In-Depth Treatment of Photocatalytic Character of a Reaction
Effect of O2
- It is able to scavenge electrons at the surface of UV-irradiated TiO2, thereby allowing the separation of the photogenerated charges. This process is essentially equivalent to decreasing the recombination rate of electrons and holes.
- O2 can react with alkyl radicals or, more generally, organic radicals, yielding peroxyl radicals en route to the mineralization of the organic precursor.
- The reduced form of O2, viz. O2._, the radical anion superoxide, can react with an organic radical cation — resulting from the reaction of holes with electron-rich organic pollutants —which is one of the primary steps in the chemical degradation events.
- One of the consequences of this multiple involvement of dioxygen is that the photocatalytic reactors used for treating water should allow O2 (air) to easily access the TiO2 surface.
- In other words, the rates of gas-to-liquid and liquid-to-solid transfers should be maximized. This condition can be achieved by one or several of the following means: bubbling air in the water producing a turbulent flow of the water in contact with air limiting the water film thickness at the TiO2 surface [the use of TiO2-coated rotating disks or of TiO2-coated beads (or small tubes) floating onwater, etc.
Effect of H2O2
Adding H2O2 to the water to be photocatalytically treated can also be viewed as a way of increasing dioxygen concentration at the TiO2 surface because H2O2 is disproportionated to H2O and O2 over UV-irradiated TiO2.
However, hydroxyl radicals can also react with the added H2O2 instead of reacting with the organic pollutants:
Therefore, the net effect depends on the type of water, the TiO2 specimen, and other experimental conditions. Reported beneficial effects are less than one order of magnitude.