Page 24 - Multifunctional Photocatalytic Materials for Energy

Metal oxide powder photocatalysts 13

environmental contamination. Organic chemicals are also present as pollutants
in groundwater and surface water, such as wells, ponds, and lakes. To protect the
water resources and achieve quality drinking water, pollutants need to be removed.
Numerous processes, such as biological and chemical oxidation reactions, adsorption
onto supported substrates, ultrasonic irradiation, and electrochemical devices, have
been used widely to destroy or remove these toxins. Among these techniques, the
photocatalytic detoxification of organic pollutants has attracted considerable attention
because of its numerous advantageous features, such as use of very small amounts of
catalysts, regeneration of catalysts, utilization of natural sunlight for environmental
remediation, and clean energy production.

2.5.2 Deodorizing and air purification

During deodorizing processes, hydroxyl radicals accelerate the breakdown of any vol-
atile organic compounds (VOCs) by breaking the molecular bonds. This process helps
combine the organic gases to form a single molecule that is not very harmful to the
environment, plants, and human beings, thus enhancing efficient cleaning of the air.
Some of the examples of odor molecules are tobacco, formaldehyde, nitrogen dioxide,
gasoline, urine, and fecal odors as well as many other hydrocarbon molecules in the
atmosphere [14,17,23].
The air purifier developed with TiO 2 can remove soil and smoke, pollen, bacte-
ria, viruses, and harmful gases, as well as halt the free bacteria in the air by filtering
out ~99.9% of them, with the help of the highly oxidizing effect of photocatalysts,
i.e., metal oxides (TiO 2 , etc.). The photocatalytic activity of metal oxides can be ap-
plied for the elimination or reduction of polluted compounds in air such as cigarette
smoke, automobile smoke, NOx, as well as volatile compounds arising from various
industries and construction sites. Also, high photocatalytic reactivity can be applied
to protect lamp housing and walls in tunneling, as well as to prevent white tents from
becoming sooty and dark. Atmospheric constituents such as greenhouse gases, chlo-
rofluorocarbons (CFCs), substituted CFCs, and nitrogenous and sulfurous compounds
undergo photochemical reactions either indirectly or directly in the presence of sun-
light. In a polluted region, these pollutants can ultimately be efficiently removed using
a suitable metal oxide. Deodorization and air purification may help to keep the air pure
and free from bad odor. These processes can help keep plants and human beings fit
and healthy [14,23].

2.5.3 Self-cleaning, self-sterilizing, and antifogging surfaces
In most industrial countries, many exterior walls of buildings are soiled by automo-
tive exhaust fumes and smoke emitted by industries, which contain oily components.
When the original building materials are coated with a photocatalyst such as TiO 2 , a
protective layer of titanium creates a self-cleaning effect on the building by becoming
antistatic, super-oxidative, and super-hydrophilic. The hydrocarbon from automotive
exhaust is oxidized, and the dirt on the walls washes away with rainfall, keeping the
building exterior clean and shining at all times [14,15,23].