Development and Application of Nanomaterials-based Photocatalytic Technology for Improvement of Urban Air Quality

Abstract

Air pollution is recognized as the biggest environmental health risk in urban cities, as air pollution is pervasive and hard to escape. As one of the notorious atmospheric pollutants, nitrogen oxides (NOx, NO + NO2) not only promotes the formation of ozone and secondary aerosols but also have direct adverse health effects on human beings. As a typical densely populated modern metropolis, Hong Kong can serve as a reference for world cities in terms of air pollution control. Even though air quality in Hong Kong Environment Protection Department (HKEPD) has improved over recent decades, the roadside NO2 level in Hong Kong still exceeds the Hong Kong Air Quality Objectives (HKAQO), European Union Air Quality Directives (EUEQD) and the most stringent World Health Organization Air Quality Guidelines (WHOAQG). Nanomaterial-based photocatalysis that only relies upon solar energy excitation provides a sustainable solution for air pollution redemption. Generally, photocatalysts developed in the laboratory are in powder form which is not appropriate for real-world applications. However, these limitations can be overcome by coating photocatalysts on the surfaces of various substrates to immobilize those powders as films. More importantly, photocatalytic coatings are available to be supported on different substrates without changing or affecting existing settings. In this study, enhancement of NOx photocatalytic degradation ability and solar light utilization were implemented in a reformative Titanium Dioxide (TiO2) film. Hydrogen peroxide solution was utilized to peptize the crystallized nanoparticles around 5-6 nm at room temperature instead of the traditional calcination process at high temperatures, which limited the commercialization due to the expensiveness of heating. Moreover, the nanosized TiO2 film was expected to provide more active sites for reactions, which contributes to a promising photocatalytic degradation ability. Based on ISO 22197-1 evaluation standards, the as-developed photocatalytic coating possesses a NOx degradation rate of 4.402 mg*m-2h-1 when applied on the concrete surface, which was higher than Degussa (Evonik) P25 and other commercial coating products at the same conditions. An artificial weather resistance test investigation implies the photocatalytic coating will provide a strong bonding interaction with substrate materials which is beneficial to the lifetime of the coating. Further investigating from a 180-day field trial in a roadside environment in Hong Kong, the as-developed coating concrete specimen presented about 5% of attenuation in the first 30 days and sustained 13.9%-18.5% photocatalytic activity after the entire 180-day outdoor exposure. The application of photocatalytic coatings is supposed to convert the roadside NOx compounds to NO2- and NO3- which are harmless in small quantities and would be washed away by water droplets. In response to practical demands, functional nanomaterials-based photocatalytic technology has been gradually promoted as a green strategy for improvements in the air quality of megacities all over the world.