Electroreduction Activity of Hydrogen Peroxide on Pt and Au

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Electroreduction Activity of Hydrogen Peroxide on Pt and Au Electrodes Xiao Li,† Dodi Heryadi,‡ and Andrew A. Gewirth*,† Department of Chemistry and Fredrick Seitz Materials Research Laboratory and National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received April 3, 2005. In Final Form: July 15, 2005 Hydrogen peroxide electroreduction on both catalytically active Pt and inactive Au surfaces are studied by using both surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculations. SERS measurements on Pt show the presence of Pt-OH at negative potentials, which suggests that hydroxide is formed as an intermediate during the electroreduction process. Additionally, the O-O stretch mode of H2O2 is observed on Pt, which shifts to lower energy as potential is swept negatively, indicating that the O-O bond is elongated. For comparison, there is no variation in the energy of the same O-O mode on Au surfaces, and there is no observation of Au-OH. DFT calculations show that H2O2 adsorption on Pt(110) results in the dissociation of O-O bond and the formation of Pt-OH bond. On Au, O-O bond elongation is calculated to occur only on the (110) face. However, the magnitude of the elongation is much smaller than that found on Pt(110).

I. Introduction Electrocatalytic dioxygen reduction on Pt surfaces is of great importance for fuel cell, metal-air batteries, and corrosion processes.1 Because of its complex kinetics and the need for better fuel cell cathodes, considerable attention has been attracted to this field.2 On Pt surfaces, O2 electroreduction proceeds via a four-electron pathway:

O2 + 4H+ + 4e- f H2O

(1)

While on Au in acid, O2 reduction is believed to proceed via a H2O2 intermediate:

O2 + 2H+ + 2e- f H2O2

(2)

H2O2 + 2H+ + 2e- f 2H2O

(3)

in which the 2e- reduction of H2O2 in eq 3 is often the limiting step in the O2 reduction scheme and occurs only with large overpotentials.1 While details of the four-electron pathway on Pt have been intensively examined for several decades,3,4 there remains considerable uncertainty regarding the exact mechanism. On the basis of Tafel slope, Markovic and co-workers postulated that O2 electroreduction proceeds via a series mechanism in acid solution, in which the ratelimiting step is the first electron transfer to make * To whom correspondence should be addressed. Tel: 217-3338329; fax: 217-333-2685; e-mail: [email protected]. † Department of Chemistry. ‡ National Center for Supercomputing Applications. (1) Adzic, R. Recent Advances in the Kinetics of Oxygen Reduction. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 197-242. (2) Dresselhaus, M. Basic Research Needs for the Hydrogen Economy; U.S. Department of Energy, Office of Basic Energy Sciences: Washington, DC, 2004. (3) Tarasevich, M. R.; Sadkowski, A.; Yeager, E. Oxygen electrochemistry. In Comprehensive Treatise of Electrochemistry; Conway, B. E., Bockris, J. O. M., Yeager, E., Kahn, S. U. M., White, R. E., Eds.; Plenum: New York, 1983; Vol. 7, pp 301-98. (4) Wang, J. X.; Markovic, N. M.; Adzic, R. R. J. Phys. Chem. B 2004, 108, 4127-4133.

superoxide.4,5 Subsequent (rapid) coupled proton-electron transfer steps would then lead to the peroxide, hydroxide, and finally water. Oxygen reduction on Pt surfaces in acid media has been studied extensively.1,6-12 O2 reduction on Pt is sensitive to not only the structure of the surface but also to the supporting electrolyte.13 The reduction activity decreases in the order ClO4- > HSO4- > Cl-, which is correlated with the increasing strength of adsorption of these anions.14 In weakly adsorbing HClO4, the activity for oxygen reduction discerned from the half-wave potential decreases in the sequence (110) > (111) > (100),15 while in H2SO4 solution, the reaction rate order is (110) > (100) > (111).16 Site blocking measurements suggest that O2 may be adsorbed on a bridge site.17 The catalytic difference among various Pt alloy surfaces has also been investigated by using both experimental and calculational approaches.5,8,18,19 (5) Markovic, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N. Fuel Cells 2001, 1, 105-116. (6) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, 1788617892. (7) Zinola, C. F.; Castro Luna, A. M.; Triaca, W. E.; Arvia, A. J. Electrochim. Acta 1994, 39, 1627-32. (8) Christoffersen, E.; Liu, P.; Ruban, A.; Skriver, H. L.; Norskov, J. K. J. Catal. 2001, 199, 123-131. (9) Markovic, N.; Gasteiger, H.; Ross, P. N. J. Electrochem. Soc. 1997, 144, 1591-1597. (10) Zubimendi, J. L.; Andreasen, G.; Triaca, W. E. Electrochim. Acta 1995, 40, 1305-14. (11) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 121-229. (12) Blizanac, B. B.; Lucas, C. A.; Gallagher, M. E.; Arenz, M.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2004, 108, 625-634. (13) Karbainov, Y. A.; Kovedyaeva, E. I.. Zh. Anal. Khim. 1991, 46, 328-33. (14) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 508, 41-47. (15) Markovic, N. M.; Adzic, R. R.; Cahan, B. D.; Yeager, E. B. J. Electroanal. Chem. 1994, 377, 249-59. (16) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. J. Phys. Chem. 1995, 99, 3411-15. (17) Adzic, R. R.; Wang, J. X.. J. Phys. Chem. B 1998, 102, 89888993. (18) Xu, Y.; Ruban, A. V.; Mavrikakis, M.. J. Am. Chem. Soc. 2004, 126, 4717-4725. (19) Balbuena, P. B.; Altomare, D.; Agapito, L.; Seminario, J. M. J. Phys. Chem. B 2003, 107, 13671-13680.

