Photocatalytic coatings promise façades that clean themselves and air that gets cleaner. The chemistry is genuine and well documented. The engineering question is whether the effect persists on a real building, and that is where specifications should focus.

The mechanism

Titanium dioxide in the coating acts as a semiconductor photocatalyst. Under ultraviolet light, electrons are promoted from the valence band to the conduction band, leaving holes behind. Those charge carriers migrate to the surface and drive redox reactions: electrons reduce oxygen to superoxide radicals, holes oxidise surface water and hydroxyl groups to hydroxyl radicals. Those radicals break down organic dirt and convert nitrogen oxides into nitrate that rain washes away. A second effect, photoinduced hydrophilicity, spreads water into a sheet rather than droplets, so rain lifts loosened dirt evenly instead of streaking.

What testing shows

ISO 22197-1 is the reference method for NOx removal performance. Published results for well formulated coatings show meaningful removal efficiency under standard conditions, with laboratory maxima around 77 percent reported for optimised systems, and measurable performance persisting at lower light intensity and lower NO concentration than the standard test uses.

The durability question

Accelerated weathering studies report activity losses within roughly 14 percent after 300 hours, retaining about 80 percent of initial performance, which is encouraging. The dominant real world failure mode is not chemical exhaustion but physical: cracking or detachment of the coating removes the photocatalyst from the surface entirely. Long term value therefore depends on adhesion and film integrity as much as on catalytic activity, which means substrate preparation and correct application matter more than the headline efficiency figure.

Practical specification advice

Require test evidence to ISO 22197-1 with the test conditions stated, not a bare percentage. Ask for weathering data, since initial activity without durability data tells you little. Confirm the coating suits the substrate and its movement, because façade cracking defeats the technology. And set expectations honestly with the client: photocatalytic coatings reduce cleaning frequency and contribute modestly to local air quality; they do not eliminate façade maintenance.

MENA Industrial Supplies supplies photocatalytic coatings with technical documentation for UAE façade and infrastructure projects.