The kinetics of the electrochemical reduction of perchlorate ions on rhodium

The reduction of ClO₄⁻ ions in acid media was studied at smooth polycrystalline rhodium electrodes by voltammetry, chronoamperometry and impedance spectroscopy. On the basis of the experimental results, a new mechanism for the electrocatalytic reduction of ClO₄⁻ ions has been proposed. According to this reaction scheme, the reduction process is starting with the adsorption of perchlorate ions at free active sites on the metal surface. It is also assumed that the whole process starting from perchlorate ion occurs on the surface and the role of desorption of intermediates can be neglected. On the basis of the kinetic model an expression has been derived for the impedance of the electrode, which fits the experimental data quite well.     

Electrochemical behavior of activated-carbon capacitor material loaded with nickel oxide

In order to enhance specific capacitance and energy density of carbon-based supercapacitor, some nanometer-scale amorphous particles of nickel oxide were loaded into activated-carbon by suspending the activated-carbon in a Ni(NO₃)₂ solution followed by neutralization. A hybrid type electrochemical capacitor was made and tested, in which the activated-carbon loaded with nickel oxide was used as cathode material and activated-carbon was used as anode material. Although the BET surface area of the activated-carbon decreased upon nickel oxide loading compared to that of the starting material, its specific capacitance increased 10.84%, from 175.40 to 194.01 F g⁻¹ and the potential of oxygen evolution on the composite material electrode was 0.076 V higher than that of the pure activated-carbon electrode, in the electrolyte of 6 mol/L KOH solution, so the hybrid capacitor had larger energy density. Similar to the pure activated-carbon electrode, no obvious change appears on the specific capacitances of the composite material electrode at various discharge currents and the composite material electrode exhibiting good power characteristics.     

Electrochemical reduction of nickel ions from dilute solutions

Electrochemical reduction of nickel ions in dilute solution using a divided GBC-cell is of interest for purification of waste waters. A typical solution to be treated is the effluent from steel etching processes which contain low quantities of nickel, chromate and chromium ions. Reduction of chromate in a divided GBC-cell results in effluent containing Cr³⁺ ions. This paper examines the limitations to further treatment of the effluent by electrochemical reduction of nickel ions. Nickel current efficiency has been determined by varying temperature, pH, current density and concentration of supporting electrolyte. It was found that the presence of a complexing agent, e.g. NH₄ ⁺ is necessary. To deposit nickel, complete removal of Cr³⁺ ions from the solution is inevitable. Optimum nickel current efficiencies are obtained at pH 5 and current density 10 mA cm^–2.     

Exceptional ion-exchange selectivity for perchlorate based on polyaniline films

Polyaniline (PANI) films were electrochemically synthesized on graphite electrode and their ion-exchange selectivities for ClO₄⁻ were investigated. Cyclic voltammetry (CV) results showed that PANI-H₂SO₄ film represented higher selectivity for ClO₄⁻ than PANI–HCl and PANI–HClO₄. Furthermore, the CV records of PANI–H₂SO₄ film in different acidic solutions demonstrated that ClO₄⁻ exchange was performed in a lower potential range (from −0.025 V to 0.097 V), which is distinguished from SO₄²⁻ and Cl⁻. Additionally, chronoamperomertry (CA) studies of PANI–H₂SO₄ film in 0.1 M various acidic solutions (H₂SO₄, HCl and HClO₄) confirmed that the diffusion coefficient values in HClO₄ (13.319/18.184 × 10⁻⁶ cm² s⁻¹) were the biggest during doping/de-doping process. The exceptional ion-exchange selectivity for ClO₄⁻ was explained by the process of dominant accumulation, quick insertion of anions, and the sluggish mobility of ClO₄⁻ inside the membrane. Results of present work demonstrated that PANI may serve as a promising membrane for ClO₄⁻ removal by ion-exchange process.     

Study of the hypophosphite effect on the electrochemical reduction of nitrobenzene on Ni

The effect of the hypophosphite ion on the electrochemical reduction of nitrobenzene on Ni was evaluated from a cyclic voltammetric study and from constant potential electrolysis in an aqueous-ethanol alkaline medium. The results were compared with the data obtained in an hypophosphite-free solution. It was found that in an hypophosphite containing solution an unusual selective reduction of nitrobenzene to nitrosobenzene occurs. It is the first time that nitrosobenzene is detected as the reaction product of the nitrobenzene electrochemical reduction in an aqueous-ethanol solution. It is proposed that the Ni modified surface which is formed upon hypophosphite oxidation is responsible for the non reducibility of nitrosobenzene. The effect of the electrode potential on the nitrobenzene electrolysis on a Ni modified electrode was analysed. It was concluded that the highest nitrosobenzene yield (33%) and selectivity (82%) is achieved at −1.1 V. It was also found that the formation of nitrosobenzene leads to an electrode poisoning effect in the electrolysis process.     

