Facile Fabrication and Unexpected Electrocatalytic Activity of

Aug 16, 2008 - Caixia Xu , Yan Zhang , Liqin Wang , Liqiang Xu , Xiufang Bian , Houyi Ma and Yi ... Jin-Yi Wang , Yong-Yin Kang , Hui Yang and Wen-Bin...
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13970

J. Phys. Chem. C 2008, 112, 13970–13975

Facile Fabrication and Unexpected Electrocatalytic Activity of Palladium Thin Films with Hierarchical Architectures Jintao Zhang, Cuicui Qiu, Houyi Ma,* and Xiuyu Liu Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed: June 1, 2008; ReVised Manuscript ReceiVed: July 6, 2008

Cobalt thin films composed of a large number of nanopetals were fabricated on the glassy carbon (GC) substrate by cyclic voltammetric deposition of Co2+ ions on a glass carbon electrode (GCE). The hierarchical Co nanostructures were further used as the sacrificial template to acquire Pd (or Pt) thin film electrocatalysts with hierarchical architectures through the galvanic replacement reaction between Co nanopetals and chloropalladite (or tetrachloroplatinate). The as-prepared Pd (or Pt) thin films contain quantities of nanoparticles and many hollow Pd aggregates in the range of submicrometer to micrometer scale. The hollow Pd aggregates were found to burst in acidic solutions at potentials more negative than the hydrogen evolution potential since Pd absorbed too much hydrogen. As an electrocatalyst for the formic acid oxidation, the Pd thin films presented much higher catalytic activity than the Pt thin films with a similar architecture. An important reason is that the formic acid oxidation at the Pd nanostructures proceeds via a non-CO reaction pathway, while the reaction at the Pt nanostructures involves formation of CO-adsorbed species, which has been confirmed by the CO stripping voltammetric curves on Pd or Pt thin films. The as-prepared Pd thin films with hierarchical architectures are expected to be a promising electrocatalyst in direct formic acid fuel cells (DFAFCs). Introduction In recent years, palladium nanostructures with special shapes and morphologies have received increasing attention since fabrication of such structures provides good opportunity to tailor physical and chemical prosperities of palladium at a nanoscale level.1-4 Pd is a very popular metal used extensively in the fields of catalysis,5,6 hydrogen storage,7 and chemical sensors.8 Due to the low price, and the superior catalytic performance in the formic acid oxidation as compared to Pt-based catalysts,9 much effort has been devoted to developing simple and practical techniques to prepare Pd electrocatalysts with controllable micro/ nanostructures, high activity, and enhanced performance for the direct formic acid fuel cell (DFAFC) applications.9-11 From a practical catalysis viewpoint, a large specific surface for nanostructures is most desired. For this purpose, a common strategy is to decrease catalyst particle sizes; however, too fine powder catalysts can cause serious operational problems such as difficulties in loadings and aggregation of the particles. Another feasible strategy to generate a large and accessible surface area of catalyst is to fabricate the nanostructured catalysts with hollow or micro/nanoporous structures. Provided that Pd nanostructures are processed into hollow ones, their performance can be tuned or improved for most of these applications.4 It is demonstrated that hollow metallic nanospheres exhibit catalytic activities quite different from their solid counterparts in addition to the advantages of low density and the use of less material.12 For example, hollow Pd spheres show good catalytic activity in Suzuki cross-coupling reactions and can be reused many times without loss of catalytic activity.13 Generally, hollow Pd and other noble metal nanostructures are obtained by using various sacrificial templates, such as silica * Corresponding author. E-mail: [email protected]. Phone: +86-53188364959. Fax: +86-531-88564464.

