Structure of Water at the Electrified Platinum−Water Interface: A

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J. Phys. Chem. C 2008, 112, 4248-4256

Structure of Water at the Electrified Platinum-Water Interface: A Study by Surface-Enhanced Infrared Absorption Spectroscopy Masatoshi Osawa,*,† Minoru Tsushima,†,‡ Hirokazu Mogami,§ Gabor Samjeske´ ,†,‡ and Akira Yamakata† Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan, CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan, and Graduate School of EnVironmental Science, Hokkaido UniVersity, Sapporo 060-0801, Japan ReceiVed: October 27, 2007; In Final Form: NoVember 23, 2007

Surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode is used to examine the structure of water on a polycrystalline Pt electrode in H2SO4 and HClO4 as a function of applied potential. The electrode surface covered with CO is used as the reference in recording spectra, which enables us to obtain the absolute infrared spectrum of the interfacial water layer (monolayer or bilayer) in contact with the surface with negligible interference from the bulk water. The spectrum of the interfacial water is largely different from that of bulk water and changes around the potential of zero charge of the electrode. The spectral changes are ascribed to the potential-dependent reorientation of water molecules from a weakly hydrogenbonded oxygen-up orientation at the negatively charged surface to a strongly hydrogen-bonded nearly flat orientation at the positively charged surface in agreement with theoretical simulations reported in the literature. Clear experimental evidence of the formation of a stable ice-like structured water on the positively charged surface is reported.

1. Introduction The structure and properties of water adsorbed on metal surfaces has been the subject of numerous experimental and theoretical investigations due to their importance in many scientific and technological disciplines including surface science, electrochemistry, heterogeneous catalysis, corrosion, and biochemistry.1-4 Significant progress has been made in studies in ultrahigh vacuum (UHV) environment, owing to a number of powerful surface analytical tools.1,2 It has been established that an icelike structure is formed on hexagonally close-packed surfaces of transition metals such as Pt, Ni, and Ru at low temperature ( 0.8 V is ascribed to the oxidation of the Pt surface, and the cathodic peak at 0.75 V on the reverse negative-going sweep is due to the reduction of surface oxide. The waves at 0.05 < E < 0.4 V originate from the underpotential deposition of hydrogen (Hupd) at hollow sites.40 The coverage of Hupd saturates around 0.05 V and additional hydrogen atoms, the so-called overpotentially deposited hydrogen (Hopd), are adsorbed at atop sites at more negative potentials. Recombination of two Hopds results in H2 evolution at E < 0.05 V.41 Cyclic voltammetry also revealed that the adsorption of Hupd is totally suppressed

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Figure 1. Cyclic voltammogram for a Pt thin film electrode in 0.1 M H2SO4 recorded at 50 mV s-1.

Figure 2. SEIRA spectra of water on the Pt electrode in 0.1 M H2SO4 recorded at potentials indicated and referenced to the single-beam spectrum of the CO-covered surface as the reference. The upward and downward bands correspond to the species on the CO-free and COcovered Pt surfaces, respectively. The spectra were offset for clarity.

in the CO-saturated solution (data not shown); that is, the surface is fully covered by adsorbed CO. Figure 2 shows a set of SEIRA spectra of the Pt electrode surface in 0.1 M H2SO4 acquired at several potentials. The spectral acquisition is not disturbed by H2 evolution in the ATR configuration,41 but the large ohmic drop prevented the polarization to potentials more negative than -0.04 V. The spectra were referenced to the single-beam spectrum of the electrode covered by CO, and hence the C-O stretching modes of CO adsorbed at atop and bridge sites of Pt are observed as the downward bands at 2070 and 1880 cm-1, respectively. Although the electrode surface was fully covered by CO, additional downward bands assignable to ν(OH) and HOH bending, δ(HOH), modes of water are observed at 3656 and 1630 m-1, respectively. It is worth noting that these water bands, characteristic of water free from hydrogen bonding, are commonly observed on CO-covered metal electrodes at almost the same frequency and disappear when CO is oxidized.30,36,42,43 Accordingly, these bands are ascribed not to water adsorbed on the surface but that on top of the CO adlayer. Support of this is given by the frequency of the δ(HOH) band, which is different from that for water in contact with the surface as will be described later. On the other hand, the upward bands are ascribed to the species on the COfree surface. Two ν(OH) bands and a δ(HOH) band of water are found at ∼3500 and ∼3000 cm-1 and at ∼1610 cm-1, respectively (shown by thick arrows). The additional bands at 1200-950 cm-1 that grow in intensity as the potential is made more positive are assigned to S-O stretching modes of sulfate

