Catalytic selective hydrogenation is a reaction widely used in petrochemical industry, particularly to produce intermediate olefins for polymer production. The main drawback of these industrial conversion processes, carried out in fixed beds, is their low selectivity at high conversion levels: injection of the whole hydrogen stream along with the hydrocarbon feed at the inlet of the reactor does not allow an efficient control of the reaction selectivity. The use of a packed bed membrane reactor (PBMR) in order to improve the distribution of hydrogen throughout the length of the reactor has been suggested as a possible solution to this problem. Consequently a new method to improve the hydrogen distribution properties of MFI zeolite membranes, which are potential candidates for a PBMR, is presented in this paper. The chemical composition of these new membranes were analysed and their performances were assessed before and after a chemical modification. After the modification treatment, the membranes were able to distribute hydrogen with a very low loss of hydrocarbon by counter diffusion.

To study the role of strain effects on catalytic properties in the case of supported particles, core–shell PdNi bimetallic nanoparticles were prepared by chemical synthesis and studied in hydrogenation of buta-1,3-diene. Multitechnique characterisation (EDS, TEM, EXAFS) results indicate good homogeneity of the bimetallic particles in terms of size and composition. The core–shell structure of NiPd bimetallic particles and the compressive stress induced on Pd atoms in the top layer due to its larger atomic radius compared with Ni, have been evidenced by EXAFS analyses. According to the literature, Pd activity for buta-1,3-diene hydrogenation is amplified when Pd is strained at the surface of Ni single crystal faces, mainly in the case of (110) planes. This compressive stress is supposed to produce surface reconstructions with undulated Pd row structures. In our case, no amplification of the Pd activity for buta-1,3-diene hydrogenation performed in liquid-phase conditions was observed for a monolayer of Pd strained on 5 nm Ni particles. Consequently, the compressive strain on the Pd atoms at the surface of the nanoparticles seems not to play a major role in the amplification of the activity for buta-1,3-diene hydrogenation. Surface reconstructions observed on extended surfaces to relax the surface stress seems to be the key point for Pd activity amplification. For supported particles, we propose that this relaxation phenomenon does not occur for such small three-dimensional particles.

Pt-M/Al2 O3 bimetallic catalysts with M = Ge , Au or Pd were prepared and characterised by H2 -O2 volumetric titration and IR(CO) spectroscopy. The effects of the second metal on the sulfur resistance of platinum were investigated by the hydrogenation of orthoxylene. Compared to platinum monometallic reference catalyst, the bimetallic catalysts ranking in terms of hydrogenation activity was PtPd > PtAu = Pt >> PtGe . Correlation between these catalysis results and electronic properties extracted from IR(CO), or orthoxylene hydrogenation without sulfur, shows that sulfur resistance of platinum can not be exclusively related to an electronic effect and an electrodeficient character of Pt surface atoms. The mechanism of sulfur poisoning is discussed.

Complexation with nitrite ions in aqueous medium was used to prepare highly dispersed Pd° particles supported on alumina. It has been possible to obtain catalysts with different Pd loading keeping the particle size lower than 1nm as shown by HRTEM, chemisorption, or extended X-ray absorption fine structure (EXAFS) characterizations, and as a consequence with different particle surface density. We observed that electronic properties measured by IR(CO) or X-ray photoelectron spectroscopy (XPS) measurements are strongly influenced by this last parameter and that the origin of these variations can be related to the different strengths of the interactions of the palladium precursor with the alumina surface sites as shown by low temperature CO adsorption followed by FTIR spectroscopy. On increasing Pd loading, saturation of unsaturated Lewis surface sites follows their acid strength: the more acidic ones are the first to react as germination sites, followed by the medium and finally the weaker sites. These different interactions may explain the variation of electronic properties of such small reduced particles which appears to be very sensitive to the chemical nature of their initial germination site on the oxide support. On the other hand, no such effects were observed on changing the type of alumina (δ or γ) support, in agreement with the principle of the synthesis method which tends to limit strong interaction with the support. Comparison with Pd° particles with the same particle size and particle surface density but prepared with the well known grafting of acetylacetonate precursor shows marked differences in terms of electronic properties followed by CO FTIR. This result illustrates the possibility to prepare highly dispersed Pd particles with different physico-chemical properties.

