in Aqueous Solution: Adlayer Phase Transitions - ACS Publications

Infrared reflection-absorption spectra (IRAS) measured for the C-O stretch (νCO) of ... potential (E) are examined in comparison with extant IRAS and...
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Langmuir 1999, 15, 2931-2939

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Coverage-Dependent Infrared Spectroscopy of Carbon Monoxide on Palladium(100) in Aqueous Solution: Adlayer Phase Transitions and Electrooxidation Pathways Shouzhong Zou, Roberto Go´mez,† and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received December 31, 1998. In Final Form: February 15, 1999 Infrared reflection-absorption spectra (IRAS) measured for the C-O stretch (νCO) of carbon monoxide dosed on ordered Pd(100) in aqueous 0.1 M HClO4 as a function of the coverage (θCO) and the electrode potential (E) are examined in comparison with extant IRAS and adlayer structural data on Pd(100) in ultrahigh vacuum (UHV). A noteworthy feature of the electrochemical adlayer, as for the UHV system, is the presence of a single νCO band blue shifting markedly (by ca. 100 cm-1) with increasing coverage up to saturation (θCO ≈ 0.8), indicative of uniformly bridging CO coordination. Extrapolation of the θCOdependent νCO frequencies to surface potentials corresponding to the UHV-based system, however, yielded reasonable concordance only at high coverages. The discrepancies at lower θCO are suggested to arise from solvation effects on the local CO-site surface potentials. Examination of the θCO-dependent νCO peak frequencies (νPCO), integrated absorbances (Ai), and bandwidth (∆ν1/2) for the dosed electrochemical adlayers reveals marked alterations at θCO ≈ 0.5. The unusual nonmonotonic form of the Ai - θCO and ∆ν1/2 - θCO dependencies are discussed in relation to the behavior observed for the UHV-based system. The observed similarities suggest the occurrence of a phase transition for the electrochemical system as in the well-characterized UHV case, involving the formation of phase/antiphase adlayer domains for θCO > 0.5. The relation between the νPCO - θCO behavior observed for adlayers formed by dilute CO solution dosing and by progressive electrooxidative removal from an initially saturated layer is also considered. Unusually, the hystersis observed between these plots is only minor, and restricted to intermediate coverages (θCO ∼ 0.4-0.6). This behavior indicates that the CO domains formed upon partial adlayer electrooxidation are small and/or spontaneously dispersed. The possible connection between metal surface oxidation and CO adlayer oxidation on Pd(100) is considered in the light of the IRAS and voltammetric data.

Introduction Understanding the chemisorption and reactivity of carbon monoxide and related small molecules on ordered monocrystalline metal surfaces in solution is an important contemporary objective in electrochemical surface science, mirroring the longstanding significance of such adsorbates at analogous metal-ultrahigh vacuum (UHV) interfaces. Such studies are facilitated greatly by the now wellestablished suitability of infrared reflection-absorption spectroscopy (IRAS) as a sensitive means of scrutinizing in-situ electrochemical systems, together with the widespread availability of corresponding vibrational and other structural information for CO on UHV-based surfaces.1 We have been interested for some time in exploring CO adsorption and electrooxidation on low-index Pt-group electrodes by means of IRAS, especially in comparison with UHV-based systems as a means of elucidating the influences of the electrochemical double layer on the chemisorbate properties.1a,2-8 Most of the systems examined so far involve multiple coverage-dependent CO binding sites, especially on platinum and rhodium surfaces, whose occupancy (along with the overall adlayer structure) can be altered markedly by the presence of * Corresponding author. Telephone: (765) 494-5466. Fax: (765) 494-0239. E-mail: [email protected]. † Permanent address: Departament de Quı´mica Fı´sica, Universitat d’Alacant, Apartat 99 E-03080, Alacant, Spain. (1) For overviews, see: (a) Weaver, M. J.; Zou, S, In Spectroscopy for Surface Science; Clark, R. J. H., Hester, R. E., Eds.; Advances in Spectroscopy 26; Wiley: Chichester, England, 1998; Chapter 5. (b) Iwasita, T., Nart, F. C. Prog. Surf. Sci. 1997, 55, 271. (c) Korzeniewski, C. Crit. Rev. Anal. Chem. 1997, 27, 81. (d) Korzeniewski, C.; Severson, M. W. Spectrochim. Acta 1995, 51A, 499.

solvent and electronic/ionic charge.2,3,7,8 Low-index iridium surfaces, especially Ir(111), provide an interesting exception in that single-site (probably atop/nearly atop) binding usually prevails throughout the wide range of accessible CO coverages (θCO) in aqueous electrochemical as well as UHV environments.4 This circumstance provides an opportunity to explore the influence of the electrochemical double-layer upon the θCO-dependent vibrational properties, specifically the C-O stretching (νCO) vibration, in the absence of “chemical” effects associated with bindingsite differences. Indeed, we have recently undertaken a detailed examination of the Ir(111)/CO system with this objective in mind.4a (2) (a) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391. (b) Chang, S.-C., Weaver, M. J, Surf. Sci. 1990, 238, 142. (c) Jiang, X.; Weaver, M. J. Surf. Sci. 1992, 275, 237. (d) Weaver, M. J. Appl. Surf. Sci. 1993, 67, 147. (3) (a) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (b) Yau, S.-L.; Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. Soc. 1991, 113, 6049. (4) (a) Tang, C.; Zou, S.; Severson, M. W.; Weaver, M. J. J. Phys. Chem. B 1998, 102, 8796. (b) Tang, C.; Zou, S.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem., in press. (c) Go´mez, R.; Weaver, M. J. J. Phys. Chem. B 1998, 102, 3754. (d) Go´mez, R.; Weaver, M. J, Langmuir 1998, 14, 2525. (5) (a) Zou, S.; Go´mez, R.; Weaver, M. J. Surf. Sci. 1998, 399, 270. (b) Zou, S.; Go´mez, R.; Weaver, M. J. J. Electroanal. Chem., submitted for publication. (6) Weaver, M. J.; Zou, S.; Tang, C. J. Chem. Phys., submitted for publication. (7) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 102, 10166. (8) (a) Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J. Chem. Phys. 1994, 100, 6750. (b) Kizhakevariam, N., Villegas, I.; Weaver, M. J. J. Phys. Chem. 1995, 99, 7677. (c) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Surf. Sci. 1995, 336, 37. (d) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Langmuir 1995, 11, 2777. (e) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 5842.

