Title: Smart coatings: Degradation of priority pollutants on TiO2 based photocatalytic materials in indoor and outdoor environments-Principles and mechanisms

Abstract

The heterogeneous photo-catalysis using semiconductors e.g.TiO2 is a promising technology for the degradation of environmental pollutants. Preliminary evidence indicates that materials and paints enriched with TiO2 degrade, upon activation with UV-light, NOx gases and volatile organic compounds (VOCs) e.g. benzene, toluene at concentrations typical for the urban environment. The photocatalytic TiO2-materials and paints developed are primarily for use outdoors, on facades in high traffic roads. Due to its band gap of 3.2 eV, TiO2 is effective only in the UV-region (ca. 5%) of the solar spectrum and with wavelengths <380 nm. Hence, efforts made to increase the area of activity of TiO2 using visible light, which will expand its application to improve the quality of indoor environments. The photocatalytic activity of TiO2 depends on the lifetime of charge carriers - positive holes and electrons - produced on its surface. Recombination of positive holes and electrons occurs in an extremely short time with most charge carriers recombining at the surface of the semiconductor before undergoing redox reactions. In this case no reaction takes place. One way to reduce or inhibit recombination is to blend/dope TiO2 with transition metals, which create traps for electrons and /or positive holes and block the charge carriers by reducing the recombination rate. Doping (change/modification of the crystalline structure of TiO2) causes a bathochromic (red) shift, which results in a reduction in the energy gap leading to increased absorption in the visible light region. Many transition metals such as V, Cr, Fe, Mn, Ni, Co, Cu, Zn have been explored to reduce the energy gap and facilitate the transfer of electrons to the conduction band and thus extend the spectral range of modified TiO2 to the area of visible light. In our experiments, 0.1% (w/w) and 1% (w/w) Mn-TiO2 admixtures were prepared and the ability of the modified photo-catalysts to degrade NO by both solar and indoor illumination was evaluated. Principles and mechanisms of the photocatalytic reaction at the air/catalyst interface and the possible formation of undesired by-products through the photocatalytic reaction of TiO2 with organic paint matrices are discussed.