Michael J. Weaver

Electro-oxidation mechanisms of methanol and formic acid on Pt-Ru alloy surfaces

Abstract Voltammetry combined with single-potential alteration infrared spectroscopy (SPAIRS) were used to study the extent of adsorbed CO produced at Pt, Ru and Pt-Ru alloy electrodes during methanol and formic acid oxidation in acidic supporting electrolyte. The addition of even small atomic fractions of Ru to Pt surfaces caused a decrease in the quasi-steady-state level of CO on the surface for both reactions. This result is consistent with the bifunctional mechanism proposed previously: Ru sites nucleate oxygen containing species at ≈0.2-0.3 V lower potential than on the pure Pt surface; the adsorption of methanol occurs on Pt ensembles producing adsorbed CO; in the case of formic acid, adsorption is equally facile at Pt-Pt, Pt-Ru and Ru-Ru sites, with dehydration producing adsorbed CO; the further electro-oxidation of CO is catalyzed by oxygen-containing species nucleated onto nearby by Ru atoms. The improved efficiency of the alloy surfaces for oxidation of adsorbed CO at low potential shifts the rate limiting step to the adsorption step, which results in very low coverages of the surfaces by adsorbed CO.

Carbon monoxide adlayer structures on platinum (111) electrodes: A synergy between in‐situ scanning tunneling microscopy and infrared spectroscopy

The spatial structure of compressed carbon monoxide adlayers on Pt(111) in aqueous acidic solution has been explored by means of in‐situ scanning tunneling microscopy (STM) along with infrared reflection–absorption spectroscopy (IRAS). Besides offering a detailed structural picture of this electrochemical interface in comparison with the well‐studied Pt(111)/CO system in ultrahigh vacuum (uhv) environments, the real‐space structural information provided by STM allows an assessment of the obfuscating influence of dynamic dipole coupling upon IRAS binding‐site assignments. In turn, the latter data provide an important crosscheck on the validity of binding‐site assignments deduced from the STM images. Emphasis is placed on the structures formed from near‐saturated CO solutions, encouraged by the electrode potential‐induced adlayer phase transition at ca. 0 V vs SCE observed previously under these conditions by IRAS. At potentials below 0 V, a hexagonal close‐packed (2×2)–3CO adlayer is observed, with a CO co...

The adsorption of sulfate on gold(111) in acidic aqueous media: adlayer structural inferences from infrared spectroscopy and scanning tunneling microscope

The potential-dependent adsorption of sulfate on ordered Au(111) from acidic aqueous electrolytes has been examined in situ by means of IR reflection-absorption spectroscopy (IRAS) and by atomic-resolution scanning tunneling microscopy (STM) in order to explore further the nature of the adsorbate bonding and the structural changes attending the formation of the ordered adlayer at high potentials, as observed recently using STM. Solution conditions that encompassed aqueous sulfuric acid, sulfuric acid/sulfate electrolytes of varying pH and dilute sulfate in excess perchloric acid were selected in order to facilitate comparisons with recent adsorbate compositional data extracted from chronocoulometric and radiotracer measurements which utilized the last mentioned type of electrolyte. Essentially the same ordered adlayer structures were deduced by STM to form at suitably high potentials (≳ 0.8 V/SCE) in both sulfuric acid and the mixed sulfate/excess perchloric acid media. The adlayer, which exhibits a (√3 × √7) symmetry, involves a fractional sulfate coverage of 0.2, in accordance with chronocoulometic and radiotracer data. The possibility that the ordered sulfate adlayer incorporates coadsorbed hydronium cations is discussed; such coadsorption is suggested by the presence of additional tunneling maxima in the STM images. The IRAS data display a prominent SO stretching band νso at 1155–1220 cm−1, the potential-dependent intensity of which correlates with sulfate surface concentrations reported earlier. The appearance of this νso feature is also insensitive to the electrolyte conditions, including pH, consistent with its assignment to adsorbed sulfate rather than bisulfate. The νso frequency exhibits only a slight (ca. 5 cm−1) downshift upon forming the ordered adlayer, indicating that adsorbate ordering incurs no marked changes in sulfate speciation or surface bonding.

Surface-enhanced Raman scattering at gold electrodes: dependence on electrochemical pretreatment conditions and comparisons with silver

Abstract The dependence of the intensities for surface-enhanced Raman scattering (SERS) at gold electrodes on the nature of the electrochemical roughening procedure is explored for the model adsorbates benzonitrile and thiocyanate in order to establish “optimal” conditions for practical applications as well as to shed light on the fundamental factors involved. The pretreatment employed involves consecutive positive-negative potential sweeps in aqueous KCl, followed by rinsing and surface transfer; this has been shown previously to yield unusually stable as well as intense SERS. For roughening conditions yielding the greatest SERS intensities, scanning electron microscopy showed the surface to be covered by gold particles of around 100 nm diameter. Some illustrative comparisons are also made with corresponding SERS intensities and stabilities at silver electrodes.

