J. Phys. Chem. 1993,97,6484-6491
6484
Infrared Spectroscopy as a Probe of CO Adsorption at Pt(335) under Aqueous Electrochemical Conditions Chung S. Kim, Wade J. Tornquist,+and Carol Korzeniewski' Department of Chemistry, University of Michigan, Ann Arbor, Michigan 481 09
Received: February 11, 1993; In Final Form: April 1, I993
Experiments measure the infrared spectral features for carbon monoxide adsorbed at Pt(335) (Pt(S)-[4( 111) X (loo)]) under aqueous electrochemical conditions. The coverage and potential dependence of the C-O stretching vibrational mode is measured for CO adlayers formed by dosing under potential control from electrolyte solutions containing dilute levels (ca. 4 X le5 M) of dissolved CO. Infrared bands assignable to CO chemisorbed at sites on the step edge and sites on the terrace in both on-top and bridging coordination environments are observed. At low coverages, spectral features appear that are attributable to CO at on-top and bridging sites on the step edge. Plots of on-top CO peak energy versus electrode potential are unexpectedly nonlinear over the range of low coverages, with slopes that approach zero at potentials in the classical hydrogen adsorption region and increase to large values at potentials in the double layer region. At intermediate coverages, spectral features appear that are assignable to on-top and bridging CO at both terrace and edge sites. The effects of dipole-coupling between terrace and edge CO adsorbates become apparent at intermediate coverages, as the intensity of the higher energy feature for on-top CO at terrace sites is enhanced at the expense of the lower energy feature for on-top CO at edge sites. At the highest coverages, dipole-coupling effects become a major factor in determining the appearance of the infrared spectral features; the on-top CO vibrational bands are identical to features observed for CO at Pt( 111) under analogous electrochemical conditions, while the spectral features of bridging CO coincide with those observed for CO at Pt( 100) in the high-coverage limit. Potentialand coverage-dependent structural transformations of the adsorbed CO adlayer are discussed within the context of models derived in earlier studies which probed CO adsorption at stepped platinum surfaces through the use of imaging and vibrational spectroscopic techniques.
Introduction An area of growing importance in electrochemistry concerns probing adsorption and reactivity at liquid-solid interfaces by using electrodes that are prepared from single-crystal materials with exposed surfaces of atomically well-defined structure.'" Research in this area developed with the refinement of procedures for preparing clean and well-ordered surfaces for study under electrochemical condition^.',^-^ Important work linked the techniques for preparing and characterizing metal surfaces in ultrahigh-vacuum (uhv) environments with classical electrochemical experiment^.'^^^^ With the emergence of methods for annealing crystallinesamples on the bench top,2,3969single-crystal materials have become more widely used in electrochemicalstudies and their greater accessibility has stimulated in situ spectroscopic experiments. Notable among in situ spectroscopic techniques is infrared spectroscopy,2J0which in recent years has proved powerful for examiningbonding and reactivity at electrodeswith well-defined surface structure.2J1-22 Many of these studies focused on examining infrared spectra of carbon monoxide chemisorbed at the low index planes of platinum and rhodium and demonstrated the usefulness of this system for probing the effect of the doublelayer environment and surface electronic structure on adsorbate bonding and reactivity.I1-l7 Other experiments explored the electrocatalytic reaction pathways of a series of small organic molecules and provided new insights into the surface chemistry.1a22 As a result of these efforts and key quantitative studies which employed polycrystallinee l e ~ t r o d e s improved , ~ ~ - ~ ~ procedures for controlling adsorbate surface coverage, reducing spectral interferences from bulk solution species, and evaluating mechanistic surface electrochemistry have emerged. These studies have
* Corresponding author.
Department of Chemistry, Eastern Michigan University, Ypsilanti, MI
48197.
