Direct electrochemical reduction of spinach and desulfovibrio desulfuricans Norway ferredoxins

Electrochemical techniques were used to study the reduction-reoxidation of spinach and Desulfovibrio desulfuricans ferredoxins in the presence of poly- -lysine. A peak corresponding to the reduction of native molecules was detected by differential pulse polarography and voltammetry at the basal-plane pyrolytic graphite electrode. The existence of reduction-reoxidation process was evidenced by cyclic voltammetry. The redox potential values of −0.42±0.01 V (spinach ferredoxin), −0.37±0.01 V (D. desulfuricans ferredoxin I), −0.41±0.01 V (D. desulfuricans ferredoxin II) were obtained. No changes in redox potential values were detected in the pH range 5–9.5. The presence of poly- -lysine promotes the electron transfer between the electrode and the ferredoxin molecules. It is suggested that several factors such as electrostatic interactions and the close environment of clusters can influence the electron transfer rates of heterogeneous exchange between the electrode and the ferredoxin molecules.     

Electrosorption and inhibition properties of p-norborn2-yl phenolate ions at the mercury/solution interface

The electrosorption properties of p-norborn-2-yl phenolate ions in alkaline solutions were investigated by ac polarographic and electrocapillary measurements.

Two adsorption regions were found. At low bulk surfactant concentrations the adsorption at the positively charged electrode (−0.2 E −0.6 V) is predominant while at higher surfactant concentrations the adsorption at the negatively charged electrode (−0.6 E −1.0 V) is more pronounced. At E = −0.40 V the adsorption parameters were determined (a ≈ 2; ΔG°_A = −32.5 ± 1 kJ mol⁻¹. Between −0.6 E −1.0 V one potential of maximum adsorption for all concentrations does not exist and therefore the adsorption parameters could not be calculated.

At E = −0.40 V progressive two-dimensional nucleation with a nucleation order of 3 was observed which corresponds well with the high attraction constant.

The electrode reaction S₂O²⁻₈ + 2e⁻ → 2 SO²⁻₄ is inhibited by norborn-2-yl phenolate ions in the potential range −0.2 E −0.6 V. In the second potential range of capacity decrease the electrode process is much less retarded. At E = −0.40 V, in a similar manner as described for neutral molecules, a linear dependence of the log k_s (k_s apparent rate constant) on ln c_A and π (π = surface film pressure), respectively, has been found.     

Theoretical approach to faradaic admittance of hydrogen absorption reaction on metal membrane electrode

Faradaic admittance of hydrogen absorption reaction (har) on metal membrane electrode has been derived on the basic of “two models” of the har under the permeable boundary condition by using an extension of Armstrong's kinetic approach. In the case of the har through adsorbed phase (model A), two limiting cases of the Faradaic admittance are considered depending upon the magnitude of rate of hydrogen transfer from bulk to metal surface. One involves the diffusion-controlled har for fast rate of hydrogen transfer and the other is the interface-controlled har for slow rate of hydrogen transfer from bulk to metal surface. Depressing and/or tailing of the second semicircle, often found in the complex-plane impedance spectra for hydrogen evolution reaction (her), is fairly explained by the diffusion-controlled har. In the case of the direct har without passing through the adsorbed phase (model B), the Faradaic admittance is differentiated by the absence of adsorption capacitance from that based upon the model A.     

Simultaneous removal of 3d transition metals from multi-component solutions by activated carbons from co-mingled wastes

The main purpose of the present work was to study the simultaneous removal of 3d transition metals from multi-component solutions by novel porous material obtained from carbon-containing liquid and solid waste. The activated carbon was prepared from co-mingled natural organic waste: 25% sunflower husks, 50% petroleum waste and 25% low-grade bituminous coal. The porous carbon material was obtained via stages of pre-oxidation with binary eutectic Na/K carbonates (in order to avoid melting and coke formation), followed by “step by step” carbonization at 100–400 °C in an inert atmosphere and activation with steam at 850 °C.

