G. Bergeret

Catalytic hydrodechlorination of hexachlorobenzene on carbon supported Pd-Ni bimetallic catalysts

Abstract The study of the catalytic activity of carbon supported Pd-Ni catalysts for the hydrodechlorination of hexachlorobenzene was carried out in the liquid phase. The degree of dechlorination was found proportional to the surface Pd concentration, which is enhanced by the segregation of this element at the surface of the bimetallic particles. It is also shown that isolated Pd atoms located at the surface of Ni rich bimetallic particles are more active than those lying in larger ensembles. Finally bimetallic Pd-Ni catalysts containing only between 20 and 50 Pd atom%, although less active than pure Pd, lead to 75% of useful compounds, i.e. benzene, mono and dichlorobenzene.

The atomic structure of platinum aggregates encaged in Y zeolite. Structure in the course of benzene hydrogenation

Abstract The atomic structure of 10-A-diameter platinum aggregates encaged in Y-type zeolite has been studied by the radial electron distribution method from X-ray data. When the particles are free from adsorbates the structure is distorted and the interatomic distances are contracted with respect to the normal fcc structure of bulk platinum. On hydrogen adsorption the structure undergoes a total relaxation, and the aggregates exhibit the normal fcc structure and interatomic distances. Benzene adsorption produces a weak relaxation but the structure is still distorted and contracted because the benzene overlayer coordinates the surface atoms less efficiently than dissociated hydrogen. In the course of benzene hydrogenation at 300 K with P H 2 P C 6 H 6 = 3 the structure is intermediate between that of the particles covered with hydrogen and that of the particles covered with C6H6; both reactants are therefore adsorbed on the surface. The structure is similar when the reaction is carried out with an excess of benzene ( P H 2 P C 6 H 6 = 1 ); in this case the surface remains partially covered with irreversibly adsorbed hydrogen which does not participate in the hydrogenation reaction.

A new method of direct synthesis of bimetallic phases: Silica supported PdCu catalysts from mixed acetylacetonates

Abstract The direct synthesis on a silica support of well defined, crystalline, mixed PdCu bis-acetylacetonates has been achieved and the structure of these new compounds was determined by powder X-ray diffraction. These clusters have been deposited on a silica support and their thermal decomposition under helium was followed by in situ X-ray diffraction, thermo-gravimetry and mass spectroscopy. Microcrystalline metallic alloys with a narrow distribution of composition and particle size in the 2–4 nm range are obtained. Infrared experiments of CO chemisorption confirm the homogeneity of composition of the particles but conclude to a partial coverage of palladium by copper.

Supported Pd-Cu catalysts prepared from bimetallic organo-metallic complexes: relation between surface composition measured by Ion Scattering and reactivity

The direct synthesis on a silica support of mixed Pd-Cu acetylacetonates clusters is achieved by crystallization from solutions of the pure compounds. The stochiometric clusters are identified by X-ray diffraction. After elimination of the organic ligands under argon followed by in situ XRD, alloys are formed for all the studied compositions. Low energy ion scattering shows that the surface of the particles has a higher Cu concentration than the bulk. As expected, these alloys have a lower activity than pure palladium for the hydrogenation of butadiene, but only butenes are formed even at 100 % conversion of the reactant. This selectivity is explained by the preferential migration of Cu atoms on given sites of the surface as demonstrated by Monte Carlo simulations.

Sulphur resistant palladium-platinum catalysts prepared from mixed acetylacetonates

Well-defined mixed acetylacetonates precursors were synthesised on a silica support and identified by their X-ray differaction spectra. After a calcination treatment to decompose the ligands and a reduction in hydrogen, bimetallic particles of 3.5 to 5 nm have been obtained. Analytical microscopy shows that these catalysts have a perfectly homogeneous composition. Low energy ion scattering experiments show that the surface of the particles is palladium enriched, as expected from Monte Carlo simulations. However, in the simple hydrogenation of cyclooctadiene to cyclooctene, where Pd is two orders of magnitude more active than Pt, it is observed that the rate is not proportional to the Pd surface concentration. This is interpreted as an ensemble effect on the chemisorption of the reactant. A study of the hydrogenation of tetralin in the presence of H 2 S shows a maximum of sulphur resistance for the Pd 20 Pt 80 atomic composition.

Influence of small amounts of rhodium on the structure and the reducibility of Cu/Al2O3 and Cu/CeO2-A12O3 catalysts

The influence of small amounts of rhodium on the structure and the reducibility of copper-based catalysts deposited on alumina and ceria-alumina supports has been studied by XRD and H2-TPR techniques. For both supports, the addition of rhodium does not affect the reduction mechanism of copper, which has been shown to involve small clusters/isolated ions at lower temperatures and large particles at higher temperatures. However, rhodium addition inhibits the formation of large size copper particles in favor of small ones, thus making easier the reduction of the catalyst at low temperature.