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Apr 3, 2013 - Reversible surface segregation of Pt in Pt3Au/C catalysts was accomplished through a heat treatment under a CO or Ar atmosphere, which ...
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Reversible Surface Segregation of Pt in a Pt3Au/C Catalyst and Its Effect on the Oxygen Reduction Reaction Kug-Seung Lee,†,‡ Hee-Young Park,† Hyung Chul Ham,† Sung Jong Yoo,† Hyoung Juhn Kim,† EunAe Cho,† Arumugam Manthiram,§ and Jong Hyun Jang*,† †

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea § Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ‡

S Supporting Information *

ABSTRACT: Reversible surface segregation of Pt in Pt3Au/C catalysts was accomplished through a heat treatment under a CO or Ar atmosphere, which resulted in surface Pt segregation and reversed segregation, respectively. The Pt-segregated Pt3Au/C exhibited a significantly improved oxygen reduction reaction (ORR) activity (227 mA/mgmetal) compared to that of commercial Pt/C (59 mA/mgmetal). For the Pt-segregated Pt3 Au/C, the increased OH-repulsive properties were validated by a CO bulk oxidation analysis and also by density functional theory (DFT) calculations. Interestingly, the DFT calculations revealed that the binding energy for Pt-segregated Pt3Au (111) surfaces was 0.1 eV lower than that for Pt (111) surfaces, which has been previously reported to exhibit the optimum OH binding energy for the ORR. Therefore, the reversible surface segregation is expected to provide a practical way to control the surface states of Pt−Au bimetallic catalysts to enhance ORR activity. In addition, the Pt-segregated Pt3Au/C showed excellent electrochemical stability, as evidenced by its highperformance retention (96.4%) after 10 000 potential cycles, in comparison to that of Pt/C (55.3%).

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INTRODUCTION The oxygen reduction reaction (ORR) has been intensively investigated for the commercialization of fuel cells because the kinetics of the ORR is sluggish and because the ORR requires catalysts with high electrochemical stability. Significant improvements in ORR catalysts have been reported where transition metals, including Ni, Co, Fe, Cu, Pd, and Y, have been incorporated into Pt-based multimetallic electrocatalysts. The ORR enhancements in such cases have been recently explained using density functional theory (DFT).1−8 To further decrease the amount of expensive Pt required in ORR catalysts, numerous investigations of M-core/Pt-shell structures have been reported, and enhanced ORR activities of the core/shell catalysts have been validated using a d-band model. When the Pt−Pt distance in a shell is decreased by the influence of transition metal cores with smaller lattice sizes, the Pt d-band center is lowered, and as a result, the reactants and intermediates adsorb less strongly. However, the long-term stability of the core/shell catalysts has been brought into question because the conventional core metals (M = Ni, Co, Fe, Cu, and their alloys) are not stable at the potentials of interest (>0.7 V) in acidic media, and they can be exposed to the surface during operation of the fuel cell due to the tendency of oxygen to bind more strongly to the core metals than to Pt.9 Interest in Pt−Au bimetallic catalysts has recently increased because of their OH-repulsive properties and excellent electrochemical stability in acidic media.9−11 The Pt−Au bimetallic © 2013 American Chemical Society

catalysts are expected to exhibit enhanced ORR performance with lower surface coverage of OH, whereas the surface of Pt can be more extensively poisoned through strong OH binding at high potentials. However, the ORR performance of Pt−Au catalysts has been mostly reported to be lower than or comparable to that of Pt catalysts.12−17 The promising stability of Pt−Au catalysts is attributed to the stability of Au itself and to the improved stability of Pt via the prevention of the place-exchange mechanism.9 In addition to their use as ORR catalysts, Pt−Au bimetallic catalysts have also attracted interest for their use as catalysts for the oxidation of formic acid18−20 and as bifunctional catalysts for rechargeable Li−air batteries.21−23 In a recent paper, our group reported that a CO atmosphere can induce a surface Pt enrichment in PtAu alloy catalysts.24 The ORR performance of the surface-enriched catalyst was comparable to the general ORR performance of Pt/C. In this article, as a remarkable advancement in ORR performance of Pt− Au bimetallic catalysts and its validation by simulation, we report the adsorbate-induced reversible surface segregation of a carbonsupported Pt3Au alloy catalyst (Pt3Au/C). The reversible surface segregation of Pt was induced through heat treatments performed in different atmospheres (air, CO, and Ar) to control the composition of several of the top layers, which allowed the Pt surface area and its electronic structures to be tuned without Received: March 29, 2013 Published: April 3, 2013 9164

