Contribution a l'etude des effets catalytiques exerces par les composes soufres a l'electrode a gouttes de mercure. Cas de la thiouree

The influence of thiourea and some of its N-alkyl-derivatives on the reduction of various metallic cations (Ni²⁺, Co²⁺, Co²⁺) has been studied in neutral medium at the dropping mercury electrode. The polarographic curves exhibit kinetic prewaves reaching a 2^?_e amplitude at high concentration. In addition fairly large extra-currents can be detected at verynegative potentials. By resorting to classical and pulse polarography and to macro-electrolysis, it has been demonstrated that these currents are related to both the electroreduction of thiourea with production of sulphide and cyanide ions and catalytic discharge of water itself. The reaction mechanism is discussed in terms of the structure of the organic compound and the properties of the electrode metal surface, in connection with similar phenomena observed in the presence of thiocyanate ion (reduction of the thione group) and sulfur proteins (catalytic hydrogen evolution).     

Etude par resonance paramagnetique electronique de noirs de carbone traites par l'azodiisobutyronitrile en absence d'oxygene

Various carbon blacks were treated in absence of oxygen with azobisisobutyronitrile (AIBN). Samples of non-graphitized carbon blacks (Spheron 6, P33, Sterling FT) and graphitized carbon blacks (Graphon, P33 (HTT)) were studied by ESR before and after the AIBN treatment. The line width, g-factor, and the contributions of localized and conduction carrier spins were determined. The results obtained lead the authors to conclude that in the process of fixation of the products of the thermolysis of the AIBN no radicals are involved.     

Etude par thermoluminescence de l'hydration de l'aliminate tricalcique

Thermoluminescence shows that the hydration of tricalcium aluminate is deeply influenced by structural defects of the solid. Hydration reaction excites centers of the solid detected by thermoluminescence; this energy storage is related to trapped electrons. Hydration of tricalcium aluminate in the presence of gypsum is not influenced by traps but thermoluminescence allows to corroborate the well-known mechanism of this reaction.     

Adsorption de l'acide maleique et de l'hydrogene sur le platine-platine en presence de cuivre adsorbe

According to a previous study, the catalytic hydrogenation of maleic acid on platinum catalyst is inhibited by copper adsorption. A copper adatom deactivates five accesible atoms of platinum. A cyclic voltammetry study of hydrogen and maleic acid adsorptions on platinum and on platinum modified by copper adatoms accounts for this result.Indeed a copper adatom inhibits the adsorption of one hydrogen atom on one platinum atom and of one maleic acid molecule on 5 ± 1 platinum atoms.     

Electrolyse d'acides dans l'acetronitrile—relation entre les transpositions et l'adsorption

The anodic oxydation of the anions cyclopropylacetate, cyclobutanecarboxylate and allylacetate in acetonitrile leads to products of addition to the solvent. These addition products are mixtures of cyclopropylcarbinyl and cyclobutyl compounds and of unsaturated non-cyclic compounds, of which the relative amounts differ form one carboxylate to another.These results are interpreted in terms of competition between desorption and the isomerisation of the carbocations which are initially formed. The carbocations are adsorbed on the electrode in the non solvated state and can be considered as free species undergoing isomerisations only during their lifetime in the adsorbed state. Once desorbed, they are trapped extremely rapidly by the solvent and can no longer isomerize.     

Etude potentiostatique et spectroscopique de l'aluminium recouvert par une couche d'oxyde: effet de differents anions

The effect of different anions on the formation and the breakdown of the passivating film on aluminium was studied by means of stationary and transient potentiostatic measurements associated with analysis of the surface region of the metal by photoelectron spectrometry (ESCA). The progressive penetration of chloride ions into the oxide layer, dependent on the electrode potential was pointed out. The perchlorate ion was shown to be reduced at the film-solution interface.     

Examen de l'influence d'une activation sur un coke de saccharose basse temperature par des methodes de resonance magnetique

The nuclear dynamic polarization method (NDP) was used for a determination of the nature of the interactions between the nuclei of liquids or gases adsorbed on a saccharose charcoal and its paramagnetic centers. Various chemical processes of activation used by different authors to increase the adsorbing surface unfortunately modify the electronic properties of the charcoal (vanishing of the paramagnetic centers). A new method of preparation of a low temperature active charcoal is described. E.P.R. experiments made on this charcoal lead to the conclusion that this new process does not modify fundamentally its paramagnetic properties. NDP investigations on molecules adsorbed on such charcoal show a molecular sieve effect, hence an increase of the porosity. Furthermore, such experiments made with various fluorine compounds show a scalar interaction between charcoal electrons and fluorine nuclei, that does not exist between electrons and protons. Differences between effects for various molecules are also investigated.     

