J. Phys. Chem. B 2000, 104, 6049-6052
6049
In Situ Infrared Reflection Absorption Spectroscopic Studies of Coadsorption of CO with Underpotential-Deposited Lead on Pt(111) in an Aqueous Acidic Solution Nagahiro Hoshi,† In Tae Bae, and Daniel A. Scherson* Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, OH 44106-7078 ReceiVed: March 2, 2000
The effects of underpotential-deposited lead on the adsorption of CO on Pt(111) surfaces have been investigated in 0.1 M HClO4 by in situ infrared reflection absorption spectroscopy (IRAS). Lead coverages of about 0.4 on Pt(111) electrodes polarized at 0.1 V vs RHE prevent CO from adsorbing on multiply bonded sites, reducing the overall coverage to about a third of that observed in the absence of coadsorbed Pb under otherwise identical conditions, yielding a single, sharply-defined feature in the infrared reflection absorption spectra characteristic of atop-bonded CO. However, the integrated absorption band of atop CO at 0.1 V in the presence of coadsorbed Pb was found to be about 3 times higher than that predicted from the corresponding spectral features observed for Pb-free surfaces at that specific coverage corrected for dipole-dipole coupling interactions. Although much of this effect may be due to a Pb-induced decrease in the dielectric screening within the CO adlayer, the gains in intensity are too large to be explained solely on this basis, pointing to chemical factors, such as Pt-Pb charge polarization, as an important factor.
Introduction The integrated intensity, Ai, of infrared bands of species adsorbed on metal surfaces is often influenced by the coverage and also by the presence of other coadsorbates. Theoretical considerations predict that in the case of pure chemisorbate layers, Ai is proportional to Rv(1 + ReU ˜ )-2,1 where Rv represents the vibrational polarizability of the adsorbate, Re the electronic polarizability of adjacent coadsorbates, which may include the same species, and U ˜ is a geometric parameter that depends on the spatial arrangement of adsorbates within the adlayer, known as the lattice sum term. It becomes evident from this (and other) model(s)2 that Ai should decrease as Re increases, as illustrated most clearly by the much higher initial slopes of Ai vs coverage of CO adsorbed on Ir(111) observed in UHV compared to aqueous environments.3 Much of our current understanding of this dielectric screening of the dynamic dipole moment by neighboring species has been derived from studies involving CO adsorption of Pt4 and Ir3 single-crystal surfaces, in both ultrahigh vacuum (UHV) and electrochemical environments. This work examines changes in the infrared absorption features of CO adsorbed on Pt(111) in 0.1 M HClO4 aqueous solutions induced by coadsorption of underpotential-deposited Pb. As will be shown, adsorption of Pb on Pt(111) polarized at 0.1 V vs RHE reduces the overall CO coverage to about a third that observed at the same potential in the absence of Pb, and prevents adsorption of CO on multibonded sites. The latter is clearly evidenced by the occurrence of a single sharp band in the infrared reflection absorption spectrum centered at 2044 cm-1 ascribed to CO adsorption on atop sites. This behavior resembles that observed much earlier by Chang and Weaver for CO coadsorbed with irreversibly bound Bi on Pt(111) in the same electrolyte.5 Analysis of the integrated absorption band of atop CO in the presence and absence of coadsorbed Pb yielded values of Ai† Permanent address: Department of Applied Chemistry, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
(CO(atop)) in excess of those estimated for the atop feature for the Pb-free system, suggesting that the absorptivity of atopadsorbed CO is enhanced in the presence of the coadsorbed metal on the surface. Experimental Section The spectroelectrochemical cell used for these measurements is similar to that reported by Nart et al.6 In this specific design, the IR beam impinges onto the electrode surface from below through a 60° CaF2 dove prism window. This arrangement allows voltammetric curves to be recorded in the hanging meniscus configuration, thereby avoiding contributions due to non-single-crystal areas of the specimen. Once the electrochemical characterization is completed, the well-oriented surface can be pressed against the prism for spectral acquisition. In situ infrared reflection absorption spectroscopy (IRAS) measurements were performed with a Brucker FTIR instrument with p-polarized light at 4 cm-1 resolution. Experiments were carried out in aqueous 0.1 M HClO4 (Baker Ultrapure reagent) diluted in 18 MΩ water (Barnstead) solutions at room temperature employing well-oriented Pt(111) single crystals (ca. 1 cm in diameter) prepared by annealing in an O2/ H2 flame followed by cooling in cold Ar.7 As is customary, the surface of the crystal was protected with a drop of ultrapure water during transfer to the spectroelectrochemical cell. Electrochemical data were recorded using a PAR 173 potentiostat/galvanostat and a PAR 175 universal programmer in the same cell where the IRAS measurements were performed. A reversible hydrogen electrode in the same solution (RHE) and a circular band Pt foil were used as reference and counter electrodes, respectively. Once the good quality of the surface had been verified, CO was bubbled into the solution for 15 min, and then removed by purging with N2, while the electrode was polarized at either 0.1 or 0.4 V vs RHE. A small aliquot of a 3 mM Pb2+ solution prepared by dissolving PbO (Johnson Matthey 99.9999%) in 0.1 M HClO4 was then added to the
10.1021/jp0008391 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/31/2000
6050 J. Phys. Chem. B, Vol. 104, No. 25, 2000
Hoshi et al.
