Strong surface Lewis acid sites lead to distortion of the ozone molecule affording its decomposition with formation of a free oxygen molecule and a surface oxygen atom that remains attached to the site.4
O3 adsorbed onto TiO2 surface acts as a very strong electrophylic
O 3 ˙ - + H 2 O
→ OH + OH- + O2
The above reported mechanism gives rise to the formation of a hydroxyl radical per each trapped electron (reactions 6 and 7), whereas three electrons are needed for the generation of a hydroxyl radical when oxygen acts as electron trap (see reactions 8-11).