contributed important tools for examining processes at electrified interfaces, and they provide a means to compare in a systematic way vibrational spectroscopic information obtained under electrochemical and uhv conditions. The present work extends these infrared techniques to examine the chemicial bonding of CO adlayers at the high index Pt(335) (Pt(S)-[4(111) X (lOO)] (Figure 1))surfaceplaneunderaqueous electrochemical conditions. The high-index planes of platinum have been examined in studies of adsorption and reactivity under ~ h and electrochemi~al~'3~ v ~ ~ ~conditions. ~ Of the experiments which employed vibrational spectroscopy, Pt(335) has supported the most detailed investigations. Initial work by H a ~ d e and n~~ co-workers used infrared reflection-absorption spectroscopy in combination with temperature-programmed desorption (TPD) to probe the coverage dependence of the CO adlayer structure. TPD experimentsrevealed two coveragedependent CO adsorption states and infrared experiments identified vibrational bands associated with CO bonded linearly to single platinum surface atoms (on-top or terminal coordination) at sites on the terrace and sites on the step edge. The infrared studies observed strong variations in theintensity and the position of thevibrational bands which could be accounted for by considering dynamic dipole+ coupling interaction^.^^.^ Our goal of the present study is to measure the coverage dependence of the CO spectral features under aqueous electrochemicalconditions with the aim of probing how the double-layer environment affects the occupancy and chemical bonding of CO at edge and terrace sites. Hayden and co-workers' initial infrared studies motivated subsequent vibrational experiments which aimed to explore local field effects at stepped surfaces by electroreflectancevibrational spectroscopy (EVS).34-41EVS is a uhv technique whichmeasures electromodulated infrared reflection-absorption spectra by scanning a diode laser sourcethrough an infrared absorption transition while applying an oscillating electric field across the adosrbate layer. The EVS signal measures changes in the adsorbate
0022-3654/93/2097-6484S04.00/0 0 1993 American Chemical Society
CO Adsorption at Pt(335)
Figure 1. Schematic diagram of the R(335) surface plane.
oscillator strength brought about by the applied electric field and is recovered by using phase sensitivedetection. EVS bandshapes are derivative-likewith zero crossing at the peak frequency of the infrared reflection-absorption band under zero applied field. EVS experiments which probed CO adsorption at Pt(335) documented the field dependent shift in C-O stretchingvibrational frequency as a function of CO surface coverage at 300 K.34 These studies identified vibrational bands associated with on-top CO at edge and terrace sites and observed strong, coverage dependent variations in the signal. The field-dependent response was large for CO at edge sites and unexpectedly approached zero for CO at terrace sites. A unipolar band was recorded for terrace CO at intermediatecoverages, and the unusual response was attributed to dynamic dipole coupling effects between edge and terrace CO dipoles. Factors which influenced the local electric field and electronic structure of CO at edge sites and at terrace sites were discussed in the context of how they might contribute to the variations in the field dependent signals. The emergence of EVS stimulated questions concerning the connection between the field-dependent adsorbate vibrational properties measured under uhv conditions and the potential dependent vibrational propertiesmeasured under electrochemical conditions. The earliest EVS experiments explored the fielddependent response of on-top CO at Ni(100) and Ni(ll0) at saturation coverage and 300 K.41 These studies showed how the field-dependent shift in C-0 stretching vibrational frequency measured by EVS (the “Stark tuningrate”) could bequantitatively related to the potential-induced shifts measured by infrared spectroscopy under electrochemical conditions. These experiments found a close correspondencebetween the tuning rates for CO at monocrystalline nickel in uhv and the potential induced shift in C-O stretching vibrational frequency measured for CO at polycrystalline platinum under electrochemical conditions. In recent years, more comprehensiveelectrochemicaland uhv studies have revealed conditions under which important differences are observed. At the Pt(ll1) surface plane, the C-O stretching vibrational frequency of terminal CO at saturation coverage is a strong function of electrochemical potential,12a.b.d,f whereas the corresponding field-dependent shift for this system measured in uhv is a factor of 4 Further, at Pt(335), the uhv tuning rate for CO bound to Pt( 111) terrace sites approaches zero.34A second goal of the present studies is to contribute corresponding vibrational data to further evaluate the connection between fielddependent vibrational properties of adsorbates in uhv and in electrochemical environments by documenting the potential dependence of the infrared spectral features for CO adsorbed at Pt(335) under aqueous electrochemical conditions. The most recent studies which probed the coverage dependence of the CO adlayer structure at Pt(335) used electron energy-loss spectroscopy (EELS) in combination with TPD.33 These experiments observed CO in both two-fold bridging and on-top (terminal) environments at sites on the terrace and sites on the step edge. The CO population at edge sites and at terrace sites was distinguishable on the basis of TPD spectra, and EELS data provided a measure of the bridging-to-terminal CO population ratio. By using this experimental data together with knowledge of the adlayer structures for CO at the closely related Pt( 112) (Pt(S)-[3( 111) X (loo)]) surface from electron stimulated
The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6485 desorption ion angular distribution (ESDIAD) experiments?la these studieshave provided detailed coveragedependentstructural models of the CO adlayer at Pt(335). The present work documents the coverage and potential dependence of the infrared spectral features for CO adsorbed at Pt(335) under aqueous eleutrochemicalconditions. In agreement with uhv vibrational studies, these in situ experiments observe infrared bands assignable to vibrational modes of CO adsorbed at sites on the step edge and sites on the terrace in both on-top and bridging coordination geometries. Potential-dependent alterations in the CO site occupancy are observed, and at low coverages, these alterations can be correlated with nonlinearities in plots of on-top CO peak energy versus electrode potential ( ~ ~ c 0 -plot^^^-^^). E The potential-dependent shift in edge CO vibrational frequency measured from these plots is significantly larger than values reported for low coverages of CO at Pt( 111) under similar experimental conditions. These differences are discussed within the context of the electric field enhancements predicted by Greenler and co-workers for Pt(335) edge sites4* and the uhv tuning rates measured by EVS.34 At high coverages, the spectral features for on-top CO at terrace sites and bridging CO at edge sites become dominant on account of dipole-coupling effe~ts,~ZJ~a,a and these findings demonstrate the importance of considering dipole-coupling interacttions when deriving models of the interface on the basis of in situ infrared spectra.
