Comportement electrochimique de la galene (PbS) dans les conditions de pH proches de celles de la flottation

The electrochemical behaviour of lead sulphide mineral (galena) has been investigated by cyclic voltammetry and controlled potential electrolyses. In the pH range 5–9, the oxidation leads—at + 0.5 V vs sce—to the formation of elemental sulphur and lead species: Pb²⁺, Pb(OH)₂ or Pb(OH)Cl depending on the pH, and on the nature of the electrolyte.

The reactions involved in the overall anodic/cathodic cycle have been interpreted; at pH 9, galena is reduced at − 1.3 V, to lead metal and HS⁻ ions.     

The sulphur---sulphide electrode in molten salts—I: Chronopotentiometric behaviour in lithium chloride---potassium chloride eutectic

By means of cathodic chronopotentiometric and potentiometric studies of (nominally) polysulphide solutions in LiCl---KCl eutectic at temperatures between 400 and 440°C, the predominant solute species present at concentrations around 10⁻² M was shown to be polysulphide S²⁻_x. This conclusion is in harmony with earlier spectral and electrochemical studies in LiCl---KCl eutectic.

The chronopotentiometric oxidation of sulphide ion S²⁻ in the same melt is rather complex, but it appears to be controlled by the extensive formation of a polymeric sulphur film, although a mixture of various soluble polysulphides is ultimately formed. Various detailed mechanisms that can account for the observed behaviour are discussed.     

Electrodeposition and dissolution of yttrium-hexacyanoferrate layers

The electrodeposition and dissolution of yttrium-hexacyanoferrate [YHCNFe(II)] were investigated by electrochemical quartz crystal microbalance technique (EQCM). The electrodeposition was carried out by potential cycling or stepping from solutions of Y(NO₃)₃ and K₃[Fe^III(CN)₆] of different concentrations. The ratio of the reactants was also varied. No deposition was found in dilute solutions (c < 10⁻³ mol dm⁻³). The increase of concentrations led to an intense deposition of YHCNFe(II) in the course of reduction of [Fe^III(CN)₆]³⁻. At high concentrations of the reactants a coagulation deposition of YHCNFe(III) at open-circuit has also been detected. During the reduction the first phase is the nucleation which requires saturation or oversaturation in respect to the reacting species near the gold surface. The growth phase is much faster than the formation of nuclei, and its rate depends on the concentration and the concentration ratio of the species. The composition of the deposits has been determined by total reflection X-ray fluorescence (TXRF) spectrometry. From the molar ratio of atomic constituents (K, Y and Fe) of the slightly soluble deposit (solubility: 5 × 10⁻⁵ mol dm⁻³) formed after reduction of Fe(III) a formula K_0.46Y_1.18[Fe^II(CN)₆] can be derived. This value is in good accordance with the molar mass calculated from the results of EQCM experiments which also revealed that the deposit contains ca. 2 mol H₂O/mol YHCNFe(II). The solubility of YHCNFe(III) is substantially higher (s = 2 × 10⁻³ mol dm⁻³), and according to the results of TXRF measurements, its composition is Y[Fe^III(CN)₆]. The reoxidation of YHCNFe(II) takes place in two steps. The first one is a partial oxidation which is accompanied by the desorption of K⁺ ions from the layer. During further oxidation a fast dissolution occurs due to the high solubility of YHCNFe(III).     

Electrochemical characterization of 8-hydroxyquinoline-5-sulphonate/aluminium(III) aqueous solutions

