Measurement of Benzenethiol Adsorption to Nanostructured Pt, Pd

pubs.acs.org/Langmuir © 2010 American Chemical Society

Measurement of Benzenethiol Adsorption to Nanostructured Pt, Pd, and PtPd Films Using Raman Spectroelectrochemistry Michael B. Pomfret, Jeremy J. Pietron,* and Jeffrey C. Owrutsky Chemistry Division, U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375 Received October 28, 2009. Revised Manuscript Received January 13, 2010 Raman spectroscopy and electrochemical methods were used to study the behavior of the model adsorbate benzenethiol (BT) on nanostructured Pt, Pd, and PtPd electrodes as a function of applied potential. Benzenethiol adsorbs out of ethanolic solutions as the corresponding thiolate, and voltammetric stripping data reveal that BT is oxidatively removed from all of the nanostructured metals upon repeated oxidative and reductive cycling. Oxidative stripping potentials for BT increase in the order Pt < PtPd < Pd, indicating that BT adsorbs most strongly to nanoscale Pd. Yet, BT Raman scattering intensities, measured in situ over time scales of minutes to hours, are most persistent on the film of nanostructured Pt. Raman spectra indicate that adsorbed BT desorbs from nanoscale Pt at oxidizing potentials via cleavage of the Pt-S bond. In contrast, on nanoscale Pd and PtPd, BT is irreversibly lost due to cleavage of BT C-S bonds at oxidizing potentials, which leaves adsorbed sulfur oxides on Pd and PtPd films and effects the desulfurization of BT. While Pd and PtPd films are less sulfur-resistant than Pt films, palladium oxides, which form at higher potentials than Pt oxides, oxidatively desulfurize BT. In situ spectroelectrochemical Raman spectroscopy provides real-time, chemically specific information that complements the cyclic voltammetric data. The combination of these techniques affords a powerful and convenient method for guiding the development of sulfur-tolerant PEMFC catalysts.

Introduction The sulfur tolerance of catalysts for proton-exchange membrane fuel cell (PEMFC) anodes must be improved if PEMFCs are to be used in the industrial, military, and transportation sectors. Hydrogen derived from reformed hydrocarbon fuels is likely to contain significant amounts of H2S and organosulfur compounds. The performance of catalysts in PEMFCs degrades significantly after a few hours of exposure to 50 ppm gas-phase H2S.1 The development and application of in situ methods for studying sulfur-containing species on electrode materials are vital to understanding and alleviating poisoning in PEMFCs using reformed fuels. Raman spectroscopy is a viable technique for in situ investigation of catalyst and fuel cell systems, as it can report with molecular specificity on the state of the electrochemical interface as a function of electrochemical potential.2-5 In the present study, benzenethiol (BT) is employed as a model adsorbate in spectroelectrochemical Raman studies of nanostructured Pt, Pd, and PtPd electrodes, and the relative sulfur resistance of Pt, Pd, and PtPd are contrasted. In situ spectroelectrochemistry enables elucidation of the BT desorption process from different metal catalysts as a function of potential and with molecular specificity.

*To whom correspondence should be addressed. Tel: 202.767.8135; fax: 202.767.3321; Email: [email protected]. (1) Mohtadi, R.; Lee, W. K.; Cowan, S.; Van Zee, J. W.; Murthy, M. Electrochem. Solid State Lett. 2003, 6, A272. (2) Abernathy, H. W.; Koep, E.; Compson, C.; Cheng, Z.; Liu, M. L. J. Phys. Chem. C 2008, 112, 13299. (3) Matic, H.; Lundblad, A.; Lindbergh, G.; Jacobsson, P. Electrochem. Solid State Lett. 2005, 8, A5. (4) Pomfret, M. B.; Marda, J.; Jackson, G. S.; Eichhorn, B. W.; Dean, A. M.; Walker, R. A. J. Phys. Chem. C 2008, 112, 5232. (5) Pomfret, M. B.; Owrutsky, J. C.; Walker, R. A. Anal. Chem. 2007, 79, 2367. (6) Stanislaus, A.; Cooper, B. H. Catal. Rev. Sci. Eng. 1994, 36, 75. (7) Wang, J. X.; Springer, T. E.; Adzic, R. R. J. Electrochem. Soc. 2006, 153, A1732.

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Heterogeneous hydrogenation catalysts6 and catalysts for the hydrogen oxidation reaction (HOR)7;the anode reaction in PEMFCs8;adsorb molecular hydrogen from the gas or liquid phase and dissociate it into adsorbed hydrogen atoms. Sulfurtolerant hydrogenation catalysts are intriguing candidates for sulfur-tolerant HOR catalysts, because the elementary reaction steps involved are similar on both. Sulfur-tolerant hydrogenation catalysts comprising supported precious metals have been the subject of considerable investigation, particularly supported platinum-palladium (PtPd) alloy catalysts.9-17 Results of early investigations of the sulfur tolerance of small (∼1.0-1.5 nm, as estimated from reported particle dispersion values) PtPd catalysts supported on γ-Al2O3 during hydrogenation reactions suggested that intermetallic interactions between Pt and Pd result in electron-poor surface Pt atoms that resist sulfur adsorption.9,10 Most other studies of sulfur tolerance at supported PtPd have involved the use of supports that are acidic either in the Brønsted sense (i.e., they contain acidic protons) or in the Lewis definition (i.e., they are electron-withdrawing in nature), both of which serve to reduce the electron density at the supported metal nanoparticle catalysts. Electron-withdrawing supports used include SiO2-Al2O3,11,12 Al2O3-B2O3,13 various acidic (8) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45(PII), S0167. (9) Jan, C. A.; Lin, T. B.; Chang, J. R. Ind. Eng. Chem. Res. 1996, 35, 3893. (10) Lin, T. B.; Jan, C. A.; Chang, J. R. Ind. Eng. Chem. Res. 1995, 34, 4284. (11) Navarro, R. M.; Pawelec, B.; Trejo, J. M.; Mariscal, R.; Fierro, J. L. G. J. Catal. 2000, 189, 184. (12) Thomas, K.; Binet, C.; Chevreau, T.; Cornet, D.; Gilson, J. P. J. Catal. 2002, 212, 63. (13) Yasuda, H.; Kameoka, T.; Sato, T.; Kijima, N.; Yoshimura, Y. Appl. Catal., A 1999, 185, L199. (14) Jiang, H.; Yang, H.; Hawkins, R.; Ring, Z. Catal. Today 2007, 125, 282. (15) Matsui, T.; Harada, M.; Bando, K. K.; Toba, M.; Yoshimura, Y. Appl. Catal., A 2005, 290, 73. (16) Jongpatiwut, S.; Li, Z. R.; Resasco, D. E.; Alvarez, W. E.; Sughrue, E. L.; Dodwell, G. W. Appl. Catal., A 2004, 262, 241. (17) Jongpatiwut, S.; Rattanapuchapong, N.; Rirksomboon, T.; Osuwan, S.; Resasco, D. E. Catal. Lett. 2008, 122, 214.