Experimental Section Electrolyte solutionswere prepared from perchloric acid (Baker Ultrex) or sulfuric acid (Aldrich, 99.999% purity) using distilled water that was further purified by using a Barnstead Nanopure cartridge system (Barnstead Nanopure 11)followed by oxidation processing under ultraviolet irradiation (Barnstead Organic-pure). Carbon monoxide was obtained from Matheson in 99.5% purity. The Pt(335) singlecrystal disk (7-mm diameter by 2 mm thick) and Pt( 111) single-crystaldisk (about 9 mm diameter by 2 mm thick) were obtained from the Materials Preparation Facility at Cornel1 University oriented to within *lo. The orientation was verified in our laboratory by X-ray back diffraction. The Pt(335) surface consists of (1 11) terraces four rows wide and (100) steps one atom high. A schematic model is shown in Figure 1 with the step atoms shaded for greater clarity. The macroscopic (335) surface plane is rotated 14.23’ from the (111) terrace planes. Electrical contact to the platinum crystals was made through a platinum wire spot welded to the back of the disk. Prior to electrochemical experiments, the crystal was flame annealed by heating to redness in a hydrogen flame. The crystal was cooled in a quartz chimney under a stream of ultra pure hydrogen [Scott Specialty gases, 99.999% purity] for approximately 3 0 4 0 s and then immersed in ultrapure water. All subsequent manipulations were performed with the crystal surface covered by ultrapure water. The electrochemicalcell used for spectroscopic measurements was constructed from Kel-F using the design previously reported by Seki and c o - ~ o r k e r s . The ~ ~ trapezoidal calcium fluoride window was purchased from Solon Technologies, Solon, OH. A platinum wire counter electrode and a saturated calomel (SCE) reference electrode were employed. The reference electrode was located in a compartment which was separated from the main sample chamber by a glass stopcock and connected via a Luggin capillary. The working electrode was assembled by mounting the platinum crystal disk on the end of a hollow glass plunger and sealing the edges of the disk with Teflon tape. The cell potential was controlled by a Pine AFRDE4 potentiostat (Pine Instruments, Grove City, PA). Infrared spectroscopic measurements were performed using a Digilab FTS-40Fourier transform infraredspectrometerequipped with a liquid nitrogen cooled MCT detector. The electrochemical
Kim et al.
6486 The Journal of Physical Chemistry, Vol. 97, No. 24, 1993
cell was positioned in the sample compartment such that infrared radiation was specularlyreflected from the surface of the working electrode. Spectral resolution was 4 cm-1. The working electrode surface was dosed with carbon monoxide by adsorption from an aqueous electrolyte solution containing dilute levels of CO (ca. 4 X l e 5 M).IZd Dosing was performed with the working electrode positioned in the spactroelectrochemical cell. The spectroelectrochemical cell was filled with a premeasuredvolume of fresh electrolytesolution that was degassed by bubbling with argon. Just prior to cell alignment, the dilute CO dosing solution was prepared by adding a volume of COsaturated electrolytesolution, measured by using a 1-mLsyringe, into the fresh electrolyte solution contained within the electrochemical cell. Dosing was accomplished by pulling the electrode away from the infrared transparent window for a time period which varied between 0.5 and 10 min. The electrode was then returned to its position, and the thin layer re-formed. In all experiments reported, dosing was performed with the electrode held at -0.20 V, in the so-called classical hydrogen region, where atomic hydrogen is assumed to be the predominant madsorbed species. Spectra were obtained by using the single potential alteration12s24b (or staircaselo) method. With the electrode held at a potential of 4 - 2 5 V, 1024 interferograms were collected and madded then transformed to a single beam spectrum. This spectral acquisition process was repeated as theelectrode potential was stepped in sequence to more positive values. The singlebeam spectrum obtained with the electrode held at +0.6 V was used as the reference spectrum, as it provided a background free of adsorbed CO on account of its removal from the surface by electrooxidation. Surface coverages were determined by converting the spectra to absorbance units and computing the integrated absorbance of the band arising from the CO2 oxidation product at 2343 cm-1.12d Coverages are reported relative to saturation (Ole-) to maintain consistency with theinitial infrared work of Hayden and mworker~.'~ However, in more recent work, Lambert and Tobin determined that the saturation coverage of CO at Pt(335) corresponds to B = 0.63.34a All potentials are reported with respect to the SCE.
