In-situ Growth Of Hydrotalcite With Manganese Based Metal Organic Frameworks And Their Application In Photocatalytic CO₂ Reduction

Excessive emissions of carbon dioxide（CO₂）seriously affect the balance of carbon cycle in nature,leading to a variety of serious environmental problems,especially the greenhouse effect.Therefore,the conversion of CO₂ into high value-added hydrocarbons and fuels is one of the important topics in basic research.At present,using clean and renewable solar energy to drive CO₂ conversion is a promising method.Layered double hydroxides（LDHs）have typical two-dimensional nanostructures.Due to their wide range of photoresponse,fast electron transfer channels and abundant metal active species,they have attracted extensive attention in photocatalytic CO₂ reduction.However,the serious self accumulation of LDHs leads to the burial of a large number of metal active species,which affects the catalytic performance of hydrotalcite materials.In recent years,it has been suggested that LDHs be immobilized onto metal organic frameworks（MOFs）,zeolite imidazole framework（ZIFs）,metal foam and other templates to solve the above problems.Among them,MOFs can provide abundant and evenly distributed metal centers,and it has become an attractive research direction to construct LDHs composites using MOFs as self sacrificial templates.Therefore,in this paper,the MOFs template is used to precisely control the partial epitaxial transformation to construct the hierarchical structure MOFs@LDHs compound material.It shows excellent performance in photocatalytic CO₂ reduction.The structure-activity relationship between its structure and catalytic performance was explored by various characterization methods.1.MIL-100@NiMn-LDH highly selective photoreduction CO₂ to CH₄ under irradiation above 500 nm.Firstly,the manganese based metal organic framework MIL-100（Mn）was synthesized by hydrothermal method with trimesic acid as ligand and manganese nitrate as metal salt.Then the hierarchical structure MIL-100@NiMn-LDH was successfully prepared by partial etching method using MIL-100（Mn）as template,and the ultrathin NiMn-LDH nanosheets were in-situ grown on the surface of MIL-100（Mn）.Morphology and chemical structure of MIL-100@NiMn-LDH were characterized by SEM,HRTEM,XRD,XPS and other methods.The experimental results show that MIL-100@NiMn-LDH still retains the regular octahedral structure of MIL-100（Mn）,and the staggered growth of NiMn-LDH is tightly anchored on its surface,with a thickness of about 1 nm.MIL-100@NiMn-LDH shows excellent performance of high selectivity for CH₄ production in photocatalytic CO₂ reduction reaction.The selectivity of CH₄ is as high as 88.8%,and the selectivity of by-product H2 is inhibited to 1.8%under the wavelength of λ>500 nm.That is because the ultra-thin LDHs nanosheets grown on the MIL-100（Mn）outer surface can expose abundant surface active sites and promote the adsorption and activation of CO₂.In addition,the in-situ grown NiMn-LDH ultrathin nanosheets are rich in metal and oxygen defects,which can raise efficiency of photogenerated electron holes segregation and migration while changing the energy band structure.The catalyst MIL-100@NiMn-LDH showed good structural stability after five cycles.2.MIL-100@CoMn-LDH highly yied photoreduction CO₂ to syngas.Because of its unique electronic structure,metal Co shows weak adsorption on the intermediate products of CO₂ reduction process,and can selectively generate CO.Therefore,in order to efficiently prepare syngas,active metal Co was introduced to successfully construct the hierarchical structure MIL-100@CoMn-LDH.The basic morphology and crystal structure of MIL-100@CoMn-LDH were analyzed by SEM and XRD.The microstructure and chemical composition of MIL-100@CoMn-LDH were analyzed by HRTEM and XPS.It was applied to the photocatalytic reduction of CO₂ to syngas.The yield of CO is as high as 14.02 μmol h^-1 when the light wavelength is more than 400 nm,and the ratio of CO to H2 is 1:1.The tightly combined CoMn-LDH and MIL-100（Mn）can effectively improve the transport and separation efficiency of photogenerated carriers,thus achieving high conversion of CO₂.In addition,different metal elements as active sites can selectively produce different products in the photocatalytic reduction of CO₂.In the cycle experiment,the conversion of CO₂ did not decrease significantly,which proved that the composite had good catalytic stability.