Photocatalysis and water purification and electricity generation
Only forty-six percent of people in Africa have safe drinking water - water most often comes from open holes dug in the sand of dry riverbeds, and it is invariably contaminated.
Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions. Its practical application was made possible by the discovery of water electrolysis by means of titanium dioxide. The commercially used process is called the advanced oxidation process (AOP). There are several ways the AOP can be carried out; these may (but do not necessarily) involve TiO2 or even the use of UV light. Generally the defining factor is the production and use of the hydroxyl radical.
Solar water disinfection is a type of portable water purification that uses solar energy to make biologically-contaminated (e.g. bacteria, viruses, protozoa and worms) water safe to drink. Water contaminated with non-biological agents such as toxic chemicals or heavy metals require additional steps to make the water safe to drink.
There are three primary subsets of solar water disinfection:
1.Electric. Solar disinfection using the effects of electricity generated by photovoltaic panels (solar PV).
2.Heat. Solar thermal water disinfection.
3.UV. Solar ultraviolet water disinfection.
While Solar irradiation, UV-A in particular, can directly inactivate water-borne micro-organisms, photocatalysis based approaches have gained increased traction in recent years. Titanium dioxide, in its anatase and rutile phases is notably the most studied material for such applications. In photocatalyzed water decontamination the anti-microbial activity of solar irradiation is boosted by the presence of a stable semiconductor oxide (TiO2 or otherwise) in which electron-hole pairs (excitons) are photogenerated by irradiation exceeding the material's optical band-gap. While exciton recombination is predominant, this process has been shown to facilitate the photo-oxidation of pollutants at the photocatalyst surface.
Photocatalytic water splitting is an artificial photosynthesis process with photocatalysis in a photoelectrochemical cell used for the dissociation of water into its constituent parts, hydrogen (H2) and oxygen (O2), using either artificial or natural light. Theoretically, only solar energy (photons), water, and a catalyst are needed.
Hydrogen fuel production has gained increased attention as oil and other nonrenewable fuels become increasingly depleted and expensive. Methods such as photocatalytic water splitting are being investigated to produce hydrogen fuel, which burns cleanly and can be used in a hydrogen fuel cell. Water splitting holds particular interest since it utilizes water, an inexpensive renewable resource. Photocatalytic water splitting has the simplicity of using a powder in solution and sunlight to produce H2 and O2 from water and can provide a clean, renewable energy, without producing greenhouse gases or having many adverse effects on the atmosphere.
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