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Before addressing the four-electron reduction of oxygen directly, we focus first on the two-electron reduction of peroxide to water on Pt and Au surfaces. If the series mechanism is indeed operative on Pt, then this reactivity will be important in the overall reaction scheme. Research examining peroxide association with Pt in acid media has focused on oscillatory behavior in current at very high overpotentials20-24 and has also used rotating ring disk (RRDE) measurements to examine the reactivity of Pt single-crystal faces.25-27 The RRDE measurements showed that peroxide reduction on all three low Miller index faces of Pt was facile immediately following removal of the irreversibly adsorbed hydroxide formed at positive potentials on Pt surfaces. Interestingly, peroxide was detected in rotating ring disk measurements at high overpotentials on some Pt surfaces during oxygen reduction because of inhibition of the four-electron pathway by adsorbed hydrogen.16 Previously, we examined peroxide reduction on a Bi underpotential deposition modified Au(111) electrode and concluded that a Bi-OH intermediate played a central role in peroxide reduction reactivity.28 There is no direct physical measurement examining the role of this or any other intermediate for peroxide reduction on Pt surfaces. Any detailed understanding concerning mechanism will require such observations. In this paper, we utilize surface-enhanced Raman spectroscopy (SERS)29 to examine peroxide reduction on Pt and Au surfaces. The technique allows examination of the low-energy vibrational region (below 1000 cm-1) where in-situ IR30 and infrared-visible sum frequency generation (SFG)31,32 spectroscopies are problematic because of a combination of low source intensity and inappropriate window materials. Additionally, the surface selection rule, operative in both IR and SFG, only allows examination of dipoles with a component normal to the electrode surface. Since O2 (and by extension, H2O2) is thought to adsorb with the O-O bond parallel to the surface,17 these species will be invisible to IR and SFG. However, the use of SERS comes with a corresponding cost. In particular, SERS must be performed on roughened polycrystalline surfaces, making detailed comparisons between structure and spectroscopy difficult. Additionally, there is still uncertainty regarding the SERS enhancement mechanism. While the use of SERS to examine Au, Ag, and Cu surfaces has been known for decades, the technique has recently been extended to examine other noble metal surfaces. In the case of Pt, enhancement is obtained by depositing a few monolayers of Pt atop a roughened Au (20) Fetner, N.; Hudson, J. L. J. Phys. Chem. 1990, 94, 6506-9. (21) Venrooij, T. G. J. V.; Koper, M. T. M. Electrochim. Acta 1995, 40, 1689. (22) Lingane, J. J.; Lingane, P. J. J. Electroanal. Chem. 1963, 5, 411. (23) Nakanishi, S.; Mukouyama, Y.; Nakato, Y. J. Phys. Chem. B 2001, 105, 5751-5756. (24) Nakanishi, S.; Mukouyama, Y.; Karasumi, K.; Imanishi, A.; Furuya, N.; Nakato, Y. J. Phys. Chem. B 2000, 104, 4181-4188. (25) Li, Y. J.; Lenigk, R.; Wu, X. Z.; Gruendig, B.; Dong, S. J.; Renneberg, R. Electroanalysis 1998, 10, 671-676. (26) Grgur, B. N.; Markovic, N. M.; Ross, P. N. Can. J. Chem. 1997, 75, 1465-1471. (27) Semenkina, T. M.; Elfimova, G. I.; Bogdanovskii, G. A. Zh. Fiz. Khim. 1978, 52, 1713-16. (28) Li, X.; Gewirth, A. A. J. Am. Chem. Soc. 2003, 125, 7086-7099. (29) Tian, Z.-Q.; Ren, B. Encycl. Electrochem. 2003, 3, 572-659. (30) Weaver, M. J. Top. Catal. 1999, 8, 65-73. (31) Richmond, G. L. Chem. Rev. (Washington, D.C.) 2002, 102, 26932724. (32) Tadjeddine, A.; Pluchery, O.; Le Rille, A.; Humbert, C.; Buck, M.; Peremans, A.; Zheng, W. Q. J. Electroanal. Chem. 1999, 473, 2533.

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surface.33 A major advance in this regard was the demonstration that pinhole-free Pt monolayers could be achieved by first forming a conformal underpotential deposition (upd) monolayer of Cu on the Au(poly) surface, followed by exchange between this upd layer and solution PtCl42-.34 The Pt monolayers provide the reactivity of Pt, while the underlying Au provides the SERS enhancement. Definitive proof that the so-constructed Pt/Au layers were indeed pinhole-free came from an examination of the vibrational spectroscopy of the interaction of CO with the Pt/Au electrodes, since CO bound to Au exhibits a different spectral signature than that associated with Pt,33 a behavior reproduced with the samples used here. However, there has been no examination of peroxide or oxygen electroreduction on this material. Ab initio calculations have been widely applied to molecule-surface systems over the last few decades.35 Compared with the cluster-based methods and embeddedadsorbate method (EAM), the supercell calculations used in this paper feature faster convergence and excellent agreement with experimental results.36 Recently, successful calculations of this type have been performed between a series of small molecules and metal surfaces,37 and we have recently used these to examine the interaction between peroxide and modified Au surfaces28,38,39 as well as SCN and Au.40 The interaction between dioxygen and Pt electrode surfaces has been addressed computationally through numerous calculations.18,41-43 Relevant to the field of oxygen reduction, Anderson has performed MP2-level calculations examining electron transfer between dioxygen and a Pt atom or atoms as a model for the surface with which it is associated.44-46 Anderson finds that the lowest energy pathway involves electron transfer to O2 associated with Pt followed by a series of one-electron transfers to the product leading to water: (i) Pt-O2 + H+(aq) + e- f Pt-OOH (ii) Pt-OOH + H+(aq) + e- f Pt-(OHOH) (iii) Pt-(OHOH) + H+(aq) + e- f Pt-OH + H2O (iv) Pt-OH + H+(aq) + e- f Pt-OH2.44,46,47 Generally, the computational results suggest the presence of Pt-OH during the dioxygen reduction process on a Pt surface.24,46 Recently, Panchenko et al. used DFT calculations to examine the interaction of O2 and several putative intermediates of O2 electroreduction on Pt(ijk) surfaces.48 (33) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Anal. Chem. 2001, 73, 5953-5960. (34) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173-L179. (35) Over, H.; Tong, S. Y., Chemically adsorbed layers on metal and semiconductor surfaces. In Handbook of Surface Science; Holloway, S., Richardson, N. V., Eds.; Elsevier: Amsterdam, 1996; Vol. 1, pp 425502. (36) Brivio, G. P.; Trioni, M. I.. Rev. Mod. Phys. 1999, 71, 231-265. (37) Lundqvist, B. I.; Bogicevic, A.; Carling, K.; Dudiy, S. V.; Gao, S.; Hartford, J.; Hyldgaard, P.; Jacobson, N.; Langreth, D. C.; Lorente, N.; Ovesson, S.; Razaznejad, B.; Ruberto, C.; Rydberg, H.; Schroder, E.; Simak, S. I.; Wahnstrom, G.; Yourdshahyan, Y. Surf. Sci. 2001, 493, 253-270. (38) Li, X.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 5252-5260. (39) Li, X.; Gewirth, A. A. J. Raman. Spectrosc. 2005, 36, 715-24. (40) Li, X.; Gewirth, A. A. J. Am. Chem. Soc. 2003, 125, 1164711683. (41) Sljivancanin, Z.; Hammer, B. Surf. Sci. 2002, 515, 235-244. (42) Watwe, R. M.; Cortright, R. D.; Mavrikakis, M.; Norskov, J. K.; Dumesic, J. A. J. Chem. Phys. 2001, 114, 4663-4668. (43) Xu, Y.; Mavrikakis, M. J. Phys. Chem. B 2003, 107, 9298-9307. (44) Anderson, A. B. Electrochim. Acta 2002, 47, 3759-3763. (45) Anderson, A. B.; Albu, T. V. J. Am. Chem. Soc. 1999, 121, 1185511863. (46) Anderson, A. B.; Albu, T. V. J. Electrochem. Soc. 2000, 147, 4229-4238. (47) Bockris, J. O. M.; Abdu, R. J. Electroanal. Chem. 1998, 448, 189-204. (48) Panchenko, A.; Koper, M. T. M.; Shubina, T. E.; Mitchell, S. J.; Roduner, E. J. Electrochem. Soc. 2004, 151, A2016-A2027.

