In Situ Infrared Reflection Absorption Spectroscopy of Carbon

Jul 11, 2007 - Paramaconi Rodríguez , Gonzalo García , Enrique Herrero , Juan M. Feliu , Marc T. M. Koper. Electrocatalysis 2011 2 (3), 242-253 ...
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Langmuir 2007, 23, 9092-9097

In Situ Infrared Reflection Absorption Spectroscopy of Carbon Monoxide Adsorbed on Pt(S)-[n(100)×(110)] Electrodes Kosuke Mikita, Masashi Nakamura, and Nagahiro Hoshi* Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba UniVersity 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522 Japan ReceiVed March 12, 2007. In Final Form: May 10, 2007 Structural effects on the adsorption of CO have been studied using infrared reflection absorption spectroscopy (IRAS) on Pt(S)-[n(100)×(110)] surfaces (n ) 2, 5, 9) that have densely packed kink atoms in the step. Coverage and potential dependence of the IRAS spectra are scrutinized. On-top and bridge-bonded CO are found on all of the surfaces examined. CO is adsorbed on only kink at low coverage (θCO e 0.2). Adsorbed CO on kink gives an IR band at lower frequency than that on step. CO is adsorbed on both kink and terrace at 0.3 e θCO. Water is adsorbed on the terrace of Pt(510) n ) 5 and Pt(910) n ) 9 at low CO coverage, but water is not found on Pt(210) n ) 2 of which the first layer is composed of only kink atoms. It is suggested that coadsorbed water on the terrace enhances the activity for the oxidation of adsorbed CO on the kink remarkably.

Introduction Infrared reflection absorption spectroscopy (IRAS) is one of the most powerful methods to investigate the structure of an adsorbate in a solid-liquid interface.1 CO has a high cross section of IR light, and its simple diatomic structure presents a good model for the double layer structure in a solid-liquid interface. Pt electrodes have high activity for fuel cell reactions, whereas CO is a poisonous adsorbate on Pt electrodes.2 Elucidation of the nature of adsorbed CO will contribute to the development of new electrocatalysts with a high tolerance of CO. Thus many papers have reported IRAS spectra of CO on Pt electrodes. A single-crystal electrode plays a key role in the determination of the structure of an active site for CO oxidation. IRAS spectra of adsorbed CO were measured on the low index planes of Pt in aqueous solutions.3-10 On-top and 3-fold CO is observed on Pt(111) at full coverage at lower potentials.5,9,10 Three-fold CO converts to bridge-bonded CO at higher potentials.5,9,10 Pt(100) adsorbs on-top and bridge-bonded CO, on which bridge-bonded CO converts to on-top CO with the increase of positive potentials.7 Pt(110) gives a single band of on-top CO.4 The IRAS study was extended to stepped surfaces of Pt, such as Pt(S)-[n(100)×(111)], Pt(S)-[n(111)×(111)], and Pt(S)[n(111)×(100)].11-17 All of the surfaces are composed of flat terraces and linear step lines. At low coverage, adsorbed CO on * To whom correspondence should be addressed. E-mail: hoshi@ faculty.chiba-u.jp. (1) Bewick, A.; Pons, S. In AdVanceds in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1985; Vol. 12, Chapter 1. (2) Markovic´, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117-229. (3) Leung, L.-W. H.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988, 92, 6985-6990. (4) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 230, 222-236. (5) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142-162. (6) Kitamura, F.; Takeda, M.; Takahashi, M.; Ito, M. Chem. Phys. Lett. 1987, 142, 318-322. (7) Kitamura, F.; Takeda, M.; Takahashi, M.; Ito, M. J. Phys. Chem. 1988, 92, 3320-3323. (8) Furuya, N.; Motoo, S.; Kunimatsu, K. J. Electroanal. Chem. 1988, 239, 347-360. (9) Kitamura, F.; Takeda, M.; Takahashi, M.; Ito, M. Surf. Sci. 1989, 223, 493-508. (10) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1990, 101, 1648-1660. (11) Watanabe, S.; Inukai, J.; Ito, M. Surf. Sci. 1993, 293, 1-9. (12) Rodes, A.; Gomez, R.; Feliu, J. M.; Weaver, M. J. Langmuir 2000, 16, 811-816.

