Oxidation of Carbon Monoxide at a Platinum Film Electrode Studied by

In Final Form: August 13, 1999. Fourier transform infrared reflection absorption spectroscopy with the attenuated total reflection technique. (ATR-FTI...
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Langmuir 1999, 15, 8757-8764

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Oxidation of Carbon Monoxide at a Platinum Film Electrode Studied by Fourier Transform Infrared Spectroscopy with Attenuated Total Reflection Technique Yimin Zhu, Hiroyuki Uchida, and Masahiro Watanabe* Laboratory of Electrochemical Energy Conversion, Faculty of Engineering, Yamanashi University, Takeda 4-3, Kofu 400-8511, Japan Received June 28, 1999. In Final Form: August 13, 1999 Fourier transform infrared reflection absorption spectroscopy with the attenuated total reflection technique (ATR-FTIR), coupled with cyclic voltammetry (CV) measurement, is used to observe the oxidation process of adsorbed CO at Pt film sputtered on a silicon prism. The interesting bipolar shape of the linearly bonded CO band is observed at high coverage of CO, although no CO band is included in the reference spectrum. This asymmetric shape is ascribed to Fano resonance. In addition to a linear CO and a bridged CO, a new absorption band, presumably assigned to a carboxyl radical, was detected. This band may be formed by a heterogeneous reaction between adsorbed CO and H2O on the Pt surface in the hydrogen adsorption potential region. The adsorbed carboxyl radical was oxidized at a less positive potential than the adsorbed CO, which can be ascribed to a presumable origin for the pre-peak that appeared in a CV reading prior to the oxidation of such a linear or bridged CO. This oxidation led to the rearrangement of CO ad-layers, especially at the high coverage of CO. In the case of the low coverage of CO, the conversion from the bridged CO to the linear CO is ascribed to the potential induced electronic effects of the electrode surface on the adsorption states. A consumption of adsorbed H2O and a production of CO2 were also clearly indicated by the spectroscopy when COOH or CO disappeared from the surface.

1. Introduction The electrochemical oxidation of carbon monoxide (CO) on noble metals, such as Pt, Pd, Rh, or Ir, those with foreign adatoms, and their alloys has been extensively investigated1-31 because CO behaves as a poisoning species to the anodic reaction in fuel cells. The CO remains in hydrogen that is derived from steam-reformed hydrocar* To whom correspondence should be addressed. Telephone: +81-55-220-8620. Fax: +81-55-254-0371. E-mail: mwatanab@ ab11.yamanashi.ac.jp. (1) Vogel, W.; Lundquist, J.; Ross, P.; Stonehart, P. J. Electrochim. Acta 1975, 20, 79. (2) Dahr, H. P.; Christner, L. G.; Kush, A. K. J. Electrochem. Soc. 1987, 134, 3021. (3) Sun, S. G.; Clavilier, J. J. Electroanal. Chem. 1987, 126, 95. (4) Paffett, M. T.; Ticcianelli, E.; Pafford, J.; Gottesfeld, S. Abstracts of Fuel Cell Seminar, Long Beach, CA, 1988; Oct. 23-26, pp 126-129. (5) Zurawski, D.; Wasberg, M.; Wieckowski, A. J. Phys. Chem. 1990, 94, 2076. (6) Lemons, R. A. J. Power Sources 1990, 29, 251. (7) Beden, B.; Lamy, C.; De Tacconi, N. R.; Arvia, A. J. Electrochim. Acta 1990, 35, 691. (8) Gasteiger, H.; Markovic, N.; Ross, P. J. Phys. Chem. 1995, 99, 8945. (9) Igarashi, H.; Fujino, T.; Watanabe, M. J. Electroanal. Chem. 1995, 391, 119. (10) Oetjen, H.-F.; Schmidt, V. M.; Stimming, U.; Trila, F. J. Electrochem. Soc. 1996, 143, 3838. (11) Grgur, B. N.; Zhuang, G.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 1997, 100, 19538. (12) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 275. (13) Motoo, S.; Watanabe, M. J. Electroanal. Chem. 1976, 69, 429. (14) Watanabe, M.; Motoo, S. Denki Kagaku 1976, 44, 602. (15) Motoo, S.; Shibata, M.; Watanabe, M. J. Electroanal. Chem. 1980, 110, 103. (16) Motoo, S.; Watanabe, M. J. Electroanal. Chem. 1980, 111, 261. (17) Watanabe, M.; Shibata, M.; Motoo, S. J. Electroanal. Chem. 1985, 187, 161. (18) Watanabe, M.; Uchida, M.; Motoo, S. J. Electroanal. Chem. 1987, 229, 395. (19) Watanabe, M.; Igarashi, H.; Fujino, T. Design of CO Tolerant Anode Catalysts for Polymer Electrolyte Fuel Cells; Abstracts of The 1997 Joint International Meeting of ECS and ISE, Paris, Aug 31-Sept 5, The Electrochemistry Society, Pennington, NJ, 1997; Vol. 97-2, p 1238.

