Electrochemical Synthesis, Voltammetric Behavior, and

Jan 30, 2008 - Jinan 250100, China, School of Chemistry and Chemical ... Testing Center, Shandong Normal UniVersity, Jinan 250014, China, and College ...
2 downloads 0 Views 328KB Size
Download PDF
2456

J. Phys. Chem. C 2008, 112, 2456-2461

Electrochemical Synthesis, Voltammetric Behavior, and Electrocatalytic Activity of Pd Nanoparticles Wei Pan,†,‡ Xiaokai Zhang,§,| Houyi Ma,*,†,‡ and Jintao Zhang‡ Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong UniVersity, Jinan 250100, China, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China, Analysis and Testing Center, Shandong Normal UniVersity, Jinan 250014, China, and College of Physics and Electronics, Shandong Normal UniVersity, Jinan 250014, China ReceiVed: October 17, 2007; In Final Form: December 3, 2007

Size-controlled Pd nanoparticles were electrochemically synthesized in large quantities at room temperature through the direct electroreduction of bulk PdCl42- ions in the presence of poly(N-vinylpyrrolidone) (PVP), a water-soluble polymer. The technological key to electrochemical synthesis of Pd particles is that PVP greatly promoted the formation process of Pd particles and inhibited the electrodeposition process of Pd onto the Pt cathode at the same time besides effectively stabilizing the formed Pd nanoparticles. The as-synthesized Pd nanoparticles display the voltammetric behavior significantly different from the bulk Pd and also present a high catalytic activity toward the electrooxidation of methanol.

1. Introduction Pd is an important transition metal with high catalytic activity,1,2 although it is not used as widely as Pt at the present time.3 Recently, much attention has been focused on the preparation4 of Pd nanoparticles and their application in the fields of catalysis,5 hydrogen storage,6 and chemical sensors7 because of the large surface area-to-volume ratio, relatively lower price than Pt, and especially the unique function in absorption of hydrogen.8,9 Small Pd and other noble metal particles are usually prepared by chemical reduction of metal salts, however, electrochemical synthesis based on the use of surfactants10,11and polymers12 as the stabilizing agents for metal nanoparticles provides a simple yet effective method. Reetz et al.10a demonstrated for the first time that it was feasible to electrochemically prepare the sizeselective transition metal nanoparticles via the direct electroreduction of metal ions in organic electrolyte solutions. This type of electrochemical method was further developed by our group to prepare in large quantities well-dispersed silver and gold nanoparticles in aqueous electrolyte solutions.12 Herein selecting a short-chain poly(N-vinylpyrrolidone) polymer (PVPK17) as the stabilizer for Pd nanoclusters, we synthesized size-controlled Pd nanoparticles through the electrochemical reduction of bulk Pd(II) ions directly from the aqueous solutions at the cathode/ solution interface, which makes possible large-scale preparation of the well-defined Pd particles in comparison with Reetz’ sacrificial anode method.10 Besides, the main advantages of the present method include simple operation, high yield, and absence of undesired byproducts. One of the key problems for acquirement of metal nanoparticles is how to minimize the metal deposition process on the * Corresponding author. E-mail: [email protected]. Phone: +86-53188364959. Fax: +86-531-88564464. † Key Laboratory for Colloid and Interface Chemistry of State Education Ministry. ‡ School of Chemistry and Chemical Engineering. § Analysis and Testing Center. | College of Physics and Electronics.

