Anal. Chem. 1997, 69, 2349-2353
Vibrational Coupling as a Probe of Adsorption at Different Structural Sites on a Stepped Single-Crystal Electrode Chung S. Kim† and Carol Korzeniewski*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Adsorption of carbon monoxide at step and terrace sites on a Pt(557) ≡ Pt(s)-[6(111) × (100)] electrode was detected with infrared spectroscopy. Vibrational coupling between adsorbates provided insights into the assembly of molecules at the different structural sites. The intermolecular coupling was weak at low coverages as CO ordered along the steps. For coverages between 40 and 70% of saturation, separate bands assignable to CO on steps and CO on terraces appeared. Coupling across this coverage range was markedly weaker on Pt(557) than on the structurally related Pt(335) ≡ Pt(s)-[4(111) × (100)] electrode surface. The results indicate that, after the steps fill, CO populates the terraces on Pt(557) at random rather than by ordering in alignment with the steps. At coverages below saturation, vibrational bands assignable to CO molecules at step and terrace sites are affected differently by changes in electrode potential. The potentialinduced spectral changes for the terrace CO bands are similar to those of Pt(111)/CO, but the step CO bands show deviations from this trend at hydrogen adsorption potentials. The surfaces of solid catalysts have high coverages of steps and kinks (e.g., low coordination sites). Compared to atomically smooth terrace planes, these sites have been shown to reduce the activation energy for bond cleavage and increase the strength of adsorption.1,2 On transition metal surfaces, several analytical methods have been used to investigate the special geometric and electronic influences of low-coordination atoms.1-4 Due to its versatility, infrared spectroscopy has been used under a variety of conditions to probe adsorption and reactivity at different structural sites on metal surfaces (cf. refs 5-14). Early work focused on the detection of CO adsorbed at steps and † Present address: Naval Research Laboratory, Chemistry Division, CODE 6174, Washington, DC 20375-5320. (1) Somorjai, G. A. J. Phys. Chem. 1990, 94, 1013. (2) Somorjai, G. A. Surf. Sci. 1991, 242, 481. (3) Johnson, D. F.; Weinberg, W. H. Science 1993, 261, 76. (4) Henderson, M. A.; Szabo, A.; Yates, J. T., Jr. J. Chem. Phys. 1989, 91, 7245. (5) Greenler, R. G.; Burch, K. D.; Kretzschmar, K.; Klauser, R.; Bradshaw, A. M.; Hayden, B. E. Surf. Sci. 1985, 153, 338. (6) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G. Surf. Sci. 1985, 149, 394. (7) Jansch, H. J.; Xu, J.; Yates, J. T., Jr. J. Chem. Phys. 1993, 99, 721. (8) Xu, J.; Yates, J. T., Jr. J. Chem. Phys. 1993, 99, 725. (9) Borguet, E.; Dai, H.-L. J. Chem. Phys. 1994, 101, 9080. (10) Wang, H.; Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. J. Chem. Phys. 1995, 103, 2711. (11) Sinniah, K.; Reutt-Robey, J. E.; Robinson-Brown, A.; Doren, D. J. J. Chem. Phys. 1994, 101, 764.
