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Carbon-Supported Pd-Pt Nanoalloy with Low Pt Content and Superior Catalysis for Formic Acid Electro-oxidation Han-Xuan Zhang, Chao Wang, Jin-Yi Wang, Jun-Jie Zhai, and Wen-Bin Cai* Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials and Department of Chemistry, Fudan UniVersity, Shanghai 200433, China ReceiVed: January 28, 2010; ReVised Manuscript ReceiVed: March 01, 2010
Inspired by the “third-body” effect and the d-band center theory, a series of carbon black-supported PdxPt1-x (denoted as PdxPt1-x/C, with atomic fraction x ) 0.5-1) nanocatalysts were synthesized and screened in order to pinpoint the optimal composition targeted for the electrocatalytic oxidation of formic acid. The effective alloying of these two elements is better demonstrated by the compositional dependent synergetic electrocatalysis identified for the as-synthesized PdxPt1-x/C nanoparticles rather than by the XRD characterization. Preliminary screening indicates that Pd0.9Pt0.1/C is the optimum catalyst for the desired reaction among all tested PdxPt1-x/C samples. This high performance of the Pd0.9Pt0.1 nanoalloy can be ascribed to the effectively inhibited CO poisoning at largely separated Pt sites and appropriately lowered d-band center of Pd sites. 1. Introduction Nowadays, the direct formic acid fuel cell (DFAFC) with the electro-oxidation of formic acid (FA) as its anode reaction is becoming a promising green power source for portable device applications owing to its advantages of higher open circuit voltage, lower fuel crossover, safer fuel, and simpler chargetransfer over direct methanol fuel cell.1,2 From the basic research viewpoint, the electro-oxidation of FA on Pt-group electrodes has been a model reaction in establishing the relationship between the surface composition and/or electronic property and the surface reactivity. Therefore, theory-guided synthesis and screening of a carbon-supported nanocatalyst with the optimal electrocatalytic activity at reduced cost for the electro-oxidation of FA is not only a test of fundamental theories, but also essential for the ultimate commercialization of DFAFC. The electrocatalysts commonly used for FA oxidation are either Pt-based2 or Pd-based.1 On pure Pt surfaces, CO poisoning due to the dehydration of FA on at least two or more contiguous Pt atoms (the ensemble3) hinders the direct dehydrogenation oxidation of FA at lower overpotentials. In light of the so-called “third-body” effect,2,4-6 this obstacle can be largely overcome by modifying Pt surfaces with foreign metal adatoms,2,7,8 decorating Pt submonolayer on Au nanoparticles,9-12 or forming Pt-based alloys.10,13,14 Nevertheless, these approaches are not in harmony with the cost-effective strategy as most of the precious noble Pt or Au atoms in the nanocatalysts are not involved in the electrocatalysis, and moreover, some of the above designed structures are not stable once operated at relatively higher potentials, leading to the irreversible loss of electrocatalytic activity. Pd, which costs much less than Pt and Au (the current price of Pd is ca. one-quarter that of Pt, and two-fifths that of Au) and is far less prone to the dehydration of FA (and thus less CO poisoning), is another attractive anode catalyst material in DFAFC.1,15 However, the electrocatalytic activity and durability of the Pd nanocatalysts have yet to be substantially enhanced before they can meet the stringent demand of higher power * To whom correspondence should be addressed. E-mail: wbcai@ fudan.edu.cn. Fax: +86-21-65641740. Phone: +86-21-55664050.
