Underpotential Shift Research Articles

The Effects of Organic Adsorbates on the Underpotential and Bulk Deposition of Silver on Polycrystalline Platinum Electrodes

The effects of various heterocyclic coadsorbates (pyridine, pyrazine, 2,2′‐bipyridyl, 4,4′‐bipyridyl, 4‐phenylpyridine, 4‐mercaptopyridine, and 2‐mercaptopyridine) on the electrodeposition of silver onto polycrystalline platinum electrodes in 0.10M have been studied using cyclic voltammetry. Adsorbates bonding through a nitrogen heteroatom significantly hinder both the silver underpotential (UPD) and bulk deposition processes giving rise to a diminution in the underpotential shift in the former and an overpotential in the latter. The diminution of the underpotential shift suggests a strong interaction of the adsorbate adlayer with the platinum substrate. In addition, the existence of a significant overpotential necessary for initiation of bulk silver deposition suggests a significant interaction between the UPD silver monolayer and the adsorbate adlayer. A Pt/Ag/adsorbate intermediary structure is proposed based on this voltammetric evidence. The sulfur‐containing adsorbates completely inhibit silver UPD as well as the formation of a film on the polycrystalline electrode. The only observable processes were bulk deposition (at overpotentials of the order of 125 mV) and subsequent bulk stripping of the deposited silver. The effects of having a polycrystalline surface have been assessed by comparing these results to those from studies of silver deposition onto Pt(111) surfaces in the presence of the same coadsorbed species.

Thermodynamic derivation and model calculations of the metal underpotential dependence on electron work function differences

The relationship between the work function of the substrate+adsorbate system and the underpotential shift is derived from a thermodynamic argument. An alternative explanation of the correlation formulated in the literature between the underpotential shift and the work function difference of the metals involved is proposed. A simple model is used to illustrate the meaning of the equations derived. Abrupt adsorption-desorption behaviour is predicted for those electrochemical systems where the work function shows a pronounced minimum as a function of the coverage, provided that this is a first-order contribution to the chemical potential of the system.

Study of electrochemical gold adsorption on polycrystalline platinum substrates

The underpotential shift of adsorbed gold is about 0.06 V. Adatom deposition on a platinum surface may take place via ionization of the substrate metal. It may be assumed that during the oxidation of adsorbed gold, corrosion of the substrate metal catalysed by adsorbed gold also takes place. Contrary to earlier results, the number of hydrogen adsorption sites covered by one gold adatom is only about one.

Influence of competing adsorbates on the underpotential deposition of copper on Pt(111)

We present a survey of the voltammetry for the underpotential deposition (UPD) of copper from aqueous sulfuric acid solution on a flame annealed Pt(111) electrode pretreated with or in the presence of aqueous solutions of a variety of adsorbales: because of its sensitivity to the presence of competing adsorbales, variations in the Cu UPD voltammetric features are used to infer substrate / adsorbate interactions. The adsorbales studied included both ionic and molecular species and were: sulfate, cyanide, thiocyanate and cis-1,2-dicyanoethylene-1,2-dithiolate anions; dimethylformamide (DMF), acetonitrile, dimethylsulfoxide (DMSO), pyridine (from 0.1 M H 2SO 4 and PDW (pyrolytically distilled water)); formic acid, acetic acid, hydroquinone (HQ); pyrocatechol, 2,2 - ́ bipyridyl (0.1 M H 2SO 4 and PDW), 4,7-dihydroxy-1,10-phenanthroline, and 1,3-propanedithiol. Anionic adsorbates caused a diminution in the underpotential shift (relative to that in sulfuric acid alone), dependent on the strength of adsorption of the anion. Neutral adsorbales gave a variety of responses in terms of underpotential shifts and generally produced changes in the voltammetric peak shapes. Sulfur containing ligands gave rise to the most dramatic effects, generally causing a large reduction in the underpotential shift and in some cases (e.g. propanedithiol) complete inhibition of the UPD process. DMSO, acetonitrile, and DMF did not appreciably affect the potential of UPD but caused substantial kinetic effects in terms of variations in peak heights and widths. For adsorbates that could undergo protonation (e.g. pyridine, bipyridine and dihydroxy phenanthroline) the observed behavior was strongly dependent on pH, reflecting the fact that protonation of the nitrogen can lead to profound alteration in the mode of binding. Deposition in the presence of formic or acetic acid suggests the presence of discrete coexisting domains.