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zeolites,12,14,15 and fluorine-doped Al2O3.16,17 The data suggest that the bimetallic interaction between Pt and Pd in PtPd catalysts does not render the metals adequately sulfur tolerant. When PtPd catalysts are dispersed on supports that are not electron-withdrawing, sulfur tolerance can derive from other factors: Navarro et al. interpreted infrared (IR) spectra of CO adsorbed on SiO2-Al2O3-supported PtPd catalysts as indicating that small, electron-deficient Pt clusters on larger Pd particles were simultaneously hydrogenation-active and sulfur-tolerant;11 Matsui et al. used X-ray absorption spectroscopy (XAS) to show that a strong interaction between PtPd nanoparticles and nonacidic SiO2 supports distort the crystal structure of PtPd catalysts. They posited that this distortion renders the metal particles electron-deficient and thus confers the sulfur tolerance of the catalyst.15 The deficiency induced by such bonding distortions may be responsible for the sulfur tolerance of PtPd catalysts on non-electron-withdrawing supports reported earlier.9,10 Density functional theory (DFT) calculations of H2, H2S, and atomic sulfur adsorption energies on Pt, Pd, and PtPd particles show that both H2 and H2S adsorb more strongly on PtPd than on Pt but the enhancement of H2 adsorption is greater, and that sulfur tolerance of PtPd catalysts arises from more successful competition for active sites by H2 over H2S.14 The mechanisms responsible for the sulfur tolerance of supported PtPd catalysts in hydrogenation reactions are still a subject of study and debate. In evaluating PtPd bimetallic catalysts as sulfur-tolerant catalysts for the HOR, the influence of electrochemical potential must be considered. Several examples of in situ and operando electrochemical XAS of fuel-cell catalysts have been reported recently. 18-22 While powerful and extremely useful for delineating details such as metal-metal and metal-adsorbate bond lengths and catalyst particle size, the technique requires powerful X-ray sources, as well as considerable preparation time for each sample, and yields results that are often difficult to interpret. In the past two decades, Raman spectroscopy has emerged as a powerful tool for evaluating processes at electrochemical interfaces.23-28 Chan et al. used in situ Raman spectroscopy to characterize the potential-dependent formation of oxides at Pd, Rh, Ir, and Pt electrodes and to differentiate electrochemical from gas-phase oxidation of the metals.29 Gomez et al. demonstrated that electrochemical processes at nanoparticles deposited on macroscopic electrodes could be characterized using Raman spectroscopy, including the adsorption and desorption of adsorbates and their electrochemical oxidation.30,31 Raman spectroscopy has been used to study processes in operating fuel cells, including fuel utilization on solid oxide fuel cell (SOFC) anode (18) Gatewood, D. S.; Ramaker, D. E.; Sasaki, K.; Swider-Lyons, K. E. J. Electrochem. Soc. 2008, 155, B834. (19) Gatewood, D. S.; Schull, T. L.; Baturina, O.; Pietron, J. J.; Garsany, Y.; Swider-Lyons, K. E.; Ramaker, D. E. J. Phys. Chem. C 2008, 112, 4961. (20) Roth, C.; Benker, N.; Buhrmester, T.; Mazurek, M.; Loster, M.; Fuess, H.; Koningsberger, D. C.; Ramaker, D. E. J. Am. Chem. Soc. 2005, 127, 14607. (21) Scott, F. J.; Roth, C.; Ramaker, D. E. J. Phys. Chem. C 2007, 111, 11403. (22) Teliska, M.; Murthi, V. S.; Mukerjee, S.; Ramaker, D. E. J. Electrochem. Soc. 2005, 152, A2159. (23) Ren, B.; Liu, G. K.; Lian, X. B.; Yang, Z. L.; Tian, Z. Q. Anal. Bioanal. Chem. 2007, 388, 29. (24) Tian, Z.-Q.; Ren, B. Annu. Rev. Phys. Chem. 2004, 55, 197. (25) Weaver, M. J. Topics Catal. 1999, 8, 65. (26) Weaver, M. J. J. Raman Spectrosc. 2002, 33, 309. (27) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. J. Chem. Soc. Chem. Commun. 1973, 80. (28) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J.; Paul, R. L.; Reid, E. S. J. Raman Spectrosc. 1976, 4, 269. (29) Chan, H. Y. H.; Zou, S.; Weaver, M. J. J. Phys. Chem. B 1999, 103, 11141. (30) Gomez, R.; Solla-Gullon, J.; Perez, J. M.; Aldaz, A. J. Raman Spectrosc. 2005, 36, 613. (31) Gomez, R.; Solla-Gullon, J.; Perez, J. M.; Aldaz, A. ChemPhysChem 2005, 6, 2017.

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surfaces4,5 and water content in PEMFC electrolytes.3 Most germane to the present study, Zheng et al. used Raman spectroscopy to show that thiourea adsorbed through its sulfur atom to roughened Pt electrodes only at potentials where no platinum oxides were present and otherwise was decomposed to adsorbed sulfur and a nonadsorbed product.32 Gao et al. delineated adsorption and electro-oxidative pathways of sulfur on Au electrodes immersed in concentrated sulfide solutions using real-time, in situ, surface-enhanced Raman spectroscopy (SERS), observing the protonation and deprotonation of adsorbed sulfur species and the formation of surface polysulfide species as a function of electrode potential.33 In the present study, films of Pt, Pd, and PtPd nanoparticles are electrochemically deposited on indium-tin oxide (ITO) electrodes, and the adsorption as well as electrochemical oxidation and reduction of chemisorbed BT are evaluated. Large metal nanoparticles;on the order of 102 nm in most cases;comprise the films, so the effects of the support can be ignored and the interaction between the adsorbate and the metal or mixed metal are evaluated exclusively. The relative resistance to sulfur binding of the three catalyst compositions will depend on particle morphology and surface chemistry and, in the case of the PtPd particles, intermetallic interactions. Benzenethiol is a Ramanactive, sulfur-containing molecule that was used as a model probe of the Au-electrolyte interface34,35 and to investigate SERS effects at nanostructured Pd and Pt23,36;although BT on Pd has not been studied in an electrochemical environment. The potential-dependent desorption of BT from Pt, Pd, and PtPd, as well as the formation, adsorption, and desorption of other interfacial chemical species, are characterized electrochemically in acidic electrolyte and correlated with in situ Raman spectroscopy.

