Study On Improving Photocatalytic Hydrogen Production Performance Of G-C₃N₄ Based Composite Catalyst By Hole Transfer

The development of today's society still depends on the traditional energy system.The rapid development of the economy has caused a serious shortage of energy.In addition,the use of traditional fossil fuels produces a large number of toxic gases,liquids,solid wastes and other pollutants,which seriously pollutes the social environment on which human beings live.The search for new and clean energy sources has become a common problem all over the world.Solar energy is a renewable and clean energy source,and hydrogen(H2)is a clean and renewable resource with pollution-free products,which is considered as an important energy source in the future.Therefore,photocatalytic hydrogen production technology is considered as one of the most simple and effective technologies to solve energy and environmental problems,and the core of photocatalytic hydrogen production technology is to develop a cheap,efficient and pollution-free semiconductor photocatalyst.Among many semiconductor catalysts,graphite carbon nitride(g-C3N4)has a suitable band gap(2.7 eV),good stability,wide source of raw materials,low price and non-toxicity compared with other catalysts,so it has a wide range of applications.However,g-C3N4 has low specific surface area and low visible light utilization.Moreover,the application of g-C3N4 in photocatalytic decomposition of aquatic hydrogen is affected by its fast photogenic carrier recombination rate.Based on this,this paper modifies g-C3N4 with different molecules,by photogenerated hole transfer,accelerating the separation of photogenerated electrons and holes,and improving the photocatalytic activity of g-C3N4.The main research contents are as follows:1.Prepare g-C3N4 by calcination at 550? by thermal condensation polymerization.g-C3N4 was synthesized with dihydropyridine(diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate,DHPE)by hydrogen bond and?-? conjugation to obtain g-C3N4/DHPE(CN/DHPE)composite catalyst.In the CN/DHPE composite catalyst,DHPE molecule acts as an oxidation-reduction agent to transfer photogenic holes and effectively inhibits photogenic electron and hole recombination,so that more electrons can be used for photocatalytic decomposition of aquatic hydrogen.XRD and TEM results show that the introduction of DHPE does not change the structure and morphology of g-C3N4.PL,TRPL and electrochemical test results confirmed that the introduction of DHPE can indeed inhibit the photogenerated electron-hole recombination,prolong the photogenerated electron and hole recombination time.In the application of photocatalytic decomposition of aquatic hydrogen,when 4 wt%DHPE was introduced,the photocatalytic decomposition rate of aquatic hydrogen reached the maximum value(1345.0 ?mol h-1 g-1),which was 4.7 times that of unmodified g-C3N4(286.0 ?mol h-1 g-1).Moreover,in the long-term cyclic reaction test(20 h,5 test cycles),the hydrogen production rate of the CN/DHPE composite catalyst did not decrease significantly,indicating that the CN/DHPE photocatalyst has a good stability.2.A novel and efficient g-C3N4 matrix photocatalyst system was constructed by a simple and economical method.N-hydroxyphthalimide(NHPI)molecules modify the surface of g-C3N4 by hydrogen bonding and ?-? conjugation.NHPI molecules can be used as redox agents to transfer photogenic holes.The effective energy level matching between g-C3N4 and NHPI molecule makes the photogenic cavity transfer from the valence band of g-C3N4 to NHPI molecule to form NHPI+free radical.The photogenic charge compound is inhibited,and promotes the separation of photogenic electrons and holes.Therefore,more electrons are used for photocatalytic decomposition of aquatic hydrogen reaction.Through FT-IR and XPS characterization,it can be seen that NHPI and g-C3N4 have been successfully combined.XRD and TEM results showed that the structure and morphology of g-C3N4 were not changed by the introduction of NHPI.The results of PL,TRPL,EPR and electrochemical tests confirmed that the introduction of NHPI can indeed inhibit the photoelectron and hole recombination and prolong the photoelectron and hole recombination time.In the application of photocatalytic decomposition of aquatic hydrogen,under the optimal experimental conditions,when 2 wt%NHPI was introduced,the photocatalytic decomposition rate of aquatic hydrogen of g-C3N4/2 wt%NHPI(CN/2 wt%NHPI)composite catalyst reached the maximum(1145.4 ?mol h-1 g-1),which was 4.2 times higher than that of g-C3N4(274.0 ?mol h-1 g-1).The stability test of the catalyst showed that the photocatalytic activity of the CN/2 wt%NHPI composite catalyst did not decrease significantly under the long-term action,and the CN/NHPI composite catalyst has good stability and efficient photocatalytic performance.In general,a series of efficient and stable g-C3N4 matrix photocatalyst systems were constructed by modifying g-C3N4 with different molecules.The molecules introduced as redox agents can be used to transfer photogenic cavitation.By transferring g-C3N4 photogenic cavitation,the photogenic electron-cavitation pair composite time can be prolonged,and the photocatalytic activity of g-C3N4 matrix composite catalyst can be enhanced.