Investigation on electrochemical properties of cerium doped lead dioxide anode and application for elimination of nitrophenol

The present work focused on the investigation of electrochemical properties of cerium doped lead dioxide anode, i.e. Ti/Ce–PbO₂. SEM, AFM, XRD and XPS were used to characterize the morphology, crystal structure and elemental states of the modified anode. Electrochemical impedance spectroscopy (EIS) was also utilized to study the electrochemical property of Ti/Ce–PbO₂. The electrochemical activity of the Ti/Ce–PbO₂ anode was investigated by means of bulk electrolysis and compared with that of a PbO₂ anode. The accelerated life test and oxidants determination were also conducted. The results indicated that the incorporation of cerium improved the electrocatalytic activity and stability of PbO₂ anode. The service life of Ti/Ce–PbO₂ electrode was much longer than that of traditional lead dioxide electrode. The electrochemical activity obtained from degradation of o-nitrophenol (o-NP) outperformed the traditional lead dioxide electrode as well. The Ti/Ce–PbO₂ electrode is considered a promising anode for the treatment of organic pollutants.     

Inhibition of PbO2 formation during lead electrowinning

The effect of the addition of arsenic compounds on the inhibition of PbO₂ formation on the anode, thus promoting oxygen evolution in the electrowinning process of lead, has been investigated. Various possible anode reactions involving arsenic have been discussed. Preferential adsorption of arsenic compounds on the electrode surface seems to be the most probable reason for the inhibition of PbO₂ formation.     

Effect of CeO2 and graphite powder on the electrochemical performance of Ti/PbO2 anode for zinc electrowinning

In the zinc hydrometallurgy process, the key to reduce the voltage of the corrosion is to reduce the oxygen evolution potential of the anode. In this experiment, the effects of doped CeO₂ and graphite powder (GP) on the electrochemical properties of Ti/PbO₂ were investigated. Ti/PbO₂, Ti/PbO₂-CeO₂, Ti/PbO₂-GP and Ti/PbO₂-CeO₂-GP anode were prepared by direct current (DC) plating. The surface morphology and composition of these different Ti/PbO₂ electrodes before and after polarisation were analysed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion mechanism was analysed by linear sweep voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The cell voltage, current efficiency, service life and failure mechanism of Ti/PbO₂ anodes were observed and analysed by simulating electrowinning experiments. The results show that Ti/PbO₂-CeO₂-GP anodes is denser than other electrodes. Ti/PbO₂-CeO₂ (12?g/L)-GP (8?g/L) with the lowest anode potential is 1.5390?V. The potential of the lowest oxygen evolution peak of the Ti/PbO₂-CeO₂ (12?g/L)-GP (8?g/L) electrode is 0.824?V. The Ti/PbO₂-CeO₂-GP anode had a minimum cell voltage of 2.85?V and a current efficiency of 95.8%. The Ti/PbO₂-CeO₂-GP has a minimum corrosion rate of ??0.0320?g·A^?1·h^?1 and a longest anode lifetime of up to 127?h. The best electrode for the surface coating is Ti/PbO₂-CeO₂ (12?g/L)-GP (8?g/L).     

Effects of ultrasound on electrochemical oxidation mechanisms of p-substituted phenols at BDD and PbO2 anodes

The effects of low-frequency (40 kHz) ultrasound are investigated with regard to the effectiveness and mechanisms of electrochemical oxidation of p-substituted phenols (p-nitrophenol, p-hydroxybenzaldehyde, phenol, p-cresol, and p-methoxyphenol) at BDD (boron-doped diamond) and PbO₂ anodes. Although ultrasound improved the disappearance rates of p-substituted phenols at both the BDD and PbO₂ anodes, the degree of enhancement varied according to the type of p-substituted phenol and type of anode under consideration. At the BDD anode, the %Increase values were in the range 73-83% for p-substituted phenol disappearance and in the range 60-70% for COD removal. However, at the PbO₂ anode, the corresponding %Increase values were in the range 50-70% for disappearance of p-substituted phenols and only 5-25% for COD removal, much lower values than obtained at the BDD anode. Further investigations on the influence of ultrasound on the electrochemical oxidation mechanisms at BDD and PbO₂ anodes revealed that the different increase extent were due to the specialized electrochemical oxidation mechanisms at these two anodes. The hydroxyl radicals were mainly free at the BDD electrodes with a larger reaction zone, but adsorbed at the PbO₂ electrodes with a smaller reaction zone. Therefore, the enhancement due to ultrasound was greater at the BDD anode than at the PbO₂ anode.     

