Promoting Effect of Ni in PtNi Bimetallic Electrocatalysts for the

Nov 4, 2010 - Promoting Effect of Ni in PtNi Bimetallic Electrocatalysts for the Methanol Oxidation Reaction in Alkaline Media: Experimental and Densi...
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J. Phys. Chem. C 2010, 114, 19714–19722

Promoting Effect of Ni in PtNi Bimetallic Electrocatalysts for the Methanol Oxidation Reaction in Alkaline Media: Experimental and Density Functional Theory Studies Qian Jiang,†,‡ Luhua Jiang,† Hongying Hou,†,§ Jing Qi,†,‡ Suli Wang,† and Gongquan Sun*,† Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100039, China ReceiVed: May 1, 2010; ReVised Manuscript ReceiVed: September 21, 2010

Carbon-supported bimetallic PtmNin electrocatalysts with different Pt/Ni atomic ratios were synthesized through a modified polyol process. The as-prepared electrocatalysts were characterized by X-ray diffraction, transmission electron microscopy, voltammetry techniques, and single-cell tests. It was revealed that the PtmNin bimetallic nanoparticles were uniformly distributed on carbon supports with average diameters of about 3 nm. Pt and Ni were partially alloyed, indicated by the decreased Pt lattice constants compared with that of pure Pt. The results of the electrochemical measurements showed that the PtmNin/C catalysts, compared with the Pt/C, have superior specific activity toward the methanol electrooxidation reaction (MOR) in alkaline media as well as a higher power density in a direct methanol fuel cell test with the Pt3Ni1/C as the anode catalyst. Density functional theory studies further revealed that the electronic structure of Pt was modified by Ni due to the charge transfer from Ni to Pt atoms in PtmNin clusters, leading to a weakened CO adsorption on PtmNin binary clusters than on Pt itself. This provides an explanation for the enhanced MOR activity of the PtmNin/C catalysts. 1. Introduction Electrocatalysis in alkaline media is a subject of growing interest for its importance in the development of alkaline direct methanol fuel cells.1-4 The use of alkaline media in a fuel cell has many advantages. It is known that the reaction kinetics is more facile for both the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR) in alkaline than in acidic media.5,6 This is partly due to the weak adsorption of considered ions in alkaline media.7,8 In addition, the hydroxide groups in alkaline media further facilitate the methanol dehydrogenation process and the oxidation removal of CO species during the MOR.9,10 Besides, the stability of the electrode material is improved in alkaline media as compared with that in strong acidic environments.11 Presently, Pt is still the best electrode material for lowtemperature proton-exchange membrane fuel cells (PEMFCs) in both its activity and its stability. Methanol oxidation on Pt involves major steps of methanol adsorption and successive dehydrogenation to intermediates, such as CO,10,12,13 and CO oxidation to the final product of CO2. Although CO adsorbs to a lesser extent in alkaline media, it is still the main poisonous species based on some spectroscopic and electrochemical results.14-16 Ru is well-known for its ability to provide OH species for CO oxidation removal by the bifunctional mechanism in acidic media.17,18 However, in alkaline media, no significant improvement in the activity of the Pt/Ru catalysts for the MOR was observed at moderate temperature.19 Nickel and other transition metals as addictives to Pt were widely reported for their superior activity toward the ORR.20-22 Recently, it was * To whom correspondence should be addressed. Tel: +86 0411 84379063. Fax: +86 0411 84379063. E-mail: [email protected]. † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences. § Present address: Universite de Provence-CNRS, UMR 6264, Laboratoire Chimie Provence, Marseille, France.

reported that the addition of Ni to Pt or PtRu could also improve the activity of Pt toward the MOR whether in acid or in alkaline media.11,23-27 However, the promoting mechanism of Ni to Pt in Pt/Ni or PtRu/Ni is not fully developed yet. It was reported that a synergistic effect exists between Ni and Pt or PtRu alloy catalysts.23,24 This proposal was supported by an observation on the bonding energy shift of the Pt in Pt/Ni nanoparticles by XPS measurements,11,25 suggesting that the electronic structure of Pt was modified due to electron transferring from Ni to Pt. A similar electronic effect was also proposed by Liu et al.26 to explain the improved CO tolerance during the hydrogen oxidation reaction on a MoOx@Pt catalyst with a core-shell structure. Some other studies proposed that the improved MOR activity of the PtNi binary catalyst could be explained by the bifunctional mechanism because Ni in alkaline media exists in an oxidized state and the Ni oxide species may offer OH species to remove the intermediate COad.27,28 The aim of this work is to investigate the MOR activities on the PtmNin bimetallic catalysts in alkaline media and, especially, the promoting effect of Ni on Pt. Carbon-supported PtmNin bimetallic electrocatalysts with different Pt/Ni atomic ratios were synthesized by a polyol method and characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), voltammetry techniques, and also DMFC single-cell tests under fuel cell working conditions. More importantly, with the help of density functional theory (DFT) studies, we further explore the interactions between Ptm/PtmNin clusters and the adsorbed CO, as the main poisoning intermediates during MOR, to gain further insights into the promoting mechanism of Ni to Pt. 2. Experimental Section 2.1. Preparation of Electrocatalysts. Carbon-supported PtmNin/C electrocatalysts were synthesized via a modified polyol process as described in previous papers.29,30 A typical process

