What is a photocatalyst?

Photocatalysis is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400 – 750 nm) or infrared radiation (750 – 2500 nm). A catalyst is not changed or consumed in the overall chemical reaction.

There are various materials that show photocatalytic capability, and titanium dioxide (TiO2) is said to be the most effective.

When a photocatalyst is exposed to light in the presence of water vapor, two highly reactive substances are formed: hydroxyl radicals [OH] and a superoxide anion [O2-1]. It allows for the oxidation of airborne Volatile Organic Compounds (VOCs) and toxic organic matter into carbon dioxide and water at room temperature when exposed to light. It does not require any advanced technology to operate effectively.

Toxic VOC + Light + Water + Photocatalyst

=

Non-Toxic inorganic Gas + Photocatalyst

Photocatalysts have the following advantages over any current air purification technologies:

  • Full break-down of pollutant rather than a simple transfer into a filter
  • Purification works at ambient temperature and pressure
  • Based on more than 30 years of peer-reviewed research
  • Economical, cheap and low energy consumption
  • Adapted for a large range of pollutants (VOC, bacteria, pathogens, mold)

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:

 

Relative Power of Chemical Oxidants

CompoundOxidation Potential (volts)Relative Oxidizing Power
(CI2 = 1.0)
Hydroxyl Radical2.82.1
Sulfate Radical2.61.9
Ozone2.11.5
Hydrogen Peroxide1.81.3
Permanganate1.71.2
Chlorine Dioxide1.51.1
Chlorine1.41.0
Bromine1.10.80
Iodine0.760.54

Source: U.S. Environmental Protection Agency

Stain Decomposition Test

Test Procedure

  • The inside of the blue dotted line is coated with TiO2.
  • The red ink is an organic ink that can be decomposed through photocatalysis.
  • A UV light with intensity of 1mW/cm2 was used for this test.

Immediately after illumination

10 minutes later

20 minutes later

30 minutes later

40 minutes later