the stepped surfaces gives an IR band at lower frequency than that on Pt(111) and Pt(100).11,13,15-17 This fact shows that CO is adsorbed on the step atoms on stepped surfaces. CO is adsorbed on the terrace as well as the step at intermediate coverage. Adsorbed CO on the terrace gives an IR band at higher frequency than that on the step: adsorbed CO on the step provides a shoulder band. At full coverage, the IR band on the step cannot be distinguished from that on the terrace because of the strong dipole-dipole coupling of adsorbed CO.11-17 A kinked-stepped surface, Pt(321), has a higher oxidation capability of CO than Pt(111) in the UHV system.18,19 Pt(S)[n(100)×(110)] surfaces, which contain protruded atoms in the step (Figure 1), also have a high activity for electrochemical reactions especially in CO2 reduction.20 We define the protruded atoms in the step as kink atoms (Figure 1). Kink atoms on Pt(S)-[n(100)×(110)] surfaces have a lower coordination number (5) than step atoms on Pt(S)-[(n-1)(111)×(110)] surfaces (6) which have linear step lines. Adsorbed CO on kink atoms may have special features that are not found on step atoms. There has been, however, no report on IRAS spectra of CO adsorbed on kink atoms in aqueous solution. In this paper, we report IRAS spectra of CO adsorbed on Pt(210) ()2(100)-(110)), Pt(510) ()5(100)-(110)), and Pt(910) ()9(100)-(110)) electrodes (Figure 1) in 0.1 M HClO4. Coverage and potential dependence of the spectra are discussed. Experimental Section A single crystal bead of Pt was prepared with the method reported previously.14,21 The cross sectional area of the crystal was between (13) Lebedeva, N. P.; Rodes, A.; Feliu, J. M.; Koper, M. T. M.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 9863-9872. (14) Hoshi, N.; Tanizaki, M.; Koga, O.; Hori, Y. Chem. Phys. Lett. 2001, 336, 13-18. (15) Kim, C. S.; Korzeniewski, C. Anal. Chem. 1997, 69, 2349-2353. (16) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484-6491. (17) Kim, C. S.; Korzeniewski, C.; Tornquist, W. J. J. Chem. Phys. 1994, 100, 628-630. (18) McClellan, M. R.; Gland, J. L. Surf. Sci. 1981, 112, 63-77. (19) McClellan, M. R.; McFeely, F. R. J. Vac. Sci. Technol. A 1983, 1, 10701073. (20) Hoshi, N.; Kawatani, S.; Hori, Y. J. Electroanal. Chem. 1999, 467, 6773. (21) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205-209.

10.1021/la7007182 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007

CO Adsorbed on Pt(S)-[n(100)×(110)] Electrodes

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Figure 1. Hard sphere models of Pt(S)-[n(100)×(110)]. Black spheres indicate kink atoms.

Figure 2. Coverage dependence of IRAS spectra of CO adsorbed on Pt(S)-[n(100)×(110)] electrodes in 0.1 M HClO4. Sample and reference potentials are 0.1 and 0.8 V(RHE), respectively.

