Study On Molecular Modified G-C₃N₄-based Catalyst And Its Photocatalytic Performance For Hydrogen Production

Nowadays,the development of industry and society is very fast,has got into the rapid lane.Development needs to consume energy,Today’s energy is still mainly rely on the traditional burning fossil fuels,as is well to known,the process of using fossil fuel has brought a series of environmental pollution problems.For example,more CO2 accumulation from the reaction can bring greenhouse effect,acid rain which caused by SO2,it can evolute to produce acid and ozone depletion.These seriously restrict the long time and high-quality development of human society.In order to alleviate the current the problems of energy shortage and environmental pollution we faced,it is urgent to find and develop green,renewable and clean energy.In many areas of experimental energy like tidal,nuclear,wind and solar,solar energy as the source of all things,endless,is in line with the trend of energy development choice.Photocatalytic technology is to use solar energy to convert it into chemical energy through appropriate materials.At the same time,hydrogen(H2)has a high energy density,and the products after combustion reaction have no pollution and no toxic to the environment,So it is regarded as a candidate for future energy.In summary,the photocatalytic hydrogen evolution technology has attracted the attention of researchers.Its core is to develop semiconductor catalyst materials which are low cost,high efficiency,nice stability and no contamination.Since 2009,graphite phase carbon nitride(g-C3N4),a nonmetallic semiconductor catalyst,has aroused intense research among scientists.The g-C3N4 has unique and excellent properties,including easy availability of original materials and inexpensive,handy preparation operation,befitting electronic band structure,response to visible light and good performance in physical and chemical stability.Everything has two sides.Although it has many advantages as mentioned above,its visible light response range is narrow.photo-generated carriers are liable to recombine,and the charge separation efficiency is not high,which seriously affects its practical application prospects in photocatalytic hydrogen production technology.Therefore,in order to settle these difficulties.it is essential to decorate pure g-C3N4 to improve the efficiency of photocatalytic hydrogen production and meet the application standards.Based on this,this article choose the appropriate molecules to modify g-C3N4,designing several g-C3N4 matrix composite catalysts.Through the results of the modified samples photolysis aquatic hydrogen and the data of characterization,we attempt to propose photo-generated hole transfer mechanism and construction of type Ⅱ heterojunction which can explain the mechanism of photo-generated carriers separation and the transferred process in photocatalytic reaction.The specific research contents of this paper are as follows:1.In chapter 2,a novel composite catalyst(CN/L-trp)was designed.Using urea as the precursor,g-C3N4 was synthesized by thermal condensation and then modified by biological small molecule L-trp(CN/L-trp)through hydrogen bond and π-πconjugate interaction,and g-C3N4/L-trp(CN/L-trp)photocatalyst was obtained.Under visible light irradiation,the best CN/10 wt%L-trp sample show the highest hydrogen production efficiency,can arrive to 1046.0 μmol h-1g-1,which is four times that of pure g-C3N4(260.2 μmol h-1 g-1).The stability test results also show that CN/10 wt%L-trp has long-term photocatalytic activity.Levorotary-tryptophan(L-trp)as a biomolecule has an outstanding reductive function.Electrochemical tests show that L-trp can serve as redox mediators to effectively transfer hole to accelerate the hole transfer kinetics.This can availably facilitate the spatial separation of the photoproducted holes and electrons and increase the efficiency of H2 production.Moreover,according to the PEC and EIS test results,the modified catalyst exhibits higher current density and lesser charge transfer resistance.2.In chapter 3,we have constructed highly efficient visible light driven g-C3N4/FePc(which named CN/FePc)type-Ⅱ heterostructures through π-π interaction to decorate Iron(Ⅱ)phthalocyanine(FePc)on the surface of the g-C3N4.Iron phthalocyanine(Ⅱ)(FePc)molecules as photosensitizer,on the one hand,can indeed extend the scope of visible light absorption to enhance the photocatalytic activity.On the other hand,it has 18 electronic large π conjugate system.By means of ultrasonic assisted method and magnetic stirring,the strong π-π interaction and the appropriate energy level match between the two semiconductor(g-C3N4/FePc)were used to build a stable Type-Ⅱ heterojunction CN/FePc composite catalysts.The reduction reaction of water and oxidation reaction of sacrificial agent were separated.They carried out in the conduction band and valence band of the two semiconductors respectively.This way can better to realize the real separation,effective transfer and utilization of photoelectron-hole pairs in space.Under optimal experimental conditions,the CN/4 wt%FePc composite catalyst exhibited the highest photocatalytic activity for hydrogen production from reduced water with an output value of 41.73 μmol h-1,which was 3.4 times that of g-C3N4(12.18 μmol h-1).Moreover,the g-C3N4/4 wt%FePc Type-Ⅱheterostructured photocatalyst exhibits no obvious loss of activity for H2 evolution after five consecutive cycles of continuous.Based on the test results,a possible mechanism of photocatalytic hydrogen production was provided.