The adsorption of the 3d transition metals: copper (II), cobalt (III), nickel (II), iron (III), and chromium (III), on novel activated carbons has been investigated using multi-component model solutions. Experiments have been carried out on the thermodynamics of the simultaneous adsorption of the 3d transition metals in a static mode. The total metal removal combines the process of metal hydroxide precipitation in the solution with the metal cation adsorption on negatively charged carbon surface in a single operation unit. The carbon/metals interaction at the surface of spent adsorbents is discussed.     

ac Impedance of Faradaic reactions involving electrosorbed intermediates—I. Kinetic theory

The evaluation of the electrochemical adsorption behaviour of chemisorbed intermediates generated in a multi-step Faradaic reaction at appreciable net currents is important for understanding the reaction mechanism of the overall process involved. Measurement of ac impedance of the reaction at controlled potentials provides an important experimental route to the required information about the “overpotential-deposited” ad-atom species. Interpretation of the measurements however, requires further examination. Based on an extension of Armstrong's treatment, it is shown that interpretation of ac impedance measurements directly in terms of the components of an intuitively assumed equivalent circuit is rarely correct; only in the case of underpotential-deposition of an ad-species, where no continuous Faradaic currents pass, in such an approach satisfactory. Kinetic analysis is given for the behaviour of a multistep process with examples from the cathodic H₂ evolution reaction where electrochemical and H-recombination desorption pathways are involved. The kinetic analysis enables the steady-state adsorption pseudocapacitance, C_φ, for H to be evaluated as a function of overpotential. Its behavior is clearly distinguishable from the quantity C_p commonly written as the pseudocapacitance element in the equivalent circuit for this type of reaction.     

Solvation of ions—XX. The ferrocene—ferricinium couple and its role in the estimation of free energies of transfer of single ions

The ferrocene-ferricinium electrode (Pt/Foc, Fic⁺) was investigated in water, acetonitrile, ethanol, DMSO and DMF using single scan cyclic voltammetry and phase sensitive ac polarography. The oxidation-reduction is pseudo-reversible in all five solvents with an electrochemical rate constant of approximately 10⁻² cm/s. In all solvents a slow irreversible chemical step involving the ferricinium cation follows electron transfer, so that slow cyclic voltammetry or polarography rather than potentiometry is preferred if ferrocene is to be used as a reference electrode in non-aqueous solvents.

The Strehlow assumption, ΔG_tr(Foc) = ΔG_tr(Fic⁺ gives very different free energies of transfer of single ions from non-aqueous solvents to water when compared with the TATB assumption that ΔG_tr(Ph₄As⁺) = ΔG_tr(Ph₄B⁻). This discrepancy is likely to be because ferricinium is only a moderately large cation, so that ΔG_tr(Fic⁺) is less positive than ΔG_tr(Foc) for transfer to water. The discrepancy is not because of abnormal electrochemical behavior of the Pt/Foc, Fic⁺ electrode in water or other solvents. Values of E° vs nhe, H₂O in a variety of solvents based on the TATB assumption are presented.     

Evaluation of cysteine as environmentally friendly corrosion inhibitor for copper in neutral and acidic chloride solutions

The efficiency of cysteine as a non-toxic corrosion inhibitor for copper metal in 0.6 M NaCl and 1.0 M HCl has been investigated by electrochemical studies. Potentiodynamic polarization measurements and electrochemical impedance spectroscopy “EIS” were used to study the effect of cysteine on the corrosion inhibition of copper. Inhibition efficiency of about 84% could be achieved in chloride solutions. The presence of Cu²⁺ ions increases the inhibition efficiency to 90%. Potentiodynamic polarization measurements showed that the presence of cysteine in acidic and neutral chloride solutions affects mainly the cathodic process and decreases the corrosion current to a great extent and shifts the corrosion potential towards more negative values. The experimental impedance data were analyzed according to a proposed equivalent circuit model for the electrode/electrolyte interface. Results obtained from potentiodynamic polarization and impedance measurements are in good agreement. Adsorption of cysteine on the surface of Cu, in neutral and acidic chloride solutions, follows the Langmuir adsorption isotherm. The adsorption free energy of cysteine on Cu (−25 kJ mol⁻¹) reveals a strong physical adsorption of the inhibitor on the metal surface.     

