Ce-MOFs And Its Derived Catalysts For CO₂ Separation And Photocatalytic Reductions

The rapid development of society intensifies the consumption of non renewable energy.As a result,the concentration of greenhouse gases dominated by CO₂ rapidly increases,and many climate problems rise.Therefore,how to reduce the concentration of CO₂ in the air has become a hot topic in research fields.At present,the methods of reducing CO₂ concentration are divided into physical adsorption method and chemical conversion method.Physical adsorption method mainly uses porous materials to physically adsorb CO₂,or uses the large specific surface area and different pore diameters of porous materials to realize CO₂ selective adsorption,so as to complete CO₂-based gas separation.As a new kind of porous material,metal organic framework materials（MOFs）not only have large specific surface area and regular pore structure,but also can use different functional groups within organic ligands for functional modification to further improve the CO₂ adsorption and separation properties of the materials.Chemical conversion method is to directly convert CO₂ into high value-added gas products such as CO and CH₄ through chemical reactions.Among them,photocatalytic CO₂reduction is a chemical method with the mildest reaction conditions and in line with green and sustainable development.CO₂ green recycling can be realized by using solar energy and semiconductor catalyst.At present,a large number of literatures show that MOFs and their derivatives are excellent photocatalytic CO₂ reduction catalyst,and the derived metal oxides can form heterojunction structure with g-C₃N₄,which improves the photocatalytic performance.Although there are many studies on the use of MOFs and their derivatives in CO₂ separation and photocatalytic reduction,the types of organic ligand functional groups of MOFs,the types of MOFs derivatives and the methods for the combination with g-C₃N₄ still need to be deeply studied to further clarify the structure-activity relationship between the structure and properties of MOFs and their derivatives.Based on the above discussion,in this paper,we focus on Ce-MOFs and its derivatives.The specific work is as follows:1.Benzoic acid Ce-MOFs with different functional groups for CO₂/CH₄separationMOFs can be directly modified by the functional groups within organic ligands.Therefore,the study of regulating the types of ligand functional groups,and its effect on CO₂/CH₄ separation performance are significant for screening and designing MOFs based catalysts for CO₂ adsorption and separation.Therefore,we prepared four kinds of benzoic acid Ce-MOFs（Ce-BDC,NH₂-Ce-BDC,NO₂-Ce-BDC and Ce-BTC）with different functional groups by simple oil bath method to explore the effects of different functional groups on CO₂/CH₄ adsorption and separation.The CO₂ adsorption performance at different temperatures shows that when the specific surface area is close,the functional group modification can improve the CO₂ adsorption performance of MOFs,and the adsorption performance of-NH₂ is better than that of-COOH.All functional groups improve the CO₂/CH₄ separation coefficient.The adsorption heat value is obtained by Clausius Clapeyron equation,and the binding ability of functional groups to CO₂ follows:-NH₂>-NO₂>-COOH.However,the adsorption and separation performance of CO₂ is affected not only by functional groups,but also by the specific surface area of the material.Thanks to high adsorption heat and large specific surface area,NO₂-Ce-BDC has the highest CO₂ adsorption performance.TG showed that the thermal stability of Ce-MOFs prepared by changing ligands was good,and there was no skeleton collapse at 150℃.2.Preparation of CeO₂/g-C₃N₄ catalyst and its photocatalytic CO₂ reduction performanceCalcination of urea-impreganted MOFs is a convenient method to directly synthesize metal oxide/g-C₃N₄ composite catalysts,but there are few studies on the effects of type of ligands in calcined MOFs on catalytic photocatalysis.Therefore,in this work,we calcined two Ce-MOFs/g-C₃N₄ composites at different temperatures to study the effects of ligands on the properties of catalysts and photocatalytic CO₂ reduction.Specifically,we selected Ce-BTC and Ce-Ui O-66 for comparsions,and put the pre-prepared g-C₃N₄ into MOFs,followed by calcination at 270℃and 450℃,respectively,to obtain different catalysts.The photocatalytic performance test shows that the catalytic performance of the composite catalyst obtained from the calcination with Ce-Ui O-66 as precursor is better than that of Ce-BTC,because the band gap energy of the former is lower.We found that CeO₂ can promote the decomposition of g-C₃N₄,therefore,there is almost no g-C₃N₄ in the composites calcined at450℃,impressively,the catalytic performance is better than that calcined at 270℃.Various kinds of characterization demonstrate that the presence of terephthalic acid in Ce-Ui O-66 and pyromellitic acid（or their incompletely decomposed products）in Ce-BTC reduces the photogenerated electron hole separation efficiency and increases the band gap energy.Although g-C₃N₄ expands the light absorption edge of the material,the negative effect of phenylcarboxylic acid ligands is dominant,resulting in its catalytic performance is inferior to the catalyst calcined at 450℃.3.Preparation of hierarchical structured cerium oxide with rich oxygen vacancies and its photocatalytic CO₂ reduction performanceBecause Ce³⁺/Ce⁴⁺in ceria can be converted,ceria has rich oxygen vacancies.The theoretical calculation shows that the existence of oxygen vacancy can reduce the band gap energy of ceria and increase the light absorption range,which is often used in the synthesis of photocatalysts.Therefore,the preparation of hierarchical structured cerium oxide with rich oxygen vacancies is very beneficial to the ceria based composite catalyst and further improve the photocatalytic CO₂ reduction performance.In this work,we selected Ce-BTC as the template and impregnated Ce-BTC with urea.After calcination,ceria with porous,enriched oxygen vacancy and hierarchical structure was obtained.XRD and BET results showed that the crystal structure of ceria was not changed after urea impregnation,and the specific surface area did not decrease significantly.Raman and XPS spectra showed that the oxygen vacancy concentration of ceria after urea impregnation was significantly higher than that prepared in the absence of urea impregnation.The existence of oxygen vacancy reduces the band gap energy of ceria and improves the separation efficiency of photogenerated carriers as well.Porous and hierarchical structures improve the utilization efficiency of light,and have smaller electrochemical impedance,which is conducive to photogenerated electron transfer.