media to yield a final concentration of 0.5 mM Pb2+. All voltammetric data were obtained with the single crystal in the hanging meniscus configuration. Spectral data, representing the coadded average of 400 scans, were collected after Pb-CO coadsorption by pressing the Pt(111) electrode polarized at either 0.1 or 0.4 V against the window. The potential was then stepped to 0.8 V, which is positive enough for full CO oxidation to ensue, and after about 60 s, a new series of spectra were acquired. All IRAS spectra are displayed in the form -∆R/R ) -[R(Eref) - R(Esam)]/R(Eref) vs wavenumber, where R(Eref) is the average reflection spectrum at the reference potential Eref ) 0.8 V and R(Esam) the corresponding spectrum at either 0.1 or 0.4 V. On the basis of this convention, contributions due to products (dissolved carbon dioxide) and reactants (adsorbed CO) point in either negative or positive directions, respectively. Results and Discussion Figure 1 shows the cyclic voltammogram of Pt(111) in 0.1 M HClO4 obtained in the spectroelectrochemical cell in the hanging meniscus configuration. These curves were recorded after adsorbing CO to saturation coverages at 0.1 V (panel a) and 0.4 V (panel b) followed by N2 purging and, subsequently, adding lead into the solution to a concentration of 0.5 mM with the CO-covered Pt(111) electrodes polarized at the specified potentials. In each case, the potential was scanned first in the negative direction to 0.05 V vs RHE and, then, positively to ca. 1.0 V (see the solid line). The curves observed for the CO/ Pt(111) interface polarized at 0.4 V were very similar to those obtained in the absence of lead in the solution, except that the CO coverage estimated from the CO oxidation peak, without capacitive corrections, was slightly smaller, 0.59, than that observed in the Pb-free solution, 0.63. In contrast, polarization at 0.1 V yielded, in addition to a much more diminished oxidation peak centered at 0.70 V (see below), a prominent feature centered at ca. 0.55 V. According to Borup et al.,8 who studied coadsorption of Pb and CO on Pt(111) in the same electrolyte, this latter peak originates from a Pb-derived redox process and, thus, is not directly related to CO oxidation. Support for this view, as shown originally by these authors, was obtained from voltammetric experiments in which the upper limit was restricted to 0.6 (see the inset, Figure 1), for which the feature at 0.55 V was observed in the first and all subsequent cycles. On the basis of the shape and relative size of the stripping features reported in Figure 2, ref 8, for saturation coverages of CO as a function of Pb coverage, θPb, the actual lead coverages obtained in our work for CO/Pt(111) polarized at 0.1 V would be larger than 0.43 but definitely smaller than 0.50 (see below). Once CO was electrochemically oxidized, by scanning the potential to 1.0 V, the peak centered at 0.55 V (see the dotted line, panel a) disappeared, yielding the characteristic profile of Pb UPD on Pt(111) in 0.1 M HClO4 reported by other authors,9 regardless of whether the electrode had been polarized at 0.1 or 0.4 V. A voltammetric behavior virtually identical to that shown in Figure 1 was found by bubbling CO into a leadcontaining solution while polarizing the electrode at either 0.1 or 0.4 V, providing evidence that the observed phenomenon is a function of the applied potential, but independent of the sequence in which the two-adsorbate interface is assembled. Curve b in panel A of Figure 2 shows IRAS spectra-of CO adsorbed on Pt(111) in neat (lead-free) 0.1 M HClO4, at 0.1 V, using the spectrum obtained at 0.8 V as a reference. In agreement with data reported in the literature,10 these spectra are characterized by two positive-pointing features at 2065 and
Figure 1. Cyclic voltammograms of Pt(11) in 0.1 M HClO4 obtained after adsorbing CO to saturation coverages at 0.1 V (panel a) and 0.4 V (panel b) followed by N2 purging and, subsequently, adding Pb2+ into the solution to a concentration of 0.5 mM with the CO/Pt(111) electrodes polarized at the specified potentials (solid lines). In each case, the potential was scanned first in the negative direction to 0.05 V vs RHE and then positively to ca. 1.0 V (see the solid line). The cyclic voltammograms in dotted lines were obtained during the second and subsequent scans initiated at 1.0 V vs RHE. Inset: cyclic voltammogram of CO/Pb/Pt(111) surfaces prepared as in panel a in this figure, but with the upper potential limit restricted to 0.6 V vs RHE.
