What is photocatalytic oxidation?
Oxidative decomposition means that if titanium oxide is exposed to light, organic substances on its surface will be broken down by oxidation, eventually forming carbon dioxide and water. This has applications in fields such as deodorization, elimination of volatile organic compounds (VOCs), elimination of soiling, antibacterial action and sterilization.
Titanium oxide is said to be an optical semiconductor, and it changes from an insulator to a conductor by absorbing the energy of light. This occurs because electrons are excited from the valence band to the conduction band, causing the formation of holes in the valence band . The Excited electrons react with oxygen in the atmosphere Anions forming superoxide (O • two -), and the holes react with Moisture in the atmosphere producing Hydroxyl Radicals (• OH). These are Extremely Active Species reactive oxygen, and decompose and Oxidize Organic Substances ( Figure 1 ).
Figure 1. Schematic representation of photocatalytic reactions (a) water splitting; (b) biomass oxidation; (c) photo-induced reforming of biomass.
The most powerful advanced oxidation systems are based on the generation of hydroxyl radicals. The hydroxyl radical is an extremely powerful oxidation agent, second only to Fluorine in power (2.23 in Relative Oxidizing Power). Following is a listing of common chemical oxidants, placed in the order of their oxidizing strength:
| Exhibit XIII-11 Relative Power of Chemical Oxidants |
||
| Compound | Oxidation Potential (volts) |
Relative Oxidizing Power (CI2 = 1.0) |
|---|---|---|
| Hydroxyl Radical | 2.8 | 2.1 |
| Sulfate Radical | 2.6 | 1.9 |
| Ozone | 2.1 | 1.5 |
| Hydrogen Peroxide | 1.8 | 1.3 |
| Permanganate | 1.7 | 1.2 |
| Chlorine Dioxide | 1.5 | 1.1 |
| Chlorine | 1.4 | 1.0 |
| Bromine | 1.1 | 0.80 |
| Iodine | 0.76 | 0.54 |
Source: U.S. Environmental Protection Agency
Organic Decomposition Example
Photocatalyst decomposition capabilities of the treated surface weaken the adhesiveness of matter that contains oils. Rainwater spreads uniformly over the surface and floats away grime, for self-cleaning functionality.
Utilizing the strong oxidation strength of hydroxyl radical, photocatalytic oxidation can effectively disinfect, deodorize, and purify air, water, and different surface area.
Stain Decomposition Test
Test Procedure
- Inside of blue dots line is coated with TiO2.
- The red ink is diluted one-twentieth with water
- A UV light with intensity of 1mW/cm2 was used for this test
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| Immediately after illumination |
10 minutes later | 20 minutes later | 30 minutes later | 40 minutes later |
