Titania-zirconia

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-... 

P. Beatrice, C. Pliangos, W.L. Worrell, and C.G. Vayenas, The electrochemical promotion of ethylene and propylene oxidation on Pt deposited on Yttria-Titania-Zirconia, Solid State Ionics 136-137, 833-837 (2000). 

Electron micrographs (scanning and transmission) showed that tungsten carbide is well dispersed on the surface of each support as nanosized particles (20 - 50 nm) as typified by the images in Figs. 3 (a b). However, BET surface area decreased in the order alumina > silica > titania > zirconia. With highest surface area obtained for each support being 240,133,18 and 9 m g respectively. 

Common catalyst compositions include oxides of chromium or molybdenum, or cobalt and nickel metals, supported on silica, alumina, titania, zirconia, or activated carbon. 

N. Kiratzis, P Holtappels, C. E. Hatchwell, M. Mogensen, and J. T. S. Irvine, Preparation and characterization of copper/yttria titania zirconia cermets for use as possible solid oxide fuel cell anodes, Fuel Cells 1,211-218 (2001). 

Catalysts for low-temperature gasification include combinations of stable metals, such as rathenium or nickel bimetallics and stable supports, such as certain titania, zirconia, or carbon. Without catalyst the gasification is limited (Krase et al., 2000). Sodium carbonate is effective in increasing the gasification efficiency of cellulose (Minowa et al., 1997). Likewise, homogeneous, alkali catalysts have been employed for high-temperature supercritical water gasification. 

ALUMINA, TITANIA, ZIRCONIA, LANTHANA GEL METAL RESISTENCE... 

The leaching of catalyst components into the aqueous phase during the reaction represents a possible disadvantage of the process. Therefore, the choice of catalyst support materials has to be limited to those that exhibit long-term hydrothermal stability (e.g. carbon, titania, zirconia). 

It also became evident that a great variety of catalysts, potentially exhibiting a large flexibility, could be prepared via solid solutions. Three different degrees of freedom can be varied in a controlled fashion the chemical nature of the host matrix AO, the chemical nature of the guest cation M, and the dopant concentration x. Furthermore, solid solutions can be formed not only by cubic oxides but also by alumina, titania, zirconia, and others. Thus, another degree of freedom is added, namely, the different crystal structures. 

For most of the oxide-supported monolayer oxides (e.g. vanadia, molybdena and tungsta supported on alumina), titania, zirconia and silica surface species are... 

Heat treating lemp. (X) Alumina ) Slit-shaped Cylinder-shaped Titania ) Zirconia )... 

Some dense inorganic membranes made of metals and metal oxides are oxygen specific. Notable ones include silver, zirconia stabilized by yttria or calcia, lead oxide, perovskite-type oxides and some mixed oxides such as yttria stabilized titania-zirconia. Their usage as a membrane reactor is profiled in Table 8.4 for a number of reactions decomposition of carbon dioxide to form carbon monoxide and oxygen, oxidation of ammonia to nitrogen and nitrous oxide, oxidation of methane to syngas and oxidative coupling of methane to form C2 hydrocarbons, and oxidation of other hydrocarbons such as ethylene, methanol, ethanol, propylene and butene. 

As to ceramic membranes [3,4] the focus has been so frir in particular on amorphous porous aluminas and silicas. Other inorganics studied include titania, zirconia, non-oxide ceramics (carbides), and microporous carbons. 

Catalysts containing niobia supported on various oxides have been the subject of considerable recent interest [1-4]. The molecular structures and reactivity of niobium oxides supported on alumina, titania, zirconia and silica have been intensively investigated over the last few years. Niobia supported on silica has been shown to be active for the dehydrogenation and dehydration of alcohols, photo-oxidation of propene and oxidative decomposition of methyl tertiary butyl ether. Titania supported niobia is active for the selective catalytic reduction (SCR) of NO by NH3. 

Titania, zirconia, and alumina samples11681 with periodic 3-D arrays of macropores were synthesized also from the corresponding metal alkoxides, using latex spheres as templates. 

It should be stressed that this is a very simplified model. For example, the magnitude and size of the electrical charges on the pore wall and particle surface will be important in cases where pore and particle size are not too different. No data are available on initial membrane formation. A similar situation exists to explain the positive effect of some additions (e.g. PVA to boehmite solutions). The trends in experimental observations and the qualitative model discussed above holds also for the formation of other types of mesoporous membranes (titania, zirconia, silica) [6] (Chapter 7). 

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. 

Silica and silica-titania/zirconia membranes with high quality combining high separation factors and high permeation values were first reported by Uhlhorn [12,58] and were further developed and analysed by de Lange et al. [43,46,47,60]. Further optimisation has been undertaken by Verwey and coworkers [59] in the same group. 

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