0.30 and 0.35 cm2. The crystal was polished with diamond slurries down to 0.3 µm. The polished surface was annealed in H2/O2 flame at about 1300 °C for removing distortions caused by the mechanical polishing and then cooled down to room temperature in the atmosphere of Ar/H2 with a volume ratio of 9:1. The annealed electrode was transferred to an IRAS cell with its surface protected with ultrapure water. Electrolytic solutions were prepared using ultrapure water treated with Milli Q plus low TOC (Millipore) and suprapure grade chemicals (Merck). The purity of Ar and CO was higher than 99.9999% and 99.95%, respectively. CO was passed through 1 M KOH solution for removing traces of iron carbonyl that might be contained in the gas cylinder. IRAS spectra were measured with an FT-IR spectrometer FT/ IR-6100FV(JASCO). Infrared light (p-polarized) was incident to the electrode surface through a CaF2 prism with an angle of incidence of 60°. The reflected light was detected using an MCT detector cooled with liquid nitrogen. Spectra were averaged over 128 scans with a resolution of 4 cm-1. At full coverage, IRAS spectra were measured according to the following procedure: 1. The solution (0.1 M HClO4) was saturated with CO in advance. 2. A single-crystal electrode was immersed in the solution for 15 min with the potential held at a sample potential ES at which CO is adsorbed. 3. After removing dissolved CO by the purge of Ar, the electrode was pushed to the window, and sample spectra were collected. 4. The potential was stepped to the reference potential ER (0.8 V (RHE)) at which adsorbed CO is oxidized completely. Reference spectra began to be collected exactly 60 s after the potential step. After the crystal was annealed in H2/O2 flame, the procedure ( steps 2-4) was repeated at various ES between 0.1 and 0.5 V (RHE). The coverage of adsorbed CO was controlled by changing CO concentration in the solution: a prescribed volume of CO saturated solution was added into an Ar saturated solution in an IRAS cell. When IRAS spectra were measured at low CO coverage, sample spectra were collected from 0.1 V successively with the increase of the potential. CO coverage (θCO) was estimated with the use of the integrated band intensity of CO2 produced from the oxidation of adsorbed CO according to Weaver et al.10 Integrated band intensity of CO2 of the Pt(S)-[n(100)×(110)] electrode is compared with that of Pt(111) which is fully covered with adsorbed CO. Coverage of

Figure 3. IR band frequency of on-top CO plotted against the total coverage of CO (θCO) at 0.1 V (RHE) on Pt(S)-[n(100)×(110)] electrodes. Circles: Pt(210) ()2(100)-(110)). Squares: Pt(510) ()5(100)-(110)). Triangles: Pt(910) ()9(100)-(110)). CO on Pt(S)-[n(100)×(110)], θCO,Pt(n10), is calculated by the following equation: θCO,Pt(n10) ) (dPt(111)/dPt(n10))(ICO2,Pt(n10)/ICO2,Pt(111)) θCOsat,Pt(111) where d, ICO2, and θCOsat,Pt(111) represent the density of atoms at the first layer, the integrated band intensity of CO2, and the saturation coverage of CO on Pt(111), respectively. CO2 tends to be diffused out of the thin layer between the electrode surface and CaF2 window during the collection of the reference spectra. For the reduction of error due to the loss of CO2, we set the starting time for the data collection of the reference spectra to exactly 60 s after the potential step. IRAS spectra are displayed in the form of absorbance ()-log R(ES)/R(ER)) vs wavenumber. Positive-going bands arise from adsorbates at ES.

Results and Discussion Pt(S)-[n(100)×(110)] electrodes give voltammograms characteristic of their orientation in H2SO4 solution.22 We measured voltammograms of the prepared electrodes in 0.5 M H2SO4 to confirm the crystal orientation. The voltammograms were identical with those reported previously;22 the surfaces were welldefined.

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Figure 4. Potential dependence of IRAS spectra of CO adsorbed on Pt(S)-[n(100)×(110)] electrodes in 0.1 M HClO4. Reference potentials are 0.8 V (RHE). Each spectrum is measured after the electrode is freshly annealed. Inset shows magnification of the band of bridge-bonded CO.

Figure 5. Potential dependence of integrated band intensity of on-top and bridge-bonded CO on Pt(S)-[n(100)×(110)] electrodes. Open squares: CO coverage. Filled circles: integrated band intensity of on-top CO. Filled squares: sum of integrated band intensities of asymmetric and symmetric bridge-bonded CO.

