Bridgid N. Wanjala, Rameshwori Loukrakpam, Jin Luo, Peter N. Njoki, Derrick Mott, and Chuan-Jian Zhong* Minhua Shao* and Lesia Protsailo

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1 17580 J. Phys. Chem. C 2010, 114, Thermal Treatment of PtNiCo Electrocatalysts: Effects of Nanoscale Strain and Structure on the Activity and Stability for the Oxygen Reduction Reaction Bridgid N. Wanjala, Rameshwori Loukrakpam, Jin Luo, Peter N. Njoki, Derrick Mott, and Chuan-Jian Zhong* Department of Chemistry, State UniVersity of New York at Binghamton, Binghamton, New York Minhua Shao* and Lesia Protsailo UTC Power, 195 GoVernor s Highway, South Windsor, Connecticut Tetsuo Kawamura Toyota Motor Engineering & Manufacturing North America, Incorporated, 1588 Woodridge AVenue, Ann Arbor, Michigan ReceiVed: July 22, 2010; ReVised Manuscript ReceiVed: August 30, 2010 The ability to control the nanoscale size, composition, phase, and facet of multimetallic catalysts is important for advancing the design and preparation of advanced catalysts. This report describes the results of an investigation of the thermal treatment temperature on nanoengineered platinum-nickel-cobalt catalysts for oxygen reduction reaction, focusing on understanding the effects of lattice strain and surface properties on activity and stability. The thermal treatment temperatures ranged from 400 to 926 C. The catalysts were characterized by microscopic, spectroscopic, and electrochemical techniques for establishing the correlation between the electrocatalytic properties and the catalyst structures. The composition, size, and phase properties of the trimetallic nanoparticles were controllable by our synthesis and processing approach. The increase in the thermal treatment temperature of the carbon-supported catalysts was shown to lead to a gradual shrinkage of the lattice constants of the alloys and an enhanced population of facets on the nanoparticle catalysts. A combination of the lattice shrinkage and the surface enrichment of nanocrystal facets on the nanoparticle catalysts as a result of the increased temperature was shown to play a major role in enhancing the electrocatalytic activity for catalysts. Detailed analyses of the oxidation states, atomic distributions, and interatomic distances revealed a certain degree of changes in Co enrichment and surface Co oxides as a function of the thermal treatment temperature. These findings provided important insights into the correlation between the electrocatalytic activity/stability and the nanostructural parameters (lattice strain, surface oxidation state, and distribution) of the nanoengineered trimetallic catalysts. Introduction One of the major challenges for the commercialization of proton exchange membrane fuel cells (PEMFCs) is the development of a catalyst system that is robust, active, and cost effective. Alloying Pt with other transition metals is one approach to address this challenge Studies have shown that the overpotential for oxygen reduction reaction (ORR) at the cathode currently accounts for about 70% of the energy losses. 15 Different supported and unsupported Pt-group electrocatalysts with binary PtM composition where M ) Sc, Y, Au, Co, Ni, V, Fe, Cr, Pd, W, Ag, Ti, and Mn or ternary composition PtM1M2 where M1M2 ) V, Fe, Ni, Cr, Co, and Cu 1-14,16-33 have been prepared using different techniques. Examples include nanoengineered synthesis approaches 1-4,6-8,18,19 and related techniques, 20 coprecipitation, 22,23,27,34 incipient wetness method, 21 electrochemical synthesis, 33 sputtering, codeposition, 38,39 electrodeposition, 14 etc. In the studies of Pt-based catalysts with bimetallic and trimetallic compositions, the enhanced catalytic activity in these systems has been attributed to numerous factors including the bifunctional nature of the catalyst surface, 3,8,39 * To whom correspondence should be addressed. cjzhong@ binghamton.edu; minhua.shao@utcpower.com. lattice shrinking where there is a change in the Pt-Pt interatomic distance, 8,21,41 number of Pt nearest neighbors, d-band center shift, 42 Pt metal content on the particle surface, 41 and Pt skin effect/segregation. 9-11,25,37,43-46 For example, annealed polycrystalline Pt 3 Co nanocrystals with a Pt skin have been found to exhibit higher catalytic activity than the bimetallic Pt 3 M surfaces where M ) Fe, Ni, Co, and Ti in a study conducted by Stamenkovic et al. 9,47 High-temperature annealing is also reported to enrich Pt on extended Pt alloy systems by promoting surface segregation of Pt. 48,49 Size 39,50-52 and shape/morphology effects, 26,53 catalyst composition and particularly surface composition, oxidation state of Pt, and the second metal have been reported to have some effects on the activity of electrocatalysts. 27 The effect of cobalt dissolution in the PtCo/C system was shown to neither detrimentally reduce the cell voltage nor dramatically affect the membrane conductance. 31 The study on PtNiCo/C electrocatalyst systems was reported for methanol oxidation reaction (MOR), whereby it was prepared by coprecipitation method. 23 The catalyst has also been studied for ORR in a phosphoric acid fuel cell (PAFC). 54 It has also been reported as a catalyst for PEMFCs whereby it was prepared by the incipient wetness technique in a study conducted by Seo et al /jp106843k 2010 American Chemical Society Published on Web 09/23/2010

