J. Phys. Chem. 1996, 100, 4237-4242
4237
Potential-Dependent Metal-Adsorbate Stretching Frequencies for Carbon Monoxide on Transition-Metal Electrodes: Chemical Bonding versus Electrostatic Field Effects Shouzhong Zou and Michael J. Weaver* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393 ReceiVed: NoVember 9, 1995X
The dependence of the metal-carbon vibrational frequency (νM-C) upon electrode potential (E) for saturated CO adlayers on palladium, platinum, rhodium, and iridium film electrodes is examined in comparison with that for the well-studied intramolecular (C-O) vibration (νCO) by means of surface-enhanced Raman spectroscopy (SERS) in order to evaluate the likely roles of chemical bonding versus the electrostatic field in the electrochemical Stark effect. In each case, the dνM-C/dE values are negative, from ca. -10 to -20 cm-1 V-1, contrasting the positive dνCO/dE values, ca. 30 to 60 cm-1 V-1, observed for adsorbed CO on these Pt group metals. The findings are compared with the predictions of theoretical treatments which account variously for the roles of the interfacial electrostatic field (i.e., the classical vibrational Stark effect) and potential-dependent chemical bonding (i.e., metal-adsorbate orbital overlap). It is necessary to invoke that the latter factor is exerting a major role in the surface-adsorbate interactions in order to account for the observed νM-C -E dependences.
Introduction An intriguing and much-discussed feature of vibrational spectra for adsorbates at metal-solution (i.e., electrochemical) interfaces is the significant and even substantial dependence of the band frequencies upon the applied electrode potential. Of particular interest are systems where the adsorbate coverage and orientation are constrained to remain fixed as the potential is altered, thereby avoiding variations in dipole coupling and other structure-dependent contributions to the band frequencies. In this case, the frequency-potential (ν-E) dependencies, while generically termed “Stark tuning”, may originate from potentialdependent metal-adsorbate bonding as well as from the influence of the variable electrostatic field exerted across the chemisorbed layer, i.e., a vibrational Stark effect. Although it has been argued that such “chemical bonding” and “electrostatic field” contributions can be considered to be formally equivalent in a complete description of electronic polarization,1,2 considerable attention has been given to the likely relative importance of these factors in specific chemisorption systems, especially involving carbon monoxide.3-8 Most experimental information on such electrochemical Stark tuning effects refers to intramolecular adsorbates, such as the C-O stretching frequency, νCO, of carbon monoxide. This situation reflects the common usage of infrared reflectanceabsorption spectroscopy (IRAS) which, although employed increasingly to examine adsorbates at well-defined monocrystalline electrodes,9 is restricted in practice largely to band frequencies above ca. 800 cm-1. Nevertheless, IRAS studies of νCO-E dependencies for CO adsorbed on transition metal electrodes have been informative regarding the nature of Starktuning effects.2,10 In particular, the marked sensitivity of the observed Stark-tuning slopes to both the electrode material and adsorbate binding geometry,10 along with their values that are typically larger than expected for isolated oriented (i.e., nonbonded) CO,2 implicates a likely importance of metal-chemisorbate bonding in the phenomenon (vide infra). In addition to intramolecular adsorbate modes, surfaceenhanced Raman spectroscopy (SERS) has been utilized to X
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4237$12.00/0
obtain Stark-tuning information for metal-adsorbate vibrations in some electrochemical systems, exploiting the sensitivity of this technique even in the low-frequency (
