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Heterogeneous photochemical catalytic oxidation mainly refers to the use of semiconductors, such as TiO 2 , ZnO, etc.
, to oxidize and degrade organic matter through photocatalysis, which is a hot spot in recent research
.
The research on using semiconductor materials to catalyze the photodegradation of organic matter in water began in the past ten years
4.
2.
1 Reaction mechanism of TiO 2 photocatalytic oxidation
The reason why semiconductor materials can be used as catalysts is determined by their own photoelectric properties
.
According to the definition, semiconductor particles contain an energy band structure, which is usually composed of a low-energy valence band full of electrons and an empty high-energy conduction band, separated by a forbidden band
In the photocatalytic oxidation reaction in aqueous solution, water molecules lose electrons mainly in semiconductors.
OH - and organic matter themselves can also act as photo-induced hole capture agents.
The water molecules are changed to generate hydroxyl radicals with strong oxidation ability.
OH, ·OH are the most reactive oxidants in water, and they have almost no selectivity to the substrate
.
Photoinduced Electron trapping agent is mainly adsorbed on the Ti02 2 surface oxygen, both recombination of electrons and holes can be suppressed, but also the oxidant, the reaction product may have hydroxylated oxide, are another source of surface hydroxyl groups
When the energy of the photon (HV) higher than the band gap of the semiconductor, the semiconductor valence band electrons from the valence band to the conduction band, photo-generated electrons (E - ) and the photogenerated holes (H + ), the specific mechanisms are as follows:
Ti02 2 + HV → E - + H + (4-1)
OH aqueous solution - , both water and organic molecules can act as capture agents photogenerated holes to form a free hydroxyl strong oxidation ability, the specific mechanism is as follows:
H + + OH - → · OH
h + +H 2 O→·OH+H + (4-3)
The capture agent of photogenerated electrons is mainly O 2 adsorbed on the surface of the catalyst and metal ions in water.
The specific mechanism is as follows:
E - + O 2 → O · 2- (4-4 of)
·O 2- +H + →HO 2 · (4-5)
2HO 2 ·→O 2 +H 2 O 2(4-6)
H 2 O 2 + O · 2- → · OH + OH - + O 2(4-7)
Organ+·OH+O 2 →CO 2 +H 2 O+other products (4-8)
M n-+ (metal ion) + NE - → M 0 (4-9)
The order of oxidation potential of several strong oxidants is as follows:
F 2 >·OH>O 3 >H 2 O 2 >HO 2 ·>MnO>HClO>Cl 2 >Cr 2 O 7 2- >ClO 2
It can be seen that the hydroxyl radical has a very high oxidation potential and is a strong oxidant
.
·OH oxidation potential is 2.
In summary, the steps of the photocatalytic oxidation reaction can be described as follows: reactants, O 2 and water molecules are adsorbed on the surface of TiO 2 ; after light irradiation, TiO 2 generates electrons and holes; electrons and holes respectively diffuse to TiO 2 particles Surface; electrons, holes, oxygen and water molecules form hydroxyl radicals; hydroxyl radicals and reactants undergo oxidation reactions
