Nitric Oxide and Carbon Monoxide Adsorption on Polycrystalline

Indeed, the infrared reflection−adsorption spectroscopy (IRAS) of CO adsorbed .... the hydrogen adsorption/desorption process at 0 to −0.3 V and t...
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Langmuir 1997, 13, 6713-6721

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Nitric Oxide and Carbon Monoxide Adsorption on Polycrystalline Iridium Electrodes: A Combined Raman and Infrared Spectroscopic Study Shouzhong Zou, Roberto Go´mez,† and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received August 1, 1997. In Final Form: October 3, 1997X The adsorption of nitric oxide and carbon monoxide on polycrystalline iridium in aqueous 0.1 M HClO4 was probed by in-situ surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRAS) with the primary objective of probing the molecular and dissociative chemisorption of the former adsorbate in this electrochemical environment. The latter adsorbate was examined partly as a means of characterizing the microscopic nature of the iridium electrode, given the well-documented sensitivity of the CO intramolecular vibration to the surface structure. The SERS vibrational technique was harnessed by utilizing ultrathin iridium films electrodeposited on a gold substrate. The availability of this method enables vibrational features associated with surface-adsorbate as well as intramolecular bonds to be scrutinized, facilitating detection of adsorbed NO fragments. Saturation adsorption of CO yields a single C-O stretching (νCO) band at ca. 2040-2070 cm-1 in both the Raman and infrared spectra, suggestive of exclusive atop (or near-atop) binding as on monocrystalline Ir surfaces. Significant (15-20 cm-1) discrepancies in the potential-dependent νCO frequencies measured by SERS and IRAS are evident, however, indicating that the ensemble distribution of iridium surface microenvironments sensed by these techniques is dissimilar. However, the νCO frequencies are approximately consistent with those evaluated for corresponding iridium-vacuum interfaces once the differences in surface potentials are taken into account. In contrast to the νCO band, the frequency of the metal-CO (νM-CO) vibration decreases with increasing electrode potential, in harmony with SERS findings on other transition-metal surfaces and theoretical bonding expectations. Adsorption of NO yields a weaker N-O stretching (νNO) band at 17801810 cm-1 in both the SERS and IRAS spectra, indicative of the presence of (probably atop) molecular NO chemisorption. The SERS lower-frequency region is dominated by a well-defined band at 570 cm-1, attributed to a metal-oxygen stretch from the chemisorbed oxygen fragment formed by NO dissociation. This assignment arises in part from the markedly different potential-dependent stability as well as intensity of the 570 cm-1 band in comparison with the νNO feature. Supporting evidence includes the observation of a near-identical vibrational band upon oxygen dissociation (and NO adsorption) on Ir surfaces in vacuum and for the present Ir films upon gas-phase O2 dosing. While the extent of NO dissociation on Ir cannot be estimated quantitatively, consideration of band intensities suggests that the coverages of molecular NO and the atomic-oxygen fragment are not greatly different. Comparisons are briefly made with NO adsorption on other transition-metal surfaces in electrochemical and vacuum environments.

Introduction Given the considerable attention devoted to the adsorption and reactivity of nitric oxide as well as carbon monoxide on transition-metal surfaces in gas-phase, especially in ultrahigh vacuum (UHV) enviroments, it is of obvious interest to probe the vibrational spectroscopic properties of these archetypically simple molecules at related electrochemical interfaces. Indeed, the infrared reflection-adsorption spectroscopy (IRAS) of CO adsorbed at metal-aqueous interfaces has attracted widespread attention in recent years, encouraged in part by the opportunities for exploring chemisorbate bonding at ordered monocrystalline electrodes in relation to corresponding metal-UHV interfaces,1 as well as the important role played by adsorbed CO in catalytic electrooxidations of organic molecules.2 While NO has received much less attention, recent electrochemical and infrared spectral studies have mapped out its adsorption and reactivity on low-index platinum and rhodium surfaces,3 and related investigations on low-index iridium and palladium electrodes have also recently been pursued.4,5 † Permanent address: Departament de Quı´mica Fı´sica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain. X Abstract published in Advance ACS Abstracts, November 15, 1997.

(1) For overviews, see: (a) Weaver, M. J. J. Phys. Chem. 1996, 100, 13079. (b) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391. (2) Iwasita, T.; Nart, F. C. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH Publishers: Weinheim, 1995, p 124.

