An in situ UV-vis and IR spectroelectrochemical study of the deposition of a hydroquinone anion salt on platinum electrodes from dichloromethane solutions

Cyclic voltammetry with a platinum electrode of hydroquinone (BQH₂) solutions in dichloromethane, containing tetrabutylammonium perchlorate supporting electrolyte, shows a sharp asymmetric irreversible oxidation peak at about ?0.3 V (SCE). This feature is seen, in addition to the expected features in this system, when the cycle is extended to potentials more negative than ?0.6 V (SCE). Cyclic voltammetry, in situ UV-vis and infrared spectroelectrochemistry have shown that hydroquinone anion (BQH^?) is formed at negative potentials and this appears to arise via surface decomposition of hydroquinone to p-benzosemiquinone (BQH) followed by reaction of BQH with the p-benzoquinone radical anion (BQ^?). The sharp asymmetric peak in the cyclic voltammograms is due to oxidation of the hydroquinone anion in the insoluble tetrabutylammonium salt on the electrode surface. The oxidation of BQH^? appears to occur via disproportionation of (BQH) and leads to BQH₂ and p-benzoquinone (BQ) as the products.     

Study of the corrosion behavior of titanium and some of its alloys for biomedical and dental implant applications

The influence of alloying elements and the potential on the corrosion resistance of Ti and other Ti-based biomedical implant alloys under simulated physiological conditions is presented. Ti and its following alloys: Ti10Mo, Ti10Mo10Al, Ti7Al4.5V and Ti5Al4.5V and Ti5Al2.5Fe were studied. Electrochemical impedance spectroscopy was used and the experimental results were compared with those obtained by potentiostatic and potentiodynamic techniques. All the measurements were made in Ringer’s solution at pH 7.8 and at different potentials (E_corr, ?650, ?500, 0, +200 and +400 mV vs. SCE). Under these conditions, all the Ti-based alloys exhibited spontaneous passivity. It was evident that Mo, V and Fe improved passivity and limited the active corrosion of the β-phase of Ti while Al enrichment of the α-phase was found to be detrimental to the passivity and corrosion resistance of Ti.     

Electrochemical mimicking of reduced rubredoxin by iron(II) complexes of cysteine peptide thiolate in aqueous micelle solution

The electrochemical properties of Fe(II) complexes with a number of cysteine-containing oligopeptides were investigated in aqueous Triton X-100 (10%) micelle solution. The complexes with an aromatic ring such as [Fe(Z-cys-Pro-Leu-cys-Gly-NH-C₆H₄-p-F)₂]^2?, [Fe(Z-cys-Pro-Leu-cys-Gly-NH-CH₂-CH₂-C₆H₄-p-F)₂]^2? and [Fe(Z-cys-Pro-Leu-cys-Gly-Phe-OMe)₂]^2? exhibit their redox potentials at ?0.28, ?0.30 and ?0.26 V vs. SCE, respectively, in aqueous Triton X-100 (10%) micelle solution with good reversibility. The potentials are very close to that of native rubredoxin (?0.31 V vs. SCE in aqueous solution). A para-substituent effect was observed for the redox potentials of [Fe(Z-cys-Pro-Leu-cys-Gly-NH-C₆H₄-p-X)₂]^2? (X=OMe, H, F and CN) in aqueous micelle solution. The most positively shifted potential was found at ?0.26 V vs. SCE for the complex with the strongest electron-withdrawing cyano (CN) substituent. The results show that the complexes with an aromatic ring are quite stable in such aqueous micelle solutions.     

