7771
J . Phys. Chem. 1991, 95, 7771-7777 HNOj-HZO ice is not possible. However, during the preparation of this paper we received a preprint from Hanson and Ravishankara2*who found the reaction probabilities of y I = 0.3 for the CION02 HCl reaction and of y2 = 0.006 for the C10N02 H 2 0 reaction on a " N A T surface that was prepared by reacting CIONOz with H 2 0ice at 200 K and allowing the H N 0 3 product to build up in the substrate. The surface area and the bulk density of their substrate were not measured. The HN03-covered H 2 0 ice was assumed to be nonporous. Their value of y I = 0.3 is in excellent agreement with our value of 0.27 prior to correcting for internal surface area. If we correct their data for internal surface area by using our measurements of porosity and BET surface area and an estimated average ice thickness of 11 pm in their work, we obtain y I = 0.2, which is a factor of 2 larger than our corrected value of 0.1 f 0.02. After correction for the internal diffusion, their value for y2becomes 0.002. Hanson and Ravishankara argue that their "NAT" surface may be about 48-52 wt % H N 0 3 by referring to the H N 0 3 partial vapor pressure measurement by Hanson and Mauersberger.'* If we assume their "NAT" surface is about 48 wt %, y2 would be a factor 2 greater than our data shown in Figure 8. These differences are not significant in view of the difficulties associated with the reaction probability measurements. To our knowledge there is no previous measurement of the HCI sticking coefficient on type I PSCs. Thus, our result represents the first determination.
+
+
(25) Tolbert, M. A.; Rossi, M. J.; Malhotra, R.; Golden, D. M. Science 1987, 238, 1258.
(26) Tolbert, M. A,; Rossi, M. J.; Golden, D. M. Geophys. Res. Lett. 1988, I S . 841. (27) Watson, L. R.; Van Doren, J. M.; Davidovits, P.; Worsnop, D. R.; Zahniser, M. S.;Kolb, C. E. J . Geophys. Res. 1990, 95, 5631. (28) Hanson, D. R.; Ravishankara, A. R. J . Geophys. Res. 1991,%, 5081.
The possible effect of our preliminary results on calculated ozone loss in the polar stratosphere has been discussed b r i e f l ~ .Time ~ constants for heterogeneous processes on PSCs as suggested by Turco et al.' are given by
t = 4/yws
(11)
where y is the HCI sticking coefficient or C10N02 reaction probability, w is the molecular velocity of HCI or CION02, and S is the surface area of type I PSCs per unit volume. For these calculations, we assume S to be 1 X lo-' cm2/cm3 for fully developed PSCs3and use the measured ys for HCI adsorption and for reactions 1 and 2. The results are shown in Figure 9. A typical lifetime of a PSC is about a few days (approximately IO6 s).~ The time constant for HCI adsorption ranges from 1 X IO5 s at 40 wt 3'% H N 0 3 to about 2 X lo7 s a t 54 wt %. This suggests that only PSCs composed of H20-rich NAT could readily incorporate HCI. The heterogeneous chemical process responsible for converting inactive chlorine to active forms is the CIONOl + HCI CI2 H N 0 3 reaction. The time constant for this process is about a few hours. Finally, it should be noted that the CIONOz H 2 0 HOC1 H N 0 3 reaction on type I PSCs is probably too slow to play a role in the polar stratosphere.
-
+
+
-
+
Acknowledgment. This research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We are grateful to Roland H. Smith for his assistance in collecting infrared spectra and to D. R. Hanson and A. R. Ravishankara for the preprint of their manuscript prior to publication. Registry No. CION02, 14545-72-3; HCI, 7647-01-0.
Adsorption of Acetylene on Rhodium- or Platinum-Modified Silver and Gold Electrodes: A Surface-Enhanced Raman Study Hannah Feilchenfeld*lt and Michael J. Weaver Department of Chemistry, Purdue University, West Lafayette, lndiana 47907 (Received: May 31, 1990; In Final Form: March 19, 1991)
Surface-enhanced Raman spectra (SERS) of acetylene adsorbed on rhodium- and platinum-modified silver and on rhodium-modified gold were determined at room temperature in both acidic and neutral supporting electrolytes. In acidic media the spectra exhibited intense bands in the vicinity of 1100 and 1500 cm-I; similar peaks were observed on unmodified gold, but not on unmodified silver. In a neutral environment the same bands appeared on rhodium-modified gold, and sometimes on unmodified gold; they were never detected on silver or on silver modified by transition metals. The 1100- and 1500-cm-1 bands were assigned to sp2or sp3rehybridized forms of acetylene, possibly produced by catalytic hydrogenation on the surface of the electrodes. Their intensities reversibly increased at negative potentials, suggesting a potential related formation and desorption of the new species. Weak bands, attributed to unhybridized acetylene molecules aa-bonded to the surfaces, were occasionally observed in the 1800-2300-cm-' spectral region. No signals appeared on polished electrodes, even when these were modified by thin layers of rhodium or platinum, and the intensity of the peaks decreased gradually when increasing amounts of transition metal were deposited on the roughened substrates. The bands were therefore ascribed to surface enhancement phenomena rather than to bulk Raman scattering.
