Pd-Pt/Al

Pd-Pt/Al2O3 catalysts are prepared by controlled surface redox reactions, the refilling or the direct redox reaction. By the refilling method, the reduction of platinum salt is made by hydrogen preadsorbed on the palladium parent catalyst. By the direct redox reaction, the metallic palladium of the parent catalyst acts directly as reducer. The characterization of bimetallic catalysts by different techniques (metal accessibility, IR, EXAFS) points out a platinum selective deposit over palladium according to the preparation method under consideration. By using preadsorbed hydrogen, platinum is deposited on low coordination sites of palladium, whereas by direct redox reaction, the deposit occurs preferentially on the palladium faces. The knowledge of the bimetallic species structure allows to explain the catalytic results for the 18O/16O isotopic exchange

Deactivation by coking of supported palladium catalysts. Effect of time and temperature

International audienceThe coking reaction with cyclopentane was investigated on silica supported palladium catalysts in the temperature range 400-500 degrees C at various times. After reaction, the catalysts were characterized by temperature programmed oxidation by the fouling effect of coke on the reaction of cyclohexane dehydrogenation and by infrared spectroscopy of chemisorbed CO. TPO profiles of coked catalysts show the presence of two oxidation peaks corresponding to coke deposition on the metallic function: the first one near 250 degrees C which would correspond to coke deposited on palladium atoms of high coordination number, depends very little on the temperature or the time of the coking reaction. On the other hand, the second TPO peak near 400 degrees C which would involve palladium atoms of low coordination number, is enhanced by an increasing time or temperature of the reaction. These results clearly indicate that low coordinated palladium atoms are more resistant to the deactivation by coke under reforming condition

Preparation and characterization of Pd-Pt/Al

Bimetallic Pd-Pt/Al2O3 catalysts were prepared by controlled surface reactions. The reduction of PtCl62- can be made by using as reductant hydrogen preadsorbed on a parent monometallic Pd/Al2O3 catalyst (refilling), or metallic palladium itself (direct redox reaction). Two precursors (Pd(NH3)4 Cl2 and Pd(Acac)2) were used to prepare the (1 wt % Pd) parent catalyst. The platinum loading is within the range 0.13-0.14 wt %. The characterization of the catalysts was made by hydrogen chemisorption, infrared spectroscopy of adsorbed CO (as probe molecule) and E.X.A.F.S. The two preparation methods lead to different kind of platinum deposit, whatever the palladium parent catalyst. By using preadsorbed hydrogen, platinum can be deposited in decoration on low coordination sites (comers, edges,...) of palladium particles. By direct redox reaction, the deposit of platinum atoms occurs preferentially on the (111) faces of palladium particles. However some sintering can occur, especially during hydrogen treatment in aqueous medium. That sintering is enhanced by chloride ions

Characterization of bimetallic rhodium-germanium catalysts prepared by surface redox reaction

International audienceThe preparation of bimetallic rhodium-germanium/silica and rhodium-germanium/alumina catalysts was investigated by controlled surface reaction. Their catalytic performances were measured for two gas phase reactions (toluene hydrogenation at 323 K and cyclohexane dehydrogenation at 543 K) and for a liquid phase reaction (citral hydrogenation at 343 K). Elemental analysis of bimetallic catalysts showed that germanium can be deposited by redox reaction between hydrogen activated on a parent monometallic rhodium catalyst and germanium tetrachloride dissolved in water (catalytic reduction method). EDX microanalysis of rhodium-germanium/silica catalysts indicated that rhodium and germanium were deposited in close contact on the silica support. However, on alumina-supported catalysts, germanium deposition occurred also separately on the support. For the different test reactions, the catalytic properties of rhodium were strongly altered by the addition of Germanium. On alumina-supported catalysts, interesting catalytic effects were observed in citral hydrogenation when not only close contact exists between both metals but when, in addition, the second metal was deposited on the support in the close vicinity of rhodium

Preparation of alumina supported palladium-platinum catalysts by surface redox reactions. Activity for complete hydrocarbon oxidation

International audienceThe complete oxidation of hydrocarbons (methane, propane and propene) and the O-18/O-16 isotopic exchange were investigated on supported palladium-platinum catalysts. The bimetallic catalysts were prepared by controlled surface reactions (the "refilling" RC and the "direct redox reaction" RD) and by coimpregnation CI. The preparation methods lead to different kinds of platinum deposit, which involve different behaviours in hydrocarbon oxidation and in isotopic exchange. Even if the structure of bimetallic catalysts is modified under oxygen, the results show that the bimetallic catalysts keep a memory effect of their preparation method under the reaction conditions used in this wor

Citral hydrogenation over alumina supported Rh-Ge catalysts - Effects of the reduction temperature

International audienceTwo series of alumina supported Rh-Ge catalysts were prepared either by surface redox reaction (catalytic reduction (CR)) or by a simple impregnation of germanium on rhodium/alumina (SI samples). They were characterized by temperature-programmed reduction (TPR) and by their activity for the liquid phase hydrogenation of citral used as model reaction. The addition of germanium to rhodium by CR allows to preferentially hydrogenate citral to the unsaturated alcohols (geraniol and nerol), whereas the saturated aldehyde (citronellal) is the main product on the monometallic rhodium catalyst. The citral conversion and the selectivity to unsaturated alcohols increase with the germanium content and go through a maximum as a function of the reduction temperature of catalysts. At higher germanium loadings, the activity decrease observed after a reduction at 500degreesC is explained by the reduction of germanium species deposited on the support in the course of the CR preparation procedure. For reduction temperatures in the 150-350degreesC range, bimetallic catalysts exhibit better hydrogenating properties in the solvent isopropanol in comparison with n-heptane while the reverse occurs after a reduction at 500degreesC. The CR catalysts are more active and selective to unsaturated alcohols than bimetallic catalysts prepared by the SI method. However, the exposure of CR bimetallic catalysts to ambient air deteriorates their catalytic properties for citral hydrogenation

Study of supported bimetallic Pd-Pt catalysts. Characterization and catalytic activity for toluene hydrogenation

International audienceBimetallic Pd-Pt catalysts were prepared either by a surface redox reaction (RC catalysts) or by coimpregnation (CI catalysts). RC bimetallic catalysts show a higher sulfur resistance in the course of toluene hydrogenation in the presence of thiophene compared to monometallic palladium and CI bimetallic catalysts. A characterization of the RC catalysts by IR and EXAFS allows to propose a structure of the Pd-Pt crystallites which would explain these results