Photocatalytic activity of nanostructured titanium dioxide from diffusion flame synthesis

Vapour fed flame aerosol synthesis has been employed to synthesize TiO2 photocatalyst by oxidation of titanium tetrachloride (TiCl4). The effect of precursor loading on particle growth and rutile phase formation was studied using laminar premixed flame with (Ar + TiCl4)/CH4/O2/N2 configuration and synthesis temperature of around 900 °C. The increase in precursor flow rate, as key synthesis parameter, from 2.5 to 29.5 mmol h−1 increased the average particle diameter, dBET, from 11 to 22 nm and decreased the specific surface area from 145 to 71 m2 g−1 respectively. The samples were anatase-rich (up to 98 wt.%) and of increasing rutile fraction, when the primary TiO2 particles exceeded the critical size of dc ≈ 11.6 nm. Degradation rate of aliphatic dichloroacetic acid (DCA) and aromatic 4-chlorophenol (4-CP), used as model pollutants for evaluation of photocatalytic activity, was higher for diffusion flame aerosol powders than for commercial pyrogenic TiO2 products, P25 and P90, Evonik, of comparable sizes and anatase fractions. This effect was attributed to a lower degree of agglomeration resulting in a higher specific surface area. Photocatalytic activity of nanopowders is exclusively coupled with the anatase fraction. A beneficial effect due to accompanying rutile was not detected. The photocatalytic efficiency of nanopowders was found to decrease strongly with increase in average particle sizes larger than dc but was not positively affected by d lower than dc.