S0743-7463(97)00853-6 CCC: $14.00

A number of studies from this laboratory have demonstrated the utility of surface-enhanced Raman spectroscopy (SERS) for examining CO and other adsorbates at Pt-group transition metal surfaces in electrochemical6 and gas-phase7 enviroments. The surfaces, formed by electrodepositing ultrathin films of the desired metal onto SERS-active gold, display similar electrochemical and infrared spectral behavior as for adsorbates on conventional polycrystalline electrodes.6a,b,d,8 The advantages of SERS compared with IRAS for examining adsorbate vibrational spectra on such surfaces include the ability to examine metal-adsorbate and other low-frequency vibrations as well as intramolecular modes. This has enabled, for example, the unusual electrode potential-dependent (3) (a) Rodes, A.; Go´mez, R.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Electrochim. Acta 1996, 41, 729. (b) Rodes, A.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Pe´rez, J. M.; Aldaz, A. Langmuir 1995, 11, 3549. (c) Go´mez, R.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M. J. Electroanal. Chem. 1995, 393, 123. (d) Rodes, A.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1993, 359, 315. (e) Villegas, I.; Go´mez, R.; Weaver, M. J. J. Phys. Chem. 1995, 99, 14832. (4) (a) Go´mez, R.; Weaver, M. J. J. Electroanal. Chem., in press. (b) Go´mez, R.; Weaver, M. J. Submitted for publication in Langmuir. (c) Go´mez, R.; Weaver, M. J. Submitted for publication in J. Phys. Chem. (5) Go´mez, R.; Zou, S.; Weaver, M. J. In preparation. (6) For example: (a) Leung, L.-W. H.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 5113. (b) Leung, L.-W. H.; Weaver, M. J. Langmuir 1988, 4, 1076. (c) Zhang, Y.; Gao, X.; Weaver, M. J. J. Phys. Chem. 1993, 97, 8656. (d) Zou, S.; Weaver, M. J. J. Phys. Chem. 1996, 100, 4237. (7) For example: (a) Wilke, T.; Gao, X.; Takoudis, C. G.; Weaver, M. J. Langmuir 1991, 7, 714. (b) Tolia, A. A.; Williams, C. T.; Takoudis, C. G.; Weaver, M. J. J. Phys. Chem. 1995, 99, 4599. (c) Williams, C. T.; Tolia, A. A.; Chan, H.; Takoudis, C. G.; Weaver, M. J. J. Catal. 1996, 163, 63.

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behavior of the metal-carbon stretch for adsorbed CO to be examined6d as well as the catalytic oxidation of this adsorbate in both electrochemical and gas-phase enviroments.6a,b,7 The sensitivity of SERS to metaladsorbate vibrations should be of particular value for examining NO adsorption since dissociative chemisorption to yield oxygen and nitrogen fragments, as well as molecular metal-NO bonding, is commonly seen to occur on transition metals in UHV.9 Indeed, this laboratory has recently employed SERS to detect chemisorbed fragments from NO adsorption on Rh and Pd surfaces in the gas phase under reactive ambient-pressure conditions.7b,c We have recently developed an improved metal-film electrodeposition procedure, utilizing constant-current conditions in phosphate electrolyte, which has enabled iridium surfaces to be examined by SERS for the first time. (Related tactics have also been found to yield more uniform films of the previously studied metals Pt, Rh, and Pd.10) Our interest in iridium stems partly from our recent IRAS examinations on monocrystalline Ir electrodes4,11 together with the opportunity to compare its adsorption behavior with other Pt-group electrodes. The adsorption of CO on iridium surfaces exhibits relatively simple vibrational spectroscopic behavior, with atop bonding (i.e., to a single Ir atom) apparently predominating on various single-crystal (and polycrystalline) Ir surfaces in both vacuum12 and electrochemical4,11 environments. In contrast, NO adsorption on these surfaces can exhibit apparently bridging as well as atop binding geometries and varying degrees of dissociation, depending on the metal substrate geometry and temperature.4b,c,13 We report here a survey of the vibrational properties of both NO and CO on polycrystalline iridium electrodes by combined SERS and IRAS measurements. The Raman spectral data, in particular, enable the identification of NO dissociative as well as associative chemisorption on iridium electrodes, while the infrared spectra provide additional information on the N-O and C-O intramolecular vibrational behavior. We also compare briefly the spectral behavior of NO on iridium with results for other Pt-group metals. Experimental Section Most details of the experimental arrangement for electrochemical Raman spectroscopy are described in refs 14 and 15. Briefly, the Raman excitation was at 647.1 nm, with a power of ca. 20-40 mW on the surface. The scattered light was collected (8) (a) Motoo, S.; Shibata, M.; Watanabe, M. J. Electroanal. Chem. 1980, 110, 103. (b) Lin-Cai, J.; Pletcher, D. J. Electroanal. Chem. 1983, 149, 237. (9) Studies of temperature-dependent NO dissociation on iridium include: (a) Kanski, J.; Rhodin, T. N. Surf. Sci. 1977, 65, 63. (b) Zhdan, P. A.; Boreskov, G. K.; Boronin, A. I.; Scheelin, A. P.; Egelhoff, W. F., Jr.; Weinberg, W. H. J. Catal. 1979, 60, 93. (c) Ibbotson, D. E.; Wittrig, T. S.; Weinberg, W. H. Surf. Sci. 1981, 110, 294. (d) Davis, J. E.; Kapeboom, S. G.; Nolan, P. D.; Mullins, C. B. J. Chem. Phys. 1996, 105, 8362. (10) Zou, S.; Weaver, M. J. Unpublished results. (11) Jiang, X.; Chang, S-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7453. (12) (a) Marinova, T. S.; Chakarov, D. V. Surf. Sci. 1989, 217, 65. (b) Kisters, G.; Chen, J. G.; Lehwald, S.; Ibach, H. Surf. Sci. 1991, 245, 65. (c) Martin, R.; Gardner, P.; Nalezinski, R.; Tu¨shaus, M.; Bradshaw, A. M. J. Electron. Spectrosc. Relat. Phenom. 1993, 64/65, 619. (d) Lyons, K. J.; Xie, J.; Mitchell, W. J.; Weinberg, W. H. Surf. Sci. 1995, 325, 85. (e) Lauterbach, J.; Boyle, R. W.; Schick, M.; Mitchell, W. J.; Meng, B.; Weinberg, W. H. Surf. Sci. 1996, 350, 32. (f) Reinalda, D.; Ponec, V. Surf. Sci. 1979, 91, 113. (13) (a) Cornish, J. C. L.; Avery, N. R. Surf. Sci. 1990, 235, 209. (b) Gardner, P.; Martin, R.; Nalizinski, R.; Lamont, C. L. A.; Weaver, M. J.; Bradshaw, A. M. J. Chem. Soc. Far. Trans. 1995, 91, 3575. (14) Wilke, T.; Gao, X.; Takoudis, C. G.; Weaver, M. J. J. Catal. 1991, 130, 62. (15) Gao, X.; Zhang, Y.; Weaver, M. J. Langmuir 1992, 8, 688.