2 Thermal Treatment of PtNiCo Electrocatalysts J. Phys. Chem. C, Vol. 114, No. 41, Ternary catalysts, 55 e.g., trimetallic nanoparticle catalysts PtVFe and PtNiFe, have been found to exhibit catalytic activities in fuel cell reactions, which are 4-5 times higher than that of pure Pt catalysts. 1,6 However, an overview of the literature indicates that little is available for understanding how the detailed nanoscale phase and strain properties of the catalysts correlate with the electrocatalytic properties in terms of thermal treatment temperatures of the nanoengineered catalysts. The strain effect of the interatomic distances on the catalytic activity is considered to be an important aspect for the mechanistic understanding of the ORR electrocatalysis of Ptbased alloy nanoparticles. 56 Alloying Pt with elements of smaller atomic radius such as Co and Ni, as described in this work, should reduce the interatomic distances near the Pt sites and thus enhance the electrocatalytic activity. Recent computational studies have also demonstrated the lowering of the d-band center of Pt through compressive strain and electronic effect induced by the transition metals. While the effects of treatment temperature on the electrocatalytic activity and stability have been reported previously, 57 the understanding of how lattice shrinking and nanocrystal structures can be manipulated by thermal temperature and how they affect the electrocatalytic activity and stability has received little attention. In general, heat treatment is considered to induce changes in particle size, morphology, dispersion of the metal on the support, alloying degree, active surface site formation, catalytic stability, and surface electronic properties, with the optimum treatment temperature being strongly dependent on the individual catalyst. 57 There is an increasing need for an in-depth understanding of the correlation between the nanoscale phase and faceting structures and the electrocatalytic properties. In this report, we describe the results of an investigation of the thermal treatment on lattice shrinking and nanocrystal faceting properties for PtNiCo/C catalysts, which are nanoengineered in terms of size and composition. To the best of our knowledge, there is no prior report of such a detailed study of the PtNiCo nanocatalysts in terms of the lattice parameter, nanocrystal faceting, surface oxidation states, and atomic distributions as a function of the thermal treatment temperature in enhancing the electrocatalytic activity for ORR, which is the focus of the present work. Experimental Section Chemicals. Platinum(II) acetylacetonate (Pt(acac) 2, 97%) and nickel(ii) acetylacetonate (Ni(acac) 2, anhydrous, >95%) were purchased from Alfar Aesar, Cobalt(III) acetylacetonate (Co(acac) 3, 99.95%) was purchased from Strem Chemicals. 1,2- Hexadecanediol (CH 3 -(CH 2 ) 13 -CH(-OH)-CH 2 -OH, 90%), octyl ether ([CH 3 (CH 2 ) 7 ] 2 O, 99%), oleylamine (CH 3 (CH 2 ) 7 CHd CH(CH 2 ) 8 NH 2, 70%), oleic acid (CH 3 (CH 2 ) 7 CHdCH(CH 2 ) 7 COOH, 99+%), and Nafion solution (5 wt %) were purchased from Aldrich. Optima-grade perchloric acid was purchased from Fisher Scientific. Other solvents such as ethanol and hexane were purchased from Fisher. All chemicals were used as received. Synthesis. The general synthesis of PtNiCo nanoparticles involved the use of three metal precursors, Pt II (acac) 2,Ni II (acac) 2, and Co III (acac) 3, in controlled molar ratios. These metal precursors were dissolved in an octyl ether solvent. A mixture of oleylamine and oleic acid was also dissolved in solution and used as capping agents. 1,2-Hexadecanediol was used as a reducing agent for reduction of the Pt, Ni, and Co precursors. The general reaction for the synthesis of the (oleylamine/oleic acid)-capped PtNiCo nanoparticles involves a combination of thermal decomposition and reduction reactions. The composition of the Pt 0 n1ni 0 n2co 0 n3 nanoparticles, where n1, n2, and n3 represent the atomic percentages of each metal, is controlled by the feeding ratio of the metal precursors. The nanoparticle product can be collected by precipitation method. Possible byproducts include CH 2 -(CH 2 ) 13 -CH(-OH)-CH(dO) (aldehyde), CH 2 -(CH 2 ) 13 -CH(-OH)-COOH (carboxylic acid), acac, and CO, which were soluble in the solvent and discarded after precipitating out the nanoparticles. PtNiCo nanoparticles of different compositions were synthesized by manipulating the relative concentrations of metal precursors such as platinum(ii) acetylacetonate, nickel(ii) acetylacetonate, and cobalt(iii) acetylacetonate and capping agents such as oleylamine and oleic acid. In a typical procedure for the synthesis of Pt 36 Ni 14 Co 49, for example, 4.2 g of 1,2- hexadecanediol (4.88 mmol), g of cobalt acetylacetonate (Co(acac) 3, 1.44 mmol), g of nickel acetylacetonate (Ni(acac) 2, 0.73 mmol), g of platinum acetylacetonate (Pt(acac) 2, 0.95 mmol), 3 ml of oleylamine (6.4 mmol), 3 ml of oleic acid (9.4 mmol), and 450 ml of octyl ether were added to a 3-neck 1 L flask under stirring. The solution was purged with N 2 and heated to 105 C. The solution appeared to have dark green color. At this temperature, N 2 purging was stopped and the solution was kept under N 2. The mixture was heated to 280 C and refluxed for 40 min. The solution appeared black in color. After the reaction mixture was allowed to cool to room temperature, the solution was transferred to a large flask under ambient environment. The product was precipitated by adding ethanol ( 1000 ml). The yellow-brown supernatant was discarded. The black precipitate was completely dried under nitrogen and dispersed in a known amount of hexane ( 100 ml). The trimetallic composition was based on the linear correlation between the trimetallic feed ratios and the nanoparticle product composition for a series of trimetallic ratios. The details will be described in another synthesis-focused report. Catalyst Preparation. Catalyst preparation included the assembly of PtNiCo nanoparticles on carbon black and thermal treatment. The assembly was accomplished by a process of loading the nanoparticles onto carbon black materials through interactions between the capping shells and the carbon surface. A typical procedure included the following steps. First, 480 mg of carbon black (Ketjen Black) was suspended in 400 ml of hexane. After sonicating for 3 h, 320 mg of Pt 36 Ni 15 Co 49 was added into the suspension. The suspension