output in working DFAFCs.16-20 It is well recognized that the d-band center of a metal relates to the bond strength of the adsorbates on its surface and thus influences the catalytic properties.4,21-23 Kibler et al. demonstrated that the electrocatalysis of FA at a Pd overlayer on a metal single crystal surface could be significantly enhanced as the d-band center of Pd shifted down with an appropriate value due to the modest lattice compressive strain.24 As for the Pt-Pd bimetallic nanocatalysts, they have been intensively investigated as the catalysts for oxygen reduction reactions (ORR)25-28 or electro-oxidation of FA.29-33 For the latter, Pd-modified Pt nanoparticles,29,30 unsupported Pt-Pd bimetallic nanoparticles,31 and supported Pd-Pt nanoalloys with Pt/Pd molar ratios ranging from 4:1 to 1:2 have been synthesized for the oxidation of FA.32,33 However, the conclusions are rather controversial regarding the optimal bimetallic composition, and the catalysts either contained relatively high Pt contents or had very limited improvement for electrocatalysis as compared to Pd/C.29-33 Surprisingly, no prior reports have been found on screening PdxPt1-x/C with much lower Pt contents and yet with superior electrocatalysis toward FA oxidation, in light of the existing theories, i.e., the “third-body” effect and the d-band center theory. In this work, inspired by the above-mentioned effects, a series of Pd-based PdxPt1-x (x ) 0.5-1) nanoparticles dispersed on carbon black support have been initially synthesized and screened for FA electro-oxidation, and the unprecedented synergetic activity is pinpointed to the Pt/Pd atomic ratio as low as ca. 1/9. The “third-body” effect for the optimal catalyst is illustrated with ball models, and the effect of the d-band center shift of Pd in PdxPt1-x demonstrated by positive correlation of different electrochemical measurements. 2. Experimental Section 2.1. Preparation of the PdxPt1-x/C (x ) 0.5-1) Catalysts. Syntheses of PdxPt1-x/C (x ) 0.5-1) nanocatalysts were carried out according to a modified protocol initially for Pd/C34 targeted for FA oxidation and Pt1Pd1/C, Pt1Pd1.5/C, and Pt1Pd3/C25 targeted for ORR. In contrast to the polyol31 and microemulsion35 syntheses reported in the literature, the present method
10.1021/jp100835b 2010 American Chemical Society Published on Web 03/10/2010
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TABLE 1: The Transferred Volumes of Pd(II) and Pt(II)-Containing Solutions for the Synthesis of a Series of PdxPt1-x/C (x ) 0.5-1) Catalysts
Pd0.5Pt0.5/C Pd0.6Pt0.4/C Pd0.7Pt0.3/C Pd0.8Pt0.2/C Pd0.9Pt0.1/C Pd0.95Pt0.05/C Pd/C
Na2PdCl4 (0.05 M)/µL
K2PtCl4 (0.05 M)/µL
EDTA (0.1 M)/µL
753 903 1050 1205 1354 1430 1506
753 603 456 301 152 76 0
753
is much simpler and does not consist of tedious decontamination procedures. The preparation process was as follows: desired volumes of 0.05 M Na2PdCl4 and 0.05 M K2PtCl4 (see Table 1 for details) and 753 µL of 0.1 M EDTA were added to 26 mL of H2O. Such a mixture was kept at 60 °C for 40 min under vigorous stirring. After the solution was cooled to room temperature, the solution pH was adjusted to ca. 9-10 with NaOH solution. Then, 32 mg of Vulcan XC-72 carbon was added to the mixture. After the suspension was sonicated and stirred for another 30 min, 6 mL of 0.05 M Na2CO3 containing 18 mg of NaBH4 was dropwisely added to the suspension at 0.3 mL/min controlled by a peristaltic pump. The resulting suspension was further stirred at room temperature for 2 h, and then filtered and washed with a copious amount of Milli-Q water, and eventually vacuum-dried at 70 °C overnight. Unfortunately, this protocol yielded a much lower Pt weight loading on Vulcan XC-72 carbon than expected in the absence of Pd, and thus a commercial Pt/C catalyst (20 wt % from Johnson Matthey company) was used in the tests. 