Synthesis of Alumina- and Alumina/Silica-Coated Titania Particles in an Aerosol Flow Reactor

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 TiCl 4 with O 2 at 1300 and 1500 °C and were passed through a coating region where they were mixed with AlCl 3 or mixtures of AlCl 3 and SiCl 4 . Uniform and dense coatings of Al 2 O 3 were obtained at both 1300 and 1500 °C using AlCl 3 inlet concentrations that corresponded to Al 2 O 3 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 Al 2 O 3 /SiO 2 were obtained at 1300 °C when the SiO 2 /Al 2 O 3 weight ratio was less than 2:1. It is proposed that coating takes place in two modes: (a) chemical vapor deposition of AlCl 3 and SiCl 4 on the particle surface and (b) gas-phase reactions of the metal chlorides to form Al 2 O 3, SiO 2, or Al 2 O 3 /SiO 2 particles that subsequently collide with the titania particles and sinter into the coating.