Ethanol Oxidation on Well-Ordered PtSn Surface Alloy on Pt(111

Aug 13, 2013 - Surface and subsurface structures of PtSn surface alloy on Pt(111) were determined using in situ scanning tunneling microscopy (STM) an...
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Ethanol Oxidation on Well-Ordered PtSn Surface Alloy on Pt(111) Electrode Masashi Nakamura,†,* Risa Imai,† Naoto Otsuka,† Nagahiro Hoshi,† and Osami Sakata‡ †

Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan ‡ Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, Kouto 1-1-1, Sayo-gun, Hyogo 679-5148, Japan S Supporting Information *

ABSTRACT: Surface and subsurface structures of PtSn surface alloy on Pt(111) were determined using in situ scanning tunneling microscopy (STM) and X-ray diffraction. Different ordered structures of the PtSn alloy layer were observed by STM in HClO4 at coverage of θSn ≤ 0.23. Superstructure of (√3 × √7)R19.1° with small domain size was formed at θSn = 0.23. This structure promoted the catalytic activity for the ethanol oxidation reaction with high durability. X-ray structural analysis showed that the ratio of Sn in the subsurface was below 3(2)%, The PtSn alloy layer was mainly formed at the surface of the Pt(111) electrode. The Sn atoms protruded by 0.02 nm from the Pt layer, which was similar to the surface structure of Pt3Sn(111). One Pt atom in the (√3 × √7)R19.1° structure contacts to one or two surrounding Sn atoms, which lead to the highest activity for the EOR. ethanol oxidation reaction (EOR).10−14 Studies on Sn-modified single crystal Pt electrodes show that the EOR activity depends on the surface structure of Pt substrate and Sn coverage.13,14 However, conventional modification causes dissolution of the adsorbed Sn by the potential cycle up to Sn oxidation potential in H2SO4.13 Moreover, since Sn is randomly adsorbed on welldefined surface, the active site for the EOR is unclear. For the PtSn alloy, the surface structure has been studied by X-ray diffraction and low energy electron diffraction (LEED).15,16 Although p(2 × 2) structure on Pt3Sn(111) is stable in H2SO4, the upper potential limit was restricted to the dissolution potential of Sn (0.6 V vs RHE).17 Surface alloys are good candidates of high-performance electrocatalysts because the specific compositions and structures of their surface layers can promote alcohol oxidation reactions. Well-ordered PtSn surface alloys were prepared on Pt(111) under UHV conditions.18−20 Two different structures, p(2 × 2) and (√3 × √3)R30°, were revealed by LEED and scanning tunneling microscopy (STM). In this study, well-defined PtSn surface alloys were prepared by thermal annealing of Sn-modified Pt(111) in Ar/H2 atmosphere. Surface and subsurface structures of the PtSn surface alloy on Pt(111) were determined by using electrochemical STM and X-ray diffraction. We found some superstructures of the PtSn surface alloy, which are not

1. INTRODUCTION Bimetallic alloy materials are widely used as electrocatalysts for alcohol oxidation and oxygen reduction reactions in fuel cells.1−5 Bimetallic alloy electrodes activate electrochemical reactions by structural and electronic perturbational effects. For example, the methanol oxidation on PtRu and the oxygen reduction reaction on PtNi are enhanced by the bifunctional mechanism and modification of the d-band structure, respectively.4,5 Therefore, the activity of alloy catalysts strongly depends on their composition and surface structure.5−7 In many Pt-base metal alloy electrodes, “Pt-skin” is exposed on the surface layer, as a result of stabilization by penetration of the foreign metal into the subsurface and dissolution of the base metal.5,8 Bimetallic surface alloys are also significant for fundamental surface science because of the novel properties that are not present in any of the parent metals.9 Surface alloying allows the formation of a well-defined alloy layer as well as the control of the coverage of the foreign metal on the substrate. A welldefined surface alloy structure is useful for understanding reaction mechanisms and active sites on surfaces. Many ordered surface alloy systems are formed on substrates by thermal annealing of vapor deposited layers under ultrahigh vacuum (UHV) conditions.9 However there are few applications for electrocatalysts composed of well-defined surface alloys. Ethanol, which can be obtained from biomass, is one of the most suitable fuels for polymer electrolyte fuel cells because of its higher energy density and lower toxicity compared to methanol. PtSn alloy and Sn-modified Pt electrode activate the © XXXX American Chemical Society

Received: July 2, 2013 Revised: August 13, 2013

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density of hydrogen desorption according to the following equation

observed under UHV, and discuss here the correlation between the surface structure of the alloy layer and the EOR activity.

