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detailed studies of surface segregation and surface intermediates in the oxidation of Ni-Cr alloys. Dick Brundle (IBM, Almaden) presented a stimulating discussion of basic and applied studies of oxidation of Pd/Sn alloy surfaces and their use to initiate Cu plating onto epoxy. Mark Paffett (Los Alamos Natl. Lab) outlined studies of the chemisorption of H2and CO at well-defined Sn/Pt(111)surface overlayers and thin alloys. In addition to the above studies of single-crystal and planar bimetallic surfaces, there were excellent talks concerning the catalytic, chemisorption, structural, and electronic properties of high surface area bimetallic catalysts. Gary Haller (Yale University) described EXAFS and XANES characterization of supported Ru/Cu catalysts. Terry King (Iowa State University) presented ‘Wu NMR studies of Cu-Ru/Si02 catalysts. Greg Griffin (University of Minnesota) spoke on the direct synthesis of higher alcohols using copper/cobalt/ZnO catalysts. Jozsef Margitfalvi (Hungarian Acad. Sci.) described anchoring techniques using tin and lead alkyls to prepare supported bimetallic (Pt-Sn, Pd-Sn, Ni-Sn, and Ni-Pb) catalysts. Vladimir Ponec (Leiden University) gave a stimulating review of hydrocarbon reactions over a series of Pt/Re
catalysts. Dick Gonzalez (University of Illinois, Chicago) reviewed detailed studies of the structural, chemisorption, and catalytic properties of supported Pt-Ru clusters. John Michel (Du Pont, Wilmington) discussed the interesting effects of alkali promoters on the microstructure of Pd-Re catalysts. In the course of the symposium, there were many useful discussions between “high surface area types” and “single crystal types” of scientists interested in bimetallic and alloy surfaces, and the strong similarities in the phenomena occurring at these two types of surfaces could be clearly seen.
Bruce E. Koel* Department of Chemistry and Biochemistry and the Cooperative Institute for Research in Environmental Sciences University of Colorado Boulder, Colorado 80309-0215 Charles T. Campbell Department of Chemistry Indiana University Bloomington, Indiana 47405
Adsorption and Electrooxidation of Carbon Monoxide on Rhodium- and Ruthenium-Coated Gold Electrodes As Probed by Surface-EnhancedRaman Spectroscopy+ Lam-Wing H. Leung and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received December 11,1987. I n Final Form: February 5, 1988 Surface-enhancedRaman (SER) spectra are reported as a function of electrode potential for carbon monoxide adsorbed from CO-saturated 0.1 M HCIOl media on thin (ca. one to three equivalent monolayer) films of rhodium and ruthenium electrodeposited onto gold. As for our recent findings for similar platinum and palladium overlayers (ref 4), the results illustrate the virtues of such thin coatings on roughened gold as a straightforward means of imparting SERS to adsorbates at transition-metal surfaces. The surface state was monitored during positive-going, and subsequently negative-going,potential excursions from C-O stretching vibrations, vco,observed in the 1900-2150-cm-’region and from low-frequency bands identified , surface-oxide, ma, vibrations. At relatively negative potentials, a single major with Surface-CO,v M ~ and vco band is observed on rhodium and ruthenium, at ca. 2045 and 2030 cm-’, respectively, ascribed to linearly bound CO; an additional weaker feature at 1900-1930cm-’on the former surface is attributed to bridge-bound CO. Altering the potential to more positive values just prior to CO electrooxidation, in the region 0.5-0.7 V vs SCE on both surfaces, yielded additional intense higher frequency vco features at 2110 and 2140 cm-’ on rhodium and ruthenium, respectively. These bands are indicative of CO bound to “oxidized”surface sites. The onset of extensive surface oxidation under these conditions was also indicated by the appearance of broad vM-0 bands around 450-550 cm-‘. The disappearance of the vco and ~4 features coincided with ~ The SER voltammetric CO electrooxidation and was accompanied by development of the Y M bands. results are compatible with potential4ifference infrared spectra obtained under similar conditions, although the highest frequency vco bands are apparently too weak to be detected in the latter case. The dependence of the vco band frequencies on the rhodium, ruthenium, and also platinum and palladium films upon the electrode potential over a wide range, ca. -0.8 to 0.8 V vs SCE, is also examined by utilizing SER measurements in basic as well as acidic media. Although the frenzy, particularly concerning enhancement mechanism details, that characterized much of the earlier research in surface-enhanced Raman scattering (SERS) has now subsided, the technique retains considerable promise as a sensitive means of obtaining detailed +Presented a t the symposium on “Bimetallic Surface Chemistry and Catalysis”, 194th National Meeting of the American Chemical Society, New Orleans, LA, Sept 1-3, 1987; B. E. Koel and C. T. Campbell, Chairmen.
0743-7463/88/2404-1076$01.50/0
structural information for adsorbed species, especially in electrochemical environments.’ An obstacle to the more widespread application of this technique to problems of chemical significance is that the SEW effect is limited for (1) Recent reviews include the following: (a) Chang,R. K.; Laube, B. L. CRC Crit. Rev. Solid State Mater. Sci. 184,12,1. (b) Moskovits, M. Rev. Mod. Phvs. 1985,57,783. (c) Weitz, D. A: Moskovita, M.;Creinhton, J. A. In Chemistry and Structure at Interfaces-New Laser and Oitical Techniques; Hall, R. B., Ellis, A. B., Eds.: V C H Deerfield Beach, FL, 1986; p 197.
