Alexandra Teleki, Department of Mechanical and Process Engineering, Swiss Federal Institute of Technology Zurich, Particle Technology Laboratory, Sonneggstrasse 3, Zurich, CH-8092, Switzerland, Martin C. Heine, Particle Technology Laboratory, Swiss Federal Institution of Technology Zurich, Sonneggstrasse 3, Zurich, 8092, Switzerland, Frank Krumeich, Laboratory for Inorganic Chemistry, ETH Zurich, Wolfgang-Pauli-Str. 10, Zürich, CH-8093, Switzerland, Kamal M. Akhtar, Millennium Inorganic Chemicals (a Cristal Company), 6752 Baymeadow Drive, Glen Burnie, MD 21060, and Sotiris E. Pratsinis, Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, Zurich, CH-8092, Switzerland.
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.