Influence of lead dioxide surface films on anodic oxidation of a lead alloy under conditions typical of copper electrowinning

Cyclic voltammograms, current transients at constant potential and potential decay transients have been used to study the formation of lead dioxide surface films in the presence of cobalt ions and their role in decreasing the oxidation rate of a lead alloy under steady state conditions typical of copper electrowinning. The observations in the present work indicate, consistent with the surface film model, that the formation of a continuous PbSO₄ + α-PbO₂ film on the surface of the lead alloy in the presence of cobalt ions hinders further oxidation of the metal. The protectiveness of the film is dynamic in the steady state; the film is continuously forming and dissolving. Also studied was the potential of the oxygen evolution reaction on α-PbO₂ and β-PbO₂ in 170 g L⁻¹ H₂SO₄ with and without cobalt ions. The steady state potential for oxygen evolution on β-PbO₂ in 170 g L⁻¹ H₂SO₄ at 285 A m⁻² decreased in the presence of cobalt ions and the steady state potential of β-PbO₂ was essentially the same as that of (i) the Pb–Ca–Sn alloy and (ii) α-PbO₂. The implication is that the potential of the Pb–Ca–Sn alloy is determined by the α-PbO₂ and/or β-PbO₂ on its surface.     

Electrochemical oxidation of 1,3,5-trimethoxybenzene in aqueous solutions at gold oxide and lead dioxide electrodes

A kinetic study of the electrochemical oxidation of 1,3,5-trimethoxybenzene (TMB) by direct electron transfer at treated gold disk was combined with results of electrolysis in order to produce total degradation into CO₂ and H₂O at Ta/PbO₂ anode. The oxidation of TMB at gold electrode was studied by cyclic voltammetry. The cyclic voltammogram shows one irreversible anodic peak (I) corresponding to the oxidation of adsorbed TMB molecules. The proposed mechanism is based on the hypothesis of two-electron oxidation of TMB molecule leading, via intermediate of a radical cation, to the formation of the 2,4,6-trimethoxyphenol (TMP) and an adsorbed polymeric film. The TMP molecule undergoes a rapid oxidation leading to the formation of 2,6-dimethoxy-p-benzoquinone (DMBQ) as a major product. Degradation of TMB was studied by galvanostatic electrolysis using Ta/PbO₂ anode. The influence of initial TMB concentration and applied current density was investigated. Measurement of total organic carbon (TOC) and analysis by HPLC were used to follow this degradation. The experimental data indicated that the removal of TMB follows a pseudo first-order kinetic. The efficiency of the electrochemical process increases at lower current density and higher TMB initial concentration while it decreases with the TOC removal progress.     

Influence of cobalt ions on the anodic oxidation of a lead alloy under conditions typical of copper electrowinning

The influence of cobalt ions in the solution on the anodic oxidation of a commercial Pb–Ca–Sn alloy under conditions typical of copper electrowinning was studied using cyclic voltammograms and potential decay transients. The cobalt ions changed the structure, morphology and chemical composition of the surface film from a loose porous film to a thin dense film. This change of surface film is the cause for the decreased rate of oxidation for the lead anode in the presence of cobalt ions. The steady state potential of the alloy during anodic oxidation decreased with (i) increasing cobalt ion concentration, (ii) increasing rotation speed, (iii) increasing temperature, (iv) decreasing acid concentration and (v) decreasing current density. The steady state potentials were lower in the presence of cobalt ions. This decrease in the steady state potential may indicate that oxygen evolution is more rapid on the more compact film formed in the presence of cobalt ions. Alternatively, an additional pathway of oxidation may be provided by the cobalt ions.     

Morphologic study of electrochemically formed lead dioxide

Various electrochemical methods with different conditions were used to prepare lead dioxide (PbO₂). The observation revealed that the morphology of deposited PbO₂ could be controlled by simply changing deposition parameters. Under the condition of oxygen evolution, which dominates the electrode process, uniformly distributed high porous structured PbO₂ was formed. The results indicated that large current density or high potential polarization should be one of the most important and necessary factors for forming high surface area PbO₂ deposit. Only β-PbO₂ was identified by X-ray diffraction measurement for the deposit prepared by present methods and solution. One potential application of this method is to prepare nanoscaled PbO₂ parallel lines.     

