Electrochemical behaviour of indium in H2SO4

The kinetics of the electrodissolution of indium and the hydrogen evolution on indium in H₂SO₄ of various pH values have been investigated. For electrodissolution, Tafel slopes of 22 mV per decade and a reaction order of 1 with respect to OH^– have been obtained. For hydrogen evolution, Tafel slopes were 20 mV per decade; the reaction was first order with respect to H⁺. An electrodissolution mechanism based on three consecutive one-electron transfer steps with the last electron transfer step rate-determining is consistent with the experimental results.     

Invariance with pH of enthalpies of activation for O2 reduction at Pt electrodes in acid solutions

The apparent enthalpies of activation, ΔH^*_a, for O₂ reduction at Pt electrodes are determined in acid solutions of different pH. In the high current density region, characterized by a Tafel slope close to − 120 mV, ΔH^*_a,h is lower than ΔH^*_a,l in the low current density region characterized by a Tafel slope close to − 60 mV. Although ΔH^*_a,l > ΔH^*_a,h it is concluded that at every pH the enthalpies of activation at the zero Galvani potential difference are the same in both current density regions. Therefore, irrespective of the different Tafel slopes, the same mechanism is operative in both regions. In all solutions, ΔH^*_a for either current density region are independent of pH when they are determined at constant potentials vs rhe. This invariance with pH is unexpected in view of the suggested variance of the Galvani potential difference with pH. Alternative causes for the invariance are discussed.     

Electrochemical studies with a Cu-5wt.%Ni alloy in 0.5 M H2SO4

The electrochemical behavior of the annealed Cu-5wt.%Ni alloy in 0.5 M H₂SO₄ was studied by means of open-circuit potential (E_OCP) measurements, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and quasi-stationary linear potential sweep. The hydrodynamics of the system was also studied. This material is constituted by a single α₁ phase. The anodic behavior of a Cu-Ni alloy in H₂SO₄ consists fundamentally on the electrodissolution of Cu, its main component, and the formation of a sulfur-containing passive layer. The presence of Ni decreases the rate of Cu oxidation, mostly at high positive potentials. The impedance spectra, obtained for the unrotating electrode, can be interpreted in terms of a simple charge-transfer reaction across a surface layer. When the electrode is rotated, the occurrence of an inductive loop evidenced the existence of an adsorbed layer. All the resistance estimated from the proposed equivalent circuits diminished with the electrode rotation rate, emphasizing the influence of ion transport in the overall electrode process. The system presented two anodic Tafel slopes: 40 mV dec⁻¹ for E<255 mV and 67 mV dec⁻¹ for E>275 mV. A Tafel slope of 40 mV dec⁻¹ evidences that copper dissolution can be interpreted in terms of the mechanism proposed by Mattsson and Bockris. The second Tafel suggests that at potentials more positive than 275 mV, copper dissolves according to a mechanism that considers the disproportionation of adsorbed Cu(I) species.     

Platinum crystallite size considerations for electrocatalytic oxygen reduction—I

An examination of the activity contributions for atoms at corners, edges and planes of platinum electro-catalyst crystallites was carried out for the reduction of molecular oxygen in 1M H₂SO₄. Varying the mean crystallite diameters between 30 and 400 Å caused at least a 200-fold and 30-fold change in the surface densities of corner and edge sites, respectively. Deliberate sintering of the platinum crystallites caused crystallite growth, with a reduction of surface and lattice defects. The catalysts showed a uniform activity for oxygen reduction of 0·018 ± 0·003 mA/real cm² Pt at 900 mV (nhe) with a Tafel slope of 65 ± 5 mV at 50 and 70°C. Platinum atoms at corners, edges, kink sites or dislocations are not more active than atoms on the crystallite faces for this reaction.     

The influence of chloride ions on the kinetics of iron dissolution

The anodic dissolution of pure iron was studied in oxygen-free solutions at high concentrations of chloride and hydrogen ions at 25°C under potentiostatic steady-state conditions. In the range c_H⁺ ? 1 M the following kinetic data were obtained: Tafel slope b₊ ? +100 mV, electrochemical reaction order related to C_H⁺, n_+.H⁺ = +1·1, and electrochemical reaction order related to the chloride ion concentration, n_+,Cl^? = +0·6. These values cannot be correlated to the currently proposed mechanisms of iron dissolution, another mechanism is suggested for the described conditions. The correlations to known mechanisms are discussed.     

