ARTICLE pubs.acs.org/Langmuir
Surface Structures and Electrochemical Activities of Pt Overlayers on Ir Nanoparticles Kug-Seung Lee,†,§ Sung Jong Yoo,†,§ Docheon Ahn,‡ Tae-Yeol Jeon,† Kwang Hyun Choi,† In-Su Park,† and Yung-Eun Sung*,† †
World Class University Program of Chemical Convergence for Energy & Environment, School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-744, South Korea ‡ Pohang Accelerator Laboratory, Pohang 790-784, South Korea
bS Supporting Information ABSTRACT: Pt overlayers were deposited on carbon-supported Ir nanoparticles with various coverages. Structural and electrochemical characterizations were performed using transmission electron microscopy (TEM), X-ray diffraction, high-resolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), cyclic voltammetry (CV), CO stripping voltammetry, and N2O reduction. The surface of Ir nanoparticles was covered with Pt overlayers with thickness varying from the submonolayer scale to more than two monolayers. Surface analyses such as CV and CO stripping voltammetry indicated that the Pt overlayers were uniformly deposited on the Ir nanoparticles, and the resultant Pt overlayers exhibited gradual changes in surface characteristics toward the Pt surface as the surface coverage increased. The distinct CO stripping characteristics and the enhanced Pt utilization affected electrocatalytic activities for methanol oxidation. The electrochemical stability of the Pt overlayer was compared with a commercial carbon-supported Pt catalyst by conducting a potential cycling experiment.
1. INTRODUCTION Pt-based electrocatalysts have been found to have good performance when applied to low-temperature fuel cells including polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs); however, the use of Pt makes these fuel cells expensive to produce. Therefore, many attempts have been made to reduce the use of Pt in fuel cell electrodes. Alloying Pt with transition metals has been extensively studied with the objective of enhancing electrocatalytic activities and reducing the Pt content in catalysts. However, these alloying methods are not satisfactory to reduce the Pt loading. Non-noble catalysts including transition metal chalcogenides, transition metal oxides, and macrocycles have been suggested as alternatives to Pt; however, their activities are too low as compared to that of Pt. A reasonable approach for minimizing the use of Pt is the formation of core/shell structures via surface modification of metal nanoparticles. In this approach, precious or active metals (Pt or Pt alloys) are deposited on cheaper or inactive metal nanoparticles so that the utilization of Pt can be improved. Change in molecular adsorption (such as CO, O, and OH) properties on the Pt overlayer by alteration of electronic r 2011 American Chemical Society
and/or geometric structure in the core/shell architectures is also worthy of consideration, because the adsorption properties can significantly affect electrochemical activities. Atomic-scale surface modification of the catalysts has been suggested in order to improve the efficiency of utilization of active materials and to enhance electrocatalytic activities in processes such as methanol oxidation,1-4 CO oxidation,5,6 hydrogen oxidation,7,8 and oxygen reduction.9-12 To build up core/shell structures with monolayer-scale overlayer thicknesses, spontaneous (or replacement) depositions were pioneered by Adzic and co-workers.7,8,13-15 A sacrificial Cu monolayer was deposited on core particles using well-known under-potential deposition (UPD) methods, and the Cu monolayer was replaced by the Pt overlayer. The replacement can definitely be ascribed to a difference in the standard potentials of the Pt precursor and Cu. Redox reactions occur in which electrons from Cu atoms are transferred to the Pt ions on the surface of the core particles. In this case, therefore, the sacrificial Received: September 23, 2010 Revised: November 24, 2010 Published: February 01, 2011 3128
dx.doi.org/10.1021/la103825s | Langmuir 2011, 27, 3128–3137
Langmuir Cu monolayer plays the role of a reductant. Attempts to form the core/shell structures directly (without sacrificial layers) onto core particles using UPD techniques have not been reported, and they appear unfeasible, because for UPD to occur, the work function of the substrate material must be higher than that of Pt, the overlayer material.16 The Cu-replacement method can provide information on the effect of core materials on electrochemical activities as a model catalytic system, while the Pt overlayer can be deposited uniformly on the surface of core particles. However, the electrochemical deposition is difficult to apply to commercialization of catalysts synthesis, although a scale-up synthesis was recently reported.17 Other methods involve heat treatment of alloyed bimetallic catalysts. Temperature increase in an inert or reductive atmosphere can induce a Pt-dominant surface because of the difference between the segregation energy of the two metals alloyed.18-21 The application of this method is limited to core materials that have lower segregation energy than Pt, and the heat treatment often causes particle agglomerations. From the practical viewpoint of synthesis, it is necessary to apply chemical syntheses to the formation of core/shell catalysts. In a chemical synthetic method, a reducing agent is introduced into a solvent containing metal precursors, in which the reduction of the metal precursors occurs because of the difference between the standard potentials of metal precursors and the reducing agent. In order to form core/shell structures, it is of central importance to choose an appropriate reducing agent in a given synthetic system. If the difference between standard potentials of the metal (Pt, in this case) precursors and the reducing agent is too large, facilitating the fast reduction of Pt, then the reduced Pt atoms can readily agglomerate with each other to produce independent Pt particles. In addition to the choice of a suitable reducing agent, the surface states of core particles including metallic/oxide phases and impurity contents can significantly influence the Pt overlayer structures because of their different surface energies and lattice parameters. In this regard, Au, the noblest metal, is the most favorable core material for forming uniform overlayer structures and improving the utilization of overlayer materials. Indeed, Au has been widely employed as a seed or core material.1-3,5,10,11,22 However, Au is too expensive, difficult to synthesize with small diameters (less than 3 nm), and not conducive to specific activities.22-24 Ir may be a strong alternative material: it is less expensive than Au, can be easily synthesized with small particle diameters (less than 2 nm),25-27 and is electrochemically stable in acidic media.28,29 The small size of core particles is advantageous from the viewpoint of achieving high surface area. In this study, Pt overlayers are prepared on carbon-supported Ir nanoparticles to form core/shell structures, and their surface structures, electrocatalytic activities for methanol oxidation, and electrochemical stability are examined. The thickness of the Pt overlayer deposited on Ir nanoparticles was varied from the submonolayer scale to more than two monolayers to clarify the effect of Ir core particles on the properties of the Pt overlayers.
