Sensitivity of Compressed Carbon Monoxide Adlayers on Platinum(111)

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Sensitivity of Compressed Carbon Monoxide Adlayers on Platinum(111) Electrodes to Long-Range Substrate Structure: Influence of Monoatomic Steps Antonio Rodes, Roberto Go´mez, Juan M. Feliu,* and Michael J. Weaver*,† Departamento de Quı´mica Fı´sica, Universidad de Alicante, E-03080 Alicante, Spain Received August 16, 1999 Infrared spectra are reported for saturated CO adlayers on highly ordered Pt(111) and related stepped surfaces in aqueous 0.1 M HClO4 with the objective of ascertaining the importance of long-range substrate structure to the surface-binding configurations of the compressed adlayers. On Pt(111) in the presence of CO in solution, a pair of CO-stretching (νCO) bands are obtained throughout the accessible potential region below the onset of adsorbate electrooxidation that are diagnostic of atop/3-fold hollow coordination, fingerprinting the (2 × 2)-3CO (θCO ) 0.75) compressed adlayer also characterized earlier by scanning tunneling microscopy (STM). This finding differs from some earlier reports, which indicate a structural conversion to an atop/bridging CO binding arrangement at higher potentials: this transition was seen to be triggered here only by the onset of adsorbate electrooxidation. The corresponding spectral fingerprint obtained in the absence of CO in solution (i.e., for the irreversibly adsorbed case) indicates a mixture of 3-fold and 2-fold bridging along with atop CO, suggesting the presence of less compressed adlayer domains, consistent with the known lower packing densities (θCO ≈ 0.65 to 0.7). The introduction of even occasional (110) or (100) steps, specifically for Pt(17,17,15) and -(17,15,15) (i.e, n(111) × (110) and n(111) × (100) where n ) 16], yields significantly different νCO spectra, especially for the irreversibly adsorbed case which indicate the predominant presence of bridging rather than 3-fold hollow CO, most clearly at higher potentials. The presence of higher (110) step densities attenuates as well as broadens further the multifold νCO bands, eventually (for n ) 2) yielding a spectral fingerprint closely akin to that of Pt(110) itself. Increasing the (100) step density (to n ) 5,2) also removes the multifold νCO features associated with the (111) terraces, bridging νCO bands indicative of bonding on the (100) step sites becoming prevalent. The importance of long-range substrate order in determining the compressed CO adlayer arrangements on Pt(111) terraces is assessed in the light of these findings.

Introduction Ever since the advent of flame-annealing methods for preparing well-ordered monocrystalline samples for electrochemical characterization,1 platinum surfaces have occupied a centrally important position in the developing field of in-situ electrochemical surface science.2 Encouraged by the emergence of infrared reflection-absorption spectroscopy (IRAS) as a powerful technique for vibrational characterization of adsorbates in electrochemical as well as ultrahigh vacuum (UHV)-based environments, the chemisorption of carbon monoxide has received particular attention as a benchmark system for scrutinizing both structural and catalytic properties of monocrystalline metal electrodes.3 In particular, the well-known sensitivity of the vibrational properties of chemisorbed CO to both the surface binding geometry and adsorbate intermolecular interactions, combined with the uniquely detailed structural information available for the corresponding UHV-based systems, endow such adlayers with † Permanent address: Department of Chemistry, Purdue University, West Lafayette, IN 47907.

(1) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (2) For general overviews see: (a) Weaver, M. J.; Gao, X. Annu. Rev. Phys. Chem. 1993, 44, 459. (b) Weaver, M. J. J. Phys. Chem. 1996, 100, 13079. (3) For reviews see: (a) Nichols, R. J. In Adsorption of Molecules at Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; Chapter 7. (b) Korzeniewski, C.; Severson, M. W. Spectrochim. Acta 1995, 51A, 499. (c) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271. (d) Weaver, M. J.; Zou, S. In Spectroscopy for Surface Science Advances in Spectroscopy 26; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, U.K. 1998; Chapter 5.

