Ignacio Villegas

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...

Dipole–dipole coupling effects upon infrared spectroscopy of compressed electrochemical adlayers: Application to the Pt(111)/CO system

Experimental infrared spectra for CO adlayers on Pt(111) electrodes having known real‐space structures as deduced by scanning tunneling microscopy are compared with predictions extracted from conventional dipole–dipole coupling models in order to test the validity of such treatments for compressed electrochemical adlayers, especially with regard to band‐intensity transfer effects. The specific structures considered are (2×2)–3CO and (√19×√19)R23.4°–13CO hexagonal adlayers; the former is especially close packed (θCO=0.75) with a pair of threefold hollow and one atop CO per unit cell, while the latter has a lower coverage (θCO=13/19) and involves largely asymmetric binding sites. The comparisons between dipole‐coupling theory and experiment include infrared spectra for various 13CO/12CO mixtures, thereby exploiting the well‐known systematic alterations which are induced in the degree of coupling for a given adlayer. Consistent with an earlier assessment (Ref. ) the conventional dipole–dipole treatment can a...

Scanning tunneling microscopy and infrared spectroscopy as combined in situ probes of electrochemical adlayer structure. Cyanide on Pt(111)

Abstract The spatial structure of irreversibly adsorbed cyanide adlayers on a Pt(111) electrode is deduced by means of in situ scanning tunneling microscopy (STM) combined with infrared reflection—absorption spectroscopy (IRAS). The latter data suggests atop-like Ptue5f8CN coordination. The STM images display a (2 ×2 )R30°−7CN structure, with cyanides bound in symmetric atop sites surrounded by hexagonal “clusters”, of CN, likely bound in near-atop sites. This arrangement, stable over the applied potential range −0.5 to 0 V versus SCE, gives way to a (2×2) structure and disordered adlayers at lower potentials. These findings are compared with previous structural information deduced from ex situ electron diffraction.

Local structure and phase transitions within ordered electrochemical adlayers : some new insights from in situ scanning tunneling microscopy

The application of in situ atomic-resolution scanning tunneling microscopy (STM) to the elucidation of local structure and potential-induced phase transitions within ordered adlayers on monocrystalline metal electrodes is discussed, and illustrated primarily for compressed CO adlayers on Pt(111). The value of the additional structural information on adsorbate binding sites that can be obtained from infrared reflection-absorption spectroscopy (IRAS) as well as from potentiodynamic STM imaging tactics is emphasized. The atomic-level and nanoscale structural alterations wrought upon the Pt(111)/CO adlayer by bismuth predosing are described as a further illustration of the synergetic value of the spatial and surface-bonding information provided by STM and IRAS. A comparison is made between the potential-dependent adlayer characteristics of the compressed CO adlayers with iodide adlayers on gold and platinum low-index electrodes. Possible reasons for the marked differences in the phase-transition behavior of these nonmetallic adlayers are briefly discussed, along with comparisons with analogous adlayers in ultrahigh vacuum environments.

Nature of the atomic-scale restructuring of Pt(100) electrode surfaces as evidenced by in-situ scanning tunneling microscopy

Elucidating the atomic and nanoscale structures of single-crystal metal electrodes is of central importance in electrochemistry. Until recently, most information along those lines was obtained by low-energy electron diffraction and related methods following transfer into ultrahigh vacuum (uhv) [1,2]. The recent advent of scanning tunneling microscopy (STM) as an in-situ probe is offering important new insight into the structural properties of the electrode substrates themselves as well as ordered adsorbate layers [3]. Examinations of surface restructuring, i.e. the occurrence of metal atomic arrangements which differ from the ideal bulktermination structure, are forming an important part of current research efforts [2,3]. Most studies along these lines are concerned with metal reconstructions, whereby the top layer of surface atoms rearrange to form ordered terraces with unit cells that are quite different from the (1 X 1) symmetry. A related, yet quite distinct, form of restructuring entails longer-range disruption of the surface lattice, possibly to form periodic superstructures, but perhaps involving retention of the local unit-cell geometry. While examples are as yet less plentiful, the local-probe nature of the STM makes the technique well suited to the examination of the latter form of restructuring, which can escape detection when using diffraction-based methods. A recent example of restructuring with retention of the terrace (1 X 1) geometry is seen by STM on Au(ll0) electrode in iodide media, where periodic cleavage along the (li0) direction is observed so to yield terrace “channels” bordered by monoatomic steps at high potentials, where an ordered iodine adlayer is present 141. Another case, examined initially by Vitus et al. [5& involves the formation of dense arrays of 20-40 A substrate “islands” on initially uniform Pt(100) terraces upon in-situ replacement of an iodine adlayer, formed

Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: some implications for ionic and chemisorbate solvation at electrode surfaces

Abstract The utility of infrared reflection-absorption spectroscopy (IRAS) for examining structure and bonding for model electrochemical interfaces in ultrahigh vacuum (UHV) is illustrated, focusing specifically on the solvation of cations and chemisorbed carbon monoxide on Pt(111). These systems were chosen partly in view of the availability of IRAS data (albeit limited to chemisorbate vibrations) for the corresponding in-situ metal-solution interfaces, enabling direct spectral comparisons to be made with the “UHV electrochemical model” systems. Kelvin probe measurements of the metal-UHV surface potential changes (ΔΦ) attending alterations in the interfacial composition are also described: these provide the required link to the in-situ electrode potentials as well as yielding additional insight into surface solvation. Variations in the negative electronic charge density and, correspondingly, in the cation surface concentration (thereby mimicking charge-induced alterations in the electrode potential below the potential of zero charge) are achieved by potassium atom dosage onto Pt(111). Of the solvents selected for discussion here — deuterated water, methanol, and acetonitrile — the first two exhibit readily detectable vibrational bands which provide information on the ionic solvation structure. Progressively dosing these solvents onto Pt(111) in the presence of low potassium coverages yields marked alterations in the solvent vibrational bands which can be understood in terms of sequential cation solvation. Comparison between these spectra for methanol with analogous data for sequential methanol solvation of gas-phase alkali cations enables the influence of the interfacial environment to be assessed. The effects of solvating chemisorbed CO are illustrated for acetonitrile; the markedly larger shifts in CO frequencies and binding sites for dilute CO adlayers can be accounted for in terms of short-range coadsorbate interactions in addition to longer-range Stark effects. The differing degrees of selective solvation of cations versus CO for water and acetonitrile are also explored. While the former solvent removes all specific effects of K+ upon CO, these short-range interactions partly remain upon acetonitrile solvation. The close connections between the UHV-based findings and the behavior of the in-situ electrochemical systems are also discussed.

Model electrochemical interfaces in ultra-high vacuum: solvent-induced surface potential profiles on Pt(111) from work-function measurements and infrared Stark effects

Abstract The influence of various solvents upon the interfacial-potential profile on Pt(111) has been investigated by means of work-function changes and infrared frequency Stark shifts attending sequential-molecular dosing in ultra-high vacuum (UHV) at a suitably low temperature (ca. 100 K) with the primary objective of assessing the role of surface solvation in related electrochemical systems. The solvents examined — dichloromethane, benzene, acetone, acetonitrile, methanol, and ammonia — span a range of polarity and other solvating properties. These species were dosed onto both clean and CO-saturated Pt(111), the Stark shifts being evaluated for the Cue5f8O stretching mode of terminally co-ordinated carbon monoxide. Marked decreases (≥ 1 eV) in the work function, Φ, and hence in the surface potential, φ, are observed on the addition of most solvents onto clean Pt(111). Milder yet still substantial Φ decreases are also observed for solvent dosage upon CO-saturated Pt(111). These latter Φ changes correlate approximately with the observed v CO frequency downshifts, suggesting that the latter property is also sensitive to the solvent-induced electrostatic interfacial field. The functional form of both the Φ decreases and the corresponding v CO frequency downshifts induced by solvent dosage provide insight into the dosage-dependent potential profile and its relationship to both the monolayer and multilayer solvent structure. The present findings are also briefly compared with corresponding v t CO − Φ data obtained for potassium atom dosing, where the surface potential is altered instead by varying the surface electronic charge in the presence of a given solvent. The underlying factors responsible for the surprisingly large solvent-induced surface potential shifts are discussed in detail, and the likely importance of the surface electronic charge distribution as well as solvent dipole orientation and adsorbate-metal charge sharing is pointed out.

