Achille De Battisti

Adsorption of acetonitrile at the mercury/aqueous solution interface. Comparison of constant-potential and constant-charge analysis

Summary The adsorption of acetonitrile (ACN) on Hg from aqueous solutions of NaF has been studied by means of both electrocapillary and capacity curves. Full analysis of data has been carried out at constant charge as well as at constant potential. Both types of analysis have been found to give essentially the same picture of the adsorption process. Isotherms have been found to be congruent with respect to both of the electrical variables in the region more negative than the point of maximum in adsorption, whereas they are non-congruent with respect to both in the positive region. A Langmuir isotherm is obeyed in the former region, while in the latter the system is described by a Frumkin isotherm with positive interaction constant, corresponding to lateral repulsion, increasing as the charge or the potential becomes more positive. Reasons for this behaviour are discussed. The capacity of the inner layer has been also determined at constant amount adsorbed, and hypotheses have been made on the detailed structure of the inner layer in the presence of ACN.

Electron transfer reactions at conductive diamond electrodes

The electrochemical behavior of the iron(III)/iron(II) and the ferri/ferro–cyanide redox couples in aqueous media has been investigated at conductive diamond, to obtain information on the properties of this electrode material. The investigation has been carried out at as prepared and oxidized diamond surfaces, under conditions of different concentration and temperature. The electron transfer kinetics has been followed by cyclic voltammetry, quasi-steady polarization (low-field conditions) and AC impedance, and the results have been interpreted on the basis of the Hush model, attempting its extension to the ferri/ferro–cyanide redox couple as well.

Electro-oxidation of Some Phenolic Compounds by Electrogenerated O3 and by Direct Electrolysis at PbO2 Anodes

The oxidative degradation of phenolic compounds (4-chlorophenol and 4-nitrophenol) was studied using different electrochemical systems involving ozone formation at PbO 2 anodes: (i) direct electrolysis at constant current; (ii) ex-situ use of O 3 and (iii) combined use of anodically generated stream of O 3 /O 2 fed into the cathode where H 2 O 2 is electrogenerated by O 2 reduction. We show that the latter advanced oxidation method gives the best results: it is a Fenton-type degradation of the target pollutants taking place in the cathodic compartment by reason of the highly oxidizing environment brought about by radicals that are formed mainly in the reactions of 0 3 with OH ― and HO 2 ― .

MICROSTRUCTURE AND ELECTRICAL-PROPERTIES OF IRO2 PREPARED BY THERMAL-DECOMPOSITION OF IRCL3.X H2O - ROLE PLAYED BY THE CONDITIONS OF THERMAL-TREATMENT

Abstract Some features of the thermal decomposition of IrCl 3 ·3 H 2 O in an oxygen atmosphere are presented. The formation of IrO 2 phases has been followed by X-ray diffractometry, and the onset of definite reflections has been documented for samples pyrolyzed at 350° C, although moderate weight losses are observed by thermogravimetry at this temperature. The effect of the thermal history of iridium oxide coatings on their structure has been studied by means of electrical resistance measurements. An explanation of the results has been attempted in terms of impurity segregation to the oxide grain boundaries.

Adsorption of succinonitrile at the mercury-aqueous solution interface: Unusual role of a fixed dipole in the adsorbed molecule

Abstract The adsorption of succinonitrile (SN) on a polarized Hg electrode has been studied from 0.25 M NaF aqueous solutions by means of both capacity and electrocapillary curves. Adsorption isotherms have been found to be strictly congruent with respect to both charge and potential. The charge of maximum adsorption is at −4.6 μC cm−2 and ΔGads0, the free energy of adsorption at zero coverage, decreases more rapidly as the charge is made positive with respect to OmaxM than on the other side. The adsorption obeys a Frumkin isotherm with a=−1.87 corresponding to attraction and saturation concentration of 4.8×10−10 mol cm−2 corresponding to an area per molecule of about 34.5 A2. Experimental results suggest that SN adsorbs flat on the surface but, unlike acetonitrile and propionitrile, the CN groups are slightly turned towards the solution. This is responsible for the absence of electronic interaction of the CN groups with the metal at positive charges, which is apparent with the other two nitriles. Implications of the particular structure of the adsorbed molecule on the other adsorption parameters are discussed in detail.