unique attributes for the microscopic level characterization of Pt-group metal-solution interfaces.4,5 The Pt(111)/CO electrochemical system has engendered much attention, reflecting its archetypical importance along with the multifaceted structural scrutiny afforded the corresponding Pt(111)/CO UHV interface, including detailed IRAS measurements.6 Comparison between IRAS data for the electrochemical and solvent/charge-free UHVbased system reveals several marked differences, the former exhibiting C-O stretching (νCO) bands indicative of bridging as well as atop coordination at low coverages, and significantly (30-50 cm-1) lower νCO frequencies than observed in the latter environment.4,9,10 Such structural dissimilarities can be ascribed both to the effects of solvent coadsorption and the lower surface potentials that typify the aqueous electrochemical system.3d,4,9-12 These factors have been clarified by UHV-based “double-layer modeling” measurements, whereby the effects of the solvent and (4) (a) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (b) Chang, S.-C.; Weaver, M. J.; J. Phys. Chem. 1991, 95, 5391. (5) Tang, C.; Zou, S.; Severson, M. W.; Weaver, M. J.; J. Phys. Chem. B 1998, 102, 8796. (6) (a) Schweizer, E.; Persson, B. N. J.; Tu¨shaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49. (b) Tu¨shaus, M.; Schweizer, E.; Hollins, P.; Bradshaw, A. M. J. Electron Spectrosc. Relat. Phenom. 1987, 44, 305. (7) (a) Persson, B. N. J.; Tu¨shaus, M.; Bradshaw, A. M. J. Chem. Phys. 1990, 92, 5034. (b) Tu¨shaus, M.; Berndt, W.; Conrad H.; Bradshaw A. M.; Persson, B. Appl. Phys. 1990, A51, 91. (8) Bradshaw, A. M.; Schweizer, E. In Spectroscopy of Surfaces; Advances in Spectroscopy 16; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, U.K. 1988; Chapter 8. (9) Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (10) (a) Weaver, M. J.; Zou, S.; Tang, C. J. Chem. Phys. 1999, 111, 368. (b) Weaver, M. J. Surf. Sci. 1999, 437, 215.

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interfacial charge on the Pt(111)/CO infrared spectra can be scrutinized progressively and in unique detail.11,12 These Pt(111)/CO adlayer structural differences extend even to saturated CO coverages. In-situ scanning tunneling microscopy (STM) along with IRAS identified a pair of adlayer structures for the Pt(111)/CO acidic aqueous interface in equilibrium with solution phase CO.13 At low potentials, a (2 × 2)-3CO adlayer was identified from STM, featuring two three-fold-hollow and one atop CO per unit cell.13 These coordination geometries yield characteristic νCO bands at ca. 1780 and 2070 cm-1, respectively, blueshifting as usual toward higher potentials.13 The coverage of this compressed adlayer, θCO ) 0.75, is higher than that usually achieved in UHV, even at low temperatures, most likely facilitated by the relative stabilization of multifold CO coordination at the negatively charged electrochemical interface. At higher electrode potentials, close to the onset of CO electrooxidation, a transformation to a lower-coverage compressed structure (θCO ) 0.68) was observed, having a (x19 × x19)R23.4°-13CO unit cell.13 While this structure is spatially more complex, featuring asymmetric as well as symmetric bridging and atop CO binding geometries, the prevalence of dynamic dipole-dipole coupling results again in only a pair of νCO bands, now at ca. 1850 and 2075 cm-1, respectively.13,14 While the lower-frequency νCO band intensities in both structures are suppressed due to dipole coupling with other adlayer vibrational components,14 the marked (ca. 60-70 cm-1) difference in the multifold νCO band frequency in the two adlayer structures renders their identification by means of IRAS quite straightforward. Interestingly, both these hexagonal compressed CO structures are notably different to those implied from lowenergy electron diffraction7 (or STM15) data for saturated Pt(111)/CO UHV adlayers at low temperatures, which indicate the formation of more complex periodic (phaseantiphase) domain-wall structures.7 Following the original STM/IRAS report,13 corroborating evidence for the formation of a compressed (2 × 2) adlayer and its potential-induced conversion to the (x19 × x19) structure has been obtained from these and less direct means by several other laboratories.16-18 An interesting, yet outwardly unexplained, feature of this system evident from these studies is that the potential at which the (2 × 2) w (x19 × x19) adlayer transition is observed (by IRAS or otherwise) is somewhat variable, values from ca. 0.3 to above 0.6 V (where CO electrooxidation starts in earnest) being obtained.13,16-19 While the adlayer structural change may therefore be connected with the onset of adsorbed CO electrooxidation (vide infra), this is apparently not a requirement. Thus the same phase transition has been noted by in-situ IRAS for Pt(111)/CO in a number of nonaqueous solvents at comparable electrode potentials, (11) (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. Langmuir 1995, 11, 2777. (d) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 5842. (12) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 10166. (13) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (14) Severson, M. W.; Stuhlmann, C.; Villegas, I.; Weaver, M. J. J. Chem. Phys. 1995, 103, 9832. (15) Song, M.-B.; Yoshimi, K.; Ito, M. Chem. Phys. Lett. 1996, 263, 585. (16) Yoshimi, K.; Song, M.-S.; Ito, M. Surf. Sci. 1996, 368, 389. (17) Akemann, W.; Friedrich, K. A.; Stimming, U. quoted in: Akemann, W.; Friedrich, K. A.; Linke, U.; Stimming, U. Surf. Sci. 1998, 402-404, 571. (18) Lucas, C. A.; Markovic, N. M.; Ross, P. N. Surf. Sci. 1999, 425, L381. (19) Tang, C.; Weaver, M. J. Unpublished observations.