On the performances of lead dioxide and boron-doped diamond electrodes in the anodic oxidation of simulated wastewater containing the Reactive Orange 16 dye

The performances of the Ti-Pt/β-PbO₂ and boron-doped diamond (BDD) electrodes in the electrooxidation of simulated wastewaters containing 85 mg L⁻¹ of the Reactive Orange 16 dye were investigated using a filter-press reactor. The electrolyses were carried out at the flow rate of 7 L min⁻¹, at different current densities (10-70 mA cm⁻²), and in the absence or presence of chloride ions (10-70 mM NaCl). In the absence of NaCl, total decolourisation of the simulated dye wastewater was attained independently of the electrode used. However, the performance of the BDD electrode was better than that of the Ti-Pt/β-PbO₂ electrode; the total decolourisations were achieved by applying only 1.0 A h L⁻¹ and 2.0 A h L⁻¹, respectively. In the presence of NaCl, with the electrogeneration of active chlorine, the times needed for total colour removal were markedly decreased; the addition of 50 mM Cl⁻ or 35 mM Cl⁻ (for Ti-Pt/β-PbO₂ or BDD, respectively) to the supporting electrolyte led to a 90% decrease of these times (at 50 mA cm⁻²). On the other hand, total mineralization of the dye in the presence of NaCl was attained only when using the BDD electrode (for 1.0 A h L⁻¹); for the Ti-Pt/β-PbO₂ electrode, a maximum mineralization of 85% was attained (for 2.0 A h L⁻¹). For total decolourisation of the simulated dye wastewater, the energy consumption per unit mass of dye oxidized was only 4.4 kWh kg⁻¹ or 1.9 kWh kg⁻¹ using the Ti-Pt/β-PbO₂ or BDD electrode, respectively. Clearly the BDD electrode proved to be the best anode for the electrooxidative degradation of the dye, either in the presence or absence of chloride ions.     

Anodic oxidation and electro-Fenton treatment of rotenone

Rotenone, a widely used botanical insecticide submitted to strong restrictions regarding its environmental hazards, was studied as a target compound for electro-Fenton (EF) treatment in aqueous-acetonitrile mixture (70:30) of pH 3.0. In this system, the degradation of organic pollutants occurs by attack of hydroxyl radicals (OH) which are produced from the reaction of added ferrous catalyst (Fe²⁺) and hydrogen peroxide (H₂O₂) electrogenerated by oxygen reduction at carbon felt cathode. The degradative efficiency of EF system was comparatively studied versus anodic oxidation method (AO) in absence and presence of H₂O₂. It was found that only EF is sufficiently powerful to induce fast and efficient mineralization of rotenone and its degradation intermediates.The mineralization of rotenone was found to depend largely on organic solvent type, metal ion catalyst, applied current and initial rotenone concentration. The best operative conditions are achieved using aqueous-acetonitrile mixture of pH 3.0 in the presence of 0.2 mM Fe²⁺ catalyst with a current intensity of 100 mA. Under these optimized conditions, 30 min were sufficient to completely degrade rotenone in 100 mL of a 20 mg L⁻¹ solution. A nearly complete mineralization (∼96% of COD removal) was achieved after 8 h treatment.Rotenone removal kinetic was found to obey the pseudo-first order model and the absolute second order rate constant (k_Rot = 2.49 × 10⁹ M⁻¹ s⁻¹) for the reaction between the substrate and OH was derived.HPLC-MS and HPLC-DAD analysis were applied to identify and follow the evolution of rotenone oxidation products. Three stable aromatic intermediates were observed and two of these were identified as 12aβ-hydroxyrotenone and hydroquinone. Subsequent attack of these intermediates by OH radicals leads to the formation of aliphatic carboxylic acids such as succinic, acetic, oxalic and formic, quantified by ion-exclusion chromatography.     

Anodic oxidation of p-methoxytoluene in acetic acid on graphite

The direct oxidation of p-methoxytoluene in acetic acid medium was studied on a graphite anode for the preparation of p-anisaldehyde and p-anisyl acetate. The effect of the water contents of the solvent on the reaction selectivity was determined for solvent mixtures containing from 0 to 20 vol % water. In anhydrous medium p-anisyl acetate was the main product. When the water contents increased the yield of p-anisaldehyde increased. It was shown that the higher the proportion of water, the lower the p-methoxytoluene oxidation rate. The formation of products of consecutive oxidation steps decreased as the temperature increased from 60 to 90 °C.     

