Adsorption and Electro-Oxidation of Carbon Monoxide at the Platinum

The vibrational spectrum of carbon monoxide at the Pt/acetonitrile interface as a function of applied potential has been investigated using the surfac...
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Langmuir 2002, 18, 2737-2742

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Adsorption and Electro-Oxidation of Carbon Monoxide at the Platinum-Acetonitrile Interface as Probed by Surface-Enhanced Raman Spectroscopy Peigen Cao,† Yuhua Sun,† Jianlin Yao,‡ Bin Ren,‡ Renao Gu,*,† and Zhongqun Tian*,‡ Department of Chemistry, Suzhou University, Suzhou 215006, People’s Republic of China, and State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry and Institute of Physical Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China Received October 20, 2001 The vibrational spectrum of carbon monoxide at the Pt/acetonitrile interface as a function of applied potential has been investigated using the surface-enhanced Raman spectroscopy technique. The electrolyte is 0.1 M LiClO4. The bands observed at ca. 506 and 2055 cm-1 are attributed to the platinum-CO (νPt-C) and intramolecular C-O (νC-O) stretching vibrations, respectively, suggesting linearly adsorbed CO on platinum. The νPt-C band has an experimental Stark tuning rate of -4 cm-1/V, while the slope of νC-O band frequency versus potential approaches zero before the onset of electro-oxidation of COads, possibly resulting from low surface coverage of CO on platinum. The roughened Pt electrode surface has a high electrocatalytic activity, on which CO electro-oxidation occurs at ca. -0.7 V (vs Ag/Ag+). The main product of COads oxidation is confirmed to be carbonate due to the existence of trace water in the double-layer region as a source of oxygen for the reaction. It has also been found that the solvent acetonitrile can exert a chemisorbed decomposition reaction on the roughened Pt surface, while the adsorption of CO can significantly inhibit this reaction.

Introduction Adsorption and electro-oxidation of carbon monoxide at a platinum electrode from aqueous solutions have been studied extensively in view of the electrocatalytic properties of platinum.1-5 To obtain a molecular-level picture of the interface, specifically, the structure and density of surface adlayers, adsorbate-substrate bonding, and the dynamics of the surface adlayer, in situ spectroscopy techniques have been employed in this area combined with electrochemical measurements. The most commonly used spectroscopy techniques include infrared reflectanceadsorption spectroscopy (IRAS)2,3,6-8 and sum frequency generation (SFG).9,10 Nevertheless, these investigations are mostly restricted to the probing of the vibrational frequency shifts of intramolecular modes rather than the metal-adsorbate surface modes. The main reason blocking * To whom correspondence should be addressed. Renao Gu: Tel, 86+512+5112813, 5112645; Fax, 86+512+5231918; E-mail, [email protected]. Zhongqun Tian: Tel, 86+592+2181906; E-mail, [email protected]. † Suzhou University. ‡ Xiamen University. (1) Korzeniewski, C.; Pons, S.; Schmidt, P. P.; Severson, M. W. J. Chem. Phys. 1986, 85, 4153. (2) Wasileski, S. A.; Weaver, M. J.; Koper, M. T. M. J. Electroanal. Chem. 2001, 500, 344. (3) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 5942. (4) Thomas, V. D.; Schwank, J. W.; Gland, J. L. Surf. Sci. 2000, 464, 153. (5) Zaera, F.; Salmeron, M. Langmuir 1998, 14, 1312. (6) Mucalo, M. R.; Cooney, R. P. J. Chem. Soc., Faraday Trans. 1991, 87, 1221. (7) Brandt, R. K.; Sorbello, R. S.; Greenler, R. G. Surf. Sci. 1992, 271, 605. (8) Bae, I. T.; Sasaki, T.; Scherson, D. A. J. Electroanal. Chem. 1991, 297, 185. (9) Peremans, A.; Tadjeddine, A. J. Electroanal. Chem. 1995, 395, 313. (10) Baldelli, S.; Eppler, A. S.; Anderson, E.; Shen, Y. R.; Somorjai, G. A. J. Chem. Phys. 2000, 113, 5432.