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These authors found that H2O2 was unstable relative to OH or H2O and atomic O on all three low Miller index faces of Pt. There is as yet no direct experimental confirmation for the presence of OH or any other species on the Pt surface during the electroreduction event, although the presence of OH has been inferred from UVvisible reflectance spectroscopy.49 In this paper, we use SERS to examine the interaction between H2O2 and a Pt surface. DFT calculations are applied to investigate the interaction of H2O2 on Pt(111), (100), and (110) surfaces. For comparison, H2O2 adsorption on Au(111), (100), and (110) surfaces is also examined. II. Methodology All solutions were prepared from ultrapure water (Milli-O UVplus, Millipore Inc, 18.2 MΩcm). Reagent grade H2SO4 (Baker, UltrexII) and H2O2 (30%, Fisher) were used for preparing solutions. The working electrode for cyclic voltammetric (CV), rotating disk (RDE), and SERS measurement was a polycrystalline Au disk (Monocrystals Inc.) with a diameter of 1 cm. The crystal was annealed for 3 min in a hydrogen flame and was quenched by ultrapure water before experiments. The counter electrode was a gold wire (Alfa, 99.9985%, 0.5 mm in diameter), which was flamed every time prior to use. The reference electrode was a Ag/AgCl cell maintained in a separate compartment connected to the cell via a capillary salt bridge to minimize contamination from the reference electrode. All potentials are reported with respect to NHE in this paper. The solutions were deoxygenated by N2 for 1 h and a N2 atmosphere was maintained in the cell during all experiments. Rotating disk electrode data was obtained using a Pine Model MSRX rotator equipped with a collet to hold the gold crystal. Surface-enhanced Raman spectroscopy (SERS) was used to obtain vibrational information from Pt films deposited on SERSactive Au.50,51 Pinhole-free SERS active Pt/Au samples were synthesized as previously described.33 Voltammetry and the spectroscopic behavior of these films with respect to CO adsorption was as previously described, confirming that the films were pinhole-free. Details of the SERS instrumentation and cell have been reported previously.28 N2 was sparged and maintained in the cell during all experiments to exclude the interference of O2 from air. The spectrum acquisition time was typically 30 s. Spectra were obtained between 200 cm-1 to 1600 cm-1 at 0.1 V intervals between -0.1 and 1.0 V. The system was allowed to equilibrate for 2 min at each potential before acquiring spectra. The spectral resolution was estimated to be 3 cm-1. The baseline for each SERS spectrum was corrected prior to presentation. Density functional theory calculations with the generalized gradient approximation (GGA-PW91)41,42 were performed on periodic structures to examine peroxide adsorption on Pt and Au surfaces using CASTEP.52,53 Ultrasoft pseudopotentials were used to describe the electron-core interactions of Pt, Au, O, and H. Valence states include the 5d and 6s shells for Pt and Au, 2s and 2p for O, and 1s for H. The electronic wave functions were expanded in a plane wave basis set with an energy cutoff of 340 eV for Pt42 and 400 eV for Au.53 For total energy calculations, a Monkhorst-Pack k-point sampling scheme with 12 k-points in the supercell was applied. All Pt and Au surfaces were modeled by a (2 × 2) unit cell consisting of 3 layers and 12 metal atoms. The vacuum region between slabs was 10 Å. In all cases, surface modifications were applied to only one face of the slab. For H2O2 adsorption on the metal surfaces, the topmost metal layer was allowed to relax along with H2O2 during the adsorption process, while the other atoms were constrained at their equilibrium positions calculated without the adsorbate. Calculations for (49) Kolb, D. M. NATO ASI Ser., Ser. C 1986, 179, 301-30. (50) Weaver, M. J.; Zou, S.; Chan, H. Y. H. Anal. Chem. 2000, 72, 38A-47A. (51) Zhang, Y.; Gao, X.; Weaver, M. J. J. Phys. Chem. 1993, 97, 865663. (52) Cerius2. Cerius2 Modeling Environment, release 4.8; Accelrys Inc.: San Diego, CA, 2002. (53) Yourdshahyan, Y.; Zhang, H. K.; Rappe, A. M. Phys. Rev. B 2001, 63, 1405.

Figure 1. Cyclic voltammogram (scan rate ) 25 mV/s) from a Pt thin film on SERS active Au(poly) in solution containing (a) 0.1 M H2SO4 and (b) 10 mM H2O2 + 0.1 M H2SO4. (c) Rotating disk electrode (RDE) voltammogram (rotation speed ) 400 rpm, scan rate ) 25 mV/s) from Pt/Au in solution containing 10 mM H2O2 + 0.1 M HS2O4. The inset in (c) shows the KouteckyLevich plot of the inverse reduction current (1/i) as a function of the square root of the inverse of the rotation rate (1/ω0.5). isolated molecules were performed in the same supercell arrangement as above. Calculations were performed using an SGI Origin 2000 computer at the National Center for Supercomputing Applications (NCSA).

III. Results and Analysis 3.1. Electrochemical Behavior. Figure 1a shows the CV obtained from a Pt thin film on a SERS active Au surface (Pt/Au) in 0.1 M H2SO4 in the region between -0.1 and 1.0 V. The CV exhibits the features expected for polycrystalline Pt in acid media and is substantively different from that found on Au.33 Figure 1b shows the CV obtained from the Pt/Au sample in 0.1 M H2SO4 + 10 mM H2O2. At potentials above 0.75 V, the onset of Pt oxidation on the surface is observed.33 The current associated with H2O2 reduction begins to flow around 0.8 V.24 After reaching maximum ca. 0.7 V, the H2O2 reduction current decreases because of mass transfer effects. The small peak around 0.1 V is attributed to the adsorption of H, which is also observed absent H2O2.54 Figure 1c shows the RDE voltammogram obtained at a rotation speed of 400 rpm and a scan rate of 25 mV/s in a solution containing 0.1 M H2SO4 + 10 mM H2O2. The potential at which the maximum current occurs shifts negatively as the reduction current increases with the increase of rotating speed, which is associated with the IR drop at the surface.55 Interestingly, the reduction current is constant below 0.2 V, a behavior analogous to that found on the Pt(110) surface, which is also the most active face for this electroreduction process.26 On the other (54) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Ca, X. W.; Tian, Z. Q. Surf. Sci. 1998, 406, 9-22. (55) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001; p 718.