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Results and Discussion Voltammetry. Figure 2 shows cyclic voltammograms for Pt(1 11)and Pt(335) in aqueous acid electrolytesolutions. Features of the Pt( 111) voltammogram have been well-documented,3,7.8,9a,c.12a.b,d.35b,36.38 and the voltammogram is presented here to calibrate our single-crystal electrochemistrytechniques against those of other workers in the field. The Pt( 111) voltammetric features (Figure 2A) are consistent with those observed for the ordered surface in aqueous perchloric acid electrolytt~?,149aC~"".~~ Similarly, features of the Pt(335) voltammetry in 0.1 M H2SO4 shown in Figure 2B are in qualitative agreement with previous voltammetric studies of this surface which employed aqueous sulfuric acid electrolytes.38b There is a sharp feature in the classical hydrogen region centered at about -0.05 V (SCE), and features associated with the unusual adsorption statecharacteristic of Pt( 111) voltammetry (ca. +0.4 V vs SCE)7-9are absent. The sharp feature at -0.05 V has been attributed to hydrogen adsorption/desorption at sites of (100) symmetry (sites at the ~ t e p e d g e ) .This ~~~ featurealsoappear sat-0.05 Vin thePt(335) voltammogram recorded in aqueous perchloric acid solution (Figure 2C). Further, the Pt(335) voltammetry in 0.1 M HC104 shows features at potentials above +0.4 V that are coincident with the unusual adsorption state at Pt( 111). In contrast to Pt(1 11) voltammetry in perchloric acid, however, the Pt(335) features above +0.4 V are shifted to more positive potentials and the peak current is greatly diminished. Experiments which measured the voltammetry of a series of related high index
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c/cvw#a Figure 2. Cyclic voltammograms for flame annealed Pt( 111) and Pt(335) singlacrystalelectrodesin aqucouselectrolytesolutionsat50mV/e. Scansstartfrom0.0VvsSCE. (A)Pt(lll)inO.l MHCIOd;(B)Pt(335) in 0.1 M HzSO4; (C) Pt(335) in 0.1 M HCIO4.
platinum surfaces from the [llO] zone in perchloric acid electrolytes observed similar voltammetric features at potentials associated with the unusual adsorption and Clavilier has predicted that the appearance of these features may be generally characteristic of surfaces with (1 11) terrace planes!. Coverage Depenhce of Cubon Monoxide Spectral Featarea. Infrared spectra of CO adsorbed at Pt(335) under aqueous electrochemical conditions are shown in Figures 3 and 4. These spectra document the coverage dependence of the CO spectral features at 4 . 2 and +O. 1 V recorded immediatelyafter COdosing at-0.2V. PreparingtheCOmonolayeratpotentialsin thedoubla layer region (0.0-0.3 V vs SCE), where water rather than atomic hydrogen is assumed to be the predominant madsorbed species, was difficult on account of appreciable CO oxidation at these potentials and because of rearrangements in the CO adlayer that were not completely reversible when the potential was stepped back to -0.20 V. Similar behavior was reported by Chang and WeaverlZcfor CO adsorbed at Pt( 100). Spectra shown in Figures 3 and 4 are subdivided into three regions ( A X ) on the basis of CO surface coverage. In these spectra, a coverage- and potential-dependent vibrational band appears between 2006 and 2066 cm-l and is assignable to the
The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6487
CO Adsorption at Pt(335)
L
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WAVENUMBERS(cm- 1) Fiopre 3. Single-potential alteration infrared ~ p e c t r of a~ adsorbed ~ ~ ~ CO ~ ~at Pt(335) as a function of CO surface coverage. Spectra correspond to CO at low, intermediateand high (A, B, and C, respectively) surface coverages, and were obtained with the electrodeat +0.1 V vs SCE in 0.1 M HClO,. The reference single beam spectrum was obtained with the electrode at a potential of +0.6 V, where the surface is free of adsorbed CO on account of its oxidation. Surface coverages were determined by converting the spectra to absorbance units and computing the integrated absorbance of the band arising from the COz oxidation product at 2343 cm-l.lZdCoverages are reported relative to saturation (e/@-) to maintain consistency with the
initial infrared work of Hayden and co-worker~.~~ 1788
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C-O stretching vibrational mode of on-top C0.32 A second coverage- and potential-dependent spectral feature appears in therange 1788-1883 cm-l and is assignable to theC-0 stretching mode of CO adsorbed at 2-fold bridge sites.I2 In comparing the CO infrared spectral features measured under aqueous electrochemical and uhv conditions, two important differences are observed. First, in the uhv environment, a coverage-dependent vibrational band assignable to on-top CO appears between 2065 and2100cm-l, shiftedapproximately 40-60 cm-l higher in energy compared to spectral features measured at the corresponding CO surface coverage in the electrochemicalenvironment. Second, a spectral feature assignable to CO adsorbed at 2-fold bridge sites was not detected in infrared experiments conducted under uhv conditions.
