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

Abstract

This study reports the oxidation/mineralization of formaldehyde (FA), a well-known hazardous volatile organic compound (VOC), using TiO2 doped with nitrogen (N-TiO2: N/Ti molar ratio=1) to improve its photoelectronic properties. The prepared N-TiO2 photocatalyst is deposited over ceramic beads and placed in a packed-bed tube reactor to induce the photocatalytic oxidation (PCO) of FA under the following conditions [FA concentration (100–500 ppm), flow rate (100–500 mL min-1), O2 (0–21%), relative humidity (RH: 0–100%), and ultraviolet (UV)-A illumination (32 W light source)]. The PCO performance in dry conditions shows 100% degradation of 100 ppm FA 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 should be attributable to its enhanced light reactivity with lower bandgap energy (3.10 eV) and longer charge carrier life time (1.34 ns) relative to P25 (3.25 eV and 1.04 ns) and pure (anatase) TiO2 (3.25 eV and 1 ns). The photocatalytic activation of N-TiO2 is optimized in slightly humid conditions (RH 20%). 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.