spheres,13,14 porous anodic aluminum oxide,15 and mixedsurfactant liquid crystal,16 to control the shapes, structures, and morphologies of the products, which involve laborious processes in the preparation and removal of the templates.17 To date, there has been only limited success due to their intrinsic difficulties in fabrication. As a result, it is desirable to develop the easier and more convenient routes to fabricate hollow Pd nanostructures for their applications in different areas. Recently, a facile synthetic method based on the galvanic replacement reactions proposed by the groups of Xia4,18,19 and Wan12,17 has drawn our attention. In this paper, this type of method is further developed to construct high-efficiency Pd (or Pt) thin-film electrocatalysts with hollow hierarchical micro/nanostructures. At first, a large amount of Co nanopetals was electrodeposited onto the glassy carbon surface by using the cyclic voltammetric method. Some deposited Co was stripped from the foreign substrates and replated later during the repeated potential cycling, forming flower-like Co nanoaggregates.20 This is a key step to the subsequent fabrication of hierarchical Pd (or Pt) architectures by surface replacement reaction between Co thin films and H2PdCl4 (or K2PtCl4). Interestingly, the as-prepared Pd thin films with hierarchical architectures presented an unexpected high catalytic performance for the formic acid electrooxidation, much higher than the Pt thin films with identical microstructures. The new fundings are helpful for better understanding of how the morphology and structure of metal nanocatalysts affect their catalytic properties. The electrochemical studies further demonstrate that the formic acid oxidation on the hierarchical Pd architectures proceeded via a non-CO reaction pathway, without formation of strongly adsorbed CO species like that on the Pt surface. The novel hierarchical Pd architectures are expected to act as a promising electrocatalyst in the DFAFC.

10.1021/jp804828k CCC: $40.75  2008 American Chemical Society Published on Web 08/16/2008

Fabrication and Electrocatalytic Activity of Pd Thin Films

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Experimental Section The aqueous solutions were prepared with AR reagents and ultrapure water (>18 MΩ cm). The glass carbon electrodes (GCEs) were ground with emery papers of decreasing particle sizes to no. 3500 finish at first, then polished with 0.05 µm alumina slurry. Before use, the electrodes were cleaned ultrasonically in HNO3 (1:1), washed with ultrapure water, and finally rinsed with absolute ethanol in proper order. The nanostructured Co thin films, which contained a large amount of Co nanopetals and many flower-like Co nanoaggregates, were directly formed on a clean GCE with a continuously sweeping potential at a rate of 50 mV s-1 between -0.95 and -0.60 V (vs. SCE) for 25 cycles in 0.1 mol dm-3 Na2SO4 + 10 mmol dm-3 Co(CH3COO)2 mixed solution. Assuming 100% faradaic efficiency for the deposition process, the total amount of Co deposited was estimated at ∼30 µg cm-2 by integrating the charge needed to reduce Co(II) ions. The GCE covered with Co thin films was immersed into 27.5 mmol dm-3 H2PdCl4 (or K2PtCl4) solution for 15 min, allowing complete replacement of Co by Pd (or Pt). After thoroughly rinsing with ultrapure water, the GCE covered with Pd (or Pt) deposition was placed in 1.0 mol dm-3 H2SO4 for carrying out the continuous potential cycling in the designated potential range until it gave steady cyclic voltammograms. In this way, the Pd (or Pt) thin films with hierarchical architectures were obtained on the GCE. Electrochemical measurements were performed with IK 2005 and CHI 760C electrochemical workstations at ambient temperature (∼22 °C). The electrochemical cell had a threeelectrode configuration. A bright Pt plate (1.0 cm × 2.0 cm) served as the counter electrode, and a saturated calomel electrode (SCE) was selected as the reference electrode. The reference electrode was led to the surface of the working electrode through a Luggin capillary. The electrooxidation of formic acid was carried out in 0.1 mol dm-3 H2SO4 + 0.2 mol dm-3 HCOOH mixed solutions. The electrolyte solutions were deaerated by N2 bubbling for 10 min prior to the experiments and a blanket of N2 was maintained throughout the experiment. Surface morphologies of metallic architectures were observed by field emission scanning electron microscope (FESEM, Hitachi S-4800), transmission electron microscopy (TEM, Hitachi H-7000, 100 kV), and high-resolution TEM (HRTEM, JEOL-2100, 200 kV), respectively. EDS analyses were recorded with an INCAX-sight energy dispersive X-ray spectrometer equipped on the FESEM. For the TEM measurements, the Pd and Pt products were dispersed in ethanol at first, and several drops of the suspension were then put on a Formvar-covered copper grid, followed by natural evaporation of the solvent in the air. Results and Discussion Morphology of Co Thin Films. Nanostructured Co thin films were directly formed on the GC substrate by electrochemical deposition with cyclic voltammetry of Co2+ ions, and their surface morphology was characterized by FESEM. It is observed from a typical large-scale SEM image shown by Figure 1a that the Co films are composed of quantities of nanopetals (or nanoflakes) and many submicron-sized “islands”. However, the real structure of the films is not so simple. On closer inspection from the enlarged images of an island-like large particle (Figure 1b) and a randomly selected region between the “islands” (Figure 1c), it is found that that the whole thin film is actually constructed of a large number of nanopetals, and moreover, each submicron-sized “island” is an aggregate of hundreds of nanopetals, which appears in the form of a flower-like structure.

