Noxer block

Noxer blocks are concrete pavers that feature a 5-7 mm thick coating of titanium dioxide (TiO₂) atop a cement mortar layer. Titanium dioxide is a heterogeneous photocatalyst that uses sunlight to absorb and render oxides of nitrogen (NO and NO₂) harmless by converting them to nitrate ions (NO₃^-), which are then either washed away by rain or soaked into the concrete to form stable compounds.^[1][2]

Mechanism

When titanium dioxide is exposed to ultraviolet radiation from sunlight, it absorbs the radiation and electron excitation occurs. The following reactions then occur on the surface of the titanium dioxide crystals:

Photolysis of water: H₂O → H⁺ + OH (hydroxyl radical) + e^-

O₂ + e^- → O₂^- (a superoxide ion)

The overall reaction is therefore:

H₂O + O₂ → H⁺ + O₂^- + OH

The hydroxyl radical is a powerful oxidizing agent and can oxidize nitrogen dioxide to nitrate ions:

NO₂ + OH → H⁺ + NO₃^-

The superoxide ion is also able to form nitrate ions from nitrogen monoxide:

NO + O₂^- → NO₃^-

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.

Noxer blocks have replaced ordinary paving in around 30 towns in Japan, originally having been tested in Osaka in 1997 and can also be found underfoot in the City of Westminster (London).

The noxer blocks aim to reduce these pollution levels and therefore lower the amount of photochemical smog.

1. Ultraviolet radiation is absorbed by the titanium dioxide, which causes the photolysis of water into superoxide ions and hydroxyl radicals.
2. Nitrogen oxides react with the superoxide ions and the hydroxyl radicals to form nitrate ions.
3. The nitrate ions are absorbed into the block and form stable compounds.

References

1. ^ Makins, Stephanie (November 2001). "Paved with titanium". Chemistry Review 11 (2): 22–23. 
2. ^ Frazer, Lance (April 2001). "Titanium Dioxide: Environmental White Knight?". Environmental Health Perspectives 109 (4): A 174–177. http://www.nanoes.com/Articles/Environmental/innovations.pdf.