10.1021/la9817720 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/24/1999

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Also a recent focus of electrochemical IRAS studies in our laboratory has been low-index palladium surfaces.5 Although the use of flame-annealing pretreatment tactics, commonly employed for Pt, Rh, and Ir crystals, is less reliable for palladium, a simple electropolishing method is available which yields apparently well-ordered surfaces (vide infra).9 The θCO-dependent νCO properties on Pd(111) and Pd(110) are complex, due to the formation of diverse adlayer structures featuring multiple binding geometries10,11 and adsorbate-induced surface reconstruction,12 respectively. However, the corresponding vibrational behavior on Pd(100) in UHV is ostensibly simple, only a single νCO band being observed, attributable to bridging CO, which blue shifts markedly (by ca. 100 cm-1) with increasing coverage up to saturation.11,13 Analysis of lowenergy electron diffraction (LEED) patterns14 along with vibrational13 and other data15,16 indicate the formation of an ordered c(2 x2 × x2)R45° adlayer by θCO ) 0.5, transforming sharply to more complex “domain-wall superlattice” phases toward higher coverages while maintaining essentially bridging CO coordination. Unlike Pd(111), the Pd(100)/CO system yields θCO-dependent vibrational behavior that is largely temperature-insensitive.11,13 Complications due to surface reconstruction are also absent for Pd(100), the bulk-termination (1 × 1) structure being retained in both the absence and presence of chemisorbate. Given this outwardly straightforward yet markedly θCOsensitive νCO behavior on Pd(100) in UHV, the comparative examination of the Pd(100)/CO-aqueous electrochemical system by means of in-situ IRAS is of interest. The results of such a study are reported here. The Pd(100)/CO electrochemical interface displays similar θCO-dependent vibrational fingerprints and hence chemisorbate coordination geometries to the UHV case. While the electrochemical νCO band frequencies vary somewhat from the UHV-based values, attributable partly to different surface potentials, a marked adlayer phase transition at θCO ≈ 0.5 is evident from the various νCO spectral parameters, which are comparable in the two environments. In addition, νCO spectra obtained for partly electrooxidized CO layers on Pd(100) are unusually similar to θCOdependent spectra attained via solution CO dosing. These and other findings suggest that CO adlayer oxidation on Pd(100) is coupled with metal surface oxidation. Experimental Section The in-situ electrochemical IRAS measurements were conventional and were undertaken largely as described elsewhere.17,18 Briefly, the infrared spectrometer was an IBM (Bruker) IR-98 vacuum FT instrument, with a Globar light source and an MCT narrow-band detector. Some measurements utilized instead a Mattson RS-2000 N2-purged spectrometer. The spectral (9) Chierchie, T.; Mayer, C. Electrochim. Acta 1988, 33, 341. (10) Tu¨shaus, M.; Berndt, W.; Conrad, H.; Bradshaw, A. M.; Persson, B. Appl. Phys. A 1990, 51, 91. (11) Szanyi, J.; Kuhn, W. K.; Goodman, D. W. J. Vac. Sci. Technol. 1993, A11, 1969. (12) Raval, R.; Haq, S.; Blyholder, G.; King, D. A. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 629. (13) (a) Ortega, A.; Hoffman, F. M.; Bradshaw, A. M. Surf. Sci. 1982, 119, 79. (b) Hoffman, F. M.; Surf. Sci, Rep. 1983, 3, 107. (c) Uvdal, P.; Karlsson, P.-A.; Nyberg, C.; Andersson, S.; Richardson, N. V. Surf. Sci. 1988, 202, 167. (14) Berndt, W. Bradshaw, A. M. Surf. Sci. 1992, 279, L165. (15) Behm, R. J.; Christmann, K.; Ertl, G.; Van Hove, M. A. J. Chem. Phys. 1980, 73, 2984. (16) Yeo, Y. Y.; Vattuone, L.; King, D. A. J. Chem. Phys. 1997, 106, 1990. (17) Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (18) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 241, 143.

