Nitrate reduction on single-crystal platinum electrodes

The structure sensitivity of the reduction of nitrate has been studied on a series of single-crystal platinum electrodes by cyclic voltammmetry and in situ FTIRAS in sulfuric and perchloric acid solutions. The nitrate reduction is a structure-sensitive reaction on single-crystal platinum electrodes. However, this structure sensitivity is essentially controlled by other species (hydrogen, sulfate) that interact strongly with the electrode surface rather than by a structure-sensitive nitrate adsorption, dissociation or reduction.Voltammetric and spectroscopic data point to adsorbed nitric oxide (NO) as the main stable intermediate of the nitrate reduction to ammonia. No evidence for the formation of N₂O was found. On surfaces with sufficiently wide (1 1 1) terraces in the absence of specifically adsorbed sulfate, an oxidizable nitrate reduction product is detected voltammetrically, which may tentatively be attributed to the formation of a small amount of hydroxylamine earlier during the voltammetric scan.     

Anodic, cathodic and cyclic voltammetric deposition of ruthenium oxides from aqueous RuCl3 solutions

Anodic, cathodic and cyclic voltammetric (CV) deposition of ruthenium oxides from aqueous RuCl₃ solutions have been investigated using stationary and rotating disk electrodes (RDE) in this work. The CV deposition behavior was examined using a RDE to differentiate the contribution of current from the reactions of ruthenium ions in the electrolyte and ruthenium oxides already adsorbed on the electrode. The results indicate that the CV growth of ruthenium oxides within the potential range of aqueous electrolyte decomposition is attributed to the anodic oxidation of ruthenium ions in the electrolyte. Cathodic deposition occurs only at potential negative than −0.30 V versus saturated calomel electrode (SCE) when H₂ evolves on the electrodes. Anodic deposition of ruthenium oxides can be obtained effectively in the potential range of ca. 0.9-1.1 V versus SCE, depending on the pH value of the electrolyte. The optimum anodic and cathodic deposition potential for maximum deposition efficiency is 1.0 and −0.9 V versus SCE, respectively, in the electrolyte solution of pH 2.     

Electrolytic reduction of oxygen at solid electrodes in aprotic solvents-the superoxide ion

Cyclic voltammetric behavior of oxygen reduction to superoxide ion in DMF and CH₃ CN in presence of tetrabutyl ammonium hexafluorophosphate as supporting electrolyte at glassy carbon, rhodium, palladium, lead and tungsten electrodes was investigated. The first reduction step to superoxide ion was found to approach reversibility with an average rate constant of 2.2 × 10^?2 cms^?1 at the glassy carbon electrode whereas at rhodium and palladium, the behavior was irreversible exhibiting high cathodic currents on the anodic scans which presumably is caused due to surface disproportion of superoxide ion at these electrodes. At lead and tungsten electrodes one irreversible reduction step was observed in both aprotic solvents.     

Effect of cathodic reduction on catalytic activity of amorphous alloy electrodes for electrooxidation of sulfite

Active electrode materials for a new zinc electrowinning process, in which the thermodynamic cell voltage is about half that of the conventional process by replacing oxygen evolution by anodic oxidation of SO₂ produced in the zinc smelting process have been studied. Immersion in HF solution and subsequent cyclic voltammetry (CV) in sulfuric acid are known to be effective surface activation treatments of the amorphous alloy electrodes. The galvanostatic cathodic reduction (CR) treatment was applied to obtain further activation for sulfite oxidation for HF- and CV-treated electrodes prepared from amorphous nickel-valve metal-platinum group metal alloys. This treatment has been found to be effective in enhancing the activity. Among the amorphous Ni-40Nb alloys containing platinum group elements, the platinum-containing electrode showed the highest catalytic activity, which was higher than that of platinized platinum. Furthermore, the electrocatalytic activities of CR-treated electrodes prepared from amorphous alloys containing platinum and rhodium, and platinum and ruthenium were higher than that of the electrode containing only platinum. According to XPS analysis of the amorphous Ni-40Nb-1Pt-1Ru alloy specimen the enrichment of platinum and ruthenium occurred by CV treatment, and a small amount of oxidized platinum and ruthenium species remained on the electrode surfaces, but most of them were cathodically reduced to the metallic state by CR treatment. High catalytic activities for sulfite oxidation can be attributed to the metallic state of platinum and ruthenium contained in the alloy electrodes, even though the activity of these electrocatalysts is higher than that of pure Pt or Ru.     

