Study On The Preparation Of CdS、ZnIn₂S₄ Phase Junction Photocatalysts And Their Performance Towards Toluene Selective Oxidation

Selective oxidation of toluene to produce value added benzaldehyde is an important chemical reaction.Due to the high energy barrier for C（sp³）–H bond activation,the current industrial toluene oxidation process generally suffered from harsh reaction conditions and low product selectivity,which is high energy consumption in the reaction and separation process.Photocatalytic technology has attracted much attention due to its advantages such as direct use of solar energy and mild reaction conditions.The research core of photocatalytic technology is to develop efficient and stable photocatalysts.Metal sulfide is a kind of semiconductor with strong visible light response and various energy band structure have been widely used in photocatalytic water decomposition,carbon dioxide reduction,ammonia synthesis and organic synthesis reactions.However,the low separation efficiency and slow migration of photogenerated carriers limit their further application.In this thesis,cubic-hexagonal CdS and cubic-hexagonal ZnIn₂S₄ phase junction photocatalysts were prepared by phase transformation and in-situ growth method,respectively,and used in photocatalytic selective oxidation of toluene to benzaldehyde.The composition,structure,morphology,band structure of photocatalysts were characterized by various means,and the carrier separation efficiency,oxygen activation capacity were investigated.The"structure-activity relationship"between the structure and properties of the photocatalysts was preliminarily investigated,and the possible reaction mechanism was proposed.The main research contents and innovative results are as follows:1)Based on the theory of heat induced phase transformation,the CdS nanoparticles with cubic-hexagonal phase junction with the adjustable ratio of two phases were prepared by one-step hydrothermal method.It shows excellent photocatalytic activity for selective oxidation of toluene to benzaldehyde at ambient temperature and pressure.After 2 hours of reaction,the conversion of toluene and the selectivity of benzaldehyde were 34.4%and 97.4%,respectively.Compared with single phase CdS,the outstanding photocatalytic performance of CdS phase junction is attributed to the existence of phase junction which promoting the separation of photogenerated carriers and improving the activation capacity of oxygen.The possible mechanism of photocatalytic oxidation of toluene by CdS phase junction was proposed by investigating the energy band structure,carrier migration direction and reaction process.2)Cubic-hexagonal ZnIn₂S₄ phase junction nanosheets with controllable ratio of two phases were prepared by in-situ hydrothermal growth method,which shows excellent photocatalytic activity for selective oxidation of toluene to benzaldehyde at mild conditions.After 1 hour of reaction,the conversion of toluene was 19.7%,the selectivity and formation rate of benzaldehyde were 96.8%and 3769.1μmol·g^-1·h^-1,respectively.The excellent photocatalytic performance of ZnIn₂S₄ phase junction is attributed to that the in-situ phase junction not only improved the specific surface area of the material but also provided an efficient carrier migration path,thus improving the efficiency of oxygen adsorption-activation and carriers separation.3)Based on the energy band structure of photocatalysts,carrier migration direction and reaction process,we proposed the possible mechanism of photocatalytic selective oxidation of toluene into benzaldehyde over CdS,ZnIn₂S₄:The carriers are produced after the photocatalyst excited by light;Carriers migrating to the surface activate toluene and oxygen into benzyl radical and superoxide radical,respectively;The benzyl radical was oxidized to benzaldehyde by superoxide radical and oxygen.