Coverage‐dependent dipole coupling for carbon monoxide adsorbed at ordered platinum(111)‐aqueous interfaces: Structural and electrochemical implications

Surface infrared spectra in the terminal C–O stretching region are reported for various 12CO/13CO mixtures adsorbed on an ordered Pt(111) electrode in 0.1 M HClO4 as a function of CO coverage θ (0.05≲θ≲0.6) and electrode potential E. Contributions to the coverage‐induced shift in the C–O stretching frequency νtCO arising from dynamic dipole–dipole coupling ΔνD, and from static dipole and ‘‘chemical’’ effects ΔνC, are obtained from the mixed isotope spectra. Similar to the corresponding ultrahigh vacuum (UHV) system, the former provides the largest contribution, ΔνD being 30–35 cm−1 for saturated CO layers (θ≊0.6). Substantial differences in the νtCO ‐θ (and ΔνD ‐θ) dependencies were observed for CO adlayers formed by dosing from suitably low solution concentrations (∼2×10−5 M), and from saturated irreversibly adsorbed layers by means of partial electrooxidative stripping.Using the latter ‘‘stripping’’ procedure, only small (≲5 cm−1) decreases in νtCO and ΔνD were observed as θ is decreased over the range ...

Mechanisms of formic acid, methanol, and carbon monoxide electrooxidation at platinum as examined by single potential alteration infrared spectroscopy

Abstract Surface infrared spectra have been obtained as a function of potential and time during the voltammetric oxidation of formic acid and methanol on polycrystalline platinum in order to probe the possible role of adsorbed carbon monoxide in the electrooxidation mechanisms. The procedure involves obtaining a sequence of single-beam infrared spectra using an FTIR spectrometer during potential sweep or following potential-step perturbations, and referencing these to a spectrum obtained at the initial potential or after complete oxidation had occurred. Using this “single potential alteration infrared” (SPAIR) technique, individual spectra for the C-O stretch of adsorbed carbon monoxide, νCO, as well as the O-C-O stretch for the CO2 product could be obtained under voltammetric conditions in as little as 3.5 s, and repetitively every 7 s. Such SPAIR spectra obtained during either potential sweep or step excursions indicate that the onset of both formic acid and methanol oxidation coincides with the oxidative removal of adsorbed carbon monoxide. The electrooxidation of CO irreversibly adsorbed from solution carbon monoxide was also examined using this approach. The integrated absorbance of the νCO band was generally found to be proportional to the CO coverage (determined from either the CO2 band intensity or the voltammetric charge) during electrooxidation. The dependence of the νCO peak frequency upon coverage during potentiostatic oxidation, however, is sensitive to the timescale over which the process proceeds. The adsorption kinetics of CO formed from formic acid and methanol were evaluated from the time dependence of the νCO band intensity following suitable potential step sequences to potentials where CO electrooxidation is suitably slow. These data suggest that while adsorbed CO may act as an adsorbed intermediate for methanol oxidation, it probably acts as a chemisorbed poison for formic acid oxidation.

Coverage- and potential-dependent binding geometries of carbon monoxide at ordered low-index platinum- and rhodium-aqueous interfaces : comparisons with adsorption in corresponding metal-vacuum environments

Abstract Infrared spectra for carbon monoxide adsorbed at ordered low-index platinum- and rhodium-aqueous interfaces as a function of electrode potential, E , and CO coverage, θ co are compared with each other and with vibrational spectra obtained at corresponding metal-ultrahigh vacuum (UHV) surfaces in order to explore the influences of the doublelayer environment on the CO adlayer structure. Earlier results for Pt(111), (100), (110), and Rh(100) electrodes in aqueous perchloric acid are supplemented by additional data in neutral and alkaline electrolytes in order to expand the accessible potential range. New results are also reported for Rh(111) and Rh(110) for each of these conditions. The form of the electrochemical infrared spectra in the C-O stretching ( v CO ) region, especially the relative intensities of the characteristic terminal and twofold bridging v CO bands, tends to be most similar to spectra obtained at the metal-UHV surfaces at high θ CO and when the former are evaluated at relatively positive potentials. Altering the potential in the negative direction on most electrodes at high θ CO favors increasingly CO binding in bridging rather than terminal sites, especially on the rhodium surfaces. This is consistent with the greater extent of dπ-2π ∗ metal-CO back donation expected under these conditions. Bridging CO coordination is also favored increasingly at most electrochemical interfaces towards lower θ CO , these effects of water and/or hydrogen coadsorption are compatible with the behavior of analogous UHV systems. The v CO frequency- E slopes at constant adsorption site occupancy vary substantially with the CO binding geometry, being ∼ 30–35, 40–45, and 50–60 cm −1 V −1 for terminal, twofold and threefold bridging sites, respectively, at high θ CO . These findings, together with the observed increases in d v CO /d E towards lower θ CO . are consistent with the anticipated variations in back bonding. At least at high θ CO , the observed smaller v CO frequencies for the electrochemical relative to the corresponding UHV interfaces are largely compatible with the lower surface potentials that characterize the former, along with differences in site occupancy.

