Enhancement in Ethanol Electrooxidation by SnOx Nanoislands

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Enhancement in Ethanol Electrooxidation by SnOx Nanoislands Grown on Pt(111): Effect of Metal OxideMetal Interface Sites Wei-Ping Zhou,*,† Stephanus Axnanda,† Michael G. White,†,‡ Radoslav R. Adzic,† and Jan Hrbek† † ‡

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States Department of Chemistry, Stony Brook University, Stony Brook New York 11974, United States ABSTRACT: An integrated surface science and electrochemistry approach has been used to prepare and characterize SnOx/ Pt(111) model catalysts and evaluate their electrochemical activity for the ethanol oxidation reaction (EOR). Nanoislands of SnOx are deposited onto the Pt(111) by reactive layer assisted deposition in which Sn metal is vapor deposited onto a Pt(111) surface precovered by NO2. X-ray photoelectron spectroscopy (XPS) shows that the SnOx islands are highly reduced with Sn2+ being the dominant chemical species. After exposing the SnOx/Pt(111) surface to H2O or an electrolyte solution, XPS provides evidence for a significant amount of H2O/OH adsorbed on the reduced SnOx surfaces. Electrochemical testing reveals that the catalytic performance of Pt(111) toward ethanol electrooxidation is significantly enhanced with SnOx islands added onto the surface. The enhanced EOR activity is tentatively attributed to the efficient removal of COads-like poisoning species at Pt sites by oxygen-containing species that are readily formed on the SnOx nanoislands. Moreover, the strong dependence of the EOR activity on SnOx coverage provides experimental evidence for the importance of SnOxPt interface sites in the EOR.

1. INTRODUCTION Liquid fuel powered fuel cells are promising as convenient power sources for portable electronics, transportation auxiliary power, and midscale applications. The most attractive liquid fuel, considering its availability, energy density, and environmental impact, is ethanol.1 However, further development of ethanolpowered fuel cells is seriously hindered by slow kinetics and inefficient oxidation of ethanol to CO2 on available electrocatalysts.24 As a result, considerable effort has been focused on the development of electrocatalysts to enhance ethanol oxidation activity and, at the same time, to advance understanding of the reaction mechanism.28 Platinum-based electrocatalysts that are promoted by SnOx deposits have recently been shown to have high performance for the ethanol oxidation reaction (EOR).38 The enhanced activity of Pt/SnOx nanocatalysts was suggested to be the result of a bifunctional mechanism in which activated water provided by SnOx at low potentials promoted the oxidative removal of COlike surface poisoning species on Pt sites.3,610 However, the performance of the Pt/SnOx nanocatalysts were reported to be strongly dependent on the preparation method and activation conditions.3,5,10 Moreover, the complexity of powder nanocatalysts has led to some controversy regarding the nature of the active phase and the optimal surface composition.36,10,11 Therefore, an issue of fundamental importance for developing Pt/SnOx binary and ternary catalysts is to understand the possible correlation between the surface composition and the chemical properties of SnOx and EOR activity. Metal oxide nanostructures deposited on metal single crystals are often used as “inverse” models of more complex powder catalysts r 2011 American Chemical Society

found in heterogeneous catalysis but rarely in electrocatalysis.8,1216 Fundamental studies of the electronic and structural properties of such inverse catalyst systems revealed that the metal/oxide interface plays a crucial role in enhancing catalytic activity.1317 Moreover, the chemical properties and catalytic activity of the metal oxide thin film could be “modified” by varying the film thickness and metal substrate materials.12 With large numbers of low-coordination and defect sites, small oxide nanoparticles are found to be more active than larger bulk-like islands.15,16 Therefore, introducing the metal oxide nanoparticles on metal surfaces could be an alternative approach for enhancing the activity of mixed metal/metal oxide catalysts. The growth of monolayer and multilayer tin oxide thin films on a Pt(111) surface has been previously reported by oxidation of Sn/Pt(111) surface alloys1820 and bulk Pt3Sn(111) alloys.21,22 After exposure of a Pt3Sn(111) surface to oxygen at high temperature a (4 4) overlayer structure was observed. However, the Sn 3d XPS spectra did not show any significant change before and after oxidation, suggesting that the Sn atoms that are bound to the oxygen atoms are still in the Pt/Sn alloy environment.22 Batzill et al. studied the oxidation of Pt/Sn surface alloys by NO2.19,20 They reported a (4 4) surface structure for monolayer and multilayer coverages of tin oxide grown on Sn/Pt(111) surface alloys. On the basis of STM and XPS studies, a StranskiKrastanov growth model was proposed in which a wetting oxide layer is completed before formation of crystalline Received: April 22, 2011 Revised: June 28, 2011 Published: July 02, 2011 16467