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Figure 1. Anodic-cathodic cyclic voltammograms at 20 mV s-1 obtained in 0.1 M HClO4 for electrochemically roughened gold (dashed trace) and following iridium electrodeposition (solid trace). See text for iridium deposition procedure and other details. by a SPEX Triplemate spectrometer equipped with a Photometrics PM512 CCD detector and a CC200 camera controller. The electrode substrate was a 4 mm diameter gold disk sheathed in Teflon, which was electrochemically pretreated to yield SERSactivity as described in ref 16. The procedure for Ir electrodeposition is described in the next section. The electrochemical infrared measurements were made largely as outlined in ref 17, utilizing an IBM (Bruker) IR 98-4A vacuum instrument. The gold electrode substrate, modified by Ir deposition identically to the SERS procedure, was a somewhat larger (9 mm diameter) gold disk than used in the SERS measurements so to optimize the infrared signal-to-noise. Carbon monoxide (99.9%) and nitric oxide (99.0%) were obtained from Matheson. The perchloric acid supporting electrolyte was prepared from 70% HClO4 (GFS Chemicals, double distilled). Other chemicals were reagent grade or better and all solutions were prepared by using water purified by a Milli-Q Plus system (Millipore). All measurements were made at room temperature (23 ( 1 °C), and electrode potentials are reported versus the saturated calomel electrode (SCE).

Results and Discussion 1. Cyclic Voltammetry. The procedure for electrodeposition of iridium onto gold was adapted from ref 18. This utilized a quiescent solution of 4 mM Na2IrCl6 in 0.7 M Na2HPO4 (pH ) 8.5). The deposition was performed in a constant-current fashion, normally with a cathodic current density of 3 mA cm-2 (at ca. -1.0 V vs SCE) for 2-3 min unless otherwise indicated. (The current efficiency for iridium deposition is fairly low, most of the charge being consumed by H2 evolution.) The electrode was then transferred to 0.1 M HClO4 for electrochemical and spectroscopic characterization. Typical cyclic voltammograms obtained for the roughened (SERS active) gold electrode before (dashed trace) and after (solid trace) iridium deposition are shown in Figure 1. The formation of an iridium film on gold is evidenced (16) Gao, P.; Gosztola, D.; Leung, L.-W. H.; Weaver, M. J. J. Electroanal. Chem. 1987, 233, 211. (17) Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (18) Hansel, G. Metalloberflache 1967, 21, 238.

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by the appearance of cathodic-anodic current features characterizing the hydrogen adsorption/desorption process at 0 to -0.3 V and the compression of the cathodic peak near 0.9 V due to the removal of gold oxide. The extent of these changes increased with the deposition time, i.e., the faradaic charge passed during the electrodeposition. The voltammogram for this iridium film is essentially the same as those observed with ordered Ir(110) electrodes19a and iridium films prepared by other methods,19b except the presence of a residual gold oxide reduction peak. Unlike the deposition of other transition metals on gold,6b this reduction peak could not be compressed completely by increasing the deposition time. (Indeed, the peak grows upon cycling between -0.3 and 1.3 V, along with attenuation of the hydrogen adsorption/ desorption features, indicative of anodic loss of Ir metal.) It has been shown that the onset of iridium oxidation occurs at about 0.5 V in acidic media, 0.4 V less positive than that of gold.19 Vigorous oxidation of the iridium film, at potentials markedly exceeding 0.5 V, can also change the surface morphology, exposing more gold sites. The voltammogram shown in Figure 1 was obtained after an 8 min deposition at 3 mA cm-2, i.e., about 3-fold longer than as normally used for SERS and IRAS experiments. The residual gold oxide reduction peak at 0.9 V is nonetheless clearly discernible. However, as indicated in the SERS data for adsorbed CO (vide infra), a much shorter deposition (2-3 min) is enough to completely block CO adsorption on such gold sites. With the iridium-coated gold in 2 M H2SO4 cycled between 0.1 and 1.3 V several times, the iridium film can be removed gradually and the substrate gold recovered. Utilizing this iridium electrostripping technique and assuming the final oxidation state is Ir(IV), we determined the thickness of the iridium film obtained by a 3 min deposition to be about 2-3 monolayers. A representative voltammogram of an Ir film electrode in CO-saturated 0.1 M HClO4 solution is shown in Figure 2 (dashed trace). The solution was obtained by bubbling CO into an argon-purged solution for a few minutes. The voltammogram of the “blank” solution is presented for comparison (solid trace). Saturation CO adsorption is discerned from the complete blockage of hydrogen adsorption/desorption and the smaller double-layer charging currents. The broad anodic wave centered at 0.6 V is due to CO electrooxidation. It has been shown that NO is strongly adsorbed on Pt, Rh, and Ir monocrystalline electrodes in acidic nitrite solutions.3,4b Figure 3 shows a typical voltammogram for Ir in 0.1 M HClO4 + 2 mM NaNO2. (Note that the current scale is 5-fold less sensitive than that in Figures 1 and 2.) The main features are the reduction currents starting at ca. 0.3 V, increasing gradually with negative-going potential, and an irreversible oxidation peak at around 0.9 V. The voltammogram is featureless between these two regions. Voltammograms for the Ir film obtained in NO-saturated 0.1 M HClO4 solution (i.e., after purging NO into 0.1 M HClO4 for several minutes) are essentially the same as those obtained in acidic nitrite, except that the oxidation and reduction current peaks are ca. 2-fold larger in the former solution. Nishimura et al.20 have studied the electroreduction of nitrite ions on porous Pt in sulfuric acid solutions by means of differential electrochemical mass spectroscopy (DEMS). They concluded that N2 was formed at -0.15 to 0.35 V. Ammonium cations were detected at lower potentials in some early studies.21 The present irreversible oxidation (19) (a) Motoo, S.; Furuya, N. J. Electroanal. Chem. 1984, 167, 309. (b) C ˇ ukman, D.; Vukovic´, M. J. Electroanal. Chem. 1990, 279, 283. (20) Nishimura, K.; Machida, K.; Enyo, M. Electrochim. Acta 1991, 36, 877.