S0003-2700(96)01306-6 CCC: $14.00
© 1997 American Chemical Society
terraces of single-crystal materials in ultrahigh vacuum (UHV).5,6 These studies showed that the adlayer vibrational modes are strongly affected by dipole-dipole and other vibrational coupling forces.5,6,15-17 As a result, when both types of sites are occupied, each adlayer vibrational mode consists of both COs and COt motion, where the subscripts “s” and “t” signify CO bound to step and terrace sites, respectively.15 Only in the absence of COsCOt coupling will there exist completely independent COs and COt vibrational modes. Adsorbate coupling in adlayers formed at stepped surfaces has been most extensively studied for CO adsorption on platinum. Experiments in UHV have shown that the platinum-CO heat of adsorption is higher at step and kink sites than on close-packed (111) terrace planes.18,19 As a consequence, CO preferentially adsorbs at low-coordination sites on platinum at ambient temperatures and begins to occupy terrace planes after the step and kink sites essentially fill.4,6,19 In both electrochemical and UHV environments, it has been possible to detect Pt-COs vibrational bands without interference from CO on terraces at low CO coverages. For higher coverages, between about 30 and 60% of saturation, separate bands assignable to COs and COt vibrations have been resolved.6-8,10,12,13,20-22 The vibrational modes detected at intermediate coverages are not associated with pure COs or COt motion, but the extent of coupling is not great. Since for parallel dipoles the dipole coupling interactions scale as d-3,16,17,23,24 where d is the distance between CO molecules, the intermolecular interactions can be weak at intermediate coverages. Near saturation, dipole coupling becomes strong, and separate COs and COt bands have not been resolved. Although coupling between adsorbates limits infrared spectroscopic detection of molecules at different structural sites on surfaces, the spectral band shifts and intensity alterations provide (12) Reutt-Robey, J. E.; Doren, D. J.; Chabal, Y. J.; Christman, S. B. J. Chem. Phys. 1990, 93, 9113. (13) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Chem. Phys. 1994, 101, 9113. (14) Shin, J.; Korzeniewski, C. J. Phys. Chem. 1995, 99, 3419. (15) Brandt, R. K.; Greenler, R. G. Chem. Phys. Lett. 1994, 221, 219. (16) Leibsle, F. M.; Sorbello, R. S.; Greenler, R. G. Surf. Sci. 1987, 179, 101. (17) Greenler, R. G.; Leibsle, F. M.; Sorbello, R. S. Phys. Rev. B 1985, 32, 8431. (18) Collins, D. M.; Spicer, W. E. Surf. Sci. 1977, 69, 85. (19) Luo, J. S.; Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. Surf. Sci. 1992, 274, 53. (20) Lambert, D. K.; Tobin, R. G. Surf. Sci. 1990, 232, 149. (21) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484. (22) Kim, C. S.; Korzeniewski, C.; Tornquist, W. J. J. Chem. Phys. 1994, 100, 628. (23) Persson, B. N. J.; Ryberg, R. Phys. Rev. B 1981, 24, 6954. (24) Severson, M. W.; Stuhlmann, C.; Villegas, I.; Weaver, M. J. J. Chem. Phys. 1995, 103, 9832.
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information about adlayer intermolecular forces and the spatial arrangement of adsorbates. Vibrational coupling has been used to advantage in the identification of active sites on supported platinum catalysts.25 On platinum electrodes, adsorbate intermolecular coupling has served as a probe of CO island formation26-28 and CO assembly at steps and terraces.13 In the present study, the ordering of CO molecules at different structural sites on a Pt(s)-[6(111) × (100)] ≡ Pt(557) electrode was detected by observing the effects of vibrational coupling on infrared spectra. The adlayer intermolecular coupling was weak at the lowest coverages, as CO ordered along step edges, and increased gradually as the steps filled. At intermediate CO coverages, the coupling between COs and COt molecules was markedly weaker on Pt(557) than on structurally related Pt(335). These results suggest that, after the steps fill, CO populates the terraces on Pt(557) at random, rather than in registry with the steps. EXPERIMENTAL SECTION Electrolyte solutions were prepared from perchloric acid (Aldrich, redistilled, 99.999% purity) using distilled water that was further purified with a Barnstead Nanopure II cartridge system followed by oxidation processing under ultraviolet irradiation (Barnstead Organic-pure). The stock dosing solution was prepared from natural carbon monoxide gas (Matheson, 99.5%). The Pt(557) disk electrode (10 mm diameter × 2 mm thick) was purchased from Aremco (Ossining, NY). The orientation, specified to within (1°, was verified in our laboratory by X-ray back-diffraction. To facilitate electrical contact, a thin platinum wire was spot welded to the back of the crystal. Before electrochemical experiments, the front