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The Journal of Physical Chemistry C tin oxide.19,20 However, XPS studies indicated formation of a mixed tin oxide surface composition with Sn4+ and Sn2+ and metallic Sn species even after repeated oxidationannealing cycles of a Sn/Pt surface alloy by NO2.19,20 Herein, we report for the first time an electrochemical study of the EOR on SnOx supported on a Pt(111) surface. The SnOx/ Pt(111) model electrode was prepared in ultrahigh vacuum (UHV) by vapor deposition of Sn onto Pt(111) under reactive gas environment (NO2 here) and then characterized with various techniques including XPS, low-energy ion scattering spectroscopy (ISS), low-energy electron diffraction (LEED), chronoamperometry (CA), and cyclic voltammetry (CV). The SnOx/Pt interface of the model electrode closely approximates real Pt/SnOx powder catalysts in terms of both reactivity and composition of reactive sites. Moreover, depositing SnOx on a Pt singlecrystal substrate allows us to focus on the promotional effect of the tin oxides in the ethanol catalytic processes. We show that the EOR activity of the Pt(111) surface is greatly enhanced after depositing SnOx. Moreover, the strong dependence of the EOR activity on SnOx coverage provides experimental evidence for the importance of SnOx/Pt interface sites in the EOR reaction mechanism.

2. EXPERIMENTAL METHODS The UHV apparatus consists of a surface analysis chamber, a transfer chamber, and a high-pressure chamber where an electrochemical reaction cell was attached along with an in vacuo transfer system. The model SnOx/Pt(111) electrodes can be transferred between these chambers, which are separated by gate valves, without exposure to air.23,24 The surface analysis chamber, with a base pressure below 1.5 1010 Torr, contains LEED (SPECS GmbH), evaporator (Omicron GmbH), quadrupole mass spectrometry with a liquid-nitrogen-cooled shroud (Hiden Analytic), and a hemispherical electron/ion analyzer with multichannel detector (Thermo Fisher Scientific Instruments; Alpha110) equipped with a twin-anode X-ray source (XR3) for XPS and an ion source (EX03) for ISS. Photoemission spectra were measured at a constant pass energy of 20 eV using Mg KR X-ray radiation (hν = 1253.6 eV). The Avantage software (Thermo Fisher Scientific Instruments) was used to record and process the XPS and ISS data. The binding energy was calibrated by setting the Pt 4f7/2 level to 71.1 eV for a clean Pt(111) surface.25 A differentially pumped ion gun was used for ISS measurements. Ion beam currents measured at a positive bias were less than 10 nA/cm2 with a 0.5 KeV Ne+-ion beam at a background Ne pressure of 5 108 Torr. The angle of incidence (angle between the surface and the ion beam) of the Ne+ ions was 45°, and the takeoff angle was 90° to avoid shadowing effects; the average backscattering angle was 135°. Sputtering of the surface by Ne+ ions was kept to a minimum by using low ion energies (0.5 keV) and short exposure times. The Pt(111) disk (Princeton Scientific, 99.99%) was 4 mm thick with a 8 mm diameter and oriented to better than (1°. The Pt(111) crystal was mounted onto a home-built sample holder by Ta wires spot welded to the sides of the crystal. The sample holder provides capabilities for resistive heating to 1100 K, cooling to 250 K with liquid N2, and sample transfer between the UHV surface science and electrochemical chambers. A K-type (chromelalumel) thermocouple was spot welded to the back of the crystal for temperature measurements. The Pt(111) electrode was cleaned by cycles of argon-ion bombardment and