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Figure 2. Anodic-cathodic cyclic voltammograms at 20 mV s-1 for iridium-coated gold in 0.1 M HClO4 (solid trace) and CO-saturated 0.1 M HClO4 (dashed trace). Iridium film thickness was about 3 equivalent monolayers.

peak has also been observed during previous studies on monocrystalline Pt and Rh surfaces. When Rh and Pt single crystals are in contact with acidic nitrite solutions, the NO formed either adsorbs molecularly or partly dissociates, depending on the chemical nature and crystallographic structure of the metals.3 To determine if any irreversibly adsorbed redox-active species are present on the Ir surface, we performed “emersion” experiments following the strategy described in ref 3a. These were executed by first immersing the Ir film in the acidic nitrite solution for 1-5 min, rinsing the electrode with ultrapure water, and transferring it into an electrochemical cell containing 0.1 M HClO4. The resulting voltammogram is comparable to those obtained for a fresh electrode in 0.1 M HClO4 (see Figure 3, dashed trace). This observation is somewhat different to those obtained for Rh3a,c and Pt3a,d low-index surfaces, for which the voltammograms exhibit clear features attributed to redox reactions involving irreversibly adsorbed NO. 2. SERS of Adsorbed CO. Given that the C-O (νCO) and metal-carbon stretching (νM-CO) band frequencies are very sensitive to the adsorption environment and that CO is unassailably adsorbed in molecular form at room temperature, we describe first the vibrational spectroscopy of this archetypical diatomic adsorbate on the Ir film electrode. The adlayer was formed by bubbling CO into the solution for several minutes at a fixed electrode potential. The Ir film yielded strong SERS bands within both the νM-CO (300-600 cm-1) as well as the νCO (17002200 cm-1) frequency regions. A selected sequence of SER spectra as a function of potential for CO adsorbed on the Ir film is shown in Figure 4. The electrode potential was (21) (a) Gadde, R. R.; Bruckenstein, S. J. Electroanal. Chem. 1974, 50, 163. (b) Janssen, L. J.; Pieterse, M. M.; Barendrecht, E. Electrochim. Acta 1977, 22, 27.

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A

Figure 3. Anodic-cathodic cyclic voltammograms at 20 mV s-1 for iridium-coated gold in 0.1 M HClO4 (dashed trace) and 0.1 M HClO4 + 2 mM NaNO2 (solid trace). Iridium film thickness was about 3 equivalent monolayers.

held initially at -0.2 V, at which the first spectrum was recorded, and then altered in a staircase fashion, usually in 0.1 V increments. Each spectrum was recorded using a 10 s acquisition time. To illustrate the reversibility of the spectra with respect to the potential alteration, the data taken during sequential positive- and negative-going potential excursions are shown, stacked upward with increasing time. Corresponding spectra encompassing the intramolecular C-O (νCO) and the metal-CO (νM-CO) vibrations (parts A and B of Figure 4, respectively) were obtained separately to obtain a satisfactory frequency resolution. The positive potential limit in Figure 4 was chosen to be 0.6 V since the electrooxidative removal of adsorbed CO, as signaled by attenuation of the νCO and νM-CO bands, proceeds at higher potentials. Indeed, a broad feature centered at ca. 550 cm-1 appears under these conditions, attributed to iridium oxide. This same band also appears in Ar-sparged (i.e., CO-free) 0.1 M HClO4 under these conditions. Considering first the high-frequency region (Figure 4A), there is only one νCO band observed at 2060-2070 cm-1. This feature can be assigned to atop (i.e., terminal) adsorbed CO from its frequency in comparison with those for similar coordination geometries on Pt and Rh.6 As usual, the frequency increases toward higher potentials. A νCO band around 2110 cm-1 due to CO adsorbed on residual (uncovered) gold sites, which we have observed on other metal-coated gold electrodes,6a,d,22 is absent here. This indicates that the gold surface was covered essentially completely by the Ir film (vide supra). In contrast to other transition metals,6a,d,22 a νCO band originating from bridging CO (normally seen below 2000 cm-1) is absent, suggesting the occurrence of CO coordination solely in atop (and/or near-atop) geometries on iridium. Similarly exclusive atop coordination of CO has been observed by vibrational spectroscopies on Ir(110) and (111) surfaces in both electrochemical4b,11 and UHV environments.12 The corresponding low-frequency spectra also display only one band (Figure 4B), a strong feature at 510-515 cm-1, which can confidently be assigned to the νM-CO stretch also arising from atop CO, partly on the basis of its relatively high frequency. Clearly evident from Figure 4B is that in contrast to the νCO band the νM-CO frequency (22) Zhang, Y.; Weaver, M. J. J. Electroanal. Chem. 1993, 354, 173.