these techniques may be the rather low detecting sensitivity in the low-frequency region below ca. 700 cm-1, where typical metal-adsorbate vibrations are located. However, the application of laser Raman techniques, particularly the advent of surface-enhanced Raman spectroscopy (SERS), has enabled one to obtain information for metal-adsorbate vibrations in addition to the intramolecular modes in some electrochemical systems. Most such SERS data have been gathered for halide11 and pseudo-halide12 adsorption at silver and to a lesser extent gold electrodes.13 However, the breadth of applications of SERS in surface science has been limited heretofore, as far as we know, to coinage metallic substrates (Cu, Ag, Au). Of particular interest and also importance in further developing the SERS technique is the extension to the study of transition metals such as platinum used in electrocatalysis in many technologically important processes. The strategies involve coating onto SERS-active Ag or Au electrodes with ultrathin films of platinum14 or other transition metals15 by means of electrochemical deposition. Valuable information has been obtained by this unique method. However, problems arise from the existence of “pinholes” in the ultrathin deposition film, which may complicate the interpretation of the spectra. Therefore, a more reliable and also difficult approach would be to obtain the surface Raman spectra directly from a bare Pt surface without the complexity of the sandwiched configuration (adsorbate/Pt/SERS-active metal). With the development of the Raman instrumentation, particularly the advent of the CCD cameras having (11) Gao, P.; Weaver, M. J. J. Phys. Chem. 1986, 90, 4057. (12) Yi, H.; Wu, G. Z. Spectrochim. Acta, Part A 1989, 45, 123. (13) Bron, M.; Holze, R. Electrochim. Acta 1999, 45, 1121. (14) Zou, S.; Williams, C. T.; Chen, E. K.-Y.; Weaver, M. J. J. Am. Chem. Soc. 1998, 120, 3811. (15) Aramaki, K.; Fujioka, E. Corrosion 1996, 52, 83.

10.1021/la015638i CCC: $22.00 © 2002 American Chemical Society Published on Web 02/05/2002

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essentially photon-noise-limited signal detection, combined with a proper surface roughening pretreatment by Tian et al., the SERS spectra of some organic adsorbates have been acquired at the bare Pt electrode16 and other transition metals, such as nickel17 and iron.18 The surface enhancement factor (SEF) for Pt was calculated experimentally in the range of 10-120.19 Tian and co-workers have been employing this newly discovered property of platinum to examine SERS of a variety of inorganic and organic adsorbates at this electrode from aqueous solutions, including hydrogen, cyanide, thiocyanate, halides, carbon monoxide, methanol, pyridine, and some aromatic species.19-21 A related objective is to monitor electrochemical kinetics and mechanisms by using SERS combined with faradaic electrochemical techniques, other than to test the versatility and suitability of this strategy. In this work, we report here the adsorption and electrooxidation of carbon monoxide at platinum electrodes in acetonitrile as part of these ongoing studies. The motivation to choose this system is based on the following facts. First, the widely used smooth Pt electrode in the IRAS and SFG investigations is different from the roughened material used in practice.8 The ease of studying the surface bonding, investigating highly roughened surfaces with dark color by using SERS techniques, may provide a way to bridge the gap between the systems of fundamental research and practical applications. Second, a relatively small number of studies have been reported in the literature concerning the vibrational spectra of adsorbed CO in the presence of nonaqueous solvents partly because of the low solubility of CO in these aprotic solvents.22 Therefore, the present study should be meaningful and informative by comparison with those studies on the smooth Pt surfaces and from aqueous solutions. Particularly, the ability of SERS to obtain information of the vibrational mode regarding the metal-adsorbate interaction, νPt-C, in addition to the intramolecular mode, νC-O, would be useful to provide a better understanding of the adsorption and electro-oxidation of carbon monoxide at the bare roughened Pt electrodes. In addition, the influence of the competitive adsorption of CO on the dissociative adsorption reaction of the solvent CH3CN is also discussed. Experimental Section The measurements were made in 0.1 M LiClO4 acetonitrile solutions at room temperature. The anhydrous salt used in this work, lithium perchlorate (analytical reagent grade), was dried by heating under vacuum up to 60 °C for 6 h before use. Acetonitrile (HPLC grade, 0.003% water) was distilled twice from calcium hydride and stored before use in sealed containers over Woelm alumina. A three-compartment spectroelectrochemical cell was used to perform the in situ Raman measurements. The cell was dried in a vacuum oven at 120 °C before use. The working electrode was a polycrystalline Pt (99.99%) rod embedded in a Teflon sheath, with a geometric surface area of 0.1 cm2. A large Pt ring served as the counter electrode. All the potentials, unless specified, are reported versus a Ag/Ag+ reference electrode in CH3CN contain(16) Ren, B.; Huang, Q. J.; Cai, W. B.; Mao, B. W.; Liu, F. M.; Tian, Z. Q. J. Electroanal. Chem. 1996, 415, 175. (17) Huang, Q. J.; Yao, J. L.; Mao, B. W.; Gu, R. A. Chem. Phys. Lett. 1997, 271, 101. (18) Cao, P. G.; Yao, J. L.; Ren, B.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 2000, 316, 1. (19) Tian, Z. Q.; Ren, B.; Mao, B. W. J. Phys. Chem. B 1997, 101, 1338. (20) Ren, B.; Xu, X.; Li, X. Q.; Cai, W. B.; Tian, Z. Q. Surf. Sci. 1999, 427, 157. (21) Gu, R. A.; Cao, P. G.; Yao, J. L.; Ren, B.; Tian, Z. Q. J. Electroanal. Chem. 2001, 505, 95. (22) Huang, J. M.; Korzeniewski, C. J. Electroanal. Chem. 1999, 471, 146.

