Colloidal Synthesis and Characterization of Carbon-Supported Pd−Cu

Jun 25, 2010 - ZhiShan Luo , Maria Ibáñez , Ana M. Antolín , Aziz Genç , Alexey Shavel , Sandra .... E. Sánchez-Miguel , M.J. Tenorio , J. Morère , A...
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4144 Chem. Mater. 2010, 22, 4144–4152 DOI:10.1021/cm100155z

Colloidal Synthesis and Characterization of Carbon-Supported Pd-Cu Nanoparticle Oxygen Reduction Electrocatalysts Nancy N. Kariuki,* Xiaoping Wang, Jennifer R. Mawdsley, Magali S. Ferrandon, Suhas G. Niyogi, John T. Vaughey, and Deborah J. Myers Chemical Sciences and Engineering Division Argonne National Laboratory, Argonne, Illinois 60439, USA Received January 18, 2010. Revised Manuscript Received June 4, 2010

The ability to control the size and composition of metal or alloys nanoparticles is important in preparing catalysts. This paper reports a colloidal synthesis methodology for the preparation of monodisperse palladium-copper (Pd-Cu) alloy nanoparticles with an average diameter of 3 nm for the as-prepared particles and 5-10 nm upon removal of the capping agents. Our approach involves the use of metal precursors, capping agents, and reducing agents in controlled ratios for nanoparticle formation in a single organic phase, followed by deposition of the capped nanoparticles on high surface area carbon and removal of the capping agents via heat treatment in either oxidizing or reducing atmosphere. The results of characterizations using transmission electron microscopyenergy dispersive X-ray analysis (TEM-EDX), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), temperature programmed oxidation and reduction combined with mass spectrometry (TPO/TPR-MS), powder X-ray diffraction (XRD), and cyclic voltammetry (CV) are discussed. The resulting high-surface-area-carbon-supported Pd-Cu catalysts (PdCu/C) showed high activity for the oxygen reduction reaction (ORR) in acidic electrolyte. Our study revealed composition and heat-treatment dependent ORR activity. Introduction The polymer electrolyte fuel cell (PEFC) has great potential as a high efficiency energy conversion device for transportation and stationary applications. One of the major challenges to widespread commercialization of this technology is the high cost of the currently used platinum electrocatalyst for the cathode oxygen reduction reaction (ORR). The search for alternative ORR catalysts is therefore an important and active research area for fuel cell technology. While substantial progress has been made in the development of nonplatinum group metal ORR electrocatalysts,1-5 this class of materials has activity and stability considerably lower than those of Pt. An alternative approach is to replace Pt with less expensive, catalytically active, and relatively stable noble metals and to alloy the noble metals with base metals to enhance their stability and activity via electronic modification. Palladium is the second most active metal for the oxygen reduction *Corresponding author [email protected].

(1) Jasiniski, R. Nature 1964, 201, 1212. (2) Jahnke, H.; Sch€ onborn, M.; Zimmermann, G. Topics in current chemistry 1976, 61, 133. (3) Feng, Y. J.; Alonso-Vante, N. Phys. Status Solidi B 2008, 245(9), 1792. (4) Johnston, C. M.; Piela, P.; Zelenay, P. Handbook of Fuel Cells: Advances in Electrocatalysis, Materials, Diagnostics and Durability, Vielstich, W.; Gasteiger, H. A.; Yokokawa, H., Eds. John Wiley: 2009; Vol. 5 & 6. (5) Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P. Science 2009, 324(5923), 71. (6) Vracar, L. M.; Sepa, D. B.; Damjanovic, A. J. Electrochem. Soc. 1986, 133(9), 1835. (7) Kinoshita, K., Electrochemical Oxygen Technology.: John Wiley & Sons: New York, 1992.

pubs.acs.org/cm

reaction,6,7 however its mass activity is approximately five times lower than that of Pt. Binary Pd-base metal (BM) (where BM = Co, Ni, Fe, Cu, W, Mo) systems have been identified as promising PEFC cathode electrocatalysts, with enhanced activity for ORR and stability compared to Pd alone.8-14 The origin of the enhanced activity has been linked to modification of the electronic structure of Pd upon bonding with the alloying metal.15 In addition to enhancing activity, dissolution potentials of the noble metals may be shifted to higher potentials, thus stabilizing the electrocatalyst against dissolution in acidic medium.16,17 Alloy catalysts are typically prepared using traditional approaches which involve impregnation of metal precursors and postdeposition treatment at high temperatures in reducing atmospheres to promote alloy formation.10 However, catalyst particles resulting from this preparation (8) Savadogo, O.; Lee, K.; Mitsushima, S.; Kamiya, N.; Ota, K. I. J. New Mater. Electrochem. Syst. 2004, 7(2), 77. (9) Shao, M. H.; Sasaki, K.; Adzic, R. R. J. Am. Chem. Soc. 2006, 128 (11), 3526. (10) Wang, X. P.; Kariuki, N.; Vaughey, J. T.; Goodpaster, J.; Kumar, R.; Myers, D. J. J. Electrochem. Soc. 2008, 155(6), B602. (11) Fouda-Onana, F.; Savadogo, O. Electrochim. Acta 2009, 54(6), 1769. (12) Xu, C. X.; Zhang, Y.; Wang, L. Q.; Xu, L. Q.; Bian, X. F.; Ma, H. Y.; Ding, Y. Chem. Mater. 2009, 21(14), 3110. (13) Sarkar, A.; Murugan, A. V.; Manthiram, A. J. Phys. Chem. C 2008, 112(31), 12037. (14) Sarkar, A.; Murugan, A. V.; Manthiram, A. J. Mater. Chem. 2009, 19(1), 159. (15) Shao, M. H.; Liu, P.; Zhang, J. L.; Adzic, R. J. Phys. Chem. B 2007, 111(24), 6772. (16) Greeley, J.; Norskov, J. K. Electrochim. Acta 2007, 52(19), 5829. (17) Ma, Y. G.; Balbuena, P. B. J. Phys. Chem. C 2008, 112(37), 14520.