cathode during electrochemical synthesis according to Rodrigues-Sanchez et al.13 Considering that a metal is readily electrodeposited onto a foreign metal substrate with similar lattice structure, in this paper, we purposefully selected Pt which has identical lattice structure, similar atomic radii, and comparable lattice energy14 with Pd as the cathode materials to investigate how the stabilizing agent (PVP) promotes the formation process of Pd particles, thereby giving reliable evidence that the nanoparticle stabilizer plays a more important role than the cathode materials do during electrochemical synthesis of metal nanoparticles. In addition, electrochemical measurements indicated that the as-synthesized nanosized Pd particles displayed voltammetric characteristics quite different from those of bulk Pd, with few problem of massive hydrogen absorption,15,16 and also exhibited a high electrocatalytic activity toward the oxidation of methanol. These new findings are of fundamental importance to have a better understanding of electrochemical formation mechanism of nanoparticles and also helpful for the development of Pd nanoparticles as the catalysts in direct methanol fuel cells (DMFCs). 2. Experimental Section 2.1. Electrochemical Synthesis of Pd Nanoparticles. Electrochemical synthesis of Pd nanoparticles was performed in a two-electrode cell. A rotating Pt electrode made from a 2.0 mm diameter Pt rod (Aldrich, 99%) was used as the cathode, and a 1.0 cm × 2.0 cm Pt plate was used as the anode. The rotation speed of the cathode was controlled at 1000 rpm. All chemicals were of analytical grade and were used as received from the suppliers. The electrolytic solutions were basically composed of KNO3, H2PdCl4, and poly(N-vinylpyrrolidone) (PVP) (Alfa Aesar, K17: M h W ≈ 8000), in which PVP served as the stabilizer for Pd nanoparticles. The electrolysis was carried out in the galvanostatic manner at room temperature (∼22 °C). The current density and electrolysis time were chosen depending on the experimental requirements. The shape and size of the as-synthesized Pd nanoparticles were characterized by a Hitachi H-800 TEM operated at 100 kV accelerating

10.1021/jp710092z CCC: $40.75 © 2008 American Chemical Society Published on Web 01/30/2008

Electrochemical Synthesis of Pd Nanoparticles voltage. Samples were prepared by adding ethanol to a fraction of the Pd colloid electrochemically synthesized, and a droplet of it was dropped on a carbon-coated copper grid. 2.2. Electrodeposition of Pd in Solutions with and without PVP. All electrochemical measurements were carried out in a conventional three-electrode cell at ambient temperature by using a CHI 760C electrochemical workstation. A Pt wire of 0.8 mm diameter (Aldrich, 99.9%) embedded in the epoxy resin mold was used as the working electrode, and only its cross section was allowed to contact the electrolytes. Before each experiment, the Pt electrode was ground with emery papers of decreasing particle size to #2000 finish, then rinsed with triple distilled water and absolute ethanol in proper order. A 1.0 cm × 2.0 cm Pt plate and a saturated calomel electrode (SCE) were used as the counter electrode and the reference one, respectively. The electrolytic solutions were deaerated by N2 bubbling for 10 min prior to each measurement, and a blanket of N2 was maintained throughout the whole experiment. Cyclic voltammograms (CVs) for the Pt electrode in 0.1 mol dm-3 KNO3 + 5.0 × 10-4 mol dm-3 H2PdCl4 mixed solutions with and without PVP were acquired through linear potential scan from the open-circuit potential (Eocp) to the designated negative vertex potential (-0.40 V vs SCE), then to the given positive vertex potential (1.40 V vs SCE), and finally backward to the initial potential at a scan rate of 50 mV s-1. 2.3. Voltammetric Behavior and Electrocatalytic Activity of Pd Nanoparticles. Pd nanoparticles were used as unsupported catalysts or carbon-supported catalysts. In the former case, a few drops of ethanol suspension that contained the Pd nanoparticles collected by centrifugation was first placed on the surface of a glass carbon electrode (GCE), then a drop of 5% Nafion solution (DuPont) was put on the electrode surface after the ethanol evaporated at ambient temperature. In the latter case, Pd nanoparticles were supported on high surface area Vulcan XC-72 carbon powders (Cabot, ∼250 m2 g-1) by admixing a dispersion of Pd colloid with a suspension of Vulcan carbon supports in ethanol. The mixture was at first sonicated for 30 min, followed by 48 h vigorous stirring. Subsequently, the carbon-supported catalysts were filtrated, washed repeatedly with triple distilled water, and dried at room temperature (∼22 °C) under ambient conditions or at 80 °C in vacuum for 24 h in proper order. Voltammetric behavior of Pd nanoparticles and their electrocatalytic activity were characterized at room temperature in 0.5 mol dm-3 H2SO4 solution and 1.0 mol dm-3 KOH + 1.0 mol dm-3 CH3OH mixed solution, respectively. The electrolytes were deoxygenated before each electrochemical measurement. The unsupported Pd electrocatalysts were used directly, whereas the working electrodes of carbon-supported Pd nanoparticles were prepared according to the following procedures: (i) dispersing the catalyst powders in 5% Nafion solution under strong ultrasonic vibration, and (ii) putting a drop of the dispersion onto a GCE embedded in a Teflon mold, followed by natural evaporation of solvent in air. A Pt plate of 1.0 cm × 2.0 cm was employed as the counter electrode, and a reversible hydrogen electrode (RHE) and an SCE were selected as the reference electrodes in acidic and alkaline solutions, respectively. Electrochemically active surface areas (EASAs, i.e., real surface areas) of Pd nanoparticles were estimated from the charge needed to form surface oxide monolayers according to the oxygen adsorption measurement method suggested by Woods et al.17 and Trasatti and Petrii.18 All current densities for the Pd nanoparticle electrodes have been normalized by the real surface areas.

