Preparation Of Molybdenum-based Co-Catalyst And Study On Its Photocatalytic Hydrogen Production Performance From Water Splitting

The use of solar energy photocatalytic decomposition of water to produce hydrogen is considered to be a clean and sustainable strategy to solve the global environmental pollution and energy crisis.Semiconductor materials such as Ti O2 have been widely used in photocatalytic decomposition of water to produce hydrogen.However,to some extent,most of the current photocatalysts have problems such as low light absorption efficiency,high photogenerated carrier recombination rate,low stability and low photocatalytic activity.Ternary sulfides have attracted more and more attention in the field of photocatalytic water decomposition and hydrogen evolution due to its unique electronic structure and high light absorption characteristics.However,ternary sulfides also have problems such as low photo-generated carrier separation efficiency and high surface hydrogen evolution overpotential.The above problems can be solved to a certain extent by means of semiconductor recombination,co-catalyst modification and morphology control.Molybdenum-based compounds have received extensive attention due to their unique electronic structure and metal-like properties and they are used as co-catalysts for the photocatalytic water decomposition and hydrogen evolution reaction.Based on the above analysis,in this paper,we designed and prepared two molybdenum-based compounds:molybdenum carbide（Mo2C）and carbon-molybdenum oxide（Mo O2/C）,and used them as a co-catalyst to modify the ternary sulfide photocatalyst（Zn In2S4）.The influence of material composition,preparation process,morphology,etc.on the activity of Zn In2S4 photocatalytic hydrogen evolution reaction was systematically studied,and the catalytic reaction mechanism was preliminarily clarified.The main research contents are as follows:1)First,using ammonium molybdate and dicyandiamine as raw materials,Mo2C was prepared through the high-temperature solid-phase method;then,Mo2C/Zn In2S4 composite photocatalytic material was synthesized by an in-situ method.The composition,morphology,structure of the material and the relationship between the above characteristics and the photocatalytic hydrogen evolution performance have been systematically studied.The results show that the Mo2C material has metal-like properties,high conductivity,high surface work function and low hydrogen evolution overpotential.After compounded with Zn In2S4,it can effectively promote the separation and migration efficiency of the photo-generated charge of Zn In2S4.In addition,Mo2C can also provide abundant active sites for the hydrogen evolution reaction,effectively reduce the overpotential of the hydrogen evolution reaction happened on Mo2C/Zn In2S4,When the ratio of dihydrodiamine and ammonium molybdate is 2:1 and the loading of Mo2C is 10wt%,the sample has the highest hydrogen evolution efficiency.The efficiency is about 1.33mmol h^-1·g^-1,which is 5.1 times to that of pure Zn In2S4.2)Using graphite phase carbon nitride and ammonium molybdate as precursors,a Mo O2/C cocatalyst was synthesized through high-temperature solid-phase reaction,and Mo O2/C@Zn In2S4 photocatalytic composite material was constructed by in-situ hydrothermal method.When the Mo O2/C promoter content is 5wt%,the composite material reveals the highest hydrogen evolution efficiency,which is 7 times to that of pure Zn In2S4,and it is similar to the hydrogen evolution efficiency of the platinum modified Zn In2S4 catalyst.Based on the research on the morphology,structure,electrochemical performance and hydrogen evolution efficiency of composite materials,the mechanism of the high photocatalytic activity of the materials is initially proposed:On one hand,the introduction of a nitrogen-doped carbon layer into Mo O2 can effectively inhibit the agglomeration of Mo O2 in the high-temperature solid-phase reaction,and on the other hand,the graphite phase carbon layer can effectively improve the conductivity of the material;When Mo O2/C is used as the co-catalyst for the hydrogen evolution reaction of Zn In2S4,an more effective heterojunction interface between Mo O2/C and Zn In2S4 constructed through the in-situ recombination process,which is conducive to the effective transfer of electrons;In addition,the large surface work function difference between Zn In2S4 and Mo O2/C causes the formation of a Schottky-like interface between the two materials,which can drive electrons moving from Zn In2S4 to Mo O2/C,thereby the carrier separation efficiency of the photocatalyst greatly improved,and the overpotential of hydrogen evolution on the surface of Zn In2S4 reduced.Therefore,the photocatalytic hydrogen evolution activity of the composite has been significantly improved.When the mass ratio of ammonium molybdate to g-C3N4 is 1:2,the Mo O2/C content is 5wt%,and the hydrogen production efficiency of the composite sample reaches 1.83 mmol h^-1·g^-1,which is 7 times to that of the pure sample Zn In₂S₄(0.27 mmol h^-1·g^-1).