Introduction
The surface chemistry of acetylene adsorbed onto transition metals has been the subject of intensive investigation, albeit with rather contradictory results. Studies by normal Raman (NR) and surface-enhanced Raman spectroscopy (SERS), both in ultrahigh vacuum at various temperat~resl-~ and in electrochemical systems: indicate that while the adsorbed acetylene undergoes various 'Permanent address: Department of Organic Chemistry, The Hebrew University of Jerusalem, 91 904 Jerusalem, Israel.
0022-365419 112095-777 1 $02.50/0
degrees of rehybridization, the molecule remains essentially whole.14 Polymerization on the surface has sometimes been ( I ) (a) Moskovits, M.; DiLella. D. P. In Surfuce Enhunced Roman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982; p 243. (b) Manzel, K.; Schulze, W.; Moskovits, M. Chem. Phys. Lett. 1982, 85, 183. (2) Pockrand, I.;Pettenkofer, C.; Otto, A. J. Electron Spectrosc. Reluf. Phenom. 1983, 29, 409. (3) Bobrov, A. V.; Kimel'fel'd, J. M.; Mostovaya, L. M. J. Mol. Saucr. 1980, 60, 43 I .
0 1991 American Chemical Society
7772 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 In contrast, high-resolution electron energy loss spectroscopy, low-energy electron diffraction, and related studies of acetylene adsorbed on a variety of well-defined crystallographic surfaces' show complete rehybridization of the carbon-carbon bond from sp to sp2 or sp3, often associated with decomposition of the molecule on the surface into different species such as vinylidene, ethylidyne, and other fragments. We recently examined by SERS the adsorption of acetylene and other alkynes onto silver or gold electrodes and found these compounds to form UT complexes on the surface.* We have now extended our research to catalytically active metals such as rhodium and platinum. Most transition metals do not by themselves constitute SERS-favorable substrates; attempts to detect Raman signals from adsorbates on the surface of various metals, whether these are in the form of polycrystalline electrodes?JO thin films evaporated onto various substrates,"J* coll~ids,'~J* or alumina- and silica-supported layers,"J5 yield relatively weak signals. Measurements carried out with roughened silver or gold substrates modified by electr~lyticl~-'~ or vapor" deposition of a transition-metal film generally show a quenching of the Raman signals when compared with unmodified silver or gold. Only in isolated instances could new bands be observed and assigned to molecules bound to the overlying metal rather than to the silver or gold substrate.'kJ1d The SERS technique has, therefore, been considered unsuited to the investigation of the bonding between adsorbates and transition metals. Nevertheless, we have recently found experimental conditions which allowed us to observe strong Raman bands from carbon monoxide adsorbed on platinum-, palladium-, rhodium-, and ruthenium-modified gold as well as from pyridine adsorbed on rhodium-modified silver.20 In the present study we were able to obtain intense SERS bands from acetylene adsorbed on silver and gold electrodes modified by thin layers of rhodium or platinum, and to examine the catalytic
(4) (a) Parker, W. L.; Siedle, A. R.; Hexter, R. M. J . Am. Chem. SOC. 1985, 107,264. (b) Parker, W. L.; Siedle, A. R.; Hexter, R. M. Langmuir 1988,4999. ( 5 ) Hanson. D. M.; Udagawa, Y.; Tohji, K. J . Am. Chem. Soc. 1986,108, 3884. (6) Patterson, M. L.; Weaver, M. J. J . Phys. Chem. 1985, 89, 5046. (7) See for instance: (a) Mate, C. M.; Kao, C.-T.; Bent, B. E.; Somorjai. G. A. Surf. Sei. 1988,197, 183 and references therein. (b) Sheppard, N. J. Electron Spectrosc. Relat. Phenom. 1986, 38, 175 and references therein. (8) Feilchenfeld, H.; Weaver, M. J. J . Phys. Chem. 1989, 93, 4276. (9) Bilmes, S.A. Chem. Phys. Lett. 1990, 171, 141. (IO) (a) Fleischmann, M.; Graves, P. R.; Hill, 1. R.; Robinson, J. Chem. Phys. Lett. 1983,95, 322. (b) Furtak, T. E.; Miragliotta, J. Surf. Sci. 1986, Musiani, M.M.; Fleischmann, M.; Mao, B.; Tian, 167, 381. (c) Mengoli, G.; 2.Q.Electrochim. Acta 1987, 32, 1239. ( I I ) (a) Lopez-Rim, T.;Gao, Y.; Vuye, G. Chem. Phys. Lett. 1984,111, 249. (b) Ciao, Y.; Lopez-Rios, T. Phys. Reu. Lett. 1984, 53, 2583. (c) Lopez-Rim, T.; Gao, Y. J . Vac. Sci. Techno/., B 1985,3,1539. (d) Gao, Y.; Lopez-Rim, T.Surf. Sci. 1988, 198, 509. (e) Lopez-Rim, T.; Gao, Y.Surf. Sci. 1988, 205, 569. (12) (a) Yamada, H.; Yamamoto, Y.; Tani, N. Chem. Phys. Lett. 1982, 86, 397. (b) Yamada, H.; Yamamoto, Y. Surf. Sci. 1983, 134, 71. ( I 3) Parker, W.L.; Hexter, R. M.; Siedle, A. R. Chem. Phys. Lett. 1984, 107, 96. (14) Benner, R. E.; Von Raben, K. U.; Lee, K. C.; Owen, J. F.; Chang, R. K. Chem. Phys. Lett. 1983, 96,65. (IS) (a) Parker, W. L.; Hexter, R. M.; Siedle, A. R. J. Am. Chem. SOC. 1985, 107, 4584. (b) Parker, W.L.; Siedle, A. R.; Hexter, R. M. J . Catal. 1986, 99, 482. (!6) (a) Moerl, L.; Pettinger, B. SolidSrare Commun. 1982.43, 315. (b) Pettinger, B.; Moerl. L. J . Electron Spectrosc. Relat. Phenom. 1983, 29, 383.
