Benchmarking of photocatalytic coatings performance and their activation towards pollutants degradation

The use of organic-based photocatalytic coatings is a promising technology for improving air quality by eliminating inorganic and organic pollutants and for creating self-cleaning and antibacterial surfaces. The most common photocatalyst is titanium dioxide, in nanoparticulate form, which is embedded in a film-forming agent. In the present work, we investigate the performance of different photocatalytic coatings containing TiO2 nanoparticles (Aeroxide (R) P25 TiO2), towards the photo-degradation of a dye (methylene blue) as model reaction in a lab-scale UV photo-reactor. The TiO2 nanoparticles (similar to 21 nm) are tested as aqueous slurry and further used for preparation of photocatalytic coatings following two approaches: i) dispersion of P25 in a commercial acrylic paint aimed at outdoor use as a proof-of-concept, and ii) full formulation of acrylic-based photocatalytic coatings from raw materials, engineered with different pigmentation degrees defined by the lambda parameter (pigment volume concentration relative to critical pigment volume concentration). The TiO2-acrylic coatings prepared with both methods show activity towards dye degradation in UV-irradiation tests. After the first 50-60 min of UV exposure a significant increase (up to 70-100%) in the apparent rate constant for methylene blue photo -degradation sets in for the photocatalytic coatings suggesting that the coatings are activated in the early stages of UV exposure. Scanning-electron-microscopy analysis confirms that the activation mechanism involves the creation of a porous surface at the outermost layer of the coating, gradually exposing TiO2 photocatalyst, as result of partial binder photo-degradation. Water contact angle (WCA) measurements, performed with a digital microscope, reveal that the kinetic transition is accompanied by a sudden drop of the WCA, indicating an enhanced wettability that is attributable to the hydrophilicity OH-rich TiO2 surface becoming accessible with the increased porosity. Use of a high lambda can induce initial porosity that lowers the relative magnitude of this initial activation.