Effect of crystallisation on the reaction kinetics of nickel reduction by hydrogen

The reaction kinetics of precipitation from aqueous solution is not only a function of the concentration of reagents but also depends on the properties of the solid particles formed in the process. These property changes arise from the increasing influence of surface properties in comparison to volumetric bulk-properties as the particle size decreases. The ratio α of the active surface area to the actual surface area of the particles in the system is used in this work to evaluate the reaction activity of the particles. The investigation of the reaction kinetics of nickel reduction without sampling during the process of the reaction was successfully carried out in the experiment. The overall kinetics of nickel reduction have been suggested, where the constants relate to the main processes of nickel precipitation from the solution. The significant agglomeration reduces the deactivation of the nickel particles in the precipitation process, while breakage and crystal growth decrease the activation. The activations of dry and wet seeds are insignificantly different.     

Electrochemical reduction of cerium oxide into metal

The Fray Farthing and Chen (FFC) and Ono and Suzuki (OS) processes were developed for the reduction of titanium oxide to titanium metal by electrolysis in high temperature molten alkali chloride salts. The possible transposition to CeO₂ reduction is considered in this study. Present work clarifies, by electro-analytical techniques, the reduction pathway leading to the metal. The reduction of CeO₂ into metal was feasible via an indirect mechanism. Electrolyses on 10 g of CeO₂ were carried out to evaluate the electrochemical process efficiency. Ca metal is electrodeposited at the cathode from CaCl₂–KCl solvent and reacts chemically with ceria to form not only metallic cerium, but also cerium oxychloride.     

Electrochemical reduction of trinitrotoluene on core-shell tin-carbon electrodes

In this work, we studied the electrochemical process of 2,4,6-trinitrotoluene (TNT) reduction on a new type of electrodes based on a core-shell tin-carbon Sn(C) structure. The Sn(C) composite was prepared from the precursor tetramethyl-tin Sn(CH₃)₄, and the product contained a core of submicron-sized tin particles uniformly enveloped with carbon shells. Cyclic voltammograms of Sn(C) electrodes in aqueous sodium chloride solutions containing TNT show three well-pronounced reduction waves in the potential range of −0.50 to −0.80 V (vs. an Ag/AgCl/Cl⁻ reference electrode) that correspond to the multistep process of TNT reduction. Electrodes containing Sn(C) particles annealed at 800 °C under argon develop higher voltammetric currents of TNT reduction (comparing to the as-prepared tin-carbon material) due to stabilization of the carbon shell. It is suggested that the reduction of TNT on core-shell tin-carbon electrodes is an electrochemically irreversible process. A partial oxidation of the TNT reduction products occurred at around −0.20 V. The electrochemical response of TNT reduction shows that it is not controlled by the diffusion of the active species to/from the electrodes but rather by interfacial charge transfer and possible adsorption phenomena. The tin-carbon electrodes demonstrate significantly stable behavior for TNT reduction in NaCl solutions and provide sufficient reproducibility with no surface fouling through prolonged voltammetric cycling. It is presumed that tin nanoparticles, which constitute the core, are electrochemically inactive towards TNT reduction, but Sn or SnO₂ formed on the electrodes during TNT reduction may participate in this reaction as catalysts or carbon-modifying agents. The nitro-groups of TNT can be reduced irreversibly (via two possible paths) by three six-electron transfers, to 2,4,6-triaminotoluene, as follows from mass-spectrometric studies. The tin-carbon electrodes described herein may serve as amperometric sensors for the detection of trace TNT.     

The occurrence of perchlorate during drinking water electrolysis using BDD anodes

Electrochemical studies were carried out to estimate the risks of perchlorate formation in drinking water disinfected by direct electrolysis. Boron Doped Diamond (BDD) anodes were used in laboratory and commercially available cells at 20 °C. The current density was changed between 50 and 500 A m⁻². For comparison, other anode materials such as platinum and mixed oxide were also tested. It was found that BDD anodes have a thousandfold higher perchlorate formation potential compared with the other electrode materials that were tested. In long-term discontinuous experiments all the chloride finally reacted to form perchlorate. The same result was obtained when probable oxychlorine intermediates (OCl⁻, ClO₂⁻, ClO₃⁻) were electrolysed in synthetic waters in the ppm range of concentrations. The tendency to form perchlorate was confirmed when the flow rate of drinking water was varied between 100 and 300 L h⁻¹ and the temperature increased to 30 °C. In a continuous flow mode of operation a higher chloride concentration in the water resulted in a lower perchlorate formation. This can be explained by reaction competition of species near and on the anode surface for experiments both with synthetic and local drinking waters. It is concluded that the use of electrodes producing highly reactive species must be more carefully controlled in hygienically and environmentally oriented applications.     