IRAS spectra of adsorbed CO were measured in 0.1 M HClO4 in which the anion is not adsorbed on Pt electrodes strongly. LEED measurements reported faceting of Pt(S)-[n(100)×(110)] surfaces in UHV.23 However, we revealed that the Pt(310) n ) 3 electrode has an unreconstructed (1×1) structure in 0.1 M HClO4 with the use of in situ surface X-ray diffraction.24 Adsorbed CO does not affect the topmost structure of Pt(310) n ) 3.24 In the LEED measurements, the surfaces were cleaned by a combination of Ar ion sputtering and oxygen heat treatment combined with annealing at 950 °C. In our study, the surfaces were flame-annealed at about 1300 °C and then cooled in an Ar/H2 atmosphere. The difference in the pretreatment procedure may result in the different surface structures. Coverage Dependence. Figure 2 shows the coverage dependence of IRAS spectra of adsorbed CO on Pt(S)-[n(100)×(110)] electrodes at 0.1 V (RHE). Two bands are observed around 2000 and 1800 cm-1. These bands can be assigned to on-top and bridge-bonded CO according to the previous reports7,9,11 A negative going band around 2340 cm-1 is due to CO2 that is produced by CO oxidation at the reference potential (0.80 V). The band frequency of on-top CO is plotted against the total coverage of adsorbed CO (θCO) in Figure 3. At low coverage θCO e 0.2, the band frequencies on Pt(510) n ) 5 and Pt(910) n ) 9 are almost the same as that on Pt(210) of which first layer is composed of only kink atoms (Figure 1). The band frequencies on Pt(510) n ) 5 and Pt(910) n ) 9 abruptly shift to a higher (22) Furuya, N.; Koide, S. Surf. Sci. 1990, 226, 221-225. (23) Blakely, D. W.; Somorjai, G. A. Surf. Sci. 1977, 65, 419-442. (24) Hoshi, N.; Nakamura, M.; Nakahara, A.; Sumitani, K.; Sakata, O. Extended Abstract of International Conference on Electrified Interfaces 2007, D9 (Sahoro, Hokkaido, Japan).

Table 1. Band Frequencies of Adsorbed CO on High Index Planes of Pta electrode

E, V (RHE)

θCO

νon-top, cm-1

νbridge, cm-1

5(100)-(110) 6(100)-(111)11 4(100)-(111)11 5(111)-(100)13 5(111)-(110)13

0.1 0.2 0.2 0.1 0.1

0.1 0.08 0.04 0.10 0.11

1997 2020 2020 2002 2009

1784 1820 1820 1801 1786 (3-fold CO)

aν on-top: band frequency of on-top CO. νbridge: band frequency of bridge-bonded CO (3-fold CO on 5(111)-(110)).

value at θCO ) 0.3 and then increasing more slowly than that of Pt(210) n ) 2 at 0.3 < θCO (Figure 3). The band shape becomes asymmetric on Pt(510) n ) 5 and Pt(910) n ) 9. On Pt(210) n ) 2, the band frequency shifts almost linearly with the increase of the coverage. According to previous reports on Pt(S)-[n(100)×(111)]11 and Pt(S)-[n(111)×(100)],13,14-16 CO is adsorbed on step at low coverage: the band frequency of CO at the step is lower than that at the terrace. These facts show that CO is adsorbed on kink atoms of Pt(S)-[n(100)×(110)] at θCO e 0.2. The abrupt shift and asymmetry of the band indicate that CO is adsorbed on the terrace as well as kink at 0.3 e θCO on Pt(510) n ) 5 and Pt(910) n ) 9. At higher θCO, the CO band on the kink cannot be distinguished from that on the terrace clearly because of the strong dipole-dipole interaction of adsorbed CO. On Pt(S)[n(100)×(111)] and Pt(S)-[n(111)×(100)] surfaces, however, on-top CO at the step gives a clear shoulder peak in the on-top CO band at the terrace at 0.3 e θCO e 0.5.11,13,15-17 Absence of the shoulder peak on Pt(S)-[n(100)×(110)] may be attributed to stronger dipole-dipole coupling between CO on the terrace and