8-Hydroxyquinoline-5-sulphonate/Al(III) aqueous solutions were studied both by potentiometric titrations and voltammetric measurements, in order to obtain the number, the stoichiometry and the stability constants of the complexes formed at equilibrium, and to evaluate the redox and (electro)kinetic properties of the free ligand and of the metal/ligand complexes. The complexes formed in 0.2 m (Na)Cl aqueous solution (stability log beta values ± standard deviation) are AlL⁺ (8.95 ± 0.05), AlL₂⁻ (17.43 ± 0.03) and AlL₃³⁻ (24.58 ± 0.05), where “L” denotes the free ligand in the completely deprotonated form (L²⁻, pK_a1 = 3.910 ± 0.008, pK_a2 = 8.319 ± 0.004). AlL₃³⁻ is the predominant Al(III) species in a very wide range of pH, metal and ligand concentrations and metal-to-ligand ratios. The free ligand shows an oxidation wave at 0.62 V versus SCE. The proposed oxidation mechanism includes a first reversible one-electron oxidation of the ligand, followed by a coupling reaction and by a second reversible one-electron oxidation, and finally by a decomposition reaction. The addition of Al(III) lowers the intensity of the oxidation wave due to the formation of the redox-inactive complex AlL₃³⁻. A residual low signal was attributed to the free ligand produced by the complex dissociation, AlL₃³⁻ = AlL₂⁻ + L²⁻. All the kinetic parameters involved in the ligand oxidation and in the complex disruption were calculated on the basis of the agreement between experimental and simulated linear sweep and cyclic voltammetries. Correctness of the mechanisms proposed was further confirmed “a posteriori” by the agreement between potentiometric and linear sweep voltammetric results. The low residual signal observed in the presence of fully formed complex was attributed to the free ligand produced by the complex dissociation, having a kinetic constant estimated 0.2 s⁻¹.     

The anodic dissolution of copper in hydrochloric acid solutions

The electrodissolution of copper in hydrochloric acid solutions at the rotating ring-disk electrode was found to be controlled by both mass transfer and reaction in the apparent-Tafel region in HCl concentrations of between 0.1 and 1.0 M. The proposed mechanism describes the adsorption of CuCl_ads on the corroding copper surface and the diffusion of CuCl⁻₂ from the copper surface. The reaction in the limiting-current region was found to be controlled by the diffusion of Cl⁻ to the copper surface through a porous CuCl layer that forms on the surface. The thickness of this porous layer is dependent on the stirring conditions, and independent of the Cl⁻ concentration. Cu²⁺ is also produced at the Cu surface during electrodissolution. A mechanism describing the formation of a porous film of CuCl on the surface, the diffusion of Cl⁻ through this film and the formation of Cu²⁺ has been proposed.     

Iron(III) protoporphyrin IX—single-wall carbon nanotubes modified electrodes for hydrogen peroxide and nitrite detection

Iron(III) protoporphyrin IX (Fe(III)P), adsorbed either on single-walled carbon nanotubes (SWCNT) or on hydroxyl-functionalized SWCNT (SWCNT-OH), was incorporated within a Nafion matrix immobilized on the surface of a graphite electrode. From cyclic voltammetric measurements, performed under different experimental conditions (pH and potential scan rate), it was established that the Fe(III)P/Fe(II)P redox couple involves 1e⁻/1H⁺. The heterogeneous electron transfer process occurred faster when Fe(III)P was adsorbed on SWCNT-OH (11 s⁻¹) than on SWCNT (4.9 s⁻¹). Both the SWCNT-Fe(III)P- and SWCNT-OH-Fe(III)P-modified graphite electrodes exhibit electrocatalytic activity for H₂O₂ and nitrite reduction. The modified electrodes sensitivities were found varying in the following sequences: S_{SWCNT-OH-Fe(III)P} = 2.45 mA/M ≈ S_{SWCNT-Fe(III)P} = 2.95 mA/M > S_Fe(III)P = 1.34 mA/M for H₂O₂, and S_{SWCNT-Fe(III)P} = 3.54 mA/M > S_Fe(III)P = 1.44 mA/M > S_{SWCNT-OH-Fe(III)P} = 0.81 mA/M for NO₂⁻.     

The electrochemistry of tellurium in methylene chloride

It was found that TeCl₄ in methylene chloride (containing tetrabutylammonium perchlorate as a supporting electrolyte) is reduced to TeCl₂, Te⁰ and Te⁻² in potential applied. Two reduction waves are observed during the reduction of TeCl₄ at a platinum rde (Ep₁ = 0.08 ± 0.02 V, Ep₂ = −1.10 ± 0.02 V) due to the reduction of TeCl₄ to Te and Te⁻² respectively. A cathodic deposition of tellurium from TeCl₄ is followed by anodic stripping wave (Ep₁ = 0.42 ± 0.02 V), corresponding to the oxidation of Te to TeCl₂. It has been shown that reduction of TeCl₄, or TeCl⁻²₆ to Te causes coating of the electrode with metallic tellurium, on whose surface chloride ions are strongly bonded. It was found that when the electrolysis solution contains excess of chloride ion with respect to Te^IV the peak potentials of the cathodic waves shift to more cathodic values (Ep₁ = 0.30 ± 0.02 V, Ep₂ = −1.25 ± 0.02 V). After cathodic deposition of at least a monolayer of tellurium from such a solution two anodic waves appear (Ep₁ = 0.12 ± 0.03 V, Ep₂ = 0.24 ± 0.03 V) which are both due to the oxidation of Te to Te^II.     