Dissolution du nickel en transpassivite 2eme partie: etude du degagement d'oxygene simultane et influence du pH

The oxygen discharge was studied in the transpassive range of nickel on 0·5M H₂SO₄. The potential range is separated into two parts, the experimental data indicating that, above 1650 mV/sce the oxygen discharge occurs alone and between 1360 and 1650 mV/sce two reactions namely nickel dissolution and oxygen discharge occur simultaneously. A model is proposed where two OH^? ions are simultaneous adsorbed on the first formed layer of Ni(OH), a fraction of the surface is covered by Ni(OH)₃. The oxygen discharge is also a multistep reaction. Between 1360 and 1650 mV/sce, the dissolution of metal and the oxygen discharge are in competition at all times, the surface is covered simultaneously by Ni(OH)₂ and Ni(OH)₃.The calculated curves are in good accord with the potentiostatic curves, the influence of pH is correlated with the experimental data.     

Combination of Ozone with Activated Carbon as an Alternative to Conventional Advanced Oxidation Processes

The objective of this study was to compare the efficiency of O₃/granular activated carbon (GAC) to enhance ozone transformation into ·OH radicals, with the common advanced oxidation processes (O₃/OH^?, O₃/H₂O₂). The results obtained with model systems under the given experimental conditions showed that the system O₃/OH^? (pH 9) and O₃/H₂O₂ (pH 7, [H₂O₂] = 1·10^?5 M) are more efficient than O₃/GAC (pH 7, [GAC] = 0.5 g/L) to enhance ozone transformation into ·OH radicals. However, in Lake Zurich water the O₃/GAC process has a similar efficiency as O₃/H₂O₂ for ozone transformation into ·OH radicals. The results also show that the presence of GAC during Lake Zurich water ozonation leads to (i) removal of hydrophilic and hydrophobic micropollutants, (ii) reduction of the concentration of CO₃ ^2?/HCO₃ ^?, and (iii) decrease of the concentration of dissolved organic carbon (DOC) present in the system.     

Studies on the mechanism of the hydrogen electrode reaction by means of deuterium tracer—II. The reaction mechanism

The exchange reaction between deuterium gas and light water was conducted with Ni, Rh, and Pt catalyst. Through analysis of isotopic composition of the gaseous hydrogen during the exchange reaction, the exchange rates of the individual steps, H₂ ? 2H(a) and H(a) + B ? H⁺B + e, of the hydrogen electrode reaction were determined at various hydrogen pressures and solution pH's, where H(a) is a hydrogen adatom and B = H₂O or OH^?. The rates of the steps were widely different among the metals studied but, throughout these metals, the hydrogen pressure dependence of the rate of the first step was with the power of 1·0–0·9, and that of the second step, 0·2–0·5. Both the rates were independent of solution pH. Generally, the former step is virtually rate-determining under low hydrogen pressures, whereas the latter takes over the role with increasing hydrogen pressure. Under ordinary pressures, the first step is virtually rate-determining on Pt but neither step is singly rate-determining on Rh and Ni.     

Hydrogen evolution on RuxTi1−xO2 in 0.5 M H2SO4

Hydrogen evolution from 0.5 M H₂SO₄ on Ti electrodes coated with a Ru_xTi_1−xO₂ (x=0.04-0.5) layer has been studied by potentiostatic polarisation, cyclic voltammetry and ac-impedance spectroscopy. The results indicate that after a certain activation period the performance of such an electrode coating is comparable to platinum. The addition of small amounts of sodium molybdate increased the capacitance of the electrode and a reduction of the performance was observed. Increasing the temperature of the pure electrolyte from 20 to 75 °C caused an increase in the rate of the hydrogen evolution and in addition an increase of the electrode capacitance. The electrodes have been found to be rather tolerant to chloride and Fe²⁺ ions, and could hence be promising candidates as catalyst materials for solid polymer water electrolysis systems. From steady state measurements the Tafel slopes were found to change from −105 mV/decade for pure titanium to about −40 mV/decade for the (RuTi)O₂ coated electrodes. The exchange current densities were calculated from the steady state curves, as well as from impedance data, to be about 10⁻⁴ A cm⁻² after activation.     