The Underpotential Adsorption/Deposition and Stripping of Mercury on Gold in Dilute Sulfuric Acid

The underpotential deposition of mercury on gold was studied by potentiostatic techniques at a rotating gold ring-disc electrode. Underpotential mercury deposition occurred at potentials more positive than 680 mV (against a dynamic hydrogen electrode). Anodic stripping voltammetry at the disc with ring collection showed that a monolayer coverage of underpotential mercury was equivalent to approximately 370pc/cm2. The stripping process yielded several anodic peaks and produced largely HgII, at least 16% of which remained adsorbed on the bare gold electrode. Potential-step experiments at the disc and ring-shielding data indicated that the underpotential shift was about 500 mV and that the deposition process involved both adsorbed (35-47% of the theoretical monolayer coverage) and soluble mercury(II) species. The adsorbed mercury had a formal positive charge of 0.40-0.46. The most anodic stripping peak (at 1200 mV) of underpotential mercury may be used to determine nanomolar levels of mercury by anodic stripping voltammetry, by using an overvoltage of less than 100 mV for preelectrolysis.

Electroreduction of Ni 2+ and Fe 2+ in a fused electrolyte : Formation of a two-component alloy and the role of the underpotential deposition

It is shown that the simultaneous electroreduction of Ni 2+ and Fe 2+ ions dissolved in a fused electrolyte (LiCl+KCl) at 503°C is controlled by ion diffusion and by metal interdiffusion. These transport properties are studied by chronopotentiometry. During the reduction step two transition times are observed; these times depend on the diffusion fluxes of Ni 2+ and Fe 2+ ions. At the end of this reduction pulse, oxidation of the alloy coating by Ni 2+ ions gives rise to a potential jump which characterizes the metal interdiffusion. At short times a large increase of the interdiffusion rate is observed; this effect is attributed to a rapid transport process occurring in the very first atomic layers of the coating; it presents some similitude with the underpotential shift observed in metal deposition at room temperature.

Deposition and Stripping Properties of Mercury on Iridium Electrodes

The reduction of mercury on a fresh iridium surface is reversible with a half‐wave potential of +162 mV (vs. SCE). A Hg(O) monolayer is formed by underpotential deposition (UPD) at +600 mV during the initial scans of a repetitive cyclic voltammogram. This monolayer acts as bulk Hg(O) during further cycles, and the reduction half‐wave potential then shifts towards +410 mV. Anodization of the electrode at a potential > +1000 mV returns the reduction wave to its original position. The peak for the oxidation of bulk Hg(O) from the surface occurs at +430 mV followed by a peak for the underpotential stripping (UPS) of the Hg(O) monolayer at +650 mV. The quantity of Hg(O) for the monolayer corresponds to a 40% coverage of the Ir surface. The underpotential shift for the bulk and monolayer stripping of Hg(O) from Ir was found to be 210 mV. No indications of a Hg‐soluble compound formation between Hg and Ir were found.

Mono- and multilayer formation studies of thallium on glassy carbon electrode

Detailed investigations on the mono- and multilayer formation of thallium onto a glassy carbon electrode reported. The monolayer region shows two peaks—one obeying Kolb's correlation[D. M. Kolb et al., J. electroanal. Chem. 54, 25 (1974)] and the other occuring with a large underpotential shift. The monolayer peak obeying kolb's correlation follows adsorption-desorption kinetics and the other one progressive nucleation-growth kinetics. The multilayer formation of thallium occurs by 3D nucleation-growth kinetics at higher concentrations of thallium and is independent of the presence or absence of underpotentially deposited thallium or nature of the substrate.

Interpretation of the potentiodynamic response during the underpotential deposition of silver on polycrystalline gold

A potentiodynamic study was made of the underpotential deposition (UPD) of silver on gold using the rotating ring-disk electrode technique. The implications of analyzing the potentiodynamic data without subtracting the residual charging currents are examined. Our interpretation of the dynamic electrosorption valency data is consistent with a simple relationship between the negative shift in point of zero charge, UPD coverage and the underpotential shift. Furthermore, under equilibrium conditions, the charging current accompanying potential scan can be related quantitatively to this predicted pze shift. The potentiodynamic disk current peak structure can be interpreted in terms of the adsorption isotherm by making use of concurrent mass flux data obtained from ring electrode shielding data. Generally, complicating factors such as deposition kinetics, double layer changes and parallel chemisorption processes prevent the analysis of current-potential data obtained at a single electrode in the absence of independently obtained mass flux data. Kinetic models that involve the coupling of mass transport, adsorption and charge transfer are analyzed. A mixed control model involving charge transfer, adsorption and mass transport agrees with experimental results.

