Study On The Photocatalytic Performance Of CdS Towards Toluene Selective Oxidation:Synergetic Effect Of Defect And Heterojunction

Selective oxidation of toluene for the production of highly value-added benzaldehydes is an important topic in the field of chemical research.Because the activation of C（sp³）–H bond in toluene requires relatively high energy thus the involvement of pre-activation process or high temperature and high pressure in the industrial production process,which requires a large amount of energy consumption and causes environmental pollution problems.Photocatalytic toluene selective oxidation technology using clean and inexhaustible solar energy as energy source and the reaction was conducted under mild condictions（normal temperature and pressure）has aroused tremendous attention of researchers.The core of photocatalytic technology is stable and efficient photocatalyst.Cadmium sulfide（CdS）is a kind of photocatalyst with appropriate energy band structure and responsive to visible light,which has been widely used in photocatalytic water decomposition,carbon dioxide reduction and organic synthesis.However,due to serious combination of charge carriers and deactivation due to self-oxidation,their application is greatly constrained.In this thesis,CeO₂/CdS and CdIn2S4-CdS photocatalyst was prepared by controlling the morphology of catalyst,constructing heterojunction and introducing defects at the same time.The prepared photocatalysts were evaluated in the selective oxidation of toluene to benzaldehyde under visible light irradiation.At the same time,the compositon,morphology and structure of the catalyst were characterized by various means,and the structure-activity relationship between the catalyst structure and performance was studied,also the corresponding possible mechanism was proposed.The main research contents and innovative results are as follows:1)Oxygen-defected CeO₂/CdS photocatalyst was prepared successfully using octahedral Cd₃（C₃N₃S₃）₂（achieved by precipitation method）and oxygen-defected CeO₂as precursor.All of the obtained photocatalysts maintained the regular octahedral structure of the precursor.When the molar ratio of CeO₂ to CdS is 1:10,the catalyst display the highest photocatalytic activity for selective oxidation of toluene,and the formation rate of benzaldehyde is 2298.7μmol g^-1 h^-1.Its high activity could be attributed to the existence of loaded oxygen vacant CeO ₂ promoted the separation of photogenerated carriers and the formation of reactive oxygen species.At the same time,CeO₂ nanoparticles were compounded on the octahedral CdS surface,which provided more active sites for toluene catalytic oxidation.2)An precursor decomposition plus ion exchange method was investigated for the preparation of heterogeneous CdIn₂S₄-CdS photocatalyst with S defects using octahedral Cd₃（C₃N₃S₃）₂ and In（NO₃）₃ as precursor.High photocatlytic activity towards toluene selective oxidation was achieved in normal condition with illumination of visible light with toluene conversion rate reached 80.3%,and selectivity of benzaldehyde keep at 98%.The results show the co-existence of S defects and heterojunction can produce a synergetic effect which promotes the separation of photogenerated carriers and the activation of oxygen to generate a large number of oxygen active substances（superoxide radicals）.At the same time,the selective adsorption of toluene and the preferred desorption properties of benzaldehyde in reaction process over the CdIn₂S₄-CdS photocatalyst makes benzaldehyde be produced with high selectively.The comprehensive performance of this photocatalyst is better than that of the similar photocatalysts reported at present.