Letter pubs.acs.org/acscatalysis
Oxygen Reduction Reaction Activity for Strain-Controlled Pt-Based Model Alloy Catalysts: Surface Strains and Direct Electronic Effects Induced by Alloying Elements Masato Asano, Ryutaro Kawamura, Ren Sasakawa, Naoto Todoroki,* and Toshimasa Wadayama Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan S Supporting Information *
ABSTRACT: Surface strain and electronic interactions (i.e., strain and ligand effects) play key roles in enhancing the oxygen reduction reaction (ORR) catalytic activity of Pt-based alloy catalysts. Herein, we evaluate the ORR activity enhancement factors for Pt(111)-shell layers on Pt25Ni75(111) single-crystal surfaces prepared by molecular beam epitaxy under ultrahigh vacuum (UHV). Scanning tunneling microscopy images of the pristine surfaces collected under UHV revealed periodic surface modulations, known as Moiré patterns, suggesting that the topmost Pt(111)-shell layers are compressively strained by the influence of the underlying Ni atoms. The correlation between the ORR activities and estimated strains for 3-ML- and 4-ML-thick Pt shells (where ML represents monolayer), each having −1.7% and −1.2% strained Pt-shells, correspond well to the strain-based theory predictions. On the other hand, a 2-ML-thick Pt shell, with −2.8% strain, exhibits a remarkable ORR activity enhancement, i.e., 25 times higher than the pristine Pt(111): the enhancement factor anomalously deviates from the value predicted by the strain-based theory. Therefore, the activity enhancement of the 2-ML-thick Pt sample can be ascribed to a ligand effect induced by the Ni atoms just below the topmost Pt(111)-shell layer. The results obtained in this study provide a fundamental insight into the ORR activity enhancement mechanisms of Pt-based electrocatalysts. KEYWORDS: oxygen reduction reaction, platinum-based alloy catalysts, strain and ligand effect, polymer electrolyte fuel cell, scanning tunneling microscopy
T
also contribute to the activity enhancements of the alloy catalysts. Generally, the separation of the two aforementioned effects is quite difficult, because the ligand and strain effects simultaneously impact the catalytic activity. Therefore, a comprehensive understanding of the surface strains and electronic changes of the topmost Pt shell induced by the underlying alloying elements would be indispensable toward developing highly ORR-active alloy catalysts. Model catalyst studies using well-defined single-crystal alloy surfaces enable us to discuss the ORR mechanisms of the practical alloy catalysts. Indeed, surface analyses of model catalyst surfaces and their electrochemical performance have provided crucial information for understanding the ORR properties of Pt-based alloy catalysts.18−22 In practice, welldefined bimetallic surfaces prepared by molecular beam epitaxy (MBE) are preferable for surface analysis, and, thereby, for discussing the activity and durability of the alloy catalysts.23 Epitaxially grown metal monolayers on single-crystal metal or alloy substrates with different lattice parameters exhibit periodic geometrical patterns in their scanning tunneling microscopy
he oxygen reduction reaction (ORR) is a key catalytic reaction at the cathode electrode in polymer electrolyte fuel cells (PEFCs). Traditionally, Pt nanoparticles (NPs) supported on carbon materials are applied as ORR catalysts, because of the relatively high activity for the ORR among homogeneous metal catalysts.1 However, because of the rarity and cost of Pt, the development of inexpensive, highly active, and more durable catalysts is needed. Alloying Pt with nonnoble metals such as Ni, Co, or Fe is one of the effective methods for reducing Pt usage:2−11 configurations involving pure Pt topmost surface layers (Pt shells) over alloy cores have exhibited intrinsic activity enhancements, relative to Pt. The ORR activity enhancement for the Pt-based alloy catalysts can be explained on the basis of electronic (ligand) and geometrical (strain) effects, which modify the d-band electronic properties for the topmost surface Pt-shell layers.12 Recent theoretical studies have demonstrated that the strain effect dominantly influences the catalytic activity for practical alloy catalysts with upper three atomic shell-layers.12−14 Furthermore, volcano relationships between the strains (or Pt−Pt bond distances) and activity enhancements of the alloy catalysts have been frequently used to explain the ORR mechanism.5,15−17 However, because of the relatively wide distribution of the Pt-shell thicknesses in practical alloy NP catalysts, the ligand effect exerted by the underlying core alloying elements should © XXXX American Chemical Society
Received: May 24, 2016 Revised: July 11, 2016
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DOI: 10.1021/acscatal.6b01466 ACS Catal. 2016, 6, 5285−5289
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ACS Catalysis (STM) images, depending on the degree of mismatch,24,25 suggesting that lateral surface strains are derived from the lattice mismatch. Accordingly, strains can be quantitatively evaluated on the basis of the pattern periodicities. In this work, we experimentally evaluated the ORR activity enhancement factors for strain-controlled Pt(111) model shells fabricated on a Pt25Ni75(111) single-crystal alloy substrate by MBE. The results obtained provide new insights for understanding the enhanced ORR activity of Pt-based alloy catalysts. Figures 1a−d summarizes the ultrahigh vacuum (UHV)STM images and corresponding surface models of pristine Pt25Ni75(111) and n-monolayer-Pt deposited Pt25Ni75(111) (nML-Pt/Pt25Ni75(111); n = 2−4) surfaces. The surface models were constructed considering these annealing processes. First, for the clean Pt25Ni75(111) surface, part of subsurface Pt atoms
segregate to the topmost surface by annealing at 1173 K. Accordingly, the surface Pt composition can be larger (Pt:Ni = 50:50) than the bulk composition (25:75).26 Furthermore, annealing at 673 K for flattening the Pt-shell should induce interdiffusion of Pt in the shell and Ni in the substrate. Therefore, effective Pt-shell thickness can be thinner than the deposited thickness. The clean Pt25Ni75(111) substrate surface shows atomically flat terraces with 100 nm widths. The images of the nML-Pt/Pt25Ni75(111) series indicate periodic geometrical patterns, shown by hexagonally shaped white dotted lines in the images, having height modulations of less than a half the diameter of a Pt atom. The periodic geometrical arrangements, known as Moiré patterns, generally stem from a lattice mismatch between the topmost surface and second atomic layers.24 The Moiré patterns in Figure 1 suggest that a uniform lateral compressive strain in the long-range order is induced in the topmost surface Pt shell. Therefore, we can estimate the surface strain induced by the lattice mismatch using the Moiré equation, which is described by eq 1:27 aMoire ́ =
(ashell × asubst) |ashell − asubst|
(1)
where aMoiré is the distance between the centers of the hexagonal patterns estimated from the UHV-STM images, as shown in Figures 1b−d. The terms ashell and asubst are lattice constants for the topmost surface Pt-shell layer and the substrate Pt25Ni75 alloy,28 respectively. The value of ashell can be estimated experimentally from eq 1. The lattice strains of the Pt-shells then can be calculated using eq 2: strain (%) =
(ashell − aPt) × 100 aPt
(2) 28
where aPt is the lattice constant for bulk crystalline Pt. The surface strains for the nML-Pt/Pt25Ni75(111) estimated by eq 2 are summarized in Table 1. The periodicity of the Table 1. Structural Parameters Estimated from Moiré Patterns for UHV-STM Images of nML-Pt/Pt25Ni75(111) (see Figures 1b−d) sample 2ML-Pt/ Pt25Ni75(111) 3ML-Pt/ Pt25Ni75(111) 4ML-Pt/ Pt25Ni75(111) a
aMoiré (nm)
asubsta (nm)
ashell (nm)
strain (%)
8.2 ± 0.5
0.366
0.381 ± 0.001
−2.8 ± 0.3
6.6 ± 0.4
0.366
0.385 ± 0.002
−1.7 ± 0.4
6.0 ± 0.2
0.366
0.387 ± 0.001
−1.2 ± 0.2
Values determined from ref 26.