Cao et al. ing 0.1 M LiClO4 and 0.01 M AgNO3 (0.268 V versus a saturated calomel electrode (SCE), see ref 23). An EG&G model 173 potentiostat was employed to control the applied potentials. The working electrode should be roughened to obtain a SERS-active surface for the Raman measurements. Briefly, the Pt electrode was first mechanically polished successively with 0.5 µm aluminum powder down to 0.05 µm to a mirror finish followed by ultrasonic cleaning with Milli-Q water. Then, a square wave of 1.5 kHz with upper and lower potentials of 2.4 and -0.2 V (SCE) was applied to the electrode in 0.5 M H2SO4 for 5-10 min. The potential was then polarized at -0.2 V until the electroreduction of the surface Pt oxides was completed. An additional potential cycle between -0.3 and 1.25 V (SCE) at a scan rate of 500 mV/s was employed to the electrode in order to remove the unstable atoms or clusters, which then yielded a stable Pt surface for the acquisition of the SERS spectra. The roughened Pt electrode was then immersed in acetonitrile and rinsed by the solvent before use in order to eliminate the contamination of water. The acetonitrile solution was purged with dry CO for about 10 min until a saturated adsorption quantity of CO was obtained on the Pt surface. The electrode was then transferred to the spectroelectrochemical cell containing the acetonitrile solution (without CO) for the Raman measurements. SERS data were obtained using a confocal microprobe Raman system (LabRam I from Dilor, France). The excitation wavelength was 632.8 nm from an inner air-cooled He-Ne laser with a power of 10 mW and a spot of size 3 µm at the sample surface. The slit and pinhole used were 200 and 800 µm, respectively. The acquisition time was 100 s, and the accumulation was 4 times for each spectrum. With a holographic notch filter and a CCD detector, the system has an extremely high detecting sensitivity. A 50× long working-length objective (8 mm) was used so that it was not necessary for the objective to be immersed in the solution.

