Photo-catalytic concrete is a formulation of concrete used as
pavers and other structural concrete that includes
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolub ...
(TiO
2) as an admixture or superficial layer. Titanium dioxide is a
heterogeneous photocatalyst
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the abil ...
that uses sunlight and moisture to absorb and render
oxides of
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
(
NO and
NO2) into
nitrate ions (NO
3−), which are then either washed away by rain or soaked into the concrete to form stable compounds.
History
The technique of photocatalytic concrete was firstly used in architecture during construction of the Italian Church of Dio Padre Misericordioso. Richard Meier construction also known as «Jubilee Church» is confined to 2000 anniversary of the Christianity that was celebrated in 2000. In order to avoid frequent cleanings of new church's concrete «sails», new development was used - white self-cleaning coating of the walls. However, back then it wasn't known that this colouring plaster containing titanium dioxide and white pigment absorbs exhaust gases and other elements of city fog.
This discovery raised question of wide application of similar materials in urban construction. According to studies, the air at a distance of 2.5 m from a facade coated with titanium dioxide contains 70% less various combustion products than other city buildings. So the people inhale less harmful substances while passing by the buildings treated this way.
Options for using photocatalytic cement to cover asphaltic roads are also being considered. As an experiment it was used at 230-meter stretch of highway near Milan. Measurements allowed to see that at this road average traffic load of 1000 vehicles per hour, the reduction of nitrogen oxides in the air at ground level was 60%.
Some were skeptical about this discovery: in their opinion, it is necessary to reduce the level of harmful substances emissions, and not to eliminate their consequence - smog. Moreover effectiveness of practically all catalysts weakens as time passes.
Mechanism
When
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolub ...
is exposed to
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
radiation from
sunlight, it absorbs the radiation and
electron excitation
Electron excitation is the transfer of a bound electron to a more energetic, but still bound state. This can be done by photoexcitation (PE), where the electron absorbs a photon and gains all its energy or by collisional excitation (CE), where ...
occurs. The following reactions then occur on the surface of the titanium dioxide crystals:
Photolysis of
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
:
: H
2O → H
+ + OH (
hydroxyl radical
The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...
) + e
−
: O
2 + e
− → O
2− (a
superoxide
In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...
ion)
The overall reaction is therefore:
: H
2O + O
2 → H
+ + O
2− + OH
The hydroxyl radical is a powerful
oxidizing agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
and can oxidize
nitrogen dioxide
Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the productio ...
to
nitrate ions:
: NO
2 + OH → H
+ + NO
3−
The
superoxide
In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...
ion is also able to form nitrate ions from
nitrogen monoxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its c ...
:
: NO + O
2− → NO
3−
The oxidation of NO
x to nitrate ions occurs very slowly under normal atmospheric conditions because of the low concentrations of the reactions. The photochemical oxidation with the aid of titanium dioxide is much faster because of the energy absorbed by the coating on the block and also because the reactants are held together on the surface of the block. The reaction using titanium dioxide shows a greater oxidizing power than most other metal-based catalysts.
Photo-catalytic blocks have replaced ordinary paving in around 30 towns in
Japan, originally having been tested in
Osaka
is a designated city in the Kansai region of Honshu in Japan. It is the capital of and most populous city in Osaka Prefecture, and the third most populous city in Japan, following Special wards of Tokyo and Yokohama. With a population of ...
in 1997 and have been used in the
City of Westminster (London).
The aim of these blocks is to reduce atmospheric
pollution
Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...
levels and therefore lower the amount of photochemical
smog.
References
{{reflist
Catalysts
NOx control
Concrete