Experimental Section Materials. Chloroplatinic acid hexahydrate (H2PtCl6 3 6H2O, Sigma-Aldrich, >37.5% Pt basis), palladium(II) chloride (PdCl2, Alfa Aesar, 99.999%), BT (g98%, Sigma-Aldrich), sulfuric acid (H2SO4, Fisher Scientific, 95-98%, ACS plus grade), perchloric acid (HClO4, GFS Chemicals, double-distilled), potassium perchlorate (KClO4, GFS Chemicals, ACS reagent grade), and ethanol (Warner-Graham Co., 200 proof absolute) were all used as received. Indium-tin oxide (ITO)-coated glass slides (Delta Technologies, Ltd.) were cleaned before use by 15 min sonication successively in detergent, deionized water, acetone, and ethanol. Marine epoxy (Loctite) was used as received. Preparation and Characterization of Metal Nanoparticle Films. Platinum (Pt), palladium (Pd), and platinum-palladium (PtPd) films were electrochemically deposited from metal salt solutions in 0.5 M H2SO4 on clean ITO-coated glass in a threeelectrode electrochemical cell, comprising ITO-coated glass as the working electrode, Pt gauze as the counter electrode, and a fritted Ag/AgCl/3 M KCl reference electrode (Bioanalytical Systems). Potentials were applied with a Pine Instruments AFCBP1 potentiostat controlled by a PC and Pinechem electrochemical software. As various Ag/AgCl reference electrodes were used in the electrodeposition, electrochemical cleaning, cyclic voltammetry, and spectroelectrochemical experiments, reference electrode potentials for each experiment were calibrated versus a reversible (32) Zheng, J.-Z.; Ren, B.; Wu, D.-Y.; Tian, Z.-Q. J. Electroanal. Chem. 2005, 574, 285. (33) Gao, X.; Zhang, Y.; Weaver, M. J. Langmuir 1992, 8, 668. (34) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (35) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570. (36) Abdelsalam, M. E.; Mahajan, S.; Bartlett, P. N.; Baumberg, J. J.; Russell, A. E. J. Am. Chem. Soc. 2007, 129, 7399.

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Pomfret et al. hydrogen electrode (RHE) to eliminate any slight discrepancies in reference potentials. Films of Pt particles ranging from ∼200 to 500 nm in size were electrochemically deposited onto ITO-coated glass from solutions of 5 mM H2PtCl6 3 6H2O in 0.5 M H2SO4 (aq) by adapting a method developed by Duarte et al.37 Two potential steps were applied to the ITO electrode. A 30 s step at -150 mV is used to initiate nucleation followed by a 2 min step at 350 mV to sustain Pt particle growth. In some cases, this two-step sequence was performed twice to increase Pt coverage. Nanoparticulate Pd films were fabricated by depositing irregularly shaped Pd particles ranging from 50 to 500 nm in size onto ITO-coated glass by a cyclic voltammetric method adapted from Xu and Lin.38 The ITO electrode, immersed in 5 mM PdCl2/0.5 M H2SO4 (aq), is cycled positively at 100 mV/s from the open-circuit potential (OCP), typically around 850 mV, to 1450 mV followed by a negative-going sweep to 0 mV and finally returning to the original OCP, for a total of 20 cycles. Platinum-palladium films were deposited onto ITO electrodes from a mixed-metal salt solution consisting of 2.5 mM PdCl2/2.5 mM H2PtCl6 3 6H2O/ 0.5 M H2SO4, by the same cyclic voltammetric method used to deposit Pd films. The morphology of the Pt, Pd, and PtPd films was determined by scanning electron microscopy (SEM, Leo Supra 55 scanning electron microscope), using an accelerating voltage of 20 kV and with the aperture set at 30 μm. The chemical state of the nanoparticle films was determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha spectrometer) with monochromatic Al-KR radiation (1486.6 eV). All measurements had a step size of 0.15 eV and dwell time of 0.1 s. Each individual spectrum was recorded in fixed analyzer transmission (FAT) mode with pass energy of 20 eV and an accumulation of 10 scans. All spectra were processed and fitted with Unifit 2007 software, using the minimum number of peaks consistent with the best fit. BT Adsorption. To make in situ spectroelectrochemical cells, metal-on-ITO electrodes were sealed or “potted” in plastic vials with marine epoxy, which was subsequently allowed to dry overnight. The potted electrodes were electrochemically cleaned by cycling between 0 mV and 1350 mV versus RHE at 200 mV/s for ∼50 cycles in 0.1 M HClO4 and subsequently characterized by cycling 3 times each at scan rates of 50 mV/s and 20 mV/s. The films were then rinsed in Nanopure H2O, followed by ethanol, and then immediately soaked in a ∼0.01 M solution of BT in ethanol for 1 h. After the benzenthiol soak, excess BT was rinsed away with clean ethanol, after which Raman spectroscopy (electrode in air) and spectroelectrochemistry (electrode under electrolyte) were immediately performed. Raman Spectroscopy and Spectroelectrochemistry. The assembled spectroelectrochemical cell (0.1 M HClO4, platinum wire counter electrode, Ag/AgCl reference electrode (World Precision Instruments, Inc., FLEXREF)) was placed under the objective lens (20
) of a Raman microscope (Renishaw Ramascope). The 514 nm line of an argon-ion laser focused through the objective lens of the microscope served as the illumination source, and Raman scattered light of that beam was collected through the same objective lens. Spectra were obtained with a resolution of 4 cm-1 and comprise 25 10-s coadded scans taken at a constant potential. The first spectrum was taken with the cell at OCP, typically around 550 mV, and followed by a series of spectra recorded at various potentials from 200 mV to 1300 mV in 150-200 mV increments. Spectra were taken first in ascending order of potential increments then in descending order to track changes in the metal films and in the chemisorbed BT as metal oxides were formed and then reduced on the metal nanoparticle films. After spectroelectrochemical experiments, electrodes were rinsed of electrolyte with ultrapure water (Millipore) and (37) Duarte, M. M. E.; Pilla, A. S.; Sieben, J. M.; Mayer, C. E. Electrochem. Commun. 2006, 8, 159. (38) Xu, Y. H.; Lin, X. Q. J. Power Sources 2007, 170, 13.

