Langmuir 1986, 2, 179-183
179
Adsorption and Electrooxidation of Carbon Monoxide at the Gold-Aqueous Interface As Studied by Surface-Enhanced Raman Spectroscopy M. A. Tadayyoni and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received M a y 21, 1985. I n Final Form: November 12, 1985
Carbon monoxide adsorbed at gold electrodes in acidic and neutral media has been examined by surface-enhanced Raman spectroscopy (SERS). A C-0 stretching mode, Y C O , was obtained in CO-saturated solutions at frequencies between 2080 and ca. 2150 cm-1, depending on the electrode potential. Negative of 200 mV vs. SCE in 0.1 M HCIOl and negative of -100 mV in 0.1 M NaC104, the band intensity is approximately constant, with the peak frequency decreasing with increasing negative potential by roughly 50 cm-l V-’. The vco frequency in the vicinity of the potential of zero charge, 2105 cm-’, is close to that found for CO adsorbed at gold-gas interfaces and suggests that the carbon is bound to a single surface gold atom. Vibrational bands for adsorbed CO at significantly different frequencies were obtained, however, by using electrochemical surface infrared spectroscopy. At potentials positive of 200 mV in 0.1 M HC104, the SERS YCO band intensity decreases and the frequency increases sharply from 2110 to 2130-2150 cm-’ at 500 mV; these changes are partly irreversible in that higher frequency components remain upon returning to more negative potentials. The 2130-2150-cm-’ features are ascribed to CO bound to partly oxidized surface sites. The lower frequency form produced by predosing at more negative potentials is apparently electrooxidized at noticeably less positive potentials than the high-frequency adsorbate on the basis of both SERS and electrochemical data. The gold-aqueous interface provides an interesting substrate a t which to examine electrocatalytic processes in view of its strongly adsorptive properties combined with a wide polarizable potential range. In order to provide a molecular-level picture of the interface, it is clearly desirable to supplement electrochemical adsorption and kinetic data with spectroscopic information. Either in situ Raman or infrared spectroscopy can be employed, both having distinct advantages and limitations.’*2 Although gold is known t o display surface-enhanced Raman scattering (SERS),’ few such studies have been undertaken a t this surface since the spectra have usually been found to be much weaker than a t the preeminent SERS metal, silver. However, we have recently reported a simple procedure, based on potential sweep oxidation-reduction cycles in 0.1 M KC1 followed by electrode transfer, by which gold electrodes can be mildly roughened so to yield both intense and stable SER spectra with 647-nm excitation for a miscellany of adsorbate^.^ Indeed, the spectra exhibit not only comparable intensity to those typically seen a t silver but have a much greater stability. Thus the SER spectra a t gold typically exhibit reversible potential-dependent intensities, even for adsorbates (e.g., oxyanions, benzene, etc.) that do not exhibit stable or even detectable spectra a t silver electrode^.^ This property is attributed to an inherent stability of the “SERS-active” sites at gold so that stable spectra can be obtained even for low coverages of weakly bound species. In contrast, silver electrodes often require the presence of high coverages of strongly bound adsorbates to retain their SERS a ~ t i v i t y . ~ (1)For recent reviews, see: (a) Chang, R. K.; Laube, B. L. CRC Crit. Reo. Solid State Mater. Sci. 1984,12,1.(b) Burke, R.L.; Lombardi, J. R.; Sanchez, L. A. Adu. Chem. Ser. 1982,201,69. (c) Chang, R. K., Furtak, T. E., Eds. “Surface-Enhanced Raman Scattering”; Plenum; New York, 1982. (2)Bewick, A.; Pons, S. In “Advances in Infrared and Raman Spectroscopy”; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1985. (3)Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M. J. Langmuir 1985,I, 173. (4)(a) Weaver, M.J.; Barz, F.; Gordon, J. G., 11; Philpott, M. R.Surf. Sci. 1983,125,409. (b) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G., 11; Philpott, M. R. J . ElectroanaL Chem. 1984,160,321.
