Enhancing Oxygen Reduction Reaction Activity via Pd−Au Alloy

Oct 26, 2010 - Minhua Shao , Amra Peles , Krista Shoemaker , Mallika Gummalla , Peter N. Njoki , Jin Luo , and Chuan-Jian Zhong. The Journal of Physic...
2 downloads 0 Views 2MB Size
Download PDF
pubs.acs.org/JPCL

Enhancing Oxygen Reduction Reaction Activity via Pd-Au Alloy Sublayer Mediation of Pt Monolayer Electrocatalysts Yangchuan Xing,*,† Yun Cai, Miomir B. Vukmirovic, Wei-Ping Zhou, Hiroko Karan,‡ Jia X. Wang, and Radoslav R. Adzic* Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States

ABSTRACT New Pt monolayer electrocatalysts were prepared using galvanic displacement of a copper monolayer deposited at underpotentials on a Pd core. By performing underpotential deposition twice, two monolayers were deposited, forming a core-shell structure with double shells. The double shells consist of an outermost shell of Pt monolayer and a sublayer shell of Pd-Au alloy. It was found that by adjusting the compositions of the alloy sublayer, it is possible to mediate the oxygen reduction reaction (ORR) activity of the Pt catalysts. An alloy with 10% (atomic) Au was found to be the most active among the catalysts tested. Furthermore, the catalysts showed good cycling stability that may be due to stabilizing effect of Au. Since different alloys can be used as the sublayer for mediation, this work may open up various opportunities to tailor electrocatalysts for best ORR activity. SECTION Energy Conversion and Storage

of a PtML for the ORR.21 A volcano plot has been found for the activity of a PtML as a function of the calculated d-band center of the substrates. It has also been demonstrated that Pt alloys of PtNi and PtFe can improve ORR activity as a result of segregation of Pt to the surface,22 whose interaction with the atomic layers underneath was attributed to the reduction of Pt-OH formation.23,24 Segregation of Pt to form a Pt “skin” on the surface produces a catalytic effect similar to a PtML, despite that the majority of Pt is still inside the alloy. However, achieving surface segregation in an alloy is often complicated by heat treatment, which can lead to other physicochemical changes in the catalyst. Cu UPD has been shown to be a versatile technique to achieve PtML on different metals without forming bulk alloys. PtML has been obtained on both extended crystal surfaces as well as nanoparticles, (e.g., Pd, PtCo, PdFe, etc.).7-9,25-28 When the PtML is formed on a particle substrate, a core-shell structured catalyst is obtained. The shell (PtML) and the core (substrate metal) can be different metals, allowing combinatorial tailoring of the catalysts. Most PtML catalysts obtained from Cu UPD have been shown to display enhanced ORR activity with respect to pure Pt.8,29 The design of core-shell structured catalysts can be various because of the many metals and alloys that can be used as cores. In this Letter we report a new design of the core-shell catalyst consisting of double shells instead of a

H

ighly active electrocatalysts for oxygen reduction reaction (ORR) continue to be an important research area in polymer electrolyte membrane (PEM) fuel cells.1-4 The sluggish ORR5 of current electrocatalysts has hindered the development of viable PEM fuel cells as a power source. Pt and its alloys are still the best electrocatalysts for ORR, but the noble metal is costly and has limited resources on the earth. There is a need to reduce its usage to the minimum.6 One way to reduce Pt usage is to make a thin Pt layer on a support metal that is less noble. This has been achieved by using copper underpotential deposition (UPD) and subsequent galvanic replacement of Cu by Pt.7-9 The atomically formed Pt monolayer (PtML) significantly increases Pt utilization since almost every Pt atom in the monolayer is on the surface and can participate in electrocatalytic reactions. More importantly, the interactions between the atomic PtML and the support metal have been demonstrated to provide a synergistic effect in improving ORR kinetics.10-14 When Pt is deposited on a foreign metal, either lattice compression or expansion can occur to the Pt layer due to lattice mismatch.8 For example, Pt atoms deposited on a Ru substrate would have a compressive strain, but they would have a tensile strain when deposited on Au.8 This substrate induced structural change directly affects their electronic structure as indicated in the shift of the d band.15-20 Consequently, binding of adsorbates on Pt can be either enhanced or weakened, providing a means to tailor the electrocatalytic activity of Pt catalysts. In our previous studies with single crystal electrodes we have demonstrated the effect of the substrate on the activity

r 2010 American Chemical Society

Received Date: September 15, 2010 Accepted Date: October 22, 2010 Published on Web Date: October 26, 2010

