Effect of silver catalyst on the activity and mechanism of a gas diffusion type oxygen cathode for chlor-alkali electrolysis

Application of a gas-diffusion type oxygen cathode will contribute to energy saving in chlor-alkali electrolysis. For this purpose the development of gas-diffusion electrodes with high performance and durability is essential. We have investigated the performance for oxygen reduction and the mechanism of its on gas-diffusion electrodes with and without Ag catalyst in order to develop such oxygen cathodes with high performance and durability. It has been found that an electrode with no catalyst, that is, carbon support only in the reaction layer, shows electrochemical activity for oxygen reduction in 32 wt % NaOH at 80 °C and 1 atm O₂, but loading of 2 mg cm^–2 Ag of particle size 300 nm, not only improves the activity by about 100 mV but promotes the four-electron reduction to produce OH^–, while H₂O₂ is the predominant reaction intermediate in the absence of the Ag catalyst. The production of H₂O₂ has been demonstrated by conducting CV measurements to detect H₂O₂ in the anodic scan after a cathodic sweep up to 0.3 V vs RHE. It has been shown that the gas-diffusion type oxygen cathode with Ag catalyst has the high performance and durability necessary for chlor-alkali electrolysis.     

High Diffusion Permeability of Anion-Exchange Membranes for Ammonium Chloride: Experiment and Modeling

It is known that ammonium has a higher permeability through anion exchange and bipolar membranes compared to K⁺ cation that has the same mobility in water. However, the mechanism of this high permeability is not clear enough. In this study, we develop a mathematical model based on the Nernst–Planck and Poisson’s equations for the diffusion of ammonium chloride through an anion-exchange membrane; proton-exchange reactions between ammonium, water and ammonia are taken into account. It is assumed that ammonium, chloride and OH⁻ ions can only pass through membrane hydrophilic pores, while ammonia can also dissolve in membrane matrix fragments not containing water and diffuse through these fragments. It is found that due to the Donnan exclusion of H⁺ ions as coions, the pH in the membrane internal solution increases when approaching the membrane side facing distilled water. Consequently, there is a change in the principal nitrogen-atom carrier in the membrane: in the part close to the side facing the feed NH₄Cl solution (pH < 8.8), it is the NH₄⁺ cation, and in the part close to distilled water, NH₃ molecules. The concentration of NH₄⁺ reaches almost zero at a point close to the middle of the membrane cross-section, which approximately halves the effective thickness of the diffusion layer for the transport of this ion. When NH₃ takes over the nitrogen transport, it only needs to pass through the other half of the membrane. Leaving the membrane, it captures an H+ ion from water, and the released OH⁻ moves towards the membrane side facing the feed solution to meet the NH₄⁺ ions. The comparison of the simulation with experiment shows a satisfactory agreement.     

Dimerization of ions in aqueous solution of lithium chloride in adiabatic conditions

Dimerized forms of salt ions: Li₂OH⁺ and Cl ₂ ^– , are contained in aqueous solution of lithium chloride, and their concentration increases with an increase in the concentration of the solution, when electron-acceptor additives are added, and when the solution is alkalized. The amount of dimerized impurities should decrease in the series of alkali metal chlorides, including HCl, from lithium to cesium.Translated from Khimicheskie Volokna, No. 6, pp. 35–37, November–December, 1994.     

Kinetics of Ion Formation in the Volumetric Reaction of Methane with Air

The formation of ions in the volumetric reaction of methane and hydrogen with air is analyzed. In methane–air mixtures, the highest concentrations are observed for NO⁺, NO₃ ^–, CO₃ ^–, CO₄ ^–, OH^–, and NO₂ ^– ions. In hydrogen–air mixtures, maximum concentrations are characteristic for NO⁺, OH^–, H₃O⁺, O^–, and O₂ ^– ions. In both rich and lean mixtures after ignition there is a long time interval during which the ion concentrations are far from equilibrium. The duration of this interval and the ion concentrations in it depend on the initial parameters of the mixture and the air–fuel ratio. Key words: volumetric reaction of methane with air, ion-molecular reactions, kinetics, ions.     

