Synthesis And Study Of Graphitic Carbon Nitride（g-C₃N₄）-based Photocatalysts For Photoreforming Of Lignocellulose Into Hydrogen And Value-added Chemicals

With the continuous reduction of fossil resources and the increasingly severe environmental problems,the use of renewable resources to supply energy and chemicals has been paid more and more attention.Photoreforming lignocellulose is a promising technology for obtaining hydrogen energy and value-added chemicals.The key to this technology is to find photocatalysts that are non-toxic,easy to synthesize,inexpensive and have visible light or even longer wavelength response.Graphitic phase carbon nitride（g-C₃N₄）has attracted much attention due to its unique structure and properties.However,the photocatalytic activity of the pure g-C₃N₄is greatly inhibited due to the difficulty in separating photogenerated charge and the poor utilization rate of photoquantum.In order to optimize the performance of g-C₃N₄and further improve its photocatalytic activity,this academic dissertation adopted a variety of methods to modify g-C₃N₄,prepared three different g-C₃N₄-based photocatalysts,and used them in photoreforming of lignocellulose into hydrogen and value-added chemicals.Specific research contents are listed as following:1.The Na-doped,cyano-functionalized g-C₃N₄-based photocatalyst CN-Na_xwas successfully prepared by thermal condensation of dimelamine precursor and Na I through a facile molten salt procedure.The morphology,chemical structure,elemental composition,light absorption and energy band position of the photocatalyst were studied by various characterization methods.The H₂production activity of the photoreformingα-cellulose was also investigated.The photocatalytic activity test of CN-Na_xshowed that the H₂yield of the optimal catalyst CN-Na₆reached 8.93μmol in 12 h,which was about 14 times that corresponding to the pure g-C₃N₄（0.61μmol）.It also produced value-added chemicals such as mannose,rhamnose,glucuronic acid,galacturonic acid,glucose and arabinose.Based on the above experimental results,a mechanism for enhancing the activity of photoreformingα-cellulose to generate H₂and value-added chemicals using photocatalyst CN-Na₆was proposed:Na⁺adsorbed hydroxyl-OH,making CN-Na_xmore hydrophilic.As the active site,cyano-C≡N further enhanced the separation and migration rate of photogenerated carriers.As a sacrificial substance,α-cellulose consumed h⁺to produce more e^-,reduces H⁺to produce more H₂,and greatly improves the H₂production activity;ESR test results showed that h⁺and·O₂^-are active substances in the photocatalytic oxidation ofα-cellulose.2.The 2D/2D Z-type heterojunction CN-B_xof g-C₃N₄-based two-dimensional nanosheets,which were co-doped with B and Na and functionalized with cyanide group,was successfully constructed by electrostatic self-assembly strategy.The morphology,chemical structure,element composition,light absorption and energy band positions of the composite photocatalyst were systematically studied by a series of characterization methods,and the H₂production activity of the photoreforming lignocellulose was also investigated.The results showed that the H₂yield of CN-B_2:2was 15.78μmol in 12 h,about 20 times that of pure g-C₃N₄（0.78μmol）,and produced value-added chemicals such as mannose,ribose,rhamnose,glucuronic acid,galacturonic acid,glucose,xylose and arabinose.Based on the above experimental results,a mechanism for enhancing the activity of photoreformingα-cellulose to generate H₂and value-added chemicals using photocatalyst CN-B_2:2was proposed:B atoms replaced partial C atoms in g-C₃N₄to form an electrophilic center,which accelerated the photogenerated e^--h⁺pair separation.As the active site,cyano-C≡N and Z-type heterostructure further improved the carrier separation and migration rate.α-cellulose,as a sacrificant,consumed h⁺to produce more e^-,thus reducing H⁺to produce more H₂,which greatly improved the H₂production activity.The results of ESR test suggested that·OH,·O₂^-and h⁺are active substances in the photocatalytic oxidation ofα-cellulose.3.The in-plane g-C₃N₄-based heterojunction CN-C_xwas successfully synthesized by thermal conjugation strategy.The morphology and microstructure of the synthesized CN-C_0.40photocatalyst were analyzed by SEM and TEM image characterization methods.The photoabsorption,energy band position and photoelectric properties of the photocatalysts were systematically studied by various characterization techniques.The H₂generation activity of photoreforming lignocellulose was also examined.The results showed that when the best photocatalyst CN-C_0.40was underλ>630 nm illumination and without any cocatalyst,the H₂production ofα-cellulose was the highest after 24 h,and the H₂production was 48.80μmol.And some value-added chemicals such as mannose,glucuronic acid,cellobiose,galacturonic acid,galactose,arabinose,lactic acid,formic acid and5-hydroxymethylfurfural are produced,and the conversion rate ofα-cellulose is at least 26%.Based on the above characterizations,a mechanism for enhancing the activity of photoreforming lignocellulose to generate H₂and value-added chemicals using photocatalyst CN-C_0.40was proposed:The introduction of carbon rings made CN-C_xhad stronger light-harvesting ability.At the same time,an internal electric field with heptazine edges was formed,which effectively improved the separation and migration efficiency of photogenerated carriers.As a sacrificial substance,lignocellulose consumed h⁺to generate more e^-,reduces H⁺to generate more H₂,and greatly improved the H₂production activity;ESR test results showed that·OH,·O₂^-and h⁺are active substances in the photocatalytic oxidation of lignocellulose.