Electrochemical reduction of acetone on mercury electrode in aqueous sulphuric acid solution

Electrochemical reduction of acetone in aqueous H₂SO₄ solution was studied on Hg electrode by galvanostatic, potentiodynamic and polarographic methods. Electrocapillary curves show that acetone molecule adsorbs to a monolayer at concentrations larger than 0·5 M in a potential range of 0 > V > ? 1·0 V (nhe). It orients with the positive end of the dipole toward electrode surface and extends an attractive interaction to its neighbours.Kinetics oberved by galvanostatic pulse method and polarography leads the following conclusions; (1) adsorbed acetone molecule undergoes elecrochemical reduction at potentials more negative than ?1·0 V (nhe), (2) reaction rate is the first and second order with respect to the amount of adsorbed acetone and proton activity, and (3) one electron transfer step determines the reaction rate (Tafel slope is 0·12 V). Reaction intermediate is concluded from the potentiodynamic study to be isopropanol radical, (CH₃)₂??OH, whose amount is proportional to that of adsorbed acetone and to a_H+. From the above results, the rate of the electrochemical reduction of acetone is H₂SO₄ solution is concluded as determined by the step, (CH₃)₂?OH(a) + H⁺ + e^? → (CH₃)₂CHOH.     

Synthesis and magnetic properties of a new hexanuclear iron(III) compound

A hexanuclear Fe(III) compound with new core topology, [Fe₆(OMe)₂(DHMP)₄(AD)₂] (1, H₃DHMP = (E)-2-((1,3-dihydroxy-2-methylpropan-2-ylimino)methyl)phenol, H₂AD = adipic acid), is reported. The hexanuclear Fe(III) core can be viewed as two Fe₃(DHMP)₂ units connected through double μ₂-OMe^? bridges, featuring adjacent Fe···Fe distances of 3.105 and 3.109 Å within unit and 3.111 Å between units, respectively. Variable temperature magnetic studies indicate antiferromagnetic exchanges between adjacent Fe(III) ions and which mediated by μ₂-OMe^? bridges is stronger than those by DHMP alkoxo bridges. Magnetic-structural correlation is also proposed.     

A mononuclear copper(II) complex of a protonated unsymmetrical dinucleating Schiff base ligand

Reaction of the unsymmetrical and potentially dinucleating Schiff base ligand HL¹ with copper(II) salts has given mono- and dinuclear metal complexes. The structure of a mononuclear complex of diprotonated HL¹, [Cu(H₂L¹)(OH₂)](ClO₄)₃·H₂O·CH₃OH_, has been determined.     

Surface, kinetics and electrocatalytic properties of the Ti/(Ti + Ru + Ce)O2-system for the oxygen evolution reaction in alkaline medium

Ti-supported (Ti + Ru + Ce)O₂ electrodes, prepared at 450 °C, were characterised by XRD, open-circuit potential (E_oc), capacity data (C) and morphology factor (φ) determinations. XRD measurements showed mixed oxides present a low degree of crystallinity. E_oc-data and CV-spectra support surface electrochemistry of mixed oxides is governed by the Ru(III)/Ru(IV) redox couple. In situ surface characterisation revealed the active surface area increases on increasing nominal CeO₂-content. φ-Values remained in the 0.18-0.3 interval supporting the coatings have a low electrochemical porosity. Kinetics was studied recording polarisation and chronopotentiometric curves, which permitted to determine the Tafel slope and reaction order (with respect to OH⁻), in the low and high overpotential domains. Tafel slope data, b, presented a dependence on overpotential and oxide composition indicating the OER electrode mechanism depends on these variables. A unit reaction order with respect to OH⁻ was found for all electrode compositions investigated. The theoretical analysis of the electrode mechanism permitted to analyse the changes in the experimental Tafel slopes taking into account modifications in the apparent electronic transfer coefficient, α_ap. Analysis of the true and apparent electrocatalytic activities revealed the O₂-evolution reaction rate is affected by oxide composition due to morphologic effects.     

