Kinetics of anodic formation and cathodic reduction of MnO2 in the sulfate electrolyte solutions

The anodic formation of manganese dioxide is studied voltammetrically in a wide range of potential scan rate (V = 0.001–8 V/s). Using the diagnostic criteria of cronovoltammetric method, based on the original experimental data, the mechanism of electrooxidation of manganese ions in the acidic medium with subsequent reaction of disproportionation of the product of irreversible electrode reaction and hydrolysis yielding manganese dioxide is proposed. The kinetics of cathodic reduction of electrolytic manganese dioxide in the 0.5 M Na₂SO₄ solution is studied under the steady-state and non-steady-state potentiodynamic polarization conditions. From the experimental data, it is found that, in the acidic medium (pH 1–3), the mechanism of the electrode process changes depending on the cathodic potential scan rate: at the scan rate V < 0.5 V/s, MnOOH forms via one-electron transition leading, in its turn, to the partial deactivation of electrode surface with subsequent disproportionation of manganite. At the relatively high potential scan rates, manganite has no time to form, and the two-stage reduction via one-electron transitions at each stage is well pronounced. The parameters of the electrode processes are calculated.     

Corrosion and adsorption properties of electrochemical manganese dioxide in the acidic and weakly alkaline sulfate solutions

Corrosion and adsorption properties of manganese dioxide, which was anodically deposited on platinum, its powder, and compacted tablets of manganese dioxide in the solutions with various pH values are studied using various experimental methods. It is shown that the corrosion rate of manganese dioxide decreases with increasing pH of solution, and the process proceeds by the electrochemical mechanism with conjugate chemical reactions of the formation and decomposition of the product of transition stage. The adsorption properties of manganese dioxide powder with respect to copper ions increase with increasing pH of solution, whereas the amount of manganese ions passing to the solution almost vanishes at pH 9.     

Electrochemical synthesis of layered manganese oxides intercalated with tetraalkylammonium ions

Thin films of birnessite-type layered manganese oxides with various interlayer spacings have been prepared on a platinum electrode by a one-step electrochemical procedure. The process involves a potentiostatic oxidation of aqueous Mn(2+) ions at around +1.0 V (Ag/AgCl) in the presence of tetraalkylammonium cations with different alkyl chain lengths. X-ray diffraction indicates that the films deposited with tetrabutylammonium (TBA), tetrapropylammonium (TPA), and tetraethylammonium (TEA) ions are composed of a single phase where unhydrated tetraalkylammonium ions are accommodated as a monolayer between manganese oxide layers. The interlayer spacing of the products increases in an order of TEA < TPA < TBA. The film deposited with tetramethylammonium (TMA) is a mixture of two phases relating to hydrated and unhydrated guest cations, the former being predominant probably as a result of less hydrophobic property of TMA compared to that of other tetraalkylammonium ions. The TBA(+)-intercalated Mn oxide film-coated electrode exhibits a good charge/discharge property in a KCl solution between 0 and +0.8 V. In this case, TBA(+) ions between the Mn oxide layers are rapidly replaced with K(+) in solution by ion exchange, accompanying a shrinkage of the interlayer. The incorporated K(+) ions as well as protons play an important role in the electrochemical conversion between Mn(4+) and Mn(3+) in the oxide layer. In the TBACl solution, the interlayer TBA(+) ions can be excluded electrochemically during the positive-going scan, concomitant with the oxidation of Mn(3+) sites. This causes an anodic current and the accompanying shrinkage of the interlayer. On the reverse scan, however, the compressed interlayer does not allow the incorporation of bulky TBA(+) ions from the electrolyte, with virtually no cathodic current observed. Such an obvious difference in electrochemical behavior between the two electrolytes can be recognized by considering that most of the Mn oxide surface is present inside the layered structure, not on the external surface. This indicates that the layered structure is formed over the entire film.     