Published on Web 06/25/2010

r 2010 American Chemical Society

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method are typically large with wide size distributions and also lack composition homogeneity. These attributes make correlations of particle size and composition with activity difficult. In our preceding work on Pd-Cu ORR electrocatalysts,10 carbon supported Pd-Cu bimetallic catalysts with nominal molar ratios of (Pd:Cu) ranging from (9:1) to (1:9) were prepared by coimpregnation followed by reduction with hydrogen at temperatures between 300 and 800 °C. The ORR activity obtained from these catalysts varied with the heat treatment temperature and the Pd-Cu precursor ratio, which was attributed to variations in the degree of alloying, nanoparticle size, and bimetallic composition. The highest ORR activity from the study was shown by the bimetallic compositions with Pd-Cu molar ratios of (1:1) and (1:3) heattreated at 600 °C. It was anticipated that the ORR activity would be enhanced further by using alternative catalyst preparation methods that can offer a higher degree of alloy homogeneity with a smaller particle size and the formation of desired nanostructures and alloy phases at low temperatures. In this context, colloidal preparation methods18-22 offer an attractive approach to prepare multimetallic alloy compositions with a high degree of homogeneity and controlled particle size. Organic monolayer-protected alloy nanoparticles have been explored to address some of the challenges in nanoscale catalyst preparation.19,20,23 These types of nanoparticles serve as the building blocks for catalytic materials by taking advantage of diverse attributes, including size monodispersity, processability, solution dispersibility, stability, and self-assembly. This paper focuses on colloidal preparation of organic monolayer-capped bimetallic compositions with Pd-Cu molar ratios of (1:1) and (1:3). These ratios were selected due to their high ORR activity enhancement relative to the other compositions studied in our previous study.10 By manipulating or controlling the relative concentrations of the two metal precursors, capping agents, and reducing agents, organic monolayerstabilized nanoparticles in which the alloy Pd-Cu cores are encapsulated with a monolayer shell of amines and acids were successfully synthesized in a single organic phase. The average diameters of as-prepared nanoparticles were well-controlled at ∼3 nm with narrow particle size distributions. The composition of the binary nanoparticle cores was controlled by manipulating the feed ratios of the metal precursors used for the synthesis. This size- and composition-controlled synthesis is, to our knowledge, the first example of one-pot synthesis of organic monolayer stabilized Pd-Cu alloy nanoparticles for application in fuel cell electrocatalysis.

Experimental Section

(18) Raghuveer, V.; Ferreira, P. J.; Manthiram, A. Electrochem. Commun. 2006, 8(5), 807. (19) Luo, J.; Kariuki, N.; Han, L.; Wang, L. Y.; Zhong, C. H.; He, T. Electrochim. Acta 2006, 51(23), 4821. (20) Luo, J.; Wang, L. Y.; Mott, D.; Njoki, P. N.; Kariuki, N.; Zhong, C. J.; He, T. J. Mater. Chem. 2006, 16(17), 1665. (21) Bonnemann, H.; Nagabhushana, K. S. J. New Mater. Electrochem. Syst. 2004, 7(2), 93. (22) Bradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993, 5(3), 254. (23) Ibanez, F. J.; Zamborini, F. P. J. Am. Chem. Soc. 2008, 130(2), 622–633.

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Nanoparticle synthesis and electrode preparation. Chemicals. Palladium(II) acetate (Pd(OAc)2, 97%), copper(II) acetate (Cu(OAc)2, 97%), 2-ethoxyethanol (C2H5OCH2CH2OH), 90%), oleylamine (CH3(CH2)7CHdCH(CH2)8NH2, 70%), and oleic acid (CH3(CH2)7CHdCH(CH2)7COOH, 99þ%) were purchased from Aldrich and used as received. Other chemicals, such as ethanol (ACS reagent grade) and hexane (ACS reagent grade), were purchased from Fisher. Carbon black (XC-72R, Cabot) was used as the nanoparticle support material. Synthesis. The general synthesis of alloy Pd-Cu nanoparticles involved the use of two metal precursors, palladium acetate and copper acetate, (PdII(OAc)2 and CuII(OAc)2), in controlled molar ratios. The precursors were dissolved in 2-ethoxyethanol, which was also used as the reducing agent for the Pd- and Cuprecursors at elevated temperature. A mixture of oleylamine and oleic acid in controlled molar ratios was used as the capping agent. The composition of the (n1:n2) Pd-Cu nanoparticles, where n1 and n2 represent the atomic content of each metal, was controlled by controlling the feed ratio of the metal precursors. The nanoparticle product was isolated by centrifugation. In a typical procedure for the synthesis of (1:3) Pd-Cu, for example, 0.3644 g of Cu(OAc)2 (2.0 mMol), 0.1562 g of Pd(OAc)2 (0.7 mMol), 400 mL of 2-ethoxyethanol, and 5.2 mL of oleic acid (16.4 mMol) were purged with argon (Ar) for 30 min before heating to 80 °C under magnetic stirring. The solution mixture appeared greenish-blue. At this temperature, 1.8 mL of oleylamine (5.5 mMol) was added and the reaction mixture heated to 120 °C. The solution immediately turned a bright blue color upon addition of oleylamine and then faded to near clear before slowly turning to a dark brown color, an indication that the reduction was taking place. The reaction was continued under reflux conditions under an argon blanket for 3 h. The reaction mixture was allowed to cool to room temperature and the nanoparticle powder isolated by centrifugation. The black waxy product was dispersed in hexane (5 mL) and precipitated out by adding ethanol (40 mL) and centrifuging. The purified nanoparticle product was dried under N2 and redispersed in hexane. Preparation of electrocatalysts. Carbon black was dispersed into hexane by agitation for g3 h in an ultrasonic bath. A controlled amount of Pd-Cu nanoparticles was added into the carbon suspension and the suspension was further agitated for 30 min, followed by stirring for about 15 h to adsorb the particles onto carbon. The suspension was then allowed to stand to settle the powders, which were isolated by decanting. The carbon-supported Pd-Cu powders were collected and dried at room temperature under N2 or Ar. The loading of Pd-Cu on the carbon support was controlled by monitoring the weight ratio of Pd-Cu nanoparticles versus carbon. Typical preparations resulted in a Pd metal loading of 10 wt %. Activation of catalysts. Catalyst activation was achieved in a one-step heat treatment in reducing atmosphere by flowing H2 (Linde, 99.99%) through the carbon-supported nanoparticles placed in a tube furnace at elevated temperatures (300, 400, and 500 °C). Preparation of catalyst electrode. Catalyst ink was prepared by mixing the powdered catalyst with 5 wt % Nafion solution (Aldrich) in a weight ratio of PdCu/C to dry Nafion of approximately 2.5 and with methanol (as a dispersant) at a ratio of one gram of methanol per ∼7 mg catalyst. The ink was stirred for >12 h to obtain a homogeneous dispersion. The ink was then deposited onto a polished (0.05 μm Al2O3 paste) glassy carbon rotating disk electrode (GC-RDE) (Pine Instruments Co., geometric area: 0.196 cm2) and dried at room temperature to form a

