Basics of Photocatalysis, Mechanism and Applications

Photocatalysis

Photocatalysis is the activity occurring when a light source interacts with the surface of semiconductor materials, called photocatalyst. It is the amalgamation of photochemistry and catalysis. The word “photocatalysis” is derived from the Greek language and composed of two parts:

  • The prefix “photo” means light
  • Catalysis” may a substance that can increase the rate of the reaction.

The basic principle is that the organic molecules come into contact with the surface of the photocatalyst under UV light irradiation, which leads to occur a series of oxidation and reduction (redox) reactions. During this process, there must be at least two simultaneous reactions occurring, oxidation from photogenerated holes, and reduction from photogenerated electrons. The generation of reactive hydroxyl radicals is responsible for the degradation of recalcitrant compounds.

 

Types of Photocatalysis

There are two types of photocatalytic processes:

  1. Heterogeneous: In heterogeneous photocatalysis, the catalyst and the substrate are in different phases.
  2. Homogenous: In homogenous photocatalysis, the catalyst and the substrate are involved in a single-phase.

 

What is the Photocatalyst?

A photocatalyst is defined as a substance that is activated by adsorbing a photon and can accelerate a reaction without being consumed.

How does photocatalyst work:

A photocatalyst is a material that absorbs light to bring it to a higher energy level and provides such energy to a reacting substance to make a chemical reaction occur. Current and past research has investigated several types of photocatalysts such as:

  1. Titanium Dioxide: TiO2
  2. Zinc Oxide: ZnO
  3. Magnesium Oxide: MgO
  4. Tungsten(VI) Oxide: WO3
  5. Iron(III) Oxide or Ferric Oxide: Fe2O3
  6. Silicon Carbide: SiC
  7. Gallium Arsenide: GaAs
  8. Zinc Sulfide: ZnS
  9. Gallium Phosphide: GaP
  10. Cadmium Sulfide: CdS

However, Titanium dioxide is by far the most widely used. It has been widely used as a photocatalyst in many environmental and energy applications due to its efficient photoactivity, high stability, low cost, and safety to the environment and humans. However, its large bandgap energy, ca. 3.2 eV limits its absorption of solar radiation to the UV light range which accounts for only about 5% of the solar spectrum.

 

Mechanism of Photocatalysis

The reaction basically depends on light (photon) intensity and the catalyst. Only a few steps will let you know the whole mechanism of photocatalysis.

  1. When light incidents on the surface catalyst.
  2. The catalyst absorbs light at a specific wavelength with the corresponding promotion of an electron from the valence band to the conduction band, which leaves a hole (electron deficiency) in the valence band after moving of electron from the valence band to the conduction band. This can be schematically represented as TiO2 + hν = e− cb (TiO2) + h+ vb (TiO2),where cb is the conduction band and νb is the valence band.
  3.  The holes would be left in the valence band of the catalyst. These holes in the valence band can oxidize donor molecules and react with water molecules to generate hydroxyl radicals (The hydroxyl radicals have strong oxidizing power responsible for the degradation of pollutants).
  4. A reaction takes place between electrons from the conduction band and dissolved oxygen species to form superoxide ions in which these electrons propel the redox reactions (Saravanan et al., 2017).
                                                                                                                Figure: Mechanism of photocatalysis

A general, summarization of possible reactions occurring during photocatalysis is presented below:

Applications of photocatalysis:

Widely used in a variety of applications and products in the fields as below:

  1. Removal of heavy metals
  2. Degradation of toxic pollutants
  3. Degradation of specific contaminants
  4. Sludge treatment
  5. Water splitting
  6. Removal of harmful gases
  7. No disinfection by-products
  8. Metal reduction
  9. building exterior self-cleaning
  10. Reduction of color and odor
  11. Removal of pollution
  12. Sterilization

 

 

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