Spectra obtained by electrochemical infrared reflection absorption spectroscopy (EC-IRAS) for carbon monoxide (CO) adlayers formed by partial CO dosing on various ruthenium-decorated platinum nanoparticle films are reported. The need to achieve a well distributed rather than aggregated metal nanoparticle array is demonstrated, given that such nanoparticle aggregates induce complex dielectric behavior. The strategy here is to use an "organic glue matrix" (short chain SAMs) between the nanoparticles and the gold substrates. The observed promotion in CO electrooxidation by the existence of a Ru island on Pt nanoparticles, of interest to fuel-cell catalysis, showed a strong relationship with Ru surface concentrations, consistent with previous studies on single crystal or polycrystalline bimetallic surfaces. Two distinctive CO infrared bands, one for the Pt-CO and one for Ru-CO domain were found after the dipole coupling of CO within the two CO domains was minimized. Interestingly, those two CO bands showed independent electrooxidation behavior with electrode potential changes. Also, it is shown that the electrooxidation of CO on large Ru islands is less facile than on small Ru islands. In addition, the activity of commercial Pt/Ru alloy nanoparticles to CO stripping was tested and IRAS spectra were reported as a comparison to our Ru-decorated Pt nanoparticles.

Illustrative quantum-chemical calculations for selected atomic and molecular chemisorbates on Pt(111) (modeled as a finite cluster) are undertaken as a function of external field, F, by using Density Functional Theory (DFT) with the aim of ascertaining the sensitivity of the field-dependent metal-adsorbate binding energetics and vibrational frequencies (i.e., the vibrational Stark effect) to the nature of the surface coordination in electrochemical systems. The adsorbates selected--Cl, I, O, N, Na, NH(3), and CO--include chemically important examples featuring both electron-withdrawing and -donating characteristics. The direction of metal-adsorbate charge polarization, characterized by the static dipole moment, mu(S), determines the binding energy-field (E(b-F) slopes, while the corresponding Stark-tuning behavior is controlled primarily by the dynamic dipole moment, mu(D). Significantly, analysis of the F-dependent sensitivity of mu(S) and mu(D) leads to a general adsorbate classification. For electronegative adsorbates, such as O and Cl, both mu(S) and mu(D) are negative, the opposite being the case for electropositive adsorbates. However, for systems forming dative-covalent rather than ionic bonds, as exemplified here by NH(3) and CO, mu(S) and mu(D) have opposite signs. The latter behavior, including electron-donating and -withdrawing categories, arises from diminishing metal-chemisorbate orbital overlap, and hence the extent of charge polarization, as the bond is stretched. A clear-cut distinction between these different types of surface bonding is therefore obtainable by combining vibrational Stark-tuning and E(b)-F slopes, as extracted from experimental data and/or DFT calculations. The former behavior is illustrated by means of potential-dependent Raman spectral data obtained in our laboratory.

The electrochemical reduction of nitrous oxide on iridium single-crystal electrodes constitutes a paradigmatic example of sensitivity to the surface structure. Ir(111) and Ir(110) electrodes catalyze the process, while Ir(100) is almost totally inactive. An electroreduction mechanism is proposed consisting of the initial dissociative adsorption of N2O yielding oxygen adatoms followed by the reduction of the oxygen adatoms by electrosorbed hydrogen (rate-determining step). The proposed mechanism explains the main features of the voltammetric reduction waves on the active basal planes and provides an explanation for the lack of activity of the Ir(100) electrode. The latter is consistent with the observation that the adsorption/desorption of hydrogen occurs as a sharp phase transition (θH = 0 ↔ θH = 1), the N2O reaction being quenched at lower potentials.

