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

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