bons or is produced on anode catalyst surfaces as a byproduct during the electro-oxidation of small organic molecules. For the cell operation at an elevated temperature (e.g., 200 °C) for phosphoric acid fuel cells (PAFCs), the poisoning effect is negligible in the presence of 1% CO in reformed gases, even on a pure Pt anode catalyst, because of a sufficiently lowered CO coverage.2 However, when the operating temperature is brought lower than 100 °C, the anodic reaction can hardly occur under the steady-state condition because the catalyst surface sites are almost fully covered with CO, even in the presence of parts per million levels.4,9,10 It has been demonstrated that the oxidation reaction is enhanced dramatically by foreign adatoms such as the IVth, Vth, or VIth group metals in the Periodic Table, in a manner similar to alloying regardless of the CO partial pressure. This enhancement can be explained by the bifunctional mechanism proposed by Watanabe and Motoo or their co-workers, considering (20) Zawodzinski, T. A.; Springer, T. E.; Gottesfeld, S. On the Meaning of CO Tolerance Exhibited by Catalysis in Polymer Electrolyte Fuel Cells; Abstracts of The 1997 Joint International Meeting of ECS and ISE, Paris, Aug 31-Sept 5, The Electrochemistry Society, Pennington, NJ, 1997; Vol. 97-2, p 1228. (21) Sun, S.; Cai, W.; Wan, L.; Osawa, M. J. Phys. Chem. B 1999, 103, 2460 . (22) Kazarinov, V. E.; Andreyev, V. N.; Shelpakov, A. V. Electrochim. Acta 1989, 34, 905. (23) Kazarinov, W. E.; Andreyev, W. N. Elektrokhimiya 1972, 6, 927. (24) Stonehart, P. Electrochim. Acta 1973, 18, 63. (25) Couto, A.; Perez, M.; Rinco, A.; Gutierrez, C. J. Phys. Chem. 1996, 100, 19538. (26) Gomez, R.; Weaver, M. J. Langmuir 1998, 14, 2525. (27) Lin, W.; Iwasita, T.; Vielstich, W. J. Phys. Chem. B 1999, 103, 3250. (28) Ianniello, R.; Schmidt, V. M.; Stimming, U. Stumper, J.; Wallau, A. Electrochim. Acta 1994, 39, 1863. (29) Leiva, E. P. M.; Santos, E.; Iwasita, T. J. Electroanal. Chem. 1986, 215, 357. (30) Kunimatsu, K.; Seki, H.; Golden, W. G.; Golden, J. G. Langmuir 1986, 2, 464. (31) Wieckowski, A.; Rubel, M.; Guitrrez, C. J. Electroanal. Chem. 1995, 99, 3411.

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intrinsic properties of the adatoms such as an affinity for adsorption of oxygen-containing species or a donation of electrons to neighbor Pt sites due to the electron negativity.12-17 However, a large overpotential of ca. 0.4 V is still required for the oxidation, although the effect of the adatoms has led to a decrease of >0.4 V in comparison with that on pure Pt. On the other hand, work in our laboratory has also shown that the activity of hydrogen oxidation at Pt electrocatalysts can be maintained at room temperature in the presence of 100 ppm of CO, when Pt is alloyed with Ni, Co, Fe, Mo, or Sn as well as Ru.19 We ascribe this enhancement to the lowered CO coverage that is due to the increase in 5d vacancy produced by alloying, as indicated by X-ray photoelectron spectroscopy (XPS) analysis. Ross and co-workers claimed that the tolerance of the Pt-Mo alloy to CO poisoning can be ascribed to holes appeared on the alloy surface mesh consisting of both metal atoms.11 Gottesfeld and co-workers explained the tolerance as being due to an increased CO-free site, caused by the enhanced CO oxidation by alloying, in the hydrogen oxidation potential.20 The CO tolerarnce mechanism is still elusive, yet is very important to establish for the design of new alloy catalysts used in polymer electrolyte fuel cells (PEFCs). Use of in situ spectroscopic measurements, such as Fourier transform infrared (FT-IR) spectroscopy or X-ray absorption spectroscopy (XAS) is important to clarify the mechanism from the information of the adsorption states or catalyst states, respectively. In IR measurements, the high sensitivity and the high wavenumber resolution as well as the similar time resolution to the cyclic voltammograms (CVs) are very important. To clarify the differences of adsorbates and/or adsorption states between the pure Pt and alloy electrodes, we have recently constructed a system for FTIR with attenuated total reflection technique (ATR-FTIR). As the preliminary experiment, attention is focused on the pure Pt electrocatalyst. We believe that there must be some important differences of adsorbates and/or adsorption states not only between the pure Pt and alloy electrodes but also operational conditions such as CO partial pressure or the anode potential. To understand the CO tolerance in practical PEFC conditions, it is very important to obtain information regarding the intermediate species of CO oxidation as well as water molecules coadsorbed with CO. Vibrational spectroscopic techniques can provide information on bonding structures of adsorbed molecules on the surface. The behavior of adsorbed water on Pt, Au, Rh, and Ag electrode surfaces has been investigated by using infrared reflection absorption spectroscopy (IRRAS).32-43 Most of the studies using IR spectroscopy involve an external reflection setup in which the light beam has to pass through at least 103 layers of water or (32) Bewick, A.; Kunimatsu, K. Surf. Sci. 1980, 101, 131. (33) Bewick, A.; Russell, J. W. J. Electroanal. Chem. 1982, 132, 329. (34) Bewick, A.; Russell, J. W. J. Electroanal. Chem. 1982, 132, 337. (35) Habib, M. A.; Bockris, J. O’M. Langmuir 1986, 2, 388. (36) Kunimatsu, K.; Bewick, A. Indian J. Technol. 1986, 24, 407. (37) Kunimatsu, K.; Samant, M. G.; Seki, H. J. Electroanal. Chem. 1989, 258, 163. (38) Brooker, J.; Christensen, P. A.; Hamnett, A.; Fe, R. Faraday Discuss. 1992, 94, 339. (39) Parry, D. B.; Samant, M. G.; Seki, H.; Philpott, M. R.; Ashley, K. Langmuir 1993, 9, 1878. (40) Rusell, A. E.; Lin, A. S.; O’Grady, W. E. J. Chem. Soc., Faraday Trans. 1993, 89, 195. (41) Shigaya, Y.; Hirota, K.; Ogasawara, H.; Ito, M. J. Electroanal. Chem. 1996, 409, 103. (42) Kitamura, F.; Nanbu, N.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1998, 452, 241. (43) Iwasita, T.; Xia, X. J. Electroanal. Chem. 1996, 411, 95.