Zou et al. resolution was (4 cm -1. As usual, the electrochemical thinlayer cell utilized a CaF2 window beveled at 60° to the surface normal. The Pd crystal (0.8 cm diameter, 2 mm thick) was procured from the Materials Preparation Facility at Cornell University, and was oriented within 1°. The surface pretreatment involved electropolishing, which entailed anodizing at 0.5 mA with a Pt mesh counter electrode, for 5-10 s in a solution of 0.5 M LiCl + 0.2 M Mg(ClO4)2 in methanol.9 (This procedure probably removes several metal layers.5) The surface was then rinsed thoroughly with ultrapure water and transferred rapidly to an electrochemical cell. Cathodic-anodic cyclic voltammograms for the formation and removal of underpotential deposited (upd) copper were obtained as a check upon the surface state. A pair of sharp reversible cathodic/anodic current peaks are evident at 0.25 V vs SCE, suggestive of an ordered surface. [Strong evidence supporting this contention is obtained from the observation of essentially identical upd copper peaks on Pd electrodes formed by expitaxial electrodeposition onto ordered Pt(100).19] Following the pretreatment procedure, the electrode was emersed at 0.45 V vs SCE, rinsed again with ultrapure water, and transferred immediately to the infrared spectroelectrochemical cell. Carbon monoxide (99.3%) was obtained from Airco, 13CO (99%) from Cambridge Isotopes, perchloric acid (70%, double distilled) from G. F. Smith, and copper sulfate from G. T. Baker. Solutions were prepared by using water purified by a Milli-Q Plus system (Millipore). All measurements were made at room temperature (23( 1 °C), and all electrode potentials are reported here vs the saturated calomel electrode (SCE).

Results Anodic Voltammetry. It is instructive at the outset to examine the voltammetric oxidation of adsorbed CO, not only to establish suitable conditions for in-situ IRAS but also to characterize, at least qualitatively, the electrooxidation kinetics. Traces a-c in Figure 1A are representative anodic voltammograms at 20 mV s-1 for the oxidation of different dosages of adsorbed CO on Pd(100) in 0.1 M HClO4. The adlayers were formed by exposing the surface to dilute (ca 0.01 mM) CO in 0.1 M HClO4 for controlled times (e5 min) at 0.2 V before brief argon sparging to remove the solution CO (cf. ref 17). Similar voltammetric results were obtained for adlayers formed by solution CO dosing at 0-0.3 V. (Lower potentials were usually avoided due to the formation of adsorbed atomic hydrogen, likely leading to H absorption and possible Pd surface degradation.) The dotted trace is a corresponding cyclic voltammogram obtained in the absence of CO. Trace a, obtained for a saturated adlayer, exhibits a sharp peak centered at 0.55 V, characteristic of CO oxidation on Pt-group surfaces. Traces b and c, referring to low-coverage CO adlayers (ca. 0.3 and 0.05, respectively), both exhibit current onsets at somewhat (0.1-0.15 V) lower potentials than observed for the saturated adlayer. However, these initial anodic currents are significantly smaller than those obtained for metal surface oxidation in the complete absence of CO at the same electrode potentials, ca. 0.3-0.5 V. This suggests that the initial oxidation of the Pd surface is impeded in the presence even of dilute CO adlayers. As in previous studies from this laboratory, lowercoverage adlayers were also formed by partial electrooxidative removal from an initially saturated layer, in this case by applying potential pulses to 0.5 V for a few seconds. The θCO-dependent νCO spectra for such “partially electrooxidatively stripped” layers in comparison with those formed by direct solution dosing to the desired coverage are examined below. Representative anodic voltammograms for three subsaturated adlayers formed by the (19) Go´mez, R.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Surf. Sci. 1995, 344, 85.

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Figure 2. Infrared absorbance spectra for irreversibly adsorbed CO on Pd(100) at 0.2 V vs SCE in 0.1 M HClO4 for a sequence of coverages, θCO, as indicated, formed by (A) direct solution dosing and (B) partial electrooxidative removal from a saturated adlayer.

Figure 1. Representative voltamograms (20 mV s-1) for the oxidation of irreversibly adsorbed CO at various coverages on ordered Pd(100) in 0.1 M HClO4 formed by (A) direct CO solution dosing and (B) partial electrooxidative removal from a saturated adlayer (see text). Key to approximate θCO values: (A) a, 0.8 (saturated); b, 0.3; c, 0.05; (B) a, 0.8; b, 0.55; c, 0.35; d, 0.05. The dotted trace in both parts A and B was obtained in the absence of adsorbed CO.

former method are shown as traces b-d in Figure 1B, compared again with that for a saturated CO layer (trace a). Somewhat similar to the “direct dosing” case (Figure 1A), the current-potential peaks for voltammetric adlayer oxidation in Figure 1B shift to lower potentials for smaller initial θCO values. A subtle difference, however, is that the current-potential onsets for the subsaturated layers in Figure 1B are closely similar to that observed for metal surface oxidation in the absence of CO. Of course, the experimental distinction between anodic currents associated with CO adlayer oxidation and metal oxide formation is ambiguous under these conditions. Indeed, this factor obfuscates the reliable extraction of θCO values from such voltammograms without additional double-layer information.20 Nevertheless, these differences in the voltammetric oxidation of subsaturated layers formed by direct CO dosing and partial electrooxidative removal are suggestive of significant, if subtle, adlayer structural dissimilarities. (20) Go´mez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48.

