Enhanced Oxygen Reduction Activity of Platinum Monolayer on Gold

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Enhanced Oxygen Reduction Activity of Platinum Monolayer on Gold Nanoparticles Minhua Shao,*,† Amra Peles,‡ Krista Shoemaker,† Mallika Gummalla,‡ Peter N. Njoki,§ Jin Luo,§ and Chuan-Jian Zhong§ †

UTC Power, South Windsor, Connecticut 06074, United States, ‡United Technologies Research Center, East Hartford, Connecticut 06118, United States, and §Chemistry Department, State University of New York at Binghamton, Binghamton, New York 13902, United States

ABSTRACT The increase in oxygen binding energy was previously proposed to account for the lower oxygen reduction activity of a Pt monolayer supported on Au(111) single crystal than that on Pd(111) and pure Pt(111) surfaces. This singlecrystal based understanding, however, cannot explain the new finding of a 1.6-fold increase of oxygen reduction activity on Pt monolayer-modified 3-nm Au nanoparticles (Pt/Au/C) in comparison with that on Pt/Pd/C with a similar particle size. The Pt/Au/C catalyst also has an activity higher than that of a state-of-the-art 2.8-nm Pt/C catalyst. Our new experimental results and density functional theory calculations demonstrate that a significant compressive strain in the surface of the core nanoparticles plays a role in the observed activity enhancement. SECTION Surfaces, Interfaces, Catalysis

T

he slow kinetics of the oxygen reduction reaction (ORR) remains one of the serious challenges for mass production of low temperature fuel cells due to the necessity of high Pt loadings in state-of-the-art electrocatalysts.1-3 Recent efforts in ORR electrocatalysis have focused on improving the catalytic activity of Pt alloys,4 minimizing Pt content by utilizing a core-shell structure,5-8 and replacing Pt with less expensive materials.9 Pd-based substrates modified by a Pt monolayer have shown a higher specific activity than pure Pt both at extended and nanoscale surfaces due to a slightly weakened oxygen binding energy (BEO), i.e., lower surface reactivity.10-12 Pt monolayer-modified Au single crystal surfaces, on the other hand, have a lower activity than pure Pt due to a stronger BEO resulting from a large tensile strain in the surface.11 Contrary to the single crystal activities, we report here a 1.6-fold higher ORR activity on Pt monolayer-modified 3 nm Au nanoparticles (Pt/Au/C) than on Pt/Pd/C with a similar particle size. This catalyst also has an activity higher than that of a state-of-the-art 2.8 nm Pt/ C catalyst. This new finding, however, is not explainable simply by the previous single-crystal based understanding. Here we present new experimental evidence and density functional theory (DFT) calculation results to demonstrate that a significant compressive strain in the surface of the core nanoparticles may play a role in the observed ORR activity enhancement. In general, the activity of Pt monolayer catalysts strongly depends on the electronic properties of the core and amount of strain in the surface.11 Bulk Au is not a good substrate for Pt monolayer electrocatalysts due to the large lattice mismatch (4.1%) with Pt resulting in a strong BEO.11 In contrast, bond length contraction is expected to occur at Au nanoparticle

r 2010 American Chemical Society

surfaces, generating a significant strain, which is dependent on particle size and surface adsorption site.13 The effect of gold particle size is considered to be an important parameter controlling catalyst activity.14,15 Here we studied the surface strain effect from the core on the ORR activity of Pt monolayer catalysts. The as-synthesized Au nanoparticles display an average size of 3 nm, with a high level of monodispersity in size (Figure S1, Supporting Information). After heat treatment at 300 °C with carbon support, the particle size of Au does not show a notable change (Figure S2). The ORR activity of Pt/Au/C was compared with the activities of Pt/Pd/C and state-of-the-art Pt/ C. To minimize the particle size effect on the catalytic activity,2 we studied Pd/C and Pt/C nanoparticles with a particle size similar to Au/C. Figure 1 shows the X-ray diffraction (XRD) patterns of Au/C (30 wt %, homemade), Pd/C (20 wt %, BASF) and Pt/C (30 wt %, TKK) with average particle sizes of 3.0, 3.3, and 2.8 nm, respectively, calculated from Scherrer formula using the (220) peaks in the XRD patterns. The particle sizes calculated from XRD data agree well with the transmission electron microscopy (TEM) measurements (Figure S2). The Pt monolayer was deposited on Au/C and Pd/C via a Cu underpotential deposition (UPD)-Pt-displacement method.10 Figure 2a shows a typical Cu UPD curve on the 3 nm Au/C nanoparticle surfaces in a N2-saturated HClO4 solution (black dashed line). All potentials are given with respect to a reversible hydrogen electrode (RHE). One pair of well-defined current peaks is observed at 0.53 V, together with another pair of Received Date: November 22, 2010 Accepted Date: December 15, 2010 Published on Web Date: December 22, 2010