Fuel cells—II. Propane and propylene on Adams' platinum catalyst

In order to control the quantity of previously adsorbed hydrogen in fuel electrodes to be used with hydrocarbons, the use of Adams' platinum catalyst was investigated. Preferred procedures for electrode preparation were developed. Adams' platinum oxide powder was intimately mixed with a silver powder and pressed to form a disk, which, fixed within a cell, was reduced with hydrogen in glacial HOAc and then treated with a water-proofing wax solution. Reduction could be stopped at a definite stage and the reduced electrode could be protected from air throughout all manipulative and measuring procedures. The potential behaviour of 15 electrodes reduced to differing degrees was studied in helium, propane, propylene and hydrogen at 80°C.

During flushing with propylene, the open-circuit potential increased quite rapidly after an induction period, which was related to the original degree of electrode reduction. The steady potential reached increased with the degree of reduction. The potential of an electrode initially saturated with hydrogen gradually decreased with propylene flushing, approaching a steady value.

The good discharge behaviour observed for propylene far exceeded that due to hydrogen adsorbed during electrode preparation. Propylene flushing for more than 20 h was necessary to reach a steady potential. On interruption of the discharges, the potential returned to almost the initial potential. The steady potential may represent an equilibrium potential between propylene, electrode and electrolyte, and may be called a “propylene potential”.     

Adsorption and anodic oxidation of methanol on iridium and rhodium electrodes

Adsorption and electro-oxidation of methanol on smooth iridium and rhodium electrodes have been studied. The regularities obtained were compared with the results of previous measurements on smooth platinum. The adsorption of methanol on iridium has been established as characterized by regularities peculiar to a surface with an exponentially distributed inhomogenity of adsorption sites (Freundlich isotherm, linear change of the activation energy of adsorption with logarithm of surface coverage). The adsorption regularities for rhodium are more complex. The character of the isotherms on iridium and rhodium, as well as on platinum, does not depend on the nature of adsorbed neutral particles (methanol, hydrogen etc) and is apparently determined by the electrode surface properties. As follows from kinetic regularities (influence of potential, concentration and pH of solution, surface coverage) the rate-determining step of steady-state methanol electro-oxidation on iridium and rhodium is the oxidation of carbonaceous chemisorbed particles by adsorbed OH radicals.     

Adsorption of acetic acid on platinum,gold and rhodium electrodes

The adsorption of acetic acid has been studied on platinum, gold and rhodium electrodes from aqueous electrolytes. On the platinum electrode, the evidences have been presented that in the double layer region of the electrode potentials: (i) the undissociated acetic acid molecule is adsorbed, (ii) the adsorption process is reversible and (iii) the adsorption occurs in the second ad-layer of the interfacial region, ie on the top of the chemisorbed water molecules. In the hydrogen region, the slow (reductive) chemisorption of this compound has been observed.The similarity in the character of the adsorption of acetic acid in the double layer region of the three electrodes investigated have been shown.     

EMF measurements of cells employing metal—metal oxide electrodes in aqueous chloride and sulphate electrolytes at temperatures between 25–250°C

EMF measurements are reported on chemically prepared platinum—platinum oxide, iridium—iridium oxide, rhodium—rhodium oxide and zirconium—zirconium oxide electrodes over a temperature range of 25–250dgC and a pH range of 1–12 using chloride and sulphate aqueous electrolytes. The effects of temperature, pressure, concentration and time are discussed. The platinum—platinum oxide electrode is shown to have an apparent oxygen and sulphate response at 25°C in concentrated sulphuric acid. A less than ideal pH response is observed for all the oxide electrodes in air-equilibrated solutions. Electron microscopic studies provided information on the nature of the electrode surface.     