Electrically generated lead(IV) acetate and manganese(III) acetate as reagents for coulometric redox titrations in acetic acid

Summary The conditions were investigated for electrochemical generation of lead(IV) acetate in acetic acid by oxidation of lead(II) acetate on a lead dioxide electrode and on a platinum electrode. Bivalent manganese ions are quantitatively oxidized on a platinum electrode to the tervalent state in the same solvent. Coulometric titration methods for the determination of small amounts of hydroquinone in acetic acid were developed on the basis of the electrically generated oxidizing agents. Better results were obtained in titration with electrogenerated manganese(III) acetate. The titration end-point was detected by the biamperometric method. Suitable redox indicators were found.

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Zusammenfassung Die Bedingungen zur elektrochemischen Pb(IV)-Acetatherstellung aus Pb(II)-Acetat in Eisessig durch Oxydation an der Blei(IV)-oxid-Elektrode und an der Pt-Anode wurden untersucht. Auch Mn(II)-Ionen wurden in diesem Lösungsmittel quantitativ zu Mn(III) an der Pt-Elektrode oxydiert. Mit diesen auf elektrochemischem Wege erhaltenen Oxydationsmitteln wurden coulometrische Titrationsmethoden für die Bestimmung kleiner Hydrochinonmengen ausgearbeitet. Bessere Resultate wurden mit Mn(III)-Acetat erhalten. Die Endpunktbestimmung wurde durch biamperometrische Anzeige oder mit Hilfe visueller Redoxindikatoren durchgeführt.

     

Electrochemical study of heteronuclear complex [Cp(CO)2MnCu(μ-C=CHPh)(μ-Cl)]2

The electrochemical behavior of the heteronuclear dimeric vinylidene complex [Cp(CO)₂MnCu(μ-C=CHPh)(μ-Cl)]₂ (I) is studied using polarography, cyclic voltammetry, and controlled-potential electrolysis on mercury and platinum electrodes in acetonitrile. The dimer is reduced in two two-electron stages: at the first stage, metal-halogen bonds are broken and binuclear monomers [Cp(CO)₂MnCu(μ-C=CHPh)(Cl)]^? are formed; at the second stage, mononuclear manganese complexes Cp(CO)₂Mn=C=CHPh, metallic copper, and chloride ions are formed. The two-electron oxidation of dimer [Cp(CO)₂MnCu(μ-C=CHPh)(μ-Cl)]₂ also results in the formation of binuclear monomers, which decompose to complex Cp(CO)₂Mn=C=CHPh, Cu²⁺ ions, and Cl^? ions.     

Etude du comportement électrochimique du manganèse dans le tampon borate

The anodic behavior of the manganese electrode in borate buffer solutions have been studied by potentiostatic and galvanostatic electrochemical techniques. Three distinct steps determined on the charging curves are attributed to the formation of Mn (OH)₂, MnOOH and MnO₂. The manganese dioxide formed anodically in pH 11 borate solution may be reduced cathodically with high faradaic efficiency.     

Electric field-directed assembly of gold and platinum nanowires from an electrolysis process

Gold and platinum nanowires have been grown on a silicon dioxide surface between two microfabricated electrodes from an electrolysis process under an AC signal with a DC offset. In the process, the anode electrode acted as a sacrificing layer and oxidized to metal ions. The metal ions were reduced to particles and aligned along the electric field forming nanowires connected with the cathode. Greater than 10 V and 12 V DC offset were required for growing gold and platinum nanowires, respectively. Other factors affecting the growth of the nanowires include frequency of applied bias and gap between the electrodes.     

Initial Stages of Copper Electrocrystallization on a Glassy-Carbon Ring–Disk Electrode from Sulfate Electrolytes of Various Acidity: Potentiostatic Current Transients

Initial stages of copper electrocrystallization on glassy carbon from sulfuric acid electrolytes of pH 0.3 and 3.7 are studied by the method of potentiostatic current transients on rotating and stationary ring–disk electrode. The rate of copper deposition in a 0.5 M Na₂SO₄ + 0.01 CuSO₄ (pH 3.7) solution is marginally higher than in a 0.5 M H₂SO₄ + 0.01 CuSO₄ acid electrolyte (pH 0.3) at the expense of adsorption of sulfate and hydroxide ions on the substrate surface and the copper crystals. Regularities governing the multistage discharge of copper ions, the formation of the new phase nuclei, and the deposit dissolution are analyzed. The results of the study are compared with the data on the kinetics of copper electrocrystallization on a platinum electrode. The acceleration of the copper deposition on glassy carbon in the acid solution of pH 0.3, as compared with platinum, is due to accelerated discharge of copper ions and increased number of univalent copper ions in the near-electrode layer of solution. The oxygen-containing surface groups of glassy carbon (quinone–hydroquinone, carbonyl, etc.) are probably active centers for the discharge of copper ions and three-dimensional nucleation.     