Infrared Spectroscopy. Of central interest here is the examination of νCO spectra for CO adlayers on Pd(100) in acidic aqueous solution as a function of the chemisorbate coverage and electrode potential. Similarly to previous IRAS studies of CO chemisorption from this laboratory (e.g., ref 17), absolute νCO spectra were obtained by acquiring a set of interferometer scans (typically 100) for the interface containing irreversibly adsorbed CO at the desired electrode potential, subtracted from which was a corresponding “reference spectrum” obtained after stepping the potential to a value (0.6 V) where the adlayer is entirely oxidized. Aside from removing solvent and other bulk-phase spectral interferences, this potential-difference infrared (PDIR) tactic18 also enables the adlayer coverages to be obtained accurately from the measured absorbance of the 2343 cm-1 feature due to solution CO2 produced by adsorbed CO electrooxidation,20,21 along with the known surface atomic density of Pd(100), 1.32 × 1015 atom cm-2. While this procedure strictly requires a calibration with another adlayer of known coverage (taken here as Pt(110), for which θCO ≈ 1.020), it circumvents the double-layer corrections required in order to extract the CO surface concentrations from the voltammetric data. (Note, however, that closely concordant saturated coverages, θsat CO, for various low-index Pt-group electrodes are obtained by spectrophotometric and Coulombic means once the latter is corrected properly for coupled double-layer effects.20) This infrared spectrophometric procedure applied to Pd(100) yields θsat CO equal to 0.80 ( 0.03, over the potential range 0-0.3 V. Figure 2A shows a typical set of infrared absorbance spectra obtained for irreversibly adsorbed CO at an increasing sequence of θCO values, as indicated, produced again by direct dosing with dilute CO for controlled times (21) Weaver, M. J.; Chang, S.-C.; Leung, L.-W. H.; Jiang, X.; Rubel, M.; Szklarczyk, M.; Zurawski, D.; Wieckowski, A. J. Electroanal. Chem. 1992, 327, 247.

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Figure 3. Peak νCO frequencies vs CO coverages, θCO, for adlayers formed on Pd(100) at 0.2 V by direct solution dosing (filled circles) and progressive electrooxidative removal (filled diamonds). The open circles and squares are corresponding UHV-based data obtained at 300 and 80 K, taken from ref 13a. The dashed trace is obtained from the electrochemical νPCO-θCO data for dosed CO by extrapolation to θCO-dependent surface potentials corresponding to those for the Pd(100)/CO-UHV interface (see text).

( 0.5 being accompanied by the appearance of a low-frequency “shoulder”, before the band narrows again at coverages approaching saturation, θCO > 0.7 (Figure 2A). A plot of νPCO vs θCO for CO adlayers dosed onto Pd(100) at 0.2 V is shown in Figure 3 (filled circles). The θCO-dependent νCO behavior of CO adlayers formed instead by progressive partial electrooxidative removal from a saturated dosed layer provides an interesting comparison. A typical set of such data, obtained (as above) by applying short (e a few seconds) potential pulses to 0.5 V, is shown in Figure 2B. Inspection of these spectra alongside the corresponding νCO-θCO set obtained for dosed adlayers (Figure 2A) indicates an overall similarity, although the electrooxidatively stripped adlayers exhibit higher νPCO values at intermediate coverages. A plot of νPCO vs θCO obtained for progressively electrooxidized adlayers is also included (filled diamonds) in Figure 3. Clearly evident is a hysteresis at intermediate θCO values between the νPCO-θCO curves formed by direct CO dosing (filled circles) and partial electrooxidative removal (filled dia-

Zou et al.

Figure 4. Integrated absorbance of the νCO band vs CO coverage for adlayers formed on Pd(100) by dilute solution dosing at 0.2 V (filled circles) compared with corresponding data for the Pd(100)/CO-UHV interface at 80 K (open squares), taken from ref 13a.

monds), referring therefore to progressively increasing and decreasing coverages, respectively. Nevertheless, the two sets of spectra, as well as the νPCO values, are closely similar at lower, θCO < 0.35, as well as higher coverages, θCO > 0.65. The occurrence of such large νCO red shiftss upon progressive electrooxidative adlayer removal is unusual, only small ( 0.5.13-16] Comparison between these UHV-based data and the “equivalent variable-potential” νPCO-θCO plot (dashed trace) reveals a significant disparity at low coverages, although an approximate concordance is evident at higher θCO values (Figure 3). This comparison is admittedly clouded somewhat by uncertainties (ca. 0.1-0.2 eV) in ΦM for clean Pd(100)31 as well as in Eabs(ref). However, unaffected by these uncertainties, the “equivalentpotential” νPCO-θCO trace exhibits a significantly (1.5-2fold) higher slope than the UHV-based νPCO-θCO data. Such variable-coverage analyses for CO adlayers are often complicated by differing coordination geometries and/or in the θCO values. However, a comparison for Ir(111)/CO, featuring exclusive atop/near-atop CO coordination, indicates a reasonable agreement (within ca. 5-10 cm-1) between the νPCO values in aqueous electrochemical and ambient-temperature UHV environments at low (ca. 0.050.2) as well as high θCO values using the above procedure.4a As already noted, the Ir(111)/CO and Pd(100)/CO systems share the desirable (and relatively unusual)

(29) For example, see: Wagner, F. T. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1993; Chapter 9. (30) Trasatti, S. Electrochim. Acta 1991, 36, 1659.