We have been employing this newly discovered property of gold to examine SERS of a variety of inorganic and organic adsorbates a t this electrode, including inorganic anion^,^*^ ethylene, and other alkenes,6 alkynes: and aromatic specie^.^ A related objective is to monitor electrochemical kinetics and mechanisms by using SERS combined with faradaic electrochemical techniques. The electrooxidations of iodide5 and ethylene6* a t gold have recently been examined in this manner. As part of these ongoing studies, we report here the adsorption and electrooxidation of carbon monoxide a t gold electrodes as studied by SERS combined with cyclic voltammetry. Carbon monoxide is the archetypical adsorbate for examining adsorbate-metal surface interactions. Numerous investigations a t metal-gas interfaces have demonstrated the sensitivity of the C-0 stretching frequency to the nature of the surface bonding,8 so that comparisons with vibrational spectra for carbon monoxide a t metal-electrolyte interfaces should help elucidate surface interactions in the latter type of environment. Indeed, carbon monoxide adsorbed a t various electrodes, including gold, has recently been examined by using potentialmodulated infrared spectros~opies.~ Only very weak Raman spectra have been seen for carbon monoxide adsorbed on silver electrodes,1° even though intense signals are observed a t silver a t cryogenic temperatures.” An additional reason for selecting this system here is that carbon monoxide readily undergoes electrocatalytic (5) Tadayyoni, M. A.; Gao, P.; Weaver, M. J. J. Electroanal. Chem. 1986,198,125. (6) (a) Patterson, M. L.; Weaver, M. J. J. Phys. Chem. 1985,89,1331. (b) Patterson. M. L.:Weaver, M. J. J . Phvs. Chem. 1985.89.5046. (7)Gao, P.’; Weaver, M. J.’J. Phys. Chem. 1985,89,5040.’ (8)For a review, see: 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. (9)(a) Beden, B.; Bewick, A.; Kunimatsu, K.; Lamy, C. J. ElectroanaL Chem. 1982,142,345. (b) Nakajima, H.; Kunimatsu, K.; Aramata, A.; Kita, M. J . ElectroanaL Chem., submitted for publication. (c) Corrigan, D.; Weaver, M. J., unpublished results. (10)Mahoney, M. R.; Howard, M. W.; Cooney, R. P. J . ElectroanaL Chem. 1984,161, 163. (11)(a) DiLella, D. P.; Gohin, A.; Lipson, R. H.; McBreen, P.; Moskovits, M. J. Chem. Phys. 1980,73,4282.(b) Wood, T. H.; Klein, M. V.; Zqemer, D. A. Surf. Sci. 1981,107,625.
0743-7463/86/2402-0179$01.50/0 0 1986 American Chemical Society
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oxidation to carbon dioxide at gold,12 enabling the reactivity of adsorbed carbon monoxide as sensed by SERS to be compared with that obtained electrochemically. We have recently undertaken the first study of this type for simple inorganic electrode reactions involving adsorbed intermediates.13 Such comparisons, along with the corresponding surface infrared data! should shed light on the mechanism of this important electrooxidation reaction.
2128 2150
I
Experimental Section T h e electrochemical cells a n d the laser Raman system used for the S E R S measurements have been described in detail elsewhere.14 Laser excitation was provided by a Spectra Physics Model 165 Kr’ laser operated a t 647.1 nm, and the scattered light was collected by means of a S P E X Model 1403 double monochromator. T h e gold electrode consisted of a 0.2-cm-radius disk imbedded in a 0.6-cm-radius Teflon sheath. It was mechanically polished successively with 1.0- and 0.3-pm alumina and rinsed with distilled water immediately before use. Electrode roughening, so to yield SERS-active gold, involved applying a succession (15-25) of potential sweep oxidation-reduction cycles in 0.1 M KCl at 500 mV s-l from -100 to 1100 mV vs. the saturated calomel electrode ( s ~ e ) Cyclic .~ voltammograms were obtained either with a PAR Model 174A or a Model 173/175 potentiostat system, along with a Nicolet Explorer I digital oscilloscope coupled to a Houston 2000 X-Y recorder. Carbon monoxide (99.8% purity) was obtained from Matheson Gases. Other chemicals were reagent grade; water was purified by means of a Milli Q system (Millipore Corp.). All electrode potentials are quoted vs. the SCE, and all measurements were made at room temperature, 23 k 1 “C.