CO Adsorbed on Pt(S)-[n(100)×(110)] Electrodes

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Figure 6. Voltammograms of Pt(S)-[n(100)×(110)] electrodes in 0.1 M HClO4. Red lines show anode stripping voltammograms of CO adsorbed at 0.1 V (RHE). Black lines depict voltammograms without adsorbed CO. Scanning rate: 0.05 V s-1. Table 2. Band Frequency at 0.1 V (RHE) and Stark Tuning Rate (dν/dE) of Adsorbed CO on Pt(S)-[n(100)×(110)] Electrodes at Full CO Coveragea

Pt(910) n ) 9 Pt(510) n ) 5 Pt(210) n ) 2

νon-top, cm-1

νas-bridge, cm-1

νs-bridge, cm-1

dνon-top/dE, cm-1 V-1

2054 2054 2067

1885 1881

1862 1857 1860

30 22 70

aν on-top: band frequency of on-top CO. νas-bridge: band frequency of asymmetric bridge-bonded CO. νs-bridge: band frequency of symmetric bridge-bonded CO.

CO on the kink: adsorbed CO on the terrace may be densely packed around the kink of Pt(S)-[n(100)×(110)]. Table 1 summarizes band frequency of adsorbed CO at θCO ∼ 0.1, at which CO is adsorbed only on the kink or step. The band frequency of on-top and bridge-bonded CO on Pt(510) ()5(100)-(110)) is lower than those on the stepped surfaces such as Pt(11 1 1) ()6(100)-(111))11 and Pt(322) ()5(111)(100)).13 Because the coverage of CO on Pt(510) ()5(100)(110)) is almost the same as those on Pt(322) ()5(111)-(100)), the difference in the band frequencies is attributed to the difference in the back-donation.25 The coordination number of a kink atom (6) is less than that of a step atom (7). A lower coordination number of the kink may promote the back-donation of electrons to the 2π* orbital of CO, causing the red shift of on-top CO on the kink. A study in UHV, however, shows that there is no difference between the frequencies of CO adsorbed on step and kink sites.26 The kinked-stepped surface examined in UHV is Pt(432), which has a (111) terrace and a kinked step. Pt(432) was not flame-annealed; the surface might be facetted, as is the case of Pt(S)-[n(100)×(110)] surfaces in UHV.23 Different terrace structures and surface pretreatments may cause the different structural effects on the band frequency. The lower band frequency at the kink site might indicate that the C-O bond strength of CO at the kink is weaker than that at the step. However, a recent DFT calculation shows that there is no apparent correlation between the binding energy of CO and (25) Mehandru, S. P.; Anderson, A. B. J. Phys. Chem. 1989, 93, 2044-2047. (26) Greenler, R. G.; Burch, K. D.; Kretzschmar, K.; Kauser, R.; Bradshaw, A. M.; Hayden, B. E. Surf. Sci. 1985, 152/153, 338-345.

Figure 7. Potential dependence of IRAS spectra of CO adsorbed on only kink atoms of Pt(S)-[n(100)×(110)] electrodes in 0.1 M HClO4. θCO ) 0.2. Reference potentials are 0.8 V (RHE).

