Mechanistic implications of the effect of TiO2 accessibility in TiO2-SiO2 coatings upon chlorinated organics photocatalytic removal in water

The photocatalytic removal rates r of dichloroacetic acid (DCAA), 4-chlorobenzoic acid (4-CBA) and 4-chlorophenol (4-CP) in water were compared for TiO(2) samples used either as a powder or as a coating on a fiber glass tissue, SiO(2) being the binder. From SEM-EDX measurements it was deduced that SiO(2) prevails over TiO(2) in the coating top layers and (18)O(2)-Ti(16)O(2) isotopic exchange showed that the accessibility of O(2) to TiO(2) was markedly reduced when TiO(2) was thus coated. The unfavorable effect of the restricted TiO(2) accessibility on r was drastic for DCAA, much less pronounced for 4-CBA, and still smaller for 4-CP. It is inferred that DCAA can be attacked only when it directly interacts with TiO(2), whereas 4-CP can also react within the near-TiO(2) surface water layers. The 4-CBA intermediate behavior is in line with the structural similarities of 4-CBA with DCAA and 4-CP.