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Figure 2. SERS spectra obtained from Au(poly) in solution containing 10 mM H2O2 + 0.1 M HS2O4. The anodic potential sweep from -0.1 V to 1.0 V is shown on the left, while the cathodic sweep is shown on the right.

Figure 3. SERS spectra obtained from Pt/Au(poly) in solution containing 0.1 M HS2O4. The anodic potential sweep from -0.1 V to 1.0 V is shown on the left, while the cathodic sweep is shown on the right.

two low Miller index faces of Pt, the peroxide electroreduction current decreases because of H adsorption.26 The inset to Figure 1b shows the Koutecky-Levich plot of the inverse reduction current (1/i) at -0.1 V as a function of the square root of the inverse of the rotation rate (1/ ω0.5) as the rate is varied from 0 to 3000 rpm. The nearzero intercept (0.01 mA-1cm-2) of the linear regression line in the plot indicates that the current at low potentials is truly diffusion-limited.55 Similar behavior is also observed on Pt single-crystalline surfaces.26 3.2. SERS Spectra of H2O2 Electroreduction on Pt/ Au and Au Surface. SERS is used to investigate species at the interface during the electrocatalytic event. For comparison, the bare Au surface is first examined, even though this surface is inactive for H2O2 electroreduction over the range of potentials examined here.28 3.2.1. SERS from Au(poly). Figure 2 shows SERS from Au in solution containing 10 mM H2O2 + 0.1 M H2SO4 between -0.1 and 1.0 V. On the left side of the panel is the anodic scan, whereas the cathodic scan is shown on the right. Two peaks (labeled 1 and 2) are observed in the region between 700 and 1050 cm-1.56 Peak 2 at 975 cm-1 is the symmetric S-O stretch of sulfate.57,58 The intensity of νs(SO42-) decreases as the potential is scanned positively into the surface oxidation region, a behavior observed previously.58 The diminution of SERS intensity at positive potentials is possibly associated with (a) changes in surface conditions giving rise to SERS enhancement, (b) replacement by a surface water or oxide species, or (c) changes in sulfate packing on the Au surface. Interestingly, the intensity of the symmetric Cl-O stretch from perchlorate increases with increasingly positive potential on the same surface.28 Peak 1 at 860 cm-1 is the characteristic ν(O-O) mode of H2O2.59,60 Both the intensity and frequency of this mode are invariant with potential, consistent with the inability of Au to reduce H2O2 except at very negative potentials (E < ∼-0.1 V). 3.2.2. SERS from Pt/Au in 0.1 M H2SO4. Figure 3 shows the SERS spectra from Pt/Au in solution containing 0.1 M H2SO4 obtained at potentials between -0.1 and 1.0 V.

Two peaks (peaks 2 and 3) are observed in the 200-1100 cm-1 region. Peak 2 at 975 cm-1, the symmetric S-O stretch of sulfate,57,58 exhibits lower intensity at high potentials similar to the situation observed on the Au electrode. Peak 3 at 575 cm-1 is assigned to ν(Pt-OH), reported previously on Pt.51 The assignment of this band to a Pt-OH species is based on the downshift of the band with solvent deuteration.51 Although this feature is associated with Pt oxidation, it is present throughout the entire potential region studied, consistent with previous results.61 The origin of this potential invariance is unknown. Interestingly, both Weaver and we report the presence of this band under oxygen-free conditions. Figure 5 shows a plot of the potential dependence of the relative area under the 575 cm-1 band. The plot shows that the Pt-OH intensity is greatest at positive potentials and then diminishes as the potential is made more negative. Finally, we observe no features at lower energy, consistent with Weaver’s second report on Pt oxidation.61 3.2.3. SERS from Pt/Au in 10 mM H2O2 + 0.1 M H2SO4. Figure 4a shows the SERS spectra obtained from Pt/Au in a solution containing 10 mM H2O2 and 0.1 M H2SO4 between -0.1 and 1.0 V. Only the spectra obtained during the cathodic scan are shown. Peak 2 at 975 cm-1 is the νs(SO42-) mode, which exhibits the same behavior as found for both Pt/Au and Au in solutions absent H2O2. Peak 1, assigned to ν(O-O) of H2O2 on the Pt/Au surface, is observed in the range from 840 to 860 cm-1. However, unlike the situation on Au, this band does not exhibit constant frequency.28 Shown in Figure 4b is the potential dependence of the frequency of the ν(O-O) mode overlaid with the CV recorded during the SERS experiments. At potentials below 0.5 V, the frequency of ν(O-O) remains ca. 840 cm-1 during both anodic and cathodic scans. At 0.6 V, a 10 cm-1 jump to 850 cm-1 occurs. Above 0.8 V, the frequency shifts up to 858 cm-1. ν(O-O) of free H2O2 occurs at 871 cm-1.59 Even though appreciable peroxide reduction current is observed below 0.6 V, peroxide is still observed in the SERS, a result that will be discussed below. Peak 3 at 570 cm-1 is assigned to the ν(Pt-OH) mode, which was also observed in solutions absent H2O2.51 The potential dependence of the area under this peak in the presence of H2O2 is different from what was observed in the absence of H2O2. Figure 5 shows the potential dependent behavior of this band in the presence of H2O2. In contrast to the situation absent H2O2, the 575 cm-1 band has relatively little intensity at positive potentials

(56) No other potential dependent peaks were observed in the region between 200 and 1100 cm-1. (57) Brown, G. M.; Hope, G. A. J. Electroanal. Chem. 1995, 382, 179-82. (58) Feng, Z. V.; Li, X.; Gewirth, A. A. J. Phys. Chem. B 2003, 107, 9415-9423. (59) Giguere, P. A.; Srinivasan, T. K. K.. Chem. Phys. Lett. 1975, 33, 479-82. (60) Vacque, V.; Sombret, B.; Huvenne, J. P.; Legrand, P.; Suc, S. Spectrochim. Acta, Part A 1997, 53A, 55-66.

(61) Zou, S.; Chan, H. Y. H.; Williams, C. T.; Weaver, M. J. Langmuir 2000, 16, 754-763.

