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The feasibility of in situ coating of titania particles with silica using a high-temperature, gas-phase process was demonstrated. Titania was produced from the reaction of TiCl4 and O2 in a hot-wall, tubular, aerosol reactor and directly coated in the gas phase via the reaction of O2 with SiCl4 vapor. Rough SiO2 coatings were obtained at 1300 °C while uniform, dense coatings were obtained at all conditions examined for 1500 °C. The presence of water in the reactor significantly influenced the morphology of the coatings and resulted in smooth, dense, and uniform coatings at 1300 °C. Coating thicknesses could be controlled from 5 nm to roughly 100 nm, corresponding to growth rates on the order of 10–100 nmys. The characteristics of the coatings depended upon the concentration of SiCl4 and the coating temperature. These process variables influenced the coating mechanism, growth rate, and densification which directly influenced the coating uniformity and thickness.

In situ coating of titania aerosol particles with alumina and alumina−silica mixtures by chemical vapor deposition, cluster deposition, and sintering was demonstrated in a hot-wall, continuous flow tubular reactor. Titania particles with diameters in the range 0.2−0.8 μm were produced by the reaction of TiCl4 with O2 at 1300 and 1500 °C and were passed through a coating region where they were mixed with AlCl3 or mixtures of AlCl3 and SiCl4. Uniform and dense coatings of Al2O3 were obtained at both 1300 and 1500 °C using AlCl3 inlet concentrations that corresponded to Al2O3 mass loading of less than 1 wt %. The coating thickness could be controlled from 5 to 20 nm which corresponded to deposition rates up to 50 nm/s. Uniform and dense coatings of Al2O3/SiO2 were obtained at 1300 °C when the SiO2/Al2O3 weight ratio was less than 2:1. It is proposed that coating takes place in two modes:  (a) chemical vapor deposition of AlCl3 and SiCl4 on the particle surface and (b) gas-phase reactions of the metal chlorides to form Al2O3, SiO2, or Al2O3/SiO2 particles that subsequently collide with the titania particles and sinter into the coating.