Performance characteristics of a new type of lead dioxide-coated titanium anode

A new type of PbO₂-coated metal anode, useful for electrochemical processing requiring the use of high-overpotential anodes, was prepared by applying to a titanium substrate an undercoating of Ti-Ta oxides, then covering the undercoating with an intermediate coating of stress-free -PbO₂ deposit, and finally covering the intermediate coating with a Ta₂O₅ particle-admixed -PbO₂ deposit. The anode thus produced was found to be superior to the anodized Pb or platinized Ti anode in respect of oxygen overvoltage, and corrosion resistance, even in solutions containing organic solvents. The possibility for use in chromium plating is also indicated.     

Electrochemical noise analysis of O2 evolution on PbO2 and PbO2-matrix composites containing Co or Ru oxides

Electrochemical noise measurements have been performed on various electrode materials during oxygen evolution reaction. Fluctuations of both electrode potential and electrolyte resistance, recorded under galvanostatic polarization, have been analysed in order to separate ohmic and non-ohmic contributions. The anode materials compared in this study were PbO₂ and composite oxides consisting of a PbO₂ matrix and variable amounts of catalytic dispersed phases (Co₃O₄, RuO₂ and hydrous Co oxides); the composite oxide layers were generally obtained by anodic codeposition of fine particles (typically 0.1-1 μm in size) with a PbO₂ matrix electrochemically grown by Pb²⁺ oxidation. The power spectral density (PSD) of the electrolyte resistance fluctuations is very similar for PbO₂ and composite electrodes, suggesting a similar gas evolution profile. On the contrary, the potential PSD is markedly different for PbO₂ and composites containing catalytic particles. In the latter case, the potential fluctuations are entirely due to ohmic effects, while non-ohmic components are clearly dominant in the case of O₂ evolution on pure PbO₂, probably because of the much higher activation overpotential. Furthermore, on PbO₂ electrodes a plateau was obtained at intermediate frequencies (0.1-10 Hz), which was tentatively explained by bubble coalescence phenomena.     

The electrochemistry of Magnéli phase titanium oxide ceramic electrodes Part II: Ozone generation at Ebonex and Ebonex/lead dioxide anodes

Uncoated Ebonex is not a suitable anode material for the generation of ozone. By contrast, Ebonex electroplated with PbO₂ can be highly stable in the forcing conditions essential to the formation of ozone and gives current yields comparable to lead dioxide on other substrates in both acid and neutral electrolytes.     

Comparative studies on the electrocatalytic properties of modified PbO2 anodes

A set of modified PbO₂ anodes doped with the oxides of bismuth and cobalt were prepared by means of electrodeposition in nitrate solutions. Of them, Ti/Bi-PbO₂ anode displayed excellent electrocatalytic performance. Cyclic voltammetric experiments were carried out to get a better understanding of the electrocatalytic properties of these anodes. The Ti/Bi-PbO₂ anode had the highest overpotential of 1.75 V (vs. SCE) for oxygen evolution. Both XRD patterns and scanning electronic microscopy (SEM) images demonstrated that the incorporation of Bi could diminish the size of the crystal particles. Oxidants such as hydroxyl radical, hydrogen peroxide were determined, and their amount was proportional to the electrocatalyitc activities of modified PbO₂ anodes. Electrocatalytic oxidation of o-nitrophenol was conducted by using these anodes as anode and stainless steel sheet as cathode. Ti/Bi-PbO₂ anode displayed not only excellent electrocatalytic performance but also low energy consumption. The Ti/Bi-PbO₂ anode is a promising anode for the treatment of organic pollutants.     

Effect of manganese (II) added to the electrolyte on the properties of lead anodes used in oxidation of ferrous sulphate to ferric sulphate

The present paper deals with the possibilities of increasing the corrosion resistance of lead anodes during anodic oxidation of ferrous sulphate to ferric sulphate. This was achieved by preliminary passivation of lead anodes in a sulphuric acid and ferrous sulphate solution to which manganous sulphate was introduced. Owing to the passivation of lead anodes by the 20 mA cm^–2 current a potential of 2.2 V vs SEH is established and on their surface, apart from PbO₂, MnO₂ is also formed. On the basis of voltametric measurements of lead anode oxidation, analysis of potential variations during their selfdepassivation, X-ray microanalysis and scanning analysis of the passivated electrode surface the role of MnO₂ in increasing the corrosion resistance in sulphuric acid solutions containing ferrous sulphate was determined. Also a mechanism explaining the effect of MnO₂ formed on the lead anode surface on limitation of the destruction process of the passive oxide layer during oxidation of the ferrous to ferric ions was suggested.     