The fabrication of lead dioxide layers on a titanium substrate

It is shown that stable and active lead dioxide on titanium anodes (including mesh electrodes) may be fabricated by appropriate pretreatment of the substrate and a strategy involving the anodic deposition of two PbO₂ layers—a thick underlayer with the lead dioxide doped with fluoride and iron and a top layer with the lead dioxide doped with bismuth. Periods on open circuit, especially in solutions containing an oxidisable organic or inorganic species must, however, be avoided since all forms of PbO₂ are reducible unless protected by an anodic current; the rates of the reduction does depend on the medium, the dopants in the PbO₂ and the morphology of the layer. It is also shown that, as at gold substrates, the morphology of the PbO₂ layer on titanium (consequently, their properties including adhesion and electrocatalytic activity) depend strongly on the deposition conditions including the bath composition (including Pb(II) concentration, acid concentration, concentration and choice of dopant ion), temperature, current density and deposition charge.     

Energy storage capacity investigation of pulsed current formed nano-structured lead dioxide

An electro-deposition method was used for the preparation of nano-structured lead dioxide. The lead dioxide films prepared were used as positive electrodes of lead acid batteries. Different parameters such as pulse time (t_on), pulse height, and relaxation time (t_off) were optimized to obtain higher capacity. Depend on the pulse conditions, a range of different morphologies of various porosities and connectivity was obtained. The resulting batteries were discharged to a cut off voltage of 1.75 V by a pulsed current method. The energy storage ability of the prepared lead acid batteries shows a close relation with the morphology of cathode materials. Maximum capacity was observed when pulse and relaxation time was equal to 0.1 and 5 s, respectively, at a current density of 25 mA cm⁻². A change in morphology of lead dioxide from aggregated globular structure to nanofiber was occurred. It was found that the high surface area as well as high connectivity between particles resulted in increased discharge capacity. Analysis of electrochemical impedance spectroscopy (EIS) data revealed that the charge transfer resistance is decreased by a change in morphology from bulk globular to nanofiber as the energy storage test showed. The time dependence of impedance behavior of a sample prepared at t_on = 0.1 s and t_off = 5 s at 25 mA cm⁻² was investigated and the results are discussed.     

Cr(III) oxidation with lead dioxide-based anodes

A procedure for preparing PbO₂-based electrodes with a titanium substrate is proposed. A platinum underlayer is first deposited on Ti by metal organic chemical vapor deposition (MOCVD), followed by the electrodeposition of the PbO₂ layer. The prepared Ti/Pt/PbO₂ anodes were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) before being used for oxidation of Cr(III) in sulphuric acid. The current efficiency was determined for that electrodes and the results were compared with those obtained with Pb/PbO₂ and Ebonex^®/PbO₂ electrodes with different pH conditions. The Ti/Pt/PbO₂ were found to have a very good electrochemical behaviour (current efficiency: φ=0.93 for pH 2), and may be used as dimensionally stable anodes for the oxidation of Cr(III).     

The electrochemical oxidation of aqueous sulfur dioxide: A critical review of work with respect to the hybrid sulfur cycle

Literature regarding the mechanism of the electrochemical oxidation of aqueous sulfur dioxide to sulfuric acid has been critically evaluated to provide a detailed understanding of the reaction under various applied conditions. This reaction is of high relevance to the hybrid sulfur cycle for large scale hydrogen production, as well as other industrial applications such as flue gas desulfurisation. Widespread disagreement in the literature and non-reproducible behaviour of the electrochemical oxidation reaction has been found in this review to often be a result of poorly defined electrode preconditioning procedures. It has also been found that the mechanistic pathway of the oxidation reaction is heavily influenced by the electrode material, solution pH and the applied anodic potential. These factors are thought to influence adsorption and the reductive formation of sulfur species at low potentials.     

