Redox and electrocatalytic activity of copper in base at unusually low, premonolayer potentials

The assumption involved in this work is that the formation of monolayer (α) oxide species, which occurs on copper in base at 60 °C at ca. 0.5 V (RHE), is of little relevance to the electrocatalytic behaviour of the metal at low potentials. For many processes occurring at the interface in question the critical potential is ca. −0.1 V (RHE). This is the value where several electrocatalytic oxidation and reduction processes commence or terminate under potential sweep conditions; hydrous oxide films are reduced close to this value and it is possible to produce an active (but transient) state of the metal surface which exhibits a reversible redox response in this region. The results are rationalized in terms of the incipient hydrous oxide/adatom mediator model of electrocatalysis, and the nature of the active site material at the interface is discussed. Received: 25 January 1999 / Accepted: 8 April 1999     

Auto-inhibition of hydrogen gas evolution on gold in aqueous acid solution

It was demonstrated recently that dramatic changes in the redox behaviour of gold/aqueous solution interfaces may be observed following either cathodic or thermal electrode pretreatment. Further work on the cathodic pretreatment of gold in acid solution revealed that as the activity of the gold surface was increased, its performance as a substrate for hydrogen gas evolution under constant potential conditions deteriorated. The change in activity of the gold atoms at the interface, which was attributed to a hydrogen embrittlement process (the occurrence of the latter was subsequently checked by surface microscopy), was confirmed, as in earlier work, by the appearance of a substantial anodic peak at ca. 0.5 V (RHE) in a post-activation positive sweep. Changes in the catalytic activity of a metal surface reflect the fact that the structure (or topography), thermodynamic activity and electronic properties of a surface are dependent not only on pretreatment but also, in the case of the hydrogen evolution reaction, vary with time during the course of reaction. As will be reported shortly, similar (and often more dramatic) time-dependent behaviour was observed for hydrogen gas evolution on other metal electrodes. Electronic Publication     

The effect of thermal pretreatment on the electrochemical response for palladium in aqueous media

Previous work on the electrochemistry of palladium in aqueous acid solution demonstrated the existence of two multilayer hydrous oxide reduction peaks, one at ca. 0.24 V and another at ca. 0.55 V vs. RHE, plus the presence of a reversible active surface state transition at ca. 0.24 V. In the present work with thermally activated palladium it was observed that, in agreement with the hydrous oxide reduction behaviour of the system, there is a second active state transition at E≥ca. 0.45 V. In most of its reactions in aqueous acid solution, apart from its unusual capacity to absorb hydrogen, palladium exhibits properties very similar to those of platinum; however, palladium seems to be more prone to dissolution and subsurface oxygen formation. Also the premonolayer oxidation responses of these two metals are often different as the more active state of the palladium surface is not as readily generated as that of platinum. The electrocatalytic properties of palladium, as reported earlier, correlate quite well with the hydrous oxide and premonolayer oxidation behaviour of this electrode system. Electronic Publication     

Some unusual features of the electrochemistry of silver in aqueous base

Multilayer oxide films were grown on silver in base by repetitive potential cycling; however, the type of oxide obtained, as assessed on the basis of its reduction behaviour, was dependent on the lower limit of the oxide growth cycles. Using limits of 1.03–2.60 V (RHE) the oxide film produced was assumed to be predominantly Ag₂O; reduction of the latter yielded a cathodic peak at ca. 0.8 V and a surface layer of silver microparticles of diameter ranging from ca. 100 to 227 nm which, although relatively stable, were prone to rapid, extensive reoxidation. Altering the oxide growth limits to 0.7–2.60 V resulted in the growth of a different type of oxide deposit which is assumed to be AgOH; reduction of the latter occurred in a negative sweep in a random manner, i.e. in the form of cathodic spikes extending to potentials as low as ca. –0.5 V. Both types of silver oxide species are assumed to be involved in premonolayer oxidation and electrocatalysis at silver in base and the nature of the former process is discussed in some detail. Electronic Publication     