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2457

Figure 1. Cyclic voltammograms (CVs) for a Pt electrode in 0.1 mol dm-3 KNO3 + 5.0 × 10-4 mol dm-3 H2PdCl4 mixed solutions without (a) and with (b) PVPK17. Scan rate: 50 mV s-1.

3. Results and Discussion 3.1. PVP’s Role in Inhibiting the Pd Deposition Process. It is a common phenomenon that the reduction of metal ions at a cathode will lead to formation of a new solid phase on the foreign solid substrate when a sufficient amount of electric current passes through the electrolyte solution containing the metal ions. This process is most commonly referred to as electroplating or electrochemical deposition. Theoretically, deposition of Pd onto the Pt cathode should take place with ease and even directly lead to the epitaxial growth of Pd thin films on the Pt substrate since both metals have identical lattice structure, similar atomic radii, and comparable lattice energy.14 Fabrication of ultrathin Pd overlayers on single crystalline Pt surfaces via electrochemical deposition methods has been studied intensively by Kolb et al.1,19,20 Our recent study has demonstrated that nanostructured Pd thin films can be directly formed on the Pt surface by continuously sweeping potential in the aqueous solution that contains H2PdCl4.16 Here, the electrodeposition of Pd on the Pt electrode surface in the mixed solutions of KNO3 and H2PdCl4 was proven by the unusual voltammetric behavior of the Pt electrode. For electrochemical reduction process of Pd(II) ions at the Pt electrode, the prominent feature of cyclic voltammograms (CVs) measured in the PVP-free electrolyte (Figure 1a) is that the CVs present a pair of sharp current peaks between -0.34 and -0.30 V in the hydrogen region and a wide reduction peak around 0.33 V in the oxide region. The continuous potential cycling led to formation of nano- and submicrometer-sized Pd particles on the Pt electrode, and moreover, the dissolution and replating of the palladium repeatedly proceeded.16 The resulting large Pd particles are able to show the standard electrochemical behavior for hydrogen absorption-desorption on and into palladium in acidic solutions.21 We therefore have reason to attribute two sharp peaks to the hydrogen absorption-desorption

