MOF-Based In Situ Derived Single-atom Catalysts And Their Catalytic Applications

Developing sustainable and efficient energy conversion technologies is one of the global themes facing the humanity.Exploring and developing high-performance catalysts that are versatile and inexpensive are important to address global issues such as energy crisis and environmental pollution.In practical industrial catalysis applications,more than 80%of the reaction process is accelerated by catalysts to increase production efficiency.Metal particles with different sizes exhibit different catalytic behaviors for different catalytic reactions.This is associated with the particle size,morphology,interaction between metal with support,and interaction between metal with reactants.Studies have shown that when the size of the catalyst is reduced from the nanoscale to the sub-nanometer scale and down to a single atom with atomic size,it generally exhibits high activity,high selectivity and high stability.Single-atom catalysts realize maximum exposure of atoms,thereby increase the number of active sites.The unsaturated coordination environments,quantum size effects,and enhanced metal-support interactions increase the intrinsic activity of the active sites.The above two aspects synergistically lead to the uniqueness of the single-atom catalyst.In addition,highly dispersed of metal active sites can maximize the metal utilization,which not only improves catalytic activity but also greatly reduces catalyst costs.Although the single-atom catalyst is still in the developing stage,its outstanding properties endow it with great potential for catalytic applications.Single atom catalysts are prone to agglomerate due to their high specific surface energy,which presents many challenges in material preparation.How to design and synthesize the new highly-active single-atom catalysts with a clear active center and high metal loading has gradually attracted wide attentions of researchers.With the development of advanced characterization techniques and the help of computational models,single-atom catalysts provide an opportunity to elucidate the relationship between catalyst structure and performance at atomic scale.By precisely controlling the structure of single-atom catalysts,it provides potential for designing non-precious metal catalysts with comparable catalytic activity relative to noble metals.Meanwhile,the single-atom catalysts serve as the bridge between heterogeneous catalysis and homogeneous catalysis,and provide great possibilities for overcoming the difficulties encountered in homogeneous catalysis.In just few years,single-atom catalysis has quickly become a research front in the field of catalysis,bringing people into a new era of heterogeneous catalysis and homogeneous catalysis.Herein,this paper aims to develop the more reasonable and controllable method for the preparation of single atom catalysts,to achieve precise regulation of metal-organic framework(MOF)-derived atom-scale catalysts,and to explore single atoms in combination with catalytic reaction results.The structure-activity correlation mechanism of the catalyst mainly includes the following aspects:1.Synthesis of ligand-assisted MOF-derived Ru single-atom catalysts and their application in selective hydrogenation of Quinoline.Here we report the precise control of isolated single ruthenium site supported on nitrogen-doped porous carbon(Ru SAs/N-C)through a coordination-assisted strategy.This synthesis methods based on the utilization of strong coordination between Ru3+ and the free amine groups at the skeleton of UiO-66-NH2,the amino-functionalized Zr-terephthalate metal-organic framework(MOF),which plays a critical role to access the atomically isolated dispersion of Ru sites after the following pyrolysis process.Without the assistant and anchoring effect of amino groups,the Ru precursor is prone to aggregate during the pyrolysis process at high temperature,finally forming Ru clusters encapsulated by the UiO-66(Zr-terephthalate MOF).The atomic dispersion of Ru on N-doped carbon and the specific Ru-N bond for anchoring the Ru sites can be verified by the spherical aberration correction electron microscopy and x-ray absorption fine structure(XAFS)measurements.What is more important,this single Ru sites with single-mind coordination gesture can serve as a semi-homogeneous catalyst to effectively catalyze chemoselective hydrogenation of functionalized quinolones.2.Synthesis of 2D MOF-derived Co single-atom catalysts and their application in efficient O-silylation of alcohols with silanesWe report a viable strategy for accessing stable Co single sites on ultrathin two-dimensional nitrogen-doped carbon(Co SAs/2D N-C)derived from the 2D metal-organic frameworks(MOF).This strategy is based on the pyrolysis of 2D bimetallic Zn/Co metal-organic frameworks by selectively removing Zn atoms to afford Co-based 2D MOF skeleton.The dopant of volatile Zn nodes extend the spatial distance of adjacent Co ions and prevent their aggregation during high-temperature pyrolysis,forming well-defined Co-N4 sites distributing on the 2D substrates.The atomic dispersion of Co atoms can be distinguished by spherical aberration correction electron microscopy and X-ray absorption fine structure measurements.More importantly,these ultrathin nanosheets functionalized by the isolated Co atoms possess more density of active sites and accessible surface compare with 3D nanostructures derived from 3D MOF,exhibiting exceptional catalytic performance in the oxidation of silanes under mild condition.