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five-layer slab were fixed at corresponding bulk positions, the upper two layers were fully relaxed using the conjugate gradient method until residual forces on all the constituent atoms became smaller than 5 × 10−2 eV/Å.

considerable growth of the particles. In addition, theoretical calculations were performed to verify the enhancement of the ORR performance of the catalyst as a result of surface segregation.

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EXPERIMENTAL METHODS Catalyst Preparation. The Pt3Au/C catalyst was synthesized by dispersing the carbon support, a Pt precursor (H2PtCl6·6H2O), an Au precursor (HAuCl4·3H2O), and oleylamine into anhydrous ethanol, followed by the addition of borane-tert-butylamine (Sigma-Aldrich) as a reducing agent. The metal content in the Pt3Au/C catalyst was controlled to be 40 wt %. After being washed and filtered, the catalysts were dried in a vacuum oven overnight. To remove the surfactant (oleylamine) on the catalyst surface, the Pt3Au/C catalyst was heated in a tube furnace under an air atmosphere for 1.5 h (air sample). For the surface segregation of Pt, the air-treated Pt3Au/C was heated under a CO atmosphere for 10 h (air−CO sample). The CO-treated Pt3Au/C was subsequently heat treated under an Ar atmosphere for 2 h to achieve the reverse surface segregation (air−CO−Ar). All heat treatments were performed at 200 °C. Catalyst Characterization. Electrochemical analyses were performed in a conventional three-electrode electrochemical cell using a glassy carbon rotating disk electrode (RDE, 6 mm diameter), Pt wire, and a saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively. All electrochemical measurements were reported versus a reversible hydrogen electrode (RHE). The ink slurry was prepared by mixing the catalysts with 5 wt % Nafion solution (Aldrich) and 2-propanol. The ratio of components in the catalyst ink was 1 mL of Nafion solution and 10 mL of 2-propanol per 0.1 g of catalyst. The catalyst ink was dropped onto the RDE using a micropipet and then dried in an oven. After the electrochemical cell was purged with Ar for 20 min, CV was performed at potentials of 0.05−1 V vs. RHE with a scan rate of 20 mV/s in 0.1 M HClO4 under Ar purging. The ORR activity was measured using linear sweep voltammetry at potentials of 0.2−1.1 V with a scan rate of 5 mV/s in O2-saturated 0.1 M HClO4 (1600 rpm). CO bulk oxidation was performed using a CV method (1 mV) in 0.1 M HClO4, where the electrolyte was saturated with CO for 10 min with the potential held at 0.05 V vs. RHE (1600 rpm). All electrochemical experiments were performed at 25 °C. Potential cycling for the catalyst stability test was performed at potentials of 0.6−1.1 V with a scan rate of 50 mV/s for 10 000 cycles in O2-saturated 0.1 M HClO4 solution. DFT Calculation. The calculations reported herein were performed on the basis of spin polarized density functional theory (DFT) within the generalized gradient approximation (GGA-PW9)25 as implemented in the Vienna Ab-initio Simulation Package (VASP).26 The projector augmented wave (PAW) method with a planewave basis set was employed to describe the interaction between core and valence electrons.27 An energy cutoff of 350 eV was applied for the planewave expansion of the electronic eigenfunctions. For the Brillouin zone integration, we used a (2 × 2 × 1) Monkhorst−Pack mesh of k points to calculate geometries and total energies. For Pt3Au (111) and Pt (111) model surfaces, we constructed a five-atomiclayer slab with a hexagonal 4 × 4 unit cell. The slab is separated from its periodic images in the vertical direction by a vacuum space corresponding to seven atomic layers. The lattice constant for bulk Pt is predicted to be 3.98 Å, which is close to the experimental value of 3.92 Å. While the bottom two layers of the

