Self-cleaning Concrete: Action

The self-cleaning action of concrete is due to photocatalytic reaction that occurs on the concrete surface. This promising technique utilizes nano-sized particles of semiconductor materials like TiO2, ZnO, WO3, FeTiO3 and SrTiO3 to carry out photo-induced oxidation process to disassemble volatile organic compounds (VOC) into CO2 and H2O and completely mineralize NOx and SOx. Due to its high efficiency and the generation of harmless byproducts, the concept of the photocatalytic process has therefore caught the attention of many researchers and environmentalists all over the world. When the semiconductor is exposed to UV light or even sunlight, the REDOX reaction occurs at the surface of the catalyst particle generating ●OH or hydroxyl radicals and O-2 or oxygen ions. These two powerful oxidizing agents will then disintegrate and rearrange the structure of the VOC and convert them into CO2 and H2O. As sunlight hits the surface, most organic and some inorganic pollutants are neutralized. They would otherwise lead to discolored concrete surfaces.

The primary ingredient of smog, ozone, is an extremely reactive gas molecule that reacts chemically with lung tissues, causing decreased lung function, respiratory infection, lung inflammation and aggravation of respiratory illnesses. The raw ingredients for ozone are nitrogen oxides (NOX)—produced primarily by internal combustion engines—and volatile organic compounds (VOCs)—
hydrocarbons that have evaporated from chemical plants, refineries, factories, gas stations and paints. Ozone is created when the two raw ingredients are
combined in the presence of heat and sunlight. Italcementi tests have demonstrated that a road paved with concrete made with the photocatalytic cement can reduce NOX levels by 20 to 80%, depending on atmospheric conditions. A building with photocatalytic precast concrete cladding can do the same. Because the proprietary compound oxidizes both NOX and VOCs, it combats ozone at the source. The other chemicals known to be oxidized using photocatalytic cement include:
* Inorganic compounds such as SOX, CO, NH3 and H2S
* Organic compounds such as alcohol, acids and aromatics
* Chlorinated organic compounds such as dioxins and chlorobenzene
* Pesticides such as diazinon and atrazine

The titanium-based catalyst is not spent as it breaks down pollution, but continues to work. Typical products are oxygen, water, carbon dioxide, nitrate and sulfate.

Question:

Mention three applications of semiconductor photocatalysis