Electrolyte effects on oxygen reduction kinetics at platinum: A rotating ring-disc electrode analysis

A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H₂SO₄ ～ CF₃SO₃H > H₃PO₄ > HClO₄. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH^? ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.     

Electrocatalytic hydrogenation processes controlled potential—1. The electrocatalytic reduction of nitric acid

Methods are described to control the catalyst potential during the electrocatalytic hydrogenation of nitric acid in a slurry electrode cell. The influence of traces of GeO₂ on the hydrogenation reaction is studied with a platinum electrode. Deposition of Ge on the platinum surface limits the amount of adsorbed hydrogen and enhances the nitric acid reduction. This changes the mixed potential of the hydrogen-nitric acid system on the platinum (catalyst) surface. A similar potential change occurs, when the partial hydrogen pressure in the system is altered.     

Pt修饰的Pd/石墨烯纳米复合材料的制备及对乙二醇氧化反应的电催化活性

以氧化石墨(GO)和Pd(NO₃)₂为原料,通过化学还原法制备Pd纳米粒子-石墨烯(Pd/G)纳米复合材料,然后以H₂PtCl₆作为Pt前体,在Pd纳米粒子的表面恒电位沉积Pt,制备不同Pt负载量的Pd/G(Pt-Pd/G)电极.利用场发射扫描电镜(FE-SEM)、透射电镜(TEM)和X射线能谱仪(EDX)对材料的微观结构进行了表征和分析.结果显示石墨烯上的金属粒子分散均匀,平均粒径约7.2nm.电化学测试结果显示Pt-Pd/G电极对乙二醇电化学氧化反应具有良好的催化性能.当纳米粒子的Pt:Pd原子百分比为1:42时,其反应峰电流密度分别为Pd/G和Pt/G电极的3.0倍和2.7倍.少量的Pt沉淀可显著改进Pd/G电极的催化活性.本研究采用的修饰方法简单,修饰效果明显,可应用于其他金属纳米复合材料的异金属修饰.     

Application of polyethyleneimine-quinone modified electrodes for voltammetric measurements of pH

A modified polymer, ie benzylated polyethyleneimine (PEI), coated on a glassy carbon electrode in which negatively charged quinones have been incorporated, has been used for voltammetric measurement of pH. A linear dependence of the average of the peak potentials as a function of pH was observed. The system has been successfully applied also to ultramicroelectrodes.     

Electrochemical synthesis and characterization of new electrodes based on poly‐hematoxylin films

The poly‐hematoxylin films undoped (p‐HX) and doped with strontium (p‐HX‐Sr) were prepared on the platinum electrodes (Pt) using cyclic voltammetry. The influence of experimental parameters, such as: anodic potential and number of voltammetric cycles on the films synthesis was investigated by electrochemical impedance spectroscopy (EIS). The results showed that, the optimal conditions to obtain the poly‐hematoxylin films on platinum electrode from 10^?2 mol L^?1 NaNO₃ solution, were found by recording 10 voltammetric cycles at an anodic potential of 1.5 V. Electrocatalytic activity of the poly‐hematoxylin‐platinum (p‐HX/Pt) and poly‐hematoxylin‐strontium‐platinum (p‐HX‐Sr/Pt) electrodes was tested for electrochemical degradation of benzocaine using UV‐Vis spectrophotometry. Both, the mechanisms of hematoxylin (HX) polymerization, as well as benzocaine degradation were proposed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Electrocatalytic activities of metal electrodes in anodic oxidation of hydrazine in alkaline solution