2137
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Results Following electrochemical roughening, the gold electrode in nitrogen-purged 0.1 M HC104 over the polarizable potential region (ca. -200 to 1100 mV vs. SCE) yielded only a weak SERS band at 178 cm-l. This is attributed to an adsorbed perchlorate-surface vibrational mode.3 Traces of a band a t 260 cm-’ were sometimes also obtained, probably due to residual adsorbed chloride from the electrode re treatment.^ The 178-cm-l mode increased in intensity at more positive potentials, as expected from the greater perchlorate specific adsorption. Bubbling in carbon monoxide, so to produce a saturated solution (ca. 1 yielded a well-pronounced band around 2095-2120 cm-*, along with a very weak feature at 290 cm-’ over the potential region -200 to 200 mV. Similar results were also obtained in 0.5 M H2S04,except that a band at 185 cm-’ was obtained, attributed to a sulfate-surface mode. The high-frequency band is clearly attributable to the C-0 stretching vibration, uco, for adsorbed carbon monoxide, whereas the 290-cm-l band is probably due to the corresponding carbon-surface vibration.s The vco frequency is consistent with that expected for “linearly bonded” CO (i.e., that bound to a single surface metal atom);8no bands were detected in the frequency region, ca. 1650-2000 cm-l, associated with “bridge-bonded’’ C0.8 Representative SER spectra in the 2050-2200-~m-~ region are shown for a sequence of electrode potentials in Figure 1A. The spectra were recorded in ascending order (1-5)) starting at 100 mV (1)and proceeding to 700 mV (4)before returning to 100 mV (5). Significant steady-state (12) (a) Roberts, J. L.; Sawyer, D. T. J. ElectroanaL Chem. 1964, 7, 315. (b) Roberts, J. L.; Sawyer, D. T. Electrochim. Acta 1965, 10,989. (c) Stonehart, P. Electrochim. Acta 1967, 12, 1185. (d) Gossner, K.; Mizera, E. J . Electroanal. Chem. 1979, 98, 37. (e) Gibbs, T. K.; McCallum, C.; Pletcher, D. Electrochim. Acta 1977,22,525. (0 Farrugia, T. R.; Fredlein, R. A. Aust. J. Chem. 1984, 37, 2415. (13) Farquharson, S.; Milner, D.; Tadayyoni, M. A.; Weaver, M. J. J . ElectroanaL Chem. 1984, 178, 143. (14) Tadayyoni, M. A,; Farquharson, S.; Li, T. T.-T.; Weaver, M. J. J. Phys. Chen. 1984, 88, 4701.
0
a00
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Figure 1. Potential-dependent S E R spectra in the 2050-2200cm-’ frequency region for gold electrodes in aqueous 0.1 M HC104 saturated with CO. (A) Initial electrode potential is 100 mV (spectrum l),altered sequentially to 300 (2), 500 (3), and 700 mV (4) and returned to 100 mV ( 5 ) . (B) Effect of removing solution CO by bubbling in N2 while a t 100 mV following the sequence shown in (A). After recording spectrum 6, the potential was shifted sequentially to 300 ( 7 ) ,500 (8),and 700 mV (9) and returned to 100 mV (10).