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Figure 1. Scanning electron micrographs of (a) Pt, (b) Pd, and (c) PtPd alloy nanoparticle films electrodeposited onto ITO. dried in air for 5 min, and Raman spectra of the dry films were recorded for comparison to the results obtained with the electrodes immersed in electrolyte. Electrochemical Stripping of BT Films. Electrochemical stripping of BT from the metal films was achieved by cycling between 0 mV and 1350 mV at 100 mV/s in 0.1 M HClO4 until voltammetric waves recover either to their appearance prior to BT exposure or to a steady-state condition.

Results and Discussion SEM, XPS, and Electrochemical Characterization of Clean Films. Scanning electron micrographs of Pt, Pd, and PtPd films are shown in Figure 1a-c, respectively. The Pt films feature rough but approximately spherical Pt particles primarily 200-500 DOI: 10.1021/la904107j

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nm in diameter. The Pd films feature irregularly shaped particles ∼50-500 nm in diameter, similar to those observed by Xu and Lin.38 The PtPd films feature approximately bimodal distributions of cuboctohedral particles, with one population of particles between ∼20 and 50 nm in diameter and the second ∼100-200 nm in diameter. Nanoparticulate metal electrocatalysts as well as supported catalysts are typically sized between 1 and 10 nm. While the ∼100nm-scale particles in all three film compositions are not representative of typical catalyst particle sizes, the large Pt and PtPd nanoparticles feature rough domains on the nanometer scale (as shown in higher magnification SEM images in the Supporting Information) and have similar electrochemical properties to those of smaller electrocatalyst nanoparticles, as discussed below. Higher-magnification images of the Pd particles (also shown in Supporting Information) reveal smoother domains on the ∼5-10 nm scale than those present in the Pt and PtPd particles, indicating that the large Pd nanoparticles may not mimic the electrochemical behavior of smaller catalytic nanoparticles quite as well. Comparison of the electrochemical behavior of the Pd nanoparticles to that of nanoparticles of different metal compositions in this study is still reasonable, but differences in domain sizes should be kept in mind when interpreting electrochemical results. Initially, experiments were attempted in which preformed metal nanoparticles in the 2-5 nm range were directly deposited on ITO, but poor electrode coverage (low particle number density) yielded poor signal-to-noise ratios in Raman experiments. The electrochemical deposition method rapidly and reproducibly provides adequate particle coverage, and the large particles feature large scattering cross sections that yield good signal-to-noise ratios. The chemical state of the electrodeposited particles was explored with X-ray photoelectron spectroscopy (XPS) to determine whether the PtPd mixed metal films alloy or segregate into distinct domains. X-ray measurements on Pt, Pd, and PtPd films (data provided in Supporting Information) revealed a shift in the Pd 3d binding energy from 335.2 eV for Pd alone to 335.3 eV in the PtPd film; also, the Pt 4f binding energy shifts from 71.2 eV for Pt alone to 71.1 eV for Pt in PtPd. The ∼þ0.1 eV shift in the binding energy of Pd in PtPd, and an ∼-0.1 eV shift for Pt are interpreted as evidence of donation of electron density from Pd to Pt, suggesting that either an alloy is formed38 or the metal domains are intimately mixed on the nanoscale and are in electronic communication with one another. Such an interaction raises the d-band center of Pt, allowing it to bind more strongly to sulfur, which is electron withdrawing.39,40 The PtPd films contain approximately equal amounts of Pt and Pd as measured by XPS, so the near-surface stoichiometry of the particles is consistent with the Pt/Pd ratios in the deposition solutions (in which [Pt4þ] ≈ [Pd2þ] within ∼1%). Characteristic cyclic voltammetric data obtained for the electrochemically cleaned metal films are shown in Figure 2a-c. The voltammogram for the Pt film, Figure 2a, has the characteristic features for underpotential deposition and stripping of hydrogen (Hupd) between 50 mV and 400 mV versus RHE, as well as features corresponding to formation of platinum oxide species during positive-going voltammetric scans at potentials positive of ∼600 mV and the stripping of platinum oxides between ∼1100 mV and 600 mV on negative-going scans.41 The voltammogram (39) Pillay, D.; Johannes, M. D. J. Phys. Chem. C 2008, 112, 1544. (40) Pillay, D.; Johannes, M. D. Surf. Sci. 2008, 602, 2752. (41) Angerstein-Kzlowska, H. In Comprehensive Treatise of Electrochemistry, Yeager, E., Bockris, J. O. M., Conway, B. E., Sarangapani, S., Eds.; Plenum: New York, 1984.

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Figure 2. Cyclic voltammograms of clean (a) Pt, (b) Pd, and (c) PtPd alloy nanoparticle films in 0.1 M HClO4 (aq). Scan rate: 20 mV/s.