0 1988 American Chemical Society
SERS on Rh- and Ru-Coated Au Electrodes most practical purposes to silver, gold, and copper surfaces. While these metals, especially gold, are of considerable electrochemical significance, it is clearly important to develop means of extending the applicability of SERS to a wider variety of interfacial materials. One approach involves the deposition of films of the material of interest onto a SERS-active substrate, the overlayer being sufficiently thin so that Raman-scattering enhancement is imparted to species adsorbed at the former, otherwise SERS-inactive, material. We have demonstrated the applicability of this strategy for several types of electrodeposited films at the gold-aqueous interface," taking advantage of the especially stable and intense SERS that can be obtained at gold surfaces5 as well as the tractable electrochemical properties of this metal. This has included the examination of SERS of adsorbates at underpotential deposited (upd) monolayers of mercury, thallium, and lead2 and of redox-active nickel and manganese oxide films on golds3 In view of the importance of transition metals as electrocatalytic materials, it is of interest to ascertain if this approach can be utilized to obtain SERS at such surfaces. We have recently reported that thin (ca. one to three monolayer) films of platinum and palladium electrodeposited onto gold yield stable SERS for adsorbed carbon m ~ n o x i d e . The ~ sensitivity of the C-0 stretching frequency, vCo, to the surface chemical environment enables the S E W signals to be identified as arising from CO bound to the transition-metal overlayer rather than to residual gold sites. The potential-dependent vco frequencies are similar to those obtained from potential-difference infrared (PDIR) spectra for the same surfaces, although the SERS-active CO underwent electrooxidation at significantly higher overpotentials." Low-frequency SERS bands associated with metal-oxide, vM0, as well as metal-C0, v M ~ O vibrations , were also observed. Given the high sensitivity, frequency resolution, and wide frequency response of SERS, it is of interest to employ this technique to examine the adsorption and electrooxidation of carbon monoxide on other, more easily oxidized, transition-metal films with a view toward elucidating details of the surface structures involved. Such a study for rhodium and ruthenium films on gold, using both SERS and PDIR methods in conjunction with cyclic voltammetry, is reported here. Rhodium and ruthenium were chosen in view of the stability of the metal overlayers on gold as well as their susceptibility to oxidation and favorable electrocatalytic properties. Experimental Section Details of the laser Raman system employed for the electrochemical SERS measurements are chiefly as described elsewhere! Laser excitation was provided by a Spectra-Physics Model 165 Kr+ laser operated at 647.1 nm, with a power of 70-80 mW focused to a ca.2-mm spot at the electrode surface. The Raman-scattered light was collected with a 50-mm-diameterN0.95 camera lens (DO Industries Model DO-5095) into a SPEX Model 1403 scanning double monochromator with a band-pass of 5 cm-', spectra being typically recorded with a scan rate of 0.5 cm-' s-'. Details of the (2) Leung, L.-W. H.; Weaver, M. J. J. Electroanal. Chem. 1987,217, 367. (3) (a) Desilvestro, J.; Corrigan, D. A,; Weaver, M. J. J.Phys. Chem. 1986, 90,6408. (b) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J . Electrochem. SOC.1988, 135, 885. ( c ) Gostzola, D.; Weaver, M. J., in preparation. (4) Leung, L.-W. H.; Weaver, M. J. J. Am. Chem. SOC.1987,109,5113. (5) (a) Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M. J., Langmuir 1985, I , 173. (b) Gao, P.; Gosztola, D.; Leung, L.-W. H.; Weaver, M. J. J.ElectroanaL Chem. 1987,233,211. (6) Tadawoni. M. A.: Farauharson. S.:Li. T. T.-T.: Weaver. M. J. J. Phys. Che&-l984,88,4701. .
Langmuir, Vol. 4, No. 5, 1988 1077
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E , V vs SCE Figure 1. Anodiecathodic cyclic voltammograms in 0.1 M HCIOl for electrochemicallyroughened gold (dotted trace) after rhodium deposition (dashed trace) and after bubbling in CO (solid trace). The sweep rate was 0.05 V 8-l. Rhodium deposition involved a faradaic charge of 2.0 mC cm-2, corresponding to ca. three equivalent monolayers (ca. 6 X lo4 mol cm-2). surface infrared measurements are chiefly as provided in ref 7 and 8. The infrared spectrometer was a Bruker-IBM IR 98-4A Fourier transform instrument, with a globar light source and a liquid-nitrogen-cooled MCT narrow-band detector (Infrared Associates Model HCT-MA). Either a PAR Model 174A or 1731179 potentiostat was used for electrode potential control. The gold electrode, although stationary, was of rotating disk construction (Pine Instrument Co.), consisting of a Cmm-diameter gold disk embedded in a 12-mm Teflon sheath. A somewhat larger, 9-mm-diameter,electrode was used for the surface infrared measurements in order to accommodatethe IR light beam. The latter was utilized in a thin-layer geometry, formed by pushing the electrodeup to the calcium fluoridewindow. After mechanical polishing, the electrodes were electrochemically roughened so to yield SERS activity by means of oxidation-reduction cycles in 0.1 M KC1 as described in ref 5b, followed by rinsing and transferral successively to the electrodeposition and spectroelectrochemical cells. The electrodeposition of the rhodium and ruthenium (see Results section) employed rhodium(II1) chloride trihydrate (RhC13.3H20) and ruthenium(II1) chloride trihydrate (RuC13. 3H20)(Aldrich). Carbon monoxide (99.8%)was obtained from Matheson Gases. Other chemicalswere reagent grade; water was purified by means of a Milli-Q system (Millipore Corp.). All electrode potentials are quoted w the saturated calomel electrode (SCE),and all measurements were made at room temperature, 23 1 OC.
