Study On Preparation And Catalytic Performance Of Metal-organic Framework Derived High Dispersion Metal Catalysts

Man-made excessive emissions of carbon dioxide（CO₂）from fuel combustion have caused energy and environmental problems.In recent years,CO₂conversion technology is a very effective to alleviate the above-mentioned problems which include electrochemistry,biochemistry,photoelectrochemistry,and photocatalysis.Among them,photocatalytic and electrocatalytic CO₂reduction are a promising method which have the advantages of economy,renewable,clean,and safety.They are seen as a potential way to achieve a net reduction in CO₂and produce value-added chemicals and fuels when combined with renewable energy sources.However,CO₂is very inert and not easy to convert,so it is necessary to develop a highly active catalyst to activate and convert it.Highly dispersed supported metal catalysts usually exhibit high catalytic activity due to their maximum utilization of active metal sites.Metal organic frameworks（MOFs）are a new type of crystalline porous materials,whose metal atoms can be separated at the atomic level,and are one of the most promising candidates for the preparation of highly dispersed metal catalysts.However,due to the easy aggregation of metal atoms,the synthesis of highly dispersed supported metal catalysts based on MOFs still faces huge challenges.Therefore,it is necessary to develop suitable substrates to stabilize metal atoms.Graphitized carbon nitride has attracted wide attention because of its abundant nitrogen atoms and can be used to stabilize the substrate of metal atoms.In this thesis,MOFs and urea were used as precursors to obtain highly dispersed supported metal catalysts for photocatalytic and electrocatalytic CO₂reduction by calcination at high temperature.（1）A series of highly dispersed supported metal catalysts（named X-Co-C₃N₄）with Co anchored on g-C₃N₄were prepared by one-step heat treatment of cobalt-based MOFs and urea in air.Due to the highly dispersed and exposed limited sites,as well as good light trapping and abundant mesopores,the 25-Co-C₃N₄exhibits the best performance of photocatalytic reduction of CO₂under visible light irradiation.The rate of photocatalytic reduction of CO₂to produce CO is 395.4μmol·g^-1·h^-1,far exceeding pure g-C₃N₄(4.9μmol·g^-1·h^-1).In addition,through this simple synthesis strategy,highly dispersed metal catalysts with Fe and Mn anchored on g-C₃N₄（named25-Fe-C₃N₄and 25-Mn-C₃N₄,respectively）were obtained,respectively,indicating the universality of the method.（2）Carbon and nitrogen materials with Ni metal particles loaded on carbon black were prepared by high temperature pyrolysis of Ni-MOF and urea in nitrogen,and used for electrocatalytic reduction of CO₂.At electric potential of-0.67 V～-1.07 V（vs RHE）,the Faraday efficiency of CO was more than 90%;at electric potential of-0.87 V（vs RHE）,Faraday efficiency of CO was up to 95%.The maximum CO partial current density at electric potential of-1.07 V（vs RHE）was-26.7 mA·cm^-2.The excellent performance of electrocatalytic reduction of CO₂was investigated in the mobile electrolytic cell structure.The selectivity of CO was as high as 98%at the electric potential of-0.5 V,-0.6 V and-0.7 V（vs RHE）,and the current densities were-102 mA·cm^-2,-138mA·cm^-2,-163 mA·cm^-2,respectively.