Results The cyclic voltammogram of a roughened Pt electrode in acetonitrile shows a large double-layer region over which no obvious faradaic process occurs. The potential range which is suitable for adsorption studies is over 3.0 V, in comparison with the results from an aqueous acidic solution which is commonly used in IR studies, where there is only an approximately 500 mV region suitable for investigations.24 Even when an alkaline solution is used, the potential region only extends to about 1.5 V.19 This is mainly due to the bound by the oxidation to carbon dioxide at positive potentials and the desorption of CO to allow hydrogen evolution at negative potentials. For studying potential-dependent properties of the spectra, the organic solutions clearly have a major advantage. As is described in the Experimental Section, carbon monoxide was preadsorbed onto the roughened Pt electrode before it was finally transferred to a fresh acetonitrile solution containing 0.1 M LiClO4. The testing solution contains no carbon monoxide. Figure 1A-C shows typical potential-dependent SERS spectra in three frequency regions, 430-700, 950-1200, and 1700-2180 cm-1, for the above preadsorbed roughened Pt electrode in acetonitrile containing 0.1 M LiClO4. Data were acquired in descending order (top to bottom, as shown), starting at -1.7 V and proceeding to -0.3 V before returning to -2.3 V. The results demonstrate clearly the advantage of the confocal microprobe Raman system over conventional Raman spectrometers. To the best of our knowledge, it is the first time that one is able to study the SERS spectra of CO adsorbed on a roughened Pt electrode in a nonaqueous solution as a function of applied potential. The confocal optical configuration plays a key role, particularly (23) Larson, R. C.; Iwamoto, R. T.; Adams, R. N. Anal. Chim. Acta 1961, 25, 371. (24) Papoutsis, A.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 1987, 234, 315.

Adsorption and Electro-Oxidation of CO

Figure 1. Representative potential-dependent surface Raman spectra for CO adsorbed onto a roughened platinum electrode in three frequency regions: 430-700, 950-1200, and 17002180 cm-1 (A, B, and C, respectively). Potentials were changed stepwise first positively till -0.3 V and then negatively till the extremity (see text). Laser line, 632.8 nm; acquisition time, 100 s; accumulation, 4.

to eliminate the interference of a strong Raman signal from the bulk solution phase, which is even more serious in nonaqueous solvents. The present observation of goodquality surface Raman spectra is attributed greatly to a proper surface roughening pretreatment depicted in the Experimental Section. A primary feature of this roughened Pt electrode is that it can yield stable Raman spectra and can even be reused over a long time period as long as it is subjected to an electrochemical cleaning procedure of cycling in sulfuric acid before starting a new experiment. The SEF for Pt was calculated to be ca. 10-120, depending