B

Figure 4. Sequence of surface-enhanced Raman (SER) spectra in high- and lower-frequency regions (A and B, respectively) obtained on iridium film in CO-saturated 0.1 M HClO4 as a function of electrode potential, as indicated. Potential excursion was initially positive-going from -0.2 V. Spectra are stacked upward in time.

diminishes with increasing potential. The average dνM-CO/ dE value, -7 cm-1 V-1, is comparable to those found for surface-CO vibrations on other Pt-group transition metals by using SERS.6d This effect, implying that the surfacechemisorbate binding becomes stronger as the electrode surface is charged increasingly negative, is consistent with theoretical bonding expectations that predict increasingly strong metal-CO back donation under these conditions.23 Note that the νM-CO frequencies on Ir are several tens of wavenumbers higher than those for CO adsorbed on Rh and Pt6d implying a stronger metal-carbon bond on Ir according to Badger’s rule.24 (23) Head-Gordon, M.; Tully, J. C. Chem. Phys. 1993, 175, 37. (24) (a) Badger, R. M. J. Chem. Phys. 1934, 2, 128; 1935, 3, 710. (b) Herschbach, D. R.; Laurie, V. W. J. Chem. Phys. 1961, 35, 458.

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Figure 5. Sequences of infrared absorbance spectra for saturated adsorbed CO on Ir film in CO-saturated 0.1 M HClO4 as a function of electrode potential. Spectra stacked upward with time. See text for experimental procedure.

3. Comparison with Corresponding Infrared Spectra for Adsorbed CO. Considering the suitability of IRAS for examining intramolecular adsorbate vibrations on a variety of metal substrates, including smooth polycrystalline and single crystal as well as roughened (SERS-active) surfaces, it is of interest to compare the SERS results on Ir overlayers with the corresponding insitu infrared data. Figure 5 contains a representative set of infrared absorbance spectra in the νCO region. The experimental conditions, including CO-saturated 0.1 M HClO4 and the use of a (SERS-active) Ir film electrode, were essentially the same as those utilized to obtain the SERS results in Figure 4. The spectra were obtained by recording 100 interferograms at each “sample” potential, starting from -0.2 V and increasing by 0.2 V increments in a stepwise fashion after each set of interferograms were taken. Subtracted from each set is a corresponding “reference” spectrum taken immediately afterward at a potential (0.8 V) where CO electrooxidation is complete, to eliminate bulk-phase spectral interferences in the usual manner.17 The spectra in Figure 5 show only a single band at around 2040-2060 cm-1. The abrupt decrease in intensity and frequency at 0.6 V results from the onset of electrooxidation of adsorbed CO. (Note that adsorbed CO removal commences at a lower potential in the IRAS (Figure 5) than the SERS data (Figure 4). This is because the thin-layer configuration obliged for the former measurements provides only a very limited supply of solution CO to replace adsorbate lost by slow electrooxidation.) The νCO frequency again increases with the electrode potential (Figure 5), yielding a Stark-tuning slope of ∼30 cm-1 V-1. A comparison of the νCO frequency-electrode potential data obtained by SERS and IRAS for saturated CO adsorption on the Ir film at potentials below the onset of CO electrooxidation is shown in Figure 6 (filled and open circles, respectively). Both the νCO-E slopes and the νCO frequencies obtained by these two techniques are seen to be noticeably different, the former quantity being larger and the latter values being ca. 15-20 cm-1 lower when measured with IRAS. Significant disparities in the potential-dependent νCO frequencies as determined by

Figure 6. Peak frequencies of νCO band for CO adsorbed on iridium film obtained by SERS and IRAS as indicated, plotted versus electrode potential. Data are taken from Figures 4A and 5.