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even though CO electrooxidation does not commence until much higher potentials.20 Given the continuing interest in the Pt(111)/CO adlayer system, we have now examined electrochemical infrared spectra obtained for saturated CO adlayers on both highly ordered Pt(111) terraces and on surfaces containing low as well as higher densities of either (110) or (100) oriented steps. These measurements take advantage of the Pt crystal preparation tactics developed by Clavilier et al., utilizing accurately cut and polished single-crystal beads prepared by melting Pt wires.21 Besides yielding Pt(111) surfaces featuring reproducibly large terraces, resulting from accurate crystal orientation together with the flameannealing pretreatment, this in-house procedure can readily enable a systematic sequence of higher-index Pt faces to be scrutinized, including those featuring low yet variable densities of ordered monatomic steps.35,36 (The presence of ordered monatomic steps for most high-index faces featuring either (110) or (100) oriented steps, known from earlier studies in UHV,22 are indeed confirmed from STM measurements in air.23) While there are several detailed IRAS studies reported for CO chemisorption on stepped Pt electrodes,3c,24,25 the response of the infrared spectral fingerprints on Pt(111) terraces interrupted by a systematically varying step density is a largely unexplored issue. The salient findings are presented herein. The results uncover an interesting sensitivity of the infrared spectral fingerprint, and by implication the CO (20) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5378. (21) Clavilier, J.; Armond, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (22) (a) Blakely, D. W.; Somorjai, G. A. Surf. Sci. 1977, 65, 419. (b) Ross, P. N., Jr. J. Chim. Phys. 1991, 88, 1353. (23) Herrero, E.; Orts, J. M.; Aldaz, A.; Feliu, J. M. Surf. Sci. 1999, 440, 259. (24) (a) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484. (b) Kim, C. S.; Korzeniewski, C.; Tornquist, W. J. J. Chem. Phys. 1994, 100, 628. (c) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Chem. Phys. 1994, 101, 9113. (d) Kim, C. S.; Korzeniewski, C. Anal. Chem. 1997, 69, 2349. (25) (a) Nakamura, M.; Ogasewara, H.; Imukai, J.; Ito, M. Surf. Sci. 1993, 283, 248. (b) Wanatabe, S.; Inukai, J.; Ito, M. Surf. Sci. 1993, 293, 1. (26) Clavilier, J.; El Achi, K.; Pitit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295, 333. (27) Iwasita, T.; Nart, F. C.; Vielstich, W. Ber. Bunsen-Ges.. Phys. Chem. 1990, 94, 1030. (28) Weaver, M. J. Appl. Surf. Sci. 1993, 67, 147. (29) Further evidence for the occurrence of such infrared intensitystealing effects even for oscillator components having markedly (ca. 200-250 cm-1) different singleton frequencies has been obtained recently for intermixed CO/NO adlayers on Ir(111) and other electrode surfaces.30 (30) (a) Weaver, M. J.; Tang, C.; Zou, S.; Severson, M. W. J. Chem. Phys. 1998, 109, 4135. (b) Tang, C.; Zou, S.; Severson, M. W.; Weaver, M. J. J. Phys. Chem. B. 1998, 102, 8546. (c) Tang, C.; Zou, S.; Weaver, M. J. Surf. Sci. 1998, 412/413, 344. (d) Tang, C.; Zou, S.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem. 1999, 467, 92. (31) Go´mez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48. (32) Climent, V.; Go´mez, R.; Feliu, J. M. Electrochim. Acta 1999, 45, 629. (33) Note that the θCO measurements undertaken in refs 31 and 32 employ CO charge displacement tactics, involving current transient measurements following dilute CO introduction to precisely evaluate the double-layer corrections to the ensuing anodic coulometry.31 There is evidence, however, that the use of higher, near-saturated CO concentrations and longer (>1 min) CO exposure times can yield slightly higher θCO values, again determined from anodic coulometry.39 (34) Note that the saturated calomel reference electrode (SCE) employed in refs 4 and 9 has a potential ca. 0.3 V higher than the RHE used here in 0.1 M HClO4. (35) (a) Clavilier, J.; El Achi, K.; Rodes, A. J. Electroanal. Chem. 1989, 272, 253. (b) Clavilier, J.; El Achi, K.; Rodes, A.; Chem. Phys. 1990, 141, 1. (c) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 289, 245. (36) Clavilier, J.; Rodes, A.; El. Achi, K.; Zamakhchari, M. A. J. Chem. Phys. 1991, 88, 1291.