The disparity between the infrared spectral features of CO adsorbed under aqueous electrochemical and uhv conditions can be attributed, in part, to differences in the surface potential, as similar findings have been reported for CO adsorbed at the low index planes of platinum and r h o d i ~ m . ~ J ~Theseearlier -l~ studies explained that the substantially lower surface potentials found in the electrochemical as compared to the uhv environment alter the CO vibrationalenergy and siteoccupancy such that the former environment gives rise to lower COvibrational energies and makes bridging coordination geometries favorable. Surface potential arguments cannot explain the disparityentirely,however, as strong signals for bridging CO at R(335) have been observed in EELS experiments, even down to low CO coverages (8 1 0.075).33834b At low coverages (Figures 3A and 4A), the on-top CO
6488 The Journal of Physical Chemistry, Vol. 97, No. 24,1993
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Kim et al. coverage range, and the twovibrational bands are better resolved, having roughlyequal intensity at 50% of Uhv studies attribute the low-energy band to on-top CO at edge sites and the high-energy band to on-top CO at terrace sites. That CO occupies botb sites at intermediate coverages was verified by TPD experimentswhich observed a high-temperature featureassignable to edge CO and a lower temperature feature which coincides with CO desorption from terrace sites (from sites having local symmetry consistent with the (1 11) surface lane).^"^^ The uhv studies showed that the positions and relative intensities of the COvibrational features are heavily influenced by dipolbcoupling effects. In particular, coupling between edge and terrace CO dipoles causes coverage-dependent alterations in the relative intensities of the vibrational bands. The higher energy band is allowed to 'borrow" intensity from the lower energy band on account of edge-terrace CO coupling, and computational models have demonstrated this effect quantitatively for CO at Pt(335).32*34"940 That the two bands are less readily distinguished in the aqueous electrochemicalenvironment than in uhv could be a consequence of the presence of madsorbed speciesin the former, as hydrophobic interactions with co-adsorbed water are known to promote CO island formation,2J2and the close promixity of CO dipoles within the island could increase the effective dipolecoupling interactions. It is also important to consider the influence that co-adsorbed hydrogen can have on the structure of the CO adlayer. Co-adsorbed hydrogen is present during dosing under electrochemicalconditions, and competition for adsorption sites can alter the structure of the CO adlayer compared to what it is when CO is dosed onto clean platinum in uhv. Indeed, at potentials where madsorbed hydrogen is present, the potential dependence of the edge COvibrational features (see below) differs substantially from what has been observed previously for CO adsorbed at noble metals under uhv or electrochemicalconditions. The influence that madsorbed hydrogen might have on the CO overlayer structure is discussed quantitatively in the context of how the spectral features are altered by changes in electrode potential (see below). Dipole-mupling interactions also appear to influence bridging CO spectral features at intermediate coverages. At potentials in the double-layer region, a lower energy bridging band appears asashoulderon the 1851-1861-cm-l band(Figure3B) throughout the intermediate coverage range. At potentials in the classical hydrogen region, the high-energy band is dominant and a shoulder appears at lower energy for two coverages (Figure 4B). At high CO surface coverages (Figures 3C and 4C), the lowenergy shoulder on the vibrational band for on-top CO is no longer apparent, and a single feature is observed, assignable to terrace CO which is strongly coupled to edge CO dipoles. In looking back at the uhv studies, TPD spectra indicate that both edgeand terrace CO are present at high coverages, yet the infrared experiments detect only the high energy spectral feature attributable to terraceCO. Theabsenceof edgecovibrationalfeatures is thought to be a consequence of strong dynamic coupling between edge and terrace CO dipoles, and quantitative dipolbcoupling models accurately predict these high-coverage effect^.^^^^^.^ The uhv experiments showed the importance of considering dipolecoupling when interpreting vibrational spectra of CO adsorbates at surfaces which contain a high density of defect sites such as those employed in practical catalytic application^.^^ The present experiments show that these physical interactions become important under aqueous electrochemicalconditions as well as that they should be considered when deriving models of the interfacial environment on the basis of infrared spectra. It is also notable that the bandwidth of the terrace CO feature narrows at high coverages, and the bandwidth is similar at potentialsin the classical hydrogen and the double-layer regions. These effects have been observed for CO at Pt(ll1) under aqueous electrochemical conditions, and broadening at low coverages has been attributed
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F'igure 5. Plot of on-top CO band position as a function of adsorbed CO iurface coverage for potentials at -0.2 and +0.1 V vs SCE (crosses and filled squares, respectively).
