Enhanced Oxygen Reduction Activity of Platinum Monolayer with a

Jul 16, 2014 - United Technologies Research Center, East Hartford, Connecticut 06118, United States. ‡. UTC Power, South Windsor, Connecticut 06074,...
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Enhanced Oxygen Reduction Activity of Platinum Monolayer with a Gold Interlayer on Palladium Minhua Shao,*,† Amra Peles,§ and Jonathan Odell‡ †

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong § United Technologies Research Center, East Hartford, Connecticut 06118, United States ‡ UTC Power, South Windsor, Connecticut 06074, United States ABSTRACT: The activity of Pt monolayer supported on conventional (cuboctahedral) Pd nanoparticles for the oxygen reduction reaction was improved by 2-fold by introducing an Au submonolayer between the Pd core and Pt shell. By controlling the shape of the Pd cores (cubic and octahedral), we are able to distinguish the role of Au on the activity improvement at different facets. The Au interlayer can enhance the Pt monolayer activity toward oxygen reduction on both (100) and (111) surfaces, with the enhancement on (100) much more pronounced. The larger enhancement degree at the (100) sites may be due to the larger decrease of oxygen binding energy caused by Au interlayer (0.275 eV) than that at (111) sites (0.075 eV).

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INTRODUCTION

(100) being much more pronounced. The density functional theory (DFT) calculations were conducted to explain this observation.

Core−shell catalysts have attracted significant attention for various chemical reactions due to their higher utilization of costly noble metals and activity improvement caused by the electronic and structural effects from the core materials.1 Pt overlayers supported on other less expensive metals, such as Pd and its alloys,1a,d,f,2 have shown higher activity than pure Pt for the oxygen reduction reaction (ORR), which is the cathodic reaction in fuel cells. The slow kinetics of ORR requires high Pt loading in the catalyst layer in order to achieve a desirable performance.3 The Pt monolayer catalyst is one of the most promising concepts to lower the noble metal loading in the fuel cells due to its full utilization of Pt atoms.1a It has been shown that the activities of Pt monolayer catalysts strongly depend on the properties of the core, including the compositions,1a,f,4 particle sizes, and crystalline orientations (shapes).5 Recent studies have demonstrated that the ORR activity of Pt monolayer can be further tuned by an interlayer.6 For instance, with a Pd9Au alloy sublayer between the Pt monolayer and Pd nanoparticle core, the ORR activity can be enhanced by 70%.6b However, the role of the sublayers in the activity enhancement is unclear. In this study, we find that the ORR activity of Pt monolayer supported on conventional (cuboctahedral) Pd nanoparticles is improved by 2 times by introducing an Au submonolayer between the Pd core and Pt shell. By controlling the shape of the Pd cores (cubic and octahedral), we were able to distinguish the role of Au on the activity improvement at different facets. Our results demonstrate that the Au interlayer can enhance the Pt monolayer activity toward oxygen reduction on both (100) and (111) surfaces, with the enhancement on © 2014 American Chemical Society

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EXPERIMENTS Synthesis of Pd Cubes and Octahedra. The synthesis of shape-controlled Pd nanocrystals (cubes and octahedra) is described elsewhere.7 To synthesize Pd cubes, 60 mg of Lascorbic acid (Aldrich), 5 mg of KBr (Aldrich), 185 mg of KCl (J.T. Baker), 57 mg of Na2PdCl4 (Aldrich), and 105 mg of poly(vinylpyrrolidone, PVP) (MW = 55 000) were mixed in 11 mL of aqueous solution. The mixture was heated at 80 °C in air under stirring for 3 h. To synthesize Pd octahedra, 180 mg of citric acid (Aldrich), 105 mg of PVP, and 57 mg of Na2PdCl4 were dissolved in 8 mL of water and 3 mL of ethanol. The mixture was heated at 80 °C in air under stirring for 3 h. Carbon black suspension was prepared by dispersing 80 mg of Ketjen Black in 5 mL of water and sonicating for 1 h. The synthesized Pd nanocrystals then were added to the carbon black suspension under stirring, resulting in a 20% metal loading. The carbon-supported metal powders were collected by centrifugation and dried in an oven overnight. Conventional Pd nanoparticles supported on Vulcan obtained from BASF were also used in this study (Pd/C). Electrochemical Measurements. A uniform catalyst ink was prepared by dispersing 15 mg of Pd/C in a mixture Received: April 3, 2014 Revised: July 14, 2014 Published: July 16, 2014 18505

