Gisèle Dalmai-Imelik

ESR study of deep-bed calcined NH4Y and aluminum deficient zeolites

Abstract Electron spin resonance and irradiation by γ-rays at 77 °K or at room temperature were used to study the removal of alumina from the Y type zeolite framework by heat treatment in a deep bed geometry. X and Q band ESR spectra were recorded at 77 °K and showed respectively 12 and 16 lines corresponding to an isotropic hyperfine interaction with two aluminum atoms (I = 5 2 ) (c ∥∼- c ⊥ ∼- a iso Al = 10.0 Oe ) , the eleven hyperfine lines being split by a uniaxial symmetry of the g tensor ( g ∥ = 2.0125, g ⊥ = 2.0030, and g iso = 2.0062). The paramagnetic center was identified with a V center, namely a hole of an electron trapped on an oxygen atom bonding two nearby aluminum atoms. This center is localized on alumina which has been removed from the zeolite framework. The removal of alumina occurred only when the samples were heated above 400 °C in the presence of both water vapor and ammonia. It is suggested that the mechanism of migration involves the formation of aluminic acid in a dimer form and a reorganization of the lattice leaving no aluminum atom vacancies. A special emphasis is placed on the heat treatment conditions.

Study by electron microscopy and electron diffraction of formation of nickel epitaxially grown catalysts

Abstract In order to study the influence of the crystallographic arrangement at the surface of a metal-supported catalyst on its catalytic properties, catalysts have been prepared with well faceted nickel crystallites presenting particular faces at their surface. Catalysts were obtained by reduction by hydrogen of nickel antigorite evacuated at different temperatures. The morphology and structure of nickel antigorite, the parent of the catalyst, has been determined by electron microscopy and electron diffraction at different temperatures. Rolled or flat sheets were observed. The texture of the catalysts obtained after reduction was determined. The crystallites of nickel are hexagonal platelets with either their (111) or their (110) plane parallel to the sheet support, depending on the conditions of preparation and reduction. Study of the orientations of (111) type or (110) type was done.

Evolution of adsorbed species during C2H2 adsorption on Ni(111) in relation to their vibrational spectra

Acetylene adsorption at room temperature on Ni (111) has been studied by low energy electron diffraction (l.e.e.d.), thermal desorption, work function changes and high resolution electron energy loss spectroscopy (h.r.e.l.s.). The existence of two adsorbed states has been demonstrated depending upon exposure.At low exposure, C2H2 is associatively adsorbed but a large rehybridization occurs, characterized by a carbon–carbon bond order close to 1.15. According to its hybridization state C2H2 should be adsorbed on a triangular site through two σ bonds and one π bond with nickel atoms.With increasing exposure, large modifications occur. In addition to self-hydrogenated species, benzene should be formed as shown by comparison of the vibrational spectra of adsorbed C2H2 and adsorbed C6H6. This conclusion is in agreement with the benzene formation as detected after contact between C2H2 and Ni/SiO2 catalysts with (111) exposed faces.