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on the different surface roughness factors ranging from about 20 to 400. A detailed discussion of the SEF and the SERS mechanism for platinum can be found in ref 25. For the purpose of convenient discussion, the potentials investigated have been divided into three regions: (i) -1.7 to -0.7 V, (ii) -0.7 to -0.3 V, and (iii) -0.3 back to -2.3 V (see Figure 1). As can be seen in the spectra, two bands are evident at ca. 506 and 2055 cm-1 in the low- and highfrequency regions, respectively, when the Pt electrode was polarized at -1.7 V. The 506 cm-1 band shifts to lower values in frequency with the positive-going potentials, while the high one remains constant in the first potential region. By comparison with the SERS result of CO at Ptcoated gold electrodes26 as well as electron energy loss spectra (EELS) obtained from the Pt(111)/CO system under ultrahigh vacuum,27 the band in the low-frequency region is attributed confidently to the νPt-C stretching mode. The frequency value of this band approaches that obtained from aqueous solutions by Li,28 where the νPt-C band was observed at ca. 485-489 cm-1 in sulfuric solutions and at ca. 505-511 cm-1 in NaOH solutions. An additional band was also observed by Li28 under certain conditions, but clearly not in the present study, at ca. 440 cm-1. These two bands were attributed to “linearly bonding” and “bridging bonding” CO, respectively. Hence, it is reasonable to assume that CO is mainly linearly adsorbed onto Pt judging by the present data. The fact that there is no appearance of the bands in the frequency region of ca. 1650-2000 cm-1, associated with bridgebonded CO, supports this assignment.29 The calculation of the Stark tuning rate of the νPt-C stretching band yields a dνCN/dE value of -4 cm-1/V, which is also consistent with results of studies in aqueous solution.30 The relatively small tuning rate is mainly due to the counteraction of contributions from the donation of 5σ of COads to the vacant d-orbital of Pt, which strengthens the Pt-C stretching vibration, and the decrease of the dπ-2π* back-donation, which weakens this band.31 The constancy of the νPt-C band intensity upon potential reveals a constant surface coverage of carbon monoxide in potential region i. However, one should be more careful in the assignment of the high-frequency band at 2055 cm-1, although we can assign it, as usual, to the C-O stretching vibration, νC-O, which is consistent with the frequency expected for linearly bonded CO.29 Closer examination of the potential dependence of this band in potential region i gives some different results from those of the previous IR studies for the same system. Specifically, the full width at half-maximum (fwhm) of the high-frequency band is rather large (ca. 52 cm-1) and slightly increases with variation of potential to positive values, for instance, ca. 61 cm-1 at -0.7 V. This may indicate an increasingly strong interaction in the surface species or increasingly adsorption onto different surface sites of the platinum electrode. The most striking feature is perhaps the independence of the 2055 cm-1 band frequency on the applied potentials in potential region i. A detailed analysis of this band is given in the Discussion. Considering then the second potential region, from -0.7 (25) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Surf. Sci. 1998, 406, 9. (26) Zhang, Y.; Weaver, M. J. Langmuir 1993, 9, 1397. (27) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Langmuir 1995, 11, 2777. (28) Li, X. Q. M. Sc. Thesis, Xiamen University, Xiamen, P. R. China, 1998; Chapter 3. (29) Sheppard, N.; Nguyen, T. T. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5, p 67. (30) Huang, Q. J.; Li, X. Q.; Yao, J. L.; Ren, B.; Cai, W. B.; Gao, J. S.; Mao, B. W.; Tian, Z. Q. Surf. Sci. 1999, 427, 162. (31) Zou, S. Z.; Weaver, M. J. J. Phys. Chem. 1996, 100, 4237.

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Discussion

Figure 2. Dependence of the frequency and integrated band intensity versus applied potential for (a) νPt-C and (b) the highfrequency band located in the 1700-2180 cm-1 region (see text).

to -0.3 V, a marked decrease in the νPt-C intensity was observed by altering the potential to -0.7 V. This is indicative of the occurrence of electro-oxidation of COads. A complete electro-oxidation of COads is evident for the disappearance of the νPt-C band upon scanning potential to ca. -0.3 V. In the high-frequency region, the variation of the band frequency versus potential is nonlinear. The frequency first decreases slightly and then increases abruptly to ca. 2081 cm-1 as potential is increased to -0.3 V. Furthermore, although CO electro-oxidation occurs at -0.7 V, the 2052 cm-1 band intensity increases slightly. Therefore, another contribution may incorporate into this band. The potential dependence of the integrated band intensity and frequency for νPt-C and the high-frequency band determined from the data in Figure 1 are shown in parts a and b of Figure 2, respectively. The potential range is limited to potential regions i and ii. A more interesting feature in the spectra in potential region ii is the clear observation of a new band at ca. 1086 cm-1 (see Figure 1B) in the middle-frequency region. The appearance of this band is accompanied by the disappearance of the νPt-C vibration, indicating the correlation between the two bands. The intensity of the new band approaches its maximum upon scanning potential to -0.3 V, while the frequency remains constant. For the purpose of a better understanding of the behavior of the two new bands at ca. 1086 and 2081 cm-1, the potential was scanned back to more negative values. An irreversible spectral behavior was observed (see Figure 1). The Pt-C vibration was not detected over the whole range of potential region iii, suggesting the completely elimination of carbon monoxide from the Pt surface. The 1086 cm-1 band first increases in intensity slightly and then keeps its band intensity over the more negative potential range. On the contrary, marked changes occur in the high-frequency region. As the potential is altered back to negative values, the peak frequency of the 2081 cm-1 band shifts linearly to lower values, yielding a Stark tuning rate of 50 cm-1/V.