Multilayer hydrous oxide growth on copper in base and its correlation with previously reported electrocatalytic data for this electrode system

A repetitive potential cycling procedure was used to produce a specific multilayer hydrous oxide film on copper in base at 60 °C. Such a deposit undergoes reduction in a quasi-reversible manner at ca. −0.1 V (RHE), i.e. at a potential that is unrelated to Pourbaix data for copper but, as demonstrated previously, is of major significance with regard to the electrocatalytic behaviour of this electrode system. In accordance with the incipient hydrous oxide/adatom mediator model of electrocatalysis, an active surface state of the metal (Cu*) is assumed to be involved both in electrocatalysis and as a primary product in the hydrous oxide reduction reaction. While the latter process occurs very rapidly at −0.1 V, it is not usually reversible as it is accompanied by subsequent rapid loss of the active state of the metal. The same general approach was used previously to explain the hydrous oxide and electrocatalytic behaviour of a range of noble metals. Received: 28 August 1999 / Accepted: 11 November 1999     

Electrochemical study of polyaniline deposited on a titanium surface

The electrochemical synthesis of polyaniline on a titanium surface in aqueous sulfuric acid solutions with various concentrations of added aniline has been investigated by cyclic voltammetry. By utilizing a more cathodic potential range (up to −0.6 V) for the cyclization than is usual (up to −0.2 V) on Pt and Au electrodes, the new voltammetric waves have been deconvoluted from the already well-known ones for polyaniline. By simultaneous electrochemical and in situ Raman spectroscopic measurements, the Raman bands of polyaniline electrodeposited on a Ti electrode, were assigned for potentials of −0.15 V and −0.6 V. It was found that the new monitored waves were closely related to the so-called “middle” peaks and appear only when the polyaniline reaches an overoxidized state. Received: 7 August 1997 / Accepted: 4 November 1997     

The effect of severe thermal pretreatment on the redox behaviour of gold in aqueous acid solution

Severe thermal pretreatment of gold wire electrodes in an inert gas atmosphere resulted in the appearance of dramatic premonolayer oxidation responses, which in some instances commenced at 0.25 V (RHE), for the resulting active gold electrodes in aqueous acid media. Similar behaviour was reported earlier for platinum and gold activated by cathodic pretreatment in acid solution; these active noble metal surfaces are evidently more susceptible to oxidation than bulk copper. Such behaviour was attributed to the effect of surface disorder; many of the metal atoms at the interface are assumed to be in a very active, metastable state possessing quite low lattice stabilization energy. Premonolayer oxidation responses are again correlated with electrocatalytic behaviour and the existence of unusual chemisorption behaviour for molecules reacting at highly active states of metals is outlined. Electronic Publication     

A comparative study of a polyindole-based microelectrochemical transistor in aqueous and non-aqueous electrolytes

The behaviour of a polyindole-based microelectrochemical transistor in aqueous and non-aqueous electrolytes is described. The polyindole film was grown onto two closely spaced (100 μm) platinum microelectrodes by anodic oxidation of indole (10 mM) from 0.1 M tetrabutylammonium perchlorate in dichloromethane at 1.1 V vs. Ag/AgCl. The polymerization was carried out for a sufficiently long time in order to connect both Pt microelectrodes, which operated as a transistor when immersed in an electrolytic solution. In this transistor, one microelectrode was a “source” and the other a “drain”; the Ag/AgCl wire reference electrode was used as a “gate”. The drain current (current between source and drain) was modulated by varying the gate potential (potential between source and gate) at a fixed drain potential (potential between source and drain). The transconductances of the transistor were estimated as 0.98 mS/cm and 20.6 mS/cm of channel width (separation between two microelectrodes) in aqueous and non-aqueous solutions, respectively. Received: 6 April 1999 / Accepted: 24 August 1999     