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consisting of 3 mL of 2-propanol and 60 μL of 5% Nafion (Aldrich) in 12 mL of water. A certain volume of the suspension (typically 10 μL) was deposited on a polished rotating disk electrode (RDE, Pine Instruments) made of glassy carbon. The capping agents and some impurities on Pd surfaces could be removed using a nondestructive method at room temperature by keeping the electrode potential at −0.05 V for 1 min.8 A Pt gauze and reversible hydrogen electrode (RHE) were used as a counter and reference electrode, respectively. After this step, the electrode was potential cycled between 0.08 and 0.8 V (vs RHE) for five cycles in a N2-saturated 0.1 M HClO4 solution at 50 mV s−1. The electrode was then transferred to a solution consisting of 50 mM H2SO4 (GFS Chemicals) and 50 mM CuSO4 (Aldrich) to conduct Cu underpotential deposition (UPD). The electrode, covered with a Cu monolayer, was immersed in a 1.0 mM K2PtCl4 (Johnson Matthey) + 50 mM H2SO4 solution for about 2 min to complete the Pt−Cu displacement reaction. The Au sublayer was deposited by immersing the Pd/C sample covered with a Cu monolayer in a 0.1 mM HAuCl4 (Johnson Matthey) + 50 mM H2SO4 solution for about 10 s. After rinsing thoroughly with water, the Cu UPD was repeated on the Au-decorated Pd/ C sample and then displaced with Pt. The oxygen polarization curves were measured in O2-saturated 0.1 M HClO4 at a scan rate of 10 mV s−1. The Koutecky−Levich equation was used to calculate the kinetic current at 0.9 V. Milli-Q UV-plus water (Millipore) was used to prepare all the electrolytes. CV and ORR polarization curves were obtained with an EG&G Princeton 273 potentiostat and CHI 660D electrochemical workstation at room temperature. The electrochemical surface areas were estimated by integrating the Cu UPD charges on Pd nanocrystals assuming 460, 490, and 420 μC cm−2 on conventional, octahedral, and cubic samples, respectively. All potentials were given with respect to RHE. Density Functional Theory Calculations. The details of ab initio density functional theory (DFT) calculations can be found in ref 1e. The only difference is that Au interlayers are placed between Pt pseudomorphic layers and Pd substrate for the Pt/Au/Pd system.

Figure 1. Cyclic voltammetry curves of conventional Pd/C with and without Au and Pt decoration. (A) Voltammetry curves of Pd/C with and without an Au submonolayer in 0.1 M HClO4 solution with a scan rate of 50 mV s−1. (B) Voltammetry curves of Pd/C with and without a Au submonolayer in a 0.05 M H2SO4 + 0.05 M CuSO4 solution with a scan rate of 5 mV s−1. (C) Voltammetry curves of Pt-monolayerdecorated Pd/C and Au/Pd/C in a 0.1 M HClO4 solution with a scan rate of 50 mV s−1. The currents were normalized to the geometric area of the rotating disk electrode (0.196 cm2).