Osawa et al. or bisulfate adsorbed on the electrode surface.44-46 The absence of the characteristic bands of bulk water at 3400 and 1645 cm-1 20,21 and also the potential dependent shift of the (bi)sulfate bands implies that the signals from the bulk solution were successfully cancelled out. As demonstrated in Figure 2, the spectrum of the interfacial water is potential-dependent. The very broad band centered around 3000 cm-1 emerges at ∼0.2 V and grows as the potential is made more positive. The other water bands at 3500-3570 and 1610 cm-1 are also potential-dependent, but the superposition of the downward bands of the reference spectrum obscures the shapes and frequencies of the upward water bands. To remove the interference from the downward bands, the spectra in Figure 2 were deconvoluted by assuming Gaussian function and the downward bands were subtracted from the original spectra. To minimize artifacts caused by the deconvolution, averages of the spectral parameters (frequency, peak intensity, and width) for each downward water band were used for the numerical background subtraction. The result is shown in Figure 3, where the spectra in the hydrogen absorption region (E < 0.4 V), double-layer region (0.4 < E < 0.8 V), and surface oxidation region (E > 0.8 V) are depicted in blue, red, and black, respectively, to facilitate the comparison with the CV (Figure 1). This figure clearly shows the potential dependence of the water vibrations at ∼3500, 3000, and 1600 cm-1. The weak structure at 3600-3700 cm-1 is an artifact produced by the deconvolution (the residue, cf. Figure 4). At -0.04 V where H2 evolves, a ν(OH) band is observed at 3550 cm-1 with an asymmetric shape extending toward the lower frequency region. As the potential is made more positive, the band is shifted to lower frequencies (3480 cm-1 at 0.25 V) and becomes slightly broader. Concomitant with the red shift of the ν(OH) mode, the corresponding δ(HOH) mode also is red-shifted (1613 cm-1 at -0.04 V to 1600 cm-1 at 0.4 V) accompanied by the decrease in intensity. Additionally, the very broad ν(OH) band at ∼3000 cm-1 appears at 0.15 V and grows in intensity at more positive potentials (which is also evident in the original spectra in Figure 2). At potentials higher than ∼0.3 V, the relatively sharp ν(OH) band at ∼3500 cm-1 shifts to higher frequencies (3570 cm-1 at 0.8 V) and a new weak δ(HOH) band emerges at 1645 cm-1. A few more weak bands appear to exist in the δ(HOH) region. The broad band at ∼1720 cm-1 may be assigned to the asymmetric bending mode of hydronium ion,20 but other bands are not clearly identified due to the wavy baseline. The intensities of the water bands at ∼3540, ∼3000, and 1645 cm-1 are nearly constant at 0.5 < E < 0.8 V. The first and the last bands totally disappear at high potentials, implies that these two bands arise from the same state of water. We assumed that water on top of the CO adlayer has absorption bands only at 3656 and 1630 cm-1 in the numerical background subtraction. However, there exists a possibility of the presence of more absorption bands that are obscured by the upward bands. To check the validity of this assumption, potential difference spectra were calculated from the single-beam spectra of the CO-free surface by using the spectrum at 1.2 V as the reference. The reference was chosen due to the absence of the bands at 3500-3600 and 1500-1700 cm-1 at this potential (Figure 3). The result is shown in Figure 4, which is essentially identical to that reported by Futamata et al. (Figure 2A in ref 31). Note that the water bands in the absolute spectra (Figure 3) are found in the potential difference spectra at almost the same frequencies and no other new bands are found except for the bands at ∼3400 (shoulder denoted by asterisk) and 2080

Interfacial Water at Pt-Acid Interface

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Figure 3. Absolute infrared spectra of water at the Pt electrode in 0.1 M H2SO4, calculated from Figure 2 by numerically subtracting the downward bands. See text about the numerical subtraction.