Cathodic stripping voltammetry of trace Mn(II) at carbon film electrodes

A sensitive voltammetric method is presented for the determination of tract levels of Mn (II) using carbon film electrodes fabricated from carbon resistors of 2 Ω. Determination of manganese was made by square wave cathodic stripping voltammetry (CSV), with deposition of manganese as manganese dioxide. Chronoamperometric experiments were made to study MnO₂ nucleation and growth. As a result, it was found to be necessary to perform electrode conditioning at a more positive potential to initiate MnO₂ nucleation. Under optimised conditions the detection limit obtained was 4 nM and the relative standard deviation for eight measurements of 0.22 nM was 5.3%. Interferences from various metal ions on the response CSV of Mn(II) were investigated, namely Cd(II), Ni(II), Cu(II), Cr(VI), Pb(II), Zn(II) and Fe(II). Application to environmental samples was demonstrated.     

Immobilization of methylviologen between well-ordered multilayers of manganese oxide during their electrochemical assembly

Methylviologen dications (MV2+) were immobilized between layers of manganese oxide during their electrochemical assembly by an anodic route in a homogeneous aqueous Mn2+ solution. This approach yielded a well-ordered multilayer film on a platinum substrate as a result of dense packing of planar MV2+ molecules to stabilize the layered framework. A grazing angle in-plane X-ray diffraction study revealed that the manganese oxide sheets and the molecular planes of inserted MV2+ ions are oriented parallel to the electrode surface. Cyclic voltammetry of the product film indicated an electron transfer from the underlying Pt substrate to inserted methylviologen ions through the manganese oxide sheets.     

Electrodeposition of manganese and molybdenum mixed oxide thin films and their charge storage properties

Manganese and molybdenum mixed oxides in a thin film form were deposited anodically on a platinum substrate by cycling the electrode potential between 0 and +1.0 V vs Ag/AgCl in aqueous manganese(II) solutions containing molybdate anion (MoO(4)2-). A possible mechanism for the film formation is as follows. First, electrooxidation of Mn2+ ions with H2O yields Mn oxide and protons. Then, the protons being accumulated near the electrode surface react with MoO(4)2- to form polyoxomolybdate through a dehydrated condensation reaction (by protonation and dehydration). The condensed product coprecipitates with the Mn oxide. Cyclic voltammetry of the Mn/Mo oxide film-coated electrode in aqueous 0.5 M Na2SO4 solution exhibited a pseudocapacitive behavior with higher capacitance and better rate capability than that of the pure Mn oxide prepared similarly, most likely as a result of an increase in electrical conductivity of the film. Electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy clearly demonstrated that the observed pseudocapacitive behavior results from reversible extraction/insertion of hydrated protons to balance the charge upon oxidation/reduction of Mn3+/Mn4+ in the film.     

Electrochemical synthesis of self-doped polyaniline and its use to the electrooxidation of ascorbic acid

Self-doped polyaniline (PAN) film on platinum electrode surface has been synthesized via electrochemical copolymerization of aniline with orthanilic acid (OAA). Fourier transform infrared, UV–Vis, and elemental analysis indicate the formation of the copolymer and that the copolymer has the structure of a head-to-tail coupling of aniline and OAA units. It was found that the internal doping of PAN with OAA can extend the electroactivity of PAN in neutral and even in alkaline media. The obtained self-doped PAN (PAN-OAA)-coated platinum electrode is shown to be a good surface for the electrooxidation of ascorbic acid (AA) in phosphate buffer solution of pH 7. The anode peak potential of AA shifts from 0.63 V at bare platinum electrode to 0.34 V at the PAN-OAA-modified platinum electrode with greatly enhanced current response. A linear calibration graph is obtained over the AA concentration range of 5–60 mM using cyclic voltammetry. Rotating disk electrode voltammetry and chronoamperometry have been employed to investigate the electrooxidation of AA. The PAN-OAA-modified platinum electrode shows good stability and reproducibility.     