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Figure 6. Top (top row) and side (bottom row) views of (a) Pt(110), (b) Pt(100), and (c) Pt(111) surfaces. The blue lines outline the (2 × 2) unit cell used. In side view, the vacuum region is not shown. The gray balls represent Pt atoms. Table 1. Comparison of Calculation for Pt and Au Surfaces Pt(110) Pt(100) Pt(111) Au(110) Au(100) Au(111)

Efermi (eV)

M-M (Å)a

elongationb

-7.699 -8.615 -9.108 -6.209 -6.432 -6.742

2.659/3.813 2.696 2.704 2.829/3.967 2.836 2.847

yes no no partial no no

a The atom-atom distance along the unit cell direction on the metal surface. b O-O bond elongation following H2O2 adsorption on the surface.

Figure 4. (a) SERS spectra obtained from Pt/Au(poly) in solution containing 10 mM H2O2 + 0.1 M H2SO4 between -0.1 and 1.0 V. The cathodic potential sweep is shown. (b) Potential dependence of Raman shift of peak 1 ν(O-O) overlayed with the cyclic voltammogram during the SERS experiment. The pink and blue color indicates the Raman shift in the anodic and cathodic scan, respectively. The error bar is the instrumental resolution of 3 cm-1.

Figure 5. Potential dependence of the relative SERS area of peak 3, ν(Pt-OH), in solution with (O) and without (0) H2O2. The filled symbol represents the anodic scan, while the open one is from cathodic scan. The error bar is the relative SERS noise of area. The solid (with H2O2) and dotted line (without H2O2) are provided as a guide for the eye

but then becomes more intense beginning around ca. 0.7 V. The different behaviors observed with and without peroxide suggest that the presence of peroxide changes the activity of this moiety on the surface, especially at negative potentials. 3.3. DFT Calculation Results. The SERS provides considerable new information about the status of peroxide on the Pt surface during electroreduction. First, the O-O bond of peroxide elongates on the Pt surface during the electroreduction process, as evidenced by the decrease in frequency of the ν(O-O) mode on the cathodic sweep. Second, intact peroxide is observed in the SERS experiment, even at relatively negative potentials. Finally, the SERS suggests that Pt-OH plays a role as a possible intermediate in the peroxide electroreduction process. To

understand these behaviors and the interaction between H2O2 and the Pt surface, we undertook density functional calculations to examine the interplay between singlecrystalline surfaces of Pt and peroxide. While the SERS data were obtained from roughened Pt surfaces, Hamelin has shown that the electrochemical response from polycrystalline Au surface can be modeled by a superposition of responses from the three low Miller index faces.62 Thus, we model the response from the SERS-active Pt electrodes by using the three low Miller index faces. Calculations on Au surfaces are also performed for further understanding of the different catalytic activities found between these two metal surfaces. 3.3.1. Calculation Results for Bare Pt and Au Surfaces. Figure 6 shows top (top row) and side (bottom row) views of the slabs used for calculations of the (a) Pt(110), (b) Pt(100), and (c) Pt(111) surfaces. The blue line outlines the (2 × 2) unit cell used in all calculations. The optimized geometry for each slab is found by minimizing the total energy, and Table 1 shows the calculated properties of each slab. The calculations on bare Pt (110), (100), and (111) surfaces yielded an equilibrium Pt-Pt distance of 2.66/3.81, 2.70, and 2.70 Å, respectively, slightly smaller than the experimental value of 2.77 Å (2.77/3.92 Å for Pt(110))63 For bare Au(110), (100), and (111), the corresponding calculated distances are 2.83/3.97, 2.84, and 2.85 Å, respectively. These numbers are close to the experimental values of 2.89/4.08, 2.89, and 2.89 Å.64 The comparison between experimental and calculated results for bare Au(111) have been reported previously.40 3.3.2. Calculation Results for H2O2 Adsorption on Pt and Au Surfaces. Calculation of the free peroxide yielded a calculated O-O and O-H bond length of 1.462 Å and 0.978 Å, respectively, and an O-O-H and dihedral H-OO-H angle of 100.4° and 115.0°, respectively.28 These (62) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1. (63) Lide, D. R. CRC Handbook of Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, FL, 1999. (64) Cosandey, F.; Madey, T. E. Surf. Rev. Lett. 2001, 8, 73-93.

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Table 2. Comparison of Adsorption Energy and Geometric Properties of H2O2 on Pt and Au Surfaces surface property

Pt(110)

Au(110)

Pt(100)

Au(100)

Pt(111)

Au(111)

EFermi (eV) ∆EFermi (eV)a -Ead (eV)b O-O (Å)c O-H (Å) O-M (Å) ∆M (Å)d qMe qOc qH

-7.153 0.546 1.402 3.588 0.978 2.160 0.260 0.33 -0.71 0.41

-5.746 0.463 1.981 2.752 0.977 2.158 0.145 0.29 -0.75 0.41

-7.821 0.794 1.448 1.476 0.982 2.515 0.388 -0.09 -0.50 0.51

-5.798 0.634 0.177 1.463 0.978 2.521 0.00 -0.05 -0.56 0.57

-8.251 0.857 0.643 1.463 0.99 2.40 0.01 -0.43 0.47 0.51

-6.413 0.328 0.233 1.459 0.987 3.105 0.00 0.14 -0.48 0.57

a ∆E b c fermi ) Efermi(H2O2/M) - Efermi(M). Ead ) E(H2O2/M) - E(H2O2) - E(M). For free H2O2, the O-O and O-H bond lengths are 1.469 Å and 0.980 Å, the calculated Mulliken charges of O and H are -0.57 and 0.57, respectively. d The vertical shift distance of the metal atom which is directly contacting with O. e The calculated Mulliken charge of Pt or Au atoms which is directly contacting with O.

