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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.

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.

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 (νCO) 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 νCO 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 ¹³C NMR data for the C/Pt−methanol systems in relation to the infrared properties.

An analysis is outlined that utilizes electrode potential-dependent measurements of the intramolecular vibrational frequency (νc) 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, Eeq, to the point where νc equals that measured at the corresponding metal−UHV interface yields Eeq values that are closely concordant with the work function, Φ, for the latter surfaces. This Eeq−Φ 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 νc for uniform saturated adlayers. Interestingly, a closely similar Eeq−Φ 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.

The possibility that adsorbed carbon monoxide may act as a reaction intermediate rather than merely a catalytic poison for formic acid electrooxidation on Pt-group metals in aqueous perchloric acid is explored by monitoring the time-dependent effects of reactant ¹³C/¹²C isotopic substitution on the adsorbate vibrational properties, as discerned from surface-enhanced Raman spectroscopy (SERS). The electrodes examined, polycrystalline rhodium and iridium (deposited as ultrathin films on gold to yield optimal SERS activity), exhibit substantial formation of adsorbed CO at potentials below and close to the onset of formic acid electrooxidation, as discerned from the well-known C−O (νCO) stretches at 1850−2000 cm^-1 and the metal−CO vibrations at 450−500 cm^-1. As in earlier infrared studies, the former vibrations provide a sensitive means of monitoring adsorbate ¹³CO/¹²CO replacement from the characteristic ~45 cm^-1 difference in isotopic νCO frequencies. These SER vibrational features are used to monitor the rates of adsorbed ¹³CO/¹²CO replacement triggered by abrupt switches in the reactant isotopic composition brought about either by adding a large excess of H¹²COOH to the H¹³COOH-containing solution or by utilizing a spectroelectrochemical flow cell. At electrode potentials below the onset of formic acid electrooxidation on rhodium, only slow ¹³CO/¹²CO isotopic exchange was observed, requiring several minutes (or longer) for extensive substitution. The kinetics are markedly (10−100 fold) slower than those observed when using solution CO rather than formic acid. However, altering the potential to values (above 0.2 V versus SCE) beyond the onset of formic acid electrooxidation yielded relatively rapid, albeit incomplete, isotopic CO exchange. The turnover frequencies, ~0.01−0.1 s^-1, deduced on this basis can indeed account for a significant or even substantial fraction of the electrocatalytic conversion rates of formic acid to CO2, indicating that the adsorbed CO can act as a reaction intermediate under some conditions. Qualitatively similar results were obtained for electrooxidation on iridium. The implications of these findings to the conventional “dual-pathway” mechanism for such catalytic electrooxidations are briefly discussed.

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.

The electrode potential-dependent formation, stability, and nature of oxide layers formed on iridium, palladium, rhodium, and platinum in acidic aqueous solution are explored by means of in-situ surface-enhanced Raman spectroscopy (SERS). The technique utilizes recently developed procedures by which “pinhole-free” ultrathin (ca. 3−5 monolayers) transition-metal films are prepared by electrodeposition onto a SERS-active gold substrate, yielding polycrystalline Pt-group surfaces with negligible spectral interferences from the underlying gold. The oxide films formed by water electrooxidation exhibit broad yet distinctive metal−oxygen (νM-Ox) lattice modes primarily in the range 500−800 cm^-1. Of particular interest is the nature and stability of oxides produced upon exposure to a chemical oxidant, nitric oxide, in acidic solution as sensed by SERS, in relation to the conventional electrochemical oxide. A distinctive oxide species identified by a sharp 570 cm^-1 νM-Ox feature is formed on iridium by aqueous NO exposure, which exhibits a remarkable resistance toward reduction in comparison with oxide produced by water electrooxidation. This anomalous stability is evident not only in aqueous solution but also at elevated temperatures in dry gaseous hydrogen. The film is tentatively identified as crystalline IrO2 on the basis of its vibrational fingerprint. A spectrally distinct oxide species also displaying unexpected kinetic stability is evident on palladium exposed to NO-containing electrolyte at higher electrode potentials, although no such species are discernible on rhodium or platinum electrodes. However, the rhodium oxide is seen to survive to lower potentials in NO-containing electrolyte than in the absence of NO. These findings are ascribed to the ability of nitric oxide to act as an efficient oxygen-atom source upon dissociative chemisorption, and which, unlike water, does not require electrooxidation to form metal oxide. Another significant, although much less effective, source of oxide from aqueous solution was found to be dissolved dioxygen.