Ag-Pd and Au-Pd (alloyed or core-shell) catalysts were synthesised by radiolytic reduction (gamma rays or electron beam). Selective hydrogenation of buta-1,3-diene was performed with these different nanoparticles deposited on alumina in order to investigate composition and structure effects on catalytic performances. Au-Pd and Ag-Pd nanoparticles exhibit very different catalytic behaviours: whereas only hydrogenation activity is modified on Au-Pd systems without any change in selectivity toward butenes formation, important variations in selectivity into butenes are observed for Ag-Pd nanoparticles deposited on alumina support.

Two series of Rh/spinel catalysts supported on alumina were prepared. In the first series, magnesium, nickel, and aluminum nitrates were co-impregnated over gamma-alumina beads (200 m(2) g(-1)), dried, and further calcined at 1000 degrees C to obtain MgxNi1-xAl2O4/Al2O3 supports (,approximate to 100 m(2) g(-1)), where x ranged from 0 to 1. These supports were impregnated with aqueous solutions of Rh nitrate to obtain 0.1-0.2 wt% Rh catalysts. We prepared the second series by coating alumina beads with Mg acetate. The support was dried and calcined at 1000 degrees C. The MgAl2O4 spinel was formed by solid-solid reaction between magnesia and alumina. M-Al2O4 was impregnated with different Rh precursor salts (nitrate, chloride, acetate) to obtain 0.2-0.8 wt% Rh loading. Supports and catalysts were characterized by BET area, pore volume, lXRD, TEM and SEM, CO2 chemisorption (basicity), and dimethyl-3,3-but-1-ene isomerization (acidity). Rh dispersion was measured by H-2 chemisorption. The second series of catalysts was also characterized by FTIR of adsorbed lutidine and DRIFT. The activity of the catalysts was evaluated in the ethanol steam reforming at 700 degrees C under 1 or 11 atm (H2O/ethanol molar ratio of 4, space velocity 24,000 h(-1)). Acidic and basic properties of the catalysts are crucial parameters inasmuch as they control the primary selectivity for ethylene or acetaldehyde. To avoid ethylene formation, which leads to a significant carbon deposit, all acidic sites should be neutralized. The effects of the precursors used in the support and catalyst preparation were investigated. The second preparation method gave less acidic materials with very high performances (activity and stability) in the ethanol steam reforming reaction. The morphology of the support, with a Mg-deficient spinel layer (thickness of about 8-9 nm) intimately covering all of the alumina grains (around 40 nm in size), can explain the neutralization of most acidic sites. Nitrate precursors should be avoided, since as we obtained the most acidic materials and the poorest stability. Rh acetate led to neutral catalysts with interesting performances in the ethanol steam reforming, and Rh chloride allowed the preparation of well-dispersed Rh catalysts. Although the catalysts prepared with Rh chloride were moderately acidic, they were very active and stable. (c) 2005 Elsevier Inc. All rights reserved.

119Sn Mössbauer spectroscopy at 77 K was used to observe surface tin atoms on PtSnx/SiO2 catalysts. Two families of PtSnx/SiO2 catalysts with fully reduced Pt(0) and Sn(0) atoms were prepared following the Surface Organo-metallic Chemistry on Metal strategy. These two families of catalysts exhibit about the same Sn/Pt composition, but very different metallic particle diameters, each with narrow particle size distribution, so that, the ratio between surface tin atoms and “bulk” tin atoms was significantly different for the two families. In addition to the well known “Pt–Sn” bulk alloy with small quadripole splitting, a new Pt–Sn species with large quadripole splitting and isomer shift in the range of Sn(0) was observed. The relative proportion of this species with respect to “Pt–Sn” bulk alloy is greater for small particles than for the larger ones, which suggests that this species is a surface tin atom present in a “Pt–Sn” alloy.

Highly dispersed supported bimetallic PdPt catalysts were prepared by incipient wetness co-impregnation of alumina with platinum bis-acetylacetonate Pt(Acac)(2) and palladium bis-acetylacetonate Pd(Acac)(2). The resulting Pt4Pd/Al2O3 and PtPd/Al2O3 catalytic systems dedicated to hydro-dearomatization in presence of sulfur were characterized by X-ray absorption spectroscopy, transmission electron microscopy and volumetric H2O2 titration. Direct structural evidence is given of the presence of nanometer bimetallic particles. Moreover, the complete set of results indicates a " cherry- like" structure for the bimetallic clusters with a preferential distribution of palladium atoms at the cluster stuface. Catalytic results shows that the optimum composition leading to the appropriate sulface structure must be adjusted as a function of the size of the clusters.

