Structural Regulation And Photocatalytic Performance Of MOFs-Derived 3D Hierarchically Porous Materials

Over the past few decades,the fast-growing energy demands and environmental pollution have aroused people's awareness of the exploration of renewable and sustainable energy sources.Solar-driven photocatalytic conversion of H₂O or CO₂ to yield H₂,CO,CH₄ and other clean energy fuels have been considered as a promising approach to meet the challenge of energy and environmental crisis.However,the current photocatalytic reaction systems still suffer from low reaction efficiency due to the high stability of the substrate molecules(for example,the C=O bond energy in CO₂ is ca.750 k J mol^-1)and the complex photocatalytic reaction process.In this regard,it is of urgent need to develop highly active and selective photocatalytic systems to effectively convert solar energy into chemical fuels.Metal-organic frameworks(MOFs),are a class of porous crystalline materials assembled by metal ions/clusters and organic ligands.Recently,MOFs have been widely used as precursors to prepare metal or carbon-based porous materials with unique morphology thanks to their structural diversity and controllability,as well as the high dispersion of metal nodes.On the basis of these premises,this thesis aims to develop a series of novel and highly efficient MOFs-based catalysts for photocatalytic conversions of CO₂ or H₂O.The relationship between the structure and photocatalytic performance of the catalysts,and the reaction mechanisms have been investigated in detail.The main contents and the experimental findings are as follows.A self-templated strategy was developed for the preparation of carbon-doped Cu O hollow spheres(C-Cu O HSs)through thermal transformation of a hierarchical MOF.The hierarchical Cu-MOFs not only act as template to form interior voids during the thermal transformation,but also serve as precursors to dope C atoms into the Cu O lattice.The as-synthesized C-Cu O HSs exhibit remarkable photocatalytic performance with a H₂ evolution rate of 67.3 mmol g^-1h^-1and apparent quantum efficiency of 25.3%in the present of eosin-Y photosensitizer.The good catalytic performance of C-Cu O HSs may be attributed to the hierarchical porous structure and modulated electronic structure of Cu O by C-doping with well exposed reactive sites,high water adsorption capability,and low water reduction reaction barrier.Heteroatoms doped Co P hierarchical double-shelled nanocages(HDSNC)were fabricated via a MOF-templated approach for highly efficient visible-light-driven CO₂ reduction.The unique hierarchical double-shelled hollow architectures can greatly shorten the charge transfer distances and also expose abundant reactive sites.Moreover,transition metal atoms doping is able to reduce the CO₂ activation energy barrier through stabilizing the*COOH intermediates and promote the CO desorption by destabilizing the CO*adduct.As expected,the Fe-CoP HDSNC achieves an unprecedented catalytic efficiency in visible-light-driven CO₂ reduction with an up to 3.25%apparent quantum yield and 90.3%CO selectivity.More importantly,the Fe-CoP HDSNC is also highly effective under diluted CO₂ atmosphere,suggesting the practicability of the present photocatalytic system.In order to further improve the photogenerated carrier transport efficiency,we report a novel topological transformation strategy for the fabrication of 2D ultrathin nanosheet assembled multi-shelled catalysts by using multilayer core-shell ZIFs as the self-template.The synthesis process mainly involves the preparation of multilayer core-shell ZIFs precursor and the in situ solvothermal treatment.Beneficial from the structural virtues of MOF precursors,the shell number,and cavity size as well as the composition of hierarchical multishell materials can be precisely controlled.The two-dimensional ultrathin nanosheet and hollow structure endow the hierarchical multi-shell materials with abundant active sites,enhanced photogenerated carrier transport efficiency and light absorption ability.As a result,the hierarchical multi-shell materials exhibit excellent photocatalytic performance in CO₂ reduction reaction.Moreover,the catalytic activity enhances increase significantly with an increase in the number of shell.Under the irradiation of visible light,the as-prepared nanosheet assembled four-shell hollow Zn Co-OH achieves an unprecedented catalytic efficiency with a CO production rate up to 134.3?mol h^-1.Achieving highly efficient photocatalytic CO₂ conversion,especially without the assistance of sacrifice reagent or extra alkaline additives,remains a critical issue.We developed a novel in-situ transformation strategy for the fabrication of 3D ordered macroporous N-doped carbon supported Cd S quantum dots(3DOM Cd SQD/NC)for efficient photocatalytic CO₂reduction.The as-obtained 3DOM Cd SQD/NC exhibits excellent activity and selectivity in the photocatalytic CO₂ reduction coupled with amines oxidation under the irradiation of visible light,without the addition of any sacrificial agents and alkaline additives.Notably,such a CO production rate is ca.20 times higher than that of bulky Cd S and superior to that of most of the state-of-the-art photocatalytic systems even with the assistance of sacrificial agents.Mechanistic studies reveal that the 3DOM NC matrix serves as efficient electron mediator to facilitate the photon-generated electron transfer.Theoretical calculations show that the highly dispersed Cd S QDs on the NC skeleton could significantly promote the adsorption of substrate molecules and depress the CO₂ activation energy barriers through stabilizing the*COOH intermediate,thus greatly contributing to the achieved high activity.