An electroanalytical study of the drug proflavine

A square wave adsorptive stripping voltammetric (SWAdSV) method was developed for the determination of proflavine. The electrochemical behaviour of proflavine was investigated by cyclic (CV) and square wave voltammetry (SWV) at the hanging mercury drop electrode (HMDE) and carbon paste electrode (CPE). Different parameters were tested to optimize the conditions of the determination. Better results were obtained by square wave voltammetry using CPE where two oxidation and a reduction peak, appeared, at 0.19, 0.94 and 0.20 V, respectively. The peak at 0.19 V is quasi-reversible and deposition dependent. Linearity was observed in the range of (0.2–23.4) × 10⁻⁸ M (r = 0.998) during the anodic scan and in the range of (1.17–117) × 10⁻⁸ M (r = 0.999) during the cathodic scan. The second peak at 0.94 V is irreversible and deposition independent. The linearity of this peak was observed in the range of (1.29–11.7) × 10⁻⁸ M (r = 0.998). The method was applied to the analysis of bovine serum and gave satisfactory results. Correspondence: S. Th. Girousi, Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University, Thessaloniki 54124, Greece     

Spectroelectrochemistry of conducting polypyrrole and poly(pyrrole–cyclodextrin) prepared in aqueous and nonaqueous solvents

Conducting polypyrrole (PPy) and poly(pyrrole-2,6-dimethyl-β-cyclodextrin) [poly(Py-β-DMCD)] films were prepared by electrode potential cycling on a gold electrode in aqueous and nonaqueous (acetonitrile) electrolyte solutions containing lithium perchlorate. The resulting products were characterized with cyclic voltammetry, in situ UV–Vis spectroscopy, and in situ conductivity measurements. For the electrosynthesis of poly(Py-β-DMCD), a (1:1) (mole–mole) (Py-β-DMCD) supramolecular cyclodextrin complex of pyrrole previously characterized with proton NMR spectroscopy was used as starting material. A different cyclic voltammetric behavior was observed for pyrrole and the poly(Py-β-DMCD) complex in aqueous and nonaqueous solutions during electrosynthesis. The results show that in both solutions in the presence of cyclodextrin, the oxidation potential of pyrrole monomers increases. However, the difference of oxidation potentials for films prepared in aqueous solution is larger than for the films prepared in nonaqueous solution. In situ conductivity measurements of the films show that films prepared in acetonitrile solution are more conductive than those synthesized in aqueous solutions. Maximum conductivity can be observed for PPy and poly(Py-β-DMCD) films prepared in nonaqueous solution in the range of 0.10 < E _Ag/AgCl < 0.90 V and 0.30 < E _Ag/AgCl < 0.90 V, respectively. In situ UV–Vis spectroelectrochemical data for both films prepared potentiodynamically by cycling the potentials from −0.40 < E _Ag/AgCl < 0.90 V in nonaqueous solutions are reported. This paper is dedicated to Prof. Alan Bond on the occasion of his 65th birthday in recognition of his numerous contributions toward electrochemistry.     

Adsorption of metal anions of vanadium(V) and chromium(VI) on Zr(IV)-impregnated collagen fiber

The metal anions of vanadium (V) and chromium (VI) in aqueous solution can be effectively adsorbed by Zr(IV)-impregnated collagen fiber (ZrICF). The maximum adsorption capacity of V(V) takes place within the pH range of 5.0 to 8.0, while that of Cr(VI) is within the pH range of 6.0 to 9.0. When the initial concentration of metal ions was 2.00 mmol L⁻¹ and the temperature was 303 K, the adsorption capacity of V(V) on Zr-ICF was 1.92 mmol g⁻¹ at pH 5.0, and the adsorption capacity of Cr(VI) was 0.53 mmol g⁻¹ at pH 7.0. As temperature increased, the adsorption capacity of V(V) increased, while that of Cr(VI) was almost unchanged. The adsorption isotherms of the anionic species of V(V) and Cr(VI) can be fit by the Langmuir equation. The adsorption rate of V(V) follows the pseudo-first-order rate model, while the adsorption rate of Cr(VI) follows the pseudo-second-order rate model. Furthermore, ZrICF shows high adsorption selectivity to V(V) in the mixture solution of V(V) and Cr(VI). Practical applications of ZrICF could be expected in consideration of its performance in adsorption of V(V) and Cr(VI).     