electrooxidation of CO does not commence until around 600-700 mV (as indicated by current flow). Marked changes in the SER spectra in Figure 1A typically occurred, however, even at less positive potentials. Thus the intensity of the vco band decreases and the peak frequency increases markedly, from 2110 to around 2140-2150 em-’, as the potential is altered from 100 toward 700 mV (1-3); a broad asymmetric band having a peak frequency of ca. 2125 cm-’ is recovered upon return to 100 mV (5). A fraction (ca. 10-20%) of the vco band intensity is retained if the solution CO is then removed by purging with nitrogen (Figure 1B; spectrum 7). This uco band intensity does decrease upon subsequently altering the potential once again to more positive potentials, although a small signal still survives upon return to 100 mV even after positive potential excursions as far as 1000 mV. This suggests that a portion of the CO is irreversibly adsorbed, and another fraction is only slowly electrooxidized. The effect of nitrogen purging prior to a positive potential excursion was also examined. Although a small fraction of the vco signal survived removal of the solution CO, the intensity decreased sharply as the potential was made more positive and disappeared beyond 200 mV. Bubbling in CO again following such a positive potential excursion to 1000 mV and return to 100 mV in the absence of solution CO yielded a “low-frequency’’ vco band as in spectrum 1 of Figure l A , rather than that containing higher frequency components as in spectrum 5. This indicates that the formation of the latter “high-frequency”
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Langmuir, Vol. 2, No. 2, 1986 181
Carbon Monoxide Adsorption and Electrooxidation
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400
600
800
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Figure 2. Plot of peak frequency for C-0 stretching mode, UCO, of adsorbed carbon monoxide at gold electrodes against electrode potential, E , in 0.1 M HC104 (T), 0.1 M NaClO, ( O ) ,and 0.1 M KF (A).Error bar shown indicates the approximate uncertainty in the vco values. Straight line shown has a slope of 50 cm-’ V1.
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variant of adsorbed CO, with vco 2150 cm-’ at ca 500-700 mV, requires positive potential excursions in the presence of solution CO. Surface-enhanced Raman spectra were also obtained for adsorbed CO in neutral and alkaline media. In CO-saturated neutral 0.1 M NaC10, as well as 0.1 M KF, similar vco bands were obtained as in acid media, except that the signal disappeared at potentials positive of about -100 to -200 mV, again recovering upon returning to more negative potentials. Some hysteresis in the band frequency was obtained, similar to that obtained for acidic media. The band intensity remained roughly constant with increasing negative potential to the cathodic limit, ca. -650 mV, determined by hydrogen evolution. The vco frequencies at a given electrode potential were essentially independent of pH in this potential region. Although the polarizable potential range available at each pH is somewhat limited, these data can be combined to yield an approximate frequency-potential dependence as plotted in Figure 2. The straight line shown has a slope of 50 cm-l V-I. In weakly alkaline media (pH 10-ll), however, only weak uco bands were obtained from -100 to -700 mV with peak frequencies varying in the narrow range 2118-2108 cm-’, and the bands were eliminated entirely by pH 12. Addition of 5 mM NaBr to a CO-saturated 0.1 M NaC104 solution removed the vco band entirely, yielding instead a band at 180 cm-’ due to an adsorbed bromidesurface stretching mode.3 Nonetheless, by using smaller bromide concentrations, around 0.5 mM, the vco band reappeared at more negative potentials, ca. -550 mV, where bromide desorption proceeds as evidenced by a decrease in the 180-cm-’ band intensity. Addition of chloride ions also yielded a similar effect, although higher concentrations (ca. 0.1 M) were required as expected from the weaker adsorption of chloride compared to bromide ions. Similar competitive adsorption with halides has been noted for adsorbed ethylene at gold using SERSGa It is of interest to examine qualitatively the potentialdependent SER spectra for adsorbed CO in comparison with the CO electrooxidation kinetics as sensed by faradaic current flow. In neutral as well as acidic media, the uco band is extremely weak at potentials even significantly negative of the region, around 600 mV, where the elec-
Figure 3. Anodic-cathodic cyclic voltammograms for roughened (Le., SERS-active)gold electrode from 0 mV vs. SCE at 50 mV s-l in 0.1 M HCIOl (dashed curve) and after saturation with CO (solid curve). Electrode area was 0.125 cm’.
