Preparation Of WS₂/h-C₃N₄ Composite Photocatalyst For Biocatalyzed Artificial Photosynthesis

Construction of biocatalayzed artificial photosynthetic system by incorporating the photochemical regeneration of coenzyme NADH with the redox enzymatic process to produce various fuels and chemical has an important significance for solving environmental and energy problems.To achieve this,fabricating and employing catalysts with high visible light activity was the key point.Graphite phase carbon nitride(g-C3N4)is a kind of polymeric semiconductor material with excellent visible light response,which enable g-C3N4 a wide range application as photocatalyst in photocatalytic coenzyme regeneration and other fields.However,due to the problems such as easy electron-hole recombination and low quantum yield of g-C3N4,the further application of the material is limited.In order to solve these problems,WS2/g-C3N4 composites were prepared by load WS2 on g-C3N4 through a simple ultrasonic-assisted hydrothermal method.The WS2/g-C3N4 composites was then used for photocatalytic regeneration of NAD+ to NADH,which were then coupled with dehydrogenases for sustainable bioconversion of CO2 to methanol under visible light irradiation.The main research work is as follows:1)A series of WS2/g-C3N4 composites with different ratios of WS2 were prepared by simple ultrasonic-assisted hydrothermal method.Their photocatalytic performance was evaluated by NADH photoregeneration and rhodamine B degradation experiments.The experimental results show that the photocatalytic performance of 5 wt%WS2/g-C3N4 is the highest,and the NADH regeneration yield reached about 35%within 4 hours,which was about 3-4 times higher that of pure g-C3N4.By coupling this NADH photoregeneration process with the cascade enzymatic CO2 reduction reaction catalyzed by formic acid dehydrogenase,formaldehyde dehydrogenase and alcohol dehydrogenase,a methanol productivity reached to 372.1 ?molh-1 gcat-1,which is 7.5 times that obtained using pure g-C3N4.For further application demonstration,the activity of WS2/g-C3N4 composite catalyst toward photodegradation of Rhodamine B(RhB)was evaluated.RhB removal ratio approaching 100%was achieved in 1 hour by using the WS2/g-C3N4 composite catalyst with 5 wt%of WS2,at an apparent degradation rate approximately 2.6 times higher than that of pure g-C3N4.2)To explore the mechanism accounting for the enhancement in the activity of the WS2/g-C3N4 composites,the morphological characteristics and physicochemical properties of g-C3N4,WS2,and the WS2/g-C3N4 composites were fully characterized by XRD,FT-IR,TEM,EDS,XPS,BET,TGA,PL,etc.It was speculated that a heterojunction structure was formed between WS2 and the g-C3N4.Considering the both the valence band and conduction band of g-C3N4 are higher than that of WS2,while the conduction band of WS2 is higher than the valence band of g-C3N4,upon light irradiation,the excited electrons on the conduction band of g-C3N4 will migrate to the conduction band of WS2,and the holes in the valence band of WS2 will move to the valence band of g-C3N4;thus recombination of electrons and holes was decreased and the photo-harvesting efficiency was enhanced.