Electrochemical beaviour of the solid ionic conductor Ag7I4PO4

Electrochemical investigation of the solid superionic conductor Ag₇I₄PO₄ (0.8 AgI + 0.2 Ag₃PO₄) at room temperature and at 40°C were performed by means of cyclic voltammetry, cyclic chronoamperometry and cyclic chronocoulometry, normal pulse polarography and ac polarography. It was shown that the Ag⁺ ↔ Ag redox process on Pt and Ag working electrode occurs with a certain overvoltage, ie that for Ag⁺ → Ag⁺ oxidation and the return of Ag⁺ ions into the electrolyte a certain overvoltage is necessary. From the determined values of the exchange current one estimates the redox process as a rather fast one. The silver working electrode is electrochemically inactive, while only cathodically deposited silver is electrochemically active and can be oxidized to Ag⁺ ions. Chronoamperometry and chronocoulometry show that there is a certain difference in the behaviour of Pt and Ag working electrodes due to uneven passivating anodic processes. On the basis of measurements of faradaic and capacitance currents and their dependence on frequency, diagrams of complex impedance of the Pt/Ag₇I₄PO₄ interface at various anodic and cathodic polarizations of the Pt electrode were plotted. The dependence of the serial capacity of the interface on the dc potential and temperature are discussed.     

Modification of faujasites to generate novel hosts for “ship-in-a-bottle” complexes

Zeolites X, Y, and DAY have been modified by a post-synthetical dealumination procedure to generate mesopores that are completely surrounded by micropores. In these novel host materials several bulky transition metal salen complexes have been occluded via the “ship-in-a-bottle” synthesis approach. Both the host materials and the “ship-in-a-bottle” catalysts have been characterized by FTIR spectroscopy and nitrogen adsorption. Additionally, the “ship-in-a-bottle” catalysts have been characterized by thermogravimetric analysis and ICP-AES spectroscopy. The catalysts have been tested in the stereoselective epoxidation of R-(+)-limonene and (−)--pinene with molecular oxygen as oxidant. The best results so far — 100% conversion, 96% selectivity and 91% de — were achieved with the immobilized (R,R)-(N,N′)-bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethylene-1,2-diaminocobalt complex in the epoxidation of (−)--pinene.     

The adsorption of coumarin at solid metal electrodes

The effect of coumarin on the “deposition potential” of a number of metals at solid metal surfaces has been compared with that at a dropping mercury electrode. These results have been used to interpret the adsorption of coumarin on different metal surfaces and to suggest that Tl⁺ and Pb²⁺ ions are specifically adsorbed on mercury and that Tl⁺, Co²⁺, Fe²⁺, Ni²⁺ and Zn²⁺ are specifically adsorbed on their parent metals.     

ISOTHERMAL AND NON-ISOTHERMAL MULTICOMPONENT ADSORPTION KINETICS

The model equations in the relaxation form for the multicomponent kinetics of isothermal and non-isothermal adsorption, taking into account all major distinctive features of the interphase heat and mass exchange inside porous grains and at their surface (see points 1 to 4 below) for P (“pore”) and S (“solid”) models of mass transfer within porous grains of the adsorbent, have been obtained.

First for isothermal and non-isothermal kinetics in the mixed kinetics region of mass and heat exchange in the absence natural mutual diffusion and natural thermal-diffusion the essential influence effective mutual diffusion and effective thermal-diffusion is shown.

Recommendations on the use of model equations of adsorption kinetics for describing isothermal and non-isothermal adsorption dynamics of multicomponent mixtures in the inner-diffusion and mixed (outer- and inner-diffusion) kinetic region of heat and mass exchange are made.     

An electrochemical and Raman spectroscopic investigation of synergetic effects in the corrosion inhibition of copper

The synergetic inhibition of copper corrosion by benzotriazole (BTA) and benzylamine (BZA) in chloride and cyanide media is assessed by voltammetric and ac impedance measurements. BZA enhances the performance of the strong inhibitor BTA by accelerating the growth of a protective surface layer; used alone, BZA is ineffective as an inhibitor. The competitive adsorption of aggressive anions and inhibitors is studied by surface enhanced Raman scattering (SERS). The dramatic increase in corrosion caused by low concentrations of cyanide ions is shown to be due to the displacement of BTA from the copper electrode surface; comparisons are made with the behaviour of mercaptobenzoxazole (MBO).     