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thin-film RDE.24 The loading on the electrode was ∼22 μg Pd/ cm2 for all catalysts. Catalyst Characterization. Temperature programmed reduction (TPR) and temperature programmed oxidation (TPO). TPR and TPO experiments were carried out to elucidate the organic capping agent decomposition temperatures and products formed during heat treatment of PdCu/C to remove the capping agent. The experiments were carried out on a commercial microreactor system (Zeton Altamira, model AMI-100) with mass spectrometric detection (Dycor Dymaxion quadrupole mass spectrometer) of gas effluent composition in the mass unit range from 1 to 100. A temperature ramp rate of 5 °C/min and a gas flow rate of 20 mL min-1 were used. Approximately ten milligrams of sample in a quartz U-tube reactor were pretreated in a 99.999% Ar flow at 120 °C for 30 min to remove adsorbed water before the reduction or oxidation experiments. The reduction experiments, TPR, used 3% H2/Ar as the treatment gas and a temperature ramp from room temperature to 600 °C. The oxidation experiments, TPO, used 5% O2/He and a temperature ramp from room temperature to 300 °C. After the TPR or TPO, the sample was cooled to room temperature and the purge gas was switched from H2/Ar or O2/He to Ar. Transmission electron microscopy (TEM). The size and composition of the particles were analyzed using a Philips CM30T electron microscope (200 kV), which was equipped with an energy-dispersive X-ray analyzer (EDX). For TEM measurements, samples were made into dilute suspensions in isopropanol and were drop-cast onto a carbon-coated gold grid followed by solvent evaporation in air at room temperature, leaving the catalyst on the carbon-coated TEM grid. The size distribution of the metal nanoparticles in the supported catalysts was obtained by directly measuring the size of at least 100 randomly chosen particles in the TEM images. EDX measurements were made on about ten different regions on the TEM grid. In most cases X-ray signals were collected from a spot size of 400 nm to obtain average particle composition. In some cases, a smaller spot size was used to determine if there were variations in the particle composition. Fourier transform infrared spectroscopy (FTIR). The interactions between adsorbed capping agents and metal nanoparticles were analyzed using a Perkin-Elmer Spectrum 100, with attenuated total reflectance (ATR) detector. FTIR spectra of the nanoparticles were obtained by evaporating a hexane nanoparticle dispersion on the diamond window of ATR cell. FTIR spectra of pure samples of oleic acid and oleylamine capping agents were obtained by placing a drop of the capping agent onto the diamond ATR cell. X-ray photoelectron spectroscopy (XPS). The oxidation state of Pd and Cu in the as-prepared alloy nanoparticles was analyzed using a Kratos AXIS-165 surface analysis system with a monochromatic Al Ka radiation as the excitation source. The binding energies were calibrated by referencing the C 1s peak (284.6 eV). XPS spectra of the nanoparticles were obtained by evaporating a hexane nanoparticle dispersion on an indium substrate. X-ray diffraction (XRD). The alloy structure of the catalyst particles was examined by carrying out powder X-ray diffraction using a Siemens D5000 X-ray powder diffractometer with a Cu KR radiation source (λ = 1.54056 A˚). The 2θ Bragg angles were scanned over a range of 5-80° at a rate of 0.2° min-1 with a 0.02° angular resolution. (24) Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 495(2), 134.

Kariuki et al. Electrochemical measurements. Cyclic voltammograms (CVs) of thin-film rotating disk electrodes (TF-RDE) were measured in both deaerated and O2-saturated 0.1 M HClO4 electrolyte solution using a CHI 660A electrochemical workstation (CH Instruments, Inc.). The steady-state CVs were obtained by rotating the TF-RDE at 1600 rpm using an electrode rotator (Pine Instruments Co., AFMSRX). The 0.1 M HClO4 electrolyte was prepared from 70% HClO4 (GFS Chemical, Inc., veritas double-distilled) and DI water (DI) (>18 MΩ cm-1, Millipore). The counter electrode was Au wire located in a separate fritted compartment. The reference electrode was a Hg/ Hg2SO4 electrode with a filling solution of 0.5 M H2SO4. Both the Ar (for deaeration) and O2 gases used were ultrahigh purity (99.999% Ar, 99.99% O2, Linde). The values of the potentials given in this paper are referenced to the standard hydrogen electrode (SHE). All electrochemical experiments were performed at room temperature (22 °C). The activity of the catalysts are reported as kinetic current (ik), extracted from the raw TF-RDE data using the Koutecky-Levich eq 1.25 ik ¼ ðid • iÞ=ðid - iÞ

ð1Þ

Where: id is the diffusion limited current and i is the measured current, in the steady-state CVs. The kinetic current was normalized to the amount of Pd in the TF-RDE to indicate the mass activity. To compare the intrinsic activity, the kinetic current was normalized to the electrochemically active surface area (ECA) to indicate specific activity. The ECA of the catalyst was estimated from the charge associated with hydrogen adsorption and desorption on Pd.26