碱木素电化学氧化的研究

用二氧化铅电极对竹类碱木素的碱性水溶液进行电化学氧化。根据氧化前后甲氧基含量及其红外光谱（IR）和核磁共振氢谱（1H－NMR）分析表明，二氧化铅电极的电氧化能脱除甲氧基，氧化醇羟基，破坏芳香环，使其羧基含量增加，而酚羟基的含量随氧化时间的延长有一个增加到减小的过程。凝胶色谱法（GPC）测量的结果表明，碱木素在氧化过程中，分子量分布变化很大，其平均分子量随着氧化时间的延长而先增高后降低，表明氧化过程中，同时发生了聚合反应和降解反应。用色谱质港联用仪（G－MS）检测到了降解产生的四种脂肪族化合物，没有得到芳香族的低分子化合物。     

Electrochemical treatment of effluents: A preliminary study of anodic oxidation of simple sugars using lead dioxide-coated titanium anodes

The electrolysis of simple sugar solutions, containing 0.5% NaCl or Na₂SO₄, using lead dioxide-coated titanium mesh anodes in a simple electroflotation cell indicated that electrode corrosion and dissolution of polluting Pb²⁺ ions from the electrode material were negligible even after prolonged use; the sugars were concomitantly electrooxidised at such anodes. The rate of electrooxidation followed linear removal kinetics and the relative ease of electrochemical destruction was in the order sucrose>maltose～glucose. Such oxidation processes represent added advantages in the use of these electrodes in electrochemical treatment of effluent containing the sugars. Cyclic voltammetric analysis of the glucose solution indicated that oxygen evolution on the lead dioxide-titanium anode appeared to be promoted by the presence of the sugar. A much more detailed study is required to give further insight into the mechanism of the anodic oxidation of these compounds.     

Electrocrystallization of lead dioxide: Analysis of the early stages of nucleation and growth

The kinetics of the early stages of the electrocrystallization of lead dioxide onto a vitreous carbon electrode is studied via the analysis of the early rising portion of the current-time transients (CTT's). The CTT's are recorded when the rate of charge-transfer across the electrode/electrolyte interface is the sole controlling mechanism for crystal growth. Induction times for the nucleation of the 3D growth centres are determined as a function of the applied potential. A possible controlling mechanism for the onset of the nucleation of the 3D growth centres is thus considered to be the need for the formation of at least a monolayer of deposit and/or adsorbed layers.It has been shown that CTT's that are recorded during the charge-transfer controlled growth processes allows the possibility of observing the formation of two distinct phases of a deposit, a phenomenon that would not be observed if CTT's are recorded when diffusion is the controlling mechanism for growth. The electrocrystallization of lead dioxide is shown to proceed, at least initially, via the formation of two distinct phases of PbO₂.     

Observed electrochemical oscillations during the oxidation of aqueous sulfur dioxide on a sulfur modified platinum electrode

The electrochemical oxidation of aqueous sulfur dioxide has been investigated on a sulfur modified polycrystalline platinum electrode. It has been found that time and potential-based current oscillations occur on a modified electrode in the oxidation region >0.7 V vs. SHE. The oscillation is specific to catalytic oxidation on sulfur modified platinum and is not observed on an unmodified electrode. Using i–R compensation techniques, the oscillation has been found to stem from electrochemical origins; the interfacial potential being an essential variable. The influence of other controllable variables including holding potential, sulfur dioxide concentration and sulfuric acid concentration have also been investigated for their effects on the oscillation. The electrochemical oscillation is explained here in part by the intermediate formation of dithionate. Two possible mechanisms, including a dual pathway mechanism, are explored.     

Methanol oxidation at platinized lead coatings prepared by a two-step electrodeposition-electroless deposition process on glassy carbon and platinum substrates

Platinized lead deposits, Pt(Pb), have been formed on glassy carbon (GC) and platinum electrodes by a two-step process, whereby a controlled amount of Pb was electrodeposited onto the substrates and was subsequently coated with a thin Pt layer upon immersion of the Pb/GC or Pb/Pt electrodes into a chloroplatinic acid solution. The spontaneous surface replacement of Pb by Pt resulted in Pt(Pb)/GC or Pt(Pb)/Pt electrodes which consisted of dispersed Pt(Pb) particles and displayed typical Pt surface electrochemistry in deaerated acid solutions. When tested as methanol oxidation anodes, these electrodes exhibited enhanced electrocatalytic activity both during voltammetric and constant potential experiments. This behaviour is attributed to an electronic effect of the underlying Pb onto the Pt surface layer.     

