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.
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:
- H2O → H+ + OH (hydroxyl radical) + e−
- O2 + e− → O2− (a superoxide ion)
The overall reaction is therefore:
- H2O + O2 → H+ + O2− + OH
- NO2 + OH → H+ + NO3−
- NO + O2− → NO3−
The oxidation of NOx 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.