Photocatalysts in construction

As well as removing hazardous organic compounds, NOx emissions and air pollution, photocatalysts are very effective at killing a variety of bacteria and some viruses. NBS Information Specialist Michael Smith looks at how they work.

Titanium dioxide (TiO₂) has historically been widely used as a white pigment in paints, cosmetics and foodstuffs. It exists in three crystalline forms: rutile, anatase, and brookite. It is also a photocatalyst; a semiconducting material which can be chemically activated by light. Of the three crystalline forms, anatase shows the highest photo-activity.

For a long time titanium dioxide was a considerable problem, especially in its application as a pigment, often resulting in the phenomenon of 'paint chalking', where the organic components of the paint are decomposed as result of the photocatalytic processes. In modern industrial products TiO₂ is almost exclusively used in the form of rutile crystals, however, and where it is used as a photocatalyst, it is always in the anatase form.

How does photocatalytic titanium dioxide work?

Technical version
The principle of photocatalysis by titanium dioxide is described as follows: TiO₂ is a semiconductor which when it is irradiated by photon energy (h=3.2 eV or λ=385 nm), redox reactions start. The band gap energy is excited and an electron is promoted from the valence band to the conduction band. This generates an electron-hole pair (electron e- and hole h+), such that:

TiO₂  TiO₂ (e- ; h+)

For most materials that are electrically conductive (metals), these two immediately recombine. However, for semiconductors, they survive for longer periods.

The hole and the electron created can migrate and initiate redox reactions with water and oxygen, and then degrade mineral or organic molecules adsorbed on the surface of the photocatalyst.

Non-technical version
If titanium dioxide of the anatase type is exposed to UV light at very low contact angles it gains the unique property of 'attracting' rather than repelling water (super- hydrophilicity). The water lies flat on the surface in sheets instead of forming droplets.

Furthermore, UV illumination leads to the formation of powerful agents with the ability to oxidize and decompose many types of bacteria, organic and inorganic materials to their constituent parts (H₂O and CO₂).

Destruction of pollutant molecules by TiO₂ catalyst action

In the last 15 years photocatalysis has become more attractive to industry in the development of technologies for purification of water and air. Compared with traditional advanced oxidation processes the technology of photocatalysis is known to have some advantages:

• Low-cost materials
• Ease of setup and operation at ambient temperatures
• Low consumption of energy and consequently low costs
• A wide spectrum of organic contaminants and NOx emissions can be treated
• No chemical reactants used and no side reactions produced
• Very low maintenance.

Construction uses of photocatalysts

Coatings and claddings
Through photocatalysis, cement or concrete structures can destroy most organic and inorganic pollutants that come into contact with their surfaces, minimizing discolouration.

The cement itself is often not photocatalytic. In fact it is usually only the final application of cement or a paint coat. This effectively makes the finish self-cleaning and helps in fighting air pollution and harmful emissions.

Treated cement effectively destroys airborne pollutants, cutting down on urban organic pollution.

Researchers have calculated that, in a large city, covering 15% of visible urban surfaces with photocatalytic products would reduce air pollution by approximately 50% – Italcementigroup press release.

Glazing
Self-cleaning glass uses thin film titanium dioxide coating. The film can be applied by spin coating of organo-titanate chelated precursor (for example, titanium iso-tetrapropoxide chelated by acetyl-acetone), followed by heat treatment to burn the off organic residues and to form the anatase phase.

The first stage of the self cleaning process utilises the photocatalytic property, reacting with daylight to break down organic dirt.

The second stage utilises the hydrophilic property. Here, instead of forming droplets, rainwater hits the glass and spreads evenly, running off in a 'sheet' and taking the loosened dirt with it, also drying quickly without leaving streaks.

The TC24 'Coatings on Glass' committee of the International Commission on Glass, works to develop test methods to evaluate photocatalytic self-cleaning coatings for glass.

Wastewater treatment
Pilot projects have demonstrated that photocatalytic detoxification systems can effectively kill faecal coli-form bacteria in secondary wastewater treatment.

Reactors, where the titanium dioxide is fixed on a glass, ceramics or metal surface, are the main component of thin-film fixed-bed reactors, shown below.

Thin-film fixed-bed reactor

In this reactor type industrial waste water is passed over TiO₂ coated material (glass, polystyrene, methacrylate). The reactors trap and chemically oxidize organic compounds, converting them primarily to CO₂ and water. These reactors operate at room temperature and under negligible pressure. Thus, they can be readily integrated into new and existing heating, ventilation, and air conditioning systems.

Tiles
TiO₂ coated ceramic tiles are considered to be very effective against organic and inorganic materials, as well as against bacteria. These tiles are used in hospitals and care facilities to reduce the spread of infections, and in public and commercial facilities and schools to improve hygiene conditions. Furthermore, because these tiles show hydrophilic behaviour, water forms a uniform sheet over the surface. Grease, dirt and other staining materials can easily be swept away with a stream of water. In exterior applications this characteristic makes these tiles self-cleaning.

How much does it cost to use photocatalysts?

Given that the area which interacts with the atmosphere is only the surface, the photocatalytic principle is not used in structural applications, but only where it is possible to maintain limited thicknesses, from centimetres to a few millimetres; for instance, paint, finish plaster, or as the surface of a manufactured block.

The most significant figure is the cost per square metre of the photocatalytic surface, making the cost remarkably low. To transform the façade of a 5-storey building into a photocatalytic surface may only add an extra £100-200 to the cost of a traditional paint or plaster. Industry sources estimate that paving in photocatalytic blocks only costs between 10 - 20% more than traditional paving.

Colours other than white

Products made of photocatalytic concrete are mainly achieved by cements that are doped with photocatalytic titanium dioxide, therefore only white and grey products are commonly available.

Until now, the technology for coloured photocatalytic concrete did not exist. However, the supposition that adding standard inorganic colour pigments to photocatalytic concrete may reduce or eliminate the photocatalytic activity has now been disproved.

Recently, a method was developed to create a stable iron oxide/TiO₂ compound, in cooperation with the University of Turin. Using this method a colour range of yellow, red and black photocatalytic iron oxides has been created.

Problems with photocatalytic coatings

Titanium dioxide coatings usually cannot decompose thick non-transparent deposits, which do not allow light to pass through, such as paint, silicone, construction waterstop materials or render and mineral dust.

Michael Smith is a member of the Construction Information Service editorial team. He is a mechanical engineering and building services specialist, chartered information specialist (MCLIP) and chartered environmentalist (CEnv).

Related NBS information:

Articles:

• Cements – standards, third party certification, and chromium (VI)
• Organically coated steel sheeting
• Self-healing building products

Selected links:

• Concrete, cement, binders
• Coatings, paints, varnishes

March 2011

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