The utilization of surface-attached gold nanoparticles as templates for generating Pt-group particles displaying near-optimal surface-enhanced Raman scattering (SERS) characteristics is described. Essentially epitaxial transition metal coatings down to the monolayer level can be prepared, most readily by the spontaneous replacement of an electrochemically deposited copper layer by the desired Pt-group metal. The and essentially pinhole-free nature of the coated nanoparticles is demonstrated from the form of the SER spectra for chemisorbed carbon monoxide and ethylene. The potential of the present strategy for synthesizing relatively monodispersed "core-shell" nanoparticles using a myriad of coating materials, also displaying SERS activity, is pointed out.

A comparison is made between electrochemical infrared reflection-absorption spectra for carbon monoxide adlayers formed on carbon-supported platinum nanoparticle films by direct chemisorption from solution CO and via methanol dissociation. In addition to the importance of the C/Pt materials as electrocatalysts in methanol-based fuel cells, clarifying the nature of the extent of CO formation from the latter solute is motivated by the use of methanol as a source of chemisorbed CO at these and related interfaces. As in previous studies, commercial C/Pt materials were employed, having metal loadings from 10 to 60%, corresponding to nanoparticle diameters from ca. 2 to 9 nm. Ultrathin C/Pt films with excellent infrared as well as voltammetric characteristics were prepared by physical deposition onto gold. Absolute C-O stretching (nu(CO)) bands as a function of electrode potential, CO coverage, and nanoparticle size were obtained upon solution CO dosing, as usual, by subtracting a reference spectrum measured following CO electrooxidation. Such "absolute" absorbance spectra, however, could not readily be obtained in the presence of methanol solute, obliging the utilization instead of "bipolar" potential-difference infrared (PDIR) spectra where the reference spectrum is also acquired within the potential region where the adlayer is stable. The interpretation of such bipolar PDIR data is complicated for the present systems by the broad asymmetric shape of the component nu(CO) bands, exacerbated for thicker films by their anomalous optical properties. Nevertheless, a detailed understanding of the methanol PDIR spectra, including interpretation of unexpected potential-dependent band intensities and complex bipolar band shapes, was achieved by comparison with corresponding absolute as well as bipolar spectra from solution CO dosing. This analysis, along with cyclic voltammetry, also indicates that the methanol dissociation yields only intermediate-coverage CO adlayers. These spectral findings invite a reassessment of some conclusions from an earlier infrared study (Rice, C.; Tong, Y. Y.; Oldfield, E.; Wieckowski, A.; Hahn, F.; Gloaguen, F.; Leger, J.-M.; Lamy, C. J. Phys. Chem. B 2000, 104, 5803), including a comparison of C-13 NMR data for the C/Pt-methanol systems in relation to the infrared properties.

The sensitive detection and characterization of carbohydrates by means of a strategy based on surface-enhanced Raman spectroscopy is demonstrated. Spectra are obtained after injecting a small amount of saccharide solution onto a roughened silver substrate, with subsequent deposition of silver colloid. The sensitivity achieved by this two-step approach enables high-quality Raman spectra to be obtained for small amounts of aqueous saccharides (5 microL of a 10(-2) M solution) utilizing minimal laser power and small signal acquisition times (a few seconds). Spectral "fingerprints" obtained for seven structurally similar monosaccharides demonstrate clearly an effective means by which each sugar can be identified. The application to more complex analyses is demonstrated for monosaccharide mixtures and a disaccharide, whereby the SERS fingerprints aid in the determination of components.