*
Results and Discussion Cyclic Voltammetry. Thin films of rhodium were electrodeposited onto the roughened gold electrode from quiescent solution of 3 X lo4 M RuC13in 0.5 M HC104 by holding the electrode potential at 0.1 V vs SCE for 5 2 mm. This procedure was in part similar to that given in ref 9. The electrode was then transferred to 0.1 M HC104 for spectroelectrochemical characterization. Figure 1 shows a typical cyclic voltammogram at 0.5 V s-' for the oxidation of the rhodium overlayer and its subsequent reduction (dashed trace). A corresponding voltammogram obtained for the unmodified gold surface is shown as the dotted trace in Figure 1. The characteristic anodic-cathodic voltammogram arising from the formation and removal of gold oxide (dotted trace), especially the reduction peak at ca. 0.9 V, is largely replaced upon rhodium deposition by corresponding features reminescent of conventional rho(7) Corrigan, D. s.; Weaver, M. J. J. Phys. Chem. 1986, 90,5300. (8) Corrken. - . D. S.:. Leung. -. L.-W. H.: Weaver. M. J. Anal. Chem. 1987.
59, .2252. (9) Rand, D. A. J.; Woods,
R. J. Electroanal. Chem. 1973, 44,83.
1078 Langmuir, Vol. 4, No. 5, 1988 I
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E , V vs SCE Figure 2. Anodic-cathodic cyclic voltammograms obtained at 0.05 V s-' for electrochemically roughened gold electrode with electrodeposited ruthenium in 0.1 M HC104(dashed trace) after bubbling in CO (solid trace). Thin films of ruthenium were electrodeposited with a faradaic charge of 2.0 mC cm-2,corresponding to three equivalent monolayers. See text for details of ruthenium electrodeposition. dium electrodes,10 with the oxide reduction yielding a broader cathodic wave around 0.0 V (dashed trace). The effective thickness of these overlayers could be controlled via the faradaic charge, qf,passed during electrodeposition. Optimal SRS behavior was obtained by employing a qf value of 2.0 mC cm-2, corresponding to roughly three equivalent monolayers of rhodium (ca. 6 X 10"mol cm-2). Complete suppression of the cathodic peak at 0.9 V, indicating the entire removal of residual gold sites, required thicker films; these, however, yielded weaker SERS (vide infra). This behavior is similar to that reported for the platinum and palladium overlayers on roughened golde4 The overlayer surfaces were observed to yield stable voltammetric behavior over an extended time period (ca. 6 h). Essentially the same voltammetric response (but without SERS activity) was observed with a mechanically polished gold electrode. Thin films of ruthenium were deposited in the same manner except that a solution containing 5 X 10-4M RuC13 and 0.5 M HC104was used and the electrode was held at a potential of -0.1 V v8 SCE for 51.5 min. With the above procedure, metal deposition was typically arranged so to yield a qf value of 2.0 mC cm-2, again corresponding to about three equivalent monolayers. Typical cyclic voltammetric responses of the ruthenium film in 0.1 M HC104 (dashed trace) are shown in Figure 2. Although less well defined than for the rhodium film, the voltammetric features characteristic of ruthenium oxide formation and reduction are similar to those reported previously.ll The addition of carbon monoxide, by bubbling through the previously deaerated 0.1 M HCIOl solution for a few minutes, typically yielded the anodic-cathodic voltammograms shown as solid traces in Figures 1 and 2, respectively. In both cases a sharp onset of anodic current, correspondingto CO eledrooxidation, is seen at potentials in the vicinity of the onset of anodic oxide formation. Similar voltammetric behavior for CO electrooxidation on conventional (bulk) rhodium electrodes has been observed (10) For example: Bilmee, S.A.; De Tacconi, N. R.; Arvia, A. J. J . Electroanal. Chem. 1988,143, 179. (11) Michell, D.;Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1978, 89,11.
(12) Sheppard, N.; Nguyen, T. T. In Advances in Infrared and Ramun Spectroscopy; Clark,R. J. H., Heeter, R. E., Eds.;Heyden: London, 1978; Vol. 5, p 67.
lw; 9
89 7
u cm-' 1900
Raman shift,
Figure 3. Representativesequence of potential-dependent SEX spectra in the C-O stretching, VCO, region for CO adsorbed on rhodium-coated gold electrode. Electrolyte was 0.1 M HC104 saturated with CO. Spectra were obtained in numbered sequence for potentials (vs SCE) as indicated. Laser excitation was 70 mW at 647.1 nm; monochromator scan rate was 0.5 cm-' s-l. Rhodium deposition as in Figure 1.
