Anodic characteristics of amorphous ternary palladium-phosphorus alloys containing ruthenium,rhodium, iridium,or platinum in a hot concentrated sodium chloride solution

Amorphous ternary palladium-based alloys containing platinum group metals as an additional element were prepared by rapid quenching from the molten state and their anodic characteristics were investigated in a 4 mol dm^?3 NaCl solution of pH 4 and 80° C. The amorphous alloys containing sufficient quantities of rhodium, platinum or iridium were passivated by anodic polarization and their corrosion rates at high current densities in the chlorine evolution region were extremely low. This fact was attributed to the formation of a highly protective passive film due to both the transformation to the amorphous structure and the addition of rhodium, platinum or indium. The electrocatalytic activities for chlorine evolution of amorphous alloys were higher than those of pure platinum group metals except palladium. In particular, the amorphous Pd₄₁Ir₄₀P₁₉ alloy had the desired stable, high electrocatalytic activity for chlorine evolution and the high overvoltage for oxygen evolution.     

Anodic characteristics of amorphorous palladium-base alloys in sodium chloride solutions

The anodic characteristics of a variety of amorphous palladium-base alloys were examined with a view to their use for the production of sodium hypochlorite by electrolysis of dilute sodium chloride solutions at ambient temperature. The corrosion resistance of palladium-metalloid alloys was obtained by alloying with platinum group metals and/or valve metals. Among these alloys, rhodium-containing alloys showed high electrocatalytic activities for chlorine evolution. Surface activation treatment was, however, necessary to obtain sufficiently high activities for chlorine evolution at low overpotentials. Surface-activated amorphous alloys possessed considerably higher current efficiency for chlorine evolution in comparison with currently used anodes.     

Electrochemical intercalation of aluminium chloride in graphite in the molten sodium chloroaluminate medium

Aluminium chloride intercalation in graphite was studied by anodic oxidation of compacted graphite (rod) and graphite powder electrodes in sodium chloroaluminate melt saturated with sodium chloride at 175 °C. The studies carried out by employing both galvanostatic and cyclic voltammetric techniques had shown that the intercalation reactions take place only beyond the chlorine evolution potential of +2.2 V vs. Al on both the electrodes. The extent of intercalation reaction was directly related to the anodic potential and probably to the amount of chlorine available on the graphite anodes. In the case of graphite powder electrode, a distinctly different redox process was observed at sub-chlorine evolution potentials and this was attributed to the adsorption of chlorine on its high surface area. This finding contradicts a report in the literature that the intercalation reactions occur at potentials below chlorine evolution in the chloroaluminate melt.     

The anodic characteristics of manganese dioxide electrodes prepared by thermal decomposition of manganese nitrate

Manganese dioxide electrodes were prepared by a thermal decomposition of manganese nitrate solution on a titanium or a platinum substrate, and their anodic characteristics were investigated mainly in 1N H₂SO₄ and 1N KOH. The platinum-supported manganese dioxide electrode shows the good anodic characteristics with a relatively low overvoltage for oxygen evolution while the use of the titanium-supported one as an anode needs further modification to reduce its high resistivity resulting from the thick film of titanium dioxide in spite of good adhesion of the oxide film with the titanium substrate.The primary water or hydroxide ion discharge step is rate-controlling in the anodic evolution of oxygen on the platinum-supported manganese dioxide electrode in both acidic and alkaline solutions. The oxygen overvoltage is raised and the mechanical strength of the catalytic oxide on the platinum substrate is weakened by the anodic polarization in acidic solution. These phenomena are explicable on the basis of an increase in the oxygen content in the oxide. In conclusion, the presented results suggest that the manganese dioxide film is a practical usable material for the anodic evolution of oxygen as well as chlorine, especially in alkaline solutions.     

The surface film formed on amorphous Pd-Ti-P alloy by anodic polarization in 2 m NaCl solution

X-ray photoelectron spectroscopy was used to investigate the effect of the composition and thickness of surface film on the electrocatalytic properties for chlorine gas evolution on amorphous Pd-Ti-P alloy in NaCl solution. The amount of charge for gas evolution exhibited a wavy change with an increase in polarization potential. The gas evolution became active with an increase in palladium content of the surface film and slowed down with increases of titanium and phosphorus contents of the film. However, despite the fact that the formation of surface film consisting mainly of titanium as a cation in the potential region higher than 1.6 V (sce), the catalytic activity for gas evolution increased, suggesting the change in the gas evolution mechanism in the higher potential region.     