The adsorption of benzotriazole (BTAH) on copper in aqueous electrolytes is examined by means of a vibrational analysis utilizing surface-enhanced Raman spectroscopy (SERS) as a function of solvent deuteration, pH, and electrode potential in order to examine adsorbate speciation, surface bonding, and the potential-dependent onset of Cu^IBTA phase film formation. In acidic media at pH > 1, the monoprotonated acid BTAH constitutes the predominant adsorbate, being replaced by the diprotonated BTAH2⁺ species by pH 0, and the basic BTA^- form in alkaline electrolytes. The analysis of adsorbate speciation is facilitated by the presence of several coupled ring modes yielding vibrational bands in the 1050−1350 cm^-1 region that are sensitive to deuteration of the triazole ring nitrogen(s). Insight into the adsorbate molecular geometry and mode of surface attachment is obtained by examining the adsorption-induced alterations in frequency, bandwidth, and relative intensity of the various benzotriazole ring modes. These changes are especially prevalent for triazole ring vibrations and some deuteration-sensitive modes, the latter apparently associated with Fermi resonance effects. The former torsional/bending ring vibrations also exhibit potential-dependent frequencies (i.e., an electrochemical Stark effect), reflecting copper−triazole electronic interactions. Taken together, the SER spectral features for BTAH and BTA^- suggest surface attachment via a pair of triazole nitrogens with a tilted (or vertical) orientation; the single nitrogen binding site anticipated for BTAH2⁺ is consistent with the milder vibrational perturbations observed for this adsorbate. The transformation from adsorbed benzotriazole to a Cu^IBTA film at higher potentials is readily diagnosed from the SER spectra, including the observed lack of a D/H vibrational effect on the latter species. The structure of benzotriazole films in air is also examined; emersion of adsorbed BTAH from solution spontaneously yields a Cu^IBTA layer via a coupled O2-induced electrochemical pathway. The thermal stability of copper-benzotriazole films in ambient-pressure O2 and N2 from 25 to 300 °C was also probed with SERS.

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 θCO-dependent 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 (%@mt;sys@%ν%@sx@%CO%@be@%P%@sxx@%%@mx@% ), integrated absorbances (Ai), and bandwidth (Δν1/2) for the dosed electrochemical adlayers reveals marked alterations at θCO ≈ 0.5. The unusual nonmonotonic form of the Ai − θCO and Δν1/2 − 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 %@mt;sys@%ν%@sx@%CO%@be@%P%@sxx@%%@mx@% − 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.

The surface-enhanced Raman spectroscopy (SERS) properties of thiocyanate adsorbed on platinum, palladium, rhodium, and iridium electrodes, formed by pinhole-free ultrathin film deposition onto SERS-active gold, are compared with corresponding data obtained on copper, silver, and gold, and on underpotential (upd) silver and mercury layers on gold, to assess the sensitivity of the anion coordination modes to the chemical nature of the metal surface. A crucial virtue of SERS for this purpose lies in the observability of each of the intramolecular adsorbate modes plus the metal surface−adsorbate stretching vibration, νm-a. The latter modes, inaccessible to other in-situ vibrational methods, provide particularly straightforward insight into metal-thiocyanate binding. In contrast to gold and upd mercury surfaces, which favor N-coordination only at far-negative potentials, both S- and N-binding modes are fingerprinted on Pd, Rh, and Pt surfaces from the characteristic νm-a bands. These bands, whose relative frequencies are sensitive primarily to the lead-in atomic mass, are observed throughout the range of potentials (typically −0.4 to 0.4 V vs the saturated calomel electrode) where “reversible” potential-dependent spectral behavior was obtained. Such coexisting modes of surface coordination are also indicated from the appearance of a pair of C−S stretching (νCS) vibrations indicative of S- and N-binding from the known behavior of bulk-phase metal complexes. Less diagnostic in this regard is the C−N stretching (νCN) vibration, which exhibits only a single broad band at ca. 2115−2135 cm^-1 over the above-mentioned range of potential on the Pt-group surfaces. The virtually potential-independent νCN frequencies observed on these metals, (dνCN/dE) ≤ 10 cm^-1 V^-1, contrast with the larger “Stark-tuning” slopes found on silver and copper. The potential-insensitive adsorbate frequencies and coordination modes, together with the higher νm-a frequencies compared with those on the coinage metals, are consistent with the occurrence of largely covalent transition-metal surface bonding.