Sur l'electrosorption d'ethylxanthate au cours de la polarisation anodique d'electrodes de platine et de galene

The main results concerning the galena flottation by xanthates which are relevant from an electrochemical point of view are presented. The usual kinetic theories were unable to explain the results obtained in previous electrochemical studies on xanthate oxidation at various electrodes. A modified treatment is reported. It reconciles data indicating the same phenomenology as that taking place when a monolayer is chemisorbed as well as quantitative results suggesting the presence of a thick layer. The result previously obtained with platinum and galena electrodes can thus be quantitatively interpreted. It is concluded that the complexity of the chronoamperometric data derives more from the adsorbed layer structure than from the reaction scheme. The latter is composed of the xanthate physisorption followed by chemisorption. The adsorbed phase with tens of monolayers is assumed to be analogous to a lamellar phase of a liquid crystal. It is electrically conducting and its thickness is not dependent on the substrate. However. the nature of the latter controls the oxidation rate and the possible occurrence of passivation. The electrochemical results and those obtained by other technics on dispersed systems are well can be interpreted by the proposed model.     

Etude du mecanisme de l'inhibition du zinc en milieu HCl n par les sels de phosphonium

The different steps of mechanism for zinc corrosion inhibition in a 1N HCl solution with phosphonium salts Ph_{4 ? y} P⁺nBu_y, X^? (0 ? y ? 4) are clarified according to the nature of the substituting groups.The secondary type of inhibition, involving the reduction products of these salts, ie phosphine does not appear when y is greater than 1. In this case, the electrostatic adsorption and chemisorption are the predominant steps.     

Cinetique de formation d'une couche de passivation - II. Etude theorique de l'oxydation intensiostatique d'un alliage binaire

In an earlier paper we have presented a theoretical scheme accounting for the electrochemical behavior of metals under constant anodic current and with formation of compact oxide film. The kinetics of growth of this resulting oxide film, which insulates the metal from the electrolyte, was supposed to be governed both by the electron exchange reaction and by the diffusion of point defects (vacancy, interstitial) across the oxide film. In this paper, this theoretical scheme is used to describe the electrochemical behavior of binary alloys under similar conditions and with possible formation of a composite oxide. The variation of interfacial concentration of defects versus time is derived and we have drawn the chronopotentiometric curves corresponding to different types of point defects in the oxide and to different concentrations of the elements in the alloy.     

Influence de l'adsorption du depolarisant ou d'un produit de la reaction electrochimique sur les courants polarographiques—XVIII. Etude de la dimerisation de radicaux mercuroorganiques par voltammetrie a variation lineaire de tension

Compounds with the general formula ArHgX, adsorbed on a mercury electrode are reduced to free radicals, which dimerize on the surface of the electrode. A study by linear potential sweep voltammetry shows that the dimerization involves two free radicals and establishes the validity of the theory elaborated previously for that type of reaction.     

Formation electrochimique d'alkylantimoine par reduction d'alkyldiiodoantimoine

In acid and basic aqueous-alcoholic media, in a mixture of acetic acid and sodium acetate and in dimethylformamide, alkylantimony diiodides are reducible in a two-electron step with formation of an alkylantimony insoluble in all solvents. In basic media, oxidation into an alkylantimony (V) compound is observed. Electrochemical behaviour and stability of alkylantimony are discussed.     

Comportement electrochimique d'oxocarbones et d'aci-reductones en relation avec le transport d'electrons dans les processus biologiques—VII. Reduction de l'acide rhodizonique en milieu aqueux acide

The hypothesis of an ECE1 mechanism may justify the electrochemical behavior observed for the reduction of dihydrated rhodizonic acid. In a first step, RDZ, 2H₂O is irreversibly reduced via a two-electron process without preceding dehydration. The following chemical step is an acid-catalysed dehydration of the monohydrated intermediate species and produces 1,4-tetrahydroxybenzoquinone (THQ) which is reduced via a two-electron process at a potential less negative than RDZ, 2H₂O. According to the hypothesis of an ECE1 mechanism a value of k = k′ (H⁺) with k′ = 145 ± 7 s^?1 is found for the rate constant of the chemical step. The monohydrated intermediate species is reducible via a two-electron process, and without preceding dehydration, at a potential more negative than RDZ, 2H₂O.     

