Gas-phase synthesis of titania by TiCI, oxidation in the presence of dopants (SiCI,, POCI,, and BCI,) was investigated in an aerosol reactor as a function of temperature (1300-1700 K) and dopant concentration (0-25 mol% of TiCI,). The addition of dopants, most notably silicon and phosphorus, drastically altered the morphology of titania particles from polyhedral to spheroidal, increased the extent of aggregation, increased the specific surface area, reduced the primary particle size, and decreased the rutile content. The observed morphology/crystallinity changes were explained in terms of ionic radii and valence of the dopant element.

The oxidation rate of TiCI4 vapor was measured by FTIR spectroscopy between 700" and 1000°C in a furnace aerosol reactor. The reaction rate was found to be first order with respect to TiC14 and to follow the Arrhenius form. The apparent activation energy for the reaction was 88.8 f 3.2 kJ/mol and the preexponential factor 8.26 X lo4 s-'. A kinetic mechanism for the oxidation of TiCI4 was proposed that was consistent with the observed dependence of oxygen concentration on the apparent rate constant. [

The role of aluminum as dopant in gas phase synthesis of titania powders was experimentally investigated in an aerosol flow reactor between 1300 and 1700 K. Titania was produced by vapor phase oxidation of titanium tetrachloride in the presence of dopant aluminum trichloride vapor. The presence of aluminum altered the particle morphology from polyhedral to irregular crystals. Energy dispersive analysis and transmission electron microscopy indicated that the powders were mixtures of crystalline titania and amorphous alumina. Analysis by XPS indicated significant enrichment of aluminum on the particle surface. Some aluminum titanate (up to 17% by volume) was formed at 1700 K when a high concentration of AlCl3 was used (AlCl3/TiCl4 ≥ 0.07). Measurements of lattice parameters by x-ray diffraction indicated that aluminum formed a solid solution in titania. While titania synthesized in the absence of aluminum was about 90% anatase, the introduction of aluminum resulted in pure rutile at AlCl3/TiCl4 = 0.07. The effects of aluminum on titania phase composition and morphology are explained by the creation of oxygen vacancies in the titania crystallites and by the enhancement of the sintring rate of titania grains.

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