1780 cm-1, ascribed to CO adsorbed on atop and 3-fold hollow sites, respectively, and one negative-pointing peak at 2343 cm-1 attributed to solution-phase CO2. Also in accordance with other authors (see, for example, Figure 8 in ref 11) is the diffusion of adsorbed CO from 3-fold to bridge sites at more positive potentials, 0.4 V vs RHE in our work (see curve a, panel B, Figure 2), for which the stretching frequency is 1840 cm-1. A number of spectral changes were observed upon coadsorbing Pb on preadsorbed CO/Pt(111) surfaces, while the electrode was polarized at 0.1 V (see curve b, panel A, Figure 2). Particularly evident is the disappearance of the peak ascribed to CO bonded to 3-fold hollow sites as well as the ca. 11 cm-1 shift in the frequency of the atop band toward lower energies from 2065 to 2044 cm-1. Both these effects have also been
Coadsorption of CO and Underpotential-Deposited Pb
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TABLE 1: Integrated Intensities of Solution-Phase CO2 and Adsorbed CO and Site Occupations and Absorptivities of Atop CO in the Absence of Dipole-Dipole Effects for Pt(111), in Neat and Lead-Containing 0.1 M HClO4 Initially Polarized at 0.1 V vs RHE without Pb2+ with Pb2+
Ai(CO2)
Ai(CO(atop))
θCO(atop)
Ai(CO(multi))
θCO(hollow)
(CO(atop))* a
0.19 0.065
0.17 0.25
0.25 0.25
0.070 -
0.5 -
0.32 1.0
a (CO(atop))* ) Ai*(CO(atop))/θCO(atop), where Ai*(CO(atop)) is the integrated intensity of the absorption band associated with CO(atop) in the absence of dipole-dipole effects (see the text for details).
Figure 2. In situ IRAS of CO adsorbed on Pt(111) in 0.1 M HC1O4 at 0.1 V (panel A) with (curve a) and without (curve b) added Pb2+ into the solution using the spectra recorded at 0.8 V as a reference (see the text for details). Panel B shows similar curves obtained under the same conditions, except that the potential was held at 0.4 V vs RHE.
observed for CO adsorbed on Bi-saturated (θBi ) 0.21) Pt(111) surfaces for saturation CO coverages.5 This phenomenon is especially surprising, as the coverage of atop CO in this case remains invariant upon lead coadsorption (Vide infra). A sizable reduction in the intensity of the 1840 cm -1 peak was also found in in this work for a CO/Pt(111) electrode polarized at 0.4 V after injection of lead into the electrolyte (curve b, lower panel, Figure 2); this effect, however, was not explored further. Rather accurate estimates of the relative coverages of CO, irrespectiVe of the nature of the adsorption site, can be obtained from the integrated intensities of the CO2 feature at 2343 cm-1, Ai(CO2) (see Table 1). In fact, on the basis of the outcome of several IRAS experiments, the relative intensities of all bands were found to differ by less than 10%. For example, Ai(CO2) in curve a, upper panel, is about 3 times larger than that in curve b in the same panel, indicating that coadsorption of lead on CO/Pt(111) polarized at 0.1 V decreases the total CO coverage by about two-thirds. Since θCO on bare Pt(111) at this potential is about ca. 0.75, the corresponding value for the Pbmodified surface would be about 0.25. This observation is qualitatively, i.e., without corrections for capacitiVe effects,13 consistent with the decrease in the charge under the oxidation peak at 0.7 V observed for Pt(111) in the presence of adsorbed lead (see Figure 1) compared to the otherwise bare surface. It may be noted that a saturation coverage of CO of 0.25 for a Pb coverage of about 0.45, as found in this work, is in excellent
quantitative agreement with the model put forward by Borup et al.,8 which assumes that the CO adsorption capacity on Pt(111) decreases approximately linearly with increasing lead coverage (see Figure 5 in ref 8). This value is smaller than that estimated on the basis off the charge under the voltammetric peak at 0.7 V, providing strong evidence that a sizable fraction of the charge under that peak is not due to CO oxidation, but to Pb-UPD stripping, with possible contributions due to capacitive effects. Despite the sizable overall decrease in the total amount of adsorbed CO, the integrated intensity for CO adsorbed on atop sites, Ai(CO(atop)) increased by close to 50%. A similar phenomenon has been observed by Weaver and co-workers for CO coadsorption on Bi-modified Pt(111) in the same electrolyte.5 In particular, for θBi ) 0.21, Ai(CO(atop)) for θCO of 0.21 and 0.30 was, respectively, 8-38% larger than A(CO(atop)) for adsorbed CO at saturated