previously,'0 although there are apparently no reporta for the corresponding ruthenium system. SERS within the YCO Frequency Region. Upon bubbling CO into the 0.1 M HC10, solution, we found that both rhodium- and ruthenium-coated gold surfaces yielded readily measurable surface-enhanced Raman (SER) spectra within the U M ~ (35&700-cm-') O as well as the uco (170&2200-cm-') frequency regions. A typical sequence of spectra as a function of potential in the latter region for CO adsorbed on the rhodium film is shown in Figure 3. These were recorded by starting a t -0.2 V (bottom lefthand spectrum) for the numbered sequence of positiveand then negative-going potentials shown. (For clarity, not all spectra obtained in this sequence are shown.) Each spectrum took about 10 min to be recorded. At the least positive potentials, E I0.4 V, two um bands are obtained, a weak feature at about 1900 cm-' and a stronger band at about 2025-2055 cm-', both having peak frequencies and intensities that increase toward more positive potentials. The characteristic uco feature for CO adsorbed on unmodified gold, appearing at 2110-2130 cm-I for potentials below ca. 0.5 V in 0.1 M HC104,13 is virtually absent in these spectra, indicating the paucity of residual gold surface sites. Altering the potential to between 0.5 and 0.7 V resulted in the appearance of an additional sharp band at about 2105-2110 cm-l (Figure 3). All three bands disappeared a t potentials more positive than 0.7 V, where voltammetric electrooxidation of CO was observed (Figure 1). Altering the potential back to progressively leas positive values largely reversed these changes, although the reappearance of the 2050-cm-' band was delayed until 0.3 V is reached and the weak 1900-cm-' feature is irreversibly lost. The frequencies of the major vCo band in Figure 3, 2025-2055 cm-l, are consistent with CO bound to surface (13) Tadayyoni, M. A.; Weaver, M. J. Langmuir 1986,2, 179.
SERS on Rh- and Ru-Coated Au Electrodes rhodium atoms in a terminal ("linear") configuration.'2 The substantially lower frequepcies that are characteristic of CO bound in such a manner to rhodium relative to gold surfaces can be attributed to the greater dir metal-C0 back-dopation expected for the former meta1.12J4 The low-frequency vco band, 1900-1930 cm-l, is identified with bridge-bound CO, following the interpretation of similar features observed in infrared spectra obtained at rhodium-gas interfaces.12 While both these features are commonly observed for CO on rhodium s ~ r f a c e s , ~ the ~J~J~ sharp 2110-cm-' band appearing at potentials between 0.5 and 0.7 V is more unusual. Band pairs with the high- and low-frequency partners at 2100 and 2030 cm-', respectively, have often been observed for CO adsorbed on dispersed rhodium and are attributed to coupled vibrations for a Rh1(C0)2species.le However, the lack of synergy in the potential-dependent appearance of the 2110- and 2050cm-l features in Figure 3 argues against this assignment for the present spectra. Most likely, the 2110-cm-' feature arises from CO bound to "oxidized" rhodium sites. These sites may be where rhodium has undergone formal oxidation to form Rh(1) or Rh(II1) and/or adjacent to sites where surface oxide has formed (vide infra). In either case, one would expect higher vco frequencies than for the linearly adsorbed CO since the extent of dir metal-to-ligand back-donation will thereby be diminished.12 A similar high-frequency vco feature has been observed at a supported rhodium surface exposed to gaseous CO and O2at elevated temperatures." Further evidence in support of this assignment of the 2110-cm-' band is seen from the reappearance of this feature at 0.5 V upon altering the potential in a negative direction (Figure 3). Readsorption of CO can occur under these conditions since the electrooxidation is largely suppressed. The cathodic removal of surface oxide should not occur until more negative potentials are reached (Figure l),accounting for the delay in the reappearance in the 2040-cm-' band until potentials more negative than 0.4 V are reached (Figure 3). Corresponding potential-dependent SERS measurements for CO adsorption and electrooxidation on ruthenium-coated gold yielded several comparable features. A representative set of SER spectra for this system is shown in Figure 4, obtained for a sequence of potentials from -0.3 V (bottom left-hand spectrum) to 1.0 V and return. Two bands are observed at the initial, least positive potentials (E I0.4 V), a relatively intense band at about 2030-2055 cm-' and a weak feature at 2110-2130 cm-', the peak frequencies increasing toward more positive potentials. The latter feature is almost certainly due to CO bound at residual gold sites on the basis of its freq~ency.'~ Unfortunately, the ruthenium deposition was insufficiently uniform to enable this feature to be suppressed entirely without forming sufficiently thick overlayers (greater than or equal to five equivalent monolayers) so to quench the SERS activity giving rise to the major, 2030-2050-cm-', band. At potentials positive of 0.4 V, an additional feature at about 1975 cm-' is observed. This is accompanied by the appearance of a sharp high-frequency band at about 2140 (14) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986; p 291. (15) For example: Dubois, L. H.; Somorjai, G. A. Surf. Sci. 1980,91, 514. (16) For example: (a) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957,61,1504. (b) Yates, J. T.; Duncan, T. M.; Worley, S. D.; Venghan, R. W. J. Chem. Phys. 1979, 70, 1219. (c) Yates, J. T.; Kolasinski, K. J. Chem. Phys. 1983, 79, 1026. (17) Kiss, J. T.; Gonzalez, R. D. J . Phys. Chem. 1984, 88, 898.