Results and Discussion Size, morphology, and composition of capped Pd-Cu nanoparticles. The nanoparticle colloidal product was easily dispersed in a nonpolar solvent, such as hexane. The formation of stable dispersions of the Pd-Cu colloids in nonpolar organic solvents serves to verify the presence of capping molecules with hydrophobic end groups. Interesting dispersion phenomena were also observed. For the compositions with high Pd content, a high ratio of oleylamine to oleic acid was necessary to produce colloidal dispersions that were stable in hexane (often termed “soluble”). For example, in the synthesis of (1:3) Pd-Cu, a ratio of oleylamine to oleic acid of 1 to 3 resulted in colloids that were readily dispersible in hexane, while a similar ratio of oleylamine to oleic acid resulted in agglomeration for the (1:1) Pd-Cu. When the ratio of oleylamine to oleic acid was changed to 1 to 1 for synthesis of the (1:1) Pd-Cu, the resulting colloids were readily dispersible in hexane. This observation is relevant in trying to understand the role of the capping agents in controlling the nucleation and particle growth. The number of nuclei formed in the initial growth stages and their controlled growth are crucial in achieving monodisperse colloids. Palladium has a strong affinity for amine-terminated surfactant molecules, forming palladium-amine (25) Bard, A. J.; Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications. 2nd ed.; John Wiley: New York, 2000; p 341. (26) Umeda, M.; Kokubo, M.; Mohamedi, M.; Uchida, I. Electrochim. Acta 2003, 48(10), 1367.

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Figure 1. TEM image and size distribution histogram of as-prepared, capped (1:1) Pd-Cu nanoparticles.

surfactant complexes.27,28 On the other hand, oleic acid tightly binds to Cu forming strong Cu-oleate complexes that in turn reduce the reactivity of Cu for the nucleation process. Indeed, addition of oleylamine to the reaction mixture containing PdII(OAc)2, CuII(OAc)2, and oleic acid at 80 °C was accompanied by an immediate color change from dark green to a light brown color, which intensified to a dark brown color, an indication that reduction was taking place. It is therefore likely that Pdrich nuclei are formed in the beginning of the reduction reaction. Hence, the nucleation step and the number of nuclei formed depend on the ratio of Pd to oleylamine. Further growth around the nucleation centers occurs via consumption of Pd and Cu precursors from the surrounding solution.29,30 Oleic acid is an excellent capping agent that can bind strongly to the surface of metals with native oxides through the carboxyl group. Congruent with the complexation behavior mentioned previously, oxidized surface Cu atoms are therefore most likely stabilized by oleic acid while surface Pd atoms are stabilized by oleylamine. This correlation is supported by the FTIR results presented in Figure S1 and the respective discussion in the Supporting Information (SI). Controlling the concentration of the metal precursors and surfactant molecules is critical for obtaining monodisperse bimetallic colloids. In this work, palladium to oleylamine, and copper to oleic acid ratios of 1 to 8 were found to yield Pd-Cu nanoparticles that were readily dispersed in hexane. The nanoparticle dispersions obtained were a dark brown color and were stable for months. The oxidation states of Pd and Cu in the as-prepared Pd-Cu nanoparticles were examined by X-ray photoelectron spectroscopy (XPS). The results (Figure S2 and respective discussion in the SI) suggest that the nanoparticles consist primarily of metallic Cu and Pd. Figures 1 and 2 show representative TEM images and size distribution histograms of as-prepared Pd-Cu nanoparticles with Pd to Cu molar ratios of (1:1) and (1:3), respectively. Particle size distributions were determined (27) Houdayer, A.; Schneider, R.; Billaud, D.; Ghanbaja, J.; Lambert, J. Appl. Organomet. Chem. 2005, 19(12), 1239. (28) Shimazaki, Y.; Tashiro, M.; Motoyama, T.; Iwatsuki, S.; Yajima, T.; Nakabayashi, Y.; Naruta, Y.; Yamauchi, O. Inorg. Chem. 2005, 44(17), 6044. (29) Rogach, A. L.; Talapin, D. V.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. Adv. Funct. Mater. 2002, 12(10), 653. (30) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O.; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125(30), 9090.

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Figure 2. TEM image and size distribution histogram of as-prepared, capped (1:3) Pd-Cu nanoparticles. Table 1. Particle size and composition as determined by TEM-EDX for the as-prepared Pd-Cu nanoparticles Sample name

Particle diameter (nm)

Nominal composition (atom % Cu)

Bimetallic composition determined by EDX (atom % Cu)

(1:1) Pd-Cu (1:3) Pd-Cu (2:1) Pd-Cu

3.1 ( 1.5 3.4 ( 1.7 3.2 ( 0.7

50.0 75.0 35.0

49.1 ( 2.7 73.1 ( 2.8 34.9 ( 2.2

by analysis of ∼2000 particles in the TEM images using computer software (Gatan Digital Micrograph). Table 1 is a summary of particle size and bimetallic composition, as determined by EDX. While particle size for the two compositions varied slightly, the data demonstrated an average particle diameter of about 3 nm. Nanoparticle composition from selected areas closely matched the nominal composition used in the synthesis. To validate the size- and composition-controllable colloidal synthesis, Pd-Cu nanoparticles with Pd to Cu molar ratio of (2:1) was also synthesized. TEM results showing monodisperse particles with an average particle diameter of 3 nm are shown in Figure S3 in the SI. For utilization of the nanoparticles in Figures 1 and 2 as electrocatalysts, the as-prepared nanoparticles need to be supported on carbon black (Vulcan XC-72R) and the capping agents need to be removed. Figure 3 shows a representative image of the (1:3) Pd-Cu nanoparticles after deposition on carbon, following the procedure described in the Experimental section, and before removal of capping agents. The image shows that the particles uniformly distributed on the carbon support and that the spacing between the particles was retained. Catalyst activation: TPR/TPO. Heat treatment in oxidizing gas is a common method used for removing capping agent from nanoparticles derived from molecular encapsulation.31,32 However, a consequence of oxidative treatment for alloy catalysts is the possibility of alteration of catalyst crystallographic structure through phase segregation,33,34 necessitating an additional heat treatment step at high temperatures in reducing gas to revert to the (31) Paulus, U. A.; Endruschat, U.; Feldmeyer, G. J.; Schmidt, T. J.; Bonnemann, H.; Behm, R. J. J. Catal. 2000, 195(2), 383. (32) Luo, J.; Jones, V. W.; Maye, M. M.; Han, L.; Kariuki, N. N.; Zhong, C. J. J. Am. Chem. Soc. 2002, 124(47), 13988. (33) Venezia, A. M.; Liotta, L. F.; Deganello, G.; Schay, Z.; Horvath, D.; Guczi, L. Appl. Catal., A 2001, 211(2), 167. (34) Xiong, L. F.; Manthiram, A. J. Mater. Chem. 2004, 14(9), 1454.

