Albert Renouprez

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.

Study of bimetallic Pd-Pt clusters in both free and supported phases

We study PdPt bimetallic clusters in both free and supported phases. These clusters have been produced with a laser vaporization source. Free clusters directly produced by the source are studied by time of flight mass spectrometry and photofragmentation technique. We observed a sequential evaporation of Pd atoms in the mixed clusters consistent with a palladium segregation process. This tendency has been also observed on supported particles from which the structure and the composition are determined by high resolution transmission electron microscopy and energy dispersive x‐ray analysis. A main result is that each particle has the composition of the massic rod vaporized in the source. The supported particles are well crystallized and exhibit truncated octahedron shapes. Experimental observations are well explained using a modified tight binding model. Indeed, within this model, we found that the equilibrium shape is strongly related to the variation of the cohesive energy with atomic coordination number. ...

New supported palladium-chromium catalysts: characterization and catalytic properties

New silica-supported Pd-Cr catalysts have been prepared either by using mixed inorganic salts of the two metals as precursors or through the deposition of Cr(CO)6 on a monometallic palladium catalyst. Analytic electron microscopy shows that palladium-enriched particles are formed. The EXAFS experiments show that when the reduction temperature reaches 870 K, part of the chromium is reduced and interacts with palladium. In the partial hydrogenation of 1,3-butadiene, these catalysts are less active but 100% selective for the formation of butenes. The explanation, proposed on the basis of hydrogen chemisorption and confirmed by XPS data, is a modification of the electronic structure of palladium, leading to a variation of the adsorption coefficients of the diene and alkenes, compared to the values measured on monometallic palladium catalysts.

Structure and catalytic activity of palladium-platinum aggregates obtained by laser vaporisation of bulk alloys

Abstract The European emission legislation concerning the pollution abatement of the exhaust gas from light cars imposes a reduction of the concentration of aromatics in gasoline, down to 2%. The hydrogenation of these hydrocarbons can be performed on Pd or Pt catalysts. However, these catalysts are poisoned by traces of sulphur — 100–300 ppm — remaining in the fuels. It is claimed in the literature that bimetallic Pd–Pt catalysts are less affected by this poisoning than the pure metals. To verify this point, both Pd–Pt and pure metal aggregates were synthesised by laser vaporisation of bulk alloys and deposited on alumina. Analytical microscopy and EXAFS have shown that these clusters have a very narrow composition distribution and form alloys. The study of their activity in the hydrogenation of Tetralin, a model molecule, in the presence of variable amounts of H2S, has shown that Pt is more active than Pd at low sulphur concentration, whereas Pd becomes more active for the highest H2S content. Contrary to what is claimed in the literature, no synergetic effect has been found found by alloying these two metals; actually their activity is the simple additivity of that of the two metals.

Characterization and reactivity of Pd–Pt bimetallic supported catalysts obtained by laser vaporization of bulk alloy

Abstract Bimetallic Pd–Pt clusters produced by laser vaporization of bulk alloy have been deposited on high surface alumina. Energy dispersive X-ray (EDX) analysis and transmission electron microscopy (TEM) show that they have a perfectly well-defined stoichiometry and a narrow range of size. Therefore, they constitute ideal systems to investigate alloying effects towards reactivity. Pd–Pt alloys are already known for their applications in the hydrogenation of unsaturated hydrocarbons, especially aromatics, because this system is highly resistant to sulfur and nitrogen poisoning. In this context, the catalytic properties of this system have been investigated in the hydrogenation of tetralin in the presence of hydrogen sulfide. Preliminary results show that this model catalyst is more sulfur-resistant than each of the pure supported metals prepared by chemical methods.

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.