(31) This estimate is reasonably consistent with work-function data for other Pt-group surfaces as well as with electrochemical νCO data,6 although a sizable variation in Φ ((0.2 eV) values for Pd(100) and Pd(111) is evident from the literature.32

Figure 8. Infrared absorbance difference spectra for saturated CO adlayers on Pd(100) for 13CO/12CO isotopic mixtures, with 12CO percentages as indicated (see text).

lower wavenumber feature for 30% 12CO.13b The lack of a lower wavenumber band in Figure 8 suggests the occurrence of stronger coupling within the ambienttemperature electrochemical adlayer. Discussion Comparisons with Coverage-Dependent Vibrational Parameters in UHV: Adlayer Structural Implications. To rationalize and hopefully elucidate the underlying adlayer structural reasons for the decidedly unusual θCO-dependent behavior of all four νCO spectral parameterssνPCO, Ai , ∆ν1/2, and (dνPCO/dE)sit is insightful to undertake comparisons with IRAS data for the corresponding UHV-based system. As discussed previously,2,4a,6,8e an instructive procedure is to correct the νCO frequencies for the differences in interfacial electrostatic field in the electrochemical and UHV environments. This effect is clearly evident in the Stark-tuning behavior observed for the electrochemical adlayers (Figure 7) and reflects the influence of both a variable electrostatic field across the chemisorbed layer and electronic chargedependent metal-CO bonding.26 To a first approximation, such differences can be nullified by adjusting, or extrapolating, the electrochemical νPCO values to an electrode potential equivalent to the work function, Φ (“surface potential”), of the UHV-based system. The nature of the procedure and the assumptions involved have been discussed recently in detail.6 Briefly, the conversion between the electrochemical and vacuumbased potential (or work-function) scales can be undertaken with a knowledge of the “absolute” potential of the reference electrode, Eref(abs), i.e., its value on the vacuum scale, by using30

EM ) ΦM/e - Eref(abs)

(1)

Coverage-Dependent Infrared Spectroscopy

property of featuring only a single CO binding geometry throughout the accessible coverage range. A marked difference between these two systems in UHV, however, is that the latter,15,33 but not the former,34 involves substantial work-function increases, upon CO adsorption, indicative of greater metal-adsorbate charge back-donation on palladium.35 Given this situation, the “local potential” experienced by the CO molecules at low θCO on Pd(100) can differ markedly from the average value determined by Φ or E measurements. One can imagine that such “polarized” CO molecules would be solvated by appropriately oriented water molecules at the Pd(100) electrochemical interface, so to “screen” and hence alter this local potential. (Indeed, some evidence along these lines has been obtained from work function and vibrational spectral measurements for CO/water coadsorption measurements in UHV.36,37) Unlike at high chemisorbate coverages where the electrochemical inner layer contains almost exclusively CO, such coadsorption effects at low θCO may therefore be responsible for the disparities observed between the electrochemical and UHV-based νCO frequencies at equivalent surface potentials. Despite these complications, the markedly nonlinear form of the νPCO-θCO data at fixed electrode potential is instructive, in particular the sharp νPCO increases observed for θCO > 0.5 (Figure 3). While the overall νPCO-θCO behavior in UHV is approximately linear, sharp if relatively small (ca. 10 cm-1) θCO-induced νPCO increases are nonetheless evident for coverages just above 0.5.11,13a,b Infrared spectra obtained in a vacuum under equilibrium CO pressures at near-ambient temperatures show the transition more distinctly.11,13b These spectral changes in UHV coincide with an adlayer phase transition for θCO g 0.5 as deduced originally by low-energy electron diffraction (LEED) and binding-energy measurements.38 (Current understanding of the Pd(100)/CO adlayer phase behavior in UHV is considered below.) The presence of sharp θCO-induced νCO frequency increases, along with band shape changes (vide infra), at comparable CO coverages at the electrochemical interface suggests that a similar adlayer structural transformation is also occurring for this system. Moreover, the approximate concordance observed between the equivalent-potential electrochemical and UHV-based νPCO-θCO data at moderate and high coverages provides strong evidence that the CO binding geometry is the same in both environments;6 i.e., it involves 2-fold-bridging coordination.13,14 Further insight into the occurrence of adlayer phase transitions at the Pd(100)/CO-aqueous interface can be gleaned by examining the θCO-dependent νCO absorbances and band shapes, again especially with respect to the corresponding behavior of the UHV-based system. Such comparisons are also more direct than for the νCO frequencies themselves since neither the Ai nor ∆ν1/2 values are affected intrinsically by the surface potential. At least for systems that feature a single coverage-invariant chemisorbate binding site, such as in the present case, the Ai-θCO dependence provides insight into local CO (32) Fischer, R.; Shuppler, S.; Fischer, N.; Fauster, Th.; Steinmann, W. Phys. Rev. Lett. 1993, 70, 654. (b) Wandelt, K.; Hulse, J. E.; J. Chem. Phys. 1984, 80, 1340. (c) Demuth, J. E. Surf. Sci. 1997, 65, 369. (33) Conrad, H.; Ertl, G.; Koch, J.; Latta, E. E. Surf. Sci. 1974, 43, 462. (34) Ku¨ppers, J.; Plagge, A. J. Vac. Sci. Technol. 1976, 13, 259. (35) (a) Nieuwenhuys, B. E. Surf. Sci. 1981, 105, 505. (b) Ishi, S.-I.; Ohno, Y.; Visuranathan, B. Surf. Sci. 1985, 161, 349. (36) Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J. Chem. Phys. 1994, 100, 6750. (37) Ellis, T. H.; Kruus, E. J.; Wang, H. Surf. Sci. 1992, 273, 73. (38) Tracy, J. C.; Palmberg, P. W. J. Chem. Phys. 1969, 51, 4852.

