Efficient formaldehyde photo-oxidation and reaction path study on oxygen vacancy engineered TiO<sub>2</sub>

With the intemperate depletion of energy resources and the ever-increasing emission of volatile organic compounds (VOCs), environmental issues will suffer even further. As a main contributor of VOCs, formaldehyde is quite volatile and poisonous. Short-term exposure to an environment that exceeds the standard concentration of formaldehyde (>0.1 mg/m³) would result in nausea, chest distress and headaches. While long-term exposure to formaldehyde concentrations in excess of the environmental standard can lead to lung cancer, pancreatic cancer, leukemia and even teratogenic effects. Therefore, researchers have been anchored their hopes on the complete conversion of formaldehyde into nontoxic and harmless compounds, such as CO₂ and H₂O. The traditional methods for removing formaldehyde from the environment include condensation, biological treatment and physical adsorption. However, these methods have problems with secondary pollutants and exhibit poor remediation performance at low concentrations of formaldehyde (ppm levels, namely, 1 ppm=1 mg/m³). Photocatalytic oxidation is an efficient green technology that can absorb the photons in sunlight by utilizing semiconductor photocatalytic materials. When the photon energy is higher than or equal to the semiconductor band gap (E_g), electrons can be transferred to the conduction band and left holes in the valence band. These photo-induced carriers, composed of photo-excited electrons and holes, can effectively trigger redox reactions that accomplish the removal of target pollutants. Along with the seminal report by Fujishima describing photocatalytic water splitting to produce hydrogen by employing TiO₂ in 1972, interests are emerging in the development of TiO₂-driven pollutant treatment. TiO₂ is a typical photocatalytic semiconductor that is strongly oxidized, non-toxic and chemically stable. However, the photo-aroused electrons in TiO₂ easily recombine with the holes, leading to a low quantum efficiency (<5%). The electron-holes separation rate of a photocatalyst during a photocatalytic process is affected by factors like its phase structure, particle size, crystallinity and specific surface area. Recently, some studies have reported a series of non-stoichiometric photocatalytic semiconductor materials, such as ZnO_1−x, SnO_2−x, BiO_2−x and TiO_2−x. These materials contain a large amount of vacancy defects, which can result in lattice distortion and introduce defect energy levels that accelerate the separation of charge carriers and thus further improve the photocatalytic performance. However, three problems are commonly observed with these reports: (1) The synthetic methods are complex and related to the use of strong acids, strong bases or poisonous and harmful chemicals; (2) the prepared materials cannot effectively degrade low concentrations of formaldehyde; and (3) the mechanisms and reaction paths based on formaldehyde degradation have not been determined. To enhance the photocatalytic behavior and clarify the reaction mechanism and route involved in low concentrations of formaldehyde degradation, in this paper, TiO₂ with surface oxygen vacancy was successfully prepared by calcination of P25 TiO₂ under vacuum condition, and the resulted TiO₂ with oxygen vacancy (O_v-TiO₂) exhibits superior photocatalytic activity towards formaldehyde oxidation in a continuous-flow reactor. The removal rate of formaldehyde over illuminated O_v-TiO₂ reaches 95.05%, which is 1.31 times higher than that of pristine TiO₂ with a formaldehyde removal rate of 72.52%. The experimental results showed that the introduction of O_v can not only extend the light-responsive range, but also facilitate the separation of photo-generated electron-hole pairs, therefore, enhancing the photocatalytic performance of TiO₂. In-situ infrared spectroscopy and DFT calculation indicated that the presence of O_v can improve the adsorption and activation of formaldehyde molecules. The rich delocalized electrons at the O_v sites of TiO₂ can migrate to molecular formaldehyde, which then actively fracture the C=O bonds of formaldehyde, sharply diminishing the accumulation of organic intermediates such as formic acid. This study highlights the influence of oxygen vacancies steered TiO₂ on the photo-oxidation reaction process, affording a new idea for the surface vacancy design of photocatalyst and photocatalytic treatment of gaseous pollutants. We see creating this kind of oxygen vacancies engineered TiO₂ with excellent photocatalytic behavior and repeatability can be as the key goal for future application-oriented work.