Figure 4. Potential difference spectra of water at the Pt electrode in 0.1 M H2SO4 calculated by using the single-beam spectra at 1.2 V as the reference. Asterisk shows the ghost band created by the calculation.

cm-1. It is apparent from Figure 3 that the shoulder is a ghost produced by the high-frequency tail of the broad band centered at ∼3000 cm-1 in the reference spectrum at 1.2 V. On the other hand, the band at 2080 cm-1 is assigned to Hopd adsorbed at atop sites.41 This band is very close to that of atop CO and thus invisible in Figure 2. From the comparison of absolute and potential difference spectra, the validity of the numerical background subtraction procedure is confirmed.

The potential difference approach is superior to background subtraction with the CO-covered surface as the reference in some aspects. For example, the systematic changes in vibrational frequency and band intensity are clearer in the potential difference spectra due to the better signal-to-noise ratio and the stable baseline. It also became clear that the weak structures at 3600-3700 and 1500-1800 cm-1 in Figure 3 are not the absorption bands of interfacial water. The most notable is that the hydronium band observed at ∼1720 cm-1 in the absolute spectra is totally missing in the potential difference spectra. Nevertheless, problems of the potential difference approach should also be noted. The most serious is the creation of ghost bands as mentioned above. More ghost bands can be created by using other reference potentials (Supporting Information, Figure S1). On the other hand, the broad ν(OH) band at ∼3000 cm-1 is weakened and obscured in the double-layer region. Futamata et al.31 did not notice this broad band, probably because their potential difference measurements were limited to the double-layer region where the spectral changes are small. These problems can lead to misunderstanding. From this point of view, the grasp of the global spectral features from absolute spectra is primarily important for detailed analysis of the spectra. Sulfate ion (existing mostly as bisulfate ion in strong acid) is known to specifically adsorb on Pt and could affect the structure of the interface. As is found in Figure 2, the adsorption of (bi)sulfate occurs at potentials more positive than 0.15 V and desorbed from the surface in the surface oxidation region (E > 0.8 V). It is worth noting that the potential at which (bi)sulfate starts to adsorb is close to the pzc of polycrystalline Pt (∼0.18 V vs SHE,47 which is ∼0.25 V vs RHE in 0.1 M H2SO4). The intensities of the two (bi)sulfate bands at 1180-1200 and ∼1100 cm-1 are plotted in Figure 5 as a function of the applied potential. The former band continues to increase up to

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Figure 5. Potential dependence of the integrated band intensities at 1180-1220 (b) and 1100-1116 cm-1 (O) for (bi)sulfate adsorbed on the Pt electrode from 0.1 M H2SO4. Dashed trace represents the cyclic voltammogram of the Pt electrode (same as in Figure 1). The solid curve represents the coverage of Hupd calculated from the voltammogram.

Osawa et al.

Figure 7. (a) Infrared spectra of perchlorate adsorbed on the Pt electrode from 0.5 M HClO4; (b) potential dependence of the integrated intensity of the band at 1110 cm-1.

saturates at ∼0.5 V. This behavior is similar to that for adsorbed (bi)sulfate. Since the vibrational frequency of this band is identical to that for the ion in the bulk solution and independent of the applied potential within the spectral resolution used (4 cm-1), perchlorate ion is believed to be adsorbed only weakly. 4. Discussion

Figure 6. Absolute spectra of water at the Pt electrode in 0.5 M HClO4, recorded at the potentials indicated and calculated as in Figure 3.