Effect of Substrate Structure on CO Adlayers

Figure 1. Infrared absorbance spectra obtained in C-O stretching region for saturated CO adlayers on well-ordered Pt(111) in aqueous 0.1 M HClO4 in (A) the presence and (B) the absence of ca. 1 mM CO solution. Spectra acquired (each 100 interferograms, at 8 cm-1 resolution) at increasing potentials vs RHE as indicated (see text).

microscopic adlayer structure, to both the Pt(111) terrace width and the step orientation. Experimental Section The Pt surfaces, diameter ca. 4.5 mm, were prepared by using the procedures outlined in ref 21, and were pretreated immediately prior to the measurements by means of flame annealing followed by cooling in a H2/Ar atmosphere.26 The electrolytes were prepared from concentrated perchloric acid (Merck Suprapur) and ultrapure water (Millipore Milli Q system). The electrolyte was deaerated by bubbling argon (Air Liquide N50); carbon monoxide was also from Air Liquide (N47). The electrochemical IRAS measurements utilized a Nicolet Magna 850 FTIR spectrometer equipped with a narrow-band MCT detector. The spectroelectrochemical cell27 featured a prismatic CaF2 window beveled at 60°. A Pd/H2 reference electrode was used; all electrode potentials quoted herein are versus the reversible hydrogen electrode (RHE).

Results and Discussion Pt(111) Terraces. The protocol primarily followed here was to acquire IRAS data (each 100 interferograms, at 8 cm-1 resolution) for a given Pt surface at an increasing positive sequence of electrode potentials, usually commencing at 0.1 V vs RHE, in 0.1 V increments up to 0.6 V, followed by a corresponding reference spectrum obtained at a sufficiently high potential (0.9-1.0 V) so that CO electrooxidative removal was complete, to yield potential-difference infrared spectra for chemisorbed CO in the usual fashion.9 Two solution conditions were employed, involving spectral measurements either (a) in the presence of CO-saturated 0.1 M HClO4, or (b) after removing the solution CO by Ar purging at 0.1 V. We will refer to these conditions as yielding reversibly and irreversibly adsorbed saturated layers, respectively. Figure 1A and B show representative potential dependent infrared absorbance spectra obtained in the νCO frequency region (1700-2200 cm-1) on Pt(111) for these two solution conditions. The reversible adlayer (Figure 1A) consistently yields a pair of νCO bands, located at 2065-