face of the crystal was polished with 0.05 µm alumina, after which the electrode was sonicated to remove polishing debris. The platinum wire was threaded through a glass tube and pulled until the disk was about 1.5 cm from the end of the tube. The electrode assembly was secured to a clamp on a ring stand while the disk annealed (5-7 min) in a hydrogen/air flame. As described previously,13,29 the glowing disk was cooled in a three-neck vessel above a pool of ultrapure water that was under continuous purge with ultrahighpurity argon (Scott Specialty Gases, 99.999%). The disk was submerged in the water just as the red glow disappeared. After quenching, the back of the disk was pulled against the glass tube and the front face suspended in ultrapure water while the edges were sealed with Teflon tape.13,29 A drop of ultrapure water protected the exposed platinum during transfer to the electrochemical cell. The electrochemical cell30 contained a platinum ring counter electrode and an external reference electrode compartment, which connected to the sample chamber via a wetted stopcock and Luggin capillary. In all experiments, the external compartment held a saturated calomel reference electrode (SCE). The cell body was constructed from Kel-F. A trapezoidal CaF2 window (Solon Technologies, Solon, OH) with bevel cuts at 60° mounted to the front face. In all experiments, potentials are reported with respect to the SCE. (25) Paul, D. K.; Beebe, T. P., Jr.; Uram, K. J.; Yates, J. T., Jr. J. Am. Chem. Soc. 1992, 114, 1949. (26) Chang, S. C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (27) Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391. (28) Korzeniewski, C.; Severson, M. W. Spectrochim. Acta 1995, 51A, 499. (29) Shin, J.; Tornquist, W. J.; Korzeniewski, C.; Hoaglund, C. S. Surf. Sci. 1996, 364, 122. (30) Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437.
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Figure 1. Cyclic voltammogram of flame-annealed Pt(557) in 0.1 M HClO4 at 50 mV/s. The scan starts at 0.0 V. Arrows indicate the direction of the scan.
At the start of each experiment, the electrochemical cell was filled with a measured volume of 0.1 M HClO4. The working electrode was transferred to the cell, and a cyclic voltammogram was recorded to verify surface cleanliness and order. Voltammograms were obtained at a scan rate of 50 mV/s over the potential range -0.25 to 0.60 V (vs SCE). A representative voltammogram of Pt(557) in 0.1 M HClO4 is shown in Figure 1. Following the initial potential cycle, the working electrode was held at -0.20 V while an aliquot of CO-saturated 0.1 M HClO4 was added to the cell to bring the CO concentration in the cell to about 5 × 10-5 M. The electrode was held in this solution for 0.5-15 min, depending upon the desired CO surface coverage.26 After the dosing period, the electrode was positioned against the CaF2 window, and the CO in bulk solution was removed by purging with argon. Infrared spectra were obtained with a Digilab FTS-40 Fourier transform infrared spectrometer equipped with a liquid nitrogencooled MCT detector. Interferograms were recorded at a resolution of 4 cm-1, and a triangular apodization function was used in the Fourier transformation. A variable angle reflection accessory (Spectra-Tech, Model 502), modified to accommodate the spectroelectrochemical cell, was used to position the infrared beam focus on the polished surface of the working electrode and to collect the specularly reflected radiation. Spectra were recorded by holding the electrode at the indicated potentials while 1024 interferograms were coadded, signal averaged, and Fourier transformed to give (sample) single-beam spectra. A reference single-beam spectrum was recorded at the end of each experiment with the electrode at +0.6 V (vs SCE), where CO was removed from the surface by oxidation to CO2. Absorbance spectra were computed from the ratio of sample and reference single-beam spectra. As described previously,21,26 surface coverages were determined from the integrated absorbance of the CO2 peak at 2342 cm-1 and are reported as fractions relative to saturation (θ/θmax). RESULTS Voltammetry. Figure 1 shows a cyclic voltammogram of a freshly annealed Pt(557) electrode in 0.1 M HClO4. Features in the hydrogen adsorption and double-layer regions are consistent with previous reports of Pt(557) voltammetry in 0.1 M perchloric acid.31 The voltammogram provides an indication of the quality of the crystal and the surface preparation techniques. The sharp, symmetrical waves at -0.10 V are associated with hydrogen
Figure 2. Spectra of CO adsorbed at different coverages on a Pt(557) electrode maintained at a potential of 0.1 V (vs SCE) in 0.1 M HClO4. The surface coverages are expressed as a fraction of saturation.