10.1021/jp1039755  2010 American Chemical Society Published on Web 11/04/2010

PtmNin Bimetallic Electrocatalysts was described as follows. The required amount of precursors (H2PtCl6 · 6H2O and Ni(NO3)2 · 6H2O) was dissolved in 30 mL of 1 M NaOH/ethylene glycol (EG) solution and was further stirred for 0.5 h to get a homogeneous slurry. The metal salts were reduced by EG through heating of the slurry in an oil bath at 180 °C for 4 h under N2 protection. The black colloid was then cooled to room temperature. After that, the ultrasonically dispersed Vulcan XC-72 carbon was added for precipitation. The resulting mixture was filtered and washed with a large amount of distilled water before being dried in a vacuum oven at 75 °C for 8 h. The as-prepared electrocatalysts were further thermally treated at 200 °C under a pure N2 atmosphere for 2 h. By adjusting the amount of the metal precursors, we synthesized electrocatalysts with different Pt/Ni atomic ratios, which are denoted as Pt1Ni1/C, Pt2Ni1/C, and Pt3Ni1/C (which stand for the nominal Pt/Ni atomic ratios of 1:1, 2:1, and 3:1, respectively). The total metal loading (Pt + Ni) of the catalysts was fixed at 40 wt %. A 40 wt % Pt/C and a 40 wt % Ni/C catalyst were also prepared via a similar process for comparison. 2.2. XRD, TEM, and EDX Characterizations. XRD measurements were performed on a Rigaku X-2000 diffractometer using Cu KR radiation with a Ni filter. The tube current was 200 mA with a tube voltage of 40 kV. The 2θ angular regions between 15° and 85° were explored at a scan rate of 5° min-1. TEM analysis was carried out on a JEOL JEM-2000EX microscope operating at 100 kV. Elemental analysis was performed on a scanning electron microscope (JSM-6360 LV) with an energy-dispersive X-ray spectroscope (EDX, Oxford INCA). The compositions of the metals were tested in different regions of one sample, and the values were averaged. 2.3. Voltammetry and Single-Cell Tests. A CHI 760B potentistat/galvanostat was used for electrochemical measurements in a standard three-compartment electrochemical cell. The catalyst ink was prepared as follows: 5 mg of the as-prepared catalyst was dispersed in 2 mL of ethanol containing 40 µL of 5 wt % Nafion. A 20 µL portion of the catalyst ink was then dropped onto the glassy carbon disk (5 mm in diameter) with a metal loading of 0.1 mg cm-2. A Pt foil was used as the counter electrode, and a Hg/HgO/OH- (MMO, -0.114 V vs NHE) electrode, immersed in 1 M NaOH solution, was used as the reference electrode. CV measurements were performed at room temperature in 1 M NaOH solution with/without 1 M CH3OH, depending on the purpose. Chronoamperometry measurements were performed in 1 M NaOH solution containing 1 M CH3OH with the potential holding at -0.2 V. CO stripping was performed via a procedure as follows. The mixture of CO (5%) and N2 (95%) was bubbled into 1 M NaOH solution for 20 min while the potential was held at -0.6 V versus MMO, after which pure N2 was bubbled instead of the gas mixture for more than 30 min to remove the dissolved CO in the solution. The CO stripping voltammograms were then recorded at a scan rate of 20 mV s-1. All potentials in this work were referenced to the MMO electrode. The membrane electrode assembly used in the single-cell test was fabricated by sandwiching the anode and cathode part on either side of a PBI-based membrane.31,32 Both electrodes were prepared by painting the catalysts on the carbon paper. The metal loadings were 2 mg cm-2 for the anode and 1 mg cm-2 for the cathode (40 wt % Pt/C from Johnson Matthey Inc.). The test was performed on a fuel cell test platform (Arbin Corporation, U.S.A.) with a 1 mL min-1 feeding of 1 M CH3OH containing 1 M KOH solution at the anode and 0.2 MPa O2 at the cathode at the temperature of 75 °C. The electrode area is 4 cm2.

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Figure 1. XRD patterns of (a) Pt/C, (b) Pt3Ni1/C, (c) Pt2Ni1/C, (d) Pt1Ni1/C, and (e) Ni/C.