2.2. Materials Characterizations. The compositions of the PdxPt1-x/C catalysts were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on a Hitachi P-4010. The crystalline structures were determined by X-ray diffraction (XRD) on a D8 Advance X-ray diffractometer with Cu KR radiation from 10° to 90°, and the particle sizes and dispersions on carbon support were characterized by transmission electron microscopy (TEM) on a JEOL JEM-2010 microscope. 2.3. Electrochemical Measurements. The electrocatalysis measurements were performed with a CHI 660B electrochemistry workstation. The electrolyte was 0.5 M H2SO4 with or without 0.5 M HCOOH. The working electrode was a thin layer of Nafion-impregnated catalyst cast on a glassy carbon rotating disk electrode (GC-RDE, 3 mm diameter, 0.07 cm2) prepared as follows: A catalyst slurry was first prepared by mixing 1 mL of C2H5OH, 2 mg of catalyst, and 120 µL of Nafion (5 wt %, Aldrich) with ultrasonication for 30 min. A 5.6 µL sample of this catalyst ink was transferred onto a freshly polished GCRDE. A Pt gauze was used as the counter electrode, and the Hg/Hg2SO4 electrode served as the reference electrode; however, all potentials in the following were referred to the saturated calomel electrode (SCE). High-purity N2 was used for the deaeration of the solutions. The working electrode surface was electrochemically cleaned by cycling its potential in 0.5 M H2SO4 at 50 mV s-1 in between -0.2 and 0.85 V for two cycles. For assessing the electrocatalytic activity of a cleaned working electrode, cyclic voltammetry was run in 0.5 M H2SO4 + 0.5 M FA at 50 mV s-1 with the initial and final potentials at -0.2 V and the positive potential limit at 0.65 V. For the durability test, a working electrode was first reactivated at 0.25 V for 10 s36 and then the current was recorded as function of time at 0.2 V. In CO stripping measurements, CO (>99.9% purity) was first
Figure 1. XRD patterns of the series of PdxPt1-x/C nanoalloy catalysts with x from 0.5 to 1.0.
bubbled in the electrolyte for 20 min at 0.0 V, followed by Ar bubbling for 40 min to remove the dissolved CO at the same potential, and the voltammograms were recorded between -0.2 and +1.0 V at a scan rate of 10 mV s-1. All electrochemical experiments were carried out at 25 ( 1 °C. 3. Results and Discussion 3.1. The Architecture and Morphology of the PdxPt1-x/C Catalysts. To obtain a high degree of alloying PdxPt1-x associated with reproducible performance, we extended the approach reported by Li et al.25 to fabricate the PdxPt1-x/C catalysts with a wider compositional range, in which EDTA was used as an effective chelator for Pd and Pt ions to ensure the coreduction of Pd(II) and Pt(II) species by NaBH4 at room temperature. Figure 1 shows the XRD patterns for a series of PdxPt1-x/C catalysts, where the four peaks correspond to the planes (111), (200), (220), and (311), featuring the face-centeredcubic (fcc) crystalline structure. Thermodynamically, these two metals can form alloys of any compositions with the fcc structure under suitable conditions. Nevertheless, such alloy structure for the PdxPt1-x/C samples cannot be readily identified from the XRD patterns since Pt and Pd solids have very close lattice constants. Fortunately, the synergetic responses (rather than the simple coaddition of the responses of two distinguished metals) of the PdxPt1-x/C catalysts as a function of x in the following electrochemical measurements provide more convincing evidence for the formation of a solid-solution-like alloy structure in the as-synthesized PdxPt1-x/C samples (vide infra). Particle sizes and distributions of the carbon-supported metallic nanoparticles were also critical parameters influencing the catalytic activity of the materials.37,38 The mean diameters of the nanoparticles were evaluated from the XRD patterns for PdxPt1-x/C catalysts according to the Scherrer formula:
