Support Effect And Catalytic Mechanism Of Visible-light-driven Oxidative Homocoupling Of Alkynes Over Supported Copper Nanopartical Catalysts

Energy-saving and environment-friendly visible light-induced photocatalysis has promising application prospects to replace traditional thermocatalysis,which suffers high energy consumption and high pollution.Copper-catalyzed oxidative homocoupling of alkynes（OHA）is a classical thermocatalytic reaction,but there are some problems,such as high reaction temperature,the need for alkaline additives or ligands,and the controversy of Cu⁺/Cu²⁺active species and so on.Recently,metallic copper nanoparticles（Cu NPs）have been found to have catalytic activity for visible light-induced OHA reactions,but how to maintain the metallic state of Cu NPs in an oxidizing atmosphere to obtain stable local surface plasmon resonance（LSPR）effect and photocatalytic activity is a major challenge.In this paper,different series of supported Cu NP photocatalysts were prepared by using light-responsive semiconductor materials and Cu-containing oxides with three-dimensional macroporous（3DOM）as supports.The effects of different supports on the structure,photocatalytic OHA activity and oxidation stability of supported Cu NP photocatalysts were systematically studied,and the photocatalytic OHA mechanism was revealed by combining kinetic study and free radical trapping experiments.The main results are as follows:1.The semiconductor-supported Cu NP catalysts exhibit different photocatalytic OHA activities,which decrease in the order Cu/Ti O₂>Cu/Zn O>Cu/BN>Cu/Ti N>Cu/Si C>Cu/C₃N₄.The Cu/Ti O₂ catalyst achieved the highest 1,3-diyne yield（93%）after 3 h reaction.However,due to the weak metal-support interactions,the active Cu NPs are easily oxidized by air and detached from supports.Relatively,the Cu/Zn O catalyst shows better stability,and the yield of diyne can still reach 62%after 4 cycles.2.The support-confined Cu NP catalysts derived from Cu Al₂O₄ spinel with 3DOM structure were prepared by macroporous template method,which showed high photocatalytic activity.The optimal 3DOM-Cu Fe₂O₄-supported Cu NP catalyst（Cu Fe₂O₄-3D-210）exhibited more stable photocatalytic performance.After simple recovery by a magnet,the catalyst can still achieve 72%diyne yield after five cycles without H₂ treatment,which is significantly superior to the previously reported Cu Fe₂O₄-SG-250 catalyst.The excellent photocatalytic performance can be attributed to the unique pore structure of the support,the enhanced basicity and the strong interaction between Cu NPs and the 3DOM support.3.Kinetic analysis shows that the deprotonation of alkynes is not a rate-limiting step,and the basic site on the surface of Cu Fe₂O₄-3D-210 catalyst promotes the cleavage of C-H bond at the end of alkynes to form active Cu⁺-phenylacetylene intermediate.Mechanism research shows that both O₂^·-and alkynyl radicals participate in the photocatalytic and thermocatalytic catalyzed OHA reaction,and O₂^·-species with basicity and oxidation capability play a key role in the reaction.The activation of O₂ to produce highly active O₂^·-species induced by LSPR effect of metallic Cu NPs under visible light is more efficient and energy-saving than the thermocatalytic O₂ activation on Cu⁺sites at 120 ^oC.