Moiré pattern for the 2ML-Pt/Pt25Ni75(111) surface is 8.2 ± 0.5 nm, which corresponds to a surface strain of −2.8% ± 0.3% for the topmost Pt(111) shell. Similarly, the strains for the 3ML- and 4ML-Pt/Pt25Ni75(111) surfaces can be estimated as −1.7% ± 0.4% and −1.2% ± 0.2%, respectively. Figure 1e depicts a ball model of the Moiré pattern that would result from the atomic arrangement of a Pt25Ni75(111) substrate and a −1.2% strained Pt(111) layer. The model is very consistent with the pattern for 4ML-Pt/Pt25Ni75(111) shown in Figure 1d. Figure 2a presents the cyclic voltammograms of the clean Pt(111) and nML-Pt/Pt25Ni75(111) materials. The voltammetric features of the nML-Pt/Pt25Ni75(111) samples in the
Figure 1. (a−d) UHV−STM images for clean Pt25Ni75(111) surface (panel (a)), the 2ML-Pt/Pt25Ni75(111) surface (panel (b)), the 3MLPt/Pt25Ni75(111) surface (panel (c)), and the 4ML-Pt/Pt25Ni75(111) surface (panel (d)): insets are atom models of corresponding surfaces. (e) Top view of ball model of the Moiré pattern, resulting from the atomic arrangement of the Pt25Ni75(111) substrate (blue) and the −1.2% strained Pt(111) lattice (red). 5286
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ACS Catalysis
Figure 3. Degrees of ORR enhancement at 0.9 V for the nML-Pt/ Pt25Ni75(111), relative to Pt(111), as a function of surface strains. Theoretical data are adopted from ref 30.
Actually, according to their predictive theory modified by Stephens et al.,33 a −2.0% strained Pt layer would show ca. 30 times higher ORR activity than pure Pt. In this study, the correlations between the strain and activity enhancements for the 3ML and 4ML-Pt/Pt25Ni75(111), each having −1.7% and −1.2% strained Pt-shells, almost correspond to the theoretical predictions, suggesting that the activities of the 3ML- and 4MLPt surfaces originate from surface strains induced by lattice mismatches. In contrast, although the predicted activity enhancement factor for the −2.6% strained Pt layer (2MLPt/Pt25Ni75(111)) is only ca. 5, the 2ML-Pt surface exhibits ca. 25 times higher activity than clean Pt(111). As mentioned previously, the ORR activity for a Pt-based alloy catalyst can be enhanced by ligand and strain effects. The ligand effect can be effective for modifying the electronic properties of the topmost surface Pt-shell when the surface Pt-shell thickness is just one atomic layer of Pt.34 In this study, the nML-Pt/Pt25Ni75(111) surfaces were fabricated through Pt epitaxial growth followed by thermal annealing under UHV at 673 K. Thus, the effective Pt-shell thickness might be thinner than the nominal deposited thickness through interdiffusion of the surface Pt and substrate Ni atoms (surface models depicted in insets of Figure 1). Therefore, the ligand effect induced by the Ni atoms located in the topmost layers, as well as second layers, would contribute to the anomalous ORR activity enhancement (×25) for the 2-MLPt sample. Next, we performed X-ray photoelectron spectroscopy (XPS) measurements for the nML-Pt/Pt25Ni75(111). Figure 4 summarizes the peak positions of the Pt 4f 7/2 bands for the surfaces. The peak positions shift to lower binding energies with increasing Pt-shell thickness, suggesting that direct electronic effects exerted by the Ni atoms to the topmost (111) shells decrease. Close inspection of the Pt 4f chemical shifts, however, clearly shows that the peak positions of the 2ML-Pt surface are much closer to the energy of the substrate Pt25Ni75(111). Note that the obtained core level (CL) spectra of Pt include signals due both to the Pt25Ni75(111) substrate and surface Pt-shells, because the escape depth of the photoelectrons from the bulk Pt metal is ca. 1.6 nm.35 Therefore, considering the discrete Pt 4f shifts between the 2ML-Pt and 3ML-Pt(111) shells, the electrochemical properties of the 2ML-Pt surface might be dominated by a direct electronic interaction from the substrate Ni atoms to the
Figure 2. (a) Cyclic voltammograms recorded in N2-purged 0.1 M HClO4. (b) Linear sweep voltammograms for ORR recorded at 1600 rpm in O2-saturated 0.1 M HClO4.