coverages on bare Pt(111). As found by Villegas and Weaver,11 on the basis of independent in situ scanning tunneling microscopy (STM) measurements, the occupancy ratios of atop and hollow sites for CO adsorbed on Pt(111) at saturation coverages in the absence of coadsorbed metals, i.e., 1:2, is opposite that obtained on the basis of the relative integrated intensities of the features in the IRAS spectra. This behavior is caused by dynamic dipole-dipole coupling among neighboring adsorbed CO within densely packed overlayers, which elicits “intensity transfer” from the low-frequency band to the high-frequency band, and would be expected to be especially pronounced for the highly compressed (2×2)-3CO superstructure (θCO ) 0.75).4 Theory predicts that the sum of integrated intensities for the species in the two sites is constant, and therefore, it becomes possible to calculate the absorptivity of CO(atop) in the absence of dipole-dipole effects (CO(atop))* ) Ai*(CO(atop))/θCO(atop), where Ai*(CO(atop)) is the integrated intensity of the absorption band associated with CO(atop) in the absence of dipole-dipole effects. From Table 1, the sum of Ai values in the absence of lead is 0.24; hence, given that θCO(atop) is 1/3 of the total, or 0.25, (CO(atop))* is ca. 0.32, which is a factor of 3 smaller than that observed with Pb on the surface, for which θCO(atop) also happens to be fortuitously the same, i.e., 0.25. The most likely explanation for the enhanced absorptivity of atop CO in the presence of coadsorbed Pb may be found in a marked decrease in the dielectric screening of the CO dynamic dipole induced by addition of coadsorbed Pb. As noted in the Introduction the absorbance intensity of a band for an adsorbate ˜ )2. In the presence of a is inversely proportional to (1 + ReU second coadsorbate such as solvent molecules, the electronic polarizability term must be modified to read c1Re + c2Re′, where c1 and c2 are the relative fractional coverages of CO and solvent, respectively and Re′ is the electronic polarizability of the solvent.13 Hence, if one removes the solvent from the adlayer, or replaces it by a medium with lower electronic polarizability, the intensity of the band will increase. Solvent-induced decreases in band intensities, without complications derived from adsorbate migration, have been found for NO adsorption of Pt(111) at
6052 J. Phys. Chem. B, Vol. 104, No. 25, 2000 low coverages in UHV, for which coadsorption of water leads to a 50% reduction in the intensity of the stretching feature.14 However, as pointed out by a reviewer, it is doubtful that Pb coadsorption may lead to total CO desolvation. This factor, in addition to the rather surprising shift in the frequency of atop CO upon Pb coadsorption for virtually the same site occupation (see Table 1), points to chemical effects, such as Pt-Pb charge polarization, as responsible for a significant fraction of the observed intensity enhancement. Acknowledgment. This work was supported in part by grants from DARPA and NSF. We thank one of the reviewers for his/her insightful comments. References and Notes (1) Persson, B. N.; Ryberg, R. Phys. ReV. B 1981, 24, 6954. (2) Xu, Z.; Sherman, M. G.; Yates, J. T.; Antoniewicz, P. R. Surf. Sci. 1992, 271, 249.
Hoshi et al. (3) Tang, C.; Zou, S.; Severson, M. W.; Weaver, M. J. J. Phys. Chem. B 1998, 102, 8796. (4) (a) Severson, M. W.; Stuhlmann, C.; Villegas, I.; Weaver, M. J. J. Chem. Phys. 1995, 103, 9832. (b) Severson, M. W.; Weaver, M. J. Langmuir 1998, 14, 5603 and references therein. (5) Chang, S.-C.; Weaver, M. J. Surf Sci. 1991, 241, 11. (6) da Cunha, M. C. P. M.; Weber, M.; Nart, F. C. J. Electroanal. Chem. 1996, 414, 163. (7) (a) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245. (b) Hoshi, N.; Suzuki, T.; Hori, Y. J. Phys. Chem. 1997, 101, 8520. (8) Borup, R. L.; Sauer, D. E.; Stuve, E. M. J. Vac. Sci. Technol., A 1994, 12, 1886. (9) (a) El Omar, F.; Durand, R. J. Electroanal. Chem. 1984, 178, 343. (b) Adzic, R. R.; Wang, J.; Vitus, C. M.; Ocko, B. M. Surf Sci. Lett. 1993, 293, L876. (10) (a) Kitamura, F.; Takahashi, M.; Ito, M. Surf. Sci. 1989, 223, 493. (b) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (11) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (12) Gomez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf: Sci. 1998, 410, 48. (13) Korzeniewski, C.; Huang, J. Anal. Chim. Acta 1999, 397, 53. (14) Villegas, I.; Gomez, R.; Weaver, M. J. J. Phys. Chem. 1995, 99, 14832.
