Nesrine

Addition of hydrogen peroxide is not necessary when titanium dioxide is used as the metal oxide for the catalytic degradation of dyes. This is so because the active oxidant, the hydroxyl radical, is generated in situ, when two distinct crystalline forms of TiO2 are irradiated in aqueous solutions. However when other metal oxides are used the active hydroxyl radicals are generated solely via the metal assisted decomposition of hydrogen peroxide.

The amount of the .OH radicals measured near TiO2 surfaces has been determined to be much higher than concentrations in the solution. The .OH then diffuse from the TiO2 surfaces, where they were formed, to the bulk solution. When irradiated, Rutile TiO2 produces much lower quantities of .OH radicals than Anatase.

Interestingly, when H2O2 is added to pure Anatase, the .OH radical generation decreases, but .OH radical concentrations increases when rutile and rutile-containing TiO2 are spiked with peroxide. This difference could be explained as follows: at the Anatase surface, trapped holes adsorb .OH radicals, while at the Rutile surface, Ti-peroxo (Ti-OO-Ti) is produced by the combination of two trapped holes which correspond to the adsorbed H2O2, the latter complex can work as a catalyst for the .OH radical generation from water.

Regards

Steve

Steve A. Carr Thanks for this quick respond.

I have another issue concering ZnO , it sometimes oxidize organic dyes without the use of hydrogen peroxide.

ZnO can be used as photocatalyst. Thus the radical species as well as direct reaction on catalyst surface can take place without addition of external oxidants.

Regards

G