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DOI: 10.1021/jz1015789 |J. Phys. Chem. Lett. 2011, 2, 67–72

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voltammetry curves of Au/C without (black dashed line) and with a Pt monolayer (red solid line) are compared in Figure 2b. A typical polycrystalline Pt feature is observed with a Pt monolayer on the Au surface. The loading of Pt in Pt/Au/C as determined by the charge associated with Hupd is about 80% of that obtained by calculating the Cuupd charge, indicating the Au surface is almost completely covered by an atomically thin layer of Pt atoms. The formation of a nearly complete Pt monolayer is also confirmed by the Cu UPD curve of Pt/Au/C, in which only the Cu UPD peaks associated with Pt (0.57 and 0.47 V) are observed, while peaks at 0.53 and 0.43 V that were observed on Au/C are no longer present, as shown in Figure 2a. The ORR activities of several different catalysts were measured by a thin-film rotating disk electrode (TF-RDE) (Figure S3), and the results are compared in Figure 3a. The Pt loadings on the electrode for Pt monolayer catalysts were obtained by integrating the Cu UPD charges assuming that the number of Pt atoms is equal to that of Cu atoms. The Pt mass activity of Pt/Pd/C is 0.75 A mg-1 at 0.9 V, which is 3-4 times higher than that of Pt/C (0.2 A mg-1), in good agreement with that reported by Wang et al.19 The Pt mass activity of Pt/Au/C is 1.2 A mg-1, which is 1.6-fold higher than that on Pt/Pd/C. Since almost all of the Pt atoms in a Pt monolayer catalyst are involved in the ORR and only ∼40% of Pt atoms are on the surface of a 2.8 nm Pt nanoparticle, a comparison of specific activities normalized to the Pt surface available for the reaction (electrochemical-active area) is thus appropriate (Figure 3b). The specific activities of Pt/C, Pt/Pd/C, and Pt/ Au/C are 0.24, 0.31, and 0.51 mA cm-2, respectively. These results indicate that the ORR activities of Pt atoms on the surface of ∼3 nm size particles follows the order of Pt/C < Pt/ Pd/C < Pt/Au/C. This order, however, is different from that for extended surfaces of single crystals.11 The specific activities at 0.8 V normalized to Pt(111) derived from ref 11 are also plotted in Figure 3b, which shows that the increase of ORR activities on the (111) facet follows the order of Pt/Au(111) < Pt(111) < Pt/Pd(111). Our results emphasize the importance of nanosize effects. In order to elucidate the factors that affect the variation in ORR activity, we performed theoretical investigations of the relationship between particle geometry and surface reactivity. The nanoparticles were modeled with a truncated octahedral shape ranging in size from 1.5 to 5 nm such that their sizes correspond to closed shell polyhedra with a so-called magic number of atoms.20 The structural optimization was performed using the embedded atom method (EAM) potentials as implemented in the LAMMPS code.21 The description of surface reactivity was obtained through the bonding strength of intermediates of the reaction using first-principles DFT calculations.22 We first examined the surface reconstruction of nanoparticles. The average surface strains with respect to bulk lattice interatomic distances are plotted in Figure 4a for 9-fold coordinated atoms on {111} facets, 8-fold coordinated atoms on the {100} facets, and e7-fold coordinated atoms at edges and vertices for Pt, Pd, and Au nanoparticles with various particle sizes. The amount of the strain clearly depends on the location of the atoms, with the largest amount of strain

Figure 1. Comparison of XRD patterns of Au/C, Pd/C, and Pt/C nanoparticles.

Figure 2. Cyclic voltammetry curves of Au/C with and without a Pt monolayer on it. (a) Voltammetry curves of Au/C with and without a Pt monolayer on it in a nitrogen-saturated 0.05 M H2SO4 þ 0.05 M CuSO4 solution. Scanning rate = 5 mV s-1. (b) Voltammetry curves of Au/C with and without a Pt monolayer on it in a nitrogensaturated 0.1 M HClO4 solution. Scanning rate = 50 mV s-1.

broad peaks at 0.43 V. The cathodic peaks are associated with Cu2þ reduction on the Au surface while the anodic peaks correspond to Cu stripping. These Cu UPD features are similar to those observed on a bulk polycrystalline Au electrode,16 but are very different from that on 3 nm Au nanoparticles reported by Jin et al.17 and Sasaki et al.18 The Cu UPD peaks were barely seen in their voltammetry curves, which might be due to impurities adsorbed on the Au surfaces. The cyclic


