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RAFFINAGE:SURFACES+LPI:SNA:MAO:CGE; A series of Ir-Pd/SiO2-Al2O3 catalysts has been prepared by incipient wetness co-impregnation of acetylacetonate precursors and treated by reductive thermal decomposition. The decomposition has been followed by combined thermogravimetry, differential thermal analysis, and mass spectrometry, showing a marked difference between the bimetallic and monometallic cases. The catalysts characterization by transmission electron microscopy and single-nanoparticle energy dispersive X-ray spectroscopy reveals that the particles are bimetallic and exhibit a size-composition correlation. More generally, the enrichment of larger particles with the less cohesive element (Pd in the present case) via Ostwald ripening is here claimed to be inherent to the thermal activation treatments used in the preparation of bimetallic catalysts. The catalysts have been tested in the hydroconversion of tetralin, which is considered as a model molecule for gas oil upgrading by selective ring opening. At 350 degrees C, 4 MPa H-2, and in the presence of ppm amounts of H2S, hydrogenation, dehydrogenation, and ring-opening/contraction products (ROCPs) are formed. While the activity increases with the Pd content, the selectivity to ROCPs reaches a maximum for the Ir55-Pd45 composition and increases with the sulfur concentration. The structural and catalytic results are combined to propose a qualitative model relying on metal-acid bifunctionality and size-composition correlation. While the particle size drives the number of surface sites, the corresponding surface composition governs the hydrogenation efficiency of these sites. Therefore, by controlling the mean metal composition of the catalyst, it is possible to tune the activity-selectivity balance. (C) 2012 Elsevier Inc. All rights reserved.

International audience; The mechanism of zirconia textural stabilization by means of basic reflux was studied. In agreement with earlier studies, zirconia was modified by silicon extracted from the glass material in the same manner as if silicon compounds (e.g., TEOS, SiO2) had been added to the reaction mixtures. The reflux in a Teflon bulb without the addition of silicon did not lead to improved textural properties. EXAFS study of both refluxed and nonrefluxed specimens showed their similarity and probably some dehydration of the precipitate. The FTIR, NMR, and XPS spectroscopic characterizations suggested that silicon was strongly segregated on the surface, probably in the form of silicate moieties anchored on zirconia. A reflux stage was necessary to achieve efficient spreading of the silicate over the surface of the precipitate. For the solids calcined at 1173 K, the surface stabilization corresponded to the formation of a “metastable monolayer” coverage, similar to that observed for zirconias doped by other oxoanions, such as molybdate or tungstate. Isooctane hydrocracking and o-xylene hydrogenation were studied in the presence of Pt supported on the oxides as prepared. Hydrodesulfurization of thiophene was carried out in the presence of Mo and NiMo sulphides using the same supports. In all three reactions studied, the digested zirconia support demonstrated properties similar to those of pure silica.

The role of carbonaceous species on the activity of hydrotreating catalysts has been controversial for a long time. On one hand, carbon deposit is known to be one cause of catalyst deactivation, and on the other, the presence of organic thiocompounds (DMDS, tertiononyl-pentasulfide, etc.) or gas–oil during the sulfidation is recognized to provide an activity enhancement. In this work, we attempted to investigate the effect of carbon on the HDS properties of an industrial CoMo catalyst. For this purpose two carbon-containing catalysts were compared to a carbon-free catalyst. Carbon introduction was performed either by impregnating the oxide precursor with a C20 gas oil cut and subsequent drying, or by contacting the precursor with a solution of a commercial coleseed oil and further coking. After activation by H2/H2S, the catalytic properties of the catalysts were evaluated in thiophene conversion and real feedstock desulfurization. In both cases, and for the two preparations, a positive effect of C was observed. Several characterizations were performed (TEM, XPS, TPO, BET-BJH, EXAFS) in order to determine the role of carbon, starting from several assumptions. From these results, it was concluded that at least one identifiable beneficial effect of carbon is a geometrical one. However, structural effects may not be excluded.

