Photocatalysis

A catalyst is a substance that modifies the rate of a chemical reaction without being consumed in the process. Catalysts are commonly found in our daily lives. For example, the enzymes found in our bodies serve as catalysts to accelerate bodily chemical reactions. The catalytic converter in the car exhaust system uses heat-activated platinum-iridium catalyst, which oxidizes carbon monoxide and hydrocarbon pollutants in the exhaust gases, to carbon dioxide and water.

"Photo" is derived from a Greek word which means "light". Thus, the word "photo-catalyst" implies that the catalyst requires radiant energy to modify the rate of a chemical reaction. Titanium dioxide is a commonly used photo-catalyst. Titanium Dioxide is highly stable and non-toxic (and harmless to us). Being non-toxic, TiO2 has been an approved food additive in USA since 1968 and in Japan since 1983. It is widely used around the world in various industrial materials, beauty products, paints, colorants, etc..

1. When the photocatalyst is exposed to radiant energy (UV and/or visible light), electricity is generated. It is now in an excitation state and a hole (h+) is generated while an electron (e-) is intensified.
2. An electron, in the presence of oxygen in the air, is transformed into superoxide anion. The hole, on the other hand, is transformed into hydroxyl radical by moisture in the air. These are known as active oxygen.
3. Organic substances like germs and microbials, have molecule structures that are held together by carbon-oxygen or carbon-hydrogen bonds.
4. When organic substances come in contact with the active oxygen, these bonds will be broken. And the residue is harmless carbon dioxide (CO2) and water vapour (H2O).
5. Active oxygen is ever present on the photocatalyst surface in the presence of light. All organic matters will be oxidized very rapidly, resulting in the formation of CO2 and H2O.
6. As such, the active oxygen will not be freely suspended in the air. Thus humans will not absorb the superoxide anion and hydroxyl radical. Neither will they accumulate in an enclosed area.

Due to the nature of a photocatalyst, there are many areas in which the application of a photocatalyst can be beneficial.

For indoor air quality control, photocatalyst are used to decompose any noxious volatile organic compounds. This is important, especially for some manufacturing processes, which will emit harmful gaseous pollutants into the air. For a similar reason, photocatalyst are used in waste water treatment plants to remove noxious organic substances from the waste water.

Offensive odours like ammonia and hydrogen sulphide can also be easily removed with the presence of a photocatalyst. Since photocatalyst can break down and kill bacteria, it can be used in areas where germs and viruses are plentiful. This will ensure a sterile environment.

To highlight the areas of applications, below are some examples on how a photocatalyst can help improve our environment.

Indoor Air Quality is a major concern for homeowners as well as building owners. Besides having good and well maintained ventilation systems, deploying photocatalyst air cleaners or coating photocatalyst on walls and ceilings will help in improving indoor air quality. Furniture, carpets, adhesives, body care products, etc., will constantly emits volatile organic compounds. These VOCs can be effectively removed by the photocatalysis action.

Waste water contains many organic and inorganic compounds. Using photocatalyst can help remove the organic compounds that are present. Since photocatalyst uses clean light energy, the whole process of water treatment can be more cost effective than before. Hospitals: Hospitals and clinics are constantly filled with germs and viruses. One effective way to reduce these harmful organisms is to keep cleaning and wiping the floors, walls and ceilings with antiseptic cleaning solution. However, the most cost effective method is to coat the interior walls, ceilings and windows with a photocatalyst concoction. This will help to reduce the bacterial count continuously, without having to use any antiseptic solution.

Commercial or residential buildings get dirty very quickly due to air pollution and rain. If the façade of these buildings are coated with a photocatalyst, oil particles and other organic pollutants are quickly disintegrated before they can dirty the building. When the rain comes, the super-hydrophilic effect of the photocatalyst-coated surface will help remove whatever dust particles are left over. This effectively makes it a self-cleaning building.

With the brief discussion above, it is clear that the potential of a photocatalyst is very great. Not only does it use only light energy (sunlight or florescent lights), it has properties that allows for cleaner and healthier air as well as a much cleaner environment. Although the photocatalysis principle has been around since the beginning of time, it is only in the last few decades that mankind started to discover the use of it in our daily lives. But it's still not too late for us to harvest on the gifts of nature to improve our lives with a cleaner, healthier and better environment.