Nanoscale phenomena in surface electrochemistry : some insights from scanning tunneling microscopy and infrared spectroscopy

Abstract Examples of nanoscale phenomena at electrochemical interfaces, examined recently in our laboratory by means of in-situ scanning tunneling microscopy (STM) and infrared reflection–absorption spectroscopy (IRAS), are discussed with the overall objective of assessing some spheres of applicability of these approaches for exploring substrate and adlayer atomic-/molecular-level properties which propagate over nanoscale distances. While the direct spatial structural information obtainable by in-situ STM makes this technique of obvious importance for exploring nanoscale phenomena, the infrared probe offers not only chemical specificity, but also the fast (∼10 −11 xa0s) spectral timescale enables fluxional adlayer structures to be probed. Four types of applications along these lines are considered. First, recent STM evidence for adsorbate-induced nanoscale restructuring on ordered monocrystalline electrodes is discussed, specifically for Pt(100) and Au(110) surfaces. The occurrence of restructuring with retention of the substrate unit cell (i.e. without reconstruction) is outlined. Apparent similarities in the long-range Br-induced restructuring observed on Au(110) with the adsorbate-induced reconstruction of (110) surfaces in UHV are also noted. Second, the likely occurrence of CO-induced reconstruction of Pd(110) electrodes, as probed by IRAS, is described as an example of the value of spectral comparisons between electrochemical and UHV systems for elucidating surface structure in the former environment. Third, the utility of dipole–dipole coupling analysis for elucidating the nanoscale spatial structure of both CO adlayers and metal substrates is discussed. The specific topics chosen, CO islands formed during adlayer electrooxidation and the microscopic nature of Pt/Ru alloy catalysts, illustrate the usefulness of such analyses in both ordered monocrystalline and polycrystalline surface environments. Lastly, applications of UHV-based IRAS measurements for exploring double-layer charge–solvent interactions are briefly outlined. The results uncover a long-range influence of ionic charge upon inner-layer solvent orientation and also clarify the roles of the solvent molecules in curtailing short-range ion–adlayer interactions and screening the otherwise-complex electrostatic-field effects upon adsorbate vibrational properties.

Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: interfacial cation solvation by ammonia on Pt(111)

Abstract Infrared reflection-absorption spectroscopic (IRAS), along with work-function (Φ), measurements are reported for ammonia dosed onto potassium-predosed as well as clean Pt(111) in ultrahigh vacuum (UHV) with the objective of assessing the nature of interfacial cation solvation, which is of fundamental relevance to electrochemical systems. Ammonia was selected as a strongly coordinating as well as chemisorbing solvent, for comparison with other dipolar hydrogen-bonding media examined previously in this manner. In the absence of potassium, ammonia chemisorption at 90 K yields several vibrational features in the Nue5f8H stretching ( v NH ) and especially the symmetric Hue5f8Nue5f8H bending ( δ s HNH ) regions: their identification with distinct chemisorbed, second-layer and multilayer ammonia states is aided by comparison of temperature-dependent spectra with earlier temperature-programmed desorption (TPD) data. These chemisorbate states were absent for ammonia dosed onto a saturated CO adlayer. In the presence of predosed potassium, distinctly different v NH and δ s HNH spectral features were obtained for initial ammonia dosages (corresponding to small NH 3 K stoichiometries, ≈1). Comparison with vibrational spectra for bulk-phase K + ue5f8NH 3 species support the occurrence of K + -induced reorientation of ammonia, involving also NH 3 -surface hydrogen bonding. Further ammonia exposure yielded frequency downshifts in the δ s HNH mode, ostensibly similar to that observed for second-shell solvation of gas-phase Na + and indicative of primarysecondary shell H bonding. However, completion of the K + primary solvation shell requires markedly higher ammonia exposures, reflecting further the competition between K + - and surface-ammonia interactions. In contrast to other solvents, Φ was observed to be essentially independent of the K coverage (≈3.0 eV) in the presence of ammonia multilayers. Comparison with solution-phase electrochemical data suggests strongly that solvated electrons along with K + are formed at the Pt(111)ue5f8UHV interface under these conditions.