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Figure 3. Comparisons of Pt mass activities (a) and normalized specific activities (b) of Pt and Pt monolayer catalysts. The mass activity and specific activities for nanoparticle catalysts were measured at 0.9 V by the rotating disk electrode (RDE) method at 1600 rpm. The specific activities were normalized to that of Pt/C and Pt(111) single crystals for nanoparticle and single crystal catalysts, respectively. The specific activities of single-crystal catalysts at 0.8 V were taken from ref 11.

the particle size decreases. Overall, Pt and Pd nanoparticles are the most and least strained, respectively. The calculated geometric changes in Au nanoparticles are consistent with the surface atomic contraction revealed by coherent diffraction study.13 It has been observed that the ORR specific activity of a Pt/C catalyst decreases significantly with increases in the Pt surface area per mass of catalyst, the so-called particle size effects. A 10-fold drop in the specific activity for samples with an averaged 3 nm particle size compared to polycrystalline samples is observed.2 We find that this loss of activity correlates with the reactivity of the Pt particle surface, which is dependent on adsorption site. For example, comparing the calculated binding energies of oxygen at the bridge sites (-1.53 eV at a [100]-[111] edge and -1.32 at a [111]-[111] edge) of a 1.65 nm truncated cuboctahedron Pt particle with those at face-centered cubic (fcc; -1.16 eV) and hexagonal close packed (hcp; -0.89 eV) sites on the (111) facets, the edge sites bind 32% and 13.5% stronger than fcc and 47.2% and 27.7% stronger than hcp sites. The much stronger BEO at edge and vertex sites indicates that these sites are easily oxidized due to the under-coordination of the atoms at these sites.23 Consequently, the removal of intermediates represent the rate-limiting step for ORR at these sites. In addition to the increased reactivity at the edge and vertex sites, the reactivity of {111} facets is expected to be modified as well because of the compressive strains in the nanoparticles. The study of surface reactivity of large nanoparticles by DFT methods is prohibitive due to computational cost. To circumvent this, we use the information about particle surface geometry obtained by EAM potentials to construct periodic slab models of the most prevalent {111} surface with strain corresponding to that induced at {111} facets at particle sizes identified in our experiments. It is known that the equilibrium surface composition of the two-component catalyst depends on the chemical environment of the catalyst. We find that Pt/Au is thermodynamically stable in the presence of adsorbed oxygen. In vacuum, Au tends to segregate to the surface.24 The calculated segregation energies of gold from the sublayer are -0.33 eV/ atom in vacuum and þ0.22 eV/atom with coadsorbed oxygen

Figure 4. Strains in the nanoparticle surfaces. (a) Particle sizedependent surface strain for Pt, Au, and Pd nanoparticles at different facets. The particles' {111} and {100} facets and edges (in gray) are illustrated in the inset in the upper left corner. (b) Chemisorption energies of O at the most stable fcc sites on Pt, Pt/Pd, and Pt/Au at different levels of strain on the {111} surface show direction of change compared to single crystals. The circles represent the values at bulk lattice parameters of the supporting metal, and squares represent values corresponding to 2.8 nm Pt, 3 nm Au, and 3.3 nm Pd particle strain at {111} facets.

existing at the edges and vertices of the particles, while the strains in {111} and {100} facets are approximately 2-fold smaller than the under-coordinated sites. Also, all the nanoparticles show compressive strains that increase as


