Enhancement of Photocatalytic Oxidation NO by Nitrogen Doped Defective TiO2

Semiconductor photocatalytic technology can effectively remove low atmospheric concentration of NOx by using solar energy, which has a wide application prospect. The purification efficiency of semiconductor photocatalysis depends on the surface interface structure of the material. NOx purification efficiency can be effectively improved by constructing oxygen vacancy and single atomic structure sites on the surface of the material. While in the photocatalytic oxidation of NO, surface oxygen vacancy can activate O2 to form reactive oxygen species (ROS), which can improve the efficiency of NO oxidation to produce nitrate products. However, as the reaction progresses, O2 tends to fill the surface oxygen vacancy, resulting in the quenching of the active site and subsequent reduction of catalyst activity. In this paper, nitrogen doped defective titanium dioxide photocatalyst (TiO2-x-yNy) was synthesized by solution-gel method, and the surface oxygen vacancy was repaired by introducing nitrogen element to promote the continuous activation of O2. The photocatalytic performance of TiO2-x-yNy was investigated by simulating the photocatalytic purification of low concentration NO by visible light. X-ray powder diffraction (XRD) results showed that the crystal phase of TiO2 was not changed by the introduction of defects and the doping of nitrogen atoms. X-ray photoelectron spectroscopy (XPS) results showed that the electron cloud density of Ti 2p orbital increased and the characteristic peaks of Ti3+ (457.7 eV) and N-Ti bond (400.3 eV) were observed in N 1s spectrum. Electron paramagnetic resonance (EPR) results confirmed the existence of oxygen vacancies, and the characteristic triple peak of TiO2-x-yNy was attributed to N. In ultraviolet-visible spectroscopy (UV-Vis) diffuse reflectance results, the trailing absorption phenomenon of oxygen vacancy appeared in both samples, and the absorption shoulder appeared in TiO2-x-yNy due to the introduction of N. The photocurrent test showed that TiO2-x-yNy had higher photocurrent density, and nitrogen doping was beneficial to carrier separation efficiency. Electrochemical impedance spectroscopy (EIS) test results showed that TiO2-x-yNy had higher surface conductivity, and nitrogen doping could effectively improve the carrier dynamic separation efficiency of the material. The photocatalytic efficiency of TiO2-x-yNy to remove low concentration (600×10-9) NO under visible light was up to 72% (20 min), while that of TiO2-x was only 55%. Moreover, TiO2-x-yNy could still maintain 70% of NO removal after 5 cycles. The active species capture experiment showed that the main active species of the reaction were electron and superoxide radical (·O2–). Nitrogen doping significantly enhanced the ability of the catalyst to activate molecular oxygen, resulting in more·O2– production with TiO2-x-yNy. The results of NO-temperature programmed desorption (NO-TPD) and O2-temperature programmed desorption (O2-TPD) showed that the adsorption of NO was significantly improved after nitrogen doping, and the overflow temperature of lattice oxygen decreased, and the formation energy of oxygen vacancy was lower, which was conducive to the formation of surface oxygen vacancy. Meanwhile, O2 was preferentially adsorbed at the oxygen vacancy to be activated. In situ EPR and differential calculation results showed that the activation of O2 molecules needed to charge from oxygen vacancy electronics, leading to a significant reduction in electron density. The multi-electronic center N could transmit electronic to repair oxygen vacancy, allowing the oxygen vacancy site to continuously activate the molecular oxygen, and to capture hole promote carrier separation, thus improving the photocatalytic NO purification efficiency of TiO2-x-yNy. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results showed that the products of the two samples were mainly bidentate nitrate, partial nitrite and a small amount of monodentate nitrate. The active site for O2 activation was the surface oxygen vacancy, and the deposition rate of nitrate on the catalyst was faster after N doping. Therefore, the reaction mechanism of photocatalytic purification of NO by TiO2-x-yNy was as follows: O2 was preferentially adsorbed at the surface oxygen vacancy and obtained electrons from the surface oxygen vacancy to reduce to·O2–. The generated·O2– attacked the free NO on the surface and generated nitrate. The loss of electrons at the surface oxygen vacancy leaded to the destruction of the original potential balance between the oxygen vacancy and the nitrogen multi-electron center, which enabled the electron transfer from nitrogen to the surface oxygen vacancy to realize the repair of the surface oxygen vacancy, thus promoting the continuous activation of molecular oxygen and improving the photocatalytic NO oxidation efficiency. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.