0.8 V and then decreases by desorption, while the latter band reaches a maximum at ∼0.4 V and decreases at more positive potentials. There exist several explanations for the different potential dependences of the two bands; the shift of the sulfatebisulfate equilibrium,44,45,48 and the change in adsorption sites49 or orientation (coordination number).50 Although this issue is beyond the scope of the present study, it is noted that the average of the two band intensities is well correlated with the adsorbed amount of (bi)sulfate measured by radiochemical assay.51 That is, the coverage of (bi)sulfate is almost constant in the doublelayer region. The absence of remarkable spectral changes for the interfacial water in the double-layer region may be correlated with the constant coverage of (bi)sulfate. The coverage of Hupd calculated by integrating the cathodic waves at 0.05-0.4 V also is shown in the figure (solid curve) for reference. 3.2. In Perchloric Acid. The absolute spectra of the interfacial water at the Pt surface in 0.5 M HClO4, measured and calculated by the same procedure as in H2SO4, are depicted in Figure 6. The spectra are essentially identical to those observed in H2SO4 except for the slightly higher frequency of the sharp ν(OH) band (3560-3590 cm-1) and the presence of a weak band at 3370 cm-1. An additional band assignable to a ν(Cl-O) mode of perchlorate adsorbed on the surface was observed at 1100 cm-1 as shown in Figure 7. This band emerges around 0.2 V close to the pzc, increases its intensity with increasing potential, and

Owing to the use of ATR configuration and background subtraction, the infrared spectrum of the interfacial water on the Pt electrode was found to largely differ from that of bulk water. Since the ordering of water molecules at the interface is lessened with increasing distance from the surface and completely disappears within three to four layers,10-16,19 the observed spectra can safely be ascribed to the ordered layer. The absence of the broad ν(OH) band characteristic of disordered water molecules (liquidlike water) around 3400 cm-1 52-55 also supports this discussion. Remembering that infrared absorption enhancement sharply decays with increasing distance from the surface,37 the first layer in contact with the surface gives larger absorption than the second and higher overlayers (Supporting Information, Figure S2). However, the assessment of the exact spatial range probed by ATR-SEIRAS is still a matter for discussion, and a question remains whether the obtained spectra are only of the first layer or also contain contributions from the second and higher overlayers. The observation of the free water on top of the CO adlayer in Figure 2 (downward bands) indicates that the contributions from the second and higher overlayers are included in the absolute spectra in Figure 3 that were calculated from Figure 2. On the other hand, the absence of the 1720-cm-1 band of hydronium ion in Figure 4 indicates that the contributions from overlayers are much less in the potential difference spectra, because hydronium ion is discharged to yield adsorbed hydrogen at E < 0.4 V and should not exist at the electrode surface (also at the positvely charged surface due to electrostatic repulsion). Since significant differences were not observed between the absolute and potential difference spectra of water, we think that the observed spectra are mainly of the first layer (or bilayer) in contact with the surface. For analyzing the spectra, the following fundamental aspects of water vibration1,52 should be kept in mind: Water monomer has symmetric and antisymmetric ν(OH) modes at 3720 and 3660 cm-1 and a δ(HOH) mode at 1595 cm-1. The ν(OH) modes are red-shifted and become broader and more intense by hydrogen bonding, while the δ(HOH) mode is blue-shifted. In the strongly hydrogen-bonded network system, the distinction between the symmetric and antisymmetric ν(OH) modes breaks down due to intermolecular coupling. Rather, a spectrum has

Interfacial Water at Pt-Acid Interface

Figure 8. Schematic drawing of the possible structures of water at the negatively (left) and positively (right) charged Pt surface, deduced from the SEIRA spectra.