RESULTS AND DISCUSSION Structural Characterizations. We confirmed that the nanoparticles for the as-prepared Pt3Au/C sample were evenly distributed on the carbon support with an average diameter of 2.5 nm, as shown in Figure 1. When the as-prepared sample was

Figure 1. TEM images of Pt3Au/C catalysts: (a) as-prepared and heat treated under (b) air, (c) air and CO, and (d) air, CO, and Ar.

heated under an air atmosphere to eliminate the surfactant on the nanoparticle surface, slight agglomeration of the particles was observed (air sample), and the particle diameter could not be measured accurately because of the presence of connected particles with a rodlike structure. The surface segregation of Pt was subsequently achieved via a second heat treatment performed under a CO atmosphere (air−CO sample). The last heat treatment was performed under an Ar atmosphere to facilitate a reverse surface segregation of Pt (air−CO−Ar sample). The average diameters were 3.1 and 3.5 nm for the air−CO and air−CO−Ar samples, respectively. Although the particles grew slightly and agglomerated (Figure S1, Supporting Information), the nanoparticles in the heat-treated samples were still highly dispersed on the carbon supports. The crystallographic structures were similar for all of the samples except the as-prepared sample, as shown in Figure 2. The X-ray diffraction (XRD) peak position of the (220) reflection was measured to be 66.00°, 66.92°, 66.93°, and 66.91° for the asprepared, the air, air−CO, and air−CO−Ar samples, respectively. The (220) peaks of the Pt3Au/C catalysts were located at lower angles than that of pure Pt (67.46°, PDF# 1818032), which indicates the formation of a Pt−Au alloy. On the basis of Vegard’s law, 75% of the Au atoms in the catalysts were estimated to be alloyed with Pt atoms. The electronic structure of the Pt3Au nanoparticles was examined using X-ray photoelectron spectroscopy (XPS). 9165

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32.6, 30.9, and 26.2 m2/g for the air, air−CO, and air−CO−Ar samples, respectively. The surface Au composition was measured to be 10%, 6%, and 13% for the air, air−CO, and air−CO−Ar samples, respectively, using the experimental charge values for Au−oxide reduction and Hdes charge (Figure S3, Supporting Information). All of the Pt3Au/C catalysts showed enhanced ORR performance compared to that of Pt/C (EAS: 51.6 m2/g), as shown in Figure 3b, and the air−CO sample exhibited the best performance. The half-wave potentials in the ORR polarization curves after iR correction were determined to be 0.906, 0.922, 0.938, and 0.925 V for the Pt/C, air, air−CO, and air−CO−Ar catalysts, respectively. These results indicate that the addition of Au into Pt catalysts significantly enhanced the ORR activity (air sample) and that the catalytic activity could be further enhanced through surface modification with Pt segregation (air−CO sample). Kinetic current (ik) was calculated using the Koutecky−Levich equation (1/i = 1/ik + 1/il) where i is the measured current and il is diffusion limiting current. Panels c and d of Figure 3 show that the mass activity (jk,mass) of the Pt3Au/C catalysts were enhanced as much as 5-fold (based on Pt) and 3.8-fold (based on total metal) compared to that of the commercial Pt/C catalyst. The specific activity (jk,area) was enhanced as much as 7-fold. The decrease in activity after the heat treatment under an Ar atmosphere suggests that the catalytic activity depends on the surface composition. Because the error range of the kinetic current densities was about 8%, the ORR activity change by heat treatments can be regarded as significant. To examine the surface structure of the catalysts and to ascertain the reason for the enhanced ORR activity, CO bulk oxidation was performed using RDE, as previously reported by

Figure 2. XRD profiles of as-prepared and heat-treated Pt3Au/C catalysts (air, air−CO, and air−CO−Ar samples).