Characterization of Redox Sates of Nickel Hydroxide Film Electrodes by In-Situ Raman Spectroscopy.

Abstract : Thin films of nickel hydroxide deposited on gold electrodes have been characterized in detail by in-situ surface Raman spectroscopy in conjunction with electrochemical techniques. Raman spectra were obtained for film thicknesses varying from less than one equivalent monolayer to several hundred monolayers, as determined from the faradaic charge for the cyclic voltammetric oxidation of Ni(OH)2. For the thinnest films, Raman bands at 455/cm and at 480 and 560/cm were obtained for the reduced and oxidized films, respectively, using 647.1 nm excitation at roughened gold. These signals, identified with Ni-OH and Ni-O vibrations from deuterium isotope data, were diagnosed as arising from surface-enhanced Raman scattering (SERS) in view of their absence for the reduced film when using the latter conditions for thicker oxidized films, which were consistent with resonance Raman scattering (RRS). Keywords: In situ, Surface raman spectroscopy, Nickel hydroxide, Films, Electrodes, Cyclic voltammetry.

Real-time FTIR spectroscopy as an electrochemical mechanistic probe: Electrooxidation of ethanol and related species on well-defined Pt (111) surfaces

The electrooxidation pathways of ethanol and acetaldehyde in 0.1M HClO4 on ordered Pt (111) surfaces in comparison with disordered Pt (111) and polycrystalline Pt were examined by means of FTIR spectra obtained in real time during slow (2–10 mV s−1) thin-layer voltametric sweeps. This “single potential alteration infrared” (SPAIR) procedure enables the potential-dependent quantity of the various reaction products, acetic acid, acetaldehyde, and CO2 to be determined quantitatively, and the role of CO and other CO2-producing adsorbates in the reaction kinetics to be assessed. For ethanol electrooxidation on ordered Pt (111), acetic acid and acetaldehyde are formed, along with smaller amounts of CO2 beyond ca. 0.3 V (vs. SCE) where oxidation of adsorbed CO occurs; acetic acid is the major product during the reverse potential sweep. Only small CO coverages (θ ≈ 0.1) are obtained for ethanol concentrations of 0.05-0.5 M. Substantially higher CO coverages are obtained for disordered Pt (111) and especially polycrystalline Pt. The product distributions are not altered greatly from ordered Pt (111). Markedly different behavior, however, was observed for acetaldehyde electrooxidation on ordered Pt (111) in that little acetic acid is formed until near the end of the positive-going potential sweep, 0.8 V. A greater quantity of adsorbed CO (θ ≈ 0.15) is present, but this undergoes complete electrooxidation by 0.45 V. This marked reaction inhibition is attributed instead to the presence of another (or other) adsorbed species, which is observed to undergo oxidation to CO2 only between 0.5–0.7 V. The inhibition of acetic acid formation is much less marked on disordered Pt (111), and essentially absent on polycrystalline Pt. Comparisons are also made between the electrooxidation kinetics for some related unifunctional reactants, including formic acid and isopropanol, on ordered and disordered Pt (111) surfaces. Markedly smaller amounts of adsorbed CO are generally formed from organic species on the former surface; some possible electrocatalytic consequences are discussed.

Validity of double-layer charge-corrected voltammetry for assaying carbon monoxide coverages on ordered transition metals : comparisons with adlayer structures in electrochemical and ultrahigh vacuum environments

Abstract A coulometric procedure enabling the reliable and accurate evaluation of saturated CO coverages, θ sat CO , on Pt-group transition-metal electrodes is outlined, and applied to CO adlayers on ordered low-index platinum, rhodium, and iridium surfaces in acidic aqueous media. Along with voltammetric data, the method utilizes previously described measurements of the charge displaced upon CO adsorption. The reverse of this charge, Q dis , together with the “background” charge Q b flowing between a suitable pair of electrode potentials in the absence of CO, constitutes the overall “double-layer” correction Q dl to the total voltammetric charge Q tot measured for the electrooxidation of adsorbed CO between the same potentials. Significantly, the Q dis as well as the Q b component of Q dl typically constitutes moderate or even large corrections to Q tot , so that the deduced θ sat CO values are noticeably (20–30%) smaller than some voltammetric-based estimates reported earlier. However, the revised coulometric θ sat CO values are in consistently good agreement with the corresponding coverages obtained by means of an infrared spectrophotometric procedure. These θ sat CO values are compared with adlayer structural information obtained recently from in situ scanning tunneling microscopy along with infrared spectroscopy, and also with structural data for corresponding adlayers in ultrahigh vacuum (UHV). In most cases, the electrochemical and UHV-based θ sat CO values are not greatly different (within 5–10%), even though the CO binding site arrangements are often dissimilar in these two environments. The role of the electrode potential in affecting θ sat CO under some conditions via alterations in binding-site energetics, however, is noted for the Pt(111)/CO system.