Photocatalysts

A catalyst is a chemical that changes the rate of a chemical reaction but does not participate in the reaction. Photocatalyst is a general term for chemical substances that can catalyze under the excitation of photons.

There are many materials that can be used as photocatalysts, including titanium oxide (TiO₂), zinc oxide (ZnO), tin oxide (SnO₂), zirconium dioxide (ZrO₂), cadmium sulfide (CdS) and other oxide sulfide semiconductors, among which titanium dioxide is the most popular nano-photocatalyst material in the world due to its strong oxidizing ability, chemical stability and non-toxicity.

Applications:

The discipline of photocatalysis is an emerging field of research. Once breakthroughs are made in the research and application of photocatalysts, the two major survival problems—environment and energy faced by human beings in 21st century will be solved.

• Clean enery: The first application of photocatalysts is solving energy problems by using solar photocatalytic decomposition of water to produce hydrogen. The brief reaction process is as follows:

H₂O → H₂ + 1/2 O₂

• Air and water purification: Ambient photocatalysis can be applied to solve pollution problems. The photocatalytic reaction has a strong photo-oxidation-reduction function, which can oxidize and decompose various organic compounds and some inorganic substances, can destroy bacterial cell membranes and solidify virus proteins, kill bacteria and decompose organic pollutants, and organic pollutants. Decomposed into non-polluting water (H₂O) and carbon dioxide (CO₂), it is competent in sterilization, deodorization, mildew proof, anti-fouling and self-cleaning, and air purification. Common photocatalysts are nanoscale TiO₂. The brief reaction process is as follows:

C₆H₆ + 15/2 O₂ → 6 CO₂ + 3H₂O

• Household applications: Glass, ceramics, etc., which are loaded with photocatalyst TiO₂, are commonly used antibacterial and deodorizing materials in hospitals, hotels and at homes.
• Building materials: A titanium oxide film is coated on the surface of the exterior wall of the building, the guardrail of the highway, the street lamp, etc., and the photocatalyst titanium oxide generates strong oxidizing ability and hydrophilicity under the irradiation of sunlight, and the surface can be self-cleaned.
• Textile industry: The addition of nano-photocatalysts can enhance the durability of the material. In addition, it can enhance the anti-ultraviolet radiation effect and thus have an antioxidant effect.
• Medical hygiene: Photocatalysts can be used for disinfection of domestic water. In addition, photocatalysts can inactivate certain cancer cells by modifying blood porphyrins.The main application areas are as shown in Figure 1 below.

Figure 1. The applications of photocatalysis

Classification:

• Homogeneous photocatalysis: When the reactant and the photocatalyst are in the same phase, the reaction may be referred to as a homogeneous photocatalytic reaction. Currently ozone and light-Fenton systems (Fe + and Fe + / H₂O₂) are the most commonly used homogeneous photocatalysts.
• Heterogeneous photocatalysis: The photocatalyst may be referred to as a heterogeneous photocatalyst when the reactants to be catalyzed are in different phases. Heterogeneous photocatalytic design of many types of reactions, including mild or complete oxidation, dehydrogenation, hydrogen transfer, ¹⁸O₂-¹⁶O₂ and deuterium-alkane isotopic exchange, metal deposition, hydrolysis, gaseous pollutant removal. Transition metal oxides and semiconductors are the most common heterogeneous photocatalysts.

References

1. Wu, CH. (2006). Decolorization of Reactive Red 2 by advanced oxidation processes: Comparative studies of homogeneous and heterogeneous systems. Journal of hazardous materials. 128 (2-3): 265-72.
2. Linsebigler. (1995). Photocatalysis on TiO₂ Surfaces: Principles, Mechanisms, and Selected Results. Chemical Reviews. 95 (3): 735-758.
3. Kudo. (2004). Strategies for the Development of Visible-light-driven Photocatalysts for Water Splitting. Chemistry Letters. 33 (12): 1534-1534.