Titania metal oxide catalysts

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-... 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

The most active catalyst is chromium oxide [7]. Silica (Si02) or aluminosilicates (mixed Si02/Al203) are used as the support material. The support is sometimes modified with titania (Ti02). The chromium oxide (Cr Os) catalyst was originally developed by Phillips Petroleum Company and is referred to as Phillips catalyst. Other metal oxide catalysts were developed primarily at Standard Oil of Indiana, the best known among them being the molybdenum oxide (Mo Os) catalyst. 

Neither of the base-metal oxide catalysts tested were active at low temperatures (see Figures 5 and 6), but CuO supported on alumina and silica exhibit rather low formation of acetaldehyde. This corresponds to the results presented by Rajesh and Ozkan (1993) who have tested the activity of catalysts containing either oxides of copper or chromium and also a combination of these two metal oxides supported on y-alumina pellets. The formation of acetaldehyde is slightly higher over CuO-Mn02. Catalysts supported on titania show the lowest light-off temperature for all the base-metal catalysts tested, but also the highest formation of acetaldehyde. 

Metal oxide catalysts are extensively employed in the chemical, petroleum and pollution control industries as oxidation catalysts (e.g., oxidation of methanol to formaldehyde, oxidation of o-xylene to phthalic anhydride, ammoxidation of propylene/propane to acrylonitrile, selective oxidation of HjS to elemental sulfur (SuperClaus) or SO2/SO3, selective catalytic reduction (SCR) of NO, with NHj, catalytic combustion of VOCs, etc.)- A special class of metal oxide catalysts consists of supported metal oxide catalysts, where an active phase (e.g., vanadium oxide) is deposited on a high surface area oxide support (e.g., alumina, titania, ziiconia, niobia, ceria, etc.). Supported metal oxide catalysts provide several advantages over bulk mixed metal oxide catalysts for fundamental studies since (1) the number of surface active sites can be controlled because the active metal oxide is 100% dispersed on the oxide support below monolayer coverage,... 

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]. 

The most commonly used SCR catalysts are composed of metal oxides such as titania and vanadia. These are called base metal oxide catalysts to distinguish them from catalysts containing precious metals such as platinum. In base metal oxide catalysts, the vanadium controls the reactivity, but it also catalyzes SO2 oxidation. Therefore, for high-sulfur applications, the vanadium content of the catalyst elements should be minimized (Behrens et al., 1991 A). By reducing the residence time in the catalyst, i.e., increasing the area velocity. SO2 oxidation to SO3 can be reduced. At the lower gas flows associated with lower loads, less SO3 is often generated because the lower gas temperatures decrease SO3 production more than the lower area velocities increase it (Cohen, 1993). 

TEXTURE AND SURFACE PROPERTffiS OF SUPPORTED METALLIC OXIDE CATALYSTS Na-DOPED, TITANIA AND ALUMINA-SUPPORTED VANADIA... 

The present research showed a dependence of various ratios of rutile anatase in titania as a catalyst support for Co/Ti02 on characteristics, especially the reduction behaviors of this catalyst. The study revealed that the presence of 19% rutile phase in titania for CoATi02 (C0/RI9) exhibited the highest number of reduced Co metal surface atoms which is related the number of active sites present. It appeared that the increase in the number of active sites was due to two reasons i) the presence of ratile phase in titania can fadlitrate the reduction process of cobalt oxide species into reduced cobalt metal, and ii) the presence of rutile phase resulted in a larger number of reduced cobalt metal surface atoms. No phase transformation of the supports further occurred during calcination of catalyst samples. However, if the ratios of rutile anatase were over 19%, the number of active sites dramatically decreased. 

Out of the metal oxides, sulfated titania and tin oxide performed slightly better than the sulfated zirconia (SZ) catalyst and niobic acid (Nb205). However, SZ is cheaper and readily available on an industrial scale. Moreover, it is already applied in several industrial processes (7,8). Zirconia can be modified with sulfate ions to form a superacidic catalyst, depending on the treatment conditions (11-16). In our experiments, SZ showed high activity and selectivity for the esterification of fatty acids with a variety of alcohols, from 2-ethylhexanol to methanol. Increasing... 

In conclusion, these data do not allow concluding whether or not Titania nanotubes form better catalysts due to their intrinsic nanostructure, and not simply because they have a high geometrical surface area and provide a good dispersion of supported catalysts. These properties may be found in other Titania based catalysts not having a ID nanostructure. On the other hand, it is also clear from above comments that most of the studies up to now were justified essentially from the curiosity to use a novel support more than from the rational design of advanced catalysts, which use the metal oxide nanostructure as a key component to develop... 

We focus attention here on titania (Ti02) for the following reasons. The first is that titania is a widely used oxide support for both metal particles and metal oxides, and used in some cases also directly as catalyst (Claus reaction, for example). The second is that it possesses multifunctional properties, such as Lewis and Bronsted sites, redox centres, etc. The third is that it has several applications both as a catalyst and an advanced material for coating, sensors, functional films, etc. The fourth is its high photocatalytic activity which make titania unique materials. 

The focus of these studies has been on identifying mild activation conditions to prevent nanoparticle agglomeration. Infrared spectroscopy indicated that titania plays an active role in dendrimer adsorption and decomposition in contrast, adsorption of DENs on silica is dominated by metal-support interactions. Relatively mild (150° C) activation conditions were identified and optimized for Pt and Au catalysts. Comparable conditions yield clean nanoparticles that are active CO oxidation catalysts. Supported Pt catalysts are also active in toluene hydrogenation test reactions. 

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