The effects of nitrogen-doping on photocatalytic mineralization of TiO2 nanocatalyst against formaldehyde in ambient air

ABSTRACT A series of proto-type photocatalytic air purifier (AP (Nx-Cy)) systems are built with a nitrogen-doped TiO2 (N-TiO2)-impregnated honeycomb (HC) filter for photocatalytic decomposition of 0.5–5 ppm formaldehyde (FA: CH2O) vapor under varying conditions and UV-LED light (1 watt). The binary codes of Nx and Cy in AP systems are used as the composition identifiers to represent N/Ti molar ratios (0 to 20) and N-TiO2 concentration (2 to 20 mg mL–1), respectively. The AP (N10-C10) is found as an optimum unit with the highest capability to boost the catalytic conversion of CH2O to CO2 (yield = 89.2% over 10th cycles and the clean air delivery rate (CADR) of 9.45 L min–1 in dry air). The superior charge carrier lifetime (τa: 1.70 ns) of N10-C10 over others (e.g., 1.37 ns for pure TiO2) should indicate the influential role of N-defects (No) in reducing the bandgap (3.10 eV) and in creating defect-related oxygen vacancy (OVs-Ti3+) states as predicted by the density functional theory (DFT) simulation. The photocatalytic oxidation pathway of CH2O, when assessed by diverse approaches (e.g., in-situ diffuse reflectance infrared Fourier transform, electron paramagnetic resonance, and DFT analyses), is found to involve several energetically favorable intermediate steps (such as exothermic covalent adsorption of CH2O to bridged O/OH groups on TiO2-OV {110} surface in the form of CH2O2 followed by catalytic dehydrogenation/oxidation reactions to yield CO2 through direct route: CH2O2/HCOO– + •OH → H2O + CO2). These steps are supported by the calculated density of states (DOS) for chemically active Ti-atom on {101} surface with N-impurity. The presence of No-defects and OVs is expected to influence the reaction energetics and intermediates for efficient mineralization in humidified conditions by lowering the activation barriers. This study offers valuable insights into the design and construction of an advanced photocatalytic system for efficient mineralization of aldehyde VOCs in ambient air.