3238

DOI: 10.1021/jz101297r |J. Phys. Chem. Lett. 2010, 1, 3238–3242

pubs.acs.org/JPCL

Figure 2. Typical CV curves of Cu UPD on Pd and Pd90Au10 surfaces in dearated 0.05 M CuSO4 /0.05 M H2SO4 electrolyte, and the CV curves of PtML on Pd90Au10/Pd/C and nanoparticle Pt/C (20 wt %) in 0.1 M HClO4 electrolyte at 20 mV/s scan rate. Figure 1. Illustration of the core-shell structure of alloy sublayer nanoparticle catalysts. Both the alloy sublayer and PtML were obtained with Cu UPD.

single one. The two shells were made on a Pd nanoparticle core, with the outermost shell being a PtML and the one underneath being an alloy sublayer of (Pd-Au)ML (Figure 1; abbreviated Pd-Au hereafter). This core-shell catalyst was obtained by performing Cu UPD twice on an E-TEK Pd/C catalyst (20 wt % Pd on Vulcan carbon black). The first Cu UPD was used to deposit the Pd-Au alloy layer and the second Cu UPD to deposit the PtML on top of the alloy layer; the Pd-Au alloy layer therefore becomes a sublayer to support the PtML. To study the sublayer mediation effect on the catalytic activity of the PtML, three alloy sublayers were made with different compositions in the Pd-Au alloy at 5% Au, 10% Au, and 20% Au (atomic percentages based on proportions of precursor metal salts used). Their ORR activities were investigated with a rotating disk electrode (RDE) in 0.1 M HClO4 at room temperature. All of the catalysts showed improved ORR activities over those without an alloy sublayer. However, the 10 at % Au sublayer catalyst displays the best activity. In addition, the core-shell catalysts showed very good cycling stability that may be due to the stabilizing effect of Au.30-33 Figure 2 shows the cyclic voltammograms in CuSO4 electrolyte for the original Pd/C catalyst surfaces as well as the Pd90Au10 surfaces after the first replacement of the Cu monolayer. The curve for the Pd nanoparticle surfaces shows a typical voltammogram characterizing the redox peaks on Pd. The CV curve for the Pd90Au10 layer shows a broader shoulder, which is indicative of the presence of Au. The reduction peak for the later also moved to a higher potential, indicative of Au incorporation in the Pd. The Pd90Au10 alloy sublayer has 10% Au, and is enough to exhibit the Au effects. The shoulders become narrower for the 5% Au and wider for the 20% Au catalysts, indicative of the composition changes (see Figure 1S in the Supporting Information). After a second

r 2010 American Chemical Society

Figure 3. ORR activities of different catalysts obtained with electrode rotating speed at 1600 rpm in 0.1 M HClO4, showing the best activity with an alloy sublayer of Pd90Au10 and the worst without a sublayer.

Cu UPD and its galvanic replacement by PtML, the CV curve shows a PtML surface (see also Figure 2). A typical transmission electron microscopy image is shown in Figure 2S (see Supporting Information). The image was obtained after the electrochemical measurements, and it can be seen that the catalyst particles (7.8 nm) are fairly well distributed on the support carbon. ORR activity was obtained from RDE measurements at different electrode rotating speeds. The current-potential curves of the ORR activity for the Pd90Au10 catalyst recorded at four different disk rotating speeds are shown in Figure 3S in the Supporting Information. Typical ORR curves were observed for all the catalysts, and their electrocatalytic ORR

3239

DOI: 10.1021/jz101297r |J. Phys. Chem. Lett. 2010, 1, 3238–3242

pubs.acs.org/JPCL

Figure 5. Stability of ORR of the catalyst with Pd90Au10 alloy sublayer, showing a small deactivation of the catalyst after 5000 cycles.

Figure 4. ORR mass activities for the sublayer-mediated catalysts, showing Au alloy sublayers increased the activity of the PtML, with Pd90Au10 catalyst being the most active.