(c) Watanabe, T.; Yanagihara, N.; Honda, K.; Pettinger, B.; Moerl, L. Chem. Phys. Lett. 1983, 96, 649. ( I 7) (a) Pemberton, J. E.; Coria-Garcia, J. C.; Hoff, R. L. Langmuir 1987, 3, 150 and references therein. (b) Guy, A. L.; Pemberton, J. E. Langmuir 1987, 3, 777. (c) Kellog. D. S.;Pemberton. J. E. J . Phys. Chem. 1987, 91,
1126. (18) Kester, J. J. J . Chem. Phys. 1983, 78, 7466. (19) (a) Fleischmann, M.; Tian, Z. Q.J . Electroanal. Chem. 1987, 217, Li. L. J. J . Electroanal. Chem. 1987, 385. (b) Fletschmann, M.; Tian, Z. Q.; 217, 397. (c) Fleischmann, M.; Tian, Z. Q.J . Electroanal. Chem. 1987, 2/7. 411. (20) (a) Leung, L.-W. H.; Weaver, M. J. J. Am. Chem. Soe. 1987. 109, 5113. (b) Leung, L.-W. H.; Weaver, M. J. Langmuir 1988, 4, 1076. (c) Feilchenfeld, H.;Weaver, M. J. Chem. Phys. Lett. 1989, 161, 321.
Feilchenfeld and Weaver 0
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E
a"
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.-
-02-0.4-
-06
-04
-02
00
02
04
0.6
E , V vs SCE Figure 1. Cyclic voltammograms obtained in 0.1 M HClO, with roughened electrodes: unmodified silver (solid trace); silver modified by
about 1.5 equivalent monolayers of rhodium (dotted trace); silver modified by about 3 equivalent monolayers of rhodium (dashed trace). The sweep rate was 0.1 V s-'.
reaction occurring on the surface.
Experimental Section Details on the scanning Raman spectrometer have been reported earlier;2' it comprised two Spectra Physics Model 165 Ar+ and Kr+ lasers and a Spex 1403 scanning double monochromator. The 514.5-nm Ar+ laser line was used for excitation at silver electrodes and the 647.1-nm Kr+ line at gold electrodes. The incident laser power was 40-60 mW at the sample. The spectral band-pass was 5 cm-', and the monochromator scan rate was 1 cm-' s-I. The silver and gold electrodes were 4 mm diameter rods sheathed in Teflon (Pine Instrument Co.). They were cleaned by sonification in acetone and water and by mechanical polishing with successively 1.0-, 0.3-, and 0.05-pm alumina (Buehler). The surface of the silver electrode was electrochemically roughened by a series of five oxidation-reduction cycles in 0.1 M NaC1, stepping the potential between -0.60 and 0.15 V versus a saturated calomel electrode (SCE).22a The gold electrode was also pretreated by a series of oxidation-reduction cycles as previously described.22b Acetylene (from Arco Inc.) was purified as reported before;23 traces of acetone were removed by passage through two traps at -75 OC. The gas was dissolved to saturation by allowing it to bubble through the electrolyte for 20-30 min; the bubbling was always carried out before introduction of the electrode intD the solution. The water was purified with a Milli-Q system (Millipore Corp.). All solutions were purged with nitrogen before being used. The potentials were determined versus SCE, and all measurements were carried out at room temperature, 23 f 1 O C . Results The bonding of acetylene to the roughened surface of un-
modified silver and gold was previously examined by SERS in neutral In order to study its adsorption onto transition metals, we now used three different substrates: rhodium-modified silver, platinum-modified silver, and rhodium(21) Tadayyoni, M. A.; Farquharson, S.; Li, T.T.-T.; Weaver, M.J. J . Phys. Chem. 1984,88, 4701. (22) (a) Leung, L.-W. H.; Gosztola, D.; Weaver, M. J. Langmuir 1987, 3 , 45. (b) Gao. P.; Gosztola, D.; Leung, L.-W. H.; Weaver, M. J. J . Electroanal. Chem. 1987, 233, 21 1, (23) (a) Conn, J. B.; Kistiakowski, G. B.; Smith, E. A. J . Am. Chem. Soc. 1939, 61. 1868. (b) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. In Purification of Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1980; p 83.