2.4. Theoretical Methods. The DFT calculations were preformed with the Gaussian 03 program.33 The three-parameter hybrid functional of Becke (B3LYP) was used with the exchange term described by the exchange functional of Becke and the nonlocal correlation functional described by the LYP expression.34,35 The Los Alamos LANL2DZ effective core pseudopotential (ECP)36,37 and the corresponding valence double-ζ basis set was adopted for Pt and Ni atoms, whereas the 6-311+G** basis set was used for all C and O atoms. For the bare PtmNin clusters, we carried out full geometry optimization with different geometries and spin states. The geometry with the lowest energy was regarded as the ground state and was further considered for CO adsorption studies. The binding energy (EB) of a bare PtmNin cluster consisting of m Pt atoms and n Ni atoms is calculated as follows

EB ) -(EPt/Ni - mEPt - nENi)/(m + n) where EPt/Ni, EPt, and ENi are the energy of the PtmNin cluster, an isolated Pt atom, and an isolated Ni atom, respectively. The CO adsorption energy is calculated with the following equation

Eads ) -(EM-CO - EM - ECO) where EM-CO, EM, and ECO are the calculated energy of CO adsorbed on Ptm or binary PtmNin clusters, the isolated bare Ptm or binary PtmNin clusters, and the CO molecule, respectively. 3. Results and Discussion 3.1. Physical Characterization of the PtmNin/C Electrocatalysts. The crystalline structures of the Pt/C and the PtmNin/C electrocatalysts were characterized by XRD, as shown in Figure 1. The diffraction peaks at about 40, 46, 67, and 81°, as shown in Figure 1, pattern a, indicate a face-centered cubic structure of Pt for the Pt/C catalyst. For the bimetallic PtmNin/C catalysts, the corresponding peaks shift to higher angles to some extent. The shift of diffraction peaks may indicate an at least partial alloy formation between Pt and Ni. The lattice constants of Pt in the PtmNin/C catalysts estimated by Vegard’s law are listed in Table 1. The decreased lattice constants of Pt in PtmNin/C catalysts, compared with those of Pt, suggesting the replacement of Pt atoms by Ni atoms, further proved the formation of the PtNi alloy.38 Carbon-supported Ni catalyst was also synthesized in order to investigate its catalytic property. The XRD pattern of the as-synthesized Ni/C catalyst is shown in Figure 1, pattern

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TABLE 1: Summary of the Calculated Average Diameters of the Nanoparticles, the Lattice Constants, and the Atomic Ratios of Pt to Ni for the Catalysts average diameter (nm) catalysts

XRD

TEM

Pt/C Pt3Ni1/C Pt2Ni1/C Pt1Ni1/C

2.6 2.4 2.2 2.9

3.0 2.8 2.7 3.3

a

nominal actual lattice constant atomic ratio atomic ratio (Å) of Pt:Ni of Pt:Nia 3.920 3.896 3.845 3.811

3:1 2:1 1:1

2.98:1 2.07:1 1.13:1

Analyzed by the EDX technique.

e. The three diffraction peaks at 44, 52, and 73° could be assigned to the Ni(111), Ni(200), and Ni(220) diffraction indexes of the face-centered cubic structure of Ni. No diffraction peaks for Ni oxide species were detected in the XRD pattern for the Ni/C. The average metal particle sizes of the PtmNin/C, calculated through the Scherrer’s equation39 by analyzing the Pt(220) peak, are listed in Table 1. The average particle sizes are approximately 3 nm for the Pt/C and PtmNin/C catalysts. Because the particle sizes are nearly identical for all the catalysts, the influence of the particle size on their catalytic activities could be excluded. TEM images of the Pt/C and the PtmNin/C catalysts presented in Figure 2 confirmed that the metal nanoparticles of the Pt/C and the PtmNin/C catalysts are about 3 nm in size and are well distributed on the carbon support without obvious agglomeration. The compositions of the as-synthesized PtmNin/C catalysts, characterized by the EDX technique, are also listed in Table 1. The real atomic ratios of Pt to Ni for Pt3Ni1/C, Pt2Ni1/C, and Pt1Ni1/C are 2.98:1, 2.07:1, and 1.13:1, respectively, which are very close to the nominal values. 3.2. Activity of the PtmNin/C Catalysts toward the MOR. Base CVs for the Pt/C and PtmNin/C electrocatalysts in 1 M NaOH solution are shown in Figure 3. As can be seen from

Figure 3. Base CV measurements of Pt/C, Pt1Ni1/C, Pt2Ni1/C, and Pt3Ni1/C catalysts in 1 M NaOH solution at room temperature. The scan rate is 50 mV s-1.