to be discussed in terms of two individual OH oscillators. It is also well-known that the ν(OH) is remarkably red-shifted from the gas-phase spectrum for water molecules hydrogen-bonded to other water molecules by donating hydrogen atoms (hydrogen donors), while the shift is not so significant for hydrogen acceptors.52,53 4.1 Interfacial Water in the Hydrogen Adsorption Region. It has been believed that water molecules are hydrogen-bonded to adsorbed H atoms via oxygen atoms (i.e., oxygen-down orientation).25,32,50,56 In this case, the first water layer is hydrogen acceptors in the hydrogen bonding with adsorbed H atoms and hence their vibrations are not perturbed so greatly. However, the oxygen-down orientation facilitates hydrogen bonding with the second layer as hydrogen donors. Therefore, the ν(OH) band of the first water layer should be broadened and largely redshifted from that of free water. Different from the expectation, interfacial water exhibited a relatively sharp ν(OH) band at 3500-3550 cm-1. The frequency is between those for free water on top of the adsorbed CO (3656 cm-1) and for hydrogenbonded water in the bulk (3400 cm-1), indicating that interfacial water molecules are weakly hydrogen-bonded. On the other hand, the frequency of the δ(HOH) band (1600-1613 cm-1) is lower than the corresponding band of bulk water (1645 cm-1) and also of free water on top of adsorbed CO (1630 cm-1). Rather, it is very close to that of water monomer (1595 cm-1). The extremely low vibrational frequency cannot be explained solely by weak hydrogen bonding and suggests the interaction of oxygen lone pair with the surface.1,57 The observed spectral features can be explained by assuming that water in contact with the surface is oriented by directing two hydrogen atoms toward the surface to some extent as schematically depicted in Figure 8 (left). The tilted oxygen-up orientation sterically prevents hydrogen bonding among water molecules of the first layer and allows the interaction of one of the oxygen lone pairs with the surface. Although the remaining lone pair directed toward the solution phase can be used for hydrogen bonding with the second layer, the water molecules of the first layer are hydrogen acceptors, and hence the ν(OH) vibration is not perturbed so significantly from free water as observed. This orientation is consistent with the most theoretical simulations.3,10-16 The tilted oxygen-up orientation is determined by the interplay between the electrostatic interaction of the water dipole with the electric field and the interaction of the oxygen lone pair with the surface. Although other simulations17,18 predict a vertical orientation in which hydrogen atoms directly interact with the surface, this orientation is not likely because no ν(OH) bands characteristic of OH‚‚‚Pt bonding were observed in the expected spectral range (2850-2935 cm-1).1,58,59 The observed spectral features strongly suggest that water does not form strong hydrogen bonds with adsorbed hydrogen atoms; that is, the hydrogen-adsorbed Pt surface is hydrophobic like a hydrogen-terminated Si surface. This is supported by the

J. Phys. Chem. C, Vol. 112, No. 11, 2008 4253 fact that the spectral features in the Hupd and Hopd regions are identical to those observed on a negatively charged Au(111) surface on which hydrogen atoms are not adsorbed.20,21 The result is consistent with the UHV studies, which suggested that low coverage of hydrogen has little or no influence on the adsorption/desorption properties of water.2 No interaction with Hupd is understandable when it is considered that Hupd atoms are located at hollow sites40 and are very small. More interesting is that Hopd atoms at atop sites also do not affect the water structure. Since the coverage of Hopd is roughly estimated to be 0.6∼0.7 at -0.04 V,41b a considerable number of the interfacial water molecules were expected to be repelled from the interface by Hopd atoms. However, no clear decrease in band intensity was observed in the spectra. Note that the water detected here is in contact with the surface via an oxygen lone pair and not located on top of Hopd, as is evidenced by the low δ(HOH) frequency. The results seem to suggest that coverage of water in contact with the surface is 0.3-0.4. As the potential was increased from -0.04 to 0.4 V, both ν(OH) and δ(HOH) bands were red-shifted and the latter band became weaker. On the basis of the surface selection rule,22,37 the weakening of the δ(HOH) band is explained in terms of the reorientation of water to more flat orientations caused by the decrease in the negative surface charge (the weakening of the electric field at the interface). The removal of water from the interface by the adsorption of anions (Figures 5 and 7) also contributes to the weakening to some extent but cannot fully account for the result because the anion coverage is rather small even at saturation [ 0.8 V) regions. In the hydrogen adsorption region, water molecules were weakly hydrogenbonded. The result was explained by assuming a tilted oxygenup orientation of water. The hydrogen-adsorbed Pt surface was hydrophobic, and water did not interact with adsorbed hydrogen (both Hupd and Hopd atoms). With the increase of the potential across the pzc, water molecules were reoriented to a more flat orientation and formed an icelike structure by strong hydrogen bonding with the surrounding water molecules. Although coadsorbed supporting anions partly break the hydrogen bonding in the icelike structure, the icelike structure was stable over the double-layer region. The icelike structure was more stable on