Adsorption of butane-1,4-diol at the Hg–aqueous solution interface. Transition with polarization between two ideal adsorption models

The adsorption of butane-1,4-diol on a polarized Hg electrode from both NaF and Na2SO4 solutions has been studied by means of electrocapillary and capacity curves. Analysis has been carried out both at constant charge and at constant potential. Irrespective of the choice of the electrical variable, adsorption has been found to conform to congruent Langmuir isotherms on the negative side of the adsorption maximum, and to non-congruent isotherms on the other side. Results suggest that molecules adsorb flat on the surface. Non congruence may be described in terms of a saturation coverage linearly decreasing with electric field. The inner layer capacity at constant amount adsorbed has been determined up to saturation coverage over a wide range of electric field. Results are discussed in terms of a polarization-dependent adsorbate–solvent interaction on the surface. A detailed molecular model of the adsorption layer is suggested.

Electrochemistry of the aqueous europium(III)/europium(II) redox couple at conductive diamond electrodes

In the present paper, the electrochemical behavior of the Eu(III)/Eu(II) redox couple has been investigated at conductive diamond thin film electrodes. Measurements have been carried out by means of cyclic voltammetry, quasi-steady polarization curves (low-overpotential approximation) and ac impedance. The experimental picture indicates that the kinetics of electron transfer are more complex than that suggested by a classical outer-sphere mechanism.

Depth profiles and electrochemical properties of IrO2 electrocatalysts stabilized with TiO2

IrO2/TiO2 mixed-oxide electrocatalysts of different composition have been studied by Rutherford backscattering spectrometry (RBS) and cyclic voltammetry. Obtained results indicate that enrichment of titanium oxide in the outermost part of the coatings occurs. Comparison with data for the RuO2/TiO2 system suggests that the enrichment is partially controlled by the concentration of the noble-metal ion in the precursor salt mixture and by the nature of the anions present in it, while it is practically independent of the nature of the noble-metal ion itself. Plots of voltammetric charge against iridium concentration in the coatings exhibit maxima for a sample with 30 mol% IrO2. An interpretation of the electrochemical behaviour on the basis of the shape of composition depth profiles has been attempted.

X-Ray diffraction characterization of iridium dioxide electrocatalysts

An X-ray diffraction (XRD) line-broadening analysis of coatings of pyrolytic iridium dioxide supported on amorphous silica microbeads is reported. The influence of the temperature of pyrolysis and of the annealing treatments on the average crystallite sizes of the IrO2 and Ir phases have been studied in detail. The values obtained for IrO2 are lower than those obtained from an analgous RuO2 system. However, the analysis suggests that in both cases the crystalline phase grows within an amorphous microcrystalline environment where impurities are mostly segregated in the amorphous phase and then released to the atmosphere. The lattice parameters are also calculated. Compared with the reference values, slightly greater values are obtained both for IrO2 and for Ir.

The electrical double layer between mercury and aqueous NaBF4 solutions: II. Adsorption of NaBF4 from pure salt solutions

Summary The adsorption of BF4− ions on polarized Hg electrodes has been studied from aqueous solutions of pure NaBF4 for concentrations ranging from 0.01 to 1 mol l−1 using both capacity and electrocapillary measurements. Adsorption resembles features observed with other structure-breaking ions. BF4− is rather weakly adsorbed but shows approach to saturation at high positive charges and high concentrations. The adsorption equilibrium at intermediate charges may be described by a Temkin isotherm. The charge dependence of the free energy of adsorption may be considered as linear at not too positive charges. The components of the inner layer capacity have been evaluated and the large effect of specific adsorption on the parameters of the inner layer is discussed in terms of possible increase in the thickness and decrease in the apparent dielectric constant of the inner layer as a result of ion-solvent interaction.