Figure 3, the hydrogen adsorption/desorption regions (-0.8 to -0.5 V) were well-defined for the Pt/C catalysts but decreased for the PtmNin/C catalyst compared with that of the Pt/C. The electrochemical surface areas (ESAs) calculated from the hydrogen desorption area were 37.7, 33.6, 28.9, and 22.8 m2 g-1 for the Pt/C, Pt3Ni1/C, Pt2Ni1/C, and Pt1Ni1/C, respectively. It should be noted that the calculated ESA values of Pt-based catalysts in alkaline media were lower than the corresponding values reported previously in acidic media.40,41 This is probably due to the large amount of hydroxide species in alkaline media adsorbed on the catalyst surface and blocking the hydrogen adsorption/desorption because the OHad adsorption starts at very negative potentials even beginning from the hydrogen adsorption/desorption region.42 The ESA decreased with the increasing of the Ni content in the PtmNin/C catalysts, which may be ascribed to the decreased Pt sites for hydrogen adsorption. The CV measurements for the Pt/C and the PtmNin/C catalysts in 1 M NaOH solution containing 1 M methanol are shown in

Figure 2. TEM images of (a) Pt/C, (b) Pt3Ni1/C, (c) Pt2Ni1/C, and (d) Pt1Ni1/C.

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Figure 5. Chronoamperometry tests of the Pt/C, Pt1Ni1/C, Pt2Ni1/C, and Pt3Ni1/C catalysts in 1 M NaOH and 1 M CH3OH solution at room temperature.

Figure 4. (a) CV measurements of Pt/C, Pt1Ni1/C, Pt2Ni1/C, and Pt3Ni1/C catalysts in 1 M NaOH and 1 M CH3OH solution at room temperature with the scan rate of 50 mV s-1. The current densities are normalized to the geometric area of the electrode. (b) Current densities at -0.2 V in (a) are taken but normalized to the mass of Pt.

Figure 4a. The MOR currents are normalized to the geometric surface area of the electrode (0.196 cm2). It can be seen from this figure that the current densities at -0.2 V during the forward scanning are 41.1, 35.3, 42.3, and 48.5 mA cm-2 for the Pt/C, Pt1Ni1/C, Pt2Ni1/C, and Pt3Ni1/C, respectively, following the order of Pt1Ni1/C < Pt/C < Pt2Ni1/C < Pt3Ni1/C. The currents are only slightly higher for the Pt3Ni1/C and the Pt2Ni1/C than for the Pt/C. The current densities normalized to the Pt mass (measured from the EDX results) were also compared and are plotted in Figure 4b. For the Pt3Ni1/C, Pt2Ni1/C, and Pt1Ni1/C catalysts, the Pt-mass normalized current densities at -0.2 V during the forward scanning are 559, 512, and 477 mA mg-1Pt, respectively. These values are all higher than that of the Pt/C, with an increment of about 36, 25, and 16% for the Pt3Ni1/C, Pt2Ni1/C, and Pt1Ni1/C, respectively. Chronoamperometry measurements performed in 1 M NaOH solution containing 1 M methanol are shown in Figure 5. The potential were held at -0.2 V during the measurements. It is shown that the current densities (normalized to the geometric surface area of the electrode) decrease slowly for both the Pt/C and the PtmNin/C catalysts. However, the Pt3Ni1/C and the Pt2Ni1/C catalysts exhibited higher current densities than the Pt/C catalyst during the measurements, indicating that the Pt3Ni1/C and the Pt2Ni1/C catalysts were more active for the MOR than that of Pt/C, which is consistent with the CV results. The degradation rates for the Pt/C, Pt1Ni1/C, Pt2Ni1/C, and Pt3Ni1/C catalysts are basically the same, with a value of around 0.10 mA cm-2 min-1, indicating that the stability of the PtmNin/C catalysts is

Figure 6. CV measurements of the Ni/C catalyst in 1 M NaOH solution (a) without or (b) with 1 M CH3OH at room temperature. The scan rate is 50 mV s-1.

comparable to that of the Pt/C catalyst. Actually, as discussed by Park et al.,11 the Ni hydroxide layer on the surface could protect it from corrosion even in acidic media. To investigate the property of Ni and its activity toward the MOR in alkaline media, we first carried out the cyclic voltammetry measurement of the Ni/C electrocatalysts in 1 M NaOH solution (Figure 6, curve a) and then in 1 M NaOH containing 1 M methanol (Figure 6, curve b). It can be observed from curve a in Figure 6 that an anodic peak appeared at about 0.5 V, which was attributed to be the formation of surface NiOOH species.43,44 Further scanning to more positive potentials, the oxygen evolution on Ni occurred, indicated by the sharp increase of the anodic current. When methanol was involved, as shown in curve b in Figure 6, a significant current peak for methanol oxidation could be observed with the onset potential of 0.45 V. It should be noted that the MOR started nearly at the same potential as that when the surface NiOOH began to form, as can be clearly seen from the inset in Figure 6. Nickel was reported previously as an active MOR catalyst in alkaline media,45 and a redox mechanism was proposed by Fleischmann et al. in which methanol was oxidized on Ni through the reaction with NiOOH to form Ni(OH)2,46,47 as shown in the following equation:

NiOOH + MeOH f Ni(OH)2 + intermediates Our CV results agree well with the redox mechanism above. However, it should be pointed out that the MOR on Ni proceeded at a potential higher than 0.45 V, which is much

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Figure 7. CO stripping measurements of the Pt/C, Pt3Ni1/C, Pt2Ni1/C, and Pt1Ni1/C catalysts performed in 1 M NaOH solution with the scan rate of 20 mV s-1 at room temperature. The red solid line indicates the first stripping cycle, whereas the black dashed line is the second cycle.

higher than the MOR peak potentials for the PtmNin/C catalysts (which are lower than 0.1 V; see Figure 4). This excludes the redox mechanism to be the reason for the activity enhancement on the PtmNin/C bimetallic catalysts. To further investigate the promoting effect of Ni to Pt, CO stripping experiments were performed in 1 M NaOH solution with the preadsorption of CO at the potential of -0.6 V. As shown in Figure 7, for the Pt/C and PtmNin/C catalysts investigated, no obvious hydrogen desorption peaks in the first cycle (red solid line) were observed, indicating a saturated CO adsorption on the catalysts. No obvious CO oxidation peaks in the second cycle (black dashed line) were observed, indicating that the electrolyte was free of dissolved CO. A sharp CO oxidation peak was observed for Pt/C at about -0.26 V, whereas a relatively broad peak was observed for the PtmNin/C catalyst. The broad CO oxidation peak for the PtmNin/C catalyst, in contrast with the sharp peak on the Pt/C catalyst, may be due to the inhomogeneities of the catalyst or the formation of the PtNi alloy, as also observed on a PtRu alloy catalyst.48 The integrated charge for CO stripping was usually used to calculate the ESA of the catalysts. However, some uncertainties in alkaline media, such as the unknown CO-bonding type,49 CO stripping charge correction in respect to contributions from the double-layer charging, and metal oxide formation, still exist.50 The problem was even more significant for the PtNi alloy catalyst in alkaline media because the hydroxide adsorption or oxide formation on Ni overlaps with CO adsorption and oxidation. Thus, the ESA values calculated from this method were much lower than the corresponding values calculated from the hydrogen desorption method. Nevertheless, comparing with the Pt/C catalyst, the decreased CO stripping currents and peak areas of the bimetallic PtmNin/C catalysts may be ascribed to (i) the decreased Pt sites for the CO adsorption and/or (ii) the electronic effect between Pt and Ni, as proposed in ref 23, resulting in the decreased/weakened CO adsorption on the bimetallic catalysts’ surface. Single-cell tests were performed with the Pt/C and Pt3Ni1/C as the anode catalysts. As shown in Figure 8, the open-circuit

Figure 8. Single-cell tests performed with the Pt/C and the Pt3Ni1/C as the anode catalyst, respectively. The metal loading for the anode is 2 mg cm-2. The cathode catalyst is 40 wt % Pt/C (Johnson Matthey), Inc.; the Pt loading is 1 mg cm-2. The anode feed is 1 M NaOH + 1 M CH3OH with a flow rate of 1 mL min-1; the cathode feed is O2 under the pressure of 0.2 MPa. The cell temperature is 75 °C.

voltage (OCV) of the single cell was as high as 1 V with the Pt3Ni1/C as the anode catalysts, which was 50 mV higher than that with the Pt/C as the anode catalyst. It also should be noted that the OCV values were much higher than the single cell tested in acidic media. The high OCV values presented here may indicate the faster kinetics of both the MOR and the ORR in alkaline media. The maximum power densities with Pt3Ni1/C and Pt/C as the anode catalysts are 19 and 13 mW cm-2, respectively, with the anode feed of 1 M KOH and 1 M methanol at 75 °C. The cell performance presented here is lower than our previously reported results of an alkaline direct ethanol fuel cell (DEFC).31 The reasons may be due to the lower alkaline and methanol concentration used in this work as well as the carbonization problem because CO2 as a main methanol oxidation product could react with the KOH in the solution, blocking the electrode. It is known that the MOR on the Pt surface involves the dehydrogenation of methanol to intermediates, such as CO,

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TABLE 2: Summary of the DFT Calculation Results: The Optimized Geometric Structures, the Binding Energy (EB), and the Mulliken Charge for Bare Ptm and PtmNin Clustersa

a

The dark blue and light blue balls in the structures represent the Pt and Ni atoms, respectively.