The influences of the electrolyte and the physical conditions on ozone production by the electrolysis of water

The electrosynthesis of ozone at ambient temperature can be achieved using anodes (preferably PbO₂) in aqueous, buffered electrolytes (neutral to slightly acidic media). Traces of highly adsorbing anions raise the oxygen overpotential and the O₃ yield, whilst anions with a high charge/radius ratio as well as certain transition metal ions have detrimental effects. Ozone is evolved at high electrode potentials despite corrosion of the (PbO₂, Pt or Au) anode. The same is true in general, when competing electrooxidations occur (e.g. with ClO ₃ ^– ). At fixed temperature and current density the 0₃ yield at a PbO₂ anode decreases when the pressure is raised from 1 to 10 atm.     

Effect of electrochemical preparation methods on structure and properties of PbO2 anodic layer

PbO₂ anodic layers were formed by direct oxidation of lead electrodes galvanostatically (G) and potentiostatically (P), in attempt to study PbO₂ as positive active material for lead-acid batteries. The structural and electrochemical properties of PbO₂ anodic layers, prepared by the two methods, were examined and found to be different. Comparative studies indicated that PbO₂-G electrode exhibited a longer discharge time, corresponding to a higher capacity, and a lower charge transfer resistance than that of PbO₂-P. Through SEM observation it was established that the PbO₂-G had a compact structure, while the PbO₂-P surface showed a porous structure. Discharge capacity was related to the film structure and internal resistance of PbO₂ layer. The compact structure with higher degree of interconnected particles lead to lower internal resistance thus higher capacity, and the porous surface layer would result in high transfer resistance and diminished capacity. It was concluded that preparation methods could controllably affect the structure of PbO₂ films, which further played important roles in discharge capacity.     

Spectroscopic and electrochemical investigations of lead–lead dioxide glasses and vitroceramics with applications for rechargeable lead–acid batteries

A new class of glass and vitroceramic electrodes with applications for rechargeable batteries was obtained by a melt quenching method. The structural characterization of the samples having the xPb·(100−x)PbO₂ composition, where x=0, 10, 20, 30, 40 and 50 mol% Pb, was performed by UV–vis and FTIR spectroscopies investigations.UV–vis and FTIR data reveal that the excess of lead content in the host matrix generates the transformation and/or disintegration of [PbO₆] octahedral structural units into [PbO₄] structural units or Pb²⁺ ions and non-bonding oxygen ions centers.The electrochemical performances of the glass and vitroceramics electrodes were investigated by cyclic voltammetry. The shapes of the cyclic voltammograms and redox peaks depend on the electrolyte solution concentration and the lead content in host matrix. Differences between these waves are determined by the type of electrochemically active species existing in the glass or glass ceramics. The improved performance of the vitroceramic electrodes is attributed to the presence of the lead metallic phase that seems to offer an easier route for the charge process of the electrodes. Thus, the presence of these phases generates more electrochemically active species, inhibits the secondary reactions implying PbO, takes up the ionic conduction of the larger electrolyte solution and increases the charge/discharge rate of the electrochemical processes.     

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.     

A comparison of electrochemical degradation of phenol on boron doped diamond and lead dioxide anodes

This work compares two electrode materials used to mineralize phenol contained in waste waters. Two disks covered with either boron doped diamond (BDD) or PbO₂ were used as anodes in a one compartment flow cell under the same hydrodynamic conditions. Efficiencies of galvanostatic electrolyses are compared on the basis of measurements of Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD). Galvanostatic electrolyses were monitored by analysis of phenol and of its oxidation derivatives to evaluate the operating time needed for complete elimination of toxic aromatics. The experimental current efficiency is close to the theoretical value for the BDD electrode. Other parameters being equal, phenol species disappeared at the same rate using the two electrode materials but the BDD anode showed better efficiency to eliminate TOC and COD. Moreover, during the electrolysis less intermediates are formed with BDD compared to PbO₂ whatever the current density. A comparison of energy consumption is given based on the criterion of 99% removal of aromatic compounds.     

