Investigation Of Performance And Mechanism Of Zn-Ti Layered Double Hydroxide For Photocatalytic Toluene Degradation

In modern industry,vehicle exhaust emissions,oil storage and distribution,solvent use,and many other processes have led to rapid increases in VOCs（volatile organic compounds）levels in the atmosphere.VOCs can accelerate stratospheric ozone depletion and form photochemical smog,causing severe environmental problems.Some VOCs,such as toluene,have carcinogenic,teratogenic and mutagenic effects and seriously endanger human health.Therefore,it is extremely urgent to develop efficient technology to degrade VOCs.Among various techniques of degradation VOCs,photocatalysis has attracted extensive attention because of green,energy-saving and mild reaction condition.However,due to the limitation of band structure and surface electronic structure of photocatalyst,photocatalysis will face some problems such as low purification efficiency of pollutants,generation of toxic intermediates,and deactivation of the catalyst during photocatalytic degradation of benzene series.Especially,it is difficult to improve the reaction efficiency and reveal the reaction mechanism due to the high ring-opening energy barrier and sophisticated reaction process of benzene ring.To solve these problems,it is necessary to develop photocatalysts with high activity,stability,and selectivity and to use advanced characterization methods to conduct in-depth research on the reaction process,reveal the reaction mechanism,and finally achieve efficient,stable,and safe purification of pollutants.In this thesis,Zn-Ti LDH,which can degrade toluene efficiently and stably,was prepared by controlling temperature and ion ratio.The degradation efficiency of the sample（75.2%）was significantly improved compared with P25（10.9%）.Further characterization found that the key factors promoting the photocatalytic degradation of toluene were not the specific surface area and charge separation efficiency of the samples but the generation of active free radicals and the selective generation of intermediate products.Therefore,this paper further explored the reaction mechanism of photocatalytic toluene degradation from the generation of active free radicals,activation and transformation of intermediates,and ring-opening process.Combining the results of electron spin resonance with theoretical calculation,it is found that oxygen can be better adsorbed and activated on the Zn-Ti LDH surface,promoting the generation of more superoxide radicals.The intermediates on the catalyst surface and in the gas phase during toluene degradation were studied by in situ DRIFTS,high-performance liquid chromatography,and gas chromatography-mass spectrometry.It was found that benzoic acid was the main intermediate on the catalyst surface during photocatalytic degradation,and benzaldehyde was the main intermediate in the gas phase.The interaction between the two intermediates and the photocatalyst surface was further investigated by theoretical calculation.It was found that hydrogen atoms on the surface of Zn-Ti LDH would attract oxygen atoms on the characteristic groups of the two main intermediates by forming hydrogen bonds,thus achieving effective activation of the two intermediates.Hence,it can be further confirmed that the ring-opening reaction of these two intermediates is more likely to occur on the surface of Zn-Ti LDH,thus achieving efficient and stable degradation of toluene.