Surface organometallic chemistry has been used to deposit tin atoms on an alumina-supported cobalt-molybdenum sulfide. The EXAFS spectra at the Mo and Co K-edges were not changed significantly by tin doping, which means that the initial morphology of sulfide phases was preserved during the synthesis. Sn-119 Mossbauer spectroscopy indicated that the main neighbors of the deposited tin atoms were sulfur and oxygen atoms with no significant formation of metallic tin particles. Tin seems thus to have been deposited either on the sulfide stabs through sulfur bridges or on the support via oxygen bonding. According to the infrared spectroscopy, tin doping led to the blocking of different surface sites with slight electronic structure modification. Those surface sites are sulfur vacancies on sulfide stabs, Al3+ or hydroxyl groups on the support, or "interfacial" sites: support hydroxyl groups H-bonded with sulfide phases.The impact of tin doping on the activity and selectivity of the sulfide phases was studied by catalytic tests carried out using a synthetic FCC gasoline feed. Three main reactions were found to be involved: hydrodesulfurization of suffur-containing molecules, double-bond isomerization and hydrogenation of olefins. Olefin hydrogenation was preceded by double-bond isomerization reaction, which gave quickly thermodynamically equilibrated composition. No clear correlation could be made between tin doping and isomerization performance and catalyst support was proposed as the main actor for this reaction. Small amounts of tin had little effect on the hydrogenation activity, but higher loadings resulted in an important decrease in activity. Contrary to the case of olefin hydrogenation, thiophene hydrodesulfurization activity decreased drastically at a low tin loading, and continued to decrease but more slightly at higher loadings. By comparison of the activity variations and the characterization results, the sulfur vacancies were found to play a major role in the hydrodesulfurization process. Interfacial sites were assumed to be involved in one of the key steps in the olefin hydrogenation pathway. (C) 2004 Elsevier B.V. All rights reserved.

Catalyseur pour effectuer la reaction de vaporeformage, eventuellement en combinaison avec la reaction d'oxydation partielle, de l'ethanol et d'autres alcools plus lourds en un gaz riche en hydrogene, constitue:a) d'un support comprenant une solution solide contenant un premier element aluminate a structure spinelle mixte de formule MxM'(1-x) Al2 04, x etant compris entre 0 et 1 inclus.b) et d'une phase active deposee sur le dit support et qui contient un ou plusieurs metaux du groupe VIII dans lequel la teneur en metaux M et/ou M' est comprise entre 0 et 15 %poids.

Different methods of platinum deposition in the porous framework of a tubular ceramic membrane have been investigated. Metal loading, localization, and dispersion were studied using electron microscopy techniques. Results shows the characteristics of the platinum deposit deeply depended on the membrane structure, Pt precursor nature, and application procedure.

Surface and catalytic properties of alumina-supported bimetallic Pd–Sn catalysts prepared via a colloidal oxide synthesis have been studied. In-depth characterization of the supported metallic particles has been performed by techniques such as XPS, FTIR(CO), or LEIS, indicating a strong surface enrichment by Sn and suggesting a modification of the electronic properties of Pd ensembles. EXAFS results also demonstrate the core shell structure of the supported particles in agreement with our previous study of the oxidation state of tin species by 119Sn Mössbauer spectroscopy. This in-depth characterization of the bimetallic catalysts allows us to demonstrate the influence of both PdxSny alloy formation and particle aggregation state on the selectivity of buta-1,3-diene hydrogenation.

Opening cyclic or polycyclic alkanes by carbon–carbon bond cleavage can be a good route for the valorization of certain alkanes. A key point of any proposed catalytic system would be the catalyst poisoning. In this paper, we determine experimentally the influence of the time on stream on the distribution of products when 1,4-dimethylcyclohexane is introduced together with hydrogen over a Ir/SiO2 catalyst. A simplistic model of selective catalyst poisoning is proposed to explain the influence of the time on stream on the activity and product selectivity.