The voltammetric electrooxidation rates of formic acid, formaldehyde, and methanol in acidic electrolyte on carbon-supported platinum nanoparticle films with varying particle diameters (d) in the range of ca. 2-9 nm are examined with the objective of comparing the nanoparticle size sensitivity for these related yet distinct electrocatalytic processes. The reaction rates on the larger nanoparticles (d > 4 nm) are similar to those observed on polycrystalline Pt when normalized to the same microscopic Pt surface area. As noted previously, the rates of methanol electrooxidation decrease for Pt nanoparticle diameters below 4 nm. However, formic acid electrooxidation exhibits the opposite behavior, with rates increasing markedly for d < 4 nm, while formaldehyde electrooxidation displays little sensitivity to the Pt nanoparticle size. However, the extent of chemisorbed CO formation from all three reactants, as deduced from voltammetric and infrared spectral data, diminishes with decreasing d, the CO coverages for a given nanoparticle size being in the order methanol < formic acid < formaldehyde. These nanoparticle-size-dependent electrocatalytic and CO adsorptive findings are consistent with the occurrence of a Pt site "ensemble effect", where reactant dehydrogenation to form CO, and also in the case of formaldehyde and especially methanol to yield the reactive intermediate en route to CO2 production, is impeded by the sharply decreasing availability of contiguous Pt terrace sites for d < 4 nm. This structural model is consistent with infrared measurements using CO as a nanoparticle structural probe, which show a rapidly decreasing proportion of terrace relative to edge Pt sites for d < 4 nm, in harmony with atomic packing considerations. The markedly enhanced electrocatalyic rates for formic acid oxidation on the smaller nanoparticles are attributed to the lack of a "Pt site ensemble" requirement for this process, coupled with decreased CO poisoning: unlike the other two reactions, oxygen addition (from coadsorbed-OH) is not necessarily required in order to produce CO2 from formic acid.

Some recent applications of surf ace-enhanced Raman spectroscopy (SERS) to the characterization of adsorbates at metal surfaces in electrochemical and ambient gaseous systems taken from studies in our laboratory are described with the objective of illustrating the virtues of SERS for examining catalytically relevant interfaces. Emphasis is placed on studies on Pt-group metal surfaces, prepared by ultrathin film deposition on to SERS-active gold substrates, reflecting their unique catalytic significance. The surface oxidation of Pt-group metals in gaseous oxygen and aqueous electrochemical environments is compared, as exemplified for rhodium. The occurrence of a more facile oxidation pathway with gaseous 02 in the presence of water at near-ambient temperatures is demonstrated, and shown to involve a catalytic electrochemical mechanism, as distinct from the thermal oxidation route in dry 02 which requires elevated temperatures. The central role of the surface potential in such oxidation pathways in both gaseous and conventional electrochemical systems is pointed out. The catalytic activity of such oxidized Pt-group surfaces in gaseous and electrochemical systems towards the oxidation of methanol and other small organic molecules is outlined on the basis of related SERS data. The application of time-dependent SERS as a means of diagnosing the presence of adsorbed reaction intermediates is exemplified for the case of carbon monoxide formed during formic acid electrooxidation on rhodium. The formation of reactive pi-bound ethylene, and also non-reactive ethylidyne, on Pt-group electrodes upon ethylene chemisorption is also briefly described. Copyright (C) 2002 John Wiley Sons, Ltd.

The application of Density Functional Theory (DFT) using variable-field finite-cluster models to the description of electrode-chemisorbate systems is considered, with the particular objective of assessing the relationships between potential-dependent metal-adsorbate vibrational frequencies and the chemical nature of the surface bond. The validity of employing finite-cluster models in variable homogeneous fields to describe potential-dependent electrode-chemisorbate bonding is discussed, including the choice of bulk-phase adsorbate reference states for ionic systems. The bond length-dependent charge polarization, as reflected in the so-called static (μS) and dynamic (μD) dipole moments, plays a central role in determining the field (F)-dependence of the metal-adsorbate binding energies, Eb, and vibrational force constants, KMA, respectively. A distinction between ‘ionic’ and ‘dative covalent’ surface bonding, based on whether the signs of μS and μD are the same or opposite, respectively, is demonstrated by means of DFT calculations for selected atomic and molecular adsorbates on Pt(111), and interpreted in terms of bond length-dependent charge polarization and orbital overlap. A basic consequence of these behavioral differences between μS and μD is to deny the occurrence of any uniform correlation between the Eb–F and KMA–F behavior (i.e. between the potential-energy well depth and well ‘stiffness’, respectively). On the other hand, combined potential-dependent bond-energy and bond-frequency data should therefore be invaluable in ascertaining the nature of the surface coordination. A more uniform relationship, however, is observed between F-dependent force constants and equilibrium bond lengths. The applicability of these notions to the interpretation of Stark-tuning (i.e. frequency–potential) data for intramolecular as well as metal-adsorbate vibrations is also briefly discussed.

