3.1.4. TiO2
Titania is mainly used as a photocatalyst [72–77]. Beltrán et al.
[78] showed that TiO2 can be also used as an active catalyst in the
ozonation of organic molecules such as oxalic acid. The authors
suggested that both ozone and oxalic acids are adsorbed at pH
2.5 (similarly to gas phase reactions) on the surface of the catalyst,
followed by a subsequent reaction between the two moieties.
Unfortunately, in the literature there is no direct proof of ozone
adsorption on metal oxides in the presence of water. In another
paper, Addamo et al. [79] reported photocatalytic oxidation to be
very effective at pH 10 in the removal of oxalic acid in water in
the presence of oxygen. On the other hand, Yang et al. [38] studied
ozonation of nitrobenzene on nano-TiO2 and observed that
nano-TiO2 is catalytically active if present in the form of rutile
and not anatase. Adsorption of nitrobenzene was found to play an
important role and degradation of this compound proceeded via
a radical pathway. According to the authors radicals are formed
on the hydroxylated surface of nano-TiO2, and the process is especially
effective at a pH close to pHPZC, which indicates that neutral
hydroxyls are responsible for the formation of hydroxyl radicals.
Earlier work [34] showed no ozone decomposition on anatase-TiO2
at a pH close to its pHPZC. Ye et al. [80] studied the degradation
of 4-chloronitrobenzene (CNB) during catalytic and photocatalytic
ozonation in the presence of TiO2. Both photocatalytic and catalytic
processes revealed similar efficiency of the decomposition of
CNB, but photocatalysis resulted in higher mineralization of CNB
[80]. Catalytic activity of TiO2 was also investigated in the process
of the removal of pharmaceuticals such as carbamazepine and
naproxen [81]. TiO2 had an effect on the mineralization of ozonation
by-products and their composition. It was observed that in the
presence of the catalyst the generation of hydroxyl radicals takes
place at pH 5. On the other hand, at a neutral pH, the inhibition of
hydroxyl radicals formation was noted in the presence of TiO2.