Oxygen reduction on copper in neutral NaCl solution

The reduction of oxygen on copper in neutral unbuffered 1 mol dm^?3 NaCl has been studied using rotating ring-disc electrodes at six oxygen concentrations equivalent to atmospheres of 2% O₂ + N₂ to 100% O₂. Steady-state potentiostatic measurements show that the reaction is first order with respect to [O₂] and that, following adsorption of O₂, the first electron transfer is rate determining. In 50% O₂ + N₂ and 100% O₂, a cathodic oxygen reduction peak is observed in both potentiodynamic and potentiostatic experiments at a disc potential of ?0.3 to ?0.4 V/SCE. The reaction is dominated by the overall four-electron reduction to OH^?, with only small amounts of peroxide detected by the ring electrode at disc potentials corresponding to the formation of the cathodic oxygen reduction peak. Tafel slopes increase with [O₂] and vary from ?0.13₅ V in 2% O₂ + N₂ to a limiting value of ?0.16 V to ?0.18 V in air, 50% O₂ + N₂ and 100% O₂.The results are explained by a mechanism involving oxygen reduction on two types of surface site with different reactivities. The most catalytic surface is believed to comprise Cu(0) and Cu(I) sites, where the Cu(I) species is stabilized as Cu(OH)_ads and/or submonolayer Cu₂O. The less catalytic site consists of Cu(0) only. Oxygen reduction is believed to proceed by a series pathway involving an adsorbed peroxide intermediate on both sites. Peroxide is reduced to OH^? prior to desorption at Cu(0) sites, but some is released before being reduced at Cu(0)/Cu(I) sites. Surface coverage by catalytic Cu(0)/Cu(I) species is favoured by a higher interfacial pH and more positive disc potentials.     

Mechanism of the catalysis by Alcaligenes eutrophus H16 hydrogenase of direct electrochemical reduction of NAD+

The mechanism of direct electrochemical reduction of NAD⁺ into NADH catalysed by Alcaligenes eutrophus H16 hydrogenase was analysed in thin layer electrochemical cells with platinum and carbon electrodes. Two phases can be distinguished in the catalytic reaction occurring on platinum electrodes. In the potential range from approximately ?0.620 to ?0.675 V (SCE) direct electron transfer occurred via the diaphorase-like dimer of the hydrogenase. Below ?0.69 V versus SCE the hydrogenase used hydrogen species adsorbed onto the platinum electrode, but no molecular hydrogen was required for this second catalytic phase. The mechanism was quite similar to those which had been previously determined for Rhodococcus opacus hydrogenase. This confirmed the very great similarity of the two enzymes, even if the maximum NAD⁺ reduction rate of 0.36 mM min^?1 obtained here remains lower than those reached with R. opacus hydrogenase in a previous study. Careful analysis of the experimental data obtained on a carbon electrode led to the conclusion that no direct electron transfer was observed on this material under the operating conditions used. On the other hand, the voltammetric experiments performed on a carbon electrode in a thin layer cell showed clearly the occurrence of a catalytic current due to the hydrogenase-catalysed reduction of NAD⁺ by molecular hydrogen. This may be a useful tool for further analysis of the hydrogenase kinetics.     

The reduction of l-cystine hydrochloride at lead using static and rotating disc electrodes

The reduction of the disulphide, l-cystine hydrochloride to the l-cysteine hydrochloride thiol, in 0.1 mol dm^?3 HCl at 298 K, has been studied at pre-treated, circular, 0.50 cm² lead disc electrodes using steady state linear sweep voltammetry, non-steady state voltammetry and controlled potential coulometry. The diffusion coefficient for l-cystine hydrochloride was approximately 4.8 × 10^?10 m² s^?1 from the three techniques. Reduction of the disulphide was irreversible and hydrogen evolution occurred as a competitive reaction at approximately ?1.35 V vs. SCE. Analysis of the mixed control kinetics, using a Koutecky–Levich approach, allowed the relative roles of charge transfer and mass transport to be resolved. Anomalously high Tafel slopes, of typically ?183 mV, were observed due to disulphide adsorption. The charge transfer kinetics are consistent with the first electron gain being rate determining while reaction orders are +1 with respect to both the disulphide and proton concentrations. The mechanism of l-cystine hydrochloride reduction has been critically discussed.     