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high-temperature annealing (including repeated heating of the electrode in oxygen at 107 Torr). The surface quality was monitored by XPS and LEED. Oxygen (99.998%, Matheson), NO2 (99.95%, Matheson), and CO (99.9%, Matheson) were admitted to the system via leak valve for background dosing, and the exposures are given in Langmuir (1 L = 1 106 Torr/s). Sn (99.98%, Alfa-Aesar) was vapor deposited by heating an ultrapure ingot of tin metal. The ingot of Sn metal was placed in a ceramic tube with a Ta pocket at the bottom of the tube that could be resistively heated. Calibration of the relative ISS scattering signals for Pt and Sn was performed on a well-defined p(2 2) Sn/Pt(111) surface that has a sharp LEED pattern and a known surface coverage of Sn (0.25 ML). Determination of the Sn coverage was based on the attenuation of the Pt 4f XPS peak intensity and calculated inelastic electron mean free paths.26 The Sn coverage is reported with respect to the number of Pt (111) surface atoms (1.51 1015 atoms/cm2), i.e., 1 MLE corresponds to one Sn adatom per substrate surface atom. Electrochemical measurements were carried by immersing the electrode into a modified three-electrode electrochemical cell that was directly attached onto the UHV chamber. The electrochemical cell is similar to that of previously reported design.23,24 An Autolab 12N potentiostat was used for electrochemical measurements. Chemicals used were H2SO4 (Optima, Fisher Scientific), ethanol (Sigma Aldrich), and ultrapure water (Millipore). Before each EC measurement, the solution was deaerated with ultrapure Ar (5N) for about 45 min in order to remove dissolved oxygen. A Ag/AgCl/satd NaCl electrode was used as a reference electrode, but potentials are quoted with respect to the reversible hydrogen electrode, RHE, at the same temperature.

3. RESULTS AND DISCUSSION 1. UHV Preparation and Characterization of SnOx/Pt(111) Surfaces. In the present work, we employed reactive-layer-

assisted deposition (RLAD) to prepare SnOx/Pt(111) surfaces, namely, the Sn was deposited on a Pt(111) surface precovered by NO2 at 250 K and then heated to 600 K under an atmosphere of NO2 (at 5 108 Torr). While a (4 4) LEED pattern was reported on the oxidized Sn/Pt(111) surface alloy,19,20 the SnOx/Pt(111) surface prepared by RLAD exhibits only a very weak (1 1) LEED pattern with high background, suggesting growth of three-dimensional SnOx islands rather than an ordered two-dimensional thin film. The RLAD has been previously reported for successfully preparing a wide range of supported nanoparticles of transition metal carbides and oxides on metal substrates.27,28 Growth of ceria and titania on Au(111) prepared by this method has been recently reported.29 The final product compound left on the surface is in the form of an ensemble of nanoparticles, and alloy formation is usually avoided.29 Core-level XPS measurements were used to obtain chemical state information on the tin oxide deposits as well as to monitor the total amount of Sn deposited. Figure 1a displays Sn 3d5/2 spectra for UHV-prepared SnOx/Pt(111) surfaces. The Sn 3d5/2 data show a line shape that can be fitted to two overlapping peaks using a least-squares fitting routine. The larger peak with low binding energy (486.1 ( 0.1 eV) can be attributed to Sn2+, while the smaller peak at higher binding energy (487.2 ( 0.1 eV) can be attributed to Sn4+ species.20,3032 The core-level binding energy difference between Sn2+ and Sn4+ is consistent with literature reported values.20,3032 To further verify the oxidation state of 16468

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Figure 1. Sn 3d5/2 XPS (a) and ISS spectra (b) of the SnOx/Pt(111) surface with 1.2 MLE (A), 0.7 MLE (B), and 0.4 MLE (C) of Sn deposits prepared by the RLAD method.

Sn, we also prepared a (2 2)-Sn/Pt(111) surface, and the XPS Sn 3d5/2 spectrum displayed a single peak centered at 485.4 eV for metallic Sn (indicated by the dotted line in Figure 1a). The RLAD method leads to formation of mainly reduced SnOx on Pt, with the Sn2+ signal accounting for 8590% of the detected Sn signal. Additional evidence for this assignment is the O1s to Sn 3d5/2 peak intensity ratio which is only 1.21.6 after correction