adsorption/desorption on (100) symmetry sites.31-33 The peak heights in this region have been shown to increase with the surface step density.31-33 In the double-layer region, peaks at ∼0.4 V are associated with anion-dependent adsorption states characteristic of ordered (111) terraces.31,34 These peaks are strongest for Pt(111). The current densities for the waves at -0.1 V and +0.4 V in Figure 1 are intermediate between the Pt(111) and Pt(335) responses reported in our earlier studies.21 Infrared Spectroscopy. Surface Coverage Effects. Figures 2 and 3 show infrared spectra of CO adsorbed at a Pt(557) electrode over a wide range of coverages. Spectra were obtained with the electrode held at 0.1 V, a potential in the classical double-layer region. Bands appear for CO in atop (2015-2070 cm-1) and bridging (1800-1877 cm-1) coordination environments.26,27,35 In the low coverage (θ/θmax ) 0.05-0.26) spectra of Figure 2, the band at 2017 cm-1 can be assigned to atop CO at edge sites on Pt(557), consistent with previous vibrational band assignments for CO adsorbed at stepped platinum surfaces.6-8,12,13,20-22 For bridge-bonded CO, a single feature appears at coverages below 5% saturation. The position (1800 cm-1) is intermediate between the vibrational mode energies for CO in two- and threefold bridging sites. On a Pt(111) electrode, bands for two- and threefold bridging CO are potential dependent but appear at about 1850 and 1773 cm-1, respectively.35 In accord with the quantitative models of edge CO adsorption on Pt(335) derived by Luo and co-workers,19 the 1800 cm-1 band in Figure 2 most likely arises from CO in twofold bridging sites along step edges. The low energy of the mode is attributable to a combination of strong surface-CO bonding at step sites4,6,19 and weak intermolecular coupling at the low CO coverages.16,26 A second bridging CO feature appears in Figure 2 for θ/θmax ) 0.16-0.26, toward the (31) Markovic, N. M.; Marinkovic, N. S.; Adzic, R. R. J. Electroanal. Chem. 1988, 241, 309. (32) Rodes, A.; Clavilier, J. J. Electroanal. Chem. 1993, 344, 269. (33) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245. (34) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211. (35) Villigas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648.
Figure 3. Same as Figure 2, but for higher CO surface coverages.
high-energy side of the 1800 cm-1 band. The new band is assignable to CO in twofold bridging sites and may indicate the start of CO adsorption on the terrace planes. At θ/θmax ) 0.36, the atop CO band broadens, and by θ/θmax ) 0.49, the band splits into different peaks. Infrared spectroscopy studies of CO adsorbed on stepped platinum surfaces have assigned the higher energy band in the pair to vibrational modes of CO on terrace sites and the lower energy band to modes of CO bound to step edges.6-8,10,13,20-22 Similar COs and COt site assignments have been made for the two bridging CO features (1800-1860 cm-1).7,8,22 As the coverage increases from 0.49 to 0.75, the two atop CO peaks undergo a transition from a state of approximately equal intensity to one in which the higher energy (COt) feature is dominant. A similar intensity shift occurs for the two bridging CO bands in each spectrum. The coveragedependent intensity shifts are characteristic of intermolecular coupling.24,26,36-38 In the coupled system, the higher energy mode of each pair has the most in-phase C-O stretching vibrational motion and hence a larger dynamic dipole moment normal to the surface.37 As a result, the high-energy C-O stretching mode has the strongest band intensity.37 At high coverages (θ/θmax > 0.75), the spectra display effects of strong intermolecular coupling. Atop and bridging CO features appear as single bands at 2067 and 1877 cm-1, respectively. The bandwidths narrow near saturation coverage, as the adlayer orders and inhomogeneous broadening is reduced. A plot of atop CO peak position vs CO surface coverage for adsorption at Pt(557) is shown at the top of Figure 4. The triangle symbols indicate the position of the lower energy (assigned to COs) vibrational bands. The closed squares track the position of the higher energy (assigned to COt) bands. A corresponding plot for Pt(335)/CO is displayed at the bottom of Figure 4. Comparison of the two plots indicates that the range of coverages over which COs and COt bands appear simultaneously is wider for Pt(557) than for Pt(335). As discussed below, this response can (36) Hayden, B. E. In Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum Press: New York, 1987; Vol. 1, p 267. (37) Hollins, P.; Pritchard, J. Prog. Surf. Sci. 1985, 19, 275. (38) Willis, R. F.; Lucas, A. A.; Mahan, G. D. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1983; Vol. 2, p 59.