Comparative studies on the electrogenerated chemiluminescent and amperometric behavior of the Ru(bpy)32+ system on a paraffin-impregnated graphite electrode and a glassy carbon electrode

The electrogenerated chemiluminescent (ECL) and amperometric behavior of Ru(bpy)₃²⁺ system on a paraffin-impregnated graphite electrode (PIGE) and a glassy carbon electrode (GCE) was studied by different electrochemical techniques. Under conventional cyclic voltammetric (CV) conditions, two anodic ECL peaks (EP1 and EP2) (vs. SCE) were observed at 1.18 and 1.37 V for a PIGE, and at 1.20 and 1.44 V for a GCE. The EP1 normally occurred, but not the EP2. A complicated mechanism was involved in the formulation of EP2. A detection limit as low as 1×10^?9 mol l^?1 of C₂O₄^2? was obtained on the PIGE by the CV method at 100 mV s^?1. Strong ECL signals were found on both electrodes in either an oxalate-containing aqueous solution or an organic solution when a cyclic square wave (CSW), between two suitable potentials was used. However, the GCE showed higher reproducibility than the PIGE for continuous CSW (n=10) measurements. Chronoamperometry was also applied by using a potential step from 0.15 to 1.85 V. It took 389 ms on a PIGE and 837 ms on a GCE to reach each maximal ECL intensity.     

Effect of pH on direct electron transfer between graphite and horseradish peroxidase

The effect of pH on the kinetics of the electroreduction of H₂O₂ catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to ?50 mV versus SCE (under steady-state conditions and with an RDE system) and at ?50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H₃O⁺] was shown to enhance significantly the current of the bioelectroreduction of H₂O₂ due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages (E<0.2 V) H₃O⁺ does not affect the rate of the enzymatic reduction of H₂O₂, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k_s, changed from a value of 0.54±0.05 s^?1 in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0±1.7 s^?1 in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k_s with increasing [H₃O⁺] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.     

High performance RuPd catalysts for CO2 reduction at gas-diffusion electrodes

The reactivity and selectivity of CO₂ reduction has been examined at gas-diffusion electrodes made of Ru, Pd and their alloy (Ru:Pd = 1:1). It was found that the current efficiency for the formation of formic acid at the gas-diffusion electrodes with RuPd catalysts is 90% at ?1.1 V vs. NHE, where the current density is 80 mA.cm^?2. In addition, no CO formation was observed at gas-diffusion electrodes with RuPd catalysts.     

Electroreduction of peroxodisulfate anion at a Cd(0001) single-crystal plane electrode

Electroreduction of peroxodisulfate anion on an electrochemically polished Cd(0001) plane in aqueous NaF solution with different additions of Na₂S₂O₈ was studied by rotating disc voltammetry. The rate of electroreduction of S₂O^2?₈ depends on the electrode potential and base electrolyte concentration, i.e. on the thickness of the diffuse layer. The kinetic current densities at E=const have been obtained by the Koutecký–Levich method and used for the determination of the apparent rate constant values of electroreduction of S₂O₈^2? anion. The ψ₀ potential values obtained according to the Gouy–Chapman–Grahame (GCG) model have been used for the construction of corrected Tafel plots, which were linear at E?ψ₀<?1.1 V (SCE). The value of the charge transfer coefficient α decreases somewhat with the concentration of the base electrolyte solution (0.18≤α≤0.22), but the value of α is practically independent of the concentration of S₂O₈^2? ions if c_NaF=const.     

Reduction of silver complexes: towards an ever more elaborated insight in the influence of the type of ligand on the reaction rate

The present work deals with the investigation of the electrochemical reduction of silver thiosulphate (1,2-Ag(S₂O₃)₂^3?), thiocyanate (1,3-Ag(SCN)₃^2?) and 1,8-dihydroxy-3,6-dithiaoctane (1,2-Ag(DTO)₂⁺) complexes. The influence of the ligand type on the charge transfer rate is explained by the changing positions of the density distributions of electronic energy levels of the three complexes. The basics for this approach are the theories of energy band models (EBMs). An experimental methodology is developed to determine the energy density distributions. A Ti/TiO₂ substrate, obtained by galvanostatically anodising Ti, is put forward as an appropriate substrate for this investigation, and its semiconducting properties are determined. On this substrate, charge transfer (CT) controlled currents can be measured in a sufficiently large potential domain for the three systems. A method of pre-plating is optimised such that the overall semiconducting character of the substrate is kept during the monitoring of the (quasi-)stationary current/voltage diagrams. The active surface areas, necessary for the calculation of the current density/potential curves, are calculated. The positions of the energy density distributions, obtained by the derivation of the current density/voltage diagrams, of the three complexes, show that thiosulphate exhibits the smallest density of accepting energy levels in the given potential domain. For potentials above 0.5 V vs. SCE, the DTO complex has the largest density of vacant energy levels, but for lower potentials the situation is reversed.     

