Effect of calcination temperature on photocatalytic activity of TiO₂. Photodecomposition of mono- and polyazo dyes in water

The presented studies have focused on the influence of TiO₂ properties, such as crystalline phase, crystallite size and surface area, on the effectiveness of degradation of azo dyes in water under UV irradiation. Two monoazo dyes: Acid Red 18 (AR18, C₂₀H₁₁N₂Na₃O₁₀S₃) and Acid Yellow 36 (AY36, C₁₈H₁₄N₃NaO₃S), and one polyazo dye Direct Green 99 (DG99, C₄₄H₂₈N₁₂Na₄O₁₄S₄) were applied as model compounds. The photocatalysts were prepared from a crude titanium dioxide obtained directly from the production line (sulfate technology) at the Chemical Factory "Police" (Poland). The crude TiO₂ was calcinated in air for 1-4h at the temperatures ranging from 600 to 800°C. The BET specific surface area of TiO₂ decreased gradually with increasing the calcination temperature. The crude TiO₂ exhibited specific surface area of 277 m²/g. In case of the catalysts heated at 600, 700 and 800°C the BET surface area amounted to 62.3-53.3, 33.4-26.8 and 8.9-8.3 m²/g, for the calcination time of 1-4h, respectively. The crystallite size of anatase increased with increasing heat treatment temperature and ranged from 19 to 53 nm, for the temperatures of 600-800°C, respectively. The catalysts annealed at 600 and 700°C contained primarily anatase phase (94-97%), whereas the photocatalysts heated at 800°C were composed mainly of rutile (97-99%). The highest effectiveness of azo dyes degradation was obtained in case of the photocatalyst calcinated for 1h at 700°C. The photocatalyst was composed mainly of anatase (97%) with crystallite size of 27 nm. The most effectively photodegraded was AR18, having the molecular weight of 640.4 g/mol. The most difficult to degrade was AY36 exhibiting the lowest molecular weight from all the dyes used (375.4 g/mol).