trooxidation of solution CO commences as deduced by cyclic voltammetry (cf. ref 12e). Although the sharp drop in the CO intensity seen, for example, in 0.1 M HC104 (Figure 1A) when the potential is altered positive of 100 mV might be due to CO desorption, this more likely arises from CO electrooxidation. .Thus as noted above the vco intensity remains approximately constant over a wide range of negative potentials in 0.1 M NaC104, and there is no reason to expect the surface-C0 bond to become markedly weaker as the electrode charge becomes positive. (The potential of zero charge, pzc, at polycrystalline gold in the absence of specific ionic adsorption is around -50 mV vs. SCE15). For simple reactions involving a single type of adsorption site the reactivity of the adsorbate sensed by SERS is anticipated to equal that for the preponderant adsorbed intermediate responsible for the sustained reaction of solution species. In such cases, diminution of the steady-state adsorbate coverage by electrooxidation should occur only when sufficient faradaic current is flowing so to cause substantial diffusion depletion of the incoming reactant a t the electrode surface.13 Such diffusion polarization does not occur for CO electrooxidation at gold, even in quiescent solution, until at least 800 mV in 0.1 M HClO,. These results therefore infer that CO preadsorbed at more negative potentials is electrooxidized a t ca. 200-400 mV less positive potentials than those required to electrooxidize adsorbed CO involved in the sustained oxidation of solution CO. Evidence supporting this contention is obtained from cyclic voltammetry. Figure 3 shows a pair of anodic-cathodic cyclic voltammograms at roughened (i.e., SERS active) gold obtained at 50 mV s-’ starting from 0 mV. The dashed curve was obtained in 0.1 M HClO,, and the solid curve after saturating the solution with CO. An anodic peak is observed around 300 mV for the latter, well prior to the onset of sustained oxidation of solution CO; the peak is absent on the return scan. Essentially the same electrochemical response was obtained on a “smooth” gold surface, i.e., that was mechanically polished and subjected to successive oxidation-reduction sweeps at 200 mV s-l from -200 to 1200 mV in 0.1 M HClO,. The appearance of the prepeak, however, normally required some electrode “activation” by prior potential cycling in the supporting electrolyte, so that the electrooxidation of solution CO was relatively facile. Variation in the sweep rate (15) Clavilier, J.; Huong, N. C. R . Hebd. Seances Acad. Sei. 1971,272, 1404;J . Electroanal. Chem. Interfacial Electrochem. 1973, 41, 193.