Electrochemical anion doping of poly[(tetraethyldisilanylene) oligot 2,5-thienylene)] derivatives and their p-type semiconducting properties

Chemically synthesized poly[(tetraethyldisilanylene)oligo(2,5-thienylene)] derivatives (DS/mT; m = 3 to 5) have been successfully anion-doped by electrochemical oxidation. Band-gap energies of 2.52, 2.65, 2.82 and 3.27 eV were evaluated for DS5T, DS4T, DS3T and DS2T respectively. The DS5T, DS4T and DS3T films exhibited electrical conductivities of the order of 10^?3 to 10^?4 S cm^?1 when doped with BF₄^?. The work functions of the films changed from 5.1 to ca. 5.5 eV with electrochemical anion doping. In cyclic voltammograms of the polymer films for anion doping and dedoping, an anodic peak potential and a cathodic one (E_pc.) shifted to the positive direction as the number m of thienylene units decreased. E_pcs at a sweep rate of 100 mV s^?1 were about 0.8, 0.9 and 1.0V for DS5T, DS4T and DS3T respectively. Reversible electrochemical doping and dedoping of the DS5T film were feasible when the potential was cycled between 0 and 1.2 V. At potentials more positive than 1.2 V, however, both overoxidation of the oligo(thienylene) unit and Si-Si bond cleavage took place, leading to decreases in conductivity and work function of the film.     

Potential-dependent electrochemiluminescence of luminol in alkaline solution at a gold electrode

The behavior of luminol electrochemiluminescence (ECL) at a polycrystalline gold electrode was studied under conventional cyclic voltammetric (CV) conditions. At least six ECL peaks were observed at 0.28 (ECL-1), 0.56 (ECL-2), 0.95 (ECL-3), 1.37 (ECL-4), ?0.43 (ECL-5) and 1.00 (ECL-6, a broad wave after the reverse scan from +1.66) V (vs. SCE), respectively, on the curve of ECL intensity versus the potential. These ECL peaks were found to depend on the presence of O₂ and N₂, the pH of the solution, KCl concentration, scan rate, and potential scan ranges. The emitter of all ECL peaks was identified as 3-aminophthalate by analyzing the CL spectra. It is believed that ECL-1 at 0.28 V was correlated to luminol radicals produced by the electro-oxidation of luminol anion and ECL-2 at 0.56 V was caused by the reaction of luminol radical anions with gold oxide formed on the electrode surface. ECL-1 and ECL-2 could be strongly enhanced by O₂ and O₂^?. ECL-3 at 0.95 V was likely to be due to the reaction of luminol radical anions with O₂ oxidized by OH^?. ECL-4 at 1.37 V suggested that OH^? was electro-oxidized to HO₂^? at this potential and then to O₂^?, which reacted with luminol radical anions to produce light emission. ECL-5 at ?0.43 V seems to be due to the reaction of luminol with ClO^? electrogenerated at higher positive potential and HO₂^? electrogenerated at negative potential. ECL-6 was attributed to the reaction of luminol radical anions and ClO^? electrogenerated at higher positive potential. The results indicated that luminol ECL can be readily initiated by various oxygen-containing species electrogenerated at different potentials, leading to multi-channel light emissions. Furthermore, the present work also reveals that ECL-2 is a predominant ECL reaction route at a gold electrode with higher potential scan rates under CV conditions.     