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for the XPS sensitivity factors for the O 1s and Sn 3d5/2 levels. Note that annealing of the as-prepared SnOx/Pt(111) surface to 700 K under NO2 atmosphere (at 1 107 Torr) did not result in any significant change of the Sn2+/Sn4+ intensity ratio. These results suggest that reduced Sn2+ oxides are stabilized on a Pt(111) surface prepared under UHV conditions, whereas SnO2 is the more stable form of bulk tin oxide.20 A significant amount of reduced CeOx stabilized on Au(111) has been previously reported.13,14,29 In addition, XPS shows no evidence of the oxidation of Pt during the SnOx preparation process. Low-energy ion scattering spectroscopy (ISS), a very sensitive probe of surface atomic composition, was used to measure the relative surface concentrations of Sn and Pt. The choice of Ne+ ions for ISS was dictated by the ability to resolve the elements Pt and Sn adequately in the outermost surface layer. The ISS results obtained with 0.5 keV Ne+ (Figure 1b) exhibit scattering peaks corresponding to Sn and Pt surface atoms,33 indicating a partial coverage of SnOx on the Pt(111) surface (see numerical values in Table 1). With a tripling of Sn deposits (from 0.4 MLE to 1.2 MLE), ISS measurements only show a 11 atom % increase in the covered Pt surface sites. This result strongly suggests that SnOx deposits grow as 3-D islands on Pt(111) (VolmerWeber growth mode) when prepared by the RLAD method.28,29 2. Electrochemical Study of SnOx/Pt(111) Surfaces. After surface preparation and characterization, the SnOx/Pt(111) electrode was transferred in vacuo from the UHV chamber into an electrochemical cell. Figure 2 shows a representative CV of a Pt(111) surface covered by SnOx in 0.1 M H2SO4 solution. The positive potential window was set at 0.50 V since a loss of Sn was detected by XPS after exposing the SnOx/Pt(111) surface to higher potentials. The CV features related to H adsorption and desorption (Hads/des) on bare Pt(111) were not clearly observed in the presence of SnOx, which may be due to the superimposed pseudocapacitive current generated from the reversible conversion of OHSnOx to SnOx. We also did not observe any features related to OHads/des that were previously reported on a (2 2) Pt/Sn surface alloy at 0.28 V/0.15 V.34 The featureless CV curve obtained for the SnOx/Pt(111) surface suggested that the conventional method of measuring Hads/des charge could not be used to calculate the concentration of Pt surface sites. Figure 3 compares polarization curves for ethanol oxidation on a 0.4 MLE SnOx/Pt(111) surface and a clean Pt(111) surface in 0.5 M ethanol in 0.1 M H2SO4 solution at room temperature. The CV obtained from the SnOx/Pt(111) surface does not change in the subsequent cycles in the measured potential region. The Pt(111) surface evinces a low activity with an onset potential at about 0.42 V. By comparison, the 0.4 MLE SnOx/Pt(111) surface exhibits a significantly higher activity as well as a lower onset potential ∼0.25 V. Moreover, the SnOx/Pt(111) surface also demonstrated an increased current density relative to that of Pt(111) surface in the potential range of 0.250.50 V in both positive and negative scan directions. Chronoamperometric (CA) (i.e., current versus time) data were recorded to test the activity and stability of SnOx/Pt(111) surfaces with different amounts of Sn deposited. The UHVprepared surfaces were immersed in an ethanol-containing solution at 0.45 V under potential control to record the CA data. Figure 4 shows the CA plots for ethanol oxidation on three SnOx/Pt(111) surfaces and a clean Pt(111) surface. Clearly, all three SnOx/Pt(111) surfaces exhibit higher current density (normalized to geometric area), i.e., higher activity, at 0.45 V than the Pt(111) surface over the entire reaction time. However, 16469



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Table 1. Total Amount of Sn Deposits, Fraction of Pt(111) Surface Covered by SnOx, and Current Density (normalized to geometric surface area) for Ethanol Oxidation (measured at 1 and 150 s reaction time) current density (μA cm2) at 0.45 V samples

XPS total Sn (MLE)

ISS (SnOx coverage) (atom %)

1s

150 s

Pt(111)

0

0

70.9

1.6

SnOx/Pt(111) (A)

1.2

48

158.7

22.5

SnOx/Pt(111) (B) SnOx/Pt(111) (C)

0.7 0.4

40 37

282.3 439.3

73.5 98.7

Figure 2. Cyclic voltammetry of the 0.7 MLE SnOx/Pt(111) surface in argon-saturated 0.1 M H2SO4 solution. Sweep rate is 50 mV/s.

Figure 4. (a) Ethanol oxidation current densities measured for SnOx/ Pt(111) surfaces with three different amounts of Sn deposits and clean Pt(111) in 0.5 M CH3CH2OH in 0.1 M H2SO4 at 0.45 V vs RHE. The current densities shown correspond to SnOx/Pt(111) surfaces with Sn deposits of 0.4 MLE (red line), 0.7 MLE (blue line), and 1.2 MLE (green line) and Pt(111) (black line). (b) Current density for the EOR on a Pt(111) surface partially covered with SnOx at 150 s and at 0.45 V. The fraction of Pt surface sites covered by SnOx was obtained from the ISS data.