Relation entre les proprietes acides de graphites naturels et leur activite catalytique dans l'isomerisation de l'oxyde de styrene

The surface properties of two natural graphites have been investigated by catalytic activity in the isomerisation of styrene oxide to phenylacetaldehyde and by programmed thermodecomposition (TDP) combined with mass spectrometry gas analysis. The oxygen and water vapour influence has been studied in both cases.The two catalysts are respectively a natural Ceylon graphite (GC solid) with a 3.0 m²/g BET area and a natural Madagascar graphite (GM solid) which has been ground and has a 10.6 m²/g BET area (Table 1).The catalytic isomerisation runs were carried out at 305°C and the results are summarised in Table 2 for different gas mixture compositions. Oxygen directly favours the conversion (TTG) while water vapour mainly influences the selectivity (S).The thermodecomposition products of these solids are mainly CO and CO₂ as shown on Fig. 1. However the relative percentage of CO₂ which is related to the acidic properties of graphites (24) is very different, 16% of the BET area for the GC solid, but only 2% for the GM solid (Table 3). When the catalyst CC is pretreated at 300°C in oxygen instead of nitrogen, the TDP spectrum remains the same but the number of sites remarkably increases as shown in Table 4. Moreover the comparison of Tables 2 and 4 shows a parallelism between isomerisation activity and acidic properties. For this reason, we further investigated the influence of gaseous atmosphere by studying the TDP of the two graphites, first degassed at 850°C and then pretreated at 300°C with air or nitrogen in presence (or not) of water vapour. Important modifications are observed in the spectra (Figs. 2 and 3). Oxygen increases the total number of acid sites. The effect of water vapour is weaker. However, for the GM solid it increases the relative percentage of strong acid sites, i.e. desorbing at the lowest temperatures. For the GC solid, the effect is opposite.These results suggest that only a fraction of the acid functional groups is active for isomerisation, specifically those desorbing CO₂ near the reaction temperature. This assumption is also supported by the fact that a used catalyst presents such groups, while a fresh one does not, as shown on Fig. 4, which means that catalytic sites have been developed during reaction. Their number is increased when oxygen is present in the gaseous phase, and their stability is modified by the presence of water vapour.     

Etude theorique de mecanismes de regeneration du deuxieme ordre en voltammetrie en couche mince ou en surface—application a l'etude de la regeneration catalytique de l'iode par les iodates

A theoretical study of current-potential curves in thin layer or in surface potential sweep voltammetry is presented for second order regeneration mechanisms. The theory is applied to the study of the catalytic reduction of iodate by thin layer potential sweep voltammetry. A trace of iodine is added to the iodate solution and the electroreduction of iodine to iodide initiates the catalytic reduction of iodate. The rate constant of the reaction of iodate with iodide which is determined is in good accord with values given in the literature.     

Possibilites de la diffusion centrale dans l'examen d'etats alteres du ciment

After a review of the idea of specific surface in relation to x-ray diffuse scattering, we examine in a comparative way the data gathered on normal cements and on cements having been subjected to different types of modifications. We show that these modifications are very evident on pure kinds (of cement), but are much more uncertain on concrete.     

Texture poreuse et reactivite d'un carbone vitreux et d'un coke de saccharose—I. Examen de l'oxyreactivite et de la carboxyreactivite

Vitreous carbon (in shape of plates and balls) and a saccharose coke, both heat-treated at 1000°C were examined. The variations of the oxyreactivity and of carboxy-reactivity were determined against burn-off for various granulometric portions at different temperatures. Though the average reactivity of the carbons is comparable, differences in structure were detected. In particular, for vitreous carbon balls, the carbon reactivity is greater for the core of the particles than for their periphery. This is most likely due to the treatment undergone by this material at the very last stage of its preparation to mold it to the desired shape. Experimental data are presented which suggest that texture of vitreous carbon balls and of saccharose coke present some similarities.