Adsorption of Acetylene on Ag and Au Electrodes
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7773
T
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Raman shift , cm-l Figure 2. Metal-CI stretching vibration: (a) on unmodified gold (in 0.1 M HCIO,, at 0.3 V vs SCE); (b) on rhodium-modified gold (about 3 equivalent monolayers of rhodium, in 0.1 M HCIO,, at 0.3 V vs SCE); (c) on unmodified silver (in 0.1 M NaCI, at 0.0 V vs SCE); (d) on gold modified by an underpotential deposited layer of silver (in 0.1 M NaCIO,, at 0.5 V vs SCE).
modified gold. The Raman spectra were obtained both in neutral and in acidic supporting electrolytes. For comparison, the SERS spectra of acetylene adsorbed onto unmodified silver and gold in an acidic medium were also recorded. Rhodium and Platinum Deposition on Silver and Gold Electrodes. The electrodeposition of rhodium and other transition metals on gold has been described On silver the transition-metal plating was carried out by a similar procedure: the roughened electrode was dipped into a 3 X IO4 M solution of RhCI3 in 0.5 M HC104 at a potential of about 0.05 V vs SCE, or into a 5 X M solution of H2PtCI, in 0.5 M H2S04 at about 0.15 V vs SCE. The amount of rhodium or of platinum equivalent to one monolayer on a smooth silver or gold surface was calculated to be roughly equal to 2 X mol Its deposition required a faradaic charge of about 0.6 mC cm-2 for rhodium and 0.8 mC for platinum. The surface area of electrochemically roughened silver has been determined previously by differential capacitance measurement^;^^ it was found to be 1.5-2.0 times larger than that of a polished electrode. A similar roughness factor was assumed for gold surfaces. The amount of rhodium or of platinum deposited could thus be estimated from the charge passed during the procedure. At the end of the electrodeposition, the electrode was thoroughly rinsed with water and transferred to the electrochemical SERS cell containing the solution to be examined. Typical cyclic voltammograms of roughened silver before and after rhodium deposition are shown in Figure 1. As can be seen, the characteristic silver oxidation peak at about 0.15 V progressively disappeared with rhodium deposition. Voltammograms obtained for gold electrodes have been published previously;20b they showed a similar decrease of the gold oxide reduction peak a t 0.9 V with increasing amounts of rhodium. The electrolytic deposition of the transition metals did not proceed entirely smoothly, and the mass of rhodium or platinum required to "cover" the silver or gold surface, i.e., to eliminate the Ag oxidation or the AuO reduction peaks, varied with the rate of deposition. The slower the procedure, the less rhodium or platinum was required, indicating that a reduced deposition rate yielded a more uniform transition-metal overlayer. The SER spectrum of a gold electrode freshly modified by rhodium deposition showed a characteristic shift of the Au-CI stretching frequency from 260 cm-' on unmodified gold to 295 (24) Hupp, J . T.; Larkin, D.; Weaver, M . J . Surj. Sci. 1983, 125, 429.
a I
2300
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Raman shift ,cm-l Figure 3. SER spectra of (a) blank rhodium-modified silver electrode, (b) acetylene adsorbed on unmodified silver, and (c) acetylene adsorbed on rhodium-modified silver. The amount of rhodium in (a) and (c) was about 1.5 equivalent monolayers. All measurements were in 0.1 M
NaCIOo at 0.2 V vs SCE. cm-', as well as a significant increase in the band intensity (Figure 2). These changes can probably be assigned to the formation of a Rh-CI bond on the surface, as the bulk stretching vibrations of Rh-CI have been reported in the 295-345-cm-l region.2s The Ag-CI band was clearly seen at 235 cm-l on unmodified silver and at 250 cm-' on the surface formed by underpotential deposition of a silver monolayer on roughened gold (Figure 2), but no similar signal was detected for rhodium-modified silver electrodes. The m e t a l 4 bands were probably due to residual CI- adsorbed a t the electrode surface during its preparation; they typically disappeared at negative potentials or upon acetylene adsorption. Adsorption of Acetylene on Rhodium-Modified Silver in Neutral or Acidic Electrolytes. The SER spectrum obtained from acetylene adsorbed on unmodified silver in a neutral supporting electrolyte has been reported and discussed earlier* (Figure 3b). It is characterized by medium-intensity stretching bands at 1820, 1980, and 2140 cm-l and by an additional strong broad peak at about 1500 cm-l which was attributed to sp2-rehybridized adsorbate molecules. No frequency shifts were observed in these bands after deposition of a thin rhodium overlayer on the silver electrode and acetylene adsorption from a neutral solution. The broad 1500-cm-' band was split into two overlapping features of variable relative intensities, at 1460 and 1550 cm-l (Figure 3c). No additional peaks were detected in the spectrum after rhodium deposition. The band intensities, however, were substantially affected by the rhodium overlayer. At low rhodium thickness, e.g., for deposition of 1.5 equivalent monolayers (Figure 3c), the signals were significantly stronger than on unmodified silver. With four or more equivalent monolayers of rhodium the intensities started to decrease. The signal intensities were somewhat potential dependent: in particular, the 1810-1 820-cm-' band, clearly observed at positive potentials, reversibly disappeared when the potential became negative, exactly as on unmodified silver.8 The spectra determined in acidic solutions (0.1 M HC104 saturated with acetylene) differed considerably from those obtained (25) Clark, R . J . H.; Williams, C. S.Inorg. Chem. 1965. 4, 350.
