The influence of the support material of low loaded (< 2 V nm -²) vanadia catalysts on selectivities, activation energies and turn over frequencies in the oxidative dehydrogenation of propane and the combustion of propene was investigated. CeO 2 , TiO 2 , Al 2 O 3 , ZrO 2 and SiO 2 supported catalysts were prepared by saturation wetness impregnation in toluene. Characterization with temperature programmed reduction and Raman spectroscopy revealed a high dispersion of surface vanadia species for all investigated catalysts. The impact of heat and mass transfer limitations on the catalytic performance has been thoroughly excluded. Selectivities towards propene as well as activation energies strongly depend on the support material. For all catalysts, propene selectivity increases with temperature. Deconvolution of the reaction network of ODP into decoupled reactions of different reactants for at least three of the catalysts is not possible, because of a significant impact of the oxidation state of the catalyst on the reaction. Except for the CeO 2 supported catalyst, the contribution of the bare support material on the activity can be neglected.
Two preparation methods (wet saturation impregnation with vanadyl acetylacetonate and ball-milling of γ-Al 2 O 3 with crystalline V 2 O 5 ) were applied to produce two sets of VO x /γ-Al 2 O 3 catalysts with increasing vanadium loadings. Materials from both sets were characterized by XRD, SEM, Raman, TPR and UV/Vis-DRS to elucidate the influence of the preparation method on the morphology of VO x species and on their behavior in the oxidative dehydrogenation of propane (ODP). Edge energies from UV/Vis-DRS experiments were correlated with activity and selectivity data; the lower the edge energy, the lower catalytic activity and the higher propene selectivity. Both preparation methods result in materials with comparable physico-chemical and catalytic properties although very different precursors were used. Especially the calcination step during catalyst preparation seems to transform the different precursor molecules to dispersed VO x species by an equilibration process on the support aterial surface. m
Der Einsatz hydrophober ZSM-5-Zeolithe führte trotz der geringeren H 2 O-Beladungskapazität zu NH 3 -Beladungen, die weit unter denen der hydrophileren ZSM-5 Zeolithe mit Si/Al = 50 und 11 lagen. Die NH 3 -Beladungskapazität nahm ± wie für die trockene NH 3 -Adsorption ebenfalls gefunden ± mit steigendem Si/Al-Verhältnis ab. Daher ist anzunehmen, dass auch unter Koadsorptionsbedingungen mit steigendem Si/Al-Verhältnis und einer damit zunehmenden Hydrophobizität die NH 3 -Adsorptionskapazität abnimmt.
The target of this work was to investigate the phase development in the catalyst system consisting of TiO 2 (Anatase) and V 2 O 5 (Shcherbinaite) under several gas atmospheres. Thus a set of V 2 O 5 / TiO 2 specimens was prepared by ball milling and exposed to subsequent annealing in air and feed gas in the temperature range from 400 to 700°C. The XRD-results showed that the initial phases Anatase and Shcherbinaite remain stable for all atmospheres containing oxygen. In the temperature range above 525°C the formation of a Rutile solid solution (Rutile-ss) containing VO x species takes place. However, under reducing conditions (lower oxygen partial pressure) the reduction of V 2 O 5 to V 2 O 3 was found by X-ray diffraction measurements. There is no miscibility up to 1300°C followed by the formation of V 2 TiO 5 (Berdesinskiite). SEM images underline the reduction by monitoring the change in morphology with respect to the V-containing phases. TiO 2 remains without much alteration. The two phases V 2 Ti 7 O 17 and V 2 Ti 3 O 9 (Schreyerite) as described in mineralogy have not been observed in these experiments. The knowledge of phase relations helps to find the appropriate processing conditions and to understand the aging phenomena of catalysts.
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