The electrocatalytic activities of various metals and alloys in the anodic oxidation of hydrazine in alkaline solution have been studied by means of palladium membrane method in which the contact side of the membrane was electrodeposited with a thin layer of the electrocatalytic metals. The electrode materials studied can be divided into two groups. In the first group, platinum, rhodium, cobalt, cobalt—phosphor and cobalt—boron, anodic current of hydrogen oxidation on the diffusion side decreased remarkably with an increase of the electro-oxidation of hydrazine on the contact side. The anodic oxidation of hydrazine occurs through the preliminary stepwise dehydrogenation on this group metals.On the other hand, the amount of sorbed hydrogen in the palladium, gold, nickel and nickel—phosphor electrodes increased with an increase of the electro-oxidation of hydrazine on the contact side. Thus, the anodic oxidation of hydrazine on the latter group metals may proceed through the anodic formation of the intermediate radicals which readily decompose into hydrogen and the related compounds.     

Voltammetric and galvanostatic studies of hydrous and anhydrous iridium oxide films

Electrolytically grown hydrous oxide films on iridium wire electrodes have been thermally treated from 473 to 773 K. Anhydrous oxide films formed by this treatment have been subjected to cathodic polarization at the potential of the hydrogen evolution reaction, square-wave pulsing of potential from –0.25 to +1.25 V with respoect to SCE and to anodic galvanostatic polarization in 0.5 mol dm^–3 H₂SO₄. Cathodic pretreatment caused an increase of the voltammetric charge in the oxide formation region while the square-wave pulsing formed a hydrous oxide film whose voltammetric charge was superimposed on the charge of the anhydrous oxide film. Both procedures restored the hydrophilic nature of the electrode/solution interface. Potential-time curves during anodic galvanostatic polarization served as a diagnostic criterion for the stability and the state of the oxide film.     

Kinetics of oxygen reduction at RuO2-coated titanium electrode in alkaline solution

Oxygen reduction at RuO2-coated titanium electrodes prepared by thermal decomposition was investigated by employing cyclic voltammetry and rotating-disc electrode techniques. Cyclic voltammetric results indicated that oxygen reduction was catalysed by at least some hydrous oxyruthenium species, (i.e., Ru(iii)) at low polarization range (E >–0.45V) and by at least some low oxide states of ruthenium species at high polarization (E<–0.45V). On the basis of measurements using a rotating disc electrode together with polarization curves, Tafel slopes and stoichiometric number determinations, two mechanisms for oxygen reduction on an RuO2-coated titanium electrode are proposed.     

Electrochemical surface nanopatterning by selective reductive desorption from mixed metal surfaces

We report on a novel strategy to the functionalisation of electrode surfaces based on the preparation and patterning of mixed metal electrodes using metal selective electrodesorption of a sacrificial alkanethiol. Plain palladium (Pd) and plain polycrystalline gold (poly-Au) electrodes were used initially to determine metal specific potential windows within which electrodesorption of the short alkanethiol mercaptoethanol could be achieved. We found that stripping of mercaptoethanol from gold was achieved at potentials lower than −0.800 V, whilst stripping from palladium was achieved at more positive potentials i.e. around −0.650 V. Mixed metal electrodes were prepared by electroplating for short period of times palladium onto poly-Au electrodes. The resulting surfaces were characterised electrochemically in 1 M H₂SO₄ and clearly exhibited reduction peaks for both gold and palladium oxide formation. The mixed metal electrodes were coated with mercaptoethanol, which was further selectively removed from Pd by cyclic voltammetry in NaOH in the Pd-specific potential window. The presence of bare Pd domains revealed following electrodesorption was confirmed by subsequently adsorbing the electroactive alkanethiol 6-ferrocenylhexanethiol onto the freshly revealed Pd. Cyclic voltamogramms exhibited sharp redox peaks that could only be attributed to the successful immobilisation of 6-ferrocenylhexanethiol onto fresh Pd domains. Control surfaces, i.e. MCE fully coated Pd/Poly-Au electrode, exposed to 6-ferrocenylhexanethiol did not exhibit significant voltammetric features, attesting to the efficient patterning of the mixed metal electrode by employing metal specific reductive desorption of short alkanethiols. The possibility to pattern electrode surfaces in such way will find application in the field of diagnostics, and also in heterogeneous catalysis where Pd-Au alloys have received an increased interest in the recent years.     