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another solvent before striking the surface. Very recently, Osawa and co-workers reported a series of IR spectra obtained on an evaporated gold electrode surface in aqueous electrolyte solution, utilizing surface-enhanced IR-RAS with the ATR technique.44-46 The enhanced IR absorption arises from an electric field at the metal surface that is produced by the incident IR radiation through the excitation of a localized plasmon of the metal.46 Because the enhancement is the largest at the metal surface and decays sharply within a distance of a few monolayers from the surface, the solid/liquid interface can be investigated selectively. In this work, we report a study of CO oxidation at the sputtered Pt film electrode on a silicon prism surface in 0.1 M perchloric acid by using the in situ ATR-FTIR technique. We observe the oxidation of the CO ad-layer and another adsorbed radical (carboxyl) in addition to a linear CO and a bridged CO. We also note the behavior of the adsorbed water molecules. In particular, an interesting bipolar band of the linear CO is also observed. 2. Experimental Section The configuration of the spectro-electrochemical cell used in the present work is similar to that described in ref 44. The working electrode is a thin Pt film sputtered on the flat plane of a silicon hemicylindrical prism (1 cm in radius and 2.5 cm long) that was applied by a Ar-sputtering pure Pt target (Tanaka Kikinzoku Kogyo K. K., a purity of 99.9999%) at room temperature with an ULVAC sputtering apparatus (SH33D). The thickness of the Pt film was measured with an ultramicrobalance (with a precision of 0.1 µg), which resulted in a thickness of ∼7 nm. The electrode showed sufficient electronic conductivity. The Pt-coated prism was attached to the spectro-electrochemical cell by sandwiching an O-ring. A copper foil inserted between the cell body and the edge of the Pt film was utilized to connect the electrode and a potentiostat (EG&G Princeton Applied Research, model 263). The geometrical surface area of the Pt working electrode is ∼1.77 cm2. Platinum gauze and a reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. All the potentials in this paper are quoted against RHE. Water was purified by Nanopure (Barnsted) and then distilled with the addition of a small amount of KMnO4 and NaOH. Reagent grade HClO4 (Kanto Chemical Company) was dissolved in the resulting pure water to obtain 1 M HClO4. The solution was then pre-electrolyzed according to previously published methods,47,48 followed by further dilution with the purified water to form a 0.1 M HClO4 solution. This electrolyte solution was deaerated with high purity nitrogen. Before the collection of spectro-electrochemical data, fast potential sweeps (500 mV/s) between 0.05 and 1 V were applied to the electrode for surface cleaning, and then a slow potential sweep (20 mV/s) was applied to obtain the electrochemical surface area. The total number of Pt atoms on the electrode surface (Nt) was determined by integrating the charge in the region of hydrogen adsorption/desorption in the CV. The typical roughness factor thus obtained was ∼3, taking a conventional value of 210 µC/cm2 for the oxidation of hydrogen atoms adsorbed on a unit area of smooth Pt surface. After determining the Nt, CO was adsorbed on the Pt electrode by purging 1% CO in H2 gas at a constant potential of 0.05 V for various time intervals. Different CO coverages were obtained by controlling the adsorption time period. Then, CO in the bulk solution was removed by purging with N2 while keeping the potential constant at 0.05 V until a linear potential sweep (20 mV/s) and the simultaneous spectrameasurements were started. The coverage of adsorbed species (44) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (45) Ataka, K.; Osawa, M. Langmuir 1998, 14, 4, 951. (46) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (47) Uchida, H.; Ikeda N.; Watanabe, M. J. Electroanal. Chem. 1997, 424, 5. (48) Toda, T.; Igarashi, H.; Watanabe, M. J. Electrochem. Soc. 1998, 145, 4185.

