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.
Mostly rutile titania nanoparticles (doped with Al) about 40 nm in diameter made by flame spray pyrolysis (FSP) were coated at considerable production rates, 30 g/h, in-situ with 2 – 4 nm thick SiO2 films by mixing the freshly-formed TiO2 aerosol with hexamethyldisiloxane (HMDSO) vapor just downstream of the flame. A hollow ring delivering HMDSO vapor (precursor to SiO2) through 16 jets was judiciously positioned in cross-flow above the enclosed flame coating freshly-made Al-doped rutile TiO2 particles. The as-prepared powders were characterized by (scanning) transmission electron microscopy (STEM and TEM), energy dispersive X-ray analysis, X-ray diffraction, DC plasma optical emission spectroscopy (DCP-OES) and nitrogen adsorption. The coating quality was assessed further in the photocatalytic oxidation of isopropanol to acetone. The coating thickness was controlled by the silica content. The particles were encapsulated by smooth and homogeneous coatings at above 10 wt% SiO2 contents. Premature or early HMDSO vapor injection resulted in segregated SiO2 and patchy or poorly-coated TiO2 particles. The anatase promotion of silica in TiO2 was significantly reduced as the distance of HMDSO vapor injection point from the FSP burner was increased. Well-coated TiO2 particles exhibited limited IPA photooxidation. In contrast, spraying and combusting equivalent mixtures of Si/Al/Ti precursors in the enclosed FSP reactor (without delivering HMDSO through the hollow ring) resulted in particles segregated in amorphous (SiO2) and crystalline (TiO2) domains and thus high photocatalytic activity.