was sonicated for 5 min, followed by stirring for 15 h. The suspension was evaporated slowly for 8 h by purging N 2 while stirring. The powder was collected and dried under N 2. Thermal treatment involved removal of organic shells and annealing of the alloy nanoparticles. All samples were treated in a tube furnace using a quartz tube. The PtNiCo nanoparticles supported on carbon (PtNiCo/C) were first heated at 260 C inn 2 for 60 min for removing the organic shells and then treated at various temperatures in the range between 400 and 926 C in 15% H 2 /85% N 2 for 120 min during the calcination process. Measurements and Instrumentation. The catalysts were characterized using several techniques. Transmission Electron Microscopy (TEM). TEM was performed on a Hitachi H-7000 electron microscope (100 kv) to obtain the particle size and its distribution. For TEM measurements, nanoparticle samples were diluted in hexane solution and drop cast onto a carbon-coated copper grid followed by solvent evaporation in air at room temperature. Direct Current Plasma-Atomic Emission Spectroscopy (DCP-AES). DCP-AES was used to analyze the composition, which was performed using an ARL Fisons SS-7 Direct Current

3 17582 J. Phys. Chem. C, Vol. 114, No. 41, 2010 Wanjala et al. Plasma-Atomic Emission Spectrometer. The nanoparticle samples were dissolved in concentrated aqua regia and then diluted to concentrations in the range of 1-50 ppm for analysis. Calibration curves were made from dissolved standards with concentrations from 0 to 50 ppm in the same acid matrix as the unknowns. Detection limits, based on three standard deviations of the background intensity, are 20, 2, and 5 ppb for Pt, Ni, and Co. Standards and unknowns were analyzed 10 times each for 3 s counts. Instrument reproducibility, for concentrations greater than 100 times the detection limit, results in <(2% error. ThermograWimetric Analysis (TGA). TGA was performed on a Perkin-Elmer Pyris 1-TGA for determining the weight of the organic shell. Typical samples weighed 4 mg and were heated in a platinum pan. Samples were heated in 20% O 2 at a rate of 10 C/min. X-ray Powder Diffraction (XRD). XRD was used to study the lattice constants and particle sizes of the catalysts. Powder diffraction patterns were recorded on a scintag XDS 2000 θ-θ powder diffractometer equipped with a Ge(Li) solid state detector (Cu KR radiation). Data was collected from 2θ ) 20 to 90 at a scan rate of 0.02 per step and 5 s per point. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed to identify the oxidation states of Pt, Co, and Ni in the surface of the catalysts. XPS measurements were made by using a Physical Electronics Quantum 2000 scanning ESCA microprobe. This system uses a focused monochromatic Al KR X-ray ( ev) source for excitation and a spherical section analyzer. The instrument has a 16 element multichannel detection system. The X-ray beam used was a 100 W, 100 µm diameter beam that is rastered over a 1.4 mm by 0.2 mm rectangle on the sample. The X-ray beam is incident normal to the sample, and the X-ray detector is at 45 away from the normal. The binding energy (BE) scale is calibrated using the Cu 2p 3/2 feature at ev and Au 4f 7/2 at ( 0.05 ev for known standards. The sample experienced variable degrees of charging low-energy electrons at 1 ev, 20 µa, and lowenergy Ar + ions were used to minimize this charging. The percentages of individual elements detected were determined from the relative composition analysis of the peak areas of the bands. X-ray Absorption Spectroscopy (XAS). XAS measurements of Pt and Co were carried out at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL) using Beamlines X18B and X19A. XAS data were processed and fitted using the IFEFFIT analysis package to obtain the local electronic and structural properties of the catalysts. The samples for XAS measurements were prepared by coating the catalysts powder on a carbon paper with a Pt loading of 1 mg/cm 2. Measurements of both extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) were performed with the catalyst samples. Electrochemical Measurements. Electrochemical measurements involved cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements performed using an EG&G 273 instrument in three-electrode electrochemical cells at room temperature. All electrolytic (0.1 M HClO 4 ) solutions were deaerated with high-purity nitrogen. The polarization curves were recorded after 50 potential cycles in the range of V with a scanning rate of 100 mv/s in a nitrogen-saturated 0.1 M HClO 4 solution. The electrochemical active areas (ECAs) of the catalysts were obtained by integrating the charge in the H UPD range in the CVs. The oxygen reduction activity was measured by the rotating disk electrode (RDE). The potentials are given with respect to reversible hydrogen electrode (RHE). Results and Discussion Discussion of the results in this section will focus on addressing how the size, surface, and phase properties of the catalysts change with thermal treatment temperature and how the activity correlates with temperature and the nanoscale structural parameters for a given composition. Discussions of the experimental results are divided into three sections. In the first section, the results obtained for characterization of the size, composition, and morphology of the as-synthesized PtNiCo nanoparticles and the carbon-supported catalysts are described. In the next few sections, the nanoscale phase, surface, and electronic structures of the catalysts are discussed based on experimental data from XRD, XPS, and XAS characterization. The electrocatalytic properties of the catalysts for ORR are discussed in terms of the activity and stability in the last section. 