Titania-alumina formation

The freshly reduced catalysts were characterized by X-ray diffraction using a Rigaku Miniflex X-ray diffractometer(1800) with CoK, radiation. Metal and metal oxide phases were obtained and peaks corresponding to the different supports were also observed. The titania support was found to be of pure einateise form. The titania-alumina support showed no characteristic peak of either of the oxides indicating the formation of the mixed oxide. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

The deactivation constant, obtained by fitting the data to a first order law equation (8), decreases with the reduction temperature for all cases, and is especially low for the impregnated alumina-titania catalyst. These results suggest the formation of carbon deposits and deactivation of catalysts occurs due to the metal activity, The contribution of the acid sites to deactivation seems to be negligible, despite the fact that alumina-titania supports present higher acidity than alumina or titania single oxides (9). 

The main functions of a carrier or support are usually to lend mechanical strength, increase stability to sintering and provide a larger active surface area than would otherwise be available. There is evidence that, in many instances, compound or complex formation takes place between the catalyst and the support, with a consequent effect on the catalytic properties. The most commonly used support materials are silica, alumina, silica-alumina, titania, silicon carbide, diatomaceous earths, magnesia, zinc oxide, iron oxide and activated carbon. 

That bonds are formed between particles is inferred by the fact that the gel layers are able to bear considerable stresses. These bonds are sensitive to the presence of stresses and allow stress relaxation to occur. The relation between stress relaxation and cracking on one hand and particle shape on the other hand is not known. The relative ease of preparing y-alumina membranes might be due to the relative ease of rearrangement of the particles and easy stress relaxation in plate-shaped boehmite particles and the isomorphous transitions to plate-shaped y-alumina at about 300°C, the transition also being accompanied by a relatively small volume change [2-4]. With spherical particles (titania, zirconia) stress relaxation might be more difficult. The easier formation of defect poor composites of alumina and titania (with spherical particles) supports the beneficial effect of plate-shaped particles. 

H. Verweij, K.N.P. Kumar, K. Keizer and A.J. Burggraaf, Formation and formation mechanisms of alumina and titania membranes, in Yi Hua Ma (Ed.), Proceedings of the Third International Conference on Inorganic Membranes (ICIM3) July 10-14,1993. Worcester Polytechnic Institue, Worcester, MA 016109, USA, pp. 27-35. 

Support modification has been reported earlier in the open literature [5,6,7,8,9]. Zirconia modification of silica supports was used to prevent the formation of unreducible cobalt-silicates [5]. Zr, Ce, Hf, or U modification of titania supports was reported to prevent the formation of cobalt-titanates during regeneration [6]. To increase the porosity of titania supports, they were modified with small amounts of binders, e.g. silica, alumina or zirconia [7]. Lanthanum oxide promotion of alumina was reported to be beneficial for improved production of products with higher boiling points [8], and zirconia modification of alumina supports was carried out to decrease the interaction of cobalt with alumina [9]. All these modified supports were either used for fixed bed cobalt based Fischer-Tropsch synthesis catalysts or they were used for slurry phase cobalt catalysts, but not tested under realistic Fischer-Tropsch synthesis conditions in large scale slurry bed reactors. 

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