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2075 cm-1 and 1785-1805 cm-1, blueshifting as usual toward higher potentials. As mentioned above, this spectral fingerprint is diagnostic of the presence of a (2 × 2)-3CO (θCO ) 0.75) adlayer, the higher- and lowerfrequency νCO features being indicative of the presence of atop and 3-fold-hollow CO, respectively.13 The average Stark-tuning slopes (dνCO/dE) for these features, 25 and 45 cm-1 V-1, reflect the higher values commonly observed for multifold versus atop CO.4a,28 As detailed in ref 14, the ca. two- to three-fold higher absorbance of the atop νCO feature can be rationalized with the 2:1 site occupancy in favor of the multifold CO geometry on the basis of intensity stealing predicted from dynamic dipole-dipole coupling within the highly compressed adlayer.29,30 Consequently, the smaller intensities of this and other multifold νCO features discussed below with respect to the atop band belie their major contribution to the adlayer structures. Significantly, the (2 × 2) spectral fingerprint was consistently retained on the highly ordered Pt(111) surface examined here in CO-saturated 0.1 M HClO4 throughout the electrode potential region, ca. 0.1-0.5 V, in the absence of any CO electrooxidation. However, any slight occurrence of CO oxidation as sensed directly from the appearance of the 2345 cm-1 CO2 band was usually accompanied by the substitution of the 1800 cm-1 threefold-hollow band by a feature at 1850 cm-1, diagnostic of bridging CO. This process was seen to be initiated usually between 0.5 and 0.6 V, although the onset potential depends somewhat on the measurement timescale and other experimental conditions. This observed stability of the (2 × 2) adlayer throughout the range of potentials below the onset of CO electrooxidation is distinctly different to some earlier findings,4a,13,16 where its conversion to the (x19 × x19) structure featuring two-fold-bridging CO has been observed in the range 0.3-0.5 V (vide supra). It is worth noting in this context, however, that the (2 × 2) w (x19 × x19) transformation potential in CO-saturated 0.1 M HClO4, as gleaned from IRAS measurements, was found to be sensitive to the particular Pt(111) sample used as well as to the details of the flame annealing pretreatment procedure, values varying from 0.3 to 0.6 V vs RHE being obtained.19 However, given the highly ordered nature of the Pt(111) crystal employed here, likely yielding a lower surface defect/step density than for most of the larger (ca. 1 cm) samples commonly employed for IRAS measurements, the present results are likely to reflect the nature of compressed CO adlayers on large Pt(111) terraces. This central issue is considered further below, after considering the behavior of irreversibly adsorbed layers on Pt(111) surfaces containing periodic steps. Figure 1B shows corresponding potential-dependent spectra also obtained on ordered Pt(111), but now after CO dosing at 0.1 V followed by argon purging to remove the solution CO, i.e., for an irreversible saturated adlayer. Comparison with Figure 1A reveals subtle yet significant spectral differences. At the lower potentials, the symmetric 1785-1795 cm-1 band due to three-fold-hollow CO is replaced with a weaker feature characterized by a broad tail toward higher frequencies. This spectral fingerprint strongly suggests that the uniform (2 × 2) adlayer arrangement maintained in the presence of solution CO is replaced at least in part with lower-θCO domains featuring bridging rather than three-fold-hollow CO. This interpretation is supported by the lower CO coverages, θCO ≈ 0.65-0.68, deduced under solution-free CO conditions from anodic coulometry with appropriate doublelayer corrections,31,32 although slightly higher coverages

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Figure 2. Infrared absorbance spectra obtained for saturated CO adlayers on ordered Pt(17,17,15) (A) and -(17,15,15) (B) in CO-saturated 0.1 M HClO4 at the increasing potentials indicated.

are typically obtained for the near-saturated CO solutions and longer (ca. g1 min) exposure times employed here.32 Such θCO values are close to that for the (x19 × x19) adlayer, 0.685; most likely, then, patches of this or related structural arrangements coexist with small (2 × 2) domains. Nonetheless, the lower-potential (0.1-0.3 V) νCO fingerprint in Figure 1B is subtly different to spectra reported previously for saturated irreversibly adsorbed CO on Pt(111) in acidic aqueous media, which exhibit a single symmetric multifold νCO band at 1830-1845 cm-1 (over the potential range 0.1-0.5 V).4a,9,34 As discussed further below, these spectral differences may reflect the influence of randomly distributed surface steps or other defects on the Pt(111)/CO adlayer structure examined in these earlier measurements. Interestingly, precisely this previously observed νCO spectral form is evident at the highest potential, 0.5 V, shown in Figure 1B; it is a clearcut bridging νCO band at 1845 cm-1 replacing the broad asymmetric feature seen at lower potentials. However, this marked transition in the multifold band structure, which occurs just below 0.5 V, is accompanied by a slight electrooxidative θCO decrease as signaled from the appearance of a weak yet discernible CO2 band at 2345 cm-1 (not shown in Figure 1B). Influence of Periodic (110) and (100) Steps. A selected set of infrared spectra obtained in CO-saturated 0.1 M HClO4 under the same conditions as in Figure 1A, but now for the pair of stepped surfaces Pt(17,17,15) and -(17,15,15) are shown in Figures 2A and B, respectively. These surfaces feature 16-atom wide (111) terraces punctuated by monatomic (110) and (100) steps, respectively. Unlike some vicinal faces, these surfaces do not undergo faceting in UHV;22 indeed, the presence of uniform terrace-step arrangements is supported by the systematic changes in the voltammetric features observed with varying nominal step density within both these crystallographic zones.35,36 The potential-dependent spectra for a reversible saturated CO adlayer on Pt(17,17,15) (Figure 2A) are comparable to the corresponding Pt(111) data (Figure 1A), suggesting that the (2 × 2)-3CO adlayer structure is not affected greatly by the presence of periodic