Electrochemical waste water treatment using high overvoltage anodes. Part I: Physical and electrochemical properties of SnO2 anodes

The electrochemical performance of SnO₂ as an anode material with high oxygen gas-evolution overpotential was investigated in view of its application for electrochemical oxidation of bio-refractory organics in waste waters. The influence of the doping agent (Sb, F, Cl) and doping level on the oxygen-evolution reaction was studied in terms of Tafel slope, oxygen-overpotential and exchange current densities for the Ce³⁺Ce⁴⁺ reaction. Tafel slopes of about 300 mV decade^–1 were found and the oxygen evolution overpotential was 600mV higher than that of platinum. While the doping level had no significant influence on Tafel slopes and oxygen overpotentials the stability of the SnO₂ electrode increased with charge carrier density. The oxidation of phenol was investigated as a test for the oxidizing power of the new anode material when compared to Pt or PbO₂. The rate of phenol removal was much higher for SnO₂ than for PbO₂ or Pt.     

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.     

Electrochemical oxidation of simple indoles at a PbO2 anode

Simple indoles undergo electrolytic degradation at potentials near +1 V vs the normal hydrogen electrode at a Pb/PbO₂ anode. Oxidation is first order in both current and indole concentration. The reaction is characterized by low CO₂ yields and high TOC values, that is, most of the carbon of the starting material remained in the solution after electrolysis. Monomeric, isolable oxidation products were not found even at high conversion. These results are consistent with the intermediacy of hydroxyl radicals, which are produced at the surface of the Pb/PbO₂ anode by electrolysis of water, initiating the polymerization of the starting materials to water soluble products with high net current efficiency.     

Anodic Oxidation of Unsaturated Aliphatic Ethers in aqueous electrolytes

2,5-dihydrofuran 1 was galvanostatically oxidized in aqueous electrolytes on Pt and PbO₂ anodes at current densities of 10 to 50 mA cm⁻². Under acidic conditions maleic dialdehyde 2 was obtained on both electrodes with a current efficiency (c.e) of up to 50%. In the alkaline region (pH 10) 4-hydroxy crotonic aldehyde 3 (Pt) or 2,5-dihydrofuranone 4 (PbO₂) become the main products with c.e.'s of 29% and 27%, respectively, depending on the anode material. The influence of pH and of the anode material are discussed in detail. 2,3-dihydrofuran 7 , vinyl ethyl ether 8 and for comparison, furan 9 were studied in addition.     

Uber ein chemisches verfahren zur herstellung von elektrochemisch aktivem blei-IV-oxid

There are experiments described where it is possible to produce active electrode mass due to direct influence of ozone. Ni-II-hydroxide can be turned into NiOOH when it is dry. MnO₂ can be precipitated by passing O₃ into a Mn-II-salt solution. Only for PbO₂ the situation is not so easy to handle. Within pure lead-II-salt solutions below pH 3 there will be no precipitation. Only at a higher pH and a suitable buffering PbO₂ appears as a reaction product. Technically economic procedures are reached by slurrying lead powder or low valent lead oxides under alkaline circumstances. Some detailed results of an extensive research program are being presented, which have been made in order to pursue the relation between chemical synthesis, structure and electrochemical activity of the PbO₂. Specific capacity of max. 567 coul/g of PbO₂-material have been measured. The molecular water content (presence of protons) within the solid structure plays, similar to the former tested MnO₂, but in a much more sensitive way, an important role for the electrochemical properties of the gained substances. The life of a PbO₂-particle in the positive electrode of the lead battery becomes more understandable due to the recognized results.     

Deactivation and regeneration of Pt anodes for the electro-oxidation of phenol

Electro-oxidation tests with different electrolytes (Na₂SO₄, NaCl, H₂SO₄) and anode types (Pt, Ti lined with Ir and Ta oxides, PbO₂, activated carbon) were performed on aqueous solutions containing phenol to assess the mechanism and nature of electrode deactivation phenomena. For the Pt electrode, the nature of the electro-deposited organic species was investigated by ATR-FTIR and FESEM-EDS analyses, which showed adsorption of intermediate oxidation products (e.g. benzoquinone, hydroquinone) is likely responsible for the early deactivation stages. Conversely, in the longer term, formation of polymeric films is promoted. Potentiostatic tests showed that anode regeneration can be achieved by anodic polarisation above 1.1 V (vs Hg/Hg₂SO₄). This reactivation was found to be easier in the presence of significant amounts of chloride ions. Conversely, the deactivated state is maintained for the Ti/IrO₂/Ta₂O₅ electrode even though anodic polarisation at high positive potentials is applied. Cyclic voltammetric curves on PbO₂ electrodes did not provide satisfactory results as the intensity of the lead-dioxide reduction peak was so high that peaks for phenol oxidation were hardly detectable. Finally, the activated carbon based electrode was found to be promising as it enables simultaneous adsorption of the organic pollutant and oxidation of the pollutant itself to constitute a sort of self-regenerating adsorber unit.