SERS and IRAS on a given surface have also been seen for CO adsorbed on other metals, specifically Pt6d and Au.25 These differences indicate that the ensemble of surface sites, inevitably present on a polycrystalline surface, sensed by SERS is somewhat different to IRAS. This finding is not in itself too surprising since it is well-known that the extent of the SERS enhancement is sensitive to the nanoscale morphology on which the surface sites are located.26 In the case of Pt films, however, the peak νCO frequencies measured by SERS are slightly (10-12 cm-1) lower than the corresponding IRAS values, opposite to the (larger) observed disparity with the present Ir film. On the other hand, the bandwidths of the SERS and IRAS νCO features on Ir are comparable (full width at half maximum, fwhm, of 40 and 45 cm-1, respectively). Incidentally, the large fwhm values of these νCO bands on the Ir film suggest the presence of a significant distribution of microscopically distinct bonding environments. More systematic studies of frequencies and bandwidths for different adsorbates and substrates, however, would be necessary in order to rationalize these observations on a firm basis. Despite such differences and uncertainties, it is nonetheless of interest to compare the frequencies of both the νCO and νM-C bands with their counterparts for Ir singlecrystal surfaces in both electrochemical and UHV environments. The νCO frequencies (measured at 0 V vs SCE) and the dνCO/dE values for saturated CO adlayers at the Ir(111)-aqueous interface evaluated by in-situ IRAS are 2068 cm-1 and 20 cm-1 V-1, respectively;11 the corresponding values on Ir(110) are 2062 cm-1 and 23 cm-1 V-1.4b Interestingly, the νCO frequencies evaluated for the latter surface (which is anticipated to be a crude model for a polycrystalline sample) are close to those obtained here by SERS (Figure 6). (25) Corrigan, D. S.; Gao, P.; Leung, L.-W. H.; Weaver, M. J. Langmuir 1986, 2, 744. (26) For example, see: Moskovits, M. Rev. Mod. Phys. 1985, 57, 783.

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Comparison of the potential-dependent νCO frequencies with related spectra obtained for metal-UHV interfaces can be enlightening. As discussed in several previous papers from this laboratory,3e,17,27 a reasonable consistency between corresponding electrochemical and UHV-based νCO (and other adsorbate) frequencies is often found by taking into account the differing surface potentials between these two environments and measuring (or extrapolating) E-dependent band frequencies in the former environment to the surface potential characteristic of the latter. Employing again the CO-saturated Ir(110) surface as a crude facsimile of the polycrystalline Ir film, the work function for this interface, Φ , is estimated to be around 5.7 eV.28-30 By taking the conversion constant between the electrode potential (vs SCE) and work function (i.e., versus vacuum) scales, Ek, as 4.8 eV,31 extrapolating the SERS νCO-E data to the potential (0.8 V vs SCE) roughly equivalent to the above Φ value yields a νCO value of 2077 cm-1. This value is not greatly different, although lower, than the νCO frequency measured by IRAS in UHV for a saturated CO adlayer on Ir(110), 2086 cm-1 12d or on a polycrystalline (evaporated) Ir film, 2093 cm-1.12f (A lower extrapolated νCO frequency, 2068 cm-1, is obtained by extrapolating the present electrochemical IRAS νCO-E data in the same manner.) It is worth mentioning that an unusual characteristic of CO adsorption on singlecrystal iridium surfaces in both electrochemical4b,11 and UHV environments12 is the large (up to ca. 100 cm-1) frequency upshifts observed with increasing coverage. Consequently, significant variations in the νCO frequencies between, say, different surface morphologies may arise primarily from differences in the effective CO packing densities. Undertaking the same extrapolation procedure for the present νM-CO vibration yields a frequency at 0.8 V vs SCE of 508 cm-1. This value is again comparable, although now slightly higher than, the νM-CO frequency, about 495 cm-1, observed by electron energy loss spectroscopy (EELS) for saturated CO on Ir(110) in UHV.12a While such comparisons are clearly tentative given the different nature of the present Ir films to Ir(110) or other ordered monocrystalline surfaces, the findings at least provide evidence that the CO bonding is not greatly different in these environments. 4. NO Adsorption. As mentioned above, the voltammogram of Ir-coated gold in 0.1 M HClO4 + 2 mM NaNO2 solution is featureless in the potential region, 0.3-0.8 V (Figure 3), similar to corresponding data for single-crystal Pt-group electrodes.3,4b Adlayers containing molecularly adsorbed nitric oxide have been observed by means of IRAS on Pt,3 Rh,3 and Ir4b low-index single-crystal electrodes in this potential region, as signaled by the appearance of characteristic N-O stretching (νNO) bands in the frequency range ca. 1600-1850 cm-1. Along these lines, we attempted to detect adsorbed NO on the Ir film in 0.1 M HClO4 + 2 mM NaNO2 solution. No νNO features were observed in the SER spectra under these conditions. However, increasing the effective NO solution concentration by employing instead an NO- sparged 0.1 M HClO4 (27) (a) Chang, S.-C.; Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1989, 93, 5341. (b) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (c) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5378. (28) The Φ value for clean Ir(110) is estimated to be about 5.5 eV,29 and saturated CO adsorption increases Φ by about 0.2 eV.30 (29) Nieuwenhuys, B. E.; Meijer, D. Th.; Sachtler, W. M. H. Surf. Sci. 1973, 40, 125. (30) Nieuwenhuys, B. E. Surf. Sci. 1981, 105, 505. (31) This value arises by selecting an average estimate for Ek , 4.6 V vs normal hydrogen electrode (NHE),1b and adding ca. 0.2 V to convert to the SCE reference scale.