for the Pd film (Figure 2b) also features a hydrogen region between ∼80 mV and 330 mV, but the processes in this region are considerably more complex at Pd than at Pt; both Hupd and subsurface sorption/evolution of hydrogen occur in this potential region at Pd electrodes.42 In general, the hydrogen region is consistent with voltammetry at polycrystalline Pd42,43 and at supported Pd nanoparticles.44,45 Palladium oxide formation occurs at potentials positive of ∼600 mV on positive-going sweeps, and the oxide is reductively stripped between ∼850 mV and 600 mV on negative-going sweeps, which agrees with values reported for oxidation processes on polycrystalline Pd43,46,47 and on supported Pd nanoparticles.44,45 At potentials negative of the oxide stripping peak, the current corresponding to double-layer charging of Pd (∼600-300 mV) is noticeably shifted below the zero current axis by a steady-state cathodic current of ∼-0.2 mA. This cathodic current is due to reductive proton insertion into the ITO film, which generally starts at ∼400 mV vs RHE in acid electrolyte.48 This cathodic process is also evident for the Pt film (Figure 2a), but with a lower offset (-0.14 mA) than for the Pd film (-0.2 mA in Figure 2b). The data in Figure 2 are not normalized for microscopic surface area, so that direct comparison of surface areas of the as-prepared films can be readily made. The cyclic voltammetric data for an electrochemically cleaned PtPd film are shown in Figure 2c. A combination of both Pt and Pd voltammetric features is evident (42) Breiter, M. W. J. Electroanal. Chem. 1977, 81, 275. (43) Perdriel, C. L.; Custidiano, E.; Arvia, A. J. J. Electroanal. Chem. 1988, 246, 165. (44) Papageorgopoulos, D. C.; Keijzer, M.; Veldhuis, J. B. J.; de Bruijn, F. A. J. Electrochem. Soc. 2002, 149, A1400. (45) Solla-Gullon, J.; Rodes, A.; Montiel, V.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 2003, 554, 273. (46) Bolzan, A. E.; Martins, M. E.; Arvı´ a, A. J. J. Electroanal. Chem. 1983, 157, 339. (47) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1972, 35, 209. (48) Armstrong, N. R.; Lin, A. W. C.; Fujihira, M.; Kuwana, T. Anal. Chem. 1976, 48, 741.

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Figure 3. Cyclic voltammograms of clean Pt (solid line), Pd (dashed line), and PtPd alloy (dotted) nanoparticle films in 0.1 M HClO4 (aq) normalized to their electrochemical surface areas for comparison of electrochemical processes at each film. Scan rate: 20 mV/s.

particularly at potentials corresponding to oxide formation and stripping, as has been reported elsewhere for PtPd alloy nanoparticles.44,45 As with the Pt and Pd films, there is a cathodic offset of the double-layer charging region of the voltammetry due to reductive proton insertion into ITO; here, this cathodic offset current is ∼-0.1 mA. Figure 3 depicts the data for the Pt, Pd, and PtPd films normalized for their electrochemical surface areas, enabling direct comparison of the electrochemical processes between different film compositions. It is particularly clear on the PtPd nanoparticle film that the oxide stripping wave derives from stripping oxide from both Pt and Pd atoms at the surface, showing that both surface Pt and Pd atoms are electrochemically active. Ex Situ Electrochemical Stripping of BT: Electrochemical Removal of Sulfur. Sulfur-containing species can be oxidatively stripped from Pt surfaces by repeated voltammetric cycling to potentials that alternately drive formation and reductive stripping of platinum oxides at the metal surface.49-53 Oxidative desorption of benezenethiol from Pt likely proceeds by a mechanism similar to that for most sulfur species adsorbed at Pt49,51,53,54 according to eq 1: Ph-S-Pt þ 3H2 O f Pt þ PhSO3 - þ 6Hþ þ 5e -

ð1Þ

where Ph represents the phenyl group in BT. The pathway represented in eq 1 involves oxidation of benzenethiolate to benzenesulfonic acid, which then desorbs from the electrode. Films freshly exposed to BT were electrochemically cleaned by repeated cycling between 0 mV and 1350 mV at 100 mV/s until the voltammetric waves either appear as they did before the BT soak or recover to a steady-state condition. Cyclic voltammetric data of electrochemically cleaned Pt, Pd, and PtPd films before and after exposure to BT are shown in Figure 4. The initial voltammetric scan after BT exposure is shown, as well as the last voltammetric scan after oxidatively stripping as much of the BT monolayer as can be removed, as indicated by the successive voltammetric scans reaching a steady state. Platinum films achieve a steady state after ∼12 scans, Pd films after ∼25 scans, and the alloy films after ∼22 scans; (49) Foral, M. J.; Langer, S. H. J. Electroanal. Chem. 1988, 246, 193. (50) Lamy-Pitara, E.; Bencharif, L.; Barbier, J. Electrochim. Acta 1985, 30, 971. (51) Loucka, T. J. Electroanal. Chem. 1971, 31, 319. (52) Marcus, P.; Protopopoff, E. Surf. Sci. 1985, 161, 533. (53) Quijada, C.; Rodes, A.; Vazquez, J. L.; Perez, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 394, 217. (54) Pietron, J. J. J. Electrochem. Soc. 2009, 156, B1322.

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Figure 4. Cyclic voltammograms of electrochemically cleaned (a) Pt, (b) Pd, and (c) PtPd alloy nanoparticle films in 0.1 M HClO4 before (solid line) and after exposure to BT (dashed line) and after oxidative stripping of BT from the film (dotted line). Scan rate: 50 mV/s.

however, platinum films only recover ∼85% of the activity for hydrogen adsorption/desorption as compared to the clean films. Marcus and Protopopoff showed that, at low fractional monolayer coverages of sulfur (where fractional monolayer coverage, θs, is ∼1.05 V. The BT affords some protection of Pd from oxidative (55) Protopopoff, E.; Marcus, P. Surf. Sci. 1986, 169, L237.

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Figure 6. Raman spectra of a Pt nanoparticle electrode film that has been exposed to 0.01 M BT solution for 1 h. The arrow shows the direction of the potential increments that start at 200 mV vs RHE and proceed to 700 mV, 1000 mV, 1300 mV, then back down to 1000 mV, 700 mV, and 200 mV. Figure 5. In situ Raman spectra of a clean Pt nanoparticle electrode film taken during a stepped potential sweep. The arrow shows the direction of the electrochemical potential increments that start at 200 mV vs RHE and proceed to 700 mV, 1000 mV, 1300 mV, then back down to 1000 mV, 700 mV, and 200 mV. The inset shows the low-frequency region of the Raman spectra taken at 1300 mV and 200 mV.