2458 J. Phys. Chem. C, Vol. 112, No. 7, 2008 on the Pd islands or large particles. On closer inspection, the CVs still show another pair of weak peaks at -0.18 and -0.16 V, which should be associated with the formation of adsorbed hydrogen and its electrooxidation at the Pd thin film surface, respectively.16 The preceding CV profile was greatly changed with adding PVP polymers to the aqueous solution, especially two sharp characteristic peaks were suppressed to a considerable degree and the previous cathodic peak corresponding to reduction of the palladium oxides almost disappeared, as shown by Figure 1b. These distinct differences in CV profiles before and after addition of PVP polymers reveal such a result that PVP polymers are able to strongly inhibit the electrodeposition process of Pd on the Pt surface even if Pd has a strong tendency to deposit on the Pt surface. This is of great importance to give an in-depth understanding of the electrochemical formation mechanism of metal particles with the help of stabilizers. Rodriguez-Sanchez et al. 13a proposed that electrochemical reduction of metal ions in the electrolyte containing a stabilizer (capping molecule) involved two competing processes: (i) the particle formation through reduction and stabilization by the stabilizer and (ii) formation of metal film on the working electrode surface via electrodeposition. The latter process must be effectively inhibited in order to maximize the amount of colloidal metal particles produced.22 According to the viewpoint, it is necessary to choose a conductive material whose lattice structure, atomic radii, and other structural parameters are quite different from those of the metal to be electroreduced as the cathode; otherwise, once a metal has high tendency to deposition onto the cathode substrate, no particles will be obtained. In fact, the CV results presented in Figure 1 are not in agreement with the viewpoint. The main reason is that stabilizers play a more important role in decreasing the electrodeposition trend of metal on the foreign metal substrate than the cathode materials do. We believe that the technological keys to the synthesis of noble metal nanoparticles lie in the choice of an ideal nanoparticle stabilizer rather than cathode materials. As expected, Pd nanoparticles were prepared through direct electrochemical reduction of Pd(II) ions at the Pt cathode under the condition where PVP served as the nanoparticle stabilizer. 3.2. Large-Scale Preparation of Pd Nanoparticles through Direct Electroreduction. Electrochemical synthesis of Pd nanoparticles was conducted in the galvanostatic manner while the rotation speed of the cathode was controlled at 1000 rpm. In the previous studies, the metal ions were converted from a sacrificial anode,10,11 but here the metal ions came directly from the electrolyte. In the overall electrolysis process the bulk Pd(II) ions were electrochemically reduced at the cathode/solution interface, forming metal clusters stabilized by PVP rather than electrodepositing onto the cathode. PVP greatly enhanced the rate of particle formation relative to the Pd electrodeposition perhaps because of the excellent stabilizing effect for the Pd nanoparticles. The electrodeposition process of Pd was also lowered by the application of the rotating cathode since the Pd clusters electrochemically produced were transferred from the cathode region to the bulk solution before the deposition took place. Thus, the use of PVP as the stabilizer, together with application of the rotating cathode, makes large-scale preparation of well-dispersed Pd nanoparticles to be performed in a simple pattern of electrochemical reduction. Figure 2a,c shows typical TEM images of Pd particles electrochemically synthesized at applied currents of 50 mA and 100 mA, respectively. The Pd particles are well-dispersed spherical particles. Their mean diameter and particle diameter

Pan et al.

Figure 2. TEM images ((a) 50 mA, (c) 100 mA) and the corresponding particle size distribution ((b) 50 mA, (d) 100 mA) of Pd nanoparticles electrochemically synthesized at different applied currents in the mixed solutions of 0.1 mol dm-3 KNO3, 5.0 × 10-4 mol dm-3 H2PdCl4, and 15 g dm-3 PVPK17. Scale bars in Figure 2a,c are 100 nm.

distribution were determined by measuring the sizes of at least 300 particles. From the two histograms shown by Figure 2b,d, it was determined that the mean diameters of particles prepared at 50 mA and 100 mA are 22.2 nm (with standard deviation (SD) of (4.8 nm) and 7.6 nm (with SD of (2.2 nm), respectively. The results clearly show that the particle size can be controlled by variation of the applied currents. On the basis of the basic theory of electrochemical nucleation, the dependence of particle size on current density can be explained as follows: 10a,22 The critical radius of clusters of adatoms before particle formation at the cathode/solution interface is inversely dependent on the overpotential, which in turn is directly related to the current density, so as the current density is increased, the critical radius will decrease. The higher the current density, the higher the overpotential, and thus the smaller the size of the nanoparticles produced. The PVP concentration has a great influence on the mean diameter of Pd nanoparticles under otherwise identical conditions. By comparing Figure 2a and Figure 3a,c, it is easy to find that the mean diameter of Pd particles gradually decreased when the PVP concentration increased from 15 g dm-3 to 30 g dm-3. The Gaussian fit further shows that the mean particle size reduced from 22.2 to 10.1 nm, and the particle size distribution was also improved at the same time. On the contrary, when the PVP concentration decreased to 5 g dm-3, the Pd particles were quite unstable and would sedimentate quickly soon after the synthesis. Accordingly, it is necessary to maintain the enough PVP concentration (∼10 g dm-3) so that the PVP macromolecules can effectively stabilize the Pd nanoparticles. Our previous study has shown that the PVP chain length strongly affects the shape, size, and the stability of Ag12a and Au12b nanoparticles. Especially, the longer the PVP’s chain length and the higher the stability of the as-prepared Ag and Ag nanoparticles under otherwise equal conditions.12 However, the PVP polymers with long polyvinyl chain, such as PVPK90 and PVPK30, are disadvantageous to electrochemical synthesis of Pd nanoparticles. Usually, only a small amount of large