Photoredox processes in the Cr(VI)–Cr(III)–oxalate system and their environmental relevance

Irradiation of the [Cr(C₂O₄)₃]³⁻ complex or the chromate(VI)–oxalate mixture, or the ternary system composed of Cr(III), Cr(VI) and oxalate, leads to chromium photoreductions in consequence of the ligand to metal charge transfer (LMCT) excitations induced by artificial solar radiation. In the case of the Cr(III) complex, the photoreduction involves the innersphere electron transfer leading to the formation of the Cr(II) species and the C₂O₄⁻ radicals. On reacting with molecular oxygen, Cr(II) is oxidised to Cr(III) catalysing thereby the oxalate substitution reaction. Moreover, under specific conditions, Cr(II) can be also oxidised to Cr(VI). Chromate(VI) is not photoreducible, but in the presence of oxalate, or other sacrificial electron donor, the outersphere photoinduced electron transfer (PET) produces Cr(V) species and the C₂O₄⁻ radicals. This initiates a series of thermal reactions leading to the formation of Cr(III) and oxidized oxalate (CO₂). In the system composed of [Cr(C₂O₄)₃]³⁻ and chromate(VI), the acidic medium and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions assist the Cr(VI) photoproduction. When an approximately neutral solution equilibrated with the ambient air is irradiated intermittently, Cr(VI) is consumed and/or produced, accordingly to the time sequence of exposure and dark periods. The oscillations of Cr(VI) concentrations are accompanied by continuous oxidation of oxalate, playing the role of the sacrificial electron donor. The effects of solution pH, molecular oxygen, concentrations of reagents and cations on the reaction rates were investigated. The results of this paper revealed that the Cr(III)/Cr(VI) system under environmental conditions behaves as the photocatalytic one catalysing the oxidation of oxalate or other organic matter by molecular oxygen, contributing thereby to the abatement of pollution.     

The influence of pH on reaction pathways for O2 reduction on the Au(100) face

Oxygen reduction on the Au(100) face was studied by the rotating disk-ring electrode technique in solutions of anions which adsorb strongly on gold (HSO₄⁻/SO₄²⁻ and/or OH⁻) over the entire pH range. The specific adsorption of OH⁻ anions, which is a pH dependent process, is found to play the key role in determining the reaction pathways. In the absence of OH⁻ adsorption, for pHs below 6, the reduction of O₂ begins as a 2e-process. Due to the increase in local pH during O₂ reduction, the reaction pathway turns into a 4e-reduction at a certain potential depending on the pH of the solution. For pHs higher than 6, O₂ reduction begins as a 4e-process in the potential region where specifically adsorbed OH⁻ anions are present.     

Electrocatalysis of the charge-transfer process for the Sb(III)/Sb(V) couple at a platinum electrode in concentrated HCl

Electrocatalysis of the electrochemical oxidation and reduction of Sb(V) and Sb(III) by adsorbed I⁻ and I₂ at a rotating Pt disc electrode was investigated in 12 M HCl. The heterogeneous rate constant in the absence of adsorbed halogen was increased by 200x for a maximum surface coverage by halogen. This large rate increase cannot be predicted on the basis of an electrostatic argument and a mechanism based on electron-transfer bridging is proposed.     

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl_1−xMn_xO₃ (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl_1−xMn_xO₃ samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m² g⁻¹ ranges, respectively. Redox properties and the content of Mn⁴⁺ were derived from temperature programmed reduction (TPR) with H₂. Two reduction steps are observed by TPR for pure LaMnO₃, the first attributed to the reduction of Mn⁴⁺ to Mn³⁺ and the second due to complete reduction of Mn³⁺ to Mn²⁺. The presence of Al in the LaAl_1−xMn_xO₃ solid solutions produces a strong promoting effect on the Mn⁴⁺→Mn³⁺ reducibility and inhibits the further reduction to Mn²⁺. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO₃, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.     