Model dipole−dipole coupling calculations of infrared spectra for various ordered CO packing arrangements on Pt(111) have been undertaken as a function of the spatial dimensions of isolated nanoscale adlayer domains. Comparisons of some of these predictions with coverage (θCO)-dependent infrared spectra obtained upon partial adlayer oxidation on low-index Pt-group surfaces in aqueous electrochemical environments enable approximate estimates of densely packed CO island sizes to be obtained. The evidence for CO island formation, described in several earlier reports, is based on the observation of C−O stretching bands, νCO, whose frequency, band shape, and relative intensity exhibit markedly smaller alterations upon progressive electrooxidative removal in comparison with corresponding νCO frequency-coverage data obtained for increasing θCO by means of dosing with dilute CO solutions. The dipole-coupling calculations indicate a significant sensitivity of the νCO frequency and related parameters to island size even for domains containing sizable numbers (100−200) of CO molecules, suggesting the value of the procedure for providing at least rough estimates of nanoscale island dimensions. While the domain size-dependent νCO spectra should be dependent inevitably upon the microscopic CO packing geometry, the (relative) changes in the νCO frequency for atop (i.e. terminal) adsorbate are predicted to be dependent on the island dimensions, yet relatively insensitive to the adlayer structure, suggesting the practical utility of this parameter for island size diagnosis. The resulting estimates of average island dimensions, along with their stability as gleaned from experimental time-dependent spectra, are seen to vary widely on different low-index Pt-group electrodes. Interestingly, the metastable CO domains formed on Pt(100), estimated to contain 20−50 atop CO's, have dimensions roughly compatible with the restructured substrate “mesas” seen to form by scanning tunneling microscopy.

The nature and magnitude of corrections to published estimates of the potentials of zero charge Epzc for Pt(111)−aqueous interfaces obtained by the “CO charge-displacement” strategy are assessed by comparing such in-situ electrochemical data with surface charge−potential information extracted from work function−surface composition data for related interfaces in ultrahigh vacuum (UHV). The corrections involve estimating the excess metal charge densities remaining upon displacement of the aqueous inner layer by chemisorbed CO. While the resulting adjustment to the so-called potential of zero total charge %@mt;sys@%%@ital@%E%@rsf@%%@sx@%pzc%@be@%t%@sxx@%%@mx@% for the Pt(111)−aqueous 0.1 M HClO4 interface is only small (ca. 25 mV), a larger correction to the corresponding “free-charge” value %@mt;sys@%%@ital@%E%@rsf@%%@sx@%pzc%@be@%f%@sxx@%%@mx@% is deduced, yielding %@mt;sys@%%@ital@%E%@rsf@%%@sx@%pzc%@be@%f%@sxx@%%@mx@% ≈ 0.2 V versus SHE. A closely concordant estimate of %@mt;sys@%%@ital@%E%@rsf@%%@sx@%pzc%@be@%f%@sxx@%%@mx@% is also extracted from UHV-based work function data. The substantial discrepancies in such Epzc values for the Pt(111)−acidic aqueous interface with those extracted by means of an ex−situ/electrode immersion procedure are also briefly considered in the light of the above analysis.

The adsorption of carbon monoxide and nitric oxide on an ordered Ir(110) electrode surface in aqueous 0.1 M HClO4 has been probed by voltammetry together with in-situ infrared reflection−absorption spectroscopy (IRAS). Exclusively atop coordination of both CO and NO is suggested from the relatively high C−O and N−O stretching (νCO, νNO) frequencies observed, 1980−2060 and 1820−1840 cm^-1, respectively, that upshift with increasing coverage. Adsorption of NO as well as CO is essentially molecular, with near-unity saturation coverages, as deduced from voltammetry as well as infrared spectrophotometry. The potential-dependent νCO frequencies for the saturated CO adlayer are closely compatible with that for the corresponding Ir(110)/CO interface in ultrahigh vacuum (UHV) once the differences in surface potential are taken into account. In contrast to the case of the latter system, however, the electrochemical Ir(110)/CO interface exhibits a pair of νCO bands at intermediate CO coverages (θCO), suggestive of a difference in substrate-induced adlayer domains in the two environments. Closely similar θCO-dependent νCO spectra and voltammetric oxidation profiles were obtained for adlayers formed by either partial electroxidative stripping from a saturated adlayer or by direct dosing from a dilute CO solution. This unusual behavior indicates that extensive CO “islands” are not formed by partial adlayer electrooxidation, in contrast to the behavior of most ordered low-index Pt-group electrodes, suggesting that the substrate morphology features nanoscale domains rather than large terraces. The νCO and νNO frequencies for saturated adlayers on Ir(110) and (111) are similarly red-shifted from the gas-phase νCO and νNO values. However, the νCO−E and especially the νNO−E dependences (“Stark-tuning” slopes) are markedly larger than the predicted gas-phase values. The larger dνNO/dE values are ascribed to more extensive potential-dependent dπ−2π* back-donation for adsorbed atop NO compared with CO.