The anodic evolution of oxygen and chlorine on foreign metal-doped SnO2 film electrodes

The anodic evolution of oxygen and chlorine on foreign metal-doped SnO₂ film electrodes were studied, in connection with their physical and chemical properties. In general, the anodic characteristics of noble metal-doped electrodes were much better than those of base metal-doped electrodes. The oxygen evolution reaction on noble metal-doped electrodes reflected each characteristic of the doped noble metals and the active center for this reaction was considered to be on the noble metal sites. On the other hand, the chlorine evolution reaction on such electrodes was found to be independent of the kind of doped noble metals and the probable mechanism for this reaction was proposed in which the spillover of Cl radical from noble metal to Sn sites was involved.     

The anodic characteristics of the massive β-MnO2 doped with noble metals in sodium chloride solution

The anodic characteristics of the massive β-MnO₂ doped with a slight amount of noble metals were investigated in NaCl aqueous solution. The doping of noble metal, especially Pd, decreases the chlorine overvoltage enormously. By the kinetic considerations, it was clarified that noble metal sites dispersed on the oxide surface serve effectively as an active site for the chlorine evolution reaction. Furthermore, it was suggested that new effective catalysts for the chlorine evolution might be developed by using such noble metal-doped β-MnO₂ systems.     

Anodic chlorine/nitrogen co-doping of reduced graphene oxide films at room temperature

Room temperature simultaneous doping of reduced graphene oxide films with oxygen, nitrogen and chlorine was performed through anodic polarization in a neutral nitrogen-deaerated KCl electrolyte. The thermodynamic electrochemical windows of water, dissolved nitrogen and chlorine anions were analyzed on the basis of the Pourbaix diagram. Anode polarization demonstrated that the nitrogen, water and chlorine anions can be oxidized at an applied potential of 1.7 V vs. NHE. The oxidative products, i.e. oxygen, nitrate anion and hypochlorous acid, can react with the reduced graphene oxide surface. X-ray photoelectron spectroscopy proved the chlorine–nitrogen co-doping of the treated film, along with an increase of oxygen groups. Surface structure evolution was also confirmed by Raman and Fourier-transform infrared spectroscopies. The anodic doping can be hindered by covering the reduced graphene oxide surface with sulfate anions or forming stable carbon–nitrogen bonds. Incorporation of oxygen, nitrogen and chlorine also helps to enhance the supercapacitance of the doped film.     

On the development of metallic inert anode for molten CaCl2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl₂–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe₂O₃, Cr₂O₃, Co₃O₄, NiFe₂O₄) in pure CaCl₂ or CaCl₂–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe₂O₃ increased. Ni coated with NiO film had much higher stability during anodic polarization.     

Anodic characteristics of amorphous nickel-value metal alloys containing small amounts of platinum group elements in 0.5 M NaCl

By utilizing the characteristics of amorphous alloys capable of possessing specific electrocatalytic activity and corrosion resistance by alloying and surface activation treatment, an attempt was made to find amorphous alloys which are active as the anode material for production of sodium hypochlorite by electrolysis of seawater. Amorphous Ni-Nb and Ni-Ta alloys containing only 0.5–2.0 at % palladium and other platinum group metals showed a very high activity for chlorine production by electrolysis of 0.5M NaCl at 30°C when they were previously immersed in 46% HF for surface activation. The current efficiency of these surface-activated alloys for chlorine evolution considerably exceeded that of the currently used, most active Pt-Ir/Ti electrode for electrolysis of seawater. The surface activation treatment resulted in preferential dissolution of alloy constituents unnecessary for the electrocatalytic activity, i.e. nickel and valve metals, with a consequent enrichment of electrocatalytically active platinum group elements in the surface-activated layer. The corrosion weight loss of the surface-activated amorphous alloys under the steady state conditions for chlorine production was undetectable by a microbalance.     

Microstructure of new composite electrocatalyst and its anodic behavior for chlorine and oxygen evolution

The first layer of active coating made from a rutile-structured solid solution of ruthenium and titanium dioxides having an average crystal grain size of 30 nm was thermally deposited on an adequately prepared titanium metal substrate. Then, at a temperature of 500 °C, the second layer was formed on the first layer from a mixture of amorphous particles of metallic platinum and rutile-structured iridium dioxide nanocrystals having an average crystal grain size of 26 nm.Rutile phase nanocrystals are characterized by a high density of chaotically distributed dislocations and high internal microstrain values. The coatings exhibit a compact granular morphology without cracks on the surface. Their catalytic activity is similar to that of conventional DSAs for the anodic oxidation of chloride ions from both concentrated and dilute sodium chloride solutions. The anodic current efficiency both during chlorate formation and active chlorine production was several percentage points higher in electrolyzers containing these anodes than in those containing DSAs. The catalytic activity of anodes having these coatings is about 50 mV lower than that of DSAs and about 350 mV higher than that of lead/antimony alloy electrodes, for oxygen evolution from acid sulfate solutions (0.5 mol dm^?3 H₂SO₄) characteristic of processes for the production of some metals. An accelerated corrosion test showed that the stability of the double-layer anodes is about twelve-fold higher than that of conventional DSAs.     