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Figure 3. As Figure 2, but for saturated CO adlayers in COfree 0.1 M HClO4; i.e., after the removal of solution CO.

(110) steps. The multifold νCO band is nonetheless clearly broadened and significantly (10 cm-1) redshifted on the dilute stepped surface. Interestingly, both the frequency and band shape of this multifold band on stepped Pt(111) are very similar to those observed earlier on nominally Pt(111) terraces,4a,9 suggesting a role of defects in influencing the latter. However, the atop/three-fold-hollow νCO fingerprint observed on Pt(17,17,15), as on Pt(111), in COsaturated solution, is maintained up to the onset of CO electrooxidation, close to 0.6 V. The potential-dependent spectra on the (100) stepped surface, Pt(17,15,15) (Figure 2B), are closely similar to those on Pt(17,17,15). A significant difference, however, is the appearance of a weak yet discernible νCO feature at 1845-1880 cm-1, blueshifting toward higher potentials. As discussed below, this band probably arises from twofold-bridging CO located on the (100) steps rather than on the (111) terraces. Infrared spectra for saturated irreversibly adsorbed CO layers on both Pt(17,17,15) and -(17,15,15), i.e., in the absence of solution CO, exhibit more noticeable differences when compared with Pt(111). Typical data, again obtained over the range 0.1-0.5 V where the adlayer is typically stable toward electrooxidation, are shown for these two stepped surfaces in Figure 3A and B, respectively. Comparison with corresponding spectra for the reversible adlayer case (Figure 2A,B) shows that the three-foldhollow band is largely replaced by a broader, less distinct feature that is centered at frequencies 1820-1840 cm-1, suggesting the predominant presence of two-fold-bridging CO. The difference is especially prevalent at the highest potential shown, 0.5 V, where a clear-cut bridging CO band at 1840 cm-1 is obtained (Figure 3A,B), even though no significant CO electrooxidation has occurred by this point. The weak higher-frequency feature on Pt(17,15,15) assigned to bridging CO on the (100) steps (cf., Figure 2B) is also clearly resolved from the terrace bridging CO band at 0.5 V, although some spectral overlap occurs at lower potentials (Figure 3B). Comparison between these spectra for irreversibly adsorbed CO on Pt(17,17,15) (Figure 3A) with those on Pt(111) (Figure 1B), at least at lower potentials, implicates a role of the substrate steps in altering the multifold νCO

Effect of Substrate Structure on CO Adlayers

Figure 4. Comparison of infrared absorbance spectra obtained for saturated CO adlayers at 0.1 V vs RHE in CO-saturated 0.1 M HClO4 for a series of stepped Pt surfaces in the (n(111) × (110)) (A) and (n(111) × (100)) (B) zones, as indicated.

