Preparation And Photoreduction Of CO₂ Based On Cerium-based MOF Composite Photocatalysts

Photocatalytic reduction of CO₂technology can convert CO₂into high value-added fuels(such as methane,methanol and formic acid)driven by sunlight.At the same time,it has the unique advantages of no secondary pollution and simple operation,which provides a new idea to alleviate the pressure of CO₂emission.The key to realize this technology is to find efficient photocatalysts.As a rapidly developing new material,MOFs materials have attracted extensive attention in the field of catalysis due to their large specific surface area,adjustable structure and unique photoelectric properties.Due to the variable valence state of Ce atom,the Ce-MOF material constructed with cerium metal salt as precursor has more abundant oxygen vacancies.In this paper,the preparation and modification of Ce-MOF based photocatalyst were carried out with the goal of high-efficiency photocatalytic reduction of CO₂,and the formation mechanism of modified Ce-MOF based photocatalyst and the reaction mechanism of photoreduction of CO₂were studied.(1)Bi₃Nb O₇/Ce-MOF composite photocatalyst was prepared by solid-state synthesis,and the effects of different Bi₃Nb O₇loading on the performance of photoreduction of CO₂were investigated.The results show that Ce elements are evenly distributed on the surface of Bi₃Nb O₇nano layer,the interface between Ce-MOF and Bi₃Nb O₇is in close contact,and the type-I heterostructure is successfully constructed based on electronic coupling.Through the electrical impedance(EIS)and photocurrent(PC)test results,it is found that the photogenerated carrier separation effect of the composite photocatalyst system is significantly greater than that of pure Ce-MOF,and the composite catalyst has the best carrier separation efficiency when the load of Bi₃Nb O₇is 36 wt%,so the catalyst at this ratio has the highest photocatalytic reduction performance of CO₂.Under the irradiation of simulated sunlight,the maximum yield of methane and methanol can reach 143.79?mol·h^-1·g^-1and 112.11?mol·h^-1·g-1.(2)In order to further improve the photoreduction performance of cerium-based MOF composites,Bi₂Mo O₆/Ce-MOF composite photocatalysts were prepared by solid-state synthesis.The performance test results show that the composite shows significantly enhanced photocatalytic reduction ability,which is due to the electronic coupling between Ce atom and Bi atom and Mo atom in BMO,under which the two materials have strong interface contact.In addition,the separation and transfer mechanism of photogenerated carriers in the composite system follows the system mechanism of type-II heterojunction,which has more free electrons and holes,so it can improve the photoreduction performance.The results of in-situ DRIFTS show that compared with pure Ce MOF and pure Bi₂Mo O₆,the interaction between photogenerated electrons on the surface of the composite and CO₂reduction intermediates is inhibited during the reaction,resulting in the formation of formic acid on the composite.In Bi₂Mo O₆/Ce-MOF composite photocatalyst,when the loading amount of Bi₂Mo O₆is 36 wt%,the performance of photoreduction of CO₂is the best,and the yields of methane,methanol and formic acid are as high as 113.87?mol·h^-1·g^-1?40.59?mol·h^-1·g^-1and 73.48?mol·h^-1·g^-1,respectively.(3)In order to promote the industrial application of photoreduction CO₂,magnetically recoverable Bi Fe O₃was compounded with cerium-based MOF,and Bi Fe O₃/Ce-MOF composite photocatalyst was prepared by in-situ synthesis.It is found that there is an electronic coupling between Bi Fe O₃and Ce-MOF group interface,and under this effect,a heterostructure is formed.Under this structure,the high specific surface area of Ce-MOF and the narrow band gap characteristics of Bi Fe O₃are fully utilized,and the light absorption edge of the material is expanded from 403 nm to 710 nm,which stimulates greater potential for subsequent photoreduction reaction.In addition,the magnetic recovery test shows that compared with the traditional centrifugal washing recovery method,the recovery rate of Bi Fe O₃/Ce-MOF composite can be as high as 95.4%.At the same time,the activity test shows that it also has excellent photocatalytic reduction ability of CO₂.When the mass ratio of Bi Fe O₃to Ce-MOF is 4.7:1,the catalyst has the highest photocatalytic reduction performance of CO₂,and the yields of methane and methanol are 126.77?mol·h^-1·g^-1and 90.40?mol·h^-1·g^-1,respectively.