Electrochemical Synthesis of Pd Nanoparticles

Figure 3. TEM images and particle size distribution of Pd nanoparticles electrochemically synthesized at the applied current of 50 mA in the mixed solutions of 0.1 mol dm-3 KNO3 + 5.0 × 10-4 mol dm-3 H2PdCl4 with 20 g dm-3 (a,c) or 30 g dm-3 (b,d) PVPK17. Scale bars in Figure 3a,c are 100 nm.

Figure 4. TEM image (a) and particle size distribution (b) of some large Pd nanoparticles after the Pd colloid electrochemically synthesized at the applied current of 100 mA in 0.1 mol dm-3 KNO3 + 5.0 × 10-4 mol dm-3 H2PdCl4 + 15 g dm-3 PVPK17 mixed solution was kept for three weeks. Scale bar in Figure 4a is 200 nm.

particles (more than 100 nm in size) were obtained. Although the exact reason remains unknown, we believe that the dynamic curl and stretch of the PVP backbone play a major role in determining the size of Pd particles. The Pd colloidal solutions stabilized by adequate amount of PVPK17 exhibit the high stability and can be preserved for several months, without occurrence of any precipitate. Different from Au and Ag particles, a small quantity of Pd nanoparticles were observed to become large during the preservation. Figure 4a shows a representative TEM image for the Pd particles that were kept for three weeks. Although these particles were dispersed very well, the mean diameter of the particles increased to 77.5 nm within three weeks. This should be closely associated with the dynamic movement of PVP, which makes some isolated Pd particles have chances to contact each other. Moreover, the fusion between small particles and the recrystallization resulted in the increase of the mean particle size. 3.3. Voltammetric Characteristics and Electrocatalytic Activity of Pd Nanoparticles. Hydrogen absorption in Pd and its alloys has been a subject of intensive research for many

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2459

Figure 5. Normalized cyclic voltammograms (CVs) for unsupported (a) and non-heat-treated carbon-supported (b) Pd nanoparticles in 0.5 mol dm-3 H2SO4 solutions. The scan rate of potential is 50 mV s-1.

years.8,9 It is well-known that Pd is able to absorb large quantities of hydrogen forming Pd R-β hydride phases,21a,23 but on a bulk Pd electrode the processes of hydrogen adsorption, absorption, and evolution are usually not separated from each other,23 which is exemplified by a broad anodic peak around 0.27 V (vs RHE) in the CVs of a bulk Pd electrode in H2SO4 solution.16 In contrast, the use of Pd nanoparticles as the working electrode makes it possible to distinguish various forms of hydrogen adsorbed on the particle surface or absorbed into the interior of the particles in terms of their electrochemical properties. In order to exactly evaluate the true voltammetric behavior and electrocatalytic activity of Pd nanoparticles under different conditions, it is necessary to determine the electrochemically active surface areas (EASAs) of Pd nanoparticle electrodes.24 Cyclic voltammetric (CV) measurements were first performed on the Pd nanoparticle modified glass carbon electrodes (GCEs) in 1.0 mol dm-3 H2SO4 solutions. Taking the original CV curve of unsupported Pd catalyst (directly loaded on a GCE) as an example (see Figure S1 in Supporting Information section), the electric quantity (∼1377 µC) corresponding to the shaded area in Figure S1 reflects the amount of charge needed to reduce the palladium oxides formed on the particle surfaces in the positive-going potential scan. The EASA of the unsupported Pd catalyst was therefore determined to be ∼3.25 cm2 by dividing the amount of reduction charge (1377 µC) by its conversion factor of 424 µC cm-2. Similarly, the EASAs of the carbon-supported Pd catalysts were determined following the same procedure. In this paper, all current densities for the Pd nanoparticle electrodes were normalized by their real surface areas. Figure 5 shows two typical normalized CVs recorded in 0.5 mol dm-3 H2SO4 solutions for both unsupported Pd nanoparticles (directly loaded on a GCE) and the activated carbonsupported Pd nanoparticles (dried at ambient temperature).