A simple method for preparing metal nanoparticle films immobilized on gold substrates is described. A variety of nanoparticle films are characterized by electrochemical infrared reflection-ab sorption spectroscopy (EC-IRAS) in which the anomalous "negative-absorbance" properties commonly observed in metal particle arrays can be completely controlled. Such "anti-absorbance" nu(CO) band components obfuscating the EC-IRAS data interpretation are associated with the complex dielectric behavior induced by metal nanoparticles aggregates. To achieve a well distributed metal nanoparticle array, the substrates were pretreated with 3-mercaptopropyltrimethoxysilane before anchoring gold, platinum or platinum-ruthenium alloy nanoparticles on the gold substrates. The prepared nanoparticle films displayed excellent electrochemical properties implying facile electronic communication through the organic glue matrix between the nanoparticle arrays and the gold substrates. Coating of the gold nanoparticle arrays with platinum via copper underpotential deposition (UPD) steps furthermore demonstrates optimal electronic response between the nanoparticle arrays and the underlying substrate. These findings will facilitate better nanoparticle analysis by electrochemical and optical spectroscopic means.

An analysis is presented of the manner and extent to which the metal surface-chemisorbate bond energetics and geometries as functions of the metal and the applied field can be correlated with vibrational frequencies, with specific reference to electrochemical systems. Emphasis is placed on metal-adsorbate stretching frequencies, vM-A, using oxygen and carbon monoxide chemisorption as illustrative examples; the intramolecular stretch (vCO) of the latter adsorbate is also examined in view of the extensive experimental utilization of this vibrational mode. Results based on Density Functional Theory (DFT) are presented for finite-cluster models of Pt-group and coinage-metal (111) surfaces. The DFT calculations enable a separation between steric repulsion and orbital contributions to the potential-energy surface (PES), and additionally, in the case of CO chemisorption, between the 5sigma and 27pi* orbital components. While rough metal-dependent correlations between vM-A and the surface binding energy, -Eb, are observed, such a relationship is not expected in general. Thus for CO chemisorption, the variations in -Eb are affected more by changes in the 5sigma rather than 2pi* orbital energies, whereas these components influence the M-CO stretching frequency, vM-CO, to a comparable extent. Moreover, the metal-dependent vCO frequencies do not correlate even qualitatively with -Eb; this is because the former are dominated by 2pi*, rather than 5sigma, interactions. The factors influencing the field (F) (and hence electrode potential) dependence of Eb versus VM-CO and vCO mirror somewhat this pattern. While the field-dependent influence of the 5sigma and 2pi* interactions are offsetting, the latter affects the vM CO-F, and especially the vCO-F, behavior to a greater extent than the -Eb-F dependence. Generally, then, the lack of broad-based correlations between chemisorbate vibrational frequencies and binding energetics can be understood in terms of the differing influence of the individual interaction components on the PES well shape and depth. The description of such bonding contributions in terms of dipole-moment parameters is illustrated. Also considered are relations between vibrational frequencies and bond lengths.

