Study On Regulation And Mechanism Of Cobalt-based Composite Catalysts For Photocatalytic Hydrogen Production

In recent years,environmentally friendly photocatalytic technology has a certain application prospect in hydrogen energy development,pollution purification,medical care and other fields to solve environmental and energy problems.The supporting of the cocatalyst can improve the photocatalytic performance from the aspects of light absorption,catalytic reaction activation energy,photogenerated carrier migration and so on.This paper focuses on the design of cocatalysts.It is carried out around how to replace noble metal cocatalysts.The key is how to cooperate with inexpensive cocatalysts to promote the photocatalytic hydrogen production performance,reaching a level comparable to that of noble metal cocatalysts.Cobalt-based cocatalysts with low cost,good electrical conductivity and easy crystal phase regulation were used to modify the photocatalysts.The main research contents are as follows:（1）First,a simple two-steps method,namely hydrothermal method and ammonia calcination method,was used to develop and design a 0D/2D photocatalyst reasonably assembled from CoN nano-dots/2D g-C₃N₄ nanosheets in situ.It was found that the loading of CoN nano-dots could effectively improve the photocatalytic hydrogen production activity of 2D g-C₃N₄.Using triethanolamine as sacrificial agent,the photocatalytic hydrogen production efficiency of the compound with 10%loading capacity reached 403.6?μmol g^-1 h^-1,which was 20 times that of the monomer.The reason for the increased activity is that the cocatalyst CoN achieves the uniform dispersion of active units on the main photocatalyst by means of the ultra-small size load.The electrical conductivity of CoN is conducive to the extraction of photoelectric charge,which further accelerates the separation of photoinduced carriers and promotes the utilization of light in the catalyst.（2）In order to further improve the performance,the separation efficiency of photogenerated carriers is further improved by building a chemical bond between the semiconductor material and the cocatalyst.Taking advantage of the excellent optoelectronic properties of transition metal sulfide and using 2D g-C₃N₄ nanosheet as the mian material,the optimized 20%Co₉S₈/2D g-C₃N₄ reached the maximum hydrogen production efficiency of 4758?μmol g^-1 h^-1（triethanolamine as sacrificial agent）.It is 140 times higher than that of the monomer,about 12 times of the first system in this paper.A series of means of characterization were used to characterize the valence state and electrical properties of the material.The results show that 2D g-C₃N₄ plays the role of the light-absorbing unit to generate charges,and Co₉S₈ plays the role of an electron acceptor to reduce protons to generate hydrogen.Among them,the generated Co-N_x bond guarantees the close connection between them,which can accelerate the transfer of photoinduced carriers.It is beneficial to the separation of photoelectric charge and accelerate the kinetics of photoelectric charge.（3）Finally,in order to study the effects of the cocatalyst itself on the performance of hydrogen production,we started from the crystal structure of the cocatalyst itself.Two kinds of cobalt selenide（Co Se₂）with different crystal phase structure,namely o-Co Se₂ and c-Co Se₂,have been successfully transformed by controlling calcination conditions.The effect of crystal structure on the activity of photocatalytic hydrogen production was studied by using the difference of photoelectric properties of different crystal phases.Through characterization,it is found that c-Co Se₂ has stronger electrical conductivity and more efficient charge transfer capacity than o-Co Se₂,which is theoretically more favorable for the photocatalytic reaction.After selecting semiconductor Cd S to compound with them,it is found that the two kinds of cocatalysts could promote the photocatalytic hydrogen production.Taking lactic acid as sacrificial agent,the hydrogen production efficiency of o-Co Se₂/Cd S and c-Co Se₂/Cd S with the optimal loading of 10%were 9006.2μmol g^-1 h^-1 and 7151.2μmol g^-1 h^-1 respectively,which are 20 times and 15 times higher than those of monomer.These are close to or even higher than that of the noble metal Pt supported under the same condition.The photocatalyst supported by o-Co Se₂ show better performance.Combined with the steps of photocatalytic reaction and theoretical calculation analysis,it is found that the more appropriate hydrogen adsorption free energy at the cobalt metal site of o-Co Se₂ is the key reason for the better cocatalyst for hydrogen production.