RESULTS AND DISCUSSION The Cu UPD−noble metal displacement method was applied to deposit Au sublayer and Pt monolayer.1e Figure 1A compares the cyclic voltammetry curves for the Pd/C (BASF, 20%) in a 0.1 M HClO4 solution before and after Ausubmonolayer modification. After a Cu monolayer was deposited on the Pd surface, the electrode was immersed in a 0.1 mM HAuCl4 solution for 10 s to replace Cu atoms with Au. On the basis of the reaction between Cu atoms and Au3+ ion (Cu + 2/3Au3+ → 2/3Au + Cu2+), it is expected that one monolayer of Cu atoms result in ∼0.67 monolayer of Au on the Pd surface. The charge associated with the hydrogen adsorption decreased from 0.26 mC for the bare Pd to 0.1 mC after Au deposition. The 61% loss of charge indicates that about twothirds of the Pd surface area is covered by Au, since the latter is featureless in the hydrogen adsorption/desorption region. The Cu UPD experiments also confirmed the presence and coverage of Au atoms. Figure 1B compares the Cu UPD curves of Pd/C before and after Au deposition. There are two pairs of CU UPD peaks for the bare Pd sample at 0.5 and 0.4 V, respectively. The intensities of these two pairs of current peaks decrease after Au deposition, while a new broad current wave, which corresponds to the Cu underpotential deposition on

Au,1e,9 appears in the range of 0.56−0.68 V. By immersing the Cu-monolayer-decorated Pd and Au/Pd samples in a K2PtCl4 solution, a Pt monolayer forms assuming only Cu atoms were replaced by Pt according to the reaction Cu + Pt2+ → Cu2+ + Pt. Cyclic voltammetry curves for Pt monolayer deposited on Pd/C without (Pt/Pd/C) and with an Au submonolayer (Pt/ Au/Pd/C) are compared in Figure 1C. Typical polycrystalline Pt features were observed for both samples. The surface area of Pt/Au/Pd/C is slightly lower than that of Pt/Pd/C based on the hydrogen adsorption charge, maybe due to the incomplete coverage of Pt shell on the surface of the core. Pinholes and defects in the Pt shell may form during the Pt−Cu displacement step for both Pt/Au/Pd/C and Pt/Pd/C samples. In the case of Pt/Pd/C, the pinholes/defects do not undervalue the hydrogen adsorption charge since UPD of hydrogen can also occur on the uncovered Pd atoms. While hydrogen does not adsorb on the Au surface in the potential range studied, the imperfect Pt shell on Au/Pd/C is expected to have a lower hydrogen adsorption charge than that on Pd/C. The ORR activities of several different catalysts are compared in Figure 2. The half-wave potential shifts negatively by 30 mV with an Au submonolayer on the Pd/C surface, suggesting a much lower ORR activity than that of Pd/C. The Au-modified

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Figure 2. Polarization curves for the ORR on Pd/C, Au/Pd/C, Pt/Pd/ C, and Pt/Au/Pd/C in an oxygen-saturated 0.1 M HClO4 solution with a scan rate of 10 mV s−1. Pt loading on the electrode was 0.23 and 24 μg cm−2 for Pt/Au/Pd/C and Pt/C, respectively. Insert: Comparison of the Pt mass activities for Pt/C (TKK), Pt/Pd/C, and Pt/Au/Pd/C at 0.9 V. The currents were normalized to the geometric area of the rotating disk electrode (0.196 cm2).

