Preparation And Modification Of ZnFe₂O₄ Spinel And Its Performance For Photocatalytic Reduction Of CO₂ With Water Vapor

Four ferrite catalysts of AFe₂O₄?A=Zn,Cu,Co and Ni?were successfully prepared by sol-gel method,and characterized by scanning electron microscope?SEM?,transmission electron microscope?TEM?,X-ray diffraction?XRD?,UV–Vis diffusion reflectance spectroscopy?UV-Vis?and photocurrent test to investigate the structure,morphology,optical and electronical properties,the separation efficiency of photo-generated carriers.Also,the photocatalytic activities over these photocatalysts for photocatalytic reduction of CO₂ under visible light irradiation were studied.The experimental results showed that the morphologies of the catalysts are nanoparticles.Compared with other three ferrites,ZnFe₂O₄ has larger surface area,better light absorption and higher photoinduce carriers separation efficiency.Under visible light irradiation,all four catalysts have the ability of reducing CO₂,and ZnFe₂O₄ displays the highest photocatalytic activity for reduction of CO₂ with H₂O to H₂,CO and CH₄,whereas CoFe₂O₄ has the lowest photocatalytic performance.After 4h illumination,the yield of H₂,CO and CH₄ over ZnFe₂O₄ were 4.3?mol/g/h,0.40?mol/g/h and2.35?mol/g/h,respectively.In contrast,CoFe₂O₄ shows inferior photocatalytic activity due to smaller surface area and weaker absorption of visible light.2% ZnO/ZnFe₂O₄,5% ZnO/ZnFe₂O₄,10% ZnO/ZnFe₂O₄ and 20%ZnO/ZnFe₂O₄composites were prepared by changing the amount of zinc nitrate in the preparation process.The results showed that the loading of ZnO plays an important role in improving the photocatalytic activity of ZnFe₂O₄.The introduction of ZnO can obviously enhance the light absorption of ZnFe₂O₄ and improve the separation efficiency of photogenerated hole-electron pairs.With the increase amount of the ZnO,the photocatalytic activity of the composites first increased and then decreased,and5%ZnO/ZnFe₂O₄ nanoparticles exhibits the highest activity for photoreduction of CO₂ and the yields of CO and CH₄ could reach 1.38?mol/g/h and 3.19?mol/g/h.This may be mainly related to the suitable band structure and high photocarrier separation efficiency over composite materials.The Ce-doped ZnFe₂O₄ photocatalysts were prepared by sol-gel method,and characterized by TEM,SEM,XRD,PL,CO₂-TPD and in-situ infrared.The catalytic activities of catalysts were evaluated by the activity test.It was found that part of the Ce element is doped into the lattice of ZnFe₂O₄ in the form of Ce³⁺,and the other Ce element is carried on the surface of the ZnFe₂O₄ by CeO₂,and formed p-n heterostructures with ZnFe₂O₄.The introduction of Ce element increased the specific surface area of the catalyst,the ability of responsing to visible light and adsorbing CO₂,resulting in more m-CO₃^2-,b-CO₃^2-and b-HCO₃^-surface species,which was active species for photoreduction of CO₂ to CO and CH₄.Among all samples,the catalyst with Ce/?Ce+Fe?=0.2 has the highest photocatalytic activity.The yields of H₂,CO and CH₄ reached 19.9,1.7 and 9.9?mol/g_cat/h,respectively,which were 4.6,4.3and 4.2 times higher than those of pure ZnFe₂O₄.