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vibrational interactions,13b,27 in that it should then reflect primarily the θCO-dependent extent of dielectric screening of the oscillator dynamic dipole.27 For simple UHV-based systems, the Ai-θCO plots commonly exhibit slopes that decrease, or even become negative, toward high coverages, reflecting progressively greater electronic screening by surrounding chemisorbate molecules.39 For electrochemical CO (or NO) adlayers in aqueous solution, on the other hand, more linear Ai-θCO plots are obtained, reflecting a comparable degree of dielectric screening by coadsorbed solvent molecules at low coverages as are exerted by the juxtaposed chemisorbate molecules at high packing densities.4a,6,25 (Indeed, in some nonaqueous media, even upward-curved Ai-θCO plots are predicted and observed,39 reflecting more efficient screening by polarizable solvent molecules compared to surrounding chemisorbate.8d,39) Given this scenario, the peaked Ai-θCO plot obtained for the present Pd(100)/CO-aqueous system (filled circles, Figure 4) is notable as well as unusual. The marked AiθCO nonlinearity for θCO > 0.5 and the maximum observed at θCO ≈ 0.6 give a clear indication of marked θCO-induced changes in the oscillator structural environment. Significantly, the Ai-θCO peak corresponds to coverages where the νCO band undergoes a marked blue shift, yet retains a low-frequency shoulder (Figure 2). While the corresponding Ai-θCO behavior observed for the UHV-based system at 80 K13a is not identical (open squares, Figure 4), it too exhibits a mildly peaked shape. (Note that comparisons of absolute Ai values in the electrochemical and UHV environments, as might be suggested by the data in Figure 4, are complicated by differing optical geometries, etc.4a) As already mentioned, the νCO band shape changes are reflected in a nonmonotonic ∆ν1/2-θCO plot, exhibiting a maximum at the same coverages, θCO ≈ 0.5-0.6 (filled circles, Figure 5). Comparison with the corresponding behavior of the UHV-based system, shown as open circles (300 K) and squares (80 K), taken from ref 13b, again reveals a close similarity, at least with the former data set. (Note that the observed bandwidths for the lowtemperature UHV data are probably complicated by nonequilibrium adlayer structural effects.13a,b) The larger ∆ν1/2 values seen for the aqueous electrochemical system (Figure 5) may well arise from solvation-induced inhomogeneous broadening, as well as from stochastic effects associated with variations in the local coverage.4a,25 The UHV-based ∆ν1/2-θCO behavior has been interpreted in terms of inhomogeneous band broadening, seemingly involving a mixture of two domain phases for θCO ≈ 0.5.13b The overall compatibility in the θCO-dependent behavior of both the νCO absorbance and bandwidth parameters observed for the aqueous electrochemical and UHV-based systems suggests strongly that essentially similar adlayer phase transitions are occurring in these ostensibly different interfacial environments. However, after these UHV-based IRAS data were published in the early 1980s,13a,b a significant structural reinterpretation of the Pd(100)/CO adlayer phases has been undertaken, primarily on the basis of further analyses of LEED data.13c,14,40 We therefore now will reconsider both the UHV-based and present electrochemical IRAS data for the Pd(100)/ CO adlayer in the light of current structural understanding for the former system. The Pd(100)/CO system in UHV at 300 K is well-known to form a (2x2 × x2)R45° ordered lattice at θCO ) 0.5, in which the CO molecules are known to occupy symmetric (39) Korzeniewski, C.; Huang, J. Anal. Chim. Acta, in press. (40) Biberian, J. P.; Van Hove, M. J. Surf. Sci. 1982, 118, 443.

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bridge sites.14,15,38 The presence of some adlayer ordering at lower coverages, 0.4-0.5, is also evident from LEED.41 For θCO > 0.5, the LEED diffraction features split and shift in a fashion that was originally interpreted in terms of a continuous uniaxial compression (within a given domain) away from the hexagonal (2x2 × x2) ordered arrangement so to accommodate the higher CO packing densities.13a,b,15,38 This model therefore entails a commensurate-incommensurate phase transition for θCO g 0.5 and requires postulating that the CO’s are displaced variably from symmetric bridging sites so to minimize CO-CO repulsions. This model was preferred by Bradshaw and Hoffman in interpreting their IRAS data.13a,b A detailed review by Hoffman outlines how the θCO-dependent IRAS data, especially the bandwidths, are seemingly consistent with the above interpretation. However, even at that time (1983) an alternative adlayer structural model had been proposed by Biberian and Van Hove, involving the formation of phase/antiphase domains of the (2x2 × x2)R45° structure separated by “fault lines” (domain walls) featuring locally higher CO packing densities.40 The smaller CO-CO distances within the domain walls in this model were originally considered to be inconsistent with the single, albeit blue shifted νCO band observed also for θCO > 0.5, since the stronger adsorbate-adsorbate interactions might be expected to yield an additional higher frequency νCO band.13b However, an ensuing EELS (electron energy loss spectroscopy) study identified additional metal-CO vibrational bands that are indicative of stretching/bending motions involving tilted CO’s.13c Such tilting within the domain wall regions will minimize the COCO repulsions and also can account for the absence of an additional νCO band in the EELS and IRAS data.13c More recently, Berndt and Bradshaw have evaluated additional LEED data for θCO > 0.5 and concluded that the “faultline” model is indeed correct.14 For 0.50 < θCO < 0.67, the average domain-wall separation decreases continuously, the (2x2 × x2) domain width shrinking to zero by the latter coverage, with the highest compression region 0.67 < θCO < 0.75 featuring more densely packed periodic domain walls. A key feature of the Pd(100)/CO system for θCO > 0.5 is therefore the presence of uniformly bridging coordination yet including an increasing fraction of “domain-wall” CO’s toward higher coverages that feature smaller COCO separations and greater repulsions. Although the extent of dynamic dipole coupling, partly responsible for the θCO-induced νPCO blue shifts,13b will be milder for nonparallel (tilted) CO dipoles,27 the νCO band broadening observed for the ambient-temperature UHV system for θCO g 0.5 (Figure 5) along with the asymmetric IRAS band shapes42 is in harmony with the inferred presence of such nonequivalent CO’s. The band narrowing observed toward the highest coverages (Figure 5) is also anticipated from this structural picture since the fraction of “domain-wall” CO molecules will become dominant by θCO ∼ 0.65. Such decreasing bandwidths are expected even for compressed adlayers featuring mixtures of nonequivalent CO’s since the pronounced “intensity stealing” effects, arising from dipole-dipole coupling, anticipated under these conditions will yield band absorbances that strongly favor the highestfrequency (most blue shifted) components.43 (41) Bradshaw, A. M.; Hoffman, F. M. Surf. Sci. 1978, 72, 513. (42) See Figure 30 of ref 13b. (43) (a) Persson B. N. J. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 81. (b) Persson, B. N. J.; Hoffman, F. M. J. Electron Spectrosc. Relat. Phenom. 1987, 45, 215.