2. EXPERIMENTAL SECTION A Pt(111) disk (Surface Preparation Laboratory, The Netherlands) and a (111) facet of a single crystal Pt bead were used for the measurements of X-ray diffraction and STM, respectively. The samples were annealed in an H2 + O2 flame and then cooled down to room temperature in an Ar atmosphere. The annealed surface was protected with a droplet of ultrapure water (Milli-Q Advantage) and transferred to a cell for Sn modification. The electrolyte solution was prepared with HClO4 (Merck Suprapur), SnSO4 (Aldrich), and ultrapure water. A reversible hydrogen electrode (RHE) was used as a reference electrode for all the measurements. PtSn surface alloy was prepared by immersion in SnSO4 solution. After it was rinsed with water and then annealed at 623 K for 5 min using an induction furnace in Ar + 4% H2. Sn coverage was controlled by the immersion time and SnSO4 concentration: for example, in the case of Sn coverage of 0.20, the immersion time was 3 min and the concentration of SnSO4 was 10 μM. In situ STM measurements were carried out using a Nanoscope IIId (Bruker). W wires were electrochemically etched in 4 M KOH and coated with clear nail polish to minimize the Faradaic current. X-ray diffraction measurement was performed with a multiaxis diffractometer at BL13XU (SPring-8)21 and a fouraxis diffractometer at BL4C (KEK-PF). The X-ray energy used was 12.4 keV. Integrated intensities were measured by rocking scans, and then corrected for Lorentz and area factors. A hexagonal surface coordinate system was used for the Pt(111) crystal in which the reciprocal wave vector is Q = Ha* + Kb* + Lc*, where a* = b* = 4π/√3a, c* = 2π/√6a, a = 0.277 nm, and L is along the direction normal to the surface. Structure refinements were conducted using the least-squares method with the ANA-ROD program.22

θSn =

H Q PtH − Q PtSn

QHPt

Q PtH

(1)

QHPtSn

where and are the charge densities of hydrogen desorption on Pt(111) and Sn-modified Pt(111), respectively.14,23 The spontaneous deposition of Sn in SnSO4 decreases the redox peaks characteristics of bare Pt(111) at 0.8 V. Small redox peaks appear at 0.70 V, which are assigned to OH adsorption/desorption on Sn sites (Figure 1a).13 The charge density of the redox peaks of the surface alloy at 0.70 V is 2.3 times as high as that of Sn-modified Pt(111) (Figure 1b). This increase may be associated with the modification of oxidation processes of Sn by surface alloying. In 0.1 M HClO4, CVs of Sn-modified Pt(111) and PtSn surface alloy on Pt(111) did not change after 100 potential cycles between 0.05 and 0.9 V (not shown here). This fact indicates that the adsorbed Sn atoms are not dissolved in HClO4. Figure 2 shows potential cycle dependence of CVs of Snmodified Pt(111) and PtSn surface alloy on Pt(111) in 0.5 M

Figure 2. Potential cycle dependence of CVs of (a) Sn-modified Pt(111) and (b) PtSn surface alloy in 0.5 M H2SO4 at the first cycle (red) and after the 100th cycle (blue). Sn coverage is 0.20. Dashed lines represent voltammograms of bare Pt(111) in 0.5 M H2SO4. Scanning rate is 0.05 V s−1.

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry of PtSn Surface Alloy on Pt(111). Figure 1 shows cyclic voltammograms (CVs) of Snmodified Pt(111) and PtSn surface alloy on Pt(111) in 0.1 M HClO4. The Sn coverage (θSn) was estimated using the charge

H2SO4 at θSn = 0.20. The formation of PtSn alloy eliminates the spike peak due to the order−disorder structural transition of adsorbed (bi)sulfate at 0.45 V, while broad redox peaks of (bi)sulfate adsorption/desorption appear at 0.48 V (Figure 2b). After 100 cycles, the charge density of hydrogen adsorption/ desorption of Sn-modified Pt(111) returns to that of bare Pt(111) (Figure 2a). Sn adatoms on Pt(hkl) are dissolved above 0.64 V in H2SO4.13 Base metals in Pt alloy are often dissolved by applying positive potential at which the metals are oxidized. However, the PtSn surface alloy is not affected by the potential cycle up to 0.9 V in H2SO4 (Figure 2b). This result indicates that the Sn atoms incorporated into the surface alloy are not removed by anodic dissolution. Hayden et al. prepared PtSn surface alloy on Pt(111) under UHV conditions, which gives redox peaks corresponding to OH adsorption/desorption at 0.2−0.3 V in HClO4 and H2SO4.24 CV of Pt3Sn(111) shows redox peaks of (bi)sulfate adsorption/desorption at 0.40 V.15 These voltammetric peaks characteristics of PtSn alloys are not observed on our PtSn surface alloy prepared by the annealing in Ar/H2. The atomic structure of the PtSn alloy layer is also different, depending on