1. Size, Composition, and Morphology. The PtNiCo nanoparticles obtained after cleaning treatments as described in the Experimental Section were black and waxy powder. The powders were easily dispersed in nonpolar solvents such as hexane, which is consistent with the presence of a monolayer of surfactants on the particles surface. The nanoparticle solutions had a dark brown color and were stable. The compositions of PtNiCo in the nanoparticles were determined by using DCP-AES. PtNiCo nanoparticles of a series of controlled composition and size have been synthesized. Since the focus of this report is the study of the effect of the thermal treatment temperature on the electrocatalytic activity for oxygen reduction reaction, the results from the Pt 36 Ni 15 Co 49 /C catalysts will be discussed in great detail. Figure 1 shows a representative TEM micrograph for a sample of the as-synthesized Pt 36 Ni 14 Co 49 nanoparticles. The PtNiCo nanoparticles are highly monodispersed. The highly faceted nanocrystal feature is also evident by the observation of the lattice fringes (Supporting Information). The particle sizes are very well controlled with an average size of 3.4 ( 0.4 nm. The fact that the nanoparticles have well-defined interparticle spacing and display domains of hexagonal ordering is indicative of the encapsulation of the nanocrystal cores by the organic monolayers (oleylamine and oleic acid). High-resolution TEM data of the nanoparticles with a slightly different composition revealed nanocrysalline facets (Supporting Information). Figure 2 shows a representative set of TEM micrographs and size distributions for different samples of PtNiCo/C catalysts. As the temperature was increased, there was a gradual increase in the average size of the particles (from 4 nm at 400 C to6 nm at 926 C). However, between 500 and 800 C, the size change is relatively small. High-resolution TEM data for samples of PtNiCo/C catalysts with different compositions treated at 400 and 926 C also revealed a highly faceted nanocrystal feature (see Supporting Information). It is interesting that the lattice fringes determined from these images showed a subtle decrease from a sample treated at 400 C to a sample treated at 926 C. The trend for the reduction of the lattice fringes appears to be consistent with trends of lattice constants observed using XRD and XAS techniques as described in the latter sections. 2. Phase and Lattice Parameters. Figure 3 shows a representative XRD spectrum for a sample of as-synthesized Pt 36 Ni 15 Co 49 nanoparticles. The diffraction peak position falls in between those for the monometallic Pt and those for monometallic Ni and Co. For example, the strongest peak for Pt appears at 2θ ) 40.50, slightly higher than the pure Pt (111) peak at There is no indication of the presence of the metal phases of individual elements. We note that for PtNiCo

4 Thermal Treatment of PtNiCo Electrocatalysts J. Phys. Chem. C, Vol. 114, No. 41, Figure 1. TEM micrograph and size distribution for a sample of as-synthesized Pt 36 Ni 15 Co 49 nanoparticles. Figure 2. (A and B) TEM micrographs and size distributions for samples of Pt 36 Ni 15 Co 49 /C catalysts treated at 400 C (A) and 926 C (B). (C) Plot of the particle size vs treatment temperature. catalysts prepared by the traditional method ordered structures were reported. 23,34,58 A study on the activity and stability of ordered and disordered PtCo alloys by Watanabe et al. also showed that single-phase disordered fcc structures were formed by annealing at a certain temperature range. 59 Our as-synthesized PtNiCo nanoparticles also showed a disordered fcc structure. To assess whether the PtNiCo nanoparticles were alloyed or a mixture of different components, the XRD spectra of Pt, PtNi and PtCo, and PtNiCo were further compared. From this comparison, the as-synthesized PtNiCo is shown to be a singlephase alloy (2θ(111) ) ), not a mixture of PtNi (2θ(111) ) ), PtCo (2θ(111) ) ), and Pt (2θ(111) ) ). The question of whether the trimetallic nanoparticles are simple mixtures of monometallic/bimetallic nanoparticles was addressed by comparisons of particle size, size monodispersity, and ICP/ MS-determined compositions among monometallic, bimetallic, and trimetallic nanoparticles synthesized under the same condition. The results are indicative of the trimetallic character. This analysis was confirmed by our earlier work with PtVFe nanoparticles derived from a similar method using high- Figure 3. XRD pattern for a sample of as-synthesized Pt 36 Ni 15 Co 49 nanoparticles.

5 17584 J. Phys. Chem. C, Vol. 114, No. 41, 2010 Wanjala et al. Figure 4. (A) XRD patterns for samples of Pt 36 Ni 15 Co 49 /C catalysts as a function of treatment temperature: 400 (a), 500 (b), 600 (c), 700 (d), 800 (e), and 926 C (f). (B) Plot of lattice parameter as a function of treatment temperature. resolution EDX and TEM data. 7,18 In our preliminary work, the trimetallic composition was confirmed by the HRTEM-EDX method. Figure 4A shows a representative set of XRD patterns for samples of Pt 36 Ni 15 Co 49 /C catalysts after thermal treatment at different temperatures ranging from 400 to 926 C. The data further indicated that the PtNiCo/C catalysts of this composition are single-phase alloyed at all treatment temperatures. A shift in the (111) peak position toward higher angles with an increase of annealing temperature was observed. This was attributed to the lattice shrinking as the thermal treatment temperature is increased. As shown in Figure 4B, there is a clear decrease of lattice constant from nm for 400 C to nm for 926 C. The average size of the nanoparticles was also estimated by utilizing the Scherrer correlation between the particle diameter (D) and the peak width ( s, full width at half-maximum, λ ) nm) for Bragg diffraction from ideal single-domain crystallites. The calculated D value (3.4 nm) is very close to the value determined from TEM data (3.0 nm). Table S1, Supporting Information, summarizes the lattice constants and particle sizes obtained from TEM and XRD for Pt 36 Ni 15 Co 49 /C catalysts as a function of thermal treatment temperature. The XRD sizes estimated from the Scherrer equation (see Supporting Information) were found to be slightly smaller than the TEMestimated ones, but they displayed temperature dependence similar to the TEM data. It is important to note that the small crystal domain size made it difficult to estimate the size accurately using the Scherrer equation. Despite these differences, a clear trend in size increase as a function of temperature was evident as estimated by both XRD and TEM techniques. 