Deconvoluted Pt 4f and Au 4f signals are shown in Figure S2 (Supporting Information), and the results are summarized in Table S1 (Supporting Information). The formation of a PtAu alloy was confirmed by the shift of the Au binding energy (BE) toward lower values (pure Au: 84.0 eV for 4f7/2) due to electron transfer from Pt atoms to more electronegative Au atoms, as reported previously.28,29 Electrochemical Characterizations. Figure 3 shows the results of CV and ORR performance tests using an RDE for the Pt3Au/C catalysts. As the heat treatment step preceded, the hydrogen adsorption/desorption area decreased as a result of gradual particle growth (TEM). The electrochemical active surface area (EAS) was calculated, using the Hdes charge, to be

Figure 3. Comparisons of (a) the cyclic voltammograms tested in Ar-saturated 0.1 M HClO4 with a scan rate of 20 mV/s, (b) the ORR performance obtained in O2-saturated 0.1 M HClO4 with a scan rate of 5 mV/s, (c) the mass activity normalized with respect to metal (Pt and Au) and Pt, and (d) the specific activity normalized with respect to the Hdes surface area for heat-treated Pt3Au/C and Pt/C catalysts. The mass activity and specific activities for the catalysts were measured at 0.9 V using the RDE method with the electrode rotated at 1600 rpm. ORR activities were calculated with iR correction. 9166

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Markovic.30−33 At high positive potentials (e.g., 0.9 V), CO molecules can be oxidized sufficiently fast to reach a concentration-limited current through combination with OHad or oxides that predominantly cover the surface. When the potential is changed to more negative values (i.e., during a backward sweep), the surface coverage of OHad and oxides will be gradually decreased, and at some potential, the measured current should decrease because the amounts of surface OHad and oxides will be insufficient for bulk CO oxidation. Therefore, the oxophilicity can be determined from the potential shift for current decay. On the basis of previous reports for PtAu surface alloys,34 the variation in CO adsorption kinetics was assumed to be insignificant. In the present experiment, the air−CO sample was found to be the least oxophilic sample, and Pt/C was found to be the most oxophilic sample on the basis of CO bulk oxidation results, as shown in Figure 4. These results are in good agreement with the ORR activity trends.

systems.1−3,6,35 However, the d-band theory has not been used to predict improved ORR performance of Pt−Au bimetallic catalysts because the addition of Au or the formation of a Pt overlayer on Au can induce tensile strain on the Pt lattice such that the Pt d-band center upshifts and results in a strengthening of the adsorption of reactant and intermediate species.17,18,36−38 Instead of the lattice strain effect (d-band theory), the ligand effect might cause a decrease in BEOH. Platinum alloys in which the Pt surface sites are coordinated with metal atoms with greater electronegativities, such as Au, bind OH less strongly than does pure Pt.39 The weaker OH binding can be explained by the withdrawal of sp-electron density from the Pt site by Au, which results in a shorter OH−metal bond and consequently increases the repulsive interaction between the adsorbate and the metal d-states.40 The BEOH for the optimum ORR performance has been reported to be 0.1 eV lower than that of pure Pt.39,41,42 Interestingly, the DFT calculations in the present study showed that the BEOH was 0.1 eV lower than that of Pt (111), which indicates that the fine-tuning of several top layers of Pt3Au catalysts can lead to optimum surface states for ORR performance. Therefore, in the DFT calculations, we verified that Pt−Au bimetallic catalysts exhibit OH binding energies lower than that of Pt and, for the first time, that fine-tuning of the surface layers facilitated optimum OH binding, which strongly supports the ORR and the CO bulk oxidation results. Electrochemical Stability. The stability of the Ptsegregated Pt3Au/C (air−CO) and Pt/C catalysts was tested using potential cycling in the potential range 0.6−1.1 V for 10 000 cycles. After the potential cycling, the ORR activity of the air−CO sample decreased by only 4.6%, whereas the activity of Pt/C decreased by 45.3% (Figures 5 and S4, Supporting

Figure 4. Backward sweep curves for CO bulk oxidation on heat-treated Pt3Au/C and Pt/C catalysts. The CO bulk oxidation was performed in CO-saturated 0.1 M HClO4 at a scan rate of 1 mV/s and at a rotation speed of 1600 rpm.