2. EXPERIMENTAL METHODS 2.1. Sample Preparation. For synthesizing Ir nanoparticles on carbon (Ir/C), IrOx nanoparticles were first synthesized on carbon (IrOx/C) and then IrOx/C was reduced to Ir/C. The procedure for synthesizing IrOx/C is as follows: 0.3 g of H2IrCl6 3 xH2O was dissolved in 100 mL of EG solution containing 0.1 M NaOH. The solution was
ARTICLE
refluxed in a three-neck flask at 100 °C for 1 h in Ar atmosphere and then cooled naturally. At the same time, 0.1 g of carbon black (Vulcan XC72R) was dispersed in 600 mL of DI water through ultrasonic vibration and magnetic stirring. An adequate amount of IrOx colloid was added to the carbon-dispersed solution and mixed to obtain a mixture with 8 wt % IrOx/C. After 30 min of vigorous stirring, 300 mL of 2 M H2SO4 was slowly added to the solution. After additional stirring overnight, the solution was filtered and evaporated in a vacuum oven at room temperature. The obtained IrOx/C was reduced in a tube furnace by flowing 10% H2/Ar gas for 2 h at a flow rate of 100 cc/min to form Ir/C nanoparticles. The Pt overlayers on the Ir/C nanoparticles were synthesized as follows: a certain amount of Pt precursor (H2PtCl6 3 xH2O) was added to the Ir/C-dispersed anhydrous ethanol (a-EtOH) solution and the solution was then stirred for 30 min. The total volume of the solution was 200 mL. Adequate amounts of L-ascorbic acid (C6H8O6) were introduced to the solution. The solution was then stirred for several hours, following which it was heated at 80 °C for 1 h. After cooling to room temperature, the solution was filtered with deionized (DI) water and was then evaporated in a vacuum oven. The amounts of Pt precursor were calculated to produce Pt/Ir atomic ratios of 0.5:1, 1:1, 2:1, and 3:1. Hereafter, the Pt overlayers on Ir/C samples are designated as Pt[x]-Ir, where x denotes the Pt/Ir ratio. For comparison, a commercial catalyst (40 wt % Pt/C, Johnson Matthey; hereafter referred to as Pt/C) was tested for electrochemical characterization. 2.2. Sample Characterization. Transmission electron microscopy (TEM) images were obtained using a JEOL 2010 transmission electron microscope operated at 200 kV. Samples were prepared by placing a drop of catalyst solution onto a carbon-coated copper grid that was subsequently dried. Energy-dispersive spectroscopy (EDS) point analysis was carried out using a TEM-EDS system. X-ray diffraction (XRD) analysis was performed using a Rigaku D/MAX 2500 operated with a Cu KR source (λ = 1.541 Å) at 40 kV and 200 mA. The samples were scanned from 20° to 80° (2θ) at a scan rate of 2°/min. Highresolution synchrotron X-ray powder diffraction (HRPD) measurenents of the samples were carried out at 8C2 beamline of the Pohang Light Source (PLS). The incident X-rays were vertically collimated using a mirror and monochromatized to a wavelength of 1.5495 Å using a double-crystal Si (111) monochromator. The detector arm of the vertical scan diffractometer comprises seven sets of soller slits, flat Ge (111) crystal analyzers, antiscatter baffles, and scintillation detectors, with each set separated by 20°. Each sample of ca. 0.2 g powder was prepared by a flat-plate side-loading method to avoid a preferred orientation, and the sample was then rotated about the normal to the surface during the measurement in order to increase the sampling statistics. A step scan was performed at room temperature from 10° in 2θ with an increment of 0.02° and overlap of 1° to the next detector bank up to 131° in 2θ. In order to accurately determine the lattice parameters of the Pt overlayers, HRPD patterns were fitted by the whole-pattern profile matching method, in which the system was modeled as multiphases. The fitting procedure is as follows. We first determined the lattice parameters of Ir and C in Ir/C from whole-pattern profile matching of the HRPD pattern. The given lattice parameters of Ir and C were fixed, and then, the lattice parameter of Pt in Pt[x]-Ir/C was fitted using whole-pattern profile matching of the HRPD patterns. X-ray photoelectron spectroscopy (XPS) was performed in a multipurpose surface analysis system (SIGMA PROBE, Thermo, UK) operating at base pressures of