Study of iodide sorption to the argillite of Tournemire in alkaline media

Argillaceous rocks are considered potential host rocks for radioactive waste repositories. The concrete matrix that could be used as a barrier could react with the groundwater of the geological site, inducing a drastic change in its chemical composition and its pH (10–13). Consequently, the physicochemical properties of the rock in contact with this alkaline solution may be modified and, in turn, may induce modification on the behaviour of radioelements. This study, applied to the argillite of Tournemire, involves characterizing I⁻ sorption to an argillaceous rock in alkaline media in batch experiments under N₂-controlled conditions. I⁻ was added as a ¹²⁵I radiotracer and measured by γ spectrometry.Preliminary experiments were conducted with different solution/solid ratios (v/m=2.5, 5 and 20 ml g⁻¹) and contact times (1–14 days) in order to determine the optimal experimental conditions. The chosen v/m ratio was 5 ml g⁻¹ as the best compromise between a high K_d value and a low error of the measure. The chosen experiment duration was 1 day because I⁻ sorption was highest and to limit the effects of pyrite oxidation. One of the experiments, performed with a radio-sterilized sample to test possible effects from microorganisms, showed that they could enhance iodide retention, particularly during the first 2 contact days.The influence of pH on I⁻ sorption was tested using solutions between values of 8.3 and 12.8. The K_d values were independent of pH and very low (0.3 ml g⁻¹).Finally, the influence of the chemical composition of concrete fluids was also tested. Three solution compositions corresponding to different steps in the evolution of fluids in contact with altering concrete were used: fluid in contact with fresh concrete (pH 13.2), with moderately degraded concrete (pH 12.1) and with strongly degraded concrete (pH 11.5). Each solution contained variable amounts of sodium, potassium, calcium, silica and sulphate. I⁻ sorption was also very low (K_d0.2 ml g⁻¹). Additional experiments were conducted with alkaline solutions containing different amounts of SO₄²⁻ ions (10⁻³–10⁻² M) to test sulphate–iodide sorption competition. I⁻ retention was independent of the sulphate concentration.     

Effect of silicate on the membrane-type chlor-alkali cell

The effect of silicate in the electrolyte solution in a chlor-alkali cell on a fluorocarbon ionomer, Nafion^® 901, was studied. Calcium silicate precipitated and damaged the membrane during electrolysis when the anolyte and/or the catholyte was contaminated with Ca²⁺ and Mg²⁺. The service lifetime of the membrane was thus shortened greatly.     

Stability of anodic tantalum oxide films in NaOH solutions

The dissolution behaviour of the anodic oxide films formed on tantalum was investigated in NaOH solutions of different concentrations. In solutions of [OH^–]<3.0 M, no pronounced dissolution could be detected. However, in concentrated alkali solutions, [OH]>3 M, the dissolution occurred to different extents depending on the alkali concentration. As observed from capacitance and potential measurements, the barrier film on Ta was found to be defective owing to the incorporation of OH^– ions in the film. The nature of the barrier film and its dependence on formation rate, alkali concentration and solution temperature were investigated. Complex plane analysis illustrates the behaviour of the barrier film of different thickness, i.e. formation voltages, in NaOH solutions. The barrier film cannot be considered accurately as a perfect dielectric.     

Chromium removal using a porous carbon felt cathode

Removal of hexavalent chromium by an electrochemical process using a carbon felt electrode in a flow-through electrochemical cell was studied. Optimum operating conditions, such as pH and current density were investigated. It was found that at pH 3.5, current density 400 A m^–2 and flow velocity of 1.11 cm s^–1, the total chromium and Cr(VI) concentrations can be reduced to less than 5 mg l^–1 within 230 min operation for 5 l of solution with an initial concentration of 50 mg l^–1. It was found that the solution pH is a critical factor for the chromium removal process. To achieve high chromium removal efficiency, pH and current density must be carefully controlled.     

Determination of the hydroxyl content in gels and porous “glasses” from alkoxide hydrolysis by combined TGA and BET analysis

The hydroxyl group and water contents of alumina, mullite and alumino(boro)silicate gels and porous glasses (xerogels) are discussed on the basis of thermogravimetric, gas adsorption and density measurements. Comparison is made for powder prepared by instant hydrolysis and optically clear monoliths by slow hydrolysis, heated in various atmospheres (air, vacuum, hydrogen). The hydroxyl content appears to depend on the Al/Si ratio: 6 OH^–/nm² for alumina mesoporous glasses, 2 OH^–/nm² for aluminosilicate mesoporous glasses and 3 OH^–/nm² for microporous ones, typically.     

Detection of hydrogen profiles through anodic films on aluminium by secondary ion mass spectrometry

A Cameca Ion Analyser has been used to examine barrier-type anodic films grown on aluminium in ammonium tartrate solution, pH 2.2–10.6, at 293 K. Films formed in alkaline solution are thicker than their formation voltages would suggest, with an inner compact region and an outer, probably porous or heavily hydrated region. The degree of film hydration increases with pH, particularly in the acidic neutral range. The outer regions of films are more hydrated than the inner, particularly for formation in the neutral to alkaline range. The results are discussed in terms of current theories of anodic film formation involving the lattice transport of OH⁻ ions and protons, or dissolution-precipitation phenomena, and are shown to be consistent with the theories but unable to differentiate between them.     