DFT Calculation. We performed DFT calculations to better understand the effect of the alloying of Pt with Au and the effect of CO-induced surface segregation on the oxophilicity of the surfaces of the Pt3Au/C catalysts. For this purpose, we calculated the OH binding energy (indicated by BEOH) on the CO-induced Pt3Au(111) surfaces (here, we modeled the surface segregation by exchanging Au atoms in the top surface layer with Pt atoms in the second surface layer) and the bulk-terminated Pt3Au(111). For comparison, the BEOH for the Pt(111) was also calculated, as shown in Table 1. First, we found that the BEOH for the bulkterminated Pt3Au(111) surface (the OH radical is adsorbed at the top site of a Pt atom) was reduced compared to that of the Pt(111) surface, which implies the presence of less-oxophilic (OH) surface states and, in turn, improved the ORR activity of the Pt3Au/C catalysts. Notably, the ORR kinetics can be enhanced through a reduction of the surface OH coverage (or through an increase in the availability of free Pt sites).3 This tendency is more developed at the CO-induced surface. Additionally, the reduced OH binding energy (or less-oxophilic state) on the CO-driven Pt3Au(111) surfaces is consistent with our previously presented experimental results for CO bulk oxidation. This reduction of the OH binding energy on the Pt3Au surface may be related to the ligand effect. The d-band theory has been used to successfully predict the ORR activities on given catalytic

Figure 5. Mass activities at 0.9 V for the ORR measured before and after potential cycling. Potential cycling was performed in 0.1 M HClO4 with a scan rate of 50 mV/s and with O2-purging for 10 000 cycles. The ORR performance was obtained in O2-saturated 0.1 M HClO4 with a scan rate of 5 mV/s and a rotation rate of 1600 rpm. ORR activities were calculated with iR correction.

Information). The ORR activity decrease for Pt3Au/C catalyst with higher surface Au composition (air−CO−Ar) was similar to that of air−CO sample. The decrease in the electrochemical surface area (CV) was also significantly smaller for the surfacemodified Pt3Au/C compared to the Pt/C. The excellent stability of the Pt3Au/C (air−CO) can be attributed to its OH-repulsive properties, which can prevent surface oxidation and therefore decrease the Pt dissolution rate.43,44 Additionally, a hindered place-exchange mechanism due to subsurface Au atoms might be another reason for the high stability, as suggested by Wang and colleagues for a metallic Au/FePt3 catalyst.9 9167

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Table 1. Results for BEOH Calculations Using DFTa

a The calculations were performed under the assumption that the surface segregation occurs through place exchange between the first and second layers on the (111) crystal plane in a 4 × 4 five-layer slab, whereas the three bottom layers were assumed to maintain their composition. Representative descriptions of the top and side views for the slabs are included (gray and yellow balls represent Pt and Au atoms, respectively).

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CONCLUSION A reversible surface segregation of Pt in Pt3Au/C catalysts was attempted to control and significantly improve ORR performance. The surface segregation and the reverse surface segregation of Pt were realized through heat treatments of Pt3Au/C catalysts in CO and Ar atmospheres, respectively. All of the Pt3Au/C catalysts exhibited improved ORR performance compared to that of Pt/C, and the COtreated Pt3Au/C exhibited the best performance. The improved ORR performance of the Pt3Au/C catalysts might be attributed to OHrepulsive properties of the surfaces, which were confirmed via CO bulk oxidation analysis. DFT calculations also showed that the BEOH of Pt3Au(111) was lower than that of Pt(111) and was further decreased at the CO-induced Pt-segregated surface. Notably, the calculated BEOH for several Pt-segregated surfaces was 0.1 eV lower than that of pure Pt, which has been established as the optimum BEOH for the ORR. Therefore, the optimum surface for ORR can be achieved through control of the composition of several top surface layers of Pt−Au bimetallic catalysts. In addition, the Pt-segregated Pt3Au/C exhibited greatly improved stability relative to that of Pt/C. We believe that the optimum surface composition can be achieved using not only Pt3Au as a catalyst but also PtxAuy with a lower Pt content.

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exposed Au percentages on the surface, and results of stability tests. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Joint Research Project, funded by the Korea Research Council of Fundamental Science & Technology (KRCF), Republic of Korea (Seed-10-2), and was also supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (2012M3A6A7054283).

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ASSOCIATED CONTENT

S Supporting Information *

Size distribution of Pt3Au particles, deconvoluted XPS signals, summary of XPS results, cyclic voltammograms that describe the 9168

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