The ability to detect the Pt-C vibration band in the low-frequency region is very helpful to the detailed discussion of adsorption and electro-oxidation of carbon monoxide. In the previous IR studies, it is impossible to detect the νPt-C band, indicating the advantages of Raman measurements over IR techniques. It is of interest to compare the νPt-C frequency shift upon potential for carbon monoxide adsorbed at the platinum-acetonitrile interface with the corresponding feature seen at platinum surfaces in aqueous solutions.28 SERS investigations on the νPt-C peak frequency versus potential at bare roughened Pt electrodes yielded a dνPt-C/dE of 4-5 cm-1 for linearly adsorbed CO in acidic solutions,28 which is comparable to the present data. Weaver et al.31 reported a slightly higher absolute value of 10-20 cm-1/V for CO adsorbed at a Pt film surface deposited onto Au. These disparities presumably arise from minor differences in the surface microscopic structure of the deposited Pt film and bare platinum substrates. It is also indicative of a greater sensitivity of dνPt-C/dE to the surface structure of metallic substrates than to the solution environment. Of central interest here is the examination of the peak frequency shift upon potential in potential region i for the band during the high-frequency region and then the assignment of this band. As is described in the Results, this band can be tentatively assigned to the C-O vibration of linearly adsorbed CO. Controversy still exists, particularly for the nearly constant νC-O frequency upon variation of potential in comparison with results from IR studies.32 In the IR studies of carbon monoxide on Pt in acetonitrile by Anderson et al., a Stark tuning rate of 22 cm-1/V regarding the C-O stretching mode was observed.32,33 As we know, the Stark tuning rate can be altered by many factors, such as cation effects, surface coverage, and orientation.34 In the present study, the effect of electrolyte is not taken into consideration in that the testing solution is completely the same to that taken by Anderson. Also, the change of orientation of CO in the negative potential region possibly does not take place, which otherwise should cause the variation of the 2055 cm-1 band frequency. As a result, the surface coverage and, possibly more important, the roughened Pt substrate which is different from the smooth one usually used in the IR studies may play a key role in the behavior of the band frequency upon potential. Also, the appearance of the 2081 cm-1 band reminds us of the possibility of the chemisorbed reaction of the solvent acetonitrile on such a roughened Pt surface having high electrocatalytic activity. Before further study of the C-O band, a detailed inspection of the behavior of acetonitrile in the doublelayer region, particularly interaction with the surface, is necessary. Figure 3 shows a set of potential-dependent surface Raman spectra at a roughened Pt electrode in the region 1850-2220 cm-1 in the same solution as used in Figure 1. Note that the electrode was not preadsorbed by carbon monoxide. A broad band centered at ca. 2133 cm-1 is observed clearly at -0.2 V, which is not a component of the Raman spectrum of free or coordinated acetonitrile.35 The band is located in the same frequency range and shifts with potential in a similar way as CN- adsorbed on Pt (32) Anderson, M. R.; Blackwood, D.; Pons, S. J. Electroanal. Chem. 1988, 256, 387. (33) Anderson, M. R.; Blackwood, D.; Richmond, T. G.; Pons, S. J. Electroanal. Chem. 1988, 256, 397. (34) Ashely, K.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1989, 270, 349. (35) Cooney, R. P.; Fraser, D. B. Aust. J. Chem. 1974, 27, 1855.

Adsorption and Electro-Oxidation of CO

Figure 3. Potential-dependent surface Raman spectra at a platinum electrode in acetonitrile with 0.1 M LiClO4. Laser line, 632.8 nm; acquisition time, 100 s, accumulations, 4.