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Figure 3. TEM image of carbon-supported, capped (1:3) Pd-Cu nanoparticles.

Figure 5. MS signals for the capped (1:3) PdCu/C sample during the first TPR step (dashed line) and the second TPR step (solid line) following TPO. Units are arbitrary on the y-axis.

alloy state.19 In addition, the desired structure of PdCu catalyst for application in the acidic environment of the PEFC is one where Pd forms a protective, catalytically active skin on the surface of the alloy particle to prevent Cu leaching from the alloy. Heat treatment of Pd alloys in reducing gas has been shown to segregate Pd on the surface.35-37 In this regard, heat treatment in hydrogen gas was explored as an alternative to the two-step oxidationreduction activation for removal of the capping agents. Figures 4 and 5 show TPO and TPR profiles, respectively, for the (1:3) PdCu/C sample prior to heat treatment and the sample heat treated in reducing and oxidizing gas environments. Figure 4 compares the MS signals of selected gas products for capped (1:3) PdCu/C during the initial TPO step (solid blue line) and the TPO after initial TPR (dashed blue line). The MS signals during TPO for the catalyst with the capping agent removed by heat treatment in H2 at 500 °C for

4 h (solid red line) are included for comparison. The capped (1:3) PdCu sample (capped 1:3 PdCu-TPO) shows an O2 consumption peak at 250 °C in the initial TPO, which coincides with a CO2 evolution peak and with the sharp rise of the water content, indicating oxidative removal or decomposition of the capping agent. The TPO profiles for the sample that had undergone the prior TPR and the sample that had been heat treated at 500 °C in H2 for 4 h did not show significant O2 consumption or H2O formation. Figure 5 shows the MS signals of the predominant gas products obtained during the initial TPR (dashed line) and the second TPR following TPO (solid line) for the capped (1:3) PdCu/C sample. Signals for masses 2, 18, and 44, corresponding to H2, H2O, and CO2, respectively, were detected during the TPR. During the initial TPR, the capped (1:3) PdCu/C sample shows two CO2 peaks at 100 and 300 °C. The CO2 peak at 100 °C is believed to be related to desorption of weak carbonyl groups adsorbed and/or already existing on the carbon support, and the CO2 peak at 300 °C with decarboxylation of the capping agents assisted by the formed water. In Figure 4, no peaks attributed to capping agent removal/ decomposition were observed in the TPO profile for the sample that had first undergone TPR (capped 1:3 PdCuTPR-TPO) similar to what was observed for the sample that had been heat treated at 500 °C in H2 for 4 h. The H2 consumption peak at 100 °C during the initial TPR of the capped (1:3) PdCu/C sample is believed to be related to hydrogenation of oxy groups of the capping agents. The profile for the second TPR after the TPO step shows two small hydrogen consumption peaks at 160 and 235 °C, which can be attributed to the reduction of Pd and Cu oxides38,39

(35) Khanra, B. C.; Menon, M. Physica B 2000, 291(3-4), 368. (36) Menon, M.; Khanra, B. C. Int. J. Mod. Phys. B 2000, 14(16), 1683. (37) Wang, A. Q.; Chang, C. M.; Mou, C. Y. J. Phys. Chem. B 2005, 109 (40), 18860.

(38) Batista, J.; Pintar, A.; Mandrino, D.; Jenko, M.; Martin, V. Appl. Catal., A 2001, 206(1), 113. (39) Melian-Cabrera, I.; Granados, M. L.; Fierro, J. L. G. J. Catal. 2002, 210(2), 285.

Figure 4. MS signals during initial TPO step (solid blue line), and TPO step after initial TPR step (dashed blue line) for the capped (1:3) PdCu/C sample, and for the (1:3) PdCu/C catalyst with the capping agent removed by heat-treatment in 3% H2/He at 500 °C for 4 h (solid red line).Units are arbitrary on the y-axis.