The electrostatic field-dependent energetics of low-coverage halogen chemisorption oil (I I 11) planes of platinum, gold, silver and mercury are examined of Density Functional Theory (DFT) using finite metal clusters with the primary aim of describing such behavior in relation to potential-dependent halide adsorption at electrochemical interfaces. The general relationship between the field-dependent and electrode potential-dependent adsorption energies of such neutral and ionic species is clarified in terms of the static surface-adsorbate dipole moment, mu(S). The sensitivity of the DFT-based mu(S) values to the metal cluster size and geometry is examined. Better agreement with experimental low-coverage. Its estimates, obtained from work function-coverage data at metal-vacuum interfaces, requires metal clusters extending to at least second-nearest-neighbor atoms in the surface plane. The mu(S) DFT values for metal-halogen bonding are also compared with experimental estimates extracted from electrochemical thermodynamic data. The markedly (greater than or equal to2-3-fold) larger -mu(S) values for the latter reflect the role of inner-layer solvation in inducing greater surface bond polarization. Comparable increases in -mu(S) values calculated from DFT are also obtained by including interfacial solvent molecules modeled as dielectric spheres. The sensitivity of the dipole moment to the interfacial field as deduced by DFT is also noted in relation to observed electrochemical behavior.

Some applications of vibrational spectroscopy for characterizing chemisorbates of relevance to fuel-cell technology are examined, with particular reference to the fundamental interpretation in terms of interfacial structure and bonding. The central role of the surface potential in controlling vibrational properties is illustrated by examining chemisorbed CO in corresponding electrochemical and ultrahigh vacuum-based interfaces. Some new-found opportunities for the quantum-chemical understanding of potential-dependent chemisorbate bonding by using density functional theory (DFT) are briefly discussed, specifically the connections between vibrational frequencies, binding energetics, and surface charge polarization, and the insight into specific orbital and other interactions that can be furnished by finite-cluster calculations. Related adsorptive and electrocatalytic properties of carbon-supported Pt nanoparticle films are described, prompted by the applicability of such materials in fuel cells. The infrared characterization of size-dependent nanoparticles by using chemisorbed CO as a structural probe indicates a sharply increasing proportion of edge versus terrace Pt sites for particle diameters below 4 nm, in accordance with simple atom-packing considerations. The qualitatively different dependence of the electrocatalytic properties of such films on the nanoparticle diameter for methanol and formic acid oxidation is interpreted in terms of an ‘ensemble effect’, whereas the availability of contiguous terrace sites (apparently required for the former but not the latter process) is curtailed for small particle diameters. Finally, the value of DFT as a means of predicting infrared absorbances and Raman scattering intensities by evaluating the dynamic dipole moment and its field dependence (i.e. the dynamic polarizability), respectively, is illustrated for adsorbates of interest in fuel-cell electrocatalysis, and compared with experimental data.

The field-dependent frequency behavior of the metal-adsorbate (v(M)-co) as well as the intramolecular (vco) vibration of carbon monoxide chemisorbed in atop and threefold-hollow sites on three platinum-group (111) metal surfaces-Pt, Ir, and Pd-is explored in relation to the metal-chemisorbate (M-CO) binding energetics and geometries by means of Density Functional Theory (DFT) calculations for finite clusters. This overall objective-having particular importance in electrochemical systems-of linking field-dependent vibrational, energetic, and geometric properties of the M-CO bond, prompted by the availability of potential-dependent V-M-CO data at Pt-group electrodes from Raman spectroscopy, provides an opportunity to assess in quantum-chemical terms these surface-adsorbate binding parameters in relation to the extensively studied intramolecular CO vibration. The binding energies (-E-b) tend to increase toward negative fields (F), especially for hollow-site binding. An energy decomposition into specific orbital and steric interactions shows that this effect is driven primarily by enhanced pi -back-donation, although offset by progressively weaker a-donation along with greater surface-chemisorbate steric repulsion. Although these individual. orbital and steric interactions exert similar effects on the v(M)-co frequencies, the overall v(M-CO)-F dependencies are notably different, typically displaying a broad maximum at moderate/large negative fields (ca. -0.3 to -0.5 V Angstrom (-1)). Unlike the binding-energy behavior, these nonmonotonic v(M-CO)-F dependencies correlate roughly with the corresponding F-dependent M-CO equilibrium bond lengths, r(M-CO) A decomposition of the field-dependent v(M-CO) and r(M-CO) behavior into individual interactions exhibits close parallels, with rr-bonding acting to markedly blueshift v(M-CO) and decrease r(M-CO), being offset increasingly toward more negative fields by the effects of a-bonding and steric repulsion. In contrast, the monotonically red-shifted V-CO frequencies and the correspondingly elongated C-O band lengths, r(CO), found toward negative fields arise chiefly from the well-known effects of d pi -2 pi* back-donation. A common correlation is observed between the field-dependent vco and vco values for each of the metal-CO systems and even uncoordinated CO. The likely role of electrostatic factors in the v(M-CO)-F dependencies is also considered: the increasing M --> CO charge polarization seen toward negative fields can account qualitatively for the v(M-CO)-F maxima. A semiquantitative agreement is evident with electrode potential-dependent y(M-CO) and v(CO) vibrational data, although YM-CO-F maxima have yet to be observed experimentally.