7174 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Feilchenfeld and Weaver
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Raman shift , cm-I SER spectra of acetylene adsorbed on rhodium-modified silver
Figure 5. SER spectra of acetylene adsorbed on platinum-modified silver for various amounts of platinum (in equivalent monolayers): (a) 0; (b) 0.5; (c) 1.5; (d) 3; (e) 6; (f) 12; (g) 30. All measurements were in 0.1 M H2S04at -0.3 V vs SCE.
for various amounts of rhodium (in equivalent monolayers): (a) 0; (b) 0.5; (c) 1.5; (d) 3; (e) 7; (f) 15; (g) 35; (h) 70. All measurements were in 0.1 M HCIO, at -0.1 V vs SCE.
in neutral electrolytes. On unmodified silver the adsorbed acetylene yielded very weak signals at 1550 and 1990 cm-' or no signals at all (Figure 4a). After rhodium deposition on the roughened silver all bands in the 18W2300-cm-' region disappeared, while new bands appeared at 1125 and 1510 cm-l, with a wide shoulder at 1580 cm-I. These new bands, weak at submonolayer rhodium amounts (Figure 4b), sharply increased in intensity with growing thickness of the rhodium layer. They reached a maximum for about 2 equivalent monolayers (Figure 4c)and then gradually decreased with larger amounts of rhodium; they could still be observed clearly after deposition of about 35 and 70 equivalent monolayers (Figure 4g,h). As may be seen on the cyclic voltammogram (Figure I ) , the potential range useful for SERS measurements in acidic conditions was rather limited (roughly 0.1 to -0.3 V). At negative potentials weak currents flowed through the working electrode; for instance, at -0.1 V (Figure 4), -0.1 to -0.2 mA passed through the system. The intensities of the 1125- and ISIO-cm-' bands were only weakly potential dependent. Adsorption of Acetylene on Platinum-ModifiedSilver in Acidic Electrolyte. The spectra obtained in acidic medium from acetylene adsorbed on platinum-modified silver differed only slightly from those determined on rhodium-modified electrodes. Practically no SERS signals were detected from an unmodified roughened silver electrode dipped into 0.1 M H2S04saturated with acetylene (Figure 5a). After deposition of even submonolayer amounts of platinum (Figure 5b) very intense bands appeared at 1125 and 1510 cm-I. These frequencies were exactly the same as previously observed from acetylene adsorbed on rhodium-modified silver in 0.1 M HC104, but the 1580-c"' shoulder was less prominent with platinum than with rhodium. The 1125- and 1510-cm-l bands remained intense with increasing thicknesses of platinum up to 3 or 4 equivalent monolayers (Figure 5c.d) and then gradually decreased for heavier platinum deposits; they became undetectable with more than 25 equivalent monolayers (Figure 5f,g). The intensities of the bands detected on platinum exhibited an important potential dependence. Practically nonexistent at positive or zero potentials (Figure 6a), the peaks appeared at negative
I510 I I
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Raman shift , cm-' Figure 6. Potential dependence of the SER spectrum of acetylene adsorbed on silver modified by 0.5 equivalent monolayer of platinum. The spectra were obtained in 0.1 M H 3 0 4 , in sequence from (a) to (f) at the following potentials vs SCE: (a) 0.0 V; (b) -0.2 V; (c) -0.3 V; (d) -0.4 V; (e) 0.2 V; ( 0 0 . 3 V.
potentials (Figure 6b), reached a maximum at -0.3 V (Figure 6c), and then slightly decreased (Figure 6d). When the potential was switched back to positive values (Figure 6e,f), the bands disappeared. These potential steps were associated with changes in the current flowing through the electrochemical cell: no current passed through the system at potentials between 0.2 and -0.2 V, rather large negative currents (up to -0.7 mA) could be measured at -0.3 V or more negative potentials, and a weak positive current (0.15 mA) was detected at 0.3 V. Adsorption of Acetylene on Rhodium-ModifiedGold in Neutral and Acidic Electrolytes. We have previously described the SER
Adsorption of Acetylene on Ag and Au Electrodes
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1175
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Figure 7. SER spectra obtained in a neutral environment (0.1 M NaCIO,) from acetylene adsorbed (a+) on unmodified gold, (d-f) on gold modified by about 1 equivalent monolayer of rhodium, and (g-i) on gold modified by about 4 equivalent monolayers of rhodium. The potentials (vs S C E ) were as follows: (a, d, g) 0.3 V; (b, e, h) -0.3 V; (c, f, i) -0.6 V. For reasons of clarity, spectra a-c have been magnified 5 times. spectrum of acetylene adsorbed on unmodified gold in a neutral supporting electrolyte.* It is similar to that of acetylene on silver, showing weak bands at about 1850, 1975, and 2140 cm-I, with an additional intense peak in the 1500-1600-~m-~ region. Strong bands at 1095 and 1475 cm-l have been reported earlier for acetylene adsorbed on various metals?" under diverse experimental conditions; we sometimes observed similar featurese when acetylene was bubbled through the SERS cell in the presence of an unmodified gold electrode, or at potentials more negative than -0.6 V, usually accompanied in the latter case by currents of about -0.3 to -0.5 mA. Figure 7a-c exhibits the growth of such peaks at negative potentials, though it should be emphasized that the bands always remained weak and that their occurrence was not entirely reproducible. After thin layers of rhodium were deposited on the gold substrate (e.& about 1 equivalent monolayer, Figure 7d-f), the appearance of bands at 1095 and 1475 cm-' no longer required negative potentials; definite, though weak, peaks were detected in a neutral environment even at positive potentials (Figure 7d); they increased substantially at -0.6 V (Figure 70, while a negative current of a few tenths of a milliampere flowed through the system. The bands observed in the presence of rhodium at -0.6 V were more intense by at least 1 order of magnitude than those seen under the same conditions on unmodified gold (Figure 7c,f). With more substantial amounts of rhodium the signals became weaker (Figure 7g-i). When the measurements were carried out in acidic solutions (0.1 M HCI04), significant changes were observed in the SER spectrum. In contrast to the results obtained under neutral conditions, acetylene adsorbed on unmodified gold exhibited well-defined bands at 1095 and 1475 cm-' even at zero potential, in addition to the 1975- and 2140-cm-I features observable in neutral electrolytes (Figure 8a). The 1095- and 1475-cm-I bands increased dramatically in intensity at negative potential, and the overtones and combination bands of these peaks became clearly visible at 2165, 2545, and 2935 cm-l (Figure 8b). As in neutral electrolytes, the increase in the intensities of the signals coincided with the flow of a negative current through the system. After a thin layer of rhodium (e.g., about 2 equivalent monolayers, Figure 9) was deposited on the gold electrode, the bands observed in acidic media at 1095 and 1475 cm-l with zero or slightly positive potentials were markedly more intense than on
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Raman shift , cm-' Figure 8. SER spectrum of acetylene adsorbed on unmodified gold, in acidic environment (0.1 M HCIO,): (a) at 0.0 V and (b) at -0.3 V vs SCE.