Therefore, when Pd-Au alloys are used as the sublayer for mediation, the interplay of lateral strain (compressive and tensile) and radial contraction (compressive strain) can lead to activity improvement of the ORR catalysts, as observed in the experiments. There should be an optimal alloy composition, i.e., both too little and too much Au would decrease the ORR activity. For the results reported in this Letter, the 10 at % Au, which is estimated using the atomic radii to have about 10.5% surface coverage by Au atoms in the alloy sublayer, seems to produce the best ORR activity. A more accurate composition will be obtained in future work. While lattice contraction may be important, the interaction of the substrate with PtML could result in an electronic effect on the ORR activity through charge redistribution.7,10 As demonstrated previously,7 the formation of PtOH is significantly prohibited for the PtML on Pd nanoparticles when compared with Pt nanoparticles. A suppression of the PtOH formation of PtML on alloy sublayers is also observed from the cycling voltammograms, as shown in Figure 2 when compared with Pt nanoparticles. Hydroxyl formation is considered as a major factor affecting ORR activity because of site blockage by OH.37 Au may play an important role in protecting low-coordination sites from being oxidized. Wang et al. recently reported that Au coated with FePt3 may have stabilized the oxidation of the surface atoms in certain facets of the nanoparticles.33 Similarly, the Au in the sublayer of our catalysts may have contributed to the reduction of surface oxidation, leading to improved ORR activity of the catalysts. Stability of the core-shell catalysts was tested using the best catalyst Pd90Au10. Its ORR activities are shown in Figure 5 before and after 5000 cycles at 1600 rpm. The catalyst has less than 1% loss in ORR activity, demonstrating that the catalyst is fairly stable. The stabilization of the catalyst may be due to the reduction of surface oxidation from Au in the sublayer, as argued above. Surface oxidation may lead to catalyst dissolution. Since Zhang et al.30 reported the stabilizing effects of Au on Pt nanoparticles, several studies31-33 have also demonstrated the stabilizing effect of Au. It is suspected

activities were observed. The comparison of the ORR curves at 1600 rpm for PtML deposited on PdAu sublayers with different compositions was displayed in Figure 3. It can be seen that the activities can be ranked in a decreasing order as Pd90Au10 > Pd95Au5 > Pd80Au20 > Pd100, where Pd100 represents that catalyst without sublayer mediation. The best catalyst was the one with sublayer Pd90Au10 mediation. Both increasing Au composition and decreasing it decreased the ORR activities, although Pd95Au5 catalyst showed better activity than Pd80Au20. The improvement of the ORR activities can be further quantified in Figure 4, in which the Pt mass based kinetic currents are displayed. From Figure 4, it is obvious that all the catalysts with sublayer mediation have substantially improved ORR activities over the catalyst without a sublayer. The observed ORR activities may be attributed to surface strain and the d-band center shift of the PtML induced by the sublayer alloy. All three metals involved form face-centered cubic structures. The lattice constants of Pt, Pd, and Au are 3.920, 3.890, and 4.080 Å, respectively. The mismatch between Pt and Pd produced a lateral compressive strain when a PtML is deposited on Pd: εPt/Pd = (aPt - aPd)/aPd = 0.8%, where ε is lateral strain and a lattice constant. This is for flat surfaces. For nanoparticles, surface curvature also contributes to producing a strain.34,35 For a 7.8 nm particle, this contribution is estimated to be about ∼0.4%. Although the total surface strain of ∼1.2% is still small, contraction in surface atoms in the radial direction can make a significant contribution. The enhancement in ORR activity due to lattice contraction has been demonstrated in experiments and density functional theory.10 Huang et al. has recently reported that up to 8% reduction in bond length was found in 4 nm Au nanoparticles.36 When a PtML is deposited on Au, a lateral tensile strain can be generated in PtML at 3.92% that will expand Pt in the surface layer. Contraction due to decreasing coordination numbers of surface atoms will act in the opposite direction.

r 2010 American Chemical Society

3240

DOI: 10.1021/jz101297r |J. Phys. Chem. Lett. 2010, 1, 3238–3242

pubs.acs.org/JPCL

air at room temperature between 0.75 and 0.95 V (vs RHE) with a dwell time of 30s at each potential.

in this work that the Au in the Pd-Au alloy may have improved the catalysts by affecting the lattice contraction (both lateral and radial) and preventing some specific sites of PtML from oxidation. The latter also provides an effective way to stabilize the catalyst during cycling. In summary, this Letter demonstrates a new nanostructured ORR electrocatalyst mediated with a Pd-Au sublayer. Double shells are made using Cu UPD twice on a core metal Pd. The Au in the sublayer alloy provides a bifunctional effect in improving the catalyst activity and stabilizing the catalysts during cycling. The fact that the sublayer alloy can be of different metals and compositions may open up various future opportunities in tailoring catalysts for best ORR activity.