Langmuir, Vol. 4 , No. 5, 1988 1079
L 2100 190C Raman shift, cm-' Figure 4. Representative sequence of potential-dependent SER spectra in the YCO region for CO adsorbed on ruthenium-coated gold electrode. Electrolyte was 0.1 M HClO, saturated with CO. Spectra obtained in numbered sequence shown, with potentials as indicated. Laser excitation was 70 mW at 647.1 nm; monochromator scan rate was 0.5 cm-' s-l. Other conditions as noted in Figure 2.
cm-' (Figure 4). Both these features become intense by 0.5-0.6 V. When the potential is progressively altered to more positive values, while the two higher frequency bands become markedly weaker by 0.7 V, the 1980-1985-cm-' feature remains intense until 0.8 V and only disappears entirely by 1.0 V. In addition, the middle band undergoes a marked increase in frequency, from 2058 to 2082 cm-', when the potential is altered from 0.6 to 0.7 V (Figure 4). Altering the potential back to progressively less positive values yielded a clear-cut reappearance of only the highest frequency feature, although a weak band at about 2020 cm-' is also detected at -0.3 V. The potential-induced spectral changes during CO electrooxidation are therefore largely irreversible, at least on the relatively long time scale required to gather a comprehensive series of SER spectra by using a scanning monochromator. As for the rhodium system, the major vco bapd at 2030-2055 cm-l in Figure 4 is attributed to linearly bound CO. Similar frequencies have been observed for CO on supported ruthenium-gas-phase systems12by using infrared spectroscopy as well as on single-crystal ruthenium and bimetallic Ru/Au surfaces by using electron energyloss spectroscopy (EELS).lS The marked frequency increase seen upon altering the potential from 0.6 to 0.7 V, noted bove, is ascribed to the influence of partial surface oxidation, as observed voltammetrically in Figure 2. The major high-frequency, 2140-2150-cm-', band is identified with CO bound to oxidized ruthenium sites (cf. rhodium), Similar high-frequency bands have been observed in infrared spectra of CO at ruthenium-gas interfaces under oxidized conditions,12J9including some data for supported ruthenium-gold Supporting this assignment is the reappearance of only this high-frequency feature (18) (a) Thomas, G. E.; Weinberg, W.H. J. Chem. Phys. 1979,70,954. (b) Harendt, C.; Sakakini, B.; van den Berg, J. A.; Vickerman, J. C. J. Electrqn. Spectrosc. Relat. Phenom. 1986, 39, 35. (19) (a) Schwank, J.; Parravano, G.; Gruber, H. L. J. Catal. 1980,61, 19. (b) Brown, M. F.;Gonzalez, R. D. J. Phys. Chem. 1976, 80,1731.
1080 Langmuir, Vol. 4, No. 5, 1988
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Figure 5. Potential-difference infrared (PDIR) spectra in the vco frequency region for CO adsorbed on rhodium-coated gold electrode. Electrolyte was 0.1 M HC104 saturated with CO. Spectra were obtained by recording 100 interferometer scans at the base (reference)potential, -0.2 V vs SCE, and after stepping to the various sample potentials indicated, the former scans being subtracted from the latter. The thin-layer cavity was reformed between each pair of spectral scans. during the reverse (negative-going) potential sequence. This is expected since surface ruthenium oxide is difficult to reduce," accounting for the high degree of irreversibility in the potential-dependent SERS behavior. The assignment of the 1980-1985-cm-' band seen at potentials more positive than 0.4 V is less clearcut. We tentatively attribute this to CO adsorbed in a bridged configuration. Although such features are much less common in infrared spectra for ruthenium than for rhodium surfaces,12 such a configuration is rendered more likely by the decreases in overall CO coverage arising from partial CO electrooxidaton. Similar experiments were also performed for both the rhodium and ruthenium systems by altering the potential to more positive values immediatelyafter immersion rather than in the stepwise sequence employed in Figures 3 and 4. Generally speaking, the same potential-induced spectral changes were observed irrespective of the time scale of these perturbations. Similar, although somewhat weaker, spectra were also obtained for both surfaces during positive-going potential sequences prior to CO electrooxidation if the solution CO was removed by nitrogen purging after electrode immersion. This indicates, not unexpectedly, that irreversibly adsorbed CO is being monitored. Altering the potential to sufficiently positive values so to oxidize the adsorbed CO yields, as expected, complete irreversible loss of the vco bands upon return to more negative potentials under these conditions. Comparisons with Corresponding Surface Infrared Spectra. Given the well-known suitability of infrared spectroscopy for examining adsorbed CO, and the consequent body of data assembled by using this technique for metal-gas phase systems, it is of interest to compare the SERS results for the rhodium and ruthenium overlayers with corresponding infrared data. A representative set of potential-difference infrared (PDIR) spectra in the vco region for CO adsorbed at rhodium-coated gold is shown in Figure 5. The experimental conditions, involving COsaturated 0.1 M HC104 and an electrochemically roughened gold electrode, were similar to those utilized to obtain the SERS data in Figure 3. The spectra shown in Figure