Nanorods of cryptomelane via soft chemistry method and their catalytic activity

α-MnO₂ nanorods were obtained by a fast redox transformation of aqueous solution of potassium permanganate. The formation mechanism of 1D nanocrystals proceeds via a first pH- and temperature sensitive stage followed by cation/anion control of the nanorod growth. A high surface area and nanostructuring allowed to achieve superb catalytic activity in a CO oxidation process compared to a conventionally prepared manganese dioxide.     

Initial stages of copper electrocrystallization on polycrystalline platinum and glassy carbon in the presence of acetonitrile

The kinetics of formation of copper adlayer, three-dimensional nucleation, and the deposit growth on polycrystalline platinum and glassy carbon in the 0.5 M H₂SO₄ + 10 mM CuSO₄ + (0–2) M acetonitrile (AcN) solutions at the cathodic overvoltages is studied using the methods of cyclic voltammetry and potentiostatic current transients on a ring-disc electrode. At [AcN] = 2 M, the process of formation of copper adatoms on platinum is significantly retarded. In the solutions with high contents of AcN, the processes of Cu⁺ ion production, the formation of their complexes with acetonitrile, hydrogen evolution, copper nucleation, and the deposit growth proceed in parallel. The contribution of any process to the overall current depends on the amount of adsorbed AcN at the surface of substrate and copper deposit and on the electrode potential. At [AcN] = 2 M, an increase in the cathodic overvoltage to 0.32 V leads to an abnormal increase in the current of Cu⁺ ion production on platinum, which is caused by insufficiently rapid formation of copper atoms in the reduction of Cu⁺(AcN) _x complexes.     

Effects of applied anodic potential and pH on the repassivation kinetics of pure aluminium in aqueous alkaline solution

The repassivation kinetics of pure aluminium have been explored in aqueous alkaline solutions as functions of applied anodic potential and pH by using an abrading electrode technique and a rotating disc electrode. The repassivation rate of the abraded bare surface of pure aluminium increased with increasing applied anodic potential in aqueous alkaline solutions, while it decreased with increasing pH. These results revealed that the growth rate of the passivating oxide film is enhanced by an applied electric field, but it is lowered due to the chemical attack by hydroxyl ions. A potentiostatic anodic current decay transient obtained from the abraded electrode surface showed a constant repassivation rate in neutral and weakly alkaline solutions. In contrast, in concentrated alkaline solutions it was observed to consist of three stages: a high repassivation rate in the initial stage due to a high formation rate of the oxide film on the abraded bare surface; a zero value of the repassivation rate in the second stage due to the dissolution of the oxide film by the attack of OH⁻; a high repassivation rate in the third stage due to a lowered dissolution rate of the oxide film. The dissolution rate of the passivating oxide film was observed to depend on the removal rate of aluminate ions from the oxide/solution interface. Received: 1 April 1998 / Accepted: 3 July 1998     

阳极电沉积Ti/MnO2电极及其苯酚降解的电催化性能

应用阳极电沉积法制备钛基二氧化锰(Ti/MnO2)电极. 采用X射线衍射(XRD)、扫描电子显微镜(SEM)及能谱（EDS）表征电极结构形貌. 用V~logi及苯酚降解实验优化电极制备工艺参数：在电流密度4 mA•cm-2、Mn2+离子浓度0.5 mol•L-1及温度70 ℃条件下阳极电沉积Ti/MnO2电极. 以苯酚为目标有机物,评价该电极电催化氧化性能,降解7h降解效率达49.6%.     

