Giancarlo Sandonà

Synthesis and electrochemical properties of substituted fulleropyrrolidines.

Abstract A systematic investigation of the synthesis and electrochemical behavior of a series of fullerene derivatives containing an additional electroactive group, such as ferrocene and tetrathiafulvalene, is reported. The influence of the structure on the cyclic voltammetry is discussed.

Ferrocenyl fulleropyrrolidines: a cyclic voltammetry study

The cyclic voltammetry of some fullerene–ferrocene derivatives shows that these compounds possess very interesting electrochemical properties.

Advanced oxidation processes coupled with electrocoagulation for the exhaustive abatement of Cr-EDTA

Using Cr-EDTA as a model system, a two-step method has been investigated for the abatement of persistent chromium complexes in water. The treatment consists of an oxidative decomposition of the organic ligands by means of ozonization or electrochemical oxidation at a boron doped diamond (BDD) electrode, followed by removal of the metal via electrochemical coagulation. In the designed synthetic waste, EDTA has been used both as a chelating agent and as a mimic of the organic content of a typical wastewater provided by a purification leather plant. A crucial point evaluated is the influence of the oxidative pretreatment on the chemical modification of the synthetic waste and hence on the electrocoagulation efficacy. Because of the great stability of Cr complexes, such as Cr-EDTA, the classical coagulation methods, based on ligand exchange between Cr(III) and Fe(II) or Fe(III), are ineffective toward Cr abatement in the presence of organic substances. On the contrary, when advanced oxidation processes (AOPs), such as ozonization or electrooxidation at a BDD anode are applied in series with electrocoagulation (EC), complete abatement of the recalcitrant Cr fraction can be achieved. ECs have been carried out by using Fe sacrificial anodes, with alternating polarization and complete Cr abatement (over 99%) has been obtained with modest charge consumption. It has been found that Cr(III) is first oxidized to Cr(VI) in the AOP preceding EC. Then, during EC, Cr(VI) is mainly reduced back to Cr(III) by electrogenerated Fe(II). Thus, Cr is mainly eliminated as Cr(III). However, a small fraction of Cr(VI) goes with the precipitate as confirmed by XPS analysis of the sludge.

Zinc corrosion in NH4Cl and effect of some organic inhibitors

Abstract The corrosion of zinc in ammonium chloride, both pure and containing various organic compounds, has been studied by means of analytical and electrochemical methods as the determination of the zinc dissolved and the hydrogen evolved, I ( V ) curge recording and impedance measurements. The results obtained in a potential region near the zinc corrosion potential showed that the cathodic reaction of hydrogen discharge does not fit a simple exponential law because the Tafel coefficient appears to be electrode potential dependent. The experimental findings proved that only five of the 25 organic substances tested have been found to behave as zinc corrosion inhibitors (NDA, POELE, FORAFAC, TTH and TRITON in decreasing order of influence).

Electrochemical reduction of carbonyl compounds in aqueous media in the presence of β-cyclodextrin. Reduction of acetophenone and p-methoxyacetophenone in alkaline solution

Abstract The electrochemical reduction of acetophenone, 1, and 4-methoxy-acetophenone, 2, has been investigated in aqueous alkaline solutions, on mercury electrode, in the absence and in the presence of β-cyclodextrin (β-CD). Macroelectrolyses afforded mixtures of d,l and meso pinacols (head-to-head dimers) as preferential products and, in the case of 1, also head-to-tail dimers. These dimers showed some optical activity (enantiomeric excess of ca 24% for each diastereoisomer) when obtained by reduction in the presence of β-CD, whereas d,l pinacols did not. The same results were obtained by reduction of pre-formed 1:1 complexes with β-CD. A mechanism involving dimerization of radical anions is proposed for the formation of both type of dimers, and the asymmetric induction observed in the head-to-tail dimerization products is attributed to chiral effects exerted by β-CD on the guest molecules in this particular process.

Electrochemical behaviour of Cu-Zr and Cu-Ti glassy alloys

Abstract Hydrogen evolution is a fundamental reaction for better understanding of electrochemical activity of electrode materials. Amorphous Cu–Zr and Cu–Ti ribbons were produced by melt spinning methods and were characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. The electrocatalytic efficiency was evaluated on the basis of electrochemical data obtained from cathodic polarization curves carried out in both acid and basic medium at 25°C. The surfaces of amorphous materials were prepared, before electrochemical measurements, by immersion in HF solutions. The results were compared to those obtained on polycrystalline copper and on untreated ribbons. The HF treatment yielded a porous copper structure with a higher roughness factor which had improved electrocatalytic activity compared with that of polycrystalline copper electrode.

