Photocatalytic CO₂ Reduction With MOFs And Inorganic Nanocomposites

In recent decades,the use of fossil fuels has led to excessive CO₂emissions,resulting in environmental and ecological problems such as global temperature rise,sea level rise and desert expansion,which have attracted increasing attention worldwide.As the main product of fossil fuel combustion,CO₂is now widely recognized as a major source of greenhouse gases,and therefore,it is urgent to develop new technologies for CO₂capture and conversion.At the present stage,photocatalysis is one of the promising new green technologies that can convert greenhouse gases such as CO₂into high value-added chemicals and fuels by solar energy,which can help solve the problems of fossil fuel shortage and global environmental pollution.However,this reaction of photocatalytic reduction of CO₂involves complex multiple electron transfer pathways,resulting in numerous products,such as carbon monoxide,methanol,methane,etc.How to construct a reasonable photocatalytic system to obtain highly selective and high value-added fuels has become a great challenge within this field.So far,researchers have explored some typical construction strategies,including nanostructuring,heterostructure building,incorporation of suitable co-catalysts,modulation of energy band structure,and modulation of surface morphology structure to enhance the photocatalytic CO₂conversion properties.Currently,there are solid-gas mode and liquid-gas mode for photocatalytic CO₂conversion.Among them,the solid gas mode is to disperse the photocatalyst uniformly and then CO₂and water vapor directly contact with the catalyst for the photocatalytic conversion process,which is more in line with the concept of green chemistry without organic solvents,sacrificial agents,etc.We conduct photocatalytic CO₂reduction research in solid-gas mode,and the specific work is divided into three parts as follows:（1）Cu₂O is a p-type semiconductor with a forbidden band width of about 2.10 e V.It is favored for its suitable band gap and excellent light absorption ability.However,the photocorrosion and fast electron-hole multiplexing properties of Cu₂O limit its potential applications.The catalytic performance of Cu₂O was improved by combining Cu₂O with other materials to reduce the electron-hole complexation,thus producing higher visible light photocatalytic activation.We chose Cu₂O as the main catalyst and synthesized the composite Cu₂O/MOF-74-x（x=3h,6h,12h,18h,24h,36h）in situ by a simple solvothermal method to evaluate the photocatalytic CO₂reduction performance.After 4 h of light irradiation,the Cu₂O/MOF-74-24h composite reduced CO₂to CO with a yield of up to 10.61μmol g^-1and a selectivity of about 83%,which is about 10 times that of a single Cu₂O.the introduction of Cu₂O broadened the light absorption range of the composite.MOF-74 transferred some of the photogenerated electrons of Cu₂O,which effectively enhanced the carrier migration efficiency and inhibit its compounding.（2）The Cd S is sufficient to drive the CO generation reaction due to its good visible light absorption ability and suitable energy band structure.However,the rapid photoinduced electron-hole complexation of Cd S and the limited surface catalytic sites limit the application of pure Cd S particles.We constructed a series of heterojunction photocatalysts xwt%Cd S/Ui O-67.systematic studies showed that 16wt%Cd S/Ui O-67 significantly promoted photocatalytic CO₂reduction with a product CO yield of 24.26μmol g^-1,much higher than that of Cd S and Ui O-67.this work provides an opportunity to introduce Cd S nanoparticles on the surface of MOFs to achieve excellent CO₂photoreduction performance provides a new idea.（3）Mixed-metal MOFs combining two different metals into the same backbone are considered as a promising strategy to improve the photocatalytic performance of MOFs-based photocatalysts.In mixed-metal MOFs photocatalysts,the oxygen-bridged heterogeneous metal assemblies can not only collect solar energy but also improve photocatalytic performance by effectively separating photogenerated charges through metal-to-metal charge transfer.Therefore,the performance of photocatalytic reactions can be improved by partially replacing suitable metal ions as electron mediators in the nodes.In this paper,Fe（III）-doped NH₂-MIL-68（In）with different In/Fe ratios were prepared by a one-pot method,which improved the photocatalytic performance compared with monometallic NH₂-MIL-68（In）.（4）The sulfide itself has a narrow band gap and a high visible light absorption capacity,and it is easy and low cost to synthesize,environmentally friendly and in line with the concept of green development.The composite In₂S₃/Cd S photocatalyst was synthesized by a two-step solvothermal method and applied to the CO₂photocatalytic reduction reaction.Through perfect characterization tests,it was demonstrated that Cd S can be used as a very effective co-catalyst to enhance the photocatalytic activity of the composite In₂S₃/Cd S photocatalyst.15wt%In₂S₃/Cd S composite had the highest photocatalytic activity for CO₂reduction with a CH₄yield of 25.90μmol g^-1and a selectivity of 87%,which was about 43 times higher than that of single Cd S.