dissolution. Thus, on the time scale of the cyclic voltammetric experiment, dissolution is too slow to enable significant sulfur removal in a single sweep. After multiple cycles and removal of the majority of adsorbed sulfur species, some oxidative etching of PdO species at potentials positive of ∼1.0 V may enable removal of the final remaining surface-bound sulfur species at more dilute coverages. Further evidence for some degree of Pd dissolution in the stripping experiments is the disappearance of the cathodic current offset of the voltammetry for the Pd film (Figure 4b). Electrodeposition of Pd from the electrogenerated Pd2þ at bare ITO (in competition with the thermodynamically favored Pd electrodeposition on Pd) thereby blocking direct hydrogen reduction at ITO would explain this reduced effect.43,46 The determining factor in the rate of removal of sulfur by voltammetric stripping;in this case measured by the number of scans required to achieve a clean electrode;is the relative strength of the metal-S bond. Higher potentials are required to initiate sulfur oxidation at Pd and PtPd than at Pt, indicating the greater strength of the Pd-S bond relative to the Pt-S bond. Consequently, the stronger Pd-S bond requires more oxidation/ reduction cycles to remove BT completely from Pd. Dissolution of PdO from the alloy appears to enable more complete removal of BT from PtPd than from Pt. Potential-Dependent Raman Spectra and Adsorbate Speciation. While the cyclic voltammetric data show that sulfur (and BT) are electrochemically removed from the films at increasingly high potentials and cycle numbers in the order Pt < PtPd < Pd, the electrochemical data do not fully reveal the mechanism by which BT is removed. Is sulfur oxidized directly and does BT desorb as a benzenesulfoxy species, or are the carbon-sulfur (C-S) bonds cleaved at any point, leaving adsorbed sulfur species behind? In situ Raman spectroscopy was applied to determine specific chemical information about the interfacial processes involved in BT and sulfur removal from the films. General Features. The spectroelectrochemical cell used for the in situ Raman spectroelectrochemical experiments (shown schematically in Supporting Information) allows illumination of the electrode-electrolyte interface through a ∼2 mm layer of electrolyte and collection of the Raman scattered light through a 6814 DOI: 10.1021/la904107j

microscope objective. In situ Raman experiments were conducted on clean and BT-exposed nanostructured Pt, Pd, and PtPd films. Raman data for individual films at different potentials are compared to those obtained at the OCP of freshly BT-exposed films, with the spectra obtained at OCP serving as an internal standard for each film. Surface enhancement (SERS) factors of ∼500-1000 were estimated for BT on the nanostructured films. Abdelsalam et al. measured SERS factors in this same range for BT adsorbed on well-ordered, nanostructured Pt and Pd films.36 Given that our signal-to-noise ratios for our experiments were in the range of 2-4 in most cases, enhancement factors in these ranges may be necessary to adequately characterize the spectroelectrochemical experiments performed in this study. At all potentials, Raman spectra of clean Pt films (Figure 5) exhibit peaks at 465, 630, 932, and 1121 cm-1 (a peak at 1637 cm-1 is not shown on the scale used in the figures) corresponding to nonadsorbed perchlorate anions at and near the electrode/ electrolyte interface); impurity-related peaks are also present at various potentials. For instance, a band appears at 1335 cm-1 upon the rereduction of Pt at 200 mV that is assigned to CO adsorbed on the surface (Figure 5). The CO peak is the result of oxidation of adventitious carbonaceous species attributed to lingering impurities from the epoxy or other ambient sources of carbon. A broad band at 570 cm-1 appears at potentials positive of 850 mV (Figure 5, inset) on both the positive- and negative-going potential excursions and is assigned to PtO, in agreement with results of Chan et al.29 and Gomez et al.30,31 Appearance of this peak at 850 mV;as a rising baseline centered at 570 cm-1; correlates well with the onset of the oxide wave observed in the cyclic voltammogram of the Pt film (Figure 2a). This PtO peak is reduced to just above baseline intensity for the spectrum taken at 700 mV on the negative-going excursion, although the voltammetric data show that some oxide is still present at the Pt surface. Raman spectra for Pt films that have been exposed to the 0.01 M BT solution for 1 h and subsequently rinsed and transferred to clean electrolyte are shown in Figure 6. The spectra are dominated by bands assigned to benzene ring vibrations. The υ12, υ18a, υ1, and υ8a C-C modes;at 1000, 1021, 1064, and 1572 cm-1, respectively;are the most prominent of the benzene ring peaks and are specific to BT adsorbed on a metal surface.34,35 All BT features on Pt as well as Pd and PtPd nanoparticle films (vide infra) derive from BT adsorbed to metal nanoparticles, as BT does not adsorb to ITO in detectable quantities. In a control experiment, a bare ITO electrode was soaked in 0.01 M BT Langmuir 2010, 26(9), 6809–6817

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Figure 7. Raman spectra of a Pd nanoparticle electrode film that has been exposed to 0.01 M BT solution for 1 h. The arrow shows the direction of the potential increments that start at 200 mV vs RHE and proceed to 700 mV, 1000 mV, 1300 mV, then back down to 1000 mV, 700 mV, and 200 mV.

Figure 8. Raman spectra of a PtPd alloy nanoparticle electrode film that has been exposed to 0.01 M BT solution for 1 h. The arrow shows the direction of the potential increments that start at 200 mV vs RHE and proceed to 700 mV, 1000 mV, 1300 mV, then back down to 1000 mV, 700 mV, and 200 mV.

solution for 1 h and rinsed with ethanol. Raman spectra taken of the BT-exposed ITO film contain no evidence of BT on the ITO (data not shown). The intensities of these BT bands begin to diminish at ∼1000 mV on the positive-going sweep (Figure 6), indicating the onset of BT desorption, concurrent with Pt oxidation as shown in the electrochemical data (Figures 2 and 4) and in the Raman data for the clean film (Figure 5). Previously published work suggests that BT desorbs from Au surfaces upon oxidation of the electrode surface.35 Our Raman data suggest that the same thing happens on Pt; at 1300 mV (Figure 6), there is no spectroscopic evidence of BT on the surface or of any sulfur-containing adsorbates. As the potential is returned to more negative potentials, there is no spectroscopic evidence of BT at 1000 mV, but the C-C bands become visible again at 700 mV (Figure 6), where more than half of the Pt surface oxides are reduced back to Pt0 according to the voltammogram in Figure 4a. Upon returning to 200 mV (Figure 6), the strong C-C band at 1572 cm-1 has recovered to about one-third of its original intensity. The significance of the return of the BT Raman features in clean supporting electrolyte will be further discussed below when comparing the data for the Pt, Pd, and PtPd films. Spectroelectrochemistry performed on BT-exposed Pd films yielded strikingly different results compared to those obtained for Pt films. Raman spectra of clean Pd films at various potentials appear in the Supporting Information. Raman spectra of the BTexposed Pd film are shown in Figure 7. The intensities of the BT bands are significantly lower than those measured on the Pt film. This spectral insensitivity on Pd may be due either to lower BT surface coverage or to differences in the Pt and Pd film structures, where the latter might affect the magnitude of the SERS effect. At more positive potentials, the BT peaks diminish and completely disappear above 1000 mV. When the Pd film is reduced following initial oxidation, the BT peaks do not return; however, a broad envelope of peaks between ∼1000-1200 cm-1 appears at 1000 mV and then diminishes again at more reducing potentials. These modes are consistent with those reported for sulfur oxides adsorbed on both unsupported and supported Pd nanoparticles.56,57 At 700 mV, the sulfur oxide peaks have disappeared,