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on Pt/Au, where a negative (positive) sign indicates favorable (unfavorable) segregation. The segregation of Au in Pt/Au is kinetically hindered. However, due to competing thermodynamic effects between oxygen adsorption and Pt/Au surface segregation, the binding energy of adsorbates is lower compared to the systems that do not exhibit reverse segregation. This adsorbate-induced reverse segregation effect on the bonding strength of molecules has been observed25 and is included in our calculation of BEO on Pt/Au. Figure 4b shows the binding energy of oxygen at Pt monolayers in relation to the strain for both contracted {111} particle facets and single-crystal (111) planes for all metals. As expected, due to compressive strain in {111} facets of the nanocores, the binding strength becomes weaker (less negative BEO) compared to the single crystal surface. The BEO on Pt(111) is suggested to be a little bit too strong.26 If it is slightly weakened, as in the case for the Pt/ Pd(111) surface, the ORR activity can be enhanced remarkably.11For the ORR, the reaction kinetics on a catalyst that binds oxygen-containing species (O2, O/OH, O2-, and H2O2) too strongly is limited by the rate of removal of the adsorbates, as in the case for a Pt/Au(111) single-crystal surface that binds oxygen much more stronger than Pt(111) and Pt/Pd(111) as shown in Figure 4b. However, the strain in the Au nanocore lowers the reactivity of Pt/Au {111} facets that becomes comparable to that of a Pt/Pd single crystal. These results suggest that the compressive strain in Au nanoparticles plays a role in catalytic activity enhancement. The effect of the BEO on ORR activity was further revealed by studying Pt/Pd/ Au/C, in which a Pd interlayer was deposited on a Au/C surface prior to deposition of a Pt monolayer. The Pt specific activity and mass activity of this system are very similar to those of Pt/ Pd/C, but lower than those of Pt/Au/C, as shown in Figure 3. Our DFT calculations revealed that the BEO shifted by 0.24 eV positively by including a Pd interlayer. The large shifting of binding energy may explain the low activity of Pt/Pd/Au/C. The change of BEO may not be the only reason for the high activity of Pt/Au/C. Previous studies of the ORR on Au(hkl) single crystals in both alkaline and acidic media revealed that Au(100) and its vicinal faces exhibited a much higher activity than Au (111).27,28 A recent study by Kondo et al.29 demonstrated that the ORR activity of a Pt monolayer on Au(hkl) also depended on the crystallographic orientation of the Au crystals with the activity following the order of Pt/Au(111) , Pt/ Au(110) < Pt/Au(100) in HClO4 solution. This trend is opposite to that on Pt in the same solution, in which the ORR activity increases following the order of Pt (100) < Pt(110) ≈ Pt (111).30 Thus, the ORR activity of Pt/Au/C is more dependent on the fraction of (100) atoms rather than (111) atoms. For a 3-nm cubo-octahedral Au nanoparticle, (100) and (111) atoms comprise around 7% and 60% of the total surface atoms, respectively. The contribution of this small amount of (100) atoms to the overall activity may not be ignored in Pt/Au/C since the (100) atoms are much more active than others, but negligible in Pt/C and Pt/Pd/C. In addition to considerations of the catalytic activity, the durability of Pt monolayer catalysts is a concern for long-term operation of fuel cells due to the dissolution and atomic diffusion of Pt and core atoms. The long-term stability of Pt/


Au/C was examined using a square wave potential cycling protocol at room temperature in a 0.1 M HClO4. The catalysts were cycled by holding the potential at 0.65 and 1.0 V for 5 s each. After 5000 cycles, the mass activity loss of Pt/Au/C is ∼25%, which is comparable to that of Pt/Pd/C and Pt/C.12 In summary, a different trend in the ORR electrocatalytic activity was observed for a Pt monolayer catalyst at nanoscale regime. Our calculations showed that there were compressive strains in the nanoparticle surfaces, which were dependent on the composition and size of the nanoparticles. Even for a 5-nm particle, Pd, Pt, and Au showed compressive strains that increase with decreasing particle size, with Au and Pt having larger surface strains than Pd. Our findings also demonstrated that Pt overlayers on 3 nm Au nanoparticles exhibited higher ORR activity than that on Pd nanoparticles with a similar particle size, contrary to the single crystal results. The higher activity is believed to reflect changes in the particle reactivity in the Pt top layer due to the compressive strain in particles that modify the electronic structure of Pt and consequently their overall catalytic activity toward ORR at nanoscale. Further improvement of the ORR activity may be possible by using a smaller Au core, Au alloys with smaller lattice constant, and/or increasing the fraction of (100) sites on the nanoparticles.

EXPERIMENTAL SECTION Gold nanoparticles were synthesized by the standard and modified two-phase methods.31 Briefly, AuCl4- was first transferred from an aqueous solution of HAuCl4 (10 mM) to a toluene solution by the phase transfer reagent tetraoctylammonium bromide (36 mM). Decanethiols (DTs) were then added at a 2:1 mol ratio of DTs to Au. An excess (12 ) of an aqueous solution of NaBH4 was slowly added into the solution. The reaction was allowed to occur under stirring at room temperature for 4 h, producing a dark-brown solution of the DT-capped Au nanoparticles with an average diameter of 2 nm. The solution was subjected to solvent removal in a rotary evaporator, which was followed by multiple cleanings using ethanol. The nanoparticles were then supported on carbon black and thermally treated under controlled conditions.32 A controlled amount of Au nanoparticles was added into a suspension of carbon black, which was sonicated for 30 min, followed by stirring overnight. The carbon-supported nanoparticle powders were collected and dried under N2 and were then treated in a tube furnace under controlled temperature and atmosphere. A typical thermal treatment protocol involved heating the sample at 300 °C and under 15% H2/N2 for another half an hour. The actual loading was determined by thermogravimetric analysis and DCP-AES analysis, which yielded a value of 30% for the sample examined in this work. Approximately 14 mg of the catalyst particles were dispersed in a solvent consisting of 12 mL of water, 3 mL of isopropanol and 60 μL of 5% Nafion (Aldrich) by ultrasonication for 10 min. Ten microliters of the suspension was deposited on the precleaned glassy carbon substrate (RDE, Pine Instruments) and allowed to dry. The Pt monolayer was prepared by galvanic displacement of an underpotentially

