Search

Standard EXAFS analysis on CexZr1-xO2 mixed oxides leads to incorrect structural parameters. A comparison of XRD Rietveld analysis with an EXAFS study of a Y-doped reference compound and a Ce0.5Zr0.5O2 catalyst showed that in order to obtain reliable structural parameters with EXAFS an anharmonic pair distribution function has to be used for the analysis of the second Zr−O bond. The anharmonicity of the second Zr−O subshell points to a much weaker Zr−O bond than in the first Zr−O subshell. A lamellar type structural model explains the high oxygen storage capacity of Ce0.5Zr0.5O2. The short Zr−O1 distance corresponds to the intralayer Zr−O bond and the much weaker Zr−O2 distance to the interlayer Zr−O interaction. The oxygen storage capacity is determined by the interlayer Zr−O interaction.

The reduction behaviour of Ce x Zr1−x O2 and Pt or Pd supported on Ce x Zr1−x O2 was investigated by means of XANES measured at the Ce LIII edge and magnetic susceptibility measurements. Both techniques agree quite closely for the materials studied. The presence of a noble metal greatly improves the reduction of the Ce x Zr1−x O2 support at low temperatures. The reoxidation that occurred after evacuation of the reduced solids and heating to 500°C suggests that it is linked to a reversible surface reduction process of the sample.

Dans ce travail, l'activite catalytique a ete correlee a la structure des phases d'oxydes de tungstene presentes a la surface de systemes massiques WO 3 , WO 2 et supportes Pd/WO 3 , Pd/WO 2 . Ces systemes ont ete caracterises par DRX, XPS et XAS (XANES). Ces catalyseurs ont ete testes en reactivite du 2-methyl-pentane. L'ajout de palladium aux oxydes WO 3 et WO 2 conduit a des catalyseurs bien plus actifs qui gardent la selectivite en rearrangement de squelette mais favorisent les etapes deshydrogenation-hydrogenation des alcanes si l'on n'atteint pas une reduction trop elevee en tungstene metallique.

The reduction of a 25 wt% Co/SiO 2 catalyst for the hydrocarbon synthesis has been followed by several techniques: XRD, TPR, XPS and in situ EXAFS. Before reduction the cobalt is present as a Co 3 O 4 spinel phase. A two-step reduction of Co 3 O 4 to CoO and then to Co° is observed by EXAFS. This is consistent with XPS (surface) and TPR or XRD (bulk) studies. During CO/H 2 reaction, cobalt is always in the metallic state (EXAFS). The coordination number of cobalt has been determined at each reduction step and during CO hydrogenation reaction.

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to follow the structural evolution of the monometallic Pt Al 2 O 3 and the bimetallic PtRe Al 2 O 3 systems during reduction and sulfidation treatments. After sulfidation a second reduction has been performed in order to study the elimination of reversibly adsorbed sulfur. For the monometallic system, starting from the reduced state, the number of platinum-platinum bonds decreases significantly from 4.5 to 1.5 after sulfidation and a second reduction. In addition, a significant amount of sulfur is measured around platinum atoms (3 sulfur atoms per platinum atom). On the contrary, for the bimetallic platinum-rhenium system, the number of sulfur atoms in the vicinity of platinum atoms is smaller (1 sulfur atom per platinum atom) with no major change for the number of platinum-platinum bonds. Simultaneous measurements at the rhenium absorption edge show that the sulfur atoms are preferentially fixed by the rhenium atoms (2 sulfur atoms per rhenium atom). From these quantitative results, a structural model for both the monometallic and the bimetallic systems is proposed to describe the different behaviors.

Jusqu'a present, dans le domaine de la catalyse heterogene, l'analyse des donnees de spectroscopie d'absorption X etait partielle. En effet, le formalisme theorique associee ne prenait en compte que les processus de diffusion simple du photoelectron. La prise en compte des processus de diffusion multiple de ce photoelectron est desormais rendu possible grâce a l'avenement de codes de calcul fiables. Elle permet une modelisation bien plus fine de la structure cristallographique du materiau. L'etude du processus d'impregnation du platine sur l'alumine vient ici illustrer notre propos.