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

Abstract

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 NO_x, 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 NO_x 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.

Keywords: Co-doped photocatalyst; Exhaust purification; Photoabsorption band; Photocatalytic activity; Visible light irradiation.