Study Of Hydrogen Production By Photolysis Of Water From MX₂/g-C₃N₄

At present,with the increasing problems of global energy shortage and environmental pollution,the search for renewable and clean energy sources such as hydrogen,alcohol and natural gas is one of the important ways to protect the environment and alleviate the energy shortage situation.Hydrogen stands out among the many clean energy sources because of its clean,efficient and sustainable characteristics.The photocatalytic hydrogen production reaction can convert solar energy into hydrogen energy,which has far-reaching significance in solving the problems of energy shortage and environmental pollution.As the charge separation and surface catalytic conversion efficiency of the photocatalytic hydrogen production reaction is severely limited,the reaction requires co-catalysts,especially noble metals(e.g.Pt),and two-dimensional transition metal sulphides(TMDCs)are a promising class of non-precious metal co-catalysts.Graphitic phase carbon nitride(g-C3N4)is a polymeric non-metallic photocatalyst that has attracted much attention due to its good chemical stability,ease of preparation and visible light response.However,g-C3N4 has a low photocatalytic efficiency due to its weak light absorption,low specific surface area and rapid complexation of photogenerated carriers.Therefore,in this thesis,three different TMDCs heterojunctions based on g-C3N4-based nanomaterials were prepared to replace Pt as a cocatalyst,promote the separation and migration of g-C3N4 photogenerated carriers,improve the catalyst surface reaction efficiency and enhance the photocatalytic hydrogen precipitation performance.Details of the study are as follows:1、To enhance the low photocatalytic efficiency of g-C3N4 nanomaterials due to weak light absorption,low specific surface area,and rapid compounding of photogenerated carriers,the two-dimensional transition metal sulfide WS2 was used as a co-catalyst to replace the noble metal Pt and form heterojunctions with g-C3N4 to enhance the photocatalytic hydrogen precipitation performance of g-C3N4 nanomaterials.In this paper,a WS2/g-C3N4 composite with fewer layers was prepared as a noble metal-free catalyst for hydrogen precipitation using a one-step calcination method with a rational adjustment of the ratio of precursors.The results showed that the hydrogen precipitation rate was 0.05 mmol·g-1·h-1 for pure g-C3N4 and 4.27 mmol·g-l·h-1 for g-C3N4(Pt),and the optimized hydrogen precipitation rate was 10.42 mmol·g1·h-1 for WS2/g-C3N4.The hydrogen precipitation rate of the optimized WS2/g-C3N4 was 208 and 2.4 times higher than that of pure g-C3N4 and g-C3N4(Pt),respectively.Thus,layer less WS2 composite g-C3N4 can replace the use of noble metals and enhance hydrogen precipitation performance.2.In order to further enhance the photocatalytic hydrogen precipitation performance of g-C3N4 nanomaterials in response to the problem that it is difficult to control the layer thinning of WS2 in WS2/g-C3N4 composites,the paper prepared x-MoS2/g-C3N4 composites using different contents of ammonium tetrathiomolybdate as molybdenum source and used as hydrogen production catalysts,and obtained:the 0.3-MoS2/g-C3N4 The H2 activity of 0.3MoS2/g-C3N4 was significantly better than the results of other ratios,and its photocatalytic hydrogen precipitation rate reached 8.30 mmol·g-1·h-1.The hydrogen precipitation performance of sample 0.3A-MoS2/g-C3N4 was enhanced to 12.01 mmol·g-1·h-1 by annealing the 0.3-MoS2/g-C3N4 sample with liquid nitrogen,which was the best performance of g-C3N4 and g-C3N4(Pt)by a factor of 240 and nearly 3 times,respectively.The efficient photocatalytic activity and stability of x-MoS2/g-C3N4 was found to be attributed to its typical laminate structure with fewer MoS2 layers,which facilitates the separation of photogenerated electronhole pairs.3、In order to reduce the amount of TMDCs compounded in the photocatalytic composites and to further enhance the photocatalytic hydrogen precipitation performance of g-C3N4 nanomaterials,an attempt was made to use ReS2 as a co-catalyst instead of MoS2.x-ReS2/gC3N4 heterostructures with different ratios were successfully prepared in this thesis through a two-step calcination method by first growing ReS2 and then compounding it onto g-C3N4 photocatalysts.The results showed that the best photocatalytic hydrogen precipitation rate was achieved for the 0.06-ReS2/g-C3N4 sample ratio,and the amount of TMDCs compounded in this heterostructured photocatalyst was significantly reduced.