Effect of synthesis conditions on preparation of nickel metal nanopowders via hydrothermal reduction technique

Synthesis of nickel nanopowders from aqueous solution using hydrothermal reduction method with hydrazine hydrate as a reducing agent and Cetyl trimethyl ammonium bromide (CTAB) as a surfactant was investigated. Statistical design was used to study the effects of reaction time, concentration of nickel chloride, and concentration of surfactant on the nickel particles size. Formed nickel particles were characterized using XRD and SEM. The formation of nickel single phase was revealed from XRD patterns. On the other hand, SEM showed that the nickel particles are in nanosized ranges from 55 nm to 250 nm. The analysis of the results indicated that the reaction time and surfactant addition were the controlling factors. The reduction of nanocrystalline nickel hydroxide Ni(OH)₂ into Ni is the possible formation mechanism.     

Electrochemical reduction of trivalent uranium ions in molten chlorides

The electrochemical behaviour of UIII in molten salts was studied at 715 C under argon atmosphere in a molten equimolar NaCl–KCl mixture containing uranium ions in oxidation state III (UCl3). Three different complementary electrochemical methods were used, namely cyclic voltammetry, chronopotentiometry and square wave voltammetry. These techniques provide data to explain the electroreduction mechanism. The reduction of UIII in the NaCl–KCl mixture occurs in a single step with an exchange of three electrons, this process is controlled by the diffusion of UIII. The diffusion coefficient of U(III) was calculated using the three electrochemical methods.     

Electrochemical reduction of nickel ions from diluteartificial solutions in a GBC reactor

The electrochemical reduction of nickel ions from a dilute solution has been carried out in a gas diffusion electrode packed bed electrode cell (GBC). Particle size and electrode configuration have been found to have a significant influence on the reduction process. Electrodes with a high porosity and large pores have been found to be the best for nickel deposition. The nickel current efficiency, Ni, is reported to be dependent on the current density, volumetric flow rate, nickel and boric acid concentration, and the pH. The fall in the nickel current efficiency is caused by an increase in electrode surface pH above a certain level, caused by either high bulk solution pH or high current density, leading to possibly formation of Ni(OH)2. It has been found that under conditions of exclusively metallic nickel deposition Ni/(1–Ni) is proportional to i0.69,Q10.52,CNi0.67,CBA–0.19 and pH1.0.     

Reduction of nickel oxide on alpha alumina: coalescence of nickel to metallic crystallites

Reduction of alpha-alumina-supported nickel oxide was studied with hydrogen consumption, magnetization and XRD measurements. Rupture of Ni-O bonds at 270–350 ° C is much faster than nucleation of metallic nickel and precedes growth into crystallites. Water vapor and low hydrogen flows retard both processes. At higher temperatures, growth is more rapid than Ni-O bond rupture.     

The reduction of perchlorate by hydrogenation catalysts

Perchlorate competes with thyroid uptake of iodide, an essential nutrient for the production of thyroid hormones. Despite the extensive attempts to reduce perchlorate in aqueous solution, the process is slow and requires high temperatures even in the presence of catalysts. Therefore, perchlorate reduction under hydrogen atmosphere was employed. Monometallic and bimetallic Pt based catalysts (e.g., Pt/C, Ni-Pt/C, Co-Pt/C, and W-Pt/C) supported on activated carbon were prepared by successive incipient wet impregnation method and used for gas-phase reduction of perchlorate pre-adsorbed onto the activated carbon. The catalysts were characterized for hydrogen chemisorption and XPS. Hydrogen uptake was in the order: Co-Pt/C > W-Pt/C ≈ Pt/C > Ni-Pt/C. The least H₂ uptake by Ni-Pt/C was likely due to lower dispersion on activated carbon surface with 11.5% vs. 36% for Pt/C. There was a good correlation between hydrogen uptake by impregnated activated carbon (IAC) and perchlorate reduction. The Co-Pt/C exhibited the highest hydrogen uptake and the best perchlorate reduction while Ni-Pt/C was the least reducing system. When Co-Pt/C was used almost 90% of perchlorate was reduced at 25 °C and initial surface concentration of perchlorate of 11.47 mg g⁻¹ in 24 h. The reaction rate increased 10-folds when the reaction temperature was raised to 75 °C. In 24 h reaction time, increase of temperature from 25 to 75 °C resulted in additional 10% (Co-Pt/C) and 30% (Ni-Pt/C) increase in perchlorate reduction for Co-Pt/AC and Ni-Pt/AC, respectively, which brought the reduction efficiency close to 100%. The only reaction product that evolved was Cl⁻, indicating that the cleavage of the first oxygen atom of perchlorate was the rate-limiting step. The lowest activation energy for the reduction of perchlorate was 39.5 kJ mol⁻¹ for Co-Pt/C. Results also showed that the activation of gaseous hydrogen molecules on metal catalysts was the major reducing step, although deposited metals also participated in the perchlorate reduction directly. Results of XPS analysis revealed that during adsorption/reduction some portion of the second metal in the bimetallic catalysts was lost due to dissolution while Pt was very stable.     