The preparation of Pt-group metal films on roughened gold electrodes by utilizing spontaneous redox replacement of an underpotential-deposited (upd) copper or lead monolayer with a Pt-group metal cation solute is described. The resulting films display intense surface-enhanced Raman scattering (SERS) for adsorbates bound to the overlayer and free from substrate interferences. This strategy provides a useful alternative, at least for platinum, to the constant-current electrodeposition method commonly utilized to prepare SERS-active Pt-group metal films (Zou, S.; Weaver, M. J. Anal. Chem. 1988, 70. 2387). Similarly to related earlier studies, the film uniformity (specifically, the absence or otherwise of residual Au "pinhole" sites) was tested by employing carbon monoxide, and also ethylene, as "probe" chemisorbates, since they yield vibrational frequencies on Au that are blue-shifted from the corresponding bands for adsorbate bound to Pt-group metal sites. While a single redox replacement of upd Cu with Pt(IV) yielded incomplete surface coverage, as expected, the use of multiple (up to eight) replacement cycles produced Pt films displaying remarkably intense CO vibrational bands as well as apparently "pinhole-free" properties, although such imperfections were detected with the ethylene probe. A single upd Cu replacement with Pt(II), however, yielded a remarkably uniform Pt layer, as indicated by pinhole-free characteristics using both the CO and ethylene probes along with the voltammetric behavior. The use of additional redox replacement cycles yielded marked progressive attenuation in the SERS signals. Comparable, although less optimal, SERS behavior was obtained for Pd films prepared similarly from Pd(II). The value of the strategy for exploring catalytic as well as equilibrium adsorptive chemistry on Pt surfaces is also illustrated.

An analysis is outlined that utilizes electrode potential-dependent measurements of the intramolecular vibrational frequency (nu (c)) for chemisorbates, specifically carbon monoxide and nitric oxide, at ordered Pt-group metal-solution interfaces (i.e., the electrochemical Stark effect), in comparison with corresponding vibrational frequencies at the metal-ultrahigh vacuum (UHV) interfaces, as a means of assessing the role of local versus average surface potentials in the former systems. For saturated adlayers featuring the same (or similar) binding-site configurations in the aqueous electrochemical and UHV environments, adjusting the electrode potential, E-eq, to the point where nu (c) equals that measured at the corresponding metal-UHV interface yields E-eq, values that are closely concordant with the work function, Phi, for the latter surfaces. This E-eq - Phi correlation, which also yields an "absolute potential" of the reference electrode consistent with literature estimates, indicates that the average surface potential exerts a dominant influence upon v, for uniform saturated adlayers. Interestingly, a closely similar E-eq - Phi correlation is also obtained for dilute CO or NO adlayers (i.e., for isolated solvated chemisorbed molecules) that again feature the same binding sites (and coverages) in the electrochemical and UHV environments, despite the dominant presence of inner-layer solvent. These results are used to deduce that the "local" surface potentials sensed by the chemisorbate molecules within dilute electrochemical adlayers are closely similar to (within ca. 0.05-0.1 V of) the average values. This simple finding suggests, perhaps unexpectedly, that the dipolar fields generated in the vicinity of the nonpolar chemisorbates CO and NO by the double-layer environment do not differ significantly from the average values across the solvated inner layer.

