Titania processing

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

Acetylation of acetaldehyde to ethyUdene diacetate [542-10-9], a precursor of vinyl acetate, has long been known (7), but the condensation of formaldehyde [50-00-0] and acetic acid vapors to furnish acryflc acid [97-10-7] is more recent (30). These reactions consume relatively more energy than other routes for manufacturing vinyl acetate or acryflc acid, and thus are not likely to be further developed. Vapor-phase methanol—methyl acetate oxidation using simultaneous condensation to yield methyl acrylate is still being developed (28). A vanadium—titania phosphate catalyst is employed in that process. 

The process known as transimidization has been employed to functionalize polyimide oligomers, which were subsequentiy used to produce polyimide—titania hybrids (59). This technique resulted in the successhil synthesis of transparent hybrids composed of 18, 37, and 54% titania. The effect of metal alkoxide quantity, as well as the oligomer molecular weight and cure temperature, were evaluated using differential scanning calorimetry (dsc), thermogravimetric analysis (tga) and saxs. 

Both the Toth and Alcoa processes provide aluminum chloride for subsequent reduction to aluminum. Pilot-plant tests of these processes have shown difficulties exist in producing aluminum chloride of the purity needed. In the Toth process for the production of aluminum chloride, kaolin [1332-58-7] clay is used as the source of alumina (5). The clay is mixed with sulfur and carbon, and the mixture is ground together, pelletized, and calcined at 700°C. The calcined mixture is chlorinated at 800°C and gaseous aluminum chloride is evolved. The clay used contains considerable amounts of silica, titania, and iron oxides, which chlorinate and must be separated. Silicon tetrachloride and titanium tetrachloride are separated by distillation. Resublimation of aluminum chloride is requited to reduce contamination from iron chloride. 

The method of preparation of a support material has a tremendous effect on its properties (11). For example, zeoHtes, which are highly stmctured aluminosihcates, are known to be extremely sensitive to the conditions employed both during and after crystallization (12). Also, when siUca—titania is precipitated by a coprecipitation method using ammonia, in which localized hydroxide ion gradients are estabUshed by the precipitation process itself, the product is much more acidic than when it is precipitated using urea, which suppHes hydroxide ion slowly and uniformly during precipitation (13). 

Vapor—vapor reactions (14,16,17) are responsible for the majority of ceramic powders produced by vapor-phase synthesis. This process iavolves heating two or more vapor species which react to form the desired product powder. Reactant gases can be heated ia a resistance furnace, ia a glow discharge plasma at reduced pressure, or by a laser beam. Titania [13463-67-7] Ti02, siUca, siUcon carbide, and siUcon nitride, Si N, are among some of the technologically important ceramic powders produced by vapor—vapor reactions. 

These processes are very rapid and allow the preparation of inorganic supports in one step. This technique allows large-scale manufacturing of supports such as titania, fumed silica, and aluminas. Sometimes the properties of the material differ from the conventional preparation routes and make this approach unique. Multicomponent systems can be also prepared, either by multimetallic solutions or by using a two-nozzle system fed with monometallic solutions [22]. The as-prepared powder can be directly deposited onto substrates, and the process is termed combustion chemical vapor deposition [23]. 

The present study revealed effects of various rutile/anatase ratios in titania on the reduction behaviors of titania-supported cobalt catalysts. It was found that the presence of rutile phase in titania could facilitate the reduction process of the orbalt catalyst. As a matter of fact, the number of reduced cobalt metal surface atoms, which is related to the overall activity during CO hydrogenation increased. 

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. 

The measured BET surface areas of titania samples were in the range of 99-116 m /g. It was found that surface area of titania decreased (as shown in Table 1) with increasing %02 during calcinations process whereas the crystallite size was apparently constant. 

Various methods are applied to the synthesis of titania particles including sol-gel method, hydrothermal method [2], citrate gel method, flame processing and spray pyrolysis [1]. To utilize titania as a photocatalyst, the formation of ultrafme anatase titania particles with large crystallite size and large surface area by various ways has been studied [4]. 

Another distinguishing feature of titania prepared by flame spray pyrolysis is the draar e of anatase crystallite size with the increase of flame temperature. Generally, the increase of preparation temperature increases the crystallite size in other processes such as sol-gel method, hydrothermal method [2, 3], flame processing and conventional spray pyrolysis. The decrease of crystallite size was directly related to the decrease of particle size. Fig. 5 shows SEM and TEM images of titania particles prepared by flame spray pyrolysis. 

NO, however, can only be removed by adding a reductant, ammonia, and using a catalyst. The process is called selective catalytic reduction, or SCR. The catalyst consists of vanadia and titania and works in the temperature interval 600-700 K according to the overall reaction ... 

An interesting recent example of successful application of the SSG process combined with ensuing supercritical drying is the design of titania-silica mixed oxides for the epoxidation of bulky olefins [16-18]. This example will be used to illustrate the opportunities the combined use of SSG and SCD provide for tailoring the chemical and structural properties of mixed oxides. 

For Ti02 and Z1O2, it is well known that sulfation induces a strong increase of acidity [17] and the participation of an add mechanism could then account for this promotion of activity. This mechamsm can be described as a bifunctional process oxidation of NO to NO on Cu sites, and nitration of a product of the oxidation of decane on the acid fiinction(8). The preparation of the catalyst must have a great influence on the activity. This has been shown by the comparison of three Cu/TiC catalysts prepared in different conditions one in which titania is first treated with sulfuric acid, then by Cu acetate (denominated Cu 04/Ti02, containing 0.S wt% Cu, 0.6 wt% S), one in which Cu is... 

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... 

The technology for the production of titania slag by direct reduction smelting in electric arc furnaces (particularly for smelting of iron titanates and ilmenites having lower amounts of titania) is very old and the process has been in vogue in Canada, Norway, South Africa,... 

Submerged arc smelting process Pre-reduction of ilmenite (50% Ti02) followed by smelting in arc furnace to yield pig iron and titania slag (87% Ti02) TINFOS, Norway... 

While vanadia- on titania-based catalysts can be used for both the classes of applications, there are other types of catalysts such as those based on copper [31b], which show good performances in case of mixtures of N0/N02 (nitric acid plants), while performances are worse when applied to emissions from catalytic processes. 

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