Adsorption stages and interfacial orientations of cytidine sulphate at the mercury/electrolyte interface

The adsorption and interfacial orientations of cytidine sulphate were studied by out-of-phase ac voltammetry. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential, adsorption time, temperature and the bulk concentration of cytidine sulphate. The time dependence of the electrode impedance indicates that the formation of a compact film controlled by nucleation/growth mechanism and the data were analysed according to the Avrami equation. Moreover, the role of the anions of the indifferent supporting electrolyte in the formation of the condensed film of the adsorbed molecules has been elucidated. The stacking interactions between adsorbed molecules are affected by the nature of the anions of the supporting electrolyte and are hindered by their specific adsorption. The adsorption parameters of cytidine sulphate has been computed at various pHs and for various anions in the supporting electrolyte.     

Electrode kinetics by hydrodynamic voltammetry—study of ferrous-ferric, ferrocyanide-ferricyanide and iodide-iodine systems

Using rotated and circulatory electrolysis cells, rate parameters of electrode processes have been determined at stationary conical and cylindrical platinum microclectrodes in flowing solutions, under judiciously controlled hydrodynamic conditions. Virtually ideal laminar flow was maintained at velocities up to 700 cm/s. Normally reversible d.c. current/voltage waves were made “irreversible in shape”, due to enhancement of convective mass transfer. Thus specific electron-transfer rate constants, ranging up to an estimated limit of 10 cm/s, could be evaluated by a simple and unambiguous “wave analysis” procedure. The electro-oxidation of iodide to iodine, in perchloric acid in the absence of triiodide, is governed by second-order kinetics. A mechanism is proposed to account for this remarkable finding.     

Selective adsorption behavior of ion-imprinted magnetic chitosan beads for removal of Cu(II) ions from aqueous solution

Heavy metal ion is one of the major environmental pollutants. In this study, a Cu(II) ions imprinted magnetic chitosan beads are prepared to use chitosan as functional monomer, Cu(II) ions as template, Fe₃O₄ as magnetic core and epichlorohydrin and glutaraldehyde as crosslinker, which can be used for removal Cu(II) ions from wastewater. The kinetic study shows that the adsorption process follows the pseudo-second-order kinetic equations. The adsorption isotherm study shows that the Langmuir isotherm equation best fits for the monolayer adsorption processes. The selective adsorption properties are performed in Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II) binary systems. The results shows that the IIMCD has a high selectivity for Cu(II) ions in binary systems. The mechanism of IIMCD recognition Cu(II) ions is also discussed. The results show that the IIMCD adsorption Cu(II) ions is an enthalpy controlled process. The absolute value of ΔH (Cu(II)) and ΔS(Cu(II)) is greater than ΔH (Zn(II), Ni(II), Co(II)) and ΔS (Zn(II), Ni(II), Co(II)), respectively, this indicates that the Cu(II) ions have a good spatial matching with imprinted holes on IIMCD. The FTIR and XPS also demonstrates the strongly combination of function groups on imprinted holes in the suitable space position. Finally, the IIMCD can be regenerated and reused for 10 times without a significantly decreasing in adsorption capacity. This information can be used for further application in the selective removal of Cu(II) ions from industrial wastewater.     

Characterization of water in bacterial cellulose using dielectric spectroscopy and electron microscopy

It is shown that only 10% of the 99 wt% water present in bacterial cellulose (BC) gels, produced by Acetobacter xylinum, behave like free bulk water; the majority of the water molecules in the gels is more or less tightly bound to the cellulose. The magnitude of the diffusion coefficients of ions transported in the water phase of the BC gels as well as the information contained in freeze fracture transmission electron microscopic images of the gel structures indicates that the bulk-like water is confined in “lakes” rather than forming a continuous phase throughout the gel. Water desorption isotherms suggest that these “lakes” decrease in size with increasing oxygen concentration used during the biosynthesis process of the gels.     

