A Theoretical Study on Gas-Phase Coating of Aerosol Particles

In situ coating of aerosol particles by gas-phase and surface reaction in a flow reactor is modeled accounting for scavenging (capture of small particles by large particles) and simultaneous surface reaction along with the finite sintering rate of the scavenged particles. A log-normal size distribution is assumed for the host and coating particles to describe coagulation and a monodisperse size distribution is used for the coating particles to describe sintering. As an example, coating of titania particles with silica in a continuous flow hot-wall reactor was modeled. High temperatures, low reactant concentrations, and large host particle surface areas favored smoother coatings in the parameter range: temperature 1700-1800 K, host particle number concentration 1 x 10(5)-1 x 10(7) #/cm3, average host particle size 1 &mgr;m, inlet coating reactant concentration (SiCl4) 2 x 10(-7)-2 x 10(-10) mol/cm3, and various surface reaction rates. The fraction of silica deposited on the TiO2 particles decreased by more than seven times with a hundredfold increase in SiCl4 inlet concentration because of the resulted increase in the average SiO2 particle size under the assumed coating conditions. Increasing the TiO2 particle number concentration resulted in higher scavenging efficiency of SiO2. In the TiO2/SiO2 system it is likely that surface reaction as well as scavenging play important roles in the coating process. The results agree qualitatively with experimental observations of TiO2 particles coated in situ with silica.