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem., Int. Ed. 2005, 44, 2131. (14) Creus, A. H.; Gimeno, Y.; Diaz, P.; Vazquez, L.; Gonzalez, S.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. B 2004, 108, 10785. (15) Li, H. Q.; Xin, Q.; Li, W. Z.; Zhou, Z. Z.; Jiang, L. H.; Yang, S. H.; Sun, G. Q. Chem. Commun. 2004, 2776. (16) Wang, Y.; Ren, J. W.; Deng, K. L.; Gui, L.; Tang, Y. Q. Chem. Mater. 2000, 12, 1622. (17) Zhou, Z. H.; Wang, S. L.; Zhou, W. J.; Xin, Q.; Sun, G. Q. Chem. Commun. 2003, 394. (18) Radmilovic, V.; Gasteiger, H. A.; Ross, P. N. J. Catal. 1995, 154, 98. (19) Li, W. Z.; Zhou, W. J.; Li, H. Q.; Zhou, Z. H.; Zhou, B.; Sun, G. Q.; Xin, Q. Electrochim. Acta 2004, 49, 1045. (20) Liu, Z. L.; Gan, L. M.; Hong, L.; Chen, W. X.; Lee, J. Y. J. Power Sources 2005, 139, 73. (21) Larsen, R.; Masel, R. I. Electrochem. Solid State Lett. 2004, 7, A148. (22) Jayashree, R. S.; Spendelow, J. S.; Yeom, J.; Rastogi, C.; Shannon, M. A.; Kenis, P. J. A. Electrochim. Acta 2005, 50, 4674. (23) Cuesta, A.; Couto, A.; Rincon, A.; Perez, M. C.; Lopez-Cudero, A.; Gutierrez, C. J. Electroanal. Chem. 2006, 586, 184. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6; Gaussian, Inc.: Pittsburgh, PA, 1998. (25) Becke, A. D. Phys. ReV. 1988, A38, 3098. (26) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (27) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (28) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (29) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (30) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (31) Paulus, U. A. Ph.D. Thesis, Electrocatalysis for Polymer Electrolyte Fuel Cells: Metal Alloys and Model Systems, Swiss Federal Institute of Technology Zurich, Sweden, 2002, P76. (32) Li, H. Q.; Sun, G. Q.; Jiang, Q.; Zhu, M. Y.; Sun, S. G.; Xin, Q. Electrochem. Commun. 2007, 9, 1410. (33) Lukaszewski, M.; Kusmierczyk, K.; Kotowski, J.; Siwek, H.; Czerwinski, A. J. Solid State Electrochem. 2003, 7, 69. (34) Yang, X. L.; Liu, H. F.; Zhong, H. J. Mol. Catal. A: Chem. 1999, 147, 55. (35) Greenwood, N. N.; Earnshaw, A. Chemistry of Elements; Pergamon Press: Oxford, UK, 1984. (36) Huber, K. P.; Herzberg, G. Molecular Spectra and MolecularStructure: Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979; Vol. 4. (37) Tarasevich, M. R.; Sadkowski, A.; Yeager, E. In Comprehensive Treatise of Electrochemistry; Conway, B., Bockris, J. O. M., Yeager, E., Khan, S. U. M., White, R. E., Eds.; Plenum Press: New York, 1983; Vol. 7, p 301. (38) Griffiths, J. S. Proc. R. Soc. London Ser. A 1956, 235, 23.

C Catalyst for the

Recommend Documents