An electrochemical and analytical approach to the inhibition mechanism of an amino-alcohol-based corrosion inhibitor for reinforced concrete

Laboratory investigations were performed in order to assess the effectiveness and the inhibition mechanism of an amino alcohol-based inhibitor currently used as admixture to prevent corrosion of steel in concrete. The investigation was performed in the presence of chloride ions, using solutions simulating the concrete interstitial solution. Electrochemical measurements allowed to conclude that, an inhibitor film is formed on the surface hindering the anodic activity. Furthermore, the analytical investigation through the use of X-ray photoelectron spectroscopy (XPS) shows that the inhibitor film is able to complex with the chloride ion.     

Electrochemical costripping models and mutual interferences of mutli-transition metal systems on the surface of boron-doped diamond

Electrochemical behaviors of electrodeposition stripping of some transition metals, such as Ag, Cu, Pb, Sn, and Cd were investigated on boron-doped diamond (BDD) film with differential pulse anodic stripping voltammetry (DPASV). The results show that the metal costripping process on the surface of BDD is influenced by deposition potential of transition metals, potential of hydrogen evolution reaction, mutual interferences between two transition metals, and so on. Electrochemical costripping models and mutual interferences of binary transition metals are proposed. (i) When the hydrogen evolution potential is higher or lower than the deposition potential of two transition metals, the metals do not form alloys and compound the cathode ions in the solution, and the costripping process conforms to metal 1 stripping-metal 2 stripping. (ii) When the hydrogen evolution potential is between the deposition potential of two transition metals, the metals do not form alloys and compound the cathode ions in the solution, and the costripping process conforms to metal 1 stripping-hydrogen evolution-metal 2 stripping. (iii) When two transition metals form alloys, the costripping process conforms to metal 1 stripping-alloy striping-metal 2 stripping. (iv) When the hydrogen evolution potential is between deposition potential of two transition metals, and transition metal 2 is complexed with one kind of cathode ion in the solution, the costripping process conforms to metal 1 stripping-hydrogen evolution-metal 2 complex formation-metal 2 stripping. Moreover, the electrochemical-costripping model of two transition metals in a solution with three or more transition metals, is similar to that in the binary system. It is also found that various metals electrodeposited on BDD film in the form of single metal or alloy can be completely stripped away by the constant-potential electrolysis technique at 2.8 V for 10 s.     

Electrochemical behaviour of copper in aqueous solution containing potassium ethylxanthate

The electrochemical behaviour of copper in aqueous potassium ethylxanthate (KEX) is studied by using potentiodynamic techniques at different sweep rates, complemented with SEM and EDAX. In NaCl solutions a cuprous xanthate film is formed at low potentials, the initial stage of this reaction being the electroadsorption of KEX on copper competing with the adsorption of chloride ions and water. At low surface coverages for electroadsorbed KEX the electrodissolution of copper is partially inhibited as compared to plain NaCl solutions. As the KEX monolayer is completed, a tri-dimensional cuprous xanthate film grows in the electrode surface. On subsequent increase of the applied potential a complex anodic layer is formed leading to copper passivity. Passivity breakdown promoted by either chloride ions or electro-oxidation of the organic film can be observed when the potential exceeds a certain critical value.Facultad de Ciencias Exactas, Universidad Nacional de La Plata.     

Nitric oxide reduction at noble metal electrodes: a voltammetric study in acid solution

Features of the electrochemical reduction of nitric oxide on platinum, palladium, rhodium and ruthenium in aqueous perchloric acid solutions (0.33–1.0 M) are compared. The results from voltammetric studies (ie linear potential sweep and rotating disc electrode) using the bulk metal electrodes are described and compared with residual current voltage plots in acid electrolyte alone. In general, three nitric oxide reduction peaks are observed on the metals. The most anodic peak, at ca E = 0.15 V vs sce is attributed to the one-electron reduction of nitric oxide to an adsorbed NOH intermediate on a bare metal surface (ie one free of oxides or adsorbed hydrogen). The other two peaks occur in potential regions where adsorbed hydrogen is present on the metal surface (ca E = 0.0 and −0.20 V, respectively). The co-adsorbed hydrogen complicates the analysis and precludes an unambiguous interpretation of these two peaks. However, they apparently reflect nitric oxide reduction to nitrogen, hydroxylamine and/or ammonia. In a cathodic scan on the rhodium electrode, a current plateau is seen instead of the first (most anodic) peak, a probable consequence of oxide film formation with subsequent chemical complications. On the ruthenium electrode the first two (most anodic) peaks are not observed probably due to a relatively stable oxide layer. Reaction selectivities at metal black gas diffusion cathodes operating in an electrogenerative (ie galvanic) mode with perchloric acid electrolyte are compared with the voltammetric results at the corresponding bulk electrodes. Dinitrogen formation is observed on the platinum and rhodium black electrodes as suggested from voltammetric results. A series-parallel reaction sequence is proposed to explain the results. Limitations of using simple voltammetric techniques for predicting behavior of large scale preparative electrochemical reactors are discussed.     