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which are formed during the TPO step. The lack of these hydrogen consumption peaks in the initial TPR profile indicates that the initial organic capped Pd-Cu particles are in the reduced state and/or are protected or stabilized against reduction. These results indicate that lower temperature (235 °C) is needed for the removal of capping agents in oxidizing atmosphere compared to the reducing atmosphere (350 °C). However, as shown in Figure 5, a subsequent high temperature (>300 °C) heat treatment in reducing gas is required to convert the oxides formed during the oxidative treatment to metallic PdCu alloys. Therefore, the one-step activation in a reducing (hydrogen-containing) atmosphere was followed for all PdCu/C catalysts discussed in the remainder of this paper. Effect of heat treatment temperature on catalyst particle size. Figure 6 shows representative TEM images of (1:1) PdCu/C and (1:3) PdCu/C after heat treatment at 300 °C, 400 °C, or 500 °C in hydrogen (Linde, 99.99%), with the insets showing representative high-resolution TEM images of the 500 °C samples. A representative TEM image and particle size distribution for the (2:1) PdCu/C after heat treatment at 500 °C are shown in Figure S4 in the SI. Size analysis of particles selected randomly from several TEM images showed particle size increase with increasing treatment temperature. Table 2 gives a summary of particle size and distribution for the Pd-Cu, which shows average particle diameter increases of between 2 and 7 nm after the heat treatments. The observation of particle growth from the as-prepared to 300 °C heat treated sample indicates that the capping agents are removed from the particles in reducing atmospheres at 300 °C, resulting in particle sintering, in agreement with the interpretation of the TPR-MS data (Figure 5). As expected, further Pd-Cu particle growth is observed with increasing heat treatment temperature. The mechanism of particle growth can be explained by considering kinetic models based on thermally induced sintering of particles40 through two possible pathways: (1) Ostwald ripening or (2) particle diffusion and coalescence of small particles.41-44 In the Ostwald ripening mechanism, individual metal atoms detach and diffuse across the support until they deposit on other particles. In the particle diffusion and coalescence mechanism, whole particles diffuse across the support until they contact other particles and coalesce. For both the (1:1) PdCu/C and (1:3) PdCu/C, heat treatment at the three different temperatures did not show significant variation in the catalyst Pd to Cu ratio, which is an indication that the particle growth in this case proceeds through diffusion and coalescence mechanism as opposed to the Ostwald ripening mechanism. The size distribution depends on the interplay between the interparticle dis(40) Parker, S. C.; Campbell, C. T. Phys. Rev. B 2007, 75, (3), -. (41) Nakaso, K.; Shimada, M.; Okuyama, K.; Deppert, K. J. Aerosol Sci. 2002, 33(7), 1061. (42) Mitchell, C. E. J.; Howard, A.; Carney, M.; Egdell, R. G. Surf. Sci. 2001, 490(1-2), 196. (43) Weber, A. P.; Friedlander, S. K. J. Aerosol Sci. 1997, 28(2), 179. (44) Park, J. B.; Conner, S. F.; Chen, D. A. J. Phys. Chem. C 2008, 112 (14), 5490.

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Figure 6. TEM images of (1:1) [top] and (1:3) [bottom] PdCu/C after heat treatment in H2 at various temperatures. Insets show high resolution TEM images for the heat treatment at 500 °C. Table 2. Particle size for the catalysts heat treated in hydrogen at different temperatures Sample name

Particle size (nm) As-prepared

300 °C

400 °C

500 °C

(1:1) Pd-Cu (1:3) Pd-Cu (2:1) Pd-Cu

3.1 ( 1.5 3.4 ( 1.7 3.2 ( 0.7

7.4 ( 1.9 5.1 ( 0.3 _

7.5 ( 1.9 5.8 ( 1.6 _

10.0 ( 2.5 7.2 ( 1.6 7.0 ( 1.5

tances (or particle density) and the surface free energy45 of the metal particles. It is notable that there is no significant particle size difference between 300 and 400 °C treatments indicating that the kinetics of sintering did not differ much at these temperatures. The same type of particle growth dependence on temperature was also observed by Jiang et al.46 for carbon-supported Pd nanoparticles, where 300 and 400 °C treatments resulted in nearly identical particle sizes while 500 and 600 °C resulted in much larger particle sizes. The noticeable difference in particle growth between the (1:1) and the (1:3) PdCu/C observed here may be attributed to the differences in surface free energies.45 The driving force for coalescence is the reduction in free energy through a reduction in surface area. It appears that the (1:1) Pd-Cu particles have higher surface free energy than the (1:3) Pd-Cu resulting in a higher degree of particle coalescence at all heat treatment temperatures studied. Alloy structure of the catalyst particles. Figures 7 and 8 show XRD patterns of (1:1) and (1:3) PdCu/C catalysts, respectively, after heat treatment in hydrogen at three different temperatures (300, 400, and 500 °C). The XRD pattern of the catalyst before heat treatment is included for comparison. The patterns of as-prepared (1:1) and (1:3) samples displayed a broad peak centered between the known (111) reflections for pure Pd and Cu phases (40.3° and 43.4°, respectively), indicating that the as-prepared samples are indeed alloys with very small particle size. The narrowing of the peak width with increasing temperature can be attributed to crystallite size increases and/or increased phase uniformity of the particles.47 (45) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298(5594), 811. (46) Jiang, L.; Hsu, A.; Chu, D.; Chen, R. J. Electrochem. Soc. 2009, 156 (5), B643. (47) Friedrich, M.; Armbruster, M. Chem. Mater. 2009, 21(24), 5886.

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Figure 7. XRD patterns of (1:1) PdCu/C before and after activation in hydrogen at different heat- treatment temperatures.

Figure 8. XRD patterns of (1:3) PdCu/C before and after activation in hydrogen at different heat- treatment temperatures.

The Pd-Cu phase diagram indicates that bulk alloys with Pd atomic percent ranging from approximately 35 to 47 form a body-centered cubic (bcc) structure (CsCl-type, Pm3m) at low temperatures.48 However, until recently, the structure observed for nanoparticle Pd-Cu has been a substitutionally disordered face-centered cubic (fcc) structure. Shah and Yang49 calculated, using Corrected Effective Medium theory, that the energetically preferred structure is particle size dependent with a transition from fcc to bcc at particle sizes greater than ∼6 nm (110,000 atoms). Friedrich and Armbruster recently found a crystallite size dependence of the preferred crystal structure for nanoparticle Cu60Pd40, with the product after annealing at 230 °C exhibiting an XRD pattern consistent with a mixture of disordered fcc and ordered bcc phases.47 The XRD pattern for the (1:1) PdCu/C heat treated at 500 °C, Figure 7, shows predominant peaks centered at two-theta values of 41.68, 43.2, and 48.44°. Based on the lattice parameters of Pd (a = 3.8898 A˚; JCPDS-ICDD 46-1043) and Cu (a = 3.6150 A˚; JCPDS-ICDD 04-0836) and using Vegard’s Law, a (1:1) Pd-Cu alloy should show (111) and (200) reflections at two-theta values of 41.66 and 48.48° (copper KR radiation, 1.5406 A˚). The XRD peak at 43.2° matches closely the expected value of 43.18° for the (110) reflection of ordered bcc Cu60Pd40, as (48) Subramanian, P. R.; Laughlin, D. E., In Binary Alloy Phase Diagrams, Cu-Pd (Copper-Palladium), II ed.; Massalski, T. B., Ed. 1990; Vol. 2 pp 1454. (49) Shah, V.; Yang, L. Q. Philos. Mag. A 1999, 79(8), 2025.