3. Surface Composition. XPS characterization was employed to provide some insight into the relative surface properties of the electrocatalysts treated at different temperatures. Figure 5A-C shows a set of XPS spectra in the Pt 4f, Ni 2p, and Co 2p regions in the Pt 36 Ni 15 Co 49 /C samples annealed at 400 (a) and 926 C (b). For this set of composition, Pt 4f 7/2 and Pt 4f 5/2 bands were observed at 72.1 and 75.3 ev for 400 C and 72.1 and 75.4 ev at 926 C, respectively. Ni 2p 3/2 and Ni 2p 1/2 peaks were observed at and ev for 400 C and and ev for 926 C, respectively. Co 2p 3/2 and Co 2p 1/2 peaks were observed at and ev for 400 C and and ev for 926 C, respectively. We also note that the binding energy (BE) and intensity values in the O 1s region were found to be practically identical between the two samples, demonstrating there is any difference in terms of the degree of surface oxidation in this treatment temperature range. 60 Table 1 summarizes BE values (in ev) and the relative composition values for Pt 36 Ni 15 Co 49 /C catalysts treated at the two different temperatures. The BE values for Pt 4f 7/2 and 4f 5/2, Ni 2p 3/2 and 2p 1/2, and Co 2p 3/2 and 2p 1/2 are practically identical between the two temperatures. The compositions determined from the XPS data yielded Pt 36 Ni 12 Co 52 and Pt 39 Ni 14 Co 47 for 400 and 926 C, respectively. The difference between the bulk analysis data and the surface analysis data is negligible. On the basis of the asymmetric shapes of the spectra in the Pt 4f, Ni 2p, and Co 2p regions, there are apparently components corresponding to higher oxidation states. As shown by spectral deconvolution in Figure 5 (bottom panel), species of Pt, Ni, and Co with higher oxidation states were clearly identified, which we believe correspond to surface oxides of these metals. In Table 2, the deconvolution results for the binding energy values of Pt 4f, Co 3p, and Ni 3p peaks at two different temperatures were compared. For each metal, there appears to be higher BE components, which may be associated with higher oxidation states or shakeup peaks. For Pt, there is about 30% of the peak envelope at the higher BE region, which may be indicative of the presence of PtO and PtO 2, which seems to be independent of the temperature. For Ni, there is about 60% of the peak envelope at the higher BE region, which may be indicative of the presence of

6 Thermal Treatment of PtNiCo Electrocatalysts J. Phys. Chem. C, Vol. 114, No. 41, Figure 5. XPS spectra in the Pt 4f (A), Co 2p (B), and Ni 2p (C) regions for samples of Pt 36 Ni 15 Co 49 /C catalysts treated at 400 (a) and 926 C (b). (Bottom) peak deconvolution results: (black spherical symbols) original curves, (blue dashed lines) fitted curves, and (red dashed lines) deconvoluted peaks. (Note that the spectra for Co 2p 3/2 were normalized for comparison.) TABLE 1: Comparison of Binding Energy (BE) Values (in ev) and Relative Composition (in atomic %) for Pt 36 Ni 15 Co 49 /C Catalysts Treated at the Two Different Temperatures a 400 C 926 C bands binding energy (ev) Pt:Ni:Co (%) binding energy (ev) Pt:Ni:Co (%) Pt 4f 7/ :12: :14:47 Pt 4f 5/ Ni 2p 3/ Ni 2p 1/ Co 2p 3/ Co 2p 1/ a Standard deviation: (0.05 ev for binding energy value, and (0.05 for the composition ratio. NiO x species, which seems to be independent of the temperature. However, the relative ratios of the oxidized species seem to be quite dependent on the temperature. The ratio of the higher oxidation species vs Co 0 was found to be reduced from 30% for 400 C to 20% for 926 C (see Table 2). The basic characteristics of the deconvolution results are in agreement with the literature report on the binding energy values for Pt(4f), 23,27,60-64 Co (2p 3/2 ), 23,27,62,63 and Ni(2p 3/2 ) 23,27,60 for some binary and ternary Pt-based catalysts prepared by other methods. For example, previous reports showed Pt 4f 7/2 at ev for Pt 0, ev for Pt 2+, and ev for Pt 4+ in the PtCo and PtNi alloy based binary and ternary nanoparticle systems 23,60-63 but prepared by different techniques. In comparison, our deconvoluted Pt4f 7/2 peaks were found at 72.00, 72.95, and ev. There was an insignificant change in binding energy (an increase of ev) and an insignificant change of the oxides % ( 30%) from 400 to 926 C. For Co, the reported binding energies included ev for Co 0, ev for Co 2+ (CoO), and ev for satellites peaks (shakeup peaks). 23,62,63 Our deconvoluted Co2p 3/2 peaks were found at , , and ev for 400 C. Note that the first peak ( ev) was significantly shifted to a higher energy ( 1 ev) than the literature data, which could be due to catalyst support or alloying effect. There was a small change in BE for the first peak (an increase of 0.2 ev) and an observable change of the oxides % ( 30% to 20%) from 400 to 926 C. For Ni, Ni 2p 3/2 values were reported at ev for Ni 0, ev for Ni 2+ (Ni(OH) 2 ), ev for Ni 3+ (NiOOH), and ev for Ni 2+/3+ satellites for PtNi nanoparticles. 