Rational Design Of Single Atom Catalysts And Their Applications In Heterogenization Of Homogeneous Catalytic Reactions And Enzyme-like

Catalysis plays a very important role in the progress of human civilization and the development of the world economy.It can transform raw materials into high value-added chemical products and fuels in an efficient,green and economical way.Therefore,it is called the heart of modern industry',as widely used in various fields such as energy,chemical industry,food,medicine,electronics and so on.At present,more than 90%of chemical production processes in the world are dependent on catalysis.Catalysis science can mainly be divided into three fields:heterogeneous catalysis,homogeneous catalysis and enzyme catalysis.Heterogeneous catalysts are widely used in various fields with their high stability and easy separation,but the obvious disadvantage is their complex components,unclear structures,and low atom utilization.Heterogeneous catalysts prosess clear structure and high activity,but high cost and difficult separation greatly restrict their application.Enzyme catalysts have high specific selectivity and high activity,but the enzyme is extremely unstable and easily inactivated.Therefore,the rational design of catalysts with high efficiency,stability,high selectivity,low cost and easy separation is the key of the development of catalytic science and technology.The single-atom catalyst combines the advantages of homogeneous catalysts,heterogeneous catalysts and enzyme catalysts with clear active center,100%atom utilization,stability,easy separation and recovery,and high selectivity.It is the most promising superstar subject to establish a unified theory of catalysis with building the bridge between homogeneous catalysis and enzyme catalysis.We have strongly proved this understanding from an view of experimental point.The prepared heterogeneous single palladium atom catalyst realizes the coupling hydrogenation reaction for the first time,which only occurred in the field of homogeneous catalysis.In addition,the atomically dispersed cerium atom exhibits excellent performance in fuel cells.The active centers of single-atom catalysts and biological enzymes are similar.We have grasped this feature and experimentally proved that single atom catalysts can produce biological enzyme-like activity,which have attracted lots of researches in the field of single atom enzymes.Based on this,the application of single atom catalysts in the fields of organic catalysis,electrocatalysis,and enzyme catalysis are deeply studied and discussed.The main contents are summarized as follows:1.A single palladium site catalyst as a bridge for converting homogeneous to heterogeneous in dimerization of terminal aryl acetylenes.We utilize the unsaturated dangling nitrogen bonds on the surface of ZIF-67 to stabilize the Pd single atom with the forming of Pd-N bond.Because of the Pd-N coordination is more stable than Co-N coordination in the presence of water,the internal ZIF-67 core was gradually dissolved after the phase change process under hydrothermal conditions,leaving a stable hollow residue(Pd1/H-ZIF).XRD,HAADF-STEM and XAS results show the atomic dispersion of PdN2O2 sites in Pd1/H-ZIF,and the valence of Pd is between 0 and+2.For the reaction of phenylacetylene dimerization to conjugated diene can only be catalyzed by a homogeneous catalyst.Pd1/H-ZIF exhibits excellent catalytic selectivity and activity,which successfully achieves the heterogenization of homogeneous catalytic reactions.It is a stepwise reaction.Firstly,C-C bonds are coupled to generate 1,3-enyne.The final product conjugated diene can be obtained through a hydrogenation step,and the formation of 1,3-enyne is the rate-determining step.This work not only provides the possibility of the heterogenization of homogeneous catalytic reactions,but also provides a successful case for understanding the structure-activity relationship of catalysts at the atomic scale.2.Single atomic cerium sites with high coordination number for efficient oxygen reduction in proton-exchange membrane fuel cell.We construct the rare-earth single cerium atoms doped metal-organic frameworks with hierarchically macro-meso-microporous structure(Ce SAS/HPNC)through a continuous three-step synthesis strategy involving doping,acid leaching and gas-migration process.In the first step of synthesis,Ce-doped ZIF-8 precursors with SiO2 ball embedded were prepared.After carbonization and acid leaching,SiO2 are removed,leaving the sample with a hierarchically macro-meso-microporous structure.In order to obtain the increased density of CeN4O6 active sites,CeO2 was introduced in the gas-migration process.XRD,XAS,HAADF-STEM and EELS confirm the atomic dispersion of Ce-N4/O6 sites.The well-defined Ce SAS/HPNC with 3D hierarchical ordered porous architecture can be attributed to enhancing the mass transport and the exposing more active sites.Ce SAS/HPNC exhibits the highest power density of 0.475 W cm-2 and 0.525 W cm-2 under 1.0 bar and 2.0 bar H2/O2 in fuel cell test,respectively.Additionally,under the standard H2/O2 pressures of 1 bar,the current densities of Ce SAS/HPNC is 0.473 A cm-2 at 0.6 V,which is comparable to many reported Pt-free catalysts in MEAs.This work presents a new approach for preparation of the rare-earth SACs with increased density,additionally,providing a great promise to develop high active PGM-free catalysts in future PEMFC technologies.3.Unraveling the enzyme-like activity of heterogeneous single atom catalyst.We used ZIF-8 as a support to synthesis highly dispersed single Fe atom catalyst(Fe SAEs)by a gas phase migration strategy.XRD,HAADF-STEM and XAS confirm the atomic dispersion of Fe sites,possessing the similar FeN4 sites as natural heme-containing enzymes.EPR experiments show that Fe SAEs catalyze the decomposition of hydrogen peroxide to generate highly oxidative hydroxyl radicals(·OH)during the peroxidase reaction.Operando XAFS and DFT indicate that the generation of intermediate active species Fe=O/O=Fe=O,which involving the the activation of oxygen on a single FeN4 site.In addition,Fe SAEs can also be used as a heterogeneous Fenton-like catalyst to degrade carcinogenic organic pollutants in wastewater.What's more,its catalytic performance is still very stable after 10 cycles.