Figure 1. SEM images of the overall morphology for Co (a) and Pd (d), the magnified view for the island-like large particles of Co (b) and Pd (e), and the interspaces between the Co (c) and Pd (f) island-like large particles. EDS analysis of the as-prepared Pd thin films (g).

Although the exact mechanism still remains unclear at present, it is believed that the unusual structure of the Co thin films originates from the equilibrium between the magnetic attractive force and magnetic repulsive one of the ferromagnetic Co nanoflakes with different magnetic orientation. The magnetic dipole-dipole attraction between Co nanoflakes may lead to the formation of the Co layer with reticular structures on the GC substrate and the further self-organization of flower-like nanopetal aggregates on the Co layer.21,22 At the same time, the repeated potential cycling in the solution containing Co2+ ions plays an important role in the fabrication of flower-like nanopetal aggregates.23 It is possible that some deposited cobalt was repeatedly stripped from the GC substrate and replated later, forming the Co islands or large particle aggregates, during the continuous potential sweep. Surface Morphology and Voltammetric Properties of Pd and Pt Architectures. The nanostructured Pd (or Pt) thin films can be fabricated on the GC surface with ease by means of galvanic replacement of the Co films by Pd (or Pt). The SEM observation of the Pd films at low magnification shows a hierarchical structural feature similar to that of the Co films. As can be seen in Figure 1d, a small number of Pd “islands” with sizes ranging from submicron to micron seem to float on the bottom layer of Pd constructed by a large quantity of nanoparticles. In fact, there exist great differences in morphology between nanostructured Co and Pd thin films. The enlarged SEM image of an individual Pd “island” reveals its complicated structure: this “island” is not a solid particle rather than an aggregate of numerous Pd nanoparticles (Figure 1e). The further investigation demonstrates that these islands are hollow hierarchical nanostructures. The interspaces between island-like

13972 J. Phys. Chem. C, Vol. 112, No. 36, 2008 nanoparticle aggregates are close-packed spherical and dendritic Pd nanoparticles (Figure 1f). EDS analysis of the Pd films (Figure 1g) only gives the signals of Pd, proving that all Co particles of different sizes on the GC substrate were completely etched away in the process of the galvanic replacement reaction. In the present study, the as-prepared hierarchical Pd micro/ nanostructures through the galvanic replacement reaction are closely associated with the morphology of the Co sacrificial templates. Because of the large gap between standard reduction potentials of PdCl42-/Pd (0.591 V vs. SHE) and Co2+/Co (-0.277 V vs. SHE), the redox replacement of Co films by Pd took place rapidly and the concentration of the PdCl42- complex ions had an important influence on the formation rate of Pd nuclei.12,24 It is possible that the primary shape of individual Co micro/nanostructures restricted to a considerable degree the shape of the newly formed Pd micro/nanostructures. Formation of small Pd nanoparticles can be interpreted according to fundamentals of nucleation and growth. As for the formation of micron- or submicron-scale aggregates of Pd nanoparticles, it should involve spontaneous self-organization of small Pd nanoparticles and the possible fusion between small particles, eventually evolving into a thin shell around the outline of a flower-like Co nanopetal aggregate. As compared with the common hollow Pd nanostructures, such as nanospheres and nanotubes, the present hollow Pd nanoaggregates have high structure stability, which is an important parameter in evaluating the catalyst performance. Besides, the rough surfaces are very helpful for increasing the electrochemically active surface and enhancing catalyst utilization. The most distinguished features of the present method for fabricating the Pd thin films with hierarchical Pd architectures include simple operation, acquirement of high pure products, easy control of film morphology, and especially direct use as electrode materials. It should be noted that this method is not a specialized one and is also applicable to fabrication of hierarchical structures of other noble metals. For example, hierarchical Pt architectures with similar structure were prepared by following the same procedures (Figure S1 in the Supporting Information). Electrochemical behavior of such hierarchical Pd and Pt micro/nanostructures was investigated by cyclic voltammetry (CV) in 1.0 mol dm-3 H2SO4 solutions. In strong contrast to the CV profile of bulk Pd or the Pd thin films deposited on a foreign metal,20,25 the CVs of the GCE modified with hierarchical Pd architectures (Pd film electrode (Pd-FE for short)) in Figure 2a show two pairs of sharp redox peaks in the hydrogen region in addition to a commonly observed reduction peak for palladium oxide at ∼0.51 V (vs. SCE). The two large peaks at ca. -0.21 V are related to the hydrogen absorption/desorption process on the Pd nanoparticle aggregates since these large nanoaggregates are able to show the separation of hydrogen adsorption and absorption processes on them, whereas the two small ones at ca. -0.01 V are attributed to the hydrogen adsorption/desorption process on the Pd surface.3,25 When replacing H2SO4 with HClO4, the two small peaks at the more positive potential (ca. -0.01 V) became less sharp, indicating that the hydrogen adsorption/desorption was accompanied by the sulfate desorption/adsorption at the hierarchical Pd nanostructures. This phenomenon was generally observed on Pd single-crystal surfaces by the in situ STM.26 At the same time, the desorption/adsorption of HSO4- or SO42- ions on the Pd surfaces strongly depended on the atomic rows of terraces based on the previous research results obtained on single-crystalline Pd electrodes.27,28 The increase in amount of steps and kinks

