Denise Delafosse

Oxydation menagee du chromite de fer stoechiometrique a basse température, réduction du composé obtenu

Abstract The influence of oxygen on stoichiometric, finely crystallized iron chromite gives a compound with formula FeCr2O4,5. The overall rate of the transformation is controlled by the diffusion of iron cations in the spinel lattice and the simultaneous formation of iron vacancies. Oxidation is followed by means of thermogravimetric, radiocrystallographic, E.P.R. and electrical conductivity analysis. The system can be brought back to the initial state by reduction of the oxidized product with hydrogen. This reverse reaction occurs with the same experimental energy of activation and the same rate pressure law. The data obtained are explained by a vacancy Fe3+ ion association mechanism.

Comparative study of the hydrogen reduction of Ni2+ ions in X- and Y-zeolites

The kinetics of the hydrogen reduction of Ni2+ ions in X- and Y-molecular sieves has been studied over a wide range of temperatures. The localization of Ni2+ ions before and after their reduction has been determined from X-ray analysis.Ni2+ ions can be more easily reduced in X- than in Y-frameworks. In Y-zeolites, it is shown that the reduction of Ni2+ ions depends on the initial localization of these cations and the reduction of Ni2+ ions located on sites I requires a high activation energy. In X-zeolites the sites appear to be energetically homogeneous and the reduction of Ni2+ ions on sites I, I′ and II occurs simultaneously.

Surface properties of silica–magnesia gels

The acid-base properties of silica–magnesia gels have been examined by an i.r. spectroscopic study of the adsorption of ammonia and pyridine and by a study of the cationic exchange capacity of the gels. The basic properties appear to be related to the presence either of hexacoordinated magnesium atoms or OH groups, the latter being located on tetracoordinated magnesium atoms or on neighbouring silicon atoms. The basic centres are related to hexacoordinated magnesium atoms or to OH groups located on neighbouring tetracoordinated magnesium atoms. The acidic centres appear to result from the existence of structural Lewis sites which are formed during magnesium insertion into the network.

Effects of the oxidation state of cerium on the reactivity and sulphur resistance of Ni0/zeolite, Ni0/silica and Ni0/CeO2 catalysts in butane hydrogenolysis

Butane hydrogenolysis and resistance to sulphur poisoning have been studied over various ceria-supported nickel catalysts as compared with zeolite and silica–nickel catalysts, with or without cerium additives. In the presence of cerium in exclusively the 3+ oxidation state, the selectivity in favour of methane formation is greatly lowered and the sulphur resistance of the catalysts is weak. This has been attributed to an electron enrichment of the metal due to the presence of Ce3+ ions. When cerium in the 4+ state is present as a CeO2 phase, improved thiotolerance by the catalysts was observed and is ascribed to the oxidative properties of cerium (IV) oxide.

Reactivity and structure of nickel-exchanged Prayssac vermiculite

The reduction of both lattice Fe3+ and interlayer Ni2+ ions in Prayssac vermiculite is shown to be governed by the same diffusional process, with the removal of water during the reduction assumed to be the rate-determining step. The collapsed structure of the vacuum-activated samples accounts for the difficulty in eliminating water and agrees with the inhibiting effect observed during the reduction. The Fe3+→ Fe2+ transformation is shown to be a redox process. The very weak reactivity of Ni0 particles towards C2H4 hydrogenation agrees with the assumption that most of Ni0 are encaged as atoms in the ditrigonal cavities.

Effect of Ce3+ ions exchanged in NiX zeolites on the location and reducibility of Ni2+ ions and on the stabilization of a highly dispersed metallic nickel

Highly dispersed nickel particles, with diameters ranging from 0.7 to 3 nm, can be obtained by hydrogen reduction of Ni2+ ions exchanged in X zeolites, in the presence of Ce3+ ions. Samples with various Ce3+/Ni2+ ratios have been studied.The structure and surface properties of the solids were investigated by X-ray diffraction and e.p.r. The particle size was determined by magnetic methods.It is shown that the Ce3+/Ni2+ ratio influences the initial location of the Ni2+ ions in the different cationic sites and the redox properties of the zeolite.The effect of Ce3+ ions on the obtention of highly dispersed metallic nickel is discussed in terms of strong electrostatic field and electron-donor properties associated with Ce3+ ions.

Influence of support acidity and Ce3+ additives on the reactivity of nickel particles highly dispersed on various oxide supports

Important modifications to the reactivity of metallic nickel highly dispersed on various oxide supports have been observed. These vary according to the acidity of the support and the presence of Ce3+ additives. The loss or attenuation of hydrogen chemisorption properties and catalytic activity in butane hydrogenolysis seems to be correlated to the acidity of the support, which leads to a total or partial coverage of the metallic surface during the reduction process by hydrogen, which remains strongly chemisorbed up to high temperatures.The presence of Ce3+ additives causes an increase in the catalytic activity in the CO/H2 reaction and a significant shift of the selectivities in favour of the formation of heavier hydrocarbons from both the hydrogenation of CO and the hydrogenolysis of butane.These results are discussed in terms of strong metal–support interactions and concomitant modifications of the electronic environment of the metal.

Surface flow of adsorbable gases through magnesium chromite: Carbon dioxide surface flow

Abstract We first established that helium, which is not adsorbable through magnesium chromite, may be regarded as a “calibrating gas” in flow rate measurements. Then, we studied carbon dioxide flow through the same plug, in a range of temperatures such that the CO 2 adsorption on the solid is a chemisorption: A surface flow of the CO 2 chemisorbed molecules is determined and its value leads us to consider them as mobile at the surface.

Surface flow of adsorbable gases through magnesium chromite oxygen and ammonia surface flows

Abstract The steady state flow rates investigation of chemisorbable gases through a divided solid allowed surface diffusion energies to be assessed. Compared to the desorption energy values a qualitative classification of the chemisorbed molecules mobility can be established. It is to be noticed that the analysis of results is only possible for molecules that are undissociated by adsorption and which do not interact strongly with the surface. The measurement of the surface diffusion energies may be regarded as a more direct and valid determination of the mobilities than that deduced from thermodynamic models of adsorption hardly applicable to chemisorption. However, these flow measurements lead to a macroscopic determination. Our results show that even when a relatively high surface mobility is observed (CO2) it is of an activated type due to the jump, from site to site, of the adsorbed molecules.

Kinetics of the hydrogen reduction of nickel ions in silica–magnesia

The H2 reduction of Ni2+-substituted silica–magnesia gel was studied in the ranges 613–753 K and 1.3–13 kN m–2. The reaction is slow and requires an activation energy of 174 ± 20 kJ mol–1. The rate varies with the hydrogen pressure up to 6.5 kN m–2. The reduction is not complete under the experimental conditions. The mechanism of the overall reaction appears to comprise a nucleation process together with the migration of Ni2+ ions and desorption of the water formed.