h
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Figure 9. SER spectrum of acetylene adsorbed on gold modified by about 2 equivalent monolayers of rhodium in acidic environment (0.1 M HCIO,): (a) at 0.3 V and (b) at -0.2 V vs S C E . unmodified gold, but their relative increase at negative potentials was less pronounced. With larger amounts of rhodium (e.g., about 5 equivalent monolayers, Figure IO) the features previously seen in the 1900-2200-cm-' region for acetylene adsorbed on unmodified gold disappeared entirely, while the 1095- and 1475-cm-I bands, as well as their overtones and combination bands, decreased in intensity: they were no longer detectable at positive potentials, were barely visible at zero potential, and became much weaker than before even at negative potentials. In acidic supporting electrolytes, for unmodified gold as well as for gold modified by rhodium overlayers of different thicknesses, the intensities of the bands were extremely potential dependent. The peaks appeared rapidly at zero or negative potentials, but when the potential was stepped to 0.3 V or a more positive value, they slowly decreased in intensity and disappeared over periods
7776 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
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Figure 10. SER spectrum of acetylene adsorbed on gold modified by about 5 equivalent monolayers of rhodium in acidic environment (0.1 M HCIO,): (a) at 0.3 V and (b) at -0.3 V vs SCE.
of 0.5-1 h. The bands reversibly appeared and disappeared when the potential was stepped back and forth between -0.3 and 0.3 V, though their intensities decreased after a few cycles; the appearance or reappearance of the peaks always coincided with the passage of a negative current through the electrochemical cell.
Discussion SERS Origin of the Bands. While the vibrational spectrum of acetylene is devoid of peaks in the 1000-1600-cm-' region, intense bands have been described at about 1500 and 1100 cm-l in the normal Raman spectrum of rrans-polyacetylene.26 These were assigned respectively to the stretching frequencies of carboncarbon double bonds and to vibrations resulting from the mixing of carbon-carbon single bond stretching and carbon-hydrogen bending, reflecting a change of hybridization from sp in acetylene to sp2 or sp3 in the polymer. Similar features were reported by some authors in the Raman spectra of acetylene adsorbed at the gas-solid interface of alumina or of alumina-supported rhodium and and by one of us for adsorption on a roughened gold electrode.6 These surface bands were attributed either to formation of a metal-acetylene complex4 or to oligo- and polymerization of the acetylene,5*6all of which involve some change of hybridization in the original bond. As most of the adsorbing substrates were not propitious to surface enhancement and the surface spectraH were close to the Raman spectrum of polyacetylene,26 the new bands were assumed to be due primarily to N R scattering rather than to SERS. It was therefore of considerable interest to determine in the present case whether the bands observed in the vicinity of 1100 and 1500 cm-' were due to bulk Raman scattering or whether they resulted from some surface enhancement mechanism. Strong evidence that the bands we detected at about 1100 and 1500 cm-l were caused by SERS, and not by NR scattering, from some species newly formed on adsorption, was obtained by determining Raman spectra on the unroughened surfaces of both plain and rhodium-modified silver. No Raman bands were detected in the 1000-1 600-cm-I region from acetylene adsorbed either on a thoroughly polished unmodified silver electrode in acidic environment (0.1 M HCIO,) or on a thin rhodium layer over a smooth silver electrode under the same conditions. The rhodium deposition on the polished silver varied from amounts that would be equivalent to less than 1 monolayer to amounts corresponding (26) Harada, 1.; Furukawa, Y . ;Tasumi, M.; Shirakawa, H.; Ikeda, S. J . Chem. Phys. 1980, 73. 4746.
