L. Gatineau

Reduced forms of LaNiO3 perovskite. Part 1.—Evidence for new phases: La2Ni2O5 and LaNiO2

Low-temperature reduction of LaNiO3 under hydrogen leads to new compounds such as La2Ni2O5 and LaNiO2 before total reduction. Hydrogen consumption, X-ray diffraction and XANES demonstrate the presence of these new phases. Ni3+ in LaNiO3 becomes Ni2+ in La2Ni2O5, and a pure monovalent nickel phase is obtained in LaNiO2. After this stage nickel leaves the structure as nickel metal. Before total reduction, reversible reoxidation of La2Ni2O5 and LaNiO2 leads to the original structure of the LaNiO3 perovskite.

Reduced forms of LaNiO3 perovskite. Part 2.—X-ray structure of LaNiO2 and extended X-ray absorption fine structure study: local environment of monovalent nickel

One of the products of the reduction of LaNiO3 is LaNiO2, containing monovalent nickel. This valence state of nickel has scarcely been observed in mineral compounds. X-ray data from powder samples and extended X-ray absorption fine structure studies (EXAFS) allow one to define a tetragonal structure with lanthanum at the centre of a prism composed of oxygen atoms having a square base and nickel in the centre of the square. The Ni—O distance (nearest neighbour) was found to be 1.983 A for this monovalent nickel.

On the determination of the surface fractal dimension of powders by granulometric analysis

Abstract We investigate the physical meaning of the exponent derived from the particle size ( R ) dependence of the surface area ( S ) of a solid powder, in cases where a power law S ∼ R α is observed. In general, when the surface area is measured at constant apparent volume, α = D s − 3, as derived by P. Pfeifer and D. Avnir ( J. Chem. Phys. 79 , 3558 (1983)). D s is the surface or textural fractal dimension of the particles, at least in the dimension range spanned by the particle size. On the other hand, when S is measured at constant mass, α = D s − D m , where D m is the mass fractal dimension of the particles, and the knowledge of D m is required in order to determine D s . D m is 3 either for compact or, on the opposite, for very porous particles in which the “external” surface area is negligible with respect to the “internal” surface area. In all the other cases, where the internal morphology of the particles is unknown, D m can be measured from the particle size dependence of the apparent density of the powder, ϱ a , which scales as R D m −3 .

Clay Minerals: A Molecular Approach to Their (Fractal) Microstructure

For most users, clays are just a class of mineral materials characterized by various properties such as a very small particle size, a strong adsorption capacity, a soft touch, or a plastic behavior when wetted. For geologists and soil scientists, clays are commonly defined as the fine fraction of rocks and soils, with an upper limit for the particle size at 2 μm. It turns out that this purely granulometric definition corresponds rather closely to a particular class of hydrous silicates with layer structures, which belong to the larger group of phyllosilicates.

Iron-doped pillared laponites as catalysts for the selective conversion of syngas into light alkenes

Stable, active and shape selective catalysts for the transformation of syngas into light hydrocarbons (C2—C3 alkenes) have been prepared by ion exchanging a laponite with mixed polyhydroxycations of Fe and Al; these results clearly show that mixed AlFexOy species are active and selective sites for syngas conversion.

Energetical and Geometrical Constraints on Adsorption and Reaction Kinetics on Clay Surfaces

The Euclidean dimension of an ideal solid surface is 2. We show that the effective dimensionality experienced by reactant molecules on real surfaces may be quite different. This happens, for instance, on energetically heterogeneous surfaces where the reactant molecules spend most of their time either in mobility valleys or in isolated mobility clusters, with an effective dimensionality, at short times, close to 1.33. This considerably modifies the reaction kinetics. A cluster-limited electron transfer reaction on clay surfaces is discussed. Microporous surfaces may also have an effective dimensionality different from 2. In pillared clays, due to interpillar separation distances of the order of the molecular diameter, it is close to 1.9.

Oxidation States of Nickel in Reduced Phases of LaNiO3 by XANES and EXAFS

The reduction of LaNiO3 perovskite has been carried out within a recirculation loop by hydrogen. The degree of reduction of the perovskite was measured manometrically by the hydrogen consumption. Moreover the all consummated hydrogen has been transformed into water trapped in a nitrogen getter.

Structure of LaNiO2 by EXAFS and X-Ray Diffraction: Local Environment of Ni+1

During the reduction of LaNiO3 perovskite a new phase, LaNiO2, was produced. The oxidation state of Ni in this phase, +1, has been determined by the hydrogen consumption during the reaction and by X.A.N.E.S. and E.X.A.F.S. [1].