d ) Kλ/β cos θ
(1)
where d is the average particle size, K ) 0.89, λ ) 0.15405, θ is the angle of the (111) peak, and β is the width in radians of the diffraction peak at half height. The diameters of the PdxPt1-x nanoparticles thus obtained are around 3.4 nm (see Table 2, column 4). No significant difference in their mean diameters can be seen among the samples in consideration of the evaluation error, indicating that the synthesis method is rather
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TABLE 2: Compositional, Structural, and Electrochemical Data for PdxPt1-x/C Catalystsa x Targeted
targeted metal wt loading/%
x by ICP-AES assay
particle size (XRD)/nm
SECO/m2 g-1
Ep/V
ip/mA cm-2
i1000/mA cm-2
0.00 0.50 0.60 0.70 0.80 0.90 0.95 1.00b
20 26.2 25.0 23.8 22.6 21.3 20.7 20
0.00 0.50 0.61 0.71 0.80 0.89 0.94 1.00
3.4 3.8 3.4 3.2 3.4 3.7 3.4 3.6
85.6 72.2 81.9 70.4 85.4 83.0 87.8 89.7
0.28 0.03 0.03 0.06 0.10 0.11 0.11 0.24
15.1 25.4 32.3 52.6 67.2 87.5 78.6 42.5
3.7 5.8 5.9 9.0 8.9 13.5 5.6 5.7
a SECO, Ep, ip, and i1000 refer to the electrochemical active surface area, peak current potential, peak current density, and the highest oxidation current density at t ) 1000 s. b The performance of the as-synthesized Pd/C is higher than that of the commercial Pd/C (E-TEK).
Figure 2. TEM image and the corresponding particle size distribution histogram of the Pd0.9Pt0.1/C nanocatalyst.
good for controlling the alloy particle sizes. The TEM image of the Pd0.9Pt0.1/C catalyst with the best performance (vide infra) is shown in Figure 2, together with the histogram of the size distribution. The Pd0.9Pt0.1 nanoparticles were well dispersed on carbon black with a mean size of 3.2 ( 0.6 nm, slightly smaller than that calculated from XRD data. The other samples showed similar nanoparticle sizes and dispersions (not shown). The atomic ratios of the as-synthesized PdxPt1-x/C can be determined by dissolving them in aqua regia followed by ICP-AES analysis. As can be seen in Table 2, the Pd and Pt atomic fractions determined by ICP-AES (column 3) in the nanocatalysts were very close to those in the corresponding precursor solutions (column 1), suggestive of an essentially stoichiometric coreduction of Pd(II) and Pt(II) species during synthesis. Hence, the XRD, TEM, and ICP-AES measurements ensure a sounding comparison of the electrocatalytic activities toward FA electro-oxidation with different x of the PdxPt1-x/C samples. 3.2. Electrochemistry of the PdxPt1-x/C Nanocatalysts. The electrochemical characterization of the as-synthesized nanocatalysts was carried out by running voltammograms of PdxPt1-x/C-coated glassy carbon (GC) electrodes in 0.5 M H2SO4 at a scan rate of 50 mV s-1 (shown in Figure 3A). A fixed total number of metal atoms loaded on carbon support in all electrochemical measurements were adopted to facilitate the rational comparison of x-dependent properties for PdxPt1-x/C. In other words, the amount of Pt/C (20 wt %) loaded on a electrode was nearly doubled as compared to that of Pd/C (20 wt %), since one Pt atom weighs nearly twice as much as one
