Photocatalysts are materials that can accelerate chemical reactions when exposed to light, without being consumed or altered in the process. They have the potential to play a significant role in the degradation of air pollutants such as volatile organic compounds VOCs and nitrogen oxides NOx in urban environments. The use of photocatalysts can assist in the degradation of these pollutants through the following mechanisms:1. Generation of reactive oxygen species ROS : When photocatalysts, such as titanium dioxide TiO2 , are exposed to ultraviolet UV light, they generate reactive oxygen species ROS like hydroxyl radicals OH and superoxide anions O2- . These ROS are highly reactive and can oxidize VOCs and NOx, converting them into less harmful compounds such as carbon dioxide CO2 , water H2O , and inorganic nitrates NO3- .2. Photocatalytic oxidation PCO : In the presence of light, photocatalysts can initiate a series of redox reactions that lead to the oxidation of VOCs and NOx. This process, known as photocatalytic oxidation PCO , can effectively break down these pollutants into simpler, non-toxic compounds. For example, formaldehyde a common VOC can be oxidized to formic acid and eventually to CO2 and H2O, while NOx can be converted to nitric acid and then to NO3-.3. Enhanced air purification systems: Photocatalysts can be incorporated into air purification systems, such as filters and air purifiers, to enhance their efficiency in removing VOCs and NOx from indoor and outdoor environments. When combined with UV light sources, these systems can effectively degrade pollutants through PCO, providing cleaner air for urban inhabitants.4. Integration with building materials: Photocatalytic materials can be integrated into building materials, such as concrete, paint, and glass, to create self-cleaning and air-purifying surfaces. These surfaces can actively degrade VOCs and NOx when exposed to sunlight, reducing the concentration of these pollutants in the surrounding air and contributing to a healthier urban environment.5. Synergistic effects with other air-cleaning technologies: Photocatalysts can be combined with other air-cleaning technologies, such as adsorption and biodegradation, to achieve a synergistic effect in pollutant removal. For example, activated carbon can adsorb VOCs, while photocatalysts can degrade the adsorbed pollutants under light exposure, regenerating the adsorbent and enhancing the overall efficiency of the system.In summary, the use of photocatalysts can assist in the degradation of air pollutants like VOCs and NOx in urban environments through the generation of reactive oxygen species, photocatalytic oxidation, enhancement of air purification systems, integration with building materials, and synergistic effects with other air-cleaning technologies. This can lead to improved air quality and a healthier living environment for urban residents.