Oxidation of 2-mercaptobenzoxazole in aqueous solution: solid phase formation at glassy carbon electrodes

The redox reactions of 2-mercaptobenzoxazole (MBO) have been investigated by cyclic voltammetry at glassy carbon electrodes in aqueous solution. Four anodic and three cathodic processes could be identified. A more detailed analysis of the oxidation processes up to a potential of +0.6 V (SCE) and the corresponding reduction signals showed that the oxidation leads to bis(benzoxazolyl) disulfide (BBOD). Owing to its low solubility, the oxidation product remains at the electrode surface. This product has been identified by ex situ FTIR and XPS analysis. During the reduction of BBOD, mainly MBO is formed. The remarkable lower solubility of BBOD in aqueous solutions compared to MBO allows preparation of layers of BBOD in situ and to control the amount of deposited BBOD via the MBO solution concentration and electrolysis time. The peak potential and peak shape of the reduction signals change remarkably as the amount of BBOD increases from submonolayer coverage to coverages that correspond to multilayers. The behavior can be explained by assuming an electrochemical conversion of BBOD microcrystals, which are deposited on the electrode surface, if the amount of BBOD formed during the MBO oxidation exceeds one monolayer. Received: 21 January 1999 / Accepted: 15 March 1999     

Electrochemical behaviour of a Cu/CuSe microelectrode and its application in detecting temporal and spatial localisation of copper(II) fluxes along Olea europaea roots

The electrochemical behaviour of a Cu/CuSe electrode was studied in order to define its selectivity towards cupric ions, Nerstian response, limit of detection and response time. The chalcogenide electrode was prepared by cathodic deposition of Se and subsequent formation of a thin layer of CuSe on a copper substrate. A Cu/CuSe microelectrode was prepared using copper wire 75 μm in diameter. The dimensions and response time (<0.5 s) allowed use of this electrode in the “vibrating probe method” with the aim of measuring net influxes as well as effluxes of copper(II) ions in Olea europaea roots. The electrode potential was measured along the root at a distance of 5 μm from the surface for 5 s, and then again for 5 s at a distance of 55 μm, moving the microelectrode with respect to the root surface by steps with a frequency of 0.1 Hz. The potentials measured at the two extremes of vibration were then converted to copper(II) concentrations. Substitution of these values in Fick's law yields the flux, assuming the diffusion constant D for copper ions in aqueous solutions. The results enabled us to detect copper(II) fluxes as small as 0.05 pmol cm⁻² s⁻¹. Copper(II) influx showed marked spatial and temporal features: it was highest at about 1.5 mm from the root apex and exhibited an oscillatory pattern in time. Received: 29 September 1999 / Accepted: 11 January 2000     

The influence of different variables on the electrochemical behavior of mild steel in circulating cooling water containing aggressive anionic species

Cyclic voltammetric, potentiodynamic anodic polarization and current–time transient studies were carried out on mild steel in circulating cooling water containing Cl⁻ and SO₄⁻² ions under the effect of different variables such as coolant flow, the availability of oxygen, cooling system temperature, and cooling system pH. The anodic excursion span of mild steel in cooling corrosive solution was characterized by the occurrence of a well-defined anodic peak (A1), while the reverse sweep was characterized by the appearance of two cathodic peaks (C1 and C2). The presence of Cl⁻ and SO₄⁻² ions in cooling water enhance the active dissolution of mild steel and tend to breakdown the passive film and induce pitting attack. The data reveal that increasing flow rate and temperature of cooling solution enhances the anodic peak current density (j _A1) and shifts the pitting potential (E _pit) towards more active values. It is seen that the peak current density of the anodic peak A1 increases and the pitting potential (E _pit) displaced in the noble direction in the aerated solution compared that in de-aerated solution. The pitting corrosion of mild steel by Cl⁻ and SO₄⁻² ions initiates more readily in acidic medium (pH 2.0). It was found that the incubation time (t _i) increase and in turn the pitting corrosion decrease in the order: pH 10 > pH 6.8 > pH 2.0.     

