Titania coatings on high and low surface area spherical silica particles by a sol-gel method

Spherical particles of porous silica were coated with titania by the sol-ge l method starting from titanium isopropoxide. The silica samples used were of high and low surface area (about 250 and 19 m(2) g(-1), respectively). T he coated particles were generally calcined in air at 500 degreesC for 2 h. X-Ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AE S) were used before and after erosion with Ar+ ions at 4 keV. Silica with a high surface area was partially coated with a thin (below 1 nm) TiO2 layer , whereas the low surface area silica resulted in a better coating. In this case, XPS and AES, in combination with silica and titanium chemical analys is, suggest that the particles were coated with nearly one monolayer of tit ania. High surface area samples exhibited a small decrease of BET area afte r deposition of the titanium dioxide, whereas no significant change of BET area was observed in low surface area silica. Porosity analysis showed, in both cases, a minor decrease of micropore volume and area after the coating process. This suggests that titania oligomers contained in the titania pre cursor sol do not penetrate significantly into the micropores and therefore the pore plugging process is a rather limited one. X-Ray diffraction showe d that the titania layer is amorphous but begins to crystallize in the anat ase form after about 6 h at 700 degreesC in those samples with the larger T iO2 content.