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deposited (UPD) Cu monolayer by Pt atoms on the Au/C or Pd/ C surface from a 50 mM H2SO4 þ 50 mM CuSO4 solution. The electrode, covered with a Cu monolayer was rinsed and immersed in a 1.0 mM K2PtCl4 (Johnson Matthey) þ 50 mM H2SO4 (GFS Chemicals) solution for about 2 min to displace the Cu with Pt. To prepare a Pd monolayer on Au/ C, a solution of 1.0 mM PdCl2 (Johnson Matthey) þ 50 mM H2SO4 was used. An Ag/AgCl/KCl (3 M) leak-free electrode and Pt gauze were used as a reference and counter electrode, respectively. All potentials are quoted with respect to the RHE. All of these operations were carried out in a nitrogen atmosphere. The polarization curves were measured in a nitrogenor oxygen-saturated 0.1 M HClO4 (GFS Chemicals) solution. Electrolytes were prepared from Milli-Q UV-plus water (Millipore). An RHE and Pt gauze were used as a reference and counter electrode, respectively. All the electrochemical measurements were carried out at room temperature. The polarization curves of oxygen reduction were measured in an oxygen-saturated 0.1 M HClO4 solution at a scan rate of 10 mVs-1. The kinetic current jk at 0.9 V was derived by the Koutecky-Levich equation 1/j = 1/jk þ 1/(Bω1/2), where j is the measured current density, B is a constant, and ω is the rotation rate. The electrochemical active area was derived by the charge in the hydrogen adsorption/desorption region assuming that the charge associated with full monolayer coverage is 210 μC cm-2. The Pt loadings on the electrode for Pt monolayer catalysts were obtained by integrating the Cu UPD charge assuming all the Cu atoms were replaced by and equal to Pt atoms. The first-principles calculations are based on spin-polarized DFT using a generalized gradient approximation (GGA)33 and the projector augmented wave (PAW) method34 as implemented in the Vienna Ab-Initio Simulation Package (VASP).35 The cutoff energy for the plane wave basis set was 400 eV, and the Brillouin zone was sampled using a Monkhorst-Pack sampling technique36 with a 5 5 1 k-point grid for surface calculations. The Pt monolayer surfaces are modeled as pseudomorphic layers placed on top of the Pd(111) and Au(111) with a lattice constant corresponding to the bulk substrate, or constrained according to the amount of strain in {111} facets of 2.8 nm Pt, 3 nm Au, and 3.3 nm Pd particles. Surfaces are modeled by 4-layer slabs with a 2 2 surface cell separated by a 14 Å vacuum layer perpendicular to the surface. The top two layers were fully relaxed until Hellmann-Feynman forces were 0.01 eV/ Å. The calculated lattice parameters for bulk Pt, Pd, and Au are 3.98, 3.95, and 4.17 Å, in good agreement with the experimental values. The oxygen adsorption energy (Eads) was calculated based on the equation Eads = EsubþO - Esub - 1/2E(O2) - ΔEseg, where EsubþO, Esub, E(O2), and ΔEseg represent the energy of substrate with adsorbed oxygen, the energy of substrate alone, the energy of oxygen molecule in the gas phase, and the cost in segregation energy of keeping the Pt on the surface, respectively. ΔEseg is calculated using the approach in ref 25.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Tel: 860-7277251; fax: (þ1) 860-660-7384; e-mail: Minhua.shao@utcpower. com.

ACKNOWLEDGMENT Part of the DFT calculations in this research used resources of the National Center for Computational Sciences at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy under Contract DE-AC0500OR22725. Part of SUNY-Binghamton's work on synthesis and processing for Au/C was supported by the National Science Foundation (CBET-0709113).

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SUPPORTING INFORMATION AVAILABLE Strain calculations in nanoparticles, TEM images, and RDE data. This material is available free of charge via the Internet at http://pubs.acs.org.


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Enhanced Oxygen Reduction Activity of Platinum Monolayer on Gold

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