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(θ) was determined by the following equation,

θ ) (Nt - Nf)/Nt

(1)

where Nf is the number of Pt atoms free from adsorbed species, obtained in a manner similar to Nt. Infrared spectra were taken with Bio-Rad FTS-6000 spectrometer. Unpolarized IR radiation from a Globar source was focused at the electrode/electrolyte interface by passing through the silicon prism. The incident angle was 70° from the surface normal. The radiation totally reflected at the interface was detected with a liquid nitrogen cooled linearized narrow-band HgCdTe detector (Bio-Rad). Each spectrum was taken with a resolution of 8 cm-1, which was obtained by integrating 15 interferograms to improve the signal-to-noise radio of the spectrum. The acquisition time was ∼2.5 s per spectrum. Therefore, each spectrum corresponds to that of the averages in every 50-mV interval. The first single-beam spectrum at 0.075 V (without CO2) or the last one at 0.975 V (without CO) during the potential sweep from 0.05 to 1 V was chosen as the reference for observing CO2 or CO, respectively.

3. Results and Discussion 3.1. In Situ FTIR Spectro-electrochemical Measurement for the CO Ad-layer with Saturated Coverage. Figure 1A is a CV for the oxidation of the saturated CO ad-layer (θ ) 1, solid line) on the Pt electrode in 0.1 M HClO4 in comparison with that of no CO ad-layer (dotted line). In the positive-going scan, an increase of oxidation current starts at ∼0.58 V. After a small pre-peak near 0.65 V, a main oxidation peak of CO at ∼0.82 V is observed. All adsorbed species are eliminated from the surface in the positive-going scan. Figure 1B shows a series of IR spectra on Pt with the CO ad-layer, which is simultaneously recorded with the linear potential sweep (20 mV/s) from 0.05 to 1 V. Absorption bands at ∼2360 and ∼2065 cm-1 can be assigned to the stretch mode of CO2 and linearly (atop) bound CO,7,28,49 respectively. The latter is denoted as COL. It is clear that the CO2 band starts to increase beyond 0.58 V, although that of COL does not change. This result implies that there is another adsorbed species for the production of CO2 other than COL. It is also clear that the COL band is completely removed from the electrode surface by the oxidation beyond ca. 0.8 V. The bipolar shape of the COL band is interesting. In the general discussion, a stretching frequency increases with increasing the electrode potential. Therefore, we can expect that a bipolar band with a lower reflectance at higher wavenumber for the chemisorbed CO may be produced as a reference spectrum at a lower potential is subtracted from each spectrum at a different potential. A bipolar band of COL with an anomalous shape (i.e., a higher reflectance at the higher wavenumber) was first reported in 1994 in potential-difference IR spectra on a graphite basal plane covered with Pt particles.50 This anomalous bipolar CO band was attributed to a potential-induced migration of chemisorbed CO molecules, that is, from terraces at the reference potential (50 mV versus RHE) to edge or kink sites at the sample potential (300 or 400 mV), assuming that the CO stretching frequency is higher for terraces (2070 cm-1) than for edge or kink sites (2040 cm-1). An anomalous shape of the bipolar band has also been reported for CO chemisorbed on Pt electrodeposited by potential cycling on substrates such as Pt, glassy carbon, (49) Leung, L.-W. H.; Chang S.-C.; Weaver, M. J. J. Chem. Phys. 1989, 90, 7426. (50) Christensen, P. A.; Hamnett, A.; Munk, J.; Troughton, G. L. J. Electroanal. Chem. 1994, 370, 251.

Figure 1. In situ ATR-FTIR spectro-electrochemistry for the oxidation of the saturated CO ad-layer on a sputtered Pt film electrode in 0.1 M HClO4. (A) A CV with a scan rate of 20 mV/s; a dotted line is a reference CV on Pt film without CO ad-layer. (B) IR spectra taken during a linear sweep from 0.05 to 1 V in the scan in A. The CO2 spectra were referred to the reference spectrum taken at 0.075 V (no CO2). The CO spectra were referred to the reference spectrum taken at 0.975 V (no CO). (C) Spectral regions corresponding to the COOH and H2O absorption bands, which were referred to the reference spectrum taken at 0.975 V.

graphite, or ploypyrrole film deposited on glassy carbon,51 and was attributed tentatively to IR emission. Anomalous bipolar bands were also found for CO chemisorbed on Rh electrodeposited on glassy carbon.52 It has recently been shown that the adsorption of CO on Pt and Pd dispersed on glassy carbon produces an increase of the reflectance at the CO stretching frequency, as expected for the anomalous bipolar band, which was attributed to a different mechanism of IR adsorption or an IR emission phenomenon.53 On the other hand, a normal increase of (51) Lu, G.; Sun, S.; Chen, S.; Tian, Z.; Yang, H.; Xue, K. Electrochemistry Society Spring Meeting, Los Angeles, CA, 1996, Abstract No. 891. (52) Ortiz, R.; Hernandez, R.; Kalaji, M. XII Congreso Iberoamericano de Electroquimica, Merida (Venezuela), Book of Abstracts, 1996; p 218. (53) Christensen, P. A.; Hamnett, A.; Weeks, S. A. J. J. Electroanal. Chem. 1988, 250, 127.