hydrogen adsorption and desorption regions (0.06−0.4 V) clearly show a decrease in the electrochemical Hupd charges and the lower shift of onset potentials for the H adsorption and desorption reactions, which are common features of the Pt3Ni(hkl),18 Pt−Co single crystal alloys,29 and Pt/Ni/ Pt(111) surfaces.22,23,30 In contrast, although the onset potentials of the adsorption reactions of hydroxyl-related species are largely shifted to higher potentials for the nMLPt/Pt25Ni75(111) surfaces, the curve shapes and electrochemical charges in the hydroxyl-related regions are quite sensitive to the shell thickness, i.e., the surface strains and electronic interactions of the Pt(111) shells with the substrate Ni atoms. Figure 2b shows the linear sweep voltammograms for the ORR of the clean Pt(111) and nML-Pt/Pt25Ni75(111) surfaces. The half-wave potentials for 2ML-, 3ML-, and 4ML-Pt/ Pt25Ni75(111) shift to higher potentials by ca. 120, 105, and 70 mV, relative to the clean Pt(111), respectively. These results suggest that the ORR activity is dependent on the compressive strain of the Pt(111) topmost lattice. Note that the iRuncompensated half-wave potential of the UHV-annealed Pt(111) surface is located at slightly lower potential than that of hydrogen-flame-annealed Pt(111) measured under almost the same electrochemical experimental conditions.31 This might be due to differences in the surface annealing process. Figure 3 reveals the correlations between the ORR activity enhancements of the nML-Pt/Pt25Ni75(111) and the clean Pt(111) surfaces estimated using the Koutecky−Levich equation32 versus the estimated surface strains. For comparison, previously published strain-based theoretical volcano trends of the activity enhancements are also depicted.5,33 Strasser et al. predicted that a surface strain of −2.0% would be the most beneficial toward enhancing the ORR activity from their experimental results for dealloyed Pt−Cu alloy catalysts.5 5287
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topmost Pt(111) shell. Thus, we deduce, on the basis of the foregoing discussion, that the ligand effect for the 2ML-Pt/ Pt25Ni75(111) is the predominant contributor to the anomalous activity enhancement factor (×25), with respect to clean Pt(111). Although this study focuses on the ORR activity, the electrochemical stability is another important concern for fuel cell application. We are now under investigation for durabilities of the strain-controlled Pt/Pt−Ni(111) surface systems. In summary, we explored the ORR activity enhancements of strain-controlled nML-Pt/Pt25Ni75(111) model catalyst surfaces prepared by MBE under UHV. The surface strains of the 2ML-, 3ML-, and 4ML-Pt(111) shells, estimated from the Moiré patterns in the corresponding UHV-STM images, were −2.6%, −1.7%, and −1.2% versus the pristine Pt(111), respectively. The ORR activity enhancement factors for the corresponding surfaces were ca. 25, 17, and 6. The activity enhancements for the 3ML- and 4ML-Pt(111) surfaces show good correlation with the surface strains. However, the activity for the 2MLPt(111) shell surface deviates from the enhancement factor predicted by the strain-based predictive theory, suggesting that the ligand effect dominates the evaluated activity enhancement. The results discussed in this study provide a new insight for the mechanism of the ORR activity enhancement observed for practical alloy-core NP catalysts.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01466. Experimental and characterization details (PDF)
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Figure 4. Peak positions of the Pt 4f 7/2 band for nML-Pt/ Pt25Ni75(111) surfaces and the corresponding Pt CL spectra.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. 5288
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