Feilchenfeld and Weaver to 3 equivalent monolayers on a roughened electrode. Since the roughening procedure is known to increase the area of the silver surface by a factor of 1.5-2.0,24the amounts of rhodium deposited on the smooth electrodes roughly corresponded to about 1.5-6 equivalent monolayers, well within the rhodium thickness range for which intense Raman signals were obtained when the silver substrate was roughened. If the bands at 1100 and 1500 cm-' had been due to N R scattering from compounds formed on the surface, they would have been observed with a polished as well as with a roughened substrate. Therefore, our results clearly suggest that surface enhancement rather than N R is at the origin of these bands. Another series of observations relevant to the origin of the intense new bands was the slow weakening of the signals when increasingly thick layers of rhodium or platinum were deposited on the electrodes (Figures 4 and 5 , and compare parts f and i of Figure 7). N R scattering from substances formed on the surface should have been unaffected by changes in the amount of transition metal, while a surface enhancement originating in the underlying silver or gold substrate would have been expected to yield the observed waning of the signals when the amount of rhodium or platinum became larger and the physical distance between the adsorbate and the substrate in~reased.~' The intensity changes with growing amounts of transition metals seem, however, to have been more complex than a simple weakening of the electromagnetic influence of the silver or gold. The initial increases of intensity observed when small amounts of rhodium or platinum were deposited on the electrodes may have been caused by chemical effects occurring on the metallic surfaces. The addition of transition metals, even in submonolayer quantities, probably resulted in more efficient or faster reactions and subsequently in better coverage of the electrodes with the reaction product, yielding at the beginning more intense SERS signals. If this assumption is correct, detection of the 1100- and 1500-cm-' bands may possibly have required adsorption of the acetylene moiety at a site where the transition metal and the substrate were both available for binding, Le., at the edge of a "pinhole" present in the rhodium or platinum layer. The number of pinholes would decrease with growing rhodium or platinum thickness, leading to the disappearance of the Raman peaks. In this respect considerable differences were noted in the behavior of silver and of gold. The l 100- and 15OO-cm-' bands were easily detected on unmodified gold in acidic solutions over a wide range of potentials, and sometimes in neutral media as well, a t potentials more negative than 0.6 V, but these features were never observed for acetylene adsorbed on unmodified silver, under either neutral or acidic conditions (Figures 3b, 4a, and 5a). Their Occurrence clearly required the presence of a rhodium or platinum overlayer on the silver electrode, in an acidic supporting electrolyte. This observation could indicate that no rehybridization took place on unmodified silver, as opposed to what happened on unmodified gold. On the other hand, the appearance of intense bands after rhodium or platinum deposition could point to a stabilizing effect of the transition-metal overlayer on the roughened Ag surface, resulting in easier SERS detection, rather than to a specific chemical interaction of the transition metal with acetylene. The lack of stability of roughened silver surfaces in acid has been noticed before,*& although good SER spectra have been reported on silver at even higher acid concentrations than those used here.= Whatever the enhancement mechanism, the absence of Raman bands on unroughened electrodes and the inverse correlation between the intensity of the signals and the thickness of the transition-metal overlayer both confirm that we observed a surface rather than a bulk phenomenon. They also show that, in spite of the unexpectedly large SERS enhancements detected with the rhodium or platinum overlayers, the surface enhancement is due mainly to the silver or gold substrate. The origin of 1100- and (27) (a) Murray, C. A,; Allara, D. L. J . Chem. Phys. 1982.76, 1290. (b) Cotton, T. M.; Uphaus, R. A.; Mobius, D. J . Phys. Chem. 1986, 90,6071. (c) Gosztola, D.; Weaver, M . J . Longmuir 1989, 5, 776. (28) Plieth, W . J.; Schmidt, P.: Forster, K. J. DECHEMA-Monogr. 1988, 112. 243.
Adsorption of Acetylene on Ag and Au Electrodes
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7777
bands assigned to rehybridized acetylene was associated with a 15OO-cm-' bands we observed seems to be essentially different from measurable flow of current through the system. It is quite likely, that of the NR features reported previo~sly.~,~ under these circumstances, that hydrogen atoms were formed at Chemisorption of Acetylene. The assignment of the 1100- and the electrode, leading to the conversion of acetylene into vinyl 1500-cm-l bands to C-H bending mixed with C-C stretching and These radicals could have initiated polymerization on to C=C stretching vibrations seems to be well e s t a b l i ~ h e d , ~ ~ , ~radicals. ~ the surface or further hydrogenation of the adsorbed species. Some and the presence of these peaks in Raman spectra of adsorbed degree of polymerization of the acetylene on the electrode, as acetyleneH is usually taken to imply rehybridization from sp to sometimes surmised>6 may well have occurred in the present case. sp2 or even sp3. We therefore assumed that a similar rehybriCatalytic hydrogenation of alkynes on transition metals such as dization had occurred in our electrochemical systems whenever rhodium and platinum is w e l l - k n ~ w ngold ; ~ ~has ~ ~ been reported the 1100- and 1 5 0 0 - ~ m signals -~ were detected. Nevertheless, it to have similar catalytic proper tie^.