electrodes (see ref 36 and references therein). We therefore assign the new band to surface cyanide species. Note that a decomposition.reaction of acetonitrile then occurs on the Pt surface. A detailed discussion of this chemisorbed reaction will be given elsewhere.37 Such a decomposition reaction of acetonitrile was also observed at roughened Cu, Ag,38 and Au37 but not smooth Pt electrode surfaces.39 This may suggest a different surface chemical environment in the double-layer region resulting in different surface interactions at roughened and smooth platinum electrodes. By comparing the spectra in Figures 1C (potential region i) and 3, one can find that the frequency shift upon potential for cyanide is in contrast to the constant frequency value as seen in Figure 1C. An assignment of the 2055 cm-1 band (see Figure 1) to adsorbed cyanide is therefore unreasonable. We assume that this band is ascribed to the C-O stretching vibration, νC-O, for linearly adsorbed CO. Neither the Pt-C nor the Pt-N stretching vibration of the product, cyanide, of acetonitrile decomposition was observed in the system containing no CO. Hence, the appearance of the 506 cm-1 band should be attributed to νPt-C not for CN- but for CO adsorbed on Pt. On the contrary, we believe that the decomposition reaction of acetonitrile in potential region i is inhibited significantly by adsorption of CO. However, the behavior of acetonitrile on the roughened Pt surface should not be ignored. As is described in the Experimental Section, the inducement of carbon monoxide is carried out by immersing a preadsorbed Pt electrode in acetonitrile containing no CO, but not by immersing the Pt electrode in a CO-saturated solution while acquiring the Raman spectra. Therefore, a relatively low surface coverage of CO on Pt as indicated by the flow current of oxidation of CO was obtained. It is different from the high CO coverage (θ ) 1.0) obtained by Anderson32 and Weaver.39 As a result, the double-layer environment may differ in that the coadsorbed Li+, the strong interaction of acetonitrile with rough Pt surfaces, and possibly the trace water, which cannot be thoroughly removed from acetonitrile, could disrupt the dipole(36) Ren, B.; Li, X. Q.; Wu, D. Y.; Yao, J. L.; Xie, Y.; Tian, Z. Q. Chem. Phys. Lett. 2000, 322, 561. (37) Cao, P. G.; Sun, Y. H.; Yao, J. L.; Ren, B.; Tian, Z. Q. Manuscript in preparation. (38) Mernagh, T. P.; Cooney, R. P. J. Electroanal. Chem. 1984, 177, 139. (39) Roth, J. D.; Chang, S. C.; Weaver, M. J. J. Electroanal. Chem. 1990, 288, 285.

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coupling interactions between COads. The invariation of atop (i.e., linearly adsorbed) νC-O frequency versus potential was also observed by Kim and co-workers for surface coverage of CO below 0.3 in the hydrogen evolution region.40 Electro-oxidation of COads occurs at ca. -0.7 V, as indicated in Figure 1 by the decrease in intensity of the νPt-C band and, instead, the growth of a new band at 1086 cm-1. In the normal Raman spectra of solid Li2CO3, there is also a band at 1088 cm-1 corresponding to carbonate.41 So it may be reasonable to assign the 1086 cm-1 band to carbonate, which is the final production of COads electrooxidation. The low solubility of lithium carbonate in acetonitrile makes it possible to form an insoluble film on the Pt surface which can be detected by the Ramanor. The independence of this band frequency upon back-scanning of potential to more negative values confirms this assignment. The observation of Li2CO3 under the present conditions could be a consequence of the coadsorbed trace water as a source of oxygen for the oxidation of CO. The existence of trace water in acetonitrile was confirmed by observation of SERS bands of the OH stretching vibration corresponding to water molecules for the same testing system on a silver electrode. Comparison of the onset potential of CO electrooxidation here with that obtained from IR studies42 also gives interesting results. Anderson and Huang43 reported that CO oxidation in acetonitrile occurs at between 1.0 and 1.5 V (vs SCE), far more positive than the present data. Studies in absolute methanol solution by McQuillan and co-workers44 showed that the oxidation starts at ca. 0.9 V (vs SCE). The disparities may be caused by the usage of a smooth Pt electrode. As is known to us, a highly roughened platinum electrode has many surface defects as compared with a smooth one, such as kinks, edges, and steps, which result in a high electrocatalytic activity. Therefore, the oxidation of CO is relatively unique and easy to perform. While on a smooth Pt surface, it is triggered by oxidation from the edges of COads islands on the surface without loss of dipole-dipole coupling as proposed by Kunimatsu and co-workers.45 The slightly decrease of the νC-O band frequency by altering potential to -0.7 V can now be explained by a loss of CO surface coverage, resulting in the weakening of dipole-coupling interactions. This observation concurs with that obtained from the smooth platinum surface.45 A more interesting feature is the slight increase in intensity of this band. As the electro-oxidation of CO is occurring, the inhibition arising from CO adsorption for the chemisorbed adsorption reaction of acetonitrile is decreasing. Therefore, it is reasonable to assume that the latter is occurring simultaneously. The coadsorption of the resulting product, cyanide, probably contributes to the increase of the band intensity. To the end of CO oxidation at -0.3 V, the νC-O band disappears from the spectra; instead, the band frequency (possibly one component hiding in the pre-existing νC-O band at -0.7 V) moves to a higher value, ca. 2081 cm-1, which is confidently attributable to the C-N stretching vibration for cyanide (40) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484. (41) Buijs, K.; Schutte, C. J. H. Spectrochim. Acta 1961, 17A, 927. (42) Russell, A. E.; Blackwood, D.; Anderson, M. R.; Pons, S. J. Electroanal. Chem. 1991, 304, 219. (43) Anderson, M. R.; Huang, J. J. Electroanal. Chem. 1991, 318, 335. (44) Love, J. G.; McQuillan, A. J. J. Electroanal. Chem. 1989, 274, 263. (45) Kunimatsu, K.; Shimazu, K.; Kita, H. J. Electroanal. Chem. 1988, 256, 371.

