The Degradation Of Gaseous Acetaldehyde By TiO₂-based Composites Under Visible Light

Volatile organic compounds（VOCs）are various organic compounds with a boiling point of 50°C to 260°C at room temperature and atmospheric pressure,which are one of the main causes of the air pollution.Especially,the aldehyde gases released by interior decoration materials and furniture.They are not only important precursors for the formation of PM_2.5,but extremely toxic and diffusive,which can damage human internal organs,brain and central nervous system,and even cause cancer.Therefore,the elimination of aldehydes in indoor air is of great significance to the protection of atmospheric environment and human health.Photocatalytic oxidation technology has the advantages of being carried out under normal temperature and pressure,low energy consumption,easy operation,etc.,and it is one of the most effective methods to eliminate indoor VOCs.Among the numerous photocatalytic materials,TiO₂ has attracted extensive attention due to its wide sources,stable chemical properties,high catalytic activity,and non-toxicity.However,there are still some shortcomings when TiO₂ is used for the photocatalytic degradation of gaseous aldehyde pollutants.The shortcomings include weak adsorption capacity for gaseous pollutants,insufficient utilization of sunlight,easy recombination of internal photogenerated carriers,and deactivation of materials,etc.In view of the above problems,the structure design and modification of TiO₂ materials were carried out,which enables the TiO₂-based composites to degrade gaseous acetaldehyde efficiently and stably under visible light irradiation.Next,the reaction path and deactivation mechanism of TiO₂-based photocatalyst in the degradation of acetaldehyde gas were studied to avoid the deactivation phenomenon.The specific research contents are as follows:（1）Firstly,Ag-TiO₂ was synthesized by chemical reduction method,and then Ag-TiO₂ and g-C₃N₄ were combined by ultrasonic-heat treatment to prepare g-C₃N₄/Ag-TiO₂ composites,which were used for the photocatalytic degradation of low-concentration and mobile-phase acetaldehyde gas under visible light.The nitrogen adsorption-desorption isotherm and the dynamic adsorption experiments showed that compositing with g-C₃N₄ could improve the specific surface area of composites,thereby increasing the adsorption of acetaldehyde gas.It could be confirmed by ultraviolet-visible diffuse reflectance spectroscopy that due to the narrower bandgap of g-C₃N₄ and the surface plasmon resonance effect of Ag nanoparticles,the light absorption range of composites was effectively improved.Photoluminescence spectroscopy,photocurrent spectroscopy,electron spin resonance and trapping experiments proved that after the introduction of g-C₃N₄ and Ag nanoparticles,as the appearance of heterojunction and electron acceptor Ag,the separation efficiency of photogenerated electron-hole pairs in the composites was improved.Thereby,more reactive oxygen species for photocatalytic degradation reaction were generated.The photocatalytic degradation tests of the mobile-phase gaseous acetaldehyde under visible light irradiation showed that the g-C₃N₄/50 wt%Ag-TiO₂（C0.5AT）composite exhibited the best photocatalytic activity.Its degradation efficiency was 5.8 times that of pure TiO₂,and the mineralization efficiency was 3.7 times higher than that of pure TiO₂.Additionally,C0.5AT composite still maintained excellent photocatalytic performance after continuous irradiation for 600 minutes or five cycles.The Langmuir-Hinshelwood kinetic model study not only showed that relative humidity played an important role in the degradation of acetaldehyde,but also confirmed that the photocatalytic degradation of acetaldehyde mainly occurred on the catalyst surface.Based on the above characterization tests,the degradation mechanism was analyzed,and two possible electron transfer paths inside the composites were proposed.This work provides a new idea for designing a composite photocatalyst that can effectively remove indoor low-concentration VOCs under visible light.（2）The Ag-TiO₂ photocatalyst prepared by chemical reduction method was deactivated and regenerated during the degradation of acetaldehyde gas.The intermediate by-products of acetaldehyde oxidation were analyzed by in-situ diffuse reflectance infrared Fourier transform spectroscopy,and the reaction paths on different catalysts were determined.Through photoluminescence spectroscopy,photocurrent and electron spin resonance tests,it was found that the deactivation of Ag-TiO₂ sample during the degradation of acetaldehyde was due to the accumulation of intermediate products on the catalyst surface,which gradually hindered the migration and separation of photogenerated electron-hole pairs,and then inhibited the generation of superoxide radicals and hydroxyl radicals.Although there were more intermediate products accumulated on the surface of g-C₃N₄/Ag-TiO₂ composite,it can still degrade acetaldehyde gas efficiently and stably.The anti-deactivation ability may be attributed to the fact that two-dimensional sheet material g-C₃N₄ provided additional adsorption sites for the intermediate products.This work is helpful to understand the reaction path and deactivation/anti-deactivation mechanism of the VOCs degradation process,which also provides a valuable reference for the efficient removal and performance maintenance of VOCs in the actual photocatalytic process.