5, displayed as relative transmittance values (ARIR),were obtained by recording 100 interferometer scans (requiring ca. 70 s) at the "base" potential, -0.2 V, and again after stepping to the various "sample" potentials indicated, the former being subtracted from the latter so to remove solution and other spectral interferences. (The positive- and negative-going bands therefore correspond to features present at the base and sample potentials, respectively.) This procedure differs from the usual approach involving repeated potential modulation during the spectral acquisition; it offers the advantage of enabling meaningful spectra to be obtained for sample potentials where irreversible CO electrooxidation occurs.8 The PDIR spectrum obtained for the most positive sample potential, 0.8 V, shows only a single positive-going feature centered at 2030 cm-', corresponding to the vco feature at the base potential since the adsorbed CO is oxidatively removed upon stepping to the sample potential under these conditions. This vco frequency is essentially identical with that obtained by using SERS at the same potential, -0.2 V (Figure 3). At less positive sample potentials, a negative-going band at higher frequencies is also present. This indicates the presence of adsorbed CO at these potentials having progressively larger vco frequencies with respect to that at the sample potential, which is also consistent with the potential-dependent SERS frequencies in Figure 3. A weak negative-going band at 1910 cm-' is also seen in the PDIR spectra for sample potentials around 0.1-0.4 V (Figure 5 ) , corresponding to the appearance of the 1900-1930-cm-' SERS band under these conditions. Bands corresponding to both these forms of adsorbed CO at comparable frequencies have also been observed in PDIR spectra obtained by using a conventional rhodium electrode.20 A significant difference between the present SER and PDIR data, however, is that the high-frequency band at 2110 cm-' in the former spectra, ascribed to CO bound to oxidized rhodium sites, is not observed in the latter spectra. Given that the signal-to-noise for the SER spectra is superior to the correspondingPDIR data, it is likely that the 2110-cm-' band is too weak to be detected in the latter. In addition, the relative intensities of the various vco bands are probably not a faithful measure of the relative CO coverage with either technique, especially in SERS, where the observed signals probably reflect only the adsorbate suitably close to the surface microstructures responsible for the Raman enhancement.' Figure 6 shows a representative series of PDIR spectra in the vco region for CO adsorbed at ruthenium-coated gold. Again the experimental conditions matched that employed for the corresponding SERS measurements (Figure 4). Unfortunately, the vco signals at ruthenium were significantly wedker than for the rhodium system, necessitating the use of repeated potential modulation in order to achieve satisfactory spectra. The data in Figure 4 were obtained by using 320 interferometer scans at each potential, the potential being altered after every 32 scans. As noted above, this procedure limib the technique to the detection of features that undergo reversible potentialinduced frequency and/or intensity shiftsq8Not surprisingly, then, the sole clear-cut feature in Figure 6 is a predominantly negative-going band at frequencies of 2020-2030 cm-l that matches closely the major vco band in the corresponding SER spectra (Figure 4). The PDIR signals disappear for sample potentials beyond 0.6 V, indicating that irreversible removal of adsorbed CO occurs (20) Beden, B.; Bewick, A.; Kunimatsu, K.; Lamy, C. J.Electroanul. Chem. 1982,142, 345.
SERS on Rh- and Ru-Coated Au Electrodes
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u , cm-'
iaoc
Figure 6. PDIR spectra in the uco frequency region for CO adsorbed on ruthenium-coated gold electrode. The reference (base) potential is -0.3 V vs SCE; sample potentials are as indicated (vs SCE). Electrolyte was 0.1 M HC104saturated with CO. Spectra are an average of 320 interferometer scans at each potential, the potential being altered after every 32 scans. under these conditions. This is consistent with the electrooxidaton of CO within the solution thin layer on the PDIR time scale, which is anticipated on the basis of the voltammogram in Figure 2. No electrochemical infrared studies on CO adsorbed at ruthenium appear to have been reported previously. SERS within t h e Low-Frequency Region. More information on the state of oxidation of the metal surface and its influence on CO adsorption can be obtained by examining SER spectra in the ca. 300-700-cm-' region. Raman bands arising from surface-oxygenvibrations have been observed for platinum-coated gold4 as well as unmodified goldz1and are apparently sensitive to the effective metal oxidation state as well as the oxide structure. Representative sets of such SER spectra for rhodiumand ruthenium-coated gold in 0.1 M HCIOl saturated with CO, obtained for a sequence of positive- and then negative-going potentials under the same conditions as for the vco data in Figures 3 and 4, are shown in Figures 7 and 8, respectively. For the rhodium system, at the initial, least positive potentials (10.3 V) a band centered at about 460 cm-l and a weak feature at about 525 cm-' are obtained (Figure 7). The former band, which only appears in the presence of CO, is attributed to rhodium-CO stretching, u w , by analogy with correspondingdata for the platinum surfaces4 Altering the potential to 0.4 V and especially 0.5 V yielded a dramatic increase in the intensity and sharpness of the 465-cm-' band. Interestingly, these conditions coincide with the appearance of the 2109-cm-' feature in Figure 3, suggesting that the intense 465-cm-' band is also associated with CO bound to oxidized sites. At more positive potentials (0.6-0.8 V) this V M ~ band O weakens and then disappears, and a very broad intense feature centered at around 525 cm-l develops; its intensity increases slowly with time at a given potential. This band, which is also present in the absence of CO, is assigned to surface rhodium-oxygen stretching, VRh-0, signaling the substantial development of a rhodium oxide film. A similar feature has been observed for Ozchemisorption on Rh(ll1) by using EELS.15 Similar SER frequencies and (21) Desilvestro, J.; Weaver, M. J. J. Electroanal. Chem. 1986, 209, 377.