Figure 1. CVs of (a) Sn-modified Pt(111) and (b) PtSn surface alloy on Pt(111) with Sn coverages of 0.13 (red) and 0.27 (blue) in 0.1 M HClO4. Dashed lines represent voltammograms of bare Pt(111) in 0.1 M HClO4. Scanning rate is 0.05 V s−1. B

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Figure 3. Voltammograms of the ethanol oxidation on the PtSn alloys on Pt(111) at Sn coverages of (a) 0.17, (b) 0.23, and (c) 0.62 in 0.1 M HClO4 containing 0.2 M ethanol. Dashed lines represent voltammograms of bare Pt(111). Scanning rate is 0.05 V s−1. (d) Maximum anodic current density (jp) plotted against Sn coverage.

× √7)R19.1° structure on fcc(111) in H2SO4.25,26 However, the (√3 × √7)R19.1° structure observed on the PtSn alloy appears at potentials below 0.4 V, where no anion is adsorbed as shown in Figure 2b. This result indicates that the origin of the ordered structure is not the adsorbed anions but the surface metal atoms. At lower Sn coverage (θSn = 0.07), different ordered structure is observed as shown in Figure 4c. The unit cell estimated from the lattice vectors corresponds to 3 × 4 structure. The superstructures remain unchanged after potential cycles between 0.05 and 0.9 V. This fact clearly shows that the alloy layers are stable. The roughness of the spots on these images is below 0.02 nm, which is smaller than the layer spacing and atomic diameter. These results suggest that Sn is not adsorbed as an “adatom” but is incorporated into the atomic layer. However STM images do not always show the position of the surface atom directly, but rather it is a probe of the local density of electronic states. Therefore we cannot determine the corrugation structure of alloy layer and subsurface structure from the STM image. The domain size of the (√3 × √7)R19.1° structure is below 5 nm, whereas that of the 3 × 4 structure is about 15 nm. Although (√3 × √3)R30° and p(2 × 2) structures were reported on UHV prepared PtSn surface alloys on Pt(111), we could not observe any superstructure image for the PtSn alloy layer at θSn > 0.23. 3.3. X-ray Structural Analysis of PtSn Surface Alloy on Pt(111). We determined the out-of-plane structure of the PtSn surface alloy on Pt(111) by X-ray specular crystal truncation rod (CTR) measurement, which can determine the projected electron density along the surface normal direction. Although the STM images show the (√3 × √7)R19.1° and 3 × 4 structures, no fractional order rods were found during in-plane scans because of the small domain sizes of the superstructures. Figure 5a shows the specular CTR profile of the PtSn alloy on Pt(111) in 0.1 M HClO4 at θSn = 0.23. The STM image obtained at θSn = 0.23 shows the small island that is formed by the excess Pt atoms resulting from the formation of the PtSn alloy layer (Figure 4a). The model of the structural optimization consists of a Pt island layer, three layers of PtSn alloy and bulk Pt layers as shown in Figure 5b. The vertical positions, occupancy factors, and Debye−Waller factors of Pt and Sn are optimized by the least-squares method. Figure 5c shows the optimized model for the PtSn alloy on Pt(111) at θSn = 0.23. The occupancy factors of Pt at the island layer and at the first alloy layer are 0.17(1) and 0.72(1), respectively. The occupancy factor of Sn at the first alloy layer is 0.27(2), which is

the preparation method, as described below. The different electrochemical properties may be due to the different atomic arrangements at the topmost layer and the subsurface. We investigated the activity for the EOR on PtSn surface alloys on Pt(111) at various Sn coverages in HClO4 where no anion is strongly adsorbed on the surfaces. Figure 3 shows the EOR voltammograms and Sn coverage dependence of maximum anodic current density (jp) of the PtSn surface alloys on Pt(111) in 0.1 M HClO4 + 0.2 M ethanol. A current peak of the EOR was observed above 0.3 V, depending on the Sn coverage, and the onset potential is negatively shifted after the alloying. Small shoulder peaks at 0.7−0.8 V can be assigned to the oxidation of adsorbed carbon monoxide (CO) that is generated as an intermediate species (Figures 3b,c). Lamy et al. reported that intermediate CO is oxidized at 0.7 V on PtSn alloy electrode during the EOR.12 We estimate the activity for the EOR using jp. The EOR activity increases with the increase of Sn coverage up to θSn = 0.23, and then decreases at θSn > 0.23 as shown in Figure 3d. The EOR activity at θSn = 0.23 is twice as high as that on bare Pt(111), and the coverage of Sn with the highest EOR activity is consistent with that of the Snmodified Pt(111).14 3.2. STM Images of PtSn Surface Alloy on Pt(111). Figure 4 shows STM images of PtSn surface alloys on Pt(111)