59 In comparison, our deconvoluted Ni 2p 3/2 peaks were found at , , and for 400 C. Note that the first peak ( ev) is also significantly shifted to a higher energy ( 1 ev) than the literature data as a result of support or alloying effect. There was a small change in BE (an increase of ev) and an insignificant change of the oxides % ( 15%) from 400 to 926 C. 4. Structural and Electronic Properties. In order to gain detailed structural and electronic information on these catalysts, the EXAFS measurements were carried out with Pt 36 Ni 15 Co 49 /C catalysts treated at different temperatures. Figure 6A compares the normalized XANES spectra of the PtL 3 edge for Pt foil and Pt 36 Ni 15 Co 49 /C. The white line intensities of Pt 36 Ni 15 Co 49 /C are higher than that of Pt foil, indicating the decrease of 5d orbital filling in the Pt alloys. It has been shown that oxygen-containing species can easily chemically adsorb on nanoparticles in air resulting in a partially oxidized surface that has a 5d orbital with less electron filling. 42,65 However, the contribution of the orbital hybridization between Pt and Co (Ni) to the white line intensity increase could not be ignored. 66 The XANES spectra of the Co K edge for Co foil and Pt 36 Ni 15 Co 49 /C are compared in Figure 6B. The intensities of the pre-edge peak of Pt 36 Ni 15 Co 49 /C decrease, while the white line intensities in-

7 17586 J. Phys. Chem. C, Vol. 114, No. 41, 2010 Wanjala et al. TABLE 2: Comparison of the Deconvolution Results for Binding Energy Values (in ev) of Pt(4f 7/2 ), Co(2p 3/2 ), and Ni (2p 3/2 ) peaks for Pt 36 Ni 15 Co 49 /C Catalysts Treated at Two Different Temperatures a 4f 7/2 4f 5/2 metal treatment T, C 1 (Pt 0 ) 2 (PtO) 3 (PtO 2 ) Pt (67%) (23%) (10%) (68%) (22%) (9%) metal treatment T, C 1 (Co 0 ) 2 (Co 2+ ) 3 (Co 3+ ) Co (72%) (19%) (9%) (80%) (15%) (5%) 2p 3/2 metal treatment T, C 1 (Ni 0 ) 2 (Ni 2+) 3 (Ni 3+ ) Ni (33%) (52%) (15%) (32%) (54%) (14%) a Note: The percentages for each oxidation species (100% total) were obtained using integrated peak areas. 2p 3/2 Figure 6. Comparison of XAS spectra of Pt 36 Ni 15 Co 49 /C catalysts with corresponding metal foils. (A) XANES spectra of Pt L3 edge, (B) XANES spectra of Co K edge, (C) Fourier transforms for EXAFS spectra of Pt L3 edge, and (D), Fourier transforms for EXAFS spectra of Co K edge. All EXAFS spectra were k 3 weighted. creased dramatically compared with the Co foil. These changes demonstrate the strong hybridization between Pt and Co and the formation of Co oxides. The Pt 36 Ni 15 Co 49 /C treated at 926 C has a smaller amount of Co oxides compared with the one treated at 400 C. This finding was qualitatively consistent with the results from the XPS peak deconvolution. The shape of the spectra was significantly different between Pt alloys and Co foil, suggesting the strong interaction between Co and Pt. The Fourier transform (FT) of the Pt L3 edge and Co K edge EXAFS spectra are shown in Figure 6C and 6D, respectively. The peak centered at 2.64 Å for Pt foil results from the contributions related to the first Pt-Pt coordination shell. 67 This peak shifts to lower R values for Pt 36 Ni 15 Co 49 /C due to the Pt-Co (Ni) bonding. The peak values for Pt 36 Ni 15 Co 49 /C treated at 926 and 400 C are 2.45 and 2.52 Å, respectively. The smaller R value for Pt 36 Ni 15 Co 49 /C treated at 926 C suggests a smaller lattice constant than that treated at 400 C, which is in good agreement with the XRD data. The lack of the Pt-O peak at 1.6 Å implies that the high white line intensity of the Pt L3 edge for Pt alloys in Figure 6A may not come from the Pt oxides. The peak centered at 2.15 Å for Co foil results from the scattering from Co-Co atoms. 67,68 For Pt 36 Ni 15 Co 49 /C alloys,

8 Thermal Treatment of PtNiCo Electrocatalysts J. Phys. Chem. C, Vol. 114, No. 41, Figure 7. (A) CV curves for the catalysts treated at different temperatures. (Inset) Magnified view of the hydrogen adsorption wave region. (B) Peak potentials of the two waves (I and II) in the hydrogen adsorption region as a function of temperature. Electrolyte: 0.1 M HClO 4. The polarization curves were recorded after 50 potential cycles in the range of V with a scan rate of 100 mv/s in nitrogen-saturated solution. Pt loading ) 6.6 µg/cm 2 on the electrode. a new peak around 2.45 Å arises due to the Co-Pt binding. Interestingly, the peak at 2.15 Å is very weak for Pt 36 Ni 15 Co 49 /C treated at 400 C, suggesting that the Co atoms are distributed uniformly in the alloy units. While phase segregation was observed for the sample treated at 926 C evident from the appearance of the peak at 2.15 Å, which can be assigned to the combination of scattering from Co-Co atoms, Co-Ni atoms, and Co-Pt atoms in Co-enriched phases. Thus, the Co distribution in the alloy treated at 926 C is less uniform. The existence of broad peaks in the range of Å from Co-O in the Pt alloy samples indicates that some Co atoms on the surface of the particle form Co oxides. The slightly smaller peak area for Co-O for the sample treated at 926 C than that treated at 400 C confirms a smaller amount of Co oxides in the surface of the former. The formation of Co oxides on the alloy surface may be caused by exposing the catalysts to air after thermal treatment under hydrogen. The high-temperature treatments may cause the redistribution of Co atoms in the alloy and diffusion of Co atoms on the surface into the bulk, resulting in a smaller lattice constant and smaller amount of Co oxides on the surface. There is thus an agreement between XPS and EXAFS data. XPS analyzes the surface composition (but not one monolayer, which is why most of the Co is still metallic Co), whereas EXAFS analyzes the structure of the particle. 