Preparation And Mechanism Of Carbon Nitride Photocatalysts For Reforming Lignocellulose Into Hydrogen

The current rapid development of economic and social relies heavily on fossil resources.In view of the energy and environmental problems caused by the continuous consumption of fossil energy,it is urgent to develop sustainable green energy,such as solar energy,biomass energy,hydrogen energy and other renewable energy.Hydrogen（H₂）is a clean and efficient energy carrier with high combustion calorific value and only water in combustion products,which is expected to be a clean and efficient energy carrier candidate.Solar-driven lignocellulose photocatalytic hydrogen production is a comprehensive utilization of solar energy and biomass energy.It is a new method for producing renewable hydrogen energy and regarded as an important way to solve energy and environmental problems.The photocatalytic materials with high activity are the key bottleneck for restricting the photocatalytic hydrogen production from lignocellulose.Polymer semiconductor graphite carbon nitride（g-C₃N₄）has good interaction with lignocellulose,and possess the advantages of visible light response,simple preparation,good chemical stability and suitable energy band.However,it is difficult to separate photogenerated electrons and holes,which seriously restricts its photocatalytic hydrogen production activity.In this thesis,aiming at the problem that the poor photogenerated electron-hole separation ability of g-C₃N₄ to further limit its photocatalytic activity,the monolayer g-C₃N₄,gradient boron-doped g-C₃N₄,Ce P loaded electron-rich P doped g-C₃N₄ and single-atom Co-g-C₃N₄ were designed respectively to explore the influence of exciton binding energy,gradient built-in electric field,electron-rich element doping and single-atom effect on the photogenerated electron-hole separation efficiency of g-C₃N₄,respectively.Furthermore,the internal relationship between its structure and the hydrogen production performance of photo-catalytic reforming lignocellulose was also clarified.Thus,the regulation and optimization of the hydrogen production performance of photo-catalytic reforming lignocellulose of g-C₃N₄were realized.（1）Monolayer g-C₃N₄（Mono-C₃N₄）was prepared by stripping bulk g-C₃N₄（Bulk-C₃N₄）using nitrogen-protected aqueous ball milling.Combined with morphology and structure characterization（i.e.,AFM,FTIR.etc）,it showed that the thickness of g-C₃N₄ obtained after ball milling was determined to be 0.32 nm with a typical atomic layer,and its structure had no obvious change from that of Bulk-C₃N₄.Due to the elimination of interlayer interaction,the exciton binding energy（26.5 me V）of Mono-C₃N₄ is reduced to half of that of Bulk-C₃N₄（52.8me V）,resulting in weakened Coulomb force to improve the separation efficiency of photogenerated electrons and holes.Therefore,the hydrogen production rate of photocatalytic reforming lignocellulose for Mono-C₃N₄ increases to be 73μmol g^-1 h^-1,which is 12 times than that of Bulk-C₃N₄.The reaction path of Mono-C₃N₄ for photocatalytic cellulose reorganization is followed:the cellulose is hydrolyzed to glucose,and then isomerized and rearranged to isosaccharinic acid under alkaline and oxidation conditions.Finally,the isosaccharinic acid was decarboxylated and oxidized by photogenerated holes to 2,4,5-trihydroxylpentanoic acid.At the same time,the H₂ was produced by the reduction of H₂O by photogenerated electrons.（2）Gradient boron doped g-C₃N₄ with different boron concentrations from surface to bulk was prepared by post-heat treatment.Combined with experiments and DFT theoretical calculations,it is proved that electron-deficient boron replaces the carbon in the structure of g-C₃N₄,resulting in the gradual transformation of g-C₃N₄ from n-type to p-type semiconductor.The continuous heterogeneous interface is formed in gradient boron doped g-C₃N₄,resulting in built-in electric field from n-type to p-type semiconductor to increase of the separation efficiency of photogenerated electrons-holes.The gradient boron doped B-CN-3 sample possess the best hydrogen production activity of photocatalytic reforming cellulose,which is determined to be 65μmol g^-1 h^-1,which was 10.8 times higher than that of pure g-C₃N₄.The hydrogen production rate of photocatalytic reforming sodium lignosulfonate was 261μmol g^-1h^-1.Due to the formation of built-in electric field,the oxidation ability of photogenerated holes in gradient boron doped g-C₃N₄ was weakened,and cellulose was only oxidized to gluconic acid after hydrolysis into glucose under alkaline conditions.（3）The Ce P-loaded P-doped graphitic carbon nitride（PCe-CN）was successfully prepared by electron-rich phosphorus doped g-C₃N₄（P-CN）and subsequently modified by cerium.The related results revealed that the P atom replaces the position of C atom in the triazine ring,which reduces the band gap of g-C₃N₄,broadens the visible light absorption range and enhances the absorption intensity in the visible light range.Ce was introduced to combine with part of P in the structure of g-C₃N₄ to form Ce P.The formation of electron bridge bond P-Ce improved the conductivity of the catalyst,promoted the separation of photogenerated electrons and holes,and further enhanced the photocatalytic activity.The hydrogen production rate of photocatalytic reforming sodium lignosulfonate for P₄Ce₄-CN was determined to be 467μmol g^-1 h^-1,which was 2.3 and 5.4 times than that of P₄-CN and pure g-C₃N₄,respectively.The reaction path of P₄Ce₄-CN photocatalytic reforming sodium lignosulfonate to hydrogen is that sodium lignosulfonate is desulfonated and oxidized to various carboxylic acids by photogenerated holes.（4）Single atom Co/g-C₃N₄ photocatalyst was prepared by anchoring Co²⁺with electron-rich N in the triazine ring of g-C₃N₄.Combined with the synchrotron radiation characterization and DFT calculation,it proved that Co was coordinated with four N atoms to form Co N₄ in the single-atom Co/g-C₃N₄ structure.The doping level was also formed in the energy band structure of g-C₃N₄,which enhanced the redox performance of the catalyst.The fluorescence spectra and photocurrent tests showed that the single-atom Co/g-C₃N₄ catalyst possess higher photogenerated electron-hole separation efficiency,higher photogenerated electron concentration and longer lifetime.The hydrogen production rate of photocatalytic reforming sodium lignosulfonate for single-atom Co/g-C₃N₄ was 631μmol g^-1 h^-1,which was7.3 times than that of pure g-C₃N₄.Sodium lignosulfonate removes one sulfonic acid radical and reacts with photogenerated holes to form carboxylic acids with smaller molecular weight through oxidation and ring opening reactions.In this paper,the exciton binding energy is reduced by reducing the thickness of the sheet,the continuous built-in electric field is constructed by B gradient doping,the P-Ce electronic bridge is constructed in situ by electron-rich P,and the single atom is prepared by N-anchored metal Co to tune the structure of g-C₃N₄.The above strategies can improve the photogenerated electron-hole separation efficiency and enhance photocatalytic reformation of lignocellulose for hydrogen production for g-C₃N₄.The research results provide reference and theoretical guidance for the development of high-activity reformed lignocellulose photocatalysts for hydrogen production.