El Mustapha Belgsir

A kinetic analysis of the electro-oxidation of ethanol at a platinum electrode in acid medium

In order to establish the kinetic laws which govern the oxidation process of ethanol on smooth platinum electrodes in acid medium, electrolyses of ethanol were carried out under different experimental conditions. The influence of the initial ethanol concentration and of the potential plateau of electrolysis were investigated and allowed us to improve our understanding of the reaction mechanism. In addition, some ir reflectance spectroscopy experiments were performed to identify the adsorbed intermediates and the reaction products.

Electro-oxidation of ethanol on gold : analysis of the reaction products and mechanism

Although the electrocatalytic properties of gold are not as good as those of platinum, the oxidation of small alcohol molecules, such as ethanol, is possible at gold both in acid and alkaline media. By using programmed potential electrolysis coupled with chromatographic analysis of the products formed, the nature and the variation of their concentrations as a function of pH and time have been determined. In alkaline medium, the only product detected was acetic acid. On the other hand, in perchloric acid medium, the reaction products are highly dependent on the value of the oxidation potential, with the following successive steps: ethanol→acetaldehyde→acetic acid. A general tentative mechanism is proposed and discussed.

Kinetics and mechanisms of the electrocatalytic oxidation of glycerol as investigated by chromatographic analysis of the reaction products: Potential and pH effects

Abstract The effects of the potential and the pH on the electrocatalytic oxidation of glycerol at platinum electrodes were investigated by liquid chromatography during controlled potential long-term electrolyses. The selectivity towards the production of glyceraldehyde was found to depend greatly on the applied potential and on the pH of the electrolyte. In an acid medium, the dissociative oxidation of glycerol, leading to formic acid, oxalic acid and glycolic acid, is the main process observed when the potential of electrolysis is set in the so called “oxygen region”. Corversely, the selectivity towards the formation of glyceraldehyde is very high when the applied potential is chosen before the beginning of the oxidation of the electrode surface. In alkaline medium, very few dissociation products were detected. However the conversion rate is sensibly increased.

Electrocatalytic reduction of dioxygen at macrocycle conducting polymer electrodes in acid media

Functionalized transition metal macrocyclic monomers were synthesized in order to prepare electron-conducting polymer-based electrodes. Cobalt tetra(o-aminophenyl)porphyrin and cobalt 4,4′,4″,4′″-tetraaminophthalocyanine macrocycles were electropolymerized at a vitreous carbon electrode to give, respectively, poly(CoTAPP) and poly(CoTAPc). The electrocatalytic behaviour of such electrodes, particularly the influence of the macrocyclic skeleton towards dioxygen reduction, was examined. Both electrodes are active in oxygen reduction. However, the modified electrode containing cobalt phthalocyanine seems to be the most active. The Tafel slope determined at 20°C at the onset of the reduction wave for both modified electrodes was 58 ± 5 mV/decade, while in a higher overpotential region it was close to 116 ± 20 mV/decade. The number of electrons involved during the reduction of dioxygen was determined using the Koutecky-Levich equation. The main reaction product at a poly(CoTAPP) electrode is hydrogen peroxide, whereas at a poly(CoTAPc) electrode the product is mainly water, since the last electrode allows a four-electron reduction process to occur at higher overpotentials.

Selective oxidation of carbohydrates on Nafion®-TEMPO-modified graphite felt electrodes

The TEMPO-mediated oxidation of carbohydrates was investigated in heterogeneous conditions. The aminoxyl radicals were immobilised in a Nafion® film on graphite felt electrodes. This selectively gave rise to the corresponding uronic acids. Analytical results targeted towards the kinetics and the stability of the electrodes are also reported.

The oxidation of formaldehyde on high overvoltage DSA type electrodes

Neste trabalho a oxidacao eletroquimica do formaldeido e estudada sobre eletrodos dimensionalmente estaveis preparados por decomposicao termica de percursores (correspondentes cloretos). Foram utilizados como eletrodos de trabalho: Ti/Ir0.3Ti0.7O2, Ti/Ru0.3Ti0.7O2 e Ti/Ir0.2Ru0.2Ti0.6O2. As eletrolises foram realizadas galvanostaticamente em uma celula do tipo filtro prensa em solucoes 0,5 mol L-1 H2SO4 com concentracao inicial de formaldeido de 100 mmol L-1. A concentracao de formaldeido decresce rapidamente com o tempo de eletrolise sendo que o eletrodo ternario (Ir + Ru + Ti) e o que apresenta maior atividade catalitica. O ânodo contendo somente Ir, apesar de maior carga superficial e o de menor atividade eletrocatalitica. Para a oxidacao de acido formico, formado pela oxidacao de formaldeido, a presenca de Ir na composicao do ânodo nao favorece o processo, sendo o ânodo contendo somente Ru o mais efetivo para este processo.

Electrocatalytic oxidation of aliphatic diols on platinum and gold Part I: Effects of chain length and isomeric position

Abstract The electrochemical reactivity of propanediol, butanediol, pentanediol, hexanediol and heptanediol, on polycrystalline platinum and gold electrodes, was studied in acid and alkaline media. The effect of the number of carbon atoms, i.e. the chain length, was observed. The apparent energies of activation were evaluated systematically and discussed in relation to the molecular structure of the different diols. In the case of the butanediol isomers, an effect of the alcohol group positions on the activation energy and reaction mechanisms was clearly shown.

A kinetic study of the oxidation of glyoxal on platinum-based electrodes by chromatographic techniques

The electrocatalytic oxidation of glyoxal in perchloric acid on smooth and modified platinum electrodes was investigated by quantitative analysis of the electrolysis products using gas and liquid chromatography. A mechanistic study confirmed that glyoxal adsorption is a dissociative process leading mainly to CO2 (60%) and formic acid (40%). Modification of the electrode surface by lead adatoms allowed glyoxal oxidation to be carried out at different controlled potentials. On the other hand, glyoxal consumption was shown to be a first-order reaction. It was concluded that the most important parameters controlling the selectivity of glyoxal oxidation are the electrode potential and the nature of the foreign metal adatoms.

Electrocatalytic oxidation of aliphatic diols Part V. Electro-oxidation of butanediols on platinum based electrodes

Abstract During prolonged electrolyses, the effects of the nature of the electrolyte, of the upd-lead adatoms and of the oxidation potential on the kinetics and the selectivity of the electrocatalytic oxidation of butanediols on platinum are investigated. It is particularly shown that the sulphate anions play an important role in the reaction rates and the selectivity. The reactions, under determined experimental conditions, were found to be very selective towards the first step oxidation products (aldehydes and ketones).

Electrocatalytic oxidation of aliphatic diols Part II. Effect of stereoisomerism on the oxidation of 2,3-butanediols at a platinum electrode in acidic media

Abstract A strong and rather unexpected effect of stereoisomerism was observed during the electro-oxidation of the 2,3-butanediol stereoisomers. This first original observation appears to be dependent on the absolute configuration of the reactive species and on the superficial structure of the electrode surface. The activity of the electrode towards the oxidation of 2,3-butanediol stereoisomers was also found to be greatly affected by the nature of the anion of the supporting electrolyte. Electrolysis of each stereoisomer leads very selectively to the corresponding acetoine, whereas only traces of diacetyl and CO 2 are detected.