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Figure 2. (a) Cyclic voltammograms (CVs) for the Pd-FE measured in 1.0 mol dm-3 H2SO4 and 1.0 mol dm-3 HClO4, respectively. (b) Cyclic voltammograms (CVs) for the Pt-FE measured in 1.0 mol dm-3 H2SO4. Scan rate: 20 mV s-1.

can enhance the catalytic activity of catalysts.29 The use of hierarchical Pd micro/nanostructures enables us to separately study the hydrogen absorption or adsorption behavior on the Pd surface. In contrast, CVs of the GCE modified with hierarchical Pt architectures (Pt film electrode (Pt-FE for short)) in Figure 2b do not display any other voltammetric property different from that of bulk Pt, which shows two pairs of wellseparated peaks related to the hydrogen adsorption/desorption and the platinum oxide reduction peak at ca. -0.53 V. A small amount of Pd thin film sample was scraped off the GC substrate, placed on a copper grid, and observed by the TEM. A typical TEM image (Figure 3a) reveals that the submicron-scale Pd particles were the irregular hollow microspheres. More interestingly, when controlling the potential to be lower than the hydrogen evolution potential, some hollow Pd microspheres would break open because they absorbed too much hydrogen, as indicated in Figure 3b. The electron diffraction (ED) pattern shown in the inset of Figure 3c indicates the polycrystalline nature of Pd films, in which four diffraction rings in sequence from inner to outer can be indexed to the (111), (200), (220), and (311) of the face-centered cubic Pd panes, respectively. A lattice-resolved HRTEM image (Figure 3c) taken from the outer shell of hollow microspheres represents the lattice spacing of ∼0.23 nm, being close to that of Pd (111). As for the Pt architectures fabricated by using the same procedure, the SEM and HRTEM observations confirm that they have similar structures (Figures S1 and S2 in the Supporting Information). But the hollow Pt microspheres did not burst even if much hydrogen was observed to evolve from the Pt/GCE. As mentioned above, the Pd-FE displays unusual electrochemical properties different from the bulk Pd. The Pd films with hierarchical architectures are expected to find new applications in a variety of areas that include catalysis and hydrogen sensors. Electrocatalytic Activity of Hierarchical Pd and Pt Architectures. Catalytic activity of the as-prepared Pd-FEs and Pt-FEs was characterized by using the formic acid oxidation as a test reaction. Formic acid is considered to be an attractive