concomitant with the electrochemical reduction of PdO observed at ∼750 mV (Figures 2b and 4b). Potential-dependent Raman spectra for BT chemisorbed on PtPd films are displayed in Figure 8. As when adsorbed to Pd films, BT desorbs from PtPd at oxidizing potentials, and no BT peaks are evident in the Raman spectrum at 1300 mV. The PtPd film remains free of spectroscopic evidence of adsorbed BT when measurements are taken in situ, even after a return to reducing potentials. The broad envelope of peaks corresponding to sulfur oxide species between ∼1000 and 1200 cm-1 observed on Pd films does not appear on PtPd films at 1000 mV on the negative-going potential excursion. The absence of a (SO)x feature suggests that, if oxidized sulfur species are present on the PtPd films, their coverage is below the detection limits of our in situ experiment. Ex situ Raman spectra recorded for dry films after the electrochemical Raman experiments (Figure 9) show that some BT remains on Pd and PtPd films, as well as on Pt films. The difference derives from sensitivity limitations of the in situ experiment. In the confocal Raman experiment, the mismatch of refractive indices of aqueous electrolyte and air results in a loss of over 65% of signal intensity at an electrolyte depth of ∼2 mm,58 which is approximately the thickness of the electrolyte layer above the electrode surface in the present experiment. Benzenthiolate Surface Chemistry from Raman Spectroelectrochemistry on Pt, Pd, and PtPd Films. The Raman spectra show that Pt films are fully oxidized at potentials above 850 mV and that significant amounts of BT desorb from the film surface, consistent with the cyclic voltammetric data for the clean films. At reducing potentials, some BT reappears, though the Raman bands appear weaker than immediately after initial BT exposure. Recalling the pathway described in eq 1 above, oxidation of benzenethiolate yields benzenesulfonic acid, which is completely water soluble. It appears that most of the electrogenerated benzenesulfonic acid diffuses away, while the rest remains weakly chemisorbed or physisorbed to Pt oxides. When the spectroelectrochemical experiments are performed in neutral 0.1 M KClO4 electrolyte, the Raman peaks corresponding to BT adsorbed on Pt disappear upon application of positive potential (as they did in acid electrolyte), but return to a lesser extent upon return to reducing potentials;the C-C modes at 1572 cm-1 return to ∼20% of their maximum intensity measured at 150 mV (data shown in Supporting Information). In neutral

(56) Uy, D.; Dubkov, A.; Graham, G. W.; Weber, W. H. Catal. Lett. 2000, 68, 25. (57) Wilke, T.; Gao, X. P.; Takoudis, C. G.; Weaver, M. J. J. Catal. 1991, 130, 62.

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(58) Tian, Z.-Q.; Ren, B.; Wu, D.-Y. J. Phys. Chem. B 2002, 106, 9463.

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Figure 9. Ex situ Raman spectra of dry Pt, Pd, and PtPd alloy nanoparticle films after the spectroelectrochemical Raman experiments. Benzenethiol C-C peaks are identified by dashed lines.

electrolyte, anionic benzenesulfoxy species are created in the presence of a less protonated platinum surface oxide, and thus, less of the benzenesulfoxy species remains electrostatically adsorbed to the surface long enough to be re-reduced and readsorbed at reducing potentials. In situ Raman spectra of Pd films show no evidence of adsorbed BT when the potential is returned to reducing voltages. As described above, at 1000 mV on the negative-going potential excursion, Raman bands corresponding to adsorbed sulfur oxides appear, suggesting that the C-S bond in BT is dissociated at the oxidized Pd surface, leaving oxidized sulfur species adsorbed. While it has been established that BT electrochemically desorbs from gold electrodes as phenylthiolate,33 it has been demonstrated in UHV studies that thermal BT desorption can occur through C-S bond scission on metals such as Ni,59,60 Rh,61 and CoMo alloys,62 as well as other metals.63 It is also important to note that the C-S bond in alkylthiols is generally weaker than in aryl thiols: for example, the C-S bond in benzenethiol is 86.5 kcal mol-1, whereas for methanethiol, it is 74 kcal mol-1.64 The relative strengths of these bonds do not always correlate with their relative desorption temperatures from different metals, and other factors come into play.63 Electrochemical systems are even more complex: Zheng et al. report that thiourea adsorbs dissociatively to Pt electrodes, breaking the C-S bond and leaving only adsorbed sulfide when the electrodes are introduced to thiourea solutions at open-circuit potential (∼0.86 V versus NHE), but intact thiourea adsorbs when the electrode is immersed at 0.24 V versus NHE.32 This result suggests that surface metal oxides present at the higher potentials can promote oxidative C-S bond scission, as well as desorption of alkyl or arylsulfonates. While surface PtO species catalyze scission of the weaker C-S bonds, as in the case reported for thiourea,32 they apparently do not catalyze scission of the stronger C-S bond in BT, at least not at room temperature. Swider and Rolison reported that PtOx desulfurizes thiophenelike species on carbon black in 0.1 M HNO3 at 95 °C65; additional thermal energy is apparently needed to oxidatively desulfurize aromatic sulfur compounds. Surface PdO species apparently do catalyze C-S bond breaking in BT at ambient (59) Kane, S. M.; Huntley, D. R.; Gland, J. L. J. Phys. Chem. B 1998, 102, 10216. (60) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1994, 98, 13022. (61) Bol, C. W. J.; Friend, C. M.; Xu, X. Langmuir 1996, 12, 6083. (62) Chen, D. A.; Friend, C. M.; Xu, H. Surf. Sci. 1998, 395, L221. (63) Friend, C. M.; Chen, D. A. Polyhedron 1997, 16, 3165. (64) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493. (65) Swider, K. E.; Rolison, D. R. Langmuir 1999, 15, 3302.