Zou et al.

By implication, then, the comparably nonmonotonic ∆ν1/2-θCO dependence seen for the present Pd(100)/CO electrochemical system (Figure 5), together with the θCOdependent νPCO and Ai behavior already mentioned, constitutes strong evidence favoring the presence of similar adlayer phases in the aqueous environment. The overall ∆ν1/2 decreases seen from low to high coverages in the present system and the smooth monotonic ∆ν1/2-θCO diminutions seen for other electrochemical adlayers are consistent with the occurrence of stochastic broadening at lower θCO associated with microscopically varying COCO dipole interactions.25 Indeed, for the Ir(111)/COaqueous system a quantitative fit to the observed ∆ν1/2θCO behavior was achieved on this basis.4a,25 While inhomogeneous band broadening associated with adlayer hydration may also contribute to the larger ∆ν1/2 values observed at lower θCO , the effect was deemed to be relatively minor at the Ir(111)/CO-aqueous interface since the band shapes are largely Lorenztian rather than Gaussian.4a The last parameter noted above as displaying an unusually nonmonotonic coverage dependence, the Starktuning slope (dνPCO /dE) (Figure 7), while supporting this picture on empirical grounds, is less straightforward to interpret in terms of adlayer structure. The monotonic decreases in (dνPCO/dE) with increasing coverage that are commonly seen at Pt-group electrodes are known, at least for the Pt(111)/CO system, to be due to marked θCO-induced attenuations in the “static-chemical” component, suggesting that the effect is due to increasing competition for metal surface charge (associated probably with dπ-2π back-bonding) toward higher CO coverages.2c,d While corresponding Stark-tuning measurements in UHV are very sparse, a similar θCO-dependent effect is seen for the Pt(111)/CO-UHV interface.44 The reasons for the substantial (ca. 2-fold) jump in (dνPCO /dE) seen at θCO ≈ 0.5 may be connected with the observed alteration in the ∆ΦθCO dependence seen for the UHV system at this coverage.15 The latter effect, involving a switch in the ∆Φ - θCO slope from large positive values to zero at θCO > 0.5, signals a marked change in the incremental adlayer electrostatic properties at this point, again corresponding to the phase transition as noted above. We have already emphasized the markedly different θCO-dependent infrared behavior of the Ir(111)/CO compared with the present Pd(100)/CO system. Both adlayers apparently involve a single CO binding geometry throughout the accessible coverage range, and feature similarly large θCO-induced νPCO blue shifts associated partly with dynamic dipole coupling.4a,13a,45 At least in the electrochemical environment, however, the various θCO-dependent IRAS fingerprints for the Ir(111)/CO adlayer do not exhibit the complex functionalities noted above for the Pd(100)/CO adlayer. A plausible reason for this behavioral difference is to be found in the site-dependent binding energetics. It has been pointed out that translational motion of CO away from a symmetric atop site costs relatively little energy in comparison with a corresponding spatial displacement from a bridging site, perhaps by as much as 5-10-fold.46 Consequently, while attaining nearsaturation CO packing densities on Ir(111) (θCO ≈ 0.7) clearly requires displacement of a substantial fraction of (44) Luo, J. S.; Tobin, R. G.; Lambert, D. K. Chem. Phys. Lett. 1993, 204, 445. (45) Lauterbach, J.; Boyle, R. W.; Schick, M.; Mitchell, W. J.; Meng, B.; Weinberg, W. H. Surf. Sci. 1996, 350, 32. (46) (a) Schweizer, E.; Persson, B. N. J.; Tu¨shaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49. (b) Persson, B. N. J. Chem. Phys. Lett. 1988, 149, 278.