Kinetics of layer formation and corrosion processes of passive iron in acid solutions

The kinetics of the layer formation process and the corrosion process have been determined from the relation between the stationary (i_c,0) and non-stationary (i_c) corrosion rates and the rate of layer formation (i_ι) at passive iron in acid solutions. The rates of both processes depend on the potential difference ε_2,3 at the passive-oxide/electrolyte-solution interface. The fact that i_c and i_c,0 are independent of the electrode potential is explained by a nearly constant composition of the passive oxide in contact with the solution (Fermi level within the band gap).Linear relations, such as log i_ι⁺ = a + (α_ι⁺/α_c⁺)·log i_c, are obtained, which are explained by charge-transfer hindrance. The apparent charge-transfer coefficients are α_ι⁺ = 1·43 and α_ι^? = 0·57 for the layer formation and removal reaction, and α_c⁺ = 0·84 for the corrosion reaction. From the pH dependence (pH = 0.35–2·90) of i_ι and i_c,0 the reaction orders of the hydrogen ions are found to be approximately μn_ι⁺ = ?1 and ν_c = 0. The corrosion cd depends on the sulphate concentration, with a reaction order ν_s = ß = 0·16. From this, the kinetics of the oxide formation H₂O·aq ? OH^? · ox + H⁺·aq (preceding equilibrium), OH^?·ox?O^2?·ox + H⁺·aq (rate-determining) and the kinetics of the corrosion process Fe³⁺·ox + SO₄^2?·aq ? FeSO₄⁺·ad (adsorption equilibrium, Temkin conditions), FeSO₄⁺·ad → FeSO₄⁺·aq (rate-determining), with following dissociation of the complex can be deduced. The apparent charge-transfer coefficients are interpreted by α_ι⁺ = 1 + α_ι, α_ι^? = 1 ? α_ι and α_c⁺ = α + 2ßγ_s (ga_ι, α, true charge-transfer coefficients).     

Electrocatalytic properties of new active ternary ferrite film anodes for O2 evolution in alkaline medium

Some ternary ferrites with molecular formula, CoFe_2−xCr_xO₄ (0≤x≤1.0) have been synthesized at 70 °C by a precipitation method and were transformed into the film form at the pretreated Ni support (1.5×1.0 cm²) using an oxide-slurry painting technique. The study showed that Cr-substitution from 0.2 to 1.0 mol increased the electrocatalytic activity of the oxide towards the oxygen evolution reaction (OER), the optimum improvement in apparent electrocatalytic activity being with 0.8 mol Cr. At E=600 mV versus Hg/HgO in 1 M KOH (25 °C), the apparent oxygen evolution current density (j_a) with the catalyst, CoFe_1.2Cr_0.8O₄, was ∼80 times greater than that observed with the base oxide (i.e. CoFe₂O₄). The OER on Cr-substituted oxides showed two Tafel slopes, one (b=42±1 mV per decade) at low overpotential and the other (b=66±6 mV per decade) at higher potential. The reaction order with respect to OH⁻ concentration was ∼1.3±0.1 for each electrocatalyst. The thermodynamic parameters for the OER, namely, standard apparent electrochemical enthalpy of activation (ΔH°_el^#), standard enthalpy of activation (ΔH°^#) and standard entropy of activation (ΔS°^#) have also been determined. It was observed that values of the ΔH°_el^# and ΔH°^# decreased with Cr-substitution in the CoFe₂O₄ lattice; the decrement, however, being the greatest with 0.8 mol Cr. The ΔS°^# values were largely negative varying between ∼−61 and −126 J deg⁻¹ mol⁻¹.     