2460 J. Phys. Chem. C, Vol. 112, No. 7, 2008

Figure 6. Normalized cyclic voltammograms (CVs) for the heat-treated carbon-supported Pd nanoparticles in 0.5 mol dm-3 H2SO4 solution. Scan rate: 50 mV s-1.

Although the two CVs are basically similar in the profile and the peak potentials, there exist two distinct differences: (i) the anodic and cathodic peaks in the hydrogen region between 0 and 0.32 V, especially the large anodic peak (I), became small, and (ii) the charge-discharge current in double-layer region between 0.32 and 0.57 V increased considerably, after the Pd nanoparticles were loaded on the carbon supports. The doublelayer capacitance for carbon-supported Pd nanoparticles was determined to be ∼2000 µF cm-2 according to the method that we described recently.25 The differences in the two CVs indicate that the loading of Pd nanoparticles on carbon supports lowered to a certain degree the hydrogen storage ability of Pd nanoparticles. However, after the carbon-supported catalysts were heattreated at 80 °C in vacuum, the hydrogen storage ability of Pd particles was significantly enhanced, reaching and even exceeding the level of unsupported Pd nanoparticles. This can be reflected from the larger anodic peak around 0.12 V in the CVs shown by Figure 6. The origin of three current peaks in hydrogen region can be explained as follows:9,16,21a,23 The large anodic peak (I) at more negative potential is related to hydrogen desorption from the bulk of particles, whereas the small anodic one (II) at more positive potential is ascribed exclusively to oxidation of hydrogen adsorbed on the surface of particles.9 The formation of cathodic peak (II′) is considered to be an inverse process of hydrogen desorption, more exactly the adsorption of hydrogen on the particle surface. Grden et al.9 suggested that the changes in the amount of hydrogen manifested by the peak II are proportional to the changes of the real surface area of the Pd electrode. For Pd nanoparticles, quite a number of exterior atoms are on the particle surfaces. Therefore, it is easy to observe peak II as contrast with the bulk Pd. The similar CV results are also observed for the ultrathin Pd or Pd-Pt alloy films.9,16 Obviously, the unusual electrochemical characteristics of Pd nanoparticles provide a possibility to separately study the individual processes in the hydrogen region, without considering the problem of massive hydrogen absorption. Catalytic activity of Pd nanoparticles was examined by using the methanol electrooxidation reaction as a test reaction. The measured electrocatalytic oxidation currents were normalized by dividing by the real surface areas of Pd nanoparticles obtained at the same electrodes in 1.0 mol dm-3 H2SO4 solutions. The electrooxidation of methanol on the unsupported and the nonheat-treated carbon-supported Pd nanoparticles (dried at ambient temperature) initiated at -0.55 V (vs SCE) and reached the maximum current around -0.18 V in the positive-going potential scan (see Figure 7). The enhancement in the activity for the methanol oxidation on the carbon-supported catalyst may

Pan et al.

