I. Schifter

Thermal stability of 12-tungstophosphoric acid supported on zirconia

A freshly precipitated zirconia was impregnated with an ethanol solution of H3[W12PO40]6H2O (TPA) in order to obtain 0‐25 wt.% TPA/ZrO2. The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis, X-ray diffraction, surface area, infrared, Raman, 31 P nuclear magnetic resonance analyses, and used as catalysts in the decomposition of isopropanol. Below 673 K, TPA on ZrO2 exist as distorted intact Keggin species interacting with Zr‐OH or Zr C groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPA collapse and transforms into WO3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observed even with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loaded on SiO2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA on Zirconia. The reaction rate was 3.3 times higher in a TPA/ZrO2 calcined at 773 than that at 473 K. ©2000 Elsevier Science B.V. All rights reserved.

Hydrogen interactions and catalytic properties of platinum-tin supported on zinc aluminate

Abstract Pt-Sn/ZnAl2O4 catalysts prepared by two-step impregnation (first tin), with a Sn/Pt atomic ratio ranging from zero to 6.72, were characterized by hydrogen chemisorption, temperature-programmed reduction and tested in isobutane dehydrogenation. Metal dispersion correlates linearly with reaction rate; both parameters reach a maximum when the Sn/Pt atomic ratio is about one. The activity of the sites capable of hydrogen chemisorption, as expressed in turnover frequency number, TOF, decrease as the tin concentration is increased. From theoretical ab-inito calculations, it is proposed that tin reduces the platinum-hydrogen charge transfer responsible for hydrogen dissociation through an orbital overlapping.

Metal-support effects and catalytic properties of platinum supported on zinc aluminate

Abstract Pt/ZnAl 2 O 4 catalysts with platinum contents ranging from 0.1 to 1.17 wt.-% were characterized by temperature-programmed reduction, hydrogen chemisorption, high resolution transmission electron microscopy, electron energy loss spectroscopy and tested for isobutane dehydrogenation, using helium or hydrogen as reaction media. For low metal contents, the results suggest that platinum diffuses into the oxygen vacancies in the spinel lattice while at higher loadings, metal-metal interactions are dominant, leading to particle formation. The catalytic behaviour points to the necessity of strong Pt-H interactions to preserve activity.

High temperature treated Pt/Sn ZnAl2O4 catalysts

Abstract SnZnAl2O4 mixed supports were prepared by coprecipitation method with Sn contents of 0.5, 1.6 and 3 wt.%. Later Pt (1 wt.%) was incorporated by impregnation and samples reduced in hydrogen at 823 and 1073 K. Chemisorption data showed a decrease in hydrogen as temperature increases which is attributed to PtSn alloy formation. Catalytic activity in isobutane dehydrogenation and n-heptane dehydrocyclization seems to be dependent on the amount of metallic Pt phase. Pt/SnZnAl2O4 catalysts were thermally more stable than Pt/ZnAl2O4 preparation. Tin alters the ZnAl2O4 properties towards a higher thermal stability, but if added in more than 1 wt.%, it alloys with platinum inhibiting catalytic performance.

Platinum supported on coprecipitated SnZnAl2O4

Abstract Pt catalysts (0.2–2.5 wt.%) supported on coprecipitated 1.6 wt.% SnZnAl 2 O 4 were prepared. SnZnAl 2 O 4 spinel had the same lattice parameter as ZnAl 2 O 4 independently of Pt impregnation. The highest platinum dispersion was obtained with a 0.5 wt.% platinum content. Catalytic activity for isobutane dehydrogenation and n -heptane conversion was more sensitive to chemical composition than to platinum dispersion.

Oxidative dehydrogenation of n-butane on zinc-chromium ferrite catalysts

Abstract The role of chromium as a promoter of butadiene selectivity in n-butane oxidative dehydrogenation on ZnCrxFe2-xO4 (0

Oxidative dehydrogenation of 1 -butene to butadiene on α-Fe2O3/ZnAl2O4 and ZnFexAl2-xO4 catalysts

The oxidative dehydrogenation of 1-butene to butadiene was studied over a series of catalysts of iron impregnated on ZnAl2O4 used as a support and also Fe coprecipitated with zinc and aluminum in order to obtain ZnFexAl2-xO4 type catalysts. Results were compared with bulk α-Fe2O3, ZnAl2O4 and ZnFe2O4. X-ray diffraction (XRD) and Mössbauer spectroscopy suggest that in the impregnated catalysts, Fe ions are in strong interaction with the support. These samples have higher butadiene selectivity than coprecipitated ZnFexAl2-xO4 catalysts, in which iron is incorporated into the ZnAl2O4 spinel network. Results suggest also that iron content has a greater effect on butadiene selectivity than the zinc aluminate-iron oxide interaction.

Synthesis, characterization and activity of PtSn/Al2O3 sol-gel catalysts

Abstract Bimetallic Pt Sn/Al 2 O 3 catalysts were prepared by the sol-gel method by adding Sn(But) 4 to a homogeneous solution containing aluminum tri-sec-butoxide at pH = 9. After gelation, the solids were dried and impregnated with hexachloroplatinic acid solution. The resulting catalysts were highly dispersed (90%) and selective to gasoline yields when tested in n-heptane dehydrocyclization.

Phenylacetylene hydrogenation on Pt/TiO2 sol-gel catalysts

Comparative results on sol-gel and impregnated Pt/TiO2 catalysts are presented. Important differences between the two preparations were found, especially the high resistivity of sol-gel materials to deactivation during phenylacetylene hydrogenation.

Effects of hydrogen at high temperature on ZnAl2O4 and Sn−ZnAl2O4

ZnAl2O4 and Sn−ZnAl2O4 were synthesized by coprecipitation, sol-gel and impregnation methods. These materials were calcined and treated in H2 at 1073 K. Thermal analysis (DTA and TG), nitrogen physisorption (BET method), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used as characterization techniques.H2 treatment promoted AlxZny crystallization in the coprecipitated and impregnated samples. When tin was added to zinc aluminate, the tin acted as a protective shell against high-temperature reduction, independently of the preparation technique.ZusammenfassungMittels Kopräzipitation, Sol-Gel- und Imprägnierungsmethoden wurden ZnAl2O4 und Sn−ZnAl2O4 hergestellt. Diese Substanzen wurden kalziniert und in H2 bei 1073 K behandelt. Zur Charakterisierung wurden Thermoanalyse (DTA und TG), Stickstoffphysisorption (BET Methode), Röntgendiffraktion und Scanning-Elektronenmikroskopie eingesetzt. Die Wasserstoffbehandlung fördert die AlxZny-Kristallisation in den kopräzipitierten und imprägnierten Proben. Wird dem Zinkaluminat Zinn zugesetzt, fungiert Zinn-unabhängig von der Herstellungsmethode-als Schutzschild gegen die Hochtemperaturreduktion.