Electrodeposition of nickel-cobalt-zinc alloys from a borate bath

The electrodeposition has been studied of nickel-cobalt-zinc alloys from a borate bath containing nickel sulphate (120–140 g dm^–3), cobalt sulphate (30–46 g dm^–3), zinc sulphate (144–168 g dm^–3), boric acid (30 g dm^–3) and ammonium chloride (2 g dm^–3). The operating conditions were: current density, 2.0–5.0 A dm^–2; temperature, 30–40°C and pH, 2.4 to 5.4. Light grey, semibright, stressed films have been obtained. However, the deposits consist partially of black powder when the concentration of the various components is increased. The brightness is found to increase with decreasing temperature and pH of the solution. The total cathode efficiency increases when the pH and temperature of the solution decrease, whereas at any particular pH and temperature it first decreases, reaches a minimum and then increases with increasing current density.     

Influence of M/M ratio in surface-charging behavior of Zn–Al layered double hydroxides

Zn–Al layered double hydroxides with different M^II/M^III ratios were synthesized by the coprecipitation method and the obtained solids were characterized by powder X-ray diffraction, infrared spectroscopy, acid–base potentiometric titrations, electrophoretic mobility and modeling of the electrical double layer. As general behavior, increasing hydroxyl adsorption with increasing pH and decreasing supporting electrolyte concentration takes place between pH 7 and 12. The electrophoretic mobility of the samples are in all cases positive and it decreases when the pH becomes higher than 9. A net positive charge of the particles, with a point of zero charge (PZC) around 11.8, is deduced from electrophoretic data. A modified MUSIC model, where OH⁻/Cl⁻ exchange at structural sites and proton desorption from surface hydroxyl groups are taken into account, is used to estimate the deprotonation constants, the number of surface groups, the number of sites with structural charges an the affinity constants for Cl⁻ and OH⁻ of the samples with different M^II/M^III ratios. The modeling of potentiometric titrations and electrophoresis data indicates that anions between the layers and negatively charged groups in the particle surface balance most of the structural charge at the particles surface. It shows an important variation of the number of structural sites with the M^II/M^III ratio and an important role of surface protonation–deprotonation sites to the electrical behavior and exchange properties of LDHs.     

An electrochemical investigation of the suppression of silver dissolution in aqueous cyanide by 2-mercaptobenzothiazole

Triangular potential sweep voltammetry, potentiokinetic generation of polarization curves, and coupon corrosion tests have been carried out to determine the influence of 2-mercaptobenzothiazole (MBT) on the dissolution of silver in cyanide solutions at pH 11. MBT has been shown to be an effective inhibitor for silver dissolution at concentrations similar to those used when MBT is applied as a flotation collector. The inhibition efficiency (i.e., [1 – the ratio of the corrosion rates in the presence and absence of MBT], expressed as a percentage) in 10^–2 and 10^–3 mol dm^–3 CN was found to increase with increase in MBT concentration in the range 10^–6 to 10^–4 mol dm^–3, and with increase in time of exposure of the silver to the MBT solution. The inhibition efficiency found for 10^–4 mol dm^–3 MBT in quiescent 10^–2 mol dm^–3 CN^– solution at 23 °C was 98.9%, 99.4% and 99.99% for exposure times of 10 min, 2 h and 5 days, respectively. Surface enhanced Raman spectroscopy showed that inhibition was associated with adsorption of MBT displacing cyanide from the silver surface.     

Physiological activity of water beetle defensive agents. II. Absorption of selected anesthetic steroids and norsesquiterpenes across gill membranes of the minnowPimephales promelas Raf.

The uptake of selected steroids and norsesquiterpenes by live minnows,Pimephales promelas, was studied when the compounds were administered externally in aqueous solution. The gills of minnows absorbed 80% of the steroid removed from solution. Steroid absorption across minnow gills is apparently a nonmediated process as rate saturation could not be demonstrated. Initial absorption rates of test compounds were inversely correlated with (1) the degree of oxygenation, (2) water solubility, and (3) polarity on silica gel thin-layer chromatography. These findings support the Stein model of non-mediated transport. The majority of compounds anesthetic and toxic to minnows exerted their effects at a similar internal concentration. Anesthesia occurred at ca. 10^–6–10^–5 M and death at 10^–4–10^–3 M. Since various agents causing membrane stabilization and lysis in in vitro systems act in an identical concentration range, it is proposed that the defensive steroids and norsesquiterpenes of water beetles act via membrane stabilization and lysis.     