5. Electrocatalytic Activity and Stability. RDE and CV measurements for ORR at the Pt 36 Ni 15 Co 49 /C catalysts treated at different temperatures were examined. Values of the electrochemically active area (ECA), mass activity (MA), and specific activity (SA) were extracted from the RDE and CV measurements. Figure 7 shows a representative set of CV data for the Pt 36 Ni 15 Co 49 /C catalysts for the catalysts treated at 400 (a), 700 (b), and 926 C (c) after potential cycling in the range of V for 50 cycles to clean the catalysts. The transition metal and metal oxides on the surface were removed during the potential cycling, forming a pure Pt surface layer. As evidenced by the CV curves, the characteristics in the hydrogen adsorption/desorption region showed subtle dependence on the treatment temperature. In contrast to the relatively featureless characteristic for the 400 C sample, the higher temperature treated samples showed significant peak features at and V, which are associated with (110) and (100) nanocrystal facets, respectively The hydrogen adsorption/desorption peaks at 0.20 V become predominant for the higher temperature treated samples. As shown in the inset of Figure 7A and Figure 7B (also see Supporting Information), Figure 8. RDE curves for the catalysts treated at different temperatures. Electrolyte: 0.1 M HClO 4. The polarization curves were recorded after 50 potential cycles in the range of V with a scan rate of 100 mv/s in O 2 -saturated solution (B). Pt loading ) 6.6 µg/cm 2 on the electrode. not only the 0.20 V peak became significant at 600 C but also there appears to be an observable positive shift of the peak potential ( 50 mv) with the increase of temperature. On the basis of our recent study of 7 nm sized Pt nanoparticles and Pt nanocubes where the (100)-related peak was observed at 0.23 and 0.27 V, respectively, we believe that the observed positive shift reflects the increase of domain sizes of the (100) facets as a result of the increase in treatment temperature. 73 Since the particle sizes of PtNiCo did not change significantly from 400 to 600 C, the difference in the CV behavior may reflect the changes in the structure and electronic properties of the bulk alloy and its surface induced by the higher temperature annealing. Figure 8 shows a representative set of RDE data for the Pt 36 Ni 15 Co 49 /C catalysts treated at 400 (a), 700 (b), and 926 C (c). In the kinetic region of the RDE curves at 0.9 V there is a clear indication of the kinetic current increase with increasing treatment temperature. In Figure 9, the ECA values are plotted as a function of the thermal treatment temperature. The ECA value showed an initial gradual decrease as the temperature increased, but it remained constant for those with treatment temperatures above 500 C. The ECA decrease is apparently associated with the particle size increase as shown in Figure 2C. Table 3 summarizes a set of physical, chemical, and electrochemical data for Pt 36 Ni 15 Co 49 /C catalysts as a function of the thermal treatment temperature. In Figure 10, the mass activity data was plotted as a function of the thermal treatment temperature. There was a clear trend showing the increase of the mass activity with the treatment temperature. It reached a plateau at 800 C. The mass activity

9 17588 J. Phys. Chem. C, Vol. 114, No. 41, 2010 Wanjala et al. Figure 9. Plots of ECA for a set of Pt 36 Ni 15 Co 49 /C catalysts treated at different temperatures. The results for a commercial Pt/C catalyst are included for comparison. TABLE 3: Summary of the Physical, Chemical, and Electrochemical Data for Pt 36 Ni 15 Co 49 /C Catalysts (metal loading 25%) as a Function of Thermal Treatment Temperature a temp, C lattice parameter (nm) particle size (nm) ECA MA SA ( ( ( ( ( ( a Note: ECA ) electrochemical active area (m 2 /g Pt ); MA ) mass activity (A/mg Pt ); SA ) specific activity (ma/cm 2 ) at 0.9 V. Figure 10. Plots of mass activity for a set of Pt 36 Ni 15 Co 49 /C catalysts treated at different temperatures. The result for a commercial Pt/C catalyst is included for comparison. of Pt 36 Ni 15 Co 49 /C treated at 800 and 926 C was more than 4 times higher than that of state-of-the-art Pt/C. The increase in mass activity is apparently not associated directly with the changes in the ECA as shown in Figure 9 but more associated with the specific activity. It is evident that both the specific activity and the lattice parameter depend on the thermal treatment temperature, as shown in Table 3. The specific activity as a function of lattice parameter is shown in Figure 11, which was obtained from the temperature dependencies of the specific activity and lattice parameter (Figure 11 inset), demonstrating a correlation between the specific activity and the lattice parameter. As the lattice shrinks as a result of the increased temperature treatment, the specific activity showed an increase. This increase occurred mostly for those catalysts with large and small lattice constants. The change is relatively small for the intermediate lattice constants. Figure 11. Plot of specific activity as a function of the lattice parameter for Pt 36 Ni 15 Co 49 /C catalysts. (Inset) Plots of specific activity and lattice parameter as a function of thermal treatment temperature. The lattice shrinking played an important role in enhancing the electrocatalytic activity for catalysts treated at higher temperatures. In addition, the electronic effect from the transition metal (Co and Ni) in the subsurface may also enhance the activity of the Pt overlayer on the alloy surface, as the results of periodic density functional theory (DFT) suggested. In the DFT studies, the d-band center of the Pt overlayer formed by either the high-temperature annealing or the electrochemical dissolution of transition metals in the alloy surface was found to shift to a lower position with respect to the Fermi level due to the compressive strain and electronic effect induced by the transition metals. 