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CO stretching frequency and of reflectance were found with increasing potential on particulate metals deposited on moderately reflecting substrates.54 These authors simply interpreted the anomalous shape with the known reflectance law. It is worthwhile to point out that all these results involve the possibility to produce the bipolar bands due to the difference spectra, in which the reference spectrum itself included an IR band for the adsorbed CO. Our results on the bipolar bands are quite different from those of previous works, that is, the clear bipolar bands are observed although no CO band is included in the reference spectrum. Therefore, a different explanation should be proposed for the present result. We think that this asymmetric shape is a characteristic for Fano resonance.55 Fano resonance usually appears when a discrete phonon excitation interacts with a continuous excitation. There are many kinds of origins for a continuous excitation, which causes the resonance. In the case of semiconductor, a spin-orbit coupling gives a continuous excitation, which gives the Fano resonance.56 The Fanotype asymmetric spectrum in the spin-Peierls phase of some crystals is given by the strong spin-phonon coupling.57,58 A 4p-4d Fano-like resonance in metal (e.g.; Rh) can also be observed.59 In our system, the interaction between the phonons and the electronic excitation becomes a possible origin of Fano resonance. It should be noted that the Fano resonance decreases when the electrode potential is more positive than 0.7 V, that is, θCO decreases due to oxidation of COL. This result implies that the elimination of Fano resonance may be related to two possible changes: the first is a change in the structure of the CO ad-layer due to the oxidation; the second is a change in Pt surface due to the formation of Pt oxide. Splitting bands are observed near 1300-1500 cm-1 at less positive potentials, but they disappear beyond 0.68 V, as shown in Figure 1C. These bands cannot be observed on the clean Pt electrode surface without coexistence of CO ad-layer. It is now generally recognized that when CO is adsorbed onto a Pt electrode in acid media under the steady-state condition and at potentials close to the hydrogen reversible potential, two anodic stripping peaks relating to the CO ad-layer can be observed in the voltammogram.7,22,25,29,31 Markovic et al. ascribed the prepeak to weakly adsorbed CO.60 In our in situ IR spectral measurements, we consider that the COOH species is a presumable candidate assigned to the bands between 1300 and 1500 cm-1. The symmetric OCO stretching mode should be related to the bands including the OsH deformation of COOH.61-65 Although the asymmetric OCO (54) Ortiz, R.; Cuesta, A.; Marquez, O. P.; Marquez, J.; Mendez, J. A.; Gutierrez, C. J. Electroanal. Chem. 1999, 465, 234. (55) Fano, U. Phys. Rev. B 1961, 124, 1866 . (56) Holfeld, C. P.; Lo¨ser, F.; Sudzius, M.; Leo, K.; Whittaker, D. M.; Ko¨hler, K. Phys. Rev. Lett. 1998, 81, 874. (57) Popova, M. N.; Sushkov, A. B.; Vasil’ev, A. N.; Isobe, M.; Ueda, Y. JETP Lett. 1997, 65, 743. (58) Kuroe, H.; Seto, H.; Sasaki, J.; Sekine, T.; Isobe, M.; Ueda, Y. J. Phys. Soc. Jpn. 1998, 67, 2881. (59) Murgai, V.; Huang, Y.; Ruckman, M. W.; Raaen, S. J. Electron Spectrosc. Relat. Phenom. 1990, 50, 179. (60) Markovic, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Phys. Chem. B 1999, 103, 487. (61) Proenca, L.; Lopes, M. I. S.; Fonseca, I.; Hahn, F.; Lamy, C. Electrochim. Acta 1998, 44, 1423. (62) Colthup, N. B.; Daly, L. H. Wilberley, S. E. Introduction to Infrared and Raman Spectroscopy, Academic Press: New York, 1975. (63) Iwasita, T.; Nart, F. C.; Lopez, B.; Vielstich, W. Electrochim. Acta 1992, 37, 2361. (64) Shaw, K.; Christensen, P.; Hamnett, A. Electrochim. Acta 1996, 41, 719. (65) Socrates, G. Infrared Characteristic Group Frequencies; WileyInterscience: New York, 1980.

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stretching mode should be observed near 1600 cm-1,66 this band was hardly observed probably because of the surface selection rule. The COOH should correspond to the prepeak on the CV in Figure 1A because the CO2 band starts to increase beyond 0.58 V when the new bands start to decrease, but the COL band does not change. The COOH may be formed as shown in eq 2

PtsOH2 + PtsCO f PtsCOOH + PtsH

(2)

where the adsorbed H might be kicked out as H2 when CO is further adsorbed. Bands in the same region as that for COOH can be observed after ∼0.8 V, which may correspond to carbonate species. Carbonate species may be formed through the following subsequent steps

PtsCOOH f Pt + CO2 + H+ + ePtsCO + PtsOH2 f 2Pt + CO2 + 2H+ + 2eCO2 + PtsOH2 f PtsCO32- + 2H+

(3) (4) (5)