the internal C-O stretching frequency.27,28 Theoretical calculations are necessary for the elucidation of structural effects on C-O bond strength. Potential Dependence at Full Coverage. Figure 4 shows the potential dependence of IRAS spectra of adsorbed CO on Pt(S)-[n(100)×(110)] electrodes at saturation coverage. The following saturation coverages are obtained: 0.8 for Pt(910) n ) 9, 0.7 for Pt(510) n ) 5, and 1 for Pt(210) n ) 2. The band of bridge-bonded CO is composed of two components, as shown in the inset of Figure 4. The dipole-dipole interaction is so strong at full coverage that bridge-bonded CO at the kink will not be distinguished from the one at the terrace. Thus, we assigned the higher and lower components to asymmetric and symmetric bridge-bonded CO.11 Figure 5 presents θCO and integrated band intensity of IR bands ICO at various potentials. The onset potential of CO oxidation is 0.5 V on Pt(510) n ) 5 and Pt(910) n ) 9, because θCO begins to decrease at this potential. The onset potential of CO oxidation shifts negatively to 0.3 V on Pt(210) n ) 2. These results show that Pt(210) n ) 2, of which the first layer consists of only kink atoms, has high activity for CO oxidation at full coverage. Figure 6 shows anode stripping voltammograms of adsorbed CO. Adsorbed CO is oxidized above 0.3 V on Pt(210) (27) Koper, M. T. M.; van Santan, R. A.; Wasileski, S. A.; Weaver, M. J. J. Chem. Phys. 2000, 113, 4392-4407. (28) Koper, M. T. M.; Shubina, T. E.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 686-692.

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Figure 8. Potential dependence of integrated band intensity of IR bands. Open squares: ratio of integrated band intensity of CO2 at ES to that of 0.1 V (RHE) (ICO2(ES)/ICO2(0.1 V)). Filled squares: integrated band intensity of bridge-bonded CO on the kink. Filled circles: integrated band intensity of on-top CO on the kink. Filled diamonds: integrated band intensity of water.

Figure 9. Models of the adsorption of CO and water at low CO coverage on Pt(S)-[n(100)×(110)]. O, b, and w indicate on-top CO, bridge-bonded CO, and water, respectively.

n ) 2, whereas it is oxidized above 0.4 V on the other surfaces. These facts also support the high activity for CO oxidation on Pt(210) n ) 2. Oxidation of adsorbed CO needs oxygen-containing species at the adjacent sites. Some papers report that adsorbed CO abstracts oxygen from the oxide film such as PtOH,29,30 whereas the others claim that adsorbed CO directly reacts with water near the electrode surface.31,32 Pt(S)-[n(100)×(110)] electrodes do not provide an oxide film below 0.5 V apparently, as shown in voltammograms in Figure 6. However, hydroxyl is reported to be adsorbed above 0.3 V on Pt(100) in 0.1 M HClO4.33,34 Pt(S)-[n(100)×(110)] electrodes also have a (100) terrace; the possibility of hydroxyl formation around 0.3 V cannot be excluded. Voltammograms cannot reveal the oxidation mechanism of adsorbed CO on Pt(S)-[n(100)×(110)] electrodes. The bands of on-top CO shift to higher frequency with the increase of the potentials due to the Stark effect and the decrease of back-donation.25,35 The frequency of bridge-bonded CO, however, decreases at positive potentials as shown in Figure 4. Table 2 summarizes the band frequency and Stark tuning rates (dν/dE) on Pt(S)-[n(100)×(110)] electrodes at the potentials lower than CO oxidation. On Pt(510) n ) 5 and Pt(910) n ) 9, Stark tuning rate of on-top CO is between 22 and 30 cm-1 V-1, which is almost equivalent to those on the Pt(S)-[n(111)×(111)] series.14 The sign of Stark tuning rates of bridge-bonded CO was negative. The integrated band intensity of bridge-bonded CO on Pt(910) n ) 9 and Pt(510) n ) 5 decreases with the increase of the potential, although θCO is constant below 0.4 V in Figure. 5. The band intensity of on-top CO increases at positive potentials. These phenomena can be attributed to the site conversion of bridge-bonded CO to on-top CO. Similar phenomena are observed on Pt(100)7 and Pt(S)-[n(111)×(111)] (29) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 12938-12947. (30) Markovic, N. M.; Lucas, C. A.; Ross, P. N. J. Phys. Chem. B 1999, 103, 9616-9623. (31) Bergelin, M.; Feliu, J. M.; Wasberg, M. Electrochim. Acta. 1998, 44, 1069-1075. (32) Bergelin, M.; Herrero, E.; Feliu, J. M.; Wasberg, M. J. Electroanal. Chem. 1999, 467, 74-84. (33) Go´mez, R.; Orts, J. M.; AÄ lvarez-Ruiz, B.; Feliu, J. M. J. Phys. Chem. B 2004, 108, 228-238. (34) Climent, V.; Go´mez, R.; Orts, J. M.; Feliu, J. M. J. Phys. Chem. B 2006, 110, 11344-11351. (35) Lambert, D. K. Phys. ReV. Lett. 1983, 50, 2106-2109.