γ-Al2O3-supported Pd, Pt, and bimetallic Pd–Pt catalysts have been produced by laser vaporization of the respective bulk metals and alloys. Transmission electron microscopy and energy-dispersive X-ray analysis experiments show that these bimetallic clusters have a perfectly well defined stoichioemetry. Moreover, pure or bimetallic clusters have similar size distributions. One has taken benefit of these features to study in detail the resistance to sulfur poisoning as a function of the composition and to verify to what extent what is claimed in the literature, i.e., an improved resistance to sulfur poisoning of Pd–Pt catalysts as compared to pure metals, is verified. The catalytic properties of Pd, Pt, Pd17Pt83, and Pd65Pt35 in the hydrogenation of tetralin in the presence of varied amounts of H2S have been compared. Contrary to what is claimed, the coexistence of Pd with Pt in the same particle has no influence on the hydrogenation activity nor on the sulfur tolerance of the catalysts. Hence, it has been shown that Pt has no electronic influence on the reactivity of Pd atoms.

Pure Pd, Pt, and bimetallic Pd–Pt clusters produced by laser vaporization of bulk metals or alloys have been deposited on a high surface area alumina (γ-Al 2 O 3 ). The bimetallic clusters have a perfectly well-defined stoichiometry, as demonstrated by energy-dispersive X-ray analysis experiments. Transmission electron microscopy shows that pure or bimetallic clusters have similar size distributions. The catalytic properties of Pd, Pt, Pd 17 Pt 83 , and Pd 65 Pt 35 supported clusters have been investigated in the vapor-phase toluene hydrogenation over a wide range of reaction conditions. The activation energies are 44 ± 6 kJ/mol. The pressure dependence on toluene varies from 0 at 333 K to 0.7 at 500 K whereas this dependence on H 2 increases from approximately 0.5 at 333 K up to 2.2 at 500 K. The results obtained with the pure metals corroborate those reported in the literature for catalysts prepared by chemical methods. The activities of the bimetallic catalysts do not show any synergy between Pd and Pt but rather a perfect additivity of their individual catalytic properties. Pt has been shown to have no electronic influence on the reactivity of Pd atoms. The nonlinear variation of the turnover frequency as a function of the Pd surface concentration is interpreted on the basis of an active site composed of a pair of atoms.

The present work aims at coorelating the surface characteristics of a {open_quotes}working{close_quotes} catalyst under the conditions of the oxidative coupling of methane (OCM) with mechanistic aspects of the reaction deduced from transient experiments focused on the total oxidation pathway. Several lanthana samples with variable surface area and morphology are tested with a view to identifying the exact nature of OCM-specific active sites. The lanthana surface, characterized by XPS, TEM, XRD, FTIR, and in situ DRIFT spectroscopy, appears to be mostly decarbonated and dehydroxylated under reaction conditions. Low coordination sites localized on step edges are related to the total oxidation pathway, while basic sites associated with oxygen vacancies enable the oxygen activation leading to the initial methane activation. Structure sensitivity effects are explained within this proposal of differentiated sites on a typical OCM catalyst. 32 refs., 13 figs., 4 tabs.

Supported molybdenum oxide on zirconia catalysts were prepared by simultaneous reaction of Zr oxychloride and ammonium heptamolybdate in molten K-Na nitrate eutectic at 773 K. It was shown that Zr(IV) and Mo(VI) salts react in the molten nitrate medium leading to solids of high specific surface areas (up to 200 m2/g). The solids consist of small crystallites of tetragonal ZrO2 containing polymolybdate species on their surface. Textural properties of the catalysts and their stability to calcination in air were studied as a function of the initial Mo/Zr ratio in the reaction mixture. Surface polymolybdate species stabilize tetragonal zirconia and improve the textural properties of the system. Surface areas twice those obtained by conventional methods were observed. Catalytic activity of samples was studied by the thiophene hydrodesulfurization reaction. Due to the enhanced surface area, Mo loading could be increased up to 12.5 wt% of Mo without loss of intrinsic activity per Mo atom.

Mixed nickel-ruthenium sulfides supported over alumina, with different metal compositions, were prepared by stepwise impregnation and further sulfidation. The solids were characterized by XRD, XPS, HREM, STEM, and TPR, and their catalytic properties evaluated using the hydrogenation of biphenyl. XRD, XPS, TPR, and STEM measurements indicate that ternary nickel-ruthenium-sulfur compounds with a pyrite-like structure are synthesized up to an atomic ratio of Ni/(Ni + Ru) near 0.7. STEM analysis after tests shows that these phases are stable under test conditions. A 30-fold increase in hydrogenation activity is observed in comparison to pure ruthenium sulfide supported on alumina. This synergetic effect is ascribed to the existence of mixed NiRu sites with different electronic properties from those of either Ni or Ru sites by themselves.