A radiochemical study of the kinetics of exchange between manganese oxides and some cations in solution

The kinetics of the exchange between⁵⁶Mn-labelled manganese dioxide and cations in aqueous solution was studied by measuring the β⁻ activity acquired by the solution. The results of the exchange between a chemical γ MnO₂ and a divalent M²⁺ ion (M=Mn, Co, Cu or Zn) or a trivalent M³⁺ ion (M=Ga, Fe, In, Rh or Al) indicate a fast initial process followed by a diffusion—controlled exchange. It is assumed that M²⁺ ions exchange with Mn²⁺ ions and M³⁺ ions exchange with Mn³⁺ ions in MnO₂. The process depends on the radii of the host and substituent ions and on consideration of crystal field stabilisation energies. It seems that the γ MnO₂ studied contains more Mn³⁺ than Mn²⁺ ions. The possibility of the exchange between Mn ions and cations of a different charge cannot be ruled out. The exchange between Co²⁺ ions and MnO₂ was enhanced in presence of pyrophosphate, which stabilises Mn(III) as a complex. The fraction of Mn in different samples of MnO₂ exchanged with a given cation depends on the type and not on the surface area of the sample.     

Electrochemical performances of Li–Mn spinel synthesized from nanosized chemical manganese dioxide

Li–Mn spinel (LiMn₂O₄) is prepared from several chemical manganese dioxides (CMDs) and the electrochemical performances are compared. The CMDs are synthesized via a redox process involving the simultaneous reduction and oxidation reaction of manganese ions in starting materials to converge to MnO₂. KMnO₄ is used as an oxidizing agent and various manganese(II) salts as reducing agents in the redox process. The crystalline phase, particle size and shape, and specific surface area (SSA) of the CMDs and the Li–Mn spinels are examined. The influence of the starting manganese salts on the above properties is also investigated. The CMDs form as aggregates of nanosized particles with different crystalline phase according to the starting manganese salts. Pure Li–Mn spinel phase is obtained from the CMDs irrespective of the different crystalline phases. The charge and discharge profiles of the LiMn₂O₄ are measured and compared.     

Potential dependence of anodic current transients on the renewed gold surface in thallium-containing thiosulfate solutions

The effect of potential on the anodic current transient times τ_p, which were measured on the gold electrode surface renewed by cutting off a thin surface metal layer immediately in the thallium-containing thiosulfate solutions of compositions (M): 0.05 Na₂S₂O₃, 10^?5 to 10^?4 TlNO₃, and 0.25 K₂SO₄, is studied. It is shown that the logarithm of inverse transient times 1/τ_p linearly depends on the electrode potential. Using the microgravimetrical method, it is found that, in the studied potential range, the amount of chemisorbed thallium ions only slightly depends on the potential, and at E = 0.3 V, it is approximately 0.12 μg/cm². It is evidenced that the transients reaches a plateau, when an equilibrium surface concentration of catalyst is reached. The value 1/τ_p reflects the rate of electrochemical oxidation of preliminarily adsorbed thallium(I) ions with the formation of catalytically active thallium(III) ions, and the first electron transfer is the limiting stage of the process.     

Mechanism of Electrodeposition of Lead Dioxide from Nitrate Solutions

The lead dioxide electrodeposition (LDE) from nitrate electrolytes is studied by the rotating disk electrode, cyclic voltammetry, and electrode impedance methods. A four-stage kinetic scheme of the LDE reaction is proposed: (1) the electron transfer and generation of oxygen-containing species on the electrode surface; (2) the species interact with lead ions, forming an oxygen-containing intermediate product of Pb(III) of the type Pb(OH)²⁺, which is not fixed on the electrode surface; (3) the product is oxidized, with the transfer of the second electron, forming compounds of Pb(IV) associated with oxygen (of the type Pb(OH)²⁺ ₂); and (4) the latter decomposes via a chemical mechanism to form PbO₂. The rate-determining stage of the LDE process is essentially dependent on the potential and state of the electrode surface, concentration of Pb(II) ions in solution, and hydrodynamic conditions of experiment. The proposed scheme explains within a unified mechanism of the LDE reaction all the experimental data obtained both in this work and in the literature.