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

Preparation of bimetallic Pd-Fe / Al

Bimetallic Pd-Fe/Al2O3 catalysts were prepared by underpotential deposition (UPD) of iron on parent Pd/AL2O3 catalysts of varying dispersion. The potential of palladium particles was controlled by the supplementary redox system H+/H2. Iron introduced by this method improves the activity of small palladium particles for the hydrogenation of isoprene in the liquid phase. An explanation of this phenomenon would be a selective deposit of iron on or near the palladium sites of low coordination. However, the promoting effect of iron disappears after exposure of bimetallic catalysts to air and subsequent reduction

Préparation de catalyseurs bimétalliques Pt-Au par réaction redox : caractérisation par diffraction et par diffusion centrale des rayons X

Des catalyseurs bimétalliques Pt-Au/SiO2 ont été préparés par réaction d'oxydoréduction de surface à partir d'un catalyseur parent Pt/SiO2. L'or a été introduit sous forme de AUCl4- réduit sur le platine soit par l'hydrogène préadsorbé, soit par une oxydation directe de Pt° par AuCl4-.La caractérisation des catalyseurs par diffraction et diffusion centrale aux petits angles des rayons Xa permis de montrer que les entités bimétalliques sont constituées de particules de platine décorées de particules d'or avec diffusion des deux métaux l'un dans l'autre

The effect of precursors salts on surface state of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts

The influence of the precursors on the promoting effect of ceria on Pd/Al2O3 catalyst, when ceria is coated over alumina was studied. The reaction of propane oxidation proceeded under different feed conditions and the surface active sites were characterized by X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance spectroscopy (DRS). XPS and DRS results show that PdO/Pd0 interface are the active sites independent of the precursor, while the catalysts containing CeO2 showed formation of palladium species in the highest oxidation state, probably PdO2 (338 eV) after the oxidation of propane. Besides, the O/Al and O/Ce ratios evidenced the increase of oxygen storage in the presence of CeO2. In addition, the precursor acetylacetonate favors the oxygen storage in the lattice.<br>Estudo da influência dos precursores sobre os catalisadores de Pd/Al2O3 promovidos com céria ancorado sobre a alumina. A oxidação do propano foi feita sob diferentes condições de alimentação sendo caracterizados os sítios superficiais por Espectroscopia Fotoeletrônica de raios X (XPS) e por Refletância Difusa em ''situ''. Resultados de XPS e DRS mostraram a formação de interfaces de PdO/Pd0 como sendo os sítios ativos, independentes do precursor utilizado na preparação, enquanto que os catalisadores contendo CeO2 mostraram a formação de espécies de paládio com estado de oxidação mais altos, provavelmente PdO2 (338 eV) após a oxidação do propano. Além disso, as razões O/Al e O/Ce evidenciaram um aumento de oxigênio armazenado na presença de CeO2. O precursor acetilacetonato favoreceu o armazenamento de oxigênio na rede

Sulfur Dioxide Exposure: A Way To Improve the Oxidation Catalyst Performance

The oxidation of nitric oxide and propylene was studied over a model alumina-supported platinum catalyst. Two treatments (22 h at 250 degrees C and 2 h at 800 degrees C) involving sulfur dioxide were performed in order to understand the effect of SO2 aging. The role and stability of sulfur species stored during aging were investigated by the reduction of sulfated samples at 500 degrees C and temperature-programmed reduction (TPR) up to 800 degrees C. The low activity obtained after aging without reduction revealed the poisoning effect of surface sulfur species. The reduction at 500 degrees C released half of the surface species, which increased the catalytic activity for both NO and C3H6 oxidation. The TPR removed the most stable sulfur species and improved the activity during cooling. The catalyst aged in SO2 at high temperature showed the greatest activity for both reactions because of SO2 exposure and low sulfur storage