Titania tungsten oxide

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

Ti02 nanotubes were used to support M0O3 observing a spontaneous dispersion of molybdenum-oxide on the surface of nanotubes, which was different from that observed on titania particles.Supporting tungsten oxides a preferential orientation of the (002) planes was observed. Vanadium-oxide in the form of nanorods could be prepared using the titania nanotube as structure-directing template under hydrothermal... 

The interaction of tungsten oxide with silica is weak, but is strong with titania, and intermediate with ceria and zirconia. 

Table 18.5 Selectivity (%) of tungsten carbide on titania compared to the supported tungsten oxide precursor in FT reaction...

Catalysts composed of the following oxides have been found to be inert to the synthesis alumina, silica, molybdenum oxide (Mo2Ob), vanadium oxide (V2O3), blue tungsten oxide (W2O0), thoria, titania (TiOa), magnesia, lime, barium oxide, and strontium oxide.180 At a pressure of 150 atmospheres and a space velocity of 5000 conversions of... 

Indium tin oxide, titania, zirconia, tungsten oxide, doped oxides Ceramic coatings, transparent conductive films... 

The key property required of the inorganic species is ability to build up (polymerize) around the template molecules into a stable framework. As is already evident in this article, the most commonly used inorganic species are silicate ions, which yield a silica framework. The silica can be doped with a wide variety of other elements (heteroatoms), which are able to occupy positions within the framework. For example, addition of an aluminium source to the synthesis gel provides aluminosilicate ions and ultimately an aluminosilicate mesoporous molecular sieve. Other nonsilica metal oxides can also be used to construct stable mesoporous materials. These include alumina, zirconia, and titania. Metal oxide mesophases, of varying stability, have also been obtained from metals such as antimony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W), molybdenum (M), niobium (Nb), tantalum (Ta), and manganese (Mn). The thermal stability, after template removal, and structural ordering of these mesostructured metal oxides, is far lower, however, than that of mesoporous silica. Other compositions that are possible include mesostructured metal sulfides (though these are unstable to template removal) and mesoporous metals (e.g., platinum, Pt). 

Industrial plants have been relying on a form of SCR since the 1960s [19], however, the controlled and stable nature of industrial plant operation allows several degrees of freedom that are not possible on a vehicle. Industrial plants have relatively steady emissions output, are able to introduce gaseous NH3, can control the temperature of the catalyst to a very narrow window, and readily employ cleanup catalysts as the space constraints are not as limiting as on a vehicle. With these factors in mind, the low-cost vanadium and tungsten oxides supported on titania are the most widely used catalysts employed to selectively reduce NOx from stationary sources [20]. These catalysts have also been implemented for diesel vehicles in Europe, but they have limited thermal durability as well as the potential to emit harmful gaseous vanadium [21-23]. 

In addition, Raman spectroscopy also provides structural information about the presence of small metal oxide crystallites and surface reaction intermediates. Several extensive reviews of supported metal oxide catalysts have recently appeared in the literature, which have emphasized Raman spectroscopy vanadia [7,83-85], chromia [7,85,86], molybdena [7,87], niobia [7,88], rhenia [7,85], tungsten oxide [7], titania [85], and nickel oxide [89]. 

Incorporation of a second polymeric phase and subsequently burning or leaching it out vacates large voids in the sol-gel silicate [5,6,231] and other films and monoliths (e.g., titania, [232] and tungsten oxide, [233]). This concept was first used for sol-gel thin-layer chromatographic plates [234] and monolithic columns [235]. 

Both titania (anatase more than rutile) and, even more, zirconia (tetragonal more than monoclinic), when sulfated or covered with tungsten oxide become very active for some hydrocarbon conversion reactions such as -butane skeletal isomerization [263]. For this reason, a discussion began on whether these materials have to be considered superacidic. Spectroscopic studies showed that the sulfate ions [264] as well as the tungstate ions [265,266] on ionic oxides in dry conditions, are tetracoordinated with one short S=0 and W=0 bond (mono-oxo structure) as shown in Scheme 9.3(11). Polymeric forms of tungstate species could also be present [267]. However, in the presence of water the situation changes very much. According to the Lewis acidity of wolframyl species, it is believed that it can react with water and be converted in a hydrated form, as shown in Scheme 9.3. Residual... 

The group 5-7 supported transition metal oxides (of vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and rhenium) are characterized by terminal oxo bonds (M =0) and bridging oxygen atoms binding the supported oxide to the cation of the support (M -0-MSUpport). The TOF values for ODH of butane or ethane on supported vanadia were found to depend strongly on the specific oxide support, varying by a factor of ca. 50 (titania > ceria > zirconia > niobia > alumina > silica). 

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