Titanium dioxide (TiO2) has emerged as a highly effective photocatalyst for air purification, particularly in the degradation of volatile organic compounds (VOCs) and nitrogen oxides (NOx). Its ability to harness ultraviolet (UV) light to drive redox reactions makes it a promising solution for mitigating air pollutants in both outdoor and controlled environments. The photocatalytic process relies on the generation of reactive oxygen species (ROS), which oxidize harmful gases into less toxic byproducts such as CO2, H2O, and nitrate species. Understanding the underlying mechanisms, mass transfer dynamics, and practical implementation of immobilized TiO2 systems is critical for optimizing its performance in air purification applications.
The photocatalytic activity of TiO2 is initiated when photons with energy equal to or greater than its bandgap (approximately 3.2 eV for anatase) excite electrons from the valence band to the conduction band, creating electron-hole pairs. These charge carriers migrate to the catalyst surface, where they participate in redox reactions with adsorbed molecules. In the gas phase, oxygen molecules act as electron acceptors, forming superoxide radicals (•O2−), while water vapor or surface hydroxyl groups provide protons for the formation of hydroxyl radicals (•OH). These ROS are highly reactive and drive the oxidation of VOCs and NOx through a series of intermediate steps.
For VOCs such as formaldehyde, benzene, and toluene, the degradation pathway typically involves initial hydroxyl radical attack, leading to the formation of organic radicals. These intermediates undergo further oxidation, resulting in the cleavage of carbon-carbon bonds and eventual mineralization to CO2 and H2O. In the case of NOx, photocatalytic oxidation converts NO to NO2, which subsequently reacts with hydroxyl radicals or surface-adsorbed water to form nitric acid (HNO3) or nitrate species. The efficiency of these reactions depends on the availability of active sites, the concentration of reactants, and the residence time of pollutants on the catalyst surface.
Mass transfer limitations play a significant role in determining the overall efficiency of TiO2-based air purification systems. In gas-phase reactions, the diffusion of pollutants to the catalyst surface is often the rate-limiting step, particularly at low concentrations. The design of photocatalytic reactors must account for factors such as airflow velocity, contact time, and surface area to maximize pollutant-catalyst interactions. Porous substrates with high surface area, such as fibrous filters or monolithic structures, are commonly used to enhance mass transfer while providing sufficient UV exposure.
Immobilized TiO2 systems, including coatings and filters, offer practical advantages over powdered catalysts by eliminating the need for post-treatment separation. Thin-film coatings of TiO2 can be deposited on glass, metal, or ceramic substrates using techniques such as sol-gel dip-coating, spray pyrolysis, or magnetron sputtering. The durability of these coatings is a critical consideration, as mechanical wear, chemical poisoning, and UV-induced aging can degrade photocatalytic activity over time. Strategies to enhance stability include doping with metals or non-metals, incorporating protective layers, and optimizing the crystallinity of TiO2 to reduce charge recombination.
Filters functionalized with TiO2 nanoparticles provide another approach for air purification, particularly in HVAC systems or industrial exhaust streams. These filters combine physical adsorption with photocatalytic oxidation, enabling continuous degradation of trapped pollutants. However, the accumulation of reaction byproducts, such as nitrate deposits, can block active sites and reduce efficiency. Periodic regeneration through UV irradiation or thermal treatment may be necessary to maintain performance.
The effectiveness of TiO2 in degrading VOCs and NOx has been demonstrated in numerous studies. For example, under optimized conditions, TiO2 photocatalysis has achieved degradation efficiencies exceeding 90% for formaldehyde and benzene at concentrations relevant to indoor and urban environments. NOx removal rates are highly dependent on humidity levels, with optimal performance observed at moderate relative humidity (30–70%), where sufficient water vapor is available for hydroxyl radical formation without blocking active sites.
Long-term performance of TiO2-based systems is influenced by environmental factors such as temperature, humidity, and the presence of competing species. High temperatures can enhance reaction kinetics but may also accelerate catalyst deactivation. Similarly, inorganic gases like SO2 can compete for active sites and form sulfate deposits, reducing photocatalytic activity. Advances in material engineering, such as the development of mesoporous TiO2 or composite structures with enhanced adsorption capacity, aim to address these challenges.
In summary, TiO2 photocatalysis offers a viable solution for air purification through the degradation of VOCs and NOx. The process relies on the generation of ROS under UV illumination, with reaction efficiency governed by mass transfer dynamics and catalyst design. Immobilized systems, including coatings and filters, provide scalable and durable solutions, though long-term stability remains a key consideration. Ongoing research focuses on optimizing TiO2 formulations and reactor designs to enhance performance under real-world conditions, paving the way for broader adoption in environmental remediation applications.