Electrochemical reduction of some 5-substituted-4-thiohydantoins at the dme

The polarographic behaviour of a series of 5-arylidene-4-thiohydantoin derivatives is reported and discussed. The reduction proceeds via a CECE mechanism in acid media with the formation of a 5-benzyl-4-thiohydantoin and other products. The mechanism is confirmed through pK_a determinations and Hammett's σ-E_1/2 relations.     

The reduction of Tokyo and Sinter 75 nickel oxides with hydrogen

Industrial Tokyo and Sinter 75 nickel oxides were reduced with 35 and 80% volume hydrogen-argon mixtures between 350 and 1000 °C in a TGA apparatus. Several abnormalities were observed. At low temperature and in 35% H₂, an incubation period was observed. As the temperature was increased the rate of reduction increased up to some temperature, following which a decrease in the reaction rate was observed, which again was followed by an increase in the rates as the temperature exceeded 900 °C. It was found that reduction kinetics of these oxides strongly depend on the morphological features of the oxides. It became clear that the shrinking core model was not applicable and instead the kinetic parameters were assessed by the grain model.     

Electrochemical properties of benzylmercapto and dodecylmercapto tetra substituted nickel phthalocyanine complexes: Electrocatalytic oxidation of nitrite

Nickel tetrakis(benzylmercapto)phthalocyanine (NiTBMPc) and nickel tetrakis(dodecylmercapto)phthalocyanine (NiTDMPc) complexes were synthesized and their spectral and electrochemical properties reported. The CV showed four or five redox processes for NiTBMPc and NiTDMPc, respectively. For the first time, spectroelectrochemistry gave evidence for the formation of Ni^II/Ni^I process in a NiPc complex. The rest of the processes were ring based. The NiTBMPc complex was successfully deposited on both gold and glassy carbon electrodes by electropolymerisation while NiTDMPc complex was deposited on gold electrode only. The films were electro-transformed in aqueous 0.1 M NaOH solution to the O–Ni–O oxo bridged form. The modified electrodes were characterized using electrochemical impedance spectroscopy and the results showed typical behavior for modified electrodes. Electrodes with poly-Ni(OH)Pcs films exhibited higher charge transfer resistance values, R_p than their corresponding poly-NiPcs films counterparts. All the modified electrodes showed improved catalytic activities than the unmodified electrodes towards nitrite ions electrooxidation. Better catalytic activities were observed for the modified electrodes when they were transformed to O–Ni–O oxo bridge form. All the modified electrodes exhibited high resistance to electrode surface passivation.     

Liquid-phase synthesized mesoporous electrochemical supercapacitors of nickel hydroxide

Electrochemical supercapacitive (ES) properties of liquid-phase synthesized mesoporous (pore size distribution centered ∼12 nm) and of 120 m²/g surface area nickel hydroxide film electrodes onto tin-doped indium oxide substrate are discussed. The amounts of inner and outer charges are calculated to investigate the contribution of mesoporous structure on charge storage where relatively higher contribution of inner charge infers good ion diffusion into matrix of nickel hydroxide. Effect of different electrolytes, electrolyte concentrations, deposit mass and scan rates on the current-voltage profile in terms of the shape and enclosed area is investigated. Specific capacitance of ∼85 F/g at a constant current density of 0.03 A/g is obtained from the discharge curve.     

Electrochemical reduction of substituted pyridazines: a new access to activated pyrroles

The electrochemical two electron reduction of pyridazines, substituted by electron withdrawing groups, primarily lead to their corresponding 1,2-dihydro-derivatives. Depending on the nature of the ring substitution, these intermediates can either rearrange into 1,4-dihydro-pyridazines, or undergo electrochemical reduction to give rise to activated pyrroles by a ring contraction reaction with extrusion of nitrogen. Another way of access to the latter has been achieved by a disproportionation reaction of 1,2-dihydro-pyridazines, leading directly to the expected pyrroles and recovery of 50% of pyridazines.