Pd/C also shows a lower limiting current in the mass transport region. Considering the fact that Au is much less active than Pd in the acidic solutions,10 the lower activity and limiting current indicates that the Pd surface is partially covered by Au atoms, in good agreement with the results in Figure 1. After depositing a Pt monolayer, the ORR activities of both Pd/C and Au/Pd/C samples are improved dramatically. The half-wave potentials shift positively by 25 and 65 mV for Pd/C and Au/Pd/C, respectively, after a Pt monolayer deposition. The inset of Figure 2 compares the Pt mass activities calculated by normalizing the kinetic currents at 0.9 V to the weights of Pt, which were estimated by integrating the charges associated with Cu UPD.1a The Pt mass activity of Pt/Pd/C (0.75 A mg−1) is 3−4 times higher than that of Pt/C (0.2 A mg−1), consistent with previous reports.11 The activity of Pt/Au/Pd/C is 1.4 A mg−1, which is 7- and 2-fold higher than that of Pt/C and Pt/ Pd/C, respectively. The enhancement is similar to what was observed by Xing et al. using a Pd−Au alloy interlayer between Pt shell and Pd core. It has been known that the reaction rate of ORR highly depends on the crystallographic orientation. For instance, Pt(111) is much more active than Pt(100) in a weak-adsorption electrolyte.12 On the other hand, Au(111) is less active than Au(100) and its vicinal faces in both alkaline and acidic media.13 A recent study of Pt overlayers supported on Au single crystals demonstrated that Au(111) was a good support.14 An atomic layer thick Pt shell on Au(111) was more active than that on Au(100). We also reported that Pt monolayer on Pd octahedral nanocrystals enriched with {111} facet had a much higher activity than that on cubes.5 It will be of interest to examine the effect of Au interlayer on shape-controlled Pd nanocrystals on ORR activity. The Pd nanocubes and octadedra consisting of {100} and {111} facets, respectively, supported on high surface area carbon support (Ketjen Black) were synthesized.7 The welldefined Pd cubes and octahedra with edge length in the range of 5−6 nm, as well as Pt-monolayer-decorated Pd nanocrystals, were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) in the reports published previously.5,7 parts A and B of Figure 3 compare the

Figure 3. Cyclic voltammetry curves of Pd nanocubes (A) and octahedra (B) supported on Ketjen Black with and without Au and Pt decoration in a nitrogen-saturated 0.1 M HClO4 solution. Scan rate = 50 mV s−1.

polarization curves for the Pd cubes and octahedra before and after Au and Pt decorations, respectively. The polarization curves (dashed lines) for bare Pd cubes and octahedra are similar to those reported elsewhere.7,8 After Au decoration, the currents associated with the hydrogen adsorption/desorption for both samples decreases significantly, as expected. The coverage of Au atoms on shape-controlled nanocrystals is about 60% based on the charge of hydrogen adsorption, similar to that of conventional Pd/C. The hydrogen adsorption/ desorption areas increase after depositing a Pt monolayer for both samples. The surface areas calculated from the charges associated with the hydrogen adsorption/desorption of Pt/Pd samples are consistent with those derived from the Cu UPD charges, implying that the surface area would be underestimated if using hydrogen adsorption charges on bare Pd.5,15 Therefore, all the Pt loadings and electrochemical active surface areas were derived from the Cu UPD charges in this study.15 The ORR polarization curves of Pt monolayer on bare and Au-modified Pd cubes and octahedra are compared in Figure 4A,B. By introducing an Au interlayer between Pt and Pd, the half-wave potentials of ORR curves shift positively by 35 and 6 mV for Pd cubes and octahedra, respectively. These results demonstrate that the Au interlayer can enhance the Pt monolayer activity toward oxygen reduction on both (100) and (111) surfaces, with the enhancement on (100) being more pronounced. The Pt mass activities at 0.9 V are compared in Figure 4C. The mass activity increases from 0.63 and 2.2 to 2.0 and 2.6 A mg−1 for cubes and octahedra, respectively. The respective enhancements for Pd cubes and octahedra samples are 3 and 1.2 times; i.e., the effect of Au interlayer on ORR activity is much smaller on (111) surface than that on (100). 18507

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Figure 5. Relationship between the specific activity and oxygen binding energy for the Pt monolayer catalyst on pure Pd and with the Au interlayer for {111} facets of octahedral particles and {100} facets of cubic particles. The red dashed line represents the oxygen binding energy for Pt(111) at −1.22 eV. The data of Pd and Pt/Pd cubes and octahedra were taken from ref 5.