EQCM study of electropolymerization and redox cycling of 3,4-polyethylenedioxythiophene

Polyethylenedioxythiophene films were electropolymerized potentiostatically (E=1.1–1.3 V/SHE) at a gold electrode covering an EQCM from solutions containing monomer, lithium perchlorate and non-ionic surfactant (polyoxyethylene-10-laurylether). The ion exchange was studied by EQCM during redox cycling at a scan rate of 20–50 mV s^?1 in 0.5 M LiClO₄+0.5 M LiCl+0.5 M Na toluenesulfonate solutions in the potential range ?0.5<E<0.8 V (SHE). Mass versus charge curves display a hysteresis. During the reduction scan the ion content of the film is in equilibrium and is defined by the potential, whereas during oxidation the mass lags behind the charge due to slow water exchange. The reduction scan can be described by equilibrium theory supposing that about 40% of counter-ions remain bound in the film. During polymerization at E≥1.2 V, the mass gain calculated from the frequency change slows down more rapidly than is estimated from the current. This may be attributed to the viscosity of the growing film with acoustic decay length of 2.45 μm.     

The temperature dependence of the cyclic-voltammetry response for the Pt(110) electrode in aqueous H2SO4 solution

The under-potential deposition of H (upd H) and anion adsorption on Pt(110) in 0.5 M aqueous H₂SO₄ solution is investigated by application of cyclic-voltammetry (CV). The data show that an increase in T results in a shift of the CV peak towards less positive potentials and changes in its morphology; the peak current is not affected by T. The relation between the peak potential, E_p, and T is linear, the slope being ?4.64×10^?4 V K^?1. The variation of E_p with T allows one to determine the entropy of the process, ΔS, that is ?44.8 J mol^?1 K^?1. Comparison of this value with ΔS for H_upd adsorption on Pt(111) suggests that the sharp peak represents H_upd adsorption on the Pt(110) substrate. The temperature modification does not alter the Pt(110) surface which retains the (1×1) structure. The overall adsorption charge density does not vary upon T variation (for 273≤T≤333 K) and its average value is 213 μC cm^?2.     

The cathodic deprotection of the nitrobenzoyl group from phenyl nitrobenzoates in N,N-dimethylformamide

The reduction of phenyl benzoates with nitro substituents at the 2-, 3- and 4-positions of the benzoates in N, N-dimethylformamide is reported. The phenyl 4- and 3-nitrobenzoate are reduced in two cathodic steps. The first one, at about ?0.9 V vs. SCE, a reversible one-electron process, gives a rather stable anion radical. The second reduction step at potentials between ?1.5 and ?2.0 V vs. SCE leads to formation of the dianion, which decomposes giving free phenol in good yields ( > 80%). On the other hand, the phenyl 2-nitrobenzoate is reduced in one cathodic step. This step occurs at ?0.9 V with formation of an unstable anion radical which decomposes via C-O bond cleavage, giving phenol with a yield of ca. 80%. The mechanisms of the reduction of these compounds are discussed.     

A monomer–dimer equilibrium in self-assembled monolayers of N-ethyl-N′-octadecylviologen on the electrode surface. The influence of water content and hexafluorophosphate ion

