PHOTOCATALYTIC ACTIVITY AND SELECTIVITY OF TITANIA COLLOIDS AND PARTICLES PREPARED BY THE SOL-GEL TECHNIQUE - PHOTOOXIDATION OF PHENOL AND ATRAZINE

The photocatalyzed degradation of phenol and atrazine has been reexamined under AM1 simulated solar light irradiation of TiO2 catalysts in aqueous acid media to explore the photocatalytic activity and selectivity of titania colloids and titania specimens prepared by the sol-gel method yielding particles from nanometer size powders to millimeter spherical beads. In the photooxidation of phenol, the different TiO2 materials show small but significant variations in activity; there is no evidence for variations in selectivity with phenol because of the rather similar nature of the intermediates formed (hydroquinone, catechol, benzoquinone) and the rather analogous interaction(s) of these with the active oxidizing species (.OH radicals). By contrast, when atrazine is the test probe, there is a dramatic and significant variation in activity and selectivity in the differently prepared titania samples, as witnessed by the variations in the temporal distribution of intermediate species and product formed. This is a consequence of the three different degradation pathways and the modes of interaction between the probe and the catalyst: (i) dehalogenation, (ii) dealkylation, and (iii) deamination, affording a relevant different sensitivity to the oxidizing active oxygen species. Particular attention is focused on the activity of millimeter sized TiO2 beads in the light of their potential usefulness in photochemical reactors.