with Pd, Pt/Pd, and Pt/Au/Pd compositions and oxygen binding energies of predominant facets. The data of Pd and Pt/ Pd cubes and octahedra are taken from ref 5. Since the mobility of Au atoms is significant, it is possible that they can form 3D Au clusters easily, which may cause the instability of the Pt/Au/Pd system. Potential cycling was performed on Pt/Au/Pd/C using a square-wave signal between 0.65 and 1.0 V (5 s each) in an oxygen-saturated 0.1 M HClO4 solution at room temperature. The decay rate of ORR activity is very similar to that of Pt/Pd/C, suggesting that the positive impact of Au atoms still exists. However, the changes in structure and element distribution need further investigation during potential cycling. The slow rate of removal of strong adsorbed oxygencontaining species has been considered as the bottleneck to improve the ORR kinetics on bulk Pt surfaces.17 A slightly weaker oxygen binding energy can enhances its ORR activity dramatically.17b For instance, Pt/Pd(111) is more active than Pt(111) due to the weaker oxygen binging energy of the former, by 0.06 eV. By further weakening the oxygen binding energy, the specific catalytic activities of Pt monolayer on both cubic and octahedral Pd particles are improved by introducing a Au layer. As shown in Figure 5, the Au interlayer lowers the oxygen binding energies of Pt monolayers on Pd(111) and (100) surfaces by 0.075 and 0.275 eV, respectively. Since the strain does not change in Pt monolayer by adding an Au interlayer, the geometric effect is irrelevant to the observed oxygen binding energy changes. The decrease in oxygen binding energy can be attributed to the kinetic hindrance of surface segregation in the presence of the ORR intermediates that bind on Pt much stronger than on Au and keep Pt monolayer as the top layer. However, the bonding strength is counteracted with an energy cost to keep Pt monolayer on the top of the Au interlayer, resulting in a weaker oxygen binding energy. The specific activity follows the surface reactivity according to the Sabatier principle; indicating that the reactivity in Pt/Au/Pd on octahedral particles provides for the best kinetics among the considered catalysts. The systems of Pd(100), Pt/Pd(100), Pd(111), and Pt/Pd(111) have been discussed in ref 5.

Figure 4. Polarization curves for the ORR on Pt monolayer supported on Pd cubes (A) and octahedra (B) with and without an Au submonolayer in an oxygen-saturated 0.1 M HClO4 solution. Scan rate = 10 mV s−1. Pt mass activity comparison at 0.9 V (C).

To further understand the effect of the Au interlayer on different facets, density functional theory calculations were performed. Reactivity is expressed as bonding strength toward the oxygen and is calculated as EBO = Esub+O − Esub − 1/ 2E(O2) − ΔEseg, where Esub+O, Esub, E(O2), and ΔEseg represent the energy of substrate with adsorbed oxygen, the energy of substrate, the energy of oxygen molecule, and the energy of keeping the Pt on the surface. The segregation energy is taken into account due to the strong inverse segregation effect in the system of Pt/Au/Pd, namely, the oxidized surface is more stable with Pt monolayer on top of Au sublayer, while an empty surface is more stable with Au at the topmost layer and Pt in the sublayer. ΔEseg is calculated using the approach described elsewhere.1e,16 Note that the segregation energy is negligible for Pt/Pd catalyst. Figure 5 shows the relationship between measured specific activities on cubic and octahedral particles

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CONCLUSIONS In summary, we demonstrated that an Au sublayer between the Pt monolayer shell and Pd core can improve the oxygen 18508

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reduction activity of the latter. The degree of the activity enhancement strongly depends on the morphology of the Pd particles. The oxygen reduction activity was improved by 3 and 1.2 times on Pt monolayer supported on Pd cubes and octahedron, respectively, with an Au sublayer. The observed activity enhancement may mainly be due to the electronic effect from the Au interlayer by weakening the oxygen binding energy of Pt monolayers by 0.275 and 0.075 eV supported on Pd(100) and Pd(111), respectively. Our results further emphasize the ligand (electronic) effect on the catalytic activity of surface atoms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+852) 3469-2269. Fax: (+1) 2358-0054. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a start-up fund from the Hong Kong University of Science and Technology. REFERENCES

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