(189k) The Role of Gas-Aerosol Mixing during In-Situ Coating of Flame-Made Titania Particles

Rutile TiO₂ particles of 40 nm average diameter were made by flame spray pyrolysis (FSP) and in-situ coated with ultrathin (2 ? 4 nm) SiO₂ layers. The spray flame was enclosed by a quartz glass tube while hexamethyldisiloxane (HMDSO) vapor-laden N₂ was injected in crossflow to and downstream of the flame through a metal torus pipe ring. The as-prepared powders were characterized by transmission electron microscopy (TEM), X-ray diffraction and nitrogen adsorption. The coating quality of titanium dioxide was evaluated further by photocatalysis of isopropanol to acetone under UV light using suspensions of the above particles. The effect of mixing intensity on product particle morphology was investigated by varying the exit velocity or the number of jet outlets for HMDSO-laden N₂ gas. At high mixing intensity between the TiO₂ aerosol and HMDSO-laden jet streams, the particles were encapsulated by smooth and homogeneous coatings. Poor mixing between HMDSO vapor and TiO₂ aerosol resulted in separate SiO₂ and poorly-coated TiO₂ particles. The effect of mixing TiO₂ aerosol with the HMDSO vapor stream jets was elucidated also by computational fluid dynamics: low flowrates or few jet outlets of the HMDSO vapor stream resulted in incomplete mixing across the radius of the reactor, consistent with the experimental data. Well-coated TiO₂ particles produced with multiple coating vapor jets or high mixing intensity exhibited limited photooxidation indicating the effectiveness of their SiO₂ coatings.