Photocatalytic efficacy of air purifiers equipped with self-cleaning titanium dioxide xerogel coatings against gaseous formaldehyde: A study using DRIFTS and DFT analysis

In this research, the practical efficacy of a portable air purifier (AP) loaded with a self-cleaning titanium dioxide xerogel (TX) has been investigated for the photocatalytic degradation of gaseous formaldehyde (FA). The photocatalytic performance of the AP system is optimized through control on the synthesis variables (e.g., annealing temperatures (150–750 °C), photocatalysts sol volumes (1–10 mL), process variables (e.g., high, medium, and low air-circulation speeds), type of light sources (UV-A vs. UV-C), and FA concentrations (0.5–5 ppm)). The self-cleaning potential of the AP-TX system is validated with FA removal efficiencies of ≥96 % across diverse settings and over 10 repeated cycles. The optimum kinetic rate constant for FA (e.g., 0.0195 s−1) is attained by a TX-coated honeycomb filter annealed at 450 °C based on a Langmuir–Hinshelwood first-order kinetic model. The high photocatalytic efficacy of the AP-TX system is supported further in terms of a clean air–delivery rate (CADR) of 19.57 L/min, a quantum yield (QY) 1.93×10−3 molecules/photon, and a space–time yield (STY) of 2.57×10−2 molecules/photon/g. The formation of dioxymethylene and formate as intermediates of FA degradation is confirmed by diffuse reflectance infrared Fourier transform spectroscopy analysis. DFT simulations also suggest that the photocatalytic degradation of FA should proceed endothermically. Overall, these findings are expected to help establish advanced strategies for the fabrication of self-cleaning photocatalysts for air purification applications.