Preparation Of MPc/g-C₃N₄ Nano-Heterojunction Visible-Light Photocatalytic Materials And Mechanism Of Activity Enhancement

Photocatalytic technology for selective oxidation of alcohols can product a series of important chemical products,especially using oxygen as oxidant to achieve efficient selective oxidation of alcohols is a kind of chemical synthesis path with mild conditions and friendly environment,which is of great significance to solve the energy and environmental crisis.Therefore,it is necessary to develop efficient photocatalytic materials to promote alcohol oxidation conversion.The polymer graphite carbon nitride（g-C₃N₄）has been widely studied and applied for its advantages of non-toxicity,no secondary pollution,suitable energy band structure,visible light response,stable properties and low cost.However,the photocatalytic efficiency of g-C₃N₄ is significantly reduced due to poor charge separation,narrow absorption range of visible light and lack of activation sites.Metal phthalocyanine（MPc）is a kind of two-dimensional planar polymer molecule with similar structure to carbon nitride.It has LUMO and HOMO positions matching with carbon nitride band and absorbs visible light at 550-800 nm,which can establish effective heterojunction system with g-C₃N₄.Based on this,this paper carried out research on the heterojunction composite photocatalytic system of organic molecular semiconductor MPc and g-C₃N₄,mainly using the characteristics of MPc semiconductor to improve the charge separation of g-C₃N₄,expand the visible light range,and provide catalytic activity center,and then improve the visible light catalytic conversion performance of benzyl alcohol oxidation.The mechanism of charge separation and performance improvement is also revealed.It mainly includes the following three parts:（1）The ultrathin CoPc/P-CN nanocomposites were synthesized by phosphoric acid induced CoPc assembly design,and the mechanism of improving the photocatalytic oxidation of benzyl alcohol was explored.Through theoretical calculations and experiments,it is proved that CoPc is assembled on the surface of g-C₃N₄ by H-bonding induced by phosphate groups to achieve the load of highly dispersed CoPc.The heterojunction system showed excellent photocatalytic performance in the selective oxidation of benzyl alcohol,which was mainly attributed to the existence of high energy electron transfer mechanism between g-C₃N₄ and CoPc,which improved the charge separation of g-C₃N₄.Namely,under the condition of photoexcitation,high energy electrons of g-C₃N₄ transferred to CoPc ligand through phosphate group.The electrons of CoPc ligand are then transferred to the CoPc central metal,and a large number of single exposed Co-N₄（II）sites activate O₂ to form the main species^·O₂^-for oxidation reaction,achieving efficient selective oxidation of benzyl alcohol.（2）Ultrathin CoPc/NG/CN nanocomposites were synthesized by N-doped graphene induced CoPc assembly,and the photocatalytic oxidation mechanism of benzyl alcohol was investigated.N-doped graphene with high nitrogen content was prepared by high-temperature hydrothermal method,and CoPc/NG/CN nanocomposites were prepared by two-step hydroxy-induced assembly.The results showed that the excellent photoactivity was attributed to the increased CoPc loading and induced charge transfer by the introduction of N-doped graphene,which enhanced the interfacial charge transfer and separation of CoPc/g-C₃N₄heterojunction.In addition,the highly dispersed CoPc molecule increases the visible light absorption unit and exposes the monatomic Co²⁺site as a catalytic center for O₂activation,further facilitating photocatalytic oxidation and efficient conversion of benzyl alcohol.（3）Ultrathin boron-doped g-C₃N₄ were prepared by one-step calcination with sodium borohydride and g-C₃N₄ nanosheets.Further,ultrathin Fe Pc/BCN nanocomposites with wide spectral response and rich catalytic active centers were constructed with Fe Pc,and the enhancement mechanism of the photocatalytic oxidation of benzyl alcohol was explored.The results show that the Fe Pc/BCN nano-heterojunction system achieves efficient Z-scheme charge transfer mode,which greatly promotes charge transfer and separation.Moreover,the electrons on the LUMO of Fe Pc are further transferred to the central metal Fe²⁺to form the main species^·O₂^-for oxidation reaction,and achieve efficient oxidation performance of benzyl alcohol.In addition,the Z-scheme charge transfer mechanism can be extended to different MPc/BCN systems.Experiments show that Fe Pc and BCN have the best Z-scheme composite system,charge separation is more effectively,show better oxygen activation ability,and have better oxidation performance of benzyl alcohol.By comparing the MPc/CN heterojunction,the Z-scheme charge transfer mode is more favorable for efficient charge separation.