I
600
I
400
Raman shift ,
I
I
I
600
I
I
400
I
I
cm-
Figure 7. Potential-dependentSER spectra in the low-frequency (300-700-cm-') region for rhodium-coated gold electrode in 0.1 M HCIOl saturated with CO. Spectra were obtained at the indicated potentials (vs SCE) sequentially as numbered. Other conditions are as noted in Figures 1 and 3.
1% 490
Raman shift , cm? Figure 8. Potential-dependentSER spectra in the low-frequency region for CO adsorbed on ruthenium-caated gold electrode. Electrolyte was 0.1 M HC104saturated with CO. Spectra were obtained at the indicated potentials (vs SCE) sequentially as numbered. Other conditions are as noted in Figures 2 and 4. band shapes have also been observed for platinum and gold oxide film^;^^^' their higher frequencies and broad band shape enable spectra for the oxide layers to readily be distinguished from those due to hydroxyl specific adsorption.21 Readjusting the potential back to progressively less positive values produces little intensity decrease in the VRh-0 feature until the potential of beyond 0.3 V (Figure 7). The reappearance of the vRh< band at about 0.4 v coincides closely with the emergence of the major vco band in Figure 3 as well as with the diminution of the VRh-0
1082 Langmuir, Vol. 4, No. 5, 1988 intensity, indicating again that oxide reduction is necessary for substantial CO readsorption to take place. Comparable results were also obtained with the ruthenium overlayer (Figure 8). Two bands, at 495 and 575 cm-l, are obtained at potentials negative of 0.5 V and are both assigned to ruthenium-carbon stretching modes, vRU4. We speculate that these two vibrations may be due to distinct adsorbed species that also give rise to the pair of vco vibrations (Figure 4) attributed to CO bound to reduced and oxidized surface sites. Altering the potential to 0.6 V yields the appearance of a broad overlapping feature which dwarfs and eventually replaces the uRU-c bands at more positive potentials. As before, this feature is ascribed to surface oxide formation, specifically to a ruthenium-oxygen vibration, v R U a ; it is also present in the absence of CO. Returning the potential to more negative values yielded little change over a wide potential range and a weak reappearance of the 495-cm-' band only at -0.1 V. This again indicates the irreversibility of the anodic oxide formation and removal on the ruthenium surface. Comparison with SERS Behavior in Alkaline Media. Corresponding SERS and PDIR spectra for CO adsorbed at the rhodium- and ruthenium-coated surfaces, and for the platinum and palladium systems described previously,* were also examined in alkaline media. A marked difference with the strongly acidic media used for most measurements is that both surface oxide formation and CO electrooxidation occur at substantially more negative potentials. Not surprisingly, then the low-frequency SER spectra in basic media (e.g. 0.1 M KOH) are dominated by intense broad bands around 450-550 cm-I associated with surface-oxide vibrations. For both rhodium and ruthenium, only one band in the vco region was observed. In 0.1 M KOH, this band shifted from 1970 to 2010 cm-l as the potential was altered from -1.0 to -0.3 V on rhodium and from 1950 to 2000 cm-' on ruthenium under the same conditions. In both cases, the uco feature disappeared irreversibly at -0.2 V, corresponding to the point where voltammetric CO electrooxidation was observed to commence. Combining the vco data obtained in acidic and basic media enables the potential dependence of these bands to be examined over an unusually wide range of electrode potentials. The potential dependence of the band frequency is of fundamental interest in connection with surface-bonding models.22 Plots of the SERS peak frequency, vg0, against electrode potential, E, for these bands on the rhodium and ruthenium surface on 0.1 M KOH are given in Figure 9 (upright and inverted open triangles, respectively). Also included are corresponding data obtained in 0.1 M KOH for platinum- (open circles) and palladium-coated surfaces (open squares and diamonds), along with corresponding vgo - E data obtained with each of these surfaces in 0.1 M HC104 (solid circles). For the O for rhodium and ruthenium surfaces, only the V ~ values the major band observed in 0.1 M HC104 (associated with linearly bound CO) are plotted. For palladium, bands due to both linearly and bridge bound CO are observed in basic as well as acidic media; both these features are therefore plotted in Figure 9 (squares and diamonds, respectively). Significant differences are seen in Figure 9 between the - E behavior of each band on a given surface in acidic and basic media. While the different potential regions over which the bands are observed in these media have little overlap, the vgo - E slopes are substantially larger in 0.1 ( 2 2 ) For example; (a) Bagus, P. S.; Nelin, C. J.; Muller, W.; Philpott, M. R.; Seki, H. Phys. Rev. Lett. 1986,58,559. (b) Koneniewski, C.; Pons, S.;Schmidt, P. P.; Severson, M. W. J . Chem. Phys. 1986,85, 4153.
a
no
c
AV
*.*"
V v
1960 -4
0
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0
-
i
0
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o
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(23) For example: (a) Gilman, S. J. Phys. Chem. 1963,67,1898; 1964, 68, 70. (b) McCallum, C.; Pletcher, D. J.Electroanal. Chem. 1976, 70, 217.
