Enrique Brillas

Iron(II) catalysis of the mineralization of aniline using a carbon-PTFE O{sub 2}-fed cathode

For practical electrochemical treatment of industrial wastewaters, alternative electrolytic systems with higher oxidation power need to be developed. Mineralization of aniline in acidic solution (pH 3.0) has been studied using an undivided cell with a carbon-polytetrafluoroethylene O{sub 2}-fed cathode which continuously electrogenerates H{sub 2}O{sub 2} operating at constant current. A Ti/Pt/PbO{sub 2} electrode or a Pt sheet was employed as the anode. While anodic destruction of aniline yields poor mineralization, addition of iron(II) to the solution causes a notable increase in its mineralization rate. In the so-called electro-Fenton process, the main oxidizing agent is the OH radical which can be produced at the anode reaction layer by oxidation of water and also in the bulk solution via Fenton`s reaction between Fe{sup 2{minus}} and electrogenerated H{sub 2}O{sub 2}. A faster and complete mineralization of the pollutant in the presence of iron(II) can be easily achieved by the photoelectro-Fenton process in which the solution is irradiated with UV light of {lambda}{sub max} = 360 nm during electrolysis. The reactions involved in these processes are discussed.

Oxidized and reduced poly(2,5-di-(-2-thienyl)-pyrrole): solubilities, electrodissolution and molar mass

Abstract Oxidized poly(2,5-di-(-2-thienyl)-pyrrole was synthesized electrochemically from a 5 to 20 mM monomeric solution in 0.1 M LiClO 4 + acetonitrile (ACN). The electrochemical behaviour of films was studied in 0.1 M LiClO 4 aqueous solution, where an insoluble reduced form was found. Solubilities of both oxidized and reduced states were determined in DMSO, acetone and ACN. Their densities and electrical conductivities were also obtained. The solubility of the reduced form is about 15 times higher in DMSO and ACN than that of the oxidized form. The oxidized polymer is insoluble in ACN solutions of perchlorate salts. The high solubility of the reduced state in this medium allows a quantitative electrodissolution of the polymer following the reduction process. Results from electrochemical experiments, ultramicrogravimetry and elemental analysis indicate an average presence of 0.65 ClO 4 − ions per monomeric unit incorporated in the oxidized polymer. Both states contain soluble oligomers with from two to six monomeric units, as detected by mass spectrometry (fast atom bombardment). The molecular mass of these oligomers indicates a loss of two protons per monomer incorporated in the polymer, in agreement with an electropolymerization mechanism involving polycondensation of radicals.

Electrogeneration and solubilities of oxidized poly(2,5-di-(-2-thienyl) -thiophene)

Solutions of 2,5-di-(-2-thienyl)-thiophene (SSS) were studied by cyclic voltammetry, chronoamperometry and chronopotentiometry in 0.1 M LiClO4 + acetonitrile solution. Very reproducible weights of polymer were electrogenerated by galvanostatic experiments. Studies performed at different current densities, ranging from 0.2 to 1 mA cm−2, and for different concentrations of SSS, ranging from 5 to 20 mM, prove the faradaic nature of the electropolymerization process. The productivity of the current was 2.7 × 10−3 mgmC−1 and 1.15 electrons were consumed to incorporate one molecule of SSS into the oxidized polymer. Density, conductivity and solubilities of the oxidized poly(SSS) in DMF, CH2Cl2, CHCl3, acetone and acetonitrile were determined. UV-vis measurements of saturated solutions in such solvents indicate that the dimer is the most important soluble component. The elemental analysis shows the presence of 0.38 ClO4− counterions and 0.20 ionic couples of Li+ClO4− per monomeric unit in the oxidized polymer.

Product analysis from D2O electrolysis with Pd and Ti cathodes

Abstract The enrichment of tritium in the electrolyte and incorporation of T, Li and Pt in cathodes during the electrolysis of 0.1 M LiOD solutions with Pd and Ti cathodes in open cells have been studied. All electrolytes show an increase in their tritium activity which is explained by considering values for the T-D separation factor of all cathodes lower than 1. Accumulation of small amounts of T in the Pd bulk, proceeding from the absorption of the species pre-existing in the electrolyte, has been detected by electrolytic transfer of accumulated tritium to a 0.1 M LiOH solution, as well as by extraction of gases absorbed in the cathode, which were identified by mass spectrometry. Small quantities of Li and Pt are also incorporated in Pd and Ti cathodes, which increase by raising the current density. SIMS analysis of both cathodes show a preferential accumulation of Li and H in their surface layers and confirms the absence of T in Ti.

Kinetics and mechanisms of the oxidation by permanganate of L-alanine

The oxidation of L-alanine by permanganate ion in aqueous phosphate buffers is autocatalysed by the inorganic reaction product, a solube form of colloidal manganese dioxide temporarily stabilized in solution by adsorption of phosphate ion on its surface. The rate of the non-catalytic reaction pathway is first-order in both the oxidizing and reducing agent, is not affected by potassium chloride addition to the solution, and increases with the pH of the medium, its associated activation energy being 74.0 kJ mol–1. The rate of the catalytic reaction pathway is first order in both the oxidizing and autocatalytic agent, follows the Freundlich adsorption isotherm as far as the reducing agent concentration is concerned, is not affected by potassium chloride addition to the solution, and increases with the pH of the medium, its associated activation energy being 67.2 kJ mol–1. Mechanisms consistent with the experimental data are proposed.