The adsorption of nitric oxide and carbon monoxide on polycrystalline iridium in aqueous 0.1 M HClO4 was probed by in-situ surface-enhanced Raman spectroscopy (SERS) and infrared reflection−absorption spectroscopy (IRAS) with the primary objective of probing the molecular and dissociative chemisorption of the former adsorbate in this electrochemical environment. The latter adsorbate was examined partly as a means of characterizing the microscopic nature of the iridium electrode, given the well-documented sensitivity of the CO intramolecular vibration to the surface structure. The SERS vibrational technique was harnessed by utilizing ultrathin iridium films electrodeposited on a gold substrate. The availability of this method enables vibrational features associated with surface-adsorbate as well as intramolecular bonds to be scrutinized, facilitating detection of adsorbed NO fragments. Saturation adsorption of CO yields a single C−O stretching (νCO) band at ca. 2040−2070 cm^-1 in both the Raman and infrared spectra, suggestive of exclusive atop (or near-atop) binding as on monocrystalline Ir surfaces. Significant (15−20 cm^-1) discrepancies in the potential-dependent νCO frequencies measured by SERS and IRAS are evident, however, indicating that the ensemble distribution of iridium surface microenvironments sensed by these techniques is dissimilar. However, the νCO frequencies are approximately consistent with those evaluated for corresponding iridium−vacuum interfaces once the differences in surface potentials are taken into account. In contrast to the νCO band, the frequency of the metal−CO (νM-CO) vibration decreases with increasing electrode potential, in harmony with SERS findings on other transition-metal surfaces and theoretical bonding expectations. Adsorption of NO yields a weaker N−O stretching (νNO) band at 1780−1810 cm^-1 in both the SERS and IRAS spectra, indicative of the presence of (probably atop) molecular NO chemisorption. The SERS lower-frequency region is dominated by a well-defined band at 570 cm^-1, attributed to a metal−oxygen stretch from the chemisorbed oxygen fragment formed by NO dissociation. This assignment arises in part from the markedly different potential-dependent stability as well as intensity of the 570 cm^-1 band in comparison with the νNO feature. Supporting evidence includes the observation of a near-identical vibrational band upon oxygen dissociation (and NO adsorption) on Ir surfaces in vacuum and for the present Ir films upon gas-phase O2 dosing. While the extent of NO dissociation on Ir cannot be estimated quantitatively, consideration of band intensities suggests that the coverages of molecular NO and the atomic-oxygen fragment are not greatly different. Comparisons are briefly made with NO adsorption on other transition-metal surfaces in electrochemical and vacuum environments.

Measurements of the work-function changes, ΔΦ, on Pt(111) for continuously increasing solvent exposures θs* and in the presence of various coverages of potassium, θK, in ultrahigh vacuum (UHV) at 90 K are reported with the objective of ascertaining how the surface charge−potential properties of such “UHV electrochemical model” interfaces are altered by progressive solvation. The solvents&sbd;water, methanol, acetonitrile, acetone, and ammonia&sbd;span a range of dipolar and other solvating properties and have been utilized in related vibrational spectroscopic studies from this laboratory. Since potassium dosage yields interfacial electron transfer to form K⁺ together with surface electronic charge, the corresponding ΔΦ−θK plots for various solvent dosages extracted from the above data provide surface charge−potential (σ−φ) curves for systematically varying extents of interfacial solvation. In contrast to the large (1−3 eV) monotonic solvent-induced Φ decreases observed in the absence of ionic charge, the presence of predosed K⁺ yields initial Φ increases, associated with cation solvation, followed by Φ decreases due primarily to the ensuing metal surface solvation. Examination of the corresponding ΔΦ−θK traces obtained for these different solvent dosage regions shows that the basic charge−potential features characteristic of the solvated double layer require only ionic solvation, even though complete metal surface solvation modifies significantly the electrostatic behavior. While surface solvation by the different species examined in the absence of charge yield substantially dissimilar Φ values (i.e., differing “potentials of zero charge”), the charge−potential characteristics are relatively insensitive to the solvent. This finding, comparable to that obtained for in-situ electrochemical interfaces, indicates that the effective “interfacial solvent dielectric constant” varies by only 2-fold or less. ΔΦ−θK data obtained by K dosing after solvent addition yielded larger −ΔΦ values (i.e., smaller capacitances), consistent with more complete K⁺ solvation and/or larger K⁺− surface separations. Corresponding ΔΦ−θK data for CO-saturated Pt(111) indicates that the CO adlayer plays a role in dielectric screening. Effectively θK-independent ΔΦ responses were obtained with ammonia-solvated Pt(111), however, suggestive of the formation of solvated electrons. Specific comparisons are made between the UHV-based charge−potential behavior with that for in-situ electrochemical interfaces and for ionizable high-nuclearity Pt carbonyl clusters in nonaqueous media. The latter systems, in particular, exhibit closely similar surface charge−potential characteristics to the corresponding UHV-based Pt(111) interfaces.