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showed that the prepeak is associated with a fixed charge and is therefore almost certainly due to an irreversible oxidation of initially adsorbed species. Assuming this to be a two-electron process yielded a surface concentration around 2 X mol cm-2. This is around 10% of the value expected for a close-packed monolayer of CO. The peak could be obtained both on a smooth and roughened surface and was obtained on successive scans. A small residual peak is also retained after thoroughly bubbling out the solution CO with nitrogen. This peak, however, does not appear on subsequent scans, indicating that the irreversibly adsorbed species is removed by electrooxidation. The properties of this voltammetric prewave correlated well with those of the low-frequency uco band, suggesting that they are both associated with adsorbed CO. The coverage of the voltammetrically detected species decreased sharply with increasing initial scan potential beyond 100 mV, disappearing by about 300 mV, similarly to the intensity of the 2110-cm-’ SERS band. Moreover, the potential region where the voltammetric peak disappeared shifted negative with increasing pH (about 300 mV from pH 1 to 7) in a similar manner to that found for the uco intensity (vide supra). The reactivity toward electrooxidation of the voltammetrically detected adsorbate is therefore comparable to that of the SERS-active CO preadsorbed at the same potentials, 0-200 mV, both being more reactive (i.e., oxidized at lower overpotentials) than the adsorbed intermediate for the sustained oxidation of CO. However, the voltammetric peak might conceivably be due to adsorption of an impurity added along with CO rather than to CO itself; this feature does not appear to have been noted in earlier voltammetric studies.12
Discussion It is of interest to compare the frequencies of the uco band for carbon monoxide adsorbed a t the gold-aqueous interface with the corresponding feature seen at gold surfaces in the gas phase.16 In the latter environment the uco band generally appears at 2110-2120 cm-’. These values are very close to the vco frequencies, 2105 cm-’ (Figure 2), seen at gold electrodes at the pzc (ca. -50 mV). This potential is an appropriate point at which to compare electrochemical and gas-phase properties of gold since the surface potential due to water orientation at the former surface is probably small,17and the CO coverage is unlikely to be sufficiently high to shift substantially the pzc. These similar uco frequencies indicate that the nature of CO-gold bonding is essentially the same in the two environments and, as noted above, are consistent with the presence of CO bound via the carbon to a single gold surface atom. The extent of the uco frequency-potential dependence in the region where adsorbed CO is entirely stable toward electrooxidation (Figure 21, ca. 50 cm-’ V-’, is somewhat larger than that, 30 cm-* V-l, obtained from infrared measurements for CO adsorbed at platinum electrodes.18 Such shifts have been rationalized in terms of increased metal-CO back-donation into an antibonding 7~ orbital as
the potential becomes more negative.18a,bAlthough this “chemical” explanation has received some theoretical attention,lg recent calculations suggest that the potential dependence is due primarily to a Stark effect arising from the double-layer electric field.20 Such an “electrical” component of the uco frequency should nonetheless be minimized a t potentials in the vicinity of the pzc, at least in the absence of large surface potential contributions from nearby adsorbed CO and water molecules. The SERS results differ in some respects to those obtained for CO adsorbed at gold electrodes using potential-modulated surface infrared spectro~copy.~ In acidic media, only very weak IR bands are obtained around 2120 and 1950 The lower frequency feature is more discernable in neutral solutions, and becomes dominant in strongly basic ~ o l u t i o n s .Although ~ ~ ~ ~ the pH dependence of the higher frequency band appears to bear some relationship to the SERS band seen here, the former occurs at significantly higher frequencies (2120 cm-l at -200 mV vs. SCE). The origin of the low-frequency infrared band is uncertain at the present time,9b,cespecially since it occurs a t frequencies (1928 cm-’ a t -200 mV) that are substant i d y smaller than seen with infrared spectroscopy for CO adsorbed at gold-gas interfaces.I6 It is also of interest to compare the present results for carbon monoxide with SER spectra for cyanide adsorbed at gold electrodes in view of their similar structures and since both ligands should bind via the carbon atom. We have made detailed SERS and surface infrared examinations of cyanide at gold;21absome published results are also pertinent.21c A t potentials, 5300 mV, where surface oxidation does not occur, a single C-N stretching band, uCN, for adsorbed cyanide is obtained. For cyanide concentrations above ca. 0.1 mV, this band maintains a constant high intensity (about tenfold larger than for uco with 1mM CO), even to potentials as negative as -1000 mV. This is consistent with the probable monolayer adsorption of CNunder these conditions as deduced from capacitance-potential data21aand further suggests that the coverage of CO at gold electrodes is relatively low. In contrast to the behavior of adsorbed CO the V C N frequencies at gold electrodes are higher than that for free cyanide, 2080 cm-’, as expected in view of the greater a-donating and weaker a-accepting ability of CN- vs. C0.22 Specifically, at 0 mV, uCN = 2130 cm-’, and over the range 200 to -1000 mV the decrease of vCN with increasing negative potential is around 20 cm-’ V-l. Comparison between the observed shift of the uco and uCN frequencies upon adsorption is complicated by the higher coverage of cyanide coupled with the anticipated large negative shift of the pzc upon cyanide adsorption (cf. NCS-, ref 23) since it is appropriate to compare vco and uCN for surfaces carrying equal (or zero) electronic charges, rather than a t the same electrode potential. Assuming that a cyanide monolayer shifts the pzc from 0 to -1000 mV (cf. ref 23) and no such shift occurs with adsorbed CO, one deduces that the VCN frequency is nonetheless increased from 2060 to 2110 cm-’ upon adsorption, whereas that for vco is decreased from the gasphase value, 2138 cm-’, to 2105 cm-’.
