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 insolubl ...
(TiO
2) as an admixture or superficial layer. Titanium dioxide is a
heterogeneous
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
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 abili ...
that uses sunlight and moisture to absorb and render
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s of
nitrogen (
NO and
NO2) into
nitrate
Nitrate is a polyatomic ion
A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
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 is exposed to
ultraviolet radiation from
sunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
, it absorbs the radiation and
electron excitation occurs. The following reactions then occur on the surface of the titanium dioxide crystals:
Photolysis
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
of
water:
: H
2O → H
+ + OH (
hydroxyl radical) + e
−
: O
2 + e
− → O
2− (a
superoxide ion)
The overall reaction is therefore:
: H
2O + O
2 → H
+ + O
2− + OH
The hydroxyl radical is a powerful
oxidizing agent and can oxidize
nitrogen dioxide to
nitrate
Nitrate is a polyatomic ion
A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
ions:
: NO
2 + OH → H
+ + NO
3−
The
superoxide ion is also able to form nitrate ions from
nitrogen monoxide:
: 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
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, originally having been tested in
Osaka in 1997 and have been used in the
City of Westminster
The City of Westminster is a City status in the United Kingdom, city and London boroughs, borough in Inner London. It is the site of the United Kingdom's Houses of Parliament and much of the British government. It occupies a large area of cent ...
(London).
The aim of these blocks is to reduce atmospheric
pollution levels and therefore lower the amount of photochemical
smog
Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words ''smoke'' and '' fog'' to refer to smoky fog due to its opacity, and odor. The word was then inte ...
.
References
{{reflist
Catalysts
NOx control
Concrete