SUPPORTING INFORMATION AVAILABLE Comparison of the cyclic voltammograms of the three catalysts for their sublayers is shown in Figure S1. The TEM image of the catalyst with sublayer Pd90Au10 is shown in Figure S2. ORR activities of the alloy sublayer catalyst Pd90Au10 at different rotating speeds are shown in Figure 3S. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: xingy@ mst.edu (Y.X.); [email protected] (R.R.A.).

EXPERIMENTAL SECTION The base catalyst used in this work was a commercial Pd/C catalyst from E-TEK with 20% Pd by weight. Catalyst ink was prepared following a procedure reported elsewhere.3,4 Briefly, about 2 mg catalyst was weighed and put in a glass vial. Millipore water (>18 MΩ) was added to the vial to make a 1.0 mg/mL suspension. The vial was then put in an ultrasonic bath for 15 min for dispersion. The thus prepared ink was immediately used to prepare a thin film disk. A pipet was used to take 0.015 mL ink out of the vial and put onto the glassy carbon disk. The disk was put in a desiccator under vacuum. Upon drying, a drop (∼ 0.01 mL) of 0.05% Nafion solution was put on top of the catalyst and allowed to dry again in the desiccator. The RDE has a diameter of 0.5 cm or an area of 0.1963 cm2. Therefore, the catalyst loading on the disk was 0.015 mg Pd/cm2. Cu UPD was conducted in a compartmental electrochemical cell. CuSO4 (0.05 M) and 0.05 M H2SO4 electrolyte solution was used to deposit Cu at underpotentials. Pd-Au and Pt salts of HAuCl4/PdCl2 and K2PtCl4 were prepared separately as 0.001 M solutions in 0.05 M H2SO4. For different atomic compositions of the Pd-Au alloy, the Au to Pd atomic ratio was fixed at desirable proportionality in the precursors. The as-prepared disk was used as the working electrode in the CuSO4 compartment for depositing Cu. After the first Cu UPD on Pd/C, the disk was transferred to the compartment with Pd-Au salt solution for galvanic replacement of the Cu layer. In this way a monolayer of alloy Pd-Au was deposited on the Pd/C catalysts. The disk was then put back into the CuSO4 electrolyte compartment for a second Cu UPD on the alloy layer. The procedure is similar to the first operation, but the galvanic replacement was with Pt. This produced a PtML on the sublayer of Pd-Au alloy, forming core-shell catalysts with two shells. The study of cyclic voltammetry (CV) and ORR of the catalysts was conducted in a self-made electrochemical cell. The counter electrode was a piece of Pt foil, and the reference electrode was Ag/AgCl via a bridge. The electrolyte was 0.1 M HClO4 prepared from concentrated perchloric acid (Optima, Fisher Scientific). The electrolyte was purged with high purity argon before each CV measurement, or purged with research grade oxygen before each ORR experiment. A radiometer analytical potentiostat (VoltaLab 80) was used in this study. The stability tests were performed by cycling the catalysts in

r 2010 American Chemical Society

Notes †

On leave from the Missouri University of Science and Technology, Rolla, Missouri 65409, United States. On leave from the Medgar Evers College, Brooklyn, New York 11225, United States. ‡

ACKNOWLEDGMENT This work is supported by the U.S.

Department of Energy (DOE), Divisions of Chemical and Material Sciences, under Contract No. DE-AC02-98CH10886.