Electrochemical beaviour of the solid ionic conductor Ag7I4PO4

Electrochemical investigation of the solid superionic conductor Ag₇I₄PO₄ (0.8 AgI + 0.2 Ag₃PO₄) at room temperature and at 40°C were performed by means of cyclic voltammetry, cyclic chronoamperometry and cyclic chronocoulometry, normal pulse polarography and ac polarography. It was shown that the Ag⁺ ↔ Ag redox process on Pt and Ag working electrode occurs with a certain overvoltage, ie that for Ag⁺ → Ag⁺ oxidation and the return of Ag⁺ ions into the electrolyte a certain overvoltage is necessary. From the determined values of the exchange current one estimates the redox process as a rather fast one. The silver working electrode is electrochemically inactive, while only cathodically deposited silver is electrochemically active and can be oxidized to Ag⁺ ions. Chronoamperometry and chronocoulometry show that there is a certain difference in the behaviour of Pt and Ag working electrodes due to uneven passivating anodic processes. On the basis of measurements of faradaic and capacitance currents and their dependence on frequency, diagrams of complex impedance of the Pt/Ag₇I₄PO₄ interface at various anodic and cathodic polarizations of the Pt electrode were plotted. The dependence of the serial capacity of the interface on the dc potential and temperature are discussed.     

Influence of anion adsorption on the action of inhibitors on the acid corrosion of iron and cobalt

It has been shown by the method of current/potential (i/) curves and by differential capacitance measurements that the chemisorption of halogen anions from H₂SO₄ solutions on cobalt leads, as in the case of iron, to an increase in the overvoltage of the cathodic reaction of hydrogen-ion discharge and of the anodic reaction of metal ionization. Due to a positive charge of the cobalt surface in H₂SO₄ solutions, organic cations are poorly adsorbed and ineffective as inhibitors, but in the presence of halogen anions their adsorption increases and they exert an additional inhibitive effect upon the above reactions, although less pronounced than in the case of iron.

Hydrogen sulphide accelerates the electrochemical reactions occurring on iron and cobalt electrodes. In small concentration in acid solutions, it increases the adsorption of organic bases, which become effective inhibitors. This leads to the disappearance of the accelerating effect of hydrogen sulphide. The distortion of the anodic i/ curves (decrease in the slope in the case of inhibitors and increase in the case of stimulators) is connected with the finite rate of the establishment of adsorption equilibrium and with the removal of the adsorbed organic substance together with the ionized metal atoms. A hypothesis is advanced that the acceleration of the anodic reaction of iron and cobalt ionization in the presence of hydrogen sulphide is due to the formation of a surface catalyst from the chemisorbed HS⁻ ions and the metal atoms.     

Mechanism of the chlorine evolution on a ruthenium oxide/titanium oxide electrode and on a ruthenium electrode

The mechanism of the chlorine evolution on titanium electrodes coated with a layer of ruthenium oxide and titanium oxide under different experimental conditions, and on a ruthenium electrode, both in acidic chloride solution, has been investigated. Potentiodynamic current density—potential curves were recorded as a function of the time anodic pre-polarisation, the composition of the solution and the temperature. Moreover, potential decay curves were determined. Theoretical potential decay curves were deduced for both the Tafel reaction (2 Cl_ad→Cl₂) and the Heyrovsky reaction (Cl^? + Cl_ad → Cl₂ + e^?) as a rate determining step in the formation of molecular chlorine. They were compared with those found experimentally. The influence of possible diffusion of atomic chlorine out of the electrode was also taken into consideration. It was found that for all the electrodes investigated, molecular chlorine is formed both at anodic polarisation and on open circuit according to the Volmer—Heyrovsky mechanism, where the Heyrovsky reaction is the rate-determining step. The transfer coefficient is 0.5 for the chlorine evolution at an “ideal” ruthenium oxide titanium oxide electrode and at a ruthenium electrode.