Voltammetric behavior of C60-p-tert-butylcalix[8]arene inclusion complex film

The C₆₀-p-tert-butylcalix[8]arene inclusion complex film has been directly formed on the surface of a glassy carbon electrode, and its electrochemical behavior in acetonitrile containing tetra-n-butylammonium hexafluorophosphate as the supporting electrolyte studied. The film has a two-electron reduction wave at −1.0 V (vs Ag/AgCl), but limits to the first cathodic potential scan. Received: 16 September 1997 / Accepted: 27 October 1997     

Formation of cuprous oxide layers in Cu(II) solutions containing gluconic acid

In accordance with thermodynamic analysis, cuprous oxide layers are formed spontaneously in the Cu|Cu(II), gluconic acid system at pH > 3.7 under open-circuit conditions. A current peak of Cu₂O reduction is observed on cathodic voltammograms at ca −0.7 V, its height being dependent on the exposure time. The analysis of the charge transferred in this region yields the rate of Cu₂O formation equal to 1.25 × 10⁻¹⁰ mol cm⁻² s⁻¹. The light perturbation of Cu electrode under open-circuit conditions results in the generation of a negative photopotential, which is indicative of n-type conductivity. The threshold wavelength is equal to ∼590 nm and is consistent with a band gap of ∼2.1 eV. Anodic photocurrents, which are observed near the open-circuit potential, decrease with cathodic polarization and change their sign at ∼0.05 V. Analysis of impedance data was performed, invoking the equivalent circuit that accounts for the two-step charge transfer. In the presence of Cu₂O, some retardation of Cu(II) reduction was found to occur with a slight increase in the admittance of the double layer. The suggestion has been made that oxide layers formed in Cu(II) gluconate solutions cannot be compact and uniformly distributed over the entire electrode surface. Relevant investigations of surface morphology support this conclusion.     

Analysis of photocurrent spectra at polybithienyl film coated on Pt

Photoelectrochemical measurements have been performed at a polybithienyl (PBT) film (doping level of 1 × 10¹⁸/cm³) deposited on a platinum electrode. The cathodic photocurrents and negative slope of the Mott-Schottky plot indicate that the PBT film has the features of a p-type semiconductor. The cathodic photocurrents are interpreted in terms of the Gaertner-Butler model on the basis of the theory of the semiconductor|solution interface. The (i _ph hν)^2/n vs. hν plots taken from the photocurrent spectra show two linearities for n=1 in the wavelength range from 460 nm to 490 nm and for n=4 in the wavelength range λ > 490 nm. The band gaps of the PBT film were determined to be 2.05 ± 0.05 eV for n=1 and 1.55 ± 0.05 eV for n=4. The flat-band potential is 0.33 V (vs SCE). From the slope of the Mott-Schottky plot at the modulation frequency of 3 kHz, the dielectric constant ɛ of the film and the thickness of the depletion layer W ₀ of the PBT film were determined to be 7.4 and 0.29 μm, respectively. Received: 6 January 1999 / Accepted: 6 June 1999     