J. Phys. Chem. C, Vol. 112, No. 11, 2008 4255 the Pt electrode than on Au(111). Comparison of the spectra on Pt and Au(111) suggested that the behavior of water at the electrochemical interface is determined by the interplay among water-water, water-metal, water-anion, and water-electric field interactions. Acknowledgment. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid for Basic Research 18350038) and also by Japan Science and Technology Agency. G.S. acknowledges Japan Society for the Promotion of Science (JSPS) for a Postdoctoral Fellowship for Foreign Researchers (18.06343). Supporting Information Available: Potential difference spectra and schematic drawing showing selective observation of the first water layer in contact with the Pt surface. This material is available via the Internet at http://pubs.acs.org. References and Notes (1) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (2) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 5. (3) Guidelli, R.; Schmickler, W. Electrochim. Acta 2000, 45, 2317. (4) Lipkowski, J.; Ross, P. H., Structure of Electrified Interfaces; VCH: New York, 1993. (5) Menzel, D. Science 2002, 295, 58. (6) Held, G.; Menzel, D. Sur. Sci. 1994, 316, 92. (7) Ogasawara, H.; Brena, B.; Nordlund, D.; Nyberg, M.; Pelmenschikov, A.; Pettersson, L. G. M.; Nilsson, A. Phys. ReV. Lett. 2002, 89, 276102. (8) Feibelman, P. J. Science 2002, 295, 99. (9) Bockris, J. O’M.; Khan, S. U. M., Surface Electrochemistry: A Molecular LeVel Approach; Plenum: New York, 1993. (10) Heinzinger, K., Molecular dynamics of water at interfaces. In Structure of Electrified Interface; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1993; p 239. (11) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. J. Chem. Phys. 1984, 80, 4448. (12) Spohr, E. Chem. Phys. 1990, 141, 87. (13) Heinzinger, K.; Spohr, E. Electrochim. Acta 1989, 34, 1849. (14) Nagy, G.; Heinzinger, K. 1990, 296, 549. (15) Akiyama, R.; Hirata, F. J. Chem. Phys. 1998, 108, 4904. (16) Kovalenko, A.; Hirata, F. J. Chem. Phys. 1999, 110, 10095. (17) Ohwaki, T.; Kamegai, K.; Yamashita, K. Bull. Chem. Soc. Jpn. 2001, 74, 1021. (18) Sanchez, C. G. Surf. Sci. 2003, 527, 1. (19) (a) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Nature 1994, 368 (6470), 444. (b) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Surf. Sci. 1995, 335, 326. (20) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (21) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (22) Osawa, M.; Ataka, K.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497. (23) Nihonyanagi, S.; Ye, S.; Uosaki, K.; Dreesen, L.; Humbert, C.; Thiry, P.; Peremans, A. Surf. Sci. 2004, 573, 11. (24) Schultz, Z. D.; Shaw, S. K.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 15916. (25) (a) Bewick, A.; Kunimatsu, K. Surf. Sci. 1980, 101, 131. (b) Bewick, A.; Russell, J. W. J. Electroanal. Chem. 1982, 132, 329. (26) Habib, M. A.; Bockris, J. O’M. Langmuir 1986, 2, 388. (27) Probst, J.; Thull, R. Ber. Bunsen-Ges. 1995, 99, 158. (28) Iwasita, T.; Xia, X. J. Electroanal. Chem. 1996, 411, 95. (29) (a) Shingaya, Y.; Hirota, K.; Ogasawara, H.; Ito, M. J. Electroanal. Chem. 1996, 409, 103. (b) Hirota, K.; Song, M.-B.; Ito, M. Chem. Phys. Lett. 1996, 250, 335. (c) Shingaya, Y.; Ito, M. Surf. Sci. 1997, 386, 34. (30) Shiroishi, H.; Ayato, Y.; Kunimatsu, K.; Okada, T. J. Electroanal. Chem. 2005, 581, 132. (31) Futamata, M.; Luo, L. Q.; Nishihara, C. Surf. Sci. 2005, 590, 196. (32) Zheng, W. Q.; Tadjeddine, A. J. Chem. Phys. 2003, 119, 13096. (33) Baldelli, S.; Mailhot, G.; Ross, P. N.; Somorjai, G. A. J. Am. Chem. Soc. 2001, 123, 7697. (34) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914. (35) Osawa, M.; Ataka, K.; Yoshii, K.; Yotsuyanagi, T. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 371.

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