The practical utility of nitrogen doped TiO2 as a photocatalyst for the oxidative removal of gaseous formaldehyde

This study reports the development of nitrogen-doped TiO₂ (N–TiO₂: N/Ti molar ratio = 1) photocatalyst with the enhanced photoelectronic properties for the phtocatalytic oxidation (PCO) of formaldehyde (FA). The N–TiO₂ photocatalyst is coated with ceramic beads and placed in a packed-bed tube reactor to examine the PCO-based mineralization of FA vapor (100 - 500 ppm) under ultraviolet (UV)-A illumination (32 W light source) with the control of flow rate (100–500 mL min⁻¹), O₂ (0–21%), and relative humidity (RH: 0–100%). Accordingly, the N-TiO₂ in dry conditions showcases 100% degradation of 100 ppm FA with high stability over 5 reuse cycles (compared to the P25 (75.9%) and bare TiO₂ (69.2%)) at a flow rate of 100 mL min⁻¹ and a 21 % O₂ level (quantum yield = 1.72.E−02 molecules photon⁻¹ and space-time yield = 3.44.E−03 molecules photon⁻¹ mg⁻¹). The superior performance of N–TiO₂ may reflect the combination of N/O atoms in the crystal structure to enhance the photo-redox reactivities with the heightened valence band enegry and prolonged lifespan of charge carrier (1.34 ns) relative to 1.04 ns of P25 and 1 ns of pure (anatase) TiO₂. The PCO efficiency of N–TiO₂ increases by around 1.6 times in slightly humid conditions (74.6%: RH 20%) compared to dry conditions (47%: RH 0%) while the absence of oxygen (at 0% O₂) reduces it significantly down to 13.6%. In situ diffuse reflectance infrared Fourier transform spectroscopy and electron paramagnetic resonance studies confirm the critical role of O₂ and H₂O vapor on the enhanced FA mineralization rate (CO₂ yield = 91 %) through the generation of ^•O₂^-/^•OH oxidative radicals. This study offers deep insights into the practical utility of N–TiO₂ for the photocatalytic mineralization of aldehyde VOCs.