A spectroelectrochemical investigation of the influence of sodium diisobutyldithiophosphinate on silver dissolution in aqueous cyanide

Polarization studies have been carried out to determine the influence of diisobutyldithiophosphinate (DIBDTPI) on the dissolution of silver in cyanide solutions at pH 11. DIBDTPI was found to inhibit dissolution at concentrations similar to those used when this compound is applied as a flotation collector. The inhibition efficiency in 10^–2 mol dm^–3 CN^– was found to increase with increase in DIBDTPI concentration in the range 10^–6–10^–4 mol dm^–3, and with increase in time of exposure of the silver to the DIBDTPI solution. The inhibition efficiency found for 10^–4 mol dm^–3 DIBDTPI in quiescent 10^–2 mol dm^–3 CN^– solution at 23 °C was 64.6% and 95.0% for exposure times of 10 min and 2 h, respectively. These values are significantly less than those found previously for 2-mercaptobenzothiazole under the same conditions. Surface enhanced Raman spectroscopy showed that inhibition was associated with adsorption of DIBDTPI displacing cyanide from the silver surface. Voltammetry at 0.5 mV s^–1 indicated that adsorption of DIBDTPI involves charge transfer.     

Anodic oxidation of copper cyanide on graphite anodes in alkaline solution

The anodic oxidation of copper cyanide has been studied using a graphite rotating disc with reference to cyanide concentration (0.05–4.00 M), CN:Cu mole ratio (3–12), temperature (25–60 °C) and hydroxide concentration (0.01–0.25 M). Copper had a significant catalytic effect on cyanide oxidation. In the low polarization region (about 0.4 V vs SCE or less), cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate. At a CN:Cu ratio of 3 and [OH^–] = 0.25 M, the Tafel slope was about 0.12 V decade^–1. Cu(CN)₃ ^2– was discharged on the electrode and the reaction order with respect to the predicted concentration of Cu(CN)₃ ^2– is one. With increasing CN:Cu mole ratio and decreasing pH, the dominant discharged species shifted to Cu(CN)₄ ^3–. Under these conditions, two Tafel slopes were observed with the first one being 0.060 V decade^–1 and the second one 0.17–0.20 V decade^–1. In the high polarization region (about 0.4 V vs SCE or more), cuprous cyanide complexes were oxidized to copper oxide and cyanate. Possible reaction mechanism was discussed.     

Exploration of molten hydroxide electrochemistry for thermal battery applications

The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag⁺/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH^– = H₂O + O^2– where H₂O is the Lux-Flood acid (oxide ion acceptor) and O^2– is the Lux–Flood base. This reaction dictates the minimum H₂O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li₂O, Na₂O or K₂O. Thermodynamic calculations suggest that Li₂O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li₂O_(s) + 2 OH^– = 2 LiO^– + H₂O whereas Na₂O acts as a Lux–Flood base, Na₂O_(s) = 2 Na⁺ + O^2–. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH^– to yield oxygen, that is 2 OH^– 1/2 O₂ + H₂O + 2 e^–. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H₂O] [O^2–]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH^– to hydrogen and O^2– is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.     

Reduction Characteristics of Iodate Ion on Copper: Application to Copper Chemical Mechanical Polishing

Potentiodynamic and potentiostatic polarization, and the rotating disk electrode technique were used to study the reduction characteristics of iodate (IO₃ ^–) ion on copper (Cu). Depending on the relative concentrations of IO₃ ^– and H⁺ two pH regimes were observed. The cathodic current in the first regime (pH > ,3) was controlled by H⁺ diffusion from the solution to the metal surface. In the second regime (pH > 3 and up to 10^–2 m IO₃ ^– concentration) the cathodic current was found to be under mixed control, involving reaction control via the electrochemical reduction of IO₃ ^– and transport control via the diffusion of I₂(aq). It was concluded that IO₃ ^– was an effective oxidant for Cu chemical mechanical polishing (CMP) with strongly acidic (pH < 3) slurries but it was not convenient reagent as an oxidant for Cu CMP with weakly acidic (pH > 3) slurries.     

Chlorine evolution in a centrifugal field

Intensification of the electrochemical production of chlorine and sodium hydroxide (i.e., the chlor-alkali process) was demonstrated in a centrifugal acceleration field. This intensification was shown by comparing the cell voltages and the anode potentials for a cell operated with and without a centrifugal field applied using the linear sweep voltammetric, galvanostatic and potentiostatic polarizations. Under industrial chlor-alkali electrolysis conditions, a cell voltage reduction of up to 600 mV and an anode potential reduction of up to 360 mV at 600 mA cm^–2 were achieved by using a relative acceleration rate of 190 g with an acidic saturated NaCl solution at 80 °C. The relationships between the cell performance and relative acceleration rate, for different anode materials, temperature and NaCl concentration are reported.