Spatial distribution of atoms in gas-covered Pd-X nanoparticles (X= Ag, Cu, Ni, Pt)

A Monte-carlo (MC) simulation procedure has been developed where the pair bond energies are allowed to take into account the various coordination numbers of surface atoms and the presence of adsorbates. The pair bond energies are calculated from partial bond energies of atoms which, in turn, are calculated from modified tight binding model in the second moment approximation. The model has been applied to study the role of adsorption of hydrogen, oxygen, carbon monoxide and nitric oxide on the surface composition and surface bond geometry of bimetallic Pd-X (X = Ag, Cu, Ni, Pt) nanoparticles having fcc cubo-octahedral geometry with 201, 586, 1289 and 2406 atoms. The results are compared with the known experimental results. Importance of the results in studying reactions on supported bimetallic catalysts has been highlighted.Comment: 15 pages (Latex) + 38 figures (eps

Extended X-Ray Absorption Fine Structure in Catalysis

Applications of extended X-ray absorption fine structure and X-ray edge structure to catalysis are described. Special attention is paid to the study of supported mono- and bimetallic catalysts. The method used to collect in situ data, under reaction conditions are also considered

Application of EXAFS to Catalysis and Surface Science

The paper gives recent applications of EXAFS to the structural study of microcrystalline bimetallic particles. Special attention has been paid to the case of systems composed of neighbouring elements. Also it is shown how the use of X-ray diffraction techniques and surface science methods like ion scattering can be indispensable to interpret the EXAFS data correctly. In catalysis and surface science, experiments performed in total reflection mode can provide a detailed description of the adsorption site of atoms on a flat surface

Preparation and characterization of palladium – chromium catalysts generated from mixed salts

Palladium chromate and dichromate have been synthetized and characterized by X-ray diffraction and EXAFS. These compounds were reduced between 200 and 600° C in flowing hydrogen. Below 400° C, 20 A Pd particles supported on amorphous chromium oxide are observed. Above this temperature, EXAFS shows that a bimetallic compound is formed corresponding to the composition Pd88Cr12

An exafs and volumetric study of the formation of hydrides on Pd particles encaged in Y-zeolite

EXAFS performed on 10 Å particles of Pd in the network of a Y-zeolite has shown that in vacuum the structure of the metal is similar to that of the bulk.The lattice expansion observed on the EXAFS data when 500 mbar H2 is introduced, is slightly smaller than what is observed on the bulk upon the formation of the β-hydride.It is inferred that on small clusters, where the number of sites is reduced, the stoichiometry of the hydride is reduced to 0.4 H/Pd instead of 0.6

I. Obtention et texture d’oxydes cuivriques divisés

Différentes techniques d’études des solides (microscopie électronique, rayons X, diffusion des rayons X aux petits angles, spectroscopie IR) ont été mises à profit pour examiner les propriétés texturales de plusieurs oxydes cuivriques CuO issus de la décomposition d’un même hydroxyde de cuivre Cu(OH)2. Leur composition exacte a été déterminée. Nous avons montré que la vapeur d’eau joue un rôle essentiel dans le développement de la texture des oxydes pendant leur préparation, et dans l’évolution de leur porosité, au cours du temps. Enfin, il est notable que deux échantillons d’oxyde cuivrique de composition voisine et de morphologie identique, se distinguent pourtant par des propriétés superficielles très différentes