Langmuir 1988,4, 1083-1090 This in turn suggests that adsorbed 0 or OH,rather than HzO, may be responsible for CO electrooxidation on these surfaces. This is consistent with interpretations of some electrochemical data.1° Interestingly, the SERS results differ from those observed for CO adsorbed on platinum and palladium films under similar conditions in that no high-frequency vc0 bands are obtained prior to CO electrooxidation on the latter surfaces. This difference is not entirely surprising since the rhodium and ruthenium surfaces are substantially more susceptible to oxidation and indeed enable CO electrooxidation to proceed at slightly lower overpotentials than on the platinum and palladium films.4 Generally speaking, then, although detailed interpretation of such spectra for electrochemical mechanistic purposes is speculative at best, we believe that these spectra point to the value of SERS for examining oxidizable species on transition-metal surfaces, especially in electrochemical systems. Obtaining time-resolved SERS by means of an optical multichannel analyzer (OMA) ar-
1083
rangement would be desirable in this regard since it would enable potential-dependent SERS to be obtained concurrently with cyclic voltammetric measurements. Although we have recently utilized this approach to examine a number of reactions involving adsorbed aromatic speci e ~less , ~satisfactory ~ data were obtained with the transition-metal surfaces as a result of the signal-to-noise restrictions with our present OMA arrangement. Most centrally, however, the results obtained so far indicate that the extension of SERS to transition-metal surfaces by electrodepositing them on thin films on gold is a tactic worthy of further consideration. Acknowledgment. This work is supported by the National Science Foundation. Registry NO. Au, 7440-57-5;Rh, 7440-16-6; Ru, 7440-18-8;CO, 630-08-0; HC104,7601-90-3; KOH, 1310-58-3. (24) Gao, P.; Gosztola, D.; Weaver, M. J. Anal. Chim. Acta, in press; J. Phys. Chem., submitted.
The Role of Preparative Variables on the Surface Composition of Supported Pt-Ru Bimetallic Clusters+ Saeed Alerasool, Dirk Boecker, Bahman Rejai, and Richard D. Gonzalez* Department of Chemical Engineering, University of Illinois at Chicago, Box 4348, Chicago, Illinois 60680
Gloria del Angel, Maximiliano Azomosa, and Ricardo Gomez Department of Chemistry, Universidad Autonoma Metropolitana, Iztapalapa, P.O.Box 55-534, Mexico 09340, D.F. Received December 9, 1987. In Final Form: February 10, 1988 The effect of catalyst pretreatment on the surface composition of silica-supportedPt-Ru bimetallic clusters has been studied. Surface enrichment in Pt is consistent with a model in which a mobile PtCls2-phase is envisioned to diffuse freely across the support. During pretreatment, the mobile Pt phase nucleates atop the immobile Ru surface phase. It was found that precalcination in air at 150 O C prior to reduction results in an increase in the surface concentration of Ru. The results of a differential scanning calorimetry study show that a bimetallic assisted coreduction process occurs in which both metals are simultaneously reduced at temperatures lower than those observed for the reduction of a Pt/Si02 catalyst. The reduction exotherms for the Pt/Si02 and Ru/Si02 monometallic catalysts are centered at 154 and 124 "C, respectively. The nature of the PtC&s41ica interaction was studied by using "in-situ" UV diffuse reflectance spectroscopy. Shifts in the position of the charge-transfer bands as a result of the adsorption of PtCb2-on silica are interpreted in terms of Cl- ligand exchange with the hydroxyl groups of the support. The hydrogenation of CO was studied over supported Pt-Ru bimetallic clusters of known surface composition. A statistical model based on a Ru ensemble size between 2 and 3 correctly predicts the rate of methanation as a function of surface composition. Introduction It has been suggested that surface enrichment of one metal in preference to another in supported bimetallic clusters is strongly dependent on the relative heats of sublimation of the metals.lt2 Bimetallic clusters prepared from group Ib and group VIII metals generally substantiate these claims.- For example, supported bimetallic catalysts prepared by alloying either Ni or Ru with Cu have
* Author
to whom correspondence regarding this manuscript should be addressed. Presented at the symposium on 'Bimetallic Surface Chemistry and Catalysis", 194th National Meeting of the American Chemical Society, New Orleans, LA, Sept 1-3, 1987; B. E. Koel and C. T. Campbell, Chairmen.
been shown to have surface compositionsthat are strongly enriched in Cu. Recent studies performed by Wu et al.' using a Monte Carlo simulation method have shown that for particles having cubooctahedral symmetry the Cu atoms are preferentially located at sites that have low surface coordinations. These sites are identified with edges, steps, corners, and surface defects. The Ru atoms, on the other hand, are identified with high surface coordination sites (1) Gonzalez, R. D. Appl. Surf. Sci. 1984, 19, 113. (2) Williams, F. L.; Nason, D. Surf. Sci. 1974, 45, 377. (3) Sinfelt, J. L.; Garten, J. L.; Yates, D. J. C. J. Catal. 1972,24,283. (4) Haller, G. L.; Rasasco, D. E.; Wang J. Catal. 1983,84, 477. (5) Sinfelt, J. H. J. Catal. 1973, 9, 308. (6) King, T. S.; Donnelly, R. G. Surf. Sci. 1984,141, 417. (7) Wu, X.; Smale, M. W.; Gertstein, B. C.; King, T. S. Paper 15a;
AIChE National Meeting, New York, November, 1987.
0743-7463/88/2404-l083$01.50/00 1988 American Chemical Society