The electrocatalytic influence of underpotential Pb, Tl and Bi adsorbates on the electrochemical behaviour of benzofuroxan and o-benzoquinone dioxime on platinum

The electrochemical behaviour of benzofuroxan and o-benzoquinone dioxime on bare platinum and Pt surfaces covered by heavy metal monolayers deposited at underpotentials was studied in aqueous acid solutions. It was found that Tl, Pb and Bi adsorbates markedly catalyse the reduction of benzofuroxan. The enhancement of the overall reduction process has been interpreted in terms of the change of the reduction mechanism from chemical mechanism on bare platinum to direct electrochemical mechanism of Pt surfaces covered by the underpotential monolayers. The reduction waves of benzofuroxan and o-benzoquinone dioxime on Pt/M_ads surfaces were found to be kinetically controlled. The kinetic nature of the current was attributed to the dehydration reactions of the intermediate hydroxylamines formed in the course of the electrode reaction. It was also found that underpotential layers markedly improve the reversibility of the two-electron oxidation of o-benzoquinone dioxime. This catalytic effect has been interpreted in terms of the change of the reduction mechanism from an “inner sphere” mechanism involving adsorbed intermediates on pure platinum to an “outer sphere” one without complications from the adsorption of the reacting molecules on Pt/M_ads surfaces.     

Walden product, ion-solvent interactions and solvent structure in water-rich mixtures ionic conductances in water-sulfolane, water-acetonitrile and water-dimethylsulfoxide

Conductance measurements are reported for several salts in binary aqueous mixtures containing up to 60 mole % sulfolane, 20 mole % acetonitrile and 20 mole % dimethylsulfoxide. The variations of R = (λ^±₀η₀)_s/(λ^±₀η₀)_w with solvent composition have been compared with those observed in other water-rich mixtures. Alkali cations show R values greater than one with maxima in all the solvent mixtures. This behaviour has been discussed in terms of “sorting”, “averaging” and “steric” effects. Contrary to what happens to alkali cations, halide ions show R values greater or lesser than one according to whether the organic solvent respectively increases or decreases water structure. On these bases we suggest that conductometric behaviour of the halide ions may be indicative of the effect of the cosolvent on the water structure in water-rich mixtures and that DMSO is a water structure breaker.     

Potential dependence of the surface plasmon dispersion relation and the role of adsorption of anions at the electrochemical interface

A new theory is elaborated for the surface plasmon frequency ω_SP (k_|) dependence on the charge of the electrode. Its essential feature is to account for adsorption of anions or oxygen (primary oxidation processes) induced by anodic polarization which modify the free electron distribution near the metal surface in contact with electrolyte. It is shown that the bending of the electromodulation plot near p.z.c. (constancy of ω_SP (k_|) in the cathodic range and an almost linear decrease with anodic polarization) is entirely determined by the charge-induced adsorption. The bending point does not depend on the “strength” of the field induced adsorption varied in the model simulation but on the critical value of the charge at which the adsorption starts. The surface plasmon electromodulation plots can, thus, be used for the evaluation of this important quantity.     

Behavior of overpotential—deposited species in Faradaic reactions—II. ac Impedance measurements on H2 evolution kinetics at activated and unactivated Pt cathodes

The behavior of the “overpotential-deposited” (opd) H species in the H₂ evolution reaction (h.e.r.) proceeding with appreciable net cathodic current at activated and unactivated rotated Pt electrodes is investigated by means of impedance spectrum measurements. The experiments were designed to complement our recent study of the opd H coverage and pseudo-capacitance behavior evaluated by means of the potential-decay method. Despite many papers on theoretical aspects of involvement of adsorbed H in the kinetics and mechanisms of the h.e.r. and on characterization of upd H at noble metals, surprisingly little experimental work has been done that leads to quantitative evaluation of opd H coverage and its dependence on potential in relation to the reaction mechanism and electrocatalytic behavior. Complex-plane impedance spectrum plots are derived for the h.e.r. at Pt in acid and alkaline solutions at various potentials. The behavior is quantitatively simulated by sets of rate-constant parameters that also provide a fit of the steady-state Tafel relationships. The significance of the steady-state H adsorption pseudocapacitance is examined in relation to what is measured as the capacitance component in ac impedance determinations and to the 3-element equivalent circuit for the Faradaic impedance. The results are consistent with those derived from analysis of open-circuit potential-relaxation (potential-decay) transients at the same electrodes. The impedance behavior for active Pt in acid solution suggests the participation of a diffusion process but not involving a solution species; this process is therefore probably associated with some H sorption in the surface region of the metal as indicated by previous potential-decay studies.