vibrational band measured under electrochemical conditions appears in the range 2006-2025 cm-l. By comparison with Hayden and co-workers' infrared studies of CO adsorbed at Pt(335) under uhv conditions, this low-coverage band is assignable to CO adsorbed terminally at sites on the step edge.32 Under electrochemical conditions, the position of the on-top CO vibrational band is invariant for coveragesbetween e/@- = 04.3, and then it begins shifting to higher energy with increasing coverage (Figure 5 ) . Under uhv conditions, the on-top CO vibrational band shows a continuous and approximately linear shift with increasing coverage throughout the low coverage range e(@ /, = 0 4 . 3 9 , and quantitative models have shown that this behavior can be attributed todipole-couplinginteractionsbetween adjacent CO a d s ~ r b a t e s . ~The ~ discontinuous shift in CO vibrational frequency observed under electrochemicalconditions could be a consequence of the double-layer environment, as coadsorbed hydrogen and water could disrupt the dipole-mupling interactions. Similarly, double-layer effects appear to influence somewhat the bridging CO spectral features. At the lowest coverages, a single band attributable to bridging CO appears at about 1800 cm-l. At potentials in the double-layer region, this band broadens and shifts to higher energy as coverageis increased, and two poorly resolved bands can be discerned (Figure 3A). At potentials in the classical hydrogen region (4.25 to 0.0 V vs SCE), where coadsorbed hydrogen is assumed to be the predominant co-adsorbed species, the position of this band remains invariant with increasing CO coverage (Figure 4A). Structural models of the low-coverage CO adlayer at Pt(335) derived from recent EELS33-34b and TPD33experiments suggest that the low-coverage bridging CO spectral features observed in the electrochemical environment be assigned to edge CO. The combined EELS/TPD study by Luo and c o - ~ o r k e r derived s~~ a model to explain their low coverage data that placed all CO molecules at the step edge with equal populations of bridge and on-top CO coordinationgeometries. The model aligned the bridge and on-top CO in alternation along the step edge, filling twothirds of the edge sites, in agreement with TPD measurements. This model is applicable to interpreting the low-coverageinfrared data observed in the present studies. At intermediate coverages (Figures 3B and 4B), the infrared band for on-top CO continues to shift to higher energy with increasing coverage (2025-2057 cm-l) and a shoulder appears on the low-energy side. The plot of on-top CO band position versus CO coverage (Figure 5 ) indicates the onset of abrupt spectral alterations between @/e, = 0.35-0.6 and suggests the Occurrence of significant structural rearrangements in the CO adlayer. Further, we found it difficult to prepare a partial CO monolayer at these coverages, and this difficulty may be attributable to the apparent adlayer structural transformations. Compared to the spectral features observedin the electrochemical system, the low-temperature uhv infrared spectra achieve a more uniform progression of stable coverages over the intermediate
CO Adsorption at Pt(335) to variations in the low coverage structure of the CO adlayer brought about by co-adsorbed water and madsorbed atomic hydrogen.lM It is likely that similar doublelayer effects contribute to the band broadening observed in the current studies. For bridging CO,a single band appears at high coverages with an asymmetric feature toward the low energy side (Figures 3C and 4C). Co-adsorption effects are expected to be minimal at the highest coverages, and differences in peak position (e.g., 1877 versus 1883cm-l at saturation) can be attributed to the dissimilar surfacepotentials. Site assignmentsfor the high coverage bridging features can be made on the basis of vibrational spectra obtained in electrochemical studies which probed CO adsorption at the low index planes of platinum. At saturation coverages and -0.25 V, the bridging feature is observed at 1830 cm-l for adsorption at Pt(ll1)'" and at 1872 cm-' for adsorption at Pt(100).120By comparison, the high-energy spectral features observed for bridging C o a t highsurfacecoverageonPt(335)canbeattributed to CO at sites which have (100) symmetry (i.e., sites located at the step edge), and the asymmetry toward the low-energy side can be attributed to the presence of bridging CO at terrace sites. There is an interesting correspondence between the spectral alterations observed under electrochemicaland uhv conditions over the intermediate and high coverage ranges. The EELS/ TPD study33observes a smooth increase in the occupation of terrace sites over the intermediate coverage range, but at about 8 = 0.4(8/8,, =0.64), theseexperimentsobserveasharpincrease in the on-top CO coverage at edge sites with a corresponding decrease in the coverage of edge site bridging CO. Experiments performed under electrochemicalconditions observe a transition region between 8/8, = 0.35-0.6 (Figure 5), after which the energy of the spectral features increases more uniformly toward higher coverages. Of interest is that both studies indicate gross alterations in the adsorbed CO adlayer at about 8/Omx = 0.6. The EELS and TPD data support a model in which the CO adlayer undergoes a sudden shift at about 6/Omx = 0.64 to form a densely packed array of on-top CO aligned along the step edge such that there is one CO molecule for every