Smart Surfaces: Photocatalytic Degradation of Priority Pollutants on TiO(2)-Based Coatings in Indoor and Outdoor Environments—Principles and Mechanisms

Heterogeneous photocatalysis using semiconductor oxides such as TiO(2), provides an up-and-coming solution for the degradation of environmental pollutants compared with other technologies. TiO(2)-containing construction materials and paints activated by UV/solar light destroy the ozone precursors NO and NO(2) up to 80% and 30%, respectively. The majority of TiO(2) materials developed so far are primarily for outdoor use. In recent years, substantial efforts have been made to investigate further the photocatalytic activity of materials containing TiO(2) toward priority air pollutants such as NO, NO(2), and volatile organic compounds (VOCs) frequently accumulated at high concentration levels, particularly in indoor spaces. The intention of the investigations was to modify the titanium dioxide (TiO(2)), so that it may be activated by visible light and subsequently used as additive in building envelop materials and indoor paints. This has been achieved, to a high extent, through doping of TiO(2) with transition metals such as V, Cr, Fe, Mn, Ni, Co, Cu, and Zn, which reduce the energy gap of TiO(2), facilitating the generation of free electrons and holes, thus, extending the absorption spectral range of modified TiO(2) to the area of visible light (bathochromic shift-redshift). A substantial problem using TiO(2)-containing paints and other building materials in indoor environments is the formation of byproducts, e.g., formaldehyde, through the heterogeneous photocatalytic reaction of TiO(2) with organic matrices. This affects the air quality in confined spaces and, thus, becomes a possible risk for human health and wellbeing. This work describes the principles and mechanisms of the photocatalytic reactions at the air/catalyst interface of priority pollutants such as NO, benzene, and toluene as individual compounds or mixtures. Emphasis is placed on the reaction and recombination processes of the charge carriers, valence band positive holes (h(+)) and free electrons (e(−)), on the surface of TiO(2), and on key factors affecting the photocatalytic processes, such as humidity. A hypothesis on the role of aromatic compounds in suppressing the recombination process (h(+) and e(−)) is formulated and discussed. Furthermore, the results of the photocatalytic degradation of NO under visible light conditions using different admixtures of TiO(2) and manganese doped (Mn–TiO(2)) are presented and discussed.