band structure, shifting the adsorbate coordination from three-fold-hollow to two-fold-bridging CO. Interestingly, the presence of such dilute steps has also been found to depress significantly the saturated coverage of irreversibly adsorbed CO, from 0.67 to 0.57 or so, from CO chargedisplacement measurements combined with anodic coulometry.31b Evidently, then, the reversibly and, especially, the irreversibly adsorbed saturated adlayers are altered significantly in the presence of even dilute step densities, lower packing densities being formed that tend to involve two-fold-bridging rather than three-fold-hollow, along with atop, surface coordination. As expected, the presence of higher (110) and (100) step densities exerts more profound changes on the νCO spectra. Representative data for a sequence of three such stepped Pt surfaces obtained at 0.1 V in CO-saturated 0.1 M HClO4 are shown in comparison with Pt(111) in Figure 4A and B, respectively. In the n(111) × (110) zone (Figure 4A), in addition to the 16-atom wide surface (17,17,15) already considered, spectra are shown for Pt(443) (n ) 7) and Pt(331) (n ) 2). The 7-atom (111) terrace yields a broader and less well-defined multifold νCO feature than seen for the wider, 16-atom, terrace, and especially the Pt(111) surface itself (Figure 4A). At higher potentials, ca. 0.5 V, this broad 1775 cm-1 band is replaced by a similarly weak twofold-bridging νCO band at ca. 1835 cm-1. Corresponding measurements on Pt(443) for irreversibly adsorbed CO yield a weak broad band similar to the latter feature (1825-1835 cm-1) at all potentials. This greater propensity toward bridging coordination for irreversibly versus reversibly adsorbed CO on Pt(443) is consistent with the trends noted above for Pt(17,17,15) and Pt(111). Narrowing the (111) terrace width further to 2 atoms, Pt(331), yields not only a virtual disappearance of the multifold νCO feature but also a significant blueshift and intensification of the atop νCO band (Figure 4A). Indeed, this spectral fingerprint is closely similar to that seen on Pt(110), exclusively atop coordination being obtained.37 Apparently, then, the high density of (110) step sites (37) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 230, 222.

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together with the short (111) terraces on Pt(331) disrupts entirely the atop/multifold compressed adlayer arrangement on the latter, encouraging atop coordination on the nearby terrace as well as step-edge Pt atoms. Further insight into the marked influence that step sites exert on compressed CO adlayers is evident in the corresponding νCO spectra shown on three (100) stepped surfaces in Figure 4B, again at 0.1 V in CO-saturated 0.1 M HClO4. These include Pt(322) and (311) (for which formally n ) 5 and 2, respectively) in addition to Pt(17,15,15) (n ) 16). The Pt(322) terrace width is intermediate between those on the Pt(533) and (755) surfaces for which CO adsorption has been examined with electrochemical IRAS in detail by Korzeniewski and coworkers.24 The spectral fingerprint on Pt(322) (Figure 4B) is closely similar to corresponding data on Pt(100),38 the atop νCO feature at 2064 cm-1 being accompanied by a bridging νCO band at a frequency, 1872 cm-1, which is higher than is characteristic for Pt(111) terraces. This spectral form is seen throughout the potential range 0.1 to 0.5 V, and for saturated adlayers in the absence as well as presence of solution CO. Similar νCO features are observed under these conditions on both Pt(755) and (533).23,24 Given the trends seen with increasing (100) step density (Figure 4B), it is tempting to ascribe the 1872 cm-1 feature to bridging CO bound at the (100) step sites rather than at (111) terraces. However, it is most likely that both these binding modes contribute to the 1872 cm-1 feature, with the higher-energy step-site component dominating the band intensity via dipole-dipole coupling. Such intensity transfer effects can also account for the detectable presence of bridging CO bound to (100) sites even on the dilute stepped surface Pt(17,15,15) (vide supra, Figure 4B). The last nominally (111)-(100) stepped surface included in Figure 4B, Pt(311), exhibits significantly different spectra to Pt(322). The marked νCO band seen at 1900 cm-1 (at 0.1 V) is similar to that observed on Pt(711) (i.e., 4(100) × (111)), which has been ascribed to CO at asymmetric bridge sites on (100) facets.25b It should be borne in mind, however, that CO adlayers on nominally Pt(100) electrodes have been observed by STM to trigger extensive surface restructuring, featuring small (3-4 nm terraces) punctuated by 2-atom steps.40 The microscopic interpretation of νCO spectral features at surfaces containing (100) terraces is therefore somewhat ambiguous. Overall Implications: Nature of Compressed CO Adlayers on Pt(111). It is apparent from the foregoing that the presence of periodic steps on Pt(111) terraced surfaces exerts significant and even profound alterations in the binding-site structural arrangements for both reversible and irreversible compressed CO adlayers. In the absence of steps, an atop/three-fold-hollow νCO fingerprint is reproducibly observed in the presence of solution CO at all potentials below the onset of CO electrooxidation. The highly compressed (θCO ) 0.75) nature of this adlayer arrangement is compromised for irreversibly adsorbed layers on Pt(111), where the more complex multifold νCO fingerprint (Figure 1B) indicates instead the presence of a mixture of two-fold and threefold binding geometries. This structural modification is consistent with the lower packing densities (θCO ≈ 0.650.7) normally achieved in the absence of solution CO.31,32 (38) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095. (39) Orts, J. M.; Fena´ndez-Vega, A.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal Chem. 1992, 327, 261. (40) (a) Villegas, I.; Weaver, M. J. J. Electroanal. Chem. 1994, 373, 245. (b) Vitus, C. M.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7559.