Generally, the concentration of CO32- or HCO3- is very low in the strong acid electrolyte. Therefore, their production must be related to a heterogeneous acid-base reaction in which the adsorbed water is involved, as shown by eq 5. The IR and Raman spectra of carbonate in solutions have been reported by Oliver and Davis.67 The authors found that the solvation induces a moderate splitting of the degenerate V3 mode, which leads to two peaks at 1376 and 1438 cm-1 compared with the 1415 cm-1 value for the free carbonate. Iwasita et al.68 have studied the adsorption of carbonate on Pt(111) and Pt(110) electrodes. Three bands at 1328, 1455, and 1537 cm-1 were assigned to the adsorbed carbonate species although the authors concluded that the band at 1330 cm-1 does not belong to the adsorbed carbonate. In any case, Iwasita et al.68 concluded that the band near 1455 cm-1 is due to vibrations involving the noncoordinated oxygen atoms in monodentate coordination mode. Very recently, Markovits et al.69 reported the results from a theoretical study of the bonding of carbonate on Pt(111), which suggested that the band around 1450 cm-1 is more likely due to a bidentate adsorbed carbonate. It has been reported that the stretching frequency of carbonate species is highly sensitive to the environmental conditions.70 In our case, bands in the region just mentioned could be observed after ∼0.75 V, which supports the existence of carbonate species. An absorbance change in the interfacial water (OH stretching band near 3500 cm-1),41,43,44,46 which is potential dependent, can only be observed distinctly beyond ∼0.675 V, that is, the band shift to a smaller wavenumber and a negative value as the CO2 band becomes noticeable, and the returning to the original-like state as the CO band disappears completely. The band shift can be observed more clearly when the individual spectrum is enlarged. This behavior indicates the consumption of water during oxidation of adsorbates. However, the change in the HOH bending band near 1650 cm-1 was hardly observed, probably because of the weak signal. (66) Sexton, B. A. Appl. Phys. A 1981, 26, 1. (67) Oliver, B. G.; Davis, A. R. Can. J. Chem. 1973, 51, 698. (68) Iwasita, T.; Rodes, A.; Pastor, E. J. Electroanal. Chem. 1995, 383, 181. (69) Markovits, A.; Garcia-Hernandez, M.; Ricart, J. M.; Illas, F. J. Phys. Chem. B. 1999, 103, 509. (70) Fujita, J.; Martell, A. E.; Nakamoto, K. J. Chem. Phys. 1962, 36, 339.

FTIR Study of CO Oxidation on Pt Film

Figure 2. In situ ATR-FTIR spectro-electrochemistry for the oxidation of the CO ad-layer with intermediate coverage (0.48) on a sputtered Pt film electrode in 0.1 M HClO4. (A) A CV with a scan rate of 20 mV/s. (B) IR spectra taken during a linear sweep from 0.05 to 1 V in the scane in A. The CO2 spectra were referred to the reference spectrum taken at 0.075 V (no CO2). The CO spectra were referred to the reference spectrum taken at 0.975 V (no CO). (C) Spectral regions corresponding to the COOH and H2O absorption bands, which were referred to the reference spectrum taken at 0.975 V.

Similar results (including the bipolar band of COL) for the oxidation of the CO ad-layer with a relatively high coverage, θCO ) 0.84, were also observed (data not shown). It should be noted that the main peak potential at θCO ) 0.84 is ∼80 mV lower than that at the saturated coverage of CO. This behavior is similar to that on a low-index Pt electrode,71 which indicates that the CO oxidation occurs via the reaction with water and/or hydroxyl radicals adsorbed at adjacent sites. The more opened ad-lattice structure of CO at θCO ) 0.84 may facilitate CO electrooxidation as well as an adsorption of oxygen species on Pt sites. CO Ad-layer with Intermediate Coverage. Figure 2A is a linear sweep voltammogram for the oxidation of the CO ad-layer with intermediate coverage, θCO ) 0.48, (71) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095.

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on the Pt electrode. The preoxidation peak around 0.55 V can still be observed. A main oxidation peak of CO is at ∼0.74 V. Figure 2B shows a series of IR spectra of the CO adlayer with intermediate coverage on Pt. Bands around 2000 and 1850 cm-1 can be assigned to COL and the bridged CO (COB), respectively. The CO2 band appears after ∼0.5 V, corresponding to the pre-peak in the CV. The CO2 band rapidly increases around 0.7 V, corresponding to a steep increase in CO oxidation current in the CV. A potential-dependent band shift between 3500 and 3000 cm-1, shown in Figure 2C, indicates an alteration of the OH stretching mode of adsorbed water. Changes of the adsorption state and of the amount of water accompanying the oxidation of CO and COOH were indicated by the changes in the OH stretching band. Bands between 1300 and 1500 cm-1, which were assigned to the bands for COOH in Figure 1C, can also be observed in Figure 2C. However, more adsorbed water and less adsorbed CO may lead to a change in the adsorption sites for COOH; for example, multiple bonded adsorption or/and C-down or O-down adsorption, compared with those in Figure 1C for the saturated CO ad-layer. The positive bands around 1300 and 1450 cm-1 disappear and the positive bands around 2360 cm-1 for CO2 appear at ∼0.55 V, which may be due to COOH oxidation. The negative band around 1480 cm-1 is due to the effect of the reference spectrum, which results from the carbonate species. After ∼0.8 V, the bands in this region confirm the possibility of the existence of carbonate species. CO Ad-layer with Low Coverage. Figure 3A is a linear sweep voltammogram for the oxidation of the CO ad-layer with a lower coverage, θCO ) 0.16, on the Pt electrode. During the positive-going potential scan from 0.05 V, the increase of anodic current commences at ∼0.5 V, which corresponds to the preoxidation shoulder in the case of Figures 1A and 2A. At potentials >0.65 V, the main peak of CO oxidation appears with a peak potential of ∼0.74 V. Figure 3B shows a series of IR spectra during the potential sweep. Bands around 2000, 1800, and 1730 cm-1 can be assigned to the linear CO and the bridged (2-fold or 3-fold) CO, respectively.7,72 The CO2 band appears beyond 0.5 V. However, CO bands do not clearly change, which indicates that the oxidation of COOH occurs prior to that of CO. Similar to the results in Figure 2C, OH stretching and the bending band of the adsorbed water, and characteristic absorption bands of COOH and the carbonate species are observed in Figure 3C. 3.2. Dependencies of Adsorbate Types on Potential and on Coverage. For the high coverage of CO (θCO ) 1), the normalized band intensities for COL, COOH, or CO2 and the wavenumber for COL are shown in Figure 4 as a function of potential. At θCO ) 0.84, almost the same results were obtained. Obviously, the onset potential of the production of CO2 cannot be explained by COL oxidation (refer Figure 4A), which indicates a part of CO2 is produced by the oxidation of some adsorbates other than COL. The potential of the disappearance of COOH, the intensity of which is shown by a dashed line, coincides well with the onset potential of CO2 formation. There is no possibility of a contribution of COB to CO2 formation because at this CO coverage, no COB is observed. It has been claimed in the literature that, in addition to the dominant adsorbed CO species, a different chemical (72) Garfunkel, E. L.; Crowell, J. E.; Somojai, G. A. J. Phys. Chem. 1982, 86, 310.

