Effect of synthesis conditions on photocatalytic activity of TiO2 powders synthesized in supercritical CO2

TiO2 anatase nanoparticles have been synthesized by thermal-hydrolysis of diisopropoxititanium bis(acetylacetonate) (DIPBAT) in supercritical carbon dioxide (SC-CO2). Operational variables, such as hydrolysis reactant, pressure, temperature and residence time, have a strong influence on TiO2 properties, and therefore in the final application of the titanium dioxide. Photocatalytic activity of the obtained TiO2 has been tested in the oxidation of methylorange (MeO) in aqueous solutions. The obtained results show that synthesis pressure (in the range 10–20 MPa) does not affect the photocatalytic activity of formed TiO2 anatase. Isopropyl alcohol appears as a better hydrolysis reactant than ethanol, since the formed TiO2 presents higher photocatalytic activity. When synthesis temperature is increased from 200 °C to 300 °C, crystallite size of the particles is enhanced, revealing the improvement of crystalline quality and indeed of photocatalytic activity. The proposed synthesis process can be operated in a continuous way, and from the present results, a residence time of 1 min is enough to obtain TiO2 nanoparticles with a photocatalytic activity similar to commercial powders. TiO2 particles have been subjected to a post-calcination process at 550 °C, 700 °C and 900 °C, and the experimental results show that the anatase phase is very stable and there is not a transition phase to rutile up to 900 °C. Moreover, a post-calcination treatment is not suitable from the catalysis point of view since the photocatalytic activity decreases, and the proposed synthesis process without any heat post-treatment is appropriated.