The anodic oxidation of thiocyanate ion dissolved as KSCN in dimethylsulphoxide

The anodic oxidation of SCN⁻ ion dissolved as KSCN in dimethylsulphoxide, on platinum electrodes, was investigated at ca 25, 60 and 160°C by means of various non-stationary electrochemical techniques. At low temperatures one anodic and two main cathodic processes were found. The anodic oxidation of SCN⁻ ion yields as primary product the SCN radical, which readily produces (SCN)₂. The latter can be in part reduced back to SCN⁻ ion, because it reacts in part yielding solvated hydrogen ions which cause the second cathodic reaction. In the region of 60°C, no (SCN)₂ cathodic current is observed. In the region of 160°C the only reaction is SCN⁻ ion oxidation and the primary product polymerizes to (SCN)_x, which forms a film on the electrode, causing passivation.

On the basis of kinetic data obtained for the different reactions, a mechanism for the anodic oxidation of the SCN⁻ ion and for the passivating film formation is suggested. The second process only partly fits the Müller model for the electrochemical growth of insoluble layers.     

Galvanostatic desorption of hydrogen from palladium layers—I. Theory

The theory of the galvanostatic desorption of hydrogen held in a finite palladium layer (thickness, l 10⁻³−10⁻² cm), in a thin palladium layer (thickness, λ 10⁻⁶−10⁻⁵ cm) and in a composite finite layer of a non-noble metal protected with a thin palladium layer is developed. The diffusion equations are solved by the Laplace transform and their solutions in term of the concentration of dissolved N_i and weakly adsorbed N_w hydrogen are given. Further the Iτ-functions are estimated for small values of the current, which allows the calculation of the characteristic parameters both of the metallic layer and of the partial steps involved, ie the diffusion and the transfer. The transient overpotentials occurring during the electrochemical desorption for a reversible and an irreversible oxidation step are also calculated.     

Mechanism of gas flow through coal

Coals are fossilized plant material plus inorganic silt deposited in irregular layers and containing 1–20% void space which provides a medium which is porous to gas flow. Gas flows have been measured using discs of coal cut from several coal seams. Observed flow phenomena include molecular diffusion (low gas pressure, small pores) and bulk diffusion (higher gas pressure, larger pores). In the examples investigated, methane flows ranged from 1.2 × 10⁻¹⁰ cm² s⁻¹ atm⁻¹ for Pittsburgh-seam attrital coal to 2.0 cm² s⁻¹ atm⁻¹ for Oklahoma Hartshorne coal. Flow characteristics have been compared with gas flows observed by Knudsen through glass capillaries. This information can be applied to mine safety and to coal utilization studies.     

Bacterial pyrite oxidation: Influence of morphological, physical and chemical properties

Physical and chemical characteristics of six types of pyrite isolated from ores and coals as well as the mechanism of their oxidation by Thiobacillus ferrooxidans and Sulfobacillus thermosulfidooxidans were investigated. The effect of monocrystallinity, orientation of crystals, and microaggregation on the oxidation degree of polish rock section was clearly revealed.

The investigation of the surface of powdered pyrite particles showed the ratio S²⁻/Fe²⁺ of pyrite to be equal to 2.7. This indicates that the surface of the pyrite particles was deficient in metal or redundant in sulfur. This ratio holds in the coarse of pyrite oxidation by T. ferrooxidans, indicating that Fe²⁺ and S²⁻ are oxidized simultaneously. In the presence of S. thermosulfidooxidans the ratio S²⁻/Fe²⁺ is reduced to 1.0 for some types of pyrite indicating preferential oxidation of S²⁻. The kinetics and mechanism of bacterial oxidation of pyrites are discussed.     

The kinetics of copper dissolution in acetonitrile-water copper(II) solutions

The rate of dissolution of rotating copper discs in acidified copper(II) solutions prepared in an acetonitrile-water solvent (mole fraction ACETONITRILE = 0.·25) was measured in the temperature range 270–360 K. The rate of dissolution was found to be proportional to the square root of the rotation speed and to have an activation energy of 18 ± 2kJ mol⁻¹. The reaction is therefore considered to be diffusion controlled.

The reduction of copper(II) to copper(I) on a rotating platinum electrode, and the oxidation of copper(O) to copper(I) on a rotating copper electrode, were investigated. These two reactions together make up the dissolution reaction. From the intersection of the polarization curves, it was confirmed that the dissolution reaction was diffusion controlled. Dissolution rates calculated from the electrochemical measurements agreed with the kinetic measurements. Estimates of some electrochemical parameters in the acetonitrile mixed solvent were made.

