Indoor air quality is a growing concern due to the prevalence of volatile organic compounds (VOCs) such as formaldehyde, benzene, and toluene, which are emitted from building materials, furniture, and household products. Among various mitigation strategies, titanium dioxide (TiO2)-based photocatalytic oxidation has emerged as an effective method for VOC degradation. This approach leverages the semiconductor properties of TiO2 to break down harmful pollutants under light irradiation, converting them into less harmful byproducts like CO2 and H2O. The efficiency of this process depends on multiple factors, including reaction kinetics, environmental conditions like humidity, and the light source used for activation. Recent advancements have also explored the integration of TiO2 systems into heating, ventilation, and air conditioning (HVAC) systems for scalable indoor air purification.

Photocatalytic oxidation on TiO2 surfaces proceeds through a series of redox reactions initiated by photoexcitation. When TiO2 is exposed to light with energy equal to or greater than its bandgap (approximately 3.2 eV for anatase), electrons are promoted from the valence band to the conduction band, generating electron-hole pairs. These charge carriers migrate to the catalyst surface, where they react with adsorbed oxygen and water molecules to produce reactive oxygen species (ROS) such as hydroxyl radicals (•OH) and superoxide anions (•O2−). These ROS are highly oxidative and capable of mineralizing VOCs. Formaldehyde, for instance, undergoes stepwise oxidation via formic acid and carbon monoxide before complete mineralization to CO2. The reaction kinetics follow a Langmuir-Hinshelwood model, where the degradation rate depends on VOC adsorption and surface reaction rates. Studies indicate that formaldehyde degradation rates on TiO2 under UV irradiation can reach 70-90% within a few hours, depending on initial concentration and catalyst loading.

Humidity plays a dual role in photocatalytic VOC removal. On one hand, water vapor is essential for generating hydroxyl radicals, which are primary oxidants in the process. Optimal humidity levels typically range between 40-60% relative humidity (RH), as excessive moisture can compete with VOC molecules for adsorption sites on the TiO2 surface, reducing degradation efficiency. For example, at RH levels above 70%, formaldehyde removal rates may decrease by 20-30% due to competitive adsorption. Conversely, very low humidity (below 20% RH) limits •OH formation, slowing down the oxidation process. Additionally, humidity influences the formation of intermediate byproducts; high moisture levels can promote complete mineralization, whereas low humidity may lead to partial oxidation and accumulation of harmful intermediates like formic acid.

The shift toward energy-efficient light sources has driven research into LED-driven TiO2 activation. Traditional UV lamps (e.g., mercury vapor) are being replaced by UV-LEDs due to their lower power consumption, longer lifespan, and narrower emission spectra. UV-A LEDs (365 nm) are particularly suitable for TiO2 activation, as they align well with its bandgap energy. Studies demonstrate that LED-driven TiO2 systems can achieve formaldehyde degradation efficiencies comparable to conventional UV sources, with the added benefit of reduced energy consumption. For instance, a TiO2-coated reactor illuminated by a 365 nm LED array at 10 mW/cm2 achieved 85% formaldehyde removal over 6 hours. Further optimization involves doping TiO2 with metals (e.g., Ag, Pt) or non-metals (e.g., N, C) to extend its light absorption into the visible spectrum, enabling activation under indoor lighting conditions.

Integration of TiO2 photocatalysis into HVAC systems presents a practical solution for whole-building air purification. Two primary configurations are employed: in-duct systems and photocatalytic air cleaners. In-duct systems incorporate TiO2-coated filters or panels within HVAC ducts, where air passes over the catalyst under UV-LED illumination. This setup ensures continuous treatment of recirculated air, with studies reporting 50-70% reductions in indoor VOC concentrations over extended periods. Photocatalytic air cleaners, on the other hand, are standalone units equipped with TiO2-coated honeycomb structures or fibrous matrices, often combined with particulate filters. These devices are particularly effective in localized settings, such as offices or bedrooms. Key design considerations include airflow rate, contact time with the catalyst, and uniform light distribution to maximize VOC degradation. For example, an optimized HVAC-integrated system with a 2 m/s airflow rate and 5-second residence time achieved 60% formaldehyde removal per pass.

Despite its promise, several challenges remain in TiO2-based indoor air purification. Catalyst deactivation due to surface fouling by intermediate byproducts or dust accumulation can reduce long-term efficiency. Strategies to mitigate this include periodic thermal or photocatalytic self-cleaning cycles. Additionally, the potential release of nanoparticles from TiO2 coatings necessitates careful engineering to ensure safe operation. Advances in immobilization techniques, such as embedding TiO2 in silica or polymer matrices, have shown promise in enhancing durability and minimizing particle release.

In summary, TiO2 photocatalysis offers a viable method for indoor VOC removal, with performance governed by reaction kinetics, humidity, and light source characteristics. The integration of LED-driven TiO2 systems into HVAC infrastructure represents a scalable approach to improving indoor air quality, though further research is needed to optimize long-term stability and energy efficiency. By addressing these challenges, TiO2-based systems can become a mainstream solution for healthier indoor environments.