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Figure 6. Same as Figure 5, except θ/θmax ) 0.52.
Figure 4. Plot of atop CO band energy as a function of the fractional CO surface coverage relative to saturation for Pt(557)/CO (top) and Pt(335)/CO (bottom). 4: Vibrational mode associated with CO on step sites. 9: Vibrational mode associated with CO on terraces.
Figure 5. Spectra of CO adsorbed on a Pt(557) electrode at a coverage θ/θmax ) 0.26. Spectra were recorded as the electrode potential was stepped in sequence from -0.2 to 0.1 V, as indicated.
spectra indicate that major structural changes occur in the CO adlayer as the potential is stepped between values in the hydrogen adsorption and double-layer regions (-0.1 to 0.0 V). Figure 6 shows spectra of Pt(557)/CO at different potentials for θ/θmax ) 0.52, where atop bands for both edge and terrace CO are discernible. At the most negative potential, -0.2 V, the bridging CO band at 1831 cm-1 appears dominant. In stepping the potential from -0.2 to -0.1 V, intensity shifts from the bridging to the atop CO bands, and the bridging feature splits. At double-layer potentials (0.0-0.1 V), the atop CO bands become stronger than the bridging CO bands. The atop CO bands show signs of strong coupling at the most positive potential, +0.1 V, as the intensity of the lower energy (2030 cm-1) band in the pair decreases and the higher energy band moves to 2053 cm-1. The spectra indicate that the strong intermolecular coupling interactions shift from the bridge-bonded CO molecules at negative potentials to the atop molecules at double-layer potentials. Near saturation (data not shown), the relative intensity of the atop and bridging CO bands remained constant with changes in potential. The position of the atop CO band displayed a linear shift with potential of approximately 30 cm-1/V, similar to those of Pt(111)/CO26,27,40 and Pt(335)/CO near saturation.21
be correlated with differences in the extent of intermolecular coupling at the two surfaces. Surface Potential Effects. Figure 5 shows the effect of electrode potential on the Pt(557)/CO adlayer for θ/θmax ) 0.26. At -0.2 and -0.1 V, the dominant bridging CO feature in each spectrum indicates that the CO site occupancy is primarily bridging under these conditions.24,35,39 The weaker atop CO band appears at 2010 cm-1, and the position does not change with applied potential over this region. When the potential is stepped from -0.1 to 0.0 V, from the hydrogen region into the classical double-layer region, band intensity shifts toward the atop CO feature. In addition, the atop CO band energy increases to 2017 cm-1, and the bridging CO feature broadens and displays a shoulder toward the highenergy side of the main 1800 cm-1 peak. The atop CO band upshifts further when the potential is increased to 0.1 V. The
DISCUSSION Surface Coverage Effects. Figure 4 compares coveragedependent shifts in the atop CO band position for adsorption at Pt(557) and Pt(335) electrodes. The two surfaces show significantly different responses at low and intermediate CO coverages. The clearest differences occur between θ/θmax ) 0.4 and 0.7. On Pt(557), spectral bands assignable to COs and COt are discernible across this range, while on Pt(335), these bands couple fully for θ/θmax > 0.5.21,22 Compared to Pt(335), the wider terraces on Pt(557) (six-atom vs four-atom on Pt(335)) may allow a greater separation of COs and COt molecules and thereby diminish COsCOt coupling at intermediate coverages. Observation of COs and COt vibrational bands on Pt(557) up to about 70% saturation indicates that the occupation of terraces does not occur by successive filling of rows starting with sites immediately behind the step atoms. The response suggests that the terrace sites on
(39) Roth, J. D.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem. 1990, 288, 285.