Figure 7. Illustration of optimized geometry of H2O2 adsorption on the Pt(110) (left) and (right) Au(110) surfaces. Au, Pt, O, and H atoms are orange, gray, blue, and white balls, respectively.

numbers are close to the experimental values of 1.47 Å for O-O bond, 0.97 Å for O-H bond, 100° for O-O-H angle, and 120° for dihedral angle.65 As the next step, we added peroxide to the various surfaces described above. 3.3.2.1. Interaction between H2O2 and (110) Surfaces. Because of the close correspondence between the electrochemical reactivity of the Pt(110) single crystal and the Pt/Au surface we used, H2O2 adsorption on Pt and Au (110) surfaces is discussed first. Figure 7 shows the energy-minimized structure of H2O2 on Pt(110) and Au(110). The energy-minimized geometry adopted by H2O2 is independent of starting geometry. On the Pt(110) surface, H2O2 is adsorbed on the surface with two O atoms on two bridge sites, with the O-H bonds pointing up out of the surface. On the Au(110) surface, H2O2 is adsorbed on the surface with both O atoms on bridge sites. Table 2 shows a comparison of the H2O2 adsorption energy and geometry on the Pt(110) and Au(110) surfaces. There are several features of note. First, both the Pt(110) and Au(110) surfaces stabilize H2O2 adsorption. Second, the O-O bond length in the adsorbed peroxide is elongated on both the Pt(110) and Au(110) surfaces. On Pt(110), the O-O bond of H2O2 molecule is elongated to 3.59 Å, which implies that the molecule is dissociated. Interestingly, this calculated bond length is longer than the experimentally observed cleavage distance for the O-O bond in peroxide of 2.9 Å.66 On the Pt(110) surface, each O atom interacts with two Pt atoms on the surface with the Pt-O distance at 2.16 Å (neutral surface). These Pt-O distances are consistent with the formation of Pt-O bonds.67-69 The (65) Giguere, P. A. J. Chem. Educ. 1983, 60, 399-401. (66) Akiya, N.; Savage, P. E. J. Phys. Chem. A 2000, 104, 44414448. (67) Wang, X.; Andrews, L. J. Phys. Chem. A 2001, 105, 5812-5822. (68) Sidik, R. A.; Anderson, A. B. J. Electroanal. Chem. 2002, 528, 69-76. (69) Bare, W. D.; Citra, A.; Chertihin, G. V.; Andrews, L. J. Phys. Chem. A 1999, 103, 5456-5462.

Figure 8. Illustration of optimized geometry of H2O2 on Pt(100) (left) and Au(100) (right) surfaces. Au, Pt, O, and H atoms are orange, gray, blue, and white balls, respectively.

result suggests that H2O2 on the Pt(110) surface is unstable relative to two Pt-OH species. O-O bond elongation is also observed on the Au(110) surface. On the Au(110) surface, the O-O bond of H2O2 becomes 2.75 Å, which is, however, much shorter than the corresponding length on Pt(110). The distance between O and Au atoms on the surface is 2.16 Å which is within the range of values found for the Au-O bond distance.67 The O-H bond distance changes little upon adsorption. The calculated O-O bond length on Au(110) is not longer than the ca. 2.9 Å distance required for O-O bond dissociation. Table 2 also shows the calculated Mulliken charges on the metal and peroxide atoms which provide more evidence for the interaction between metal and O atoms. On the neutral Pt(110) surface, the charge on O atoms decreases from -0.57 e in free peroxide to -0.71 e. The negative charge on the O atoms is compensated by the increased positive charge on Pt atoms on the surface from 0.18 to 0.33 e. On the Au(110) surface, the calculated Mulliken charges for O and the Au directly contacting both O are -0.75 and 0.29 e, respectively. 3.3.2.2. Interaction between H2O2 and (100) Surfaces. Figure 8 shows the optimized geometry for H2O2 adsorbed on Pt(100) and on Au(100). On both the Pt(100) and Au(100) surfaces, H2O2 is found on atop sites with the O-O bond not parallel to the surface. Table 2 shows a comparison of the adsorption energy and geometry for H2O2 adsorption on (100) surfaces. The adsorption energy of H2O2 on Pt(100), -1.448 eV, is more negative than that on Au(100), -0.177 eV, showing that peroxide is more strongly adsorbed to Pt than Au. The O-O bond length of H2O2 gives direct information concerning the interaction between peroxide and surface metal atoms. On Au(100), the O-O bond of H2O2 is calculated around 1.46 Å, which is close to the bond length of free H2O2. On Pt(100), the O-O bond of H2O2 molecule

Electroreduction Activity of Hydrogen Peroxide

Figure 9. Illustration of optimized geometry of H2O2 on Pt(111) and Au(111) surfaces. Au, Pt, O, and H atoms are orange, gray, blue, and white balls, respectively.

is also ca. 1.47 Å. On Au(100) surfaces, the closest distance between O and Au is 2.944 Å. This distance is too long to indicate any substantial interaction between O and Au atoms. On Pt(100), the Pt-O distance is 2.515 Å, much shorter than Au-O distance on Au(100), which indicates stronger interaction between H2O2 and the Pt surface. The calculated Mulliken charges provide more information concerning the interaction between metal and O atoms. For H2O2/Pt(100), the calculated Mulliken charge on the O atom is -0.50 e, while that for the Pt atoms directly contacting the O is -0.09 e which is similar to that found for Au(100). Thus, only a weak interaction can be deduced between H2O2 and the (100) faces of Pt and Au. 3.3.2.3. Interaction between H2O2 and (111) Surfaces. Shown in Figure 9 is the optimized geometry of H2O2 on Pt(111) and Au(111). On the Pt(111) surface, each O atom is found near an atop site while the H2O2 molecule is on a bridge site. The O-O bond makes an angle of 25° with the Pt(111) surface plane. These geometries agree with that inferred on Pt(111) obtained by using site-blocking measurements17,70 and with previously reported extended Hu¨ckel calculational results.71 On the Au(111) surface, each O atom from H2O2 is at a bridge site while the whole molecule resides over an hcp site and the molecule is roughly parallel to the Au surface. Table 2 shows the geometry and properties of H2O2 adsorbed on Pt(111) and Au(111) surfaces. Even though the adsorption energy on all surfaces is negative, the calculation does indicate that H2O2 is more strongly adsorbed on Pt than Au. With the Pt(111) slab, Ead of H2O2 is -0.643 eV. Next, we examine the O-O bond distance. The O-O and O-H bond length calculated on Pt and Au surfaces is nearly identical with that in the free molecule. The distance between O and nearest Au atom is 3.11 Å in the H2O2/Au(111) system, which is well beyond the range of any substantial Au-O interaction. The distance between O atoms and the closest Pt on the surface is 2.40 Å. The calculated Mulliken charge on the O atoms is -0.43 e while that on the Pt atom below the O is 0.01 e. When H2O2 is adsorbed on Pt(111) and Au(111), the calculated Mulliken charge for O is close to that in free H2O2 molecule, which indicates the presence of only weak interactions between O and metal surface atoms. IV. Discussion The measurement and calculations reported above provide considerable insight into the mechanism of H2O2 electroreduction on both the catalytically active Pt and the catalytically inactive Au surfaces. Because the SERS data show the presence of both the intact peroxide and a (70) Marinkovic, N. S.; Wang, J. X.; Adzic, R. R. Proc. Electrochem. Soc. 1997, 97-17, 251-262. (71) Zinola, C. F.; Arvia, A. J.; Estiu, G. L.; Castro, E. A. J. Phys. Chem. 1994, 98, 7566-76.