Figure 4. STM images of PtSn surface alloys on Pt(111) in 0.1 M HClO4 at 0.4 V vs RHE. Sn coverages are (a,b) 0.23 and (c) 0.07. The tip bias and the tunneling current were (a) −0.10 V and 8.0 nA, (b) −0.10 V and 12.0 nA, and (c) −0.10 V and 6.0 nA, respectively.

in 0.1 M HClO4. At θSn = 0.23, a 3-fold rotated domains with an ordered structure are observed as shown in Figure 4a. The bright region, which covers about 15% of the area, is an island with an atomic height steps. In the high-resolution image, the lattice vectors of the unit cell, superimposed in Figure 4(b), are a = 0.5 and b = 0.7 nm with an angle of 71°. This superstructure can be assigned to the (√3 × √7)R19.1° structure of the PtSn alloy layer, which is comparable to the Sn coverage estimated using eq 1. Adsorbed anions are often arranged in the form of superstructures, for example, the (√3 C

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Figure 5. (a) Specular CTR profile of the PtSn surface alloy on Pt(111) in 0.1 M HClO4 at θSn = 0.23 at 0.4 V. Solid line is structure factor calculated from the optimized model. Schematic side views of (b) the model used for structural optimization and (c) the model of the PtSn surface alloy on Pt(111) after optimization.

(√3 × √7)R19.1° structure is formed. X-ray structural analysis shows that the PtSn alloy is mainly formed on the first layer on Pt(111). The obtained PtSn alloy layer has a high durability against the potential cycle up to the dissolution potential of Sn.

consistent with the Sn coverage estimated from CV. Sn atoms in the first alloy layer protrude by 0.23(4) Å from the Pt atoms in the surface alloy layer. Similar protrusion of Sn in the surface layer is observed on Pt3Sn(111).15 The occupancy factors of Sn at the second and the third alloy layers are below 0.03(2), which indicates that the PtSn alloy layer is only formed on the surface layer. STM results indicate that the domain sizes of the superstructures depend on Sn coverage, being smaller at θSn = 0.23 than at θSn = 0.07. The incorporation of Sn into the surface layer causes lattice expansion against the subsurface layer because the atomic radius of Sn is larger than that of Pt. This lattice expansion inhibits the formation of large domain size of the superstructures at high Sn coverage. The negative shift of the onset potential of anodic current indicates the enhancement of catalytic activity at the initial step for the EOR. The EOR is a complex multistep reaction,27−29 although theoretical calculations of the EOR on PtSn alloy clusters suggest that α-hydrogen (α-H) is adsorbed at the Pt sites next to the Sn atoms during initial dehydrogenation of ethanol.30 The α-H adsorption is a rate-determining step of dehydrogenation of ethanol. The alloying of Sn stabilizes the αH adsorption by a positive ligand effect.30,31 Adsorbed OH species on the Sn site will promote to oxidation of adjacent adsorbed CO and other intermediate species according to the bifunctional mechanism.4 More experiments, such as analysis of the reaction products, are necessary for the elucidation of the reaction path of the EOR. An increase in the Sn coverage leads to an increase in the number of active sites for the dehydrogenation. However, at Sn coverage above 0.25, the number of Pt sites next to the Sn atoms decreases. One Pt atom in the (√3 × √7)R19.1° structure contacts to one or two surrounding Sn atoms, which lead to the highest activity for the EOR.

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ASSOCIATED CONTENT

S Supporting Information *

Structural parameters determined by X-ray diffraction. This material is available free of charge via the Internet at http:// pubs.acs.org.

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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 X-ray measurements were supported by JASRI/SPring-8 (2012A1208) and KEK/PF (2012G168). This work was supported by JSPS KAKENHI Grant 24651131.

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REFERENCES

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4. CONCLUSIONS PtSn surface alloy on Pt(111) was prepared by annealing in Ar/ H2 atmosphere. The surface and subsurface structures were determined by STM and X-ray diffraction. STM results exhibit two different ordered structures, (√3 × √7)R19.1° and (3 × 4), for the PtSn alloy layer. PtSn surface alloy has the highest activity for the ethanol oxidation reaction at θSn = 0.23 where D

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