Titania formation

Figure 6.17. Powder formation in the gas phase by oxidation of gaseous metal halides at high temperature and the effect of impurities on the reaction (a) Reactions of chlorides with oxygen to powders of the oxides. Impurities (in parentheses as chlorides) may accelerate or inhibit titania formation, (b) Particles size distributions of the titania products of the oxidation reactions. Impurities also affect the size distributions. ...

The gel combustion method consists of the addition of an organic reactant or a polymer to the liquid medium containing the precursor and calcining the gel formed. For instance, TiO(NOs)2 was mixed in water with oxalyldihydrazide and heated at 350 °C after dehydration and frothing sparks appear, leading to titania formation ... 

An interesting pH and temperature dependence of titania formation was also identified. At higher temperatures, the titania nanopartides for spermidine-templated titania became smaller in diameter, which was predicted to be caused by a faster nucleation rate. Interestingly, there was no notable temperature dependence for the significantly faster spermine-templated titania predpilation over the range of 5-80 C at this temperature, the reaction was kinetically faster, negating any temperature effects. A similar trend was observed for pH dependence, where... 

Penn R L and Banfieid J F 1999 Formation of rutiie nuciei at anatase (112) twin interfaces and the phase transformation mechanism in nanocrystaiiine titania Am. Miner. 84 871... 

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-... 

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]. 

The reason for the formation of anatase phase at such a high temperature might be explained as following. The as-prqiared ultrafine titania particles are liquefied at sufficimtly high temperature because melting point of nanoparticlra are lower than that of bulk titania (1850 C). The liquid titania particles are supercooled and became metastable states. The residence time in the flame is only in the order of miU-second so that the metastable phase has no time to become thermodynamically stable phase, rutile. 

The chlorination of titanium dioxide (titania) is thus entirely feasible at 900 °C. Similarly, many other metal oxides can be converted to metal chlorides by reaction with chlorine in the presence of carbon. It should be noted that carbon itself is not easily chlorinated as the standard free energy of formation of carbon tetrachloride is positive at temperatures above 500 °C. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

The reduction of the catalyst precursor with sodium formate resulted in a lower Pd dispersion than the catalyst prepared by hydrogen reduction, the particle size is much larger in the former catalyst. The mesoporous carbon supported Pd catalysts are near to those of Pd on titania with respect to their enantiodifferentiating ability. Besides the metal dispersion, the availability of the Pd surface in the pores for the large modifier molecules seems to be the determining factor of the enantioselectivity. 

N-doped titania demonstrated promising activity under visible light for the degradation of Rhodamine B in aqueous solutions [85], In thin-films of N-doped Ti02, the formation of nitride phases enhanced the catalytic properties of the film [86]. 

Uchida, M., Kim, H.-M., Kokubo, T., Fujibayashi, S. and Nakamura, T. (2003) Structural dependence of apatite formation on titania gels in a simulated body fluid. Journal of Biomedical Materials Research, 64A, 164-170. 

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