Dicyanomethylene derivatives of squaric acid: electrochemical behaviour and ESR investigation

The electrochemical behaviour of mono- and bisdicyanomethylene-substituted squaric acid dianion was investigated in dimethylformamide and acetonitrile on a Pt electrode. In cyclic voltammetry the oxidation of these dianions involves two successive, chemically reversible, one-electron processes, affording the corresponding radical anions and neutral species. The redox potentials are strongly dependent upon the number of dicyanomethylene substituents. Whereas the neutral species are stable only in the time scale of the voltammetric experiments, the radical anions can be accumulated by macroscale electrolysis. The ESR spectra of these radicals are reported. The monosubstituted dianion is protonated by electrochemical oxidation of hydrogen, whereas the disubstituted ones are not. Compounds obtained from the disubstituted dianions and the methyl viologen dication are also described.

Electrochemical behaviour of 1,2-dihydroxycyclobuten-3,4-dione in dimethyl formamide

Abstract The electrochemical behaviour of 1,2-dihydroxycyclobuten-3,4-dione (squaric acid, H 2 SQ) in dimethylformamide was investigated. On a Pt electrode, the reduction involves two successive, chemically reversible, one-electron processes, affording hydrogen and the basic species HSQ − and SQ 2− respectively. The oxidation of SQ 2− thus formed occurs in two consecutive, chemically reversible, one-electron steps, affording the radical anion SQ .− and the neutral species SQ°; the neutral species SQ° is stable in the time-scale of the voltammetric experiments only. In contrast, the chemically irreversible oxidations of H 2 SQ and HSQ − appear as more complicated processes. The oxidation of hydrogen in the presence of the electrogenerated basic species was also studied. A comparison with the results obtained on an Hg electrode is made and the occurrence of a different voltammetric behaviour, as regards the reduction process, is discussed briefly.

Oxidative dimerisation of some stabilised carbanions: a mechanistic investigation

Abstract The electrochemical oxidation of carbanions (p-CH3C6H4)Z1Z2C− (1−, Z1=Z2=CN; 2−, Z1=CN, Z2=COOEt; 3−, Z1=Z2=COOEt) was studied in CH3CN+n-Bu4NBF4 by experimental and simulated voltammetry, macroscale electrolyses and product analyses, with the aim of investigating the mechanism of the dimerisation process. The carbanions, generated by cathodic reduction of the conjugated acids, show a chemically irreversible oxidation peak whose position reflects the relative basicity of the anions (3−>2−>1−). Kinetic data deriving from peak potential analysis indicate that neutral radicals are generated at the electrode, and that a fast radical–radical coupling (DIM1 mechanism) is by far the most effective dimerisation process for all the substrates. Accordingly, the corresponding homodimers are formed quantitatively after exhaustive, one-electron macroscale electrolyses, also when the oxidation is performed in the presence of a second anion, oxidisable at more positive potentials. However, at high anion to radical ratio, a significant contribution of a radical–anion coupling (DIM2) can be involved when the anion is a good nucleophile. The position of the voltammetric oxidation peak of 3− is strongly affected by the presence of alkaline cations, which cause a shift of the wave towards more positive potentials (Li+>Na+>K+>NR4+) and also a decrease of the rate of the heterogeneous electron transfer in the same order. Both these effects can be explained by ion pairing.

Protonation kinetics of anionic intermediates in the electrochemical reduction of triphenylethylene: a disproportionation mechanism

The voltammetric behaviour of triphenylethylene (T) in dimethylformamide in the presence of traces of proton donor indicates that the radical anion (T–˙) is stable, the dianion (T2–) is rapidly protonated, and the resulting carbanion (TH–) is a relatively strong base. In the presence of added water, T–˙ decays via disproportionation followed by protonation of T2–. This was indicated by both homogeneous and voltammetric kinetics. In fact, in homogeneous conditions, the rate of decay is second-order in [T–˙], enhanced by water, and inhibited by T. Voltammetric kinetics further support the proposed mechanism showing that, under particular conditions, the reaction is rate-limited by the disproportionation step. Alternative mechanisms can be disregarded on the basis of both kinetic and thermodynamic data.