Fabrication and Electrocatalytic Activity of Pd Thin Films

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Figure 3. TEM image that indicates the hollow structure (a), SEM image for the hierarchical Pd architectures that were polarized cathodically at potentials lower than the hydrogen evolution potential in H2SO4 solution (b), HRTEM image showing the polycrystalline structure of the Pd sphere shell (c), and the electron diffraction (ED) pattern (inset in part c).

energy source in direct formic acid fuel cells. Meanwhile, the electrooxidation of formic acid is useful as a model system to study electrochemical oxidation of simple alcohols (methanol, ethanol) for fuel cell applications.30 The real surface areas of Pd and Pt thin films were determined by means of the oxygen adsorption method and the hydrogen desorption method, respectively.31,32 Formic acid is the smallest organic acid, but its electrooxidation is not as simple as imagined. It is generally accepted that the formic acid oxidation on the Pt surface may proceed via the following two pathways, i.e., dehydrogenation or dehydration pathways:33 where COads represents a strongly

adsorbed CO species on the Pt surface. Figure 4 shows the CVs of the formic acid oxidation at a Pd-FE and a Pt-FE in H2SO4 solutions, respectively. For the Pt-FE, in the positive-going potential scan, the broad, plateau-like oxidation peak between 0.25 and 0.45 V (vs. SCE) is related to the direct oxidation of formic acid to CO2, and the oxidation peak centered at 0.65 V is ascribed to either the oxidation of the COads or the oxidation of formic acid on the unblocked Pt surfaces, accompanied by the surface oxidation of platinum. In the subsequent negativegoing scan, a broader oxidation peak between 0 and 0.58 V reflects the true activity of the clean Pt catalyst since both COads and Pt oxides were stripped from the Pt surface.34 The complicated voltammetric characteristics (in Figure 4a) suggest that the formic acid oxidation on the hierarchical Pt architectures should take place via the dehydration pathway, involving formation of CO poisoning intermediates. The formic acid oxidation on the Pd-FE presents a striking contrast to those on the Pt-FE: (i) onset potential for the formic acid oxidation shifted negatively to -0.15 V, about 150 mV more negative than that on the Pt-FE, and (ii) the peak currents were considerably larger in contrast with those obtained on the Pt-FE. The primary peak of formic acid oxidation on the PdFE was situated at 0.20 V in the anodic scan, whereas the

Figure 4. The electrooxidation of formic acid in an aqueous solution of 0.2 mol dm-3 formic acid and 0.1 mol dm-3 H2SO4 on the Pd-FE and the Pt-FE with different positive limits: 1.0 (a) and 0.70 V (b). Scan rate: 20 mV s-1.

secondary oxidation peak appeared at ∼0.25 V in the cathodic scan. The potential difference between the peak potentials of two oxidation peaks is only 50 mV. Obviously, the Pd nanocatalyst did not suffer from deactivation and poisoning effect compared to the Pt nanocatalysts with similar morphology. In addition, the existence of adsorbed hydroxyl species (OHads) on the Pt surface is necessary for removal of the COads according to the Langmuir-Hinshelwood mechanism, but the hydroxyl species adsorbed on the Pd are helpless to the formic acid oxidation on the Pd-FE.35 This has been confirmed by voltammetric results shown in Figure 4b, which was carried out by reducing the positive potential limit to 0.7 V under otherwise identical conditions. For the Pd-FE, the CV profile (Figure 4b) obtained in the cathodic potential sweep seemed almost the same as the previous one presented in Figure 4a. On the contrary, for the Pt-FE, the decrease of the positive potential limit led to the obvious shape changes of the secondary oxidation peak (in Figure 4b), producing a wide current plateau in the potential range between 0.28 and 0.60 V. As a result, it is concluded