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Figure 10. (a) The first voltammetric cycle after poisoning with BT of Pt, PtPd, and Pd alloy nanoparticle films in 0.1 M HClO4. (b) The positive portion of the CVs are expanded to emphasize the oxidation processes on the films. Scan rate: 50 mV/s.

temperature. As opposed to being a sulfur-tolerant catalyst, oxidized Pd acts as an electrochemical desulfurization catalyst at oxidizing potentials. It should be noted again that palladium oxides can dissolve under acidic conditions at potentials positive of ∼950 mV.43,46,47 While some electrode material may be lost via this mechanism, the broad envelope of peaks between ∼1000 and 1200 cm-1 corresponding to adsorbed sulfur oxides at 1000 mV on the negativegoing voltage excursion suggests that surface Pd is either protected from corrosion by adsorbed BT and sulfur oxides or that the corrosion of Pd is too slow to influence the chemistry on our experimental time scale. Additionally, spectroelectrochemical experiments performed on BT using neutral 0.1 M KClO4 electrolyte (conditions under which no Pd electrodissolution occurs) yielded nearly identical results as those performed using acid electrolyte (Supporting Information), providing strong evidence that corrosion is not responsible for the removal of BT from the Pd film. As on the single-metal films, BT readily adsorbs to the PtPd film and disappears as the film is electrochemically oxidized. Ex situ Raman spectroscopy performed on the dry films (Figure 9) after the electrochemical Raman experiment shows that some BT remains adsorbed to the PtPd film but in amounts that are not detectable at in situ Raman sensitivity. Traces of adsorbed CO are also evident on the dry PtPd film, as evidenced by the broad peaks at 1335 cm-1, possibly as a result of further oxidation of BTderived benzene after the oxidative desulfurization of BT. Correlation of Cyclic Voltammetric Data with Spectroelectrochemistry: Implications for Sulfur Tolerance. Since the motivating factor in this study was to develop methods to evaluate candidates for sulfur-tolerant catalysts for PEMFC anodes, it is appropriate to consider what the above results say about the relative sulfur tolerance of Pt, Pd, and PtPd. The Langmuir 2010, 26(9), 6809–6817

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Figure 11. Intensities of Raman signals for BT on Pt, PtPd, and Pd alloy nanoparticle films in 0.1 M HClO4 as a function of electrochemical potential, normalized to their intensities at 700 mV.

extensive loss of BT from both Pd and PtPd films upon long polarization to oxidizing potentials cannot be interpreted as sulfur resistance, but rather as oxidative desulfurization of adsorbed BT. While this reaction destroys BT, sulfur compounds are left behind on the metal surface. Despite the fact that Pt does not perform this catalytic destruction of BT, it is much more rapidly stripped of sulfur than Pd or PtPd when applying repeated voltammetric cycles. Therefore, we suggest that the relevant metric for comparing electrochemical sulfur resistance is the potential at which BT begins to desorb. Figure 10 overlays the first voltammetric cycles of Pt, Pd, and PtPd after poisoning with BT. All currents are normalized to the metal specific surface area for each film. No Hupd activity is evident at the Pd catalyst (Figure 10a); Pt has the most Hupd activity, and Hupd activity of the PtPd catalyst lies in-between the two pure metals. Oxide formation and BT stripping contribute to the anodic currents, which start at the most negative potentials for Pt (∼720 mV), intermediate potentials for PtPd (∼920 mV), and the most positive potentials for Pd (>1030 mV) (Figure 10b). While BT Raman bands reappear upon cycling to reducing voltages on Pt, and not Pd or PtPd, BT is more rapidly electrochemically stripped (during repeated cycling) and desorbs at a lower potential (in a given potential excursion) on Pt, and thus, Pt is determined to be more sulfur tolerant than Pd or PtPd. Careful inspection of the in situ Raman spectra as a function of potential shows that the data indicate the same relative sulfur resistance of Pt, Pd, and PtPd derived from the voltammetric data: Pt > PtPd > Pd. In the spectrum taken at 1000 mV on the initial positive-going potential excursion on Pt (Figure 6), the intensity of the strong C-C band of BT at 1570 cm-1 diminishes to PtPd > Pd that is evident in the voltammetric stripping experiments. From a practical standpoint, the BT (or sulfur) desorption potential also correlates with the BT coverage after initial exposure;lower BT desorption potential means lower initial BT coverage. While the lowest BT desorption potentials measured here are too high to be useful for complete inhibition of sulfur adsorption in a PEMFC operating at a constant voltage, lowering initial sulfur coverage at any potential enables more rapid oxidative removal of BT and other sulfur species using cyclic voltammetric methods. The enhanced sulfur removal at lower coverage derives from the bifunctional mechanism involved in the oxidative removal of sulfur (explained more extensively in the Supporting Information). Catalysts which can be rapidly stripped of sulfur would be very amenable to such regeneration. It is intriguing that, while electrochemical data for clean PtPd nanoparticle films show that both surface Pt and Pd atoms are electrochemically active (as shown clearly in Figure 3), BT adsorbed to PtPd primarily undergoes oxidative desulfurization at oxidizing potentials, as on Pd, as opposed to oxidative desorption, as on Pt. By electrochemistry alone, one only observes oxidative stripping behavior on PtPd that is intermediate between that observed on Pt and Pd separately. Why the oxidative desulfurization pathway dominates on the mixed-metal surface is unclear (rendering it effectively like an electron-poor version of Pd) but merits further investigation. The in situ Raman spectra, in addition to supporting the conclusions of the electrochemical data, also describe the actual chemical fate of BT at oxidizing potentials, which is considerably different on Pd-containing films than on pure Pt films. The combination of Raman spectroscopy and cyclic voltammetry provides a powerful and convenient method for evaluating reactions (sulfur-catalyst binding) critical to the development of sulfur-tolerant ORR catalysts for PEMFC anodes. Acknowledgment. Support for this work was provided by the Office of Naval Research. MBP is a National Research Council Naval Research Laboratory Postdoctoral Associate (2007-2009). The authors acknowledge Nimel D. Theodore and Kathryn J. Wahl (Naval Research Laboratory - NRL) for equipment use, and Debra R. Rolison (NRL) and Karen E. Swider-Lyons (NRL) for helpful discussions. Supporting Information Available: Additional data as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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