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adsorbed on Pt. Upon returning of potential to more negative values, the dependence of the νC-N frequency shift versus potential is quite similar to that obtained from Figure 3, yielding a comparable dνC-O/dE value of 50 cm-1/V. The difference lies in the relatively lower value of frequency at the same potential. This may be caused by the interaction of cyanide with the lithium carbonate film layer or both the film and the Pt surface. Summary The present study on the adsorption and electrooxidation of carbon monoxide on Pt has once again highlighted the importance of the combination of the proper surface roughening pretreatment and a confocal microprobe Raman system for the transition metals. The ability to obtain the SERS effect for Pt electrodes enables us to probe even the behavior of carbon monoxide, which has a rather small Raman scattering cross section, at the Pt/acetonitrile interface. Main observations and conclusions have been summarized as follows: 1. Potential-dependent surface Raman spectra of carbon monoxide adsorbed onto a roughened Pt electrode in 0.1 M LiClO4/CH3CN have been observed. The bands observed at ca. 506 and 2055 cm-1 are attributed to the Pt-C and intramolecular C-O stretching vibrations, respectively, suggesting linearly adsorbed CO on platinum. 2. The constancy of the band intensity of the Pt-C and C-O stretching vibration modes upon potential reveals a constant surface coverage of carbon monoxide before the onset of electro-oxidation of COads. The νPt-C band of linearly adsorbed CO has an experimental Stark tuning rate of -4 cm-1/V, while the rate of the νC-O band

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approaches zero in the same potential region. Although the nearly constant value of νC-O band frequency could be tentatively interpreted as a consequence of low surface coverage of CO on platinum, a more detailed examination of the nature of the independence of frequency is still necessary. 3. The roughened Pt electrode surface has a high electrocatalytic activity, on which the onset of CO electrooxidation occurs at ca. -0.7 V (vs Ag/Ag+), far more negative than that obtained on smooth Pt surfaces, 1.01.5 V (vs SCE). 4. The main product of COads electro-oxidation on Pt is confirmed to be carbonate due to the existence of trace water in the double-layer region as a source of oxygen for the oxidation of CO. 5. Adsorption of carbon monoxide, although with low surface coverage, inhibits significantly the chemisorbed decomposition reaction of acetonitrile on Pt. Upon consumption of CO electro-oxidation, the latter occurs. The resulting product, cyanide, may interact with the lithium carbonate film layer, which is formed by oxidation of CO, or both the film and the Pt surface. Acknowledgment. This work is supported by the Natural Science Foundation of China and the financial support of State Key Laboratory for Physical Chemistry of Solid Surfaces of Xiamen University. The Raman spectroscopy experiments were carried out at Xiamen University. The authors are grateful for the kind help of the co-workers there. LA015638I