edge Pt atom. In contrast, the present infrared experiments observe edge CO in 2-fold bridging coordination geometries across the intermediate and high coverage ranges. The difference in surface potential between the uhv and electrochemical environments is one possible reason for the disparity, as bridging coordination is favored in the latter. Also, infrared measurements may exaggerate the importance of the edge CO bridging feature on account of dipolecoupling interactions which would enhance the intensity of the higher energy Pt( l00)/CO (edge CO) bridging band relative to the low-energy Pt( 111)/CO (terrace CO) bridging feature. That the high-coverage uhv models are in need of revision is also a possibility. Potential Dependence of Carbon Monoxide Spectral Fertures. Spectra in Figures 3 and 4 show the coverage dependence of the CO spectral features at two electrode potentials. For both low and high coverages, the C-O stretching vibrational bands appear lower in energy at potentials in the hydrogen region. The same potential dependent response is observed for intermediate coverages, although the shifts are not as great. These general trends can be discerned more readily from the plot of on-top CO band position versus CO surface coverage shown in Figure 5 . For each CO surface coverage, our experiments probed the potential dependence of the on-top CO spectral features over a wide range of electrode potentials. These findings are summarized in Figure 6 which plots the peak energy of the on-top CO feature as a function of electrode potential for a series of CO surface coverages. At high coverages (Figure 6C), the C-O stretching vibrational frequency for terminal CO shifts to higher energy with increasing potential, and the slopeof the plot increases with decreasing coverage. For high coverages, this shift is essentially linear with slopes of 33 cml/V at @/e, = 1.00 and
The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6489
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43 cm-l/V at 8/BmX = 0.83. These slopes are comparable to values measured for CO at the low-index planes of platinum over a similar range of coverages.2Jh.b-dIn particular, they agree well with values obtained for CO at Pt( 111) under similar dosing conditionsin aqueous acid media, and these findingsare consistent with observing exclusively the high-energy terrace CO mode on account of dipole-coupling effects. In the intermediate converge range (Figure 6B), the V'CO-E
Kim et al.
6490 The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 I
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Figure 7. Single potential alteration infrared ~pectral~.2'~ of adsorbed CO at Pt(335) for O/O= 0.29 as a function of electrode potential in 0.1 M HCIO4. The reference single-beam spectrum was obtained with the electrode held at a potential of +0.6 V, where the surface is free of adsorbed CO on account of its oxidation. (A) Spectra obtained at potentials in the classical hydrogen adsorption potential region. (B)
Spectra obtained at potentials in the double-layer region.
plot is essentially linear in the high-coverage limit but divides into two linear regions each with different slopes in the lowcoverage limit. For e/@, = 0 . 4 7 4 3 4 , the slope is zero at potentials in the classical hydrogen region and then increases sharply with the onset of hydrogen desorption. The potentialdependent response is similar at the lowest coverages (Figure 6A), and the transition in the &-E plots can be correlated with potential dependent alterations in the CO site occupancy. A seriesof potential dependent spectra for e/6,,, = 0.29are plotted in Figure 7. These spectra show how the CO spectral features change as the potential is swept through the transition region. There is an apparent shift in CO site occupancy from a bridging to a predominantly terminal coordination geometry coincident with the sharp rise in slope. In a similar way, the potential dependent response for low coverages of CO at Pt(100) is nonlinear, and the response can be correlated with potential dependent alterations in CO site occupancy. For the Pt(100)/ CO system, the slope of the v'c0-E plot is negative over a range of potentials in the classical hydrogen adsorption region and turns positive with the onset of hydrogen desorption.lZeA shift in the site occupancy from a predominantly on-top to a bridging coordination geometry was observed at potentials in the classical hydrogen adsorption region, and the nonlinear v'c0-E plots were explained by considering how the reduction in on-top CO site occupancy lowers the C-0 stretching frequency on account of decreased dipolmupling and acts to offset the effect of electrode potentialwhich causes an increasein the C-O stretchingfrequency at increasing positive values.12eSimilar chemical and physical effects can combine to determine the nonlinear potential dependence of the on-top CO frequencies at Pt(335). In addition, the potential dependent alterations in CO site occupancy can influence the CO surface coverage at edge sites andcausevariationsin theedge CO tilt angle. ESDIAD31astudies which probed the coverage dependence of CO at the closely related Pt( 1 12) (Pt(S)-[3( 1 11) X (loo)]) surface showed the tilt angle of edge CO to be a strong function of CO surface coverage. At the lowest coverages, CO was only observed at edge sites and was tilted in a direction "down" the steps with an angle of roughly 20' from the [112]direction. At intermediate coverages, edge CO dipoles maintained a tilt direction down the step, but COCO repulsions caused tilting along the step as well. At the highest coverages, the downward tilt direction increased to 38' from the [112]direction. For the Pt(335)/CO system, any alterations in