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Figure 4. Spectral analysis of Figure 1. (A) Dependencies of normalized peak intensity for the CO, CO2 absorption bands on the potential. (B) Dependence of the CO stretch vibration frequency on the potential. (2) CO, (b) CO2; (- - -) change in the peak intensity of the COOH, based on data around 1480 cm-1.

Figure 3. In situ ATR-FTIR spectro-electrochemistry for the oxidation of the CO ad-layer with the low coverage (0.16) on a sputtered Pt film electrode in 0.1 M HClO4. (A) A CV with a scan rate of 20 mV/s. (B) The IR spectra taken during a linear sweep from 0.05 to 1 V in the scan in (A). The CO2 spectra were referred to the reference spectrum taken at 0.075 V (no CO2). The CO spectra were referred to the reference spectrum taken at 0.975 V (no CO). (C) Spectral regions corresponding to the COOH and H2O absorption bands, which were referred to the reference spectrum taken at 0.975 V.

species (the nature of which is not known) is formed when CO adsorbs on the Pt surface at potentials in the hydrogen region. The adsorbed species proposed so far are: Pts COH, PtsCHO, PtsCOOH, Pts(COads)(OHads), or Pts (COads)(H2Oads).7,23,24 According to our work, the species may be assigned to the adsorbed COOH, based on IR adsorption bands near 1300-1500 cm-1. For the CO coverages of intermediate (θCO ) 0.48) and low (θCO ) 0.16) values, the normalized band intensities for COL, COB, or CO2 and the wavenumbers for COL are shown in Figures 5 and 6, respectively, as a function of potential. At potentials 0.5 V, CO2 is produced. Although a partial contribution of CO oxidation to CO2 production cannot be completely excluded in the potential region between 0.5 and 0.6 V, the increase of CO2 prior to the

Figure 5. Spectral analysis of Figure 2. (A) Dependencies of normalized peak intensity for the CO, CO2 absorption bands on the potential. (B) Dependence of the CO stretch vibration frequency on the potential. (2) COL; (1) COB; and (b) CO2.

noticeable reduction of COL or COB indicates the presence of additional CO-related species other than COL and COB. For the low coverage of CO, 3-fold CO was observed (refer to the band around 1750 cm-1 in Figure 3B), which is consistent with the prediction that CO tends to form multiply bonded species at a low CO coverage.7 As shown in Figures 5B and 6B, the obvious transition between COL and COB can be seen, indicating that the type of CO adsorbates is sensitive to both the potential and the coverage. Changes in the CO adsorbing states have been observed as the coverage increases. In the

FTIR Study of CO Oxidation on Pt Film

Figure 6. Spectral analysis of Figure 3. (A) Dependencies of normalized peak intensity for the CO, CO2 absorption bands on the potential. (B) Dependence of the CO stretch vibration frequency on the potential.(2) COL; (1) COB; and (b) CO2.

present work, the potential-induced conversion of the adsorption type is observed, particularly at a low CO coverage. It is clear that the supply of a less positive external potential to the PtsCO/electrolyte interface causes the transition from 1-fold (COL) to 2-fold (COB) and possibly to 3-fold CO on the Pt electrode. This result agrees well with theoretical discussions published previously.72-74 The shift up in the energy of the Pt valence band lying close to the energy of the empty π*(or 2π) orbital of CO was considered by Anderson et al..72-74 This shift up strengthens the d-π* orbital mixing, and facilitates CO adsorption at highly coordinated sites. On the contrary, applied positive potentials result in the stabilization of 1-fold coordination state.74 According to the results of Figures 4-6, the following conclusions, which are consistent with our previous results obtained with electrochemical methods, can be made.9 For 1 > θCO > 0.84, COL and COOH species are the predominant adsorbates. In addition to these species, COB can be observed for 0.48 > θCO > 0.16. The COB is stable at less positive potentials (e.g., at j0.25 V) and is converted into COL as the potential is swept toward a more positive value (e.g., between ∼0.25 and 0.6 V. 3.3. Dependence of the Band Frequencies of Adsorbates on the Electrode Potential. For θCO ) 1, a relationship between the band frequency of COL and the potential is shown in Figure 4B. Almost the same relationship was obtained, for θCO ) 0.84. As the potential is swept more positive, the COL absorption band shifts linearly to higher wavenumbers with a slope of ca. 35 cm-1/V between 0.075 and 0.4 V. This value is the same as the result reported by Ianniello et al.,28 but it is somewhat higher than the value of 30 cm-1/V reported by Kunimatsu et al.30 Two models have been proposed to explain the linear potential dependence; one is based on the first-order “Stark effect”75 assuming a rigid dipole oscillator in a variable electric field, and the second is (73) Anderson A. B.; Awad, M. K. J. Am. Chem. Soc. 1985, 107, 7854. (74) Ray, N. K.; Anderson, A. B. J. Phys. Chem. 1982, 86, 4851. (75) Lambert, D. K. J. Chem. Phys. 1988, 89, 3847.

