Investigation of nano-catalysts for air pollution control in urban microenvironments : from design to application

Abstract

With the global population increasingly residing in urban environments, the issue of poor air quality has emerged as a critical concern. due to its detrimental impact on public health. This thesis of research aimed to address air pollution challenges in urban microenvironments through a comprehensive investigation from nano-catalyst design to application. Firstly, air quality observations were carried out in a roadside area of Hong Kong including Nitric oxides (NOx), Ozone (O3), carbonyls and Volatile organic compounds (VOCs), aiming to identify the characteristics of air pollutants in urban micro-environments. It was observed that under high NOx conditions at roadside location, formaldehyde (HCHO) plays a crucial role in driving O3 production as well as posing a potential health risk, highlighting the importance of removing the ambient NOx and HCHO. Accordingly, defective titanium dioxide (TiO2–x–OV) was developed as a model catalyst in the investigation of HCHO photocatalytic oxidation pathway in a well-designed continuous-flow reactor, recognizing the important step of O2 activation and the corresponding reactive oxygen species (ROS) generation in the carbon-containing photocatalytic intermediate-conversion process. Further, S-vacancies modulated Sv-ZIS/CN heterojunction was developed for the photocatalytic oxidation of air pollutants. The abundant generation of •OH radicals in the Sv-ZIS/CN sample exhibits excellent performance for the photocatalytic oxidation of both HCHO and NO under visible light. The proposed reaction mechanism suggests that the complete oxidation of HCHO realized via successive C-H bond breakage with sufficient •OH radicals via the route CH2O2 → HCOO- → CO2. In considering the nanomaterials developed in the laboratory are inappropriate for directly use, the final stage of the study focused on exploring real-world applications of nanotechnology by using both passive and active approaches. A hydroperoxide peptized super-hydrophilicity coating surface was developed as the passive way, exhibiting superior self-cleaning properties and a NOx degradation rate of 46.8%. The coating materials shows good adhesion on construction materials and maintained 13.9%–18.5% photocatalytic activity after the 180-day outdoor exposure. On the other hand, a pilot-scale investigation was conducted involving seven units of self-designed active roadside air purifiers in an urban street canyon in Hong Kong. The air cleaning effects were quantified with efficiencies of Nitrogen dioxide (NO2), Fine suspended particulates (PM2.5), Carbon monoxide (CO) and Nitric oxide (NO) to 14.0%–16.9%, 3.5–10.0%, 11.9%–18.7%, and 19.2%–44.9%, respectively. The operation of roadside purifiers also proves the potential of alleviating the O3 formation and health risks at roadside area. The operational RH is ranged from 70–90% lifetime of the air purifiers was estimated to be approximately 130 days, offering crucial information regarding the filter replacement cycle. To sum up, this thesis of study conducted a thorough research of the nanotechnology for air pollution control in urban micro-environments from field observations, catalysts design, mechanism investigation, and practical application. These findings will provide key references for the theoretical understanding, development and implementation, as well as potential environmental benefits of employing nanotechnology for air pollution control in urban areas.