Pd atom. Hence, the increased amount of carbon support resulted in a larger double layer current for Pt/C as shown in Figure 3A. As shown in the inset of Figure 3A, notably, with increasing x, the hydrogen reductive absorption and oxidative desorption peaks around ca. -0.17 V for PdxPt1-x/C became sharper, with their desorption peak potentials shifted positively from -0.19 to -0.15 V, but remained lower than that for Pd/C (i.e., -0.12 V). The hydrogen absorption and desorption peaks at relatively negative potentials may be ascribed to H species in and out of the PdxPt1-x lattice interstices accompanied by H+ electroreduction and H electro-oxidation on PdxPt1-x surfaces in the negative and positive scans, respectively. Furthermore, in the negative-direction scan, the cathodic peak potential for reducing oxides on PdxPt1-x/C tends to shift negatively. In Figure 3B, the CO stripping voltammograms on PdxPt1-x/C-coated electrodes show that in response to an increment of 0.10 in x, the onset potential for CO stripping shifts positively by ca. 25 mV, in agreement with that found for Pd-Pt alloy nanoparticles prepared by water-in-oil microemulsion.35 All these electrochemical results cannot be explained by the simple coaddition of two separated contributions of Pd and Pt phases, rather it may be better explained by the surface Pd and Pt coeffect due to the formation of PdxPt1-x nanoalloys supported on carbon. To compare the catalytic activity and durability toward FA electro-oxidation, PdxPt1-x/C-coated GC electrodes were subjected to cyclic voltammetry in 0.5 M H2SO4 solution containing 0.5 M FA. Shown in Figure 4A, in the positive scan starting from -0.2 V, the peak current density (ip) for FA oxidation
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Figure 4. (A) Cyclic voltammograms recorded at 50 mV s-1 for PdxPt1-x/C-coated on GC electrodes in 0.5 M H2SO4 solution containing 0.5 M FA; (B) variation of Ep and ip with Pd atomic fraction (x). Figure 3. (A) Cyclic voltammograms on PdxPt1-x/C catalysts in 0.5 M H2SO4 at a scan rate of 50 mV s-1; (B) CO stripping voltammograms on PdxPt1-x/C catalysts in 0.5 M H2SO4 solution at a scan rate of 10 mV s-1 (only the oxidative CO removal region is shown).
listed in Table 2 is obviously x-dependent, which decreases in the order of Pd0.9Pt0.1/C > Pd0.95Pt0.05/C > Pd0.8Pt0.2/C > Pd0.7Pt0.3/C > Pd0.6Pt0.4/C > Pd/C > Pd0.5Pt0.5/C > Pt/C (J.M.). The highest ip observed on Pd0.9Pt0.1/C is nearly twice as high as that on Pd/C and six times as that on the commercial Pt/C, which is superior to any reported carbon black-supported Pt/C, Pd/C, and PtxPd1-x/C catalysts. To calibrate the surface reactive area effect on the resulting currents, the electrochemically active surface area (SECO) for each nanocatalyst (see Table 2) was calculated from the CO stripping peak area by assuming a charge of 420 µC cm-2 for electro-oxidation of the CO monolayer on ideally smooth Pt and Pd surfaces.20 It can be seen that the difference in SECO among PdxPt1-x/C samples is rather small, suggestive of a more important contribution from the synergetic effect of Pd and Pt sites in the FA electro-oxidation (as compared to monometallic Pt or Pd nanocatalyst). This synergy was further confirmed by the negatively shifted peak potentials for FA oxidation (Ep in Table 2) in the order of Pd0.6Pt0.4/C ≈ Pd0.5Pt0.5/C < Pd0.7Pt0.3/C < Pd0.8Pt0.2/C < Pd0.9Pt0.1/C ≈ Pd0.95Pt0.05/C < Pd/C < Pt/C (J.M.) with a larger gap between Pd/C and Pd0.9Pt0.1/C, and smaller differences between PdxPt1-x/C catalysts. Impressively, such a large negative shift (up to 0.21 V) in Ep for a PdxPt1-x/C versus that for Pd/C was not observed in any previous Pd-based bimetallic nanocatalysts, although a similar change of Ep was reported on Pd (sub)monolayer modified Pt single crystals24,39,40 as well as on the Pd-Pt
Figure 5. Chronoamperometric curves recorded at 0.2 V for four selected PdxPt1-x/C catalysts (x ) 1.0, 0.9, 0.5, and 0) in 0.5 M H2SO4 solution containing 0.5 M FA; the inset is the variation of i1000 with x for the corresponding electrodes.