Electrogeneration of poly(2,5-di-(2-thienyl)-pyrrole) in acetonirile. Kinetics, productivity and composition of the oxidized form

Abstract Poly(2,5-di-(2-thienyl)-pyrrole) (poly(SNS)) was synthesized on a Pt electrode by flow of an anodic constant current density of 0.5 mA cm −2 through a solution containing the monomer SNS and LiClO 4 in acetonitrile. The obtained polymer weight was followed by ‘ex situ’ ultramicrogravimetry. Kinetics were followed as a function of temperature, monomer concentration and electrolyte concentration. Two different kinetic regions were found at lower or greater polarization times than 100 s. Productivities of the polymerization charge (as mg of polymer produced per mC of consumed charge), composition (by elemental analysis) and the subsequent number of electrons required to incorporate a monomeric unit into the polymer were determined from different samples.

Oxidation of 2,4-dibromo-6-nitroaniline in aqueous sulphuric acid solutions on a platinum electrode

The electrochemical oxidation of 2,4-dibromo-6-nitroaniline in aqueous sulphuric acid solutions at concentrations cacid > 9.0 mol dm–3 on a platinum electrode has been studied by the use of a rotating-disc electrode, cyclic voltammetry and controlled-potential electrolysis. A single oxidation process is found at cacid 11.0 mol dm–3. Both species are present in solution over the acid concentration range 11.0–13.0 mol dm–3. Species (I) has only the amino group protonated, whereas species (II) has both the amino and nitro groups protonated. Voltammetric results allow one to establish that both species follow analogous decomposition pathways to produce the corresponding p-benzoquinone. Each electroactive form is initially oxidized in a reversible one-electron step to generate its radical cation, which is subsequently deprotonated, and this irreversible reaction is the rate-determining step. The resulting radical undergoes a reversible one-electron oxidation to form a dication. Hydrolysis of this dication gives the p-benzoquinonimine derivative, which is further hydrolysed to the corresponding p-benzoquinone. The loss of one proton from the benzene ring of the initially electrogenerated radical cation from species (II) produces the electroactive species of the second process. The initial irreversible two-electron charge transfer of this species is the rate-determining step of the second process.

Electrochemical reduction of 2-cyclohexen-1-ones in a hydroethanolic medium

Electrohydrodimerization of 2-cyclohexen-1-one, 4,4-dimethyl-2-cyclohexen-1-one and 4,4-diphenyl-2-cyclohexen-1-one in buffered hydroethanolic solutions containing 50%(v/v) ethanol over the pH range 1.0–12.1 has been studied by polarography, cyclic voltammetry and controlled-potential coulometry. All substrates give two one-electron processes corresponding to the reduction of their respective protonated and unprotonated forms. At pH values 5.5, leads to the formation of the same final hydrodimers by an initial generation of the anion radical via a one-electron reduction step, followed by protonation and dimerization of the neutral radical. In the pH range ca. 5.5–8.5 the two processes of each substrate compete and their relative contribution is governed by the rate of establishment of the protonation equilibrium between their unprotonated and protonated forms. Voltammetric results show that the rate-determining step of each process depends on solution pH.

Electrodissolution of poly(2,5-di-(2-thienyl)-pyrrole) in organic and hydro-organic media

Abstract Oxidized poly(2,5-di-(2-thienyl)-pyrrole) (poly(SNS)) films on Pt were reduced in common organic solvents under chronopotentiometric conditions. Electrodissolution was observed during reduction, being followed by ultramicrogravimetry. Thin films give an overall electro-dissolution in 0.1 M LiClO 4 + acetonitrile solutions. The ratio between charges consumed during polymer stripping and those spent for its electrogeneration was 0.22–0.28. Between 0.24 and 0.34 electrons per monomeric unit were found to be required to electrodissolve the polymer. The electroreduction of oxidized poly(SNS) was also studied in aqueous mixtures of DMF, acetonitrile, acetone or methanol, containing 0.1 M LiClO 4 . In the water-rich regions, a constant weight loss of about 20% was always found, corresponding to the percentage of perchlorate ions lost during the electroreduction process, as confirmed by elemental analysis. Insoluble fractions of reduced poly(SNS) obtained in hydro-organic media contained small oligomers, from the dimer to the octamer, as detected by mass spectrometry-fast atom bombarding (MS-FAB). Scanning electron microscopy (SEM) micrographs of deposits produced in a 0.1 M LiClO 4 aqueous solution showed that reduced polymer films are spongy.

Inert carbon free radicals. 13. New free radicals of PTM (perchlorotriphenylmethyl) series with meta functionalization

Abstract The synthesis of new organic free radicals of the PTM (perchlorotriphenylmethyl) series with different substituents in the 3- and 5-positions of one phenyl ring and their α- H precursors are described. All these radicals are stable red solids with high melting points. Their electron spin resonance spectra in solution and their magnetic susceptibilities in solid are reported and commented upon. Electrochemical behavior of radical 5 by means of cyclic voltammetry is also presented.