the CO tilt angle would change the alignment between the CO dipoles and the external electric fxld across the innerlayer and causes nonlinearities in the v * c ~ plots. E Further, the severe downward tilt of edge CO at high coverages can account for the appearance of bridging features characteristic of CO in environments of ( 100)symmetry under electrochemical conditions. For Pt(l12). the [Ool] direction is 35.3O down the steps from the [ 1121 direction; therefore, CO dipoles tilting 38' down the steps are necessarily in the (001)surface plane. If similar tilting is observed for edge CO at Pt(335), then this behavior could account for the appearance of the bridging spectral features measured at high coverages in the present studies. Another model that can account for the observed potential dependent spectral features predicts electric field enhancements at Pt(335) step site^.^^ Lambert et al. haveconsidered thismodel in interpreting their uhv tuning rate measurements for edge CO at Pt(335).34 Although the edge CO tuning rates were much larger than the tuning rates measured for on-top CO at Pt(l1 l), theedge CO tuningrates werecomparable touhvvaluesmeasured for CO at nickel and to corresponding electrochemical values measured for CO at Pt( 1 1 1). Rather than arguing that the EVS results indicate an enhancement in the Pt(335) tuning rate for edge CO, these workers interpreted their uhv tuning rate for CO atPt(l11)asbeingunusuallysmall. In theelectrochemicalstudies presented here, the potential dependent shift in edge CO vibrational frequency measured at low coverages is substantially larger than corresponding values measured for all other systems. The potential-dependent shift in on-top COvibrational frequency for e/Omx = 0.47 and 0.34, at potentials in the double-layer region, is 93 and 60 cm-I/V, respectively. At the lowest coverages, the potential dependent shift in on-top CO vibrational frequency at potentialsin the double-layer region is between 75 and 80 cm-' /V. In contrast, typical potential dependent shifts measured for ontop CO at Pt(ll1) are reported to be about 40-44 cm-I/V at low coverages.ZJZa,b,d These electrochemical measurements provide some support to the proposal that electric fields at the Pt(335) surface plane are inhomogeneous and are enhanced at the step at least under aqueous electrochemical conditions. The presence of co-adsorbed species also plays an important role in determining the potential-dependent shift in edge CO vibrationalfrequency at low coverages. The zero slopes measured at potentials where co-adsorbed hydrogen is present are unexpected and may be a consequence of severalphysical and chemical effects acting in combination (i.e., chemical bonding, dipole coupling, adsorbate-induced perturbations in tilt angle). Competition between CO and co-adsorbed hydrogen for sites at the step edge likely plays a major role in determining the response, and interactions which would cause adsorbed CO to tilt away from the external field direction, or to migrate to positions along the bottom of the step where the field model of Greenler et aL4* predicts the external fields are weakest, areinteresting possibilities which could account for the observed potential dependence.
ConclusioM This work documents the coverage and potential dependence of the infrared spectral features for CO adsorbed at Pt(335) under aqueous electrochemical conditions. Spectral features associated with CO in on-top and bridgingcoordinationgeometries are readily distinguished, and assignments of CO at sites on the terrace and sites on the step edge can be made by drawing upon correspondinguhv studies. Dipolmupling interactionsinfluence the appearance of infrared spectral features in a similar way under both uhv and aqueous electrochemical conditions, but there are subtle differences at low and intermediate coverages which can be attributed to interactions between the adsorbed CO layer and the aqueous electrochemical environment. Co-adsorbedwater and hydrogen also influence the potential dependence of the infrared spectral features measured under aqueous electrochem-
CO Adsorption at Pt(335) ical conditions, as sharp discontinuities are observed in plots of on-top CO peak energy versus electrode potential at potentials which correlate with the onset of hydrogen desorption processes. The slope of v'c0-E plots is unbxpectedly large for low-CO coverages at potentials in the double-layer region, and these findings are supported by field enhancement models proposed by Greenler et aL4* The reported potential dependent spectral features for CO adsorption at Pt(335) contribute important corresponding vibrational data for evaluating the connection between field dependent vibrational properties of adsorbates in uhv and in electrochemical environments. At the highest coverages, the dynamic dipolscoupling interactions which obscure low-energyfeatures under uhv conditions are also observed under electrochemical conditions, and these findings demonstrate the importance of considering dipole-coupling effects when deriving physical models of the interface on the basis of in situ infrared spectra.
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