Oxygen reduction on a Ru x S y (CO) n cluster electrocatalyst in 0.5 M H2SO4

A ruthenium-sulfur carbonyl cluster electrocatalyst, Ru _x S _y (CO) _n , was synthesized by pyrolysis of Ru₃(CO)₁₂ and elemental sulfur in a sealed ampoule at 300 °C. The pyrolyzed compound was characterized by DSC, FT-IR, XRD and SEM (EDX) techniques. The electrocatalytic activity and kinetic parameters for the molecular oxygen reduction were determined by a rotating ring-disk electrode (RRDE) in a 0.5 M H₂SO₄ solution at 25 °C. The cathodic polarization indicates two Tafel slopes: −0.124 ± 0.002 V dec⁻¹ at low and −0.254 ± 0.003 V dec⁻¹ at high overpotentials, and first-order kinetics with respect to O₂ concentration. From the analysis of Levich plots and RRDE results, the oxygen reduction on Ru _x S _y (CO) _n was determined to proceed mostly via a multielectron transfer path (4e⁻) to water formation ( >94%). Received: 4 March 1999 / Accepted: 26 May 1999     

Investigations on the melting behaviour of triglyceride nanoparticles

Suspensions of triglyceride nanoparticles have been proposed as carrier systems for intravenous administration of poorly water soluble drugs. Such nanosuspensions can easily be produced by homogenization of the melted triglyceride in an aqueous phase. Using special emulsifier blends it is possible to obtain suspensions with an average size of the recrystallized particles below 100 nm (photon correlation spectroscopy z-average). As can be observed by transmission electron microscopy the particles are very thin platelets with thicknesses in the range of only a few molecular layers. Nanoparticles of saturated monoacid triglycerides (smaller than 200 nm) exhibit uncommon melting behaviour, which is expressed in their differential scanning calorimetry curve by multiple endothermal peaks over a temperature range of about 10 °C. This effect was attributed earlier to the particle thickness distribution in the suspension rather than to polymorphic transitions since all the material exists in the stable β modification. Here we present experimental investigations on the correlation between the melting behaviour of trilaurin nanosuspensions and the particle thickness distribution determined by analysis of difference X-ray diffraction patterns recorded at progressively higher temperatures in the melting range of the particles. Because of the weak X-ray scattering of the systems investigated synchrotron radiation was used besides conventional sources. The Fourier analysis of the difference diffraction patterns is described in detail and the advantages and difficulties in using this method are discussed. It was observed that the melting temperatures of the nanoparticles increase with increasing particle thicknesses. Simultaneously a decrease in the interplanar (001) spacing with increasing particle thickness was found. Received: 27 July 1999 Accepted: 5 October 2000     

Effect of solute hydrophobicity on phase behaviour in solutions of thermoseparating polymers

 Two-phase systems consisting of a polymer rich phase and polymer depleted phase, where the polymer is either ethyl(hydroxy ethyl)cellulose (EHEC) or Ucon (a random copolymer of ethylene oxide and propylene oxide), have been studied. Both of these polymers can be separated from an aqueous solution by either temperature increase or addition of cosolutes. The polymers are thermoseparating and phase separate in water solutions at the cloud point temperature. Two types of EHEC have been studied: one with a cloud point at 60 °C and the other at 37 °C. The Ucon polymer used in this study has a cloud point at 50 °C. Ternary phase diagrams of polymer/water/cosolute systems have been investigated. When a strongly hydrophilic or hydrophobic cosolute is added to an EHEC- or Ucon–water solution, a phase separation occurs already at, or below, room temperature. As cosolutes, hydrophobic molecules like phenol, butyric and propionic acid, and hydrophilic molecules like glycine, ammonium acetate, sodium carboxylates (acetate to valerate), were studied. The polymer rich phase formed when mixing polymer, water and cosolute was strongly enriched or depleted with hydrophobic or hydrophilic cosolutes, respectively. The two phase region increased for propionic acid, butyric acid and phenol as a result of increased cosolute hydrophobicity. The opposite occurred in the series sodium acetate, sodium butyrate and sodium valerate. The effect of temperature on the phase behaviour has also been investigated. Model calculations based on Flory–Huggins theory of polymer solutions are presented, in form of a phase diagram, which semiquantitatively reproduce some experimental results. Received: 5 July 1996 Accepted: 4 November 1996