~~
(16) (a) Yates, D. J. C. J . Colloid Interface Sci. 1969, 29, 194. (b) Bradshaw, A. M.; Pritchard, J. Proc. R. SOC.London, Ser. A 1970,316, 169. (c) Chesters, M. A.; Pritchard, J.; Sims, M. L. In “Adsorption-Desorption Phenomena”; Ricca, F., Ed.; Academic Press: London, 1972; p 277. (d) Kottke, R. A.; Greenler, R. G.; Tompkins, H. G. Surf. Sci. 1972, 32, 231.
(17) For example: Trassati, S. Electrochim. Acta 1983, 28, 1083. (18) (a) Kunimatsu, K. J. Electroanal. Chem. 1982, 140, 211. (b) Russell, J. W.; Overend, J.; Scanlon, K.; Severson, M.; Berwick, A. J . Phys. Chem. 1982,86,3066. (c) Golden, W. G.; Kunimatsu, K.; Seki, H. J. Phys. Chem. 1984,88, 1275. (d) Kunimatsu, K.; Golden, W. G.; Seki, H.; Philpott, M. R. Langmuzr 1985, I , 245.
(19) (a) Ray, N. K.; Anderson, A. B. J.Phys. Chem. 1982,86,4851. (b) Holloway, S.; Norskov, J. K. J . Electroanal. Chem. 1984, 161, 193. (20) (a) Lambert, D. K. Solid State Commun. 1984,51,297. (b) Bagas, P. A.; Muller, W., unpublished results quoted in ref 18e. (21) (a) Gao, P., unpublished observations. (b) Tadayyoni, M. A. Ph.D. Thesis, Purdue University, West Lafayette, IN, 1984. ( c ) Baltruschat, H.; Heitbaum, J. J. Electroanal. Chem. 1983, 157, 319. (22) For example: Nakamoto, K. “Infrared and Raman Spectra of Inorganic and Coordination Compounds”; Wiley: New York, 1978, p 259. (23) Weaver, M. J.; Barz, F.; Gordon, J. G., 11; Philpott, M. R. Surf. Sci. 1983, 125, 409.