Figure 3. Comparison of currentpotential polarization curves in positive- (solid line) and negative-going (dashed line) sweep directions for a 0.4 MLE SnOx/Pt(111) surface (red line) and a Pt(111) surface (black line) in 0.5 M CH3CH2OH in 0.1 M H2SO4. Sweep rate is 10 mV/s.

the activity enhancement and the current decay rate depend on the surface composition, as the current density is different for

each SnOx/Pt(111) surface at a given reaction time. At long reaction times the current decay was small, indicating that the current was almost stabilized after 150 s. Two different reaction times (1 and 150 s) were analyzed for changes of reactivity; the numerical data are listed in Table 1. The results in Table 1 and Figure 4 clearly show that the clean Pt(111) surface is incapable of producing significant current at a reaction time of 150 s. With SnOx added to the surface, a significant enhancement of catalytic activity is observed at 150 s, with a maximum current for small SnOx deposits. On the basis of the activity change between 1 and 150 s, the estimated current decay rate is similar on all three SnOx/Pt(111) surfaces with a slightly slower rate on surfaces with smaller amounts of SnOx. To emphasize the correlation 16470



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electrolyte (i.e., after activity measurement) were studied. In each case, the electrochemical chamber was evacuated immediately after measurements and the electrode was quickly transferred back to the surface analytical chamber for XPS measurements. Figure 5 shows a comparison of the Sn 3d5/2 spectra and the corresponding O 1s spectra from a 0.4 MLE SnOx/Pt(111) surface before and after exposure to H2O and electrolyte solution. For both exposures, changes in the Sn 3d5/2 line shapes indicate an increase in the Sn4+component, probably due to adsorption of H2O on the SnOx surfaces. The latter is supported by the corresponding O 1s spectra in Figure 5b. Upon exposing the SnOx/Pt surface to H2O or electrolyte solution, the O 1s spectra are dominated by a feature at 532.2 eV (Figure 5b). Note that similar changes were observed in the Sn 3d and O 1s XPS spectra before and after reaction for all three SnOx/Pt(111) surfaces studied. The O 1s peak at 532.2 eV was previously assigned to an intermediate state between adsorbed H2O and OH groups adsorbed on SnOx.31,32 The lattice oxygen and chemisorbed oxygen on SnOx could be replaced by water and/ or a OH group to form Sn(OH)x compounds.31,32 It has been suggested that a direct redox reaction of Sn(OH)4 with COads could be responsible for the enhanced activity of methanol oxidation on PtSn mixture based on the standard potentials of the Sn(OH)2/Sn(OH)4 couple (E0 = 0.075 V vs NHE at room temperature) in acidic media.9 Although the XPS results may not accurately reflect the “in-situ” chemical composition of Sn during EOR due to the possibility of postreactions occurring under UHV conditions,35 the similarities of the SnOx surface composition after exposure to H2O or the electrolyte solution undoubtedly suggest a strong interaction of SnOx surfaces with water. Note that water desorbs from metallic Sn on Pt/Sn surfaces below room temperature under UHV conditions.36 It is generally accepted that the EOR on Pt in acidic media follows parallel reaction paths, i.e.,3743 C2 H5 OH f CH3 CHO f CH3 COOH ðpartial oxidationÞ C2 H5 OH f CO þ CHx f CO2 ðcomplete oxidationÞ

Figure 5. XPS spectra of Sn 3d5/2 (a) and O 1s (b) obtained for a 0.4 MLE SnOx/Pt(111) surface after exposure to H2O (A) and after contacted with the electrolyte (B). The corresponding SnOx/Pt(111) surface after UHV preparation is also shown for comparison.

between the activity and the surface SnOx concentration, the current densities were plotted as a function of SnOx coverage (Figure 4b). Clearly, the activity exhibits a strong dependence on SnOx surface concentration. Here, the surface with a 37% SnOx coverage (0.4 MLE Sn) exhibits the highest current density which is almost a two-orders of magnitude greater than that of bare Pt(111) at 0.45 V. The CA data again demonstrate the enhanced catalytic performance of SnOx/Pt(111) surfaces seen in the potentiodynamic measurements in Figure 3. The chemical properties of the deposited SnOx following exposure to H2O vapor (i.e., before dipping into the cell) and