Figure 7. Normalized cyclic voltammograms (CVs) for electrooxidation of 1.0 mol dm-3 methanol on the unsupported Pd catalyst and on two carbon-supported Pd catalysts in 1.0 mol dm-3 KOH. Scan rate: 50 mV s-1.

be attributed to the increase of the EASAs since the aggregation between the Pd particles took place more easily when Pd nanoparticles were directly loaded on a GCE. It is more interesting that the CVs (blue line) for heat-treated Pd nanoparticles show a very high and large oxidation peak centered at the more negative potential (∼ -0.23 V) in the anodic scan followed by a high and broad secondary oxidation peak in the cathodic scan in strong contrast to those of the two non-heattreated Pd nanoparticles. Obviously, the heat treatment significantly enhances the catalytic activity of Pd nanoparticles. The possible reason is that more active sites on the Pd catalyst surface were exposed to the reactant molecules after the heat treatment. In this paper, the Pd nanoparticles were electrochemically synthesized based on the PVP’s outstanding role in stabilizing Pd nanoparticles. Seeing that the cyclic amide groups in PVPs have a strong affinity for Pd clusters, PVP macromolecules can closely wrap the individual Pd particles and even form a stabilizing shell covering Pd nanoparticles. The PVP shell is of great advantage for stabilizing the Pd nanoparticles but has a bad influence on their catalytic activity. Therefore, once the PVP shell was removed by the heat treatment, the Pd nanoparticles naturally became much more active since the methanol oxidation reaction took place on the nearly bare surfaces of Pd nanoparticles. 4. Conclusions In this paper, we demonstrate that well-controlled Pd nanoparticles can be electrochemically synthesized through the direct electroreduction of bulk Pd(II) ions in the electrolytic solutions that contain appropriate amount of PVP polymers. PVP plays multiple roles during the electrochemical synthesis of Pd nanoparticles, including greatly enhancing the formation process of Pd particles and inhibiting the electrodeposition of Pd as well as effectively stabilizing the Pd particles. The size of Pd nanoparticles can be controlled very well by adjusting the applied current and the PVP concentration. The as-synthesized displayed the voltammetric characteristics different from those of the bulk Pd, which provides a possibility to separately study the processes of hydrogen adsorption, absorption, and evolution. Pd nanoparticles were supported on Vulcan XC-72 carbon and heat-treated to remove the PVP shell. The resulting Pd catalysts were more active in the electrooxidation of methanol than the non-heat-treated Pd catalysts. Acknowledgment. This work was supported by National Natural Science Foundation of China (20573068, 20673067)

Electrochemical Synthesis of Pd Nanoparticles and the Visiting Scholar Foundation of Key Laboratory in Shandong University. Supporting Information Available: A detailed description on how to determine the electrochemically active surface areas (EASAs) of Pd nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Baldauf, M.; Kolb, D. M. Electrochim. Acta 1993, 38, 2145. (2) Hoshi, N.; Kida, K.; Nakamura, M.; Nakada, M.; Osada, K. J. Phys. Chem. B 2006, 110, 12480. (3) Baldauf, M.; Kolb, D. M. J. Phys. Chem. 1996, 100, 11375. (4) (a) Ding, J. H.; Gin, D. L. Chem. Mater. 2000, 12, 22. (b) Kim, S. W.; Park, J.; Jang, Y.; Chung, Y.; Hwang, S.; Hyeon, T.; Kim, Y. W. Nano Lett. 2003, 3, 1289. (c) Gugliotti, L. A.; Feldheim, D. L.; Eaton, B. E. Science 2004, 304, 850. (5) (a) Niwa, O.; Kato, D.; Kurita, R.; You, T.; Iwasaki, Y.; Hirono, S. Sens. Mater. 2007, 19, 225. (b) Larsen, R.; Zakzeski, J.; Masel, R. I. Electrochem. Solid State Lett. 2005, 8, A291. (c) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165. (d) Shao, M. H.; Sasaki, K.; Adzic, R. R. J. Am. Chem. Soc. 2006, 128, 3526. (6) (a) Rose, A.; Maniguet, S.; Mathew, R. J.; Slater, C.; Yao, J.; Russell, A. E. Phys. Chem. Chem. Phys. 2003, 5, 3220. (b) Liu, S. Y.; Kao, Y. H.; Su, Y. O.; Perng, T. P. J. Alloys Compd. 2001, 316, 280. (7) (a) Zhu, X. H.; Lin, X. Q. Anal. Sci. 2007, 23, 981. (b) Sun, Y.; Wang, H. H. Appl. Phys. Lett. 2007, 90, 3. (8) (a) Czerwinski, A.; Kiersztyn, I.; Grden, M.; Czapla, J. J. Electroanal. Chem. 1999, 471, 190. (b) Czerwinski, A.; Kiersztyn, I.; Grden, M. J. Electroanal. Chem. 2000, 492, 128. (9) Grden, M.; Piascik, A.; Koczorowski, Z.; Czerwinski, A. J. Electroanal. Chem. 2002, 532, 35. (10) (a) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401. (b) Reetz, M. T.; Winter, M.; Breinbauer, R.; Thurn-Albrecht, T.; Vogel, W. Chem.-Euro. J. 2001, 7, 1084. (11) (a) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (b) Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai,

