Carlos Borrás

Competitive electrochemical oxidation of p-chlorophenol and p-nitrophenol on Bi-doped PbO2

The initial stages of oxidation of aqueous solutions of p -chlorophenol (pcp) and p -nitrophenol (pnp) on Bi-doped PbO2 electrodes have been studied. From deconvolution and analysis of UV � /Vis spectra of the solutions obtained during electrochemical oxidation, benzoquinone and aliphatic acids were identified as the primary oxidation intermediates; oxidation of benzoquinone was found to be the slow step during the early stages of the electrochemical combustion process. The effect of competing adsorption of pcp and pnp on Bi � /PbO2 was also examined, and the presence of pnp in solution was found to inhibit the rate of oxidation of pcp during concurrent oxidation of both phenols. # 2003 Elsevier Ltd. All rights reserved.

Characterization of a carbon paste electrode modified with tripolyphosphate-modified kaolinite clay for the detection of lead

We report about the use of carbon paste electrode modified with kaolinite for analytical detection of trace lead(II) in domestic water by differential pulse voltammetry. Kaolinite clay was modified with tripolyphosphate (TPP) by impregnation method. The results show that TPP in kaolinite clay plays an important role in the accumulation process of Pb(II) on the modified electrode surface. The electroanalytical procedure for determination of Pb(II) comprised two steps: chemical accumulation of the analyte under open-circuit conditions, followed by electrochemical detection of the pre-concentrated species using differential pulse voltammetry. The analytical performance of this system has been explored by studying the effects of preconcentration time, carbon paste composition, pH, supporting electrolyte concentration, as well as interferences due to other ions. The calculated detection limit based on the variability of a blank solution (3s(b) criterion) for 10 measurements was 8.4×10(-8) mol L(-1), and the sensitivity determined from the slope of the calibration graph was 0.910 mol L(-1). The reproducibility (RSD) for five replicate measurements at 1.0 mg L(-1) lead level was 1.6%. The results indicate that this electrode is sensitive and effective for the determination of Pb(2+).

Electrooxidation of Aqueous p-Methoxyphenol on Lead Oxide Electrodes

Oxidation of p-methoxyphenol (pmp) in aqueous solution on bismuth-doped lead oxide was studied, and the effects of the initial pmp concentration, applied potential and hydrodynamic conditions upon the oxidation rate were identified. Under all conditions studied, the concentration decay of pmp during electrooxidation follows first—order reaction kinetics. Through analysis of rotating ring-disc currents, the faradaic efficiencies for oxidation at various concentrations of pmp in solution were determined. Using u.v.—vis. and H1RMN spectroscopy for solution analysis, it is shown that partial oxidation of pmp occurs in chloride-free aqueous solutions. The principal products were p-benzoquinone and maleic acid, with low production of CO2 up to 1000 C dm−3 charge. Mineralization to CO2 was considerably improved upon addition of chloride ions to the solution. In situ FTIR spectra of the electrode surface during electrolysis indicated that the presence of chloride ions enhances the mineralization of pmp by reaction of benzoquinone with anodically generated hypochlorite.

Electrochemical nucleation and the classical theory: Overpotential and temperature dependence of the nucleation rate

A comparison between the activation energies for critical nuclei formation obtained from both the classical nucleation theory and Arrhenius kinetic analysis has been proposed. Results obtained indicate that such comparison is hampered by temperature-induced changes of the surface energies, modifying the contact angles and correspondingly the activation energies for nucleation.

Measurement of phenols dearomatization via electrolysis: the UV-Vis solid phase extraction method.

Dearomatization levels during electrochemical oxidation of p-methoxyphenol (PMP) and p-nitrophenol (PNP) have been determined through UV-Vis spectroscopy using solid phase extraction (UV-Vis/SPE). The results show that the method is satisfactory to determine the ratio between aromatic compounds and aliphatic acids and reaction kinetics parameters during treatment of wastewater, in agreement with results obtained from numerical deconvolution of UV-Vis spectra. Analysis of solutions obtained from electrolysis of substituted phenols on antimony-doped tin oxide (SnO(2)--Sb) showed that an electron acceptor substituting group favored the aromatic ring opening reaction, preventing formation of intermediate quinone during oxidation.

Current transient study of the kinetics of nucleation and diffusion-controlled growth of bimetallic phases

A model to describe the kinetics of nucleation and diffusion-controlled growth of bimetallic phases has been developed, and analytical expressions have been obtained for the elucidation of nucleation kinetics through determination of the number density N0 of active sites and their nucleation frequency A, from experimental current transients obtained under potentiostatic control. The validity of the model has been demonstrated with the electrodeposition of Ag–Hg phases at two distinct compositions.

Electrochemical oxygen transfer reactions: electrode materials, surface processes, kinetic models, linear free energy correlations, and perspectives

This work summarizes progresses achieved in electrochemical oxygen transfer reactions from water to organic pollutant molecules on metal oxide electrodes during the past two decades. Fundamental understanding of the dynamics of the electrochemical oxygen transfer reaction is of crucial importance for the development of key concepts of electrocatalytic processes, leading to the implementation of environmental electrochemistry wastewater treatment schemes with rational design of the suitable electrocatalytic systems. We discuss the current knowledge on the electrochemical oxygen transfer reaction, emphasizing the importance of surface processes in order to generalize mechanistically the experimental results obtained on different electrode materials, describing also the practical kinetic models developed and their implications. From the information gathered in this review, it is apparent that explanations for the kinetics of the reactions in relation to the structure of the organic compounds involved is lacking, hence that new information about structure-reactivity relationships is needed. We show in particular that the open circuit decay of the concentration of radical cations, obtained from spectroelectrochemical data, allows correlating the structure of adsorbed states with reactivity during oxygen transfer reactions, pointing as well to research efforts required to understand the catalytic performance of metal oxide electrodes in decomposing organic compounds strongly adsorbed on their surfaces. Finally, some perspectives for future research in this area are briefly commented.