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The more pronounced occurrence of bridge-bound rather than three-fold-hollow coordination for irreversibly adsorbed CO evident on the dilute stepped surfaces Pt(17,17,15) and -(17,15,15), especially at higher potentials (Figure 3), along with the still-lower packing densities (θCO ≈ 0.55) attainable on these surfaces,32 provides a clear indication of the importance of long-range order in determining the compressed adlayer arrangement. This central point is underscored further by the progressive changes seen in the multifold νCO region upon increasing the (110) or (100) step density in the presence as well as absence of solution CO (Figures 3 and 4), indicating clearly a selective destabilization of the three-fold- versus twofold-bound CO in the compressed adlayer structures. Evidently, the compressive forces required to generate the (2 × 2)-3CO or related adlayer arrangements characteristic of the Pt(111)/CO electrochemical interface can be achieved only with a degree of long-range substrate order of substantially larger dimensions than the adlattice unit-cell itself. The earlier observations of the (2 × 2)3CO adlayer arrangement on Pt(111) preferentially at lower potentials13,16,17 is consistent with the expectation that three-fold CO coordination is stabilized increasingly at larger negative electrode charges from the greater dπ2π* back-bonding occurring under these conditions.11a,41,42 While the survival of the (2 × 2) arrangement on Pt(111) even up to ca. 0.5-0.6 V, as seen here, corresponds to smaller charge densities, the high potential of zero charge for the Pt(111)/CO interface, about 1.0 V,43 indicates that the electrode charge remains negative. Consistent with this picture, while this adlayer arrangement has not been observed on Pt(111) in UHV, the (2 × 2)-3CO structure (41) Mehandra, S. P.; Anderson, A. B. J. Phys. Chem. 1989, 93, 2044. (42) Koper, M. T. M.; Van Santen, R. A. J. Electroanal. Chem. 1999, 476, 64. (43) Weaver, M. J. Langmuir 1998, 14, 3932.

Rodes et al.

can be attained on Pd(111) at low temperatures,7b,44 a surface for which multifold CO coordination is known to be more stable relative to atop or bridging geometries than is the case on uncharged Pt(111).45 Presumably, then, the apparent presence of this highly compressed adlayer structure on the present ordered (111) terraces, even at potentials up to the onset of CO electrooxidation, reflects a stabilization afforded by a longer-range propagation of intermolecular forces. This stabilization, however, can be altered markedly by the presence of surface defects and steps. Given these findings, it would be of interest to undertake Monte Carlo or other model simulations in order to elucidate the manner and extent to which the propagation of dipole-dipole and other adsorbate-adsorbate interactions may influence the structure and packing densities within such compressed electrochemical adlayers. From an analytical perspective, the observed sensitivity of the infrared spectral fingerprint to the nature and degree of longer-range Pt(111) substrate order may also offer an additional means of characterizing such monocrystalline electrode surfaces beyond the appearance of the wellknown structure-sensitive voltammetric features.1,36 Acknowledgment. M.J.W. is grateful to the Ministerio de Educacio´n y Cultura (Spain) for a Visiting Scientist Fellowship (Programa de Estancias de Investigadores extranjeros en re´gimen de an˜o saba´tico en Espan˜a). This work was also supported by the DGES through project PB96-0409. LA991104U (44) Kuhn, W. K.; Szanyi, J.; Goodman, D. W. Surf. Sci. 1992, 274, L611. (45) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107.