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2461 W. C.; Wang, C. R. C. Langmuir 1999, 15, 701. (c) Mohamed, M. B.; Ismail, K. Z.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 9370. (12) (a)Yin, B. S.; Ma, H. Y.; Wang, S. Y.; Chen, S. H. J. Phys. Chem. B 2003, 107, 8898. (b) Huang, S. X.; Ma, H. Y.; Zhang, X. K.; Yong, F. F.; Feng, X. L.; Pan, W.; Wang, X. N.; Wang, Y.; Chen, S. H. J. Phys. Chem. B 2005, 109, 19823. (c) Ma, H. Y.; Yin, B. S.; Wang, S. Y.; Jiao, Y. L.; Pan, W.; Huang, S. X.; Chen, S. H.; Meng, F. J. Chemphyschem 2004, 5, 68. (13) (a) Rodriguez-Sanchez, L.; Blanco, M. C.; Lopez-Quintela, M. A. J. Phys. Chem. B 2000, 104, 9683. (b) Rodriguez-Sanchez, M. L.; Rodriguez, M. J.; Blanco, M. C.; Rivas, J.; Lopez-Quintela, M. A. J. Phys. Chem. B 2005, 109, 1183. (14) (a) Attard, G. A.; Price, R.; Alakl, A. Electrochim. Acta 1994, 39, 1525. (b) Attard, G. A.; Price, R. Surf. Sci. 1995, 335, 63. (15) Hoyer, R.; Kibler, L. A.; Kolb, D. M. Electrochim. Acta 2003, 49, 63. (16) Zhang, J. T.; Huang, M. H.; Ma, H. Y.; Tian, F.; Pan, W.; Chen, S. H. Electrochem. Commun. 2007, 9, 1298. (17) Woods, R. Chemisorption at electrodes. Hydrogen and oxygen on noble metals and their alloys. In Electroanalytical Chemistry A Series of AdVances; Bard, A. J., Ed.; Marcel Dekker: New York, Basel, 1976; Vol. 9, pp 109-117. (18) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63, 711. (19) Hoyer, R.; Kibler, L. A.; Kolb, D. M. Electrochim. Acta 2003, 49, 63. (20) (a) Kibler, L. A.; Kleinert, M.; Kolb, D. M. Surf. Sci. 2000, 461, 155. (b) Kibler, L. A.; Kleinert, M.; Lazarescu, V.; Kolb, D. M. Surf. Sci. 2002, 498, 175. (c) Kibler, L. A.; El-Aziz, A. M.; Hoyer, R.; Kolb, D. M. Angew. Chem., Int. Ed. 2005, 44, 2080. (21) (a) Guerin, S.; Attard, G. S. Electrochem. Commun. 2001, 3, 544. (b) Bartlett, P. N.; Gollas, B.; Guerin, S.; Marwan, J. Phys. Chem. Chem. Phys. 2002, 4, 3835. (22) Welch, C. M.; Compton, R. G. Anal. Bioanal. Chem. 2006, 384, 601. (23) Birry, L.; Lasia, A. Electrochim. Acta 2006, 51, 3356. (24) Diao, P.; Zhang, D. F.; Guo, M.; Zhang, Q. J. Catal. 2007, 250, 247. (25) Yong, F. F.; Ma, H. Y.; Wang, X. N.; Feng, X. L.; Huang, S. X.; Jiang, J. Z.; Chen, S. H. Electrochim. Acta 2006, 51, 3743.