REFERENCES (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

3241

Sha, Y.; Yu, T. H.; Liu, Y.; Merinov, B. V.; Goddard, W. A., III. Theoretical Study of Solvent Effects on the PlatinumCatalyzed Oxygen Reduction Reaction. J. Phys. Chem. Lett. 2010, 1, 856–861. Garsany, Y; Epshteyn, A.; Purdy, A. P.; More, K. L.; SwiderLyons, K. E. High-Activity, Durable Oxygen Reduction Electrocatalyst: Nanoscale Composite of Platinum-Tantalum Oxyphosphate on Vulcan Carbon. J. Phys. Chem. Lett. 2010, 1, 1977–1981. Lee, S. W.; Chen, S.; Suntivich, J.; Sasaki, K.; Adzic, R. R.; Shao-Horn, Y. Role of Surface Steps of Pt Nanoparticles on the Electrochemical Activity for Oxygen Reduction. J. Phys. Chem. Lett. 2010, 1, 1316–1320. Gong, K.; Su, D.; Adzic, R. R. Platinum-Monolayer Shell on AuNi0.5Fe Nanoparticle Core Electrocatalyst with High Activity and Stability for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 14364–14366. Adzic, R. R. Recent Advances in the Kinetics of Oxygen Reduction. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley: New York, 1998. Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal., B 2005, 56, 9–35. Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Klie, R.; Sasaki, K.; Adzic, R. R. Platinum Monolayer Electrocatalysts for O2 Reduction: Pt Monolayer on Pd(111) and on Carbon-Supported Pd Nanoparticles. J. Phys. Chem. B 2004, 108, 10955–10964. Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Uribe, F. Platinum Monolayer Fuel Cell Electrocatalysts. Top. Catal. 2007, 46, 249–262.

DOI: 10.1021/jz101297r |J. Phys. Chem. Lett. 2010, 1, 3238–3242

pubs.acs.org/JPCL

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

(23)

(24)

(25)

(26)

Shao, M. H.; Sasaki, K.; Marinkovic, N. S.; Zhang, L.; Adzic, R. R. Synthesis and Characterization of Platinum Monolayer Oxygen-Reduction Electrocatalysts with Co-Pd Core-Shell Nanoparticle Supports. Electrochem. Commun. 2007, 9, 2848–2853. Greeley, J; Norskov, J. K.; Mavrikakis, M. Electronic Structure and Catalysis on Metal Surfaces. Annu. Rev. Phys. Chem. 2002, 53, 319–348. Nilekar, A. U.; Xu, Y.; Zhang, J.; Vukmirovic, M. B.; Sasaki, K.; Adzic, R. R.; Mavrikakis, M. Bimetallic and Ternary Alloys for Improved Oxygen Reduction Catalysis. Top. Catal. 2007, 46, 276–284. Xu, Y.; Ruban, A. V.; Mavrikakis, M. Adsorption and Dissociation of O2 on Pt-Co and Pt-Fe Alloys. J. Am. Chem. Soc. 2004, 126, 4717–4725. Wang, J. X.; Zhang, J.; Adzic, R. R. Double-Trap Kinetic Equation for the Oxygen Reduction Reaction on Pt(111) in Acidic Media. J. Phys. Chem. A 2007, 111, 12702–12710. Wang, J. X.; Inada, H.; Wu, L.; Zhu, Y.; Choi, Y. M.; Liu, P.; Zhou, W.-P.; Adzic, R. R. Oxygen Reduction on Well-Defined Core-Shell Nanocatalysts: Particle Size, Facet, and Pt Shell Thickness Effects. J. Am. Chem. Soc. 2009, 131, 17298– 17302. Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Effect of Strain on the Reactivity of Metal Surfaces. Phys. Rev. Lett. 1998, 81, 2819–2822. Grabow, L.; Xu, Y.; Mavrikakis, M. Lattice Strain Effects on CO Oxidation on Pt(111). Phys. Chem. Chem. Phys. 2006, 8, 3369– 3374. Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93, 156801/1–156801/4. Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Modification of the Surface Electronic and Chemical Properties of Pt(111) by Subsurface 3d Transition Metals. J. Chem. Phys. 2004, 120, 10240–10246. Bligaard, T.; Norskov, J. K. Ligand Effects in Heterogeneous Catalysis and Electrochemistry. Electrochim. Acta 2007, 52, 5512–5516. Nilekar, A. U.; Mavrikakis, M. Improved Oxygen Reduction Reactivity of Platinum Monolayers on Transition Metal Surfaces. Surf. Sci. 2008, 602, L89–L94. Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Controlling the Catalytic Activity of Platinum-Monolayer Electrocatalysts for Oxygen Reduction with Different Substrates. Angew. Chem., Int. Ed. 2005, 44, 2132–2135. Toda, T.; Igarashi, H.; Watanabe, M. Role of Electronic Property of Pt and Pt Alloys on Electrocatalytic Reduction of Oxygen. J. Electrochem. Soc. 1998, 145, 4185–4188. Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M. Surface Composition Effects in Electrocatalysis: Kinetics of Oxygen Reduction on Well-Defined Pt3Ni and Pt3Co Alloy Surfaces. J. Phys. Chem. B 2002, 106, 11970–11979. Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M. Surface Segregation Effects in Electrocatalysis: Kinetics of Oxygen Reduction Reaction on Polycrystalline Pt3Ni Alloy Surfaces. J. Electroanal. Chem. 2003, 554-555, 191. Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Spontaneous Deposition of Pd on a Ru(0001) Surface. Surf. Sci. 2001, 477, L173–L179. Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Pt Submonolayers on Ru Nanoparticles. A Novel Low Pt Loading, High CO Tolerance Fuel Cell Electrocatalyst. Electrochem. Solid State Lett. 2001, 4, A217–A220.