13974 J. Phys. Chem. C, Vol. 112, No. 36, 2008 that the Pd film electrocatalysts possess the higher catalytic activity and the stronger poison resistance than the Pt film electrocatalysts with similar morphology. The CVs for the formic acid oxidation on the bulk Pd and Pt electrodes are shown in the Supporting Information (Figure S3). It is observed that the current density for the formic acid oxidation was greatly enhanced on the Pd-FE as compared to that on the bulk Pd under the same conditions by comparing Figure 4 and Figure S3 (Supporting Information). Arenz and co-workers reported that Pt-Pd single crystal bimetallic surfaces and the pseudomorphic Pd monolayer on Pt(111) exhibited higher activity for the formic acid oxidation than for the pure Pt(111) surface.36,37 It is noteworthy that several effects of bimetallic catalysts, such as the so-called d-band model,38 ensemble effect,39 and synergistic effect,40 could contribute to enhancing the catalyst performance. In this study, due to excluding the effect of catalyst support, the ultrahigh surface area of the Pd thin films and the large numbers of active sites on the surfaces should be responsible for enhancement of the catalytic activity. Mechanism for Electrooxidation of Formic Acid on Hierarchical Pd and Pt Architectures. The great differences between the voltammetric characteristics of the Pd-FE and the Pt-FE under the same conditions suggest that the formic acid oxidation on the hierarchical Pd and Pt architectures takes place via different reaction pathways. The key technological problem is how to acquire direct evidence of the CO intermediate. We focused on the anodic stripping of the CO spontaneously adsorbed on the Pd and Pt thin films at first. The irreversibly adsorbed CO adlayers were formed on the Pd (or Pt) nanostructures by exposing the Pd-FE or the Pt-FE to CO-saturated 0.1 mol dm-3 H2SO4 solution for 3 min at a potential of -0.1 V (vs. SCE). Subsequently, the electrode was taken out and immediately transferred into oxygen-free 0.1 mol dm-3 H2SO4 solutions for measuring the CO stripping voltammograms. Typical CO stripping voltammograms on the Pd-FE and the Pt-FE are shown in Figure 5a. A pronounced CO stripping peak appeared on the Pd-FE at an even higher potential (0.77 V) than that obtained on the Pt-FE (∼0.60 V), indicating the stronger CO bonding on Pd.41 At the same time, no oxidation peaks corresponding to the hydrogen desorption were observed from both voltammograms in Figure 5a, which implies that the hydrogen adsorption/desorption process on the Pd or Pt surface was completely suppressed by the CO adsorption. The oxidative removal of COads from the Pd or Pt surface can be finished only through the reaction of COads with the hydroxyl species adsorbed on the Pd or Pt surfaces to form CO2. Another type of CO stripping experiments was designed to determine whether the formic acid oxidation at the hierarchical Pd (or Pt) micro/nanostructures proceeds via the non-CO reaction pathway or the CO intermediate route. At first, the PdFE or the Pt-FE was held in 0.1 mol dm-3 H2SO4 + 0.2 mol dm-3 HCOOH mixed solution at 0.45 V (vs. SCE) for 3 min. After the potentiostatic polarization, the Pd-FE (or Pt-FE) was immersed in the solution for another 5 min. Then electrodes were rapidly rinsed with ultrapure water and transferred into oxygen-free 0.1 mol dm-3 H2SO4 solution for carrying out the CO stripping experiments. As demonstrated by the plot in Figure 5b, a very sharp anodic oxidation peak was observed with the peak potential at ∼0.45 V on the Pt surface, whose shape was very similar to the CO stripping peak in Figure 5a although the peak potential was slightly more negative. Because the adsorbed CO adlayers can be oxidized on the Pt and other noble metal electrodes at potentials around 0.4-0.8 V (vs. SCE), we have

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Figure 5. CO stripping voltammograms obtained on the Pd-FE and the Pt-FE in 0.1 mol dm-3 H2SO4. Scan rate: 50 mV s-1. (a) The irreversibly adsorbed CO layers were formed by exposing the Pd-FE or the Pt-FE to CO-saturated 0.1 mol dm-3 H2SO4. (b) The electrodes were immersed in a mixed solution of 0.1 mol dm-3 H2SO4 and 0.2 mol dm-3 HCOOH for 3 min with the potential held at 0.45 V, then immersed in the mixed solution for another 5 min under open-circuit potential.