Detailed intramolecular vibrational spectra obtained by means of surface-enhanced Raman scattering (SERS) for benzonitrile adsorbed on seven electrode surfaces-four Pt-group metals (platinum, palladium, rhodium, and iridium) and the Group IB metals (copper, silver, and gold)-are reported with the aim of exploring the metal-dependent nature of surface-chemisorbate interactions. The Pt-group surfaces were prepared as ultrathin electrodeposited films on gold, enabling the SERS activity inherent to the substrate to be imparted to the overlayer material. Benzonitrile was selected as a "model" organic adsorbate since it displays a rich array of coupled aromatic ring as well as substituent modes which collectively can provide insight into the various molecular perturbations induced by surface coordination via the nitrile substituent. The experimental spectra are compared with ab initio calculations of vibrational frequencies, bond geometries, and charge distributions obtained by means of Density Functional Theory (DFT), which yields valuable insight into the underlying structural reasons for the sensitivity of the experimental coordination-induced frequency shifts to the nature of the intramolecular mode and the metal surface. The DFT results also form an invaluable aid in making SER spectral assignments, along with providing detailed information on the coupled atomic displacements involved in each vibrational mode. Benzonitrile surface coordination was modeled in the DFT calculations by binding the nitrile group to metal atoms and small metal clusters. While the majority of the aromatic-ring SER frequencies are altered only slightly (approximately < 5 cm(-1)) upon surface coordination, several modes (especially nu(1), nu(6a)) are blue-shifted substantially (by up to 50 cm(-1)). These shifts were identified by DFT as arising from mode coupling to the nitrile substituent, especially involving the C-CN bond that is compressed upon nitrile coordination, associated with metal-adsorbate back-donation. The small (<5 cm(-1)) red-shifts seen for ring vibrations not involving coupled substituent motion apparently arise from increased antibonding aromatic electron density. The metal-dependent frequency shifts seen for these coupled aromatic vibrations as well as for the more localized C-N nitrile stretching mode are consistent with increased back-donation anticipated in the sequence d(10) < d(9) < d(8) within a given Periodic row. Overall, the findings provide a benchmark illustration of the virtues of DFT in interpreting complex vibrational spectra for larger polyatomic adsorbates.

An analysis is outlined in which the dependence of the binding energy, Eb, stretching frequency, νM–A, and equilibrium bond length, req, of metal–adsorbate bonds on the external interfacial field (F), and hence surface potential of relevance to electrochemical systems, are described in terms of potential-energy surface and bond-polarization parameters. Density Functional Theory (DFT) calculations for finite metal clusters with monoatomic adsorbates forming polar surface bonds are utilized both to examine metal-, adsorbate-, and field-dependent trends in the required parameters and to examine the applicability of the analytic relations in comparison with numerical calculations. To a very good approximation, the Eb–F dependence is described by the field-dependent static dipole moment, μS, of the metal–adsorbate bond. As expected, the field-induced changes in the potential-energy surface (PES) are determined by the r-dependent μS values, i.e., the dynamic dipole moment, μD. The factors determining the νM–A–F dependence, however, are distinctly more intricate, involving the PES anharmonicity coupled with μD, as well as being influenced significantly by the μD–r dependence in some cases. While both μS and μD are influenced importantly by surface bond polarity, the role of bond polarizability appears to be quite different, so that only a crude correlation between μS and μD is evident. These differences further underscore the disparate nature of the Eb–F and νM–A–F behavior. Nevertheless, an approximate inverse correlation between the νM–A–F and req–F dependence is usually anticipated. The broad-based utility of DFT for assessing as well as predicting the role of surface bond polarization in controlling the field-dependent electrode–adsorbate PES is pointed out.