Microstructures and optical performances of nitrogen-vanadium co-doped TiO2 with enhanced purification efficiency to vehicle exhaust.

Titanium dioxide (TiO2) is widely used to purify air pollutants in environmental engineering, but it is only activated by ultraviolet (UV) light. The metal or nonmetal single doping of TiO2 cannot observably improve the purification efficiency of TiO2 under visible light. To further increase the photocatalytic activity and purification efficiency of TiO2 on vehicle exhaust under visible light, nitrogen (N)-vanadium (V) co-doped TiO2 was first prepared. The influences of N-V co-doping on phase structures, morphology, microstructures, electronic structures, and photo-absorption performances were then observed and examined using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-visible light diffuse reflectance spectra. Purification efficiency and reaction rates of N-V co-doped TiO2 on NOx, HC, CO and CO2 in vehicle exhaust were studied using a purification test system under UV and visible light irradiations, respectively. Results indicate that N and V are synchronously doped into the crystal structures of TiO2 to replace O and Ti, respectively. N and V show the synergistic co-doping effect to suppress the grain growth of TiO2 and improve the dispersity and specific surface area of TiO2. Also, the N-V co-doping introduces more lattice distortions and defects in the crystal lattices of TiO2. Further, N presents in the form of Ti-O-N and O-Ti-N bonds, and V exists in the form of V5+ and V4+. These form the impurity energy level in the band gap to narrow the energy band of TiO2. Additionally, the N-V co-doping broadens the photoabsorption threshold of TiO2 from 387 nm to 611 nm. These results show that N-V co-doping increases the photocatalytic activity of TiO2. Finally, the N-V co-doped TiO2 shows higher catalytic purification efficiency on NOx and HC under UV and visible light. The N-V co-doping obviously increases the purification efficiency of TiO2 on CO and CO2 when exposed to visible light, and their reversible reactions are not found. The N-V co-doping of TiO2 is a feasible approach to purify vehicle exhaust under visible light irradiations.