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.     

Cyclic voltammetric study of the corrosion protection of metal surfaces by plasma-polymerized coatings

The corrosion-protective performance of plasma-polymerized (PP) films on metal substrates has been investigated by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and surface potential measurements. A PP film from cyclohexane was deposited on Fe, Ni, and Cu substrates using a radiofrequency generator. A PP film-coated metal substrate was employed as the working electrode in aerated NaCl aqueous solution and the cyclic voltammogram was measured repeatedly ten times upon application of a triangular potential between -1 and 1 V vs. the initial corrosion potential of the sample. The cyclic voltammogram depended strongly on the metal substrate: with Fe and Ni, only an anodic current was observed; with Cu, an anodic and subsequently a cathodic current appeared. For all the metals, the anodic current level in the voltammograms at the same potential sweep number decreased with increasing film deposition time. The anodic current level for Fe and Cu increased with increasing potential sweep number, but that for Ni exhibited a maximum, followed by a decrease. The anodic current level for the metal substrates decreased in the order Fe > Ni > Cu. The cathodic current for Cu decreased with an increase in the deposition time and increased with an increase in the potential sweep number. The dependence of the cyclic voltammogram on the metal substrate is discussed in terms of the PP film thickness and the positive charge on the film surface, as well as the standard reduction potential of the metal. The corrosion potential for PP film-coated metal substrates also changed with the deposition time; this change is suggested to be related to the increase in the positive charge on the surfaces.     

Effect of anions and pH on the ethanol electro-oxidation at a platinum electrode

The electro-oxidation of ethanol and its inhibition by adsorbed chloride ions have been studied by cyclic voltammetry in 0.64 M HNO₃, 0.64 M HClO₄, 0.64 M NaNO₃, 0.64 M NaClO₄, 0.43 M NaNO₃/0.072 M Na₂SO₄ and 0.43 M NaClO₄/0.072 M Na₂SO₄ solutions at 25°C. The results show that these anions and the pH influence the peak current and potential for all three anodic waves. The anion effect is more pronounced in acidic solutions than in neutral solutions. The magnitude of the effect in the absence of chloride indicates that the surface coverage of these anions increases in the order perchlorate, sulfate, nitrate. The chloride inhibition is dependent on the anions and is smallest in sulfate solutions. Mechanisms proposed in an earlier study are found to be consistent with these results. It is also suggested that the ethanol system may be useful as a probe in studies of anion adsorption properties and platinum surface states.     

An improvement of color electrophoretic paint via ultrafine modified pigment paste

The ultrafine paste used in electrophoretic paint was prepared via ultrasonic method with pigment yellow 83 (P.Y.83), octadecyl dimethyl benzyl ammonium chloride, and deionized water. The average particle size of the ultrafine paste was only 196?nm and zeta potential was 30?mV with ultrasound for 30?min and 2?wt.% cationic dispersant. The conductivity of the paint was improved as the cationic dispersant increased. Centrifugal stability of the paint with ultrafine paste reached the maximum at 2?wt.% cationic dispersant. Compared with the performances of cathodic electrophoretic paint with the ultrafine yellow paste, the anodic film was smooth, uniform, and neat with 0.5?wt.% cationic dispersant in the ultrafine yellow paste. Deposited amount of the anodic film was 27?g/m², which was higher than the cathodic film. The curing property, adhesion, and chemical resistance of the cathodic film were better than the anodic film. Properties of five color cathodic paints (blue, yellow, red, black, and green) were discussed and were compared with the film of electrophoretic paint containing commercial pastes. Except the green film, the blue, yellow, red, and black films were smooth, fine, and uniform, and also provided good L-effect. The deposited amount of these films with excellent adhesive force was about 14–19?g/m². The electrophoretic paints containing modified pigment pastes and commercial pigment pastes present good adhesion and chemical resistance.     

Preparation of 1,2,3-Triazole-Containing Resins for Adsorption of Palladium Ions

1,2,3-Triazole-group-containing resins for palladium ion adsorption in HCl media were prepared. The monomer, synthesized from acryloyl chloride and 3-(1,2,3-triazol-1-yl) propanol, and a crosslinker, ethylene glycol dimethacrylate, was polymerized in dimethyl sulfoxide via radical polymerization. The specific surface area decreased with increasing monomer:crosslinker molar ratio. In acidic media, the adsorption of palladium ions was greater than those of platinum and ruthenium ions, because stable complexes were formed between palladium ions and triazole groups. The maximum amount of palladium ions adsorbed was 0.41 mol/kg.