Understanding and improving indoor air quality via doped titanium oxide photocatalysts for photocatalytic oxidation (PCO) air purifiers

Poor indoor air quality in public buildings remains a critical environmental and public health concern. Photocatalytic oxidation (PCO) air purifiers are a popular solution to this problem and were widely installed in public buildings during the COVID-19 pandemic to remove viral particles from the air. PCO air purifiers use UV-light and titanium dioxide catalysts to oxidize volatile organic compounds (VOCs), harmful pathogens, and other pollutants to carbon dioxide and water. However, one negative consequence of PCO is that the process can result in the production of partially oxidized intermediates, such as formaldehyde. These intermediates are toxic, reduce indoor air quality, and are harmful to vulnerable populations such as children and the elderly. In light of this challenge, we are developing titanium dioxide photocatalysts that are doped with transition metals to increase photocatalytic performance through band-gap engineering. The photocatalytic activity of the doped titanium dioxide catalysts was examined in the aqueous phase via dye degradation experiments under UV light illumination. The degradation kinetics of both methyl orange and bromothymol blue reveal a volcano-type dependence in performance with an optimum dopant concentration of less than 10%. We tested the photocatalytic oxidation performance of these catalysts in the gas phase utilizing a commercially available PCO air purification system and a duct loop designed to simulate a HVAC system. PCO air purifier filters were prepared by impregnating commercially available fiberglass air filter media with the doped titanium dioxide nanoparticles. The photocatalytic performance of the doped catalysts was studied in relation to pure titanium dioxide catalysts by monitoring the indoor air quality after VOCs were introduced into the duct loop and oxidized by the PCO filter. This included monitoring the concentrations of VOCs, CO, CO₂, and formaldehyde.