alloy films.41 Obviously, those bulk Pt-supported Pd and Pd-Pt alloy film electrodes are not suited for practical applications due to cost-effective concerns. The long-term stability of the catalysts was also tested with chronoamperometry at 0.2 V, as shown in Figure 5. Again it was found that Pd0.9Pt0.1/C showed the highest oxidation current density at t ) 1000 s (i1000 in Table 1), i.e., 13.5 mA cm-2, which is 2.4 and 3.6 times as high as that observed on Pd/C and Pt/C, respectively. The inset in Figure 5 shows the dependence of i1000 on x for each corresponding electrode,
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SCHEME 1: Ball Models of PdxPt1-x Bimetallic Nanoparticles with x ) 0.9 (left panel) and 0.5 (right panel)
revealing again a “volcano-like” curve, in agreement with the above ip dependence on x. In the following, the synergy of low Pt-contented Pd-based alloys in the electrocatalytic oxidation of FA will be briefly discussed based on the surface “thirdbody” effect and the d-band center theory. 3.3. The Origins of Synergetic Electrocatalysis. In Figure 4B and the inset of Figure 5, the variations of ip, Ep, and i1000 with x present the “volcano-like” or “inverted-volcano-like” feature, indicating that the Pd/Pt atomic ratio did play a delicate role in the electro-oxidation of FA. For Pt/C, the small oxidation current at lower potentials in the forward scan was ascribed to surface CO poisoning resulting from self-dissociation of FA. By contrast, in the backward scan with CO species removed at higher potentials, the FA oxidation current on Pt/C turned much higher before it decreased due to the regaining of surface CO. Excellent electrocatalysis toward FA oxidation can thus be anticipated on appropriately engineered Pt surfaces with the least CO poisoning. Considering the dehydration of FA to form CO requires at least two neighboring Pt sites at the surface (the socalled ensemble effect3), increasing the x value in PdxPt1-x/C obviously increases the chance for Pd atoms to arrange around single Pt atoms (the “third-body” effect).4-6 Shown in the left panel of Scheme 1 is a ball model, depicting a statistical arrangement of the Pd and Pt atoms in a Pd0.9Pt0.1 alloy nanoparticle, where an effective separation of single Pt sites by the surrounding Pd atoms facilitates the direct oxidation of FA via the dehydrogenation pathway on these Pt sites. As the Pt fraction increases, dimer, trimer, and even more neigboring Pt sites at the nanoparticle surface get to show up (see the ball model in the right panel of Scheme 1 for a Pd0.5Pt0.5 nanoparticle of the same size), causing the dehydration of FA to occur on some Pt sites, which blocks the catalytic activity of these Pt sites and decreases the FA oxidation current. Notably, Pd0.5Pt0.5/C and Pd0.6Pt0.4/C showed a higher current in the negative direction scan than in the positive direction scan, to some extent, signaling the “ensemble” effect for CO formation. It should be pointed out that the Pd0.5Pt0.5/C synthesized with methanol as the reductant and SB-12 as the stabilizer had much lower oxidation currents in the positive scan,32 and unsupported Pd0.5Pt0.5 nanoparticles synthesized with the polyol method exhibited independent electrochemical responses from Pt and Pd sites,31 suggesting that our synthesis method yielded more effectively intermixed Pd and Pt atoms in the resulting nanoalloys. The “third-body” effect on surface Pt sites of PdxPt1-x nanoalloys seems not sufficient to explain the enhanced electrocatalysis, since this effect yields the least shift of Ep from that for Pt/C (ca. 0.35 V).8-12 The d-band center theory should be additionally cited to explain the observed Ep shift upon a
Figure 6. A plot of the FA oxidation peak potentials versus the first hydrogen-desorption peak potentials, showing a roughly linear correlation. The numbers labeled in the figure correspond to x values in PdxPt1- alloys.