The Use of para-Chlorobenzoic Acid (pCBA) as an Ozone/Hydroxyl Radical Probe Compound

Since OH^· radicals cannot be measured directly during an ozonation process, para-chlorobenzoic acid (pCBA) has been used recently as an OH^· radical probe compound during ozonation based on its very slow direct reaction with ozone and fast reaction with OH^· radicals. However, in experiments of this study it was shown that pCBA accelerated ozone decay. Furthermore, the formation of hydrogen peroxide was observed during this process. The formed H₂O₂ increases the decomposition of aqueous ozone and leads to enhanced formation of OH^· radicals. The chain reaction therefore changes to HO₂ ^? ion initiated decay of ozone instead of hydroxide ion, OH^?. Thus, an error in applying pCBA as a probe compound in low scavenger containing waters is likely to occur if the scavenging rate of pCBA makes up more than 5% of the total scavenging rate.     

pH-controlled the formation of 4-sulfocalix[4]arene-based 1D and 2D coordination polymers

Three manganese/4-sulfocalix[4]arene complexes, namely, {H[(C₂₈H₂₀O₁₆S₄)Mn(H₂O)₄Mn_0.5(H₂O)₂]}_n ·6nH₂O (1), {NH₄[(C₂₈H₂₀O₁₆S₄)Mn(H₂O)₄Mn_0.5(H₂O)₂]}_n · 5nH₂O (2), [(C₂₈H₂₀O₁₆S₄)Mn₂(H₂O)₈]_n · 6nH₂O (3), have been synthesized under different pH conditions. Complex 1, which exhibits a one-dimensional (1D) structure, is formed at [H⁺] = 2.0 mol L⁻¹. Reaction at pH 4 leads to another one-dimensional (1D) coordination polymer of 2. At pH 5, a two-dimensional (2D) coordination polymer of complex 3 is formed, showing clearly structural effects on pH response.     

Cathodic reduction of oxygen on copper and brass

In this investigation, the electrochemical reduction of oxygen on copper and brass has been studied using the ring-disc electrode technique in solutions containing chloride, sulphate and nitrate anions and ammonium cation. The E—I curves obtained in the forward and reverse direction of polarization for copper and brass in NaCl, Na₂SO₄ and NH₄Cl solutions are similar in nature and show two waves. However in (NH₄)₂SO₄ and NH₄Cl solutions are similar in nature and show two waves. The rate of oxygen reduction is the highest in ammonium sulphate for both copper and brass whereas it is the lowest in NH₄Cl in the case of copper. In Na₂SO₄ and NaCl solutions, the rate of oxygen reduction is higher on copper than on brass. Apparent Tafel slopes for oxygen reduction obtained for copper and brass vary from 65 mV to 240 mV depending upon the medium.The two steps observed with copper disc electrode have been identified as due to the reduction of oxygen to hydrogen peroxide and reduction of H₂O₂ to OH^? ion or water depending upon the pH of the solution. In acid chloride and sulphate media no H₂O₂ was detected, which suggests direct reduction to H₂O. The diagnostic plots of I_d/I_rυs ω^{?$\text{1}\text{2}$} employed by Bockris et al indicate that in Na₂SO₄ the reduction of oxygen to H₂O₂ takes place in a parallel reaction whereas in (NH₄)₂SO₄ both direct reduction of O₂ to water (or OH^? ion) and the reduction through intermediate H₂O₂ occur.     

Effects of H2O in EtOH–H2O disperse medium on the electrophoretic deposition of CaSiO3 fine powder

The effects of H₂O in the EtOH–H₂O disperse medium on the electrophoretic deposition (EPD) of CaSiO₃ fine powder were investigated. Fine CaSiO₃ powder with average diameter of 1·7 μm was prepared by the coprecipitation method. It was deposited on a stainless steel substrate by EPD in the disperse media with various H₂O concentrations (0–20·2 mass%) under a DC field of 50 V. The amount of the CaSiO₃ deposition increased with increasing H₂O up to 11·2 mass% but decreased rapidly beyond this concentration. The surface potential of the powder showed a similar trend as the amount of deposition against H₂O concentration. The effect of H₂O was summarized as follows: (1) the addition of positive charge on the surface of CaSiO₃ particles (2) the neutralization of the surface charge by OH⁻ caused by the dissolution of CaSiO₃ in the H₂O.