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Pt-OH species at potential where the peroxide reduction reaction occurs, we have to consider that both species are present at the interface during the reaction. The presence of the O-O stretch corresponding to the peroxide is somewhat unexpected, since SERS measurements on the Bi upd system did not reveal any peroxide present during either peroxide 28 or oxygen reduction reactivity. 38 4.1. Electrochemical and SERS Results. 4.1.1 Electrochemical and SERS Results on Au. No features were observed in the cyclic voltammogram obtained on Au(poly) because of the reduction of H2O2 within the potential region studied here. The inactivity of Au toward both peroxide and oxygen electroreduction at these potentials in acid media has been long known.5 SERS measurements also show that both the intensity and the energy of ν(O-O) of H2O2 exhibit little potential dependence which indicates that H2O2 is present on or near the Au surface throughout the potential range examined here. We found a similar result, albeit in a solution with a different electrolyte, in a previous paper.28 4.1.2. Electrochemical and SERS Results on Pt/Au. In contrast to the Au(poly) surface, voltammetry from the Pt/Au sample indicates that this surface is active for peroxide reduction at potentials below ca. 0.8 V. This behavior is similar to that found previously on Pt singlecrystal electrodes.26 On the Pt(100) and Pt(111) surfaces, the electroreduction current was found to decrease at low potentials, because of competitive adsorption of hydrogen, while such behavior was not found on Pt(110).26 On the basis of this comparison, our polycrystalline material exhibits characteristics in common with Pt(110), since the current here also does not decrease at negative potentials. The current density measured at our sample at a given rotation rate is roughly equivalent to that found for the single crystals, given the uncertainties in determining the surface roughness factor. There are two important features seen in the SERS spectra obtained from Pt. First, SERS spectra show that the frequency of ν(O-O) changes with potential on Pt/Au. An abrupt 20 cm-1 decrease in this frequency is observed at potentials below ca. 0.8 V. The decrease in frequency is associated with elongation of the O-O bond. An estimate of the magnitude of elongation is ca. 0.01 Å by using Badger’s rule, as described in the Supporting Information. This simple estimate does not take into account interactions between the peroxide and the surface and may for that reason underestimate the degree of elongation. The onset of peroxide elongation is commensurate with the potentials where peroxide electroreduction occurs. Interestingly, the ν(O-O) mode of H2O2 is observed at all potentials. The presence of this mode above 0.8 V is attributed to H2O2 which is on or near the surface but not reduced. The presence of this band at lower potentials during electroreduction is somewhat surprising, since the near-zero Koutecky-Levich intercept for peroxide electroreduction indicates that this process is mass transport limited. However, the presence of the ν(O-O) mode must indicate that peroxide is present at or near the Pt surface or at least near some sites on this surface. An exact accounting of the fate of all of the peroxide requires detailed understanding of the interrogation length of the SERS measurement into the solution (typically quoted to be ca. 1-2 nm)72 and a more intimate description of the electrode surface, which is not available in the present measurements. Nonetheless, the bond length elongation observed here does show that peroxide interacts with the Pt surface and that this interaction is commensurate with potentials (72) Weaver, M. J. J. Raman Spectrosc. 2002, 33, 309-317.

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where electroreduction occurs. RDE studies of peroxide electroreduction on Pt(ijk) show that this activity is facile on all three low Miller index faces of Pt. One possible reason for the apparent disparity with these results is the relative time scale of the SERS and electrochemical measurements; the electrochemical measurement, even when mass transport limited, occurs on a time scale that is several orders of magnitude slower than the time scale of the Raman measurement (10-8 s vs 10-15 s).55 The faster time scale of the spectroscopic measurements presented here allows detection of species that are not inferred from the slower electrochemical studies. The second interesting feature observed in the SERS is that the behavior of the Pt-OH mode on the Pt/Au surface is dependent on the presence of peroxide. Absent peroxide, this band increases in intensity at high potentials, which is consistent with the expected surface oxidation. As remarked above, this band does not disappear entirely at more negative potentials, as was found previously. However, in the presence of peroxide, the intensity of the Pt-OH stretch increases at negative potentials. The onset potential for this increase is ca. 0.7 V, which again correlates with the onset of peroxide electroreduction activity. The observation of increased intensity in this mode in the presence of peroxide suggests a role for a Pt-OH species during the electroreduction event. The experiment thus appears to describe two different behaviors for H2O2 during the electroreduction event. On one hand, it would appear that peroxide is dissociated into Pt-OH species on the Pt surface. This behavior is reminiscent of that found for the Bi/Au system, where dissociative adsorption of H2O2 was found to occur, producing two Bi-OH moieties.28 These moieties were subsequently reduced during the electroreduction process. On the other hand, we also observed a signal associated with intact peroxide, the potential sensitivity of which indicates that the peroxide must be interacting with the surface. How can there be two different behaviors for peroxide on the Pt surface? One possibility is that the heterogeneous SERS active Pt surface provides sites which evince both activities. Another possibility is that the Pt-OH and H2O2 species are both present on the surface, but their residence times are different. To provide some insight into the behavior of peroxide on Pt and Au surface, we performed calculations examining this interaction. 4.2 Calculations for Au. DFT calculations for H2O2 adsorption on both the Au(111) and Au(100) surfaces suggest that H2O2 and Au interact only weakly. In particular, the adsorption energies for peroxide on Au are low (ca. 0.2-1.4 eV). Additionally, O-O bond elongation upon adsorption, associated with reduction reactivity on Bi surfaces28 and observed experimentally on Pt (above), is not found on Au(111) or Au(100). The Au-Operoxide distances on Au(111) and Au(100) are all calculated to be above 2.5 Å, which is considerably greater than the distance expected for a Au-OH bond (2.2 Å). These results from the calculations strongly support the experimental observation of no reactivity for peroxide reduction on these surfaces. However, on Au(110), H2O2 is adsorbed with an elongated O-O bond. The calculated magnitude of elongation is below the 2.9 Å distance necessary for O-O bond cleavage.66 At the level of examining O-O bond elongation, the calculation thus suggests that Au(110) surfaces should be more active toward peroxide electroreduction relative to the other two faces of Au examined here. Experimentally, O2 electroreduction on Au in acidic solution proceeds

