Preparation And Photocatalytic CO₂ Reduction Performance Of ZIF-67-derived Cobalt-based Composites

In recent years,the synthesis of catalytic materials using metal-organic framework（MOF）as precursor has attracted more and more researchers’attention.Due to their controllable size,morphology,high specific surface area and porous structure,these materials have been extensively studied in the field of photocatalysis such as carbon dioxide reduction.Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and specific surface area test were used to test the microstructure of the materials.The composition of the catalysts was analyzed by X-ray photoelectron spectroscopy（XPS）and X-ray diffraction（XRD）.Meanwhile,the photocatalytic performance can be improved by constructing heterostructures on the basis of single MOF-derived materials through cation exchange and other methods.In this study,a variety of cobalt-based derivatives composite catalysts were prepared using ZIF-67 as the precursor system,and the photocatalytic reduction performance of carbon dioxide in gas phase was studied.The specific research contents are summarized as follows:1.Construction of Co_0.85Se/Mo Se₂ hollow polyhedron heterostructure composites.Using cobalt nitrate and dimethyl imidazole as reactants,ZIF-67 material with dodecahedral shape was prepared by simple stirring aging.The hollow dodecahedral Co_0.85Se material was synthesized by selenifying ZIF-67 by solvothermal method,and the Co_0.85Se/Mo Se₂ heterostructure composite was synthesized by loading Mo Se₂nanosheets on the surface of the Co_0.85Se matrix by solvothermal method.According to the photocatalytic performance,the loading capacity of the surface Mo Se₂ nanosheets was optimized and adjusted.The results show that the CO₂ reduction ability of Co_0.85Se/Mo Se₂ composite photocatalyst is increased by adjusting the loading amount of Mo Se₂ nanosheets on the surface.Compared with single Co_0.85Se,Co_0.85Se/Mo Se₂heterojunction structure is more conducive to the effective transfer and separation of photogenerated electrons and holes,thus significantly improving the performance of CO₂ photocatalytic reduction.At the same time,the photocatalytic activity was also affected by the amount of Mo Se₂.The maximum CO yield of Co_0.85Se/Mo Se₂composite reached 6.65μmol·g^-1·h^-1,and the yield of CH₄ reached 0.19μmol·g^-1·h^-1.2.The synthesis of Co_0.85Se-Cd Se/Mo Se₂/Cd Se sandwich composite photocatalytic material.The synthesized Co_0.85Se/Mo Se₂ composite was cationically exchanged with Cd²⁺by solvothermal route.By controlling the cation exchange ratio,partial and complete substitution of Cd²⁺can be realized,and Co_0.85Se-Cd Se/Mo Se/Cd Se sandwich structure composites and Cd Se/Mo Se/Cd Se composite photocatalysts can be prepared in a controllable manner.The results show that Co_0.85Se-Cd Se/Mo Se/Cd Se sandwich composite photocatalyst has the best CO₂ reduction ability.The maximum yields of CO and CH₄ of Co_0.85Se-Cd Se/Mo Se₂/Cd Se sandwich composites are 15.04μmol·g^-1·h^-1and 0.41μmol·g^-1·h^-1,respectively.The improved photocatalytic activity is mainly attributed to the sandwich heterostructure which is more favorable to the separation and transport of photoelectric charge.3.The synthesis of Co-Ni@N-CNT@C₃N₄ sandwich composite photocatalytic material.The C₃N₄ precursor matrix was first prepared by the hydrothermal method using melamine and solid phosphite as the reactants,and then the C₃N₄ nanosheets are prepared by high-temperature calcination method in air.The Ni doped ZIF-67nanosheets were then grown on the both sides of C₃N₄ nanosheets.The subsequent thermal treatment under H₂/Ar mixed gas leads to the formation of Co-Ni@N-CNT@C₃N₄.The special sandwiched structure can provide high visible light,accelerated charge separation,and increased active sites.After 4 hours of light,the CO and CH₄ yields of the Co-Ni（9:1）@N-CNT@C₃N₄ composite were 20.7μmol·g^-1·h^-1and 0.52μmol·g^-1·h^-1,respectively.