Although the partial oxidation of ethanol was suggested to be the main reaction pathway, recent studies indicated that CC bond breaking can readily occur at step and defect Pt sites at low potentials.3743 In fact, Chang et al.42 reported a significant coverage of COads following ethanol adsorption on a Pt(111) electrode at 0.10 V from an in situ FTIR study. Removal of the COads and CHx intermediates at low potentials is likely to be the rate-determining step for the EOR on a Pt electrode.3743 Therefore, the observed fast decay of the activity of bare Pt (111) surfaces (see Figure 4) may be due to formation of surface COads and CHx poisoning species, presumably by ethanol decomposition at surface defect sites. The electrochemical oxidation of surface COads on Pt in acidic media proceeds via a LangmuirHinshelwood-type reaction. Thus, oxidative removal of surface COads on Pt sites can be significantly enhanced if a catalyst component can activate H2O at low potentials at nearby sites (the bifunctional mechanism).44,45 The strong interactions between H2O and SnO2 surfaces can result in spontaneous breakage of one of the OH bonds.8 Density function theory (DFT) calculations also suggest that reduced oxide nanoparticles can facilitate H2O dissociation.13,14 Our XPS results provide experimental evidence of the strong interaction between water and SnOx. Therefore, we tentatively assign 16471


The Journal of Physical Chemistry C the enhanced EOR activity of SnOx/Pt(111) surfaces to the efficient removal of COads poisoning species at Pt sites by oxygen-containing species that are readily formed on the SnOx nanoparticles. A similar mechanism has been proposed for the enhanced activity of methanol oxidation on SnOx/Pt when compared with Pt.9,10,46 Formation of COads species requires SnOx modifier, which by itself, however, is inactive toward ethanol electrooxidation at room temperature.6 In addition, deposition of SnOx on a Pt surface decreases the available Pt surface sites for ethanol decomposition. Thus, it is expected that an optimized SnOx/Pt catalyst for the EOR can only be achieved with a balance between available Pt and SnOx sites on surface. The high activity and stability at low SnOx coverage not only confirms the cooperative effect of oxidemetal interface in the EOR but also indicates the special chemical properties of tin oxide islands. Batzill et al.20 suggested that tin oxide films on Pt that are only 23 layers thick exhibit bulk-like oxide properties. Thus, the SnOx structures formed at 1.2 MLE may behave more bulk-like than those formed at 0.4 MLE coverage. The “volcano” dependence of the EOR activity on the surface composition (see Figure 4b) may be attributed to the “ensemble” effect governed by the presence of active sites at the PtSnOx boundary.47,48 More work is necessary to understand the mechanisms for the current decay. Incomplete ethanol oxidation may also occur on SnOx/Pt surfaces, and adsorption of acetic acid on Pt sites could lead to activity decay.3,6 This issue may possibly be solved by introducing a third catalytic component like Rh, which is known to increase the efficiency of CC bond breaking and decrease formation of acetic acid.7,8 Further studies are in progress to investigate the interaction of Rh with SnOx and Pt and its effects on the activity and selectivity of the EOR.

4. SUMMARY We used an integrated surface science and electrochemistry approach to prepare and characterize SnOx/Pt(111) model catalysts and evaluate their electrochemical activity for ethanol oxidation. The RLAD method provides a simple and reproducible method for the formation of reduced SnOx islands on Pt. The Sn2+ signal accounts at least for 85% of the detected Sn. The reactivity of the Pt(111) toward the EOR is significantly enhanced by adding the SnOx islands onto the surface. Furthermore, the activity of SnOx/Pt catalysts strongly depends on the surface SnOx coverage, indicating the importance of the SnOx/Pt interface sites in the EOR. Upon exposing the SnOx/Pt(111) surface to H2O and electrolyte, XPS data show a significant amount of H2O/ OH adsorbed on the reduced tin oxide surfaces, which could be critical for efficient oxidative removal of COads poisoning species at Pt sites at potentials relevant to fuel cell applications. Our combined surface science and electrochemistry approach provides new insight into understanding the chemical properties of tin oxide and tin oxide/Pt interface sites in ethanol electrolysis. ’ AUTHOR INFORMATION Corresponding Author

*Phone: 631-344-7298. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported in part by the U.S. Department of Energy, Divisions of Chemical and Material Sciences, under

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contract no. DE-AC02-98CH10886. W.P.Z. and S.A. are thankful for the financial support from the LDRD program, Brookhaven National Laboratory.

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