Synthesis Of Cu₂O-based Ternary Heterojunction Catalysts And Their Performances For The Photocatalytic Reduction Of CO₂

Energy and environmental issues have become two major themes of today’s world development.The rapid development of modern industrial economy and the extensive use of fossil fuels have greatly increased the concentration of carbon dioxide（CO₂）,a greenhouse gas in the atmosphere.The proposal of the goal of"carbon peaking and carbon neutrality"guides a new direction for the resource utilization of CO₂.Artificial photosynthesis（photocatalytic conversion）is a promising technology to convert CO₂ with H₂O into high value-added chemicals,and is considered as one of the effective ways to achieve carbon neutrality.It is still a great challenge to convert extremely stable CO₂molecules with C=O bonds(dissociation energy of 750 k J mol^-1)into other energy species by photoreduction,and the construction of efficient photocatalysts is the key to realize CO₂ reduction.Among the reported photocatalysts,Cu₂O shows attractive prospects in solar energy conversion and heterogeneous photocatalysis due to its narrow band gap and high photoelectric conversion efficiency.Based on the nature of the photocatalytic CO₂ reduction reaction,in this paper,Cu₂O nanocrystals with（111）exposed crystal planes are used as carriers,and Cu₂O is modified by supporting noble metal nanoparticles（Ag,Au）or composite graphene materials（rGO）and metal-organic framework（MOF）materials,which effectively enhanced the visible light absorption efficiency and the separation efficiency of photogenerated electron and hole pairs.The modified ternary heterojunction catalyst improves the enrichment degree of photogenerated electrons and the adsorption and activation ability of reactant CO₂,and improves the photocatalytic reduction of CO₂ activity and product CH₄ selectivity.The specific research contents are as follows:In this paper,the ternary catalyst of reduced graphene oxide（rGO）-coated Ag/Cu₂O-octahedron nanocrystals（Ag/Cu₂O@rGO）was successfully fabricated by method of water bath combining with gas-bubbling-assisted membrane reduction.Supported Ag nanoparticles with low fermi energy can enrich the photogenerated electrons originated from visible light-driven Cu₂O octahedral nanocrystals.The surface extendedπbond of coated rGO nanolayers on Ag/Cu₂O can further capture photoelectrons and improve adsorption-activation capacities for reactants.Ag_n/Cu₂O@rGO catalysts with ternary rGO-Ag-Cu₂O heterojunction exhibit excellent performance during selective photocatalytic CO₂ reduction with H₂O into CH₄.Ag4/Cu2O@rGO catalyst has the highest formation rate(82.6μmol g^-1 h^-1)and selectivity（95.4%）of CH₄ product.Combined with the results of in-situ DRIFT spectra and density functional theory calculations,the photocatalytic mechanism is proposed:the protonation of CO^*intermediate is key step for selective photocatalytic CO₂ reduction into CH₄.It provides one novel strategy to development of high-efficient photocatalyst for selective CO₂conversion into C1 chemicals.MOF materials with high-density metal nodes provide abundant catalytic active sites,and the excellent CO₂ adsorption performance also makes MOF materials attract much attention in the field of photocatalytic CO₂ reduction.In this paper,a novel ternary heterojunction Au/（Cu₂O@CuBTC）photocatalyst was successfully prepared by method of water bath combining with self-assembly method.Octahedral Cu₂O nanocrystals with（111）exposed crystal plane were first encapsulated in CuBTC nanocages to form Cu₂O@CuBTC core-shell heterostructures,and then Au NPs with uniform size were loaded on the Cu₂O@CuBTC core-shell structure to form Au/（Cu₂O@CuBTC）catalyst.Cu₂O nanocrystals with specific shapes and crystallographic planes can provide Cusources for the construction of CuBTC.CuBTC can not only enhance the adsorption and activation of the reactant Cu₂O,but also act as an active component to stabilize the reaction intermediate,providing favorable conditions for the CO₂ reduction reaction.The supported Au NPs are crucial for enriching photoelectrons from visible light excitation,which is beneficial to improve the separation efficiency of photogenerated electron-hole pairs.The designed Au/（Cu₂O@CuBTC）ternary heterostructure exhibits excellent performance in the selective photocatalytic reduction of CO₂ to CH₄ with H₂O.Among the prepared catalysts,the Au₂/（Cu₂O@CuBTC）catalyst has the highest photoactivity,and the CH₄ yield reaches 46.3μmol g^-1 h^-1,which is 9.4 times higher than that of the Cu₂O catalyst,and the CH₄ selectivity is increased from 58.3%of Cu₂O to 96.6%.The unique directional charge transfer mechanism of the ternary heterostructure provides an extremely effective strategy for enhancing the photocatalytic performance.In conclusion,the Cu₂O-based ternary heterostructure catalyst designed and synthesized in this paper can effectively improve the absorption efficiency of visible light,the separation efficiency of photogenerated electron and hole pairs,and the adsorption and activation ability of the reactant CO₂.The selectivity of CH₄ product can be controlled by these photocatalysts.In-depth exploration of the reaction mechanism and electron transfer mechanism provides a strong theoretical basis and practical guidance for the further design of new high-performance catalysts.