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A

B

Figure 7. Sequences of SER spectra in high- and lowerfrequency regions (A and B, respectively) on iridium film in NO-saturated 0.1 M HClO4 as a function of electrode potential. See Figure 4 caption and text for other details.

solution (vide supra) yielded SERS bands that are attributable to molecularly adsorbed NO. Parts A and B of Figure 7 show typical potential-dependent sets of SER spectra obtained under these conditions in the N-O stretch and metal-adsorbate vibrational frequency regions, respectively. Each spectrum in the latter frequency segment (200-900 cm-1) was recorded by employing a 10 s acquisition time. In the higher frequency region (16002000 cm-1), a longer acquisition time (30 s) was used due to lower signal intensities. The potential was first held at 0.3 or 0.0 V (the results were essentially the same for these two potentials), and NO gas was bubbled into the solution for 2 min. The potential was switched subsequently to -0.2 V, whereupon the first spectrum was recorded. As for the SERS experiments with CO, the electrode potential was then altered in a staircase fashion utilizing 0.2 V increments, initially in the positive direction

Adsorption on Polycrystalline Ir Electrodes

and then back to -0.2 V. (The spectra shown in Figure 7 are again stacked sequentially upward.) The higher-frequency spectra (Figure 7A) are considered first. There is a well-defined band at ca. 1800 cm-1, accompanied by a weaker feature at ca. 1600 cm-1. By comparison of the observed frequencies with EELS and IRAS data from UHV,13 the former band can tentatively be assigned to NO adsorbed on atop sites (vide supra). (Note, however, that merely utilizing intramolecular frequencies to assign bonding geometries for adsorbed NO,32 and to a lesser extent CO,33 has recently been shown to be fraught with uncertainty in some cases.) Similarly high potential-dependent νNO frequencies, 1800-1840 cm-1, have been observed for a saturated NO adlayer on a Ir(110) electrode.4b The observation of such high νNO frequencies appears to be unique to Ir among transitionmetal surfaces, although νNO bands on Ir(100) in UHV that are located down to 1570 cm-1 have been observed for low NO exposures.13b The potential-dependent intensity of the ca. 1600 cm-1 feature in Figure 7A is similar to the ∼1800 cm-1 νNO band and is therefore tentatively assigned to molecular NO adsorbed at bridge sites. As for the νCO band, the peak frequency of the atop νNO band upshifts with increasing electrode potential. However, the Stark tuning slope, dνNO/dE ∼ 40 cm-1 V-1, is higher than that for CO (vide supra). Comparable values are found for NO adsorbed on monocrystalline transitionmetal electrodes, such as Ir(110)4b and Pt(111).3 The upper and lower potential limits (0.6 and -0.2 V, respectively) in Figure 7A are determined by oxidation and reduction of NO, respectively, although the diminutions in νNO band intensity seen for potentials below 0.4 V may well be due to the reductive removal of adsorbed NO. This potential region coincides with the featureless segment of the voltammogram in 0.1 M HClO4 + 2 mM NaNO2 (Figure 3). Experiments were also performed that involved exposing similar Ir films to ambient-pressure NO in the gas phase.34 No νNO SERS bands were detected under these conditions at either 25 °C or elevated temperatures. Given the relatively low intensities of the νNO SERS bands in the electrochemical environment, however, this result may reflect sensitivity restrictions and/or the likely greater presence of surface contaminants in the gas-phase case. Similarly to the aforementioned CO studies, we also performed corresponding IRAS experiments on iridium films in NO-saturated 0.1 M HClO4. As before, the electrode potential was first held at -0.2 V to ensure that any iridium oxide was reduced. The IRAS measurements commenced at 0.6 V, the electrode potential decreasing by 0.2 V increments between each spectrum. The subsequent spectrum taken at -0.26 V, where the adsorbed NO was completely reduced, served as a “reference”, being subtracted as usual from each “sample” spectrum to eliminate bulk-phase contributions. Figure 8 presents a selective set of such spectra. Similarly to the SERS data (Figure 7A), the main feature is a band around 1800 cm-1, with a Stark-tuning slope, dνNO/dE, of about 50 cm-1 V-1. However, the weak 1600 cm-1 band observed in the SER spectra is absent, possibly due to a strong interference from the H-O-H bending mode for bulk water that is commonly encountered in infrared measurements. (32) (a) Asensio, M. C.; Woodruff, D. P.; Robinson, A. W.; Schindler, K.-M.; Gardner, P.; Ricken, D.; Bradshaw, A. M.; Conesa, J. C.; Gonza´lezElipe, A. R. Chem. Phys. Lett. 1992, 192, 259. (b) Aminpirooz, S.; Schmalz, A.; Becker, L.; Haase, J. Phys. Rev. B 1992, 45, 6337. (c) Mapledoram, L. D.; Wander, A.; King, D. A. Chem. Phys. Lett. 1993, 208, 409. (33) Schindler, K.-M.; Hofmann, Ph.; Weiss, K.-U.; Dippel, R.; Gardner, P.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P.; Davila, M. E.; Asensio, M. C.; Conesa, J. C.; Gonza´lez-Elipe, A. R. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 75. (34) Williams, C. T.; Zou, S.; Weaver, M. J. Unpublished results.

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Figure 8. Sequences of infrared absorbance spectra for adsorbed NO on Ir film in NO-saturated 0.1 M HClO4 as a function of electrode potential.

Figure 9. Peak frequencies of νCO band for NO adsorbed on Ir film, obtained by SERS and IRAS as indicated, plotted against electrode potential. Data taken from Figures 7A and 8.

