The effects of filter coating approaches on photocatalytic abatement of formaldehyde in indoor environment using a TiO₂-based air purifier system

Titanium dioxide (TiO₂) is the most commonly used catalytic medium in the filter system of commercial photocatalytic air purifier (AP). The AP performance can be affected sensitively by the coating conditions of such medium on the filters and its physicochemical properties (e.g., crystallinity, surface reactivity, morphology, and particle size). In this research, such an intricate relationship is first investigated through a combination of ultrasonic dip-coating of TiO₂ onto 3D honeycomb ceramic (HC) filters and their subsequent calcination under various operational conditions. The photocatalytic oxidation (PCO) performance of the prepared AP is then tested against formaldehyde (FA: at 1 ppm) under ultraviolet LED light irradiation (1 W). Its PCO efficacy is greatly enhanced by the uniform distribution of TiO₂ nanoparticles (relative to the catalyst dose) to enhance light-harvesting and mass-transfer rates. The best-performing HC filter with a uniform distribution (e.g., reduced TiO₂ film clustering) is attained by adjusting the TiO₂ solution concentration (≤3 g/L) and increasing the number of dipping cycles (up to 4) while minimizing the sonication time (<15 min). Post-annealing of TiO₂-coated HC filter at 450 °C for 5 h significantly improves the optoelectronic characteristics by 35.4% (compared with commercial TiO₂) due to surface defects and anatase/rutile phase transition. At these conditions, the AP meets the World Health Organization threshold (i.e., t_0.08 value) for indoor FA after 385 seconds (quantum yield = 3.2E-03 molecules/photon, clean air delivery rate = 35.72 L/min, and kinetic rate = 317.22 μmol/h/g). As such, the PCO efficacy of the AP (TiO₂-HC) filtering system can be improved by tuning the surface reactivity and the photon-harvesting potential through the control on the crystalline characteristics of TiO₂ and its uniform coating on the HC support based on an ultrasonic dip-coating technique.