The surface characteristics of annealed and unannealed copper discs which had been etched in the copper(II) solution are discussed briefly.     

Voltammetric behavior of the C60-γ-cyclodextrin inclusion complex (1:2) on hmde in an aqueous solution

Electrochemical behavior of the water-soluble C₆₀-γ-CD (1:2) inclusion complex has been studied on the hanging mercury drop electrode. A one-electron reversible adsorptive electro-reduction and three irreversible adsorptive electro-reductions were detected by cyclic voltammetry. The amount of C₆₀-γ-CD adsorbed at saturation is 2.50 × 10⁻¹¹ mol cm⁻², the diffusion coefficient is 4.36 × 10⁻⁶cm²s⁻¹ and the standard rate constant of the surface reaction k_s are 0.745 s⁻¹, 0.612 s⁻¹ and 0.513s⁻¹, respectively.     

Formation of singlet molecular oxygen associated with the formation of superoxide radicals in aqueous suspensions of TiO2 photocatalysts

The production and decay of singlet molecular oxygen (¹O₂) in TiO₂ photocatalysis were investigated by monitoring its phosphorescence under various reaction conditions. First, the effects of additives such as KBr, KSCN, KI, H₂O₂, and ethanol on the amount of ¹O₂ produced by photo excitation of P25 TiO₂ were measured. The same additives were employed to investigate the effect on the amount of O₂⁻ produced. Comparison between the effects on ¹O₂ and O₂⁻ suggested that ¹O₂ is formed by the electron transfer mechanism, the reduction of molecular oxygens to O₂⁻ by photogenerated electrons and the subsequent oxidation of O₂⁻ to ¹O₂ by photogenerated holes. The formation of ¹O₂ decreased at pH < 5 and pH > 11, indicating that the intermediate O₂⁻ is stabilized at the terminal OH site of the TiO₂ surface in the pH range of 5 < pH < 11. Eighteen commercially available TiO₂ photocatalysts were compared on the formation of ¹O₂ and O₂⁻ in an aqueous suspension system. The formation of ¹O₂ was increased with decreasing size of TiO₂ particles, indicating that a large specific surface area causes a higher possibility of reduction producing O₂⁻ and then a large amount of ¹O₂ is formed. The difference in the crystal phase (rutile and anatase) did not affect the formation of ¹O₂.     

Electrodeposition of cesium at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide room-temperature ionic liquid

The electrochemistry of cesium was investigated at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Bu₃MeN⁺Tf₂N⁻) room-temperature ionic liquid (RTIL) by using cyclic staircase voltammetry, rotating disk electrode voltammetry, and chronoamperometry. The reduction of cesium ions at mercury exhibits quasireversible behavior with k⁰ = 9.8 × 10⁻⁵ cm s⁻¹ and = 0.36. The diffusion coefficient of Cs⁺ in this RTIL was 1.04 × 10⁻⁸ cm² s⁻¹ at 303 K. Bulk deposition/stripping experiments conducted at a rotating mercury film electrode gave an average recovery of 97% of the electrodeposited Cs. The density, absolute viscosity, and equivalent conductance of Bu₃MeN⁺Tf₂N⁻ were measured over the range of temperatures from 298 to 353 K. A polynomial equation describing the temperature dependence of the density is presented. Both the viscosity and conductance exhibited the non-Arrhenius temperature dependence typical of glass-forming liquids. The ideal glass transition temperature and the activation energies for viscosity and conductance were obtained by fitting the Vogel–Tammann–Fulcher (VTF) equation to the experimental data for these transport properties.     

Palladium-substituted lanthanum cuprates: application to automotive exhaust purification

A series of palladium-substituted La₂CuO₄, corresponding to the formula La₂Cu_{1 −x}Pd_xO₄ (x = 0−0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13–17 m²/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C₃H₆ decreased markedly when increasing the palladium content, the activity of catalysts La₂Cu_0.9Pd_0.1O₄ and La₂Cu_0.8Pd_0.2O₄ being comparable to that of a Pt-Rh/CeO₂–Al₂O₃ catalyst for NO reduction, and higher for CO and C₃H₆ oxidation.

All the La₂Cu_{1 − x} Pd_xO₄ catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd⁰ ions atomically dispersed, surrounded by Cu⁺ and Cu²⁺ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La₂Cu_{1 −x}Pd_xO₄ structure.