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calculated from the lattice parameter of a = 3.8898 A˚.47 No separate Cu-rich and Pd-rich phases were identified by TEM-EDX. XRD pattern of (2:1) PdCu/C heat treated at 500 °C (Figure S5 in the SI) showed (111) and (200) reflections centered at two-theta values of 41.40° and 48.04°, respectively. These reflections are shifted to low two-theta values by approximately 0.3-0.5° with respect to those of the (1:1) Pd-Cu alloy, consistent with the increase in Pd content in the (2:1) PdCu/C sample. The XRD patterns for the (1:3) PdCu/C in Figure 8, show predominant peak centered at two-theta value of 42.3°, which matches the expected value of 43.3° for the (111) reflection of an fcc Cu3Pd solid solution.50 The peak for the samples heat treated at 300 and 400 °C is asymmetrical, which indicates non homogeneity in the alloy composition at the two lowest heat-treatment temperatures. The (1:3) PdCu/C also contains CuO indicated by the presence of peaks at 2θ of ∼35.6° and 38.7° (JCPDSICDD 05-0661). We note that the intensities of the oxide peaks relative to the Cu3Pd peak (∼42.3°) decreases for the sample heat treated at 500 °C, possibly due to high degree of reduction and alloying at this temperature. The high-resolution TEM images of the 500 °C heat treated samples (Figure 6- inset) agree with the XRD identification of the substitutionally disordered fcc structure in both the (1:1) PdCu/C and (1:3) PdCu/C samples. Analysis of the lattice spacing observed in the high-resolution TEM images of ten particles showed an average lattice spacing of 2.14 ( 0.08 A˚ for (1:1) PdCu/C and 2.18 ( 0.05 for (1:3) PdCu/C, consistent with the spacing expected for the (111) planes of substitutionally disordered PdCu cubic alloys (2.17 A˚ for (1:1) Pd-Cu and 2.13 A˚ for (1:3) Pd-Cu, calculated from the lattice parameters of Pd (a = 3.8898 A˚; JCPDS-ICDD 46-1043), and Cu (a = 3.6150 A˚; JCPDSICDD 04-0836)). Electrochemical property of the catalysts: ORR. The (1:1) and (1:3) PdCu/C catalysts were characterized for ORR. Figures 9 and 10 show CVs at 10 mV s-1 for the (1:1) and (1:3) PdCu/C respectively, in deaerated 0.1 M HClO4 aqueous electrolyte, without electrode rotation. The reduction feature at 0.8 to 0.6 V in the cathodic scan direction is attributed to the reduction of Pd oxide formed in the anodic scan direction (broad feature at >0.6 V). The peaks in the 0.34 to 0.14 V region are associated with hydrogen adsorption and desorption on the nanocrystalline Pd surface.51 These features are used to estimate the electrochemically active Pd surface areas of the individual catalysts in a similar way as for Pt-based catalyst.26 The large cathodic and anodic peaks at more negative potentials (∼0.05 V) are due to formation of bulk Pd hydride and desorption of hydrogen from this hydride, respectively.52,53 Figure 11 shows typical CVs, for the ORR for the (1:1) and (1:3) PdCu/C catalysts, in oxygen- saturated 0.1 M (50) Presnyakov, A. A.; Dautova, L. I.; Dzhanbusinov, E. A. Fizika Metallov i Metallovedenie 1963, 16(1), 52. (51) Salvador-Pascual, J. J.; Citalan-Cigarroa, S.; Solorza-Feria, O. J. Power Sources 2007, 172(1), 229. (52) Shohoji, N. J. Mater. Sci. Lett. 1990, 9(2), 231. (53) Sittler, F.; Ramseyer, C.; Spielmann, B.; Girardet, C.; Pagetti, J. Thin Solid Films 1998, 315(1-2), 127.

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Figure 9. Cyclic voltammograms of (1:1) PdCu/C activated at different temperatures. Electrolyte: deaerated 0.1 M HClO4 . Scan rate: 10 mV s-1.

Figure 10. Cyclic voltammograms of (1:3) PdCu/C activated at different temperatures. Electrolyte: deaerated 0.1 M HClO4 . Scan rate: 10 mV s-1.

HClO4 aqueous electrolyte, with electrode rotation of 1600 rpm and a scan rate of 10 mV s-1. Table 3 summarizes the ORR mass and specific activities of the PdCu/C catalysts obtained at different potentials using the KouteckyLevich equation.25 The measured current was normalized by the amount of Pd in the thin film electrode to indicate the mass activity and to the Pd electrochemically active surface area for the specific activity. Overall, Pd-Cu catalyst with Pd to Cu atom ratio of (1:1) showed the highest ORR activity. The data for the (1:1) catalyst also shows a general trend in improvement of activity with increase in heat treatment temperature. The improvement of catalytic activity may be associated with changes in the surface structure of the alloy particles caused by heat treatment and/or a particle size effect as is well-documented for Pt nanoparticles54 and has been observed for the ORR on Pd particles in alkaline electrolyte.46 However, the (1:3) PdCu/C catalyst did not show a systematic trend in ORR activity with increasing heat treatment temperature. This lack of dependence may arise from the more heterogeneous phase composition of the as-prepared (1:3) catalyst and the concurrent effects of heat treatment of increasing the particle size, extent of alloying, and phase homogeneity. (54) Markovic, N.; Gasteiger, H.; Ross, P. N. J. Electrochem. Soc. 1997, 144(5), 1591.