change of x. According to the DFT calculation by Nørskov and co-workers,21-23 the d-band center values of Pt and Pd are -2.25 and -1.83 eV, respectively. Moreover, unlike the case of the Pt-supported Pd overlayer,24,39,40 Pt in a Pd-rich PdxPt1-x alloy was theoretically treated as an impurity, enabling the d-band center of Pd to be decreased.21-23,42 In this regard, the d-band center of Pd can in principle be finely tuned by alloying with varying amounts of Pt. On the other hand, it was reported that a proper downshift of the d-band center of a Pd monolayer on various substrates (that is, the binding of adsorbates to the surface was neither too weak nor too strong) could yield the highest ip, and a mildly negative shift of Ep for FA oxidation.24 Although theoretical computations are initially based on single crystal bulk alloys, and the downshifted scales may be different for nanoalloys, it is still reasonable to correlate the binding strength of an adsorbate to the nanoparticle surface with the d-band center, i.e., the lower the d-band center, the weaker the binding strength, and the more negative oxidation potential. With this in mind, the result in Figure 3B may be explained: with increasing Pt content in PdxPt1-x/C, the negative shift of the CO stripping peak position from 0.66 V for Pd/C to 0.6 V for Pd0.5Pt0.5/C may imply a gradually weakened binding energy of CO on surfaces. Interestingly, the plot of the FA oxidation peak potentials (adapted from Figure 3A) versus the hydrogen oxidative desorption peak potentials (adapted from Figure 4A) (shown in Figure 6) revealed the roughly monotonic tendency, in further support of the assumption that the Pd d-band center
Carbon Black-Supported PdxPt1-x Nanocatalysts can be tuned with alloying different amounts of Pt (shown in Scheme 1). Notably, although the surface CO electrooxidation is relatively complicated, requiring the OH species as the reactant pair, the monotonic tendency roughly remains for the onset potentials for CO electro-oxidation versus the FA electrooxidation peak potentials, as can be hinted by the positive shift of the CO stripping onset potential with increasing x in Figure 3B. At this point, we may conclude that the “third-body” effect on Pt and the d-band center shift of Pd are involved in contributing to the optimal electrocatalysis of Pd0.9Pt0.1/C toward FA oxidation. Future work will be done to understand the reaction pathways involved in the FA electro-oxidation on this specific alloy electrode at a molecular level. 4. Conclusions In summary, Pd-based PdxPt1-x/C nanocatalysts with a wide composition range (x ) 0.5-1) have been synthesized and screened for the electro-oxidation of formic acid. It was found that Ep for the PdxPt1-x/C anodes shifted negatively with decreasing x as compared to that for Pd/C, and the largest ip occurred at x ≈ 0.9 with a moderate shift of Ep. The x-dependent electrocatalytic activity was mainly explained based on the “third-body” effect illustrated by the comparison of the ball models of Pd0.9Pt0.1 and Pd0.5Pt0.5 nanoalloys, and on the effect of Pd d-band center shift suggested by positive correlation of different electrochemical measurements on PdxPt1-x/C electrodes. Acknowledgment. This work is supported by NSFC (Nos. 20873031 and 20833005) and STCSM (Nos. 08JC1402000 and 08DZ2270500). References and Notes (1) Yu, X. W.; Pickup, P. G. J. Power Sources 2008, 182, 124. (2) Uhm, S.; Lee, H. J.; Lee, J. Phys. Chem. Chem. Phys. 2009, 11, 9326. (3) Park, S.; Xie, Y.; Weaver, M. J. Langmuir 2002, 18, 5792. (4) Demirci, U. B. J. Power Sources 2007, 173, 11. (5) Watanabe, M.; Horiuchi, M.; Motoo, S. J. Electroanal. Chem. 1988, 250, 117. (6) Leiva, E.; Iwasita, T.; Herrero, E.; Feliu, J. M. Langmuir 1997, 13, 6287. (7) Uhm, S.; Lee, H. J.; Kwon, Y.; Lee, J. Angew. Chem., Int. Ed. 2008, 47, 10163. (8) Peng, B.; Wang, J. Y.; Zhang, H. X.; Lin, Y. H.; Cai, W. B. Electrochem. Commun. 2009, 11, 831. (9) Kristian, N.; Yan, Y. S.; Wang, X. Chem. Commun. 2008, 353.
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