good reason to believe that the peak originated from the CO stripping. This result provides direct evidence that the oxidation of formic acid on the Pt surface really forms the CO intermediates. Besides, the voltammograms also indicated that the hydrogen adsorption/desorption on the Pt surface was strongly inhibited due to the formation of adsorbed CO species. However, there is no CO stripping peak in the CO stripping voltammograms on the Pd-FE. Moreover, the hydrogen desorption peak appeared to be almost the same as that obtained on the freshly prepared Pd-FE. We also carried out the CO stripping experiments on the Pd and Pt surfaces at other potentials (0.15 and 0.30 V). The obtained results (Figure S4 in the Supporting Information) were in good agreement with those presented in Figure 5. Thus we can draw a conclusion that the electrooxidation of formic acid at the Pd nanostructures takes place through a non-CO reaction pathway. To further confirm that formic acid oxidation on the hierarchical Pd nanostructures proceeds via a direct decomposition route, the steady-state polarization experiments for the formic acid oxidation on the Pd-FE and the Pt-FE were performed by the linear potential scan with a scan rate of 0.5 mV s-1. Figure 6 shows the anodic polarization curves for the two electrodes in 0.1 mol dm-3 H2SO4 + 0.2 mol dm-3 HCOOH mixed solutions. The anodic curve for the Pd-FE displays a linear region between -0.18 and -0.10 V, with a Tafel slope of about 59 mV/decade. Assuming that the transfer coefficient (R) is 0.5, we can conclude that the formic acid oxidation on the Pd nanocatalyst is a two-electron transfer reaction. The CO stripping experiments have evidence that the electrode process did not form CO as a reaction intermediate, therefore the dehydrogenation reaction should be the main pathway for the electrooxidation of formic acid on the Pd nanostructures. In contrast with the anodic polarization of the Pd-FE, the anodic curve of the Pt-FE under identical conditions gives an apparent Tafel slope of 138 mV/decade in the linear region from -0.04 to 0.10 V. In the case that R ) 0.5, the rate-determining step should only

Fabrication and Electrocatalytic Activity of Pd Thin Films

Figure 6. Anodic Tafel curves for the Pd-FE and the Pt-FE in a mixed solution of 0.1 mol dm-3 H2SO4 and 0.2 mol dm-3 HCOOH.

involve the transfer of one electron (the calculated value: ∼0.86). Thus, the overall reaction for the formic acid oxidation on the Pt nanostructures seems to be a consecutive charge-transfer reaction. The most likely reaction pathway is via the dehydration reaction, involving the breaking of the C-H bond or the formation of hydroxyl species (the activation of water).34,42 Conclusions The Pd (or Pt) thin films with hierarchical architectures are fabricated on the glassy carbon (GC) substrates through a facile method by using Co thin films with similar hierarchical structures as the sacrificial templates to react with chloropalladite or tetrachloroplatinate. Because of unique structural features, the Pd thin films present unusual properties in hydrogen absorption and electrocatalysis. When used as an electrocatalyst for the formic acid oxidation, the hierarchical Pd architectures display significantly higher activity and stronger poison resistance than the Pt architectures with similar morphology. The CO stripping experiments and the steady-state polarization measurements clearly indicate that the formic acid oxidation on the hierarchical Pd micro/nanostructures proceeds via a nonCO reaction pathway but takes place on the hierarchical Pt micro/nanostructures via the CO intermediate route. Due to highly catalytic performance quite different from that of the bulk Pd and the Pd nanoparticles, the as-prepared Pd thin films with hierarchical architectures are expected to act as a promising electrocatalyst in DFAFCs. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20673067), the National 973 Program Projects of China (2006CB605004, 2007CB936602), and the Visiting Scholar Foundation of Key Laboratory in Shandong University. Supporting Information Available: SEM and HRTEM images of Pt thin films, cyclic voltammogramms for formic acid oxidation on bulk Pd and bulk Pt electrodes, and CO stripping voltammograms on the Pd and the Pt thin films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Piao, Y. Z.; Jang, Y. J.; Shokouhimehr, M.; Lee, I. S.; Hyeon, T. Small 2007, 3, 255–260. (2) Xiong, Y. J.; Xia, Y. N. AdV. Mater. 2007, 19, 3385–3391. (3) Liang, H. P.; Lawrence, N. S.; Jones, T. G. J.; Banks, C. E.; Ducati, C. J. Am. Chem. Soc. 2007, 129, 6068–6069. (4) Xiong, Y. J.; Wiley, B.; Chen, J. Y.; Li, Z.-Y.; Yin, Y. D.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 7913–7917.

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