Coverage-Dependent Infrared Spectroscopy

the CO’s away from strictly symmetric atop sites, the energy cost is relatively small, and no abrupt θCO-induced phase transitions are evident from the IRAS data in either the electrochemical or UHV environments.4a,45 In the Pd(100)/CO case, when θCO e 0.5 all the CO’s can be accommodated on symmetric bridging sites with sufficient CO-CO separation (at least x2, i.e., g 3.9 Å) to minimize repulsions. For θCO > 0.5, however, this situation can only be maintained by either shifting CO’s away from bridging sites (yielding noncommensurate structures), or by forming phase/antiphase domain walls featuring locally higher packing densities while maintaining similar CO coordination geometries. Apparently, the energy cost for the former alternative is greater so that the latter is followed, involving a relatively sharp adlayer phase transition. Adlayer Electrooxidation Pathways. So far, we have only discussed the infrared properties of the adlayers formed by direct dosing with dilute CO solutions. Examining the form of the θCO-dependent νCO spectra for adlayers prepared instead by partial electrooxidative stripping in comparison with the former data can yield insight into adlayer oxidation pathways.22 As already mentioned, progressive CO electrooxidative removal on Pt-group surfaces often yields near-invariant νCO frequencies and band shapes even down to low coverages.22 This behavior is symptomatic of the formation of large CO islands during adlayer removal, so that the local (microscopic) CO packing density within these domains, and hence the extent of dipole-dipole coupling, etc., remains largely unchanged. Indeed, approximate estimates of island sizes (at least for CO domain populations, N e 50-100) can be extracted by comparing the θCO-dependent νCO frequencies with dipolecoupling predictions.23 The observation of such extended CO clusters indicates that the reaction occurs via a nucleation-growth process featuring CO oxidation by coadsorbed water (vide infra) involving a “LangmuirHinshelwood” mechanism at the island edges where these species are juxtaposed.22 While some hysteresis between the dosed (increasing coverage) and electrooxidatively formed (decreasing coverage) νPCO-θCO plots are evident (Figure 3, filled circles and diamonds, respectively), the marked νPCO red shiftss observed with decreasing θCO for the latter is notable as well as unusual. Furthermore, the marked band shape changes also seen upon partial electrooxidative removal, which largely mirror those observed for corresponding dosed adlayers (Figure 2), provide a clear indication that the microscopic adlattice configuration changes markedly under these conditions. Indeed, on the basis of model dipole-coupling calculations23 we can deduce from the shape of the νPCO-θCO plot for oxidative removal (filled diamonds, Figure 3) that close-packed CO domains, if present during this process, must be small (e.g., N < 20). This deduction implies that even if the initial removal of CO from the saturated adlayer occurs from a relatively low density of nucleation sites, the local CO islands thereby formed undergo rapid spontaneous dispersion to yield lower microscopic as well as average θCO values. Such island dissipation could well be driven by the need to relieve CO-CO repulsion within the “domain-wall” regions. While the differences observed between the dosed and electrooxidatively formed layers (Figure 3) suggests that some local CO aggregation occurs in the latter case, such hysteresis is anticipated on thermodynamic grounds.47 An interesting property of the present system which may well be connected to the above behavior concerns the

Langmuir, Vol. 15, No. 8, 1999 2939

influence of the CO adsorbate on the metal surface oxidation. As noted above, even small dosed CO coverages (θCO ≈ 0.05) inhibit the initial stage of metal surface oxidation (trace c in Figure 1A). This suggests that both oxide and CO are preferentially adsorbed at similar sites, so that removal of the latter aids formation of the former. Another unusual as well as interesting feature of voltammetric CO oxidation on Pd(100) is that it only commences at potentials, g0.4 V, beyond the onset of surface oxide formation. These two pieces of information suggest that while oxide formation can occur upon partial CO removal, the former is unlikely to constitute the major oxidant for adsorbed CO, this role being provided by water or possibly adsorbed hydroxyl. However, oxide formation may nonetheless influence the spatial nature of the CO adlayer oxidation, perhaps by dissipating CO islands that would otherwise form, thereby facilitating further CO oxidation with water/hydroxyl coadsorbed at adjacent sites. It is interesting to note in this connection that the voltammetric oxidation of low-θCO adlayers formed by prior partial electrooxiative stripping occurs at significantly lower overpotentials than for the saturated adlayer (compare traces c and d with trace a in Figure 1B). This behavior is similar to, although not identical with, that for variableθCO dosed adlayers (Figure 1A). Finally, it is worth mentioning that the voltammetric and infrared spectral aspects of CO adlayer oxidation on Pd(111) are in some respects similar to those of Pd(100).5b In particular, electrooxidative stripping yields marked changes in the νCO spectral fingerprint, indicating that CO islands are largely dissipated.5b However, the θCOdependent νCO behavior is markedly more complex than on Pd(100), indicative of multiple-site CO adsorption,5b as in the UHV case.10,11 The Pd(110)/CO electrochemical system is complicated by CO-induced surface reconstruction,5a as is its UHV-based counterpart.12 This phenomenon is apparently responsible for the slow dissipation of CO domains formed by partial adlayer electrooxidation on Pd(110).5a A common feature of the voltammetric oxidation of CO adlayers on all three low-index Pd surfaces, however, is that CO removal commences at higher overpotentials than those required to initiate metal surface oxidation.5 Overall, the present findings are considered to add significantly to our understanding of both the similarities and differences between CO adlayers on ordered Pt-group surfaces in aqueous electrochemical and UHV environments. Similar to the recently studied Pd(110)/CO system,5 matching the θCO-dependent νCO behavior for the electrochemical interface with corresponding IRAS data for the corresponding UHV-based adlayer, when combined with additional structural information for the latter surface, can aid substantially in the interpretation of the former. While the structural insight obtained in this fashion is necessarily indirect and incomplete, the strategy is of broader significance in identifying additional factors that influence the structural and kinetic behavior of the more complex and, as yet, less-understood electrochemical interface. Acknowledgment. R.G. is grateful to the Ministry of Education and Science (Spain) and U.S.I.A. for a MEC/ Fulbright Fellowship. This work was supported by the National Science Foundation.

(47) Feldberg, S. W.; Rubinstein, I. J. Electroanal. Chem. 1987, 240, 1.

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