r 2010 American Chemical Society

(27)

(28)

(29)

(30)

(31)

(32)

(33)

(34)

(35)

(36)

(37)

3242

Shao, M. H.; Sasaki, K.; Liu, P.; Adzic, R. R. Pd3Fe and Pt Monolayer-Modified Pd3Fe Electrocatalysts for Oxygen Reduction. Z. Phys. Chem. 2007, 221, 1175–1190. Zhou, W.-P; Yang, X.; Vukmirovic, M. B.; Koel, B. E.; Jiao, J.; Peng, G.; Mavrikakis, M.; Adzic, R. R. Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt-Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) SingleCrystal Alloy. J. Am. Chem. Soc. 2009, 131, 12755–12762. Zhang, J.; Vukmirovic, M. B.; Sasaki, K.; Nilekar, A. U.; Mavrikakis, M.; Adzic, R. R. Mixed-Metal Pt Monolayer Electrocatalysts for Enhanced Oxygen Reduction Kinetics. J. Am. Chem. Soc. 2005, 127, 12480–12481. Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters. Science 2007, 315, 220–222. Tang, W.; Jayaraman, S.; Jaramillo, T. F.; Stucky, G. D.; McFarland, E. W. Electrocatalytic Activity of Gold-Platinum Clusters for Low Temperature Fuel Cell Applications. J. Phys. Chem. C 2009, 113, 5014–5024. Xu, J.-B.; Zhao, T.-S.; Shen, S.-Y.; Li, Y.-S. Stabilization of the Palladium Electrocatalyst with Alloyed Gold for Ethanol Oxidation. Intl. J. Hydrogen Energy 2010, 35, 6490–6500. Wang, C.; van der Vliet, D.; More, K. L.; Zaluzec, N. J.; Peng, S.; Sun, S.; Daimon, H.; Wang, G.; Greeley, J.; Pearson, J. et al. Multimetallic Au/FePt3 Nanoparticles as Highly Durable Electrocatalyst. Nano Lett. [Online early access]. DOI: 10.1021/ nl102369k. Wang, L.; Roudgar, A.; Eikerling, M. Ab Initio Study of Stability and Site-Specific Oxygen Adsorption Engergies of Pt Nanoparticles. J. Phys. Chem. C 2009, 113, 17989–17996. Jiang, Q.; Liang, L. H.; Zhao, D. S. Lattice Contraction and Surface Stress of fcc Nanocrystals. J. Phys. Chem. B 2002, 105, 6275–6277. Huang, W. J.; Sun, R.; Tao, J.; Menard, L. D.; Nuzzo, R. G.; Zuo, J. M. Coordination-Dependent Surface Atomic Contraction in Nanocrystals Revealed by Coherent Diffraction. Nat. Mater. 2008, 7, 308–313. Wang, J. X.; Markovic, N. M.; Adzic, R. R. Kinetic Analysis of Oxygen Reduction on Pt(111) in Acid Solutions: Intrinsic Kinetic Parameters and Anion Adsorption Effects. J. Phys. Chem. B 2004, 108, 4127–4133.

DOI: 10.1021/jz101297r |J. Phys. Chem. Lett. 2010, 1, 3238–3242