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based on a “chemical bonding model” with a potential dependence.76 In addition to the electric field in the inner Helmholz layer, we must consider the other physical parameters that are factors that influence the COL frequency, such as the coverage or binding type of adsorbates. The linear shift with increasing the electrode potential should appear if these factors are constant. The result obtained here for Pt suggests that this required condition is fulfilled in the potential region less positive than ca. 0.4 V. But, the linear relationship is perturbed at potentials more positive than 0.4 V. The wavenumber shift levels off between 0.4 and 0.6 V, which indicates that there may be a rearrangement of the adsorbed CO that tends to decrease dipole-dipole coupling. The rearrangement may result from the oxidation or reorientation of trace COOH species on the electrode. After the commencement of CO oxidation at 0.65 V, the wavenumber downshifts with increasing potential due to the decrease of CO coverage. For intermediate (θCO ) 0.48) and low (θCO ) 0.16) CO coverage, the relationship between COL band frequency and potential exhibits a complicated feature, as shown in Figures 5B and 6B. For intermediate coverage, a nonmonotonic frequency-potential dependence is observed. A similar feature was reported at Pt(100) by Chang and Weaver.71 This complex behavior may be related to the conversion of COB to COL with the increase of potential (Figure 5). At first, the COL wavenumber does not change noticeably in the less positive potential region of 0.2-0.3 V, where an interconversion and a reorientation is occurring, (Figure 5A). Then, the COL wavenumber decreases, perhaps because of a lowered dipole-dipole interaction among COL molecules due to a lowered total CO coverage brought about by the partial conversion of multibonding CO such as COB into COL. After ca. 0.4 V, the CO coverages of different adsorption types are kept constant, but the COL wavenumber increases linearly, which may result from the effect of the electric field in the double layer. Finally, the COL wavenumber commences to decrease after the onset of CO oxidation. The similar potential dependence of the COL wavenumber is also observed at low (θCO ) 0.16; see Figure 6B). The negative shifts of the COL wavenumber with decreasing θCO all over the potential region may result from the lowered dipole-dipole interaction among COL molecules. 3.4. Role of Adsorbed Water in the CO and COOH Oxidations. In the present work, the interaction of water with CO and COOH is clearly shown by the ATR-FTIR technique. This interaction is not easy to observe by the conventional IR techniques, particularly because of the interference of the water that exists in the bulk solution. The consumption of adsorbed H2O was observed when COOH or CO disappeared from the surface (as discussed in Section 1 regarding Figures 1-3). For this interaction, the following reactions can be proposed, where H2O acts as a source of O for the oxidation reactions:

PtsCOOH f Pt + CO2 + H+ + ePtsCO + PtsOH2 f 2Pt + CO2 + 2H+ + 2eCO2 + PtsOH2 f PtsCO32- + 2H+

(3) (4) (5)

The coadsorption of water facilitates the CO oxidation, but it is not the key factor of controlling the CO adsorption type. The Pt sites, which become free from CO or other adsorbates after reactions are covered again with H2O, as (76) Anderson, A. B. J. Electroanal. Chem. 1990, 280, 37.

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shown by the increase of the corresponding spectra in Figures 1-3. 4. Conclusions We obtained valuable information on CO oxidation at the sputtered Pt film/Silicon electrode using in situ ATRFTIR technique. The interesting bipolar shape of the COL band with the reference spectrum at ca. 1 V (no CO band) implies the mechanism of CO ad-layer interaction with the electrode surface, which may be a characteristics of Fano resonance. The results presented here indicate that the preoxidation shoulder in the CV results from the COrelated species containing a carboxyl group and that the adsorbed H2O is consumed during the CO oxidation. The potential-dependent conversion of adsorbates reflects the effect of the electronic behavior of the electrode surface on CO adsorption. The present results support the need

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for understanding the nature of CO-tolerant Pt-based alloy electrocatalysts for fuel cells, which will be published in the other papers. Acknowledgment. This work was supported by the Proposal-Based Advanced Industrial Technology R&D Program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The work was partially supported by Grant-in-Aid No. 09237104 for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces from the Ministry of Education, Science, Sports and Culture, Japan. We thank Professor Masatoshi Osawa (Hokkaido University) for his advice on the ATR-FTIR measurements. Y.-M.Z. acknowledges NEDO for fellowship support. LA990835R