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Figure 11. Steady-state cyclic voltammograms for the (1:1) and (1:3) PdCu/C activated at 500 °C in O2-saturated and deaerated electrolyte. Scan rate: 10 mV s-1. Electrode rotation rate: 1600 rpm.

The specific activity for the Pd-Cu catalysts presented here is higher than that reported for pure Pd of the same particle size9 and approximately an order of magnitude lower than that of pure Pt.55 The (1:1) Pd-Cu heat treated at 500 °C, which was found to have a mean particle size of 10 nm, has 1.5 times the specific ORR activity of 10 nm Pd particles reported by Adzic et al. (131 μA/cm2 at 0.85 V, room temperature).9 In addition, the ORR mass activity of the (1:1) Pd-Cu catalysts prepared via the colloidal technique presented in this study is up to two times higher than that reported for similar Pd-Cu compositions prepared by the coimpregnation technique.10 A similar enhancement effect of Cu on the ORR activity of Pt has been reported for Pt-Cu nanoparticle catalysts prepared using low temperature aqueous reduction procedures (three times enhancement vs Pt) or low temperature aqueous procedures followed by annealing (two times enhancement).56,57 Recently, activity enhancements of up to six times those of pure Pt nanoparticles have been observed for voltammetrically dealloyed Pt-Cu nanoparticles comprised essentially of a pure Pt shell surrounding a Pt-Cu alloy core.58,59 The catalytic enhancement in this case was attributed to geometric effects, whereby dealloying creates favorable structural arrangements of Pt atoms at the particle surface, such as more active crystallographic facets or more favorable Pt-Pt interatomic distances.60 The enhancement of ORR activity of noble metals by base metals has been attributed to changes in the electronic structure of the valence band of the noble metal which modifies the binding energy of ORR intermediates and site-blocking species.61 In previous studies, we found that (55) Mayrhofer, K. J. J.; Strmcnik, D.; Blizanac, B. B.; Stamenkovic, V.; Arenz, M.; Markovic, N. M. Electrochim. Acta 2008, 53(7), 3181. (56) Xiong, L.; Kannan, A. M.; Manthiram, A. Electrochem. Commun. 2002, 4(11), 898. (57) Tseng, C. J.; Lo, S. T.; Lo, S. C.; Chu, P. P. Mater. Chem. Phys. 2006, 100(2-3), 385. (58) Koh, S.; Strasser, P. J. Am. Chem. Soc. 2007, 129(42), 12624. (59) Liu, Z. C.; Koh, S.; Yu, C. F.; Strasser, P. J. Electrochem. Soc. 2007, 154(11), B1192. (60) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126 (14), 4717. (61) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Angew. Chem., Int. Ed. 2006, 45(18), 2897.

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Table 3. Catalytic activities of the PdCu/C catalysts activated at various temperatures Specific activity (μA/cm2 Pd)

Mass activity (mA/mg Pd) Catalyst

0.80 V

0.85 V

0.90 V

0.80 V

0.85 V

0.90 V

(1:1) Pd-Cu-300 °C (1:1) Pd-Cu-400 °C (1:1) Pd-Cu-500 °C (1:3) Pd-Cu-300 °C (1:3) Pd-Cu-400 °C (1:3) Pd-Cu-500 °C (2:1) Pd-Cu-500 °C

559 565 750 334 297 593 384

99 100 142 71 85 115 74

2 16 23 7 13 10 10

784 833 1063 753 583 796 920

139 147 202 160 166 154 176

3 24 33 16 25 17 24

the total density of states of the valence band is shifted toward higher binding energies for Pd-Cu catalysts compared to that of metallic Pd resulting in an overall valence band structure similar to that of metallic Pt.62 Conclusion We have demonstrated a size- and compositioncontrollable colloidal synthesis of monodispersed Pd-Cu alloy nanoparticles. By manipulating the relative concentrations of the metal precursors, the synthesis produces Pd-Cu nanoparticles with an average diameter of 3.3 nm and controlled binary composition. Control of the ratios of metal precursors and organic capping molecules was critical for obtaining colloidal dispersions that were stable in organic solvents. Furthermore, the Pd-Cu nanoparticles were easily supported on carbon black with retention of the small particle size, and controllable dispersion and metal weight loading. XRD showed that the colloidal synthesis technique results in cubic Pd-Cu alloys, unlike the coimpregnation technique which requires temperatures as high as 600-800 °C to promote formation of Pd-Cu alloys with Cu to Pd atomic ratios of (1:1) and higher.13 Heat treatment of the carbon-supported nanoparticles in reducing environment was effective in removing the capping agents from the particle surface resulting in (62) Wang, X. P.; Kariuki, N. N.; Niyogi, S.; Smith, M. C.; Myers, D. J.; Hofmann, T.; Zhang, Y. F.; Bar, M.; Heske, C. Proton Exchange Membrane Fuel Cells 8, Pts 1 and 2 2008, 16(2), 109.

Pd-Cu alloy nanoparticles of 5-10 nm which are catalytically active for ORR. The catalysts heat treated at 500 °C showed high ORR activity, with the (1:1) Pd-Cu composition showing higher mass and specific activity and a larger particle size compared to the (1:3) composition. The (1:1) PdCu/C catalyst also showed a trend of improvement in mass activity with increase in heat treatment temperature, even with increased particle size, reflecting a significant effect of particle size on ORR activity for this class of catalysts and/or formation of a favorable surface structure with increasing temperature. Acknowledgment. This research was conducted at Argonne National Laboratory, a U.S. Department of Energy, Office of Science Laboratory, operated by UChicago Argonne, LLC, under contract no. DE-AC02- 06CH11357. The electron microscopy was performed at the Electron Microscopy Center for Materials Research at Argonne. This research was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program. The authors would like to thank Karren More at Oak Ridge National Laboratory for the high-resolution TEM images and Chongjiang Cao at the University of Illinois at Chicago for the XPS spectra. Supporting Information Available: Additional figures, results and discussion details for the XPS and FTIR characterization, and additional references. This information is available free of charge via the Internet at http://pubs.acs.org/.