and the CO oxidation removal to the product of CO2. Thus, an improved CO oxidation removal process as well as decreased CO adsorption strength may result in an improved MOR rate. It is proposed that Ni, when alloyed with Pt, may modify the electronic structure of Pt and facilitate the MOR process by decreasing the CO poisoning. We will discuss this in detail combined with our DFT calculations in the following section. Nevertheless, Ni oxide species could form at relatively negative potentials, which may also serve as the oxygen donor for CO oxidation removal and facilitate the MOR process as well.28 3.3. Simulation and Computation of CO Adsorption on Ptm and PtmNin Clusters. To investigate further the promoting mechanism of Ni to Pt in the PtmNin/C catalysts for the enhanced MOR activity, we carried out DFT studies of CO adsorption on Ptm and PtmNin clusters. We performed the DFT studies on CO adsorption rather than on methanol adsorption because CO is the main poisoning species and CO oxidative removal is the rate-determining step during the MOR. The investigation of CO adsorption properties could be helpful to further understanding the poisoning/promoting mechanism of the catalysts. The cluster

approximation approach was chosen where a finite cluster is used to describe the structures of the catalysts. Even though the cluster approximation approach may not fully represent the actual solid surface of the Pt/C or PtmNin/C catalysts, it does allow us performing the DFT calculations and simulating the catalytic processes well enough to gain insight into the reaction mechanism. Full geometric optimizations were first performed on Ptm and bimetallic PtmNin clusters starting from different spin states and geometries. The calculated results, including the optimized geometric structures, the binding energy of a cluster (EB), and the Mulliken charge of the clusters, are summarized in Table 2. Some stable isomers are also listed in the table for comparison. It is clearly seen from Table 2 that the EB of the bimetallic PtmNin clusters are all higher than the corresponding Ptm clusters. For example, the EB are 2.328 and 2.602 eV for Pt3 and Pt2Ni1 clusters and 2.581 and 2.827 eV for tetrahedral Pt4 and Pt3Ni1 clusters, respectively. The increased EB values for the bimetallic clusters indicated that they are more stable

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TABLE 3: Summary of the DFT Calculation Results: The Optimized Geometric Structures, the Bond Lengths of M-C (dM-C) and CO (dCO), and the Vibrational Frequencies of M-C (ωM-C) and CO (ωCO)a

a

The dark blue, light blue, gray, and red balls in the structures represent the Pt, Ni, C, and O atoms, respectively.

than the corresponding Ptm clusters. It is worth mentioning that all the Pt atoms are negatively charged, whereas the Ni

atoms are positively charged, indicating a partial charge transfer from Ni to Pt atoms in PtmNin clusters. This charge

PtmNin Bimetallic Electrocatalysts transfer may change the electronic structures of the Pt as well as the surface chemical properties. The simulation of CO adsorption on the optimized Ptm and bimetallic PtmNin clusters was also carried out. The calculation results, including the stable adsorption structures of Ptm-CO and PtmNin-CO clusters, the CO adsorption energy (Eads), the bond lengths of M-C (dM-C, M stands for Pt or Ni; C is the carbon atom in a CO molecule) and CO (dC-O), and the vibrational frequencies of M-C (ωM-C) and CO (ωC-O) of Ptm-CO and PtmNin-CO clusters, are all summarized in Table 3. It can be seen that the calculated CO bond length is in the range of 1.134-1.206 Å, which agrees well with the reported experimental value of 1.15 ( 0.05 Å,51 confirming the validity of the calculation method. The calculated CO frequency is between 1719 and 2144 cm-1. Compared to the CO frequency of 2134 cm-1 in the gas phase,52 substantial red shift of the CO frequency was observed in most of the PtmNin-CO complexes. This frequency shift of CO adsorption on other metal clusters was also observed,53,54 and an assumption was being made that the origin of the frequency shift was caused by modified charge donation and back-donation.55 The adsorption strength may vary depending on the adsorption sites, so three CO adsorption sites, that is, atop sites, bridge sites, and hollow sites, on Ptm or PtmNin clusters were investigated. For bare Ptm clusters, the CO adsorption energy on atop, bridge, and hollow sites follows a decreasing order. For example, the Eads for atop, bridge, and hollow sites of the Pt3-CO cluster is 2.089, 1.979, and 1.505 eV, respectively, indicating that the adsorption on atop sites is dominant for the same cluster composition. The CO adsorption on Pt2-CO and Pt4-CO clusters, as seen in Table 3, follows the same trend. The favorable atop site adsorption of CO on Ptm clusters was also reported by other DFT studies.56 Moreover, it is important to find that the Eads for the bimetallic PtmNin-CO clusters, excluding the Pt1Ni1-CO cluster, are lower than those for the corresponding Ptm-CO clusters of the same cluster configuration. For example, the Eads for atop sites of the Pt2Ni1-CO and tetrahedral Pt3Ni1-CO clusters are 1.562 and 1.289 eV, respectively, which are much lower than the corresponding values for the Pt3-CO and tetrahedral Pt4-CO clusters (2.089 and 2.293 eV, respectively). The calculated Eads for the bimetallic Pt2Ni1-CO and Pt3Ni1-CO clusters is in the range of 0.184-1.684 eV, all lower than those for atop site adsorbed Pt3-CO and tetrahedral Pt4-CO clusters. The decreased CO adsorption strength on Pt-based alloys, such as PtRu, PtSn, PtGe, and PtMo, was also observed in other DFT studies.55,57 The origin of the strong CO adsorption on the bare Pt surface can be explained by the π-back-donation mechanism.58 In this mechanism, charge transfer from the 5σ orbital of CO molecules to metal atoms occurred together with a back-donation of charge from the d band of metal atoms to the antibonding 2π* orbital of CO molecules. For PtmNin bimetallic clusters, the electronic structure of Pt was modified by the charge transfer from Ni to Pt atoms, as discussed above. This charge transfer results in an increase of the electron density of the Pt d orbital and further inhibited the charge transfer from the 5σ orbital of CO molecules to Pt atoms. This modification of the electronic structures of Pt by Ni weakens the interactions between PtmNin clusters and adsorbed CO and thus may result in the decreased CO poisoning as well as the enhanced MOR activity. 4. Conclusions Carbon-supported bimetallic PtmNin/C electrocatalysts with different atomic ratios were synthesized via the polyol process.