Figure 10. Migration and oxidation of on-top CO on kink.

series.13 Vidal et al. state that the site conversion is triggered by adsorbed OH formation.36 The band intensity of on-top CO is enhanced up to 0.4 V on Pt(210) n ) 2, although adsorbed CO is oxidized above 0.3 V. This result was well reproducible. It is plausible that only bridgebonded CO is oxidized on Pt(210) n ) 2. Some of the remaining bridge-bonded CO may convert to on-top CO. Intensity transfer from multibonded CO to on-top CO is reported on Pt(111).37 Intensity transfer from bridge-bonded CO to on-top CO might also be enhanced on Pt(210) n ) 2 at positive potentials remarkably. Potential Dependence at Low Coverage. Coverage dependence of the IRAS spectra has shown that CO is adsorbed on only kink atoms at θCO e 0.2 (Figure 2). Figure 7 depicts the potential dependence of IRAS spectra of CO adsorbed on kink at θCO ) 0.2. The band of on-top CO decreases with the increase of the potential and finally disappears at 0.3 V on Pt(510) n ) 5 and Pt(910) n ) 9. On Pt(210) n ) 2, the band of on-top CO also shrinks at positive potentials, but it remains even at 0.5 V. As for bridge-bonded CO, more than 90% of the band remains at 0.5 V (Figure 8). The band intensity of CO2 also decreases at positive potentials, showing that θCO decreases with the increase of the potential. On-top CO is oxidized more preferentially than bridge-bonded CO on kink atoms of Pt(S)-[n(100)×(110)]. This is not the case of full coverage as shown in Figures 4 and 5. The stepped surface, Pt (11 1 1)()6(100)-(111)), shows no preferential oxidation of on-top CO.11 The band intensity of on-top CO also decreases at positive potentials on Pt(S)-[n(111)×(111)] electrodes.13 However, the decrease of on-top CO is not due to CO oxidation on Pt(S)-[n(111)×(111)], because the CO2 band is not observed at (36) Vidal, F.; Busson, B.; Tadjeddine, A. Electrochim. Acta 2004, 49, 36373641. (37) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648-1660.