^' Electrochemical cathodic is not clear which chemical changes took place in the acetylene hydrogenation of alkynes will usually result in a mixture of olefinic molecule and caused these intense signals. The adsorption of and saturated products.3z It is significant in this respect that in alkynes on unmodified silver and gold in neutral electrolytes, neutral electrolytes the 1100- and 1500-cm-I Raman bands appreviously reported? yielded Raman bands only in the 1500-1600peared on gold only as weak signals at very negative potentials and 1800-2300-cm-' regions, leading us to postulate a weak UT (-0.6 V or more negative), while in acidic solutions, where protons complexation of the molecules with the surface and some degree were abundant, the same bands could be observed at zero or of rehybridization of the bond. Most of the spectra observed positive potentials and then became substantially more intense in the current study for acetylene adsorbed on rhodium or platinum when the potential was made negative. The catalytic effect of overlayers under neutral conditions were very similar: the 1 100rhodium and platinum was also clearly visible, as shown for inand 1500-cm-I features were very weak or nonexistent, while C 3 C stance by the striking differences between the intensities of the stretching bands were present in the 1800-2300-cm-' region 1100- and 1500-cm-I bands on gold and on rhodium-modified gold (Figures 3 and 7a-c). The same kind of UT bonding was probably (Figure 7). The fact that no bands were detected a t these frepresent here too. quencies on rhodium-modified silver in neutral electrolyte may In acidic media, however, as well as on gold or modified gold be due to the different catalytic properties of silver as compared in neutral solutions at negative potentials, where the intense new to gold. bands appear at 1100 and 1500 cm-I, the formation of a complex The fast appearance of the SERS bands when the potential was between the metal and acetylene seems far less likely. The same stepped to a negative value seems to agree with the hydrogenation vibrational frequencies (1095 and 1475 cm-I) were observed on hypothesis. The much slower decrease in the intensities of the both gold and rhodium-modified gold, and again similar wavebands at positive potentials reflects the gradual desorption of the numbers (1 125 and 15 10 cm-I) were measured for both rhodiumreaction products when no further hydrogen was produced, rather and platinum-modified silver, while acetylene adsorbed on rhothan a reverse dehydrogenation reaction. Although the catalytic dium-modified silver and on rhodium-modified gold yielded difelectrochemical hydrogenation assumed here is far from proven, ferent frequencies. Specific adsorbate to transition metal bonding and further investigation is clearly indicated, such a reaction seems therefore to have played little role in determining those nevertheless seems perfectly reasonable. frequencies. A shift to higher wavenumbers when passing from the gold to the silver substrate has been observed and discussed Conclusion before6*26and was ascribed to the switch from the 647.1-nm Kr+ We have obtained intense Raman signals from acetylene adto the 514.5-nm Art laser excitation, although bands have been sorbed on thin layers of catalytically active transition metals over reported at the lower wavenumbers for acetylene adsorbed on silver or gold substrates. These bands are due to surface enalumina-supported rhodium with Art laser e ~ c i t a t i o n . This ~ hancement rather than to bulk scattering, and the presence of the suggests that the bands detected a t about 1100 and 1500 cm-' underlying silver or gold is essential for their appearance. With are in fact of identical origin on all metals and that no distinctive deposition of increasingly thick layers of rhodium or platinum the metal to acetylene bonds were formed between the adsorbate and intensity of the peaks passes through a maximum and then dethe different surfaces. In contrast, in a recent study of pyridine adsorbed on silver and on rhodium-modified silver,z0cwe observed creases. While for pyridineZk we have been able to detect measurable very clear frequency shifts after rhodium deposition, which infrequency shifts and to distinguish between the molecules adsorbed dicated a difference in the bonding or complexation of pyridine on the silver substrate and those adsorbed on the thin rhodium to silver and to rhodium. overlayer, we observed a different behavior in the present case. In the present case, the lack of sensitivity of the acetylene spectrum to the natures of the substrate and of the overlying Acetylene adsorbed on transition metals exhibits intense new bands transition metal may well show that a chemical reaction, rather at 1100 and 1500 cm-l, characteristic of a rehybridized species than a complexation, took place on the electrode. Such a reaction with frequencies essentially independent of the underlying surface. could yield a rehybridized product, identical on all metals and We suggest these bands to be due to the product of an electroonly loosely bound to the surface; the geometric structure and chemical hydrogenation taking place on the metal. Whether this vibrational frequencies of this species would only slightly depend assumption is justified or not, and irrespective of the actual reaction occurring with acetylene, the intense bands we observed in the on the nature of the underlying surface. It is notable, in this connection, that the bands attributed to the rehybridized adsorbate spectra show the potential value of the SER technique. Study were relatively sharp, their widths at half-maximum being 40 cm-l of surface processes on catalytically active metals by Raman a t 1100 cm-I and 30 cm-' at 1500 cm-I, as compared to widths spectroscopy seems to be considerably more promising than was of 60-1 50 cm-l reported previously* for the adsorption of alkynes previously believed. on unmodified silver and gold in neutral solutions; this indicates less heterogeneous bonding to the roughened surface and confirms (29) Hutchins, R. 0.;Hutchins, M.G . In The Chemistry of Functional the assumption of relatively weak adsorption to the metal. Groups, Supplemenf C; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester. The marked dependence of the band intensities on both the pH 1983;p 571. (30)Boitiaux, J. P.; Cosyns, J.; Robert, E. Appl. Caral. 1987. 32, 169. of the supporting electrolyte and the external potential applied (31) Bond, G.C.; Sermon, P. A. J . Chem. Soc., Chem. Commun. 1973, to the SERS cell suggests that an electrochemical hydrogenation 444. of the adsorbed acetylene may have taken place in the system. (32) Utley, J. H.;Lines, R. In The Chemistry of the Carbon-CarbonTriple As pointed out, the appearance of the new 1100- and 1500-cm-l Bond Patai, S., Ed.; Wilcy: Chichester, 1978;p 739.