J. Phys. Chem. C, Vol. 114, No. 46, 2010 19721 The XRD and TEM characterizations indicated that the bimetallic PtmNin nanoparticles are at least partially alloyed and uniformly distributed on the carbon support with the average particle size of about 3 nm. The electrochemical measurements showed enhanced specific activities for the MOR and decreased CO adsorption on the bimetallic PtmNin/C catalysts than for the Pt/C catalyst in alkaline media. A higher power density of the single cell using Pt3Ni1/C as an anode catalyst was obtained. DFT studies revealed that the Pt electronic structure was modified through partial charge transfer from Ni to Pt. The modification of the Pt electronic structure weakened the interaction between the bimetallic PtmNin clusters and adsorbed CO and was supposed to be responsible for the decreased CO poisoning and improved MOR activity. Acknowledgment. This work was financially supported by the National Hi-Technology Research & Development Program (2007AA05Z159 and 2009AA05Z121) and the “100-TalentsProgram” of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences. References and Notes (1) Jayashree, R. S.; Egas, D.; Spendelow, J. S.; Natarajan, D.; Markoski, L. J.; Kenis, P. J. A. Electrochem. Solid-State Lett. 2006, 9, A252. (2) Varcoe, J. R.; Slade, R. C. T. Electrochem. Commun. 2006, 8, 839. (3) Liu, J. P.; Ye, J. Q.; Xu, C. W.; Jiang, S. P.; Tong, Y. X. Electrochem. Commun. 2007, 9, 2334. (4) Yu, E. H.; Scott, K. J. Power Sources 2004, 137, 248. (5) Prabhuram, J.; Manoharan, R. J. Power Sources 1998, 74, 54. (6) Meng, H.; Shen, P. K. Electrochem. Commun. 2006, 8, 588. (7) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. J. Chem. Phys. 2001, 115, 8193. (8) Morallon, E.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1992, 334, 323. (9) Samant, P. V.; Fernandes, J. B. J. Power Sources 2004, 125, 172. (10) Spendelow, J. S.; Goodpaster, J. D.; Kenis, P. J. A.; Wieckowski, A. Langmuir 2006, 22, 10457. (11) Park, K. W.; Choi, J. H.; Kwon, B. K.; Lee, S. A.; Sung, Y. E.; Ha, H. Y.; Hong, S. A.; Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 1869. (12) Beden, B.; Kadirgan, F.; Lamy, C.; Leger, J. M. J. Electroanal. Chem. 1982, 142, 171. (13) Spendelow, J. S.; Lu, G. Q.; Kenis, P. J. A.; Wieckowski, A. J. Electroanal. Chem. 2004, 568, 215. (14) Morallon, E.; Rodes, A.; Vazquez, J. L.; Perez, J. M. J. Electroanal. Chem. 1995, 391, 149. (15) Caram, J. A.; Gutierrez, C. J. Electroanal. Chem. 1992, 323, 213. (16) Perez, J. M.; Munoz, E.; Morallon, E.; Cases, F.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1994, 368, 285. (17) Watanabe, M.; Motoo, S. J. Electroanal. Chem. Interfacial Electrochem. 1975, 60, 267. (18) Yajima, T.; Wakabayashi, N.; Uiroyuki, H.; Watanaabe, M. Chem. Commun. (Cambridge, U.K.) 2003, 82. (19) Tripkovic, A. V.; Popovic, K. D.; Grgur, B. N.; Blizanac, B.; Ross, P. N.; Markovic, N. M. Electrochim. Acta 2002, 47, 3707. (20) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (21) Mukerjee, S.; Srinivasan, S. J. Electroanal. Chem. 1993, 357, 201. (22) Stamenkovic, V. R.; Mun, B. S.; Arenz., M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241. (23) Fu, X. Z.; Liang, Y.; Chen, S. P.; Lin, J. D.; Liao, D. W. Catal. Commun. 2009, 10, 1893. (24) Skowronski, J. M.; Wazny, A. Mater. Sci. 2006, 24, 291. (25) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750. (26) Liu, Z. F.; Hu, J. E.; Wang, Q.; Gaskell, K.; Frenkel, A. I.; Jackson, G. S.; Eichhorn, B. J. Am. Chem. Soc. 2009, 131, 6924. (27) Spendelow, J. S.; Babu, P. K.; Wieckowski, A. Curr. Opin. Solid State Mater. Sci. 2005, 9, 37. (28) Shen, P. K.; Xu, C. W.; Zeng, R.; Liu, Y. L. Electrochem. SolidState Lett. 2006, 9, A39. (29) Zhou, Z. H.; Wang, S. L.; Zhou, W. J.; Xin, Q.; Sun, G. Q. Chem. Commun. 2003, 394. (30) Li, W. Z.; Zhou, W. J.; Li, H. Q.; Zhou, Z. H.; Zhou, B.; Sun, G. Q.; Xin, Q. Electrochim. Acta 2004, 49, 1045.