CO Adsorbed on Pt(S)-[n(100)×(110)] Electrodes

the potentials examined.13 Preferential oxidation of on-top CO is characteristic of Pt(S)-[n(100)×(110)] electrodes at low CO coverage. A band is observed around 1620 cm-1 on Pt(910) n ) 9 and Pt(510) n ) 5 (Figure 7). The band is assigned to in-plane deformation vibration of adsorbed water.38,39 The OH stretching vibration of water, which can be observed around 3500 cm-1, cannot be observed in this work. In the IRAS configuration, IR light is absorbed remarkably by bulk water in the frequency of the OH stretching vibration; IR light might become too weak to detect the OH stretching vibration of adsorbed water on the electrode surface. Previous studies in UHV show, however, that the water molecule is adsorbed with its molecular plane parallel to the surface at low water coverage.40 The IR band intensity of the in-plane deformation vibration of adsorbed water is abnormally higher than that of OH stretching vibration.41,42 This intensity relation is attributed to a vibronic coupling of a vibration of the water molecule with an electronic transition from water to metal through the lone pair.43 Absence of water on Pt(210) n ) 2, of which surface is composed of only kink atoms, supports that water is adsorbed on the (100) terrace. The band of on-top CO diminishes in harmony with the decrease of the water band at 0.3 V on Pt(510) n ) 5 and Pt(910) n ) 9, whereas on-top CO remains above 0.3 V on Pt(210) n ) 2. This fact suggests that water adsorbed on the terrace enhances the oxidation of on-top CO at the kink. ATR-SEIRAS measurements also show that coadsorbed water promotes the oxidation of CO on polycrystalline Pt electrodes.44 Figure 9 presents the model of the adsorption of CO and water on Pt(S)-[n(100)×(110)] electrodes. IRAS results of Figure 7, however, cannot directly exclude the possibility that CO is oxidized via an adsorbed hydroxyl (38) Iwasita, T.; Xia, X. J. Electroanal. Chem. 1996, 411, 95-102. (39) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 1066410672. (40) Yamamoto, S.; Beniya, A.; Mukai, K.; Yamashita, Y.; Yoshinobu, J. J. Phys. Chem. B 2005, 109, 5816-5823. (41) Nakamura, M.; Song, M. B.; Ito, M. Chem. Phys. Lett. 2000, 320, 381386. (42) Nakamura, M.; Ito, M. Chem. Phys. Lett. 2001, 335, 170-175. (43) Nakamura, M.; Ito, M. Chem. Phys. Lett. 2004, 384, 256-261. (44) Shiroishi, H.; Ayato, Y.; Kunimatsu, K.; Okada, T. J. Electroanal. Chem. 2005, 581, 132-138.

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group, because previous studies show the adsorption of the hydroxyl group on the (100) terrace below 0.4 V (RHE).33,34 On the assumption that on-top CO at the kink is oxidized by adsorbed water on the (100) terrace, the preferential oxidation of on-top CO can be explained with the use of the migration model of adsorbed CO on Pt(111) ()6(100)-(111)).11 If on-top CO on the kink migrates to the terrace, on-top CO can be oxidized with the water adsorbed on the terrace (Figure 10). Barrier for the migration of on-top CO might be lower than that of bridgebonded CO on Pt(S)-[n(100)×(110)], causing the higher activity for on-top CO oxidation. Adsorbed CO is not oxidized below 0.4 V on Pt(510) n ) 5 and Pt(910) n ) 9 at full coverage. The IR band of adsorbed water was negligible at full CO coverage. Absence of coadsorbed water may inhibit CO oxidation at full coverage below 0.4 V.

Conclusion Coverage dependence of IR spectra shows that CO is adsorbed on only kink atoms of Pt(S)-[n(100)×(110)] electrodes at θCO e 0.2. The IR band frequency of adsorbed CO on kink atoms is lower than that on step atoms. Potential dependence of IR spectra indicates bridge-bonded CO converts to on-top CO on Pt(S)-[n(100)×(110)] at positive potentials at full coverage. At θCO e 0.2 where CO is adsorbed on only kink atoms, water is adsorbed on Pt(510) n ) 5 and Pt(910) n ) 9, whereas adsorbed water is not found on Pt(210) n ) 2 that has no terrace site in the first layer. These results show that water is adsorbed on the (100) terrace of Pt(S)-[n(100)×(110)] electrodes. On-top CO on the kink is oxidized more preferentially than bridge-bonded CO. On-top CO diminishes in harmony with the decrease of adsorbed water on Pt(510) n ) 5 and Pt(910) n ) 9. These facts suggest that coadsorbed water on the terrace enhances the activity for adsorbed CO oxidation on the kink. Acknowledgment. This work was supported by a grant from the New Energy and Industrial Technology Development Organization. LA7007182