(40) Leung, L.-W. H.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988, 92, 6985.
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Pt(557) fill essentially at random, possibly affected by CO-CO repulsions.41 Between θ/θmax ) 0 and 0.4, the atop CO band position is nearly constant at Pt(335) but shifts up in energy with increasing coverage on Pt(557). The response of the latter system is similar to that of Pt(111)/CO at low coverages; however, the shift in the atop CO band position with increasing coverage is twice as great on Pt(111) over this range.26 The low-coverage spectral responses may be due to differences in surface step density. Since the step density for Pt(335) is about 30% greater than that for Pt(557), terrace site occupation will commence at lower coverages on Pt(557). It is possible that broadening of the atop CO band at θ/θmax ) 0.36 in Figure 2 reflects the initial CO occupation of terrace sites. The COs-COt interactions that result may be responsible for the gradual upshift of the atop CO band energy. It would be valuable to compare spectral properties of CO on Pt(335), Pt(557), and Pt(111) electrodes as a function of the absolute coverage of CO on each surface. Currently, such a comparison is limited by incomplete knowledge of the adlayer coverages at saturation. In UHV, it appears that step structures support higher atop CO coverages than (111) terrace planes.4,19 The high step-site populations likely contribute to the strong intermolecular couping on Pt(335). Surface Potential Effects. In Figures 5 and 6, the potentialdependent changes in the relative intensity of the atop and bridging CO features are consistent with responses observed for CO on other platinum surfaces.24,35,39 In situ scanning tunneling microscopy measurements have confirmed that these spectral changes correlate with a shift in the CO site occupancy from bridging toward atop as the potential is stepped positive.24,35 It appears that bridging coordination is favored at negative potentials to accommodate the increased metal-dπ f CO-2π* charge donation.27,39 For the low-coverage spectra in Figure 5, it is notable that the position of the atop CO band is constant between E ) -0.2 and -0.1 V. For all CO coverages on Pt(111), and at higher coverages
on stepped platinum surfaces, the atop CO band shifts linearly with potential at a rate of 30-45 cm-1/V.21,26,27,40 The potential (or electric field)-induced COs band shifts, measured at low coverages on stepped platinum surfaces, have displayed marked deviations from this trend in both electrochemical21 and UHV experiments.10,20 On Pt(335) electrodes, the atop COs band position is constant at negative potentials (vs SCE) but shifts linearly at a rate of about 80 cm-1/V at positive potentials. Related electric field-induced shifts of similar magnitude have been observed for Pt(335)/COs in UHV.10,20 The response of COs at negative electrode potentials has been discussed in terms of possible effects of coadsorbed hydrogen.21 However, in recent UHV experiments, hydrogen adsorption did not significantly reduce the electric field-dependent COs band shifts for Pt(335)/ CO.42 The factors that account for the unexpected potential and electric field-dependent responses for COs remain uncertain; however, the present work shows that these effects, in particular the insensitivity of COs to changes in potential at negative electrode potentials, are not limited to Pt(335)/CO.
(41) Schweizer, E.; Persson, B. N. J.; Tushaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49. (42) Wang, H.; Tobin, R. G.; Lambert, D. K. J. Chem. Phys. 1994, 101, 4277.
AC961306K
CONCLUSIONS Effects of intermolecular coupling on infrared spectra of adsorbates provide information about the arrangement of molecules at different surface sites. In the present study of CO adsorption at a Pt(557) electrode, weak coupling between CO at steps and CO on terraces over a wide range of CO coverages suggests that the terraces fill at random, possibly affected by strong CO-CO repulsive forces. The CO adlayer on Pt(557) undergoes potential-induced structural changes in a manner similar to that of CO on Pt(335). ACKNOWLEDGMENT The Office of Naval Research is gratefully acknowledged for support of this research. Received for review December 31, 1996. Accepted April 3, 1997.X
X
Abstract published in Advance ACS Abstracts, May 15, 1997.
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