Interpretation of the potentiodynamic response during the underpotential deposition of silver on polycrystalline gold

A potentiodynamic study was made of the underpotential deposition (UPD) of silver on gold using the rotating ring-disk electrode technique. The implications of analyzing the potentiodynamic data without subtracting the residual charging currents are examined. Our interpretation of the dynamic electrosorption valency data is consistent with a simple relationship between the negative shift in point of zero charge, UPD coverage and the underpotential shift. Furthermore, under equilibrium conditions, the charging current accompanying potential scan can be related quantitatively to this predicted pze shift. The potentiodynamic disk current peak structure can be interpreted in terms of the adsorption isotherm by making use of concurrent mass flux data obtained from ring electrode shielding data. Generally, complicating factors such as deposition kinetics, double layer changes and parallel chemisorption processes prevent the analysis of current-potential data obtained at a single electrode in the absence of independently obtained mass flux data. Kinetic models that involve the coupling of mass transport, adsorption and charge transfer are analyzed. A mixed control model involving charge transfer, adsorption and mass transport agrees with experimental results.

ChemInform Abstract: ADSORPTION AND ALLOY FORMATION OF ZINC LAYERS ON SILVER

AbstractDie Abscheidung von Zn auf Ag aus alkalischen Zincatlösungen wird mittels cyclischer Voltammetrie, Potentialimpulsmethoden, Röntgen‐Beugung und Rasterelektronenmikroskopie untersucht.

Adsorption and Alloy Formation of Zinc Layers on Silver

The deposition of zinc on silver from alkaline zincate solutions has been investigated using cyclic voltammetry, potential pulse methods, x‐ray diffraction, and scanning electron microscopy. Approximately one monolayer of zinc is formed in the underpotential deposition region prior to bulk deposition. The underpotential shift is 0.15V. Zinc forms at least two alloys with the substrate. One was identified as and the other as the β′ phase of. The bulk zinc deposit consists of hexagonal platelets that are oriented both parallel and perpendicular to the basal plane.

Deposition of monolayer and bulk lead on Ag(111) studied in vacuum and in an electrochemical cell

The early stages of Pb condensation on Ag(111) have been studied in ultrahigh vacuum and in an electrochemical cell. The vacuum deposited films were investigated by AES, LEED and work function measurements, and the electrolytically deposited films in situ by cyclic voltammetry, potential step experiments and reflectance spectroscopy. Despite the very different environments a similar growth behaviour is observed. In both cases deposition starts with the formation of a uniform monolayer on Ag(111), which occurs in three steps: random adsorption at very low coverages followed by a √3 × √3 superstructure and finally, a hexagonal close packed layer. From an analysis of the AES data for the vacuum deposited film a particular case of Stranski-Krastanov type is found for the subsequent growth. The formation of ordered submonolayer structures which is clearly detectable by LEED, is inferred for the electrolytically formed layer since a similar coverage dependence in the reflectance is observed. From the underpotential shift of the electrolytically formed monolayer an excess binding energy of about 0.3 eV is deduced for the Pb adatoms on Ag(111).

Electrodeposition of mercury on gold from very dilute mercury(II) solution

Flameless atomic absorption spectrometry (flameless a.a.s.) was applied to study the state of mercury deposited on a gold plate electrode from very dilute mercury(II) solution by controlled-potential electrolysis. A stable monolayer is formed on the gold electrode by the electrolysis at a potential about 200 mV more positive than the reversible Nernst potential for the reduction of mercury(II) to mercury(0). After the monolayer formation, bulk mercury is deposited on the monolayer at the reversible potential and an adatom layer is also found. The difference of activation free energies between the evaporation of mercury from the monolayer and that from bulk mercury corresponds to the underpotential shift for the electrodeposition of mercury on the gold electrode.

Electrodeposition of mercury on gold from very dilute mercury(II) solution

Flameless atomic absorption spectrometry (flameless a.a.s.) was applied to study the state of mercury deposited on a gold plate electrode from very dilute mercury(II) solution by controlled-potential electrolysis. A stable monolayer is formed on the gold electrode by the electrolysis at a potential about 200 mV more positive than the reversible Nernst potential for the reduction of mercury(II) to mercury(0). After the monolayer formation, bulk mercury is deposited on the monolayer at the reversible potential and an adatom layer is also found. The difference of activation free energies between the evaporation of mercury from the monolayer and that from bulk mercury corresponds to the underpotential shift for the electrodeposition of mercury on the gold electrode.

Underpotential deposition of metals and work function differences

The electrolytic deposition of metal atoms onto foreign metal substrates at underpotentials has been studied in aqueous and non-aqueous solutions. It is shown, that the potential difference between monolayer and bulk deposition is closely related to the difference in the work functions of substrate and deposit, causing a partial charge of the adatoms. The ionic contribution to the chemical bond, arising from this partial electron transfer, can account for the favourable deposition of the first monolayer. A formula is given which allows the prediction of the amount of underpotential shift from differences in work function. This is explained in analogy to Pauling's treatment of the chemical bond between atoms of different electronegativity.