11,72b The downshifting of the d-band center, which is also expected in our PtNiCo catalysts, results in a lower oxygen binding energy on the Pt overlayer. The oxygen binding energy on a pure Pt surface is too strong. By slightly weakening it through alloying with other transition metals, the ORR activity is expected to show a significant increase. The ORR activity of PtNiCo catalysts thus strongly depends on the amount of compressive strain in the surface in addition to the electronic effect from the Co and Ni in the subsurface. Therefore, controlling the lattice constant of the alloys by adjusting the annealing temperature and the composition is a very effective way to further tune the ORR activity of the Pt alloys. The stability of the catalysts during potential cycling was also examined in terms of the ECA and mass activity. The electrochemical measurements after potential cycling of the catalysts for 50 cycles between 0.02 and 1.2 V were performed. During the potential cycling, the transition metals on the catalyst surface leached out, forming a pure Pt surface. Thus, the effects of transition metals and alloying degree on the ECA measurements were negligible. Figure 12 shows a representative set of durability test results for normalized ECA (A) and mass activity (B) comparing Pt/C (a) and Pt 36 Ni 15 Co 49 /C catalysts treated at different temperatures. The potential was held at 0.65 V for 5 s and switched to 1.2 V and held at this potential for another 5 s. The decay rate of the mass activity is apparently dependent on the thermal treatment temperature. Overall, the ECA values for Pt 36 Ni 15 Co 49 /C catalysts showed a lower significant reduction as a function of cycling numbers in comparison with that for Pt/C catalyst. The increase in treatment temperature apparently increases the stability of the ECA value. In contrast, the mass activity for Pt 36 Ni 15 Co 49 /C catalysts showed a more significant reduction as a function of cycling numbers in comparison with that for Pt/C catalyst. This decrease in activity occurred mostly after the initial 2000 cycles. Upon further cycling, the decrease

10 Thermal Treatment of PtNiCo Electrocatalysts J. Phys. Chem. C, Vol. 114, No. 41, nanoengineering the structures and properties of the catalysts. The composition, size, and phase properties of the PtNiCo nanoparticles are controllable by our synthesis and processing approaches. The increase in the thermal treatment temperature of the carbon-supported catalysts has been shown to lead to a gradual shrinkage of the lattice constants of the alloy as revealed by HRTEM, XRD, and EXAFS and a change of surface properties as implied by the XPS and CVs of the nanoparticle catalysts. This synthesis and processing approach to the nanoengineered trimetallic nanoparticle catalysts has been shown to influence the electrocatalytic activity and stability in a controllable way. A combination of the lattice shrinkage and the change of surface properties on the nanoparticle catalysts as a result of the increased thermal treatment temperature is believed to play a major role in enhancing the electrocatalytic activity for catalysts. Moreover, detailed analyses of the oxidation states, atomic distributions, and interatomic distances revealed a certain degree of changes in Co enrichment and surface Co oxides as a function of the thermal treatment temperature. Detailed understanding of the implication of the correlation the electrocatalytic activity/stability and the nanostructural parameters (lattice, surface oxidation state, and distribution) to other trimetallic or bimetallic nanoparticle catalyst systems, which is part of our ongoing work, is expected to provide important insights into the design of active and robust fuel cell catalysts. Figure 12. Durability test results for normalized ECA (A) and mass activity (B) comparing Pt/C and Pt 36 Ni 15 Co 49 /C catalysts treated at different temperatures. of the mass activity is relatively small. The fast activity decay of Pt 36 Ni 15 Co 49 /C may be caused by dissolution of transition metals in the alloys at high potentials. Future work will focus on improving the durability of this type of catalyst. It is important to point out that from the durability data shown in Figure 12 where the normalized mass activities were plotted the actual mass activity for the catalysts remained higher than commercial Pt/C catalysts after the durability test, demonstrating that the significance of the trimetallic composition and temperature effects on the enhanced electrocatalytic activity and stability. In an earlier study, 74 heat treatment was shown to play a key role in obtaining a catalyst with proper corrosion resistance and a high surface area. For example, Pt/C and PtFe/C catalysts treated at temperatures up to 900 C in a flowing gas stream (N 2 +7% H 2 ), the degree of alloying, and the particle size were shown to be strongly dependent on the temperature. The Pt alloy catalysts were shown to exhibit an increased resistance to sintering at high temperature and improved stability against dissolution. Other related studies on heat treatment also revealed enhanced alloying, 42 improved Pt-carbon interaction, 75 modified surface adsorption of OH, 76 surface atomic rearrangement, 77 etc. In our case, the enhanced stability was attributed to the change in the lattice strain based on the experimental evidence, though we do not rule out possibilities of other surface or electronic effects. Conclusions Taken together, the findings have demonstrated that the combination of the controlled synthesis and processing of the carbon-supported PtNiCo nanoparticles and manipulation in thermal treatment temperature plays an important role in Acknowledgment. This work was supported by UTC Power and in part by the National Science Foundation (CBET ). Work at the NSLS was supported by the DOE BES grant DE-FG02-03ER We thank Mark Engelhard for XPS measurement, which was performed using EMSL, a national scientific user facility sponsored by the Department of Energy s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory. We also thank Dr. H. R. Naslund for assistance in DCP-AES analysis and Dr. I-T Bae for assistance in HR-TEM analysis. Supporting Information Available: Additional data of TEM, HR-TEM, and CV. This material is available free of charge via the Internet at References and Notes (1) Zhong, C. J.; Luo, J.; Fang, B.; Wanjala, B. 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