19722

J. Phys. Chem. C, Vol. 114, No. 46, 2010

(31) Hou, H. Y.; Sun, G. Q.; He, R. H.; Wu, Z. M.; Sun, B. Y. J. Power Sources 2008, 182, 95. (32) Hou, H. Y.; Sun, G. Q.; He, R. H.; Sun, B. Y.; Jin, W.; Liu, H.; Xin, Q. Int. J. Hydrogen Energy 2008, 33, 7172. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision A.1; Gaussian, Inc.: Pittsburgh, PA, 2003. (34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (35) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (37) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (38) Kim, H. J.; Choi, S. M.; Nam, S. H.; Seo, M. H.; Kim, W. B. Catal. Today 2009, 146, 9. (39) Radmilovic, V.; Gasteiger, H. A.; Ross, P. N. J. Catal. 1995, 154, 98. (40) Li, H. Q.; Sun, G. Q.; Li, N.; Sun, S. G.; Su, D. S.; Xin, Q. J. Phys. Chem. C 2007, 111, 5605.

Jiang et al. (41) Qi, J.; Yan, S. Y.; Jiang, Q.; Liu, Y.; Sun, G. Q. Carbon 2010, 48, 163. (42) Schmidt, T. J.; Ross, P. N.; Markovic´, N. M. J. Phys. Chem. B 2001, 105, 12082. (43) Vukovic, M. J. Appl. Electrochem. 1994, 24, 878. (44) Enea, O. Electrochim. Acta 1990, 35, 375. (45) Taraszewska, J.; Roslonek, G. J. Electroanal. Chem. 1994, 364, 209. (46) Fleischmann, M.; Korinek, K.; Pletcher, D. J. Electroanal. Chem. 1971, 31, 39. (47) El-Shafei, A. A. J. Electroanal. Chem. 1999, 471, 89. (48) Vidakovic, T.; Christov, M.; Sundmacher, K. Electrochim. Acta 2007, 52, 5606. (49) Gilman, S. J. Phys. Chem. 1962, 66, 2657. (50) Nart, F.; Vielstich, W. In Handbook of Fuel CellssFundamentals, Technology and Applications; Vielstich, W., Gasteiger, H. A., Lammy, A., Eds.; John Wiley & Sons: Chichester, U.K., 2003; Vol. 2, Chapter 21 and references therein. (51) Ogletree, D. F.; van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1987, 173, 251. (52) Ge, Q. F.; Song, C. R.; Wang, L. C. Comput. Mater. Sci. 2006, 35, 247. (53) Sadek, M. M.; Wang, L. C. J. Phys. Chem. A 2006, 110, 14036. (54) Joshi, A. M.; Delgass, W. N.; Thomson, K. T. J. Phys. Chem. C 2007, 111, 11424. (55) Liao, M. S.; Cabrera, C. R.; Ishikawa, Y. Surf. Sci. 2000, 445, 267. (56) Song, C. R.; Ge, Q. F.; Wang, L. C. J. Phys. Chem. B 2005, 109, 22341. (57) Wang, J. G.; Hammer, B. J. Catal. 2006, 243, 192. (58) Blyholder, G. J. Phys. Chem. 1964, 68, 2772.

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