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via a 4e- pathway on some stepped Au surfaces at high overpotentials, while only a 2e- process is found for all three low Miller index faces regardless of overpotential.73,74 This observation suggests that H2O2 electroreduction is not favored on smooth Au surfaces. Additionally, there is no evidence in the SERS described above for O-O bond elongation. The discrepancy between calculation on Au(110) and experiment may be resolved in two different ways. First, the calculations performed here can only generate the optimized geometry of the system and do not provide any information concerning the kinetics of the reaction. The association of bond elongation with reactivity may not be entirely accurate, especially when the bond elongation is below the dissociation limit. Second, as mentioned above, these calculations do not include the effect of electrolyte or water, nor do they model electrochemical double layer. These additional factors may play a role in cases where the bond elongation is not explicitly large. 4.3. Calculations for Pt. 4.3.1 Interaction with H2O2. DFT calculations show that H2O2 adsorption on Pt surfaces is much different from that on Au. On Pt(110), association of H2O2 with the surface results in a calculated O-O bond length of 3.6 Å. This distance is well beyond the O-O bond cleavage distance of 2.9 Å. The calculated Pt-O bond distance is 2.16 Å. Since Pt-O(H) bond distances are observed experimentally in a range between 1.9 and 2.1 Å,67-69 this result suggests that a Pt-OH bond has formed on the Pt(110) surface. For the (110) face, then, the calculation suggests that as soon as bare Pt is available for interaction with peroxide, reactivity will ensue. The bond elongation calculated for Pt(110) is not found on either Pt(100) and Pt(111). Additionally, the calculated -∆Eads for H2O2 Pt(111) is less than that found on Pt(110). The calculations suggest that the Pt(110) surface is most active for peroxide reduction on the basis of the substantial O-O bond elongation calculated. The bond elongation occurs as the peroxide is added along the (001) direction of the unit cell. This direction features a Pt-Pt distance calculated to be 3.8 Å (experimental value: 3.92 Å) The longer Pt-Pt distance apparently favors the dissociative addition of peroxide between the Pt atoms on this face. On the other two low Miller index faces, Pt-Pt distances are considerably smaller (ca. 2.7 Å), and adsorption without substantial elongation is calculated. Adsorption of peroxide across the fourfold hollow site on the Pt(100) surface (the (011) direction) would access the 3.9 Å Pt-Pt spacing, but this geometry is not found, for unknown reasons. Interestingly, the (2 × 2)-Bi underpotential deposition lattice also apparently activates H2O2 electroreduction via dissociative adsorption across two adatoms several Å distant.28 The suggestion from the calculation that the activity sequence for peroxide reduction is Pt(110) > Pt(100) > Pt(111) is not found experimentally.16,26 Instead, electrochemical results suggest an activity sequence of Pt(100) > Pt(110) > Pt(111) on the basis of the overpotential of peroxide reduction on these faces. This discrepancy between calculation and experiment may be explained through the effect of irreversible hydroxide formation on the surfaces.5 Interestingly, the oxide begins to form on the Pt(110) surface at potential >0.8 V, while the formation of oxide occurs at a relative higher potential on Pt(100).26 In the calculations performed here, there is no accounting for the activity of other species, such as water and electrolyte, at the electrode surface. (73) Strbac, S.; Adzic, R. J. Serb. Chem. Soc. 1992, 57, 835-48. (74) Xu, Y.; Mavrikakis, M. J. Chem. Phys. 2002, 116, 10846-10853.

Electroreduction Activity of Hydrogen Peroxide

That peroxide O-O bond elongation on Pt(100) and Pt(111) is not found in these calculations appears in contrast to the high reactivity for peroxide electroreduction observed on all three low Miller index faces of Pt. Peroxide reduction activity commences as soon as the potential is positive enough to remove blocking Pt-OHads species formed from decomposition of water.26 Of course, it may well be the case that bond elongation is not the only measure of activity for electroreduction. These computational results are in contrast to those of Panchenko et al. who stated that peroxide was spontaneously decomposed to either OH or H2O and atomic O on all three low Miller index faces of Pt. 48 We do not understand the origin of the discrepancy between the two calculations. While there is an apparent disconnect between the electrochemical measurements and the calculations reported here, these calculations do provide some support for the SERS results. The calculations suggest that bound H2O2 should be formed upon interaction of H2O2 with low Miller index Pt faces. The SERS measurement supports this suggestion. Unknown is the residence time of the peroxide and the influence of applied potential on subsequent reactivity. On some faces of Pt, peroxide interacts with the surface and has a finite residence time on it (which may, however, be significantly shorter than the residence time interrogated by the electrochemical measurement). The calculation suggests that the Pt(100) and Pt(111) faces may be associated with this behavior. The SERS measurement also indicated the presence of PtOH species on the surface following peroxide introduction. The calculation shows that Pt(110) addition of peroxide to the Pt surface results in cleavage of the O-O bond of H2O2, leading to Pt-OH formation on the surface. The formation of Pt(OH)2 or PtOH stabilizes the dissociative adsorption of H2O2 on the Pt surfaces. In this respect, the calculations and the experimental results are consistent. V. Conclusion We examined the interaction between H2O2 and Pt or Au surfaces by using SERS and detailed DFT calculations. SERS measurements show the presence of a Pt-OH band

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at negative potentials, suggesting that peroxide has dissociated or at least strongly adsorbed on Pt during electroreduction. Additionally, SERS measurements reveal the presence of an elongated O-O bond after removal of irreversibly adsorbed OH and commensurate with the onset of catalysis. No change in O-O bond-stretching frequency was observed on Au nor was there any evidence found for Au-OH formation associated with peroxide introduction. DFT calculations confirm the presence of Pt-OH species on the (110) face of Pt and suggest formation of a Pt-O bond between Pt and H2O2 on the other faces. The origin of the calculated increased activity for Pt(110) is the longer Pt-Pt distance accessed by the peroxide relative to the other two faces. The calculations provide an explanation for the elongated O-O bond observed in the SERS measurement. On Au, only the (110) face exhibited any propensity for O-O bond elongation; however, the calculated magnitude of elongation was considerably less than that found for Pt(110). These observations suggest that one pathway for peroxide electroreduction on Pt involves the dissociative adsorption of peroxide to form two Pt-OH species. Electron flow reduces this bond, resulting in the formation of OH-, which then associates with H+ in solution to form water. Acknowledgment. The author thanks J. O. White of the Laser Laboratory in the Frederick Seitz Materials Research Laboratory at the University of Illinois for his assistance in Raman data acquisition. The Laser Laboratory is funded by Department of Energy grant DE-FG0291ER45439 through the Materials Research Laboratory at the University of Illinois. This work was funded by the NSF (CHE-02-37683), which is gratefully acknowledged. Supporting Information Available: Derivation of the relationship between O-O bond length and stretching frequency and geometries (Cartesian coordinates) of the calculated slabs. This material is available free of charge via the Internet at http://pubs.acs.org. LA0508745