Carbon Monoxide Adsorption and Electrooxidation
The primary feature of the SER spectra for acidic COcontaining solutions at such positive potentials compared to those at 5100 mV is the partly irreversible shift in the uco frequencies toward higher values, ca. 2130-2150 cm-l, together with a loss in intensity (Figure 1A). Such frequency increases have been observed at gold-gas interfaces either in the presence of oxygen or for incompletely reduced surfaces.16aComparable frequency increases for vco have also been seen with matrix-isolated AuCO species upon binding oxygen.24 It is therefore reasonable to ascribe the 2130-2150-cm-’ features to CO adsorption on “oxidized” sites, perhaps with some surface reconstruction to form Au(I), and/or in the vicinity of adsorbed oxygen, hydroxide ions, or possibly oriented water molecules. Since extensive surface oxidation of gold does not commence in 0.1 M HC104 until around 900 mV, the oxygen (or OH) coverage will not be high under conditions where the SER spectra were obtained. Such “oxidized” sites might be those in which sustained oxidation of CO occurs, given that they predominate (at least in the SER spectra) a t potentials corresponding to the onset of sustained faradaic current. Nevertheless, as noted above, the adsorbate formed by dosing the surface with CO while confining the potential to values below 200 mV, exhibiting lower (ca. 2110 cm-’) and more mildly potential-dependent uco frequencies, appears to electrooxidize at substantially less positive potentials. It is therefore quite plausible that the CO bound to such oxidized sites do not contribute effectively to the catalytic oxidation process, acting instead as “poisoned sites. Such inactive oxidized sites might even be stabilized by the presence of adsorbed CO. The formation of such sites, as sensed by SERS, by holding the potential at values positive of ca 200 mV may then be responsible for the lack of sustained electrooxidation of CO until substantially higher overpotentials, since adsorption of the more electroactive (lower frequency) CO is severely impeded under these conditions. The reason for such reactivity differences between CO adsorbed at relatively negative and positive potentials, and presumably at reduced and partly oxidized surface sites, is unclear. It is possible, of course, that the SERS-active sites are an atypical minority which are not involved prominently in the electrochemical responses.25 These reactivity differences are, nonetheless, consistent with the (24) McIntosh, D.; Ozin, G. A. Inorg. Chem. 1977,16,51. Huber, H.; McIntosh, D.; Ozin, G. A. Inorg. Chem. 1977, 16, 975. (25) This also may be true of the adsorption sites giving rise to surface infrared spectra (such as in ref 9), especially since the electrocatalytically relevant sites themselves may constitute only a minority of the total surface.
Langmuir, Vol. 2, No. 2, 1986 183
overall electrochemical kinetics in that the electrooxidation rates of solution CO under cyclic voltammetric conditions decrease as the initial scan potential is made more posior the potential is held a t values where sustained CO electrooxidation occurs.12c The likely electrooxidation mechanism involves reaction between adsorbed CO and either an adjacent adsorbed water molecule or hydroxide ion, depending on the pH.12 It is possible that the more oxidized surface contains more tightly bound oxygen species which may react less rapidly and prevent other species from approaching the adsorbed CO. Another factor may be the increase in the surface-carbon u-bond strength (and therefore the increased difficulty in breaking this bond to form the C 0 2 product) signaled by the greater vco frequency on the oxidized surface, although a concomitant decrease in the surface-carbon r-bonding strength also occurs.26 Interestingly, recent kineticlin situ infrared studies of the deactivation of CO oxidation by O2 at Ruand Rh-Si02 catalysts2’ also exposed a connection between the appearance of higher frequency, linearly bonded, uco bands and occurrence of catalyst deactivation. This also was ascribed to the formation of more tightly bound, and less reactive, oxygen species.27 It would be extremely interesting to employ SERS to examine the adsorption and oxidation of gas-phase CO at gold; we currently are initiating such gas-phase SERS studies in our laboratory. In spite of the inevitable uncertainties in drawing conclusions concerning the chemical nature of adsorbate-surface interactions from vibrational spectroscopy, we believe that the foregoing adds significantly to the extant d e m o n s t r a t i o n ~ ~ of 3 ~the ~ J ~utility of the SERS as a molecular probe of reactive electrochemical systems. Detailed comparative studies between SERS and surface infrared spectra for nonreactive, as well as reactive, adsorbates under electrochemically characterized conditions will be required to establish the former vibrational probe as an entirely viable tool for such purposes. Nonetheless, the unique capability of SERS for rapidly obtaining surface spectra without bulk media interferences would seem to make such efforts well worthwhile.
Acknowledgment. Additional experimental work was willingly undertaken by Ping Gao. We are grateful to Dr. K. Kunimatsu for sending us ref 9b prior to publication. This work is supported in part by the NSF Materials Research Laboratory at Purdue University, the Air Force Office of Scientific Research, and the Office of Naval Research. (26) Reference 22, p 279. (27) Kiss, J. T.; Gonzalez, R. D. J . Phys. Chem. 1984, 88, 892, 898.