This paper describes the dimerization of self-assembled monolayers (SAMs) of N-ethyl-N′-octadecylviologen (1) on GC and Au electrode surfaces in the presence of 0.1 M NH₄PF₆ aqueous solutions. The ‘wet’ and ‘dry’ SAMs of 1 showed multiple redox peaks for the first reduction of 1 in the presence of NH₄PF₆, in contrast to the case of other supporting electrolytes (typically KCl, NaNO₃, Na₂SO₄ and NaClO₄) where both wet and dry SAMs of 1 exhibited a single redox wave for the first reduction. The dry SAM showed a well defined reduction peak at ?0.57 V along with a shoulder reduction peak at ?0.50 V and two oxidation peaks at ?0.50 and ?0.42 V. On the contrary, the wet SAM gave a very sharp reduction peak at ?0.50 V and a small shoulder peak at ?0.57 V in addition to two oxidation peaks like those observed for the dry SAM. The reduction peak of ?0.50 V was ascribed to the reduction of strongly hydrated dications of 1, while the reduction peak at more negative potential (?0.57 V) was attributed to the reduction of the dehydrated dications of 1. The two oxidation peaks at ?0.50 and ?0.42 V were ascribable to the oxidation of the usual radical cation monomer and the radical cation dimer, respectively. In the case of the wet SAM, upon continuous potential cycling, the sharp reduction peak of ?0.50 V clearly decreased, whereas the more negative reduction peak of ?0.57 V was highly stable. In this case, in the oxidation process, the monomer peak of ?0.50 V increased, while the dimer peak of ?0.42 V decreased. Thus it is reasonably assumed that in the wet SAM, initially the radical cations of 1 feel an aqueous environment in the monolayer where the dimerization is highly favored and at subsequent potential cycles, due to the entry of hydrophobic anions of PF₆^? into the monolayer, the pre-existent water molecules are expelled from the monolayer and under this circumstance the radical cations of 1 may feel the environment very similar to non-aqueous media where the dimerization is totally suppressed. The adsorption tendency of 1 on the electrode surface was also studied using the SAMs prepared by dissolving 1 in water+ethanol mixtures of different ratios. The appearance of multiple peaks was found to depend significantly on the alkyl chain length of asymmetric viologen. The inclusion/expulsion of solvents and anions into/from the SAM during the redox reaction were studied by the electrochemical quartz crystal microbalance (EQCM). It was found that in the presence of SO₄^2? ions ca. 17 water molecules per one SO₄^2? ion were transported to the SAM of 1 during the oxidation, whereas ca. five water molecules were transported in the presence of PF₆^? ions.     

SnO reduction in lithium cells: study by X-ray absorption, 119Sn Mössbauer spectroscopy and X-ray diffraction

We studied the reduction mechanism of SnO in lithium cells by X-ray absorption near OK and SnL_I edge spectroscopy, ¹¹⁹Sn Mössbauer spectroscopy and X-ray diffraction. The reduction mechanism is complex, involving mixed valence intermediate compounds. In the interval 0≤Li/Sn≤2 the main reaction corresponds to a partial reduction of Sn^II, which is partially reversible, regenerating the tin oxide during the charge. For Li/Sn greater than two, LiSn bonds are formed, but SnO interactions are still present. The charge within this interval also partially regenerates tin oxide, but the reversibility does not extend to Li/Sn lower than two. At low voltage and very large depth of discharge, the formation of LiSn alloys and Li₂O clearly takes place, with negligible SnO interactions.     

Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O₂ was reduced to H₂O₂ in a 2-electron reaction with the rest being reduced to OH^? in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O₂ was almost exclusively reduced to OH^? in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than ?1.2 V vs. Ag/AgCl and the second step reduction (E < ?1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around ?1.2 V. The O₂ reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O₂ reduced to OH^? in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment.     

Electrocatalytic oxidation of d-galactose in alkaline medium

The electrocatalytic oxidation of d-galactose was investigated on platinum, gold and nickel electrodes in 0.1 M NaOH medium. The oxidation of galactose on nickel takes place in the NiOOH region and leads to the cleavage of the C–C bonds. This was confirmed by HPLC analyses of electrolyzed solutions which demonstrate relatively high amounts of low molar mass carboxylic acids, i.e., formic and glycolic acids. The oxidation of galactose on platinum is initiated at very low potentials, i.e., ?0.8 V vs. SCE probably without oxygenated species on the electrode surface and gives moderate selectivity towards galactonic acid. The important effect of the lead adatoms on the electrocatalytic properties of platinum was demonstrated by the increase in the yield of galactonic acid from 34% to 67% with addition of 10^?5 M Pb²⁺. On gold, galactose oxidation begins at approximately ?0.5 V vs. SCE and is probably catalysed by the presence of hydrous gold oxides. The best yield of galactonic acid, 86%, was obtained after 6 h of electrolysis using a two potential program: ?0.1 V vs. SCE for 30 s, 1.5 V for 1 s.