Liquid phase oxidation of acetophenone to phenylglyoxal by selenium dioxide alone or with aqueous nitric acid

The liquid phase oxidation of acetophenone to phenylglyoxal using selenium dioxide was carried out at 90°C in dioxane or ethyl alcohol with 10% (v/v) of water as a solvent with a 2:1 molar ratio of acetophenone to selenium dioxide. The yield of phenylglyoxal was more than 98% based on acetophenone and the efficiency of utilisation of selenium dioxide was as high as 99%. A novel process version based on the oxidation of acetophenone by aqueous nitric acid in the presence of selenium dioxide as a selective catalyst in a redox cycle was also considered with a view to changing the use of selenium dioxide as a stoichiometric oxidising agent to that of a catalyst, and it was found to be attractive.     

A novel flow battery: A lead acid battery based on an electrolyte with soluble lead(II): Part VII. Further studies of the lead dioxide positive electrode

Extensive cycling of the soluble lead flow battery has revealed unexpected problems with the reduction of lead dioxide at the positive electrode during discharge. This has led to a more detailed study of the PbO₂/Pb²⁺ couple in methanesulfonic acid. The variation of the phase composition (XRD) and deposit structure (SEM) have been defined as a function of current density, Pb²⁺ and H⁺ concentrations, deposition charge and temperature as well as the consequences of charge cycling. Pure α-PbO₂, pure β-PbO₂ and their mixtures can be deposited from methanesulfonic acid media. The α-phase deposits as a more compact, smoother layer, which is well suited to charge cycling. While the anodic deposition of thick layers of PbO₂ is straightforward, their reduction is not; the complexities are explained by an increase in pH within the pores of the deposit. The results suggest that operating the battery at lead(II) concentrations <0.3 M and elevated temperatures should be avoided.     

Activation of monolithic catalysts based on diatomaceous earth for sulfur dioxide oxidation

The formation of the active phases during the activation process of monolithic catalysts based on V₂O₅–K₂SO₄ supported on diatomaceous earth for SO₂ to SO₃ oxidation in flue gases, has been shown to be a crucial factor to achieve satisfactory catalytic performance. As the temperature is increased from room temperature to 470°C, SO₂ and SO₃ are taken up by the green catalyst and the precursors are transformed into the active species. The role of each component of the catalyst during the activation was analyzed by studying the behavior towards SO₂ adsorption of four materials, which contained: diatomaceous earth, diatomaceous earth + V, diatomaceous earth + K, and diatomaceous earth + V + K. The influence of the potassium sulfate accessibility in the green catalyst was studied by using two different preparation methods, which gave rise to differences in the catalysts SO₂ adsorption properties and catalytic performance. Furthermore, the influence of the activation atmosphere was studied using nitrogen, oxygen or a flue gas composition. It was shown that pyrosulfate species should be formed at temperatures below 400°C, to keep the vanadium in the active 5+ oxidation state.     

The preparation of PbO2 coatings on reticulated vitreous carbon for the electro-oxidation of organic pollutants

The preparation of PbO₂ coatings on reticulated vitreous carbon (RVC) has been carried out at constant current from electrolytic baths containing aqueous Pb(II) and methanesulfonic acid (MSA, CH₃SO₃H). The morphological and structural analysis of the RVC/PbO₂ deposits carried out by scanning electron microscopy (SEM) and X-ray diffraction revealed that a thick (100 μm), homogeneous, nanostructured β-PbO₂ film can be successfully formed. As a result, three-dimensional β-PbO₂ structures were obtained, being particularly interesting for their use as anodes in wastewater treatment. The high oxidation ability of these anodes has been verified by the electro-oxidation of Methyl Orange aqueous solutions. Quick decolourisation was achieved, with total colour removal in less than 60 min at 600 mA due to the production of large amounts of reactive OH radicals from the oxidation of water at high anodic potentials. The progressive mineralisation of the solutions was also ascertained from the total organic carbon (TOC) removal, which was much quicker at a higher applied current. All the coated RVC electrodes exhibited excellent long-term stability and remained unaltered after prolonged electrolyses. In addition, novel PbO₂ composite coatings were prepared in the presence of hydrothermally synthesized titanate nanotubes (TiNT). The SEM images showed the presence of TiNT agglomerates along the PbO₂ surface, which led to higher anodic current in the cyclic voltammetries carried out with Methyl Orange solutions. It is suggested that TiNT favour the adsorption of the organic molecules, facilitating the contact with the OH radicals and thus accelerating the electro-oxidation process. This was confirmed by the faster TOC removal compared to that yielded by the RVC/PbO₂, being 45% instead of 24% at 120 min.