A comparison between the νNO frequencies obtained from the SERS (Figure 7A) and the IRAS data (Figure 8) plotted versus the electrode potential (cf. Figure 6) is shown in Figure 9. Again, there are significant disparities in the νNO-E data obtained from these two techniques on the Ir film, although milder than those seen for adsorbed CO (vide supra). Differences in the νNO bandwidths evaluated by SERS and IRAS are also evident, although the bands tend to be broad (fwhm values in the range ca. 50-80 cm-1, Figures 7A and 8). Such behavior presumably reflects the sensitivity of the νNO frequencies to the microscopic surface morphology, along with the likelihood that the NO coverages are well below saturation, as discussed below. 5. Dissociation of Adsorbed NO. Examination of the low-frequency region, say 200-900 cm-1, in the SER spectra is of particular interest for NO adsorption since it may reveal surface-adsorbate vibrational bands for nitrogen and/or oxygen dissociation fragments in addition

6720 Langmuir, Vol. 13, No. 25, 1997

to those for molecularly adsorbed NO, thereby shedding light on possible NO dissociation chemistry, which is well known on metal surfaces in UHV.9,13 As already mentioned, the low-frequency SER spectra shown in Figure 7B were obtained under the same conditions as for the corresponding νNO data in Figure 7A, in NO-saturated 0.1 M HClO4. The potential-dependent spectra in Figure 7B are dominated by an intense well-defined band at 570 cm-1, together with a broader and weaker feature at around 420 cm-1 and another weak band near 825 cm-1. In contrast to the νNO band, the amplitude of the 570 cm-1 feature does not attenuate at potentials below 0.4 V; indeed the band intensity increases at potentials approaching -0.2 V where the νNO feature is largely removed in both the SER and IRAS data (Figures 7a and 8). Especially since the latter intensity attenuation is probably due to electroreduction of adsorbed NO, this behavioral difference provides strong evidence that the 570 cm-1 band arises instead from another adsorbate, which is reduced less easily. Indeed, this 570 cm-1 feature cannot be removed cathodically in NO-saturated 0.1 M HClO4 even upon holding the potential at -0.4 V, where H2 evolution occurs on Ir. Furthermore, emersing the electrode from the NOsaturated electrolyte and transferring it to dearated 0.1 M HClO4 results in the complete loss of the νNO band but the retention of the 570 cm-1 feature, the latter only being removed at -0.4 V under these conditions. Close inspection of Figure 7B shows that the 570 cm-1 band frequency apparently decreases slightly, by 3-4 cm-1, between -0.2 and 0.6 V, ostensibly in similar fashion to the νM-CO feature for adsorbed CO (Figure 4B). While this effect might be construed as supporting an assignment of the 570 cm-1 feature to a surface vibration of molecular NO, νM-NO , at least part of the frequency shift appears to be an artifact, arising from overlap with the neighboring lower-frequency envelope. The most likely assignment of the dominant 570 cm-1 band is a metal-O stretch from adsorbed atomic oxygen, Oad, formed by NO dissociation. Several lines of evidence support this assertion. First, EELS vibrational bands at similar frequencies, 550-575 cm-1, are observed on Ir(111) and Ir(100) surfaces13,35 in UHV upon dosing either oxygen or NO, with the latter adsorbate at temperatures where dissociation of adsorbed NO is known to proceed. (This occurs by 400 K on Ir(111)13a and above 300 K on unreconstructed Ir(100).13b) These literature sources also provide evidence of the relative instability of adsorbed atomic nitrogen, Nad, at least in UHV, in that the thermally induced dissociation of adsorbed NO is accompanied by considerable N2 (but no O2) desorption along with the appearance of the 550-575 cm-1 band.13 A second piece of evidence supporting the assignment of the present 570 cm-1 band to a metal-Oad stretch is that a near-identical SERS feature is observed on the present Ir films (at around 100 °C) by dosing O2 at nearambient pressures in the gas phase.34 A similar, albeit weaker, band at ca. 570 cm-1 was also observed on the Ir film in O2-saturated 0.1 M HClO4 after a potential excursion up to about 1.0 V. Admittedly, dosing gaseous NO onto the Ir film at 25 °C under similar conditions yielded only a weak 570 cm-1 feature, with a more intense band appearing at 315 cm-1.34 The latter can be attributed to a metal-nitrogen vibration for Nad on the basis of comparisons with SER spectra for ambient-pressure NO adsorption on Pt, Pd, Rh, and Ru films.7,36 The absence of a similar band around 300 cm-1 upon exposing the Ir (35) Marinova, T. S.; Kostov, K. L. Surf. Sci. 1987, 185, 203. (36) (a) Tolia, A. A.; Willliams, C. T.; Weaver, M. J.; Takoudis, C. G. Langmuir 1995, 11, 3438. (b) Williams, C. T.; Tolia, A. A.; Weaver, M. J.; Takoudis, C. G. Chem. Eng. Sci. 1996, 51, 1673.

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film (and also Pt, Pd, and Rh films37) to NO under aqueous electrochemical conditions may well be due to spontaneous electrooxidation of Nad. Although a 570 cm-1 band was reported for molecular N2O adsorption on Ir(111) in UHV at low temperatures (85 K),13a we can essentially eliminate this adsorbate as being responsible for the present 570 cm-1 band. Thus N2O desorbs completely from Ir(111) by 120 K; furthermore, no SERS features were observed on the Ir film over the present electrode potential range in N2O-saturated 0.1 M HClO4. The presence of protons in the aqueous electrolyte suggests that the 570 cm-1 feature (Figure 7B) may arise from adsorbed hydroxyl, OHad, rather than Oad. Indeed, the presence of the former on gold and transitionmetal film electrodes has been diagnosed in some cases from the characteristic 15-20 cm-1 frequency downshift of the metal-adsorbate SERS band upon solvent deuteration.6c,38 However, no detectable (